!HYDROLYTIC!DEGRADATION!OF!NEAT!AND!MODIFIED!POLY(LACTIC!ACID)! WITH!NANOPARTICLES!AND!CHAIN!EXTENDERS!BY!WATER8ETHANOL! SOLUTIONS! ! By! ! Fabiola!Maria!Iñiguez!Franco! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! 2018! A!DISSERTATION! Submitted!to! Michigan!State!University!! in!partial!fulfillment!of!the!requirements! for!the!degree!of! Packaging!–!Doctor!of!Philosophy! ! ! ABSTRACT! ! ! !By! ! ! HYDROLYTIC!DEGRADATION!OF!NEAT!AND!MODIFIED!POLY(LACTIC!ACID)!WITH! NANOPARTICLES!AND!CHAIN!EXTENDERS!BY!WATER8ETHANOL!SOLUTIONS! Fabiola!Maria!Iñiguez!Franco! Poly(lactic!acid)!(PLA),!a!biobased!polymer,!has!been!used!for!applications!in!the! medical,! agriculture! and! food! packaging! industries.! During! its! life! cycle,! PLA! can! be! exposed! to! different! environments! including! water! and! alcohol! solutions! promoting! its! hydrolytic!degradation.!Depending!on!the!application,!degradation!can!be!an!advantage! or!a!disadvantage.!Hydrolysis!can!be!used!as!an!advantage!for!converting!PLA!back!to! its! monomer! via! chemical! recycling.! In! the! case! of! long! service! life! applications,! hydrolysis!can!be!a!disadvantage!because!controlling!the!resistance!to!degradation!is! essential.! Incorporation! of! nanoparticles! and! chain! extenders! can! be! used! to! expand! PLA! applications.! This! work! aimed! to! evaluate! the! hydrolytic! degradation! of! PLA! by! water8ethanol! solutions,! to! use! hydrolysis! in! water8ethanol! solutions! for! facilitating! chemical! recycling! of! PLA,! and! to! understand! the! effect! of! adding! nanoparticles! and! chain!extenders!on!the!hydrolysis!of!PLA.!! The! parameters! and! factors! affecting! the! hydrolysis! of! PLA! in! water8ethanol! solutions! were! studied.! Experimental! studies! of! the! concurrent! induced! crystallization! and!hydrolytic!degradation!of!PLA!films!were!performed!at!40!°C!in!contact!with!water,! 50%!ethanol,!and!95%!ethanol.!PLA!films!in!contact!with!50%!ethanol!showed!faster! degradation!due!to!a!competitive!effect!between!swelling!by!the!presence!of!ethanol,! and!the!hydrolysis!by!water!sorption!molecules!starting!the!degradation!reactions.! ! ! The!chemical!recycling!of!PLA!induced!by!hydrolysis!and!swelling!of!50%ethanol! solutions! at! moderate! temperatures! were! also! assessed.! The! kinetic! parameters! governing! the! chemical! recycling,! rate! constant,! and! activation! energy! (Ea)! were! quantified.! Analysis! of! the! molecular! weight! distribution! of! PLA! was! performed! to! estimate! the! rate! of! hydrolysis.! A! reparameterization! of! the! Arrhenius! equation! was! proposed! for! a! better! estimation! of! the! Ea! with! a! low! correlation! coefficient! between! parameters.!The!Ea!values!obtained!were!104.74!and!96.27!kJ/mol!for!water!and!50%! ethanol!solution,!respectively.! The! effect! of! adding! organomodified! montmorillonite! (OMMT)! on! the! hydrolytic! degradation! of! PLA! in! water,! 50%! ethanol,! and! 95%! ethanol! was! quantified.! The! change!in!molecular!weight,!crystallinity,!release!of!lactic!acid,!and!release!of!surfactant! were!monitored.!PLA8nanocomposite!film!exposed!to!50%!ethanol!had!a!faster!rate!of! degradation.! No! difference! in! the! rate! of! degradation! was! found! with! OMMT! incorporation!into!the!PLA.!The!clay!released!from!PLA8OMMT!films!during!hydrolysis!in! 50%!ethanol!was!0.58%!wt.!of!the!initial!amount!of!nanoclay!in!the!PLA!film!at!90!days.! Finally,!the!effect!of!the!addition!of!an!epoxy8acrylic!additive!as!a!chain!extender! on! the! hydrolysis! of! PLA! was! studied.! Addition! of! the! chain! extender! increased! the! dynamic!moduli!of!PLA!and!the!onset!decomposition!temperature.!These!results!were! attributed! to! the! branched! structure! of! PLA! resulting! from! the! chain! extender.! The! hydrolysis!of!PLA!in!50%!ethanol!solution!was!decreased!by!the!presence!of!the!chain! extender,!changing!the!degradation!mechanism!from!bulk!to!surface!erosion.! ! ! ! ! Copyright!by! FABIOLA!MARIA!IÑIGUEZ!FRANCO! 2018! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! This!dissertation!is!dedicated!to!my!beloved!parents!and!siblings! ! ! ! ! ! ! ! ! ! ! ! v! ! ACKNOWLEDGMENTS! ! I! would! like! to! thank! to! the! Mexican! Council! of! Science! and! Technology! (CONACyT)!for!the!scholarship!assistant!during!my!studies!at!Michigan!State!University,! and!also!to!the!Mexican!Secretariat!of!Public!Education!(SEP)!and!the!Government!of! Mexico!for!providing!financial!support!throughout!my!Ph.D.! I!would!like!to!express!my!sincere!gratitude!to!my!advisor!Dr.!Rafael!Auras!for! continuous!support!during!my!Ph.D.!studies,!for!his!motivation,!patience,!and!immense! knowledge.!I!could!not!have!imagined!having!a!better!advisor!and!mentor.!I!am!really! grateful!for!being!accepted!as!part!of!his!group.!His!guidance!helped!me!in!my!research,! and!he!provided!enormous!support!for!my!personal!and!professional!growth.! I!would!like!to!extend!my!gratitude!to!my!thesis!committee.!To!Dr.!Maria!Rubino! for! her! guidance! and! support.! To! Dr.! Kirk! Dolan! for! his! enormous! knowledge! and! timeless! support! in! all! of! the! parameter! estimation! topics.! Taking! his! class! was! very! valuable!for!my!thesis.!My!sincere!thanks!to!Dr.!Susan!Selke!for!her!help!and!insightful! comments!for!improving!this!research.!My!deepest!thanks!to!Dr.!Herlinda!Soto8Valdez! for!continuing!to!be!a!part!of!this!journey,!her!guidance!as!my!master’s!advisor,!brought! me!to!Michigan!State!University.! My!sincere!thanks!also!go!to!Dr.!Daniel!Holmes!for!his!help!with!the!NMR!studies,! for! his! patient! explanations! with! this! regard! and! his! support! in! establishing! methodologies! for! this! research.! I! am! grateful! to! Dr.! Gary! Burgess! for! his! time! and! contribution! in! this! thesis.! Also,! I! gratefully! acknowledge! the! help! of! the! Mass! Spectrometry! &! Metabolomics! Core! Facility! at! MSU! for! their! assistance! with! the! ! vi! ! developments! of! the! chromatography! methods.! I! am! also! thankful! to! the! faculty! and! staff!of!the!School!of!Packaging!at!MSU.! I! would! like! to! extend! my! gratitude! to! all! my! friends! Hayati,! Edgar,! Jiyon,! Jin,! Pom,! Kim,! Rebeca,! Abraham,! Javi,! Felipe! and! Marco! for! all! the! great! times! together! and!for!their!support!and!for!being!part!of!my!family!far!from!my!hometown.!! Thanks!to!Matthew!Faber!for!his!love!and!emotional!support!that!he!has!provided! in!the!last!years,!thanks!for!making!this!journey!better.! Thanks! to! my! amazing! parents,! Maria! de! Jesus! Franco! Gómez! and! Gilberto! Iñiguez!Covarrubias,!for!being!an!inspiration!in!my!life,!for!always!being!there!for!me,! and!for!their!unconditional!support!and!love.!Thanks!to!my!siblings!Pau,!Jouss!and!Gill! my!best!friends,!for!their!encourage!and!love.!I!love!my!family!I!could!not!do!this!without! them.! ! ! ! vii! ! TABLE!OF!CONTENTS! ! LIST!OF!TABLES!..........................................................................................................!xii! ! LIST!OF!FIGURES!........................................................................................................!xiv! ! KEY!TO!ABBREVIATIONS!...........................................................................................!xx! ! CHAPTER!1!.....................................................................................................................!1! Introduction!.....................................................................................................................!1! 1.0! Background!and!motivations!..........................................................................!1! 1.1! Overall!goal!and!objectives!............................................................................!7! 1.2! Dissertation!overview!.....................................................................................!8! REFERENCES!...............................................................................................................!10! CHAPTER!2!...................................................................................................................!18! Literature!Review!.........................................................................................................!18! 2.0! Introduction:!Poly(lactic!acid)!–!PLA!.............................................................!18! 2.1! Hydrolytic!degradation!of!PLA!......................................................................!21! 2.1.1! Mechanism!...............................................................................................!22! 2.1.2! Parameters!controlling!hydrolytic!degradation!.........................................!24! 2.1.2.1! pH!......................................................................................................!24! 2.1.2.2! Temperature!......................................................................................!27! 2.1.2.3! PLA!stereoisomers!............................................................................!30! 2.1.2.4! Molecular!weight!................................................................................!30! 2.1.3! Structural!changes!...................................................................................!33! 2.1.3.1! Crystallinity!........................................................................................!33! 2.1.3.2! Swelling!.............................................................................................!40! 2.2! Chemical!recycling:!Application!of!hydrolytic!degradation!...........................!43! 2.2.1! Water!........................................................................................................!44! 2.2.2! Alcohols!....................................................................................................!45! 2.2.3! Other!organic!solvents!.............................................................................!45! 2.3! Modification!of!PLA!......................................................................................!47! 2.3.1! Nanoparticles:!organomodified!montmorillonite!(OMMT)!.........................!47! 2.3.1.1! Characteristics!...................................................................................!48! 2.3.1.1.1!Structure!and!properties!................................................................!48! 2.3.1.1.2!Surface!modification!......................................................................!49! 2.3.1.1.3!Morphology!....................................................................................!52! 2.3.1.1.4!Benefits!of!PLA8nanocomposite!....................................................!54! 2.3.1.2! Hydrolytic!degradation!.......................................................................!56! 2.3.1.3! Chemical!compound!release!.............................................................!59! 2.3.2! Chain!extenders!.......................................................................................!62! REFERENCES!....................................................................................................!67! ! ! viii! ! CHAPTER!3!...................................................................................................................!81! Concurrent!Solvent!Induced!Crystallization!and!Hydrolytic!Degradation!of!PLA!by! WaterQEthanol!Solutions!..............................................................................................!81! 3.0! Abstract!........................................................................................................!82! 3.1! Introduction!...................................................................................................!82! 3.2! Materials!and!methods!.................................................................................!84! 3.2.1! Chemicals!and!Reagents!.........................................................................!84! 3.2.2! Production!of!PLA!film!..............................................................................!85! 3.2.3! Storage!experiments!................................................................................!85! 3.2.4! PLA!molecular!weight!..............................................................................!85! 3.2.5! Water!and!ethanol!sorption!......................................................................!86! 3.2.6! Differential!scanning!calorimetry!(DSC)!...................................................!87! 3.2.7! X8ray!diffraction!study!(XRD)!...................................................................!88! 3.2.8! Dynamic!mechanical!analysis!(DMA)!.......................................................!88! 3.2.9! Lactic!acid!release!...................................................................................!88! 3.3! Results!and!discussion!.................................................................................!89! 3.3.1! Molecular!weight!......................................................................................!90! 3.3.2! Water!and!ethanol!sorption!......................................................................!92! 3.3.3! Crystallinity!...............................................................................................!99! 3.3.4! Lactic!acid!release!.................................................................................!107! 3.4! Conclusions!................................................................................................!110! 3.5! Acknowledgments!......................................................................................!111! APPENDICES!..............................................................................................................!112! APPENDIX!3A:!NMR!Analysis!..................................................................................!113! APPENDIX!3B:!Optical!images!of!PLA!samples!during!hydrolytic!degradation!.......!117! APPENDIX!3C:!Diffusion!of!H2O!and!D2O!vapor!in!PLA!film!...................................!118! APPENDIX!3D:!Sorption!isotherms!for!H2O!and!D2O!vapor!in!PLA!film!..................!119! APPENDIX!3E:!Ethanol!sorption!into!PLA!films!.......................................................!120! APPENDIX!3F:!Expansion!of!PLA!in!contact!with!different!volume!fraction!of!ethanol!..!! ! !..........................................................................................................................!121! APPENDIX!3G:!Model!for!change!in!molecular!weight,!including!polymer!expansion!...!! ! !..........................................................................................................................!122! APPENDIX!3H:!Molecular!weight!distribution!(MWD)!of!PLA!films!during!hydrolytic! degradation!...............................................................................................................!124! APPENDIX!3I:!Model!for!LA!release!........................................................................!125! APPENDIX!3J:!Theoretical!diffusion!coefficients!of!LA8oligomers!...........................!127! APPENDIX!3K:!Initial!amount!of!lactic!acid!oligomers!.............................................!132! REFERENCES!.............................................................................................................!133! CHAPTER!4!.................................................................................................................!141! Chemical!Recycling!of!Poly(Lactic!Acid)!by!WaterQEthanol!Solutions!.................!141! 4.0! Abstract!......................................................................................................!142! 4.1! Introduction!.................................................................................................!142! 4.2! Material!and!methods!.................................................................................!145! 4.2.1! Chemicals!and!reagents!........................................................................!145! 4.2.2! Film!production!.......................................................................................!145! 4.2.3! Molecular!weight!....................................................................................!146! ! ix! ! 4.2.4! Nuclear!Magnetic!Resonance!(NMR)!....................................................!147! 4.2.5! Parameter!estimation:!Order!of!reaction!................................................!147! 4.2.6! Parameter!estimation:!Activation!energy!...............................................!148! 4.2.6.1! Step!1:!Re8parameterization!of!the!Arrhenius!equation!..................!148! 4.2.6.2! Step!2:!Scaled!sensitivity!coefficient!(X′)!.........................................!149! 4.2.6.2.1!Temperature!simulation!(Tsim)!.....................................................!150! 4.2.6.2.2/pH/simulation!(pHsim)!...................................................................!150! 4.2.6.3! Step!3:!Estimation!of!the!optimum!pHref!and!Tref!..............................!150! 4.3! Results!and!discussion!...............................................................................!151! 4.3.1! Hydrolysis!in!different!alcohol!solutions!.................................................!151! 4.3.2! Hydrolytic!degradation!of!PLA!in!50%!ethanol!.......................................!154! 4.3.3! Effect!of!temperature!on!the!hydrolytic!degradation!of!PLA!in!50%! ethanol:!Activation!energy.!................................................................................!163! 4.3.4! Hydrolytic!degradation!of!PLA!in!water!..................................................!169! 4.3.5! Effect!of!temperature!on!the!hydrolytic!degradation!of!PLA!in!water:! Activation!energy!...............................................................................................!174! 4.4! Conclusions!................................................................................................!177! 4.5! Acknowledgements!....................................................................................!177! APPENDICES!..............................................................................................................!178! APPENDIX!4A:!Rate!constants!and!order!of!reaction!for!PLA!films!at!70!°C!in!water8 alcohol!solutions!.......................................................................................................!179! APPENDIX!4B:!NMR!Analysis!..................................................................................!180! APPENDIX!4C:!Scale!Sensitivity!coefficient!of!hydrolytic!degradation!of!PLA!in!50%! ethanol!......................................................................................................................!183! APPENDIX!4D:!Parameter!estimation!of!the!Ea!for!the!hydrolysis!of!PLA!in!50%! ethanol!......................................................................................................................!184! APPENDIX!4E:!Parameter!estimation!of!the!Ea!for!the!hydrolysis!of!PLA!in!water!..!193! APPENDIX!4F:!MWD!of!PLA!films!during!hydrolytic!degradation!when!in!contact!with! 50%!ethanol!at!80!°C!and!different!pH!.....................................................................!195! REFERENCES!.............................................................................................................!196! CHAPTER!5!.................................................................................................................!202! Effect!of!Nanoparticles!on!the!Hydrolytic!Degradation!of!PLAQNanocomposites!by! WaterQEthanol!Solutions!............................................................................................!202! 5.0! Abstract!......................................................................................................!203! 5.1! Introduction!.................................................................................................!203! 5.2! Materials!and!methods!...............................................................................!206! 5.2.1! Chemicals!and!reagents!........................................................................!206! 5.2.2! Production!of!films!..................................................................................!206! 5.2.3! Characterization!of!PLA8OMMT!film!......................................................!207! 5.2.4! Storage!experiments!..............................................................................!208! 5.2.5! Molecular!weight!....................................................................................!208! 5.2.6! Water!and!ethanol!sorption!....................................................................!210! 5.2.7! Dynamic!mechanical!analysis!(DMA)!.....................................................!210! 5.2.8! Differential!scanning!calorimetry!(DSC)!.................................................!211! 5.2.9! Lactic!acid!release!.................................................................................!211! 5.2.10!Surfactant!release!..................................................................................!211! ! x! ! 5.2.11!Nanoclay!release!...................................................................................!212! 5.3! Results!and!discussion!...............................................................................!212! 5.3.1! Characterization!of!PLA8OMMT!film!......................................................!213! 5.3.2! Molecular!weight!....................................................................................!215! 5.3.3! Water!and!ethanol!sorption!....................................................................!218! 5.3.4! Crystallinity!.............................................................................................!224! 5.3.5! Lactic!acid!release!.................................................................................!228! 5.3.6! Surfactant!release!..................................................................................!231! 5.3.7! Release!of!clay!.......................................................................................!234! 5.4! Conclusions!................................................................................................!238! 5.5! Acknowledgments!......................................................................................!239! APPENDICES!..............................................................................................................!240! APPENDIX!5A:!Composition!of!masterbatch!and!films!after!processing!................!241! APPENDIX!5B:!Schematic!representation!of!the!cells!for!storage!experiments!......!242! APPENDIX!5C:!Crystallinity!......................................................................................!243! REFERENCES!.............................................................................................................!248! CHAPTER!6!.................................................................................................................!254! Control!of!Hydrolytic!Degradation!of!Poly(Lactic!Acid)!by!Incorporation!of!Chain! Extender:!from!Bulk!to!Surface!Erosion!..................................................................!254! 6.0! Abstract!......................................................................................................!255! 6.1! Introduction!.................................................................................................!255! 6.2! Materials!and!methods!...............................................................................!257! 6.2.1! Chemicals!and!reagents!........................................................................!257! 6.2.2! Film!production!.......................................................................................!258! 6.2.3! Molecular!weight!....................................................................................!259! 6.2.4! Characterization!of!films!.........................................................................!259! 6.2.5! Hydrolytic!degradation!study!..................................................................!261! 6.2.6! Diffusion!of!water!vapor!.........................................................................!261! 6.3! Results!and!discussion!...............................................................................!262! 6.3.1! Processing!of!PLA!films!.........................................................................!262! 6.3.2! Characterization!.....................................................................................!264! 6.3.2.1! Rheological!behavior!.......................................................................!264! 6.3.2.2! Thermal!properties...........................................................................!268! 6.3.3! Hydrolytic!degradation!study!..................................................................!271! 6.4! Conclusions!................................................................................................!277! 6.5! Acknowledgements!....................................................................................!278! REFERENCES!.............................................................................................................!279! CHAPTER!7!.................................................................................................................!285! Overall!Conclusion!and!Recommended!Future!Work!............................................!285! 7.0! Overall!Conclusion!.....................................................................................!285! 7.1! Recommended!Future!Work!......................................................................!288! REFERENCES!.............................................................................................................!290! ! ! ! xi! ! LIST!OF!TABLES! ! Table! 2.1! Ea! values! reported! in! literature! for! the! hydrolytic! degradation! of! PLA! in! Table! 3.2!Diffusion!coefficients!(D)!and!amount!of!ethanol!at!equilibrium!(M∞)!in!PLA! Table! 2.3!Common!modified!montmorillonites!used!for!PLA8nanocomposites,!adapted! Table! 2.4! Comparison! of! material! properties! between! neat! PLA! and! PLA8OMMT! Table!3C.1!Diffusion!coefficient!(D)!of!H2O!and!D2O!vapor!in!PLA!film!at!75%!relative! Table!3J.1!Parameters!to!predict!the!diffusion!coefficient!of!LA!and!up!to!5!LA8mers!in! different!environment!at!different!temperature!range.!....................................................!29! Table!2.2!Diffraction!peaks!of!α,!β,!α'8form!crystals,!as!reported!by![54858].!.................!35! from!Souza!et!al.![100].!..................................................................................................!51! nanocomposites!with!different!amounts!of!OMMT,!adapted!from!Ray!et!al.![112].!........!55! Table!3.1!Rate!constants!for!PLA!films!at!40!°C!in!water8ethanol!solutions.!.................!92! films!at!40!°C!..................................................................................................................!95! Table!3A.1!1H8NMR!peaks!and!identification!of!ethanol!in!PLA.!.................................!114! humidity,!40!°C.!............................................................................................................!118! PLA.!..............................................................................................................................!130! temperatures!in!50%!ethanol!solution!and!water.!........................................................!162! 4.3)!for!the!hydrolytic!degradation!of!PLA!in!50%!ethanol.!..........................................!163! pH!temperatures!in!50%!ethanol!solution.!...................................................................!166! ......................................................................................................................................!167! Table!4.5!Final!estimated!parameter!values!at!the!optimum!Tref.!................................!168! alcohol!solutions.!..........................................................................................................!179! (Eq.!4.7)........................................................................................................................!187! Table! 4.4.!Final!estimated!parameters!values!at!the!optimum!pHref!=!7.697!(Eq.! 4.7). Table! 4A.1! Rate! constants! and! order! of! reaction! η! for! PLA! films! at! 70! °C! in! water8 Table!4D.1!Correlation!matrix!of!the!estimated!parameters!at!the!average!pHref/=!7.33! Table! 4.1! Order! of! reaction! (η)! and! rate! constants! for! PLA! films! at! different! Table!4.2!Correlation!matrix!of!the!estimated!parameters!using!Arrhenius!equation!(Eq.! Table!4.3!Order!of!reaction!(η)!and!rate!constants!for!PLA!films!at!80!°C!and!different! ! xii! ! Table! 4D.4! Correlation! matrix! of! the! estimated! parameters! at! the! optimum! Tref/ =! Table!4D.5!Correlation!matrix!of!the!estimated!parameters!at!the!average!Tref!=!62.5!°C! Table!4D.2!Correlation!matrix!of!the!estimated!parameters!at!the!optimum!pHref/=!7.697! Table!4D.3!Correlation!matrix!of!the!estimated!parameters!at!the!average!Tref/=!62.5!°C! Table! 4D.6! Correlation! matrix! of! the! estimated! parameters! at! the! optimum! Tref/ =! 57.688!°C!for!hydrolytic!degradation!of!PLA!in!50%!ethanol!solution!(Eq.!4.5).!..........!192! Table!4E.1!Correlation!matrix!of!the!estimated!parameters!at!the!average!Tref/=!75!°C! (Eq.!4.7)........................................................................................................................!188! (Eq.!4.6)........................................................................................................................!189! 56.538!°C!(Eq.!4.6).!.....................................................................................................!190! for!hydrolytic!degradation!of!PLA!in!50%!ethanol!solution!(Eq.!4.5).!...........................!191! for!hydrolytic!degradation!of!PLA!in!50%!ethanol!solution!(Eq.!4.5).!...........................!193! 75.931!°C!for!hydrolytic!degradation!of!PLA!in!water!(Eq.!4.5).!...................................!194! films!exposed!to!water8ethanol!solutions!at!40!°C.!......................................................!217! (v/v)!solution!from!DMA,!and!change!in!Tg!(ΔTg)!with!respect!to!non8immersed!films.!223! Table!5A.1!Composition!of!masterbatch!and!films!after!processing.!...........................!241! degradation!of!PLA8C!and!PLA8OMMT!films!in!50%!ethanol!at!40!°C.!........................!247! Table!6.1!Thermal!properties!of!the!second!heat!scan!of!PLA8C!and!PLA8J!films.!.....!271! 11,!and!D/of!water!vapor!at!40!°C,!70%!RH.!................................................................!275! Table! 4E.2! Correlation! matrix! of! the! estimated! parameters! at! the! optimum! Tref/ =! Table!6.2!Hydrolysis!rate!constants!for!PLA!films!immersed!in!50%!ethanol!at!80!°C,!pH! Table!5.1!Hydrolytic!degradation!rate!constants!for!PLA8C,!PLA8OMMT!and!PLA8QAC! Table!5.2!Tg!of!PLA8C!and!PLA8OMMT!films!submerged!in!different!fractions!of!ethanol! Table! 5C.1! Crystallinity! fraction! (Xc)! determined! from! XRD! profiles! during! hydrolytic! ! ! ! xiii! ! LIST!OF!FIGURES! ! Figure! 2.2! Chemical! structure! of! L(+),! D(8)! lactic! acid,! L8lactide,! D8lactide! and! Figure!2.4!Hydrolytic!chain!cleavage!mechanisms!of!PLA!in!alkaline!(a)!and!acidic!(b)! Figure! 2.1! Number! of! research! reports! published! since! 1990! based! on! a! Web! of! Science!search!using!keywords!“PLA”,!“PLLA”,!“PDLA”,!“polylactic!acid”,!“polylactide”,! Figure! 2.5!Change!in!Mn!during!hydrolytic!degradation!of!amorphous!PLLA!with!high! and!low!molecular!weights!(PLLA1!and!PLLA,!respectively),!and!different!ratios!of!D8 lactide! (PLLA2! [1.2%! D8lactide],! PLLA3! [0.2%! D8lactide],! P(LLA8DLA)! [77/23])! in! and!“poly(lactic!acid)”.!*!Data!until!03/01/18.!..................................................................!19! stereocomplex.!...............................................................................................................!21! Figure!2.3!Hydrolytic!degradation!mechanisms,!adapted!from!Tsuji![10].!.....................!23! media,!adapted!from!De!Jong!et!al.![23].!.......................................................................!25! phosphate8buffered!solutions!at!37!°C,!adapted!from!Saha!and!Tsuji![42,!43].!.............!32! phosphate8buffered!solutions!at!37!°C,!adapted!from!Çelikkaya!et!al.![44].!..................!33! MAF!and!RAF,!adapted!from!Sonchaeng!et!al.![64].!.....................................................!37! degradation,!adapted!from!Tsuji![10].!.............................................................................!38! Figure!2.9!Alcoholysis!of!PLA,!adapted!from!Sánchez!and!Collinson![86].!...................!45! Figure!2.10!Structure!of!2:1!layered!silicates,!adapted!from!Ray!and!Okamoto![96].!...!49! adapted!from!Raquez!et!al.![95].!....................................................................................!50! polymers,!adapted!from!Alexandre!and!Dubois![108].!...................................................!54! between!1!and!20,!adapted!from!Cailloux!et.al.![152]!....................................................!63! ........................................................................................................................................!64! Figure! 2.13! General! structure! of! Joncryl®! ADR,! where! R1!–! R5! are! H,! CH3,! a! higher! alkyl! group,! or! combination! of! theml! R6! is! an! alkyl! group,! and! x,! y! and! z! are! each! Figure! 2.11! Organomodification! of! layered! silicates! with! quaternary! ammonium,! Figure! 2.6! Hydrolytic! degradation! of! PDLLA! with! different! molecular! weights! in! Figure!2.7!Schematic!representation!of!crystalline!morphology!of!semi8crystalline!PLA:! crystalline!phase!corresponding!to!CF,!and!the!non8crystalline!phase!corresponding!to! Figure!2.8!Schematic!representation!of!PLA!structure!(a)!before!and!(b)!after!hydrolytic! Figure! 2.12! Types! of! nanocomposites! derived! from! interaction! between! clays! and! Figure!2.14!Reaction!of!Joncryl®!and!PLA!end!groups,!adapted!from!Najafi!et!al.![135]. ! xiv! ! Figure!3.1!Mn!as!a!function!of!time!during!hydrolytic!degradation!of!PLA!film!immersed! in!water,!50%!ethanol!and!95%!ethanol!at!40!°C.!Values!indicated!as!*!were!considered! Figure! 3.3! Rate! constant! for! hydrolytic! degradation! of! PLA! films! fitting! first! order! into! PLA! at! different! exposure! times! versus! volume! fraction! of! ethanol! in! solvent! solution!at!40!°C.!!D2O!and!ethanol!sorption!lines!were!obtained!from!experimental!data! Figure!3.4!Tg!of!PLA!when!immersed!in!different!volume!fractions!of!ethanol!at!40!°C! from!the!immersion!DMA!results.!Insert:!Tg!as!a!function!of!p!where!Tg!values!(*)!were! Figure! 3.2! D2O! sorption! into! PLA! films! in! contact! with! D2O,! 50%! ethanol! and! 95%! ethanol! at! 40! °C.! y8axis! represents! grams! of! D2O! sorbed! divided! by! grams! of! PLA! Figure!3.5!Percent!crystallinity!during!hydrolytic!degradation!of!PLA!films!immersed!in! water,!50%!and!95%!ethanol!at!40!°C.!Trend!lines!from!fitting!Avrami!equation!(Eq.!3.3.! Numbers! I,! II! and! III! indicate! the! regions! corresponding! to! the! different! processes! of! outliers!and!therefore!no!used!for!fitting!of!the!first!order.!..............................................!91! remaining!in!the!disk!after!hydrolysis!with!H2O.!Trend!lines!are!used!as!visual!aid.!......!94! reaction!equation!with!!!from!(Eq.!3.2!(−•−).!D2O!sorption!(−)!and!ethanol!sorption!(−−)! of!PLA!immersed!in!water,!50%!ethanol!and!95%!ethanol!at!different!exposure!times.!97! estimated!when!p!>!0.5!(Tg!=!80.34p!+!52.83l!R2=0.999).!..............................................!99! PLA!crystallization.!.......................................................................................................!100! 95%!ethanol!at!40!°C.!The!numbers!on!each!profile!indicate!days!immersed.!............!104! days!immersed.!............................................................................................................!106! each!solution.!...............................................................................................................!107! Figure!3A.1!1H8NMR!spectrum!of!PLA!with!ethanol.!...................................................!113! Figure!3A.2!D8NMR!spectrum!of!PLA!with!D2O!..........................................................!116! 50%!and!95%!ethanol!at!40!°C.!...................................................................................!117! 40!°C.!...........................................................................................................................!118! Figure!3D.1!Sorption!isotherms!for!H2O!and!D2O!vapor!in!PLA!film!at!40!°C.!............!119! Figure!3.7!DSC!thermograms!of!PLA!film!during!hydrolytic!degradation!in!(A)!water,!(B)! 50%!ethanol!and!(C)!95%!ethanol!at!40!°C.!The!numbers!on!each!thermogram!indicate! Figure! 3B.1! Optical! images! of! PLA! samples! during! hydrolytic! degradation! by! water,! Figure!3C.1!Diffusion!of!(A)!H2O!and!(B)!D2O!vapor!in!PLA!film!at!75%!relative!humidity,! Figure!3.6!XRD!profiles!of!PLA!films!during!hydrolytic!degradation!in!(A)!water,!(B)!50%! ethanol,!(C)!95%!ethanol!and!in!the!3rd!day!of!immersion!in!(D)!50%!ethanol!and!(E)! Figure! 3.8! Release! of! LA! during! hydrolytic! degradation! of! PLA! films! in! contact! with! water,!50%!and!95%!ethanol!at!40!°C.!Fitted!lines!indicate!the!prediction!of!(Eq.!3.4!to! ! xv! ! Figure! 3E.1!Ethanol!sorption!into!PLA!films!in!contact!with:!(A)!50%!ethanol!and!(B)! 95%!ethanol!at!40!°C.!y8axis!is!in!grams!of!ethanol!sorbed!divided!by!grams!of!PLA!disk! Figure!3F.1!Expansion!of!PLA!in!contact!with!different!volume!fractions!of!ethanol!(%! expansion=6p>/R2=0.969,!where!p!is!the!ethanol!fraction!by!volume).!The!%!expansion! was!determined!as!100!times!the!volume!of!ethanol!sorbed!in!PLA!divided!by!the!initial! Figure!3H.1!MWD!of!PLA!films!during!hydrolytic!degradation!when!in!contact!with:!(A)! Figure! 3J.1! Theoretical! estimated! diffusion! coefficients! of! LA! and! LA8mers! in! water,! Figure!4.1!Mn!as!a!function!of!time!during!hydrolytic!degradation!of!PLA!film!immersed! in! 50%! ethanol,! 50%! methanol,! 50%! 18propanol! and! 20%! 18butanol! at! 70! °C.! Lines! indicate! fitting! of! the! first! order! reaction! Eq.! 4.2! with! k/=! 0.0352! ±! 0.0027,! 0.0294! ±! 0.0014,!0.0384!±!0.0024!and!0.0367!±!0.0017!h81,!for!50%!ethanol,!50%!methanol,!50%! used!for!the!1H8NMR!experiments.!...............................................................................!120! volume!of!the!disk.!.......................................................................................................!121! water,!(B)!50%!ethanol!and!(C)!95%!ethanol!at!40!°C!.................................................!124! 50%!and!95%!ethanol.!Fitted!exponential!decay!lines!are!provided.!...........................!131! 18propanol!and!20%!18butanol,!respectively.!...............................................................!153! ethanol!at!(A)!40,!(B)!60,!(C)!70!and!(D)!80!°C.!...........................................................!156! 720!h.!...........................................................................................................................!158! more!likely!reactions!during!PLA!hydrolysis!as!determined!by!deconvolution.!............!159! during!PLA!hydrolysis.!..................................................................................................!165! at!(A)!60,!(B)!70,!(C)!80!and!(D)!90!°C!.........................................................................!171! more!likely!reactions!during!PLA!hydrolysis.!................................................................!173! Figure!4.4!Mn!as!function!of!time!during!hydrolytic!degradation!of!PLA!film!immersed!in! 50%!ethanol!at!(A)!40,!(B)!60,!(C)!70!and!(D)!80!°C.!(!,",#,!!)