IMPROVING GAS BARRIER PROPERTIES OF SUGARCANE - BASED LLDPE WITH CELLULOSE NANOCRYSTALS By Madhumitha Natarajan A THESIS Submitted to Michigan State University in partial fulfillment of the requirements for the degree of Packaging Master of Science 202 1 ABSTRACT IMPROVING GAS BARRIER PROPERTIES OF SUGARCANE - BASED LLDPE WITH CELLULOSE NANOCRYSTALS By Madhumitha Natarajan This study was aimed at improving the gas barrier property of sugarcane - based LLDPE using cellulose nanocrystals (CNCs). Specifically, this study evaluated the effect of testing methods (isostatic versus gravimetric) on CO 2 permeability coefficient (P CO2 ) and/or O 2 permeability coefficient (P O2 ) of various bio - PE grades with different densities (LLDPE, LDPE, and HDP E) as well as the effect of CNC content on crystallinity, tortuosity factor, and gas barrier properties of bio - LLDPE sheets and films. The isostatic and gravimetric methods yielded similar P CO2 , irrespective of PE grade. However, the P CO2 n egatively correlated with PE density. Al l nanocomposites showed considerable improvement in gas barrier irrespective of the CNC content. The P CO2 of LLDPE sheets decreased by 36% by adding 10 wt.% of CNCs into the sheet. Similarly, a significant decline in both P O2 (about 50%) and P CO2 (about 33 %) of LLDPE films was obtained by adding 2.5 wt.% of CNCs into the films. Nevertheless, no correlation was established between gas permeability and percent crystallinity of LLDPE sheet since the P CO2 decreased almost linearly with increasing CNC content wh ereas the percent crystallinity of LLDPE increased only up to 2.5% CNC content and remained constant thereafter. In contrast, the tortuosity factors calculated from the diffusion coefficients increased almost linearly with CNC contents and correlated well with the gas permeability improvement in the bio - LLDPE - based nanocomposites. Consequently, the enhanced gas barrier in the nanocomposite was assigned to the tortuosity effect created by the impermeable cellulose nanocrystals rather than the changes in perc ent crystallinity. Copyright by MADHUMITHA NATARAJAN 2021 iv Dedicated to my father Mr. R. Natarajan and my mother Dr. R. Vijayala kshmi v ACK NOWLEDGEMENT S I would like to thank my major advisor Dr. Laurent Matuana for his constant support and guidance throughout this thesis. He always encouraged me to strive for the best. I am grateful to m y committee members Dr. Liu ( Biosystems and Agricultural Engineering, Michigan State University) and Dr. Lee (School of Packaging, Michigan State University) for taking time and providing valuable comments to me . I would like to thank our lab manager Aaron Walworth for training me on using different lab equipment and always help ing me with any queries in the lab. I am thank ful to School of Packaging for providing me the opportunity to pursue Master s in this prestigious university and gain exposure in the field of Packaging Science. I would like to thank m y lab mates Sonal Karkhanis and Krishnaa Balaji Venkatesan for always encouraging me to do my best. Sonal was like a mentor to me and helped me learn various lab equipment and made me comfortable in the lab when I was new . Krishnaa was always there to cheer me up and encourage me whenever I needed some motivation . Finally, I would like to thank my parents , sister Indhujha and brother - in - law Subramaniam for constantly supporting me to achieve my dreams . I would also like to thank all my friends back home who have always shown their support despite not being physically present here with me . I am very grateful to m y friends here , Anirudh, Aakas h , Parth, Ankita and others who have constantly supported me through my Master of Science program by always cheering me up and providing a listening ear when I needed . vi TABLE OF CONTENTS LIST OF TABLES ................................ ................................ ................................ ................. viii LIST OF FIGURES ................................ ................................ ................................ ............... x CHAPTER 1 ................................ ................................ ................................ .......................... 1 INTRODUCTION ................................ ................................ ................................ ................. 1 1.1 Introduction ................................ ................................ ................................ .......... 1 1.2 Objectives ................................ ................................ ................................ ............ 5 1.3 Hypothesis ................................ ................................ ................................ ............ 5 1.4 Structure of thesis ................................ ................................ ................................ 6 REFERENCES ................................ ................................ ................................ ...................... 7 CHAPTER 2 ................................ ................................ ................................ .......................... 11 BACKGROUND AND LITERATURE REVIEW ................................ ............................... 11 2.1 Introduction ................................ ................................ ................................ .......... 11 2.2 Polyethylene ................................ ................................ ................................ ......... 11 2.2.1 Synthesis of polyethylene ................................ ................................ ........... 11 2.2.2 Petroleum based and biobased polyethylene ................................ .............. 12 2.3 Variants in polyethylene ................................ ................................ ...................... 13 2.3.1 Branched polyethylene versus linear polyethylene ................................ .... 13 2.3.2 Copolymer LLDPE ................................ ................................ ..................... 14 2.4 Properties of polyethylene ................................ ................................ ................... 15 2. 4.1 Low - density polyethylene (LDPE) ................................ ............................. 15 2.4.2 High - density polyethylene (HDPE) ................................ ............................ 16 2.4.3 Linear - low - density polyethylene (LLDPE) ................................ ................ 16 2.5 Processing technologies of polyethylene ................................ ............................. 18 2.5.1 Extrusion ................................ ................................ ................................ ..... 18 2.5.2 Blown film extrusion ................................ ................................ .................. 20 2.5.3 Cast film extrusion ................................ ................................ ...................... 21 2.5.4 Extrusion blow molding ................................ ................................ .............. 22 2.5.5 Compression molding ................................ ................................ ................. 23 2.5.6 Thermoforming ................................ ................................ ........................... 25 2.6 ................................ ............................... 26 2.7 Cellulose nanocrystals (CNCs) ................................ ................................ ............ 26 2.7.1 Extraction of CNCs ................................ ................................ ..................... 27 2.7.2 Properties of CNCs ................................ ................................ ..................... 29 2.8 CNCs in bio - based polymers ................................ ................................ ............... 29 2.9 Processing technologies of composite films ................................ ........................ 30 2.9.1 Melt processing ................................ ................................ ........................... 30 2.9.2 Solvent casting ................................ ................................ ............................ 31 2.9.3 Electr ospinning ................................ ................................ ........................... 32 2.10 Gas barrier property measurement ................................ ................................ ..... 32 2.10.1 Isostatic permeability method ................................ ................................ ... 33 vii 2.10.2 Gravimetric permeability method ................................ ............................. 35 2.11 Crystallinity ................................ ................................ ................................ ........ 37 2.11.1 Measurement techniques ................................ ................................ ........... 37 2.11.1.1 Differential scanning calorimetry (DSC) ................................ ...... 37 2.11.1.2 Fourier transform infrared spectroscopy (FTIR) .......................... 38 2.12 Effect of crystallinity on barrier properties of plastics ................................ ...... 39 REFERENCES ................................ ................................ ................................ ...................... 41 CHAPTER 3 ................................ ................................ ................................ .......................... 47 EXPERIMENTAL ................................ ................................ ................................ ................. 47 3.1 Mat erials ................................ ................................ ................................ .............. 47 3.2 Samples manufacture for permeability tests ................................ ........................ 47 3.2.1 Compression molded samples for gravimetric - sorption method ................................ ................................ ................................ ........ 48 3.2.2 Blown film extrusion for isostatic permeability method ............................ 49 3.3 Property evaluation ................................ ................................ .............................. 49 3.3.1 Gas permeability by sorption experiments ................................ ................. 49 3.3.2 Tortuosity factor ................................ ................................ .......................... 52 3.3.3 Gas permeability by isostatic permeability method ................................ .... 53 3.3.4 Crystallinity of nanocomposites ................................ ................................ . 54 3.3.5 Density measurements ................................ ................................ ................ 57 3.3.6 Optical microscopy ................................ ................................ ..................... 57 3.3.7 Statistical analysis ................................ ................................ ....................... 57 REFERENCES ................................ ................................ ................................ ...................... 58 CHAPTER 4 ................................ ................................ ................................ .......................... 62 RESULTS AND DISCUSSIONS ................................ ................................ .......................... 62 4.1 Effect of testing methods (isostatic versus gravimetric) on CO 2 permeability of PE ................................ ................................ ............................... 62 4.2 CO 2 barrier improvement and its mechanisms in LLDPE/CNC nanocomposite sheets ................................ ................................ ........................... 65 4.2.1 Effect of CNC addition on the crystallinity of LLDPE .............................. 65 4.2.2 Effect of CNC ad dition on the CO 2 diffusion and solubility coefficients of LLDPE ................................ ................................ ................ 67 4.2.3 Correlations between crystallinity, tortuosity factor, and permeability coefficient ................................ ................................ .............. 68 4.3 CO 2 and O 2 barrier properties of LLDPE films with CNCs ................................ 74 APPENDIX ................................ ................................ ................................ ............................ 76 REFERENCES ................................ ................................ ................................ ...................... 88 CHAPTER 5 ................................ ................................ ................................ .......................... 91 CONCLUSION ................................ ................................ ................................ ...................... 92 5.1 Conclusion ................................ ................................ ................................ ........... 92 5.2 Future work ................................ ................................ ................................ .......... 94 viii LIST OF TABLES Table 2 - 1 Summary of properties of polyethylene. ................................ ........................ 17 Table 4 - 1 Effect of testing method on the carbon dioxide permeability (P CO2 ) of various bio - PE grades. ................................ ................................ .... 63 Table 4 - 2 Effect of CNC content on the crystallinity and permeation parameters of compression molded bio - LLDPE sheet. ................................ . 66 Table 4 - 3 Effect of CNC addition on the CO 2 and O 2 permeability of bio - LLDPE. ................................ ................................ ................................ .... 74 Table A - 1 Permeability coefficients of bio - HDPE film obtained using isostatic permeability method. ................................ ................................ ....... 77 Table A - 2 Permeability, diffusion and solubility coefficients of bio - HDPE using gravimetric method. ................................ ................................ .. 77 Table A - 3 Permeability, diffusion and solubility coefficients of bio - LDPE obtained using gravimetric method. ................................ .................... 78 Table A - 4 CO 2 permeability coefficients of bio - LLDPE film obtained using iso - static permeability method. ................................ ............................ 79 Table A - 5 CO 2 permeability coefficient values of bio - LLDPE obtained using gravimetric method. ................................ ............................... 79 Table A - 6 Measured densities of bio - PE obtained using density gradient method. ................................ ................................ ............................ 80 Table A - 7 Measured densities of bio - LLDPE with various CNC contents obtained using density gradient method. ................................ ......... 81 Table A - 8 Effect of CNC content on crystallinity of bio - LLDPE/CNC composites. ................................ ................................ ................................ ..... 83 Table A - 9 CO 2 permeability (P in 10 - 16 kg·m/m 2 sec·Pa), diffusion (D in 10 7 cm 2 /sec), and solubility (S in 10 - 3 in g/m 3 ·Pa) coefficients of neat bio - LLDPE and bio - LLDPE filled with various CNC content obtained using gravimetric method. ............................ 84 Table A - 10 Tortuosity factor of bio - LLDPE with various CNC contents. ................................ ................................ ................................ ......... 85 Table A - 11 CO 2 permeability coefficients of neat bio - LLDPE and bio - LLDPE/2.5% CNC composite films. ................................ ............................. 86 ix Table A - 12 O 2 permeability coefficients of neat bio - LLDPE and bio - LLDPE/2.5% CNC composite films. ................................ ............................. 87 x LIST OF FIGURES Figure 2 - 1 Structure of polyethylene. [ 3 ] ................................ ................................ ........... 12 Figure 2 - 2 Structure of a) HDPE b) LDPE c) LLDPE. ................................ ................... 15 Figure 2 - 3 Schematic diagram of extrusion process. ................................ ....................... 19 Figure 2 - 4 Schematic diagram of blown film extrusion process. ................................ .... 21 Figure 2 - 5 Schematic diagram of compression molding process. ................................ ... 24 Figure 2 - 6 Schematic diagram of structure of cellulose with repeating unit of cellobiose. [ 26,28 ] ................................ ................................ ................... 27 Figure 3 - 1 Infrared spectra of sugarcane - based neat LLDPE and LLDPE/5% CNC nanocomposite sheets. ................................ ...................... 56 Figure 4 - 1 Schematic diagram of the tortuosity effect. ................................ ................... 70 Figure 4 - 2 Effect of CNC content on tortuosity factor (TF) and CO 2 permeability of bio - LLDPE. ................................ ................................ .......... 71 Figure 4 - 3 Optical microscope images of bio - LLDPE sheets (left column) with various CNC contents: (a) 0%, (b) 2.5%, (c) 7.5% and (d) 13.5% as well as of bio - LLDPE films (right column) with various CNC contents: (a) 0%, (b)1%, (c) 2.5% and (d) 3.5%. ................................ ................................ ........................ 72 Figure 4 - 4 Pco 2 vs tortuosity factor. ................................ ................................ ................ 73 Figure A - 1 Effect of CNC content on density of bio - LLDPE/CNC composites. ................................ ................................ ................................ ..... 82 1 CHAPTER 1 INTRODUCTION 1.1 Introduction Most of the plastics used in the industry for various applications including packaging applications are petroleum - based and are obtained from crude oil. However, the use of petroleum - based plastics is a major contributor to global greenhouse gas emissions [1]. Therefore, there is a need for more sustainable, environmentally friendly alternatives like biobased and/or biodegradable polymers [2]. Bioplastics have a smaller carbon footprint compared to petroleum - based plastics [3]. In addition to being sustainable, biobased plastics also have physico - mechanical properties like petroleum - based polymers; and hence are ideal alternative to the latter [4] . Globally , bioplastics was valued at $4.6 billion in 2019 and is expected to reach $13.1 billion by 2027 [5]. The recent development of sugarcane - based polyethylene is a typical example of more environmentally sustainable plastics. Recently, Braskem, a Brazilian che mical company, has launched a set of biobased polyethylene derived from sugarcane ethanol that have similar properties to their petroleum - based counterparts. Polyethylene (PE) is one of the most widely used polymers in the plastics industry and has differ ent variants like high density polyethylene (HDPE), low - density polyethylene (LDPE) and linear low - density polyethylene (LLDPE). Each variant has its unique properties in terms of appearance, crystallinity, mechanical, and barrier properties. In food packaging, material with transparency is highly desired as it influences packaging aesthetics along with material having 2 good moisture and gas barrier to provide required shelf life for the packaged product. All PE variants have good water barrier, but poor gas barrier and their transparencies vary [6 - 8]. HDPE has good gas barrier compared to other variants of PE but has a milky white appearance, whereas LDPE has poor gas barrier but has excellent transparency. Since both properties are crucial for packaging applications these drawbacks limit the scope of these poly mers in packaging. LLDPE on the other hand has improved regularity in its structure at a low - density making it better than LDPE in most properties especially in terms of its gas barrier properties. LLDPE is also transparent in appearance making it better t han HDPE in terms of transparency. Consequently, LLDPE is a suitable polymer for applications requiring both good transparency and good gas barrier like produce bags, frozen food bags, as a protective pallet stretch film , etc. [7 - 10]. However, when compare d to other plastics like EVOH, PET, etc., LLDPE is poor in its gas barrier properties. Further enhancement in gas barrier properties will make it more suitable for a wide range of packaging applications. Various methods are commonly employed to enhance th e gas barrier properties of packaging materials. Coating as well as production of multilayers and laminates structures are few techniques where layers of different materials are combined to achieve desired gas barrier properties of the films [11,12]. Unfor tunately, these different layers cannot be easily separated at these multilayer films end up in the landfill. Landfill is a major issue with polymers as it i s an unsustainable waste management practice. 3 Manufacture of bio - based composite films of monolayer structure with excellent gas barrier properties could be the solution to a such problems as they are just as recyclable as their neat polymer counterparts [13]. Monolayered nanocomposite films manufactured by incorporating nanoparticles such as nano clay [14], cellulose whiskers [15], cellulose nanofibers (CNFs) [16], cellulose nanocrystals (CNCs) [17 - 19], etc. into various polymer matrices have been reported to improve the gas barrier properties of the neat polymer counterparts. The improved barrier performance of nanocomposite films has been mainly assigned to the tortuosity effect created by the presence of highly crystalline nanoparticles int o the polymeric matrix, which increases the degree of crystallinity of the neat polymer. These crystals increase the effective travel path length for permeants diffusing through the nanocomposites. This reduces the rate of diffusion, thus lowering permeati on as reported by others [17 - 19]. Unfortunately, limited work has been done on nanocomposites based on bio - PE. Recently, Bazan and coworkers investigated the mechanical, thermal properties, and micromechanics of composites made of biobased polyethylene and a combination of natural fibers (e.g., wood flour, coconut shell fibers, and basalt fibers). They reported that hybridization of cellulose fibers with basalt fibers significantly improves the properties compared to adding cellulose fibers alone. Additiona lly, the accelerated water and thermal ageing of composites samples deteriorated their strength properties compared to unaged counterparts [6]. It is well accepted that gas barrier properties of semi - crystalline polymer filled with nanoparticles is affect ed not only by the impermeable nanoparticles, but also greatly depends on its crystallinity and crystalline morphology [15 - 21]. Although a negative correlation between percent crystallinity and permeability coefficients of various permeants has been report ed for 4 nanocomposites [15 - 21], polymer crystallinity is certainly not the only factor influencing its permeability, since no correlation has also been established between level of crystallinity and permeability [22 - 24]. For example, d espite a 27% reduction in crystallinity, the oxygen and carbon dioxide permeability of the polypropylene (PP)/ethylene - propylene - diene rubber (EPDM) blend nanocomposite reduced two - fold by adding only 1.5 vol% montmorillonite - based organoclay into PP/EPDM blend [22]. The increa se in barrier property of the nanocomposite blend was attributed to the combination of two phenomena, including (i) the decrease in area available for diffusion, a result of impermeable flakes replacing permeable polymer; and (ii) the increase in the dista nce a solute must travel to cross the film as it follows a tortuous path around the impermeable flakes [22]. Similarly, a significant decline in both O 2 (about 45%) and CO 2 (about 68 %) permeability coefficients of PLA films was obtained by adding 1.37 vol % graphene oxide nanosheets (GONSs) assigned to the impermeable GONSs acting as crystallites and the strong interfacial adhesion between GONSs and PLA matrix, rather than the changes in crystallinity and crystalline morphology of PLA matrix [23]. Therefore, in addition to level of crystallinity, the role of other factors contributi ng to barrier improvement like the tortuosity factor must also be investigated to understand the mechanisms of barrier improvement in nanocomposites. 5 1.2 Objectives Therefore, this study was aimed at improving the gas barrier property of sugarcane - based LLD PE using cellulose nanocrystals (CNCs). Specifically, this study evaluated the effect of CNC content on gas barrier properties of sugarcane - based LLDPE sheets and films. Emphasis was placed on the gravimetric - sorption method to quantify the diffusion and s olubility coefficients needed to estimate the tortuosity factor and permeability coefficient, which were correlated to elucidate the mechanism involved in barrier property improvement in nanocomposites. To achieve this objective the following specific obje ctive were proposed : 1) Determine the tortuosity factor and gas permeability coefficient of bio LLDPE/CNC composites. 2) Correlate the tortuosity factor and gas permeability coefficient in order to elucidate the mechanism involved in barrier property improvement in nanocomposites. 1.3 Hypothesis This research is intended to test the hypothesis that gas barrier property of sugarcane - based polyethylene can be improved by addition of cellulose nano crystals. 6 1.4 Structure of thesis The first chapter of the thesis includes an introduction to rationalize the research. Chapter 2 covers a background on various processing techniques polyethylene, manufacturing of cellulose nano crystals and incorporation techniques. Chapter 3 will cover the material specifications, equi pment, and sample manufacturing. The results of permeability coefficient, crystallinity, tortuosity factors, diffusion, solubility of the rigid sample and permeability coefficients for the film sample composites and correlation between the permeability coe fficient and tortuosity factor along with its discussion is covered in Chapter 4. Chapter 5 gives the summary of the findings inferred from the experimental data and proposed future. 7 REFERENC E S 8 REFERENCES 1. Thompson, R. C., Moore, C. J., Vom Saal, F. S., and Swan, S. H. (2009). Plastics, the environment, and human health: current consensus and future trends. Philosophical Transactions of the Royal Society B: Biological Sciences , 364 (1526), 2153 - 2166. 2. plastics. Angewandte Chemie International Edition , 54(11), 3210 - 3215. 3. Boonniteewanich, J., Pitivut, S., Tongjoy, S., Lapnonkawow, S., and Suttiruengwong, S. (2014). Ev aluation of carbon footprint of bioplastic straw compared to petroleum based straw products. Energy Procedia , 56, 518 - 524. 4. Türünç, O., Montero de Espinosa, L., and Meier, M. A. (2011). Renewable polyethylene mimics derived from castor oil. Macromolecular R apid C ommunications , 32(17), 1357 - 1361. 5. Bioplastics m arket is projected to reach $13. 1 billion by 2027. (n.d.).< https://www.alliedmarketresearch.com/press - release/bioplastics - market.html > [accessed April 6 , 202 1 ]. 6. Bazan, P., Nosal, P., Kozub, B., and Kuciel, S. (2020). Biobased p olyethylene h ybrid c omposites with n atural Fiber: m echanical, t hermal p roperties, and m icromechanics. Materials , 13(13), 2967 . 7. Emblem, A. (2012). Chapter 13: Plastics properties for packaging materials. Emblem , A. , Emblem , H. Packaging T echnology . Woodhead Publishing. 287 - 309 . 8. Selke, S. E., and Culter, J. D. (2016). Plastics P ackaging: P roperties, P rocessing, A pplications, and R egulations . Hanser. 9. Kontou, E., and Niaounakis, M. (2006). Thermo - mechanical properties of LLDPE/SiO2 nanocomposites. Polymer , 47(4), 1267 - 1280 . 10. Ashizawa, H., Spruiell, J. E., and White, J. L. (1984). An investigation of optical clarity and crystalline orientation in polyethy lene tubular film. Polymer Engineering & Science , 24(13), 1035 - 1042. 11. Erlat, A. G., Spontak, R. J., Clarke, R. P., Robinson, T. C., Haaland, P. D., Tropsha, Y, and Vogler, E. A. (1999). SiOx gas barrier coatings on polymer substrates: morphology and gas transport considerations. The Journal of Physical Chemistry B , 103(29), 6047 - 6055. 9 12. Dole, P., Averous, L., Joly, C., Valle, G. D., and Bliard, C. (2005). Evaluation o PE multilayers: p rocessing and properties. Polymer Engineering & Science , 45(2), 217 - 224 . 13. Wang, J., Gardner, D. J., Stark, N. M., Bousfield, D. W., Tajvidi, M., and Cai, Z. (2018). Moisture and oxygen barrier properties of cellulose nanomaterial - b ased films. ACS Sustainable Chemistry & Engineering , 6(1), 49 - 70. 14. Müller, C. M., Laurindo, J. B., and Yamashita, F. (2011). Effect of nanoclay incorporation method on mechanical and water vapor barrier properties of starch - based films. Industrial Crops and Products , 33(3), 605 - 610 . 15. Bendahou, A., Kaddami, H., Espuche, E., Gouanvé, F., and Dufresne, A. (2011). Synergism effect of montmorillonite and cellulose whiskers on the mechanical and barrier properties of natural rubber composites. Macromolecular Materials and Engineering , 296(8), 760 - 769. 16. Jonoobi, M., Harun, J., Mathew, A. P., and Oksman, K. (2010). Mechanical properties of cellulose nanofiber (CNF) reinforced po lylactic acid (PLA) prepared by twin screw extrusion. Composites Science and Technology , 70(12), 1742 - 1747 . 17. Karkhanis, S. S., Stark, N. M., Sabo, R. C., and Matuana, L. M. (2018). Water vapor and oxygen barrier properties of extrusion - blown poly(lactic aci d)/cellulose nanocrystals nanocomposite films. Composites Part A: Applied Science and Manufacturing , 114, 204 - 211. 18. Liu, Y., and Matuana, L. M. (2019). Surface texture and barrier performance of poly(lactic acid) J ournal of Applied Polymer Science , 136(22), 47594. 19. Karkhanis, S. S., Stark, N. M., Sabo, R. C., and Matuana, L. M. (2018). Performance of poly( lactic acid)/cellulose nanocrystal composite blown films processed by two different compounding approaches. Polym er Engineering & Science , 58(11), 1965 - 1974 . 20. Fortunati, E., Peltzer, M., Armentano, I., Torre, L., Jiménez, A., and Kenny, J. M. (2012). Effects of modified cellulose nanocrystals on the barrier and migration properties of PLA nano - biocomposites. Carbohydrate P olymers , 90(2), 948 - 956. 21. Sanchez - Garcia, M. D., and Lagaron, J. M. (2010). On the use of plant cellulose nanow hiskers to enhance the barrier properties of polylactic acid. Cellulose , 17(5), 987 - 1004. 10 22. Frounchi, M., Dadbin, S., Salehpour, Z., and Noferesti, M. (2006). Gas barrier properties of PP/EPDM blend nanocomposites. Journal of M embrane S cience , 282(1 - 2), 142 - 148. 23. Huang, H. D., Ren, P. G., Xu, J. Z., Xu, L., Zhong, G. J., Hsiao, B. S., and Li, Z. M. (2014). Improved barrier properties of poly( lactic acid) with randomly dispersed graphene oxide nanosheets. Journal of Membrane Science , 464, 110 - 118. 24. Drieskens, M., Peeters, R., Mullens, J., Franco, D., Lemstra, P. J., and D. G. (2009). Structure versus properties relationship of poly( lactic acid). I. Effect of crystallinity on barrier properties. Journal of Polymer Science Part B: Po lymer Physics , 47(22), 2247 - 2258. . 11 CHAPTER 2 BACKGROUND AND LITERATURE REVIEW 2.1 Introduction This chapter covers the background and literature related to the scope of the research. The synthesis of polyethylene, types of polyethylene, processing techniques of polyethylene will be covered along with the extraction, processing techniques, properties of cellulose nanocrystals and processing technologies of composite films. The different barrier measurement techniques and crystallinity measurement tec hniques will also be covered. 2.2 Polyethylene Polyethylene (PE) belongs to the family of addition polymers based on ethylene. Polyethylene can be with linear or branched, it can be copolymers or homopolymers. It is widely used in the polymer industry and is used for a variety of applications like films, containers, pipes, fibers , etc. It is one of the first olefinic polymers to be used in food packaging [1]. 