IMPROVED WATER RESISTANCE AND BARRIER PROPERTIES OF POLYVINYL ALCOHOL WITH POLYURETHANE SILOXANE COATING FOR PACKAGING APPLICATIONS By Vijay Pandian A THESIS Submitted to Michigan State University i n partial fulfillment of the requirements f or the degree of Packaging Master of Science 2018 ABSTRACT IMPROVED WATER RESISTANCE AND BARRIER PROPERTIES OF POLYVINYL ALCOHOL WITH POLYURETHANE SILOXANE COATING FOR PACKAGING APPLICATIONS By Vijay Pandian Flexible high - barrier (HB) materials offer exciting potential applications with relevance to numerous key industries including packaging. Despite their promising potential applications, the existing flexible HB materials have limited appli cations due to prevailing drawbacks such as poor optical clarity and limited scalability. The objective of this study was to investigate a new scalable approach for flexible HB fabrications with good optical properties. This new approach relies on the use of hydrophobic fabrication of polymers that reduces the solubility of the permeants (oxygen, water vapors) in a film/plastic and hence the barrier properties can be improved drastically. In this study, polyvinyl alcohol (PVOH) was selected as a model pol ymer to increase its water vapor barrier properties and oxygen barrier properties at high relative humidity. PVOH is a biodegradable synthetic polymer with good optical clarity and excellent oxygen barrier under dry conditions. However, at high relative h umidity, the barrier properties are drastically reduced due to the bonding of hydrogen between water and PVOH. When a thin layer of polydimethylsiloxane polyurethane (PDMS PU) was applied onto PVOH films, the water vapor and oxygen barrier properties of P VOH were drastically improved as quantified by permeability and other analytical tests. Interestingly, the tensile strain and impact strength of the coated films were enhanced after applying the coating. Besides, the obtained coated films were optically cl ear that makes them appealing for packaging applications. This novel HB fabrication approach can be applied to other packaging plastic such as PET, PLA, LDPE, etc. Copyright by VIJAY PANDIAN 2018 iv Dedicated to my mother T he nmozhi P andian v ACKNOWLEDGEMENTS I would like to take this opportunity to sincerely express my gratitude to my advisor Dr. Muhammed Rabnawaz for the constant , never ending support for my M.S. study . I would like to thank him for accepting me as his first student at Michigan state university. He was always there when I needed him, with his immense knowledge, patient demeanor and continuous encouragement. He pushed me at times when I was about to give up and helped me to grow as a p erson. Without him this research would never be possible. Besides my advisor , I would like to thank the rest of my thesis committee, Dr. Maria Rubino and Dr. Mojgan Nejad for their insightful comments and valuable feedback that helped me to improve the qua lity of this research work. My sincere thanks go to Mr. Aaron Walworth, who gave me essential laboratory training s. Without his support, these experiment for would never be possible. I thank my lab mates (Fahad Khan , Krystal Cheng , Zhao Li ) , for the challenging discussion s and endless support. I would also like to thank my friends: Shruthi Kumarraj , Gauri Awalgaonkar , Sonal Karkhanis and Yuzhu Liu for constantly cheering me up. Last but not the least, I would like to tha nk my family for giving me all the strength through this entire process. vi TABLE OF CONTENTS LIST OF TABLES ................................ ................................ ................................ ....................... viii LIST OF FIGURES ................................ ................................ ................................ ....................... ix Chapter 1 ................................ ................................ ................................ ................................ ......... 1 Introduction ................................ ................................ ................................ ................................ ..... 1 1.1 Introduction ................................ ................................ ................................ ............................... 1 1.2 Objectiv e ................................ ................................ ................................ ................................ ... 4 1.3 Hypothesis ................................ ................................ ................................ ................................ . 4 1.4 Approach ................................ ................................ ................................ ................................ ... 5 1.5 Structure of thesis ................................ ................................ ................................ ..................... 5 REFERENCES ................................ ................................ ................................ ............................... 6 Chapter 2 ................................ ................................ ................................ ................................ ......... 9 Background and literature review ................................ ................................ ................................ ... 9 2.1 Background information on poly (vinyl alcohol) ................................ ................................ ..... 9 2.2 Semi crystalline polymers ................................ ................................ ................................ ....... 10 2.2.1 Microstructure ................................ ................................ ................................ .................. 10 2.2.2 Ta cticity of PVOH ................................ ................................ ................................ ........... 12 2.3 Physical properties ................................ ................................ ................................ .................. 13 2.4 Chemical properties ................................ ................................ ................................ ................ 14 2.5 Limitations of PVOH ................................ ................................ ................................ .............. 14 2.6 PVOH/Nano clay ................................ ................................ ................................ .................... 14 2.7 PVOH/Polymer blends ................................ ................................ ................................ ............ 15 2.8 Graphene oxide/PVOH ................................ ................................ ................................ ........... 16 2.9 Silicon oxide/PVOH ................................ ................................ ................................ ............... 17 2.10 Polydimethylsiloxane (PDMS) ................................ ................................ ............................. 18 2.11 Polyurethane siloxane coating ................................ ................................ .............................. 19 REFERENCES ................................ ................................ ................................ ............................. 20 Chapter 3 ................................ ................................ ................................ ................................ ....... 25 Experimental ................................ ................................ ................................ ................................ . 25 3.1 Sample preparation ................................ ................................ ................................ ................. 25 3.1.1 Materials ................................ ................................ ................................ ........................... 25 3.1.2 Prepar ation of PVOH films ................................ ................................ .............................. 26 3.1.3 Preparation of polyurethane siloxane (PU - S) coating solution ................................ ........ 27 3.1.3.1 Preparation of PU - S coating with PDMS - A ................................ .............................. 27 3.1.3.2 Preparation of PU - S coating with PDMS - B ................................ .............................. 27 3.1.4 Applying PU - S coating on PVOH films ................................ ................................ .......... 28 3.1.5 Coated PVOH films ................................ ................................ ................................ ......... 30 3.2 Characterization ................................ ................................ ................................ ...................... 30 3.2.1 Contact angle measurement ................................ ................................ ............................. 30 vii 3.2.2 Sliding angle measurement ................................ ................................ .............................. 31 3.2.3 Optical transmittance measurements ................................ ................................ ................ 32 3.2.4 Water uptake analysis ................................ ................................ ................................ ....... 33 3.2.5 FTIR - ATR measurements ................................ ................................ ................................ 33 3.2.6 Tensile properties ................................ ................................ ................................ ............. 34 3.2.7 Dart drop impact test ................................ ................................ ................................ ........ 34 3.2.8 Barrier properties ................................ ................................ ................................ .............. 35 3.2. 9 X - ray photoelectron spectroscopy (XPS) ................................ ................................ ......... 36 3.2.10 Scanning electron microscopy (SEM) ................................ ................................ ............ 36 3.2.11 Thermo gravimetric analysis (TGA) ................................ ................................ .............. 37 3.2.12 Cross - cut adhesion test ................................ ................................ ................................ ... 37 REFERENCES ................................ ................................ ................................ ............................. 39 Chapter 4 ................................ ................................ ................................ ................................ ....... 41 Results & Discussi on ................................ ................................ ................................ .................... 41 4.1 Water - resistance ................................ ................................ ................................ ...................... 41 4.1.1 Contact angle measurement ................................ ................................ ............................. 41 4.1.2 Sliding angle measurement ................................ ................................ .............................. 45 4.2 Optical transmittance ................................ ................................ ................................ .............. 46 4.3 Weight gain analysis ................................ ................................ ................................ ............... 48 4.3. 1 Weight gain analysis by weight gain approach ................................ ................................ 48 4.3.2 ATR - IR analysis for water gain analysis: ................................ ................................ ........ 49 4.4 Mechanical properties ................................ ................................ ................................ ............. 52 4. 4.1 Tensile test ................................ ................................ ................................ ........................ 52 4.4.2 Drop dart impact test ................................ ................................ ................................ ........ 54 4.5 Barrier properties ................................ ................................ ................................ .................... 56 4.5.1 Permeability of oxygen and water vapor ................................ ................................ .......... 56 4.6 Surface analysis ................................ ................................ ................................ ...................... 59 4.6.1 XPS analysis ................................ ................................ ................................ ..................... 59 4.6.2 SEM analysis ................................ ................................ ................................ .................... 60 4.7 Thermal analysis ................................ ................................ ................................ ..................... 61 4.7.1 TGA ................................ ................................ ................................ ................................ .. 62 4.8 Crosshatch adh esion test ................................ ................................ ................................ ......... 63 REFERENCES ................................ ................................ ................................ ............................. 64 Chapter 5 ................................ ................................ ................................ ................................ ....... 66 Conclusion ................................ ................................ ................................ ................................ .... 66 5.1 Conclusions ................................ ................................ ................................ ............................. 66 5.2 Future work ................................ ................................ ................................ ............................. 67 5.3 Impact o f this research ................................ ................................ ................................ ............ 68 REFERENCES ................................ ................................ ................................ ............................. 70 viii LIST OF TABLES Table 4.1: Contact angle measurements for PVOH and coated PVOH samples .......................... 43 Table 4.2: Sliding angle values for PVOH and coated PVOH samples ................................ ....... 45 Table 4.3: Weight gain analysis for PVOH and coated PVOH samples and also in comparison with % decrease of water gain w.r.t to neat PVOH ................................ ................................ ...... 49 Table 4.4: FTIR - ATR data for PVOH and coated PVOH samples focusing on area under the peak for OH peaks from 3200 cm - 1 to 3500 cm - 1 ................................ ................................ .......... 51 Table 4.5: Tensile strength and % of elongation data for PVOH and coated PVOH samples ..... 53 Table 4.6: Failure mass data for PVOH and coated PVOH samples ................................ ............ 55 Table 4.7: Permeability data for PVOH and coated PVOH samples for oxygen and water vapor ................................ ................................ ................................ ................................ ....................... 58 Table 4.8: Atomic compositions data obtained with XPS for coated PVOH samples. ................ 60 ix LIST OF F IGURES Figure 1.1 : Schematic presentation of PU and PU/S coating on PVOH films. .............................. 4 Figure 2.1 : Chemical structure of PVOH. ................................ ................................ .................... 10 Figure 2.2: Synthesis of PVOH from PVAc in the presence of an alkaline catalyst. ................... 10 Figure 2.3: Lamella structure in semi - crystalline polymer [7]. ................................ .................... 11 Figure 2.4: Microstructure of semi crystalline polymer [57]. ................................ ....................... 11 Figure 2.5: Stereo chemical isomers of PVOH [56]. ................................ ................................ .... 13 Figure 2.6: Illustration of intermolecula r H - bonding in the PVOH/PPG blends .......................... 16 Figure 2.7 : Structure of PDMS ................................ ................................ ................................ ..... 18 Figure 3.1: The chemical structures of materials used for the coating study. .............................. 25 Figure 3.2 : Illustration of PVOH film preparation process. ................................ ......................... 26 Figure 3.3: Dip coating process for the PVOH coating. ................................ ............................... 29 Figure 3.4 : Contact angle of a liquid drop on a flat surface [11]. ................................ ................. 31 Figure 3.5 : Sliding angle of a liquid drop on an inclined surface. ................................ ................ 32 Figure 3.6: Schematic presentation of cross - cut adhesion test [12]. ................................ ............. 38 Figure 4.1: Damage caused by water droplet on PVOH film surface ................................ .......... 42 Figure 4.2 : Contact angle images for different PVOH coated samples taken by the VCA optima instrument ................................ ................................ ................................ ................................ ..... 43 Figure 4.3: Chemical surface modification of PVOH films using PDMS ................................ .... 44 Figure 4.4: Water droplet behavior on the surface of uncoated PVOH ................................ ........ 45 Figure 4.5: Visua l difference between neat PVOH and coated PVOH samples. (A) PVOH, (B) PDMS - A/PVOH, (C) PDMS - B/PVOH. ................................ ................................ ....................... 47 Figure 4.6: % T vs wavelength for PVOH and coated PVOH samples from 200 nm to 900nm emphasizing more on the visible range (400nm to 700nm) ................................ .......................... 47 x Figure 4.7: FTIR - ATR spectra for (A) PVOH (B) PU/PVOH (C) PDMS - A/PVOH (D) PDMS - B/PVOH from 400 cm - 1 to 4000 cm - 1 ................................ ................................ ........................... 50 Figure 4.8: Graphic al presentation of tensile strength and % of elongation for PVOH and coated PVOH samples ................................ ................................ ................................ .............................. 53 Figure 4.9: Graphical presentation of failure mass for PVOH and coated PVOH samples. ........ 55 Figure 4.10 : Schematic illustration of the PU/PVOH with urethane bonds on their surface. ...... 57 Figure 4.11 : Schematic illustration of the PDMS/PVOH with PDMS showing PDMS polymer chains on the surface. ................................ ................................ ................................ .................... 57 Figure 4.12: Graphical presentation of oxygen and water vapor permeability at 23°C 75%RH for PVOH and coated PVOH samples. ................................ ................................ ............................... 58 Figure 4.13: SEM images for films of A) PVOH, (B) PU/PVOH, C) PDMS - A/PVOH, (D) PDMS - B/PVOH. The images were recorded at a magnification of 100,000x. White scale bar shown in the figure is equal to 200 nm. ................................ ................................ ........................ 61 Figure 4.14: TGA profile for PVOH, PU/PVOH, PDMS - A/PVOH and PDMS - B/PVOH .......... 62 Figure 4.15: Crosshatch adhesion test (A) Scratched film (B) Black electrical tape after attempti ng to remove crosscut coat from the scratched film. ................................ ....................... 63 Figure 5.1: An example of the application of our developed high barrier coatings in the packaging industry [2]. ................................ ................................ ................................ ................. 68 1 1.1 Introduction Polymeric films are used in packaging applications due to their lightweight , low - cost, good mechanical properties . However, polymers often have average barrier properties and are plagued with poor biodegradability [ 1] . Poly (vinyl alcohol) (PVOH) is a uniq ue polymer because it has good barrier properties at low relative humidity (RH) and is also biodegradable. However, poor water resistance of PVOH has limited its application [ 1 - 3] . Also , at high RH , PVOH barrier properties are drastically reduced. If water resistance of PVOH films are increase d , this polymer can find many applications because of its good barrier and biodegradable nature. PVOH films are optically clear , suitable for packaging applic ation s . PVOH is mainly used for vinylon pro duction [16] , as well as a packaging material for detergents, and used as water - soluble bags for collecting contaminated gowns in hospitals [18] . PVOH would have a valuable use for packaging as an alternative to non - biodegradable polymers like LDPE and HDPE. However, PVOH film lose its oxyge n barrier properties at high RH [1,5] , because they absorb water vapor at high RH that cause swelling of the film s , thus increasing their free volume and hence m ake them bad barrier to gas and water molecules [ 1 ,3,6] . In addition, PVOH films cannot be used for direct water - contact applications because they are completely dissolved in water medium. T his undesirable poor water resistance further limits the applicati ons of PVOH in packaging industry [14] . Thus, an improvement in the water resistance properties of PVOH films will increase the use of biodegradable PVOH films in various fields including packaging industry . 2 Several researche r s have worked to improve the water resistance properties of PVOH by using various methodologies including the use of nanocomposites [5], polymer - polymer blends [7], incorporation of graphene oxide [8] , and through sol - gel methods [ 9 ] . However, such approaches resulted in compromi sing other properties like optical transparency and/ or mechanical properties. The aim of this study was to improve the water resistance of PVOH films without compromising its optical and mechanical properties. Our approach is to mask the polar hydroxyl functional group ( - OH) to suppress their water absorption. Other r esearchers have previously achieved this by ch emical cross - linking using the OH groups of the PVOH [10]. Crosslinking also improved the thermal and mechanical properties of the PVOH films [ 2 , 10, 11 ] . However, the barrier properties of oxygen and water vapor and the significance of water resistance have not been addressed properly [10]. In this study, PVOH was coated with PU . PU was chosen because it binds well to PVOH films, and is also optically clear thus maintains the clarity of PVOH films. [ 12 , 13 ] . When PU coating was applied onto the PVOH film , there was little improvement in the water resistance. To improve the wate r resistance of P VOH films, hydrophobic PDMS was incorporated by grafting . PDMS is known to impr o ve the water resistance of polyurethane as reported by Rabnawaz et al . [12] . In this study, our focus was to investigate the effect of PDMS on various properti es of the PVOH films including water and oxygen barrier, water resistance, clarity, tensile and impact strengths . 