.1" v 10212:“: 3,- I‘VE. 3?, 5&3“. ‘3 ’1‘ .:‘13"!---‘ . I. u '4 {3851-321 " v sir t. .r_ . ,3 .' WI ._ .th :11} 2;} v 91*”? I 4 n , . h ,h.p",o“f ‘s‘ESLE‘GlJ ~ “ 95W: 4 r h. ' fl}, . . 4 3, 13‘ 7'3““ ”*5: .5) a? ..‘_ :;$l,{iy‘35,| i W . :3}: f4. ‘ 3 » 3 “q (:1. F |,..‘ 9:“ V ,.‘ U 5.1-). :Amntml !3{3.‘5:"“H‘ I WW5 7 i ~" A. - xx 4»;. '« J..- /»<7 (q f. 7 / MODIFICATION OF POLYLACTIDE VIA REACTIVE EXTRUSION ' by Denise Lynn Carlson u A THESIS Submitted to Michigan State University in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE Department of Chemical Engineering 1995 ABSTRACT MODIFICATION OF POLYLACTIDE VIA REACTIVE EXTRUSION by Denise Lynn Carlson The branching of polylactide (PLA) by a free radical initiated reactive extrusion process has been accomplished. The addition of between 0.066 and 0.67 % maleic anhydride (MA) onto the PLA backbone was also performed to enhance the interfacial adhesion in PLA blends. Reaction conditions were varied from 160° to 200°C with initiator concentrations between 0.0 and 0.5 %. Characterization was performed using triple detection size exclusion chromatography, melt flow index, and various thermal analysis techniques. A decrease in both molecular weight and melt viscosity indicated that PLA without initiator had extensive thermal degradation. The optimum range for branching, indicated by a high molecular weight and low melt flow index polylactide, was found to be around 170°C to 180°C and 0.1 to 0.25 % initiator. Laser scanning confocal microscopy was evaluated for potential application in assessing polymer blends. To my husband and my parents... ACKNOWLEDGMENTS I wish to thank Dr. Li Nie, Dr. Philippe DuBois, and Dr. Ramani Narayan for their input in this research. I also wish to acknowledge Dr. Joanne Whallon for her assistance on the Laser Scanning Confocal Microscope and the Biomaterials group at the Michigan Biotechnology Institute for their help and suggestions. Material for this work was provided by Cargill Central Research, Minneapolis, MN. TABLE OF CONTENTS Chapter Page Number List of Tables ............................................................................................... viii List of Figures ................................................................................................. x Nomenclature ............................................................................................... xiv 1. Introduction 1.1 Motivation ............................................................................................... l 1.2 Structure of Thesis ................................................................................... 4 2. Background and Literature Review 2.1 Terminology ............................................................................................. 6 2.1.1 Molecular Weights ..................................................................... 6 2.1.2 Chain Scission ............................................................................ 7 2.1.3 Branching and Crosslinking ........................................................ 8 2.1.4 Gelation ...................................................................................... 8 2.1.5 Grafting ...................................................................................... 8 2.1.6 Reactive Extrusion ..................................................................... 9 2.2 Molecular Weight Determination ........................................................... 11 2.2.1 End Group Analysis ................................................................. 11 2.2.2 Colligative Properties ............................................................... 11 2.2.3 Light Scattering ........................................................................ 12 2.2.4 Intrinsic Viscosity ..................................................................... 13 2.2.6 Gel Permeation Chromatography .............................................. 17 2.3 Free Radical Polymerization .................................................................. 17 2.3.1 General Mechanism of FRP ....................................................... 17 2.3.2 Reactive Extrusion of FRP ........................................................ 19 2.3.3 Maleation .................................................................................. 21 3. Materials 3.1 Polylactide ............................................................................................. 24 3.1.1 Commercial Preparation ........................................................... 24 3.1.2 Applications ............................................................................. 26 5. 7. 3.2 Lupersol 101 .......................................................................................... 28 3.3 Maleic Anhydride .................................................................................. 29 Processing and Characterization 4.1 Processing .............................................................................................. 30 4.1.1 Reactive Extrusion .................................................................... 30 4.1.2 Extrusion Conditions ................................................................. 31 4.2 Characterization ...................................................................................... 36 4.2.1 Gel Permeation Chromatography ............................................... 36 4.2.2 Intrinsic Viscosity ...................................................................... 38 4.2.3 Melt Flow Index ........................................................................ 39 4.2.4 Differential Scanning Calorimetry ............................................. 39 4.2.5 Thermal Gravimetric Analysis ................................................... 39 4.2.6 Dynamic Mechanical Analysis .................................................. 40 4.2.7 Titration .................................................................................... 41 4.2.8 Scanning Electron Microscopy ................................................. 44 4.2.9 Extraction ................................................................................. 46 4.2.10 Moisture Analysis ................................................................... 46 Free Radical Branching of PLA 5.1 Discussion of Results ............................................................................. 47 5.1.1 Effect of Extrusion Temperature ............................................... 47 5.1.2 Effect of Initiator Concentration ............................................... 48 5.1.3 Thermogravimetric Analysis ..................................................... 57 5.1.4 Differential Scanning Calorimetry ............................................ 64 5.1.5 Dynamic Mechanical Analysis ................................................. 66 5.1.6 Highly Branched Samples ......................................................... 70 5.1.7 Film Results ............................................................................. 75 5.2 Proposed Reaction Mechanism ............................................................... 76 Maleation of PLA 6.1 Discussion of Results ............................................................................. 81 6.1.1 Effect of Extrusion Temperature ............................................... 81 6.1.2 Effect of Initiator Concentration ............................................... 82 6.1.3 Thermogravimetric Analysis ..................................................... 90 6.2 Proposed Reaction Mechanism ............................................................... 91 PLA Blends 7.1 Blend Theory .......................................................................................... 95 7.1.1 Miscibility ................................................................................ 95 7.1.2 Compatibility ............................................................................ 97 7.2 Materials ................................................................................................ 97 7.2.1 Cellulose Acetate and its Derivatives ........................................ 97 7.2.2 Polypropylene .......................................................................... 98 7.2.3 Poly(vinyl acetate) .................................................................... 98 7.2.4 Ethylene vinyl acetate copolymers ............................................ 99 7.3 Equipment and Procedures ................................................................... 100 7.3.1 Solution Casting ..................................................................... 100 7.3.2 Haake Mixer ........................................................................... 100 7.4 Discussion of Results ........................................................................... 101 7.4.1 Blends of PLA with CA, CAP, CAB ...................................... 101 7.4.2 Blends of PLA with PP ........................................................... 102 7.4.3 Blends of PLA with PVA ....................................................... 102 7.4.4 Blends of PLA with EVAC Copolymers ................................. 104 7.4.5 Blends of PLA with Starch ..................................................... 104 8. Related Work 8.1 LSCM Background .............................................................................. 107 8.2 Starch Matrix with Protein Filler .......................................................... 112 8.3 Extruded Modified Starch with Talc Filler ........................................... 118 9. Conclusions and Recommendations 9.1 Conclusions ......................................................................................... 122 9.2 Recommendations ................................................................................ 125 Appendix -- Raw Data ...................................................................................... 128 Bibliography .................................................................................................... 140 vii 1.1 4.1 4.2 5.1 5.2 5.3 5.4 5.5 5.6 6.1 6.2 7.1 7.2 LIST OF TABLES Page Number Relationship of various polymer properties to molecular weight (MW) and molecular weight distribution (MWD). ...................................................... 2 Temperature settings for free radical branching. ...................................... 35 Temperature settings for maleation. ......................................................... 35 Free radical branching of PLA: MP load%, melt viscosity, and TriSEC. .51 Decomposition temperatures from thermogravimetric analysis. ............... 58 DSC results: 10°C/min to 200°C. ............................................................. 64 To averages for dynamic mechanical analysis. ......................................... 66 Highly brached sample comparison to unextruded PLA. .......................... 72 Tensile Results for PLA film. .................................................................. 75 Maleation of PLA: MP load%, melt viscosity, TriSEC, and % maleation. ................................................................................................ 85 Maleation decomposition temperatures from thermogravimetric analysis. ................................................................................................... 90 ELVAX properties. .................................................................................. 99 Glass transition temperatures for PLA/PVA blends. ............................... 103 viii A.l A2 A3 A4 A.5 Branched data from TriSEC (triple detector size exclusion chromatography). ................................................................................... 129 Ubbelhode viscometry experiments. ...................................................... 134 Polylactide film results. ......................................................................... 135 Maleation data from TriSEC. ................................................................. 136 Titration results of maleated samples. .................................................... 139 2.1 2.2 3.1 3.2 3.3 4.1 4.2 4.3 4.4 4.5 5.1 5.2 LIST OF FIGURES Page Number Flow of dilute polymer solution in a capillary. ......................................... 14 Schematic of Ubbelhode viscometer. ....................................................... 16 Commercial preparation of polylactide. ................................................... 25 Lupersol 101 [2,5-dimethyl-2,5-di-(t-butylperoxide)]. ............................. 28 Maleic anhydride (MA), [C4HZO3]. .......................................................... 29 Baker Perkins twin screw extruder (Composite Materials and Structures Center, MSU). ......................................................................................... 33 Co-rotating twin screw configuration. ...................................................... 34 TriSEC flow loop. ................................................................................... 37 Orion 960 AutochemistryI system. ............................................................ 43 SEM image formation. ............................................................................. 45 DV chromatograph showing the shift in molecular weight distribution of unextruded (higher M.W.) and extruded (lower M.W.) PLA. (normalized to concentration) ...................................................................................... 