!Filled!markers!indicate! the! main! initial! path! of! hydrolysis! reaction.! Unfilled! markers! indicate! more! likely! side! reactions.! (*)! Less! prevalent! side! reactions.! (−−−)! Lines! indicate! fitting! of! Eq.! 4.2! for! Figure!4.7!Mn!as!function!of!time!during!hydrolytic!degradation!of!PLA!film!immersed!in! water!at!(A)!60,!(B)!70,!(C)!80!and!(D)!90!°C.!(!,",#,! !)!Filled!markers!indicate!the! main! initial! path! of! hydrolysis! reaction.! Unfilled! markers! indicate! more! likely! side! reactions.! (*)! Less! prevalent! side! reactions.! (−−−)! Lines! indicate! fitting! of! Eq.! 4.2! for! Figure!4.2!MWD!of!PLA!films!during!hydrolytic!degradation!when!in!contact!with!50%! Figure!4.3!Deconvolution!of!MWD/of!PLA!for!different!hydrolysis!times!in!50%!ethanol! at!60!°C!as!shown!in!Figure!4.2B:!(A)!120!h,!(B)!192!h,!(C)!288!h,!(D)!552!h!and!(E)! Figure!4.5!Mn!as!a!function!of!time!during!hydrolytic!degradation!of!PLA!film!immersed! in!50%!ethanol!at!80!°C!and!different!pH.!(!,",#)!Filled!markers!indicate!the!main!initial! path!of!hydrolysis!reaction.!Unfilled!markers!indicate!more!likely!side!reactions.!(*)!Less! prevalent!side!reactions.!(−−−)!Lines!indicate!fitting!of!Eq.!4.2!for!more!likely!reactions! Figure!4.6!MWD!of!PLA!films!during!hydrolytic!degradation!when!in!contact!with!water! ! xvi! ! Figure! 4.8! Rate! constants! (k)! of! the! hydrolytic! degradation! of! PLA! in! 50%! ethanol! Figure!4B.1!1H,13C8gHMBC!of!PLA!exposed!to!50%!ethanol!at!80!°C!run!on!a!Bruker! Figure! 4C.1! SSC! of! hydrolytic! degradation! of! PLA! in! 50%! ethanol! using! first! order! reaction!equation!Eq.!4.2!at!(A)!40,!(B)!60,!(C)!70!and!(D)!°80!C.!Arrows!indicate!the! Figure! 4B.2! 13C! NMR! of! PLA! prior! to! hydrolysis! (top)! and! after! hydrolysis! in! 50%! ethanol! at! 80! °C! (bottom).! ! The! expansion! shows! the! carbonyl! region! and! indicates! additional!small!resonances!around!169.4!ppm!that!are!most!likely!attributable!to!PLA! Figure! 4D.1! SSC! of! hydrolytic! degradation! of! PLA! in! 50%! ethanol! using! simulated! dynamic!temperature!(Eq.!4.9)!in!Eq.!4.6!at!Tref!=!62.5!°C!and!pHref/=!7.33.!(A)!β/as!a! parameter!to!estimate!with!initial!guesses:!kref!=!0.0099!h81,/Mno/=!100,000!Da,/Ea!=!9.04! x104!J.mol81,/β!=!0.147.!(B)/β!as!a!constant!with!initial!guesses:!kref!=!0.01!h81,/Mno!=! solution!and!water!vs.!temperature.!Trend!lines!are!used!as!a!visual!aid.!...................!174! Avance!900.!.................................................................................................................!181! oligomers.!.....................................................................................................................!182! optimal!time!to!accurately!estimate!k.!..........................................................................!183! 100,000!Da,/Ea!=!9.04!x104!J.mol81.!..............................................................................!185! =!0.093!h81,/Mno!=!100,000!Da,/β!=!0.147.!....................................................................!186! using!Eq.!4.7.!...............................................................................................................!187! using!Eq.!4.6.!...............................................................................................................!189! Eq.!4.5.!Initial!guesses:!kref!=!0.007!h81,/Mno!=!100,000!Da,/Ea!=!9.04!x104!J.mol81.!.....!191! hydrolytic!degradation!of!PLA!in!50%!ethanol!solution!(Eq.!4.5).!................................!192! ethanol!at!80!°C!and!different!pH:!(A)!11,!(B)!7,!and!(C)!4.!.........................................!195! PLA!matrix.!XRD!patterns!for!OMMT!clay!powder,!PLA8OMMT!and!PLA!films.!..........!214! Figure!4D.7!Plot!of!correlation!coefficient!of!kref!and/Ea!as!a!function!of!possible!Tref!for! hydrolytic!degradation!of!PLA!in!water!(Eq.!4.5).!.........................................................!194! Figure!4F.1!MWD!of!PLA!films!during!hydrolytic!degradation!when!in!contact!with!50%! Figure!4D.2!SSC!of!hydrolytic!degradation!of!PLA!in!50%!ethanol!at!80!°C!and!different! pH!using!simulated!dynamic!pH!(Eq.!4.10)!in!Eq.!4.7!at!pHref/=!7.33.!Initial!guesses:!kref! Figure! 4D.3!Plot!of!correlation!coefficient!of!kref!and!β!as!a!function!of!possible!pHref/ Figure! 4D.4! Plot! of! correlation! coefficient! of! kref! and/Ea! as! a! function! of! possible! Tref! Figure! 4D.5! SSC! of! the! activation! energy! estimation! at! Tref! =! 62.5! °C! of! hydrolytic! degradation!of!PLA!in!50%!ethanol!using!simulated!dynamic!temperature!(Eq.! 4.9)!in! Figure!4D.6!Plot!of!correlation!coefficient!of!kref!and/Ea!as!a!function!of!possible!Tref/for! Figure!5.1!Displacement!of!diffraction!pattern!of!the!OMMT!clay!when!embedded!in!the! ! xvii! ! Figure! 5.2! Intercalation! of! OMMT! in! PLA! matrix.! TEM! bright! field! images:! (A)! PLA8 Figure!5.6!Sorption!isotherms!of!ethanol!vapor!in!PLA8OMMT,!PLA8C!and!clay!particles.! The!sorption!of!PLA8C!and!clay!correspond!to!the!nominal!composition!of!PLA8OMMT! film!(PLA:Clay!=!95:5).!PLA8C!+!Clay!is!the!mathematical!addition!of!ethanol!sorption!of! Figure!5.7!Tg/of!(A)!PLA8C!and!(B)!PLA8OMMT!films!submerged!in!different!fractions!of! Figure! 5.4! Similar! sorption! of! H2O! vapor! in! PLA! and! PLA8OMMT! films.! Sorption! Figure! 5.5! Ethanol! sorption! of! PLA8C! (%sorption=6pl! R2=0.978)! and! PLA8OMMT! (%sorption=12pl!R2=0.965)!films!in!contact!with!different!volume!fractions!of!ethanol!(p/ Figure! 5.3! First! order! reduction! of! Mn! for! PLA8C,! PLA8OMMT,! and! PLA8QAC! films! showing!that!the!faster!decay!in!Mn/was!when!the!films!were!immersed!in!50%!ethanol.! during!hydrolysis!of!(A)!PLA8C,!(B)!PLA8OMMT!and!(C)!PLA8QAC!films!submerged!in! water,!50%!and!95%!ethanol!at!40!°C.!Values!indicated!as!*!were!considered!outliers! OMMT!(x!100!000),!(B)!PLA8OMMT!(x!270!000).!........................................................!214! Lines!indicate!fitting!of!first!order!reaction!with!!!obtained!from!Eq.!5.3.!Mn!as!vs!time! and!therefore!no!used!for!fitting.!..................................................................................!216! isotherms!for!H2O!vapor!in!PLA8C!and!PLA8OMMT!films.!...........................................!218! is!the!ethanol!fraction!by!volume).!................................................................................!219! PLA8C!and!clay.!...........................................................................................................!220! ethanol!(v/v)!from!the!DMA.!Inserts:! Tg! vs!ethanol!fraction!(")l! !!experimental!data,!∗! estimated!values!that!could!not!be!experimentally!measured,!and!–!(fitted!line)!predicted! values! as! (A)!%&=−0.34"+52.34;122=0.999!(B)!%&=−0.34"+54.50;122=0.927. ......................................................................................................................................!222! degradation!in!50%!ethanol!at!40!°C.!...........................................................................!225! degradation!in!50%!ethanol!at!40!°C.!Immersed!days!are!labeled.!.............................!227! ethanol.!.........................................................................................................................!229! films!submerged!in!water,!50%!and!95%!ethanol.!Lines!are!provided!as!a!visual!aid.!233! ethanol!at!40!°C.!...........................................................................................................!236! Figure!5.10!Release!of!LA!during!hydrolysis!at!40!°C!of!(A)!PLA8C,!(B)!PLA8OMMT,!(C)! PLA8QAC! films! submerged! in! water,! 50%! and! 95%! ethanol.! Lines! are! provided! as! a! visual!aid.!Exponential!increase!of!LA!release!when!PLA!films!were!immersed!in!50%! Figure!5.11!Release!of!surfactant!from!PLA8OMMT!and!PLA8QAC!films!showing!initial! higher!release!in!95%!ethanol!but!later!the!release!was!higher!in!50%!ethanol.!Release! of! QAC! during! hydrolytic! degradation! at! 40! °C! of! (A)! PLA8OMMT! and! (B)! PLA8QAC! Figure! 5.8! DSC! plots! of! (A)! PLA8C! and! (B)! PLA8OMMT! films! during! hydrolytic! Figure! 5.9! XRD! profiles! of! (A)! PLA8C! and! (B)! PLA8OMMT! films! during! hydrolytic! Figure! 5.12! Release! of! nanoclays! from! PLA8OMMT! samples! submerged! in! 50%! ! xviii! ! Figure!5.13!TEM!images!of!released!nanoclay!particles!from!PLA8OMMT!film!to!50%! Figure!5B.1!Schematic!representation!of!(A)!glass!vial!cells!used!for!molecular!weight! experiments,! LA! release,! surfactant! release,! and! (B)! PP! tubes! for! nanoclay! release! Figure!6.1(A)!General!structure!of!Joncryl®!ADR,!where!R1!–!R5!are!H,!CH3,!a!higher! alkyl!group,!or!a!combinationl!R6!is!an!alkyl!group,!and!x,!y!and!z!are!each!between!1! Figure!6.2!Mn!of!PLA8C!and!PLA8J!films!after!processing!at!selected!residence!times.! Values! with! different! lower! case! letters! within! the! same! classification! are! statistically! Figure! 5C.2! (A)! Crystalline,! (B)! MAF,! and! (C)! RAF! during! hydrolytic! degradation! of! PLA8C! and! PLA8OMMT! films! immersed! in! 50%! ethanol! at! 40! °C.! I,! II! and! III! in! (A)! indicate!different!regions!of!PLA!corresponding!to!the!different!crystallization!processes.! The!main!points!in!(A),!(B)!and!(C)!were!obtained!using!Eq.! 5C.1,! Eq.! 5C.2!and!Eq.! 5C.4,!respectively.!Symbols!☐!and!"!in!(C)!represent!the!values!obtained!from!Eq.!5C.3. ethanol!and!the!EDS!analysis!corresponding!to!particles!in!the!image!15!d.!..............!238! experiments.!.................................................................................................................!242! Figure!5C.1!DSC!plot!of!PLA8C!film!immersed!during!60!d!in!50%!ethanol!at!40!°C.!.!244! ......................................................................................................................................!246! and!20.!(B)!Possible!structure!of!PLA!and!Joncryl®!according!to!Najafi!et!al.![18].!.....!258! different!(α=0.05).!.........................................................................................................!263! Figure!6.3!Time!dependency!of!PLA!melt!at!170!°C!for!a!time!period!of!15!min.!.......!265! frequency!at!170!°C.!.....................................................................................................!267! curves!for!a!reference!temperature!of!180!°C.!.............................................................!268! Figure!6.6!TGA!thermograms!of!PLA8C!and!PLA8J!films.!...........................................!269! Figure!6.7!DSC!curves!of!the!second!heat!scan!of!PLA8C!and!PLA8J!films.!..............!270! ethanol!at!80!°C,!pH!11.!...............................................................................................!272! 70%!RH.!.......................................................................................................................!274! order!reaction!equation!while!lines!in!(B)!are!used!as!visual!aid.!................................!277! Figure!6.10!(A)!Mn!and!(B)!Mv/Mvo!as!a!function!of!time!during!hydrolytic!degradation!of! PLA!films!immersed!in!50%!ethanol!at!80!°C,!pH!11.!Lines!in!(A)!are!fitting!of!the!first! Figure! 6.4! Mechanical! spectra! of! PLA8C! and! PLA8J! films! as! a! function! of! angular! Figure!6.5.!Van!Gurp8Palmen!plot!of!PLA8C!and!PLA8J!samples!obtained!from!master! Figure!6.8!MWD!of!(A)!PLA8C!and!(B)!PLA8J!films!during!hydrolytic!degradation!in!50%! Figure!6.9!%Weight!change!of!PLA8C!and!PLA8J!films!exposed!to!water!vapor!at!40!°C,! ! ! ! xix! ! ! AF4! CAPS! CDCl3! CF! 13C!NMR! D/ D8NMR! D2O! DMA! DMF! DSC! Ea/ EHP! EoL! FDA! FV! G*/ G′/ G″/ GFAAS! gHMBC! KEY!TO!ABBREVIATIONS! ! asymmetrical!flow!field8flow!fractionation! 38(cyclohexylamino)818propanesulfonic!acid! chloroform8D! crystalline!fraction! carbon!nuclear!magnetic!resonance! diffusion!coefficient! deuterium!NMR! deuterium!oxide! dynamic!mechanical!analysis! N,N8Dimethylformamide! differential!scanning!calorimetry! activation!energy! electrically!heated!plate! end!of!life! Food!and!Drug!Administration! free!volume! complex!modulus! storage!modulus! loss!modulus! graphite!furnace!atomic!absorption!spectrometry! gradient!heteronuclear!multiple!bond!correlation! xx! ! ! GPC! 1H8NMR! H2O! HDI! HEPES! HPLC! ICP8MS! k/ kref/ LA! gel!permeation!chromatograph! proton!nuclear!magnetic!resonance! water! hexamethylene!diisocyanate! N8(28Hydroxyethyl)piperazine8N′8(28ethanesulfonic! acid)! high8performance!liquid!chromatography! inductively!coupled!plasma!mass!spectrometry! rate!constant! rate!constant!at!Tref! lactic!acid! LDH8C12! laurate8modified!Mg8Al!layered!double!hydroxide! MAF! MALS! MK10! MK5! MMT! Mn/ Mno/ MV/ Mw/ MWD/ NMR! mobile!amorphous!fraction! multi8angle!light8scattering! montmorillonite!K10! montmorillonite!K5! montmorillonite! number!average!molecular!weight! initial!Mn! viscosity!average!molecular!weight! weight!average!molecular!weight! molecular!weight!distribution! nuclear!magnetic!resonance! xxi! ! η*/ OMMT! PA! PA6! PBAT! PBS! PBSA! PCDI! PDI/ PDLA! PDLLA! PE! PET! pHref/ pHsim/ PLA! PLLA! PMDA! PP! QAC! RAF! RH! RMSE! complex!viscosity! organomodified!montmorillonite! polyamide! polyamide!6! poly(butylene8adipate8co8terephtalate)! poly(butylene!succinate)! poly[(butylene!succinate)8co8adipate]! poly(carbodiimide)!(PCDI)! polydispersity!index! poly(D8lactic!acid)l!poly(D8lactide)! poly(DL8lactic!acid)l!poly(DL8lactide)! polyethylene! poly(ethylene!terephthalate)! reference!pH! pH!simulation! poly(lactic!acid)! poly(L8lactic!acid)l!poly(L8lactide)!! pyromellitic!dianhydride! polypropylene!! organomodifier,!surfactant! rigid!amorphous!fraction!! relative!humidity!! root!mean!square!error! ! xxii! SC! SEC! SIC! SSC! t/ T/ Tc/ Tcc/ TEM! Tg!/ TGA! THF! Tm/ TNPP! Tonset/ TPS! Tref/ Tsim/ Xc/ XRD! ! ! sensitivity!coefficient! size!exclusion!chromatography! solvent!induced!crystallization! scaled!sensitivity!coefficient! time! temperature! crystallization!temperature! cold8crystallization!temperature! transmission!electron!microscopy! glass!transition!temperature! thermal!gravimetric!analysis! tetrahydrofuran! melting!temperature! tris(nonylphenyl)phosphite! onset!decomposition!temperatures! thermoplastic!starch! reference!temperature! temperature!simulation! degree!of!crystallinity! X8ray!diffraction!study! xxiii! ! CHAPTER!1! Introduction! 1.0! Background!and!motivations! As! pressure! for! circular! and! biobased! economies! are! increasing,! production! of! plastics! from! fossil! resources! is! no! longer! considered! sustainable,! and! production! of! novel! plastics! from! renewable! resources! is! increasingly! gaining! acceptance.! Among! these! new! bioplastics,! poly(lactic! acid),! PLA,! is! one! of! the! main! polymers.! Not! only! because!it!is!produced!from!renewable!resources!such!as!corn!or!cassava!starch,!but! because!it!has!lower!environmental!footprint!and!is!also!providing!the!opportunity!to!be! disposed!to!a!new!end!of!life!scenario,!composting![1,!2].! PLA!is!a!linear!aliphatic!thermoplastic!polyester,!in!which!lactic!acid!(LA)!is!the! precursor! obtained! from! the! fermentation! of! corn! or! potato! sugar! [1].! The! world! production!capacity!of!PLA!is!150,000!metric!tons,!and!additional!production!plants!are! being! developed! [3].! PLA! is! being! used! in! a! number! of! broad! and! diverse! industries! such!as!medical,!pharmaceutical,!clothing,!packaging!and!agriculture!industries!since!it! is!biocompatible!with!the!human!body,!is!degradable,!has!good!wicking,!breathability,! optical,!mechanical!and!barrier!properties.!However,!in!using!PLA!in!these!industries!for! different! applications,! it! is! exposed! to! water! and! organic! solvents.! The! presence! of! water!and/or!organic!solvents!promotes!hydrolytic!degradation!and!structural!changes! in! PLA.! Under! moist! conditions! the! ester! groups! of! the! main! chain! are! hydrolytically! degraded!leading!to!a!decrease!in!molecular!weight!and!the!release!of!low!molecular! weight,! soluble! oligomers! and! monomers! [1,! 4].! On! the! other! hand,! when! PLA! is! ! ! ! 1! ! ! exposed! to! organic! solvents! such! as! ethanol,! the! molecules! plasticize,! swell! and! crystallization!is!induced![588].!! In!medical!applications,!PLA!has!been!widely!used!in!implants,!surgical!sutures,! controlled! delivery! systems,! and! tissue! culture! [9812].! As! a! medical! implant,! the! degradation!of!PLA!over!time!is!favorable,!avoiding!the!surgical!removal!step.!However,! the! rate! of! PLA! degradation! may! cause! defense! reactions! from! the! human! host.! For! instance,! during! the! hydrolysis! of! PLA,! the! pH! of! cells/tissue! is! reduced! due! to! the! release! of! LA! that! accumulates! leading! to! inflammation! of! the! tissue.! Toxicity! effects! may! occur! during! long8term! use! [13].! Although! a! large! amount! of! research! was! conducted! regarding! the! degradation! of! PLA! for! medical! applications! [14817],! full! understanding!of!the!degradation!mechanisms!of!PLA!in!the!medical!field!environment! is!still!needed.!! Both!the!use!of!implantable!biomaterials!and!the!importance!of!the!sterilization! techniques! to! avoid! infections! associated! with! the! medical! devices! have! been! increasing.!The!impact!of!the!sterilization!method!is!critical!for!the!success!of!the!device! since! it! will! dictate! the! material! bulk! and! surface! properties! [18].! Use! of! steam! to! sterilize! PLA! can! lead! to! hydrolysis! because! of! the! high! temperatures,! high! relative! humidity!and!pressure.!Ethanol!soaking!is!another!technique!that!has!been!applied!for! the! disinfection! of! PLA! electrospun! material! for! tissue! engineering.! However,! the! employment!of!ethanol!on!PLA!has!an!effect!on!its!physico8chemical!properties!such!as! morphology,!crystallinity,!surface!topography,!and!mechanical!properties![18,!19].!!Due! to!its!degradation,!PLA!can!be!used!for!drug!release.!In!these!surroundings,!PLA!can! be!exposed!to!aqueous!or!biological!media![20822].!The!media!in!contact!with!PLA!is! ! ! ! 2! ! ! important! since! the! interaction! with! PLA! will! dictate! the! mechanism! of! drug! release! including!drug!dissolution,!diffusion!through!the!polymer!matrix,!swelling!of!the!polymer! and!its!surface!or!bulk!erosion![22].!! PLA!is!suitable!for!fiber!and!textile!applications.!In!the!automotive!industry,!PLA! fiber!has!gained!attention!for!car!interior!parts!such!as!carpets!and!floor!mats![23].!PLA! has! been! also! investigated! as! an! alternative! for! apparel! manufacturing! due! to! its! wicking!properties!and!breathability![24].!However,!in!such!applications!PLA!could!also! be! exposed! to! organic! solvents,! which! may! change! its! morphology! and! under! high! temperature!and!moisture!conditions!will!degrade.!! In!agriculture,!PLA!has!been!considered!as!a!plastic!to!protect!soils!from!erosion,! plants!from!insects!and!birds,!and!as!a!material!for!the!controlled!release!of!herbicides! [25828].!However,!the!implementation!of!PLA!in!the!agriculture!field!is!still!in!the!early! stages! due! to! its! cost.! Studies! have! shown! that! PLA! limits! the! food! sources! for! microorganisms! to! initiate! the! biodegradation! process! at! low! temperatures! when! it! is! used!as!a!mulch!film!due!to!the!relatively!high!glass!transition!temperature!(Tg)!and!low! available!amorphous!region![25].!Thus,!commercialized!PLA8based!mulch!films,!most!of! them! made! of! PLA! blends,! have! plasticizers! or! LA! oligomers,! which! accelerate! the! hydrolysis!process!due!to!the!free!volume!that!they!provide!in!the!PLA!polymer!matrix,! allowing!faster!water!diffusion!and!accessibility!of!the!microorganisms![29,!30].!!! In! food! packaging,! PLA! can! be! found! as! films,! sheets! and! rigid! thermoform! containers!for!short!shelf!life!products![1,!2,!31,!32].!For!fresh!products!or!beverages,! PLA!is!exposed!to!humid!environments!or!it!can!be!in!direct!contact!with!food!products! with!high!content!of!water!like!juices,!but!also!with!cold8chain!dairy!products!or!alcoholic! ! ! ! 3! ! ! beverages.! To! ensure! the! safety! of! using! polymers! like! PLA! for! food! packaging! applications,! migration! studies! are! required! where! PLA! is! exposed! to! food! simulants! under!specific!conditions!of!temperature!and!time!to!simulate!food!exposure!conditions.! For! instance,! 95%! ethanol,! 50%! ethanol! or! pure! water! are! commonly! used! as! food! simulants! for! fatty,! alcoholic! and! watery! liquid! products! [33,! 34].! Thus,! PLA! has! numerous! challenges! to! overcome! the! hydrolytic! degradation! under! these! different! environmental! conditions! to! provide! the! required! properties! for! packaged! food! containers!during!their!lifetime.! The!hydrolytic!degradation!of!PLA!is!not!always!considered!as!a!disadvantage.! One! of! the! applications! of! the! hydrolysis! of! PLA! during! its! end! of! life! scenario! is! for! chemical!recycling!purposes,!that!is!a!depolymerization!method!that!converts!PLA!back! to!its!monomer!for!new!PLA!production!with!the!similar!properties!of!virgin!PLA![35,!36].! The! depolymerization! of! PLA! and! other! polymers! is! generally! performed! at! high! temperatures!using!toxic!solvents!(i.e.!toluene,!chloroform,!tetrahydrofuran!(THF))![4,!35,! 37,! 38].! Other! methods! have! been! applied! through! solvolysis! of! PLA! by! transesterification!with!alcohol!to!lactate!esters![39841].!The!use!of!an!aqueous!phase!at! high!temperatures!has!been!investigated![42,!43].!However,!it!is!important!to!control!the! temperature!since!temperatures!higher!than!the!melting!temperature!(Tm)!of!PLA!can! trigger! racemization! and! decomposition! of! LA.! Therefore,! chemical! recycling! using! “green”! solvents! at! moderate! temperatures! should! be! economically! viable! to! safely! recover!PLA.!! As! aforementioned! the! versatility! of! PLA! has! led! to! broad! applications.! Depending! on! the! application,! degradation! of! PLA! can! be! an! advantage! or! a! ! ! ! 4! ! ! disadvantage.! In! the! case! of! applications! for! long! service! life,! for! instance! as! a! packaging! material,! controlling! the! resistance! to! hydrolytic! degradation! is! important! since! the! properties! of! the! container! must! remain! intact! during! the! shelf! life! of! the! product!to!maintain!the!quality!of!the!product.!Several!studies!have!been!conducted!to! improve! PLA! properties! to! expand! its! applications,! but! also! to! avoid! the! thermal! degradation!of!PLA!during!processing.!These!achievements!have!reached!by!blending! PLA!with!nanoscale!particles!and!also!with!the!addition!of!chain!extenders!to!control!the! thermal!degradation!of!PLA!during!melt!processing![44855].! In!recent!years,!the!addition!of!nanoscale!particles!has!gained!greater!attention! due!to!the!significant!improvement!of!material!properties!since!the!interface!interactions! between!the!polymer!matrix!and!the!nanofiller!are!maximized,!enhancing!the!polymer! properties! [48].! PLA8nanocomposites! based! on! layered! nanoclays,! in! particular! montmorillonite!(MMT),!have!attracted!attention!since!addition!of!MMT!can!improve!PLA! properties,!such!as!mechanical,!thermal!and!barrier!properties![56].!Modification!of!the! surface!of!MMT!is!necessary!to!promote!the!interaction!between!the!clay!and!PLA.!For! that,! cations! located! in! the! galleries! of! the! silicate! layers! are! exchanged! by! organomodifiers! or! surfactants! making! PLA! compatible! with! the! clay,! so! good! dispersion!of!the!layered!silicates!within!the!polymer!matrix!can!be!obtained!and!PLA! properties! enhanced! [57].! PLA8nanocomposites! based! on! organomodified! MMT! (OMMT)! have! been! extensively! studied! in! terms! of! mechanical,! thermal! and! barrier! properties.!However,!the!effect!of!the!organo8clay!fillers!on!the!hydrolytic!degradation!of! PLA8nanocomposites! has! scarcely! been! addressed! since! the! impact! of! the! OMMT! in! ! ! ! 5! ! ! such! process! will! dictate! the! durability! of! PLA! materials! in! different! environments! potentially!compromising!its!end!use.! Even! though! research! involving! the! influence! of! OMMT! on! the! hydrolytic! degradation! of! PLA8nanocomposites! in! water! and! phosphate! buffered! solutions! has! been! reported! [58860],! the! applications! of! nanocomposites! also! involve! different! environments,!for!instance!organic!compounds!or!different!media!for!food!applications! (e.g.,!juices,!alcoholic!beverages,!cold8chain!dairy!products).!Applications!involving!the! direct! contact! of! nanocomposites! to! food! products! have! created! concerns! about! the! potential! release! of! nanoparticles! into! food! systems! where! their! safety! should! be! ensured![61,!62].!! Studies!have!addressed!the!release!of!nanoparticles!and!organomodifiers!from! different! nanocomposites! exposed! to! different! conditions,! using! polymers! such! as! polypropylene!(PP)!and!polyamide!(PA)![63,!64].!Theories!have!been!used!to!describe! the! migration! process! of! nanoparticles! from! polymeric! packaging! materials! into! food! systems![65].!However,!PLA8nanocomposites!have!to!be!treated!differently!since!PLA!is! a! polymer! susceptible! to! hydrolytic! degradation! under! moist! conditions! and! the! exposure!to!certain!compounds!leads!to!structural!changes!that!could!affect!the!release! behavior!of!PLA8nanocomposite!compounds.! Understanding! the! effects! of! different! environments! including! water8solvent! solutions!on!the!hydrolysis!of!PLA!and!the!effect!of!engineered!nanoparticles!is!crucial! to! evaluate! the! stability! of! PLA! during! its! lifetime! and! to! address! the! safety! of! these! materials.! To! fill! the! missing! gap,! research! must! be! conducted! to! understand! the! release! of! PLA8nanocomposite! compounds! during! the! hydrolytic! degradation! of! PLA.! ! ! ! 6! ! ! Furthermore,! the! PLA! hydrolysis! process! in! different! water8solvent! solutions! must! be! considered!based!on!the!final!application!of!PLA!since!the!impact!of!the!nanoparticles! will!dictate!the!durability!of!PLA8nanocomposites.!! The!incorporation!of!chain!extenders!into!PLA!during!melt!processing!has!being! widely! studied! since! PLA! is! susceptible! to! thermal! degradation! during! processing.! Different!factors!can!affect!the!degradation!during!processing!such!as!moisture!content,! residual! metal! catalyst,! oxygen! and! processing! temperature! [66].! Chain! extenders! reconnect! cleaved! chains,! increasing! the! molecular! weight! of! the! polymer.! The! main! mechanism! of! stabilization! in! this! case! is! the! chain! extension! resulting! from! the! formation!of!either!longer!linear!chains!or!long!branched!chains.!The!incorporation!of! epoxy! based! chain! extenders! (i.e.! Joncryl®)! into! PLA! increased! the! molecular! weight! and! improved! the! rheological,! mechanical! and! thermal! properties! [49,! 50,! 67871].! However,!the!effect!of!adding!epoxy!chain!extenders!on!the!hydrolytic!degradation!of! PLA!is!not!well!understood.! 1.1! Overall!goal!and!objectives! The! overall! goal! of! this! dissertation! is! to! evaluate! the! hydrolytic! degradation! of! PLA! by! water8ethanol! solutions,! the! use! of! this! system! to! be! applied! in! the! chemical! recycling! of! PLA,! and! the! effect! of! adding! nanoparticles! and! chain! extenders! on! the! hydrolytic!degradation!of!PLA.!To!accomplish!the!main!goal,!different!objectives!have!to! be!addressed.! 1)! To! understand! the! parameters! and! factor! affecting! the! hydrolysis! of! PLA! by! water8ethanol!solutions.! ! ! ! 7! ! ! 2)! To!understand!and!to!analyze!the!hydrolytic!degradation!of!PLA!by!water8ethanol! solutions!for!the!purpose!of!chemical!recycling!at!moderate!temperatures.! 3)! To!understand!the!effect!of!the!nanoparticles!and!the!surfactant!on!the!hydrolytic! degradation!of!PLA8nanocomposites,!and!the!connection!between!the!transport! of!water8ethanol!solutions!through!the!PLA8nanocomposite!and!the!release!of!the! nanoparticles!and!the!surfactant.! 4)! To!understand!the!effect!of!adding!chain!extenders!into!PLA!during!processing! and!its!effect!on!the!hydrolytic!degradation!of!modified!PLA.! 1.2! Dissertation!overview! To! answer! the! objectives! of! this! dissertation! this! document! is! organized! in! the! following!manner.!Chapter!2!provides!a!critical!literature!review!detailing!the!hydrolytic! degradation!mechanism!of!PLA,!the!parameters!controlling!the!hydrolysis!process,!the! chemical! recycling! of! PLA,! a! detailed! description! of! the! addition! of! nanoparticles! into! PLA! and! the! effect! on! the! hydrolytic! degradation,! and! a! review! of! the! use! of! chain! extenders!into!PLA.!! Chapter! 3! is! a! version! of! a! published! article! that! presents! the! results! to! understand! the! concurrent! effect! of! the! solvent! induced! crystallization! (SIC)! and! the! hydrolytic!degradation!of!PLA!when!it!is!exposed!to!different!water8ethanol!solutions!at! 40!°C.!At!the!end!of!the!chapter!as!an!appendix,!the!supporting!information!is!presented! that!enhanced!the!main!article.! Chapter!4!is!a!version!of!the!accepted!article!to!a!journal!that!investigates!and! analyzes!the!hydrolytic!degradation!of!PLA!in!water8ethanol!solutions!for!being!used!for! chemical! recycling! at! moderate! temperatures,! estimating! the! kinetic! parameters,! rate! ! ! ! 8! ! ! constant!and!activation!energy!that!determines!the!hydrolysis!of!PLA!in!these!solutions.! An!appendix!is!presented!at!the!end!of!the!chapter!as!supporting!information.!!! Chapter! 5! is! a! version! of! a! published! article! that! explores! the! effect! of! adding! nanoparticles! and! surfactant! on! the! hydrolytic! degradation! at! 40! °C! of! PLA8 nanocomposites!in!water8ethanol!solutions!and!the!interaction!between!PLA,!nanoclay! and! solvent.! An! appendix! is! presented! at! the! end! of! the! chapter! as! supporting! information.!! Chapter! 6! is! a! version! of! a! submitted! article! that! studies! the! effect! of! adding! chain!extenders!during!PLA!processing!and!the!effect!on!the!hydrolytic!degradation!of! PLA! in! water8ethanol! solution! at! 80! °C! with! the! purpose! of! modifying! the! hydrolysis! mechanism!of!the!polymer.!! Chapter!7!summarizes!all!the!work!in!this!dissertation!and!concludes!with!future! work!recommendations.!! ! ! ! ! ! ! 9! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! REFERENCES! ! ! ! 10! ! ! REFERENCES! ! [1]! R.! Auras,! B.! Harte,! S.! Selke,! An! overview! of! polylactides! as! packaging! materials,! Macromol.!Biosci.!4!(2004)!8358864.! [2]!E.!Castro8Aguirre,!F.!Iñiguez8Franco,!H.!Samsudin,!X.!Fang,!R.!Auras,!Poly!(lactic! acid)—Mass!production,!processing,!industrial!applications,!and!end!of!life,!Adv.!Drug! Deliv.!Rev.!107!(2016)!3338366.! [3]! E.T.! Vink,! S.! Davies,! Life! 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Physico8chemical! and! rheological! properties,! Eur.! Polym.! J.! 58! (2014)!908102.! ! ! ! ! 17! ! ! CHAPTER!2! Literature!Review! 2.0! !Introduction:!Poly(lactic!acid)!–!PLA! Conventional!polymers!are!mostly!made!from!chemicals!that!are!mainly!derived! from! fossil! fuels! and! after! their! consumption! are! discarded,! creating! environmental! concerns! as! increasing! greenhouse! gas! emissions! and! pollution.! In! this! regard,! the! development!of!biodegradable!polymers!derived!from!biobased!resources!has!gained! great!attention.!Poly(lactic!acid)!(PLA)!is!an!aliphatic!polyester!derived!from!renewable! resources! such! as! corn! and! potatoes! with! lower! environmental! impact! than! fossil! derived!polymers!and!offering!alternative!disposal!routes!(e.g.,!composting)![1].!Figure! 2.1!shows!that!the!interest!and!the!number!of!published!research!studies!of!PLA!have! increased!in!the!last!25!years. ! 18! ! ! ! Figure! 2.1! Number! of! research! reports! published! since! 1990! based! on! a! Web! of! Science!search!using!keywords!“PLA”,!“PLLA”,!“PDLA”,!“polylactic!acid”,!“polylactide”,! and!“poly(lactic!acid)”.!*!Data!until!03/01/18.! ! PLA! has! been! used! in! diverse! industries! for! different! applications,! such! as! medical,! clothing,! packaging! and! agriculture.! For! medical! applications,! PLA! can! be! found!as!suture!material,!stents!and!drug!delivery!systems![284]l!in!agriculture,!as!mulch! film! to! protect! soils! and! plants! [5].! and! in! packaging! industries! as! films! and! rigid! thermoformed!containers!for!food!and!non8food!products![6].! ! 19! ! ! PLA!properties!depend!on!the!polymer!architecture!(i.e.,!stereochemical!makeup)!and! molecular!weight![1].!PLA!is!synthetized!from!lactic!acid!(LA)!or!lactide,!which!are!chiral! molecules!having!different!enantiomers:!L8!and!D8LA!or!L8!and!D8lactide!(Figure!2.2)![7].! High! molecular! weight! PLA! is! produced! from! different! combinations! of! LA! or! lactide! enantiomers!amounts.!PLA!comprises!by!isotactic!sequences!of!L8LA!or!L8lactide!and! D8LA!or!D8lactide!are!referred!to!PLLA!and!PDLA,!respectively.!When!PLA!is!formed!by! meso8lactide,! or! a! mixture! of! L8LA! or! D8LA! (or! L8! and! D8lactide),! or! racemic! mixture! (50:50)!of!L8LA!and!D8LA!(or!L8!and!D8lactide)!is!represented!by!the!term!PDLLA.!PLA! derived!from!greater!than!92%!L8LA!can!be!semicrystalline,!while!PLA!from!50!to!92%! L8LA!is!amorphous![1].!The!stereochemical!architecture!of!PLA!controls!the!degree!of! crystallinity,!and!therefore!the!degradation!behavior.! ! ! ! 20! ! ! Figure! 2.2! Chemical! structure! of! L(+),! D(8)! lactic! acid,! L8lactide,! D8lactide! and! ! stereocomplex.! 2.1! Hydrolytic!degradation!of!PLA! ! Degradation! of! polymers! occurs! under! normal! conditions! during! their! lifetime! when!they!are!exposed!to!the!environment!and!sometimes!is!the!limiting!factor!for!their! end! use! [8].! Degradation! is! defined! as! the! loss! of! properties! leading! to! failure! of! the! material.!These!changes!occur!mainly!through!main!chain!scission!induced!by!a!variety! of!environmental!agents!such!as!temperature,!water,!microorganisms,!oxygen!or!light![8,! 9].!! Degradation! of! PLA! proceeds! by! different! mechanisms! such! as! chemical! hydrolysis,! oxidation,! microbial,! enzymatic! and! thermal! degradation.! PLA! degrades! primarily!by!hydrolysis!of!ester!groups!in!the!polymer!backbone!under!moist!conditions! ! 21! ! ! [1,! 10].! The! degradation! mechanism! and! rate! depend! on! the! chemical! and! physical! properties! of! the! material! including! diffusivity,! porosity! and! morphology.! It! is! also! important!to!consider!besides!the!material!properties,!the!media!in!which!the!polymer!is! exposed,!meaning!that!temperature,!pH,!solutes!or!microbes!can!affect!the!degradation! behavior!of!PLA!based!materials![10,!11].!! Hydrolytic!degradation!of!PLA!starts!with!chain!scission!of!the!ester!groups!in!the! presence! of! water,! according! to! the! below! reaction,! resulting! in! the! reduction! of! molecular!weight!and!release!of!soluble!oligomers!and!monomers![10]:! −566+786→−5667+76:! Hydrolytic! degradation! of! polyesters! is! partially! controlled! by! the! rate! of! water! diffusion.!Water!molecules!diffuse!first!into!the!amorphous!regions,!cleaving!the!ester! bonds,!since!the!amorphous!areas!are!less!organized!than!crystalline!regions!and!water! penetrates!more!easily.!Then,!the!hydrolysis!continues!at!the!boundary!of!the!crystalline! domains!once!the!amorphous!regions!are!degraded,!leading!to!an!increase!in!the!mass! loss!rate![10].!During!hydrolytic!degradation!the!number!of!carboxylic!acid!chain!ends! increases,!making!the!hydrolysis!a!self8catalyzed!reaction!due!to!the!accumulation!of! the!acidic!polymer!fragments!in!the!specimens![12814].!! 2.1.1! Mechanism! The! material! degradation! mechanism! of! PLA! can! be! either! surface! or! bulk! erosion.!The!difference!between!the!two!mechanisms!is!due!to!the!competition!between! the!diffusion!of!water!molecules!into!PLA!and!the!reaction!that!leads!to!the!hydrolysis!of! PLA![12].!Surface!erosion!proceeds!when!the!degradation!rate!is!much!higher!than!the! diffusion!of!water!molecules!and!the!hydrolysis!mainly!occurs!on!the!surface,!where!an! ! 22! ! ! insignificant!dependence!of!the!rate!of!weight!loss!per!unit!surface!area!on!thickness!is! exhibited! (Figure! 2.3)! [15].! In! contrast,! bulk! erosion! is! a! homogenous! degradation! mechanism! where! the! diffusion! rate! of! water! molecules! is! higher! than! the! hydrolysis! rate!of!the!ester!bonds!of!the!material![10].!During!bulk!erosion,!the!ester!bonds!cleave! randomly,!generating!carboxyl!groups!that!accelerate!the!degradation!and!a!decrease! of!molecular!weight!is!observed!over!the!degradation!time![16,!17].!The!homogeneous! degradation!mechanism!can!be!divided!into!three!stages:!(1)!absorption!of!water!into! the! PLA! matrix,! (2)! decrease! of! molecular! weight! without! weight! loss,! and! (3)! weight! loss!due!to!the!release!of!water8soluble!oligomers!and!monomers![10,!16,!18,!19].!!! ! Figure!2.3!Hydrolytic!degradation!mechanisms,!adapted!from!Tsuji![10].! ! Different!factors!can!determine!the!mechanism!of!the!hydrolytic!degradation,!for! instance,!the!thickness!of!the!material,!PLA!molecular!architecture!or!pH!of!the!media! as!will!be!discussed!in!the!next!section.!Generally,!PLA!proceeds!via!bulk!erosion!when! ! 23! ! ! thickness!is!lower!than!0.5!–!2!mm!and!surface!erosion!when!the!dimension!is!higher! than!7.4!mm![20].!When!material!thickness!is!between!0.5!–!2!mm!and!7.4!mm,!PLA! hydrolysis!is!mostly!via!core8accelerated!bulk!erosion!(Figure!2.3)!due!to!the!oligomers! and! monomers! with! high! catalytic! effect! trapped! and! accumulated! in! the! core! of! the! material!where!the!hydrolysis!begins![21].!! Molecular!architecture,!linear!or!branched,!can!affect!the!hydrolytic!degradation! mechanism!behavior!and!rate!because!of!the!number!of!terminal!hydroxyl!groups!per! unit! mass.! In! theory,! a! branched! PLA! with! higher! number! of! hydroxyl! groups! than! a! linear!polymer,!is!expected!to!have!faster!hydrolysis!reactions,!which!can!attract!more! water! molecules! and! the! cleavage! of! ester! bonds! occurs.! However,! the! terminal! hydroxyl! groups! can! form! hydrogen! bonds! with! the! next! terminal! hydroxyl! group! preventing!them!from!interacting!with!water!molecules!to!start!hydrolysis!reactions!with! the!ester!groups!around!the!terminal!hydroxyl!groups.!This!interaction!will!restrict!chain! mobility! as! well! disturbing! the! diffusion! of! water! molecules! into! the! PLA! matrix! and! therefore! delay! the! hydrolytic! degradation! resulting! in! surface! erosion! [22].! Tsuji! and! Hayashi![22]!studied!the!hydrolytic!degradation!of!linear!28arms!and!branched!48arms! PDLLA!where!the!48arms!resulted!in!degradation!via!surface!erosion!due!to!its!larger! number!of!terminal!hydroxyl!groups,!and!the!28arms!degraded!via!bulk!erosion.!! 2.1.2! Parameters!controlling!hydrolytic!degradation! 2.1.2.1$ pH$ The!hydrolysis!mechanism!depends!on!the!pH!of!the!media!due!to!the!different! susceptibility! of! the! ester! groups! in! lactic! acid! oligomers! (Figure! 2.4).! In! acidic! conditions! the! hydrolysis! proceeds! via! chain8end! scission,! whereas! in! alkaline! media! ! 24! ! ! the!mechanism!takes!place!via!backbiting!or!intramolecular!transesterification!to!form! lactide!units!and!lactic!acid!as!by8products![23].!! ! Figure!2.4!Hydrolytic!chain!cleavage!mechanisms!of!PLA!in!alkaline!(a)!and!acidic!(b)! media,!adapted!from!De!Jong!et!al.![23].! ! The!pH!of!the!media!contacting!the!polymer!influences!the!rate!of!hydrolysis!of! PLA.!PLA!chains!are!more!easily!degraded!in!strong!acidic!and!basic!media!where!the! ! 25! ! ! presence!of!hydronium!(H3O+)!and!hydroxide!(OH8)!ions!catalyzes!the!cleavage!of!ester! bonds! [12,! 