Properties of Nanocellulose and Water-Based Polymer Tricomponent Composites

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Morgenstern, Andrew
Shofner, Meisha L.
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In its current state, the world is reliant on plastics. They are economically cheap materials with versatile properties and countless applications. However, concerns regarding the ecological impacts of plastics, particularly the single-use variety, that will not degrade in landfills and the influence of fossil fuels (the source material for most polymers) on climate change have become imperative topics in global discussion. The motivation of this work is to help in the process of developing sustainable options as replacements for environmentally detrimental materials. The materials used in this study were two cellulose nanomaterial (CN) filler types: cellulose nanocrystals (CNCs) and cellulose nanofibrils (CNFs), and poly(vinyl alcohol) (PVA). PVA is the most commercially important water-soluble polymer currently in use. In addition to being a flexible, non-toxic, biodegradable, and, of course, water-soluble thermoplastic, it is also compatible with many natural materials. Cellulose is naturally produced in all plants, most forms of algae, some microorganisms, and at least one animal and serves as the primary structural component for cell walls. It is the most abundant organic polymer from biomass and is easily incorporated in soft matrix polymers as CN to reinforce properties. CNCs and CNFs mainly vary in structure. This work combines PVA, CNC, and CNF to form a nanocellulose and water-based polymer tricomponent composite or tricomponent CN/PVA nanocomposite. In this work, the two types of CN used had similar structures to those in previous work, which used CNCs and chitin nanofibers (ChNFs) with a PVA matrix to make tricomponent nanocomposites, to assess the role of hydrogen bonding between the CN particles and the PVA matrix. This allowed for a proper comparison to the previous results and an increased understanding of the design guidelines for mixed filler composites. The structure and properties of the developed composites were observed via optical microscopy, structural, thermal, and mechanical characterization techniques, namely polarized optical microscopy (POM), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and micro-tensile tensile testing (MTT). Both CNC and CNF decreased the Tg and melting/degradation onset. While CNF had more of an effect on Tg than CNC, roles were reversed for melting/degradation onset. Both CNC and CNF prevented the material from yielding and greatly decreased ductility compared to neat PVA. CNF being more prone to agglomerate and form rigid structure networks had more of an influence on ductility than CNC. Both CNC and CNF decreased transparency and increased hydrophilicity with CNF being much more prominent impacts. CNF increased crystallinity while CNC decreased it. However, most notably, there were seeming critical ratios that existed for mixed CN/PVA nanocomposites where ductility and strain at maximum engineering stress were minimized with higher (≥2.5wt%) loadings of CNF and where stiffness and strength were maximized at higher loadings of CNC. This suggests that mixed CN/PVA nanocomposites may improve the mechanical properties (where strength, stiffness, ductility, and strain at maximum engineering stress are all favored) at higher loadings of CNC. These results are consistent with previous results for CNC/chitin nanofiber composites in that synergistic improvements were achieved as certain nanofiller pairing ratios. These results open the path for further work on tricomponent CN/PVA nanocomposites which include the development of crystallinity control, dispersion and CN loading optimization, further characterization, optimization of CN loading pairings for specific properties, additional property assessment, and renewability and sustainability evaluation.
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