52 DV chromatograph showing the shift in molecular weight distribution of PLA extruded at 160°C with increasing L101 content. (normalized to concentration) .......................................................................................... 53 5.3 DV chromatograph showing the shift in molecular weight distribution of PLA extruded at 170°C with increasing L101 content. (normalized to concentration) .......................................................................................... 54 5.4 DV chromatograph showing the shift in molecular weight distribution of PLA extruded at 180°C with increasing L101 content. (normalized to concentration) .......................................................................................... 55 5.5 MP I of extruded samples showing a decrease in MFI with increasing L101 content. .................................................................................................... 56 5.6 TGA showing decomposition temperature for branched PLA with 0.1% L101. .............................................................................................. 59 5.7 TGA for pure PLA compared to extruded PLA with 0.5% L101. ............. 60 5.8 TGA for free radical branched PLA at 170°C. .......................................... 61 5.9 TGA for free radical branched PLA at 200°C. .......................................... 62 5.10 Compilation of DSC results for To comparison. ....................................... 65 5.11 Storage modulus for several samples as given by DMA. .......................... 67 5.12 Loss modulus for several samples as given by DMA. .............................. 68 5.13 Tan delta (tan 5) for several samples as given by DMA. .......................... 69 5.14 Mark-Houwink plot of the 170°C series. .................................................. 73 5.15 A comparison of light scattering chromatographs of PLA (top) and 190°/0.5 (bottom). .................................................................................................. 74 5.16 Peroxide decomposition. .......................................................................... 77 5.17 Proposed branching mechanism. .............................................................. 78 5.18 Chain scission reactions. .......................................................................... 79 6.1 Weight percent maleation as a function of initiator concentration. ........... 86 6.2 6.3 6.4 6.5 7.1 7.2 7.3 7.4 7.5 7.6 8.1 8.2 8.3 8.4 8.5 8.6 DV chromatograph of 180°C series showing molecular weight distribution. (normalized to concentration) .................................................................. 87 DV chromatograph of 200°C series showing molecular weight distribution. (normalized to concentration) .................................................................. 88 MFI of maleated samples. ........................................................................ 89 Proposed mechanism for the maleation of PLA ......................................... 93 Comparison of PP and PLA. .................................................................... 98 Poly(vinyl acetate). .................................................................................. 98 SEM micrograph of 70% CAP and 30% PLA showing immiscibility. 102 SEM micrograph of 60% PLA and 40% starch blend showing poor interfacial adhesion. ............................................................................... 105 SEM micrograph of 70% PLA and 30% starch blend with 0.5% L101 showing partial interfacial adhesion. ...................................................... 106 SEM micrograph of 70% maleated PLA and 30% starch showing good interfacial adhesion. ............................................................................... 106 LSCM image production. ....................................................................... 110 SEM micrograph of potato showing cell wall and starch granules. ......... 113 LSCM photograph of densified potato showing zein penetration. .......... 115 LSCM photograph of densified potato showing diffusion of zein along crack. .................................................................................................... 116 LSCM photograph of densified potato (inner cross-section). .................. 117 SEM X-ray peaks for the modified starch with talc blend. ..................... 120 xii 8.7 SEM micrograph showing the surface of the modified starch with talc blend. .................................................................................................... 121 8.8 SEM dot map of the X-ray analysis for Silicon. ..................................... 121 xiii NOMENCLATURE CA .................................. cellulose acetate CAB ................................ cellulose acetate butyrate CAP ................................ cellulose acetate propionate CHCl; ............................. chloroform CHzClz ............................ methylene chloride DMA ............................... dynamic mechanical analysis DSC ................................ differential scanning calorimetry (or calorimeter) EVAC ............................. ethylene vinyl acetate copolymers FDA ................................ food and drug administration FRP ................................. free radical polymerization GPC ................................ gel permeation chromatography (or chromatograph) HCl ................................. hydrogen chloride L101 ................................ lupersol 101 LSCM .............................. laser scanning confocal microscope (or microscopy) MA .................................. maleic anhydride MeOH ............................. methanol MFI ................................. melt flow index NaOH .............................. sodium hydroxide PE ................................... polyethylene PLA ................................. polylactide (or polylactic acid) PP ................................... polypropylene PS .................................... polystyrene PVA ................................ polyvinyl acetate RALLS ............................. right angle laser light scattering SEC ................................. size exclusion chromatography (or chromatograph) SEM ................................ scanning electron microscopy (or microscope) TGA ................................ thermogravimetric analysis THF ................................ tetrahydrofuran TriSEC ............................. triple detector size exclusion chromatography xiv a ....................................... Mark-Houwink parameter A; ..................................... second virial coefficient C,c ................................... concentration dn/dc ................................ refractive index increment AGm ................................. free energy of mixing AH“1 ................................. enthalpy of mixing AP ................................... pressure change ASm ................................. entropy of mixing g ...................................... gravity G’ .................................... loss modulus G” ................................... storage modulus IO ...................................... incident intensity lo ,0." ................................. scattering intensity for solution 19 so“, .................................. scattering intensity for solvent K ...................................... optical constant or Mark-Houwink parameter L ..................................... length of capillary Mi .................................... molecular weight of polymer type “i” M“ ................................... number average molecular weight Mmk ................................ peak molecular weight M" ................................... viscosity average molecular weight Mw ................................... weight average molecular weight Na ..................................... Avagadro’s number NRC. ................................. normality of hydrogen chloride Ni .................................... number of molecules of polymer type “1” n0 ...................................... refractive index of solvent 11 ....................................... refractive index of solution Nmor ................................. normality of morpholine P(0) .................................. particle scattering function Q ..................................... volumetric flow rate R ..................................... gas constant Rg ..................................... radius of gyration Ra ..................................... Rayleigh ratio r ...................................... distance rc ...................................... radius of capillary t, to .................................. time and initial time T ..................................... temperature To .................................... glass transition temperature tan 6 ................................ ratio of storage to loss moduli V ..................................... volume VH0. ................................. volume of hydrogen chloride V_mar ................................. volume of morpholine V; .................................. molar volume of polymer type “i” XV v .................................... molar volume v ....................................... scattered volume Wamplc ............................. weight of sample or ..................................... viscometer constant 1 ...................................... polymer-polymer interaction parameter 4); ...................................... volume fraction of polymer type “i” L, ..................................... incident beam wavelength 1] ..................................... solution viscosity no .................................... solvent viscosity 11,6. ................................... relative viscosity 11,9 ................................... specific viscosity [1]] ................................... intrinsic viscosity 9 ....................................... scattering angle p ...................................... density xvi C haptcr I INTRODUCTION 1.1 Motivation Polylactide (PLA) is an important biodegradable polymer which has been used in such established applications as medical implants [Gilding (1982)], sutures [Conn et al. (1974), Schmitt et al. (1967)], and drug delivery systems [Heller (1985)]. As the need for biodegradable polymers in the context of designing materials for the environment opens up new market opportunities [Narayan (1992)], polylactide polymers are finding commercial use in single-use disposal items. However, one of the limitations for using PLA is its processing instability. Gogolewski (1993) has shown that the degradation of PLA already occurs at 160°C under injection molding. Another shortcoming of PLA is its very low melt viscosity which may limit its blow molding processibility. The free radical branching of PLA could offer the opportunity for enhancement of physical and chemical properties and/or improvements of processibility by increasing the molecular weight in order to compensate for the molecular weight decrease by processing degradation and by increasing the melt viscosity. Table 1.1 shows how various properties are affected by the molecular weight and molecular weight distribution of a polymer. 2 Table 1.1: Relationship of various polymer properties to molecular weight (MW) and molecular weight distribution (MW D). Key: + property increases, - property decreases, * little change. [Yau et al. (1979)]. Property Increase Narrow MW MWD Tensile Strength + + Elongation + - Yield Strength + - Toughness + + Brittleness + - Hardness + - Abrasion Resistance + + Softening Temperature + + Melt Viscosity + + Adhesion - - Chemical Resistance + + Solubility - * The proposed free radical process is very simple and easy to manage by reactive extrusion in the presence of trace amounts of free radical initiators. Free radical polymerization via reactive extrusion has been done extensively on polypropylene and polyethylene systems leading to controlled degradation [Suwanda et al. (1989)] and branching [Suwanda et al. (1988a)], respectively. Combining PLA with natural materials and synthetic polymers provides ways of cost reduction and combined properties. Unfortunately, simple PLA composites with natural materials and polyblends have poor properties because of the lack of 3 interfacial adhesion. Introducing new functional groups onto the polylactide backbone paves the way to prepare composites, laminates, coated items, and blends/alloys with improved properties and cost effectiveness. Functionalizing the matrix polymer and the fiber/filler with highly reactive groups is perhaps the most successful strategy leading to a variety of commercial composites and alloys made by reactive processing. In this study, the addition of maleic anhydride (MA) to the PLA polymer backbone has been accomplished. The purpose of this research is to investigate the results of the free radical initiated branching of PLA extruded at temperatures ranging from 160°C to 200°C with an initiator concentration between 0.0 and 0.5 percent. Free radical initiated maleation of PLA was also done using 2 percent MA with similar temperature and concentration ranges. The modified PLA samples were characterized by several analytical methods including gel permeation chromatography (GPC), right angle laser light scattering (RALLS), melt flow index (MFI), and thermal gravimetric analysis (TGA). Based on the analytical results, the chemical modification may then be characterized as chain scission, branching, crosslinking or any combination of the three. A proposed reaction mechanism is also included. 