24826].! When! PLA! is! exposed! to! basic! conditions,! the! carbonyl! carbon! atoms! of! the! polymer! are! susceptible! to! attack! by! the! hydroxide! ions! where! the! hydrolytic!degradation!and!the!molecular!weight!reduction!are!more!significant!than!in! acid!solutions![24,!25].!! Studies!have!shown!that!the!hydrolytic!degradation!of!PLA!immersed!in!neutral! and! acid! aqueous! medium! proceeds! via! a! bulk! erosion! mechanism,! resulting! in! the! decrease! of! molecular! weight! with! degradation! time! where! the! autocatalysis! is! an! important!factor!for!this!type!of!erosion![16,!17,!20].!Yuan!et!al.![16,!17]!showed!that! PLLA! fibers! faced! bulk! degradation! in! a! neutral! medium! at! 37! °C! where! the! process! was!slow,!and!the!changes!in!properties!were!not!detected!in!a!short!time.!In!the!study! of!hydrolysis!of!PLA!at!pH!7.4!by!von!Burkersroda!et!al.![20]!a!bulk!erosion!mechanism! was!found.!During!bulk!erosion!at!neutral!pH!the!polymer!take!up!water!homogeneously! and! causes! the! degradation! of! polymer! chains! all! over! the! matrix! creating! free! carboxylic!acid!groups.!These!carboxylic!acid!groups!lead!to!a!decrease!of!pH!inside! the!polymer,!accelerating!the!degradation!inside!the!polymer!bulk.!Since!the!surface!is! at!neutral!pH,!a!pH!gradient!is!created!that!slows!down!the!degradation!of!the!polymer! at! the! surface! and! the! surface! layer! breaks! when! a! critical! osmotic! pressure! forms! inside!the!polymer!matrix!due!to!the!accumulation!of!by8products!and!then!release!of! degradation! products! [20].! In! the! case! of! PLA! exposed! at! acidic! conditions! von! Burkersroda! et! al.! [20]! studied! the! hydrolysis! at! pH! 2! where! the! degradation! was! already!acid!catalyzed!and!found!no!pH/gradient!buildup!as!in!the!case!of!neutral!pH! ! 26! ! ! and!therefore!degradation!took!place!homogenously.!So,!autocatalysis!is!an!important! factor!for!the!bulk!erosion!mechanism.! Degradation! of! PLA! in! contact! with! alkaline! solutions! proceeds! via! surface! erosion![15,!20].!In!this!case,!the!hydroxyl!ions!present!at!the!surface!of!the!material!are! probably! consumed! by! the! carboxyl! groups,! products! from! PLA! degradation! taking! place!at!the!film!surface,!without!diffusing!into!the!PLA!core![15].!Von!Burkersroda!et!al.! [23]! showed! in! the! study! of! PLA! hydrolyisis! at! pH! 13! a! linear! erosion! profile! and! a! constant!molecular!weight!throughout!the!erosion.!Yuan!et!al.![17]!reported!that!in!dilute! alkaline! solutions,! PLA! can! exhibit! surface! and! bulk! degradation! where! the! viscosity8 average!molecular!weight!and!mass!of!PLA!fibers!decreased!gradually!with!degradation! time,! but! in! concentrated! alkaline! solutions! when! PLA! has! surface! erosion,! the! viscosity8average!molecular!weight!did!not!drop.!!! 2.1.2.2! Temperature! Hydrolytic! degradation! of! PLA! also! depends! on! the! temperature! to! which! the! polymer!is!exposed.!The!rate!of!hydrolysis!of!PLA!increases!with!temperature!due!to! the!faster!polymer!chain!scission!of!the!ester!bonds![12,!27829].!When!temperatures!are! above!the!glass!transition!temperature!(Tg)!the!rate!of!degradation!is!enhanced!by!the! mobility! of! the! polymer! chains! increasing! the! diffusion! of! water! molecules! into! the! polymer! matrix! and! starting! the! hydrolysis! reactions! where! the! formation! of! carboxyl! groups!increases![18,!30832].!! At! elevated! temperatures! the! molecular! weight! and! thermal! characteristics! of! PLA!are!affected.!Mitchell!and!Hirt![29]!found!that!samples!of!PLA!at!60!°C!showed!a! higher!polydispersity!index!(PDI)!than!at!40!°C!attributed!to!the!faster!degradation!of!the! ! 27! ! ! amorphous!region.!It!has!been!shown!that!when!the!temperature!of!the!environment!in! which! PLA! is! exposed! increases,! the! thermal! properties! such! as! Tg,! melting! temperature!(Tm)!and!crystallization!temperature!(Tc)!decrease![29,!32].!The!drop!in!Tg! can! be! explained! by! the! reduction! of! molecular! weight! and! the! plasticizing! effect! of! lactic! acid! oligomers! during! hydrolysis! [32].! Tm! and! Tc! are! also! shifted! to! lower! temperatures! due! to! the! fragmentation! of! polymer! chains! along! with! the! reduction! in! molecular!weight!and!the!susceptibility!of!amorphous!regions!to!hydrolysis![29,!32].!! To! study! the! effect! of! temperature! on! the! hydrolytic! degradation! of! PLA,! the! activation! energy! (Ea)! is! commonly! used! to! describe! how! the! kinetic! rate! of! the! reactions!changes!with!temperature.!The!Ea!is!described!by!the!Arrhenius!equation!and! is! usually! estimated! by! linearizing! the! equation! using! the! natural! logarithm.! Different! authors! have! estimated! the! Ea! for! the! hydrolytic! degradation! of! PLA! in! different! environments! within! a! range! of! 36! to! 110! kJ/mol! [25,! 29,! 30,! 33838].! These! environments!include!water,!high8pressure!steam,!100%!relative!humidity!(RH),!neutral,! basic! and! acidic! media! as! shown! in! Table! 2.1.! Depending! on! the! media! and! the! environment!these!values!change.!When!PLA!is!exposed!to!temperatures!above!Tg!the! chain!mobility!is!enhanced!allowing!the!diffusion!of!water!into!the!polymer!and!therefore! affecting! the! hydrolysis! rate.! So,! it! is! expected! that! experiments! performed! above! Tg! result! in! ready! hydrolytic! degradation! and! lower! Ea.! However,! some! authors! have! estimated!the!Ea!using!a!range!of!temperature!below!and!above!Tg!that!could!affect!the! ! final!estimation.!! ! ! 28! ! ! Table!2.1!Ea!values!reported!in!literature!for!the!hydrolytic!degradation!of!PLA!in! different!environment!at!different!temperature!range.!! Environment! Temperature,!°C! Ea,!kJ/mol! Reference! 100.5! 102.7! 83.26! 83.68! 75.2! 110! 100! 58! 73! 36.85! 53.23! 69.6! 49.6! 51.05! 110.04! 93.72! 87.2! pH!7.4! 37!–!70! pH!7.4! pH!7.4! pH!10.5! 21!–!45! 37!–!97! 50!–!70! pH!2! 40!–!120! Water! 140!–!180! Water! Water! 120!–!160! 170!–!250! 180!–!350! 100%!RH! 40!–!80! High8pressure!steam! 100!–!130! ! ! 29! ! [30]! [25]! [39]! [34]! [33]! [38]! [40]! [36]! [29]! [37]! ! 2.1.2.3! PLA!stereoisomers! The!properties!of!PLA!can!be!controlled!by!the!amount!of!isomers,!and!therefore! the! hydrolysis! rate! of! the! material! as! well.! Tsuji! [41]! studied! the! effect! of! L8lactide! content,! tacticity,! and! enantiomeric! polymer! blending! on! the! hydrolysis! of! PLA! films.! Non8blended! PDLLA,! PLLA,! PDLA! films! and! PLLA/PDLA! (1/1)! blend! (stereocomplex)! film!were!processed!in!an!amorphous!state!and!exposed!to!hydrolysis!in!a!phosphate8 buffered!solution!(pH!7.4)!at!37!°C!for!24!months.!Results!showed!that!the!hydrolysis! decreased!in!the!following!order:!nonblended!PDLLA!>!nonblended!PLLA!>!nonblended! PDLA! >! PLLA/PDLA(1/1)! blend.! The! high! hydrolyzability! of! the! nonblended! PDLLA! chains! compared! with! the! nonblended! PLLA! and! PDLA! was! attributed! to! the! lower! regularity.! The! low! regularity! of! PDLLA! films! decreased! the! intermolecular! interaction! making!it!more!susceptible!to!the!attack!of!water!molecules.!The!retarded!hydrolysis!of! PLLA/PDLA/(1/1)! blend! film! compared! with! the! nonblended! films! was! ascribed! to! the! strong!intermolecular!interactions!between!PLLA!and!PDLA!chains.! 2.1.2.4! Molecular!weight! Molecular!weight!of!PLA!is!an!important!parameter!that!influences!the!hydrolytic! degradation! of! the! polymer:! the! lower! the! molecular! weight,! the! faster! the! rate! of! hydrolysis.! When! PLA! has! low! molecular! weight! the! polymer! has! higher! molecular! mobility,! and! higher! density! of! hydrophilic! terminal! carboxyl! and! hydroxyl! groups! that! enhance! degradation.! Furthermore,! with! the! high! density! of! the! catalytic! terminal! carboxyl! group! the! formation! of! monomers! and! water8soluble! oligomers! during! hydrolysis!is!probably!higher.!All!factors!mentioned!above!should!be!noticeable!at!PLA! molecular!weight!lower!than!1!x!104!g/mol![10].! ! 30! ! ! Saha!and!Tsuji![42]!studied!the!effect!of!molecular!weight!and!D8lactide!units!on! the!hydrolytic!degradation!of!PLLA!in!phosphate8buffered!solution!at!37!°C!(Figure!2.5).! Different! PLLA! films! were! exposed! to! hydrolysis! PLLA1! (number! average! molecular! weight!(Mn)/=!4.09!x!105!g/mol!and!D8lactide!=!0%),!PLLA2!(Mn/=!1.22!x!105!g/mol!and! D8lactide! =! 1.2%)! and! PLLA3! (Mn/ =! 8.07! x! 104! g/mol! and! D8lactide! =! 0.2%).! The! hydrolytic!degradation!rate!constant!k/of!PLLA!increased!with!increasing!D8lactide.!They! found!that!the!effect!of!the!amount!of!D8lactide!on!the!hydrolysis!was!higher!than!that!of! Mn!in!the!first!period!(0!–!32!weeks).!On!the!contrary,!from!32!–!60!weeks!neither!D8 lactide!nor!Mn!affected!the!rate!of!degradation.!Other!studies!on!the!hydrolysis!of!PLLA! films! and! copolymer! of! L8lactide! with! D8lactide! [P(LLA8DLA)! (77/23)]! from! Saha! and! Tsuji!were!performed!(Figure!2.5)![43].!They!found!that!the!hydrolysis!rate!was!faster! for!P(LLA8DLA)!than!for!PLLA.!The!rate!constant!increased!with!the!increasing!water! absorption.! ! 31! ! ! ! Figure! 2.5!Change!in!Mn!during!hydrolytic!degradation!of!amorphous!PLLA!with!high! and!low!molecular!weights!(PLLA1!and!PLLA,!respectively),!and!different!ratios!of!D8 lactide! (PLLA2! [1.2%! D8lactide],! PLLA3! [0.2%! D8lactide],! P(LLA8DLA)! [77/23])! in! phosphate8buffered!solutions!at!37!°C,!adapted!from!Saha!and!Tsuji![42,!43].! ! Çelikkaya!et!al.![44]!determined!the!rate!of!the!hydrolytic!degradation!of!PDLLA! with!different!molecular!weights!(Mn/=!7,300,!12,100!and!21,900!g/mol)!(Figure!2.6).!It! was! found! that! PDLLA! with! low! Mn! degraded! faster! than! PDLLA! with! higher! Mn.!This! behavior!was!explained!by!the!easy!penetration!of!water!molecules!within!the!polymer! matrix! with! low! molecular! weight! due! to! the! lower! entanglement! and! the! higher! hydrophilicity!caused!by!the!high!number!of!carboxyl!and!hydroxyl!end!groups.! ! 32! ! ! Figure! 2.6! Hydrolytic! degradation! of! PDLLA! with! different! molecular! weights! in! phosphate8buffered!solutions!at!37!°C,!adapted!from!Çelikkaya!et!al.![44].! ! ! 2.1.3! Structural!changes! 2.1.3.1! Crystallinity! PLA!crystallinity!has!been!studied!where!crystal!structures!have!been!reported! such! as! α,! α'! (δ),! β! and! γ! related! to! the! crystallization! conditions! [45].! The! crystal! structure! α8form,! which! is! an! orthorhombic! unit! cell! of! parameters! a! =! 1.066! nm,! b! =! 0.616!nm,!and!c!=!2.888!nm,!can!be!obtained!from!solution!casting,!melt!crystallization! and!annealing!of!the!glassy!state![46848].!This!crystal!form!with!a!Tm!=!185!°C!and!Tc! near! 1408155! °C! is! a! more! stable! crystal! than! the! β8form,! a! trigonal! unit! cell! of! parameters!a!=!b!=!1.052!nm!and!c!=!0.88!nm,!with!a!Tm!=!175!°C!and!Tc!?"@AB ! (Eq.!2.1)! ! where!Xt!is!the!relative!crystallinity,!t!is!time,!and!K!and!n!represent!the!crystallization! rate!constant!and!the!Avrami!exponent,!respectively.!K!is!a!growth!rate!constant!that! considers! the! nucleation! and! the! growth! rate! parameters,! while! n/ is! a! mechanism! constant!that!depends!on!the!growth!dimension!and!form!of!nucleation![65].!An!n/value! close! to! 3! is! correlated! to! a! spherical! structure! resulting! from! the! process! of! instantaneous! nucleation! controlled! by! diffusion.! Non8integral! n/ values! show! a! ! 38! ! ! combination! of! thermal! and! athermal! nucleation! mechanisms! [67].! Generally,! the! n/ values!reported!in!literature!are!around!2!and!4!for!PLA![65,!68,!69].! During!hydrolytic!degradation!PLA!suffers!structural!changes![18,!42,!70].!Zhang! et! al.! [18]! studied! the! morphological! behavior! during! hydrolysis! of! two! types! of! PLA:! semicrystalline! (1.4%! D8lactic! acid)! and! amorphous! polymer! (12%! D8lactic! acid).! The! molecular!weight!of!both!PLA!polymers!decreased!during!the!degradation!time!while!the! percentage! of! crystallinity! increased,! although! amorphous! PLA! degraded! much! faster! than! semicrystalline! PLA.! During! degradation,! α8form! crystals! were! mainly! formed! in! both!PLA!structures.!The!main!reason!for!the!formation!of!crystals!in!amorphous!PLA!is! chain! orientation! and! morphological! rearrangement! that! occurs! in! PLA! when! the! molecular! weight! decreases! during! the! degradation! process.! With! the! decrease! in! molecular!weight,!the!short!PLA!chains!have!the!ability!to!form!crystalline!structures.! The! thickness! of! PLA! materials! has! an! effect! on! the! crystallinity! during! the! hydrolysis!of!the!polymer.!It!has!been!shown!that!the!crystallinity!of!PLLA!fibers!with! different! diameters! increases! over! the! hydrolysis! process,! in! which! fibers! with! small! diameters!have!higher!crystallinity!suggesting!faster!degradation!than!fibers!with!bigger! diameters![16,!29].!During!the!degradation!time,!there!is!a!gradual!increase!in!molecular! regularity! due! to! the! decrease! of! molecular! entanglements,! which! is! reflected! in! the! crystallinity.! The! faster! the! hydrolysis! is,! the! faster! is! the! rearrangement! of! PLA! segments.!Similarly,!Tsuji!and!Ikada![26]!concluded!that!the!rate!of!weight!loss!per!unit! surface!area!of!PLLA!films!decreased!linearly!with!the!increase!in!PLLA!crystallinity.!! Temperature!is!a!main!parameter!that!influences!the!hydrolytic!degradation!rate! and!therefore!the!residual!crystallinity!of!PLA!materials.!Mitchell!and!Hirt![29]!found!that! ! 39! ! ! the!crystallinity!of!PLA!fibers!exposed!to!80!°C!at!100%!RH!increased!rapidly!between!5! and!6!days!(~!73%)!comparing!to!those!exposed!at!40!and!60!°C!(~!15%!and!~!40%,! respectively).!The!PDI!of!samples!also!was!affected!during!hydrolysis!where!samples! tested!at!40!°C!showed!a!lower!increase!in!the!PDI!than!samples!tested!at!60!°C!where! the!PDI!changed!from!2.0!originally!to!4.0!at!day!7.!The!increase!in!PDI!was!explained! by! the! degradation! of! the! amorphous! region! where! high! temperatures! drive! faster! hydrolysis.!! 2.1.3.2! Swelling! The! stability! or! durability! of! PLA! when! it! is! exposed! to! different! environments! such!as!water!and/or!organic!solvents!is!crucial!when!PLA!is!used!as!a!container,!bottle! or! other! vessel.! The! exposure! to! different! solvent! solutions! can! lead! to! structural! changes!in!PLA!that!can!modify!the!rate!of!hydrolysis.!Swelling!of!the!polymer!is!one!of! the! structural! changes! that! PLA! can! experience! when! it! is! in! contact! with! organic! solvents![71,!72].!! The!swelling!of!a!polymeric!material!occurs!when!the!solvent!molecules!diffuse! and!are!absorbed!into!the!amorphous!region,!where!the!amount!of!solvent!ad/absorbed! increases!according!to!the!affinity!with!the!polymer![72].!The!swelling!is!reflected!in!the! softening!of!the!polymer,!resulting!in!the!movement!and!the!alignment!of!the!polymer! chains,! which! may! induce! crystallization! [54,! 71].! Different! polymers! experience! SIC! when! they! absorb! organic! solvents,! such! as! poly(ethylene! terephthalate),! poly(methyl! methacrylate),!polycarbonate!of!bisphenol!A!and!poly(styrene)![73877].! SIC! is! controlled! by! the! transport! of! the! organic! solvent! in! glassy! polymers.! According! to! Ouyang! et! al.! the! transport! of! acetone! in! PET! induces! crystallization! in! ! 40! ! ! three!stages![75,!78].!The!first!stage!is!the!transport!of!the!solvent!by!diffusion,!which!is! controlled! by! the! concentration! gradient.! The! second! stage! involves! the! dramatic! increase!of!swelling!in!the!polymer!by!the!solvent!and!primary!crystallization!takes!place! where!a!large!amount!of!crystals!are!formed!by!the!release!of!internal!stress.!Finally,! the!third!stage!of!SIC!involves!secondary!crystallization!when!the!system!is!saturated! and!the!rate!of!crystallization!decreases.! The!density!of!the!polymer!is!affected!by!the!SIC.!It!has!been!shown!that!high! swelling! of! PLA! depends! on! the! organic! solvent! permeation! through! the! polymer,! inducing!a!reduction!in!the!PLA!density!leading!to!porous!structures![54].!Sato!et!al.![54]! found! that! the! density! of! amorphous! PLA! (L/D! ranged! from! 96.0/4.0! to! 96.8/3.2)! decreased! with! increasing! crystallinity! due! to! solvent! exposure! and! no! linear! relationship! was! observed! between! crystallinity! and! film! density.! Usually,! crystalline! polymers! are! denser! than! amorphous,! but! in! the! case! of! PLA! crystallized! by! organic! solvents!the!crystals!are!more!dispersed!than!amorphous!polymers.!! Tsuji! and! Sumida! [72]! studied! the! degree! of! swelling! of! PLLA! when! it! was! exposed!to!ethanol!at!25!°C!for!7!days.!They!found!that!swelling!with!ethanol!reached! an! equilibrium! after! 20! min! of! immersion! and! the! Tm/ and! the! crystallinity! of! PLA! increased!by!that!time.!Pitt!and!Zhong8wei![71]!determined!the!effect!of!water!(pH!7.4),! ethanol!and!pentanol!at!37!°C.!!They!found!that!crystallization!of!PLLA!occurred!during! degradation! in! ethanol,! causing! alignment! of! polymer! chains! after! 20! days,! indicating! the!plasticization!effect!of!ethanol,!while!the!degree!of!crystallinity!remained!unchanged! in!the!presence!of!water!during!80!days!of!exposure.!It!is!important!to!mention!that!the! degree!of!polymer!crystallinity!affects!the!rate!of!hydrolytic!degradation!since!hydrolysis! ! 41! ! ! depends! on! the! diffusion! of! water! molecules! through! amorphous! regions! to! start! the! cleavage!of!ester!bonds.!!! Tsai!et!al.![79]!showed!in!the!study!of!physical!changes!and!sorption/desorption! behavior! of! amorphous! and! semi8crystalline! PLLA! exposed! to! water,! methanol! and! ethanol!at!37!°C!that!water!in!the!glassy!structure!had!a!plasticizing!effect.!However,!the! plasticization!was!not!sufficient!to!induce!cold8crystallization.!In!the!case!of!ethanol!they! induced!cold8crystallization!in!PLLA!showing!a!complex!sorption!and!desorption!kinetics! due! to! the! progressing! cold8crystallization! on! sorption.! In! the! analysis! of! the! crystallization!kinetics!using!the!Avrami!equation!(Eq.!2.1)!the!n!values!were!found!to! be! very! low,! 0.5! and! 1! for! ethanol! and! methanol,! respectively.! These! low! values! suggested!restricted!crystal!growth!in!terms!of!chain!mobility!and!dimensionality.! When!PLA!has!been!exposed!to!different!solvents,!SIC!has!led!to!the!formation! of! different! crystals.! Tsai! et! al.! [79]! found! that! methanol! and! ethanol! induced! the! formation!of!α’8form!crystals!at!37!°C.!Sato!et!al.![54]!found!a!mixture!of!α8!and!β8form! when!PLA!was!exposed!to!60!inorganic!solvents!at!35!°C!for!24!h.!They!concluded!that! crystallinity!is!not!dependent!of!the!organic!solvent!but!on!the!degree!of!swelling.!When! films! had! crystallinity! between! 0! –! 5%! they! mainly! had! β8form! crystals.! On! the! other! hand,!samples!with!crystallinity!ranging!around!25%!formed!a!mixture!of!α8!and!β8form,! and!samples!with!crystallinity!around!40%!had!α8form!crystals.!! Other!studies!investigated!the!crystalline!complex!formation!of!new!crystals!“ε8 form”!in!PLLA!in!contact!with!low!molecular!weight!compounds,!such!as!five8membered! (cyclopentanone,! 1,38dioxolane,! C 8butyrolactone! and! THF)! and! N,N8 ring! dimethylformamide!(DMF)!below!room!temperature.!The!ε8crystals!had!an!orthorhombic! ! 42! ! ! unit! cell! with! parameters! a! =! 1.5! –! 1.6! nm,! b! =! 1.2! –! 1.3! nm! and! c/ =! 2.8! –! 2.9! nm.! Further!research!is!needed!to!fully!understand!the!swelling!and!SIC!of!PLA!by!different! solvents.!! 2.2! Chemical!recycling:!Application!of!hydrolytic!degradation! As! stated! above,! PLA! is! susceptible! to! hydrolytic! degradation,! so! this! can! be! effectively! used! for! the! chemical! recycling! of! PLA! to! its! monomer! [1].! One! of! the! preferable!recovering!routes!of!PLA!as!many!other!polymers!is!recycling,!which!can!be! either! mechanical! or! chemical! [6].! Mechanical! recycling! includes! recovering,! sorting,! regrinding! and! reprocessing! of! PLA! [80].! However,! this! process! results! in! the! deterioration!of!PLA!physical!properties!where!the!molecular!weight!is!reduced!due!to! the!exposure!to!high!temperatures!and!shear!during!reprocessing!of!PLA![81].!On!the! other!hand,!chemical!recycling!of!PLA!recovers!the!LA!by!hydrolysis!which!can!then!be! used! as! a! raw! material! for! new! PLA! production! with! the! same! properties! as! virgin! materials![81,!82].!! Different! chemical! recycling! methods! have! been! used! to! recover! PLAl! one! is! hydrolysis! or! solvolysis! to! obtain! L8LA! or! D8LA! oligomers,! and! another! is! the! depolymerization!of!PLA!to!cyclic!dimer,!L8lactide.!Some!patents!have!been!granted!for! the!depolymerization!of!PLA!by!hydrolysis!or!solvolysis![83885].!The!conditions!used!for! the!chemical!recycling!of!PLA!to!give!high!yield!of!monomers!in!short!period!and!avoid! the! removal! of! catalysts! and! additives! to! save! energy! and! cost! must! be! closely! monitored.!During!the!depolymerization!of!PLA!it!is!important!to!avoid!the!racemization! and!decomposition!of!LA,!and!also!to!consider!the!optical!purity!of!the!degraded!PLA! and!the!LA!formed![36].!Chemical!recycling!of!PLA!has!been!conducted!with!different! ! 43! ! ! solutions,! such! as! water,! alcohols! and! other! organic! solvents,! which! are! explained!in! more!detail!in!the!following!section.! 2.2.1! Water! Water! has! been! used! as! solvent! for! chemical! recycling! to! avoid! the! use! of! catalysts!and!additives.!Brake!and!Subramanian![83]!rapidly!hydrolyzed!PLA!with!water! to! water8soluble! monomers! and! oligomers! in! 3! and! 0.5! h! at! 120! and! 170! °C,! respectively.!However,!the!yield!of!formed!LA,!the!optical!purity!of!the!PLA!used!and!the! optical! purity! of! the! LA! formed! were! not! characterized.! This! is! important! since! racemization!and!decomposition!of!LA!can!occur!at!high!temperatures.!! ! Tsuji! et! al.! [36]! studied! the! hydrolysis! of! PLLA! in! the! melt! in! high8temperature! and! high8pressure! water! at! the! temperature! range! of! 180! –! 350! °C! with! no! catalyst! resulting! in! homogeneous! and! random! hydrolysis! via! bulk! erosion! mechanism.! They! investigated!the!formation,!racemization!and!decomposition!of!LA.!A!yield!of!90%!was! obtained!at!250!°C!in!10!–!20!min.!When!the!temperature!increased!to!350!°C,!the!yield! decreased!due!to!the!dramatic!racemization!and!decomposition!of!the!monomers.!Tsuji! et!al.![40]!also!investigated!the!difference!of!performing!the!hydrolysis!in!the!melt!(170!–! 190!°C)!and!in!the!solid!state!(120!–!150!°C).!The!hydrolysis!was!homogeneous!and! random!via!a!bulk!erosion!mechanism.!Crystalline!residues!were!formed!below!140!°C! but!not!at!degradation!temperatures!above!170!°C.!The!presence!of!crystalline!residues! could!slow!the!decrease!of!the!Mn!in!the!late!stages.!The!LA!yield!exceeding!95%!was! obtained!at!the!range!of!temperature!studied,!but!the!difference!was!the!time!to!get!the! maximum!yield.!! ! 44! ! ! 2.2.2! Alcohols! Chemical! recycling! by! solvolysis! of! PLA! by! transesterification! with! alcohol! to! lactate!esters!is!an!alternative!process!that!has!been!applied!by!several!authors.!The! use!of!catalysts!was!necessary!in!this!process.!!Sánchez!and!Collinson![86]!studied!the! chemical!recycling!by!alcoholysis!of!PLA!in!methanol!and!ethanol!using!zinc!acetate!as! a! catalyst! (Figure! 2.9).! They! found! that! better! yields! were! achieved! in! the! catalyzed! methanolysis!than!ethanolysis!since!the!methanol!molecule!is!a!better!nucleophile!than! ethanol! due! to! its! smaller! size! and! higher! dielectric! constantsl! therefore,! the! reaction! with!the!carbonyl!is!faster.!Leibfarth!et!al.![87]!reported!the!organocatalytic!recycling!of! postconsumer! PLA! into! lactate! esters! using! triazabicyclodecene! in! the! presence! of! alcohols!(i.e.,!methanol,!ethanol,!butanol).!The!depolymerization!of!PLA!was!completed! in!minutes!at!room!temperature,!retaining!the!stereochemistry!of!the!lactates.! ! Figure!2.9!Alcoholysis!of!PLA,!adapted!from!Sánchez!and!Collinson![86].! 2.2.3! Other!organic!solvents! Other! different! organic! solvents! besides! alcohols! have! been! applied! for! PLA! chemical!recycling.!Gironi!et!al.![88]!used!acetone!and!ethyl!lactate!without!catalysts!to! ! 45! ! ! study! the! dissolution! behavior! of! PLA! at! different! water! concentrations,! finding! that! acetone! based! solvents! were! more! effective! in! solubilizing! PLA.! One! of! their! conclusions! was! that! the! increase! of! water! concentration! in! the! solvent! phase,! determines! a! reduction! of! the! solvent! power! and! a! reduction! of! the! mass! transport! coefficient!for!the!two!solvents!tested.! ! Okamoto!et!al.![89]!degraded!PLLA!(weight!average!molecular!weight!(Mw)!=!120! 000! Da)! by! montmorillonite! K10! (MK10)! in! toluene! at! 100! °C! for! 6! h! resulting! in! oligomers!with!Mw!of!a!few!hundreds!in!a!yield!greater!than!90%.!No!isomerization!of!L8 LA!occurred!and!the!degradation!was!accelerated!by!the!addition!of!a!small!amount!of! ethanol! in! toluene! using! anhydrous! MK10! to! produce! oligomers! with! ethyl! ester! end! groups.! Chloroform,! hexane! and! o8xylene! were! used! by! Takahashi! et! al.! [90]! for! the! degradation! of! PDLLA,! PDLA! and! PLLA! using! the! enzyme! lipase! as! a! catalyst! to! produce! cyclic! oligomers.! They! found! that! adding! a! small! amount! of! water! to! the! o8 xylene!solution!significantly!accelerated!the!enzymatic!degradation!of!PLLA.!Toluene,! xylene,! and! chloroform! have! also! been! used! for! the! chemical! recycling! of! PLA! via! controlled! degradation! with! protic! (macro)! molecules! (e.g.! diols,! dipentaerythriol,! diamines,!adipic!acid!or!oligo(ethylene!glycol))!using!catalysts![91].!Although!the!use!of! enzymes! as! catalysts! can! help! the! degradation! of! PLA! at! low! temperatures,! the! application! of! enzymes! is! generally! dependent! on! factors! such! as! their! stability! and! solubility!in!an!organic!solvent!and!substrate!specificity!of!the!enzyme![90,!92].!Other! factors!can!limit!the!degradation!conditions,!such!as!temperature,!solvent!and!polymer! concentration.! ! 46! ! ! Chemical! recycling! of! PLA! based! polymer! blends! has! been! reported! by! Tsuneizumi! et! al.! [93]! for! PLA/polyethylene! (PE)! and! PLA/poly(butylene! succinate)! (PBS)!blends!using!clay!catalysts!and!enzymes!for!selective!chemical!recycling.!Two! routes!were!applied!for!PLA/PE!blends.!The!first!route!was!the!direct!separation!of!PLA! and!PE!due!to!their!different!solubilities!in!toluene!followed!by!chemical!recycling!of!PLA! using! montmorillonite! K5! clay! (MK5).! The! second! route! was! selective! degradation! of! PLA!in!the!blend!by!MK5!in!toluene!at!100!°C!forming!LA!oligomers!with!low!molecular! weight.!The!PE!was!recovered!by!the!re8precipitation!method.!Similar!procedures!were! used!for!PLA/PBS!blends.!The!first!route!was!the!direct!separation!of!PLA!and!PBS!by! their!solubility!in!toluenel!and!the!second!route!was!sequential!degradation!of!the!blend! using!lipase!to!degrade!PBS!and!then!PLA!was!degraded!by!MK5.! 2.3! Modification!of!PLA! 2.3.1! Nanoparticles:!organomodified!montmorillonite!(OMMT)! Nanocomposites!are!materials!where!at!least!one!of!the!particles!incorporated!is! of!nanometric!size.!In!general,!three!different!categories!of!nanofillers!exist,!which!are! distinguished! by! the! number! of! nanometric! dimensions! that! they! have.! Nanotubes! or! whiskers! are! elongated! nanofillers! with! two! dimensions! in! the! nanometer! range.! Isodimensional! nanoparticles! (i.e.! spherical! particles)! are! particles! where! the! three! dimensions!are!nanometric.!Plate8like!nanofiller!is!a!layered!material!with!a!thickness!on! the!order!of!1!nm!with!an!aspect!ratio!around!25!with!the!other!two!dimensions.!Layered! silicates! are! among! the! most! popular! plate8like! nanofillers,! such! as! smectic! clays,! graphene!sheets!and!layered!double!hydroxides![94,!95].!! ! 47! ! ! 2.3.1.1! Characteristics! 2.3.1.1.1!Structure!and!properties! Montmorillonite! (MMT)! is! a! clay! that! belongs! to! a! layered! smectite! group,! also! known!as!2:1!phillosilicates,!which!consists!of!several!stacked!layers!with!a!thickness!of! 1! nm! and! lateral! dimensions! from! 30! nm! to! several! microns.! The! crystal! structure! consists! of! two! silica! tetrahedral! sheets! fused! to! an! edge8shared! octahedral! sheet! of! aluminum! (Figure! 2.10)! [94,! 96].! The! aspect! ratio! is! about! 1081000! and! the! surface! area!is!in!the!range!of!750!m2/g![97].!The!parallel!layers!are!linked!together!by!van!der! Waals!forces!with!a!gap!between!the!layers!known!as!the!gallery!or!interlayer.!The!clay! layer!is!negatively!charged,!which!is!counterbalanced!by!exchangeable!cations!such!as! Na+,!K+,!Li+!and!Ca2+!located!within!the!galleries!of!the!silicate!layers.!The!cations!are! not! strongly! bound! to! the! clay! surface,! and! therefore! can! be! replaced! with! other! molecules! (e.g.,! polar! compounds)! [95,! 98].! The! general! chemical! formula! of! MMT! is! D+×F786 GHI:JD&JKLM68N67 I !where!D ! D=OPQ,5P8Q,D&8Q,>AS. !is! the! interlayer!cation![99].! ! 48! ! ! ! ~1!nm ! ! g n i c a p s ! l a s a B Exchangeable!cations Tetrahedral Octahedral Tetrahedral Al,!Mg OH O Si Li,!Na,!Rb,!Cs Figure!2.10!Structure!of!2:1!layered!silicates,!adapted!from!Ray!and!Okamoto![96].! ! 2.3.1.1.2!Surface!modification! ! MMT!is!a!hydrophilic!clay,!making!it!poorly!suited!to!interact!with!hydrophobic!or! less! hydrophilic! polymers! like! PLA,! and! therefore! a! modified! surface! is! necessary! by! exchanging! the! cations! in! the! galleries! for! organic! surfactants! including! primary,! secondary,! tertiary! or! quaternary! alkylammonium! or! alkylphosphonium! cations! [96].! Since!the!interlayer!height!of!clay!before!modification!is!relatively!small,!the!long!chain! of! alkylammonium! cations! decrease! the! surface! energy! of! MMT! and! the! interlayer! spacing!expands,!enhancing!the!interaction!between!the!clay!and!the!polymer!resulting! in!a!well!dispersed!clay!within!the!polymer!matrix!(Figure!2.11)![98].!When!a!nanoclay! ! 49! ! ! has! been! modified,! it! is! common! to! identify! it! as! an! organoclay.! For! instance,! when! MMT!has!been!exposed!to!surface!modification,!it!could!be!called!!organomodifed!MMT! (OMMT).!Table!2.3!shows!common!organomodifier!names,!structures!and!commercial! names!used!for!preparing!PLA!nanocomposites.!! ! Figure! 2.11! Organomodification! of! layered! silicates! with! quaternary! ammonium,! adapted!from!Raquez!et!al.![95].! ! ! ! ! 50! ! ! Table! 2.3!Common!modified!montmorillonites!used!for!PLA8nanocomposites,!adapted! from!Souza!et!al.![100].! Modifier! Structure! Commercial!name! Methyl,!tallow,!bis828 hydroxyethyl,!quaternary! Dimethyl,! dehydrogenated!tallow,! 28ethylhexyl!quaternary! C3H ammonium! CH2CH2OH C3H N+ T CH2CH2OH ! CLOISITE!30B/Southern! Clay!Products!! CH3 N+ HT CHCH2CH2CH2CH2CH3 CH2CH3 ! CLOISITE!25A/Southern! Clay!Products! Dimethyl,!dihydrogenated! tallow,!quaternary! C3H ammonium! Methyl,!dihydrogenated! tallow,!quaternary! C3H ammonium! Octadecyl!ammonium! H CH3 N+ HT H N+ HT H N+ CLOISITE!20A/Southern! Clay!Products! CLOISITE!15A/Southern! Clay!Products! CLOISITE!93A/Southern! Clay!Products! HT HT ! ! H NANOMER!1.30P/Nanocor! Stearyl!dihydroxyethyl! ammonium! HT H N+ ! ! C18H37 ! O O H H n n ! 51! ! NANOFIL!804/Süd8Chemie! ! Table!2.3!(Cont’d)! Modifier! Structure! Commercial!name! Dimethyl!benzyl! hydrogenated!tallow! ammonium! DELLITE!43B! ! *T!is!tallow!with!around!65%!C18,!30%!C16!and!5%!C14! The! dispersion! of! the! silicate! layers! depends! on! the! chemical! structure! of! the! organomodifier! used! in! the! organoclay.! In! the! case! of! PLA,! the! dispersion! and! de8 agglomeration!of!the!layers!within!the!polymer!matrix!depends!on!the!hydrogen8bonding! interaction!between!the!carbonyl!groups!of!PLA!chains!and!the!hydroxyl!groups!in!the! organomodifier! of! the! organoclay! [101].! Furthermore,! strong! interactions! could! also! occur!between!the!hydroxyl!end!groups!of!PLA!chains!and!the!MMT!platelet!surfaces,! or!the!hydroxyl!groups!of!the!surfactant!in!the!OMMT![102].!Therefore,!the!properties!of! the!nanocomposites!are!highly!dependent!on!the!distribution!of!the!silicate!layer!in!the! polymer!matrix.!! 2.3.1.1.3!Morphology!! ! ! The!main!techniques!for!the!preparation!of!PLA8clay!nanocomposites!are!in/situ! intercalation,! intercalation! of! polymer! from! solution! and! melt! intercalation.! In/ situ! intercalation!is!when!the!nanofillers!are!dispersed!in!liquid!monomer!and!the!monomer! is!absorbed!into!the!clay!interplanar!spaces,!followed!by!polymerization.!Intercalation!of! polymer! from! solution! requires! the! dissolution! of! the! polymer! in! a! solvent! capable! of! solubilizing! the! polymer! and! swelling! the! layers! of! the! nanoclay,! followed! by! the! ! 52! ! ! evaporation! of! the! solvent! or! precipitation! [103,! 104].! Melt! intercalation,! which! is! the! most! industrially! viable! method,! involves! the! direct! melt8mixing! of! dry! polymer! pellets! with!the!nanoclay!powders!under!shearing!action!at!a!temperature!above!the!polymer! melting!point!that!facilitates!the!diffusion!of!the!polymer!chains!into!the!clay!galleries!to! achieve!a!good!dispersion!of!silicate!layers!within!the!polymer![105,!106].! The!compatibility!between!the!nanoparticles!and!the!polymer!matrix!is!the!main! issue!to!obtain!a!good!dispersion!of!layered!silicates,!which!dictates!the!final!properties! of!the!nanocomposites.!When!the!affinity!is!low,!the!silicate!layers!are!not!intercalated! with!the!polymerl!in!other!words,!the!clay!particles!are!not!delaminated,!resulting!in!a! material! with! similar! properties! to! micro8composites! (Figure! 2.12).! On! the! contrary,! when! affinity! exists! the! results! are! either! intercalated! or! exfoliated! nanocomposite.! Intercalated! nanocomposites! are! normally! when! the! polymer! chains! are! inserted! between! the! silicate! layers,! leading! to! the! reinforcement! of! the! material.! Exfoliated! nanocomposites!occur!when!each!individual!layer!is!dispersed!within!the!polymer!and! the!structure!is!fully!delaminated,!resulting!in!the!maximal!property!enhancement![96,! 107].!Therefore,!to!achieve!miscibility!of!layered!silicates!with!biodegradable!polymers! such!as!PLA,!it!is!necessary!to!modify!the!hydrophilic!silicate!surface!to!an!organophilic! one.! ! 53! ! ! Figure! 2.12! Types! of! nanocomposites! derived! from! interaction! between! clays! and! polymers,!adapted!from!Alexandre!and!Dubois![108].! ! 2.3.1.1.4!Benefits!of!PLAQnanocomposite! ! PLA8nanocomposites!have!improved!properties!compared!to!neat!PLA,!such!as! mechanical,!thermal,!and!barrier!properties.!Mechanical!properties!are!enhanced!due!to! strong!interfacial!interaction!or!adhesion!between!the!nanoclay!and!the!polymer!matrix! and! the! level! of! intercalation! [109].! The! improvement! of! tensile! modulus,! as! a! mechanical! property,! is! also! attributed! to! the! presence! of! small! tactoids! and! the! absence!of!agglomerates,!rather!than!exfoliation![110].!Regarding!barrier!properties,!the! presence!of!clay!in!the!polymer!increases!the!dwelling!times!of!the!penetrant!around! accessible!clays,!and!therefore,!the!barrier!properties!of!the!polymer!are!improved![111].! Ray!et!al.![112]!prepared!PLA8OMMT!nanocomposites!with!loadings!of!4,!5!and! 7!wt%.!The!silicate!layers!were!intercalated!and!well!distributed!in!the!PLA!matrix.!To! ! 54! ! ! determine! the! dispersion! of! the! clays,! X8ray! diffraction! and! transmission! electron! microscope!analyses!were!performed.!All!the!nanocomposites!showed!an!increase!in! storage! modulus,! flexural! modulus,! flexural! strength,! heat! distortion! temperature,! and! gas!barrier!properties!compared!to!neat!PLA!(Table!2.4).!Maiti!et!al.![113],!studied!the! enhancement! of! PLA! properties! incorporating! OMMT! (3.8! wt%),! where! the! O2! gas! permeability!of!PLA!was!decreased!from!3.31!to!2.00!x10824!kg!m!m82!s81!Pa81.!Lewitus!et! al.! [114]! observed! that! when! OMMT! was! incorporated! at! 5%! loading! level! into! PLLA! films! the! tensile! modulus! and! the! elongation! increased! by! 30! and! 40%,! respectively,! and!the!cold8crystallization!temperature!(Tcc)!decreased!by!15!°C!due!to!the!nucleation! effect,!compared!to!neat!PLLA.!! ! Table! 2.4! Comparison! of! material! properties! between! neat! PLA! and! PLA8OMMT! nanocomposites!with!different!amounts!of!OMMT,!adapted!from!Ray!et!al.![112].! Material! properties! PLA! PLAQOMMT!(4%)! PLAQOMMT!(5%)! PLAQOMMT!(7%)! Modulus!(GPa)! Strength!(MPa)! Distortion!al!break! (%)! PPLA8OMMT/PPLA!*! 4.8! 86! 1.9! 1! 5.5! 134! 3.1! 0.88! 5.6! 122! 2.6! 0.85! 5.8! 105! 2! 0.81! *!PPLA8OMMT/PPLA:!Relative!permeability!coefficient!of!O2! ! ! 55! ! ! 2.3.1.2! Hydrolytic!degradation! PLA! properties! are! enhanced! when! layered! silicate! nanoparticles! are! incorporated! in! the! polymer! matrix! as! compared! with! the! unfilled! PLA! and! its! microcomposites.! However,! these! engineered! nanoparticles! have! an! effect! on! the! hydrolytic!degradation!of!PLA!and!depending!on!the!final!application,!the!influence!of! the!nanoparticles!on!the!hydrolysis!process!could!be!favorable!or!not.! Several!authors!have!studied!the!hydrolytic!degradation!of!PLA8nanocomposites! based!on!MMT.!Zhou!and!Xanthos![34]!studied!the!accelerated!hydrolytic!degradation! of!amorphous!(PDLLA)!and!semicrystalline!PLLA!(6%!D8lactide)!containing!unmodified! and!organically!modified!MMT!between!50!and!70!°C.!The!effects!of!polymer!type!on! the! degradation! rate! constant! showed! that! the! unfilled! PDLLA! and! its! composites! displayed!higher!hydrolysis!than!unfilled!PLLA!and!its!composites.!The!reason!was!due! to!the!amount!of!crystalline!regions!that!reduce!the!mobility!of!polymer!chains!and!FV,! and!therefore!the!water!absorption.!Nanocomposites!showed!higher!degradation!rates! than!the!unfilled!PLA,!and!the!degradation!of!the!unmodified!MMT!(microcomposites),! was!slightly!lower!than!unfilled!polymer.!The!study!demonstrated!that!high!dispersion!of! the!particles!in!the!nanocomposites!accelerates!the!hydrolytic!degradation.! The!water!uptake!of!nanocomposites!has!a!great!influence!on!hydrolysis!rates! compared! with! neat! polymers.! Zhou! and! Xanthos! [34]! observed! a! higher! amount! of! water! absorbed! by! PLA! nanocomposites! comparing! with! pristine! PLA.! When! nanoparticles!are!incorporated!in!PLA,!their!large!water!sorption!capacity!increase!the! water!sorption!of!the!nanocomposites.!Also,!organomodified!MMT,!which!was!treated! with! hydrophobic! organic! compounds,! lost! the! capacity! of! neutralizing! the! carboxylic! ! 56! ! ! groups!formed!by!the!cleavage!of!polymer!chains!during!hydrolysis.!! The!effect!of!organomodifier!or!surfactant!has!also!been!studied.!Paul!et!al.![115]! studied! the! hydrolytic! degradation! in! phosphate! buffer! solution! (pH! 7.4)! of! two! PLA8 OMMT! nanocomposites! based! on! different! surfactants,! one! more! hydrophobic! (Cloisite®25A)! and! the! other! more! hydrophilic! (Cloisite®30B).! After! five! months,! the! reduction! in! Mn/ of! unfilled! PLA! was! 41.6%,! while! the! nanocomposites! made! with! Cloisite®25A!and!Cloisite®30B!were!71.2%!and!79.2%,!respectively.!In!the!presence!of! Cloisite®30B,!the!Tg!of!PLA!started!to!decrease!after!one!month,!while!the!Tg!!of!PLA! with! Cloisite®25A! decreased! only! after! more! than! two! and! a! half! months! due! to! the! reduction!in!Mn!and!the!plasticizing!effect!of!LA!oligomers!formed!during!degradation.! The! more! hydrophilic! the! filler,! the! faster! the! hydrolysis.! However,! is! important! to! mention! that! the! initial! Mn! of! the! samples! was! not! the! same! at! the! beginning! of! the! experiments,!influencing!the!final!results.!! Fukushima! et! al.! [116]! evaluated! two! different! modified! montmorillonites! (Cloisite®30B! and! Nanofil! 804)! under! compost! degradation! conditions! at! 40! °C.! A! considerable!reduction!in!Mn!and!Mw!was!shown!by!PLA8nanocomposites,!although!the! initial!Mn!for!neat!PLA,!PLA8!Cloisite®30B!and!PLA8Nanofil!804!was!different!(72,743,! 