2.2.1 Synthesis of polyethylene Polyethylene is made from gaseous hydrocarbon monomer ethylene (C 2 H 4 ) which is produced commonly from the cracking of ethane. Ethane can be obtained from petroleum or from biobased resources. The ethylene molecule comprises of two methylene units (CH 2 ) and this is 12 linked together by a double bond. The double bond of ethylene is broken using the polymerization process and the single bond generated is used to link another ethylene molecule and this results in chain of repeating units i.e., a polymeric chain having the following structure [2]. Schematic diagram of structure of p olyethylene is shown in Figure 2 - 1. Figure 2 - 1 Structure of polyethylene . [3] 2.2.2 Petroleum based and biobased polyethylene Petroleum based polyethylene as the name suggests is developed from ethane obtained from petroleum and natural gas. These are nonre newable resources and produces synthetic polyethylene. Polyethylene is an important polymer hence these nonrenewable resources are used extensively . However, this petroleum - based polyethylene is not sustainable and contribute to a higher carbon footprint [ 4,5 ]. Recently, biobased polyethylene (BioPE) has been developed which is an alternative to the petroleum - based PE as it is more environmentally friendly. These are usually manufactured from biobased and renewable resources like sugarcane, whea t grains and sugar beet , etc. The se plant - 13 based sources are renewable feedstock which consume CO 2 in their annual growth cycle. Hence manufacturing of around 1 Ton of biobased PE from sugarcane would capture around 2.5 Tons of CO 2 from the atmosphere when solar energy is used [ 6 ] . This clearly indicates that biobased PE is contributing to a lesser carbon footprint as compared to its petroleum - based counterparts. Also, it is to be noted that BioPE exhibits similar physical and chemical characteristics as the ir petroleum - based counterparts and can be directly implemented in the manufacturing processes in the industry. 2.3 Variants in polyethylene There are variants in polyethylene which is produced during the polymerization process depending on the pressure used during the process [ 5 ]. 2.3.1 Branched polyethylene versus linear polyethylene The variants produced can be either branched or linear, homopolymer or copolymer. Each variant of polyethylene has its own unique properties. High - density polyethylene (HDPE) and low - density polyethylene (LDPE) are linear and branched variants of polyethylene with 70% to 90% crystallinity and 40% to 60% crystallinity, respectively. HDPE has a linear structure this helps it to have a greater crystalli nity producing a tighter packing of molecules which in turn leads to a higher density. Though HDPE is not a good gas barrier it has better gas barrier properties as compared to LDPE and LLDPE due to its dense packing and crystalline structure. The high cry stallinity of HDPE also makes it opaque compared to LDPE by giving it a milky white appearance. On the other hand, due to its low percent crystallinity LDPE offers good clarity and 14 has some unique properties. However, it has poor gas barrier properties due to its low percent crystallinity [1]. 2.3.2 Copolymer LLDPE Linear low - density polyethylene ( LLDPE) is linear polymer with very short branch like pendant groups, it is produced by the presence of a comonomer in the polymerization process along with a stereo - s pecific catalyst. It is also known as ultra - low - density polyethylene (ULDPE), depending on the density achieved during the polymerization process. LLDPE is intermediate in terms of its properties as compared to LDPE and HDPE. In packaging there is a need f or both clarity and good gas barrier properties, LLDPE is offers better gas barrier then LDPE but offers more clarity than HDPE hence its preferred in many packaging applications [1]. In this study LDPE has been used to improve its gas barrier properties. Schematic diagram of the structural difference among the polyethylene variants is shown in Figure 2 - 2 . 15 Figure 2 - 2 Structure of a) HDPE b) LDPE c) LLDPE . 2.4 Properties of polyethylene 2.4.1 Low - density polyethylene (LDPE) LDPE is a branched variant of polyethylene, and the branched structure of polyethylene makes it have desirable properties like clarity, heat sealability, flexibility and ease of processing. LDPE can be used in different processing techniques like cast film , blown film, injection molding, extrusion coating and blow molding. LDPE is majorly used in the flexible packaging industry. Applications of LDPE include bags for clothing and food items, containers, vapor barriers, industrial liners, household products t o name a few. LDPE has density of around 0.910 0.940 g/cm 3 and melting temperature of around 105 115 ° C. It has great properties like good impact strength, machinability, and flexibility. Also due to its lower percent crystallinity LDPE offers good clar ity as compared to HDPE but both water and gas barrier is poor compared to HDPE [1,7]. 16 2.4.2 High - density polyethylene (HDPE) High - density polyethylene (HDPE) is one of most versatile and the second most widely used polymer in the plastics industry. It is produ ced by the ethylene polymerization in a high - pressure reactor using Ziegler - Natta stereo specific catalyst. HDPE has a milky white appearance and is a non - polar linear thermoplastic. Its density usually ranges from 0.940 0.965 g/cm 3 and its melting poi nt is around 128 138 ° C. Applications of HDPE include manufacture of containers for milk, detergent, bleach, shampoo, pharmaceutical bottles, and flexible packaging applications. It has good processing capabilities and can be used in extrusion blow moldin g, injection molding, injection blow molding and blown and cast film processes. Other properties like tensile and impact strength changes with molecular weight distribution. One major disadvantage of HDPE is environmental stress cracking which is defined a s the failure of a material that is under stress and exposed to a chemical where exposure to either of them alone does not lead to a failure hence for product like detergents are HDPE copolymer is used [1,7]. 2.4.3 Linear - low - density polyethylene (LLDPE) The d ensity of LLDPE is usually 0.916 0.940 g/cm 3 . LLDPE has good mechanical properties as compared to LDPE at the same density due to its high regularity in the structure and narrower molecular weight distribution. LLDPE has a higher melting point as compare d to LDPE due to its increased stiffness. LLDPE has higher puncture resistance, tensile strength, tear properties and elongation. Applications of LLDPE include heavy duty shipping sacks, stretch /cling film, grocery snacks [1,7]. The properties of the poly ethylene variants are summarized in Table 2 - 1. 17 Table 2 - 1 S ummary of properties of polyethylene . Properties HDPE LDPE LLDPE Density (g/cm 3 ) 0.940 0.965 [1] 0.910 0.940 [1] 0.916 0.940 [1] Crystallinity (%) 70 90 [1] 40 60 [1] 35 60 [ 8 ] Oxygen permeability (10 - 17 kg·m/m 2 sec·Pa) 0.92 0.35 [ 9,10 ] 0.75 7.68 [ 9,11,12 ] 5.16 8.33 [ 13 - 15 ] Carbon dioxide permeability (10 - 17 kg·m/m 2 sec·Pa) 10.9 15 [ 9,16 ] 1.56 8.53 [ 9,11,12 ] 24.99 31.9 [ 13,14,17 ] Melting temperature ( ° C) 128 138 [1] 105 115 [1] 114 160 [ 18 ] Clarity Milk y white in appearance [1] Transparent [1] Translucent [1] 18 2.5 Processing technologies of polyethylene Polyethylene is produced using melt processing, where the first step is to convert the solid plastics usually in the form of pellets into a melt. The melt is further transformed into desired shape using various processing techniques like blown film extrusion and cast film extrusion processes for film manufacture, extrusion blow molding, compressio n molding, Injection blow molding, injection stretch blow molding for manufacture of rigid forms of polyethylene [1, 19 ]. This section of this chapter focuses on the processing techniques used for processing polyethylene with special focus on blown film ext rusion and compression molding techniques. 2.5.1 Extrusion Polyethylene is commonly processed using extrusion process to develop desired products from its melt like sheets, film, bottles , etc. The extrusion process is carried out using an extruder. An extruder is used for various machines and applications like mak ing sheets, films, and blow molder of bottles. It is also used as a part of injection molding and injection blow molding machines used in packaging. In all these applications the extruder works similar ly and only varies in its shaping operation of the melt. The extruder uses pressure, heat, and shear to uniformly transform the plastic pellets into melt [1]. The extruder comprises of a barrel which is a hollow tube having helical channels called screw. The screw is usually divided into three parts. Firstly, the feed section or solids conveying section then the compression section or melting section and lastly, metering, or pumping section. The standard single screw extruder has a right - hand helix on the screw, and it rotates in the 19 counterclockwise direction. Hopper is another important component of the extruder which helps in feeding the plastic or other components into the extruder from the feed port. The nozzle or die is the component through which the melted plastic exists the extruder [1]. Schematic diagram of the extrusion process is shown in Figure 2 - 3 . There are different types of dies that are used in the extrusion process depending on the desired shape that must be created. Annular die is used fo r blown film extrusion for production of tubular pipes or films, slit die is used in cast film extrusion to produce sheets and films and capillary die is used for rods and filaments [ 20 ]. Figure 2 - 3 Schematic diagram of extrusion process . 20 2.5.2 Blown film extrusion Polyethylene can be made into sheets and film by using blown film extrusion and polyethylene is one of the commonly used polymers in blown film production. Applications where blown film is used generally are bags of all kinds, films for ind ustrial applications like construction and agriculture. In packaging blown films are usually used in packages of cereal, meat, and frozen foods which have multilayers that need to be coextruded and is usually done suing blown film extrusion [1]. Blown film extrusion is a continuous process in which the polymer is melted, and the melt is forced through the annular die and this results into a tube which is inflated with air. This forms a bubble which then cooled. The air is blown inside the bubble to inflate the bubble and for cooling the air is always blown from the outside. In blown film extrusion, the film is biaxially oriented by stretching it both longitudinally and circumferentially during production [1]. The blow - up ratio and the speed control the properties of the blown film. The blow - up ratio is the ratio between the diameter of the final tube of the film and the diameter of the die. Various sizing and guiding devices guide the film in the blown film tow er. The stage where the film turns from molten to semi solid is called the frostline. The orientation of the film is completed by this stage though the film may still be deformable. Once the film is cool enough it is deflated using the pinch rollers and pl ates and then it is wound using the winding roller with or without treatment on the film [1]. Schematic diagram of the blown film extrusion process has been illustrated in Figure 2 - 4. 21 Figure 2 - 4 Schematic diagram of blown film extrusion process . In bl own film extrusion process once the blown film is stable and running there is very less scrap generated as compared to cast film , hence it is good process for a high - volume production. However, film quality is lower than cast films in terms of uniformity o f gauge and transparency [1]. 2.5.3 Cast film extrusion Polyethylene sheets and films can be manufactured using the cast film extrusion process. When the thickness of the film is less than 0.010 inch then it is called a film if its more than 0.010 inch then it is a sheet [ 21 ]. To produce thinner films the process used is chill roll process or cold cast process. In cast film extrusion the extrudate obtained after extrusion is forced through a slit die. The slit die creates a rectangular profile in which t he width is much higher than the thickness. 22 The cast film is produced when the melt passing through the slit die is guided onto chilled chrome rollers which helps the melt cool down to form a film and the rollers help in impact a good surface characteristi c to the film [1] . The contact of the extrudate to the first chill roll occurs tangentially and further it typically travels in an S - pattern across two or more rollers. Typically to pin the plastic onto the first chill roller an air knife is used. The dim ensions of the film are usually controlled by the various parameters like extrusion rate, die dimensions and take - off speed. For certain applications instead of chill roll process other processes like roll stack and calendaring, q uench tank and water bath process are used. Irrespective of the cooling process used, once the plastic is cooled it is wound using the nip rolls and the winder. The nip rolls help the film to get a uniform tension and feeds it to the winder. There are different types of winders ava ilable catering to different end needs [1]. 2.5.4 Extrusion blow molding Polyethylene can be made into bottles using the extrusion blow molding process. It is one of the simplest and oldest techniques used to manufacture plastic bottles. The extrusion blow mol ding process begins with forming a hollow plastic tube in the downward direction. The tube is then closed by the two halves of the mold, cutting it from the extruder. Air is blown into the mold to expand the bubble or parison using a blow pin. The blow pin is inserted in the region that will get cut off from the bottle, so the final forming of the container is done by air only. The mold is cooled using water and then it is opened at a stage when the container can retain its shape without 23 the mold. The exces s material called flash, is removed from the container neck and bottom area and from areas like handles and offset necks [1, 22 ]. Extrusion blow molding is especially useful for producing larger bottles with handles or offset necks. It is not very economic al for smaller bottles. Also coextruded bottles for different applications are made using extrusion blow molding, like the liquid detergent container with HDPE/regrind - recycle/HDPE bottle structure [1, 22 ]. 2.5.5 Compression molding Polyethylene and polyethylen e composites are commonly processed using the compression molding techniques. It comprises of different processes like resin transfer molding process, transfer molding process, compression transfer molding process. The process is chosen depending on the ty pe of product to be fabricated and material used [ 22 ]. In compression molding the raw plastic is converted into finished product by compressing them into the desired shape. The machines contain a stationary and a movable mold, the material is placed betwee n them and the mold is closed. Pressure and heat are applied to get a homogenized mixture of the materials used. Figure 2 - 5 shows the schematic diagram of the compression molding process. 24 Figure 2 - 5 Schematic diagram of compression molding process. The values of applied heat and pressure are decided based of the rheological and thermal properties of the material being used. The machine needs to be preheated to reduce the holding time. Depending on the properties desired for the product eith er slow cooling or quenching can be applied at the end of the holding time [ 23 ]. 25 2.5.6 Thermoforming Polyethylene is thermoformed for several packaging applications. Thermoforming is one of the least expensive processing techniques compared to injection and blow molding processes. Low pressure is required in the thermoforming process which further brings down the cost as cheaper materials can be used for mold manufacturing. Thermoforming is usually done to obtain smaller sized containers hence t he number of packages per cycle is large [1]. Thermoforming process comprises of three basic steps, heating of the sheet, forming the sheet, and trimming the part. Parameters like temperature, cycle time and mold designs used in thermoforming are decided based on the preliminary experimentation based on the polymer being used. The ideal radiant heater temperatures used for polyethylene are around 470 630 o C for LDPE and 510 630 o C for HDPE. The plastic softened by heat in the first step of thermoforming can be further molded by various methods. The most common three methods are drape forming, vacuum forming and pressure forming [1]. Drape forming uses gravity as the main forming force. A male mold having a positive or convex shape is used and the hot pla stic sheet is pulled down towards the mold by the application of vacuum, the hot plastic takes up the mold shape. Vacuum forming uses the air pressure as the main forming force. The mold used in vacuum forming is negative or concave shaped and the hot plas tic is forced into the shape of the mold by clamping it onto the mold and applying vacuum. The third type of forming pressure forming as the name suggests uses additional pressure to form the sheet. In this type of forming both positive and negative type o f mold can be used [1]. 