1.2 Structure of Thesis The basic concepts used in this work are not novel, but together they comprise a novel way of processing polylactide including branching and maleation. The first part of this thesis details some of the techniques which were incorporated. Chapter 2 provides background information on polymers in general, including molecular weight analysis which is extremely important for this research. A literature review on polymers which have been processed by free radical polymerization via reactive extrusion is provided as well. Previous applications include polyethylene and polypropylene. A brief description of the materials which were used is located in Chapter 3. Chapter 4 details the processing and characterization methods which were used. Specific equipment information as well as sample preparation can also be found. The heart of the work is contained in Chapter 5, the free radical branching of PLA, and Chapter 6, the maleation of PLA. These chapters discuss the results of all pertinent analytical tests. A proposed reaction mechanism is also provided. Since polylactide resin by itself may be quite expensive for commercial use, PLA blends were also formulated. Chapter 7 briefly describes blend theory and blending 5 methods. SEM micrographs of several blends are shown with a discussion of these preliminary results. Related work (Chapter 8) was done on the applications of Laser Scanning Confocal Microscopy (LSCM) for use in polymer blend systems. Traditionally, Scanning Electron Microscopy (SEM) is used to evaluate polymer morphology, but sample preparation may sometimes create artifacts in the sample. However, LSCM provides a noninvasive technique for observing polymer morphology as sample preparation is minimum. Chapter 9 contains all pertinent conclusions, as well as several recommendations for further work. ( 7haptcr 2 BACKGROUND AND LITERATURE REVIEW 2.1 Terminology 2.1.1 Molecular Weights In general, a polymer is a heterogeneous material with a wide range of molecular weights. This molecular weight distribution can be characterized by the polydispersity of the polymer. Polydispersity is defined as the ratio Mw/Mn. A wider distribution of molecular weights gives a larger polydispersity since the contribution of each molecule to the number average molecular weight, Mn, is proportional to its mass (Equation 2.1), and its contribution to the weight average molecular weight, MW, is proportional to the square of its mass (Equation 2.2). ism. 1:] Mn = m (2.1) M = -='———- (2.2) 7 :NM““"’ M = -—‘——— (2.3) .. ism, r-—l where N is the number of molecules of type i and M, is the molecular weight of molecule type i. The viscosity average molecular weight, M..., is also given (Equation 2.3), where “a” is a property of the polymer-solvent system with a value typically between 0.5 and 0.8 [Sperling]. 2.1.2 Chain Scission Polymer chain degradation, or chain scission, usually occurs when chemical bonds along the polymer backbone break. This degradation causes a reduction in the molecular weight of the polymer which results in an increase in the melt flow index. The molecular weight distribution becomes more random with a polydispersity approaching two. Since polymers with a high molecular weight have a greater number of bonds, they experience preferential chain scission. Therefore, for broad molecular weight distributions, as the molecular weight decreases, the molecular weight distribution narrows. 2.1.3 Branching and C rosslinking Free radical branching of a polymer occurs when two radical centers on the polymer backbone terminate by combination. This long chain branching process can continue until a three-dimensional network is formed. The polymer is then said to be crosslinked consisting of various levels of sol (“free” polymer) and gel (networked polymer). Long chain branching produces a high molecular weight polymer which has an increased melt viscosity. It is generally undesirable to form a crosslinked polymer via reactive extrusion as the crosslinked gel may damage the extruder. 2.1.4 Gelation The gelation of a polymer is undesirable for processing. At the gel point various phenomena occur: (1) the viscosity diverges to infinity, i.e. there is a transition from a viscous liquid to an elastic solid, (2) the weight average molecular weight diverges to infinity, and (3) an insoluble gel phase appears. 2.1.5 Grafting The grafting of polymer chains is used to enhance the properties of polymer blends. Grafting occurs when the polymer, peroxide (used as the free radical 9 initiator), and the grafting compound are processed in an extruder. In this study, maleic anhydride (MA) has been grafted to the polymer backbone. This process is commonly referred to as maleation. The addition of MA to the polymer enhances the compatibility and interfacial adhesion of various polymer blends. This is discussed in some detail in Section 2.2.3, as well as Chapter 6. 2.1.6 Reactive Extrusion Reactive extrusion refers to an extrusion process whereby the extruder is used as a chemical reactor [Brown and Orlando (1988)]. In reactive extrusion, the extruder may be considered a continuous flow reactor in which the absence of a solvent medium provides an advantage over other reactive processes. Another advantage of a reactive extruder is that several chemical process operations, such as mixing, reacting, and shaping of a material, are combined into one piece of equipment. Other advantages of reactive extrusion over conventional polymerization techniques include: (l)‘carefully controllable residence time distributions and temperature profiles; (2) the production of variable size batches with very short start-up and change over times; and (3) the extruders ability to easily process high viscosity materials [Pabedinskas et a1. (1989)]. Since very viscous materials may be reacted or produced, lower reaction temperatures may be used and a higher degree of branching may be accomplished. 10 Several types of chemical reactions may be performed by reactive extrusion [Brown and Orlando (1989)]. Bulk polymerization reactions used to prepare high molecular weight polymer from monomer or low molecular weight polymer. Graft reactions resulting in a graft copolymer of a polymer and monomer feed. Inter-chain formations of two or more polymers forming a copolymer. Coupling reactions of a homopolymer plus a branching agent to increase the molecular weight by chain extension or branching. Functionalization reactions in which functional groups are introduced to the polymer backbone. Controlled molecular weight degradation in which high molecular weight polymers are reduced to lower molecular weights. Three types of degradation may occur [Rauwendaal ( 1986)]: 0 Thermal degradation: depolymerization, random chain scission, and unzipping of substituent groups. 0 Mechanical degradation: shear and/or elongational stress. 0 Chemical degradation: such as hydrolysis or oxidation. Conventional extruders commercially available include single-screw or twin- screw. Twin-screw extruders may be intermeshing or non-intermeshing, co- ll rotating or counter-rotating [Rauwendaal (1986)]. The extruder which was used for this research was a co-rotating, intermeshing, twin-screw extruder. 2.2 Molecular Weight Determination Polymer molecular weights may be determined by several experimental methods. A brief description of some of these approaches follows. 2.2.] End Group Analysis The polymer is dissolved into a solvent and titrated for functional groups. This technique is very sensitive to impurities and is only good for low molecular weights (<5000 g/mol). 2.2.2 Colligative Properties A number of colligative properties can be measured and a corresponding molecular weight, in this case M", can be calculated. A dilute polymer solution ($0.1 wt%) is used for the following techniques: (1) boiling point elevation, (2) freezing point depression, (3) vapor pressure lowering, and (4) osmotic pressure. l2 2.2.3 Light Scattering Light scattering is a technique used for determining the weight average molecular weight. Light interacts with a molecule and is scattered. This scattered light is referred to as Rayleigh scattering and has the same wavelengh of that of the incident light beam. The information about the size and molecular weight of a polymer is experimentally determined from the light scattering intensity which is above that of the solvent background. This excess light intensity caused by the polymer molecules in solution is directly related to the MW of the polymer and the sample concentration (C). K—C-=——1——+2A.C (2.4) R9 M... P(9) ‘ The K term in equation 2.4 is an optical constant 27r2n2 dn 2 K..._ ”H 2.5 it: Na dc ( ) where n is the refractive index of the medium, K0 is the wavelength of the incident beam, N8 is Avagadro’s number, and dn/dc is the refractive index increment. The excess Rayleigh ratio, R; , gives the normalized scattering intensity with respect to the scattered volume, distance, and incident intensity, 10: R. = (1...... —I.....)*r3/I. v (2.6) The A; term is the second virial coefficient which will be set equal to zero for this study. The term P(0) in equation 2.4 is the particle scattering function. P(0) is a 13 function of the geometry and size of the polymer molecules with respect to the wavelength of the incident light. For random coil polymers, P(0) is the following: 2 -x P(9) = —.[e —(1-X)] (2.7) X- where 8 7m . 2 X=3*[—x—*Rg*sm9/2) (2.8) and RE is the radius of gyration. 2.2.4 Intrinsic Viscosity Intrinsic viscosity measurements are done in a capillary tube with a dilute solution and result in the viscosity average molecular weight (Mv). The flow rate, and hence the shear rate, through the capillary is dependent upon the distance from the capillary edge. In dilute solutions, the polymer coils are expanded and thus different shear rates are felt by the polymer resulting in an increase in frictional drag and rotational forces on the molecule (Figure 2.1). This dynamic work results in an increased solution viscosity. Figure 2.1: Flow of dilute polymer solution in a capillary. The solvent viscosity, no, and the solution viscosity, n, are both measured in the Ubbelhode capillary viscometer (Figure 2.2). The flow through the capillary controls the time for the bulb to drain. The time for the bulb to drain can be related to the viscosity of the solution using Hagen-Poisulle’s law for laminar flow: Q_ m,“ AP _ dV 8nL (it Where AP=lpg Substituting AP into Hagen-Poisulle’s law and integrating results in Where or becomes an apparatus constant equal to the following: 4 -l 0,:an g*(I§y_) 8L 1 15 The relative viscosity is the ratio of the solution and solvent viscosities: nrcl : n/nO : t/tO The specific viscosity is the relative viscosity minus one: rlsp : nrel " l The intrinsic viscosity of a solution is defined either as [n] = [1:71] C C10 01' as [77] : [ln(nrel)] C c:0 A plot of intrinsic viscosity versus concentration of both relationships should result in an extrapolation to the same point at zero concentration. Also, the sum of the slopes of the two lines is related by the Huggins equation, nsp c = [n] + k'lnlzc and the Kraemer equation, War-wire Algebraically, k + k"= 0.5 Scribe Marks Figure 2.2: Schematic of Ubbelhode viscometer. Practical Considerations To effectively use a viscometer for intrinsic viscosity measurements, several practical considerations should be met. A water bath should be used to regulate the solution temperature. The efllux time should be relatively long (generally > 100 seconds) to reduce timing errors and rninirrrize kinetic energy corrections. Small solution concentrations must be used for extrapolation to a concentration of zero, i.e. the relative viscosity me. should be between 1.1 and 1.6. Mark - Houwink Relationship An empirical relationship between the intrinsic viscosity and the molecular weight was concluded by Mark and Houwink in 1938: [n] = KM." K and a are constants for a specific solvent-polymer pair at a specific temperature. 2.2.5 Gel Permeation Chromatography In GPC, a dilute polymer solution is put through a series of columns. Larger polymer molecules have faster elution times as the smaller molecules are able to sample more of the capillaries in the packing of the columns. Calibration is done with known molecular weight standards to give molecular size versus elution time. The GPC calculates all of the molecular weight moments (Mn, MW, My, etc.) and also gives the peak molecular weight, i.e. the molecular weight which shows up most often. 