64,270! and! 47,528! Da,! respectively).! The! faster! degradation! of! PLA8nanocomposites! was!attributed!to!the!hydroxyl!groups!in!the!clay!organic!modifiers,!where!the!presence! of!Cloisite®30B!showed!higher!hydrolysis!due!to!higher!dispersion!in!PLA!than!Nanofil! 804.! However,! the! difference! in! the! initial! molecular! weight! of! the! samples! was! not! considered.!! Araújo!et!al.![117]!evaluated!the!thermal!stability!of!PLA8based!nanocomposites! ! 57! ! ! using!three!modified!MMTs.!Different!behaviors!were!shown!depending!on!the!chemical! structure! of! the! organomodifier.! The! hydrolysis! was! enhanced! by! the! presence! of! Cloisite®30B,! which! has! OH! groups! that! increase! the! hydrophilicity! and! therefore! displayed!better!dispersion!than!Dellite®43B.!Hydrolysis!of!PLA!Dellite®43B!resulted!in! less! chain! scission! due! to! the! benzene! ring! in! its! chemical! structure! making! it! more! hydrophobic!and!with!less!capability!to!absorb!water!than!Cloisite®30B!and!Cloisite®15A.! Therefore,! the! hydrolysis! rate! depends! on! the! filler! dispersion,! water! uptake! and! hydrating!ability!of!the!fillers.! Hydrolysis!of!PLA8nanocomposites!can!be!noticed!in!the!degree!of!crystallinity.! DSC! results! have! demonstrated! the! increase! of! the! crystallinity! after! the! addition! of! nanoclay!and!after!degradation![27,!114,!117].!The!incorporation!of!clay!particles!in!PLA! matrix!promotes!the!crystallization!of!PLA!indicating!a!nucleating!effect![27,!114].!The! increase!in!crystallinity!during!hydrolytic!degradation!of!PLA8nanocomposites!is!due!to! the! hydrolysis! of! PLA! chains! in! the! amorphous! regions! by! the! easy! access! of! water! molecules!into!those!regions![117].!The!increase!of!crystallinity!can!also!be!attributed!to! the! plasticization! of! PLA! by! water! molecules! and! LA! oligomer! giving! mobility! to! the! polymer!allowing!the!reorganization!of!the!chain!and!further!crystallization![115].!! Visible!changes!in!opacity!have!been!observed!as!a!consequence!of!hydrolytic! degradation!in!PLA8OMMT!nanocomposites.!Over!exposure!time!of!nanocomposites!to! hydrolysis! conditions,! the! polymer! turns! white! and! becomes! extremely! brittle.! The! changes! in! appearance! can! be! attributed! to! various! phenomena,! such! as! light! scattering! due! to! the! presence! of! water! and! degradation! products! formed! during! hydrolysis,!formation!of!holes!in!the!bulk!of!the!polymer,!or!the!increase!in!crystallinity! ! 58! ! ! due!to!the!hydrolysis!in!the!amorphous!regions!of!the!PLA!matrix![27,!115].!! 2.3.1.3! Chemical!compound!release! Nanocomposites!have!been!employed!in!a!wide!range!of!applications!due!to!the! improvements!on!polymer!properties.!However,!during!the!life!cycle!of!nanocomposites,! the!nanoclays!have!the!potential!to!be!released.!Nanoparticles!may!be!released!from! packaging!materials!in!direct!contact!with!food,!even!though!the!nanoscale!components! are! bound! to! the! material! [118,! 119].! ! After! use,! the! nanoparticles! also! could! be! released! into! the! surrounding! environment! reaching! plants,! wildlife,! or! humans! [120].! Therefore,! the! evaluation! and! characterization! of! migrants! from! nanocomposites! containing!clay!is!important!for!food!safety!considerations![121].!! Due! to! the! greater! surface! area! per! mass! of! nanoclays! compared! with! larger! sized!particles!of!the!same!nature,!ion!exchange!capacity,!and!high!absorption!ability,! nanoclays!are!more!reactive!to!biological!systems![122,!123].!Some!nanoparticles!may! cause! damage! to! biological! systems! since! they! have! the! ability! to! penetrate! cellular! barriers!inducing!oxygen!radical!generation!that!leads!to!oxidative!reactions!in!the!cell! [124,! 125].! Also,! the! organomodifiers! or! surfactants! used! in! the! nanoclays! have! the! potential!risk!to!be!toxic!to!ecosystems,!animals!and!humans![126,!127].! The! migration! process! of! nanoparticles! may! not! be! the! same! as! migration! of! small! molecules! or! additives! from! the! polymer! matrix.! Simon! et! al.! [128]! studied! the! potential! rate! of! migration! and! equilibrium! distribution! of! nanoparticles! from! polymeric! packaging!materials!based!on!physicochemical!properties!of!nanoparticles!and!polymer! materials.!The!results!indicated!that!the!migration!of!nanoparticles!takes!place!when!the! particles! are! very! small,! with! a! radius! on! the! order! of! 1! nm.! Meanwhile,! the! polymer! ! 59! ! ! matrix!should!be!low!in!viscosity!and!not!interact!with!the!nanoparticles.!On!the!contrary,! for! bigger! nanoparticles! in! polymer! matrices! with! a! relatively! high! viscosity,! such! as! organomodified!MMT,!the!migration!is!not!likely!detectable.!However,!there!is!a!lack!of! information!to!confirm!these!statements.!!! Xia!et!al.![129]!evaluated!the!release!of!OMMT!nanoclay!from!PP!and!polyamide! 6! (PA6)! nanocomposites! films! in! ethanol! at! 70! °C,! in! which! the! release! of! the! nanoparticles!was!detectable.!The!release!of!OMMT!was!verified!by!quantifying!Si!and! Al! as! nanoclay! markers! using! a! graphite! furnace! atomic! absorption! spectrometry! (GFAAS)!technique.!After!10!days!of!exposure!more!nanoclay!particles!were!released! from!PP!films!(0.15!mg/L)!than!from!PA6!(0.10!mg/L).!The!higher!release!from!PP!was! attributed!to!the!lack!of!interaction!between!the!nanoclay!and!the!polymer!matrix.!Thus,! the!interaction!between!the!nanoparticles!with!the!polymer!matrix!influences!the!release! to!the!environment.!! Different! techniques! have! been! used! for! the! detection! of! the! release! of! nanoparticles! from! PLA! nanocomposites.! For! instance,!Schmidt! et! al.! [130]! assessed! the! migration! of! nanoclay! from! a! PLA8OMMT! nanocomposite! (5%! wt.! Cloisite®30B).! Based!on!asymmetrical!flow!field8flow!fractionation!(AF4)!coupled!with!multi8angle!light8 scattering!(MALS)!analysis,!migration!of!particles!with!a!radii!range!of!508800!nm!was! detected!in!a!mixture!of!95%!ethanol!and!5%!water.!When!AF48MALS!with!inductively! coupled!plasma!mass!spectrometry!(ICP8MS)!technique!was!applied,!no!clay!minerals! were!detectable.! Besides!OMMT!clay!release,!other!nanoparticles!have!been!studied.!Schmidt!et! al.![131]!studied!the!migration!of!the!major!components!from!PLA/laurate8modified!Mg8 ! 60! ! ! Al!layered!double!hydroxide!(PLA8LDH8C12)!films!to!assess!their!suitability!for!use!as! food!contact!materials.!Three!films!were!tested!where!LDH8C12!was!introduced!into!PLA! either!by!direct!compounding!with!PLA!or!by!dispersion!of!LDH8C12!using!a!masterbatch! technique.! All! tested! films! showed! migration! of! LDH! using! acid! digestion! followed! by! ICP8MS! detecting! Mg.! Also,! migration! of! the! laurate! organomodifier! took! place.! The! results!were!within!the!migration!limits!of!the!European!Commission!for!total!migration! and!specific!lauric!acid!migration!of!≤!10!mg/dm2![132].! Studies! have! demonstrated! the! possible! release! of! the! organomodifiers! or! surfactants! when! nanoparticles! or! nanocomposites! are! exposed! to! aqueous! media.! Nigmatullin!et!al.![133]!reported!the!release!of!quaternary!ammonium!compounds!from! different! OMMTs! to! aqueous! media! by! monitoring! electrical! conductivity! for! 6! h.! The! tested! OMMTs! were! divided! into! two! groups,! organoclays! with! a! single! long! tail! of! hydrogenated! tallow! (Cloisite®10A,! Cloisite®30B)! and! with! two! long! tails! of! hydrogenated! tallow! (Cloisite®20A,! Cloisite®93A,! Cloisite®15A).! Also,! the! same! study! reported!the!migration!of!the!surfactants!when!organoclays!were!incorporated!into!PA.! The!release!of!surfactants!from!the!polymer!nanocomposite!was!slower!compared!with! the! release! from! nanoclays! where! the! release! of! surfactants! with! single! long! tail! of! hydrogenated!tallow!was!faster!than!the!surfactants!with!two!long!tails.!! Xia!et!al.![129]!studied!the!release!of!surfactant!from!two!types!of!polymer8clay! nanocomposites:!PP!and!PA6.!Nanocomposites!were!exposed!to!ethanol!at!70!°C.!The! amount!of!surfactant!released!into!ethanol!was!3.5!mg/L!from!PP!nanocomposite!and! 16.2!mg/L!from!PA6!nanocomposite.!The!release!of!the!surfactant!from!PA6!films!was! faster!than!PP!due!to!the!swelling!effect!of!ethanol!in!PA6.!The!swelling!of!the!polymer! ! 61! ! ! allowed!easy!access!of!solvent!molecules!into!the!polymer!matrix,!for!the!later!swelling! of! the! nanoclay! particles! and! then! interaction! with! the! surfactant.! Compared! with! nanoclay! particles,! the! authors! concluded! that! surfactant! molecules! can! more! easily! move!within!the!polymer!matrix,!following!the!diffusion!behavior!of!small!molecules!due! to!the!presences!of!FV!and!polymer!chain!relaxation.! 2.3.2! Chain!extenders! It!is!well!known!that!PLA!is!susceptible!to!thermal!degradation!during!processing! at! elevated! temperatures! resulting! in! an! undesired! molecular! weight! reduction! and! weight! loss! with! a! decrease! of! the! rheological! and! mechanical! properties.! Different! factors! might! affect! the! thermal! degradation! of! PLA! during! melt! processing,! such! as! moisture!content,!residual!metal!catalysts,!processing!temperature,!and!oxygen![134].! Several! authors! have! reported! the! use! of! chain! extenders! to! control! the! thermal! degradation!of!PLA!introduced!during!polymer!processing![1358141].!Chain!extenders! have!two!or!more!functional!groups!that!react!with!the!chemical!groups!formed!during! the! degradation! of! the! polymer.! The! degraded! chains! are! re8linked,! restoring! the! molecular! weight! of! the! polymer! [142].! Some! of! the! chain! extenders! that! have! been! reported! in! literature! are! tris(nonylphenyl)phosphite! (TNPP)! [135],! hexamethylene! diisocyanate! (HDI)! [143],! 1,68hexanediol! diglycidyl! ether! (HDE)! [144],! pyromellitic! dianhydride!(PMDA)![143], poly(carbodiimide)!(PCDI)![135,!145,!146]!and!epoxy!based! (e.g.!Joncryl®)![135,!136,!1428144,!147,!148].!! Epoxy! based! chain! extenders! have! proven! to! be! effective! to! maintain! and! increase! the! molecular! weight! during! PLA! processing! [135,! 136,! 147,! 149].! Joncryl®,! one! of! the! well! studied! epoxy! chain! extenders,! is! an! oligomeric! copolymer! based! on! ! 62! ! ! glycidyl! methacrylate,! styrene! and! other! acrylates! with! functionality! greater! than! four! (Figure!2.13)![141,!142].!The!molecular!weight!of!PLA!processed!with!an!epoxy!chain! extender!increases!with!the!increase!in!functionality,!but!no!further!increase!is!observed! with!a!functionality!greater!than!5![142].!When!PLA!is!degraded!during!melt!processing! due! to! hydrolysis! of! the! ester! groups,! two! new! chains! are! created,! each! with! a! carboxylic! acid! group! at! one! end! and! at! the! other! end! a! hydroxyl! group.! The! epoxy! groups! of! the! Joncryl®! chain! extender! can! theoretically! react! with! the! hydroxyl! and! carboxyl!acid!groups.!However,!studies!have!shown!that!the!reaction!is!most!favorable! with!the!carboxylic!acid!group!due!to!the!strong!polymerization!of!the!hydroxyl!groups!of! carboxylic! acid! [142,! 1508152].! A! possible! reaction! between! Joncryl®! and! PLA! end! groups!is!shown!in!Figure!2.14.! ! Figure! 2.13! General! structure! of! Joncryl®! ADR,! where! R1!–! R5! are! H,! CH3,! a! higher! alkyl! group,! or! combination! of! theml! R6! is! an! alkyl! group,! and! x,! y! and! z! are! each! between!1!and!20,!adapted!from!Cailloux!et.al.![152]!! ! ! ! 63! ! ! ! Figure!2.14!Reaction!of!Joncryl®!and!PLA!end!groups,!adapted!from!Najafi!et!al.![135].! ! ! Different!authors!have!reported!the!control!of!thermal!degradation!of!PLA!using! ! Joncryl®!comparing!to!the!addition!of!other!kind!of!chain!extenders.!Najafi!et!al.![135]! ! 64! ! ! studied! three! chain! extenders! into! neat! PLA! and! PLA8nanocomposite! (2%! wt.! Cloisite®30B):!PCDI!(2%!wt.),!TNPP!(1%!wt.)!and!Joncryl®ADR84368!(1%!wt.).!Results! showed!an!increase!in!the!molecular!weight!of!PLA!with!the!addition!of!chain!extender! having!an!impact!in!the!rheological!and!thermal!properties.!The!incorporation!of!PCDI! and!TNPP!led!to!the!formation!of!longer!linear!chains!while!Joncryl®!led!to!a!long!chain! branched! structure.! Joncryl®ADR84368! was! the! most! efficient! chain! extender! among! these.! The! presence! of! Joncryl®! gave! a! stable! viscoelastic! response,! strongly! influencing!the!rheological!properties.!The!thermal!gravimetric!analysis!showed!that!the! addition!of!PCDI!and!TNPP!increased!the!onset!temperature!for!thermal!degradation!of! PLA,! attributed! to! longer! chains! with! a! reduced! number! of! chain! ends! per! mass.! However,! the! presence! of! Joncryl®! decreased! the! onset! temperature! due! to! the! long! branched!structures!with!a!higher!number!of!ends!per!chain!than!linear!systems.!! ! Meng! et! al.! [143]! studied! the! use! of! HDI! (0.5%! wt.),! PMDA! (0.5%! wt.)! and! Joncryl®ADR84368! (0.5%! wt.)! in! PLA! and! PLA! nanocomposites.! HDI! has! bi8reactive! end! groups! per! moleculel! therefore,! extended! PLA! molecules! with! mostly! linear! structure! were! obtained.! On! the! other! hand,! the! presence! of! Joncryl®! changed! the! molecular!structure!from!linear!to!a!branched!or!cross8linked!structure!depending!on!the! concentration!of!Joncryl®!and!temperature!of!processing.!It!was!found!that!Joncryl®!and! HDI! had! much! higher! chain! extension! than! when! PMDA! was! incorporated! into! PLA.! Joncryl®! increased! the! complex! viscosity.! Also,! Joncryl®! showed! significant! thermal! stabilization!of!PLA!nanocomposites!containing!6%!Cloisite®30B.!! The!presence!of!Joncryl®!has!been!studied!in!PLA!blends!with!other!polymers,! such! as! poly(butylene8adipate8co8terephtalate)! (PBAT)! [136,! 148,! 153],! poly[(butylene! ! 65! ! ! succinate)8co8adipate]!(PBSA)![154],!and!thermoplastic!starch!(TPS).!Zhang!et!al.![149]! investigated!the!addition!of!Joncryl®ADR84370S!(1%!wt.)!into!a!blend!of!plasticized!PLA! and! TPS! and! found! the! improvement! of! the! mechanical! properties,! for! instance! the! tensile!strength,!the!yield!strength!and!the!elongation.!In!a!blend!of!PLA/PBAT!(80/20! w/w),!Al8Itry!et!al.![136]!showed!that!the!incorporation!of!Joncryl®ADR84368!(0.25,!0.5! and!1%!wt.)!improved!the!thermal!stability,!molecular!weight,!intrinsic!viscosity,!shear! thinning! and! elasticity! during! melt! processing! due! to! the! formation! of! extended! and! branched! chains.! Molecular! weight,! melt! strength! and! thermal! stability! enhancement! was!also!reported!by!Ojijo!and!Ray![154]!in!a!blend!of!PLA/PBSA!using!Joncryl®ADR8 4368,!which!changed!the!molecular!structure!from!linear!to!branched.! Studies!of!the!effect!of!Joncryl®!on!the!hydrolytic!degradation!of!PLA!is!limited!in! the!literature.!Dong!et!al.![153]!studied!the!effect!of!Joncryl®ADR84370S!(1%!wt.)!in!a! blend! of! PLA/PBAT! (80/20! w/w)! in! the! hydrolysis! of! the! material! in! alkaline! media! at! 60!°C.!The!hydrolysis!of!the!samples!was!evaluated!by!weight!loss,!molecular!weight! reduction!and!pH!variation.!They!found!that!the!hydrolysis!of!PLA!and!its!blends!went! through!a!gradual!diffusion!of!water,!fast!hydrolysis!of!the!bulk!leading!a!reduction!in! molecular! weight! and! further! hydrolysis! of! PLA! oligomers.! However,! the! addition! of! chain!extenders!in!the!blend!did!not!noticeably!retard!the!hydrolytic!degradation!of!the! PLA!matrix.!Studies!are!needed!to!understand!the!effect!of!epoxy!chain!extenders!in! the!hydrolytic!degradation!of!PLA.!! ! ! ! 66! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! REFERENCES! ! 67! ! ! REFERENCES! ! [1]! R.! Auras,! B.! Harte,! S.! Selke,! 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Properties! and! Hydrolytic! Degradation! Behavior! of! the! Poly! (lactide)/Poly! (butylene! adipate8co8terephthalate)!Blends,!International!journal!of!molecular!sciences!14!(2013)! 20189820203.! [154]!V.!Ojijo,!S.S.!Ray,!Super!toughened!biodegradable!polylactide!blends!with!non8 linear! copolymer! reactive! compatibilization,!Polymer!80!(2015)!1817.! interfacial! architecture! obtained! via! facile! in8situ! ! 80! ! ! CHAPTER!3! Concurrent!Solvent!Induced!Crystallization!and!Hydrolytic!Degradation!of!PLA! by!WaterQEthanol!Solutions! ! ! ! ! ! ! ! ! ! ! ! A!version!of!this!chapter!is!published!as:! F. Iñiguez-Franco, R. Auras, G. Burgess, D. Holmes, X. Fang, M. Rubino, H. Soto-Valdez, Concurrent solvent induced crystallization and hydrolytic degradation of PLA by water-ethanol solutions, Polymer 99 (2016) 315-323. ! 81! ! 3.0! Abstract! Poly(lactic!acid)!(PLA)!films!were!immersed!in!pure!water,!50%!and!95%!ethanol! solutions!for!up!to!180!days.!The!change!in!molecular!weight,!sorption!of!water,!sorption! of!ethanol,!and!lactic!acid!released!were!monitored.!Glass!transition!temperature!and! percent! crystallinity! as! a! function! of! ethanol! content! were! also! measured.! PLA! experienced!faster!hydrolytic!degradation!in!contact!with!50%!than!with!95%!ethanol!or! pure! water.! NMR! methodologies! were! developed! to! measure! sorption! of! deuterated! water!and!ethanol!in!PLA.!More!water!was!sorbed!in!50%!ethanol,!explaining!the!higher! hydrolysis.!!During!exposure,!PLA!experienced!solvent!induced!crystallization.!Higher! percent!crystallinity!was!found!in!films!exposed!to!50%!ethanol!with!the!formation!of!α8 form!crystals.!The!hydrolysis!of!PLA!was!correlated!with!the!release!of!lactic!acid!(LA)! during!exposure.!Mathematical!models!are!proposed!to!explain!the!concurrent!solvent! induced!crystallization!and!hydrolytic!degradation!of!PLA.! 3.1! Introduction! PLA!is!one!of!the!most!widely!used!biopolymers.!It!has!gained!attention!in!recent! years! as! a! promising! alternative! to! polymers! from! nonrenewable! resources.! PLA! is! a! linear!aliphatic!thermoplastic!polyester,!in!which!LA!is!the!precursor,!obtained!from!the! fermentation! of! corn! sugar! [1].! Currently,! the! world! capacity! for! production! of! PLA! is! 150,000!metric!tons![2].!The!versatility!of!PLA!has!led!to!broad!and!diverse!applications.! In!the!medical!field,!due!to!its!biocompatibility!with!the!human!body,!PLA!has!been!used! for!applications!such!as!sutures,!stents!and!drug!delivery!systems.[386]!In!agriculture,! PLA!has!been!used!as!a!material!for!the!controlled!release!of!herbicides![789]!or!as!a! ! 82! ! plastic!to!protect!soil!and!plants!from!erosion,!insects!and!birds![10].!In!packaging,!PLA! has!been!used!as!both!a!film!and!a!rigid!thermoform!for!containers!for!food!and!non8 food!products![1,!11813].! Depending!on!the!application,!PLA!can!be!exposed!to!different!kinds!of!media,! including!water!and/or!ethanol.!In!the!medical!field,!it!can!be!in!contact!with!aqueous! systems!or!biological!media!for!drug!delivery!systems![14816].!For!example,!disinfection! of! electrospun! materials! for! tissue! engineering! is! commonly! carried! out! by! ethanol! soaking! [17819].! In! agriculture,! for! growth! stimulation! and! the! controlled! release! of! pesticides,!PLA!is!in!a!high!relative!humidity!environment!or!in!direct!contact!with!water! [7,! 20].! In! packaging! applications,! when! PLA! is! used! for! beverage! and! fresh! food! containers,!it!could!be!in!direct!contact!with!water!or!ethanol,!or!be!exposed!to!humid! environments.!These!types!of!exposure!to!various!environmental!conditions!could!favor! PLA!degradation.!! PLA! degradation! leads! to! changes! in! mechanical! and! thermal! properties,! molecular!weight!and!morphology![21824].!PLA!in!the!presence!of!water!is!susceptible! to! hydrolytic! degradation.! The! ester! groups! are! hydrolytically! degraded,! leading! to! a! decrease!in!molecular!weight!and!the!release!of!low!molecular!weight!soluble!oligomers! and! monomers! [1,! 25].! Conversely,! it! has! been! found! that! PLA! will! plasticize! and! crystallize! in! the! presence! of! organic! solvents,! which! swell! the! polymer! matrix,! increasing!chain!mobility!and!inducing!solvent!induced!crystallization!(SIC)![23,!26828].! The!SIC!behavior!has!been!studied!in!polymers!such!as!poly(methyl!methacrylate)![29],! poly(carbonate! of! bisphenol! A)! [30],! polystyrene! [31833],! and! poly(ethylene! terephthalate)!(PET)![34836],!SIC!is!a!complex!phenomenon!involving!many!processes,! ! 83! ! such! as! diffusion! of! solvent! molecules,! swelling,! plasticization! and! crystallization.! Several!factors!can!affect!each!process!involved!in!SIC.!Examples!are!initial!crystallinity,! temperature,!molecular!weight!and!solvent!chemistry![30836].!In!a!study!of!SIC!in!PLA! ultrathin!films,!Wu!et!al.!found!that!in!the!initial!diffusion!stage,!the!solvent!molecules! interact!with!random!PLA!coil!chains,!which!increase!the!motion!of!the!PLA!segments! [28].! These! interactions! can! lead! to! either! dissolution! of! the! polymer! or! nucleation,! leading! to! the! rearrangement! of! PLA! chains! into! crystal! lattices.! Therefore,! understanding!the!concurrent!effect!of!SIC!and!hydrolytic!degradation!of!the!PLA!matrix! as!it!is!exposed!to!water8ethanol!solutions!is!of!paramount!importance!to!fully!control! and/or!develop!its!applications!and!tailor!its!uses,!which!was!the!goal!of!this!work.!! 3.2! Materials!and!methods! 3.2.1! Chemicals!and!Reagents! PLA! resin! (3.884.2%! D8LA)! was! obtained! from! NatureWorks! LLC! (Minnetonka,! MN,!USA)!with!a!weight!and!number!average!molecular!weight!(Mw,!Mn)!of!2.35!±!0.07! x! 105! Da! and! 1.21! ±! 0.08! x! 105! Da,! respectively.! Ethanol! (high8performance! liquid! chromatography! (HPLC)! grade),! acetonitrile! (HPLC! grade),! methanol! (HPLC! grade),! and!formic!acid!were!supplied!by!Sigma8Aldrich!(St.!Louis,!MO,!USA),!tetrahydrofuran! (THF)! by! Pharmco8AAPER! (North! East,! CA,! USA),! and! water! (HPLC! grade)! by! J.T.! Baker!(Center!Valley,!PA,!USA).!L(+)!LA!was!purchased!from!Supelco!(Bellefonte,!PA,! USA).!Malonic!acid!was!obtained!from!Columbus!Chemical!Industries!(Columbus,!WI,! USA).! Deuterium! oxide! (D,! 99.9%)! (D2O)! and! chloroform8D! (D,! 99.8%)! (CDCl3)! were! purchased! from! Cambridge! Isotope! Laboratories! (Andover,! MA,! USA).! N,N8 Dimethylformamide! (DMF)! was! provided! by! Avantor! Performance! Materials! (Center! ! 84! ! Valley,! PA,! USA).! Water! used! in! the! HPLC! mobile! phase! was! purified! using! Milli8Q! System!from!Millipore!Corp!(Bedford,!MA,!USA).!$ 3.2.2! Production!of!PLA!film! PLA!pellets!were!dried!at!60!°C!for!24!h!under!vacuum!(85!kPa)!and!processed! in!a!Randcastle!cast!film!microextruder!(Extrusion!System,!Inc.,!Cedar!Grove,!NJ,!USA)! with! a! screw! of! 1.5875! cm! diameter,! 24/1! L/D! ratio! extruder,! and! 34! cc! volume.! Extrusion! temperatures! were! 193,! 212,! 215,! 215! and! 210! °C! for! zone! 1,! 2,! and! 3! transfer!tube!and!die,!respectively,!with!a!rotation!speed!of!60!rpm.!The!film!thickness! was!27.9!±!9.9!μm./ 3.2.3! Storage!experiments! Ten!disks!2.0!cm!in!diameter!separated!by!glass!beads!on!a!stainless!steel!wire! were! placed! in! cells! containing! pure! water,! 50%! ethanol,! or! 95%! ethanol! by! volume,! previously!conditioned!at!40!°C.!The!total!disk!surface!area!to!fluid!volume!was!1.79! cm2/mL.! Degradation! and! release! experiments! were! conducted! at! 40! °C! using! a! migration! cell! as! recommended! by! ASTM! D4754811! [37].! Samples! were! retrieved! at! defined!times!during!6!months!to!assess!Mw,!Mn,!water!and!ethanol!sorption,!thermal! and!physical!properties,!and!LA!release.!! 3.2.4! PLA!molecular!weight! At! various! times! throughout! the! experiments,! Mw! and! Mn! were! determined! by! weighing!approximately!10!mg!of!film!retrieved!from!the!test!cells!and!dissolved!in!THF! (2!mg/mL).!A!gel!permeation!chromatograph!(GPC)!(Waters!1515,!Waters,!Milford,!MA,! USA)! equipped! with! a! refractive! index! detector! (Waters! 2414)! and! a! series! of! three! ! 85! ! columns!of!HR!Styragel®!(HR4,!HR3!and!HR2)!were!used!(each!7.8!mm!×!300!mm,! Waters!Styragel).!An!elution!of!THF!at!a!flow!rate!of!1!mL/min!was!applied!with!a!flow! rate! ramping! time! of! 5! min! and! a! total! run! time! of! 45! min.! The! temperature! of! the! detector! and! column! was! 35! °C! and! the! injection! volume! was! 100! μL.! A! calibration! curve! was! made! from! polystyrene! standards8Shodex! SM8105! (Waters,! Milford,! MA),! which! contained! a! molecular! weight! range! of! 1.37x103! to! 2.48x106! Da.! The! Mark−Houwink! constants! for! the! correction! were! K! =! 0.0164! mL/g! and! α/ =! 0.704! for! PLA!solutions!in!THF!at!35!°C.!The!measurements!were!conducted!in!triplicate.!! 3.2.5! Water!and!ethanol!sorption! Water!and!ethanol!sorbed!by!PLA!film!were!measured!at!40!°C!using!migration! cells!as!described!in!the!storage!experiments!section.!In!this!study,!the!water!(H2O)!in! the! solvents! (water,! 50%! ethanol,! 95%! ethanol)! was! replaced! with! D2O! to! avoid! contamination!from!environmental!water!and!interference!with!measurements.!Ethanol! sorbed! was! determined! using! the! 1H8NMR! (proton! nuclear! magnetic! resonance)! technique! and! water! sorbed! using! D8NMR! (Deuterium! NMR).! Samples! of! film! were! taken!periodically.!For!ethanol!sorption,!the!samples!were!rinsed!with!D2O!and!for!water! sorption!with!H2O!to!remove!the!solvent!from!the!surface.!For!ethanol!sorption,!samples! were! dissolved! in! CDCl3! with! DMF! as! the! internal! standard,! and! for! water! sorption,! samples!were!dissolved!in!THF!with!CDCl3!as!the!internal!standard.!!For!ethanol!and! D2O! sorption! measurements,! samples! were! analyzed! using! a! Varian! Inova! 600! MHz! superconducting! NMR8Spectrometer! equipped! with! a!Nalorac! 5! mm! PFG! switchable! probe! operating! at! 599.892! MHz! and! 92.069! MHz! for! 1H! and! 2H,! respectively.! ! 86! ! Experiments!were!conducted!in!triplicate.!Detailed!descriptions!of!the! 1H8NMR!and!D8 NMR!experiments!are!provided!in!the!Appendix!3A.!! Ethanol!sorption!appears!to!follow!Fick’s!law!of!diffusion.!The!analytical!solution! with!constant!concentration!assumed!on!both!disk!faces!is![38]:! ! T BXN ! (Eq.!3.1)! 12F+1 8>?" −W2F+1 8V8A H8 D=DT=1−8V8 where!!D=!is!the!amount!of!ethanol!sorbed!at!time!t!(g8EtOH/g8PLA),!DT!is!the!amount! of!ethanol!at!equilibrium!(g8EtOH/g8PLA),!t!is!time!(s),!H!is!thickness!of!expanded!disk! (m)!and!W!is!the!diffusion!coefficient!(m2/s).!The!nonlinear!regression!(nlin8fit)!function!in! estimate!of!DT!and!W.! MATLAB®!2011b!(MathWorks,!Natick,!MA)!was!used!to!fit!(Eq.! 3.1.!This!provides!an! 3.2.6! Differential!scanning!calorimetry!(DSC)! A!differential!scanning!calorimeter!(Q100,!TA!Instruments!New!Castle,!DE,!USA)! was!used!to!determine!the!glass!transition!temperature!(Tg),!melting!temperature!(Tm),! crystallization! temperature! (Tc)! and! degree! of! crystallinity! (%Xc)! of! the! PLA! samples.! The!samples!where!cooled!from!25!to!5!°C!and!then!run!at!a!temperature!range!of!5!to! 210! °C,! with! a! heating! rate! of! 10! °C/min! using! liquid! nitrogen! with! a! flow! rate! of! 70! mL/min.! The! first! heat! scans! of! the! samples! are! reported.! The! data! obtained! were! analyzed!using!the!Thermal!Analysis!Universal!2000!version!4.5A!software.!Percentage! crystallinity! was! calculated! using! the! heat! of! fusion! of! the! 100%! crystalline! for! PLA! sample!of!93.7!J/g![39].!The!measurements!were!conducted!in!triplicate.!!! ! 87! ! 3.2.7! XQray!diffraction!study!(XRD)! XRD! analyses! were! performed! using! a! Bruker! AXS! D8! Advance! X8ray! diffractometer!(Bruker!Co.,!Billerica,!MA,!USA)!equipped!with!a!Global!Mirror!filtered!Cu! Kα!radiation!source!set!at!40!kV!and!100!mA.!Samples!were!scanned!in!the!2θ!range! from!2!°!to!40!°!at!a!rate!of!0.24!°/min!and!an!increment!of!0.01!°.! 3.2.8! Dynamic!mechanical!analysis!(DMA)!! To! determine! the! Tg! of! PLA,! the! loss! factor! (tan! delta)! was! measured! as! a! function!of!temperature!when!PLA!samples!were!immersed!between!4!and!7!days!in! the! various! ethanol8water! solutions! during! testing.! A! TA! RSA8G2! Solids! Analyzer! Immersion! System! (TA! Instruments,! New! Castle,! DE,! USA)! equipped! with! a! tension! geometry!at!a!frequency!of!1!Hz!was!used.!The!samples!where!cooled!down!from!25!to! 10,!810,!810,!830,!and!860!°C!for!0,!25,!50,!75!and!95!%!ethanol!volume!and!then!heated! to!60!°C!at!a!heating!constant!rate!of!5!°C/min!using!liquid!nitrogen.!The!data!obtained! was!analyzed!using!the!TA!Instruments!TRIOS!software.! 3.2.9! Lactic!acid!release! Release!of!LA!into!the!three!different!solvent!systems!(water,!50%!ethanol!and! 95%! ethanol)! was! determined! using! the! migration! cell! at! 40! °C! as! described! in! the! storage! experimental! section.! Four! replicates! were! performed! for! each! of! the! solvent! systems.!LA!quantification!was!carried!out!since!lactide!and!oligomers!are!degradation! products!of!PLA!that!are!able!to!migrate!and!easily!decompose!to!LA.!Modified!methods! published!by!Mutsuga!et!al.!and!Di!Maio!et!al.!were!used![40,!41]./Samples!containing! 0.5!mL!of!simulant!were!taken!periodically!and!exposed!to!alkali!hydrolysis.!!For!ethanol! solutions,!the!ethanol!was!evaporated!using!a!Savant!SC110!SpeedVac!Concentrator! ! 88! ! System!(Savant!Instruments,!Holbrook,!NY,!USA)!and!then!reconstituted!with!0.5!mL!of! water.!Samples!were!then!saponified!with!50!μL!of!0.2!M!sodium!hydroxide,!followed!by! heating!for!15!minutes!in!a!water!bath!at!60!°C.!After!cooling!at!room!temperature,!50! μL!of!0.2!M!hydrochloric!acid!was!added.!LA!was!analyzed!with!an!LC/MS/MS!system! with!a!triple!quadrupole/linear!ion!trap!(AB/Sciex!QTRAP!3200,!Framingham,!MA,!USA).! Separation!was!carried!out!on!an!Ascentis!Express!C18,!2.7!μm,!100!x!2.1!mm!reverse! phase! column! (Sigma8Aldrich,! St.! Louis,! MO,! USA.)! with! a! flow! rate! of! 0.2! mL/min.! Solvents!were!A:!1%!formic!acid!and!B:!methanol.!Solvent!programming!was!isocratic! for!3!minutes!with!1%!B,!then!a!linear!gradient!to!95%!B!up!to!2!minutes,!followed!by! the!isocratic!mode!for!2!minutes.!A!linear!gradient!was!then!carried!out!in!0.01!minutes! to!1%!B!and!held!for!3!minutes!in!the!isocratic!mode.!The!oven!temperature!was!40!°C.! Mass! spectrometric! analyses! were! performed! in! the! negative8ion! mode! following! the! Ambient!Pressure!Chemical!Ionization!(APCI)!methodl!the!curtain!gas!was!set!to!20,! gas81!20!and!gas82!20,!with!a!temperature!of!650!°C.!The!calibration!curve!was!made! from!0.25!to!15!μg/mL!by!treating!the!LA!standard!solutions!in!the!same!way!as!the! samples!and!using!malonic!acid!as!the!internal!standard.! 3.3! Results!and!discussion! To!study!the!hydrolytic!degradation!and!SIC!of!PLA!by!water8ethanol!solutions,! the!change!in!molecular!weight!of!PLA!was!analyzed!during!exposure!to!determine!the! rate!of!hydrolysis!caused!by!the!cleavage!of!the!ester!bonds.!The!sorption!of!water!in! PLA!was!studied!since!hydrolysis!depends!on!the!presence!of!water!molecules!in!the! polymer!matrix.!However,!the!ethanol!molecules!in!the!solvent!solutions!influenced!the! water!sorption!in!PLA!due!to!the!swelling!effect.!Swelling!studies!were!carried!out!and! ! 89! ! the! initial! first! order! reaction! equation! for! Mn! reduction! was! modified! to! account! the! effect!of!PLA!swelling!in!the!rate!constant.!Crystallinity!studies!of!PLA!were!performed! over!the!exposure!time!to!assess!the!concurrent!SIC!by!ethanol!molecules!along!with! the!hydrolysis!of!amorphous!regions!in!the!polymer!matrix!by!water!molecules.!Finally,! LA! release! was! assessed! as! the! result! of! the! concurrent! SIC! and! hydrolysis! of! the! polymer! chains! and! a! model! was! proposed! to! predict! the! LA! release! when! PLA! is! exposed!to!water8ethanol!solutions.!! 3.3.1! Molecular!weight! Figure!3.1!shows!Mn!as!a!function!of!time!for!PLA!disks!immersed!in!water,!50%! ethanol! and! 95%! ethanol! at! 40! °C.! Molecular! weight! decreased! over! time,! indicating! hydrolysis! of! PLA.! However,! the! change! in! Mn! was! different! for! PLA! in! the! three! solutions.!A!first!order!reaction!relationship!was!fitted!to!the!experimental!data.!Table! 3.1!gives!the!rate!constants! k/(gmol81!d81)!for!each!system.!The!50%!ethanol!solution! caused! the! highest! rate! of! decrease! of! Mn! at! 0.0223! gmol81!d81!(p<0.05),! followed! by! 95%!ethanol!and!water!at!0.0133!(p<0.05)!and!0.0059!gmol81!d81!(p<0.05),!respectively.! After!120!days,!the!films!in!50%!ethanol!were!no!longer!intact!and!dispersed!as!small! fragments!in!the!solvent./The!reduction!in!Mn!during!exposure!is!attributed!to!scission!of! the! ester! bond! of! the! polymer! chains! by! water! molecules! [22,! 25].! Optical! images! of! PLA!samples!immersed!in!the!three!different!water8ethanol!solutions!during!hydrolytic! degradation!are!presented!in!Figure!3B.1,!Appendix!3B.!! ! ! ! 90! ! 14x 104 a D , n M 12 10 8 6 4 2 0 Water 50% Ethanol 95% Ethanol 0 20 40 60 100 120 140 160 180 80 time, d ! Figure!3.1!Mn!as!a!function!of!time!during!hydrolytic!degradation!of!PLA!film!immersed! in!water,!50%!ethanol!and!95%!ethanol!at!40!°C.!Values!indicated!as!*!were!considered! outliers!and!therefore!no!used!for!fitting!of!the!first!order.! ! ! ! 91! ! Table!3.1!Rate!constants!for!PLA!films!at!40!°C!in!water8ethanol!solutions.! ! k$(gmolQ1!dQ1)!for!H2O$or$D2O/ethanol$solutions$ Solvent!solution! H2O!*$ Water! 0.0059!±!0.0004!a! H2O!**$ 0.0059! D2O!***$ 0.0020!±!0.0002!a! 0.0223!±!0.0010!b! 0.0124!±!0.0007!b! 50%!Ethanol! 95%!Ethanol! 0.0230! 0.0115! 0.0133!±!0.0004!c! *Fitting! of! first! order! reaction:!DB=DBY>?"−!A ,! where! Mn! is! the! number! average! molecular!weight!at!time!A!and!DBY!is!the!initial!Mn.! **Fitting!of!first!order!reaction!with!!!from!(Eq.!3.2! 0.0111!±!0.0012!b! ***Fitting! of! first! order! reaction! kinetic.! Synthetic! data! was! used! for! calculations! assuming!a!first!order!reaction!since!only!the!first!and!last!points!were!measured.! Values! with! different! lower! case! letters! in! the! same! column! are! statistically! different! (α=0.05!Tukey8Kramer!Test)!! ! Di!Miao!et!al.!and!Sato!et!al.!evaluated!the!effect!of!organic!solvents!like!ethanol! in!contact!with!PLA,!which!swell!the!polymer!matrix!and!increase!chain!mobility![26,!41].! The!creation!of!free!volume!due!to!swelling!allows!more!water!molecules!to!diffuse!in! and!get!sorbed!in!the!PLA!matrix.!This!accelerates!the!hydrolysis!rate.!A!more!complete! understanding! of! the! diffusion! and! sorption! of! water! and! ethanol! during! hydrolytic! degradation!should!help!to!describe!the!mechanism!of!solvent!assisted!hydrolysis.!! 3.3.2! Water!and!ethanol!sorption! Diffusion!of!H2O!in!PLA!has!been!reported![42844].!D!values!of!3.53x10815!m2/s!at! 80%! relative! humidity! (RH)! and! 1.5x10811! m2/s! at! 90%! RH! in! vapor! systems,! and! ! 92! ! 2.92x10812! m2/s! in! immersion! conditions! were! reported! for! PLA! at! 40! °C.! In! our! experiment,! to! simultaneously! measure! the! rate! of! diffusion! and! sorption! of! water! in! PLA!in!ethanol!solutions!created!technical!challenges!since!the!H2O!peaks!overlap!with! PLA!resonances!in!the!1H8NMR!spectrum!as!shown!in!the!Appendix!3A.!Furthermore,! excluding!all!extraneous!sources!of!water!from!NMR!measurements!to!ensure!accurate! measurements!is!extremely!difficult.!In!contrast,!D2O!does!not!suffer!the!same!issues! and!allows!for!the!accurate!measurement!of!sorption.!Therefore,!D2O!water!was!used!to! simulate!the!diffusion!of!H2O!in!PLA.!It!is!important!to!recognize!that!D2O!will!cause!a! different!hydrolysis!rate!in!PLA!than!H2O!as!is!shown!in!Table! 3.1,!even!though!D2O! and! H2O! may! have! the! same! initial! diffusion! coefficient! as! shown! in! Figure! 3C.1,! Appendix!3C,!which!shows!the!rate!of!diffusion!of!H2O!vapor!and!D2O!vapor!in!PLA.! The!k!values!will!decrease!when!H2O!is!replaced!with!D2O!since!hydrogen!isotopes!1H! in!H2O!have!been!replaced!by!deuterium!isotopes!2H,!reducing!the!number!of!hydroxide! (deuteroxide)!ions!that!start!the!cleavage!of!the!ester!bonds.!!! Figure! 3.2! shows! the! rate! of! D2O! sorption! by! PLA.! The! concentration! of! D2O! molecules!in!PLA!was!lowest!when!the!films!were!immersed!in!D2O!and!highest!when! immersed!in!50%!ethanol.!The!higher!the!concentration!of!D2O!in!the!PLA,!the!faster! the! hydrolysis! by! D2O,! since! there! are! more! molecules! of! D2O! available! to! start! the! cleavage! of! ester! bonds.! However,! faster! cleavage! should! be! expected! if! H2O! were! used!due!to!the!reactivity!of!hydroxyl!groups,!even!though!the!sorption!of!H2O!vapor! and!D2O!vapor!into!PLA!are!similar!(Figure!3D.1,!Appendix!3D).!! ! 93! ! A L P − g O D − g / 2 , O 2 D 0.028 0.026 0.024 0.022 0.02 0.018 0.016 0.014 0.012 0.01 0.008 Water 50% Ethanol 95% Ethanol 0 20 40 60 80 100 time, d 120 140 160 180 ! Figure! 3.2! D2O! sorption! into! PLA! films! in! contact! with! D2O,! 50%! ethanol! and! 95%! ethanol! at! 40! °C.! y8axis! represents! grams! of! D2O! sorbed! divided! by! grams! of! PLA! remaining!in!the!disk!after!hydrolysis!with!H2O.!Trend!lines!are!used!as!visual!aid.! Figure! 3E.1,! Appendix! 3E! shows! immediate! ethanol! sorption! in! PLA! during! hydrolysis.!Diffusion!coefficients!were!estimated!using!(Eq.!3.1,!taking!into!account!the! swelled!thickness!of!the!PLA!films!at!equilibrium!(i.e.,!the!initial!thickness!was!27.94!μm! and!the!final!estimated!thicknesses!based!on!expansion!for!50%!and!95%!ethanol!were! 28.78!and!29.53!μm,!respectively).!Table! 3.2!shows!the!estimated!parameters.!D!for! ethanol!in!disks!immersed!in!95%!ethanol!(5.88!x!10814!m2/s)!was!almost!twice!the!D!for! ethanol! in! disks! immersed! in! 50%! ethanol! (2.55! x! 10814! m2/s).! Also,! the! amount! of! ethanol!at!equilibrium!(M∞)!was!160%!higher!in!95%!ethanol!than!in!50%!ethanol.! ! 94! ! Table!3.2!Diffusion!coefficients!(D)!and!amount!of!ethanol!at!equilibrium!(M∞)!in!PLA! films!at!40!°C! Solvent! 50%!Ethanol! 95%!Ethanol! D$x!1014!(m2/s)! M∞!(gQEtOH/gQPLA)*! 0.64!±!0.1!a! 1.47!±!0.33!b! 0.03!±!0.00!a! 0.08!±!0.00!b! Values!with!different!letter!within!the!same!column!are!statistically!different!(α=0.05!Tukey8 Kramer!Test)!! *Note:!grams!of!ethanol!sorbed!divided!by!grams!of!the!PLA!disk!used!for!the!1H8NMR! experiments! ! Experiments! were! performed! using! the! 1H8NMR! technique! to! study! the! relationship! between! ethanol! concentration! and! polymer! expansion! when! PLA! films! were!exposed!to!different!concentrations!of!ethanol!over!24!h.!Figure!3F.1,!Appendix! 3F!shows!the!relationship!between!%!expansion!of!PLA!and!ethanol!fraction!(volume!of! polymer!is!due!to!an!increase!in!void!space,!not!expansion!of!the!chains,!predicts!that! the!effect!of!PLA!swelling.!A!model!based!on!the!assumption!that!the!expansion!of!the! The!initial!first!order!reaction,!DB=DBY>?"−!A ,!can!be!modified!to!account!for! !=Z [\[]+ 0.06−[\[] "−0.06"81111! where![\!is!the!initial!void!space!in!the!PLA!matrix,![]!is!the!volume!of!the!disk,!Z!is!a! constant,!and!!"!is!the!ethanol!fraction!("!=!0,!0.5,!0.95).!This!model!incorporates!the! (Eq.!3.2)! ethanol!divided!by!total!volume).!! the!rate!constant!is:!! ! ! 95! derived! are! presented! in! !to! the! ! the! Appendix! 3G.! Fitting!DB=DBY>?"−!A effect! of! expansion! due! to! ethanol! in! the! rate! constant.! When!"=01(pure! water),! !=Z[\ [] l! and! when!"=11 (no! water),!!=0. !The! details! of! how! the! model! was! experimental!data!in!Figure!3.1!with!k/defined!in!(Eq.!3.2!gives!Z=!1.05!and![\ []=! 0.0056.!Using![]=0.00881S_`,!the!initial!void!space!([\)!in!one!disk!is!predicted!to!be! Setting! the! derivative! of!!!with! respect! to!"!in! (Eq.! 3.2! equal! to! zero! gives! the! "!≈!0.45!(Figure!3.3).!At!that!concentration!of!ethanol,!a!competitive!balance!between! maximum!rate!of!decay!in!Mn.!This!occurs!when!the!volume!fraction!of!ethanol!reaches! 4.93x1085!cm3.!Table!1!shows!the!rate!constants!using!(Eq.!3.2.!! swelling!and!hydrolysis!is!observed,!where!the!molecules!of!ethanol!swell!the!polymer! and!allow!the!maximum!concentration!of!water!into!the!polymer!to!start!the!cleavage!of! the!ester!bonds.! ! ! 96! ! Figure! 3.3! Rate! constant! for! hydrolytic! degradation! of! PLA! films! fitting! first! order! reaction!equation!with!!!from!