26 2.6 In packaging there is a need for good gas barrier properties to extend t he shelf life of food products along with clarity. As mentioned before, polyethylene is a good water barrier, but has inferior gas barrier compared to other polymers like PET, PP, EVOH [1]. Among the polyethylene HDPE has the best gas barrier, LDPE has the leas t and LLDPE is intermediate in terms of its gas barrier properties when compared to HDPE and LDPE. In terms of clarity HDPE is milky white, LDPE is transparent and LLDPE is translucent. Hence i n this study LLDPE has been chosen as the base polymer for gas barrier propert y improvement by addition of cellulose nanocrystals (CNCs). 2.7 Cellulose nanocrystals (CNCs) Cellulose nanocrystals are a type of cellulose nanomaterials extracted from lignocellulosic and is often used in packaging applications [ 24 ]. It is used for its desirable properties like renewable nature, biodegradability, low energy consumption, low cost, low - density, and more recyclability compared to inorganic fillers [ 25,26 ]. CNCs are used in other applications like wastewater treatment, biome dical industry, and electronics as well [ 27 ]. 27 2.7.1 Extraction of CNCs Bulk cellulose that is naturally occurring in the environment has both crystalline and amorphous regions in them in varying proportions, depending upon the source of cellulose. The highly crystalline regions of the cellulose microfibrils can be extracted from the bulk cellulose if it is subjected to specific combination chemical, mechanical and enzymatic treatments and the extracted highly crystalline regions result in the formation of cellulose nanocrystals (CNCs). CNCs are nearly perfect crystalline structure made up of stiff rod like particles consisting of cellulose chain segments. CNCs are also referred as nanoparticles, whiskers, nanofibers, micro crystallites. CNCs exhibit high specific strength, high surface area, modulus, and unique liquid crystalline properties [ 26 - 28 ]. Schematic diagram of cellulose is shown in Fig ure 2 - 6. Figure 2 - 6 Schematic diagram of structure of cellulose with repeating unit of cellobiose . [ 26,28 ] The process of extraction can be mechanical or chemical. Several mechanical processes like high intensity ultrasonic treatments, high pressure homogenization, micro fluidization, cryocrushing , etc. , exist and these processes work by producing shear forces which split the cellulosic fibers along the longitudinal axis and help in extraction of cellulose microfibrils. 28 However, the chemical method is better for conversion of cellulose microfibrils into CNCs because it reduces the consumption of energy and produ ces nanocrystals with improved crystallinity. The mechanical method produces ribbon like nanofiber samples with a lower crystalline fraction usually 0.05 0.55 and chemical method produces rod - like nanofiber samples with a crystalline fraction of about 0. 6. In chemical process there is strong acid hydrolysis that occurs which removes the amorphous domains that are regularly distributed along the microfibrils. The strong acids have the capability of penetrating easily into the amorphous regions which have a low level of order and hydrolyze them and leave the crystalline regions unaffected. Sulphuric acid hydrolysis is the most used process [ 28 ]. 29 2.7.2 Properties of CNCs CNCs have a highly crystalline structure which makes the structure stiff and hence it has a higher aspect ratio of around 100. Aspect ratio is defined as the ratio of length to diameter, and it helps in determination of the reinforcing capacity, which is crucial in the formation of percolated networks necessary for maintaining the properties of C NC based materials [ 24,26 ]. The variables used while manufacturing impact the properties of CNCs, for example the increases in hydrolysis time increases the length of the CNC s . The type of raw material also impacts the dimensions of CNC s for examples, CNCs obtained from wood have a length of 100 200 nm and a width of 3 5 nm while those obtained from other sources like sea plant have a length of 1000 2000 nm and width of 10 20 nm. The axial modulus of the CNCs have been reported to be 110 220 GPa and the strength of CNCs has been reported to be 7.5 7.7 GPa. 2.8 CNCs in bio - based polymers CNCs has a wide range of applications like synthesis of antimicrobials, use in medical materials, enzyme immobilization, biosensing, green cataly sis , etc. In different applications the use of CNCs is of two types, one where it is involves it to be functionalized or nonfunctionalized as synthesized CNC s and the other type is where CNCs are used in polymer nanocomposites and it acts as a reinforcing agent [ 28 ]. A polymer nanocomposite is a multiphase material where the nanomaterial reinforces the polymer phase. The nanometric size and the increased surface area of these polymer nanocomposites gives them unique properties. Incorporations of CNCs have significant 30 improvement in the mechanical properties even in low volume fractions. CNCs are also used to impart strength and modulus as it acts as a nanofiller which have a defined morphology. The fabrication of nanocomposites is done by both natural polym ers as well as synthetic polymers. Research have been done on natural polymers like starch, chitosan, natural rubber, soy protein and synthetic polymers like polyethylene, polycaprolactone, polyvinyl alcohol, polyvinyl chloride to name a few [ 24,28 ]. 2.9 Proc essing technologies of composite films The final properties of the composite films are impacted by the processing technique used. Hence it is important that the processing techniques are decided based on several factors like the intrinsic properties of th e cellulose nano crystals, the nature of the polymer matrix, interfacial characteristic properties of the cellulose nano crystals and the desired final properties of the composite film [ 29 ]. 2.9.1 Melt processing Melt processing is an important technique used for preparing nanocomposites. This is very effective method when high volume production is targeted. It is also a cheap and fast processing method. The key principle of melt processing is that the cellulose nanomaterials are dispersed in a t hermoplastic polymer melt. This can be done either by batch process or continuous process [ 30 ]. In the batch process, small amounts of the materials are added to a processing chamber where it is mixed well for a long period of time using micro extruders. Whereas, in the continuous 31 method the material is fed into a continuous processing unit where the material is melted and mixed. The continuous method is preferred for scaling up as compared to the batch process as it is better at mixing and venting the mat erial [ 30 ]. In this study both continuous and batch process has been used. To make polymer blend in lab scale the batch process is used. Traditionally the Brabender type batch mixing is used. Researchers like Iwatake and coworkers have established that co mposites prepared by batch process have shown promising results [ 31 ]. Another batch process method used for preparing composites is roll - milling process and this is usually used for mixing carbon black and other additives into a rubbery material. Some draw backs of the batch process are that it can sometimes lead to degradation and discoloration of the polymer or the cellulose. It also takes long processing time. The continuous process uses extruders. These can be counter - rotating or co - rotating twin screw e xtruders [ 30 ]. 2.9.2 Solvent c as t ing Solvent casting is a popular method for preparation of composites. When polar constituents are used , i.e., when a water - soluble polymer is used as a matrices and cellulose nano crystals are used , which are also polar, the i nteraction between them is strong. When the aqueous suspension containing these two components are mixed, a solid nano composite can be obtained by solvent casting. However, it is difficult to use solvent casting method in circumstances where hydrophilic c ellulose and hydrophobic matrices like PE, PP, PCL, and PLA are combined as there is lack of compatibility between them and this leads to poor dispersion. To improve the dispersion various 32 strategies have been developed like use of surfactants having compa tibility on one part with polymeric matrix and another with CN which chemically modifies the interface between them and increases interaction [ 30 ] . 2.9.3 Electrospinning Electrospinning also called as electrostatic fiber spinning is another processing method that uses the action of electrostatic forces to process CNs in polymer matri x . But like the solvent evaporation process electrospinning can be a challenging for insoluble polymer matrices. Another - in - shel between the spinning tip and the collector, voltage, flow rate and properties of the spinning solution [ 30 ]. 2.10 Gas b arrier property measurement In packaging it is very important to measure the gas barrier properties of the packaging material used, as this directly affects the shelf life of the food product. Especially in food and pharmaceutical packaging it is critical that the permeability of the packaging film is in accordance with the expected permeability required by the product. Two critical gases in packaging are oxygen and carbon dioxide and permeability of both gases need to be determined to design a packaging material which has the desired permeability for the given product [ 32 ]. 33 The mass transport process in polymeric materials that are important in packaging are permeability, sorption, and migration. Migration is the loss of residues and additives from the polymer and permeation is the exchange of substances through the film. To quantify these mass transport processes at least two of the three coefficients must be determined, permeability coefficient (P), diffusion coefficient (D) and solubility coefficient (S). By usin g a permeation experiment, the values of these coefficients can be estimated [ 32 ]. Various methods are used to measure the gas barrier properties of polymers. In this part of the chapter i sostatic permeability method and gravimetric method of permeability measurement will be discussed in detail. 2.10.1 Isostatic permeability method Isostatic method uses a permeation cell which has two chambers and is separated by the film being tested. One of the chambers is a high concentration chamber (HCC) and the other is a low concentration chamber (LCC). In the HCC, an atmosphere enriched permeant is generated, and these permeant molecules begin to adsorb and diffuse through the polymer till they reach the LCC. The permeant concentration at the LCC is maintained to zero by purging the permeant molecules out of the chamber using an inert gas stream [ 32 ]. The permeant flow value is zero initially, and then the permeant flow is measured as a function of time. The permeant flow starts to increase over time and increases until a transition state where it reaches a constant value. At time, the system is at a stationary state and the 34 determined as follows [ 32 ]. w here the l is the f ilm thickness , A is e xposed area , is concentration gradient. The diffusion coefficient is determined from the data at the transition rate. Following the 2. 1 and ISO method boundary condition and assuming diffusion coefficient to be independent of permeant concentration the D can obtained using the following [ 32 ] : w here t 1/2 is the time at which the permeant flow is one half of final flow. Further, using 32 ] : Commercial permeability testers l ike Permatran Mocon use the above - described isostatic method and simplifies it and measures the gas transmission rate as a function of thickness and helps in obtaining the permeability value directly by the following : 35 where GTR is the gas transmission rate, l is the thickness of the film, is the difference in partial pressure of permeant acr oss the sample [ 33 ]. 2.10.2 Gravimetric permeability method The gravimetric or sorption method measures the permeability from the steady state data unlike the isostatic method which measures permeability from the transient data [ 34 ]. Sorption experiments estimate the amount of gas absorbed or desorbed in a polymer also known as gas solubility and estimate the rate of gas diffusion also known as gas diffusivity [ 34 - 38 ]. Unlike the isostatic permeability testers which measure the gas transmission rate and directly provides the permeability coefficient, the gravimetric method provides the values of the diffusion coefficient and solubility coefficient which is very important to understand the mechanism of barrier improvement in composites and for qu antifying parameters like tortuosity factor [ 34 - 38 ]. Gravimetric method requires a thicker sample like compression molded samples, as compared to isostatic permeability method where thin films are tested. In sorption/desorption experiments the polymer sam ple to be tested is placed in a chamber and saturated with the testing gas. Once the samples are saturated, it is removed, and the weight gain is measured to see the amount of gas uptake and the percent weight gain can be obtained from the difference of th e two weights. The measured solubility of the gas is the maximum amount of absorbed of the gas. To determine the diffusion the desorption process needs to be carried out which determines the amount of gas that is lost. It can be given by the following equa tion : 36 w here, is the mass uptake at infinite time, is the amount of gas uptake at time t and l is the average thickness of the sample. If a curve is plotted for vs , the initial gradient of the curve can also be used to calculate the value of diffusion coefficient (D). Experimentally for a system in which the diffusion coefficie nt is constant, if the half time for desorption or absorption is observed, the value of this constant can be given by [ 3 5 - 38 ] : where, the value of corresponds to time at which . Furthermore, Permeability (P) is given by : where, D is diffusion coefficient and S is solubility coefficient [ 3 5 - 38 ] . 37 2.11 Crystallinity Various properties of the polymer like permeability are crystallinity. Polymer molecules prefer to move towards an arrangement that is the lowest possible energy state , which is the crystalline form. Regular repeating arrangement of mol ecules is called c rystallinity. However, crystallization takes place in small regions of the polymer mostly in the order of 1 x 10 - 9 m . For crystallization to occur the polymer chains should be able to form a parallel array and pack closely. There are vari ous sources of irregularity which prevent the polymer chain from crystallizing like stereochemical irregularity, head - to - head placement of monomer units, branching and copolymers with random placement of comonomers [1]. 2.11.1 Measurement techniques The degree of crystallinity of a polymer reflects the relative of crystalline and amorphous regions. This can be measured using different analytical methods like d ifferential scanning calorimetry (DSC), F ourier transform infrared spectroscopy (FTIR), X - ray diffraction, nuclear magnetic resonance (NMR), density gradient method to name a few. 2.11.1.1 Differential s canning c alorimetry (DSC) Differential scanning calorimetry (DSC) is a calorimetric technique. It measures the differences in heat flow between a reference and the given sample against the temperature of the sample. It is one of the most accurate methods for estimating changes in h eat capacity and enthalpy of samples. DSC is a destructive method of measurement. The DSC has a variety of applications 38 in different fields like studying corrosion, reduction, oxidation reactions by material scientists, to study physical fundamental proper ties like boiling point, enthalpy , etc. and polymer chemists often use DSC in measurement of crystallinity, rate of crystallization, polymer degradation, polymerization reaction kinetics , etc . [3 9 ] . The DSC measures the degree of crystallinity by determin ing the enthalpy of fusion from the area under the endotherm. The DSC method involves drawing a linear arbitrary baseline from the first onset of melting to the last trace of crystallinity [ 40 ]. The degree of crystallinity is given by : w here, is the weight fraction of crystallinity , is the enthalpy of fusion at melting point, is the enthalpy of fusion of the totally crystalline polymer measured at equilibrium melting point . 