2.3 Free Radical Polymerization 2.3.1 General Mechanism of FRP Free radical initiated polymerizations are one mechanism of polymer growth in which polymerization reactions occur almost instantaneously. Several polymers 18 are formed mainly by this mechanism including polyethylene, poly(vinyl chloride), and poly(methyl acrylate) [Flory (1967)]. Free radical polymerizations consist of three steps: initiation, propagation, and termination. Initiation: Typically an organic peroxide is incorporated as an initiator. Upon heating, the peroxide undergoes homolysis and decomposes to form two radical species, which are then able to react with the monomer or polymer to form another radical species. Roon—JL+2R0' In general, C—5“—>2R; Rg+M—L+RI Propagation: Propagation by the free radical mechanism occurs very rapidly. The radical species reacts with an unreactive monomer or polymer, which in turn, becomes the active center. . k . R,+M——L+R, O k '5 0 R2 + M -—";—> R 3 o k n . Rn + M P 5 R(n+l) 19 It is usually assumed that the reaction rate coefficients of propagation are independent of size and therefore are equivalent. kpl : kpz : kpn = kp Termination: Termination occurs by either combination in which the species add to each other or by disproportionation where one of the species forms a double bond. a) by combination R; + Rf, ——M P (n +m) b) by disproportionation o o kt Rn +Rm ———>" Pn +Pm 2.3.2 Reactive Extrusion and FRP Of current interest is reactive extrusion of polymers leading to either controlled molecular weight degradation or to an increase in molecular weight. Two systems of intense interest have been polypropylene (PP) and polyethylene (PE). A great deal of research and experimentation has been done on controlling the reactive degradation of PP in an extruder [Pabendinskas et al. (1989, 1994 a,b), Fritz et al. (1986)]. The reactive extrusion of PP with a free radical initiator, 20 usually an organic peroxide, has been shown to lead to chain scission and hence molecular weight degradation [Tzoganakis et a1. (l988,l989)]. This free radical initiated degradation provides an easy path for producing necessary molecular weights for specific applications. An increase in the initiator concentration degrades the high molecular weight tail and narrows the molecular weight distribution of PP [Suwanda et al. (1988 a,b), Triacca et a1. (1993)]. The changes in flow properties which result from the lower molecular weight are of much interest. As the molecular weight and viscosity decrease, melt flow properties are increased which improves the processibility of PP. The degradation of PP in an extruder has been modeled by Tzoganaskis et al. (1988) and Suwanda et a1. (1988 a,b). Pabedinskas et al. (1994 a,b) have recently tried to model this system with the explicit purpose of developing a process control strategy. In contrast to the free radical initiated degradation of PP, polyethylene free radical polymerization produces a polymer with an increased molecular weight. The reactive extrusion of linear low density polyethylene (LLDPE) and a free radical initiator leads to a high molecular weight polymer as the initiator concentration is increased [Suwanda et al. (1989)]. An increase of molecular weight should 21 improve mechanical properties. The polymerized LLDPE showed increases in MFI, yield strength, and yield modulus. 2.3.3 Maleation Reactive extrusion can be used for the functionalization of many polymers. Of specific interest over the past years has been the addition of maleic anhydride to several polymer backbones such as PP and PE. The maleation of these polymers has been generally done to improve the adhesion properties. Introducing new functional groups onto the polylactide backbone paves the way to prepare composites, laminates, coated items, and blends/alloys with improved properties and cost effectiveness. Functionalizing the matrix polymer and the fiber/filler with highly reactive groups is perhaps the most successful strategy leading to a variety of cormnercial composites and alloys made by reactive processing. Functional groups such as isocyanate, amine, anhydride, carboxylic acid, epoxide, oxazoline, are often introduced during reactive extrusion with short residence time. Combinations of hydroxyl/isocyanate [Mizuno et a1. (1978)], amine/anhydride [Lambla et al. (1989), Morita et al. (1987), Udding et al. (1988)], amine/epoxide, anhydride/epoxide, amine/lactam [Akkapeddi et al. ( 1988)], and amine/oxazoline [Sneller (1985)], provide practical routes for reactive processing. Such coupling 22 reactions provide interfacial bondings in composites, laminates, and coated items [Krishnan et a1. (1992), Argyropoulos et al. (1991)]. In polymer blends and alloys (immiscible) such coupling reactions provide control of phase size and strong interfacial bonding. A variety of functional groups has been introduced onto the surface of natural polymers [Doane et al. ( 1992), Glasser (1989)]. The free radical initiated reaction of MA with several polyolefins has led to very interesting results. Branching and/or crosslinking occurs in maleated samples of LDPE [Gaylord et al. (1982)], HDPE [Gaylord et a1. (1989)], and LLDPE [Gaylord et al. (1992)]. In an ethylene-propylene copolymer rubber (EPR), both crosslinking and degradation occur [Gaylord (1987)]. Degradation occurs in the case of maleated PP [Gaylord et al. (1983b), Callais et al. (1990), Hogt et a1. (1988)]. The degradation is greater in the presence of MA plus initiator, than in the presence of only initiator. Grafting of MA, as well as melt flow, is increased with an increase of peroxide content for PP. The free radical initiated reaction of polystyrene (PS) with various organic peroxide initiators results in degradation and molecular weight reduction. In the presence of MA; however, the extent of degradation is reduced [Gaylord et al. (1983a)]. 23 Homopolymerization of MA may play an important role in the reactions of MA with polyolefins. Cationic intermediates participate in the homopolymerization of MA, but the addition of small amounts of dimethylformamide (DMF) may prevent the reaction [Gaylord et al. (1981)]. Crosslinking which normally occurs in the maleation of LDPE is suppressed with the addition of DMF. Furthermore, the addition of DMF to PP-MA mixtures before reactive extrusion results in MA grafted PP with a higher intrinsic viscosity than a mixture without DMF. Therefore, less degradation occurs with the addition of an anti-MA homopolymerization agent such as DMF. C haptcr 3 MATERIALS 3.1 Polylactide The focus of this work is on the modification of polylactide. In this section, the preparation and uses of PLA will be described in some detail. The PLA was provided by Cargill. It has a specific gravity of 1.248. GPC results indicate that PLA has an MIn of about 122,000 and a polydispersity of 1.4 (see Table 5.1 for complete details). 3.1.] Commercial Preparation Polylactides are prepared by the ring-opening polymerization of the lactide dimer (Figure 3.1). Naturally occurring lactic acid is dehydrated to form the cyclic diester lactide. This process is also known as internal esterification. Lactic acid is a key biomass intermediate obtained from acetaldehyde or fermentation of hexoses or hexose polymers such as starch or cellulose [Sperling and Carraher (1990)]. Several catalysts may be used for the ring-opening polymerization, including tin(IV) chloride, stannous octoate, and tetraphenyltin [Van Dijk et a1. (1983)]. 24 25 During the past decade, aluminum alkoxides have also been used [Barakat et al. (1993)]. 0 CH3 / 2n Lactic Acid HO—(LH— 0 0H Catalyst 0 0 CH3 \C / \ c / n Lactide l l + 2n H20 C C / \ o / \ H3C 0 Catalyst 0 CH3 POIVIaCfide _(__ (H: _ O __ (I: + n l Figure 3.1: Commercial preparation of polylactide. 26 3.1.2 Applications Sutures Polymers of lactic acid have been used commercially for absorbable sutures. Copolymers of lactide and glycolide were synthesized as early as 1963 to produce synthetic absorbable sutures which were an improvement over catgut sutures [Conn et al. (1974), Schmitt et a1. (1967)]. Further advances of PLA sutures include dimensional stability and improved tensile strength [Schneider (1974, 1972), Yves (1970)]; however, processing via extrusion may cause a loss in inherent viscosity [Schneider (1971 )]. Drug Delivery Systems Biodegradable polymers have been found to be very efficient in the controlled release of therapeutic drugs. Lactide polymers were the first synthetic biodegradable polymers to be used in this application [Heller (1985)]. Polylactide has also been used as a semi-permeable biocompatible local delivery device for the treatment of periodontal disease [Goodson (1988), Damani (1993)]. Medical Implants Polylactide has been used to coat a sintered tricalcium phosphate implant [Eitenmuller et al. (1986)]. The PLA coating is of a certain thickness so as to 27 control the adsorption time of a therapeutically active ingredient which is contained in the porous implant. Biodegradable Packaging: Status The current applications for PLA are in the medical and pharmaceutical industries which are low volume markets able to accommodate high resin costs. However, as the need for biodegradable packaging and biodegradable items increases it becomes necessary to find a less costly way of producing PLA. Several companies are currently working on the research and development of the commercial production of PLA. Argonne National Laboratories is developing a technology for polymerizing lactic acid produced by the fermentation of potato waste. Batelle and Golden Technologies are in a joint venture for developing PLA technology for packaging applications. Cargill and Ecochem are producing lactic acid from corn and cheese whey, followed by the ring-opening polymerization to high molecular weight PLA. 28 3.2 Lupersol 101 Lupersol 101 (L101) is a difunctional di-tertiary alkyl peroxide. The free radicals generated from dialkyl peroxide decomposition are initiators in bulk and suspension vinyl polymerizations. L101 was chosen as the initiator for several reasons. A low half life has been reported by the manufacturer as 1 minute at 180°C and 13 seconds at 200°C which may result in more or complete decomposition of the peroxide at the operating temperatures and residence times. L101 is also recognized by the FDA as a food additive (Code of Regulations; Title 21 “Food and Drugs” part 170 under “Food Additives”). CH, CH, CH, CH, 1 l l I CH,—— T— o —o— C— CH,——CH,—— C —o — o — — CH, 1 | CH. CH, CH, C“. Figure 3.2: Lupersol 101 [2,5-dimethyl-2,5-di-(t-butylperoxide)] 29 3.3 Maleic Anhydride Maleic anhydride (MA), shown in Figure 3.3, was purchased from Aldrich Chemical Company. Maleic anhydride is a toxic chemical considered as corrosive and as a sensitizer. Care must be taken in handling to avoid breathing in the dust particles of MA as well as the fumes from extrusion. Also, MA may be absorbed through the skin so gloves must be worn. MA has a melting point between 540C and 560C and a boiling temperature of around 2000C, which is the maximum temperature at which any of the experiments were run. Figure 3.3: Maleic anhydride (MA), [C4H303] Maleic anhydride can be used as a coupling agent providing bonds both to a filler containing hydroxyl groups (esterification) and to the polymer matrix (through peroxide addition) [Dalvag et a1. (1985)]. Chapter 4 PROCESSING AND CHARACTERIZATION 4.1 Processing 4.1.1 Reactive Extrusion The thrust of this work is the free radical branching of PLA via reactive extrusion. Therefore, extrusion is the most important step of all the experiments. It is thus necessary to describe in some detail the reactive extruder and extrusion experiments. The extruder used was a Baker-Perkins co-rotating intermeshing twin screw extruder. Figure 4.1 shows a schematic of the extruder. The diameter of each screw is 3 cm, the length is 42 cm. There are two feed ports on the barrel, two barrel valves, and a venting port. The material was fed at the first feed port while the other feed port and venting port were kept Closed. Each screw has two sets of six mixing paddles and a Camel back discharge screw at the end. The die which was used had two 3 mm in diameter holes. The temperature was measured at three points on the barrel, at one point on the die, and at four points inside the barrel (melt temperature), defining the conditions in zone 1, zone 2, zone 3, and the die 30 31 (zone 4). The barrel could be cooled by adjusting the flow rate of the cooling water supply which was manually controlled by four valves. The extruder shafts are composed of slip-on screws, kneading paddles and orifice plug segments. The configuration of these elements was as depicted on Figure 4.2. The transversely neighboring paddles are always kept at 90 degrees to each other, while the axially kneading paddles can take on a number of orientations depending on the amount of forwarding action desired in each mixing zone. The amount of cross-sectional area available for axial flow is controlled by the barrel valves and orifice plugs. The barrel valves are triangular shaped vanes positioned over the orifice plugs which are discs with a diameter close to that of the barrel. 