3 Overall, t his study can be classified into the following seven distinct phases. Phase I - T o prove that the PDMS PU coating improves hydrophobicity of PVOH films . Phase II To prove the reduced water absorption properties of PDMS PU coated PVOH films as compared to neat PVOH. Phase III To prove that the PVOH films retain optical transparency after the coating. Phase IV To prove st ability of the coated PVOH film through thermal analysis Phase V Comparison of mechanical properties of neat PVOH films and PDMS PU coated PVOH films. Phase VI - Analysis of barrier properties of coated PV O H films. Phase VII Surface analysis of coated PVOH films. This study will also cover the potential impact of this research for environment and for industrial applications. 4 1.2 Objective The main goal of this study was to fabricate water resistant PVOH and evaluate their performance including: - W ater resistance and water barrier properties - O xygen barrier properties - M echanical properties - O ptical properties 1.3 Hypothesis We hypothesize that when a water repellent coating is applied on a PVOH film , it would reduce the water absorption. Subsequently, the free - volume increase corresponding to the water absorption will also be reduced. This anticipated reduction in the free - volume will help to increase the water vapor and oxygen barrier properties of the coated films. Therefore, we propose that a P VOH film coated with PDMS - PU (F igure 1.1B ) should have good barrier properties than neat PVOH film (Figure 1.1A ) because PDMS will make the surface highly water - resistant. Figure 1 .1 : Schematic presentation of PU and PU/S coating on PVOH films . (A) (B) 5 1.4 Approach The approach of the study was to use low surface energy PDMS (surface tension= 22.3 mN /m) [12] , to render PDMS PU coated PVOH strongly water repellent. Water has a surface energy of 72.0 mN /m and thus is strongly repellent by hydrophobic PDMS [12] . U.S. Food and Drug A dministration considers PDMS safe for food contact applications [17]. But t he biggest challenge was not being able to directly incorporate PDMS into PVOH solution , due to the lack of complementary reactive groups . In order to incor porate PDMS , first a matrix was chosen. In this case , PU which was blended with PDMS and the entire PDMS PU coating was applied on PVOH surface and cur ed. PU is chosen because of their good adhesion to PVOH and low - cost when compared with other commercial coatings . Another reason for choosing PU in this study is due to the prospect of being a biodegradable polymer by the use of aliphatic urethane. Also, PU films are optically clear and mechanically durable. 1.5 Structure of t hesis Th is thesis is structure d in a way where the rationale of this research is explained in the C hapter 1 , the Introduction and it is followed by a detailed literature review (C hapter 2 ) . Chapter 3 consists of materials and methods used during the experiments. Results and discussion are described in C hapter 4. Chapter 5 covers the summary of the key results and the potential impact of this work . 6 REFERENCES 7 REFERENCES 1. S. E. M. Selke, J. D. Culter, R. J. Hernandez; P lastics packaging; Properties, processing, a pplications and r egulations (2nd ed.), Hanser p ublishers, Munich (2004) . 2. S. Mupalanei, H. Omidian; Polyvinyl alcohol in medicine and pharmacy: a perspective J. Dev. Drugs, 2 (2013), p. 112 . 3. W. H. Hu, Z. H. Zhang, Q. G. Zhang, Q. L. Liu, A. M. Zhu; Pervaporation dehydration of water/ethanol/ethyl acetate mixtures using poly(vinyl alcohol) silica hybrid membranes; J. Appl. Polym. Sci., 126 (2012), pp. 778 - 787 . 4. M. I. Baker, S. P. Walsh, Z. Schwartz, B. D. Boyan; A review of polyvinyl alcohol and its uses in cartilage and orthopedic applications; J. Biomed. Mater. Res. B, 100B (2012), pp. 1451 - 1457 . 5. A. A. Sapalidis, F. K. Katsaros, Th. A. Steriotis , N. K. Kanellopoulos; Properties of poly(vinyl alcohol) - bentonite clay nanocomposite films in relation to polymer clay interactions; J. Appl. Polym. Sci., 123 (2012), pp. 1812 - 1821 . 6. I. Uslu, H. Dastan, A. Altas, A. Yayli, O. Atakol, M. L. Aksu; Preparati on and characterization of PVA/boron polymer produced by an electrospinning technique; e - Polymers, 133 (2007), pp. 1 - 6 . 7. G. Zhu, F. Wang, S. Dong, K. Zu, Y. Liu; Thermal, mechanical and chemical properties of hydrophilic poly(vinyl alcohol) film improved b y hydrophobic poly(propylene glycol); Polym. Plast. Technol. Eng., 52 (2013), pp. 422 - 426 . 8. J. Wang, X. Wang, C. Xu, M. Zhang, X. Shang; Preparation of graphene/poly(vinyl alcohol) nanocomposites with enhanced mechanical properties and water resistance; Polym. Int., 60 (2011), pp. 816 - 822 . 9. A. Bandyopadhyay, M. D. Sarkar, A. K. Bhowmick; Poly(vinyl alcohol)/silica hybrid nanocomposites by sol gel technique: synthesis and properties; J. Mater. Sci., 40 (2005), pp. 5233 - 5241 . 10. M. Krumova, D. Lopez, R. Benav ente, C. Mijangos, J. M. Perena; Effect of crosslinking on the mechanical and thermal properties of poly(vinyl alcohol); Poly m. , 41 (2000), pp. 9265 - 9272 . 11. A. Hasimi, A. Stavropoulou, K. G. Papadokostaki, M. Sanopoulou; Transport of water in polyvinyl alco hol films: effect of thermal treatment and chemical crosslinking; Eur. Polym. J., 44 (2008), pp. 4098 - 4107 . 8 12. M. Rabnawaz, G. Liu, H. Hu; Fluorine - f ree a nti - s mudge p olyurethane c oatings; Angew .Chem. ; volume 127 issue 43 (2015); pp. 12913 - 12918 . 13. M . Rabnawaz, G . Liu; Graft - c opolymer - b ased a pproach to c lear, d urable, and a nti - s mudge p olyurethane c oatings; Angew . Chem. ; volume 127 issue 22 (2015); pp. 6616 - 6620 . 14. - polyvinyl alcohol crosslinked film - performance and J . Environ. Polym. Degrad ., 5:2 (1997), pp. 111 117. J. Reinf. Plast. Compos., 29:4 (2010), pp. 618 629. 16. SRI c onsulting CEH r eport p olyvinyl a lcohol , published March 2007, abstract retrieved July 30, 2008. 17. "Linear Polydimethylsiloxanes" Joint assessment of commodity c hemicals, September 1994 (Report No. 26) ISSN 0773 - 6339 - 26. 18. C.C. DeMerlis, D.R. Schonekar; Review of oral toxicity of polyvinyl alcohol (PVA); Food and chemical toxicology; 41 (2003); 319 - 326. 9 2.1 Background information on poly (vinyl alcohol) Figure 2. 1 show s the chemical structure of PVOH which was synthesized first by Haehnel and Herman in 1927 [ 4 ] . Unlike other vinyl polymers (LDPE, HDPE, PP , PS ), PVOH cannot be polymerized directly from its vinyl alcohol monomer because of the unstable nature of vinyl alcohol monomer that rapidly convert s into its acetaldehyde [15] . Instead, PVOH is polymerized from vinyl acetate monomers . V inyl acetate is prepare d by reacting ethylene and acetic acid with oxygen in presence of palladium catalyst [15]. Then, poly vinyl acetate (PVAc) is prepared from vinyl ac et ate monomer. In the nex t step, PVOH is obtained by the alkaline ( NaOH, KOH or meth oxide ) hydrolysis of the acetate group of PVAc as shown in Figure 2.2 [ 5 ] . The properties of PVOH are dependent on the percent of PVAc hydrolysis . PVAc is water insoluble due to the presence of an acetyl functional group [3] . Solubility of PVOH in water increases with increasing degree of PVAc hydrolysis . Solubility is maximum for 100% hydrolyzed PVAc . Thus, 100% hydrolyzed PVAc, which is referred to PVOH, is completely water - s oluble [2, 3]. Besides, t he properties of PVOH are also dependent on the conditions such as temperature and solvents used in the synthesis of PVAc [ 6 ] . 10 Figure 2. 1 : Chemical s tructure of PVOH . Figure 2. 2 : Synthesis of PVOH from PVAc in the presence of an alkaline catalyst . 2.2 S emi c rystalline polymers 2. 2 .1 Microstructure PVOH is a semi crystalline polymer [1] . Polymers comprised of both crystalline and amorphous phases are called semi - crystalline polymer s [1,2]. In the crystalline phase, p olymer chains are arranged in an orderly manner forming the lamellae structure as shown in Figure 2.3. Meanwhile, the amorphous region has random arrangement of the polymer chains. Crystallizati on is affected by many factors that affect this process as explained by Qian in book entitled physical [ 8 ] and is briefly explained here. Crystallization of polymers containing hydroxyl groups such as PVOH is of special interest due to hydrogen bonding which impedes the crystallization process in PVOH [50]. 11 Figure 2. 3 : Lamella structure in semi - c rystalline polymer [7] . The proportions for crystalline material vary between systems contingent up on multiple factors. Common factors include molecular weight, thermal history, backbone s tiffness, intermolecular bonding, branching, tactici ty and backbone symmetry [9] . A fully - crystalline (100%) polymer is extremely rare to produce [ 9 ] . Typically, polymers are semi - crystalline, where percentag e of crystallization reaches up to 70% [ 8 ] . For PVOH, the c rystallinity varies between 0 to 54% [ 2 ] . Figure 2. 4 : Microstructure of semi crystalline polymer [57]. 12 The crystalline and amorphous regions within a polymer have varying properties. Crystalline regions are stiffer, brittle and denser whereas the amorphous regions are flexible [11] . The stiffness is attributed to the crystalline nature and expressed as a percent crystallinity of the polymer while elongation is attributed to the amorphous regions of the polyme r [11]. In addition, a morphous regions are also easy to dissolve when compared to the crystalline regions , due to the compact crystalline nature [ 1 0] . Also, p olymer s hav ing higher crystallinity absorb less er water when compare d to samples containing higher number of amorphous regions [11] . 2. 2 .2 Tacticity of PVOH Tacticity is the arrangement of pendent groups on adjacent chiral centers within a macromolecule [58]. PVOH is a semi - crystalline polymer because it is iso tactic. Assender et al. [12] and Matsuzawaw et al. [13] have investigated the effec t of tacticity on the crystallinity in POVH . Figure 2.5 below shows stereochemistry in PVOH, which are of three types namely isotactic , syndiotactic and atactic . According t o Kenney et al. [14] the ease of crystallization in PVOH varies with tactici ty in the order of isotactic > syndiotactic > atactic [14]. 13 Figure 2. 5 : Stereo chemical isomers of PVOH [56]. 2.3 Physical p roperties PVOH is a color less and odor less polymer with a high melting temperature of 180 to 228 and glass transition temperature of 75 to 85 . T he physical properties of PVOH is largely depend ent on its molecular weight aside from tacticity and degree of hydrolysis . Also , it is correlated with the method of preparation and conditions [15] . An increase in molecular weight increases PVOH r esistance to water , viscosity, and adhesive strength [16] . Also, increase in molecular weight render s PVOH films good tensile strength [16]. 14 2. 4 Chemical p roperties PVOH is stable under normal use , thus generally considered to be one of the safest polymers as well as bio - compatible. Although PVOH is chemically inert, it undergoes thermal degradation upon heating , thus ma king it less thermally stabl e. As a result, PVOH extrusion is a complicated process and a plasticize r must be added to suppress its thermal degradation [ 17]. 