2.11.1.2 Fourier t ransform i nfrared s pectroscopy (FTIR) FTIR is a nondestructive method of crystallinity measurement. It has a wide range of applications ranging from analysis of small molecules to analysis of cell and tissues. The basic principle of FTIR is that infrared spectroscopy probes molecular vibrations and functional group of the test specimen can be associated with its characteristic infrared absorption bands which further corresponds to the fundamental vibrations of the functional groups [ 39,41 ]. 39 The FTIR method helps in identification of almost all chemical groups in one sample. A change in dipo le moment of the molecule during vibration makes it absorb incident infrared light i.e., infrared active. Thus, vibrations which are symmetric do not get detected. In contrast, asymmetric vibrations are detected. This lack of sensitivity contributes to the identification of chemical groups in the samples. Especially water and amino acid molecules are not usually identified by other spectroscopy method but FTIR detects them [ 39,41 ] . Many researchers have estimated the crystallinity of polymers using the Fourier transform infrared spectroscopy (FTIR) for various polymers like polyhydroxy - alkanoates (PHAs) [ 42 ], poly( vinyl alcohol) [ 43 ], polyethylene [ 44 ], high - density polyethylene [ 45 ], polyphenylene Sulfide [ 46 ] , etc. Cole and coworkers studied results obtained from the FTIR analysis and calorimetric analysis and concluded that both methods showed excellent correlation [ 46 ]. 2.12 Effect of crystallinity on barrier properties of plastics In food packaging, gas barrier properties of the films play a crucial part in maintaining and extending the shelf life of products [ 47 ]. Different ways have been established to increase the gas barrier properties of the film like laminates and multilayer. Both these methods involve layers of different materials being combined to achieve a desired gas barrier property of the films [ 15,48 ]. A major drawback with these methods is that the different layers used in the laminates and multilayers are not easily separated in the end of the materials lifecycle. This often ends in the landfill and leads to an unsustainable waste management practice. 40 A sustainable and recyclable solution to this problem is the use of monolayer biobased composite films with excellent gas barrier properties [ 49 ]. Different types of nano particles are incorporated in the polymer matrix to produce monolayer biobased nanocomposites films. Nanoparticles like nano clay, cellulose nanofibers [5 0 ], cellulose nanocrystals [ 33, 51,52 ], cellulose whiskers [5 3 ] are often used. The improved barrier performance has been attributed to the tortuosity effect created by the presence of highly crystalline nanoparticles into the polymeric matrices, which increases the degree of crystallinity of the neat polymer. These crystals increase the effective travel path length for permeants diffusing through the nanocomposites. This reduces the rate of diffusion, thus lowering permeation as reported by other [ 33,51,52 ]. 41 REFERENCES 42 REFERENCES 1. Selke, S. E., and Culter, J. D. (2016). Plastics Packaging: Properties, Processing, Applications, And Regulations . Hanser. 2. Polyethylene (n.d) [accessed April 9, 2020]. 3. Davidson, J. D. (2014). Multiscale modeling and simulation of crosslinked polymers [Doctoral dissertation, University of Michigan]. ProQuest Dissertations Publishing. 4. Thompson, R. C., Moore, C. J., Vom Saal, F. S., and Swan, S. H. (2009). Plastics, the environment, and human heal th: current consensus and future trends. Philosophical Transactions of the Royal Society B: Biological Sciences , 364(1526), 2153 - 2166 . 5. Türünç, O., Montero de Espinosa, L., and Meier, M. A. (2011). Renewable polyethylene mimics derived from castor oil. Macromolecular Rapid Communications , 32(17), 1357 - 1361. 6. Filgueira, D., Holmen, S., Melbø, J. K., Moldes, D., Echtermeyer, A. T., and Chinga - Carrasco, G. (2018). 3D printable filaments made of biobased polyethylene biocomposites. Polymers , 10(3), 314. 7. Ronca, S. (2017). Chapter 10 : Polyethylene. Gilbert , M. Brydson's Plastics Materials . Butterworth - Heinemann. 247 - 278 . 8. Plasticfilms(n.d.) [accessed September 9, 2020]. 9. Ullsten, N. H., and Hedenqvist, M. S. (2003). A new test method based on headspace analysis to determine permeability to oxygen and carbon dioxide of flexible packaging. Polymer Testing , 22(3), 291 - 295. 10. - and nanocomposites: Effect of particle shape, size and surface treatment on polymer crystallinity and gas permeability. Macromolecular Rapid Communications , 25(17), 15 40 - 1544. 11. of chitosan coated polyethylene. Journal of Membrane Science , 403, 162 - 168. 43 12. fect of relative humidity on carvacrol release and permeation properties of chitosan - based films and coatings. Food Chemistry , 144, 9 - 17. 13. Euaphantasate, N., Prachayawasin, P., Uasopon, S., and Methacanon, P. (2008). Moisture sorption characteristic and the ir relative properties of thermoplastic starch/linear low - density polyethylene films for food packaging. Journal of Metals, Material and Minerals , 18, 103 - 109. 14. Laguna, M. F., Guzman, J., and Riande, E. (2001). Transport of carbon dioxide in linear low - dens ity polyethylene determined by permeation measurements and NMR spectroscopy. Polymer , 42(9), 4321 - 4327. 15. Erlat, A. G., Spontak, R. J., Clarke, R. P., Robinson, T. C., Haaland, P. D., Tropsha, Y, and Vogler, E. A. (1999). SiOx gas barrier coatings on polymer substrates: morphology and gas transport considerations. The Journal of Physical Chemistry B , 103(29), 6047 - 6055. 16. Carrera, M. C., Erdmann, E., and Destéfanis, H. A. (2013). Barrier properties and structural study of nanocomposite of HDPE/montmorill onite modified with polyvinylalcohol. Journal of Chemistry, Volume 2013, Article ID 679567 , 7 pages. 17. Villaluenga, J. P. G., and Seoane, B. (2000). Permeation of carbon dioxide through multiple linear low - density polyethylene films. European Polymer Journa l , 36(8), 1697 - 1702. 18. Plastic molded concepts (n.d). < https://www.pmcplastics.com/materials/lldpe - resin/> [accessed September 9, 2020]. 19. Khanam, P. N., and AlMaadeed, M. A. A. (2015). Processing and characterization of polyethylene - based composites. Advance d Manufacturing: Polymer & Composites Science , 1(2), 63 - 79. 20. Karkhanis, S.S. (2016). Strategies to manufacture poly( lactic acid) blown films without melt strength enhancers, 13 - Dissertations Publishing. 21. Wagner Jr., J. R. (2016). Chapter 9: Blown Film, Cast Film, Lamination Processes. Wagner Jr., J. R. Multilayer Flexible Packaging (Second Edition) . William Andrew. 137 - 145. 22. Rosato, D. V., Rosato, D. V., and Rosato, M. V. (2004). C hapter 14: Compression molding. Rosato, D. V., Rosato, D. V., and Rosato, M. V. Plastic Product Material a nd Process Selection Handbook. Elsevier. 439 - 454 . 44 23. Tatara, R. A. (2017). Chapter 14: Compression Molding. Kutz, M. Applied Plastics Engineering Handbook. William Andrew Publishing. 291 - 320. 24. Stark, N. M. (2016). Opportunities for cellulose nanomaterials in packaging films: A review and future trends. Journal of Renewa ble Materials , 4(5), 313 - 326. 25. Azizi Samir, M. A. S., Alloin, F., and Dufresne, A. (2005). Review of recent research into cellulosic whiskers, their properties, and their application in nanocomposite field. Biomacromolecules , 6(2), 612 - 626. 26. Liu, Y. (2018). Processing and property evaluation of poly( lactic acid)/cellulose nanocrystals extruded cast films Dissertations Publishing. 27. Grishkewich, N., Mohammed, N., Tang, J., and Tam, K. C. (2017). Recent adva nces in the application of cellulose nanocrystals. Current Opinion in Colloid & Interface Science , 29, 32 - 45. 28. George, J., and Sabapathi, S. N. (2015). Cellulose nanocrystals: synthesis, functional properties, and applications. Nanotechnology, science, and applications , 8, 45 - 54. 29. Habibi, Y., Lucia, L. A., and Rojas, O. J. (2010). Cellulose nanocrystals: chemistry, self - assembly, and applications. Chemical reviews , 110(6), 3479 - 3500. 30. Oksman, K., Aitomäki, Y., Mathew, A. P., Siqueira, G., Zhou, Q., Butylina, S ., and Hooshmand, S. (2016). Review of the recent developments in cellulose nanocomposite processing. Composites Part A: Applied Science and Manufacturing , 83, 2 - 18. 31. Iwatake, A., Nogi, M., and Yano, H. (2008). Cellulose nanofiber - reinforced polylactic acid . Composites Science and Technology , 68(9), 2103 - 2106. 32. Compared. Packaging Technol ogy and Science , 9(4), 215 - 224. 33. Karkhanis, S. S., Stark, N. M., Sabo, R. C., and Matuana, L. M. (2018). Water vapor and oxygen barrier properties of extrusion - blown poly( lactic acid)/cellulose nanocrystals nanocomposite films. Composites Part A: Applied Sc ience and Manufacturing , 114, 204 - 211. 34. Barr, C. D., Giacin, J. R., and Hernandez, R. J. (2000). A determination of solubility coefficient values determined by gravimetric and isostatic permeability techniques. Packaging Technology and Science: An International Journal , 13(4), 157 - 167. 45 35. Crank, J. (1979). The mathematics of diffusion. Oxford university press. 36. Journal of Vinyl and Additi ve Technology , 7(2), 67 - 75. 37. Matuana, L. M. , Park, C. B., and Balatinecz, J. J. (1996). Characterization of microcellular foamed PVC/cellulosic - fibre composites. Journal of Cellular Plastics , 32(5), 449 - 469. 38. Matuana, L. M. (2008). Solid state microcellular foamed poly( lactic acid): morphology and property characterization. Bioresource Tec hnology , 99(9), 3643 - 3650. 39. Robinson, J. W., Frame, E. S., and Frame II, G. M. (2014). Undergraduate instrumental analysis. CRC press. 40. Kong, Y., and Hay, J. N. (2002). The measurement of the crystallinity of polymers by DSC. Polymer , 43(14), 3873 - 3878. 41. Berthomieu, C., and Hienerwadel, R. (2009). Fourier transform infrared (FTIR) spectroscopy. Photosynthesis Re search , 101(2 - 3), 157 - 170. 42. Kansiz, M., Domínguez - Vidal, A., McNaughton, D., and Lendl, B. (2007). Fourier - transform infrared (FTIR) spectroscopy for monitoring and determining the degree of crystallisation of polyhydroxyalkanoates (PHAs). Analytical and Bioanalytical Chemistry , 388( 5 - 6), 1207 - 1213. 43. Tretinnikov, O. N., and Zagorskaya, S. A. (2012). Determination of the degree of crystallinity of poly( vinyl alcohol) by FTIR spectroscopy. Journal of Applied Spectroscopy , 79(4), 521 - 526. 44. Hagemann, H., Snyder, R. G., Peacock, A. J., and M andelkern, L. (1989). Quantitative infrared methods for the measurement of crystallinity and its temperature dependence: polyethylene. Macromolecules , 22(9), 3600 - 3606. 45. c rystallinity of the HDPE matrix in composites with cellulosic fiber using DSC and FTIR. Journal of Reinforced Plastics a nd Com posites , 19(10), 818 - 830. 46. Cole, K. C., Noël, D., Hechler, J. J., and Wilson, D. (1988). Nondestructive determination of crystallin ity by diffuse reflectance FT - IR in composites made from polyphenylene sulphide and carbon fibres. Microchimica Acta , 94(1 - 6), 291 - 295. 46 47. Chung, D., Papadakis, S. E., and Yam, K. L. (2003). Simple models for evaluating effects of small leaks on the gas barri er properties of food packages. Packaging Technology and Science: An International Journal , 16(2), 77 - 86. 48. PE multilayers: Processing and properties. Polymer Engineer ing & Science , 45(2), 217 - 22. 49. Will monolayer become the standard in flexible packaging ? (February 7, 2020). [accessed April 14, 2020]. 50. Jonoobi, M., Harun, J., Mathew, A. P., and Oksman, K. (2010). Mechanical properties of cellulose nano fiber (CNF) reinforced polylactic acid (PLA) prepared by twin screw extrusion. Composites Science and Technology , 70(12), 1742 - 1747. 51. Liu, Y., and Matuana, L. M. (2019). Surface texture and barrier performance of poly( lactic acid) cellulose nanocrystal extr Journal of Applied Polymer Science , 136(22), 47594 . 52. Karkhanis, S. S., Stark, N. M., Sabo, R. C., and Matuana, L. M. (2018). Performance of poly (lactic acid)/cellulose nanocrystal composite blown films processed by two different compoundin g approaches. Polymer Engineering & Science , 58(11), 1965 - 1974 . 53. Bendahou, A., Kaddami, H., Espuche, E., Gouanvé, F., and Dufresne, A. (2011). Synergism effect of montmorillonite and cellulose whiskers on the mechanical and barrier properties of natural rubber composites. Macromolecular Materials and Engineering , 296(8) , 760 - 767. 47 CHAPTER 3 EXPERIMENTAL 3.1 Mat erials Sugarcane - based bio - LDPE(STN7006), LLDPE (SLH118) and HDPE (SGD4960) obtained from Braskem Petrochemical (Sao Paulo, State of Sao Paulo, Brazil) were used as polymer matrices. Their respective melt flow indices were 0.6 g/10 min, 1 g/10 min and 0.7 g/10 min whereas their respective densities were 0.924 g/cm 3 , 0.916 g/cm 3 and 0.962 g/cm 3 . The biobased content of the polymers was 95%, 84% and 96% for LDPE, LLDPE and HDPE, respectively. The CNCs (2018 - FPL - CN C - 126) used in this study was obtained from the Forest Products laboratory in Madison, Wisconsin, USA. Sulfuric acid hydrolysis was used to produce freeze - dried cellulose nanocrystals around 100 300 nm long and a diameter of around 5 nm [ 1 ]. CNCs were pr ocessed in a high - intensity mixer (MX1050XTS from Waring Commercial Xtreme) for 1 minute at 22,000 rpm to reduce the fiber agglomeration and then oven - dried for at least 24 hours to remove any absorbed moisture. 3.2 Samples m anufacture for permeability tests Two types of barrier measurement techniques were used in this study to understand the mechanism of gas barrier improvement in bio - LLDPE/CNC nanocomposites. The first approach included the gravimetric/sorption method, which measures the amount of gas uptake or lost from the sample. This method required rigid samples of approximately 1.5 mm to 2 mm thick for 48 performing the tests effectively, which were prepared using compression molding process [ 2,3 ]. The second technique involved the isostatic permeability m ethod, which measures the gas transmission rate through the sample and estimates its permeability coefficient. This method required thin film samples for tests to be performed effectively, these were prepared using blown film extrusion. Karkhanis et al. pe rformed isostatic method of permeability measurement successfully for their PLA - CNC nanocomposites using similar sample preparation techniques [ 4 ]. 3.2.1 Compression molded samples for gravimetric - sorption method The bio - LLDPE pellets along with dried CNCs we re blended in a 60 ml electrically heated three - piece internal mixer/measuring head (3:2 gear ratios) with counter - rotating roller - style mixing blades (C.W. Brabender Instruments, Inc.). The mixer was driven by a 7.5 hp Intelli - Torque Plasti - Corder Torque Rheometer® (C. W. Brabender Instruments, Inc.). The polymer and CNC were mixed for 3 minutes at 170°C. The rotor speed used was 35 rpm and weight charge set at 45 g determined from the preliminary testing. The CNCs contents in the nanocomposites were fixed at 2.5%, 5%, 7.5%, 10%, and 13.5% of total material weight. A 5 kg deadweight was put on the top of the ram throughout the experiments [2 ,3 ]. The blended materials were cooled and pressed at 180 o C and 5 tons of maximum pressure for 6 minutes using the hyd raulic laboratory press (Carver, Model 120 - 10HC) to a target thickness of 2 mm using a metal spacer while processing. Similar process was utilized to manufacture control samples based on neat LLDPE, neat LDPE, and neat HDPE. 49 3.2.2 Blown film extrusion for i sostatic permeability method Bio - LLDPE pellets and dried CNCs were blended for 1 min at high intensity setting in a commercial blender ( MX1050XTS from Waring Commercial Xtreme). The blended materials were blown into films using a 19 mm single - screw extrud er (C.W. Brabender Instruments, South Hackensack, NJ, USA) with a length - to - diameter ratio of 30:1 as previously described [ 4 - 6 ]. The annular die used was of 25.4 mm in diameter and a die - opening diameter of 0.889 mm. The temperature profile of the extrude r was set to 185 - 185 - 185 - speeds of the extruder rotational screw and take up rollers were both set to 20 rpm, while an air pressure of 0.207 kPa (0.030 psi) was used to inflate the film to a blow - up ratio of 3, leading to ~ 0.062 mm thick films, measured by digital micrometer (model 49 - 70 from TMI, Ronkonkoma, NY, USA) [ 4 - 6 ]. 