(Eq.!3.2!(−•−).!D2O!sorption!(−)!and!ethanol!sorption!(−−)! ! into! PLA! at! different! exposure! times! versus! volume! fraction! of! ethanol! in! solvent! solution!at!40!°C.!!D2O!and!ethanol!sorption!lines!were!obtained!from!experimental!data! of!PLA!immersed!in!water,!50%!ethanol!and!95%!ethanol!at!different!exposure!times.! ! Hydrolytic!degradation!of!PLA!occurs!in!the!amorphous!regions!since!they!have! a!larger!void!volume!than!the!crystalline!regions!and!water!can!penetrate!amorphous! regions! more! readily! [39,! 45].! Since! hydrolysis! reduces! the! amorphous! regions,! the! degree!of!crystallinity!increases,!even!though!the!crystalline!regions!remain!unchanged! [25].!On!the!other!hand,!exposure!of!PLA!to!ethanol!causes!softening!in!the!polymer,! resulting! in! the! movement! and! realignment! of! polymer! chains,! which! induces! crystallinity![26,!46].!Therefore,!it!is!a!combination!of!structural!changes!in!PLA!including! ! 97! ! swelling! and! induced! crystallinity! when! exposed! to! different! combined! solvents! that! affects!hydrolysis.!! Figure! 3.4!shows!the!change!in!Tg!of!PLA!when!immersed!in!different!volume! fractions!of!ethanol!at!40!°C.!The!relationship!is!linear!(Figure!3.4!insert).!The!Tg!of!the! PLA!films!before!being!exposed!to!hydrolytic!degradation!was!59.8!±!0.5!°C!according! to!the!DSC!results.!PLA!films!immersed!in!100%!water!had!a!Tg!of!53!°C,!but!when!PLA! was! immersed! in! 50%! and! 95%! ethanol,! Tg! was! approximately! 36! and! 20! °C,! respectively.!This!means!that!the!higher!the!concentration!of!ethanol!in!the!solution,!the! lower! the! Tg! during! solvent! transport! into! PLA,! where! swelling! occurs! immediately! according! to! Figure! 3E.1,! Appendix! 3E.! The! plasticization! effect! of! ethanol! on! PLA! allows! the! movement! of! the! polymer! chains,! inducing! alignment! so! crystallinity! can! occur.!!! ! ! 98! ! 60 50 40 30 20 0 C o , e r u t a r e p m e T ) a t l e d ( n a t 0.5 Ethanol fraction 1 Ethanol fraction 0 0.25 0.50 0 10 20 30 Temperature, oC 40 50 60 70 ! Figure!3.4!Tg!of!PLA!when!immersed!in!different!volume!fractions!of!ethanol!at!40!°C! from!the!immersion!DMA!results.!Insert:!Tg!as!a!function!of!p!where!Tg!values!(*)!were! estimated!when!p!>!0.5!(Tg!=!80.34p!+!52.83l!R2=0.999).! ! 3.3.3! Crystallinity!! For! assessing! the! crystallinity! of! PLA! during! hydrolytic! degradation,! DSC! and! XRD!techniques!were!used.!DSC!was!applied!to!calculate!the!percent!crystallinity!and! identify!changes!in!Tm,!Tc!and!Tg!using!DSC!thermograms.!The!identification!of!the!type! of!crystals!formed!during!PLA!hydrolysis!was!performed!by!the!XRD!technique.!! Figure!3.5!shows!the!percent!crystallinity!(%Xc)!obtained!by!the!DSC!technique! during!hydrolytic!degradation!of!PLA!as!a!function!of!time.!The!crystallinity!of!samples! exposed! to! water! increased! from! 2.8! to! 7.3%! after! 6! months! of! immersion.! For! PLA! films!exposed!to!ethanol!solutions,!the!crystallinity!increased!dramatically.!After!the!first! ! 99! ! 15!days!of!immersion,!the!crystallinity!of!PLA!in!50%!and!95%!ethanol!was!the!same,! around! 26%.! After! one! month,! the! 50%! ethanol! samples! started! showing! higher! crystallinity.! The! increase! on! crystallinity! during! hydrolytic! degradation! has! also! been! observed!in!other!systems!when!PLA!has!been!exposed!to!different!environments!and! pH![21,!47,!48].! c X % , y t i n i l l a t s y r c t n e c r e P 60 50 40 30 20 10 0 Water 50% Ethanol 95% Ethanol I II III 0 20 40 60 80 time, d 100 120 140 160 180 ! Figure!3.5!Percent!crystallinity!during!hydrolytic!degradation!of!PLA!films!immersed!in! water,!50%!and!95%!ethanol!at!40!°C.!Trend!lines!from!fitting!Avrami!equation!(Eq.!3.3.! Numbers! I,! II! and! III! indicate! the! regions! corresponding! to! the! different! processes! of! PLA!crystallization.!! ! The!increase!in!crystallinity!can!be!explained!by!the!depression!of!Tg!due!to!the! process!of!SIC.!If!Tg!falls!below!the!test!temperature!(40!°C),!the!polymer!chains!have! sufficient! mobility! and! tend! to! rearrange! into! a! crystalline! structure,! which! is! a! more! ! 100! ! stable! configuration.[36]! According! to! Ouyang! et! al.! the! transport! of! acetone! in! poly(ethylene!terephthalate)!(PET)!induces!crystallization!in!three!stages![34,!36].!The! first! stage! is! the! transport! of! the! solvent! by! diffusion,! which! is! controlled! by! the! concentration!gradient.!The!second!stage!involves!swelling!by!the!solvent!and!the!third! stage!is!secondary!crystallization.!In!the!case!of!PLA,!the!second!stage!present!in!PET! is! observed! in! Figure! 3.5! as! the! region! marked! with! number! I! due! to! the! dramatic! increase! in! swelling! by! ethanol,! as! measured! by! the! change! in! film! thickness.! In! this! region,! primary! crystallization! takes! place,! where! large! amounts! of! crystallites! are! formed!as!a!result!of!relaxation!of!constraints!by!the!release!of!internal!stress,!where! chains! begin! to! fold! and! become! crystals! [34836].! When! the! system! is! in! saturation! (region! II),! the! crystallization! rate! is! slow! and! polymer! chains! can! form! small! crystals! dispersed! in! the! amorphous! region,! corresponding! to! a! secondary! crystallization! [34,! 36].!After!30!days!in!95%!ethanol,!the!crystallinity!started!to!increase!(region!III).!This! can! be! explained! by! the! preferential! hydrolysis! of! the! amorphous! regions! left! after! secondary! crystallization,! which! increases! the! net! crystalline! region! as! degradation! proceeds.!For!PLA!in!contact!with!50%!ethanol,!crystallization!due!to!hydrolysis!started! after!15!days!of!exposure,!resulting!in!a!higher!%Xc!than!films!immersed!in!95%!ethanol.! The! higher! %Xc! can! be! explained! by! the! higher! solubility! of! water! molecules! in! the! amorphous!regions,!hydrolyzing!them!faster!and!therefore!increasing!the!net!crystalline! regions.!! In! order! to! quantify! the! crystallization! kinetics,! the! Avrami! theory! [49,! 50]! was! (Eq.!3.3)! applied!according!to:! 1?"−@AB111111111111111111! ! 101! ! where!Xc!is!the!relative!crystallinity!of!the!polymer,!t!is!time,!K!the!crystallization!rate! constant,!and!n!is!the!Avrami!exponent.!As!observed!in!Figure!3.5,!the!Avrami!model! fits!regions!I!and!II!well.!The!deviation!from!the!Avrami!equation!in!region!III!is!due!to! the!hydrolysis!of!the!amorphous!region!and!not!to!the!formation!of!new!crystals.!The! Avrami!kinetic!parameters!of!crystallinity!for!50%!ethanol!were!K/=!0.018!s81!and!n!=! 0.20,! and! for! 95%! ethanol! were! K/ =! 0.031! s81!and! n! =! 0.16.! Generally,! the! n! values! reported!in!the!literature!are!around!2!and!4![49,!51,!52].!The!low!values!obtained!in!this! study! are! in! accordance! with! recently! reported! values! by! Tsai! et! al.! where! PLA! was! exposed!to!methanol!and!ethanol,!giving!values!of!1!and!0.5,!respectively![53].!This!is! an! indication! of! the! uni8dimensional! growth! of! crystals! restricted! by! diffusion! of! the! solvent.!! The!XRD!profiles!of!PLA!films!during!hydrolytic!degradation!are!shown!in!Figure! 3.6.! When! PLA! was! immersed! in! pure! water,! the! profiles! showed! only! broad! peaks! during! hydrolysis.! These! results! indicate! that! PLA! immersed! in! pure! water! remained! amorphous!during!hydrolysis.!No!formation!of!crystals!took!place,!which!is!in!agreement! with!the!3%!increase!in!crystallinity!during!degradation.!In!contrast,!some!sharp!peaks! began! to! appear! after! 3! days! exposure! to! 50%! and! 95%! ethanol! (Figure! 3.6D! and! 3.6E).! The! diffraction! peaks! observed! in! 6! for! ethanol! solutions! correspond! to! the! formation!of!α8form!crystals!(orthorhombic!unit!cell!with!parameters!a=1.06!nm,!b=0.61! nm,!and!c=2.88!nm)![54].!In!50%!ethanol,!diffraction!peaks!at!14.8°,!16.8°,!19.1°!and! 22.4°!were!observed.!In!95%!ethanol,!the!diffraction!peaks!were!in!the!same!range!as! in! 50%! ethanol.! They! were! 14.8°,! 16.7°,! 19.1°! and! 22.2°,! corresponding! to! the! 010,! 110/200,! 100/203! and! 102/210! plane! reflections,! respectively! [26,! 55858].! The! ! 102! ! appearance!of!crystals!in!PLA!after!3!days!exposure!to!ethanol!solutions!indicates!that! SIC!took!place!in!the!early!stages!corresponding!to!primary!crystallization!(region!I).!At! all!times,!the!diffraction!peaks!were!the!same,!meaning!the!same!kind!of!crystals!were! formed! during! secondary! crystallization! (region! II)! and! were! present! during! the! entire! degradation! of! the! amorphous! regions! (region! III).! Zhang,! et! al.! [59]! studied! the! morphology! behavior! of! amorphous! PLA! during! hydrolysis! in! neutral! conditions,! and! semicrystalline! PLA! in! acid! and! alkaline! environments! where! α8form! crystals! were! mainly!formed!and!remained!during!the!degradation!process.!! ! ! 103! ! A B 14.8o 16.8o 19.1o 22.4o C 16.8o 14.8o 19.1o 22.4o D 16.8o 14.8o 19.1o 22.4o E 16.8o 14.8o 19.1o 22.4o 180 d 120 d 60 d 7 d 0 d 120 d 60 d 7 d 0 d 180 d 120 d 60 d 7 d 0 d 5 10 15 20 2 theta angle 25 30 5 10 15 20 2 theta angle 25 30 ! Figure!3.6!XRD!profiles!of!PLA!films!during!hydrolytic!degradation!in!(A)!water,!(B)!50%! ethanol,!(C)!95%!ethanol!and!in!the!3rd!day!of!immersion!in!(D)!50%!ethanol!and!(E)! 95%!ethanol!at!40!°C.!The!numbers!on!each!profile!indicate!days!immersed.! Crystallinity!results!can!be!correlated!with!DSC!thermograms!of!PLA!before!and! after!being!hydrolyzed!by!water8ethanol!solutions!for!various!times!(Figure!3.7).!Before! exposure,!amorphous!PLA!showed!a!cold8crystallization!temperature!peak!(Tcc)!due!to! the! rearrangement! of! polymer! chains! inducing! crystallization! during! the! DSC! heating! process.! When! PLA! was! exposed! to! pure! water! (Figure! 3.7A),! Tcc/shifted! to! a! lower! temperature.!This!can!be!attributed!to!chain!scission!and!reduction!in!molecular!weight! ! 104! ! of! the! PLA! matrix,! which! facilitates! nucleation! and! growth! of! PLA! crystals,! improving! chain!segment!mobility![43,!56].!Another!explanation!could!be!the!production!of!locally! ordered!structures!during!hydrolysis,!promoting!the!occurrence!of!cold!crystallization!at! lower!temperatures![56].!The!double!melting!peaks!could!be!attributed!to!the!formation! of!two!different!crystalline!structures!formed!during!the!DSC!heating!process![60,!61].! Some! crystals! that! could! be! formed! during! crystallization! from! melt! are! the! α! and! β8 forms!that!have!approximately!the!same!energy!and!therefore!the!possibility!to!coexist! [57,!62].!The!high!Tm!corresponds!to!melting!of!the!more!stable!structure,!which!is!the! α8crystal,!while!the!low!Tm!is!ascribed!to!the!less!perfect!crystal!that!is!the!β8form![62].! For! PLA! exposed! to! ethanol! solutions! (Figure! 3.7B! and! C),! after! the! 3rd! day,! the! samples!did!not!show!crystallization!peaks,!meaning!that!PLA!had!been!crystallized!in! agreement!with!the!XRD!profiles,!presenting!α8form!crystals!(Figure! 3.6).!The!α8form! crystals! are! reflected! in! thermograms! displaying! only! one! melting! peak! over! time.! However,!after!60!days!of!immersion,!a!small!endothermal!peak!started!to!appear!and! became!stronger!over!time.!This!phenomenon!could!be!explained!by!hydrolysis!process,! which! is! reflected! in! the! change! of! the! molecular! weight! distribution! (MWD)! of! PLA! (Figure! 3H.1,!Appendix!3H).!After!60!days,!the!MWD/of!PLA!became!broader!due!to! polymer! chain! scission,! resulting! in! shorter! polymer! chains! and! the! presence! of! LA! oligomers!with!different!Tm’s/compared!to!α8form!crystals!originated!by!the!SIC!process! (regions!I!and!II,!Figure!3.5).! ! 105! ! A B C exo / g W , w o l f t a e H exo / g W , w o l f t a e H exo / g W , w o l f t a e H 50 100 Temperature, oC 150 0 d 3 d 30 d 60 d 120 d 180 d 0 d 3 d 30 d 60 d 120 d 0 d 3 d 30 d 60 d 120 d 180 d 200 ! Figure!3.7!DSC!thermograms!of!PLA!film!during!hydrolytic!degradation!in!(A)!water,!(B)! 50%!ethanol!and!(C)!95%!ethanol!at!40!°C.!The!numbers!on!each!thermogram!indicate! days!immersed.! ! ! 106! ! 3.3.4! Lactic!acid!release! During!hydrolysis!of!the!amorphous!regions!of!PLA,!low!molecular!weight!water8 soluble! oligomers! and! monomers! are! released.! Figure! 3.8! shows! the! release! of! LA! from! PLA! into! water,! 50%! and! 95%! ethanol! at! 40! °C.! During! the! first! 40! days! of! exposure,!the!release!of!PLA!monomers!was!higher!when!the!polymer!was!in!contact! with!50%!ethanol,!followed!by!95%!ethanol!and!then!by!water!(Figure!3.8!insert).!After! the!second!month!of!exposure,!the!release!of!LA!increased!exponentially!for!samples!in! contact!with!50%!ethanol.!At!120!days,!the!concentration!of!LA!in!50%!ethanol!was!700! μg/mL,!meaning!that!approximately!30%!of!the!PLA!was!hydrolyzed.!Not!long!after!that,! the! film! disintegrated.! Over! the! same! period,! the! percentage! of! PLA! hydrolyzed! by! water!and!95%!ethanol!was!1.4!and!0.5%,!respectively.!! L m g µ / , d i c a c i t c a L 8 6 4 2 0 800 700 600 500 400 300 200 100 0 0 Water 50% Ethanol 95% Ethanol 0 10 20 30 40 20 40 60 80 100 time, d 120 140 160 180 ! Figure! 3.8! Release! of! LA! during! hydrolytic! degradation! of! PLA! films! in! contact! with! water,!50%!and!95%!ethanol!at!40!°C.!Fitted!lines!indicate!the!prediction!of!(Eq.!3.4!to! each!solution.! ! 107! ! The!highest!release!of!LA!in!50%!ethanol!concurs!with!the!fastest!reduction!in!Mn! (Figure! 3.1),! since! polymer! chains! in! the! amorphous! regions! are! being! hydrolyzed! faster,!as!reflected!by!the!increase!in!%Xc!in!region!III!/(Figure!3.5).!! A!model!that!predicts!LA!release!is!proposed!based!on!the!process!of!chain!scission! into!progressively!lower!molecular!weights,!followed!by!diffusion!of!LA8mers!through!the! PLA!matrix,!followed!by!crossing!the!interface!into!the!fluid,!and!finally!dissolving!into! solution!is!described!by:! >?"2!A −11111! (Eq.!3.4)! LA! in! the! various! water8ethanol! solutions.! The! resulting! concentration! of! LA! in! the! 5d=e∙G[d where!5d!is!the!concentration!of!LA!in!the!solution,!e!and!2!are!constants,!G!is!the!disk! surface! area,![d!is! fluid! volume,!A1is! time,! and!!!is! the! rate! constant! in! (Eq.! 3.2.! The! experimental!data!gave!e=0.9!μg/cm2!and!2=2.232.!The!predicted!LA!release!from! development! of! the! model! is! presented! in! the! Appendix! 3I.! Fitting! (Eq.! 3.4! to! the! PLA!hydrolysis!immersed!in!water,!50%!and!95%!ethanol!are!the!curves!in!Figure!3.8.! During!LA!release!it!is!important!to!consider!that!the!migration!rate!of!molecules!within! the! polymer! matrix! depends! on! several! factors,! including! the! size! of! the! migrants,! density,! and! the! Tg/ of! the! polymer! [63,! 64].! Therefore,! changes! in! Tg! will! affect! the! release! of! LA! previously! discussed! since! Tg! will! determine! the! flexibility! of! polymer! chains! and! the! free! volume! within! the! matrix.! When! PLA! was! exposed! to! water! (Tg=! 53! °C)! (Figure! 3.4),! the! diffusion! of! LA! and! LA8mers! took! place! in! the! glassy! state! because! the! experiments! were! conducted! at! 40°C.! Below! Tg,! in! the! glassy! state,! the! polymer!is!stiff!and!therefore!less!open!to!diffusion!by!LA!molecules.!For!PLA!immersed! in!ethanol!solutions,!diffusion!of!LA!took!place!above!Tg,!in!the!rubbery!state,!where!the! ! 108! ! polymer! molecules! are! flexible! and! open! to! diffusion! [63,! 65].! The! higher! the! ethanol! content,!the!lower!the!Tg/and!the!higher!the!diffusion!rates!of!LA!and!LA8mers.!! The! main! migrants! from! PLA! are! LA! and! LA8mers,! which! are! degradation! products! from! the! hydrolysis! of! polymer! chains.! This! study! only! carried! out! the! quantification!of!alkali!decomposition!products!based!on!the!conversion!of!lactide!and! LA8mers!to!LA.!A!theoretical!prediction!for!the!rate!of!diffusion!of!oligomers!up!to!5!units! was!made!based!on!the!free!volume!theory!as!a!function!of!the!Tg!of!the!polymer!during! hydrolysis! experiments! (Figure! 3J.1,! Appendix! 3J).! Based! on! the! predictions,! the! release!of!LA!(90!g/mol)!is!faster!when!PLA!is!exposed!to!95%!ethanol.!These!results! are! not! in! full! agreement! with! the! experimental! data! since! the! estimated! diffusion! coefficient! does! not! take! in! account! hydrolysis! of! the! polymer.! It! only! considers! the! effect!of!Tg./ Predicted!values!showed!that!the!smaller!the!molecular!weight!of!the!oligomer,! the! faster! the! diffusion! through! PLA.! Therefore,! the! LA! quantified! during! release! experiments!can!be!identified!as!mostly!LA!released!from!the!PLA!matrix!and!not!from! the!alkali!hydrolysis!of!LA8mers!released!from!PLA!in!the!solution.!This!finding!could!be! supported! by! work! conducted! by! Mutsuga! et! al.! [40]! who! studied! the! long8term! migration! of! LA8mers! in! water! at! 40! °C.! Their! results! showed! that! until! three! months! exposure!to!water,!no!LA8mers!were!detected.!However,!by!six!months,!3.46!μg/cm2!of! LA8mers!up!to!13!units!were!quantified.!In!our!experiment,!the!initial!presence!of!LA8 mers!in!the!PLA!films!were!not!detected!by!MALDI8TOF!(data!not!shown).!Therefore,!at! the! early! stages! the! LA! quantified! during! the! experiments! can! be! attributed! to! the! ! 109! ! hydrolytic!degradation!of!PLA!in!contact!with!the!water8ethanol!solutions!and!release!as! mostly!LA!into!solution.!! 3.4! Conclusions! The! exposure! of! PLA! disks! to! different! water8ethanol! solutions! led! to! the! hydrolytic!degradation!of!the!polymer!with!concurrent!SIC.!Hydrolysis!was!accelerated! by!the!immersion!of!PLA!in!50%!ethanol,!which!showed!a!faster!reduction!in!Mn!than!in! 95%! ethanol! and! pure! water.! Hydrolysis! is! related! to! the! amount! of! water! molecules! available! to! start! chain! scission,! so! NMR! techniques! were! applied! to! study! water! sorption! in! PLA.! Higher! sorption! of! D2O! was! found! when! PLA! was! exposed! to! 50%! ethanol,!explaining!the!faster!hydrolysis.!A!new!model!was!proposed!to!explain!the!rate! of!hydrolysis,!accounting!for!the!effect!of!PLA!swelling!due!to!ethanol!sorption.!The!rate! of!degradation!for!50%!ethanol!was!0.0230!gmol81!d81.!This!was!the!maximum!rate!of! decay!in!Mn,/meaning!that!50%!concentration!provides!the!optimal!competitive!balance! between!swelling!and!hydrolysis.!During!PLA!hydrolysis!the!%Xc!increased,!indicating! that!SIC!occurred!in!PLA!when!exposed!to!50%!and!95%!ethanol.!In!the!crystallization! process!of!PLA,!three!different!regions!were!identified.!Regions!I!and!II!were!due!to!SIC! by!ethanol.!The!Avrami!equation!was!found!to!describe!the!crystallization!process!well.! Region!III!was!due!to!hydrolysis!of!the!amorphous!regions.!XRD!studies!showed!the! formation!of!α8crystal!during!SIC.!LA!release!was!studied!as!an!indication!of!hydrolysis! of!PLA.!PLA!immersed!in!50%!ethanol!showed!the!highest!release!of!LA,!which!is!in! accordance!with!the!fastest!decay!in!Mn!by/hydrolysis.!A!model!was!proposed!to!predict! LA!release!during!hydrolysis!when!PLA!is!exposed!to!different!ethanol8water!solutions.! ! ! 110! ! 3.5! Acknowledgments! The! authors! thank! Olivier! Vitrac! for! assistance! with! estimating! theoretical! LA! diffusion! coefficients.! I8F,! F.,! thanks! the! Mexican! National! Council! for! Science! and! Technology! (CONACYT),! the! Mexican! Secretariat! of! Public! Education! (SEP)! and! the! Government! of! Mexico! for! providing! financial! support! through! a! Ph.D.! fellowship.! RA! thanks! partial! support! of! the! USDA! National! Institute! of! Food! and! Agriculture! and! Michigan!AgBioResearch,!Hatch!project!R.!Auras.! ! ! ! 111! ! ! ! ! ! ! ! ! ! ! ! ! ! ! APPENDICES! ! 112! ! APPENDIX!3A:!NMR!Analysis! The! 1H8NMR! spectrum! of! PLA! film! with! ethanol! is! shown! in! Figure! 3A.1.! The! protons!of!ethanol!CH3!and!CH2!groups!are!located!at!1.22!and!3.70!ppm,!respectively.! The!CH2!protons!have!some!overlap!with!residual!THF!(3.72!ppm).!These!chemical!shift! values,! however,! demonstrate! that! ethanol! does! not! overlap! with! the! peaks! of! PLA! protons.! Based! on! these! results,! ethanol! quantification! in! further! experiments! was! carried!out!using!the!protons!of!the!CH3!group.!! ! Figure!3A.1!1H8NMR!spectrum!of!PLA!with!ethanol.! ! ! ! ! ! 113! ! Table!3A.1!1H8NMR!peaks!and!identification!of!ethanol!in!PLA.! Compound! Group! Chemical!Shift! Multiplicity,!coupling! (ppm)! constant,!#protons! PLA! PLA! Ethanol! Ethanol! 8O8CH8CH38COO8! 8O8CH8CH38COO8! CH38CH28OH! CH38CH28OH! 5.14! 1.56! 1.22! 3.70! q,!J=7.1/Hz,!1! d,!J=7.2/Hz,!3! t,!J=7.0/Hz,!3! q,!J=6.9/Hz,!2! ! Water! identification! using! the! NMR! technique! led! to! some! modifications! in! the! media! for! quantification! purposes.! 1H8NMR! spectra! cannot! provide! direct! evidence! of! the!real!amount!of!water!sorbed!in!the!experiments!due!to!the!inevitable!contamination! by! environmental! water,! which! would! interfere! with! the! actual! concentration! of! water! sorbed!by!the!PLA!film.!If!the!sample!is!contaminated!with!water!from!the!surroundings,! it!will!show!up!around!1.56!ppm!when!PLA!is!dissolved!in!CDCl3,!where!the!chemical! shift!changes!depending!on!the!solvent!used!to!dissolve!the!polymer!under!study![66].! Another!issue!is!that!the!peaks!of!the!water!overlap!with!those!of!the!CH3!group!of!PLA! at!1.56!ppm!using!CDCl3!as!the!solvent!according!to!Table!3A.1.!Therefore,!to!quantify! the! amount! in! the! sorption! experiments,! deuterated! water! (D2O)! was! used! (hydrogen! isotopes!1H!of!H2O!have!been!replaced!by!the!deuterium!isotope!2H)![67].! Figure! 3A.2! shows! the! D8NMR! spectrum! of! PLA! dissolved! in! THF! when! 50%! ethanol!was!added.!An!additional!peak!in!the!spectrum!at!3.44!ppm!was!observed.!!The! D2O! peak! was! tentatively! assigned! to! 4.09! ppm.! The! reason! for! the! presence! of! two! peaks! can! be! explained! by! the! equilibrium! between! water! and! D2O! with! a! proton! ! 114! ! transfer!from!the!ethanol!to!D2O!and!an!exchange!of!deuterium!between!the!hydroxyl! group!of!the!ethanol!and!water!according!to!the!following!reaction![68]:!! gA67+W86⇄gA6W+76W! Typically,!one!would!expect!a!single!peak!for!the!mixture!due!to!rapid!exchange! between!D2O!and!ethanol.!This!is!not!the!case!for!these!measurements!because!of!the! low! concentration! of! the! D2O/EtOH! mixture! in! the! NMR! tube,! with! the! lowest! concentration!detected!in!sorption!experiments!of!0.08!μL/mL!and!0.24!μL/mL!(8.88x108 5!and!1.89x1084!g/mL)!for!D2O!and!EtOH,!respectively.!It!is!worth!mentioning!that!with! sorption! experiments! using! 100%! D2O,! two! peaks! were! seen! in! the! spectrum.! In! this! situation,!it!could!be!assumed!that!there!was!a!proton!transfer!from!the!end!groups!of! PLA!in!the!presence!of!D2O.!EXSY!NMR!(Exchange!Spectroscopy)!was!carried!out!to! verify! that! both! peaks! belong! to! the! D2O! in! the! system.! Indeed,! cross8peaks! were! observed! between! the! resonances.! Hence,! for! quantitative! purposes,! the! two! peaks! were!taken!into!consideration!for!water!sorption!in!PLA!since!they!represent!the!water! content!in!the!solvent!systems!under!study.!! ! 115! ! Figure!3A.2!D8NMR!spectrum!of!PLA!with!D2O! ! ! ! ! ! 116! ! APPENDIX!3B:!Optical!images!of!PLA!samples!during!hydrolytic!degradation! Figure! 3B.1! Optical! images! of! PLA! samples! during! hydrolytic! degradation! by! water,! 50%!and!95%!ethanol!at!40!°C.! ! ! ! 117! ! APPENDIX!3C:!Diffusion!of!H2O!and!D2O!vapor!in!PLA!film! Methodology:!Diffusion!of!H2O!and!D2O!vapor!in!PLA!films!were!determined!by! gravimetric!analysis!using!an!SGA8100!from!VTI!Corp.!(Hialeah,!FL,!USA).!PLA!films! (15820!mg)!were!exposed!to!75%!relative!humidity!(p/po)!at!40!°C.! e g n a h c i t h g e W % 0.6 0.4 0.2 0 0.6 0.4 0.2 0 A B Best fit line for the experimental data Experimental Confidence bands Prediction bands 0 1000 2000 time,s 3000 4000 0 1000 2000 time,s 3000 4000 ! Figure! 3C.1! Diffusion! of! (A)! H2O! and! (B)! D2O! vapor! in! PLA! film! at! 75%! relative! humidity,!40!°C.! Table! 3C.1!Diffusion!coefficient!(D)!of!H2O!and!D2O!vapor!in!PLA!film!at!75%! relative!humidity,!40!°C.! Vapor! H2O! D$x!1013!(m2/s)! 5.09!±!0.07!a! D2O! 4.75!±!0.30!a! Values!with!same!letter!are!not!statistically!different!(α=0.05!Tukey8Kramer!Test)!! Note:!D!values!were!calculated!using!W=H8 7.21Ai/8!where!H!is!thickness!and!Ai/8!is!the! half!time! ! ! 118! ! APPENDIX!3D:!Sorption!isotherms!for!H2O!and!D2O!vapor!in!PLA!film! Methodology:! H2O! and! D2O! sorption! isotherms! for! the! PLA! films! were! determined! by! gravimetric! analysis! using! an! SGA8100! from! VTI! Corp.! (Hialeah,! FL,! USA).!PLA!films!(15820!mg)!were!exposed!to!relative!humidities!between!5!and!95%!at! 40!°C.! Figure!3D.1!Sorption!isotherms!for!H2O!and!D2O!vapor!in!PLA!film!at!40!°C.! ! ! ! ! 119! ! APPENDIX!3E:!Ethanol!sorption!into!PLA!films! / A L P − g H O E − g t , l o n a h t E 0.18 0.16 0.14 0.12 0.1 0.08 0.06 0.04 0.02 0 0 A 0 1 2 0 1 B 2 Best fit line for the experiment values Experimental Confidence bands Prediction bands 40 80 time, d 120 0 40 80 time, d 120 160 ! Figure! 3E.1!Ethanol!sorption!into!PLA!films!in!contact!with:!(A)!50%!ethanol!and!(B)! 95%!ethanol!at!40!°C.!y8axis!is!in!grams!of!ethanol!sorbed!divided!by!grams!of!PLA!disk! used!for!the!1H8NMR!experiments.! ! ! ! 120! ! APPENDIX! 3F:! Expansion! of! PLA! in! contact! with! different! volume! fraction! of! ethanol! 8 6 4 2 0 n o i s n a p x e % Best fit line for the experiment values Experimental Confidence bands Prediction bands 0 0.2 0.4 0.6 Ethanol fraction 0.8 1 ! Figure!3F.1!Expansion!of!PLA!in!contact!with!different!volume!fractions!of!ethanol!(%! expansion=6p>/R2=0.969,!where!p!is!the!ethanol!fraction!by!volume).!The!%!expansion! was!determined!as!100!times!the!volume!of!ethanol!sorbed!in!PLA!divided!by!the!initial! volume!of!the!disk.! ! ! ! 121! ! APPENDIX! 3G:! Model! for! change! in! molecular! weight,! including! polymer! expansion! new!chains.!Since!the!total!mass!before!and!after!scission!is!the!same,!the!molecular! Let! N! be! the! number! of! polymer! chains! in! the! disk! at! time!A/and!D=1be! the! number8average! molecular! weight! (Mn).! In! time! t! +!∆A ,! scission! during! hydrolytic! degradation!will!cause!n!cuts!in!the!polymer!chains,!leaving!O−F1chains!uncut!and!2! weight!at!time!A+∆A!is! O O−F + 2F1111111! D=Q∆==D= If!∆A!is!small,!F!will!be!small!compared!to!O.!The!equation!above!then!reduces!to! D=Q∆==D= 1−FO ! D=Q∆=−D= D= where![]!is! the! volume! of! the! polymer! disk.! The! quantity!O[]is! related! to! the! initial! density!of!the!PLA.!The!quantity!F[]!is!determined!by!the!concentration!S!of!the!water! in!the!disk!and!by!the!contact!time!∆A!between!the!PLA!and!water.!Then! ∆DD =−ZS∆A! where!S !is! the! concentration! of! water! and! β! is! a! rate! constant.! In! the! limit! as!∆A! =−FO=− F[] O[] 111! Eq.!3G.4! Eq.!3G.1! Eq.!3G.2! Eq.!3G.3! ! ! so!that! approaches!zero,! ! 122! ! 11! 11111111! Eq.!3G.5! Eq.!3G.6! Eq.!3G.7! Eq.!3G.8! rate!of!scission,!the!mass!of!water!occupying!the!voids!inside!the!disk!is:! If!diffusion!of!water!and!ethanol!into!the!disk!can!be!considered!fast!compared!to!the! = 1−" [\+∆[ [] lDD =−ZSlA! !D=DN>:m c]= no where!DN!is!the!initial!Mn.!If!S!is!constant!due!to!fast!diffusion,! D=DN>:mc=1111! S=_Ppp1qr1sPA>t [] where!p!is!the!volume!fraction!of!water!in!the!ethanol8water!mix!surrounding!the!disk,![\! is!the!total!volume!of!voids!initially!in!the!disk,!and!∆[!is!the!increase!in!this!volume!due! S= 1−" [\[]+∆[[] = 1−" [\[]+0.06" = [\[] + 0.06−[\[] "−0.06"8! D=DN>?"−!A ! !=Z [\[]+ 0.06−[\[] "−0.06"8 ! If!the!expansion!of!the!disk!found!earlier!(.06p)!is!assumed!to!apply!to!only!the!voids,! Eq.!3G.10! Eq.!3G.11! to!expansion!by!ethanol.!! not!the!chains,!then! Eq.!3G.9! ! ! ! 123! ! APPENDIX!3H:!Molecular!weight!distribution!(MWD)!of!PLA!films!during! hydrolytic!degradation!! ) ) w M ( g o L ( d w d / ) ) w M ( g o L ( d w d / ) ) w M ( g o L ( d w d / A B C 0 d 7 d 15 d 30 d 60 d 90 d 120 d 180 d 2 3 4 5 Log(Mw) 6 7 ! Figure!3H.1!MWD!of!PLA!films!during!hydrolytic!degradation!when!in!contact!with:!(A)! water,!(B)!50%!ethanol!and!(C)!95%!ethanol!at!40!°C! ! ! ! 124! ! APPENDIX!3I:!Model!for!LA!release! From! the! model! for! change! in! molecular! weight,!D=DN>?"−!A !with!!= Z uYuv+ 0.06−uYuv "−0.06"8 .! Fitting! this! model! to! the! number! molecular! weight! vs! time! data! for! all! environments! (water,! 50%! ethanol,! 95%! ethanol)! gave!Z=1.05!and! uYuv=0.0056.!With!this!information,!a!prediction!for!the!release!of!LA!can!be!proposed.! The!mechanism!of!LA!release!begins!with!chain!scission!inside!the!disk.!This!results!in! chains! with! very! different! molecular! weights! diffusing! through! the! PLA! matrix! at! the! relationship:! ! same!time,!crossing!the!interface!some!time!later!into!the!fluid,!and!further!dissolving! into! LA! monomers,! mostly! before! leaving! the! PLA! matrix.! ! The! increment! of! mass! of! PLA!entering!the!fluid!surrounding!the!disk!in!infinitesimal!time!lA!appears!to!follow!the! l_~lADx11! where!l_ !is! the! infinitesimal! mass,!D !is! the! molecular! weight! at! time!A ,! which! is! D=D\>?"−!A ,!and!2!is!a!constant!that!accounts!for!all!the!above!effects.!When!A!is! small,!D!is!large!and!l_!is!small.!When!A!is!large,!D!is!small!and!l_!is!large.! Since!mass!leaves!the!disk!by!diffusion,!l_!should!be!proportional!to!the!disk!surface! area!G .! It! then! enters! the! surrounding! fluid! volume![d ,! and! increases! the! LA! l5d=y∙G∙lA [d∙Dx = y∙G[d∙D\x>?"2!AlA111! Eq.!3I.1! Eq.!3I.2! concentration.!The!above!relationship!therefore!becomes:! ! ! 125! where!5d!is!the!concentration!of!PLA!in!the!fluid!and!y!is!another!constant.!Integrating! and!applying!the!boundary!condition!_=0!at!A=0:! 5d=e∙G[d where!e !is! a! constant.! The!G[d !ratio! used! in! the! experiments! reported! was! 1.79! >?"2!A −1111111111111111111111111111111! !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!Eq.!3I.3! ! cm2/mL.! ! ! ! 126! ! APPENDIX!3J:!Theoretical!diffusion!coefficients!of!LAQoligomers! To! predict! the! diffusion! coefficients! of! LA! and! LA8mers,! the! free! volume! model/theory! proposed! by! Ventras! and! Ventras! [69]! was! applied.! The! diffusion! coefficient! (D1)! can! be! determined! using! the! following! equation,! which! depends! on! 1111! available!to!more!than!one!jumping!unit.!To!calculate!the!trace!diffusion!coefficient!of!LA,! temperature,!concentration,!and!the!free8volume!characteristics!of!the!system![70,!71]:! average! overlap! factor! for! the! mixture! introduced,! since! the! same! free8volume! is! Wi=WN>?"−g∗2% >?" −zi[i∗+z8{[8∗ [|}/C where![|}/C !is! the! term! where! the! free8volume! characteristics! of! the! system! are! included,! with![|}!being! the! average! hole! free! volume! per! gram! of! mixture! and!C!the! the! mass! fraction! of! LA! in! PLA! film,!zi,! is! assumed! to! approach! 0.! Accordingly,! the! weight!fraction!of!PLA!in!PLA!film,!z8,!is!close!to!1.!{!is!the!ratio!of!critical!molar!volume! of!LA!jumping!unit!to!critical!molar!volume!of!PLA!jumping!unit,!and![i∗!and![8∗!are!the! [|}/C!was!calculated!using!the!following!equation:! [|}C =zi@iiCi @8i+%−%~i+z8[|}8C8 ! where!@ii/!Ci!and!@8i−%~i!are!LA!free!volume!parameters,!%!is!temperature!in!K,!C8!is! the! overlap! factor! for! the! free! volume! of! pure! PLA,! and![|}8!is! the! specific! hole! free! specific!hole!free!volume!of!LA!and!PLA!required!for!a!jump,!respectively.!! Eq.!3J.1! Eq.!3J.2! ! ! ! 127! ! ! !Eq.!3J.3! Eq.!3J.4! polymer.!The!following!equations!were!used!to!calculate!free!volume!parameters:! occupied! volume! and! the! specific! interstitial! free! volume! for! the! equilibrium! liquid! 111111%≥%~8111111111111! 111111%<%~8! volume! of! the! equilibrium! liquid! polymer! at! any! temperature.![|}8 !was! calculated! depending!on!the!glass!transition!temperature!of!PLA!1%~8 :! [|}8=[8N%~8 r}8+Ä8%−%~8 [|}8=[8N%~8 r}8+ Ä8−Äc8 %−%~8 where![8N%~8 !is!the!specific!volume!of!the!polymer!at!%~8,!r}8 !is!the!fractional!hole!free! volume!of!PLA!at!%~8,!Ä8!is!the!thermal!expansion!coefficient!for!the!equilibrium!liquid! polymer,! and1Äc8 !is! the! thermal! expansion! coefficient! for! the! sum! of! the! specific! r}8 =Ä8@88! Äc8=ln[8N%~8 1−r}8 [8N0%~8 C8=[8N%~8Ä8 1111! @i8/C8 [i∗=[iN0 ! [8∗=[8N0 ! [8∗ @i8C8 = 2.3035i~ 858~ 8! @88= 58~ 8! !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!Eq.!3J.10! !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!Eq.!3J.11! !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!Eq.!3J.8! !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!Eq.!3J.9! !Eq.!3J.5! Eq.!3J.6! Eq.!3J.7! 11111! ! ! ! ! 128! Di[iN −lnWN+ @iiCi ! ! ! equilibrium!liquid!of!LA!and!PLA!at!0!K,!respectively.!! Viscosity8temperature!and!density8temperature!data!for!pure!LA!was!used!to!determine! where!@i8 /C8 !and!@88 !are! PLA! free! volume! parameters,! 5i~ 8 !and! 58~ 8 !are! the! Williams8Landel8Ferry! (WLF)! constants,![iN0 !and![8N0 !are! the! specific! volume! of! WN,!@ii/Ci,!and!@8i−%~i,!using!the!following!equation![72]:! lnÖi=ln 0.1241×110:iÜ[c8/`2% [i∗ @8i+%−%~i1! where!Öi!is! viscosity,![c!is! the! LA! molar! volume! at! the! critical! temperature,!Di!is! the! molecular!weight!of!LA!and![iN!is!the!specific!volume!of!the!pure!LA!at!%.! WDN,% = DDN WD,% where!DN!is!the!molecular!weight!of!LA,!D!is!the!molecular!weight!of!the!oligomers,!Ä!is! the!solute,!polymer!type!and!the!temperature!difference,!%!and!%~.!!!!!! a!scaling!exponent!deviation!to!the!Rouse!theory,!which!depends!on!the!geometry!of! The! diffusion! coefficient! of! oligomers! up! to! five! units! of! LA! was! predicted! using! the! :áà:àâ ! Eq.!3J.12! Eq.!3J.13! scaling!law![73]:! ! ! ! ! 129! ! Table!3J.1!Parameters!to!predict!the!diffusion!coefficient!of!LA!and!up!to!5!LA8 mers!in!PLA.! Parameter! [iN0 ,!cm3/mol! [c,!cm3/mol! 2,!J/Kmol! Di,!g/mol! WN,!cm2/s! @ii/Ci,!Kcm3/g! @8i−%~i,!K! 5i~,!K81! 58~,!K! Ä8,!C81! [8N0 ,!cm3/g! @i8/C8,!cm3/g! [8N%~8 ,!cm3/g! g∗,!J/mol! zi! z8! {! Ä! Value! 69.2!! 259.5!! 8.314!! 90!! 2.146!x1089! 0.0145!! 10.46!! 3.24!! 164.9!! 7.4!x1084!! 0.7214,!0.7116!and!0.7024!*! 5.863!x1084,!5.783!x1084!and!5.708!x1084!*! 0.8105,!0.7995!and!0.7892!*!! 2.4564!x104!! 0! 1! 0.65! 5.5,!4.2!and!3.3!*! *!Values!for!water,!50%!and!95%!ethanol,!respectively.! ! 130! ! LA 10−18 2−LA 3−LA 4−LA s / 2 m , t n e i c i f f e o c n o i s s u f i D 10−19 10−20 10−21 10−22 5−LA Water 50% Ethanol 95% Ethanol 10−23 100 150 200 Molecular weight, Da 250 300 350 400 ! Figure! 3J.1! Theoretical! estimated! diffusion! coefficients! of! LA! and! LA8mers! in! water,! 50%!and!95%!ethanol.!Fitted!exponential!decay!lines!are!provided.! ! ! ! 131! ! APPENDIX!3K:!Initial!amount!of!lactic!acid!oligomers! Matrix8assisted! laser! desorption8ionization! time8of8flight! mass! spectrometry! 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[72]!J.!Vrentas,!C.!Vrentas,!Energy!effects!for!solvent!self8diffusion!in!polymer8solvent! systems,!Macromolecules!26!(1993)!127781281.! [73]!X.!Fang,!S.!Domenek,!V.!Ducruet,!M.!Réfrégiers,!O.!Vitrac,!Diffusion!of!aromatic! solutes! in! aliphatic! polymers! above! glass! transition! temperature,! Macromolecules! 46! (2013)!8748888.! ! 140! Chemical!Recycling!of!Poly(Lactic!Acid)!by!WaterQEthanol!Solutions! CHAPTER!4! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! A!version!of!this!chapter!is!published!as:! F. Iñiguez-Franco, R. Auras, K. Dolan, S. Selke, D. Holmes, M. Rubino, H. Soto- Valdez, Chemical recycling of poly(lactic acid) by water-ethanol solution, Polymer Degradation and Stability 149 (2018) 28-38.! ! 141! ! 4.0! Abstract! The!chemical!recycling!of!poly(lactic!acid)!(PLA)!by!hydrolysis!in!water8ethanol! solutions!at!moderate!temperatures!was!investigated!and!the!kinetic!parameters!of!the! hydrolysis,! rate! constant! and! activation! energy! (Ea)! were! assessed.! Hydrolysis! experiments!of!PLA!in!50%!ethanol!at!40,!60,!70!and!80!°C!were!performed.!Analyses! of! the! molecular! weight! distribution! (MWD)! of! PLA! were! performed! by! conducting! a! deconvolution! of! the! MWD/ distribution! since! the! MWD/ at! the! latter! stages! of! the! hydrolysis!experiments!was!not!normally!distributed.!Conducting!this!analysis!allowed! us!to!identify!the!more!likely!possible!hydrolysis!reactions!and!to!estimate!the!prevalent! rate!of!hydrolysis.!A!reparameterization!of!the!Arrhenius!equation!was!conducted!for!a! better!estimation!of!the!Ea/with!a!low!correlation!coefficient!between!the!parameters.!No! dependence!of!the!hydrolysis!rate!with!the!change!in!pH!for!50%!ethanol!was!detected.! For!a!better!understanding!of!the!effect!of!temperature!on!the!hydrolytic!degradation!of! PLA!in!water,!hydrolysis!experiments!were!also!performed!at!60,!70,!80!and!90!°C.!The! final!Ea/values!for!the!hydrolytic!degradation!of!PLA!in!50%!ethanol!and!in!water!were! 9.627! x104! and! 10.474! x104! J/mol,! respectively,! that! are! within! the! same! range! of! values!reported!in!literature.!The!Ea/was!9%!higher!when!PLA!was!hydrolyzed!in!water! than! in! 50%! ethanol.! This! study! presents! an! alternative! method! for! PLA! chemical! recycling!using!50%!ethanol.!!! 4.1! Introduction! Production!of!plastics!from!fossil!resources!is!no!longer!considered!sustainable,! so! that! pressure! for! a! circular! and! bio8based! economy! of! materials! with! lower! ! 142! ! environmental!footprint!and!from!renewable!resources!is!intensifying.!Among!these!new! bioplastics,!PLA!is!one!of!the!main!commercial!polymers!that!has!increasingly!gained! acceptance![1,!2].!PLA!is!a!thermoplastic!aliphatic!polyester,!based!on!the!ring!opening! polymerization!of!the!lactic!acid!(LA)!dimer!8!lactide!8!obtained!from!the!fermentation!of! sugars!derived!from!corn,!potato!or!cassava!starches![1,!2].!As!pollution!is!becoming! increasingly! a! concern,! evaluation! of! the! end! of! life! (EoL)! scenario! is! essential! to! understand!the!real!environmental!footprint!of!plastics!such!as!PLA.!PLA,!besides!being! disposed!through!all!the!regular!EoL!scenarios!(i.e.,!recycling,!incineration!and!landfill),! can!also!be!recovered!through!composting.!One!of!the!preferable!disposal!routes!for!all! polymers! including! PLA! is! recycling,! which! can! be! either! mechanical! or! chemical! [2].! Mechanical!recycling!involves!recovering,!sorting,!regrinding!and!reprocessing!of!PLA! [3].!Although!this!process!requires!simpler!technologies!and!it!is!easily!implemented,!it! results! in! the! deterioration! of! PLA’s! physical! properties! since! the! molecular! weight! is! reduced! due! to! shear! and! temperature,! as! well! as! foreign! contaminants! that! are! not! completely! removed! [4].! Alternatively,! chemical! recycling! converts! PLA! back! to! its! monomer!LA!by!hydrolysis!and!separate!contaminants,!so!LA!can!then!be!used!as!raw! material!for!new!PLA!production!with!the!same!properties!as!virgin!PLA![4,!5].! Chemical!recycling!is!a!depolymerization!method,!which!does!not!require!severe! operating! steps.! PLA! and! other! polymers! are! generally! depolymerized! at! high! temperature!using!toxic!solvents!such!as!toluene,!tetrahydrofuran,!dichloromethane!or! chloroform! [4,! 688].! However,! other! methods! have! been! developed! for! chemical! recycling!of!PLA!through!alcoholysis!by!the!solubilization!of!PLA!in!ethyl!lactate!followed! by!the!hydrolysis!of!the!lactic!acid!ester!formed![9,!10].!Solubilization!using!acetone!has! ! 143! ! been! explored! [11].! New! strategies! for! recycling! PLA! in! aqueous! phase! at! high! temperatures!has!also!been!investigated![12,!13].!However,!temperatures!higher!than! the!melting!temperature!(Tm)!of!PLA!may!trigger!racemization!and!decomposition!of!the! LA! [12].! Therefore,! hydrolysis! of! PLA! at! lower! and! medium! temperature! is! preferred.! The! hydrolysis! of! PLA! is! mostly! affected! by! the! pH! of! the! solution,! temperature,! and! crystallinity!of!the!original!PLA!material![6,!14].!! A! chemical! recycling! method! using! “green”! solvents! at! moderate! temperatures! should! be! economically! viable! and! preferred! for! recovery! of! PLA.! We! have! demonstrated!in!our!previous!work![15]!that!the!hydrolytic!degradation!of!PLA!in!water8 ethanol!solutions!was!faster!when!PLA!was!exposed!to!50%!water8ethanol!