2.5 Limitations of PVOH PVOH ha s excellent oxygen barrier properties under low RH [1] . The - OH groups present in PVOH make the molecule polar and thus oxygen has poor solubility in PVOH. P ermeability is the product of solu b ility coefficient (S) and diffusion coefficient (D) . The decrease in S for oxygen , significantly reduce s the permeabilit y , making PVOH a good oxygen barrier [1] . However , at high RH, it is known to a bsorb water molecules thereby losing its ability to provide oxygen barrier protection . The - OH bonds interact with water and in turn attains more free volume by swelling up [1] . This increase in the free volume allows oxygen gas to permeate easily within the molecule thus reducing the barrier properties of the PVOH films at high RH conditions. Due to these limitations, r esearche rs are focused on addressing this issue and dev elo ping ways to overcome it [2]. 2.6 PVOH/ Nano clay Composites are made of more than one material and are widely used in diverse areas . They offer unusual combination of strength, stiffness and weight , which are difficult to obtain from the 15 individual components of the composite . N anocomposites where one component is distributed in the matrix of another component in the dimensions below 100 nm [ 51,52,53]. The dispersed phase in such cases is called as nano fillers [19]. Bentonite clay is one of the many nano filler s used in PVOH films to improve the overall barrier and mechanical properties [19]. A well - dispersed bentonite / PVOH solution was prepared and the films were prepared by casting technique [19] . X - ray diffraction patterns revealed organized micro structure s in the bentonite / PVOH films . The study concluded that the mechanical properties of the film increased with increasing amount of nanocomposites . From Differential Scanning Calorimetry (DSC) analysis, it was observed that with increasing amount of n anofillers, the melting temperature of the material also increased , while oxygen permeability decreased . A 7 - fold decrease in oxygen permeability and 193 - were reported as compared with neat PVOH [19]. This improvement is due to the presence of the nanofillers in PVOH. A major drawback of this approach , however, is the decrease in optical clarity upon the addition of nanofillers . Optical transmittance is a critical factor particularly in the packaging industr y . In addition , t he prepared PVOH/nanocomposite material w as more brittle than PVOH alone , thus resulting in loss of impact resistance and loss of elongation . 2.7 PVOH/ P olymer b lends Blending of two or more different polymers to achieve certain properties is also a com monly used approach [20]. Blending polymer s alters properties such as miscibility, morphology, degradation and permeability of polymers [21]. Significant research has been conducted based on the 16 miscibility of different polymers [22, 23, 24] . A series of PVOH /Poly propylene glycol ( PPG ) blend s were prepared and their properties were investigated [25]. Interaction between PVOH and PPG is shown in Fi gure 2.6 . It was observed that th e hydrophobicity and elongation properties of the polymer blend s increased wi th the addition of PPG in the blends. These observations were attributed to the addition of flexible and hydrophobic PPG to PVOH. PPG being a rubbery polymer increase s the e longation performance [25] . Figure 2. 6 : I llustration of intermolecular H - bonding in the PVOH/PPG blend s The water resistance was improved as the PPG amount was increased in the b lend s. However, the barrier properties were not reported for these blends. Also , i ncreasing the PPG in the polymer blend adversely affected the ir optically transmittance and resulted in hazy films. 2. 8 Graphene o xide /PVOH Graphene was discovered in 2004 [26] , which consists of sp 2 bonded carbon atoms arranged in a hexagonal lattice l ike a honeycomb [27]. Th e incorporati on of graphene in to polymer s increases the mechanical properties of the resulting polymer [ 28, 29, 30]. Graphene/ PVOH nanocomposites have been published previously [31]. It was observed that an increase in the graphene oxide loading Intermolecular H - bonding 17 in PVOH film s enhanced their mechanical properties [31] . Also, t he hydrophobicity of the graphene oxide/PVOH films was improved. Similarly , the ba rrier properties of the film also increase d in proportion with the addition of graphen e . However, under high relative humidity conditions , the water barrier properties were not improved . Besides, i ncorporation of graphene into polymer solution s also result ed in the los s of optical transparency in polymeric films. Graphene is a relatively expensive material and its use for low - cost applications are commercially less viable. 2.9 S ilicon oxide/PVOH Sol - gel techniques have been widely used for preparing silicon oxide films. These films were prepared using alkoxy silyl groups as precursors, which undergo es hydrolysis and condensation [32]. This technique has greatly improved the field of ceramics and organically modified films [ 33, 34, 35] . Sol gel methods have been used in many different f ields, and currently more predominantly used in bioch emical applications [40, 41 ]. These sol - gel approaches result in films that are homo genous as well as thermally stable [36 - 39]. In a recent study, PVOH was blended with silica particles to prepare silica - PVOH nanocomposites [42]. In this study, s ilica was generated from the tetraethoxysilane precursor, h ydrochloric acid was used as the catalyst in this reaction in an aqueous medium. The goal of this study was to produce optically clear polymer films with substantial increase in water resistance an d mechanical properties . Transmission electron microscopy (TEM) revealed that the silica particles were uniformly distributed through out the PVOH film. It was observed that an increase in silica particles increase d the tensile strength of the material. Alt hough the tensile strength was improved, the material lost its ability to elongate. Comp a red with neat PVOH films , the w ater 18 resistance was significant ly improved but the water vapor barrier properties were not . Also , t he sol - gel technique is a compli cated multi step approach. Al though the distribution of silica particles is uniform, optical clarity was not retained because of light scattering due to the larger size of silica particles . 2.10 Polydimethylsiloxane (PDMS) Polydimethylsiloxane (PDMS) commo nly referred as silicones, belongs to the polymeric group of organosilicon compounds [49]. The chemical structure of PDMS is shown in Figure 2.7. PD MS is an inexpensive , inert , non - flammable, non - toxic and optically clear polymer . It has a wide range of applications ranging from medical devic e s , contact lenses to elastomers [49] . Figure 2. 7 : Structure of PDMS The idea of addition of PDMS to the PVOH was inspired from the previous studies reported by McCarthy [44] , Hozumi [45, 46] , and Rabnawaz et al . [54, 55]. In these studies, PDMS was used to prepare water and oil repellent film . This was due to the low surface tension of PDMS ( 2 2 mNm - 1 ) [47 ], as compared to water which has higher surface tension of 72.8 mNm - 1 [48] . The practical problems associated with PDMS is that these monolayers fail after long rubbing cycles. 19 One way to solve this problem is to incorporate PDMS into a thicker matrix, which would help to improve the wear resistance . 2. 11 Polyurethane siloxane coating Polyurethane ( P U ) coating s can be easily applied to a variety of substrates. The biggest advantage of PU coating is that it can bind well with a variety of substrates including PVOH and the thickness of these coating can be e asily tailored with excellent optical clarit y [43]. PU is an optical ly transparent, durable polymer which can easily be adhered to PVOH as a coating matrix . Thus, its use as a coating matrix in this study is justified. In this study, h examethylene diisocyanate trimer (HDIT) and propylene oxide - based triol (polyol) w ere used to prepare the PU coating. These two components were chosen because of their high react ivity and fast kinetics in form ing a clear, rigid coating. PDMS and PVOH forms two separate phases if mixed together. Hence, PDMS cannot be directly added to PVOH solution [43] , it was first made to react with isocy a nate and then wi th a polyol to form a polyurethane siloxane coating to obtain the envisioned PDMS PU coated PVOH films without any phase separations . 20 REFERENCES 21 REFERENCES 1. S. E. M. Selke, J. D. Culter, R. J. Hernandez; P lastics packaging; Properties, processing, applications and regulations (2nd ed.), Hanser p ublishers, Munich (2004) . 2. F. L. Marten. cience and technology. 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(vinyl alcohol) on crystallization and biodegradation of poly (3 - hydroxybutyric acid)/ poly ( vinyl alcohol) Polym. Degra d. Stab., 66:2 (1999), pp. 263 270. - p roperty relationships of poly (vinyl alcohol). III. Relationships between stereo - regularity, crystallinity, and water resistance in poly(vinyl J. Polym. Sci. , Part A: Polym. Chem., 4:3 (1966), pp. 679 698. 15. Manfred L. Hallensleben "Polyvinyl c ompounds, Others" in Ullmann's encyclopedia of industrial c hemistry, 2000, Wiley - VCH, Weinheim. doi : 10.1002/14356007.a21_743 . 22 16. Marten FL (2002) vinyl alcohol polymers. Encyclopedia of poly mer science and technology John wiley & Sons, Inc., USA . 17. Matsumura S (2005) Biodegradation of p oly ( vinyl alcohol) and its c opolymers. Biopolymers Online. 18. http://www.sigmaaldrich.com/catalog/product/aldrich/341584?lang=en®ion =US. 19 . A. A. Sapalidis, F. K. Katsaros, Th. A. Steriotis, N. K. Kanellopoulos; Properties of poly ( vinyl alcohol) - bentonite clay nanocomposite films in relation to polymer clay interactions; J. Appl. Polym. Sci., 123 (2012), pp. 1812 - 1821 . 20. J. S. Park, J. W. Park, E. Ruckenstein; Thermal and dynamic mechanical analysis of PVA=MC blend hydogels. Polym ., 2001, 42, 4271 4280. 21. Y. Nishio ; R. Manley; Cellulose - poly ( vinyl alcohol) blends prepared from solutions in N, N - dimethylacetamide - lithium chloride. Macromolecu les 1988, 21, 1270 1277. 22. T. Kondo ; C. Sawatari ; R. Manley ; D. G. Gray; Characterization of hydrogen bonding in cellulose - synthetic polymer blend systems with regioselectively substituted methylcellulose. Macromolecules 1994, 27, 210 215. 23. C. Sawat ari ; T. Kondo; Interchain hydrogen bonds in blend films of poly ( vinyl alcohol) and its derivatives with poly ( ethylene oxide). Macromolecules 1999, 32, 1949 1955. 24. K. Lio ; N. Minoura ; M. Nagura. Swelling characteristics of a blend hydrogel made of pol y(allylbiguanido - co - allylamine) and poly ( vinyl alcohol). Polym ., 1995, 36, 2579 2583. 25. G. Zhu, F. Wang, S. Dong, K. Zu, Y. Liu; Thermal, mechanical and chemical properties of hydrophilic poly ( vinyl alcohol) film improved by hydrophobic poly ( propylene glycol); Polym. Plast. Technol. Eng., 52 (2013), pp. 422 - 426 . 26 . K. S. Novoselov, A. K. Gei m , S. V. Morozov, D. Jiang , Y. Zhang , S. V. Dubonos , et al , Science 18 :666 669 (2004). 27 A. K. Geim and K. S. Novoselov , Nature Mater 6 :183 191 (2007). 28. S. Park and R. S. Ruoff , Nature Nanotechnol 4 :217 224 (2009). 