3.3 Property evaluation 3.3.1 Gas permeability by s orption e xperiments The desorption experiments were performed according to the gravimetric method used in previous research work [ 2,3 specimens. The samples were weighed using a four - digit precision digital balance, their thickness measured, and they were placed in a chamber that was later pressu rized with carbon dioxide (CO 2 ) at 800 psi and room temperature. The samples were left in the chamber until they were saturated with CO 2 (minimum of 16 hours). At the end of the saturation, the CO 2 - saturated samples were 50 removed from the pressure chamber a nd weighed again on the balance to determine the amount of CO 2 absorbed or measured solubility. And the solubility coefficient (S) was calculated as follows [ 7,8 ]: where p i is the partial pressure of gas and C i is the solubility of the gas in polymer given by the formula: with W t gas as the weight of the dissolved gas per unit volume of polymer (V p ) . Once the percent weight gain of CO 2 was recorded, desorption process was carried out immediately to determine the amount of gas lost at regular time intervals and estimate the diffusivity of gas (D) in the sample, which is given by the following equatio n [2 ,3,7 ]: where is the mass uptake at infinite time, is the amount of gas uptake a t time t and l is the average sample thickness. If a curve is plotted for vs , the initial gradient of the curve can also be used to calculate the value of diffusion coefficient (D). Experimentally for a system in which the diffusion coefficie nt is constant, if the half time for desorption or absorption is observed, the value of this constant can be given by [2 ,3 ,7 ]: 51 where the value of corresponds to time at which [2]. Furthermore, Permeability (P) is given by [ 9 ]: where D is diffusion coefficient and S is solubility coefficient. The solubility (S c ) and diffusion (D c ) coefficients of gas in LLDPE/CNC nanocomposites can also be estimated theoretically in terms of polymer matrix mass fractions using the following equations [2 ,3, 10 - 12 ]: where S p and D p are the is the mass fraction of CNC in the polymer matrix. 52 3.3.2 Tortuosity f actor The diffusion coefficients obtained from the sorption experiments can be used to calculate the product of Tortuosity Factor ( ), which quantify the hindrances in the diffusion path and immobilization factor ) , which is accounted to the restricted segmental mobility in the amorphous chains. The diffusion coefficient (D) can be expressed as: where D a is the diffusion coefficient in the amorphous region since crystalline regions act as an impermeable barrier for diffusion, is the tortuosity factor, is the immobilization factor, which is an empirical correction [ 13 ]. Both tortuosity factor and immobilization factor increase with crystallinity, hence can be considered as a const ant value for the same crystallinity values [ 14 ] and using this assumption some researchers have simplified the equation ( 3. 8) to the following equation [15]: 53 3.3.3 Gas permeability by isostatic permeability method The carbon dioxide ( CO 2 ) and oxygen (O 2 ) transmission rates of films were measured using the permeability testers Mocon, Permatran (Model 4/41) and Mocon Ox - Tran (Model 2/21), respectively. The procedures outlined in ASTM D 1434 and ASTM D3985 were used for the testing procedures of CO 2 and O 2 tests, respectively at room temperature (23 ± 0.1°C) and 0% relative humidity (RH). Kurek and coworkers established that the CO 2 and O 2 permeability of polyethylene at 0% RH and 95% RH were of the same order of magnitude [ 16 ]. Other research ers also had similar observations for polyethylene suggesting that relative humidity does not have a substantial effect on the gas permeability of polyethylene [ 16 - 20 ]. Thus, the isostatic permeation procedure was carried out at room temperature and 0% RH in this study. For each formulation, a minimum of 3 samples were tested with both gases. The gas permeability (P gas )was calculated using [ 4,21 ] . where GTR is the gas transmission rate, l is the thickness of the film, is the difference in partial pressure of permeant (carbon dioxide/oxygen) across the sample which is 101,325 Pa (1 atm). 54 3.3.4 Crystallinity of nanocomposites Many researchers have estimated the crystallinity of polymers using the Fourier transform infrared spectroscopy (FTIR) for various polymers like polyhydroxy - alkanoates (PHAs) [ 22 ], poly( vinyl alcohol) [ 23 ], polyethylene [ 24 ], high density polyethylene [ 25 ], polyphenylene Sulfide [ 26 ], etc. Also, the results obtained from the FTIR analysis have been validated with results of calorimetric analysis, showing an excellent correlation between both the methods [ 26 ]. Consequently, F TIR was employed in this study to quantify the percent crystallinity in bio - LLDPE induced by incorporating CNCs into the matrix. FTIR spectra of compression molded samples were collected using a Shimadzu IR Affinity IS infrared spectrophotometer (Shimadzu Corporation, Kyoto, Japan) in attenuated total reflectance (ATR) mode. The spectra were obtained with triangle apodization using 64 scans in the range of 4000 400 cm - 1 , at a wavelength resolution of 4 cm - 1 [ 5,6 ]. Spectra were analyzed by WinFIRST softwa re from Thermo Nicolet (Madison, WI). The crystallinity of PE is given by [ 24 ]: 730 cm - 1 and 722 cm - 1 correspond to crystalline bands whereas the peak at 723 cm - 1 is a ssociated with amorphous component, and corresponds to the ratio of intrinsic crystalline to amorphous band intensities of polyethylene [ 24 ]. 55 The FTIR spectra of neat sugarcane LLDPE and LLDPE/5% CNC nanocomposite (Fi g. 1) in this study showed two crystalline peaks at 718 cm - 1 and 729 cm - 1 , but no amorphous peak was observed at the 723 cm - 1 range. Instead, bands associated with the amorphous fraction occurred in the 1400 - 1250 cm - 1 frequency region [ 24 ]. Sugarcane - based HDPE also exhibits crystalline peaks at 716 - 717 cm - 1 [ 27 ]. Notice that the intensities of bands at 1400 to 1250 cm - 1 assigned to the amorphous component in LLDPE as well as those at 1376 - 1028 cm - 1 significantly decreased by adding 5% CNC into the matrix ( Fig. 3. 1). This result suggests a decrease in the amorphous fraction in LLDPE, probably due to the nucleating effect of CNC. Since the amorphous region of PE was also observed in the frequency range of 1400 1250 cm - 1 [ 24 ], an amorphous peak at 1262 cm - 1 ( Figure 3 - 1) was then used to calculate the crystallinity. Consequently, e q uation ( 3. 11) was slightly modified as follows to determine the percent crystallinity of the neat bio - LLDPE and bio - LLDPE in the nanocomposite samples: where , as obtained from the measurement specific to this study. The crystallinity of sugarcane - based LLDPE in nanocomposites was obtained by performing spectral subtraction. For obtaining the characteristic spectrum of neat LLDPE (i.e., difference data ) from the spectra of the LLDPE/CNC nanocomposites (i.e., sample data ), the characteristic spectrum of the cellulose nanocrystal (i.e., reference data ) was multiplied by the specific percent content of CNCs in the nanocomposite (i.e., subtraction factor ) for which the crystallinity will be estimated. The obtained spectrum was subtracted from that of its 56 corresponding nanocomposite for which the crystallinity of its component matrix (neat LLDPE ) will be determined. Colomw et al. performed similar procedure for obtaining crystallinity of HDPE from their HDPE/cellulose fiber composites using FTIR [ 25 ]. In summary, the following relation was used to extract the characteristic spectrum of neat LLDPE from the spectra of the nanocomposites: Figure 3 - 1 Infrared spectra of sugarcane - based neat LLDPE and LLDPE/5% CNC nanocomposite sheets. 57 3.3.5 Density measurements Densities of bio - PEs (LDPE, LLDPE, and HDPE) pellets and LLDPE/CNC nanocomposites were obtained using ColePalmer density gradient apparatus (Cole - Palmer Instrument Company, Vernon Hills, IL, USA). The tests were performed in accordance with the procedure outlined in ASTM D1505 - 10 (Method C - Continuous filling with liquid entering gradient tube becoming progressively more dense) by observing the level to which a test specimen sinks in a liquid gradient column. The density gradient column was prepared using high pressure liquid chromatography (HPLC) - grade water (750 ml) and isopropanol (500 ml) of densities 0.79 and 1 g/cm 3 , respectively. 3.3.6 Optical microscopy Microscope image of bio - L LDPE and bio - LLDPE/CNC nanocomposites were obtained at 12.5X magnification using an Olympus BX41 optical microscope (Olympus, Center Valley, PA) equipped with a camera (Olympus Qcolor3). 3.3.7 Statistical a nalysis A one - way t - cted to compare the permeability coefficients obtained from two different permeability testing methods (isostatic permeability method versus gravimetric method) and three different bio - PE types (LDPE, LLDPE, and HDPE). Similar t - tests were done to assess t he effect of CNC content on the crystallinity, diffusion coefficient, solubility coefficient and permeability coefficient of bio - LLDPE. 58 REFERENCES 59 REFERENCES 1. Stark, N. M. (2016). Opportunities for cellulose nanomaterials in packaging films: A review and future trends. Journal of Renewable Materials , 4(5), 313 - 326 . 2. Matuana, L. M., and rigid PVC/wood Journal of Vinyl and Additive Technology , 7(2), 67 - 75 . 3. Matuana , L. M. , Park, C. B., and Balatinecz, J. J. (1996). Characterization of microcellular foamed PVC/cellulosic - fibre composites. Journal of Cellular Plastics , 32(5), 449 - 469 . 4. Kar khanis, S. S., Stark, N. M., Sabo, R. C., and Matuana, L. M. (2018). Water vapor and oxygen barrier properties of extrusion - blown poly( lactic acid)/cellulose nanocrystals nanocomposite films. Composites Part A: Applied Science and Manufacturing , 114, 204 - 211. 5. Karkhanis, S. S., Stark, N. M., Sabo, R. C., and Matuana, L. M. (2018). Performance of poly( lactic acid)/cellulose nanocrystal composite blown films processed by two different compounding approaches. Polymer Engineering & Science , 58(11), 1965 - 197 4 . 6. Karkhanis, S. S., and Matuana, L. M. (2019). Extrusion blown films of poly( lactic acid) Polymer Engineering & Science , 59(11), 2211 - 2219. 7. Crank, J. (1979). The Mathematics of Diffusion . Oxford university press. 8. De Angelis M.G. (2014) Solubility Coefficient (S). Drioli E., Giorno L. Encyclopedia of Membranes . Springer, Berlin, Heidelberg . 9. Barr, C. D., Giacin, J. R., and Hernandez, R. J. (2000). A determination of solubility coefficient values determined by gravimetric and isostatic permeability techniques. Packaging Technology and Science: 13(4), 157 - 167 . 10. Van Krevelen , D.W. (1990), Properties of Polymers , Elsevier, New Y ork . 11. Michaels , A.S. , and Bixler , H.J. , (1961), Solubility of gases in polyethylene. Journal of Polymer Science , 50 (154), 393 - 412. 12. Michaels , A.S. , and Bixler , H.J. , (1961), Flow of gases through polyethylene. Journal of Polymer Science , 50(154), 413 - 439. 60 13. Horas J. A. and Rizzotto M. G. ( 1999 ). Gas diffusion in and through polypropylene. Karger - Kocsis, J. Polypropylene : an A - Z reference . Springer Science & Business Media , 2, 273 . 14. and Riande, E. (2010). Crystallinity effect on the gas transport in semicrystalline coextruded films based on linear low - density polyethylene. Journal of Polymer Science Part B: Poly mer Physics , 48(6), 634 - 642. 15. Bendahou, A., Kaddami, H., Espuche, E., Gouanvé, F., and Dufresne, A. (2011). Synergism effect of montmorillonite and cellulose whiskers on the mechanical and barrier properties of natural rubber composites. Macromolecular Mate rials and Engineering , 296(8), 760 - 769. 16. and Debeaufort, F. (2012). Barrier properties of chitosan coated polyethylene. Journal of Membrane Science , 403, 162 - 168. 17. , and Debeaufort, F. (2014). Effect of relative humidity on carvacrol release and permeation properties of chitosan - based films and coatings. Food Chemistry , 144, 9 - 17. 18. Ullsten, N. H., and Hedenqvist, M. S. (2003). A new test method based on head space analysis to determine permeability to oxygen and carbon dioxide of flexible packaging. Polymer Testing , 22(3), 291 - 295 . 19. Osman, M. A., and nd nanocomposites: Effect of particle shape, size and surface treatment on polymer crystallinity and gas permeability. Macromolecular Rapid Communications , 25(17), 1540 - 1544. 20. Carrera, M. C., Erdmann, E., and Destéfanis, H. A. (2013). Barrier properties and structural study of nanocomposite of HDPE/montmorillonite modified with polyvinyl alcohol. Journal of Chemistry, Volume 2013, Article ID 679567 , 7 pages. 21. Dimitroulas G.D. , Badeka A.B. , and Kontominas M.G. (2004). Permeation of m ethylethylketone, o xygen an d w ater v apor through PET f ilms c oated with SiOx: e ffect of t emperature and c oating s peed. Polymer Journal , 36 ( 3 ), 198 - 204. 22. Kansiz, M., Domínguez - Vidal, A., McNaughton, D., and Lendl, B. (2007). Fourier - transform infrared (FTIR) spectroscopy for monitoring and determining the degree of crystallisation of polyhydroxyalkanoates (PHAs). Analytical and Bioanalytical Chemistry , 388(5 - 6), 1207 - 1213 . 61 23. Tretinnikov, O. N., and Zagorskaya, S. A. (2012). Determination of the degree of crystallinity of poly( vinyl alcohol) by FTIR spectroscopy. Journal of Applied Spectroscopy , 79(4), 521 - 526 . 24. Hagemann, H., Snyder, R. G., Peacock, A. J., and Mandelkern, L. (1989). Quantitative infrared methods for the measurement of crystallinity and its temperature dependence: polyethylene. Macromolecules , 22(9), 3600 - 3606 . 25. and Carrasco, F. (2000). Changes in crystallinity of the HDPE matrix in composites with c ellulosic fiber using DSC and FTIR. Journal of Reinforced Plastics and Composites , 19(10), 818 - 830 . 26. Cole, K. C., Noël, D., Hechler, J. J., and Wilson, D. (1988). Nondestructive determination of crystallinity by diffuse reflectance FT - IR in composites made from polyphenylene sulphide and carbon fibres. Microchimica Acta , 94(1 - 6), 291 - 295 . 27. Bazan, P., Nosal, P., Kozub, B., and Kuciel, S. (2020). Bi obased polyethylene hybrid composites with natural fiber: mechanical, thermal properties, and micromechanics . Materials , 13(13), 2967 . 62 CHAPTER 4 RESULTS AND DISCUSSIONS 4.1 Effect of testing methods (isostatic versus gravimetric) on CO 2 permeability of PE Numerous methods, including a gravimetric technique and an isostatic permeation procedure, have been employed for measuring the permeation parameters of various permeants through polymers. Gravimetric permeation is a simple and straight forward method since it allows one to directly calculate both the solubility at steady state (S) and diffusion (D) coefficients from the sorption / desorption curve, from which the permeability (P) coefficient is calculated using equation (3. 5 ) . Con versely, the isostatic permeation procedure involves calculation of P at steady state, with D obtained from the transient state portion of the permeability experiment's flux rate profile curve. Once P and D are obtained, then S (transient state) is calcula ted using equation ( 3. 5 ) [ 1 - 3 ]. Since the gravimetric approach was selected in this study, the permeability coefficient values determined by these two procedures for the same PE/CO 2 system was compared to validate the selected permeation method. Table 4 - 1 lists the CO 2 permeability coefficients of three types of bio - PE determined by a gravimetric procedure and an isostatic permeability technique. Interestingly, the permeability coefficients obtained by the two methods were similar and no statistically sign ificant difference was observed between the two methods, irrespective of PE grade. 