4.1.2 Extrusion Conditions PLA and Lupersol 101 (L101) (also maleic anhydride, if maleation was desired) were mixed in zip-lock plastic bags before extrusion on the Baker Perkins co- rotating intermeshing twin—screw extruder. The extruder was purged (cleaned) with polyethylene before and after each run. When the PE coming out of the die was clear, it was assumed the no other material was in the extruder. The material was run directly after the purging with PE. Samples of 350 - 400 grams of PLA ' were used for purging and sample collection. Screw speed was set at 100 rpm with a constant feed rate of 5 percent. In the beginning of the extrusion, the 32 extrudate is a mixture of PE and the material, so the mixture was discarded and sample was not collected until the material appeared to be pure PLA (change in color from white/clear to light brown or tan). This usually occurred after 150 - 200 grams of material was collected. Approximately 100 - 150 grams of the material was collected for further testing. Collection of the material was stopped when the load of the extruder, which remained fairly constant throughout the reaction, started to decrease. Although there was still some material left in the extruder, it had been subjected to a longer residence time and would not have the same properties as the collected material. Tables 4.1 and 4.2 show the temperature settings for the free-radical branching of PLA and the maleation of PLA, respectively. The temperatures in the first zone are kept lower as the materials need to be moved ahead without melting in this zone. In the case of maleation, a lower temperature setting at the feeding zone (compared to branching only) was necessary to prevent PLA co-aggregation in the hopper. The melt temperatures in zones 1 and 3 ran lower than the set points for all of the materials; however, the melt temperature in zone 2 always ran higher than the set point. Tables 4.1 and 4.2 show these discrepancies. .952 .580 nausea—m 28 £532 osmoaaoov 5255 388 E3 mac—Homéofim : .v earn $5385 m BEN N oaoN _ oaoN 380m owafima xonm 3an 362a seam 93> 35m 83> 35m :8 80m mBEom 83.28 mwsi 855 .cocesmmaoo 388 E25 macsorou ”Nd oSwE .L £25 was: 558% 32m map—om Bum $55 02.30 l 1 l l llll ll— L mBoSm Bum 35 Table 4.1: Temperature settings for free radical branching. Temperature (C) Temperature Temperature Zone 1 Zone 2 Zone 3 Die 1 ( C) l Type 160 Set 180 155 160 160 Melt 145 165 156 186 170 Set 180 163 185 160 Melt 145 174 173 162 180 Set 180 170 190 160 Melt 152 186 181 166 190 Set 1 80 l 85 200 160 Melt 149 197 192 167 200 Set 200 1 90 210 l 55 Melt 158 202 199 l 77 Table 4.2: Temperature settings for maleation. Temperature (CC) Temperature Temperature Zone 1 Zone 2 Zone 3 Die ( C) T 180 Set 170 170 190 160 Melt 148 184 180 162 200 Set 170 190 210 155 Melt 146 201 199 175 4.2 Characterization 4.2.1 TriSEC Analysis Molecular weights and molecular weight distributions were determined using a TriSEC (triple detector size exclusion chromatograph) operating in THF at 25°C. The samples were dissolved in degassed THF and then filtered with a 0.45 micron filter before injection to remove undissolved contaminants which may block the system. The TriSEC system consists of: (1) Viscotek model 600 RALLS (right angle laser light scattering) detector, (2) differential viscometer/refractometer, and (3) size exclusion chromatograph. A random coil configuration for the polymer was assumed. The total injection volume was 242 [.11 with a flow rate of 1 mL/min. Figure 4.3 is a schematic of the TriSEC detector flow loop which is a closed loop operating system. Pure solvent is continuously passed through the apparatus. The solvent is degassed and then pumped into the GPC column oven. The solvent is first heated in the oven before it goes through the GPC columns. When a sample is run, it is injected into the oven, heated to the specified temperature, and then allowed to enter the GPC column. Once the solution has exited the column, it goes to the RALLS detector and then onto the parallel configuration of the differential viscometer/refractometer. If a sample is being run, the solution is collected in a waste container; otherwise, the pure solvent is recycled. 37 mOHUmfirmo mr5O ‘lllllrlllll mZDm 04mm MBmEOUmH> AAOm Mumm 50 g/10 min'to 5.39 g/10 min (entries 2 and 3 in Table 5.1). Further addition of L101 (i.e., from 0.05 up to 0.26); however, provided no additional property improvements (entries 2-5 in Table 5.1, also Figure 5.2). At 170°C and 180°C: Increasing the initiator content leads to an increase in M", Mmk, and the MP load % (screw torque); as well as a decrease in the MFI (entries 6-8 and 9-11 in Table 5.1, also Figures 5.3 and 5.4). Molecular weight 49 distributions are kept between 1.3 and 1.5 with the exception of the highly branched 170°/0.5 sample. At 170C and 0.1% L101, the extruded PLA is characterized by properties similar to the initial, not extruded PLA (entries 1 and 7 in Table 5.1). For example, compare the intrinsic viscosity for PLA at 1.04 with that of the 170°/0.1 sample at 1.08. The molecular weight values are very good in comparison with the mean values reported by Gogolewski after injection molding (Mn << 100,000). As previously stated, an initiator concentration of 0.5% at 170°C (entry 8 in Table 5.1) leads to highly branched PLA as can be seen in the high MP load %, the elastomeric properties exhibited upon extrusion, and the high molecular weight polydispersity of 550. Also, the sample was quite difficult to filter for SEC analysis: several filters had to be used for the dilute sample solution. An extraction experiment has been done showing no large scale cross-linking; however, microgel may be present along with the branching. A more detailed description of highly branched samples will be discussed in Section 5.1.6. At 190°C: An increase in the initiator concentration results in an increase in Mpcak and Mn (entries 12-14 in Table 5.1). Branching effectively occurs as shown by the increase in both Mn and MP load %, as well as a decrease in MFI. The highly branched, possibly microgel, state observed with 0.5% L101 (entry 14 in Table 50 5.1) is proof of the fact that branching is still occurring at this high temperature, even though chain scissions are generally more prevalent at high temperatures and high initiator concentrations. 2001C: At 2003C branching is counter-balanced by chain scission leading to an approximately constant Mpcak and Mn, in addition to a constant MFI between 10 and 13 g/10 min (entries 15-17 in Table 5.1). Based on the above observations, it clearly appears that between 170C and 190°C branching occurs as evidenced by increases in Mpeak, Mn, and extruder torque. Branching is also shown by an exponential decrease in MFI with initiator concentration (Figure 5.5). At temperatures T >190°C, in addition to branching, chain scissions by free radical processes appear probable along with other transesterification reactions (intra- and inter- molecular) leading to a decrease in molecular weight. Comparisons between undried and dried PLA in MFI experiments show that some initiator traces are still present inside the PLA sample and can further react at 190°C during the MFI test which may lead to new branching. In any case, the traces of peroxide always favor a decrease of MFI which is one of the objectives of this research. 51 Table 5.1: Free radical branching of PLA: MP load %, melt viscosity, and TriSEC analysis. (Standard deviations for TriSEC data are located in Appendix A. 1) ID PLA MP MFI (g/10 min)‘ TriSEC # samples load% .T [we/o undried driedz Mpeak Mn Mw/M, NW C L101 = = 1 PLA - 12.8 - 134,500 121,600 1.41 1.04 2 160 0.00 59-63 >50 - 91,900 88,800 1.30 0.82 3 160 0.05 58-60 5.4 9.3 137,900 125,800 4.63 1.33 4 160 0.14 54-56 4.9 7.8 132,100 124,600 1.81 1.14 5 160 0.26 50-52 4.1 6.4 139,500 130,400 2.54 1.17 6 170 0.00 64-68 >50 - 91,300 76,200 1.50 0.84 7 170 0.10 69-73 17.2 - 126,000 125,300 1.31 1.08 8 170 0.50 78-85 - - 180,800 133,900 5505 1.534 9 180 0.00 59-61 31.9 39.2 85,900 81,000 1.30 0.83 10 180 0.10 70-72 13.9 15.8 108,500 104,700 1.33 0.98 11 180 0.25 72-74 7.0 12.0 130,900 129,000 1.48 1.01 12 190 0.00 52-56 >50 - 88,500 82,900 1.30 0.79 13 190 0.10 66-69' 19.7 - 115,800 114,500 1.33 1.02 14 190 0.50 71-75 - - 162,300 153,600 5205 1.72 15 200 0.00 49-51 13.1 55.6 102,900 102,600 1.21 0.92 16 200 0.10 56-58 10.4 29.6 112,000 105,500 1.30 1.11 17 200 0.25 59-61 10.7 17.3 117,600 114,000 1.41 0.97 1 ASTM standard (D1238) at 190°C with 2.16 kg load 2 Samples vacuum dried overnight at 130°C (removal of residual peroxide) 3 Sample not extruded 4 Ubbelhode viscometry experiments result in IV = 1.16 for PLA and IV = 1.46 for 170°/0.5 (Table A-2 in the Appendix) 5 Sample highly branched 52 Eocgcuocou 3 uni—Enos .<.E 63.2 .530: Basic can $5.2 .azwé 8355:: Mo :33ng Ems; E822: 5 £5. 05 memaofi 8893825 >9 ”mm oeswi 38v oEBo> acumen—om S: 8.2 a: ”.2 4.: 9: _ _ — _ 0.0 r 3: $2 m S d O U S 9 ) w -3: A ( 22.: l r 2.. <2 EMFWOHNEOhfio >Q _ . - :1- . a...“ 9323538 3 coufipuos .8038 53 86588.85 533 0802 a Beads <1E Lo cousnEmE EwmoB E8208 5 a2» 05 @365 8833885 >0 ”Nd 859m 3:: oEEo> 5:5qu o.w_ 0.2 Na— «.2 v.2 Q: p p — p 6.0 IIIII I l.\ .11 rod— -ZN mm S d 0 u S 9 ) w Iodn A ( o~.o\oco_ l l V—.°\OOW— Ill .1 °.O*q no.o\ooo—l| odn .EfionEoEU >Q 54 n 2235588 9 uufifigos .2058 53 053805 5:» 0°02 S 825% Sn «o cousnEmB Ems; H2322: 5 £5. 2: wcgozm ESmeEoEo >Q ”Om oSwE 38V oEEO> 20:5qu 3: 92 a: ”.2 t: q: . _fl _ _ — _ CO I\ III DI‘ Ill Il‘ \nl \ I o.» 13: mac S d 0 U S 9 ) U 1 1oz A ( . were: I I w .93: II 10.2 m 92.2. " m _ W .P o? ESmoSEoEU >Q 55 3055588 9 nfificcos .2828 53 “388.55 5:5 Doom: a 96253 «in we :33ng Ewfia .2822: 5 £5 2: @355 8833885 >0 Him 8390.5 3:5 0828/ sarcasm 2: 3: “.2 «.2 v.2 9: _ — _ _ Cd 1 3 L3. We 8 d O U S 9 ) w 193 A ( _ / \\ n \ 2.2.8. I l fl \\ _ ( .932 II -92 _ 92.8. 2:. EfiwofiEcEU >Q 56 md .3880 S: mammoba as» E: E 88.8% a wgofi 838$ cow—5x0 mo E2 ”Wm oEwE .233 .x. mad N6 26 "d a mod U05 .. UOOON I O I Uocom .. O n BB? 032 I all Doe“: I II 35.. UooEIarl -Uoofllil. ("Em 01/8) 1m 57 5.1.3 Thermogravimetric Analysis TGA measures the change in weight of a sample due to volatilization, reaction, or absorption from the gas phase [Rauwendaal (1986)]. An increase in the decomposition temperature results in a more thermally stable product. At an initiator content of 0.1%, an increase in the extrusion temperature results in an increase in the decomposition temperature and a more thermally stable polymer as compared to unextruded, unreacted PLA which degrades more readily (entries 1, 5, 8, 10, 13 in Table 5.1, also Figure 5.6). Jamshida (1988) has proposed thermal degradation by a back-biting mechanism starting from the end groups of the PLA chain. At 0. 1% L101, PLA is branched leading to a decrease of the total number of end groups; therefore, the probability of thermally degrading side reactions occurring by this mechanism might be reduced. At an initiator content of 0.5%; however, the higher extrusion temperature has a somewhat lower decomposition temperature, indicating that the product at 170°C is more thermally stable than that at 190°C (entries 6 and 11 in Table 5.2, also Figure 5.7). An explanation of this may be as follows: a higher temperature tends to produce a lower gel content [Hamielec et al. (1990)] so the lower temperature is more cross-linked and may be more difficult to degrade. In addition to the fact that higher temperatures tend to produce less cross-linked materials resulting in more end groups available for degradation, higher temperatures seem to be more 58 favorable to chain scissions by free radical processes or intramolecular transesterification leading to the formation of oligomers. Oligomers have been shown to promote the degradation and also the thermal instability of PLA [Jamshida et al. (1988)]. Products extruded at 160C show little change in decomposition temperature at an increasing initiator content (entries 2-4 in Table 5.2). The decomposition temperatures are lower than that of pure, unextruded PLA (entry 1 in Table 5.2) which is characterized by longer chains. Once again, the extrusion at 160C causes degradation with a sharp decrease in Mn as reported by Gogolewski (1993). Table 5.2: Decomposition temperatures from thermogravimetric analysis. I ID # I Temperature ICI I wt°/o L101 Onset Value ICI I Max. Value (C)T I 1 Pure PLA - 320.0 321.2 2 160 0.05 319.4 320.5 3 160 0.14 319.1 320.1 4 160 0.26 318.8 319.7 5 170 0.1 323.8 325.1 6 170 0.5 322.1 323.3 7 180 0.0 315.4 316.52 8 180 0.1 322.0 323.2 9 180 0.25 324.3 325.5 10 190 0.1 323.9 325.2 11 190 0.5 321.1 322.3 12 200 0.0 326.4 328.0 13 200 0.1 326.5 328.0 14 200 0.25 319.5 320.5 1 Maximum rate of decomposition 2 Temperature still declining 59 .53 $3 53, <..E 8555 8.“ 288388 5289586 wagon.“ >mHLm>O 0mm 200V mcaumcmoem» omm ofim t _ om _ . _ . _ _ _m _ , _ 1 _ _ ._ _ . __ I om _ Uoomfi.“ ’ v . _ .005“ — < a mung - "_ .___ , u . mm oooom, if /u.. om“ 1 ./ . / / / . ./..I--I.-IH..HHW:UIIII llllllllllllllll ”“fliiiii Ion: mo“ 3U5I3M V91 (76) 60 .83 o\om.o 5m? <15 33.58 8 339:8 <1E 9::— uom <05 Kan oEmE comm.umcH «H oo.fi> >mfiem>o Away mezumemaemh omm . owm ofim ms _ _ " tom _ _ u “ _u°omfi . . u _ 1mm _ _ n _ . fl _ u _ -a n u , <48 wmaa . . , ,, / [mm , / // / UOONfi I/I’lnl/ [I I I lllllllllll '1' llHIIIH-Innlulul'lfl Ioofi mo“ nufitaM V91 (X) 61 comm.umcH < omm coo: a «a: 855% Egg Ba 5 <2. ”an 2%; mcaumemusmh owm p Do.fi> >mHLm>O AUov 0am Nfi.o ’ <38 mung .-————-————-——_-—— 5' mm [om 1mm 100 1mm mo« 3U5¥3M V91 (X) 62 0008 s E 3:055 863 Ba 5 <9 a.“ 2%: oomm.umcH 4» oo.fi> >mHLm>o goo. meaumemasmp omm . 0mm Cum . _ . om 3*? 1—1 0 —_—.__——_ ————__ r / _,_..___..._...._..____.._.__...