Esterified cellulose nanocrystals as reinforcement in poly(lactic acid) nanocomposites

Abstract

Biopolymers are of huge interest as they carry potential to minimize the environmental hazards caused by synthetic materials. Poly(lactic acid) (PLA) reinforced by cellulose nanocrystals (CNCs) has shown promising results. To obtain the optimum characteristics of PLA-CNC nanocomposites, a superior compatibility between PLA and CNCs is paramount. The application of chemical surface modification technique is an essential step to improve the interaction between hydrophilic CNCs and hydrophobic PLA. In this study, a time-efficient esterification technique using valeric acid was introduced to improve the compatibility between CNCs and PLA. Masterbatches were prepared using spin-coating method to ensure the maximum dispersion of CNCs through PLA and to decrease the drying time. Nanocomposites were prepared using extrusion and injection molding. The degree of substitution for modified CNCs was calculated as 0.35. Transmission electron microscopy exhibited esterified CNCs (e-CNCs) in the nanoscale with rod shape structure. Thermal stability improved by 15% in nanocomposites containing 3% e-CNCs, whereas untreated CNCs didn’t alter the thermal stability to a notable extent. A substantial increase of 200% was observed in crystallinity of nanocomposites reinforced with 3% e-CNCs. The incorporation of CNCs into PLA resulted in an increase in storage modulus and a decrease in tan δ intensity which was more profound in PLA-e-CNCs. The tensile strength of PLA-e-CNCs composites was found to be superior to composites reinforced with untreated CNCs. The results confirmed that a combination of time-efficient esterification and spin-coated masterbatch was a successful approach to uniformly disperse CNCs in PLA matrix.