(v/v).!This! was! attributed! to! the! faster! water! penetration! into! the! PLA! matrix! starting! the! chain! cleavage! after! immediate! swelling! of! the! PLA! matrix! due! to! the! presence! of! ethanol! molecules.!This!proposed!method!could!be!used!to!safely!recover!PLA.!However,!the! optimal!process!conditions!to!recycle!PLA!using!water8ethanol!solutions,!as!a!“green”! solvent,! must! be! determined! to! generate! a! commercial! opportunity.! Furthermore,! the! parameters!controlling!this!process!such!as!the!activation!energy!and!temperature!need! to!be!fully!assessed.! The! effect! of! temperature! on! the! hydrolytic! and! chemical! recycling! process! of! polymers!is!important!to!understand.!Ea!is!commonly!used!to!describe!how!the!kinetic! rate! of! the! reactions! changes! with! temperature,! and! it! is! described! by! the! Arrhenius! equation.! The! Ea! is! usually! estimated! by! linearizing! the! Arrhenius! equation! using! the! natural!logarithm!to!avoid!numerical!problems!of!having!correlation!between!factors!of! the! Arrhenius! equation! [12,! 16820].! This! method,! however,! creates! a! high! correlation! ! 144! ! between!the!pre8exponential!factor!and!the!Ea!parameters!of!the!equation.!Therefore,!in! this! study,! we! propose! a! reparameterization! of! the! Arrhenius! equation! as! previously! suggested! by! other! authors! in! other! fields! [21828].! In! this! work,! we! applied! reparameterization! of! the! Arrhenius! equation! for! estimating! the! Ea! of! the! hydrolytic! degradation!of!PLA!to!obtain!better!estimates,!low!relative!errors,!and!low!correlation! among!the!governing!parameters.!Thus,!work!aimed!to!understand!and!to!analyze!the! hydrolytic! degradation! of! PLA! by! water8ethanol! solutions! for! the! purpose! of! chemical! recycling! at! moderate! temperatures! and! to! estimate! the! kinetic! parameters,! rate! constant! and! activation! energy! that! drives! the! hydrolytic! degradation! of! PLA! in! these! solutions.!! 4.2! Material!and!methods! 4.2.1! Chemicals!and!reagents! PLA! pellets! (3.8! –! 4.2! wt.%! D8LA)! were! procured! from! NatureWorks! LLC! (Minnetonka,! MN,! USA).! CAPS! (38(cyclohexylamino)818propanesulfonic! acid),! HEPES! (N8(28Hydroxyethyl)piperazine8N′8(28ethanesulfonic! acid)),! sodium! citrate,! methanol,! 18 butanol! and! ethanol! HPLC! grade! were! obtained! from! Sigma8Aldrich! (St.! Louis,! MO,! USA).!Water!HPLC!grade!and!18propanol!were!acquired!from!J.T.!Baker!(Center!Valley,! PA,!USA),!respectively.!Tetrahydrofuran!(THF)!reagent!grade!stabilized!with!BHT!was! procured!from!Pharmco8AAPER!(North!East,!CA,!USA).!! 4.2.2! Film!production! PLA!pellets!were!dried!at!60!°C!for!24!h!under!vacuum!(85!kPa)!and!processed! in!a!Randcastle!cast!film!microextruder!(Extrusion!System,!Inc.,!Cedar!Grove,!NJ,!USA)! ! 145! ! with! a! screw! of! 1.5875! cm! diameter,! 24/1! L/D! ratio! extruder,! and! 34! cc! volume.! The! temperatures!of!extrusion!for!zone!1,!2,!and!3,!transfer!tube!and!die!were!193,!212,!215,! 215!and!210!°C,!respectively,!with!a!rotation!speed!of!49!rpm.!The!film!thickness!was! 27.9!±!9.9!μm.!! 4.2.3! Molecular!weight! To! study! the! hydrolytic! degradation! of! PLA! film! at! different! temperatures,! migration! cells! as! recommended! by! ASTM! D4754811! [29]! were! used.! Each! cell! contained!ten!disks!of!PLA!film,!2.0!cm!of!diameter,!placed!on!a!stainless!steel!wire!and! separated! by! glass! beads! [30].! Preliminary! experiments! on! hydrolytic! degradation! of! PLA!were!performed!in!different!water8alcohol!solutions:!50%!ethanol,!50%!methanol,! 50%! 18propanol! and! 20%! 18butanol! by! volume! at! 70! °C.! All! the! solutions! were! preconditioned!before!testing.!After!determining!50%!ethanol!as!the!main!solvent!to!be! used! in! the! study! of! the! Ea! of! PLA! hydrolytic! degradation,! migration! cells! containing! 50%! ethanol! by! volume! were! stored! at! 40,! 60,! 70! and! 80! °C.! All! cells! were! first! conditioned! at! the! set! temperature.! Cells! contained! water! were! also! conditioned! and! stored! at! 60,! 70,! 80! and! 90! °C.! Experiments! designed! to! control! the! pH! of! the! 50%! ethanol!solution!during!the!hydrolytic!degradation!of!PLA!were!performed!at!80!°C!with! pH!values!of!4,!7!and!11,!using!sodium!citrate!(0.1!M),!HEPES!(0.3!M)!and!CAPS!(0.1! M)! as! buffer! solutions,! respectively.! Samples! of! film! were! retrieved! periodically! to! assess!number!average!molecular!weight!(Mn),!weight!average!molecular!weight!(Mw)! and!polydispersity!index!(PDI).!Mn!was!assessed!as!previously!described!by!the!authors! [15]! where! 10! mg! of! film! were! dissolved! in! THF! (2! mg/mL)! and! tested! using! size! exclusion!chromatography!(SEC).!The!measurements!were!conducted!in!triplicate.!! ! 146! ! ! The!deconvolution!of!the!molecular!weight!distribution!(MWD)!was!carried!out!in! Fityk!(distributed!under!the!terms!of!GNU!General!Public!License)!using!nonlinear!least8 squares!curve!fitting!for!a!LogNormal!function![31,!32].!! 4.2.4! Nuclear!Magnetic!Resonance!(NMR)! PLA! samples,! after! hydrolysis! in! 50%! ethanol! at! 80! °C,! were! analyzed! by! 13C! NMR! (carbon! nuclear! magnetic! resonance)! on! a! 500! MHz! Varian! DirectDrive! 2! Spectrometer! equipped! with! 5! mm! PFG! OneNMR! probe.! The! gHMBC! (gradient! heteronuclear!multiple!bond!correlation)!experiment!was!run!on!a!Bruker!Avance!900! MHz! NMR! spectrometer! equipped! with! a! 5! mm! TCI! triple8resonance! cryoprobe! operating! at! 898.76! and! 226.02! MHz! for! 1H! and! 13C,! respectively.! ! Samples! were! dissolved!in!CDCl3!and!run!at!ambient!temperature.!!7292!transients!were!collected!for! the!carbon!NMR!with!a!1.0!second!recycle!delayl!data!were!zero8filled!to!262144,!and! 0.5! Hz! exponential! multiplication! was! used.! gHMBC! data! were! collected! with! a! 1.5! recycle!delay,!96!scans!per!increment!and!350!increments.!Linear!prediction!and!zero8 filling!were!applied!to!the!carbon!dimension,!and!unshifted!sine8bell!squared!windows! were!used!in!both!dimensions.!! 4.2.5! Parameter!estimation:!Order!of!reaction! To!calculate!the!order!of!the!reaction!for!the!hydrolytic!degradation!of!PLA,!the! general!rate!law!was!used:! −lDBlA =!DBä! Eq.!4.1! where! k! is! the! rate! constant! of! hydrolysis! (h81)! and! η! is! the! order! of! reaction.! The! parameters! k! and! η! were! estimated! separately.! One! of! the! parameters! was! set! at! ! 147! ! different! constant! values! in! a! range! of! possible! values! to! obtain! the! estimate! of! the! second!parameter!with!the!lowest!root!mean!square!error!(RMSE).!! After!data!analysis!and!to!calculate!the!hydrolysis!rate!constant,!the!first!order! reaction!equation!was!used:! DB=DBY>?"−!A ! Eq.!4.2! Eq.!4.3! where!Mno!is!the!initial/Mn.! 4.2.6! Parameter!estimation:!Activation!energy!! 4.2.6.1! Step!1:!ReQparameterization!of!the!Arrhenius!equation! The!effect!of!temperature!on!the!hydrolytic!degradation!of!PLA!can!be!described! using!the!Arrhenius!equation:! !=!\>?" −gã2% ! where! ko! (h81)! is! the! pre8exponential! factor,! R! is! the! universal! gas! constant! (8.314! J/mol#K),! and! T! is! temperature! (K).! A! reparameterization! of! the! Arrhenius! equation! is! proposed! in! this! study! to! estimate! Ea! and! to! avoid! a! high! correlation! between! parameters!ko!and!Ea!as!previously!explained![25827].!This!can!be!done!by!introducing! the!reference!temperature!(Tref)!that!corresponds!to!the!rate!constant!of!hydrolysis!(kref):! !=!åçd>?" −gã2 1%− 1%åçd ! Eq.!4.4! The!hydrolytic!degradation!of!PLA!can!be!described!by!the!change!of!Mn!as!a! function!of!time!(Eq.!4.2).!Then,!to!describe!the!effect!of!temperature!on!the!hydrolysis! of!PLA,!the!following!model!is!proposed!by!inserting!Eq.!4.4!in!Eq.!4.2,!resulting!in:! ! 148! ! DB=DBY>?" −!åçd>?" −gã2 1%− 1%åçd A ! Eq.!4.5! The! hydrolytic! degradation! can! also! be! pH! dependentl! therefore,! the! dependence!of!pH!was!introduced!in!Eq.!4.5!as!an!empirical!form!of!secondary!model! that!has!been!applied!similarly!for!water!activity!studies![33]:! DB=DBY>?" −!åçd>?" −gã2 1%− 1%åçd +Z"7−"7åçd A / Eq.!4.6! ! where!β!is!a!constant!which!reflects!the!pH!dependence!of!the!hydrolytic!degradation! and!pHref!is!the!reference!pH.! To!estimate/β,!experiments!involving!hydrolysis!of!PLA!at!one!temperature!and! different! pH’s! were! performed:! 80! °C! at! pH! 4,! 7! and! 11.! The! following! equation! was! used!to!estimate!β!and!pHref:! DB=DBY>?"−!åçd>?"Z"7−"7åçd A / Eq.!4.7! 4.2.6.2! Step!2:!Scaled!sensitivity!coefficient!(X′)! When! parameter! estimation! is! performed,! two! or! more! parameters! may! be! involved.!Therefore,!it!is!essential!to!determine!if!those!parameters!can!be!estimated! accurately,! easily! and! simultaneously.! The! sensitivity! coefficient! (SC)! indicates! the! magnitude! of! change! of! the! response! due! to! perturbation! in! parameters,! and! is! an! important!tool!to!determine!the!correlation!among!parameters.!The!SC!is!obtained!by! taking!the!first!derivative!of!the!dependent!variable!(μ)!with!respect!to!the!parameter!of! interest!(i.e./<éè=êë êgã).!However,!for!comparing!parameters!on!the!same!scale,!a! scaled!sensitivity!coefficient!(SSC)!(X′)!figure!is!often!plotted.!The!SSC!is!provided!by! ! 149! ! <´éè=gã ìîìéè1!! multiplying!the!SC!with!the!parameter!itself![34].!For!example,!the!X′/of!the!activation! energy!can!be!obtained!as!follows:!! ! ! ! ! ! ! ! ! Eq.!4.8! 4.2.6.2.1!Temperature!simulation!(Tsim)! Ea! indicates! the! sensitivity! of! the! rate! to! temperature.! ! When! temperature! is! constant,!it!is!not!possible!to!estimate/Ea,!or!to!plot!its!SSC.!Therefore,!a!new!concept! was! developed! to! show! the! temperature8dependent! SSC! when! there! are! multiple! isothermal!experiments.!So,!a!Tsim!approach!was!proposed!to!plot!X′/of!Ea/by!inserting!a! linear!increasing!dynamic!temperature!function!in!the!model:!!! %ïñó=%ò+%}−%ò Aóãô A/ Eq.!4.9! where!TL!is!the!lowest!temperature!(K),!TH!is!the!highest!temperature!(K),!t!is!time!and! tmax!is!the!maximum!time!duration.!! 4.2.6.2.2!pH$simulation!(pHsim)! For!the!SSC!of!Eq.!4.6!and!Eq.!4.7!the!same!concept!as!Tsim!was!applied!since! pH!is!changing!during!the!hydrolysis!reaction.!So,!a!pHsim!approach!was!proposed!by! inserting!the!following!function!in!the!models!to!plot!X′/when!pH!is!changing!over!time:! "7ïñó=1"7ò+"7}−"7ò Aóãô A/ Eq.!4.10! where!pHL!is!the!lowest!pH!and!pHH!is!the!highest!pH.! 4.2.6.3! Step!3:!Estimation!of!the!optimum!pHref!and!Tref! The! goal! is! to! obtain! near! zero! correlation! among! parameters! for! better! estimation! since! by! reducing! the! correlation,! the! relative! errors! will! be! reduced.! To! ! 150! ! reach!the!goal!it!was!important!to!find!the!optimum! pHref!and! Tref!as!explained!in!the! supporting!information!provided!online.!! To!estimate!Ea!in!Eq.!4.6,!assuming!it!is!constant!over!all!temperatures!and!over! all!pH,!it!was!necessary!to!estimate!β!from!Eq.!4.7!for!later!use!as!a!fixed!value!in!Eq.! 4.6.!In!order!to!estimate/β,!the!optimum!pHref!was!obtained!from!Eq.!4.7!by!plotting!the! correlation!between!kref/and!β!versus!a!possible!range!of!pHref.!The!optimum!pHref!was! used!to!estimate!the!parameters!kref,!Mno!and/β.!Once!β!was!estimated,!the!value!was! applied! as! a! constant! in! Eq.! 4.6! and! pHref! used! was! the! optimum! pHref! previously! estimated.! The!optimum!Tref!was!obtained!from!a!plot!of!the!correlation!between!kref!and!Ea/ as! a! function! of! the! possible! range! of! Tref! using! Eq.! 4.6! and! Eq.! 4.5.! For! the! final! estimation,!the!optimum!Tref/value!was!used!to!estimate!the!parameters/Mno,!kref!and!Ea.! The! nonlinear! regression! (nlinfit)! function! in! MATLAB®! 2016a! (MathWorks,! Natick,!MA,!USA)!was!used!to!estimate!all!the!parameters.!Mean!comparisons!of!the! studied! parameters! among! treatments! were! done! using! the! Tukey! HSD! test! (p<0.05)! with!JMP®!9.0!(Cary,!NC,!USA)!statistical!software.! 4.3! Results!and!discussion! 4.3.1! Hydrolysis!in!different!alcohol!solutions! ! In! our! previous! work,! the! hydrolytic! degradation! of! PLA! in! contact! with! water8 ethanol!solutions!was!evaluated.!PLA!showed!faster!hydrolytic!degradation!when!it!was! in! contact! with! 50%! ethanol! at! 40! °C! [15].! This! is! due! to! the! competitive! balance! between! the! swelling! effect! of! ethanol! expanding! the! PLA! network! allowing! the! maximum!sorption!of!water!into!the!PLA!matrix!and!the!cleavage!of!the!main!chain!of! ! 151! ! the!polymer!due!to!hydrolysis.!To!further!explore!the!swelling!effect!of!alcohol!solutions,! different!alcohol!solutions!all!at!mostly!50%!volume!with!water!were!used!to!evaluate! the! competitive! balance! between! swelling! and! water! sorption.! Figure! 4.1! shows! the! change!in!Mn!with!time/when!PLA!was!immersed!in!50%!ethanol,!50%!methanol,!50%! 18propanol,!and!20%!18butanol!at!70!°C.!A!lower!ratio!of!18butanol!was!selected!due!to! the!low!miscibility!between!18butanol!and!water.!Table!4A.1,!Appendix!4A,!shows!the! order!of!reaction!and!hydrolysis!rate!for!each!of!the!water8alcohol!solutions.!Since!the! order!of!reaction!for!all!the!solvents!was!close!to!one,!the!approach!to!estimate!the!rate! of!reaction,!k,!was!to!assume!as!a!first!order!behavior.!The!rate!constants!showed!that! the! hydrolysis! was! slower! when! PLA! was! in! contact! with! 50%! methanol! and! no! differences!were!found!with!50%!ethanol,!50%!18!propanol!and!20%!18butanol.! ! ! 152! ! a D , n M 14x 104 12 10 50% Ethanol 50% Methanol 50% 1−Propanol 20% 1−Butanol 8 6 4 2 0 0 20 40 60 80 time, h 100 120 140150 ! Figure!4.1!Mn!as!a!function!of!time!during!hydrolytic!degradation!of!PLA!film!immersed! in! 50%! ethanol,! 50%! methanol,! 50%! 18propanol! and! 20%! 18butanol! at! 70! °C.! Lines! indicate! fitting! of! the! first! order! reaction! Eq.! 4.2! with! k/=! 0.0352! ±! 0.0027,! 0.0294! ±! 0.0014,!0.0384!±!0.0024!and!0.0367!±!0.0017!h81,!for!50%!ethanol,!50%!methanol,!50%! 18propanol!and!20%!18butanol,!respectively.! ! Since! no! differences! were! found! among! the! alcohols,! except! with! methanol,! which! exhibited! slower! hydrolysis,! 50%! ethanol! was! used! to! assess! the! effect! of! temperature!on!the!hydrolytic!degradation!of!PLA!and!to!calculate!Ea.!Ethanol!was!also! selected! since! it! can! be! produced! from! renewable! resources! such! as! sugar! cane! or! ! 153! ! cornstarch,!reducing!the!negative!impact!of!using!solvents!derived!from!fossil!resources! for!the!recycling!of!PLA![35837].!Additionally,!the!boiling!point!of!50%!ethanol!solution!is! higher!than!50%!18propanol!(92!and!88!°C,!respectively)!and!around!the!same!as!20%! 18butanol! (93! °C)l! thus,! the! hydrolysis! and! recycling! can! be! performed! at! higher! temperatures! without! reaching! the! boiling! point,! which! would! create! process! complications.!Lastly,!ethanol!is!a!more!economical!choice!than!that!of!18propanol!and! 18butanol![38].!! 4.3.2! Hydrolytic!degradation!of!PLA!in!50%!ethanol! To!study!the!effect!of!temperature!on!the!hydrolytic!degradation!of!PLA!in!50%! ethanol,! PLA! was! exposed! at! different! temperatures! above! the! glass! transition! temperature!(Tg)!of!the!polymer!immersed!in!50%!ethanol!(Tg!=!36!°C![15]):!40,!60,!70! and!80!°C.!Figure! 4.2! shows!the!MWD!of!PLA!when!it!was!exposed!to!hydrolysis!in! 50%!ethanol.!When!the!hydrolysis!of!PLA!is!via!surface!erosion!the!main!peak!of!the! MWD!remains!at!the!initial!position!with!a!reduced!peak!area.!However,!when!PLA!was! exposed! to! hydrolysis! at! all! temperatures,! the! MWD/ shifted! away! from! its! original! position!at!time!0!h!towards!lower!Mn.!Canevarolo![39]!and!Tsuji!and!Ikada![40]!found! the!same!behavior!indicating!that!degradation!of!PLA!was!preferentially!carried!out!by! chain!scission!in!the!bulk.!At!all!temperatures,!while!the!hydrolysis!was!progressing,!the! MWD!peaks!broaden!and!fragments!of!PLA!chains!were!formed!due!to!chain!scission.! Also,! the/ MWD! peaks! at! some! point! of! the! process! changed! from! a! monomodal! distribution!to!double,!triple!or!n8peaks!distributions!as!seen!in!the!MWD,!indicating!the! formation! of! PLA! oligomers.! Tsuji! and! Ikada! [40]! studied! the! hydrolysis! of! different! blends! of! crystalline! and! amorphous! PLA.! They! found! that! the! formation! of! another! ! 154! ! peak!is!mostly!due!to!the!crystalline!regions!of!PLA!in!the!blends.!Also,!Tsuji!et!al.![13]! in!a!comparative!study!on!the!hydrolytic!degradation!of!PLA!in!the!solid!and!in!the!melt! found!the!formation!of!an!additional!peak!during!hydrolysis!due!to!the!degradation!of!the! amorphous!regions!and!presence!of!crystalline!residues!in!the!polymer!matrix.!So,!the! formation!of!additional!peaks!in!the!MWD!during!PLA!degradation!in!50%!ethanol!could! be!due!to!the!crystalline!residues!from!the!hydrolysis!process.!! In!our!previous!studies!the!crystallinity!of!PLA!film!immersed!in!50%!ethanol!at! 40! °C! was! analyzed! during! hydrolysis! [15].! The! degree! of! crystallinity! of! the! film! increased!from!around!3%!to!25%!after!168!h!of!immersion,!and!then!between!360!h! and!2880!h!the!crystallinity!further!increased!to!50%.!At!the!point!that!the!MWD/started! to! show! more! than! one! peak! below! 60! kDa,! the! crystallinity! of! the! film! was! 40%! corresponding!to!the!region!in!which!the!crystallinity!increases!due!to!the!hydrolysis!of! the!amorphous!regions!of!the!PLA.!This!behavior!could!reflect!complex!kinetics!where! the!ester!bonds!of!PLA!oligomer!chains!may!have!different!susceptibility!to!cleavage.! That!means!selective!scission!occurs!where!the!degradation!is!not!homogeneous!and! the! accumulation! of! different! low! molecular! weight! fractions! is! more! evident! at! prolonged! hydrolysis! time.! ! In! terms! of! chemical! recycling! of! PLA,! the! crystalline! residues!could!prolong!the!reaction!period!required!for!obtaining!a!high!yield!of!LA![418 43].!! ! 155! ! ) w M g o L ( d w d / ) w M g o L ( d w d / ) w M g o L ( d w d / A B C 0 h 168 h 360 h 720 h 1440 h 2160 h 2880 h 0 h 48 h 120 h 192 h 288 h 552 h 720 h 0 h 3 h 6 h 12 h 24 h 48 h 144 h D 0 h 4 h 8 h 12 h 24 h 48 h 72 h 120 h ) w M g o L ( d w d / 2.5 3 3.5 4 4.5 LogMw ! Figure!4.2!MWD!of!PLA!films!during!hydrolytic!degradation!when!in!contact!with!50%! 5 5.5 6 6.5 ethanol!at!(A)!40,!(B)!60,!(C)!70!and!(D)!80!°C.! To!study!the!effect!of!temperature!on!the!hydrolytic!degradation!of!PLA!in!50%! ethanol,!a!deconvolution!study!of!the!MWD!was!performed!to!understand!the!reaction! kinetics.! Different! deconvolution! functions! have! been! used! such! as! Gaussian! and! Lorentzian!functions![44848].!However,!it!has!been!shown!that!these!functions!are!not! appropriate! since! the! curves! for! solid8state! reactions! are! asymmetrical,! whereas! LogNormal!can!properly!fit!asymmetric!functions![32].!So,!the!LogNormal!function!was! used!to!perform!the!deconvolution!of!the!peaks!followed!by!the!kinetics!analysis!of!the! separated!peaks!to!calculate!the!rate!constants.!The!deconvolution!of!the!peaks!was! ! 156! ! carried!out!when!skewness!of!the!distribution!was!observed!and!when!more!than!one! peak!was!detected!in!the!MWD.!!This!occurred!when!Mn!was!below!60!kDa!where!the! MWD!distribution!deviated!from!monomodal!at!all!temperatures.!Figure!4.3! shows!an! example!of!the!deconvolution!of!the!last!5!curves!of!the!hydrolytic!degradation!of!PLA!in! 50%!ethanol!at!60!°C!from!Figure!4.2B.!Different!MWD!can!be!observed!from!120!h!to! 720! h! of! hydrolysis.! The! longer! the! exposure! of! PLA! in! solution,! the! more! different! lengths!of!polymer!chains!are!formed.!This!means!that!while!the!PLA!is!exposed!to!50%! ethanol,!selective!chain!scission!occurs!during!the!hydrolysis!reactions.! ! 157! ! 22 kDa 77 kDa 10 kDa 5 kDa A 9 kDa 17 kDa C 5 kDa 3 kDa 6 kDa B D ) w M g o L ( d w d / ) w M g o L ( d w d / 2.5 3 3.5 4 LogMw 4.5 5 5.5 2.5 3 3.5 E 5 kDa 2 kDa 6 kDa ) w M g o L ( d w d / 4.5 5 5.5 4 LogMw 2.5 3 3.5 4 LogMw 4.5 5 5.5 ! Figure!4.3!Deconvolution!of!MWD/of!PLA!for!different!hydrolysis!times!in!50%!ethanol! at!60!°C!as!shown!in!Figure!4.2B:!(A)!120!h,!(B)!192!h,!(C)!288!h,!(D)!552!h!and!(E)! 720!h.! ! The!hydrolytic!degradation!of!PLA!leads!to!random!cleavage!of!the!ester!bonds! where!longer!chains!of!the!polymer!are!more!susceptible!to!hydrolysis!than!the!shorter! chains![6].!The!hydrolysis!products!of!PLA!contain!fragments!of!water8soluble!products! like!oligomers!and!LA!fragments.!To!understand!the!hydrolysis!of!PLA,!the!proposed!Mn! from!the!deconvoluted!portions!of!the!original!MWD!were!plotted!as!a!function!of!time! ! 158! ! (Figure!4.4).!The!Mn!reduced!to!~50!kDa!when!PLA!was!immersed!for!720,!48,!24!and! 12!h!at!40,!60,!70,!80!°C,!respectively,!which!is!marked!with!a!crossed!dotted!line!(−!−)! in!Figure!4.4.!Before!that!time,!the!MWDs/were!monomodal.!After!this!point,!increased! skewness! of! the! distributions! and! more! than! one! peak! were! observed! where! the! individual!Mn!obtained!from!the!deconvolution!process!were!plotted.!! A B 1000 2000 3000 0 200 400 600 800 C D 15 x 104 10 5 0 0 15 x 104 10 5 0 a D , n M a D , n M 0 50 100 150 0 time, h 50 time, h 100 ! Figure!4.4!Mn!as!function!of!time!during!hydrolytic!degradation!of!PLA!film!immersed!in! 50%!ethanol!at!(A)!40,!(B)!60,!(C)!70!and!(D)!80!°C.!(!,",#,!!)!Filled!markers!indicate! the! main! initial! path! of! hydrolysis! reaction.! Unfilled! markers! indicate! more! likely! side! reactions.! (*)! Less! prevalent! side! reactions.! (−−−)! Lines! indicate! fitting! of! Eq.! 4.2! for! more!likely!reactions!during!PLA!hydrolysis!as!determined!by!deconvolution.! ! 159! ! To! further! understand! PLA! degradation,! potentially! different! paths! have! been! investigated.! For! instance,! recombination! reactions,! which! may! result! in! crosslinking! were!considered![49,!50].!Recombination!reactions!of!PLA!have!been!seen!when!the! polymer!has!been!exposed!to!thermal!degradation!or!degradation!by!composting![49,! 51,!52].!They!can!also!happen!when!cyclic!oligomers!recombine!with!linear!molecules! by!ring!opening!reactions!favoring!the!formation!of!longer!chains![51,!52].!In!this!study,! crosslinking!was!discarded!as!a!side!reaction!after!NMR!experiments!were!carried!out.! In!particular,!gHMBC!and!13C!NMR!were!run!on!PLA!exposed!to!50%!ethanol!at!80°C!to! probe! for! any! new! carbon! resonances! that! may! have! formed! that! correspond! to! crosslinking!(e.g.,!carbon!resonances!around!100!ppm!due!to!orthoesters).!Aside!from! peaks!attributable!to!PLA,!oligomers!and!lactide,!there!was!no!evidence!of!crosslinking! (Figure!4B.1!and!Figure!4B.2,!Appendix!4B).!Figure!4.4B,!C,!D!showed!a!reduction!in! Mn!until!50!kDa!with!a!subsequent!increase!in!Mn!at!60,!70!and!80!°C,!which!could!be! attributed! to! recombination! reactions.! However,! for! the! purposes! of! this! study,! these! paths! were! not! considered! for! determining! the! main! order! of! reaction! since! these! reactions! are! less! prevalent! side! reactions! and! the! fractional! areas! corresponding! to! their!respective!MWD!are!small.!Other!paths!were!also!not!considered!when!the!Mn!of! PLA!showed!an!abrupt!drop!during!hydrolysis!and!then!Mn!stayed!the!same!until!the! end!of!the!experiment.!! After! the! process! of! separating! the! individual! data! of! the! Mn! obtained! by! peak! deconvolution,!the!kinetic!analysis!of!selected!Mn!as!the!most!prevalent!reaction!was! carried!out!for!each!temperature.!Table!4.1!shows!the!order!of!reaction!of!the!selected! Mn! during! hydrolysis! of! PLA! immersed! in! 50%! ethanol! at! 40,! 60,! 70! and! 80! °C.! The! ! 160! ! order! of! reaction! for! each! temperature! was! close! to! a! first! order! kinetic! reaction.! Therefore,! the! rate! constants! were! calculated! using! Eq.! 4.2.! It! is! well! known! that! hydrolytic! degradation! depends! on! the! temperature,! so! the! rate! of! hydrolysis! of! PLA! increases! with! temperature.! When! the! temperature! increases,! the! time! in! which! PLA! was!degraded!to!~5!kDa!decreased!from!4!months!at!40!°C!to!about!5!days!at!80!°C.! According!to!the!SSC,!which!can!be!used!to!approximate!the!optimal!time!to!accurately! estimate!the!hydrolysis!reaction!rate,!the!hydrolytic!degradation!experiments!should!be! run! for! 1000,! 100,! 33! and! 16! h! at! 40,! 60,! 70! and! 80! °C,! respectively! (Figure! 4C.1,! Appendix!4C).!Therefore,!the!data!for!the!decrease!of!Mn!before!~50!kDa,!where!the! MWD/ does! not! exhibit! skewness! or! multiple! peaks,! which! is! marked! with! a! crossed! dotted!line!(−!−)!in!Figure!4.4,!should!be!sufficient!to!estimate!the!kinetic!parameters! for!a!first!order!reaction.!At!60!°C,!more!initial!experimental!points!should!be!taken!to! further!improve!the!estimates!since!only!two!points!were!taken!before!100!h.! ! 161! ! Table&4.1&Order!of!reaction!(η)!and!rate!constants!for!PLA!films!at!different!temperatures!in!50%!ethanol!solution!and! water.! Temperature&& (°C)& 40! 60! 70! 80! 90! η"*& k"(h51)&**& 50%&Ethanol& 1.016!±!0.050! 1.056!±!0.073! 1.039!±!0.078! 1.032!±!0.042! Water& nd! 50%&Ethanol& 0.0011!±!0.00004!a! Water& nd! 1.054±!0.0977! 0.0092!±!0.0006!b,!A!! 0.0043!±!0.0002!a,!B! 1.007!±!0.0612! 0.0331!±!0.0019!c,!A! 0.0181!±!0.0007!b,!B! 1.043!±!0.0732! 0.0553!±!0.0029!d,!A! 0.0506!±!0.0021!c,!A! nd! 1.048!±!0.0752! nd! 0.0916!±!0.0054!d! *!Fitting!of!general!rate!law!Eq.&4.1! **!Fitting!of!first!order!reaction!Eq.&4.2! Values!with!the!same!lowercase!letter!within!a!column!and!capital!letter!within!a!row!are!not!significantly!different!(α=0.05)!! Note:!Not!determined!(nd)!since!water!at!40!°C!is!below!the#Tg!of!PLA,!and!ethanol!at!90!°C!is!close!to!the!boiling!point!of! the!solution! ! 162! ! 4.3.3$ Effect$of$temperature$on$the$hydrolytic$degradation$of$PLA$in$50%$ethanol:$ Activation$energy.$ To! evaluate! the! effect! of! temperature! on! the! hydrolytic! degradation! of! PLA! for! chemical!recycling,!the!activation!energy!as!stated!by!the!Arrhenius!equation!should!be! estimated.!Commonly,!the!effect!of!temperature!on!the!hydrolysis!of!PLA!is!estimated! by!linearizing!the!Arrhenius!equation!Eq.$4.3!by!plotting!the!natural!logarithm!ln(k)!from! the! equation! as! a! function! of! the! reciprocal! temperature! to! avoid! fitting! a! nonDlinear! equation! [12,! 16D20].! However,! when! the! Arrhenius! equation! is! used! for! parameter! estimation,! high! correlation! is! found! when! Ea! and! ko! are! simultaneously! estimated,! resulting!in!high!relative!error.!Table$4.2!shows!the!correlation!matrix!for!the!hydrolytic! degradation!of!PLA!in!50%!ethanol!using!the!Arrhenius!equation!as!expressed!in!Eq.$ 4.3.!A!large!correlation!of!the!Ea!and!ko!for!the!hydrolysis!reaction!in!50%!ethanol!was! found,!with!relative!error!for!Ea!and!ko!of!~15!%!and!328%,!respectively.!!Therefore,!a! reparameterization! of! the! Arrhenius! equation! as! shown! in! Eq.$ 4.4! has! been! recommended!by!several!authors![21D28]!to!reduce!the!correlation!between!Ea!and%ko.!! Table$4.2!Correlation!matrix!of!the!estimated!parameters!using!Arrhenius!equation!(Eq.$ 4.3)!for!the!hydrolytic!degradation!of!PLA!in!50%!ethanol.$ Parameter$ ko! Ea! ! ko$ 1! 0.9998! Ea$ 0.9998! 1! Relative$error,$%$ 328.45! 14.94! During!hydrolytic!degradation!of!PLA,!the!number!of!carboxylic!acid!chain!ends! increases,!making!the!hydrolysis!a!selfDcatalyzed!reaction!due!to!the!accumulation!of! ! 163! ! the! acidic! polymer! fragments! in! the! specimens.! So,! the! pH! decreases! over! time,! for! example! from! pH=8.7! to! pH=4.6! at! 40! °C! after! 2800! h.! Besides! temperature,! the! hydrolysis!mechanism!of!PLA!may!depend!on!the!pH!of!the!media!due!to!the!different! susceptibility!of!the!ester!groups!in!lactic!acid!oligomers![14].!So,!it!is!important!first!to! study! the! influence! of! the! pH! on! the! hydrolysis! rate! constant! of! PLA.! A! parameter! estimation! approach! was! used! to! estimate! the! parameters! of!Eq.$ 4.5! and!Eq.$ 4.6.! A! detailed! description! of! the! methodology! and! the! calculation! steps! are! provided! in! the! supporting!information!available!online!and!elsewhere![53].! When!PLA!was!immersed!in!50%!ethanol!at!80!°C$ and!different!pH,!the!MWD% shifted!towards!lower!molecular!weight!as!hydrolysis!proceeded!(Figure$4F.1,!Appendix! 4F).!As!in!the!hydrolysis!of!PLA!in!50%!ethanol!without!controlling!the!pH!of!the!solution,! the!MWD%at!the!latest!stages!of!the!degradation!did!not!show!a!monomodal!distribution.! Therefore,! for! the! analysis! of! the! kinetics! the! same! deconvolution! procedure! of! the! MWD! was! performed! as! in! Figure$ 4.3,! and! then! the! analysis! of! the! Mn!selecting! the! more!likely!side!reactions!to!happen!as!in!Figure$4.4.!Figure$4.5!shows!the!change!in! Mn%of!PLA!during!hydrolytic!degradation!at!constant!pH!after!the!deconvolution!of!the! MWD.!From!Table$4.3!we!can!observe!that!the!order!of!reaction!was!close!to!one!for! the!hydrolysis!of!PLA!at!pH!4,!7!and!11.!Therefore,!Eq.$4.2!was!applied!to!estimate!the! rate!of!hydrolysis.!As!was!expected,!when!the!pH!of!the!media!was!basic!(pH!11)!the! hydrolysis! was! faster! than! at! acid! (pH! 4)! and! neutral! (pH! 7)! conditions! (Table$ 4.3).! When!PLA!is!exposed!to!basic!conditions,!the!carbonyl!carbon!atoms!of!the!polymer! are! susceptible! to! attack! by! the! hydroxide! ions! and! hydrolytic! degradation! and! molecular!weight!reduction!are!more!significant!than!in!acid!solutions![14,!54,!55].! ! 164! ! pH 11 pH 7 pH 4 14x 104 12 10 8 6 4 2 a D , n M 0 0 20 40 time, h 60 80 100 ! Figure$4.5!Mn!as!a!function!of!time!during!hydrolytic!degradation!of!PLA!film!immersed! in!50%!ethanol!at!80!°C!and!different!pH.!(!,",#)!Filled!markers!indicate!the!main!initial! path!of!hydrolysis!reaction.!Unfilled!markers!indicate!more!likely!side!reactions.!(*)!Less! prevalent!side!reactions.!(−−−)!Lines!indicate!fitting!of!Eq.$4.2!for!more!likely!reactions! during!PLA!hydrolysis.! ! ! ! 165! ! Table$4.3!Order!of!reaction!(η)!and!rate!constants!for!PLA!films!at!80!°C!and!different! pH!temperatures!in!50%!ethanol!solution.! pH' 4! 7$ 11$ η$*' 1.002!±!0.048! 1.008!±!0.044! 1.001!±!0.039! *!Fitting!of!the!general!rate!law!Eq.$4.1! **!Fitting!of!first!order!reaction!Eq.$4.2! k$(hK1)$**$ 0.0623!±!0.0032!a$ 0.0530!±!0.0020!a$ 0.2563!±!0.0139!b! Values!with!different!lower!case!letters!are!statistically!different!(α=0.05!TukeyDKramer! Test).! After!the!analysis!of!the!hydrolytic!degradation!of!PLA!in!50%!ethanol!at!different! pH! at! 80! °C,! where! the! rate! increased! exponentially! with! pH,! thereby! supporting! the! proposed! equation,! the! estimation! of! β! using! Eq.$ 4.7! was! carried! out! to! be! able! to! estimate%Ea!from!Eq.$4.6.!For!the!estimation!of%β,!it!was!necessary!to!find!the!optimum! pHref!to!have!low!correlation!between!parameters.!Since!there!is!no!information!on!the! pHref,! the! pH! average! was! initially! used! (pHref! =! 7.33).! A! final! pHref=! 7.697! was! determined!as!explained!in!the!supporting!information.!The!optimum!pHref!was!used!for! the! final! estimation! of! the! parameters! presented! in! Table$ 4.4! where! β! was! equal! to! 0.2153.!The!values!of!β!=!0.2153!and!pHref%=!7.697!were!used!for!the!estimation!of!Ea! as!a!function!of!pH!and!temperature!(Eq.$4.6).! ! ! ! ! 166! ! Table$4.4.!Final!estimated!parameters!values!at!the!optimum!pHref!=!7.697!(Eq.$4.7).$ Estimates$ RMSE,$x104$ 0.1004!±!0.0052! 1.1737! 117,920!±!2917! 0.2153!±!0.0148! ! ! Parameter$ kref,!hD1! Mno,!Da! β$ ! ! To!estimate!the!Ea!as!a!function!of!pH%using!Eq.$4.6,!it!was!necessary!to!find!the! optimum! Tref! to! have! low! relative! errors! in! the! final! estimation.! The! methodology! to! estimate!the!Tref!is!shown!in!the!supporting!information!giving!a!Tref!of!56.538!°C!with!a! correlation! coefficient! of! 4.1080! x10D5!between! Ea! and! kref%and! a! relative! error! of! 1.63! and!3.31%,!respectively.!Table$4.5$shows!the!final!estimated!parameters!for!Eq.$4.6!for! the!hydrolysis!of!PLA!in!50%!ethanol.! ! 167! ! Table&4.5!Final!estimated!parameter!values!at!the!optimum!Tref.& Solution& Tref,&°C& Parameter& kref,&h>1! Mno,&Da& Ea,&x104,&J/mol&& RMSE,&x103& 50%!Ethanol,!pH! correction!*! 56.538& 0.0093!±!0.00031& 114,480!±!1879& 9.589!±!0.1559!a& 8.1062& 50%!Ethanol!**! 57.688& 0.0076!±!0.00027& 110,840!±!1625& 9.341!±!0.1669!a& 7.2478& Water!**! 75.931& 0.0276!±!0.00078& 115,290!±!1744& 10.143!±!0.2228!b& 6.4603& *!Estimated!by!using!Eq.&4.6! **!Estimated!by!using!Eq.&4.5! Values!with!different!lower!case!letters!within!a!row!are!different!(!=0.05)! Note:!Tref!values!are!expressed!with!three!decimals!for!parameter!estimation!purposes!to!get!near!zero!correlation! between!kref!and(Ea.! ! 168! ! ! As!previously!mentioned,!the!hydrolytic!degradation!of!PLA!depends!on!the!pH! of! the! media.! However,! it! is! important! to! study! whether! the! pH! correction! previously! applied!would!affect!the!Ea!of!hydrolysis.!So,!Eq.$4.5!was!applied!to!estimate!Ea!without! considering! the! change! of! pH! during! hydrolysis.! For! the! nonAlinear! regression! estimation,!the!optimum!Tref!57.688!°C!with!the!lowest!correlation!was!used!for!the!final! estimation! of! the! parameters! as! presented! in! the! supporting! information! giving! a! correlation! coefficient! of! 1.2976! x10A5!between! Ea! and! kref*and! a! relative! error! of! 1.79! and!3.52%,!respectively.!The!activation!energy!of!the!hydrolytic!degradation!of!PLA!in! 50%! ethanol! without! considering! the! change! of! pH! during! hydrolysis! was! 9.341!x104! J/mol! (Table$ 4.5).! After! the! reparameterization! of! the! Arrhenius! equation! the! Ea! was! estimated!as!9.589!x104!J/mol!when!dependence!on!pH!was!included.!The!difference! between!these!values!of!Ea!was!not!statistically!significant!(p>0.05).!Therefore,!we!were! not!able!to!detect!dependence!on!pH*of!the*Ea!for!the!hydrolytic!degradation!rate!of!PLA.!! 4.3.4$ Hydrolytic$degradation$of$PLA$in$water$ For! a! better! understanding! of! how! 50%! ethanol! solution! contributes! to! the! hydrolytic!degradation!of!PLA!for!chemical!recycling!purposes,!the!effect!of!temperature! on! the! hydrolysis! of! PLA! was! also! studied! in! water! without! ethanol.! PLA! films! were! immersed!in!water!at!different!temperatures!above!the!Tg:!60,!70,!80!and!90!°C.!Figure$ 4.6!shows!the!MWD*of!PLA!during!hydrolysis!at!these!different!temperatures.!As!in!the! hydrolysis!of!PLA!in!contact!with!50%!ethanol!(Figure$4.2),!the!MWD!shifted!to!lower! molecular!weight!as!the!hydrolysis!reactions!took!place.!At!all!degradation!temperatures,! the!broadness!of!the!MWD*increased!and!at!the!last!stages!several!peaks!appeared!in! the! distributions.! Even! that! the! MWD* shifted! to! lower! molecular! weight! as! hydrolysis! ! 169! ! proceeded!for!PLA!in!water!and!in!50%!ethanol,!the!evolution!of!the!peaks!was!different.! For!example,!during!hydrolysis!at!70!°C!(Figure$4.6B),!a!peak!was!observed!after!192!h! of!immersion!in!water!(Mn!≈!4!kDa).!Then,!the!peak!shifted!to!lower!molecular!weight! (Mn! ≈! 3.5! kDa)* and! the! height! of! the! peak! increased! after! 264! h! being! the! more! predominant! peak.! However,! for! 50%! ethanol! (Figure$ 4.2)! this! peak! was! not! as! predominant!as!in!water,!which!may!be!attributed!to!the!difference!on!the!chain!scission! selectivity! during! PLA! hydrolysis! process! due! to! the! swelling! effect! of! the! ethanol! molecules.! Future! studies! should! be! conducted! to! understand! the! difference! in! chain! scission!selectivity!during!hydrolysis!reactions!of!PLA!in!water!and!50%!ethanol.!The! analysis!of!the!effect!of!temperature!on!the!hydrolysis!of!PLA!in!water!was!performed! following! the! same! procedure! of! deconvolution! as! in! the! hydrolysis! of! PLA! in! 50%! ethanol!to!identify!the!side!reactions!and!estimate!the!constant!rates!of!the!hydrolysis!of! PLA!in!water.!! ! 170! ! ) w M g o L ( d w d / ) w M g o L ( d w d / ) w M g o L ( d w d / ) w M g o L ( d w d / A B C D 0 h 72 h 168 h 360 h 504 h 720 h 1128 h 1440 h 0 h 48 h 120 h 192 h 264 h 0 h 12 h 24 h 48 h 72 h 120 h 0 h 6 h 12 h 18 h 24 h 48 h 72 h 2.5 3 3.5 4 4.5 LogMw 5 5.5 6 6.5 ! Figure$4.6!MWD!of!PLA!films!during!hydrolytic!degradation!when!in!contact!with!water! at!(A)!60,!(B)!70,!(C)!80!and!(D)!90!°C! ! ! Figure$4.7!shows!the!change!in!Mn!of!PLA!over!time!when!it!was!immersed!in! water!at!60,!70,!80!and!90!°C.!After!the!deconvolution!of!the!MWD,!the!more!likely!side! reactions!were!selected!following!the!same!process!as!in!50%!ethanol.!The!order!of! reaction!of!the!selected!Mn*for!each!temperature!was!close!to!1!(Table$4.1).!Therefore,! the!rate!constants!were!estimated!using!the!first!order!reaction!Eq.$ 4.2!as!shown!in! Table$ 4.1.! As! expected,! the! higher! the! temperature! the! faster! the! hydrolysis.! In! this! study,!the!hydrolytic!degradation!of!PLA!was!faster!when!it!was!in!contact!with!50%! ! 171! ! ethanol!than!in!water!at!60!and!70!°C!(p<0.05)^!however,!no!statistical!difference!was! detected! on! the! rate! of! hydrolysis! between! 50%! ethanol! and! water! when! PLA! was! exposed!at!80!°C!(p>0.05).!This!can!be!rationalized!by!the!increased!movement!of!the! PLA!chains!at!80!°C!resulting!in!the!same!rate!of!penetration!of!water!molecules!as!the! swelling! of! the! matrix! that! occurs! when! PLA! is! exposed! to! 50%! ethanol! solution.! Figure$ 4.8! shows! the! rate! constants! of! hydrolysis! in! 50%! ethanol! and! water! at! the! various!temperatures!of!the!experiments!performed.!The!trend!lines!indicate!that!the! higher!the!temperatures!of!the!hydrolytic!degradation!in!50%!ethanol!and!in!water!the! faster!the!hydrolysis,!increasing!exponentially.!To!understand!the!effect!of!temperature! on!the!hydrolytic!degradation!of!PLA!in!water,!the!estimation!of!the!Ea!was!also!carried! out.! ! 172! ! a D , n M 15 x 104 10 5 0 0 15 x 104 A B 500 1000 1500 0 50 100 150 200 250 300 C D 10 a D , n M 5 0 0 50 time, h 100 150 0 20 40 time, h 60 80 ! Figure$4.7!Mn!as!function!of!time!during!hydrolytic!degradation!of!PLA!film!immersed!in! water!at!(A)!60,!(B)!70,!(C)!80!and!(D)!90!°C.!(!,",#,! !)!Filled!markers!indicate!the! main! initial! path! of! hydrolysis! reaction.! Unfilled! markers! indicate! more! likely! side! reactions.! (*)! Less! prevalent! side! reactions.! (−−−)! Lines! indicate! fitting! of! Eq.$ 4.2! for! more!likely!reactions!during!PLA!hydrolysis.! ! ! ! ! 173! ! 0.12 0.1 0.08 0.06 1 − h , k 0.04 0.02 0 50% Ethanol Water 45 55 65 Temperature, oC 75 85 95 ! Figure$ 4.8! Rate! constants! (k)! of! the! hydrolytic! degradation! of! PLA! in! 50%! ethanol! solution!and!water!vs.!temperature.!Trend!lines!are!used!as!a!visual!aid.! ! 4.3.5$ Effect$ of$ temperature$ on$ the$ hydrolytic$ degradation$ of$ PLA$ in$ water:$ Activation$energy$ ! To! study! the! effect! of! the! temperature! on! the! hydrolytic! degradation! of! PLA! in! contact!with!water,!the!activation!energy!was!estimated!using!the!reparameterization!of! the!Arrhenius!equation!Eq.$4.5!without!considering!the!change!of!pH!during!hydrolysis! since!the*pH!did!not!have!an!effect!on!the!Ea!of!PLA!when!it!was!hydrolyzed!by!a!50%! ethanol!solution.!So,!it!was!assumed!that!it!would!not!have!an!effect!on!the!Ea!in!water.! The! procedure! for! the! estimation! was! the! same! as! in! 50%! ethanol! as! shown! in! the! ! 174! ! supporting! information.! For! the! final! estimation! of! the! parameters,! the! optimal! Tref! of! 75.931!°C!was!used!since!it!gave!a!correlation!coefficient!of!1.5516!x10A5!between!Ea! and!kref*and!a!relative!error!of!2.48!and!2.20!%,!respectively,!which!are!acceptable!and! very!low.!The!final!estimation!of!the!Ea!of!the!hydrolytic!degradation!of!PLA!in!water!was! 10.143!x104!J/mol!(Table$4.5).! Even!though!the!values!of!the!Ea!for!the!hydrolytic!degradation!of!PLA!in!water! and!in!50%!ethanol!were!different,!these!values!are!within!the!range!of!values!