________ ’— ’ / loo“ mo“ (X) lufitaM V91 63 At 180C, when increasing the initiator concentration from 0.0 to 0.25% L101, the thermal stability increases which indicates that branching also increases (entries 7- 9 in Table 5.2). With no initiator present, the sample extruded at 1800C is not as stable as unmodified PLA indicating that degradation has probably occurred (compare entries 1 and 7 in Table 5.2). This could be a result of hydrolysis, i.e. more hydroxyl groups are present resulting in easier degradation. At temperatures of 170C and 190C, samples with an initiator concentration of 0.5% (entries 6 and 11 in Table 5.2) are less thermally stable than those with an initiator concentration of 0.1% (entries 5 and 10 in Table 5.2); however, both are more stable than unextruded PLA. At so high of initiator concentration one can assume, next to the proposed microgel formation of PLA, that a large amount of chain scissions occur resulting in the formation of short chains which are less stable. Figure 5.8 shows the TGA for the PLA system extruded at 170°C. A plot for the 190C system is very similar. Figure 5.9 (entries 1 and 12-14 in Table 5.2) clearly confirms the results obtained in Table 5.1 at a extrusion temperature of 200°C, i.e. even if there is some branching which occurs, there are also chain scissions which become extremely important as the L101 content is increased. For example, the stability of PLA 64 extruded at 200C in the presence of 0.25%L101 is even worse than that of pure, unextruded PLA. 5.1.4 Differential Scanning Calorimetry The reactive extrusion of PLA with L101 did not affect the glass transition temperature, T“, which was maintained in the range of 58C to 62C for all samples, reacted and unreacted (Table 5.3, also Figure 5.10). A crystalline region was noted at about 117C to 123C for all samples. These results were somewhat surprising as a change in the T0 was expected. When the molecular weight increases, the density of the end groups decreases which leads to a decrease in the free volume, and hence, an increase in the To should result. In general, branching normally decreases the Ta, as the free volume is increased, while crosslinking increases the T(;, as the number of end groups is decreased. Table 5.3: DSC results: 10C/min to 200C Temperature (C) %L101 To (C) endothermic I transition IC I I Pure PLA - 59.2 122.6 170 0.1 59.1 121.6] 180 0.1 58.6 121.3 190 0.1 58.6 121.1 200 0.1 58.0 120.9 160 0.0 62.2 117.7 170 0.5 62.0 117.4 1 A third transition was apparent at 114°C. 65 data—=8 cu. Sm £88 93 mo Swag—=00 A: .m gamma comm umcH <2 oo.fi> >mfiem>o 20.. manomemaemk . F . b . _ ml D\ON«I|I|1I|\/l|/1(Il\/... 1m! 0\om«I/1.\IIII1/\\///J - H.\oom lVl /./..I 1 2.82 . I / bl- I I rtfi/ ... s.\omfi . , 1,13,!ng 1N1 lo rm “01:1 399H USO (MW) 66 5.1.5 Dynamical Mechanical Analysis DMA confirmed the results found by DSC: no noticeable change in TC, was apparent. At the glass transition temperature, To, the storage modulus, 6’, shows a rapid decrease (Figure 5.11), while the loss modulus, G”, and the tan delta (ratio of loss modulus to storage modulus) exhibit maximums (Figures 5.12 and 5.13 respectively). Table 5.4 shows the average TG results given by DMA. PLA 110 is pure PLA which has been compression molded at 110°C, while PLA 140 has been molded at 140C. The T0 associated with G" is generally accepted as the value which is reported for polymeric materials. Table 5.4: To averages for dynamic mechanical analysis. Temperature (C) %L101 G’ G” tan 6 PLA 110 - 61.78 69.98 75.36 PLA 140 - 61.46 69.38 75.1 190 0.1 60.88 68.25 74.71 200 0.1 61.32 69.06 74.82 170 0.5 59.92 69.71 75.36 190 0.5 59.83 68.23 74.62 67 cm .435 .3 83w 8 838mm .838 SM 8:62: 82on A 3 2&3 Cov 8888809 3. 2. no 8 mm 8 . L. 1. n . n x u . n 0 o 4’ [0,00 ./ xxx... x. // /.. / / z. .. Nd /. / ./ / / .. . / / / / I . x. 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I I 0.3; .3 03: «3| 70 5.1.6 Highly Branched Samples This section is specifically devoted to the 170°C and 190°C samples with 0.5% initiator concentration. Besides having a large weight average molecular weight and a high polydispersity, the highly branched samples have other outstanding characteristics. The weight average radius of gyration (ng) for all the other samples was between 14 and 20 nm (see TriSEC data in Table A-1 of the Appendix), but the ng for l70°/0.5 and 190°/0.5 were 649 nm and 936 nm, respectively. As previously stated in Section 5.1.2, these highly branched samples were more difficult to filter in comparison with the other samples. A material balance was done indicating that approximately 20% of the l70°/0.5 sample and about 10% of the l90°/0.5 sample were entrained in the 0.45 micron GPC filter. This material is either microgel, contaminants, or both. The Mark-Houwink parameter “a” is a polymer confirmation parameter (see Section 2.2.4). For random coil molecules, “a” usually has a value between 0.5 for a poor solvent and 0.8 for a good solvent. Typical ‘2.” values for the PLA polymers which were not highly branched were between 0.64 and 0.77 (Table A-1 in the Appendix). For polymers containing long chain branching, the “a” value can fall below 0.5, depending on the degree of branching [Viscotek (1992)]. The 71 values for 1700/05 and 1900/05 are 0.47 and 0.46, respectively, further confirming that these polymers may have long chain branching. Figure 5. I4 is a Mark-Houwink plot of the 170°C series. A linear Mark-Houwink (M-H) plot is generally found in linear standards. For a given polymer, samples which have the highest slope and intercept on a M-H plot represent the least branched structures. As seen in Figure 5.14, the l70°/0.5 sample is considerably lower than the other samples indicating a highly branched structure. The l70°/0.5 molecular weight is also greater than that of the other samples as the log (M.W.) line extends farther than that for the other samples. A M-H plot of the 190°C series showed similar results. A comparison of PLA and 170°/0.1 in Figure 5.14 shows that the M-H plot for both samples is similar in agreement with an earlier statement that these samples are almost equivalent. The percent of polymer with a molecular weight above 1,000,000 Daltons was also determined. Pure PLA and all other samples which were not thought to be highly branched had less than 1% of their total molecular weight above 1,000,000. l70°/0.5 has 11.14% and 190°/0.5 has 8.31% above 1,000,000 indicating long chain formation. Table 5.5 summarizes these findings in comparison with unextruded PLA. 72 Table 5.5: Highly branched sample comparison to unextruded PLA. Sample ng (nm) “a” % above 1,000,000 PLA 17.85 0.72 < 1 l70°/0.5 649 0.47 11.14 190°/0.5 936 0.46 8.31 Figure 5.15 shows a comparison of the light scattering chromatographs of pure PLA and of the l90°/0.5 sample. Clearly obvious is the bimodal peak in the 190°/0.5 sample which indicates that there is a considerable amount of high molecular weight polymer present in the sample in comparison to pure PLA in which there is no such peak. This phenomenon was seen in both the 170°/0.5 and the 190°/0.5 cases. 73 .2... Doc: .5 do 83 €385.32 i .m 28E 33.3 582.233 3.. 3.. 2.8 2.“ 2.. n: _ _ r _ r “6.5.. \\ m 32: II \ . . _ . \\ . -an _ ~25: ---- m @252 .I.I \\ . . <5 \\ . 1.3- . 1£~ r x 1 \ -2.» n2 BE xdmazomisg l.0 I X (4113003; A oisuinulfiio'] 74 22.9 LS Chromatogram l7l~ I I ___‘ -’.---" 'U E; Response (mV) u ‘ —.—_ Jar-“ur— i 0.1 . J JiJ‘MWWMWMM W1 ' W I Y 11.2 15.: 20.4 23 o Retention Volume (mL) ~31 1 554 r 4--.--. ___- . ....-__- _ LS Chromatogram IN . :Wwiiw/ U 3" O 1_ ‘. Response (mV) E ._.\__._‘ 1"“ ”‘d 2 a 10 , -. ME" WW I I .2 15 s Retention Volume (mL) Figure 5.15: A comparison of light scattering chromatographs of PLA (top) and l90°/O.5 (bottom). 75 5.1.7 Film Results An initial study of PLA film extrusion was done at in this investigation. A small single screw film extruder with an 18’x 3/4” barrel was used to extrude samples of pure PLA and PLA with initiator. As this was a preliminary study to evaluate the feasibility of film extrusion, only an extrusion temperature of 170°C was used. Three different compositions were extruded: (1) pure PLA, (2) PLA and 0.1% L101, (3) PLA and 0.5 L101. With 0.5 % free radical initiator, the resulting product was highly branched and did not extrude to a usable film. Table 5.6 shows the film properties of PLA and PLA with 0.1% L101. Tensile tests for strength, elongation and tensile modulus were conducted on a UTS machine SFM-20 using ASTM D882 for thin films. Table A-3 in the Appendix lists the operating conditions as well as the data for all experimental runs. Table 5.6: Tensile results for PLA film. Property PLA PLA with 0.1% L101 Film thickness T 0.003 Maximum psi 2980 +/- 190 2680 +/- 130 % elongation at break 3.6 +/- 1.3 4.1 +/- 2.0 76 5.2 Proposed Reaction Mechanism The formation of a free radical is the first step in the following proposed reaction mechanism. Figure 5.16 shows the decomposition of L101 which may generate several free radicals. The beta scission is a secondary reaction which may occur. Once the free radical initiator is formed, branching may take place. Figure 5.17 details the proposed reaction mechanism. First, hydrogen radical abstraction of the PLA polymer chain must take place. Radical coupling of these newly formed reactive polymer backbones may then occur resulting in the formation of a branched species. Chain scission of the polymer backbone may also occur resulting in the formation of a radical species which may also combine with another radical species to result in a branched polymer. 77 CH, CH, CH, CH, i 1 1 1 CH,—— C—— o — 0—.— C——- CH,— CH,— C ~——o — o “9—‘ CH, 1 ! f ‘ CH: CH, CH, CH3 Heat V CH, CH, CH, i ! l l l 2 CH,——— c— o. + -o—— C—— CH,—— CH,— C ——o- ; a CH. CH, CH, CH, 0 l i Beta Scission I} CH __ __ . % ' + . C”: T o CH—, C — CH, 3 CH, CH, CH, CH, CH, o— C— CH,—-—— CH5—~ C ——0- Beta Scission o 2 C— CH,—— CH—z— C —o- | ' I i s I CH, CH, + CH,- CH, CH, CH, . CH, CH, ' Beta Scission ! o = C———— CH,— CH—2 C ——o~ ¢ ' l 0.:C——CH2 CH—z—CZO | CH, Figure 5.16: Peroxide decomposition. 78 Hydrogen radical abstraction 0 CH, 1' s "' 4 C — o .._ C — l ————> i. 0 CH, I —f c m o —— C +- . (1) Radical coupling 0 CH, ____. 1 i (n+(n .+c_o_.+ —f c —— o — C + H I o l CH, Branching Chain Scission 0 CH, 6 i! i H ——C—O——C ——C—— H i ll . . _ —’ Branching Figure 5.17: Proposed branching mechanism. 79 Even if chain scissions could be promoted by free radical processes as proposed in Figure 5.17, they can also be promoted by intramolecular transesterification [McNeil] (1985)] (also called backbiting) and thermohydrolysis. Figure 5.18 shows both back-biting and thermohydrolysis reactions and the products which they may produce. Back-biting o ‘? .-. i h ~0—C— 0H——>. 0H 4.1—’0—0 ‘ J \\ j i _ J , '\, ,/ Cyclic Oligomers Thermohydrolysis o 0. ° — c —— + Hzo ———9 ------ OH + C ''''' Figure 5.18: Chain scission reactions. 80 When chain scission occurs, oligomers are formed and there is an increase in the hydroxyl and carboxylic end groups, both of which are favorable to promote thermal decomposition [Jamshidi (1988)]. This is in agreement with the TGA results discussed in Section 5.1.3. When branching occurs, the thermal stability is increased because there is a decrease in both end group and oligomer formation. In conclusion, there are two competing factors in the free radical extrusion of PLA: (1) branching, which favors large molecular weights, and (2) chain scission or hydrolysis, which favors small molecular weights. In this section, a proposed mechanism has been provided for both of these options. Actual characterization of the true mechanisms would be difficult, if not impossible. Chapter 6 MALEATION OF PLA 6.1 Discussion of Results The grafting of maleic anhydride to the polylactide backbone was done in an attempt to produce functional groups which would improve the interfacial adhesion of polylactide polymer blends (see Chapter 7). A concentration of 2 percent maleic anhydride was used for all experiments. 6.1.1 Effect of Extrusion Temperature Based on the free radical branching results given in Chapter 5, the extrusion temperature was not expected to play a large role in the maleation of PLA. Two temperatures were selected (180°C and 200°C) for this study with an initiator concentration ranging from 0 to 0.5% L101. Figure 6.1 shows that for the same initiator concentration, there is little or no difference in the grafting content of MA at the temperatures which were used. 81 82 6.1.2 Effect of Initiator Concentration At 180°C: With no peroxide, the addition of 2% MA has virtually no effect on the extruded PLA. Mn and Mpcak are approximately the same as the PLA which has been extruded only (compare entry 1 in Table 6.1 with entry 9 in Table 5.1). The addition of 0.1% L101 slightly increases both Mn and Mpcuk. Further addition of L101, 0.25 and 0.5 %, has a slight negative effect on the molecular weight of the samples (see entries 2-4 in Table 6.1). This decrease in molecular weight may be due to the competition between branching (which increases as initiator concentration increases and also increases molecular weight) and grafting of MA (which also increases as initiator concentration increases, but results in little or no molecular weight change). Figure 6.1 shows that an increase in free radical initiator results in an increase in the percent of maleic anhydride which is grafted (% maleation). Only small amounts of anhydride grafted to a polymer backbone are needed to improve the interfacial adhesion in a polymer blend system. Figure 6.2 is the TriSEC evolution which shows that the changes which occur in the shift of the molecular weights are very subtle and that the molecular weight distribution (the width of the peaks) remains fairly constant. 