29 M. B. LiD, Muller , S. Gilje , R. B. Kaner and G. G. Wallace , Nature n anotechnol 3 :101 105 ( 2008). 30 A. K. Geim , Science 324 :1530 1534 (2009). 23 31. J. Wang, X. Wang, C. Xu, M. Zhang, X. Shang; Preparation of graphene/ poly ( vinyl alcohol) nanocomposites with enhanced mechanical properties and water resistance; Polym. Int., 60 (2011), pp. 816 - 822 . 32. C. J. B rinker and G. W. S cherer - gel s cience ; the p hysics and c hemistry of s ol - g el p Press, 1990). 33. P. C alvert , Nature 353 (1991) 501. 34. B. M. Novak , Adv. Mater. 5 (1993) 422. 35. K. P . Hoh , H. I shida and J . L. K oenig , Polym. Comp. 11 (1990) 121. 36. Y. W ei , R. B akthavatchalam , D. C. Y ang and C. K. Whitecar , Polym. Prepr. 32 (1991) 503. 37. Y. W ei , D. C. Y ang and R. B akthavatchalam, Mater. Lett. 3 (1992) 261. 38. Y. W ei , D. C. Y ang , L. G. T ang and M. K. H utchins , J. Mater. Res. 8 (1993) 1143. 39. Y. W ei , D. C. Y ang and L. G. T ang , Macromol. Chem. Rapid. Commun. 14 (1993) 273. 40. H. H . W eetal , B. R obertson , D. C ullin , J. Brown and M. W alch , Biochem. Biophys. Acta. 1142 (1993) 211. 41. M. T. R eetz , A. Z onta and J . Simplekamp , Biotechnol. Bioengng. 49 (1996) 527. 42. A. Bandyopadhyay, M. D. Sarkar, A.K. Bhowmick; Poly ( vinyl alcohol)/silica hybrid nanocomposites by sol gel technique: synthesis and properties; J. Mater. Sci., 40 (2005), pp. 5233 - 5241 . 43. D. K. Chattopadhyay, K. Raju, Prog. Polym. Sci. 2007, 32, 352 418. 44. W. Chen, A. Y. Fadeev, M. C. Hsieh, D. Oner, J. Youngblood, T. J. McCarthy, Langmuir 1999, 15, 3395 3399. 45. D. F. Cheng, C. Urata, B. Masheder, A. Hozumi, J. Am. Chem. Soc. 2012, 134, 10191 10199. 46. D. F. Cheng, C. Urata, M. Yagihashi, A. Hozumi, Angew. Chem. Int. Ed. 2012, 51, 2956 2959; Angew. Chem. 2012, 124, 3010 3013. 47. J. Bra ndrup, E. H. Immergut, Polymer h andbook, Wiley, New York, 1989. 48. R. C. Weast, D. R. Lide, M. J. Astle , W. H. Beyer, CRC handbook of chemistry and p hysics, 70th ed., CRC Press, Boca Raton, 1990. 24 49. "Linear Polydimethylsiloxanes" Joint assessment of commodity c hemicals, September 1994 (Report No. 26) ISSN 0773 - 6339 - 26. 50. Nikolaos A. Peppas and Pa ula J. Hansen, Crystallization kinetics of p oly (viny l Alcohol), School of chemical engineering, Purdue u niversity, 1982. 51. S. Komarneni, J. Mafer. Chem. 2, 1219. 52. H. Gleiter, Adv. Marer. 1992, 4, 474. 53. B.M. Novak, Adv. Mafer. 1993, 5, 422. 54. M. Rabnawaz, G. Liu, H. Hu; Fluorine - free anti - smudge polyurethane c oatings; Angew. Chem. ; volume 127 issue 43 (2015); pp. 12913 - 1291 8. 55. M. Rabnawaz, G . Liu; Graft - copolymer - based approach to c lear, durable, and anti - smudge polyurethane c oatings; Angew. Chem. ; volume 127 issue 22 (2015); pp. 6616 - 6620 . 56. http://nptel.ac.in/courses/112104122/lecture14/14_9.htm . 57. https://en.wikipedia.org/wiki/Spherulite_(polymer_physics)#/media/File:Spherulite2.PN . 58. R.J. Young, P.A. Lovell Chapman; Introduction to polymers; London; 1991. pp. 443. 25 3.1 Sample p reparation 3.1.1 Materials The polymer used in the experiment was poly ( vinyl alcohol), M w = 89000 - 98000 g/mol , 99+% hydrolyzed ( Aldrich ) . The m on o amino propyl terminated PDMS polymers of M w = 2000 and M w = 800 - 1200 g/mol ( Gelest ) . The other PDMS polymer used was bis 3 - amino propyl terminated with M w = 2500 g/mol ( Sigma Aldrich ). The h examethylene d iisocy a nate trimer ( HDIT) and polyol used has the chemical structure shown in F igure 3.1. Figure 3. 1 : The chemical structures of materials used for the coating study. 26 3. 1.2 Preparation of PVOH films Figure 3.2 show s the schemati c illustration of the PVOH film formation. PVOH films were prepared by dissolving 1 .0 g of PVOH in 100 .0 m L deionized water (DI) in a 250 m L beaker. C old (DI) water was used to avoid cloudy appearance of the solution . The solution w as stirred and and then cooled t Then, 25 mL of the solution was poured i nto a plastic petri - dish (90 mm diameter) which was then placed in a fume hood overnight to allow evaporation of excess water . Finally, t he plastic petri - dish was placed in a for 2 h before peeling off the PVOH films carefully with a forcep /tweezer . Figure 3. 2 : Illustration of PVOH film preparation process . Heat at 90 and constant stirring 1.0 g PVOH + 100mL DI water dissolve cool Fume hood overnight pour Vacuum oven, 2h at 70 PVOH film 27 3. 1. 3 Preparation of polyurethane siloxane (PU - S) coating solution Two types of polyurethane siloxane (PU - S) coating s were prepared based on the type of PDMS used. In one case, PDMS bis ( 3 - amino propyl terminated ) was used and was referred to as PDMS - A, in the other case, two different PDMS (Mw=2000 g/mol and 800 - 1200 g/mol) were used are referred to as PDMS - B . 3. 1 . 3 .1 Preparation of PU - S coating with PDMS - A In a 20 mL vial, 2.2 mL of H examethylene d iisocy a nate trimer ( HDI T) 2.2 mL was diluted with 2 .0 m L of t etrahydrofuran (THF) (Fisher Chemicals) under constant stirring conditions. Then, 1 .0 wt. % solutions of the PDMS bis (3 - aminopropyl) terminated (PDMS - A ) w as prepared in THF ( for example, 0.05 m L of PDMS - A was diluted with 0.2 m L THF) . Th e diluted PDMS - A solution was added drop - wise into HDIT solution under stirring. Th is was followed by addition of 0.68 mL of propylene oxide - based triol (polyol) in to the PDMS - A /HDIT solution. The mixture was the n heated to under constant stirring a fter which the solution was allowed to cool down t o room temperature (23 ) . Finally, 6 mL of dimethyl carbonate (DMC) (Fisher chemicals) was added to the cooled solution . The solution was then stirred for another 2 minutes. N itrogen gas was purged into the solution to remove excess THF , leaving behind DMC based coating solution. 3. 1.3 .2 Prep aration of PU - S coating w ith PDMS - B In a 20 mL vial, 1.25 mL of HDIT was dil uted with 1.0 mL a cetone (Sigma Aldrich ) . The vial was placed under constant stirring conditions. In a separate vial A , 40 mg of p olydimethyl siloxane 28 m ono aminopropyl terminated ( Mw 800 g/mol - 1200 g/mol ) was diluted with 1.0 mL a cetone . I n vial B , 2 .0 mg of p olydimethyl siloxane m ono aminopropyl terminated ( Mw 2000 g/mol ) was diluted with 1 .0 mL of a cetone . Both solutions were then added drop - wise in to the 20 mL vial containing HDIT . The solutions were allowed to stir for 5 min utes with stir bars . After stirring, 0.7 mL of propylene oxide - based triol (polyol) w as added to the stirring solution . The solution w as , and then cool down to room temperature before adding 6 mL dimethyl carbonate (Fisher chemicals) . Finally, t he solution w as sti rred for another 2 minutes , and THF was removed by nitrogen bubbling process [13] . 3. 1.4 Applying PU - S coating on PVOH films The PU - S coating solutions was poured in to a glass petri - dish . PVOH films were dipped in the PU - S coating solution using forceps for 2 s econds on each side (2 s econds front and 2 s econds back) and were placed on the rim of a 250 mL beaker . The films were allowed to cure for 10 min utes under ambient conditions before plac ing them in the oven for 6 h ours at 120 were placed on top of the beaker to prevent films from s ticking to the glass. This process can be scaled up in the indus try by UV curing. Figure 3.3 below represents the dip coating process. 29 Figure 3. 3 : Dip coating process for the PVOH coating. 30 3. 1.5 Coated PVOH f ilms After curing, t he PDMS PU coated PVOH films were peeled of f from the 250 mL glass beaker using forceps . Some material is usually lost from the edges of the films as the cured urethane stick s to the rim of the beaker. However, the films at the center remained intact . The se films were then subjected to further character ization s . 3.2 Characterization 3.2.1 Contact angle measurement Contact angle is defined as the angle , where a solid surface meets a liquid vapor interface as shown in Figure 3. 4 . A solid surface is quantified by a liquid via Young equation [1]. The contact angle measurement is a non - destructive analytical technique used to determine the equilibrium contact angle between the solid film and water and depends on the liquid and solid interaction . [1 - 2 ] Typically, for water droplets , considered hydrophobic substrate and substrates with contact super hydrophobic [1] . 31 In this study, c ontact angle was measured for PVOH , PU coated PVOH and PU S coated PVOH films using VCA optima instrument. Samples were cut i nto 5.08 x 2.54 cm 2 and placed on a plastic petri dish. The samples were vacuum dried at 70 ° C for 1 h before testing. The se films were placed on a flat platform of the VCA optima instrument. A water droplet (approximately 5 µL) was placed on the sample surface through a syringe/ needle. The contact angle s of the water droplet s were immediately measured . This VCA optima instrument is also c apable of recording images of the water droplet. The lamp brightness was adjusted to 100% to obtain bri ght clear images. Three different spots were chosen on the surface of sample to obtain the average contact angle for statistical purposes. Figure 3. 4 : Contact angle of a liquid drop on a flat surface [ 11 ] . 3. 2.2 Sliding angle measurement Sliding angle i s defined as the angle between the horizontal plane and the sample surface at which the liquid droplet starts to slide off under the influence of gravity as shown in F igure 3. 5 . Sliding angle s w ere measured using the c oefficient of friction tester (T5001, MTS) . Samples were cut into 5.08 x 2.54 cm 2 and placed on a plastic petri dish. The samples were vacuum dried at 70 ° C for 1 h 32 before testing. The films were placed on a flat surface (glass slide 7.62 x 2.54 cm 2 ) a nd tethered using a tape to prevent films from sliding over upon inclining the stage. A water droplet (approximately 75 µL) was injected on to the film surface and the stage was titled. The inclination of the stage was controlled by the software connected t o the instrument. The angle at which the water droplet starts to slide was recorded as the sliding angle for the film. Triplicate measurements were made at three different spots on the same film and the average angle was calculated for statistical purposes . The s liding angle measurements were not obtained for neat PVOH samples as the films dissolved immediately after the water droplet was placed on the PVOH film . Figure 3. 5 : Sliding angle of a liquid drop on an inclined surface . 3. 2.3 Optical transmittance measurement s Optical transmittance was recorded using a UV/VIS spectrometer ( Lambda 25, Perkin Elmer Instruments ) to calculate the optical clarity of the PVOH films before and after the coating [3]. Samples were cut into 5.08 x 2.54 cm 2 and placed on a plastic petri dish. The samples were vacuum dried at 70 ° C for 1 h before testing. The se films were placed on the UV - Vis holder for transparency 33 measurements. T riplicate measurements were obtained for ea ch film. The wavelength range used was between 200 to 900nm . A graph of %T vs wavelength was plotted to compare the loss in optical transparency. 3.2.4 Water uptake analysi s For water gain analysis, films were cut into 5.08 × 5.08 cm 2 and were placed on the plastic petri dish. The films were vacuum dried at 70 ° C for 1 h before testing . The initial weight of each of the samples was noted to a four - decimal digit precision on a n analytical balance (OHAUS Adventure Pro). The samples were then placed in a humidity chamber (HOTPACK) . The samples were removed from the chamber after 1 hour and final weight s w ere recorded. The difference between initial and final weight is the amount of water absorbed by the film. Water uptake analysis were run in triplicates for each f ilm. 3. 