(from!4!to! 10! x104! J/mol)! reported! in! the! literature! when! PLA! was! exposed! to! different! environments! [12,! 16A20,! 55A57].! These! environments! include! 100%! relative! humidity! (40!–!80!°C)![16],!steam!high!pressure!(100!–!130!°C)![18],!water!high!pressure!(180!–! 350!°C^!140!–!180!°C)![12,!57],!pH!7!buffer!solution!(37!–!70!°C^!50!–!75!°C)![17,!56],! basic!media!of!pH!10!(50!–!70!°C)!and!acidic!media!of!pH!2!(40!–!120!°C)![19,!20].! Some!of!the!studies!were!performed!using!a!range!of!temperatures!below!and!above! the! Tg! that! could! affect! the! final! estimations.! However,! the! Tg! was! not! reported! in! solution,!but!of!the!initial!PLA!samples.!It!is!also!important!to!mention!that!the!studies! have!also!reported!Ea!values!from!the!linearized!Arrhenius!equation.! One! of! the! main! objectives! of! the! present! work! was! to! provide! an! alternative! method! for! chemical! recycling! of! PLA.! For! that,! it! is! important! to! consider! the! LAA oligomers! yield! during! the! hydrolysis! reactions.! Ea! values! have! been! reported! within! the!range!of!this!study!in!aqueous!phase!but!at!higher!temperatures!(120!–!250!°C),! obtaining!high!yields!(~!95%)!of!LA!when!hydrolysis!was!close!to!the!Tm*[13,!57].!When! PLA!is!hydrolyzed!in!the!solid!state,!crystalline!residues,!as!discussed!for!Figure$4.2,! could!prolong!the!reaction!period!required!to!obtain!high!yields!of!LA.!However,!Tsuji!et! ! 175! ! al.![13]!reported!that!when!PLA!containing!the!crystalline!residues!is!hydrolyzed!in!the! solid!state!in!water,!the!LA!yield!was!comparable!with!the!hydrolysis!in!the!molten!state! without!any!crystalline!residues!where!the!time!to!get!95%!LA!yield!at!120!°C!was!72!h! and!at!180!°C!was!2!h.!In!this!study!according!to!the!rate!constant!estimated!in!Table$ 4.1,!the!time!needed!for!95%!yield!of!LA!at!80!°C!in!50%!ethanol!should!be!128!h.!If! the!temperature!of!hydrolysis!increased!to!91!°C,!below!the!boiling!point!of!the!50%! ethanol!solution,!the!time!would!be!reduced.!Using!the!values!estimated!in!Table$4.1! the!rate!of!hydrolysis!at!91!°C!and!the!time!needed!to!get!the!95%!LA!yield!can!be! estimated!using!Eq.$4.5!giving!0.1698!hA1!and!41!h,!respectively.!The!time!of!41!h!is! comparable!with!the!values!obtained!in!water!at!120!°C!by!Tsuji!et!al.![13].!However,!it! is!important!to!further!study!the!yield!of!LAAoligomers!that!can!be!used!to!start!the!reA polymerization! of! PLA! from! chemical! recycling! since! it! is! not! necessary! to! go! to! monomers!if!the!goal!is!to!reApolymerize!PLA.!In!this!case,!the!time!needed!to!get!95%! yield!of!LAAoligomers!of!10!units!in!50%!ethanol!at!91!°C!would!be!around!29!h.!So,! this! work! presents! a! feasible! method! to! achieve! low! Mn* PLA! with! green! solvents! at! moderate!temperatures!using!low!energy!and!in!a!reasonable!amount!of!time.! Future!studies!should!be!conducted!to!estimate!the!yield!of!LAAoligomers!when! PLA! is! hydrolyzed! in! 50%! ethanol! to! reApolymerize! PLA.! It! is! important! to! study! the! crystallization! behavior! of! PLA! at! high! temperatures! when! PLA! is! immersed! in! 50%! ethanol!and!how!it!will!affect!the!yield!of!LAAoligomers!for!the!chemical!recycling!of!PLA! and! establish! the! separation! process! and! purification! of! LA! from! the! waterAethanol! solution.! It! is! expected! that! small! increases! in! temperature! can! be! translated! into! greater!gain!of!LAAoligomers!for!recycling!PLA.!! ! 176! ! 4.4$ Conclusions$ The! hydrolytic! degradation! of! PLA! by! waterAethanol! solutions! for! chemical! recycling!was!studied.!The!hydrolysis!of!PLA!at!moderate!temperatures!above!the!Tg! was!carried!out!in!50%!ethanol!solution!and!in!water!to!study!the!effect!of!temperature.* The!analysis!of!the!MWD!indicated!that!the!hydrolysis!in!50%!ethanol!and!water!was! preferentially!carried!out!by!bulk!erosion!showing!a!multiAmodal!distribution!at!the!latter! stages! of! the! hydrolysis.! The! deconvolution! of! MWD! indicated! that! multiple! reaction! pathways! followed! first! order! kinetics.! The! Ea! for! the! hydrolysis! of! PLA! in! water! was! around! 9%! higher! than! in! 50%! ethanol! at! moderate! temperatures:! 10.143! x104! and! 9.341! x104! J/mol,! respectively.! These! values! were! estimated! after! the! reparameterization! of! the! Arrhenius! equation! to! obtain! near! zero! correlation! between! parameters!and!therefore!producing!a!better!estimation.!Also,!the!Ea!for!the!hydrolytic! degradation!of!PLA!in!50%!ethanol!was!estimated!taking!in!consideration!the!change!of! pH,!however^!no!differences!were!found!in!the!estimations.!The!Ea!values!obtained!in! this! work! were! within! the! same! range! of! values! reported! in! literature.! This! study! presents! an! alternative! method! of! using! 50%! ethanol! to! hydrolyze! PLA! at! moderate! temperatures!for!PLA!chemical!recycling.!! 4.5$ Acknowledgements$ IAF,! F.,! thanks! CONACYT! (The! Mexican! National! Council! for! Science! and! Technology),!SEP!(The!Mexican!Secretariat!of!Public!Education),!and!the!Government! of! Mexico! for! providing! support! funds! for! a! Ph.D.! fellowship.! RA! thanks! the! partial! support!of!the!USDA!A!and!MI!AgBioResearch,!Hatch!R.!Auras.! ! ! 177! ! $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ ! $ APPENDICES$ $ 178! ! APPENDIX$ 4A:$ Rate$ constants$ and$ order$ of$ reaction$ for$ PLA$ films$ at$ 70$ °C$ in$ waterTalcohol$solutions$$$ Table$ 4A.1! Rate! constants! and! order! of! reaction! η! for! PLA! films! at! 70! °C! in! waterAalcohol!solutions.! Solvent$ η$*$ k$*$ k$(hT1)$**# 50%!Ethanol$ 1.0246!±!0.0054! 0.0254!±!0.0015!$ 0.0352!±!0.0027!a! 50%!Methanol$ 1.0111!±!0.0035! 0.0264!±!0.0010!$ 0.0294!±!0.0014!b! 50%!1APropanol$ 1.3246!±!0.0043! 0.0008!±!0.00004!$ 0.0384!±!0.0024!a! 20%!1AButanol$ 1.1323!±!0.0034! 0.0083!±!0.0003!$ 0.0367!±!0.0017!a! *!Fitting!of!general!rate!law!Eq.$4.1!! **!Fitting!of!first!order!reaction!Eq.$4.2! Values!with!different!lower!case!letters!within!a!column!are!significantly!different! (α=0.05)! ! ! $ 179! ! APPENDIX$4B:$NMR$Analysis$ In! order! to! investigate! the! possibility! and! extent! of! crosslinking! of! PLA! during! hydrolysis,! 13C! and! gHMBC! (gradient! heteronuclear! multiAbond! correlation)! NMR! experiments!were!carried!out!on!PLA!exposed!to!50%!ethanol!at!80!°C.!HMBC!crossA peaks! correlate! protons! to! carbons! that! are! 2A3! bonds! removed.! This! allows! determination!of!whether!there!are!any!new/unique!13C!resonances!associated!with!the! main! PLA! proton! resonances! (5.16! and! 1.58! ppm! for! the! CH! and! CH3,! respectively).! The! most! probable! structures! resulting! from! crosslinking! are! orthoesters! in! which! a! carboxylic! ester! in! PLA! reacts! with! ethanol! or! water! and! the! subsequent! hemiacetal! further!reacts!with!another!ester!to!form!the!orthoester.!The!resulting!carbon!with!three! alkoxy!groups!would!have!a!unique!carbon!chemical!shift!(~100ppm),!which!would!be! easily!detected!in!the!HMBC!experiment.!Figure$ 4B.1!shows!the!HMBC!spectrum!of! the!hydrolyzed!PLA!sample.!No!crossApeaks!attributable!to!crosslinking!were!observed.! There! are! resonances! consistent! with! PLA! and! its! oligomers! as! well! as! lactide.! Inspection!of!the!13C!NMR!confirms!these!results!(Figure$4B.2)! ! 180! ! Figure$4B.1!1H,13CAgHMBC!of!PLA!exposed!to!50%!ethanol!at!80!°C!run!on!a!Bruker! ! Avance!900.! ! !! ! 181! ! Figure$ 4B.2! 13C! NMR! of! PLA! prior! to! hydrolysis! (top)! and! after! hydrolysis! in! 50%! ethanol! at! 80! °C! (bottom).! ! The! expansion! shows! the! carbonyl! region! and! indicates! additional!small!resonances!around!169.4!ppm!that!are!most!likely!attributable!to!PLA! ! oligomers.! ! ! ! ! ! ! ! 182! ! APPENDIX$ 4C:$ Scale$ Sensitivity$ coefficient$ of$ hydrolytic$ degradation$ of$ PLA$ in$ 50%$ethanol$ t n e i c i f f e o C y t i v i t i l s n e S e a c S 15 x 104 10 5 0 −5 0 15 x 104 t n e c i i f f e o C y t i v i t i s n e S e l a c S 10 5 0 −5 0 X’k X’Mno A B 500 1000 1500 time, h 2000 2500 3000 0 200 400 time, h 600 800 C D 50 time, h 100 150 0 50 time, h 100 150 ! Figure$ 4C.1! SSC! of! hydrolytic! degradation! of! PLA! in! 50%! ethanol! using! first! order! reaction!equation!Eq.$4.2!at!(A)!40,!(B)!60,!(C)!70!and!(D)!°80!C.!Arrows!indicate!the! optimal!time!to!accurately!estimate!k.! ! ! $ 183! ! APPENDIX$4D:$Parameter$estimation$of$the$Ea$for$the$hydrolysis$of$PLA$in$50%$ ethanol$$ For! the! estimation! of! the! activation! energy! for! the! hydrolysis! of! PLA! in! 50%! ethanol,!first!it!is!necessary!to!plot!SSC,!which!assists!in!the!parameter!identifiability!of! the! model! proposed.! The! SSC! plot! is! needed! to! determine! correlation! among! all! the! parameters!of!interest.!Since!there!is!no!available!information!on!the!Tref!and!pHref!of!the! hydrolytic! degradation! of! PLA! to! estimate! Ea! using!Eq.$ 4.5! or!Eq.$ 4.6,! some! authors! have! recommended! using! the! average! temperature! in! the! case! of! Tref* within! the! experimental!temperature!range![21,!25,!28,!58].!For!this!study,!the!initial! Tref!chosen! was!62.5!°C,!the!average!of!the!temperature!range!of!study!(40!to!80!°C)!and!for!pHref! was!the!average!of!4,!7,!11,!corresponding!to!7.33,!later!discussed.!! To! estimate! Ea,! taking! the* pH! into! consideration! while! hydrolysis! reactions! are! taking!place,!first!Eq.$ 4.6$ was!used.!Figure$ 4D.1A!shows!the!SSC!for!the!parameter! estimation!using!Eq.$4.6$with!β*as!a!parameter!to!be!estimated.!From!the!X´plot,!β*and! Ea!are!identified!as!parameters!that!are!correlated!since!their!shape!is!similar!with!their! maximum! value! at! ~! 550! h! meaning! that! the! parameters! need! to! be! separately! estimated.!Therefore,!it!was!proposed!to!fix!β*as!a!constant.!Figure$4D.1B!shows!the! SSC! of! Eq.$ 4.6$ with! β* fixed* as! a! constant.! From! the! X´! plot! Mno! is! identified! as! the! parameter!that!is!estimated!with!more!accuracy!with!the!lowest!relative!error!followed! by!the!Ea!and*kref,!meaning!that!the!parameter!with!the!largest!error!would!be*kref.! ! 184! ! t n e i c i f f e o C y t i v i t i s n e S d e a c S l 12x 104 A 10 8 6 4 2 0 −2 −4 0 B X’kref X’Mno X’Ea X’ β X’kref X’Mno X’Ea 1000 time, h 2000 3000 0 1000 time, h 2000 3000 ! Figure$ 4D.1! SSC! of! hydrolytic! degradation! of! PLA! in! 50%! ethanol! using! simulated! dynamic!temperature!(Eq.$4.9)!in!Eq.$4.6$at!Tref!=!62.5!°C!and!pHref*=!7.33.!(A)!β*as!a! parameter!to!estimate!with!initial!guesses:!kref!=!0.0099!hA1,*Mno*=!100,000!Da,*Ea!=!9.04! x104!J.molA1,*β!=!0.147.!(B)*β!as!a!constant!with!initial!guesses:!kref!=!0.01!hA1,*Mno!=! 100,000!Da,*Ea!=!9.04!x104!J.molA1.! To!estimate*β,!Eq.$4.7!was!proposed!where!experiments!at!one!temperature!and! different!pH!were!performed.!Figure$4D.2!shows!the!SSC!of!Eq.$4.7$where!Mno*is!the! parameter! identified! as! the! easiest! to! be! estimated! with! the! lowest! relative! error,! followed!by*kref!and!then!by*β.!To!be!able!to!estimate*β,!the!hydrolytic!degradation!of! PLA!in!50%!ethanol!was!performed!at!80!°C!controlling!the!pH!during!the!experiment!at! three!different!values:!4,!7,!and!11.! ! 185! ! 10x 104 t n e i c i f f e o C y t i v i t i s n e S d e l a c S 8 6 4 2 0 −2 −4 0 X’kref X’Mno X’ β 20 40 60 time, h 80 100 ! Figure$4D.2$SSC!of!hydrolytic!degradation!of!PLA!in!50%!ethanol!at!80!°C!and!different! pH!using!simulated!dynamic!pH!(Eq.$4.10)!in!Eq.$4.7$at!pHref*=!7.33.!Initial!guesses:!kref! =!0.093!hA1,*Mno!=!100,000!Da,*β!=!0.147.$ ! The!correlation!coefficient!among!the!estimated!parameters!at!average!pHref!can! be! found! in! Table$ 4D.1.! The! correlation! coefficient! between! kref! and! β* was! A0.1023.! However,!to!perform!a!better!estimation!with!a!near!zero!correlation!between!kref!and!β,* an!iterative!search!for!pHref!was!conducted!until!the!correlation!coefficient!between!kref! and! β* showed! near! zero! correlation.! Figure$ 4D.3$ shows! the! plot! of! the! correlation! coefficient!between!kref!and!β*using!different!pHref*values.!The!optimum!pHref*was!7.697! with!a!correlation!coefficient!for!kref!and!β*of!A5.4879x10A5!(Table$4D.2).!! ! 186! ! Table$4D.1$Correlation!matrix!of!the!estimated!parameters!at!the!average!pHref*=!7.33! Mno# 0.5878! 1.0000! A0.1787! β# Relative$error,$%$ A0.1023! A0.1787! 1.0000! 5.21! 2.47! 6.74! pHref = 7.697 (Eq.$4.7).! Parameter$ kref# 1.0000! 0.5878! A0.1023! kref! Mno! β! ! ! β d n a f e r k f o t n e i c i f f e o C n o i t a l e r r o C 0.8 0.6 0.4 0.2 0 −0.2 −0.4 −0.6 −0.8 4 5 6 8 7 pHref 9 10 11 ! Figure$4D.3!Plot!of!correlation!coefficient!of!kref!and!β!as!a!function!of!possible!pHref* using!Eq.$4.7.! ! ! 187! ! Table$4D.2!Correlation!matrix!of!the!estimated!parameters!at!the!optimum!pHref*=! 7.697!(Eq.$4.7).! Parameter$ kref# kref! Mno! β! *5.4879x10A5! ! 1.0000! 0.5726! Mno# 0.5726! 1.0000! A0.0000*! A0.1788! β* Relative$error,$%$ A0.0000*! A0.1788! 1.0000! 5.18! 2.47! 6.74! To!estimate!the!Ea!as!a!function!of!pH!using!Eq.$4.6!with!the!final!β*=*0.2153!and! pHref*=!7.697!previously!estimated,!it!is!necessary!to!find!the!optimum*Tref.$Since!there! is! also! no! information! regarding* Tref,! the! first! value! used! for! the! estimation! was! the! average! temperature! used! in! the! experiments,! 62.5! °C.! Table$ 4D.3! shows! the! correlation!matrix!of!the!estimated!values!at!the!average*Tref.!The!correlation!coefficient! between!Ea*and*kref*was!0.2920^!however,!near!zero!correlation!needs!to!be!achieved! for!a!better!estimation.!After!the!iterative!search!of!the!correlation!coefficient!between! kref* and! Ea* was! performed! (Figure$ 4D.4),! it! was! determined! that! the! optimal! Tref* =! 56.538! °C! with! a! correlation! between! kref*and! Ea!of! 4.1080x10A5!with! relative! errors! of! 3.31! and! 1.63%,! respectively! (Table$ 4D.4).! The! nonAlinear! estimation! was! performed! using!the!optimum!Tref!resulting!in!an!Ea!=!9.589!x104!J/mol.! ! ! ! 188! ! Table$4D.3!Correlation!matrix!of!the!estimated!parameters!at!the!average!Tref*=!62.5!°C! (Eq.$4.6).$ Parameter$ kref# kref! Mno! Ea! ! 1.0000! 0.5601! 0.2920! Mno# 0.5601! 1.0000! 0.1278! Ea# 0.2920! 0.1278! 1.0000! Relative$Error,$%$ 3.46! 1.64! 1.63! a f e r E d n a k f o t n e i c i f f e o C n o i t a l e r r o C 0.8 0.6 0.4 0.2 0 −0.2 −0.4 −0.6 −0.8 40 Tref = 56.538 oC 45 50 55 60 Tref , oC 65 70 75 80 ! Figure$4D.4!Plot!of!correlation!coefficient!of!kref!and*Ea!as!a!function!of!possible!Tref! using!Eq.$4.6.! ! ! 189! ! Table$ 4D.4! Correlation! matrix! of! the! estimated! parameters! at! the! optimum! Tref* =! 56.538!°C!(Eq.$4.6).! Parameter$ kref# 1.0000! 0.5466! 0.0000*! kref! Mno! Ea! *4.1080x10A5! ! Mno# 0.5466! 1.0000! 0.1278! Ea# 0.0000*! 0.1278! 1.0000! Relative$Error,$%$ 3.31! 1.64! 1.63! To! study! whether! the! pH! correction! would! affect! the! Ea! for! the! hydrolytic! degradation! of! PLA! in! 50%! ethanol,! Eq.$ 4.5! from! the! reparameterization! of! the! Arrhenius! equation! was! applied.! The! initial! Tref! applied! for! the! estimation! was! the! average!of!the!experiments!performed.!Figure$4D.5!shows!the!SSC!constructed!by!the! Tsim*approach!(Eq.$4.9)!consisting!of!the!parameters*kref,!Mno!and*Ea.!From!the!X´plot! Mno! is! identified! as! the! easiest! parameter! to! be! estimated! and! therefore! the! lowest! relative! error! followed! by! Ea* and* kref.! Table$ 4D.5! shows! the! correlation! coefficient! among!the!estimated!parameters!using!Tref!at!the!temperature!average!of!62.5!°C.!The! correlation!coefficient!between!Ea*and*kref*was!0.2401^!however,!near!zero!correlation!is! desired! for! a! better! estimation! of* Ea.! Therefore,! an! iterative! search! for* Tref* was* performed!to!find!a!near!zero!correlation!between!Ea*and*kref.!Figure$ 4D.6$ shows!the! iteration!of!different!Tref!with!the!respective!correlation!coefficient!of!Ea*and*kref.!When!Tref* =!57.688!°C!the!correlation!between!Ea*and*kref*was!1.2976x10A5!with!relative!errors!of! 3.52!and!1.79,!respectively!(Table$4D.6).! ! ! 190! ! $ t n e i c i f f e o C y t i v i t i s n e S d e a c S l 10x 104 8 6 4 2 0 −2 −4 0 X’kref X’Mno X’Ea 500 1000 1500 time, h 2000 2500 3000 ! Figure$ 4D.5! SSC! of! the! activation! energy! estimation! at! Tref! =! 62.5! °C! of! hydrolytic! degradation!of!PLA!in!50%!ethanol!using!simulated!dynamic!temperature!(Eq.$ 4.9)!in! Eq.$4.5.!Initial!guesses:!kref!=!0.007!hA1,*Mno!=!100,000!Da,*Ea!=!9.04!x104!J.molA1.! ! Table$4D.5!Correlation!matrix!of!the!estimated!parameters!at!the!average!Tref!=!62.5!°C! for!hydrolytic!degradation!of!PLA!in!50%!ethanol!solution!(Eq.$4.5).! Parameter$ kref# 1.0000! 0.5619! 0.2401! kref! Mno! Ea! ! ! Mno# 0.5619! 1.0000! 0.1049! Ea# 0.2401! 0.1049! 1.0000! Relative$Error,$%$$ 3.63! 1.47! 1.79! 191! ! a f e r E d n a k f o t n e i c i f f e o C n o i t a l e r r o C 0.8 0.6 0.4 0.2 0 −0.2 −0.4 −0.6 45 Tref = 57.688 oC 50 55 65 60 Tref , oC 70 75 80 ! Figure$4D.6!Plot!of!correlation!coefficient!of!kref!and*Ea!as!a!function!of!possible!Tref*for! hydrolytic!degradation!of!PLA!in!50%!ethanol!solution!(Eq.$4.5).!! ! Table$ 4D.6! Correlation! matrix! of! the! estimated! parameters! at! the! optimum! Tref* =! 57.688!°C!for!hydrolytic!degradation!of!PLA!in!50%!ethanol!solution!(Eq.$4.5).! Parameter$ kref# 1.0000! 0.5529! 0.0000*! kref! Mno! Ea! *1.2976x10A5! ! Mno# 0.5529! 1.0000! 0.1049! Ea# 0.0000*! 0.1049! 1.0000! Relative$Error,$%$ 3.52! 1.47! 1.79! 192! ! APPENDIX$4E:$Parameter$estimation$of$the$Ea$for$the$hydrolysis$of$PLA$in$water$$ ! For! initial! estimations! in! water,! the! Tref! selected! was! 75! °C,! the! average! temperature!of!hydrolysis!in!water!(60!–!90!°C,!giving!a!correlation!coefficient!between! Ea! and! kref! of! A0.0721! (Table$ 4E.1).! After! the! iteration! of! different* values! of! Tref,! the! optimum!value!resulted!in!Tref!=!75.931!°C!(Table$4E.1)!with!a!correlation!coefficient!of! 1.5516x10A5! between! Ea! and! kref* and! relative! errors! of! 2.39! and! 3.43%,! respectively! (Table$4E.2).!! ! Table$4E.1!Correlation!matrix!of!the!estimated!parameters!at!the!average!Tref*=! 75!°C!for!hydrolytic!degradation!of!PLA!in!50%!ethanol!solution!(Eq.$4.5).! Parameter$ kref# kref! Mno! Ea! 1.0000! 0.4877! A0.0721! Mno# 0.4877! 1.0000! A0.0261! Ea# A0.0721! A0.0261! 1.0000! Relative$Error,$%$ 2.84! 1.51! 2.20! ! $ ! 193! ! a f e r E d n a k f o t n e i c i f f e o C n o i t a l e r r o C 0.8 0.6 0.4 0.2 0 −0.2 −0.4 −0.6 −0.8 60 Tref = 75.931 oC 65 70 75 Tref , oC 80 85 90 ! Figure$4D.7!Plot!of!correlation!coefficient!of!kref!and*Ea!as!a!function!of!possible!Tref!for! hydrolytic!degradation!of!PLA!in!water!(Eq.$4.5).! $ Table$4E.2!Correlation!matrix!of!the!estimated!parameters!at!the!optimum!Tref*=! 75.931!°C!for!hydrolytic!degradation!of!PLA!in!water!(Eq.$4.5).! Parameter$ kref# kref! Mno! Ea! *1.5516x10A5! 1.0000! 0.4871! 0.0000*! $ ! Mno# 0.4871! 1.0000! A0.0261! Ea# 0.0000*! A0.0261! 1.0000! Relative$Error,$%$ 2.48! 1.51! 2.20! 194! ! APPENDIX$4F:$MWD$of$PLA$films$during$hydrolytic$degradation$when$in$contact$ with$50%$ethanol$at$80$°C$and$different$pH$ 0 h 4 h 6 h 9 h 12 h ) w M g o L ( d w d / 0 h 2 h 4 h 6 h 9 h 12 h 24 h 35 h 48 h 72 h ) w M g o L ( d w d / 0 h 2 h 4 h 6 h 9 h 12 h 24 h 35 h 48 h 72 h ) w M g o L ( d w d / A B 96 h C 2.5 3 3.5 4 4.5 LogMw 5 5.5 6 6.5 ! Figure$4F.1!MWD!of!PLA!films!during!hydrolytic!degradation!when!in!contact!with!50%! ethanol!at!80!°C!and!different!pH:!(A)!11,!(B)!7,!and!(C)!4.! $ ! ! ! ! ! 195! ! $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ ! $ $ REFERENCES$ ! 196! ! REFERENCES$ $ [1]! R.! Auras,! B.! Harte,! S.! Selke,! An! overview! of! polylactides! as! packaging! materials,! Macromol.!Biosci.!4!(2004)!835A864.! [2]!E.!CastroAAguirre,!F.!IñiguezAFranco,!H.!Samsudin,!X.!Fang,!R.!Auras,!Poly!(lactic! acid)—Mass!production,!processing,!industrial!applications,!and!end!of!life,!Adv.!Drug! Deliv.!Rev.!107!(2016)!333A366.! [3]!C.!Chariyachotilert,!S.E.!Selke,!R.A.!Auras,!S.!Joshi,!Assessment!of!the!properties!of! poly!(LAlactic!Acid)!sheets!produced!with!differing!amounts!of!postAconsumer!recycled! poly!(LAlactic!Acid),!J.!Plast.!Film!Sheet.!28!(2012)!314A335.! [4]! K.! Hamad,! M.! Kaseem,! F.! Deri,! Recycling! of! waste! from! polymer! materials:! 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AcidAand! baseAcatalyzed! hydrolyses! of! aliphatic! polycarbonates!and!polyesters,!Catal.!Today!115!(2006)!283A287.! [55]!K.!Makino,!H.!Ohshima,!T.!Kondo,!Mechanism!of!hydrolytic!degradation!of!poly!(LA lactide)! microcapsules:! effects! of! pH,! ionic! strength! and! buffer! concentration,! J.! Microencapsulation!3!(1986)!203A212.! [56]! H.! Tsuji,! K.! Ikarashi,! In! vitro! hydrolysis! of! poly! (lAlactide)! crystalline! residues! as! extendedAchain!crystallites.!Part!I:!longAterm!hydrolysis!in!phosphateAbuffered!solution! at!37!C,!Biomaterials!25!(2004)!5449A5455.! [57]!V.!Piemonte,!F.!Gironi,!Kinetics!of!hydrolytic!degradation!of!PLA,!J.!Polym.!Environ.! 21!(2013)!313A318.! [58]! K.! Dolan,! Estimation! of! kinetic! parameters! for! nonisothermal! food! processes,! J.! Food!Sci.!68!(2003)!728A741.! ! 201! ! CHAPTER$5$ Effect$of$Nanoparticles$on$the$Hydrolytic$Degradation$of$PLATNanocomposites$ by$WaterTEthanol$Solutions$ ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! A!version!of!this!chapter!is!published!as:! F. Iñiguez-Franco, R. Auras, M. Rubino, K. Dolan, H. Soto-Valdez, S. Selke. Effect of nanoparticles on the hydrolytic degradation of PLA-nanocomposites by water- ethanol solutions, Polymer Degradation and Stability 146 (2017) 287-297.! ! 202! ! 5.0$ Abstract$ Hydrolytic! degradation! of! poly(lactic! acid)! (PLAAC)! and! PLAAnanocomposite! produced!with!organomodified!montmorillonite!at!5!wt%!(PLAAOMMT)!were!investigated! in!pure!water,!50%!and!95%!ethanol!solutions!at!40!°C.!The!change!in!molecular!weight,! crystallinity,!release!of!lactic!acid!(LA),!and!release!of!the!organomodifier!(QAC)!were! monitored.! Ethanol! sorption! and! glass! transition! temperature! (Tg)! vs! ethanol! fraction! content!were!determined.!Release!of!Al!as!a!nanoclay!marker!was!studied!when!PLAA OMMT!was!exposed!to!50%!ethanol!solution.!PLAAC!and!PLAAOMMT!films!had!faster! hydrolysis!in!50%!ethanol!than!in!95%!ethanol!and!water.!The!incorporation!of!nanoclay! did!not!have!an!effect!on!the!hydrolytic!degradation.!The!nanoclay!doubled!the!sorption! of!ethanol!into!the!film!due!to!the!sorption!of!ethanol!into!the!nanoclay!galleries.!The! nanoclay!also!acted!as!an!anchor,!restricting!the!movement!of!the!polymer!chains!as! evidenced!by!the!ΔTg!and!the!crystallization!behavior.!Hydrolysis!of!PLAAC!and!PLAA OMMT!was!related!with!the!release!of!LA!during!exposure.!The!release!of!QAC!from! PLAAOMMT! films! was! higher! in! 95%! ethanol! at! the! early! stages,! but! then! it! exponentially! increased! in! 50%! ethanol! due! to! the! faster! hydrolytic! degradation.! The! clay!released!from!PLAAOMMT!films!during!hydrolysis!in!50%!ethanol!was!0.58%!wt.!at! 90!d.!of!the!initial!amount!of!nanoclay!in!the!PLA!film.!! 5.1$ Introduction$ Global!concern!about!environmental!impacts!of!polymers!has!changed!consumer! preferences! and! led! producers! to! decrease! the! use! of! plastics! derived! from! fossil! resources.! Consequently,! the! production! of! novel! plastics! from! renewable! resources! has!increased.!Poly!(lactic!acid),!PLA,!is!a!new!bioplastic!that!has!gained!great!attention! ! 203! ! due!to!its!lower!environmental!footprint!and!additional!end!of!life!scenario,!composting! [1,!2].!PLA,!an!aliphatic!polyester,!is!manufactured!by!polymerization!of!lactic!acid!(LA)! obtained!by!fermentation!of!corn!or!cassava!starch![1,!2].!PLA!has!been!used!for!an! arrays!of!applications!in!broad!and!diverse!industries!for!example!medical![3A5],!clothing! [6],! agricultural! [7A10],! and! packaging![11A13].! However,! PLA! has! some! shortcomings! such!as!low!solvent!resistant,!brittleness,!moderate!barrier!properties,!and!low!thermal! stability![2,!14].! Several!approaches!have!been!used!to!enhance!PLA!properties!and!to!expand! its! applications.! Some! of! these! improvements! have! been! achieved! by! the! addition! of! fillers! such! as! natural! nanoclays,! in! particular,! montmorillonite! (MMT)! with! organomodifiers! (OMMT)! [15,! 16].! MMT! is! a! type! of! clay! that! belongs! to! the! layered! smectite!group!made!up!of!several!stacked!layers!with!a!thickness!of!1!nm!containing! exchangeable!cations!(e.g.,!Na+,!K+,!Li+!and!Ca2+)!in!the!interlayer!space!or!gallery![17,! 18].!The!structure!of!the!crystal!is!made!of!2!silica!tetrahedral!sheets!connected!to!an! octahedral!layer!of!alumina![16].!Since!MMT!is!a!hydrophilic!clay,!surface!modification,! by!exchanging!the!cations!in!the!galleries!for!organic!cationic!surfactants,!increases!the! affinity!of!the!nanoclay!to!polymers!like!PLA![16].!! Although! PLAAnanocomposites! offer! good! mechanical,! barrier! and! thermal! properties,!nanoparticles!could!have!an!effect!on!the!hydrolytic!degradation!of!PLA.!The! existence!of!nanoparticles!in!the!polymer!can!increase!the!amount!of!water!in!the!matrix! accelerating!the!cleavage!of!the!ester!bonds!on!the!main!chain!of!the!polymer!due!to! the!increase!of!the!hydrophilicity!of!the!polymer!by!the!existence!of!the!nanoparticles! [19].!Other!authors!have!also!shown!that!the!presence!of!the!surfactant!influences!the! ! 204! ! hydrolysis! rate! of! PLAAnanocomposite! materials.! The! faster! degradation! can! be! attributed!to!the!hydroxyl!groups!of!the!surfactants!improving!the!distribution!of!the!clay! in!the!polymer!and!resulting!in!easier!water!attack![20A22].!The!hydrolysis!rate!depends! on! the! dispersion! of! the! filler,! water! uptake! and! the! ability! of! hydrating! by! the! fillers.! Other! studies! have! addressed! the! impact! of! OMMT! on! the! hydrolytic! degradation! of! PLAAnanocomposites!in!water!and!phosphate!buffered!solutions![19,!20,!23].!! Nanocomposites! can! be! used! in! different! environments,! such! as! organic! compounds,! acidic! surroundings,! mixtures! of! solvents,! or! different! media! for! food! applications! (e.g.,! juices,! alcoholic! beverage,! coldAchain! dairy! products).! PLA! is! susceptible! to! hydrolytic! degradation! under! moist! conditions,! leading! the! reduction! of! molecular! weight! and! the! release! of! LA! monomers! and! soluble! LA! oligomers! [2,! 24].! The!presence!of!organic!solvents,!such!as!ethanol,!plasticizes!the!PLA!matrix,!inducing! the!chain!mobility!and!crystallization!affecting!the!degradation!rate!of!the!polymer![25A 31].!The!transport!of!waterAethanol!solutions!into!PLAAnanocomposites!could!differ!from! neat!PLA!where!the!release!of!nanoparticles!and!the!surfactant!could!occur!changing! the! hydrolysis! behavior! and! therefore! the! durability! and! performance! of! PLA.! Understanding! the! effect! of! nanoparticles! on! the! hydrolysis! of! PLA! in! waterAethanol! solutions!and!the!interactions!within!the!system!is!essential!for!the!development!of!safe! PLAAnanocomposites.! The! goal! of! this! work! was! to! understand! the! effect! of! adding! nanoparticles!and!surfactant!on!the!hydrolysis!of!PLAAnanocomposites!in!waterAethanol! solutions!and!the!interaction!between!PLA,!nanoclay!and!solvent.! ! 205! ! 5.2$ Materials$and$methods$ 5.2.1$ Chemicals$and$reagents$ From!NatureWorks!LLC!(Minnetonka,!MN,!USA),!PLA!resin!IngeoTM!2003D!(3.8! –! 4.2! wt.%! DALA)! was! procured.! The! relative! number! (Mn)* and! weight! (Mw)! average! molecular! weight! of! the! resin! was! 1.21! ±! 0.08! x! 105!Da! and! 2.35! ±! 0.07! x! 105! Da,! respectively.! The! nanoclay! used! was! Cloisite®30B! (OMMT)! obtained! from! BYK! Additives!Inc.!(Gonzalez,!TX,!USA)!having!73%!wt!MMT!and!27%!wt!surfactant!(QAC)! (methyl,!tallow,!bisA2Ahydroxyethyl,!quaternary!ammonium).!The!surfactant!available!on! the! nanoclay! (Tomamine™QATA2! 70%PG)! was! also! obtained! separately! at! 60! –! 70%! purity!having!30!–!40%!propylene!glycol!(Air!Products!and!Chemicals!Inc.,!Butler,!IN,! USA).!The!surfactant!consists!of!methyl!tallow!ranging!from!14!to!18!carbons!(~60%! C18^!~35%! C16^!~5%! C14).! Ethanol,! acetonitrile,! methanol! (all! HPLC! grade),! formic! acid!and!propyl!hydroxy!benzoate!were!procured!(SigmaAAldrich,!St.!Louis,!MO,!USA).! Tetrahydrofuran!(THF)!was!obtained!from!PharmcoAAAPER!(North!East,!CA,!USA).!The! HPLC! grade! water! was! obtained! from! J.T.! Baker! (Center! Valley,! PA,! USA).! Malonic! acid!was!procured!from!Columbus!Chemical!Industries!(Columbus,!WI,!USA).!L(+)!LA! was!obtained!from!Supelco!(Bellefonte,!PA,!USA).!The!water!used!for!the!LC/MS/MS! mobile!phase!was!purified!by!a!MilliAQ!System!(Millipore!Corp.,!Bedford,!MA,!USA).!! 5.2.2$ Production$of$films$ PLA! films! were! prepared! by! drying! PLA! pellets! at! 60! °C! for! 24! h! and! 85! kPa.! PLAAOMMT!(OMMT!5%!wt)!and!PLAAQAC!(surfactant!1.5%!wt)!films!were!processed!in! two! stages.! In! the! first! stage! masterbatches! were! prepared! in! a! twinAscrew! extruder! model! ZSK! 30! (Werner! Pfleiderer,! NJ,! USA).! PLAAOMMT! masterbatch! consisted! of! ! 206! ! 80%!wt!of!PLA!dried!pellets!and!20%!wt!of!Cloisite®30B!with!processing!temperatures! of! 148! –! 189! °C.! For! PLAAQAC! masterbatch! (surfactant! 2.5%! wt),! the! range! of! processing! temperature! was! 148! –! 185! °C.! In! the! second! stage! the! films! were! cast! using! a! Randcastle! microextruder! (Extrusion! System,! Inc.,! Cedar! Grove,! NJ,! USA)! having!a!1.5875!cm!diameter!screw,!34!cc!volume,!and!24/1!L/D!ratio!extruder.!PLAA OMMT!film!was!processed!by!mixing!25%!wt!of!the!masterbatch!with!75%!wt!of!PLA! previously! dried! as! mentioned.! The! extrusion! temperatures! for! PLAAOMMT! film! were! between!193!–!248!°C!and!18!rpm.!For!PLAAQAC!film!lower!temperature!range!of!143!–! 173! °C! and! 30.5! rpm! was! used! since! QAC! acted! as! a! plasticizer! during! extrusion,! lowering!the!processing!temperature.!A!control!film!was!produced!(PLAAC)!in!the!same! cast! film! extruder! as! PLAAOMMT! and! PLAAQAC! films! with! extrusion! temperatures! of! 193!–!215!°C!and!49!rpm.!Each!processing!temperature!range!was!selected!to!obtain! good!films!for!experimental!methods.!The!thicknesses!of!the!films!were!different!where! PLAAOMMT!was!the!thinnest!being!27.9!±!9.9,!10.3!±!1.5!and!35.9!±!10.6!μm!for!PLAAC,! PLAAOMMT!and!PLAAQAC,!respectively.! 5.2.3$ Characterization$of$PLATOMMT$film$ TEM!(Transmission!electron!microscopy)!and!XRD!(XAray!diffraction)!were!used! to!characterize!the!structure!of!PLAAOMMT!films.!TEM!analyses!were!performed!in!a! JEOL! 100CX! II! TEM! manufactured! by! JEOL,! USA,! Inc.! (Peabody,! MA,! USA).! To! assess! the! spatial! distribution! of! the! nanoparticles,! the! PLAAOMMT! films! were! embedded!into!a!resin!to!provide!support!for!the!film!during!the!ultramicrotomy!and!to! retain! the! spatial! organization! of! the! specimen! section! on! the! TEM! grid.! The! polymerization! of! the! resin! was! carried! out! in! an! oven! during! two! days! at! 60! °C.! ! 207! ! Microtomed! sections! (100! nm)! were! obtained! using! an! ultramicrotome! RMC! MYX! (Boeckeler! Instruments,! Inc.,! Tucson,! AZ,! USA),! and! observed! with! an! acceleration! voltage!of!120!kV!in!the!bright!field!imaging!mode.$ XRD! analyses! were! performed! by! a! Rigaku! Rotaflex! RuA200BH! XAray! diffractomer!with!a!NiAfiltered!Cu!Kα!radiation!source!at!40!kV!and!100!mA.!PLAAOMMT! and!PLAAC!films!were!analyzed!using!a!2θ!range!between!1.5°!and!6°!at!0.3!°/min!with! 0.01°!increment.!To!evaluate!the!original!interlayer!space,!clay!powder!was!analyzed.! The!interlayer!spacing!was!calculated!according!to!Bragg’s!Law![32].!! 5.2.4$ Storage$experiments$ Ten!disks!of!2.0!cm!in!diameter!of!PLAAC,!PLAAOMMT!and!PLAAQAC!films!were! inserted!in!wire!made!of!stainlessAsteel!and!the!disk!were!separated!by!glass!beads.!A! schematic!representation!of!the!cells!is!in!the!Appendix!5B!(Figure$ 5B.1).!Then!they! were!placed!in!cells!containing!different!waterAethanol!solutions!previously!conditioned! at!40!°C:!water,!50%!ethanol,!or!95%!ethanol!(v/v).!The!surface!area!to!fluid!volume! ratio!of!the!films!was!1.79!cm2/mL.!The!degradation!through!hydrolysis!of!PLA!films!and! release!tests!were!performed!using!a!migration!cell!as!indicated!in!ASTM!D4754A11!at! 40!°C![33].!Samples!of!the!film!were!retrieved!periodically!to!assess!Mw!and!Mn,!and! samples!of!the!solution!were!also!removed!to!evaluate!LA!and!QAC!release.! 5.2.5$ Molecular$weight$ Mw!and!Mn*were!tested!as!previously!described!by!the!authors![29].!In!brief,!film! (10!mg)!was!taken!from!the!vials!and!dissolved!in!THF!(2!mg/mL)!and!tested!using!gel! permeation!chromatography!(GPC).!The!measurements!were!conducted!in!triplicate.!! ! 208! ! To! calculate! the! rate! constants! for! the! change! in! Mn! of! PLA! films,! the! experimental!data!was!fitted!using!a!first!order!reaction!funded!on!the!criteria!that!the! expansion! of! the! polymer! material! is! because! of! the! increase! in! emptied! space! as! !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!Eq.!5.1! Eq.!5.2! solutions![29]:! with!! previously! demonstrated! by! the! authors! when! PLA! was! in! contact! with! waterAethanol! "#="#%&'(−*+ ! *=, -.-/+ 0.06−-.-/ (−0.06(4 ! where!"#%!is!the!initial!"#!(Da),!+!is!time!(s),!*!is!the!rate!constant!(dA1),!-.!is!the!initial! empty!space!that!is!in!the!PLA!matrix!(cm3),!-/!is!the!disk!volume!(cm3),!,!is!a!constant,! and!!(!is!the!fraction!of!ethanol!((!=!0,!0.5,!0.95).!However,!for!better!estimation!of!*,! *= (−(%.5 (−(%.65 (%−(%.5 (%−(%.65 *78%+9 (−(% (−(%.5 (%.65−(% (%.65−(%.5 *78%.65! 99999999 where!*78%,*78%.5 !and!*78%.65 !are! rate! constant! for! water,! 50%! and! 95%! ethanol,! MA,!USA)!using!the!nonlinear!regression!(nlin?fit),!so!"#%,!*78%,!*78%.5!and!*78%.65!could! (−(% (−(%.65 (%.5−(% (%.5−(%.65 *78%.5+! respectively.!Eq.$ 5.1!and!Eq.$ 5.3!were!fitted!in!MATLAB®!2016a!(MathWorks,!Natick,! the!Taylor!series!expansion!was!applied!in!this!work!following!the!same!concept!of!the! ! ! expansion!of!the!polymer,!replacing!Eq.!2!by:! Eq.!5.3! ! ! 209! ! be! estimated.! Mean! comparisons! of! the! studied! parameters! among! treatments! were! done!by!Tukey!HSD!test!(p<0.05)!using!JMP®!9.0!(Cary,!NC,!USA)!statistical!software.! 5.2.6$ Water$and$ethanol$sorption$ Water!vapor!sorption!isotherms!of!PLAAC!and!PLAAOMMT!films!were!determined! for!relative!humidity!ranging!from!5!to!95%.!Ethanol!vapor!sorption!isotherms!for!PLAAC,! PLAAOMMT! and! clay! (Cloisite®30B)! were! carried! out! at! 40! °C! with! relative! pressures! (i.e.,!partial!ethanol!vapor!pressure/saturation!ethanol!vapor!pressure)!from!0.05!to!0.70.! Isotherms!were!obtained!by!gravimetric!analysis!with!a!SGAA100!(VTI!Corp.,!Hialeah,! FL,! USA).! Between! 15! and! 20! mg! of! sample! was! used! and! the! measurements! were! conducted!in!triplicate.!! Ethanol!sorption!of!PLAAC!and!PLAAOMMT!films!in!contact!with!different!volume! fractions! of! waterAethanol! solutions! was! evaluated! at! 40! °C.! Samples! were! analyzed! using!1H!NMR!(proton!nuclear!magnetic!resonance)!in!a!Varian!Inova!600!MHz!NMRA Spectrometer,! which! was! equipped! with! a! Nalorac! 5! mm! PFG! switchable! probe! (599.892! MHz).! A! detailed! description! of! ! the! ! 1H! NMR! technique! was! previously! provided![29].!The!measurements!were!conducted!in!triplicate.!! 5.2.7$ Dynamic$mechanical$analysis$(DMA)$ The!loss!factor!(tan!delta)!vs*temperature!was!analyzed!to!determine!the!glass! transition!temperature!(Tg)!of!PLAAC!and!PLAAOMMT!films!where!they!were!submerged! in! different! waterAethanol! solutions! during! the! testing! with! a! TA! RSAAG2! (TA! Instruments,!New!Castle,!DE,!USA)!as!previously!described![29].! ! 210! ! 5.2.8$ Differential$scanning$calorimetry$(DSC)$ The!degree!of!crystallinity!(%XC),!crystallization!temperature!(Tc),!Tg,!and!melting! temperature! (Tm)! of! PLAAC! and! PLAAOMMT! films! that! had! been! immersed! in! 50%! ethanol! at! 40! °C! were! obtained! using! a! DSC! (Q100,! TA! Instruments).! The! samples! were!retrieved!periodically!and!dried!before!being!analyzed.!The!samples!were!cooled! from!25!to!5!°C!and!then!run!from!5!to!210!°C!at!10!°C/min!using!a!flow!rate!of!purge! nitrogen!of!70!mL/min.!The!first!run!of!the!heat!scan!of!the!samples!is!reported.!The! measurements!were!conducted!in!triplicate.!! 5.2.9$ Lactic$acid$release$ LA! release! from! PLAAC,! PLAAOMMT! and! PLAAQAC! films! into! water,! 50%! and! 95%!ethanol!at!40!°C!was!determined!as!previously!described!by!the!authors![29].!In! brief,!0.5!mL!of!solution!from!the!test!cells!were!retrieved!regularly!and!then!an!alkali! hydrolysis!was!performed.!Samples!with!50%!and!95%!ethanol!were!evaporated!and! later!reconstituted!with!0.5!mL!of!water.!LA!was!evaluated!using!a!LC/MS/MS!system.!!!! 5.2.10$Surfactant$release$$ The!release!of!the!surfactant!(QAC)!from!PLAAOMMT!and!PLAAQAC!films!was! studied! using! a! LC/MS/MS! system! (AB/Sciex! QTRAP! 3200,! Framingham,! MA,! USA)! having! a! triple! quadrupole/linear! ion! trap.! Separation! was! performed! on! a! Symmetry! C18!(3.5!μm,!2.1!x!100!mm)!reverse!phase!column!(Waters,!Milford,!MA,!USA)!at!a!flow! rate!of!0.3!mL/min.!The!mobile!phase!was!A:!1%!formic!acid!and!B:!acetonitrile.!The! programming! of! the! solvent! was! isocratic! for! 1.5! minutes! with! 1%! B.! Then! a! linear! increase!to!95%!B!up!to!3.5!minutes!was!programmed,!followed!by!an!isocratic!mode! for!2.5!minutes.!A!linear!gradient!was!done!for!0.01!minutes!to!5%!B!and!held!for!2!min! ! 211! ! A!isocratic!mode.!The!oven!temperature!was!kept!at!40!°C.!The!analyses!were!carried! out! in! the! positiveAion! mode! using! the! ion! spray! method.! The! calibration! curve! was! determined!from!solutions!of!0.25!to!15!μg/mL!using!propyl!hydroxy!benzoate!as!the! internal!standard.! 5.2.11$Nanoclay$release$ The! release! of! nanoclay! from! PLAAOMMT! films! into! 50%! ethanol! solution! was! determined!using!a!graphite!furnace!atomic!absorption!spectrometry!(GFAAS)!(Perkin! Elmer,! Waltham,! MA,! USA).! Al! was! selected! as! the! nanoclay! marker! element.! The! nanoclay! concentration! was! estimated! by! correlating! with! the! concentrations! of! Al! based! on! the! elemental! content! in! the! nanoclay.! The! samples! were! prepared! as! previously!described,!but!with!the!glass!vials!replaced!as!migration!cells!by!50!mL!PP! tubes!and!Teflon!beads!to!avoid!potential!migration!of!aluminum!from!the!glass.!The! experiment!was!conducted!in!quadruplicate.!! 5.3$ Results$and$discussion$ The! influence! of! nanoparticles! on! the! hydrolytic! degradation! of! PLAA nanocomposites!by!different!waterAethanol!solutions,!the!variation!in!Mn!of!PLAAC,!PLAA OMMT!and!PLAAQAC!was!analyzed!during!exposure!to!water,!50%!and!95%!ethanol! solutions! to! determine! the! rate! of! hydrolysis.! Since! water! molecules! influence! the! hydrolysis!reactions,!the!sorption!of!water!was!determined!for!PLAAC!and!PLAAOMMT! films! using! vapor! isotherms.! However,! the! presence! of! ethanol! affects! the! water! sorption!due!to!PLA!swelling.!So,!swelling!studies!were!performed!where!the!films!were! immersed! into! different! volume! fractions! of! waterAethanol.! To! study! the! possible! ! 212! ! sorption!of!ethanol!by!the!clays,!ethanol!sorption!isotherms!were!determined!for!PLAAC,! PLAAOMMT! and! clay! powder.! Crystallinity! studies! of! PLAAC! and! PLAAOMMT! were! carried! out! over! the! exposure! time! in! 50%! ethanol! to! assess! solvent! induced! crystallization! (SIC)! by! ethanol! molecules! and! to! describe! the! crystalline! and! nonA crystalline! phases! of! the! polymer! during! hydrolysis.! LA! and! QAC! release! were! evaluated!for!PLAAC,!PLAAOMMT!and!PLAAQAC!films!when!they!were!immersed!in!the! different! waterAethanol! solutions.! Finally,! the! release! of! clay! from! PLAAOMMT! film! to! 50%!ethanol!was!assessed!during!hydrolytic!degradation.!! 5.3.1$ Characterization$of$PLATOMMT$film$ XRD!patterns!of!OMMT!clay!powder,!PLAAOMMT!and!PLAAC!films!between!2θ!=! 1.5°!and!6°!are!shown!in!Figure$ 5.1.!The!spacing!between!the!layers!for!the!OMMT! powder!was!1.85!nm!