POLY(3-HYDROXYBUTYRATE-CO-3-HYDROXYVALERATE) - CELLULOSE NANOWHISKERS COMPOSITES BY SOLUTION CASTING
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In this research, renewable biobased nanocomposites based on poly(3-hydroxybutyrate-co-3-hydroxyvalerate) and cellulose nanowhiskers were prepared by solution casting. Nanocomposite structure and property investigations were characterized to understand the reinforcing mechanism of CNWs in PHBV and establish longstanding goal of predicting structure-property relationships for material design and optimization. The CNW concentration strongly influenced the extent of nanowhiskers dispersion and level of property enhancement, as determined by structural, mechanical, thermal, dielectric and rheological characterizations. CNW content of 2.3 wt% indicated significant property transitions for all the investigated properties except for rheological ones. The change was believed to be due to the transition from homogeneous CNW dispersion to agglomeration. High CNW content led to CNW agglomerations and reduced nanocomposite moduli, yield strengths and real permittivity. DSC and POM analyses showed that CNWs could alter the nucleation and growth under both non-isothermal and isothermal crystallization conditions. Although the effect of CNW on nucleation ability and degree of crystallinity of PHBV was significant and could contribute to the mechanical properties improvement, the results of this study confirmed that primarily CNW homogeneous distribution and interconnected 3-D structure governed E' enhancement. Oriented PHBV/CNW composites were fabricated in-situ under electric field. CNW were oriented along the direction of applied filed. Image analysis revealed that degree of order and orientation distribution was mainly influenced by CNW content. TEM, DMA and XRD analysis showed that low CNW concentrations resulted in higher degree of anisotropy. Furthermore, for CNW content above 5 wt% the electric field was not sufficient to align CNW given the increased viscosity of PHBV/CNW solutions, hence, restricted mobility of CNW. The superior properties observed in both randomly oriented and aligned cellulosic nanocomposites may be explained by conventional ideas of reinforcement as predicted by micromechanics theories like Halpin-Tsai, Voigt-Reuss and Cox "shear lag" models. In latter model, the distribution of orientation angle was taken into account and the predictions were consistent with experimental values. Based on good agreement between experimental nanocomposite moduli and Cox model predictions, especially at low concentrations, it was clear that superior reinforcement arises mainly from the high modulus and aspect ratio of CNWs at these concentrations. Furthermore, the critical concentration corresponding to the formation of percolated CNW structure was modeled based on excluded volume theory. The simulation was in good agreement for randomly oriented nanocomposites. From individual micromechanical properties of the nanofibers and matrix, aspect ratio, orientation and volume fraction of nanofibers, it has been demonstrated that the effective storage moduli can be simulated to obtain properties of random and aligned composite lamina.