2.5 FTIR - ATR measurements FTIR - ATR analysis were used to determine the amount of water absorbed by the films in the humidity chamber [4]. FTIR - ATR test was performed with Shimadzu (IRAffinity - 1S). The sample s were prepared in triplicates with different sample sets and were subjected to ATR - IR analysis . Similar to weight gain analysis the samples were vacuum dried at 70 ° C for 1h before starting the test. The samples were cut into 5.08 × 5.08 cm 2 . The FTIR - ATR spectr um for each sample was obtained in the wavelength range of 400 - 4000 cm - 1 with 64 scans using happ genzel 34 method . The IR spectra for the samples were obtained before and after placing th em in the humidity chamber. It was ensured that the spectra for the films were recorded immediately after removing them from the chamber to prevent further water absorption from the environment. The difference in - OH peak area was compared to determine the a mount of water absorbed during the humidity test. The FTIR - ATR w ere used as complementary tests the data obtained with weight gain analysis . Further work on the FTIR - ATRP approach for water uptake analysis will be carried out in the future. 3. 2.6 Tensile properties Tensile properties were measured using In stron (Model 5565) . Specimen for the tensile tests were prepared in the dimension of 50 .0 x 6.35 mm 2 . All the samples were vacuum dried at 70 ° C for 1 h before testing. Ten samples were tested for each sample set . The samples were prepared using a cutting board. The usual size of samples for Instron is 17.78 x 2.54 cm 2 [5] . But the samples for this method was prepared in a petri dish , where the maximum size of the samples were ~ 7.62 cm . So , the samples were cut into 50 x 6.35 mm 2 rectangles for the test. The sample s were preconditioned before running tensile test. Based on the obtained value s , the tensile strength and % of elongation w as calculated. The strength of the material was determine d based on this method . 3.2 .7 Dart d rop i mpact t est The D art d rop impact test (Lab Think) was performed for PVOH films with and without coating s. Samples were prepared in large petri - dish es ( diameter 13.97 cm ) because t he samples from small 35 petri dish es ( diameter 7.5 cm ) were not able to cover the diameter of the cav ity of the dart drop instrument . All the samples were vacuum dried at 70 ° C for 1 h before testing. The d art d rop impact was measured follow ing the standard method [6] . S amples were fixed, and the dart was released from 33.02 cm with different weights and the resulting values were calculated. A standard stair - step approach was used to increase the weight of the dart . At first, t he lowest weight dart was r eleased , and the dart weight was increased un til the film breaks. Once it break s, the sample was recorded as a fail. Then , the weight of the dart was reduced again and al lowed to fall on the film . Now the weight of the dart was again increased until it breaks again (fail s ). Thus, in total 20 test values were calculated with 10 fails and 10 passes . The failure mass was calculated by considering the different weights the dart was allowed to hit the film . Failure mass was used to calculate the energy required to break the samples as impa ct strength . 3. 2.8 Barrier properties The barrier properties were measured using Mocon instruments (OX - TRAN Model 2/21 and PERMATRAN - W Model 3/33) [8] . Films of area 3.14 cm 2 were used for permeability tests. All the samples were vacuum dried at 70 ° C for 1 h before testing. The relative humidity of the carrier gas as well a s the permeant gas was set at 75 test s. The thickness of the sample was measured using a micrometer (TMI) and the value was inputted in t o the software to calculate permea bility . The machine calculates the permeability of samples following equ . 1 and 2 below. 36 equ. 1 equ. 2 3. 2.9 X - ray photoelectron spectroscopy (XPS) A Perkin Elmer Phi 5600 ESCA system was used for the XPS analysis. XPS is a surface - sensitive technique that measures the atomic concentration of a material on the surface by quantitative spectroscopic methods . XPS spectra is capture d by first irradiating a material with a beam of X - rays and the kinetic energy of the ejected electrons (and their numbers) that escape s from the top of the surface (down to 10 nm ) are being analyze d [9] . For the XPS analysis , samples were cut into small pieces of 2.54 x 2.54 cm 2 . All the samples were vacuum dried at 70 ° C for 1 h before submitting it for testing. One particular sample prior to XPS analysis, w as rinsed with hexane to see whether PDMS is chemically graft ed to PVOH. 3.2.10 Scanning e lectron m icroscopy (SEM) Scanning electron microscopy (S EM) (JEOL 7500F SEM, JEOL Ltd., Japan) is an electron microscope that produces images by scanning the samples surface with a focused beam of electrons which is located in the c enter for advanced microscopy at Michigan state university . This study was performed to understand the surface morphology of samples. The 7500F is designed for maximum information and resolution extraction by use of multiple advanc ed secondary and back scattered electron detectors. A special energy filter for the detected electrons and electron beam 37 deceleration is a part of it. All the samples were vacuum dried at 70 ° C for 1 h before testing. The samples were cut into small pieces (0.5 X 0.5 cm 2 ). The se samples were mounted on aluminum stubs with epoxy glue. The mounted samples were placed in a desiccator overnight for curing. The samples were then coated with Osmium (Os) to make the surface conductive . These stubs were placed on the 6 stub holder stand for testing. All the samples were examined with accelerating voltage of 5 kV at 100,000x magnification. 3. 2.1 1 Thermo gravimetric a nalysis (TGA) TGA Q50 is a method of thermal analysis in which the weight loss of a sample is measured with respect to change in temperature [10] . The instrument was set to run from 25 ° C to 600°C so that it can capture the degradation of the material. This analysis helps us to under stand the composition of the material and also to know whether the thermal stability is affect ed by the presence coatings . All the samples were vacuum dried at 70 ° C for 1 h before testing. The samples were cut into small pieces and laid flat on the aluminum pan for testing. 3.2.1 2 Cross - cut adhesion test Cross - cut adhesio n test (Figure 3.6) was performed to ensure whether the PU - S coating applied on the PVOH film is adhered properly . This test method was followed by ASTM D3359 with modification appropriate for the sample. Sand paper was used to cross - cut the coated films. A tape (3M Scotch heavy duty vinyl electrical) was paste d at the center of the crosscut lines and then quickly removed. The area was examined to see if any coating has been removed. Since the coating 38 was transparent, a black tape was used to determine the detached coating. The samples were vacuum dried at 70 ° C for 1 h before testing. Figure 3. 6 : Schematic presentation of cross - cut adhesion test [12]. 39 REFERENCES 40 REFERENCES 1. Langmuir 9:8 (1993), pp. 2237 2239. 2. M. Langmuir 13:20 (1997), pp. 5494 5503. 3. Skoog, Douglas A.; Holler, F. James; Crouch, Stanley R. (2007). Principles of i nstrumental a nalysis (6th ed.). Belmont, CA: Thomson Brooks/Cole. pp. 169 173. 4. P. Griffiths, J.A. D e Hasseth ; Fourier t ransform i nfrared s pectrometry (2nd ed.). Wiley - Blackwell , 2017. ISBN 0 - 471 - 19404 - 2 . 5. ASTM Standard D 882 - t est m ethod for t ensile properties of thin plastic sheetin . , West Conshohocke n, PA. 6. ASTM Standard D 1709 - S tandard test methods for impact resistance of plastic film by the free - falling dart metho . , West Conshohocken, PA. 7. ASTM Standard E 968 - S tandard practice for heat flow calibration of differential scanning calori . , West Conshohocken, PA . 8. ASTM Standard D 3985 - S tandard test method for oxygen gas transmission rate through plastic film and sheeting using a coulometric s . , West Conshohocken, PA. 9. Electron spectroscopy for atoms, molecules and condensed matt er , Nobel l ecture, December 8, 1981 . 10. Coats, A. W.; Redfern, J. P. (1963). "Thermogravimetric a na lysis: A r Analyst. 88 (1053): 906. 924. Bibcode: 1963 Ana. Doi: 10.1039/AN9638800906. 11. http://lnf - wiki.eecs.umich.edu/wiki/Contact_angle_measurements . 12. http://www.prestogroup.com/articles/test - the - resistance - of - coating - to - separation - with - cross - hatch - tester/ 13. Y. Kemmochi, K. Tsutsumi, A. A rikawa, H. Nakazawa; Centrifugal concentrator for the substitution of nitrogen blow - down micro - concentration in dioxin /polychlorinated biphenyl sample preparatio n". J. Chromatogr. A (2002). 943 (2): 295 97. 41 4.1 Water - resistance The key objective of this study was to improve the water resistance of PVOH fil ms by applying PDMS polyurethane ( PU - S ) coatings. The water - resistance behaviors of the PVOH, PVOH PU and PU - S coated PVOH were quantified by the contact as well as sliding angle s measurements in Table 4.1 and Table 4.2. 4.1.1 Contact angle measurement The c ontact angles of water droplet on solid substrates defines the substrate water resistance. A h igher contact angle s water - resistant substrate , while contact angles <90 o represents a hydrophilic substrate. Water contact angles for the films prepared in this study are summarized in T able 4.1 . Results indicated that neat PVOH films had a contact angle of 72.9 ± 1.2 , when the contact angle was recorded immediately. After few seconds, the PVOH film started to d issolve by the water droplet on their surface . Figure 4. 1 shows the damage caused by the water droplet to the PVOH film. Thus, beyond doubt PVOH has absolutely no resistance against water. 42 Figure 4. 1 : Damage caused by water droplet on PVOH film surface The water contact angles after applying PU coatings on PVOH, was 97.6 ± 2.6 °. In addition , the water stayed on these samples for longer time without any change in the contact angle. Indicating the good water - resistance of the PU / PVOH films . Interes tingly , t he contact angle s for PDMS - A/ P VOH films and PDMS - B/PVOH films w ere increased significantly , 107. 7 ± 3 . 0 ° and 107.7 ± 2 .9 ° respectively. These contact angles strong ly indicate very good wa ter repellency of the PVOH coated films caused by PDMS. From contact angle measurements, it can be concluded that , PDMS has made the PVOH films extremely water resistant . D amaged area on PVOH film 43 Table 4. 1 : Contact angle measurements for PVOH and coated PVOH samples Contact angle (°) PVOH PU /PVOH PDMS - A/PVOH PDMS - B/PVOH NA 97.6 ± 2.6 107. 7 ± 3 . 0 107.7 ± 2 .9 PU /PVOH Coated PDMS - A /PVOH PDMS - B/PVOH Figure 4. 