(2θ!=!4.75!°).!The!dAspacing!of!the!OMMT!increased!to!3.42!nm! (2θ!=!2.6)!when!embedded!into!the!PLA!matrix.!The!increase!of!the!interlayer!spacing! and!the!displacement!of!the!peak!at!a!smaller!angle!indicated!that!the!polymer!entered! the! clay! gallery,! forming! an! intercalated! nanocomposite.! These! results! are! consistent! with!other!studies!using!OMMT!at!different!concentrations!within!a!PLA!matrix!resulting! in!intercalated!nanocomposites![17,!34,!35].!The!intercalated!structure!was!confirmed! by!the!TEM!images!(Figure$5.2)!where!the!silicate!monolayers!are!randomly!distributed,! but!not!well!exfoliated!in!the!PLA!matrix.!! ! 213! ! s p c , y t i s n e t n I e v i t l a e R d = 3.42 nm OMMT PLA−C PLA−OMMT d = 1.85 nm 1.5 2 2.5 3 3.5 2 theta angle 4 4.5 5 5.5 6 ! Figure$5.1!Displacement!of!diffraction!pattern!of!the!OMMT!clay!when!embedded!in!the! PLA!matrix.$XRD!patterns!for!OMMT!clay!powder,!PLAAOMMT!and!PLA!films.$ (A) (B)! Figure$ 5.2! Intercalation! of! OMMT! in! PLA! matrix.$ TEM! bright! field! images:! (A)! PLAA OMMT!(x!100!000),!(B)!PLAAOMMT!(x!270!000).! ! ! 214! ! 5.3.2$ Molecular$weight$ Figure$ 5.3! shows! the! Mn! vs! time! for! PLAAC,! PLAAOMMT! and! PLAAQAC! films! submerged!in!water,!50%!and!95%!ethanol!at!40!°C.!For!the!three!films!the!molecular! weight! decreased! over! time! due! to! the! hydrolytic! degradation! of! PLA.! Table$ 5.1! presents!the!rate!constants!k*(dA1)!for!PLAAC,!PLAAOMMT!and!PLAAQAC!films!exposed! to!the!different!solutions.!It!is!important!to!note!that!after!processing,!the!films!showed! different!"#%.!!However,!when!the!hydrolytic!degradation!rate!was!estimated,!the!"#%9of! each!of!the!film!was!taking!into!consideration.!For!the!three!films,!when!PLA!films!were! immersed!in!50%!ethanol,!the!rate!of!hydrolysis!was!faster!than!in!water!and!in!95%! ethanol! (p<0.05).! When! PLA! is! in! contact! with! waterAethanol! solutions,! there! is! a! competitive!balance!effect!between!the!swelling!caused!by!ethanol!molecules!and!the! water!molecules!that!diffuse!into!the!core!of!the!polymer!matrix!starting!the!cleavage!of! the!ester!bonds.!In!our!previous!work,!the!maximum!rate!of!decay!in!Mn!for!PLA!film! was! when! the! volume! fraction! of! ethanol! was! around! 0.50! where! the! swelling! of! the! polymer!was!sufficient!to!allow!the!maximum!sorption!of!water!molecules!to!begin!the! hydrolysis![29].!For!PLAAOMMT!and!PLAAQAC!the!50%!ethanol!solution!could!have!the! same!effect!as!in!PLAAC!film.!! The!addition!of!the!QAC!into!PLA!accelerated!the!rate!of!hydrolysis!when!PLA! was!in!contact!with!water!(p<0.05)!(Table$5.1).!The!QAC!acted!as!a!plasticizer!inducing! movement!of!the!polymer!chains!and!therefore!allowing!the!diffusion!of!water!molecules! trough!the!PLA!to!begin!cleavage!of!the!ester!bonds.!However,!the!presence!of!QAC! decreased!(p<0.05)!the!hydrolysis!rate!of!PLA!in!95%!ethanol.!This!may!be!explained! by! a! chemical! reaction! with! water! molecules! and! the! ammonium! from! the! QAC! ! 215! ! depleting! available! water! and! so! leaving! few! water! molecules! to! participate! in! the! hydrolysis!reaction![36].!! a D , M n a D , M n 15 x 104 10 5 0 15 x 104 10 5 0 15 x 104 Water 50% Ethanol 95% Ethanol A B C 10 a D , n M 5 0 0 20 40 60 80 100 time, d 120 140 160 180 ! Figure$ 5.3! First! order! reduction! of! Mn! for! PLAAC,! PLAAOMMT,! and! PLAAQAC! films! showing!that!the!faster!decay!in!Mn*was!when!the!films!were!immersed!in!50%!ethanol.! Lines!indicate!fitting!of!first!order!reaction!with!*!obtained!from!Eq.$5.3.!Mn!as!vs!time! during!hydrolysis!of!(A)!PLAAC,!(B)!PLAAOMMT!and!(C)!PLAAQAC!films!submerged!in! water,!50%!and!95%!ethanol!at!40!°C.!Values!indicated!as!*!were!considered!outliers! and!therefore!no!used!for!fitting.! ! 216! ! Table$5.1!Hydrolytic!degradation$rate!constants!for!PLAAC,!PLAAOMMT!and!PLAAQAC! films!exposed!to!waterAethanol!solutions!at!40!°C.$ k!(dT1)*! PLATC! PLATOMMT! PLATQAC! ! Solvent$ solution! Water! 0.0062!±!0.0007!a,!A! 0.0079!±!0.0009!a,!AB!! 0.0093!±!0.0008!a,!B!! 50%!Ethanol! 0.0218!±!0.0026!b,!A! 0.0218!±!0.0027!b,!A! 0.0229!±!0.0021!b,!A! 95%!Ethanol! 0.0125!±!0.0014!c,!A! *Fitting!of!first!order!reaction!with!*!obtained!from!Eq.$5.3! 0.0106!±!0.0011!a,!AB!! 0.0092!±!0.0007!a,!B!! Note:!Values!with!the!same!capital!letter!within!a!row!and!the!same!lowercase!letter! within!a!column!are!not!different!(α=0.05)!$ $ The!incorporation!of!nanoparticles!into!the!PLA!matrix!did!not!affect!the!reduction! of!the!Mn!when!the!polymer!was!immersed!in!each!solvent!(p>0.05).!However,!some! authors! have! reported! that! nanoparticles! have! an! effect! on! the! hydrolysis! of! PLA,! concluding! that! when! the! nanoparticles! are! within! the! PLA,! there! is! an! easier! water! attack!since!the!volume!and!surface!of!the!polymer!in!contact!with!the!nanoparticles!is! higher![19,!21].!Therefore,!even!if!the!nanoparticles!did!not!affect!the!hydrolysis!rate,!it! is! important! to! understand! whether! the! incorporation! of! nanoparticles! changes! the! solventApolymer!interaction!affecting!PLA!morphology!and!its!possible!applications.!! ! 217! ! 5.3.3$ Water$and$ethanol$sorption$ Hydrolytic!degradation!of!PLA!begins!with!the!diffusion!of!water!through!the!PLA! matrix!cleaving!the!ester!bonds!of!chains![24].!Figure$5.4!shows!similar!sorption!of!H2O! vapor!in!PLAAC!and!PLAAOMMT!films!indicating!similar!amount!of!water!molecules!sorb! in!the!polymer!matrix,!which!are!then!available!to!start!the!hydrolysis!reactions,!resulting! in!similar!hydrolysis!rates.!The!presence!of!ethanol!changes!the!interaction!of!PLA!and! clay,!so!different!sorption!of!water!and!ethanol!can!take!place.!! ! Figure$ 5.4! Similar! sorption! of! H2O! vapor! in! PLA! and! PLAAOMMT! films.! Sorption! isotherms!for!H2O!vapor!in!PLAAC!and!PLAAOMMT!films.!! Figure$ 5.5!presents!the!ethanol!liquid!sorption!of!the!PLA!films!in!contact!with! different! volume! fractions! of! ethanol! as! quantified! by! 1H! NMR.! Sorption! of! ethanol! in! PLAAOMMT! was! almost! double! that! in! PLAAC.! Therefore,! to! understand! the! higher! sorption!of!ethanol!in!PLAAOMMT!film,!sorption!isotherms!of!ethanol!vapor!in!PLAAC,! ! 218! ! PLAAOMMT!and!clay!were!carried!out!over!a!relative!pressure!ranging!from!0.05!to!0.70! (Figure$5.6).!! % , n o i t p r o S 14 12 10 8 6 4 2 0 0 PLA−C PLA−OMMT 0.2 0.4 Ethanol fraction 0.6 0.8 1 ! Figure$ 5.5! Ethanol! sorption! of! PLAAC! (%sorption=6p^! R2=0.978)! and! PLAAOMMT! (%sorption=12p^!R2=0.965)!films!in!contact!with!different!volume!fractions!of!ethanol!(p* is!the!ethanol!fraction!by!volume).! ! 219! ! Figure$ 5.6! Sorption! isotherms! of! ethanol! vapor! in! PLAAOMMT,! PLAAC! and! clay! particles.! The! sorption! of! PLAAC! and! clay! correspond! to! the! nominal! composition! of! PLAAOMMT! film! (PLA:Clay! =! 95:5).! PLAAC! +! Clay! is! the! mathematical! addition! of! ! ethanol!sorption!of!PLAAC!and!clay.! ! In! Figure$ 5.6! the! isotherms! of! PLAAC! and! clay! represent! the! ethanol! sorption! corresponding!to!the!nominal!composition!of!PLAAOMMT!film,!which!is!95%!PLA!film! and! 5%! clay! (PLA:Clay! =! 95:5).! The! PLAAOMMT! isotherm! showed! higher! ethanol! sorption!for!all!the!P/Po!range!compared!to!the!ethanol!sorbed!in!95%!PLAAC!and!5%! ! 220! ! clay.! To! understand! the! influence! of! clay! in! PLA,! the! mathematical! addition! of! the!%ethanol!sorbed!by!95%!PLAAC!film!and!5%!clay!is!shown!in!Figure$5.6!as!PLAC!+! Clay.!The!computed!%ethanol!sorbed!at!high!P/Po!was!similar!to!the!ethanol!sorption! measured!in!PLAAOMMT.!However,!at!low!P/Po!the!computed!ethanol!sorbed!was!not! similar! to! the! measured! %ethanol! in! PLAAOMMT! film.! The! reason! for! this! difference! could!be!attributed!to!clustering!of!the!clay!powder!particles!while!the!clay!layers!in!the! PLAAOMMT!film!are!intercalated!(Figure$5.2)!allowing!easy!sorption!of!ethanol!into!the! clay! galleries.! Therefore,! the! higher! sorption! of! ethanol! in! PLAAOMMT! at! volume! fractions!of!ethanol!between!0.2!and!0.5!(Figure$5.5)!could!be!attributed!to!the!sorption! of!ethanol!into!the!galleries!of!the!clay!particles.!Sorption!of!organic!solvents!by!OMMT! clays!has!been!described!by!intercalation!and!clay!swelling![37].!So,!the!higher!sorption! of!ethanol!by!PLAAOMMT!film!could!be!explained!by!the!sorption!of!ethanol!between!the! clay! galleries,! since! the! sorption! of! ethanol! in! the! PLA! portion! of! PLAAOMMT! is! the! same!as!in!PLAAC!film.! PLA! plasticizes! and! crystallizes! in! the! presence! of! ethanol,! increasing! chain! mobility.!In!our!previous!work![29],!when!PLA!was!immersed!in!waterAethanol!solutions,! the! decrease! of! the! Tg! of! PLA! films! was! larger! when! the! amount! of! ethanol! in! the! solution! augmented! and! swelling! occurred! immediately! (Figure$ 5.7A! and! Table$ 5.2).! The!interaction!of!the!PLAAOMMT!with!the!solvent!may!be!not!the!same!as!in!neat!PLA.! Figure$5.7B!and!Table$5.2!present!the!change!in!Tg!of!PLAAOMMT!when!immersed!in! different!amount!of!ethanol!fractions.!The!Tg!of!PLAAC!and!PLAAOMMT!films!were!64.7! ±!0.1!and!62.4!±!0.1!°C,!respectively,!according!to!the!DMA!results.!The!lower! Tg!of! PLAAOMMT! can! be! due! to! the! free! QAC! in! the! matrix! of! the! polymer,! which! was! ! 221! ! released!from!the!clay!gallery!during!processing,!softening!the!polymer.!For!PLAAOMMT! immersed!in!0,!25,!50,!75!and!95%!ethanol,!the!Tg*dropped!to!51.7,!47.6,!39.2,!33.4!and! 17.8!°C,!respectively.!This!indicated!that!the!Tg*of!the!films!decrease!with!the!content!of! ethanol!in!the!solution,!inducing!more!movement!of!PLA!polymer!chains.!! 3.5 3 C o , g T 2.5 ) a t l e d ( n a t 2 1.5 1 0.5 60 50 40 30 20 0 0.5 Ethanol fraction 1 A 0 0.25 0.50 60 50 40 30 20 0 0.5 Ethanol fraction 1 B 0 0.25 0.50 0.75 0.95 2.5 2 C o , g T 1.5 1 0.5 0 −20 0 20 40 Temperature, oC 60 80 0 −40 −20 20 0 40 Temperature, oC 60 80 Figure$5.7!Tg*of!(A)!PLAAC!and!(B)!PLAAOMMT!films!submerged!in!different!fractions!of! ethanol!(v/v)!from!the!DMA.!Inserts:! Tg! vs!ethanol!fraction!(()^! !!experimental!data,!∗! estimated!values!that!could!not!be!experimentally!measured,!and!–!(fitted!line)!predicted! values!as!(A)!=>=−0.34(+52.34;9D4=0.999!(B)!=>=−0.34(+54.50;9D4=0.927.! ! ! 222! ! ! Table$5.2!Tg!of!PLAAC!and!PLAAOMMT!films!submerged!in!different!fractions!of! ethanol! (v/v)! solution! from! DMA,! and! change! in! Tg! (ΔTg)! with! respect! to! nonA immersed!films.! ! PLATC$ PLATOMMT$ Ethanol$fraction$ Tg,$°C$ ΔTg,$°C$ Tg,$°C$ ΔTg,$°C$ No!immersion! 64.7!±!0.1! 0! 0.25! 0.50! 0.75! 0.95! 52.6! 44.2! 36.8! 27.3! 20.2! No!immersion*! 59.8!±!0.5! !*Values!obtained!by!DSC.! ! ! 12.1! 20.5! 27.9! 37.4! 44.7! ! 62.4!±!0.1! 51.7! 47.6! 39.2! 33.4! 17.8! 55.8!±!0.7! ! 10.7! 14.8! 23.2! 29.0! 44.6! ! Tan(delta)!peaks!in!Figure$5.7!could!indicate!interactions!between!the!polymerA nanoparticleAsolvent.!The!high!of!the!tan(delta)!peak!is!an!indication!of!the!interaction! between!the!polymer!chains!and!the!solvent![38,!39].!Jonoobi!et!al.![38]$found!that!when! the! concentration! of! nanofibers! in! PLA! increased! the! tan(delta)! peak! decreased! compared!with!PLA!(neat),!indicating!that!less!chains!of!the!polymer!were!contributing! to! the! transition.! In! the! case! of! PLAAOMMT! film! (Figure$ 5.7B),! the! peak! was! higher! when!it!was!immersed!in!water!than!when!immersed!in!ethanol!solutions.!Lower!peaks! could!indicate!that!the!sorption!of!ethanol!molecules!enhanced!the!interaction!with!the! polymer!and!the!solvent,!so!the!polymer!became!more!elastic![39].!Also,!the!increase!in! ! 223! ! breadth!proposes!more!particleApolymer!interactions!due!to!the!broader!distribution!of! relaxation!time![39].!Therefore,!when!PLAAOMMT!was!exposed!to!ethanol!solutions,!the! swelling! of! PLA! enhanced! the! interaction! of! the! polymerAsolventAnanoparticles.! This! interaction!between!the!nanoparticles!and!solvent!can!be!observed!in!Figure$ 5.7B!in! which! the! tan(delta)! peak! of! waterAethanol! solutions! did! not! reach! zero! at! the! xAaxis! when!temperature!was!high,!unlike!the!PLAAC!film!(Figure$5.7A).!! The!presence!of!the!OMMT!nanoparticles!in!the!PLA!film!could!act!as!an!anchor! restricting! the! movement! of! PLA! polymer! chains.! Klonos! et! al.! [40]! found! that! the! presence!of!nanoparticles!in!PLA!such!as!graphene!oxide,!carbon!nanotubes!and!silica,! immobilizes!the!polymer!at!the!interfaces.!It!has!also!been!studied!that!the!presence!of! metal! organic! framework! in! PLA! increases! the! toughness! of! the! polymer! matrix! [41].! This!restricted!movement!can!be!reflected!in!the!change!of!Tg!(ΔTg)!from!DMA!results! (Table$5.2)!where!the!change!in!Tg!was!lower!for!PLAAOMMT!than!for!PLAAC!films!for! ethanol!fraction!0.05)! on! further! extension! of! time.! So,! it! was! inferred! that! 8! min! was! the! minimum! time! required! to! activate! the! chain! extender! to! reconnect! chains! of! PLA! matrix! that! had! been! cleaved! during! heat! processing.! Therefore,! the! PLAAJ! film! with! 8! min! of! residence! time! was! selected! for! further! characterization!analysis!and!hydrolytic!degradation!studies!having!the!highest!Mn!with! the! lowest! residence! time! of! processing.! For! comparison! purposes! the! PLAAC! film! processed!at!the!same!conditions!as!PLAAJ!was!selected.! 6.3.2$ Characterization$ 6.3.2.1$ Rheological$behavior$$ Films!with!the!highest!Mn!after!processing!with!a!residence!time!of!8!min!were! characterized!by!their!rheological!properties.$The!time!dependency!of!viscosity!of!PLAA C!and!PLAAJ!films!was!investigated!by!oscillatory!time!sweep!tests!for!a!time!period!of! 15!min!at!170!°C.!As!shown!in!Figure$6.3,!the!complex!viscosity!(η*)!of!the!two!types!of! samples!behaved!differently!during!the!heating!period^!it!decreased!from!0.17!to!0.13! Pa.s!for!the!PLAAC,!while!an!increasing!trend!(1.5!to!1.9!Pa.s)!was!noticed!for!the!PLAA J.!The!observed!values!clearly!indicated!a!timeAdependency!for!both!samples.!The!test! indicated!a!minor!degradation!of!PLA!samples!happening!because!of!the!degradation!of! the! polymer,! whereas! the! addition! of! the! chain! extender! improved! the! stability! of! the! PLA!matrix!by!affecting!its!meltAstrength!by!means!of!chain!extension/branching![20,!32].! ! 264! ! ! Figure$6.3!Time!dependency!of!PLA!melt!at!170!°C!for!a!time!period!of!15!min.$ $ The! influence! of! the! chain! extender! on! the! mechanical! rigidity! of! PLA! films! at! 170! °C! is! illustrated! in! Figure$ 6.4.! PLAAC! film! at! melt! exhibited! highAfrequency! dependency,! and! the! loss! modulus! (G″)! was! significantly! higher! than! the! storage! modulus! (G′),! confirming! a! predominating! liquidAlike! characteristic! of! the! melt.! PLAAJ,! with!incorporated!chain!extender,!had!significantly!lower!frequencyAdependency!of!the! dynamic!moduli!compared!to!the!neat!PLA.!Figure$6.4!shows!that!the!slope!of!G′*?*ω!of! PLAAJ!at!a!terminal!range!of!frequency!is!largely!deviates!from!2,!and!it!is!less!than!1.! The! slope! in! the! lowAfrequency! range! indicates! the! blend! has! a! different! morphology! than!the!neat!polymer.!A!power!law!behavior!at!lowAfrequency!(slope!<1)!is!one!of!the! ! 265! ! characteristic!dynamic!responses!of!a!coAcontinuous!morphology![33].!Addition!of!chain! extender! markedly! improved! the! mechanical! properties! of! the! PLAAJ! matrix! as! compared! to! the! control! sample.! For! the! control! sample,! the! liquidAlike! property! predominates!(G″>G′)!throughout!the!frequency!range,!and!a!gel!point!was!attained!at! the!highest!frequency!(62.8!rad/s).!Addition!of!Joncryl!improved!the!G′!significantly,!and! the*G′!exceeded!the!G″!above!1!rad/s!with!predominating!solidAlike!property.!The!G′!of! PLAAJ! increased! almost! 1A2! log! cycles! over! the! control! sample! within! the! studied! frequency!range,!which!was!further!confirmed!by!the!low!phase!angle!(δ)!data.!A!similar! 30!times!increase!in!the!complex!shear!viscosity!of!PLA!incorporated!with!1!wt%!chain! extender!(Joncryl®4368)!in!PLA!blend!(at!an!angular!frequency!of!0.1!rad/s)!at!180!°C! was! previously! reported! [21].! ! Such! a! significant! increase! in! the! mechanical! rigidity! indicated! that! macromolecular! chain! extension! had! occurred! during! the! extrusion! compounding!process![34].!The!difference!in!storage!modulus!between!neat!PLA!and! PLAAJ!has!been!reported!by!other!authors!who!attributed!the!difference!to!creation!of!a! branched!polymer![18,!19,!22,!35].! ! 266! ! Figure$ 6.4! Mechanical! spectra! of! PLAAC! and! PLAAJ! films! as! a! function! of! angular! ! frequency!at!170!°C.! ! A!plot!of!phase!angle,!δ!against!the!complex!modulus!(G*)!eliminates!the!effect! of!shifting!along!the!frequency!axis!and!yields!temperature!independent!curves!when! time!temperature!superposition!(TTS)!holds![36].!Figure$ 6.5!illustrates!these!plots!for! both! PLA! films! at! selected! temperatures.! All! of! the! data! were! superimposed! in! one! curve!with!a!very!characteristic!shape!although!the!superposition!of!the!PLAAJ!sample! was!superior!to!that!of!the!neat!PLA!polymer.!At!low!values!of!G*,!δ!is!nearly!90°!for! PLAAC,! indicating! that! the! samples! are! viscous.! The! behavior! of! PLAAJ! was! quite! different! from! that! of! PLAAC,! and! the! difference! in! the! shape! of! the! curves! can! be! ! 267! ! related! to! the! influence! of! chain! extension! or! a! change! of! topology! from! linear! to! branched!polymer!for!PLAAJ![21,!37,!38].!! Figure$6.5.!Van!GurpAPalmen!plot!of!PLAAC!and!PLAAJ!samples!obtained!from! master!curves!for!a!reference!temperature!of!180!°C.$ ! ! 6.3.2.2$ Thermal$properties$ Figure$ 6.6! displays! the! TGA! thermograms! of! PLAAC! and! PLAAJ! films! to! investigate!the!effect!of!the!chain!extender!on!the!thermal!degradation!behavior!of!PLA.! The!onset!decomposition!temperatures!(Tonset)!of!PLAAC!and!PLAAJ!were!359.5!±!0.3! ! 268! ! and!358.4!±!0.1!°C,!respectively.!A!marginal!decrease!in!the!Tonset!was!observed!after! the! incorporation! of! the! chain! extender! in! PLA.! Although! the! values! were! close,! this! could! be! attributed! to! branched! structures! of! PLAAJ! film,! which! result! in! a! higher! concentration!of!hydroxyl!groups!as!end!groups,!making!the!polymer!less!stable!than! the!linear!PLA.!These!results!are!in!accordance!with!those!researchers!who!studied!the! thermal!stability!of!branched!PLA!with!Joncryl®!ADRA4368!and!epoxidized!soybean!oil! [18,!39].! 100 80 60 40 20 % , t h g e W i 0 200 250 PLA-C PLA-J 450 ! 350 300 Temperature, oC 400 Figure$6.6!TGA!thermograms!of!PLAAC!and!PLAAJ!films.$ ! Figure$6.7!shows!the!DSC!heating!thermograms!of!PLAAC!and!PLAAJ!films.!The! thermal!parameters!are!listed!in!Table$6.1.!Incorporation!of!the!chain!extender!did!not! ! 269! ! influence!the!Tg!of!the!PLA!significantly.!However,!the!%XC!slightly!increased!with!the! presence!of!the!chain!extender,!and!the!Tm!and!the!Tcc!for!PLAAJ!decreased!by!1!°C.! Even!that!the!difference!was!marginal,!branched!polymers!had!faster!cold!crystallization! rates! than! the! linear! PLA! due! to! heterogeneous! nucleation! effect! by! the! branching! points![35,!40A42].!Although!there!was!a!nucleation!effect!of!the!chain!extender,!the!Tm! showed! a! marginal! decrease,! which! can! be! attributed! to! the! formation! of! imperfect! crystals!in!the!PLAAJ!film!sample.!The!presence!of!chain!branches!after!processing!PLA! with! the! chain! extender! restricted! the! chain! mobility,! forming! thinner! and! imperfect! crystals!with!lower!melting!point![35].! exo / g W , w o l f t a e H 20 40 60 80 100 120 Temperature, oC PLA−C PLA−J 140 160 180 200 ! Figure$6.7!DSC!curves!of!the!second!heat!scan!of!PLAAC!and!PLAAJ!films.$ ! 270! ! Table$6.1!Thermal!properties!of!the!second!heat!scan!of!PLAAC!and!PLAAJ!films.$ Sample$ Tg,$°C$ Tcc,$°C$ Tm,$°C$ PLAAC! 59.6!±!1.2a! 126.8!±!0.4a! 150.9!±!0.1a! PLAAJ! 60.6!±!0.3a! 125.5!±!0.5b! 149.5!±!0.2b! XC,$%$ 0.2!±!0.1a! 0.7!±!0.2b! Values!with!different!letter!within!the!same!column!are!different!(α=0.05).! $ 6.3.3$ Hydrolytic$degradation$study$ To!study!the!effect!of!the!addition!of!chain!extender!on!the!hydrolytic!degradation! of!PLA,!PLAAC!and!PLAAJ!films!were!immersed!in!50%!ethanol!solution!at!80!°C!and!pH! 11.! These! conditions! were! selected! based! on! our! earlier! works! to! accelerate! the! hydrolysis!of!PLA!films![43,!44].!It!was!found!that!the!fast!hydrolysis!of!PLA!immersed!in! 50%! ethanol! solutions! especially! under! basic! conditions! and! high! temperatures! was! very! effective! for! the! degradation! [30].! Figure$ 6.8! shows! the! molecular! weight! distribution!(MWD)!of!PLAAC!and!PLAAJ!films!during!the!hydrolysis!in!50%!ethanol!at!80! °C!and!pH!11.!In!the!beginning,!before!the!hydrolysis!started!(time!=!0),!PLAAC!showed! a! monomodal! distribution! with! a! PDI! of! 2.0! ±! 0.2.! However,! PLAAJ! films! exhibited! a! multimodal! distribution! with! PDI! of! 2.2! ±! 0.1,! which! could! be! associated! with! the! presence! of! branches! with! different! hydrodynamic! volumes! as! a! consequence! of! different!chain!topologies![21,!23].!Cailloux!et!al.![23]!studied!sheets!of!branched!PLA! with!Joncryl®4300F!where!they!attributed!the!peak!at!the!lower!molecular!weight!in!the! MWD! of! branched! PLA! to! a! linear! population! and! the! peak! at! the! higher! molecular! weight!to!a!branched!chain!population.! ! 271! ! ! Figure$6.8!MWD!of!(A)!PLAAC!and!(B)!PLAAJ!films!during!hydrolytic!degradation!in!50%! ethanol!at!80!°C,!pH!11.! When!PLAAC!film!was!exposed!to!hydrolytic!degradation!in!50%!ethanol!for!24!h,! the!MWD!shifted!from!its!original!position!towards!the!lower!molecular!weight!(Figure$ 6.8A).!This!behavior!has!been!reported!as!degradation!of!PLA,!which!is!preferentially! carried! out! by! the! bulk! erosion! [45A48].! Optical! images! of! PLA! samples! immersed! in! 50%! ethanol! solution! can! be! found! in! IñiguezAFranco! et! al.! [43].! With! progress! of! hydrolysis,!the!MWD*peaks*broadened!and!at!some!point!changed!from!monomodal!to! ! 272! ! multimodal!distribution.!This!could!be!explained!by!the!fragments!of!PLA!chains!formed! due!to!chain!scission!resulting!in!different!molecular!weights!or!due!to!the!hydrolysisA resistant!crystalline!residues!where!the!Mn!decrease!slowed!down!during!the!hydrolysis! process![45,!47,!48].!Different!results!have!been!found!when!PLA!has!been!exposed!to! alkaline!solutions!at!37!°C!where!the!hydrolysis!of!PLA!proceeded!via!surface!erosion! mechanism! [49].! In! the! case! of! the! hydrolytic! degradation! of! PLAAJ! the! behavior! of! MWD! was! different! than! PLAAC! film! (Figure$ 6.8B)^! the! MWD* remained! at! the! initial! position! and! the! lower! molecular! weight! tail! increased! with! a! reduced! peak! area! as! hydrolysis!was!taking!place.!This!change!of!behavior!must!correspond!to!hydrolysis!of! PLAAJ! via! surface! erosion! where! a! significant! weight! loss! occurs! with! no! significant! molecular!weight!decrease![50].!After!24!h,!the!MWD!broadened!with!the!formation!of! new!peaks!that!can!be!attributed!to!chain!scission!and!accumulation!of!PLA!oligomers.! Tsuji!and!Hayashi![50]!studied!the!hydrolytic!degradation!of!linear!2Aarm!and!branched! 4Aarm!PLA!where!the!hydrolysis!proceeded!via!bulk!and!surface!erosion,!respectively.! The!authors!advocated!that!such!a!change!happened!due!to!the!higher!concentration!of! terminal!hydroxyl!groups!in!the!branched!PLA!that!can!form!hydrogen!bonding!among! them.!This!interaction!prevents!the!terminal!hydroxyl!group!from!interacting!with!water! molecules!where!the!cleavage!is!taking!place!in!the!ester!groups!around!the!terminal! groups! preventing! the! diffusion! of! water! into! the! polymer! core! to! start! hydrolysis! reactions.! To!understand!the!effect!of!the!branches!on!the!hydrolytic!degradation!of!PLAAJ,! water! diffusion! studies! were! performed! by! exposing! the! films! to! water! vapor! to! determine! the! D! as! previously! described! by! the! authors! [43,! 51].! Figure$ 6.9! shows! ! 273! ! the!%weight!change!profiles!of!PLAAC!and!PLAAJ!films!exposed!to!water!vapor!at!40!°C! and!70%!RH.!Experiments!at!80°C!were!excluded!because!of!the!equipment!limitation.! Also,!it!is!important!to!point!out!that!crystallization!could!occur!upon!immersion!at!80!°C! as!aforementioned!in!Figure$ 6.8A.!At!40!°C!and!70%!RH,!the!estimated!D!in!PLAAC! was!almost!1.5!times!larger!than!in!PLAAJ!(Table$6.2).!The!slower!diffusion!of!water!in! PLAAJ!could!be!due!to!the!presence!of!branches,!supporting!the!hydrolysis!mechanism! via!surface!erosion!where!there!is!an!interaction!of!water!molecules!with!the!terminal! hydroxyl!groups!in!the!surface!of!the!polymer.! e g n a h c i t h g e W % 0.6 0.5 0.4 0.3 0.2 0.1 0 0 PLA-C PLA-J 500 1000 1500 2000 time, s 2500 3000 3500 4000 ! Figure$ 6.9!%Weight!change!of!PLAAC!and!PLAAJ!films!exposed!to!water!vapor!at!40! °C,!70%!RH.! ! 274! ! Table$6.2!Hydrolysis!rate!constants!for!PLA!films!immersed!in!50%!ethanol!at!80!°C,!pH! 11,!and!D*of!water!vapor!at!40!°C,!70%!RH.!$ Film$ PLAAC$ PLAAJ$ k$(hT1)*# D#x10T13$(m2/s)**$# 0.206!±!0.014!a! 4.14!±!0.27!a! 0.065!±!0.008!b! 2.88!±!0.13!b! *! Fitting! of!Eq.$ 6.2.! For! PLAAJ,! this! is! considered! as! a! pseudoAfirst! order! reaction! for! practical!comparison!purpose.! **!D*values!were!calculated!using!Eq.$6.3.! Values!with!different!lower!case!letters!are!statistically!different!(α=0.05).! ! When! a! deconvolution! analysis! of! the! MWD! of! PLAAJ! (Figure$ 6.8A)! was! performed!using!a!LogNormal!function!as!previously!discussed!by!the!authors![48,!52],! two! main! different! Mn! peaks! were! identified.! At! time! 0! the! peaks! corresponded! to! different! Mn:! 12.28! x105! and! 2.35! x105! Da.! However,! for! the! estimation! of! k,! the! experimental!data!of!the!highest!Mn*peak!of!PLAAJ!sample!did!not!follow!the!first!order! equation! reaction! (Eq.! 2).! Therefore,! for! purposes! of! this! study! and! for! practical! comparison!reasons*a!pseudo!Mn!of!PLAAJ!film!was!used!to!fit!Eq.!2.!Applying!a!first! order!reaction!kinetics!for!PLAAC!samples!and!a!pseudo!first!order!reaction!for!PLAAJ! film,!different!rate!constants!were!estimated!(Table$6.2).!When!the!chain!extender!was! introduced!into!the!PLA!matrix,!the!Mn!increased!and!the!hydrolysis!decreased!around! 70%! (p<0.05).! The! branched! chain! extension! changes! the! hydrolysis! from! bulk! to! surface! erosion.! When! surface! erosion! happens! the! diffusion! of! water! molecules! is! lower!than!the!degradation!rate!of!the!polymer.!The!behavior!of!the!molecular!weight! ! 275! ! reduction!of!PLAAC!and!PLAAJ!films!can!also!be!seen!in!Figure$6.10B,!which!shows!the! Mv/Mvo!during!hydrolysis!in!50%!ethanol.!A!steady!reduction!in!Mv!of!PLAAJ!film!and!the! absence!of!an!abrupt!drop!were!observed!associated!with!a!surface!reaction!pattern!of! hydrolysis! [53].! The! reduction! in! Mv! was! only! 26.2%! after! 12! h! for! PLAAJ! while! the! reduction! for! PLAAC! was! 84.9%.! Such! difference! in! Mv*could! be! associated! with! the! hydrolysis! occurring! mainly! in! the! near! surface! region! for! PLAAJ! film,! in! which! the! backbone!cleavage!rate!is!much!higher!than!the!diffusion!of!water,!and!therefore!the! decrease!in!the!molecular!weight!is!not!as!fast!as!in!bulk!erosion.! ! ! 276! ! ! Figure$6.10!(A)!Mn!and!(B)!Mv/Mvo!as!a!function!of!time!during!hydrolytic!degradation!of! PLA!films!immersed!in!50%!ethanol!at!80!°C,!pH!11.!Lines!in!(A)!are!fitting!of!the!first! order!reaction!equation!while!lines!in!(B)!are!used!as!visual!aid.! 6.4$ Conclusions$ This!study!showed!the!effect!of!including!an!epoxyAacrylic!additive!on!PLA!films! and! its! effect! on! the! hydrolytic! degradation! of! PLA.! EpoxyAacrylic! compounds! have! being! widely! studied! as! a! chain! extender! for! the! thermal! stabilization! of! PLA! during! processing.! However,! not! much! information! has! been! provided! about! their! effect! on! PLA!hydrolytic!behavior.!PLA!processed!at!a!residence!time!of!8!min!with!1.5%!wt.!of! ! 277! ! chain!extender!was!adequate!to!allow!the!epoxy!groups!of!the!chain!extender!to!retard! the!PLA!degradation!and!to!provide!a!PLA!with!high!Mn!Rheological!and!thermal!data! revealed! that! chain! extension! led! to! the! formation! of! branched! chain! structures! improving! the! dynamic! moduli! and! decreasing! the! onset! temperature! of! PLA.! When! PLA!films!were!exposed!to!hydrolytic!degradation,!the!hydrolysis!mechanism!of!linear! PLAAC!and!branched!PLAAJ!films!were!different.!The!PLAAC!film!hydrolysis!proceeded! mainly! via! bulk! erosion! while! surface! erosion! was! taking! place! in! PLAAJ! films.! This! mechanism!was!identified!based!on!the!different!MWD*behavior!during!hydrolysis!of!the! films.! These! results! can! be! attributed! to! the! high! concentration! of! terminal! hydroxyl! groups!in!the!branched!PLA!that!could!form!hydrogen!bonds!between!each!other.!Thus,! preventing!the!diffusion!of!water!molecules!into!to!the!core!of!the!PLA!matrix!to!start!the! bulk! hydrolysis! reaction! process.! PLA! modified! with! chain! extender! can! prolong! the! service!life!of!the!PLA!products!by!decreasing!its!hydrolytic!degradation!during!storage! and!usage!and!changing!its!mechanism!of!hydrolytic!failure.!However,!further!studies! should!be!conducted!at!normal!shelf!life!conditions!to!study!the!fait!of!chain!extenders! compounds!during!the!hydrolytic!degradation!of!PLA!and!their!possible!migration!to!the! environment!ensuring!the!safety!of!the!modified!PLA!material.!! 6.5$ Acknowledgements$ IAF,! F.,! thanks! CONACYT! (The! Mexican! National! Council! for! Science! and! Technology),!SEP!(The!Mexican!Secretariat!of!Public!Education),!and!the!Government! of!Mexico!for!providing!support!funds!for!a!Ph.D.!fellowship.!R.!Auras!thanks!the!partial! support!of!USDA!and!Michigan!AgBioResearch,!Hatch!project!R.!Auras.! ! ! ! 278! ! ! $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ ! REFERENCES$ ! 279! ! REFERENCES$ $ [1]! R.! Auras,! B.! Harte,! S.! Selke,! 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[52]!E.!CastroAAguirre,!R.!Auras,!S.!Selke,!M.!Rubino,!T.!Marsh,!Impact!of!Nanoclays! on!the!Biodegradation!of!Poly!(Lactic!Acid)!Nanocomposites,!Polymers!10!(2018)!202.! [53]!T.!Kijchavengkul,!R.!Auras,!M.!Rubino,!E.!Alvarado,!J.R.C.!Montero,!J.M.!Rosales,! Atmospheric!and!soil!degradation!of!aliphatic–aromatic!polyester!films,!Polym.!Degrad.! Stab.!95!(2010)!99A107.! ! 284! ! CHAPTER$7$ Overall$Conclusion$and$Recommended$Future$Work$ 7.0$ Overall$Conclusion$ PLA!is!a!biodegradable!polymer!that!has!been!used!to!replace!the!use!of!fossilA based!polymers!for!specific!applications,!such!as!in!the!medical!field,!agriculture!and! food! packaging! industries! [1].! PLA! during! its! life! cycle! can! be! exposed! to! various! solvents! such! as! water! and! alcohol,! leading! to! hydrolytic! degradation! and! structural! changes![2,!3].!The!hydrolytic!degradation!of!PLA!has!widely!been!studied.!However,! the!synergetic!effect!of!different!waterAethanol!solutions!is!not!well!understood!to!control! and!develop!its!applications!and!tailor!its!uses.!! First,!the!parameters!and!factors!affecting!the!hydrolysis!of!PLA!in!waterAethanol! solutions! were! studied! (Chapter! 3).! Experimental! studies! of! the! SIC! and! hydrolytic! degradation!of!PLA!films!were!performed!at!40!°C!in!contact!with!water,!50%!ethanol,! and!95%!ethanol.!This!study!found!that!PLA!films!in!contact!with!50%!ethanol!showed! faster! degradation! than! in! water! and! 95%! ethanol.! This! result! was! attributed! to! the! competitive!balance!between!the!swelling!when!ethanol!was!present,!and!the!hydrolysis! due! to! the! sorption! of! water! molecules! to! start! the! degradation! reactions.! During! the! study,! it! was! confirmed! that! the! water! sorption! was! higher! during! hydrolysis! of! PLA! exposed!to!50%!ethanol!solution.!The!faster!hydrolysis!in!50%!ethanol!was!confirmed! by! LA! release.! Furthermore,! when! PLA! was! exposed! to! waterAethanol! solutions,! the! polymer!experienced!SIC!where!the!crystallinity!increased!dramatically!in!the!first!stage! due! to! the! primary! crystallization! where! a! large! number! of! crystals! were! formed,! and! ! 285! ! then! the! system! experienced! saturation! where! the! crystallization! rate! was! slow,! corresponding!to!secondary!crystallization.!At!the!last!stage,!the!crystallinity!started!to! increase!attributed!to!the!hydrolysis!of!amorphous!regions!being!higher!when!PLA!was! exposed!to!50%!ethanol,!which!was!in!accordance!to!the!faster!hydrolytic!degradation! of!PLA.! Depending!on!the!application,!the!hydrolytic!degradation!of!PLA!can!be!seen!as! an!advantage!or!a!disadvantage.!PLA!hydrolysis!can!be!used!for!chemical!recycling!to! recover!PLA!back!to!its!monomer!for!new!PLA!production![4,!5].!After!demonstrating! that!the!hydrolytic!degradation!of!PLA!in!waterAethanol!solutions!was!faster!when!PLA! was!exposed!to!50%!ethanol,!a!method!to!safely!perform!the!chemical!recycling!of!PLA! using! a! “green”! solvent! was! developed! (Chapter! 4).! For! that,! it! was! necessary! to! estimate! the! kinetic! parameters,! rate! constant! and! activation! energy! driving! PLA’s! hydrolytic!degradation.!The!hydrolysis!of!PLA!was!carried!out!above!Tg!in!50%!ethanol! and!water.!The!hydrolysis!in!these!two!systems!was!through!a!bulk!erosion!mechanism! proven!by!the!analysis!of!the!MWD!with!a!multimodal!distribution!at!the!latter!stages!of! the! hydrolysis.! The! analysis! of! the! deconvolution! showed! multiple! reaction! pathways! that! followed! the! first! order! reaction! kinetics.! The! Ea! was! estimated! using! a! reparameterization!of!the!Arrhenius!equation!to!obtain!a!better!estimation!with!near!zero! correlation!among!the!parameters.!The!Ea!values!obtained!were!within!the!same!range! of!values!reported!in!literature!when!PLA!has!been!exposed!to!different!media,!such!as! alkaline!media,!water!or!acidic!media.!The!results!indicated!that!50%!ethanol!could!be! used!as!a!potential!solution!alternative!to!perform!the!chemical!recycling!of!PLA.! PLA! has! some! limitations! such! as! poor! toughness! and! impact! strength! [6].! ! 286! ! Nanoparticles!have!been!incorporated!into!the!PLA!matrix!to!enhance!its!mechanical,! thermal! and! barrier! properties! [7].! PLAAnanocomposites! during! their! life! cycle! can! be! exposed! to! various! solvents! such! as! water! and! alcohol.! Therefore,! there! is! a! critical! need!to!understand!the!effect!of!the!organoAclay!filler!on!the!hydrolytic!degradation!of! PLAAnanocomposites! and! the! potential! release! of! the! nanoparticles! from! different! environments!representing!prolonged!exposures.!Chapter!5!showed!the!results!of!the! hydrolytic!degradation!of!PLAAnanocomposites!by!water,!50!and!95%!ethanol!solutions! at!40!°C.!PLAAnanocomposite!film!exposed!to!50%!ethanol!solution!had!a!faster!rate!of! degradation! and! higher! release! of! LA! than! in! 95%! ethanol! and! water! due! to! the! synergetic!effect!of!the!swelling!of!the!matrix!and!water!diffusion.!No!difference!in!the! rate!of!degradation!was!found!with!OMMT!incorporation!into!the!PLA!films.!This!was! explained! by! the! similar! water! vapor! sorption! in! neat! PLA! and! PLAAOMMT! films.! However,! the! effect! of! the! nanoparticles! in! PLA! affected! the! sorption! of! ethanol! as! almost! the! double! amount! of! ethanol! sorption! occurred! into! the! clay! galleries! without! expansion!of!the!PLA!matrix.!The!presence!of!nanoparticles!restricted!the!movement!of! the! PLA! chains! showing! similar! behavior! in! the! crystallization! process! to! neat! PLA.! During!the!hydrolytic!degradation!of!PLAAnanocomposite,!the!release!of!surfactant!was! detected,! being! higher! in! the! 50%! ethanol! system.! The! release! of! nanoclay! was! measured!by!quantifying!Al!in!the!50%!ethanol!solution!corresponding!to!304.9!ppb!of! nanoclay!released!after!90!d!of!exposure,!which!was!0.58%!of!the!initial!clay!added!into! PLA.!! PLA!is!susceptible!to!thermal!degradation!during!melt!processing![8].!Addition!of! chain! extenders! can! ameliorate! PLA’s! thermal! degradation! during! processing! [9A12].! ! 287! ! However,!studies!evaluating!the!effect!of!chain!extenders!on!the!hydrolytic!degradation! of! PLA! are! limited.! Chapter! 6! studied! the! effect! of! epoxyAacrylic! additives! (Joncryl®! 1.5%!wt.)!used!as!a!chain!extender!in!the!hydrolytic!degradation!of!PLA.!The!hydrolysis! was! performed! in! accelerated! condition,! 50%! ethanol! at! 80! °C! and! pH! 11.! The! incorporation! of! the! chain! extender! increased! the! molecular! weight! after! processing! indicating!that!the!carboxylic!end!groups!of!PLA,!as!a!product!of!the!degradation,!were! reconnected!by!the!epoxy!groups!of!the!chain!extender.!Results!revealed!that!the!chain! extension! led! to! the! formation! of! a! branched! structure.! The! change! of! the! molecular! architecture! affected! the! hydrolysis! mechanism.! The! hydrolytic! degradation! was! delayed! with! the! presence! of! chain! extender! showing! a! surface! erosion! mechanism! while! the! linear! neat! PLA! presented! bulk! erosion.! The! surface! erosion! process! was! attributed!to!the!lower!diffusion!of!water!molecules!into!the!core!of!the!polymer!due!to! the! presence! of! PLA! branches.! The! branches! increased! the! number! of! terminal! hydroxyl!groups!that!can!interact!with!each!other,!preventing!the!interaction!with!water! molecules,! and! therefore! the! diffusion! into! the! bulk! to! start! hydrolysis! reactions.! PLA! modified!by!chain!extender!can!prolong!the!service!life!of!the!material!by!delaying!the! hydrolysis!reactions!during!storage!and!use.! 7.1$ Recommended$Future$Work$ $ Future!work!should!focus!on!the!chemical!recycling!of!PLA!using!50%!ethanol! solution! to! optimize! the! commercial! and! industrial! process.! To! successfully! commercialize! this! chemical! recycling! process^! the! yield! of! LAAoligomers! during! the! hydrolysis!reactions!should!be!fully!understood.!Efficient!depolymerization!of!PLA!would! be! around! 95%! yield! of! LAAoligomers! in! a! short! period.! However,! these! studies! are! ! 288! ! missing!along!with!the!investigation!of!induced!crystallization!during!hydrolysis.!Crystal! residuals! present! or! formed! during! the! hydrolysis! can! delay! the! recover! process.! To! have!a!better!overview!of!the!recovery!process,!the!separation!of!phases,!waterAethanol,! along! with! the! LAAoligomers! from! the! solution! should! be! investigated! to! consider! the! costAbenefit!of!the!process!using!a!"green"!solvent!during!the!chemical!recycling!of!PLA.!! ! Besides! the! layered! silicateAbased! nanoparticle! studied! in! this! dissertation,! different! nanoparticles! have! been! added! into! PLA! to! improve! its! properties! such! as! sepiolite!and!halloysite!nanotubes![13].!The!study!of!the!effect!of!these!nanoparticles!on! the! hydrolytic! degradation! of! PLA! by! waterAethanol! solutions! is! recommended! to! be! investigated! to! understand! the! different! PLAAnanoparticles! interactions! for! possible! applications!of!PLAAnanocomposites.!The!transport!of!waterAethanol!solutions!into!the! different!PLAAnanocomposites!can!differ!where!the!release!of!nanoparticles!could!occur! changing! the! hydrolysis! behavior! and! therefore! the! durability! and! performance! of! the! material.! These! changes! may! influence! the! release! of! the! nanoparticles,!but! also! the! release! could! be! affected! by! the! hydrolytic! degradation! of! PLAAnanocomposites! by! waterAethanol!solutions.! This! dissertation! has! also! left! for! future! work! a! better! understanding! of! the! possible!fate!of!the!chain!extender!during!the!hydrolytic!degradation!of!modified!PLA.! Migration! studies! of! the! chain! extenders! should! be! performed! looking! for! a! possible! diffusion! of! epoxyAacrylic! chain! extenders! from! PLA! that! can! reach! different! environments! encountered! for! consumer! and! nonAconsumer! goods.! 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