63 Table 4 - 1 Effect of testing method on the carbon dioxide permeability (P CO2 ) of various bio - PE grades. Bio Polymer types Density (g/cm 3 ) P CO2 (10 - 17 kg · m/m 2 s · Pa) Provided Measured Isostatic 1 Gravimetric 1 Literature 2 LLDPE 0.916 0.915 ± 0.001 27.9 ± 0.70 a 29.3 ± 1.74 a 31.9 [ 4 ] LDPE 0.924 0.919 ± 0.001 - 22.9 ± 0.4 17 to 24 [ 5 ] HDPE 0.962 0.951 ± 0.001 8.28 ±0.4 b 8.02 ± 0.42 b 10.09 [ 6 ] 1 Similar superscript letters are not significantly different based on the t - test results at a 5% significance level. 2 Listed values are for petroleum - based polymers and superscript numbers represent the cited references . 64 However, the CO 2 permeability was affected by the PE grade, LLDPE showing the highest coefficient permeability and HDPE the lowest. Such a trend was expected and attributed to the difference in density of these three PE (Table 4 - 1). LLDPE with the lowest density had the h ighest permeability followed by LDPE and HDPE. The density is approximately inversely proportional to the free volume of the polymer and proportional to the crystallinity of the polymer [ 7 ]. Polymers with lower density have poor chain packing and lower cry stallinity. Hence it can be inferred that LLDPE having the lowest density (Table 4 - 1) would have the highest free volume leading to a higher permeability value. Wang and coworkers studied the effect of polyethylene density on gas permeability and reported similar trends [ 7 ]. Their results indicated that the permeability of polyethylene decreases by a factor of almost 5 - 6 for an approximate increase of 5% in density [ 7 ]. It is worth mentioning that the CO 2 permeability values of different PE determined by bo th methods are in accordance with the values reported in the literature (Table 4 - 1) [ 4 - 6 ], suggesting that that both methods are equally reliable techniques to measure the permeability of polymers. However, the gravimetric method was chosen for further inv estigation as this method can be used to easily obtain the diffusion coefficient needed to quantify the tortuosity effect in LLDPE/CNC nanocomposites. 65 4.2 CO 2 barrier improvement and its mechanisms in LLDPE/CNC nanocomposite sheets 4.2.1 Effect of CNC addition on the crystallinity of LLDPE The crystallinity of LLDPE from the spectra of the LLDPE/CNC nanocomposites obtained by the infrared spectra subtraction method was determined to establish correlation between crystallinity and permeability coefficie nts. The crystallinity of LLDPE increased with CNC addition level up to 2.5% and remained constant as the CNC content increased further (Table 4 - 2). The increased crystallinity can be attributed to the highly crystalline CNCs acting as an effective nucleat ing agent [ 8 - 10 ]. It should be mentioned that all nanocomposites had similar percent crystallinities since the addition of more than 2.5% CNCs did not improve LLDPE crystallinity further probably due to the retarded crystal growth caused by CNC agglomerati ons at high loading levels [ 8,9 ]. A loading level of 2.5% CNC or probably lower appeared to be the maximum concentration of CNC for an effective heterogeneous crystal nucleation in LLDPE matrix. Karkhanis and coworkers reported similar nucleating effect wh en CNCs were added to PLA matrix [ 8 ]. A maximum increase in PLA crystallinity was observed at 1% CNC content in their study and further addition of CNCs to the PLA matrix did not affect the crystallinity [ 8 ]. This is in general agreement with the results o f Clarkson and coworkers who recently reported similar trends where very small concentrations of CNC (0.05 and 0.55 wt. %) are found effective heterogeneous nucleation agent for PLAs [ 10 ]. Other researchers also validated this trend; with the addition of ab out 1% CNC into the polymer matrix showing improvement in crystallinity, which remained constant thereafter [ 9,11 - 13 ]. 66 Table 4 - 2 Effect of CNC content on the crystallinity and permeation parameters of compression molded bio - LLDPE sheet . CNC content (%) % 1 Diffusion 1 (10 - 7 cm 2 /sec) Tortuosity Factor (%) Solubility 1 (10 - 4 g/m 3 · Pa) P CO2 1 (10 - 17 kg · m/m 2 s · Pa) Percent decrease in P CO2 Experimental Predicted Experimental Predicted 0 50.0 ± 0.0 a 5.2 ±0.4 a 5.2 100 57.2 ±1.7 a 57.2 29.3 ± 1.74 a - 2.5 73.7 ± 1.4 b 4.9 ±0.2 a 5.1 107 ±5.0 54.3 ±0.9 b 52.9 26.6 ± 1.3 a 9 5 73.0 ± 0.5 b 4.6 ±0.2 a 4.9 114.1 ±4.7 57.1 ±0.6 a 54.2 26.3 ± 1.1 a 10 7.5 75.3± 0.6 b 4.3 ±0.1 a 4.8 120.8 ±3.4 53.1 ±1.0 b 49.1 23.0 ± 0.9 b 22 10 75.3± 0.4 b 3.7 ±0.2 c 4.7 143.9 ±9.6 50.4 ±1.6 c 45.4 18.7 ±1.3 c 36 13.5 74.1 ± 0.8 b 3.5 ±0.1 c 4.5 147.2 ±2.6 55.5 ±1.1 b 48.0 19.6 ±0.4 c 33 1 Similar superscript letters in each column are not significantly different based on the t - test results at a 5% significance level. 67 4.2.2 Effect of CNC addition on the CO 2 diffusion and solubility coefficients of LLDPE Table 4 - 2 summarizes the experimental ly measured CO 2 diffusion (D) and solubility (S) as well as calculated permeability (P) coefficients of LLDPE and LLDPE/CNC nanocomposites. The S and D coefficients predicted from equations ( 3. 6 ) and ( 3. 7 ) , respectively, are also listed. The experimental diffusion and so lubility coefficients decreased almost linearly as the CNC content increased in the nanocomposites. The reductions in both D and S coefficients were statistically significant (p value < 0.0001 ). Similar trends were obtained for the S and D coefficients pre dicted from equations 3. 6 and 3. 7, respectively. Nevertheless, the theoretically predicted D coefficients were slightly higher and that predicted S coefficients were slightly lower than the experimental ones. The discrepancy between the measured and predicted values increased with CNC content. This could be due to not taking into consideration in equations 6 and 7 the mass fraction of crystals available in LLDPE, which is a semi crystalline polymer, nor accounting for the sorption of gas into CNCs, if any. A poor adhesion between the polymer matrix and cellulosic fibers in composites without coupling/compatibilizer agent also contributes to the difference between the measured and predicted gas sorption parameters in cellulose - based composites [ 14 ]. Re ductions in both D and S coefficients were expected since these sorption parameters are a function of the polymer matrix mass fraction in the composites [14 - 19]. Our previous work on cellulosic fiber/plastic composites demonstrated that cellulose fibers re ject gas by acting like crystallites and, only the amorphous region in the composite (i.e., polymer matrix) absorbs the gas [14,15]. Not only increasing the fiber contents into the composites reduces the matrix mass fraction 68 available for gas diffusion in the composite, but also the fiber regions in the composites obstruct the movement of gas molecules and, therefore, increase the average length of the paths they must travel [14,15]. Thus, the decrease in the solubility of CO 2 in LLDPE/CNC nanocomposites in dicates that the less amorphous is the material, the lower is the gas solubility. Similarly, the diffusion coefficient decreased as the polymer mass fraction decreased in the nanocomposites and that the addition of CNCs increased the CO 2 diffusion time in the nanocomposites, in good agreement with results reported by other investigators [1,20 - 23]. 4.2.3 Correlations between crystallinity, tortuosity factor, and permeability coefficient Table 4 - 2 summarizes the CO 2 permeability (P) coefficients and tortuosity fa ctors of LLDPE calculated from equations ( 3. 5 ) and ( 3. 9 ) , respectively, as a function of CNC contents. As, expected the P coefficients of LLDPE decreased as the CNC content increased due to the decreased in both D and S coefficients as discussed above. Thi s indicates that the permeation process was partly controlled by both solubility and diffusion through the LLDPE matrix. The addition of CNC significantly improved the CO 2 barrier performance of LLDPE sheet up to 10 wt. % content and levelled off above this concentration. Remarkably, though the crystallinity of bio - LLDPE increased with CNC content up to 2.5% and remained constant thereafter; the CO 2 permeability coefficient in the nanocomposites on the other hand decreased almost linearly with increasing am ounts of CNCs (Table 4 - 2). This suggests that the crystallization of LLDPE matrix caused a decrease of CO 2 permeability, but not 69 in linear proportion with the decrease in amorphous volume and that the gas permeability in LLDPE appeared to be controlled by other factors in addition to level of crystallinity. In other terms, even though the polymer crystallinity played a role in improving the CO 2 barrier property of bio - LLDPE up to 2.5% CNC, it is certainly not the only factor influencing its gas barrier impr ovement above this CNC addition level. Our results are in good agreement with those of other investigators who reported no correlation between percent crystallinity and permeability coefficient [2 0 - 22 ]. Indeed, Frounchi and coworkers observed a two - fold re duction in both oxygen and carbon dioxide permeability coefficients of the PP/EPDM blend filled with 1.5 vol% organoclay compared to unfilled PP/EPDM blend, despite a 27% reduction in nanocomposite crystallinity [2 0 ]. Similar results have also been observe d by other researchers [2 1,22 ]. A significant decline in both O 2 (about 45%) and CO 2 (about 68 %) permeability coefficients of PLA films was obtained by adding 1.37 vol% graphene oxide nanosheets into PLA film, even though all the PLA 1 ]. Therefore, in addition to level of crystallinity and/or crystal morphology, the role of other factors contributing to barrier improvement like the tortuosity factor must also be investigated to understand t he mechanisms of barrier improvement in nanocomposites. Since the CO 2 permeation process (decreased permeability coefficient) in the nanocomposite was partly controlled by the gas diffusion through the LLDPE matrix, the observed decrease in diffusion coef ficient in the nanocomposites indicated an increase in the gas diffusion time as per equation ( 3. 4). The increased diffusion time was attributed to the CNCs acting as impermeable crystals for the movement of gas molecules through the polymer matrix creatin g a tortuous path for the movement of gas molecules. This mechanism known as a tortuosity effect is 70 illustrated in Figure 4 - 1 , showing a path A that represents the polymer matrix without CNCs and a path B representing the counterpart with CNCs. The diffusi on time in path B will be greater than in path A due to the presence of CNCs. which is creating a tortuous path for the movement of gas molecules, thus increasing its diffusion time. Huang and coworkers attributed the improved gas barrier performance of PL A/graphene oxide nanosheets to the tortuosity effect created by the impermeable graphene oxide nanosheets [2 1 ]. Likewise, numerous investigators have claimed this tortuosity mechanism to explain improvement in gas and/or water vapor barrier of various poly mers [2 0 - 22 ] but without quantifying it. Figure 4 - 1 Schematic diagram of the tortuosity effect. In this study, the tortuosity effect was quantified as the tortuosity factor according to equation ( 3. 9) (Table 4 - 2). The effect of CNC content on the tortuosity factor and CO 2 permeability coefficient is illustrated in Figure 4 - 2 . The estimated tortuosity factor increased with the increase in the CNC content in the nanocomposites. This result was expected since CNCs obstruct the movement of gas molecules in the nanocomposites, then more CNCs in the nanocomposites tend 71 to increase the average length of the paths the gas molecule must travel in the sample; thus, increasing the tortuosity factor. Moreover, samples of nanocomposite sheets and films with various CNC contents were observed under an optical microscope to illustrate the tortuosity effect (Figure 4 - 3 ). The images showed an increase in the number of CNC particulates present per unit area with increase in concentration of CNC; thus, the more CNC in the material, the higher is the tortuosity factor. The CO 2 permeability coefficient of LLDPE decreased al most linearly with increasing amounts of CNCs as previously discussed (Table 4 - 2 and Figure 4 - 2 ). Figure 4 - 2 Effect of CNC content on tortuosity factor (TF) and CO 2 permeability of bio - LLDPE . 15 20 25 30 35 80 100 120 140 160 -3 0 3 6 9 12 15 P CO2 (10 - 17 x kg·m/m 2 ·s·Pa) Tortuosity factor (%) CNC Content (%) %TF PCO2 72 Figure 4 - 3 Optical microscope images of bio - LLDPE sheets (left column) with various CNC contents: (a) 0%, (b) 2.5%, (c) 7.5% and (d) 13.5% as well as of bio - LLDPE films (right column) with various CNC contents: (a) 0%, (b)1%, (c) 2.5% and (d) 3.5%. 73 A str ong negative correlation was established between the tortuosity factor and the permeability coefficient. As the tortuosity factor increased the CO 2 permeability coefficient values decreased (Figure 4 - 4 ). This correlation showed a good fit with an R 2 value of approximately 95% ( Figure 4 - 4 ), clearly indicating that factors other than crystallinity also contribute to gas barrier improvement of polymers. Figure 4 - 4 Pco 2 vs t ortuosity f actor . R² = 0.9547 10 15 20 25 30 35 95 105 115 125 135 145 P CO2 (10 - 17 x kg·m/m 2 ·s·Pa) Tortuosity Factor (%) 74 4.3 CO 2 and O 2 barrier properties of LLDPE f ilms with CNCs LLDPE is used in many flexible packaging applications like heavy duty shipping sacks, stretch/cling film, grocery snacks, etc. H ence it is important to investigate the effect of CNCs addition on the gas barrier of bio - LLDPE films, in addition to the nanocomposite sheets discussed previously. The films were studied for their oxygen and carbon dioxide barrier properties, the most imp ortant gases crucial for food packaging applications, to understand the effect of CNC addition on LLDPE gas barrier enhancement (Table 4 - 3). Table 4 - 3 Effect of CNC addition on the CO 2 and O 2 permeability of bio - LLDPE. LLDPE composition Gas permeability (10 - 17 kg · m/m 2 s · Pa) 1 P CO2 % decrease in P CO2 P O2 % decrease in P O2 Control 27.9 ± 0.70 a - 31.8 ± 2.27 a - 2.5% CNC 18.7 ± 2.77 b 33 16 ± 0.72 b 50 1 Different superscript letters in each column indicate that the difference is statistically significant at values As expected, a significant reduction in gas permeability coefficients of bio - LLDPE films was obtained by adding CNCs into the matrix. Both the CO 2 and O 2 permeability coefficients of LLDPE significantly decreased by about 33% and 50%, respectively, by adding only 2.5% CNC. The enhanced gas barrier performance of bio - LLDPE/CNC film s is mainly assigned to the efficient nucleating effect of CNCs in the bio - LLDPE matrix along with the tortuosity effect created by the impermeable CNC crystals for the movement of gas molecules. The microscope images in F igure 75 4 - 3 clearly show an increase in the number of CNC particulates present per unit area with increase in concentration of CNC, confirming the to rtuosity effect. Several studies have reported similar nucleating effect on addition of CNCs to the polymer matrix [ 8,9 ]. Furthermore, these results also validate the gas barrier enhancement results achieved in the nanocomposites sheets discussed previousl y. Interestingly, the film samples required less amount of CNCs to show a similar improvement in CO 2 barrier as compared to the nanocomposite sheets. The reason for this is unclear now but this could be ascribed to the bi - directionally stretched blown extrusion film having different kinds of crystalline structures compared to uniaxially - stretched compression molding sheets. The voids in compression molded samples could also account for this discrepancy. However, this hypothesis was ruled out since the density of LLDPE/CNC nanocomposites increased with CNC content as expected from the rule of mixtures due to the higher specific gravity of CNC (~1.45) compared to that of LLDPE (0.915). The density of LLDPE increased from 0.915 g/cm 3 to 0.925 g/cm 3 , 0 .933 g/cm 3 , 0.944 g/cm 3 , and 0.950 g/cm 3 by adding 2.5%, 5%, 7.5%, and 10% CNC, respectively into the matrix. The nanocomposite sank to the bottom of the column at 13.5% CNC. 76 APPEN DI X 77 Table A - 1 Permeability coefficient s of bio - HDPE film obtained using isostatic permeability method. Samples Thickness (mm) CO 2 TR ( cc/m² day) Permeability (kg · m/m 2 sec · Pa) 1 0.0493 7052 7.82E - 17 2 0.0493 7185 7.97E - 17 3 0.0493 8701 9.65E - 17 4 0.044 0 7521 7.44E - 17 5 0.0486 7807 8.53E - 17 Average 4.81E - 02 7.65E+03 8.28E - 17 SD 2E - 03 7E+02 9E - 18 COV 0.05 0.09 0.10 S E 1E - 03 3E+02 4E - 18 Table A - 2 Permeability, diffusion and solubility coefficients of bio - HDPE using gravimetric method. Samples Diffusion Coefficient (10 7 cm 2 /sec) Solubility Coefficient (g/m 3 · Pa) Permeability (kg · m/m 2 sec · Pa) 1 4.67 2.10E - 03 9.81E - 17 2 3.89 2.05E - 03 7.98E - 17 3 3.13 2.53E - 03 7.92E - 17 4 2.68 2.30E - 03 6.17E - 17 5 2.75 2.38E - 03 6.55E - 17 6 2.79 2.08E - 03 5.79E - 17 7 4.05 2.26E - 03 9.17E - 17 8 2.43 2.22E - 03 5.39E - 17 9 3.99 2.02E - 03 8.05E - 17 10 4.74 1.98E - 03 9.38E - 17 11 2.91 2.45E - 03 7.12E - 17 12 2.30 2.14E - 03 4.93E - 17 A verage 3.65E+00 2.23E - 03 8.02E - 17 SD 8.02E - 01 2.02E - 04 1.26E - 17 COV 0.22 0.09 0.16 SE 0.27 6.72E - 05 4.21E - 18 78 Table A - 3 P ermeability, diffusion and solubility coefficients of bio - LDPE obtained using gravimetric method. Samples Diffusion Coefficient (10 7 cm 2 /sec) Solubility Coefficient (g/m 3 · Pa) Permeability (kg · m/m 2 sec · Pa) 1 5.43 4.57E - 03 2.48E - 16 2 4.5 4.72E - 03 2.14E - 16 3 5.5 4.69E - 03 2.57E - 16 4 5.3 4.69E - 03 2.48E - 16 5 5.2 4.60E - 03 2.39E - 16 6 5.8 4.52E - 03 2.60E - 16 7 5.0 4.72E - 03 2.33E - 16 8 4.2 4.54E - 03 1.90E - 16 9 4.5 4.90E - 03 2.20E - 16 10 4.3 4.88E - 03 2.10E - 16 11 4. 