83 At 200°: Results similar to the 180°C series are seen at 200°C. The sample with 0.1% L101 has slightly higher Mn and Mpcak, while the samples with 0.25% and 0.5% L101 are slightly lower (see entries 5-7 in Table 6.1). Figure 6.3 is the analogous TriSEC evolution which also shows (1) subtle shift in molecular weights, and (2) a fairly constant molecular weight distribution. General Comments Table 6.1 also shows the reduction of MP load % at increasing amounts of peroxide. The presence of maleic anhydride appears to cause the chain scission of PLA. A melt flow analysis was done at both temperatures (see Figure 6.4 and Table 6.1) indicating that the addition of increasing quantities of initiator result in higher melt flow indexes (i.e., lower melt viscosity). The observation of increased melt viscosity in the presence of peroxide alone and of reduced melt viscosity in the presence of both peroxide and maleic anhydride is not what is found in the modification of polyolefins. For example, in the modification of polyethylene, the addition of peroxide causes branching and gelation, the presence of maleic anhydride promotes further branching and gelation [Hogt (1988)]; and in the modification of polypropylene, the addition of peroxide cause scission of PP, the presence of maleic anhydride causes further scission of PP chain [Callais et al. (1990)]. 84 Results of the TriSEC analysis further show that the addition of peroxide and MA does increase the chain scission of PLA. Table 6.1 also shows that the weight average intrinsic viscosity, IV“. of the maleated samples is between 0.73 and 0.95; whereas, the IV... for pure PLA is 1.04 (much higher). A further indication of reduced chain size, probably by chain scission, is the decrease in the radius of gyration for the maleated samples (Rg between 13.6 and 15.4 nm) relative to the pure PLA sample (Rg = 17.85 nm) (see Table A4 in the Appendix). 85 22. 2.0 82 08.8 88.8 3.2 8-8 88 28 a 82. 8.8 8.2 28.8 8...» 8.8 2-8 W8 28 e 88 8.8 8.2 82 52 88.2: 8.2 8-8 2 .o 28 m 28 Rd 2.2 812 8&8 «.8 8-8 88 2: 4 $8 88 8.2 88.8 88.8 8.8 8-8 n3 2: m 28 8.8 R; 88.8 8852 8.8 8-6 2 .o 2: N 88 8.8 8.2 83 m 88.8 8.2 8-8 8.8 2: _ 8: .2 .222 ..2 .12 85 23 .x. E 8.» .2 882.8 8 a e 25 0mm 22 8.. 22 .28.... <8 2.3 <8 -- <2 2 .2 N 83 88.8. 83.5. =< .3. 8883 a 3.... a. as 2.85. . Sm 8.283% 328.83 dozen—«E .x. was .855. $283.3 :2: xx. 28— LE {.5 mo cacao—«E 2 .w 033. 86 0.7 U U 0 O O O OO 0 Fl! N L J l 1 ' l l l r - ‘0. ‘n. '7'. "1 N. "T O O C O O 3 VW%1M %L101 Figure 6.1: Weight percent maleation as a function of initi tor concentration. 87 Acocmbfioaoo 8 32.2505 .5235va Ems? 838—22 @2305 $28 3822: Uoowfi mo 88388930 >0 ”No 6.52m 38V uEBO> .835qu 2: NW .32 92 n2 0.: --.I..h-- _ xx .1 _| S. / //l \ ‘1“: i / /// \\ /x \ / // . \ x _ / . . \ . ow / \ / a . . \ /,, < \ I . x / // .\\ \ x / C, \\ 8,2 W x S ,, . w a \\ \ w / /x/ \\ \ . M“ x \ \ _ w / xx x . -98 A / .\. . 33.8. n u u i ( / x 1 2.228. | I 33.8. I o 2282' - can 08 88388020 >Q 88 8 Bfifiéocv docBEmE Ewfi? H3322: wfiaofi motom c8838 Dooom mo EfimeEoEo >9 ”00 oSwE 3.5 o820> 5:5qu Aaowmbaoocoo N: a: 3; on. o N. F p _ _ 0.3 III // ul. ‘IHII. llllll / \ / // // \\ / \ - 3 \\ / \\ / \ // \ $2 \\ / \ \ / \ \ 1oz / / 1 \ \ nofioofi l I / 2.23.2 II 52958 I in 3:. . ESonEoEU >Q (Aw) asuodsau 89 140 WWWWWHHMHW llllllll llll Ill II“ III llllllll llll W Milli“ ll“ III “II, .n. O Hill” I“ “II III UN Ill llllllll llll Ill Nil Ill“ 0.25 I 180°C 5 200°C 1 Y J»- 40 ~ 20 l 1 l I O O w \0 120 - lOO - (um! 01/8)I.~1w %L101 Figure 6.4: MFI of maleated samples (with 2% maleic anhydride). 90 6.1.3 Thermogravimetric Analysis As stated in Section 5.1.3, an increase in the decomposition temperature results in a more thermally stable product. The maleated samples have decomposition temperatures which range from 2-7°C below that of PLA (see Table 6.2). This is to be expected as the maleated samples, in general, are of lower molecular weight. An exception to this is the 180°/O.1 sample which has a slightly higher decomposition temperature. This is explained by the fact that the 180°/O.1 sample is of a higher molecular weight than the other samples. Table 6.2: Maleated decomposition temperatures from thermogravimetric analysis. ID # Temperature (C) wt°/o L101 I Onset Value (C) Max. Value (C) l 1 T’T—W—fl 316.4 2 180 0.1 317.3 322.5 3 180 0.25 313.8 318.3 4 180 0.5 309.2 314.6 5 200 0.1 314.9 319.0 6 200 0.25 314.5 319.1 7 200 0.5 309.2 314.9 91 6.2 Proposed Reaction Mechanism The formation of a radical is the first step in the maleation of polylactide. The radical formation is the same is it was for the branching of PLA (see Figure 5.16). Once the radical is formed, hydrogen abstraction can occur producing a polylactide which may react with the maleic anhydride radical. The resulting polymer radical may then combine with another radical (MA, peroxide, or polymer radicals or hydrogen) to complete the reaction (see Figure 6.5). The homopolymerization of maleic anhydride is considered by many to be another significant reaction when grafting MA onto polymer backbones [Gaylord et al. (1983b, 1989)]. Recently; however, Russell (1995) discussed a thermodynamic argument based on the ceiling temperature of poly(maleic anhydride) in which the formation and grafting of poly(maleic anhydride) during maleation in the melt (at temperatures greater than 160°C) would not occur. In the maleation of PLA, the high shear stress in the extrusion process may inhibit the homopolymerization of MA. In any case, the homopolymerization of MA without grafting is assumed to be unlikely for the conditions which were used. Another possible reaction is beta scission of the polylactide backbone by the free radical leading to an cue-formation which may react with MA (see Figure 5.17, chain scission). A similar process was proposed by De Roover et al. (1995) for 92 the maleation of PP; however, their experimental observations showed that very severe conditions were needed to favor the ene-reaction with MA including: (1) very low PP molecular mass, (2) very high concentration of MA, (3) high temperature and pressure, and (4) long reaction times. FTIR analysis did not support their (De Roover et al.) theory that an ene-reaction with MA brought about by beta scission occurred. For the maleation of PLA, only 2 % MA is added to the reaction (low concentration) and the reaction time is under 2 minutes; therefore, the possibility of an ene-reaction of PLA has been ruled out. The ketone formed by the beta scission of PLA (which corresponds to the cue-formation of PP) would be structurally unfavorable for the grafting of maleic anhydride. 93 R( )( )R ———-—> 2 R0- 0 CH, R()- + +C—o—C-j— -—————> H 0 CH, 1 I ' (1) +( ——[—C-~O~——C-]— R)H 0 CH, C=C Ii I (1) + c” 'C‘—> ‘fC—O—C—} 0,, o .0 I C—CK c \C.~ o o -0 Recombination H-,RO-,MA- 0 CH, I I +c —o —— c —} x I C—C \ X=H,RO,MA c C. o] o .0 Figure 6.5: Proposed mechanism for the maleation of PLA. 94 As in the case for the branching of PLA, chain scission of the polylactide chain may also occur (see Figure 5.17 (chain scission)) with the resulting radical becoming available for reaction with MA. Similarly, thermohydrolysis and back- biting may still happen (see Figure 5.18) leading to further degradation of the polylactide chain. Again, the above reaction is a proposed mechanism for the grafting of maleic anhydride onto the polylactide polymer backbone. Gaylord and others have been working on the maleation mechanism (with PP and PE) for several years now with no explicit results. Actual characterization of the true mechanism for PLA maleation would be difficult, requiring a much more in depth study. Chapter 7 POLYLACTIDE BLENDS A polymer blend consisting of two or more polymeric materials can be tailored to industrial needs. Polymer blends have several advantages over polymers including cost and time for development (7-10 years for a new polymer, 2-4 years for a new blend) [Meier (1991)]. Combining polylactide with natural materials and synthetic polymers provides a way of cost reduction and combined polymer properties. The objective of this part of the study is to find miscible or compatible blends of PLA with other polymers. 7.1 Blend Theory 7.1.1 Miscibility Polymer blends may be miscible, partially miscible, or immiscible. When a mixture of polymers forms a single thermodynamically compatible phase, a miscible blend is formed. A single glass transition temperature is indicative of a miscible or partially miscible blend. Electron microscopy of a miscible blend will show polymer homogeneity. Miscible blends also exhibit combined physical properties. Films of miscible blends are generally transparent. 9S 96 Immiscible blends are characterized by high interfacial tension and poor adhesion between phases. Macrophase separation generally occurs resulting in poor material properties such as tensile strength and elongation. Whether or not two polymers are miscible depends on the free energy of mixing, AG", AGm = AH", - T ASm where AH", is the enthalpy of mixing, T is the temperature, and ASm is the entropy of mixing. For the polymer blend to be miscible, AGm must be negative. The entropy and enthalpy of mixing are defined by Flory-Huggins as the following [Meier ( 1991)]: AS" =—flln¢. law, VR V, Vz AH»: _ fl VR,-m¢2§ where V is the volume, R is the gas constant, 4:; is the volume fraction of polymer i, I]; and '5 are the molar volumes, and x is the interaction parameter. For high molecular weight polymers, the entropy of mixing is very small so the free energy of mixing is determined by the enthalpy of mixing which is positive for most systems. Specific interactions such as acid-base and hydrogen bonding 97 can occur which enables AH,n to be negative and hence the polymer blends will be miscible. 7.1.2 Compatibility Polymer blends are considered compatible if a desired or beneficial result occurs when the polymers are mixed. Compatible blends are not necessarily miscible. Compatibilized blends are immiscible blends which have been altered by methods such as surface modification, grafting, or the addition of a compatibilizing agent. This modification lowers the interfacial tension and increases the adhesion between the polymers resulting in a product which has specific properties desired by industry. The modification of polylactide with maleic anhydride was performed to improve the adhesion of PLA to various polymers and fillers. 7.2 Materials 7.2.1 Cellulose Acetate and its Derivatives Cellulose acetate (CA), cellulose acetate propionate (CAP), and cellulose acetate butyrate (CAB) were chosen because of reports from literature on the miscibility of polyesters with CA, CAP, and CAB. These materials were provided by Eastman Kodak. 98 7.2.2 Polypropylene Polypropylene was selected because of its similarity to PLA in its methyl group. PP has a density of about 0.85 g/cm3 and a T0 of -17°C. CH. 0 CH, II I I +CH2_ Gil—)3 -(-—C—O—CH—-);| PP PLA Figure 7.1: Comparison of PP and PLA. 7.2.3 Poly(vinyl acetate) PVA was selected because of its structural similarity to PLA, its good theoretical background for miscibility, and its potential application in drug delivery systems. The high molecular weight PVA was provided by the Aldrich Chemical Company. It has a density of 1.191 g/cm3. H H +<|:— 240°C), it was eliminated as a possible blend material with PLA. CAP and CAB can be extruded at a temperature of 210°C. Initial studies were done with blends of 70% CAP and 30% PLA. Transparent extrudate was observed for this composition at a processing temperature of 230°C. However, SEM studies show that two phases are present, i.e., CAP and PLA are not miscible. This phenomenon can be seen in Figure 7.3. Solution casting of PLA with CAP/CAB in THF and methylene chloride did not result in transparent films. The 70/30 blend was also run in the Haake nrixer at a set temperature of 210°C for 30 minutes. The actual recorded temperature was ~ 220°C. This increase in temperature may be due to the shear forces in the mixer. The resulting resin was not transparent. Further analysis via SEM also revealed two phases. The transparent extrudate obtained from extrusion is presumably due to PLA degradation. 102 Figure 7.3: SEM micrograph of 70% CAP and 30% PLA showing immiscibility. 7.4.2 Blend of PLA with PP A blend of 70% PP with 30% PLA was attempted in the Haake mixer at 190°C for 20 minutes. The blend showed no miscibility. 7.4.3 Blends of PLA with PVA Solution casting was conducted for a range of PLA/PVA compositions. The resulting films were transparent. Initial SEM results show compatibility and possible miscibility at some compositions. These films are not very stable under the electron beam, so care must be taken to avoid charging. The 70% PLA/30% 103 PVA blend appeared to be miscible, i.e. two distinct phases are not apparent, by SEM. The 40% PVA/ 60% PLA blend; however, was immiscible as two phases were present when viewed by SEM. Small circles of PVA were apparent in the polylactide polymer matrix. DSC studies show a reduced glass transition temperature (T0) of the blends; however, the T05 for both PLA and PVA are very close (58°C and 42°C, respectively) so it is difficult to tell if there is really only one Tc, which would indicate miscibility or if there are two TGs which would be indicative of immiscibility. Table 7 .2 shows the T05 as given by DSC. Table 7.2: Glass transition temperatures for PLA/PVA blends. Sample To (°C) Pure PLA 57.8 Pure PVA 42.0 70% PLA, 30% PVA 47.2 60% PLA, 40% PVA 46.3 104 7.4.4 Blends of PLA with EVAC Copolymers Blends of 70% PLA with 30% ELVAX 150 and 350 were dissolved in CH2C12 and then cast. The resulting films were not transparent; however, they appeared to be exceptionally strong. SEM analysis revealed two phases; however, the compatibility between PLA and ELVAX 350 was very good. 7.4.5 Blends of PLA with Starch This set of experiments was done to compare the interfacial adhesion properties between starch and (1) PLA, (2) PLA with Lupersol 101, and (3) PLA, Lupersol 101, and maleic anhydride. Figure 7.4 is an SEM nricrograph of a 60% PLA/40% starch blend showing poor interfacial adhesion. Figure 7.5 is of a 30% starch/70% PLA blend with 0.5% L101 (PLA wt. basis). This micrograph shows some interfacial adhesion, but separation of the blend is readily apparent. The addition of MA provides end groups which should improve the interfacial adhesion. Figure 7.6 is an SEM micrograph of a 30% starch/70% maleated PLA blend. The maleated PLA consists of PLA, 2% MA, and 0.5% L101. The interfacial adhesion seen here is very good. It is thus concluded that the addition of even a small amount of maleic anhydride onto the polylactide backbone will improve the interfacial adhesion of polylactide 105 blends. These blends may be incorporated into single-use biodegradable disposable items in the future. Figure 7.4: SEM micrograph of 60% PLA and 40% starch blend showing poor interfacial adhesion. 