2 : Contact angle images for different PVOH coated samples taken by the VCA optima instrument This increase in water resistance is observed by masking the initially exposed OH group of the PVOH (Figure 4.3 ). The - OH groups on the PVOH form hydrogen bond with water that result s in film dissolution. T he OH groups were masked by applying an exterior coating of polyurethane. However, polyurethane is less water resistance alone, and therefore, PDMS was applied along PU onto PVOH films [1] . While poly urethane is comprised of polar urethane bo nds that can form hydrogen bond with water but due to their cross - linked structure, polyurethane once cross - link ed is insoluble in water but still can absorb moisture [1] . The application of PDMS, in the PU coating masked the urethane bonds , making it waterproof . PDMS is known to enrich on the surface of the films because of its good interaction with non - polar air. Therefore, even the addition of PDMS in the PU at 1 - 2wt% make the surface strongly water repellent. 44 Figure 4. 3 : Chemical surface modifi cation of PVOH films using PDMS PDMS 45 D amaged area on PVOH film Water droplet not sliding even at 60 ° for PVOH film 4. 1.2 Sliding angle measurement Table 4.2 summarizes the sliding angles obtained for PVOH films with different coatings. A lower water sliding angle represent s stronger water repellency surface and vice versa. It was observed that on ne a t PV OH film s, the water droplet did not slide and instead dissolved the film immediately when came in contact with the film. PU / PVOH films s howed a sliding angle of 28.2 ± 1 .2 ° . Meanwhile, t he sliding angles for PDMS - A / PVOH films and PDMS - B/PVOH were found to be 14° and 16° , respectively . Thus, in both coatings where PDMS was incorporated showed excellent water sliding angles , which corresponds to the presence of the PDMS with low surface tension . Table 4. 2 : Sliding angle values for PVOH and coated PVOH samples PVOH PU/PVOH PDMS - A/PVOH PDMS - B/PVOH NA 28.2 ± 1.2 a 13.8 ± 0. 2 b 16.3 ± 1.1 c Means with same lowercase letters are not significantly different, means with different lowercase letters are significantly different based on ttest with 5% significant level. Figure 4. 4 : Water droplet behavior on the surface of uncoated PVOH 46 4. 2 Optical t ransmittance Retaining t ransparency with excellent optical properties along with good water resistance was another key objective of t h is study because t ransparent fil ms are highly desirable for packaging applications . A UV - Vis Spectrophotometer w as used to measure the optical transmittance of the films before and after coating the PVOH films with different coating systems . Figure 4.6 summarizes the perce nt T ransparency of neat PVOH fil m vs PVOH films coated with different coatings namely PU/PVOH , PDMS - A /PVOH and PDMS - B / PVOH film s . Figure 4.5( A ) shows neat PVOH films , which are highly transparent . Once coating was applied on PVOH film, the films maintained their excellent clarity as shown in Figure 4.5 (B) and (C) . Thus, visually there is no difference between PVOH and coated PVOH samples. For comp arison, transmittance of the different coated PVOH films were measured a t fixed wavelength (550nm) and was compared against neat PVOH films . At 550 nm , the optical transmittance of PVOH film was 91.7%, PU / PVOH film was 90. 5 %, PDMS - A/PVOH was 85. 7 % and PDMS - B/PVOH film was 83. 3 %. This slight decrease in the %T for the PU/PVOH film corresponds to thicker PU/PVOH films. Also, the incorpora tion of PDMS reduced the %T by 6 - 9 %, that is typically caused by the different refractive indices of PDMS and PU that causes little scattering but visually the films were very clear. 47 A B C Figure 4. 5 : Visual difference between neat PVOH and coated PVOH samples. (A) PVOH, (B) PDMS - A / PVOH, (C) PDMS - B / PVOH . Figure 4. 6 : % T vs wavelength for PVOH and coated PVOH samples from 200 nm to 900nm emphasizing more on the visible range (400nm to 700nm) 0 10 20 30 40 50 60 70 80 90 100 200 300 400 500 600 700 800 900 % Transmittance Wavelength (nm) PVOH PU/PVOH PDMS-A/PVOH PDMS-B/PVOH ± ± ± ± 48 4. 3 Weight g ain a nalysis It is anticipated that an increase in water resistance of the films would also reduce their water gain. Therefore, the water absorption of the films at different stages (before and after the exposure of humidity) of coatings were measured. Water gain analysis were determined by using two techniques namely w eight gain analysis and IR - spectroscopy. 4. 3.1 Weight gain analysis by weight gain approach As described in Chapter 3, the coated and uncoated PVOH films were weighed before and after placing in th e conditioning chamber. The difference in weight before and after the exposure indicated the amount of water absorbed by the films. I t was expected that the coated PVOH sample s would have lower water absorption than neat PVOH films. Table 4.3 summarizes the weight gained by the different films post RH testing. The presence of coating helped PVOH retain its structural stability preventing it from dissolving . It was also observed that the PDMS - A /PVOH absorb ed 29 % less water than neat PVOH films . PDMS - B /PVOH showed very impressive reduction in water gain and absorbed 41 % less water than PVOH films. On removing the films from the chamber, it was observed that the PVOH films became too soft due to high humidity . T he samples however, retained mechanical integrity and remained stiff . 49 Table 4. 3 : Weight gain analysis for PVOH and coated PVOH samples and also in comparison with % decrease of water gain w.r.t to neat PVOH Sample Water gain in grams % decrease in water gain w ith respect to neat PVOH PVOH 0.0219 ± 0.0027 a NA PU/PVOH 0.0175 ± 0.0030 ab 20% PDMS - A/PVOH 0.0156 ± 0.0016 b c 29% PDMS - B/PVOH 0.0129 ± 0.0012 b c 41% Means with same lowercase letters are not significantly different, means with different lowercase letters are significantly different based on ttest with 5% significant level. This reduction in the water uptake is justifi able by the fact that chemically modified PVOH films being more hydrophob ic in nature, absorb less water than neat PVOH. 4. 3.2 ATR - IR analysis for water gain analysis: The ATR - IR spectrum is an excellent analytical test to determine the amount of water content in a material by tracking the change in the peak intensity of - OH peak that appears between 3200 cm - 1 and 3500 cm - 1 , but this study was performed to get preliminary data to make sure whether it aligns with the manual weight gain analysis . In this study, t he area under the - OH peak for the PVOH films with and without coat ings was compared before and after the RH test to quantify the amount of water absorbed during the test. A greater area under the peak indicates higher water absorption and vice versa. 50 Figure 4. 7 : FTIR - ATR spectra for (A) PVOH (B) PU/PVOH (C) PDMS - A/PVOH (D) PDMS - B/PVOH from 400 cm - 1 to 4000 cm - 1 51 Figure 4. 7 shows the ATR - IR spectra of the different films before and after the RH test. It is evident from the stacked IR traces that - OH peak intensity increased for the samples after the RH test suggesting water gain by the samples. Table 4.4 summarizes the percentage difference of ar ea under the - O H peak for each of the se coatings . T able 4. 4 : FTIR - ATR data for PVOH and coated PVOH samples focusing on area under the peak for OH peaks from 3200 cm - 1 to 3500 cm - 1 Sample Area under the Peak Percentage difference (%) PVOH NA PU/PVOH 77.30% PDMS - A coating 92.20% PDMS - B coating 96.10% As expected, neat PVOH absorbed the maximum amount of water with huge increase in the OH peak after RH test . PU/PVOH samples showed a less increase for the OH peak . PDMS - B/ PVOH samples showed only a minor increase in the OH peak, which was 96.1% less than the increase observed for the neat PVOH. Similarly, the OH increase for the PDMS - A /PVOH was much smaller ( 92.2% lower) than the neat PVOH. Based o n the above data it proves that the coated PVOH samples absorb significantly less amount of water. These ATR - IR results ar e also in good agreement with those obtain ed from the weight gain anal ysis. 52 4.4 Mechanical p roperties Mechanical properties of the films are very important for packaging as well as non - pack ag ing applicat i ons. Two tests were performed to determine the mechanical properties including: (i) T ensile test to determine the strength and elongation of the material (ii) D art d rop impact test to determine failure mass 4. 4.1 Tensile t est The strength and e longation of the material is determined through this tes t . Data obtained from the tensile tests is summarized in Table 4.5 . O verall the PVOH films before and after coating has good mechanical properties. P U /PVOH films were more elastic th an PVOH film s possibly due to the more flexible nature of the urethane . Ureth ane in th i s study carries HDIT, which is a highly flexible material . On e can choose stiff PU materials for coatings if desirable . PDMS - A /PVOH films ha d higher tensile strength than P DMS - B /PVOH . However, PDMS - B /PVOH had highe r % of elongation than PDMS - A /PV O H . This i ncrease in elongation properties of the films corresponds to the soft and elastic nature of PDMS. 53 Figure 4. 8 : Graphical presentation of tensile strength and % of elongation for PVOH and coated PVOH samples Table 4. 5 : T ensile strength and % of elongation data for PVOH and coated PVOH samples PVOH PU /PVOH PDMS - A/PVOH PDMS - B/PVOH Tensile Strength (MPa) % of Elongation Tensile Strength (MPa) % of Elongation Tensile Strength (MPa) % of Elongation Tensile Strength (MPa) % of Elongation 99. 8 ± 14.1 6. 1 ± 0.8 75.1 ± 14.1 8.6 ± 0. 5 59.6 ± 9. 2 9. 7 ± 1.5 12.5 ± 1.9 21. 1 ± 3. 1 6.1 8.6 9.7 21.1 99.8 75.1 59.6 12.5 0 20 40 60 80 100 120 PVOH PU/PVOH PDMS-A/PVOH PDMS-B/PVOH 0 5 10 15 20 25 30 Tensile Strength (MPa) Films % of Elongation Tensile Test % of Elongation Tensile Strength (MPa) 54 4. 4.2 Drop d art i mpact t est The drop dart impact test determines how much weight the film can resist before it breaks when subjected to immediate stress . It is important for a material to have high i mpact strength for packaging applications . Impact strength is related to the elastic nature of the material. Results obtained for PV O H films with and without coatings are shown in T a b le 4.6. For PU/PVOH, the sample has shown ~50% increase in impact resistance . Urethane used in this study , is flexible and amorphous, and thus its impact resistance is higher than PVOH alone. PDMS - B/PVOH coated samples have a failure mass of 37.5 g , which is almost twice that of PVOH film s (failure mass of 19.5 g of PVOH) . This increased failure mass correspond s to the synergistic effect of the amorphous PU as well as rubbery PDMS. For PVOH, the tensile strength is good because of its inelasticity , but their impact resistance is poor , h ence the m aterial breaks easily. 55 Figure 4. 9 : Graphical presentation of failure mass for PVOH and coated PVOH samples. Table 4. 6 : Failure mass data for PVOH and coated PVOH samples Failure mass (g) PVOH PU/PVOH PDMS - A/PVOH PDMS - B/PVOH 19.5 28.5 35.5 37.5 19.5 28.5 35.5 37.5 0 5 10 15 20 25 30 35 40 PVOH PU/PVOH PDMS-A/PVOH PDMS-B/PVOH Failure Mass (g) Films Dart drop impact test Failure mass 56 4.5 Barrier p roperties High b arrier properties are essential for polymer films in packaging applications. However , PVOH is a n excellent natural barrier to oxygen but h a s poor barrier to water vapor. Under high RH, PVOH tends to lose its oxygen barrier properties and becomes a poor barrier to both oxygen and gas molecules . 4. 