9 4.94E - 03 2.39E - 16 12 4.4 5.00E - 03 2.20E - 16 13 4. 6 4.93E - 03 2.26E - 16 14 4. 2 5.18E - 03 2.17E - 16 15 4.7 4.93E - 03 2.19E - 16 16 4.8 4.74E - 03 2.27E - 16 A verage 4.53E+00 4.78E - 03 2.29E - 16 SD 1.30E+00 1.89E - 04 1.86E - 17 COV 0.287 0.040 0.081 SE 3.25E - 01 4.73E - 05 4.66E - 18 79 Table A - 4 CO 2 p ermeability coefficients of bio - LLDPE film obtained using iso - static permeability method. Samples Thickness (mm) CO 2 TR (cc/m² day) Permeability (kg·m/m 2 sec Pa) 1 0.066 17510.39 2.60E - 16 2 0.066 19749.70 2.93E - 16 3 0.071 17703.35 2.82E - 16 4 0.071 17615.00 2.80E - 16 5 0.062 21446.97 2.99E - 16 6 0.064 17776.92 2.57E - 16 Average 0.07 18633.72 2.79E - 16 SD 0.00 1616.12 1.72E - 17 COV 0.05 0.09 0.06 SE 1.45E - 03 6.60E+02 7.01E - 18 Table A - 5 CO 2 p ermeability coefficient values of bio - LLDPE obtained using gravimetric method . Samples Diffusion Coefficient ( 10 7 cm 2 /sec ) Solubility Coefficient ( g/m 3 · Pa ) Permeability ( kg · m/m 2 sec · Pa ) 1 4.8 5.37E - 03 2.61E - 16 2 5.2 4.91E - 03 2.55E - 16 3 7.1 5.50E - 03 3.93E - 16 4 3.3 6.77E - 03 2.20E - 16 5 4.8 5.92E - 03 2.85E - 16 6 5.9 5.76E - 03 3.41E - 16 7 5.3 5.89E - 03 3.14E - 16 8 4.6 5.78E - 03 2.64E - 16 9 5.5 5.53E - 03 3.05E - 16 A verage 5.2 5.72E - 03 2.93E - 16 SD 1.055 5.03E - 04 5.21E - 17 COV 0.204 8.81E - 02 1.78E - 01 SE 0.35 1.68E - 04 1.74E - 17 80 Table A - 6 Measured d ensit ies of bio - PE obtained using density gradient method . Samples LLDPE LDPE HDPE Distance (in) Density (g/cm 3 ) Distance (in) Density (g/cm 3 ) Distance (in) Density (g/cm 3 ) 1 13.000 0.912 12.375 0.915 6.375 0.948 2 12.375 0.915 11.875 0.918 5.625 0.952 3 12.125 0.917 11.375 0.921 5.500 0.952 4 12.375 0.915 11.375 0.921 6.000 0.950 5 12.625 0.914 11.500 0.920 5.625 0.952 6 12.375 0.915 11.500 0.920 5.625 0.952 7 12.375 0.915 11.625 0.919 5.500 0.952 8 12.375 0.915 11.500 0.920 5.625 0.952 Average 12.453 0.915 11.641 0.919 5.734 0.951 Std 0.2582 0.0014 0.3370 0.0018 0.3021 0.0019 COV 0.0207 0.0015 0.0289 0.0020 0.0527 0.0020 SE 0.09130 0.00049 0.11914 0.00064 0.10679 0.00069 81 Table A - 7 Measured d ensit ies of bio - LLDPE with various CNC co ntents obtained using density gradient method. Samples 2.5% CNC 5% CNC 7.5% CNC 10% CNC Distance (in) Density (g/cm 3 ) Distance (in) Density (g/cm 3 ) Distance (in) Density (g/cm 3 ) Distance (in) Density (g/cm 3 ) 1 11.13 0.922 9.00 0.934 6.50 0.947 5.38 0.953 2 10.33 0.926 8.75 0.935 7.25 0.943 5.63 0.952 3 10.50 0.925 9.38 0.932 7.63 0.941 7.00 0.944 Average 10.65 0.925 9.04 0.933 7.13 0.944 6.00 0.950 Std 0.421 0.002 0.31 0.002 0.57 0.003 0.88 0.005 COV 0.039 0.002 0.035 0.002 0.080 0.003 0.146 0.005 SE 0.243 0.001 0.182 0.001 0.331 0.002 0.505 0.003 82 Figure A - 1 Effect of CNC content on density of bio - LLDPE / CNC composites. 0.915 0.920 0.925 0.930 0.935 0.940 0.945 0.950 0.955 0 2 4 6 8 10 12 Density (g/cm 3 ) % CNC content 83 Table A - 8 Effect of CNC content on crystallinity of bio - LLDPE / CNC composites. CNC content 0% 2.50% 5% 7.50% 10% 13.50% 1 50 76 72 75 75 73 2 50 75 73 75 76 75 3 50 71 74 76 75 75 Av erage 50 73.7 73 75.3 75.3 74.1 SD 0 2.4 0.8 1 0.8 1.4 COV 0 0.032 0.012 0.014 0.01 0.019 SE 0 1.4 0.5 0.6 0.4 0.8 84 Table A - 9 CO 2 p ermeability (P in 10 - 1 6 kg·m/m 2 s ec ·Pa ) , diffusion (D in 10 7 cm 2 /sec ) , and solubility (S in 10 - 3 in g/m 3 ·Pa ) coefficient s of neat bio - LLDPE and bio - LLDPE filled with various CNC content obtained using gravimetric method. Samples LLDPE 2.5% CNC 5% CNC 7.5% CNC 10% CNC 13.5% CNC D S P D S P D S P D S P D S P D S P 1 4.8 5. 4 2.6 5.6 5. 3 2.9 5.0 5.7 2. 9 4.5 5. 4 2.4 3.4 5.4 1.8 3.4 5. 7 1.9 2 5.2 4.9 2.5 4.7 5.2 2. 5 5.2 5.7 2.9 4.5 5. 2 2. 4 4.6 4. 5 2.0 3.4 6.1 2. 1 3 7.1 5.5 3.9 5.2 5. 7 2.9 4.4 5.5 2. 5 4.5 5.0 2. 3 2.6 4. 7 1.2 3.3 5. 5 1.8 4 3.3 6. 8 2.2 4. 3 5. 5 2. 3 5.2 5.7 2.9 4.7 5.8 2. 8 4.2 5.1 2.1 3. 7 5. 4 1.9 5 4.8 5.9 2. 9 4.7 5.5 2. 6 4.3 5.9 2. 6 3.9 5. 4 2. 1 4.0 5. 4 2.1 3. 7 5. 5 2. 0 6 5.9 5. 8 3.4 4.1 5. 8 2. 4 4.1 5. 1 2. 1 3.9 5.5 2.2 3. 8 5. 5 2.0 7 5.3 5. 9 3.1 4.0 5.5 2.2 4.1 5.4 2. 2 3.3 4. 5 1. 5 3.5 5. 3 1. 9 8 4.6 5. 8 2.6 3.7 5.2 1.9 3.5 5.5 1.9 9 5.5 5.5 3. 1 0. 2 0. 3 0.9 Av era g e 5.2 5.7 2.9 4.9 5.4 2.6 4.6 5.7 2.6 4.3 5.3 2.3 3.7 5.0 1. 9 3.5 5.5 1.9 SD 1.1 0.5 0.5 0.5 0.2 0. 3 0.5 0.1 0.3 0.3 0. 3 0.2 0.6 0.4 0. 4 0. 2 0. 3 0.9 85 Table A - 10 Tortuosity factor of bio - LLDPE with various CNC contents. Samples Tortuosity Factor 2.5% CNC 5% CNC 7.5% CNC 10% CNC 13.5% CNC 1 0.928 1.039 1.164 1.534 1.516 2 1.104 1.005 1.147 1.137 1.531 3 0.994 1.172 1.155 2.005 1.572 4 1.218 1.006 1.097 1.243 1.421 5 1.107 1.195 1.337 1.310 1.413 6 1.261 1.276 1.320 1.385 7 1.311 1.281 1.575 1.463 8 1.387 A verage 1.070E+00 1.141E+00 1.208E+00 1.439E+00 1.472E+00 SD 1.12E - 01 1.25E - 01 8.89E - 02 2.70E - 01 6.96E - 02 COV 1.05E - 01 1.10E - 01 7.36E - 02 1.88E - 01 4.73E - 02 SE 5.02E - 02 4.74E - 02 3.36E - 02 9.55E - 02 2.63E - 02 86 Table A - 11 CO 2 permeability coefficients of neat bio - LLDPE and bio - LLDPE/2.5% CNC composite films. Samples LLDPE LLDPE/2.5% CNC Thickness (mm) CO 2 TR (cc/m² day) Permeability (kg·m/m 2 ·sec·Pa) Thickness (mm) CO 2 TR (cc/m² day) Permeability (kg·m/m 2 ·sec·Pa) 1 0.066 17510.39 2.60E - 16 0.066 16776.17 2.4903E - 16 2 0.066 19749.70 2.93E - 16 0.066 13503.62 2.00452E - 16 3 0.071 17703.35 2.82E - 16 0.062 12975.35 1.8152E - 16 4 0.071 17615.00 2.80E - 16 0.064 7977.00 1.15183E - 16 5 0.062 21446.97 2.99E - 16 6 0.064 17776.92 2.57E - 16 Average 0.07 18633.72 2.79E - 16 6.46E - 02 1.28E+04 1.87E - 16 SD 0.00 1616.12 1.72E - 17 1.81E - 03 3.63E+03 5.54E - 17 COV 0.05 0.09 6.17E - 02 0.03 0.28 0.30 SE 1.45E - 03 6.60E+02 7.01E - 18 9.06E - 04 1.82E+03 2.77E - 17 87 Table A - 12 O 2 permeability coefficients of neat bio - LLDPE and bio - LLDPE/2.5% CNC composite fil ms . Samples LLDPE LLDPE/2.5% CNC Thickness (mm) OTR (cc/m² day) Permeability (kg·m/m 2 ·sec·Pa) Thickness (mm) OTR (cc/m² day) Permeability (kg·m/m 2 ·sec·Pa) 1 0.064 4524 2.25E - 16 0.064 2421 1.21E - 16 2 0.062 4551 2.19E - 16 0.062 3660 1.77E - 16 3 0.062 4741 2.29E - 16 0.062 3282 1.58E - 16 4 0.0574 5048 2.25E - 16 0.062 3743 1.81E - 16 5 0.061 7512 3.56E - 16 0.062 4037 1.95E - 16 6 0.062 9550 4.61E - 16 0.062 3419 1.65E - 16 7 0.056 12071 5.26E - 16 0.062 4210 2.03E - 16 8 0.054 10556 4.43E - 16 0.062 3730 1.80E - 16 9 0.07 5292 2.88E - 16 0.062 3447 1.66E - 16 10 0.056 5248 2.29E - 16 0.062 3150 1.52E - 16 11 0.057 5714 2.53E - 16 0.062 2723 1.31E - 16 12 0.062 6038 2.91E - 16 0.062 2688 1.30E - 16 13 0.062 5292 2.55E - 16 0.062 2567 1.24E - 16 14 0.055 8312 3.56E - 16 15 0.055 7733 3.31E - 16 16 0.070 5968 3.25E - 16 17 0.072 6976 3.91E - 16 Average 6.10E - 02 6.77E+03 3.18E - 16 6.22E - 02 3313.61 1.60E - 16 SD 5.57E - 03 2.24E+03 9.36E - 17 5.55E - 04 574 2.72E - 17 COV 0.1 0.3 0.3 8.92E - 03 0.2 1.70E - 01 SE 1.35E - 03 5.43E+02 2.27E - 17 1.39E - 04 144 7.28E - 18 88 REFERENCES 89 REFERENCES 1. Bendahou, A., Kaddami, H., Espuche, E., Gouanvé, F., and Dufresne, A. (2011). Synergism effect of montmorillonite and cellulose whiskers on the mechanical and barrier properties of natural rubber composites. Macromolecular Materials and Engineering , 296(8), 760 - 769. 2. Barr, C. D., Giacin, J. R., and Hernandez, R. J. (2000). A determination of solubility coefficient values determined by gravimetric and isostatic permeability techniques. Packaging Technology and Science: 13(4), 157 - 167 . 3. Dimitroulas G.D., Badeka A.B, and Kontominas M.G. (2004). Permeation of methylethylketone, oxygen and water vapor through PET films coated with SiOx: effect of temperature and coating speed. Polymer Journal , 36 ( 3 ) ,198 - 204. 4. Euaphantasate, N., Prachayawasin, P., Uasopon, S., and Methacanon, P. (2008). Moisture sorption characteristic and their relative properties of thermoplastic starch/linear low - density polyethylene films for food packaging. Journal of Metals, Material and M inerals , 18, 103 - 109. 5. Selke, S. E., and Culter, J. D. (2016). Plastics Packaging: Properties, Processing, Applications, And Regulations . Hanser. 6. Carrera, M. C., Erdmann, E., and Destéfanis, H. A. (2013). Barrier properties and structural study of nanocomposite of HDPE/montmorillonite modified with polyvinylalcohol. Journal of Chemistry , Volume 2013, Article ID 679567, 7 pages. 7. Wang, Y., Easteal, A. J., and Chen, X. D. (1998). Ethylene and oxygen permeability through polyethylene packaging films. Packaging Technology and Science , 11(4), 169 - 178. 8. Karkhanis, S. S., Stark, N. M., Sabo, R. C., and Matuana, L. M. (2018). Water vapor and oxygen barrier pro perties of extrusion - blown poly( lactic acid)/cellulose nanocrystals nanocomposite films. Composites Part A: Applied Science and Manufacturing , 114, 204 - 211. 9. Liu, Y., and Matuana, L. M. (2019). Surface texture and barrier performance of poly( lactic acid) ce Journal of Applied Polymer Science , 136(22), 47594. 90 10. Clarkson , C.M., Awad El Azra k, S.M, Schueneman , G.T, Snyder , J.F, and Youngblood , J.P (2020). Crystallization kinetics and morphology of small concentrations of cellulose nanofibrils (CNFs) and cellulose nanocrystals (CNCs) melt - compounded into poly(lactic acid) (PLA) with plasticizer . Polymer , 187, 122101. 11. Karkhanis, S. S., Stark, N. M., Sabo, R. C., and Matuana, L. M. (2018). Performance of poly( lactic acid)/cellulose nanocrystal composite blown films processed by two different compounding approaches. Polymer Engineering & Science , 58(11), 1965 - 1974. 12. Fortunati, E., Peltzer, M., Armentano, I., Torre, L., Jiménez, A., and Kenny, J. M. (2012). Effects of modified cellulose nanocrystals on the barrier and migration properties of PLA nano - biocomposites. Carbohydrate Po lymers , 90(2), 948 - 956. 13. Sanc hez - Garcia, M. D., and Lagaron, J. M. (2010). On the use of plant cellulose nanowhiskers to enhance the barrier properties of polylactic acid. Cellulose , 17(5), 987 - 1004. 14. Matuana, L. M., Park, C. B., and Balatinecz, J. J. (1996). Characterization of microc ellular foamed PVC/cellulosic - fibre composites. Journal of Cellular Plastics , 32(5), 449 - 469. 15. Journal of Vinyl and Additive Technology , 7(2 ), 67 - 75 . 16. Van Krevelen , D.W. (1990), Properties of Polymers , Elsevier, New York. 17. Michaels , A.S. and Bixler , H.J. (1961), Solubility of gases in polyethylene. Journal of Polymer Science , 50 (154), 393 - 412. 18. Michaels , A.S. and Bixler , H.J. (1961), Flow of gases through polyethylene. Journal of Polymer Science , 50(154), 413 - 439. 19. Matuana , L. and Li. , Q. A Factorial (2001) Design applied to the extrusion foaming of polypropylene/wood - flour composites. Cellular Polymers , 20(2), 115 - 130. 20. F rounchi, M., Dadbin, S., Salehpour, Z., and Noferesti, M. (2006). Gas barrier properties of PP/EPDM blend nanocomposites. Journal of Membrane Science , 282(1 - 2), 142 - 148. 21. Huang, H. D., Ren, P. G., Xu, J. Z., Xu, L., Zhong, G. J., Hsiao, B. S., and Li, Z. M. (2014). Improved barrier properties of poly( lactic acid) with randomly dispersed graphene oxide nanosheets. Journal of Membrane Science , 464, 110 - 118. 91 22. D. G. (2009). Structure versus properties relationship of poly( lactic acid). I. Effect of crystallinity on barrier properties. Journal of Polymer Science Part B: Po lymer Physics , 47(22), 2247 - 2258. 23. (2010). Crystallinity effect on the gas transport in semicrystalline coextruded films based on linear low - density polyethylene. Journ al of Polymer Science Part B: Polymer Physics , 48(6), 634 - 642. 92 CHAPTER 5 CONCLUSION 5.1 Conclusion This study was intended at improving the gas barrier property of sugarcane - based LLDPE using bio - cellulose nanocrystals (CNCs). Specifically, this study evaluated the effect of testing methods (isostatic versus gravimetric) on CO 2 permeability of various bio - PE grades as well as the effect of CNC content on crystallinity, tortuosity factor, and gas barrier properties of bio - LLDPE sheets and films. The iso static and gravimetric permeation methods yielded similar CO 2 permeability coefficients irrespective of PE grade and b oth methods were concluded to be equally reliable to measure the gas permeability of the polymers. The effect of density on bio - PE permeab ility coefficient was also established where the PE grade with the lowest density, i.e., bio - LLDPE was observed to have the highest permeability coefficient values among the different PE grades studied. The crystallinity of bio - LLDPE increased with CNC addition level up to 2.5% and remained constant as the CNC content increased further attributable to the crystal nucleating effect of CNC and the retarded crystal growth caused by CNC agglomerations at high loading levels, respectively. The diffusion and solubility coefficients in bio - LLDPE decreased almost linearly as 93 the CNC content increased, which caused a decreased polymer matrix mass fraction available for gas diffusion as well as an increased CO 2 diff usion time in the nanocomposites. The CO 2 permeability coefficient of LLDPE decreased almost linearly with increasing CNC content. A significant decline in CO 2 (about 36%) permeability coefficients of LLDPE sheets was obtained by adding 10 wt. % of CNCs i nto LLDPE sheet, and P CO2 values levelled off above this concentration. No correlation was established between gas permeability and percent crystallinity of LLDPE sheet since the permeability coefficient decreased almost linearly with increasing CNC conten t whereas the crystallinity of the nanocomposites increased only up to 2.5% CNC content and remained constant thereafter. In contrast, a strong negative correlation was established between the tortuosity factor and the permeability coefficient, clearly ind icating that factors other than percent crystallinity also contribute to gas barrier improvement of bio - LLDPE. The effect of CNC content on the gas barrier properties of bio - LLDPE films was also examined. As expected, a significant improvement in gas barr ier of bio - LLDPE films was obtained by adding CNCs into the matrix . The P CO2 and P O2 coefficients of bio - LLDPE films reduced by 33% and 50%, respectively, when 2.5% of CNC was added into the films, validating the gas barrier enhancement results achieved with nanocomposites sheets . 94 5.2 Future work This study investigated the effect of CNC c ontent on the gas barrier properties of sheet and film bio - LLDPE and bio - LLDPE/CNC composites . As discussed, the results showed a significant improvement in gas barrier properties of bio LLDPE by adding CNCs. This improvement will expand their scope in a w ide range of packaging applications. However, it would also be appropriate to study the effect of CNC addition on o ther properties of the resulting composite films/sheets such as optical (haze, color, clarity) , thermal properties (glass transition and melting temperatures, thermal stability) , mechanical (tensile and impact strength, toughness, creep, resilience, puncture resistance) as well as sealing (seal strength, hot tack) . These properties are important par ameters to consider while evaluating real world performance of a packaging material and will widen the scope of these composites.