'x/M l8l :08; 8.388 Sm 93% >82 2mm ”9w 2:»an >mx®«8.9 umma "Hm hfixmm mCM£Um¢Gm 3m mu} 888.9 Sam.@« .¢mmw« érrqr --__-,_.-] _ o¢¢.« "sad.“ “so Hom @ u >mxsao.a “romtzo swung emrmamrmm Ham asmmrza 121 . -, ' -1. as 28KU X188 8882 188.8U CE894 Figure 8.7: SEM micrograph showing the surface of the modified starch with talc blend. («f zaKu x195. aaaz 188.8U‘CE094 I Figure 8.8: SEM dot map of the X-ray analysis for silicon. Chapter 9 CONCLUSIONS AND RECOMMENDATIONS 9.1 Conclusions Branching The free radical branching of polylactide via reactive extrusion is a novel concept which has been applied in the past to polyolefms such as polypropylene and polyethylene, but it has never been applied to polylactides. The branching of PLA by reactive extrusion provides an in-situ method of improving the processibility of PLA which could then be used in blow molding and injection molding applications. The results of this investigation indicate that polylactide branching is favored at temperatures around l70°-l80°C with an initiator concentration of about 0.1 - 0.25 %. Highly branched systems, which may include microgelation, are favored at initiator concentrations of 0.5% at about the same temperature range. Chain scission is favored at higher temperatures (T >190°C). Without the presence of initiator, PLA undergoes rapid degradation which may be attributed to chain 122 123 scission due to thermohydrolysis and back-biting. This degradation may hinder the applicability of PLA to blow molding and injection molding processes. The goal of this research has been met. The molecular weight of extruded/processed PLA has been increased with results that are comparable to unextruded, unprocessed PLA. The melt flow index is also increased, i.e., an increase in the melt viscosity is observed which may improve the blow molding processibility of PLA. Maleation The successful branching of PLA encouraged us to study the maleation of PLA by reactive extrusion processes involving free radicals. Such maleated PLA is of prime interest in order to promote good interfacial adhesion between inorganic fillers and PLA resins. Back-titration analysis of the maleated PLA showed that between 0.066 and 0.672 % maleic anhydride was grafted to the polylactide backbone. Increasing the amount of peroxide initiator led to an increase in the grafting of MA; whereas, extrusion temperature had little effect on the maleation reaction as was expected from the branching study. l 24 Polylactide Blends Polymer blends are increasingly important from an industrial standpoint as they allow for the tailoring of resin properties. An attempt was made to blend PLA with several different substances including: (1) cellulose acetate derivatives, (2) polypropylene, (3) poly(vinyl alcohol) and ethylene vinyl alcohol copolymers (EVAC), and (4) starch. The PLA and starch blend proved to be most interesting. The interfacial adhesion between the extruded starch and PLA blend was poor; however, the addition of maleic anhydride, which grafts onto the PLA backbone, resulted in good interfacial adhesion between the substances. Related Work Laser scanning confocal microscopy is a non-invasive tool which can be used to study polymer blend morphology. LSCM is especially useful for biopolymers and their blends as many of the readily available fluoroprobes are designed for biological applications. LSCM can be beneficial in the identification of some of the characteristics in polymer blends including distribution and/or adhesion of the constituents and flow patterns without introducing artifacts usually associated with sample preparation. The LSCM which was done showed how a protein filler could be distinguished fi'om a starch matrix. Characterization of an extruded modified starch matrix with a tale filler was also attempted by LSCM. Theoretically, all of the starch in this sample has been melted to a phase which fluoresces. If this is indeed the 125 case, the parts which do not fluoresce are either air, plasticizer, or contaminants; however, there currently is not a way to make a positive identification. Therefore, LSCM was not a viable option in the extruded starch/talc blend. Scanning electron microscopy is another important fundamental tool in studying polymer blend systems. Not only were we able to evaluate the morphology of several polymer blends using SEM, we were also able to use X-ray analysis, in conjmrction with dot mapping, to view the dispersion of tale in an extruded modified starch matrix. 9.2 Recommendations Branching A patent application which covers the branching and maleation of polylactide via reactive extrusion is currently in progress. Two papers (Carlson, D.L., P. DuBois, R. Narayan, L. Nie, "Free Radical Branching of Polylactide by Reactive Extrusion" and "Maleation of Polylactide by Reactive Extrusion") are to be submitted to Polymer pending review of the patent application. The following recommendations are made for further research on the modification of polylactide via reactive extrusion. Optimization of the extrusion conditions, temperature and % initiator, should be done to maximize the processibility of 126 PLA. To aid in controlling the reaction, the maximum amount of initiator which can be added before the occurrence of crosslinking should also be determined. Various maleic anhydride concentrations should be tried in the grafting of MA to PLA. The PLA samples should be injection molded so that tensile strength and other mechanical testing can be done. Of specific interest might be polymer blend samples which compare the interfacial adhesion as related to strength. Film Extrusion A preliminary study on the feasibility of PLA film extrusion was conducted. Further work needs to be done in evaluating the temperature and initiator concentrations which will yield an optimum product. Starch / Talc System X-ray analysis via scanning electron microscopy could prove to be a useful tool in evaluating the dispersion of talc filler in a starch matrix. However, lower accelerating voltages may need to be employed in the SEM so that depth penetration into the sample is minimized. A higher magnification also may need to be used to exploit the resolution of the instrument and utilize the size of the tale particles (about 6 microns). 127 LSCM The LSCM has the capability of distinguishing several constituents of a polymer blend. To accomplish this, different fluoroprobes, which signal at different frequencies, must be used for each component. Using different lasers, one has the ability to single out each constituent. These images may then be overlaid to get the complete picture of the blend system. The LSCM may be a useful tool in evaluating polymer blend systems as discussed previously in the Conclusions. Avenues for utilizing LSCM in other polymer blend systems should be addressed. APPENDIX - RAW DATA 128 a... a... H... w... an... a... .2 3... NZ 2.. .3 8.. a... 2.. an. 8.0... 2.... s... .0... cm... 8... no... 8... 3... 8... a... 8... 3.: N38 .8. :3 .535... m3. 2... .3.. 3.2 3.... a: m... 2.... S... 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Conditions used: 10" gage length 1.0 in/min rate of grip separation 1.25" sample width 0.1 in/in*min initial strain rate 0.1 lb preload pure pla at 170C (340F) sample thickness: .005" tensile sample # max lbs max psi break lbs break psi break % break region modulus (psi) pla.200 17.43 2789 16.6 2656 4.87 grip 174.4 pla.201 17.05 2729 17 2721 2.67 center 149.7 pla.202 19.87 3180 19.72 3155 2.44 grip 184.5 pla.203 18.28 2925 18.22 2915 2.55 center 168.4 pla.204 18.77 3004 17.66 2826 3.24 grip 185 pla.205 17.56 2810 15.42 2467 4.63 center 169.2 pla.206 20.18 3229 18.75 3000 5.81 grip 175.3 pla.207 19.65 3145 19.65 3144 2.52 center 187.6 Average 18.60 2976.38 17.88 2860.50 3.59 174.26 standdev 1.21 193.13 1.51 241.08 1.32 12.32 % dev. 6.49 6.49 8.43 8.43 36.71 7.07 pla at l70C (340F) with 0.1%L101 sample thickness: .003" , tensile sample # max lbs max psi break lbs break psi break % break region modulus (psi) pla01. 100 9.68 2581 9.393 2505 4.21 center 136 pla01. 101 9.619 2565 8.496 2266 3.52 grip (top) 147.5 pla01.102 10.17 2713 9.698 2586 2.94 grip (top) 151.3 pla01. 103 9.43 2515 6.226 1660 3.46 grip (bottom) 157.5 pla01.104 10.99 2931 10.19 2718 4.35 center 163.8 plaOl.105 10.14 2703 10.14 2703 ' 2.23 grip (top) 152.9 pla01. 106 10.08 2687 9.833 2622 3.22 center 130.9 pla01. 107 10.03 2676 4.504 1201 8.89 center 139.1 Average 10.02 2671.38 8.56 2282.63 4.10 147.38 standdev 0.48 127.81 2.09 558.16 2.05 11.26 % dev. 4.78 4.78 24.45 24.45 49.94 7.64 136 .....N 8N 8... 8... 8... .... .... 8.. ....N ....N 8... ...... 8.. 8... 8... 8...... ...... 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NN. 88... 82.: ......NN. 8...-... . v. 8. 8 ...-8. 8. 28.4.8. .....2 2-3 22-3 22-3 ....-22 882 2.3 22-3 :23 N......8N N...N N...N 8... ..N. 8... 8... 8... 8N SN :.N 3.... .3 NN.N 8N .N.N .28.... ...... No... 8... ..N... N... ...... ...... 8... No... ...... ...... ..NNN .NNN .NNN 88 8.....82 8N- 8... 8. N...N. 8.... ....N. N... 8... R... 8... .N.. NNNN. N88. 8N... 8.8 .88.... 8.1 8... 8.. N.N. N... N. 2.. N8... N8... 8... .N.. 8...... 8:2 8.:. 8.8.. N N...N- 8... 8. N.N. .... ...N. 2. N8... 8... .8... .N.. 88 882 88... 82.. N N...N- E... 8.. N.N. 4. ...N. 2.. N3... 8... ..8... N. .88 882 8:: .88 _ v. 8. 8 ...-8. N8. 28. 2-8. .....2 2-3 22-3 22-3 ...-22 882 2.3 22-3 :23 N.N...aN. NNN NNN 88.. NN. N... N... 8... N...N N... :.. N4... ..NN 8.N N...N SN 282 N... No... No... NN... .N... N... ...... No... ...... ...... ...... 88 88 NN..N 8: 58.88.. 8...- 8... 8.. 2.2 8.... N...N. 2. 8.... 8... N..... ..N. 8.8 $82 8N2. N88 888.2 ....N. 8... 8.. 2 ...... ...N. 2. 8... NNN... N.-... N. 88.. 882 88: 88.. .- 8.N- R... 2... 2 N... N.N. 2.. N8... NNN... NNN... N. 8.8 822 ....NN: 88.. N 2...: .N... 8.. N2 ...: N.N. 2.. N8... 8.... .2... .N.. 88... 88: 8...: 8.8 . v. 8. 8 ...-8. N8. 28. =8. ...-2 2.3223 :23 ...-22 882 2.3 22-3 :23 N.NN........_ ....8 ...... 2.3 138 2.4 .N... .N.. N... N... .N.N ..N.N NN... 8.N .... NNN .N.. 8.4 :.N NNN .28... :... 8... No... ...... NN... NN... No... 8... No... No... 4.... .N.. 88 82 8N4 28.8... mo...- wn... a... 8.3 8.2 3.2 m... S... 2.... 8.0 an. oomoo mgr-m. ...-.$..: :..me 09:02... N... .... ... ..N. ..N. 4.N. N... .N.... N.N... N2... NN. 8:8 ..842 88282. N :...- .. ... ...... N.N. N.N. :.. NN.... N.N... NS... NN. 88. .....NN. 88428.8 N S6- 3... 8.. ..n. 8.2 m. N... .3... m3... m8... 2.. coo... coma... 83.: co... _ v.8. . E-.. N... 2.. a... ...-2 2.3223 :23 ...-22 .882 N2.3 22-3 :23 N.N...8N N..N NN.N 8... .N.. NN. .... ..N... NN... NN... NN... .... N.N N4... ....4 .2 .288 :... N..... 8... NN... NN... NN... ...... ...... ...... ...... No... 82. N4... .84 84.. .288... .N.- ...... 8.. NN.N. :.4. 8.. 2.. ...... .... 8... ..N. 84.. NNNoN. N28 S48 08...... N.N- 8... 8.. N.N. N.N. N.N. 2.. ...... 8.... .N..... N. 8.8 8.8. 8N4. 8.8 N ......- .... 8.. N.N. 4.... N.N. 2.. 8.... NN.... ..NN... .N.. 8.... 824. 88... 8.... N N.N- 8... 8. ..N. ..4. N. N... N..... N..... N8... .N.. 84.. 882 88.. .88 . v. 8. . ...-m. N... 2.. .... ...-2 2.3223 :23 ...-22 .882 N2.3 22-3 ..2.3 22.28. .88 4... 2....- 139 Table A.5: Titration results of maleated samples. Titrating against 0.004 M NaOH Normality of morpholine 0.05238 Normality of HCl 0.007179 Temperature sample moles of grams ID # (°C) %L101 Wt (g) mL HCl anhydride anhydride % anhydride“ 1 180 0.5 1.07 4.628 7.154E—05 0.007 0.6556 2 180 0.5 1.077 4.65 7.138E-05 0.007 0.6499 3 180 O 1.021 13.63 6.91E-06 0.0007 0.0664 4 180 0.25 0.914 7.792 4.882E-05 0.0048 0.5238 5 180 0.25 0.868 9.341 3.77E-05 0.0037 0.4259 6 180 0.1 1.01 11.173 2.455E-05 0.0024 0.2383 7 180 0.1 0.9 11.836 1.979E-05 0.0019 0.2156 s '200 0.1 0.996 10.651 2.83E-05 0.0028 0.2786 9 200 0.1 1.051 10.387 3.019E-05 0.003 0.2817 10 200 0.25 0.796 9.421 3.713E-05 0.0036 0.4574 1 1 200 0.25 0.849 8.927 4.067E-05 0.004 0.4698 12 200 0.5 1.004 5.445 6.567E-05 0.0064 0.6414 13 200 0.5 1.13 3.314 8.097E-05 0.0079 0.7026 * weight percent of sample which is anhydride, i.e., that has maleic anhydride functional group BIBLIOGRAPHY BIBLIOGRAPHY Akkapeddi M.K., et a], Polymer Preprints 29, 567 (1988). Argyropoulos, D.C., L. Nie, and R. 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