5.1 Permeability of o xygen and w ater vapor Table 4.7 summarizes the oxy gen and water vapor permeability data for the coated and uncoated PVOH films . As shown in the table, o verall, the presence of coating improves the oxygen and water vapor barrier properties of the PVOH film. The PVOH/PU coated samples sho wed improvement in the barrier properties relative to neat PVOH films. For example, the water vapor and oxygen barrier properties were improved by 1.5 times and 2 times higher than the neat PVOH films, respectively. This water vapor and oxygen barrier pro perties improvement of PU coated films correspond to the highly cross - linked PU films that unlike PVOH has low degree of swelling upon absorption of water. As a result, there is little free volume increase for the PU coating upon water absorption and henc e the barrier properties are high when compared with neat PVOH films. P DMS - A /PVOH barrier properties increased by ~50 times better for oxygen and ~5 times bette r for water vapor compared with neat PVOH . Similarly, PDMS - B /PVOH coated samples have 1 5 times higher barrier properties for oxygen and ~5 times high barrier a gainst water vapor . This increase in the barrier performance is attributed to the presence of a hydrophobic PDMS chains that reduce the water absorption significantly . As a result, the free v olume increase caused by the water absorption is minimum in PDMS PU coated PVOH films. Thus, these samples showed the 57 best barrier properties for both water vapors and oxygen gas compared to the neat PVOH and PU/PVOH films. Figure 4. 10 : Schematic illustration of the PU/PVOH with urethane bonds on their surface. Figure 4. 11 : Schematic illustration of the PDMS/PVOH with PDMS showing PDMS polymer chains on the surface. PDMS 58 Figure 4. 12 : Graphical presentation of o xygen and water vapor permeability at 23 ° C 75%RH for PVOH and coated PVOH samples . Table 4. 7 : Permeability data for PVOH and coated PVOH samples for o xygen and w ater vapor Test Film At 23°C, 75% RH Oxygen Permeability (kg m/m2 sec Pa) E - 18 Water Vapor Permeability (kg m/m2 sec Pa) E - 14 PVOH 1.98 ± 0.82 2.59 ± 0.12 PU/PVOH 0.79 ± 0.12 1.61 ± 0.14 PDMS - A/PVOH 0.04 ± 0.01 0.46 ± 0.17 PDMS - B/PVOH 0 .13 ± 0.04 0.50 ± 0.30 1.98 0.79 0.04 0.13 2.59 1.61 0.46 0.50 0 0.5 1 1.5 2 2.5 3 0 0.5 1 1.5 2 2.5 3 PVOH PU/PVOH PDMS-A/PVOH PDMS-B/PVOH Water Vapor Permeability (kg m/m2 s Pa) E - 14 Oxygen Permeability (Kg m/m 2 s Pa) E - 18 Films Permeability at 23 C, 75% RH Oxygen Permeability Water Vapor Permeability 59 4.6 Surface a nalysis Surface analysis stud ies were conducted to determine the chemical composition of the film surfaces at various stages of preparation . 4.6.1 XPS a nalysis XPS spectra w as used to obtain the surface compositional analysis of these films. XPS results are summarized in Table 4. 8 that quantifies the concentration of different elements on the film surfaces. As expected, PU/PVOH has the highest concentration of N1s corresponding to the Nitrogen in urethane . PDMS - A or B/PVOH coated samples , however, have low concentration of N1s concentration because these samples have PDMS enriched on their surfaces. The presence of PDMS is confirmed in PDMS - A/PVOH and PDMS - B /PVOH by the presence of Si2p in the XPS. Surprisingly, Si 2p peak though at low concentration of 4.00 appeared in the PU/PVOH . This cor responds to the possible silicone impurity from urethane formulations from supplier or a finger print could have contaminated the surface. In an attempt to investigate if there is any loss in the PDMS content upon solvent extraction, P D MS - A /PVOH samples were rinsed with excess hexane . H exane was selected as it dissolves PDMS and if there were any ungrafted PDMS on the surface, it will be washed out. As shown in Table 4.8, the Si2p concertation of the PDMS - A/ PVOH films before and after extraction remains essentially the same . This study confirmed that that the PDMS is chemical ly grafted and does not wash out from the coating. 60 Table 4. 8 : Atomic c ompositions d ata obtained with XPS for coated PVOH samples. 4.6.2 SEM analysis SEM analysis were used to analyze the surface features of the PVOH coated and uncoated samples ( SEM images are shown in Figure 4.10 ) . PVOH film ( F igure 4.10 A ) has a smooth texture as well as PU/PVOH films ( Figure 4.10 B) . Similarl y , PDMS - A/PVOH ( Figure 4.10 C) and PDMS - B /PVOH ( Figure 4.10 D) films have smooth texture indicating the absence of any phase separation of the PDMS on the surface . Though, PDMS can separate from PU m atrix to form two phase PDMS nanodomains . but in our case we used PDM S - NH 2 which graft s to PU abruptly and quantitatively and t hus phase separation wa s suppressed. SEM confirmed the absence of any ph ase separation which is also evident from the excellent clarity of the PDMS - A/PVOH and PDMS - B /PVOH films. Samples Atomic Concentration C1s N1s O1s Si2p PVOH 70.36 NA 29.64 NA PU/PVOH 64.95 8.44 22.62 4 .00 PDMS - A/PVOH 58.42 5.2 23.99 12.39 PDMS - B/PVOH 63.11 7.59 21.34 7.97 PDMS - A/PVOH after rinsing with hexane 60.88 2.5 23.84 12.77 61 Figure 4. 13 : SEM images for films of A) PVOH, (B) PU/PVOH, C) PDMS - A/PVOH, (D) PDMS - B/PVOH . The images were recorded at a magnification of 100,000x. White scale bar shown in the figure is equal to 200 nm. 4. 7 Thermal a nalysis T hermal analysis was performed by TGA to determine the thermal stability of the material and the coating decomposition temperature. (A) ( B ) ( C ) ( D ) X 100,00 0 5.0kV SEI SEM WD 4.5mm X 100,00 0 5.0kV SEI SEM WD 4.5mm X 100,00 0 5.0kV SEI SEM WD 4.5mm X 100,00 0 5.0kV SEI SEM WD 4.5mm 62 4. 7. 1 TGA TGA w as used to determine the decomposition temperature of the PVOH films. Figure 4.1 2 represents the TGA profiles for the different coatings. It can be observed that all the coatings have a similar decomposition profile with an initial weight loss at 220°C and a second weight loss at For PVOH the initial 5 to 7 % weight loss is due to moisture loss at 1 1 nd 80% weight loss happens at 220 which corresponds to the decomposition of the PVOH . For coated samples, t he initial weight loss arises from PVOH decomposition at 220 °C , which accounts for 70% of the film. The second weight loss is due to PU decomposition that accounts for the remaining 30% of the film. The PDMS content in the film is minimal and could not be detected in the TGA curves. Th us , TGA profiles sh ows that t he thermal stability of the PU coated PVOH films remained essentially the same as that of PDMS PU coated PVOH. Figure 4. 14 : TGA profile for PVOH, PU/PVOH, PDMS - A/PVOH and PDMS - B/PVOH 0.0000 20.0000 40.0000 60.0000 80.0000 100.0000 120.0000 0 100 200 300 400 500 600 700 Weight Percentage TGA PVOH PU/PVOH PDMS-A/PVOH PDMS-B/PVOH 220 ° C PVOH degradation 410 ° C PU degradation 63 4.8 Crosshatch adhesion test Crosshatch adhesion test determines the adhesion quality of a coating. The tape was placed on the scratched surface and was then take - off from the surface. Black tape was used to see the transfer of transparent coating. The tape did not show any transfer of the coating from the film (Figure 4.13 B) which proves that a strong adhesion between the urethane matrix and PV OH film. (A) (B) Figure 4. 15 : Crosshatch adhesion test (A) Scratched film (B) Black electrical ta pe after attempting to remove crosscut coat from the scratched film. No residue found 64 REFERENCES 65 REFERENCES 1. M. Rabnawaz, G. Liu, H. Hu; Fluorine - free anti - smudge polyurethane coatings; Angew.Chem.; volume 127 issue 43 (2015); pp. 12913 - 12918. 66 5.1 Conclusion s In this study, we have developed a new approach for the fabrication of water resistant PVOH films by applying PDMS polyurethane (PU PDMS) coatings. The water repellency of coated samples increased significantly as compared to the virgin PVOH films . For example, the wate r c ontact angle for the virgin PVOH films were 73º , which increased to 107 º after the fabrication. In fact, water droplet on PVOH film dissolves the film surface in two seconds . The sliding angle s of coated films were also decreased to 14º and 16º, meanwhile virgin PVOH did not show any sliding angles. The water resistance of the fabricated PVOH samples were also quantified by weight gain analysis and ATR - IR analysis , where the coated samples has excellent reduction in the water uptake. This water r esistant PVOH films were accomplished by masking the free OH group of PVO H with an exterior urethane matrix carrying water - repellent PDMS chains . These water resistant PVOH films can be used for direct water contact applications, which are not possible with unmodified PVOH. The tensile strength of the PVOH films after coatings were slightly reduced , b ut the % elongation w ere increased significantly making these films more elastic and also increase d their impact resistance. These changes in the mechanical properties correspond to the flexible nature of urethane s and PDMS used for t he coating of PVOH . The t hermal analysis confirm ed excellent thermal stability of the coated films. The PVOH films before and after the coating s remained visually clear . 67 The oxygen and water barrier properties for the coated PVOH films were improved by the incorporation of PDMS in the polyurethane formulations. For example, the oxygen barrier properties of the PDMS - B/PVOH was improved by 15 times over t he PVOH uncoated films. Similarly, the water vapor barrier properties were improved by ~5 times for the PDMS - B/PVOH compared to the neat PVOH films. A similar trend in the barrier performance was also observed for the PDMS - A/PVOH. 5.2 Future w ork This study has laid the foundation to a new fabrication approach for high barrier coatings. In the future, polyurethane will be replaced with food - grade epoxy coatings and that will be applied for coating various plastics to improve their barrier properties . Ep oxy coatings are as liner for the food can s . By replacing PU matrix with e poxy will make this coating commercially viable for food contact applications . For example, commercially, PET bo ttles are coated with epoxy materials to improve their oxygen barrier performance for applications in juices and beer packaging. Thus, one can extend this current study to PLA, PET and other plastic s to improve their water and gas barrier properties. The urethane used in this study was aliphatic urethane which has prospects of being a biodegradable polymer. This biodegradable polymer mixed with another biodegradable polymer (PVOH) along with PDMS has a high prospect of being biodegradable. In the futur e biodegradation studies wil l be done to validate the biodegradability of PU PDMS coated PVOH films. 68 Figure 5. 1 : An example of the application of our developed high barrier coating s in the packaging industry [2]. 5.3 Im pact of this research Neat PVOH films are extremely water sensitive and has poor water vapor barrier performance that limit s its applications. By coating with PU - S , the water resistance is significantly enhanced. At the same time, coated PVOH films has s ignificantly improve d gas barrier, mechanical properties as well as the films remained transparent. Such fabrication can be used for food and drug packaging where high gas and water vapor barrier per formance is required. 69 In addition, there is a need for biodegradable plastic packages to reduce landfilling problem . In this study, we used aliphatic urethane coating and PDMS on PVOH films , where all three materials are biodegradable . Thus, this study offers a new approach for enhancing bi odegradability that can be utilized in the packaging industry. 70 REFERENCES 71 REFERENCES 1. S. Mupalanei, H. Omidian; Polyvinyl alcohol in medicine and pharmacy: a perspective J. Dev. Drugs, 2 (2013), p. 112 . 2. https://www.canr.msu.edu/rabnawaz/research - area.