Modification Of Chitosan And Poly(Lactic) Acid By Cellulose Nanocrystals To Improve Nanocomposite' Properties

Nowadays,the environmental problems becoming increasingly,especially the white pollution caused by plastic packaging has spread at various regions.Chitosan(CS)and polylactic acid(PLA)are typical representatives of natural and synthetic biodegradable polymer materials.Their development and application become an effective way to protect the environment.However,poor thermal stability and poor dispersibility are great obstacles which limit them application in the polymer modification.Cellulose nanocrystals(CNCs)are a kind of bio-based nanomaterials which have abundant sources.They have high crystallinity,high strength and high Young's modulus,high specific surface area ratio and other advantages,which make their be suitable as a kind of high-performance material to modify polymer.Therefore,in this work,the modification of Chitosan and Poly(lactic)acid were used by cellulose nanocrystals to improve nanocomposite' properties.However,poor thermal stability and poor dispersibility are great obstacles which limit them application in the polymer modification.Therefore,surface modification of CNCs is the key for improving the properties of polymer/CNCs composites.To this end,when modifying the CS polymer material,CNCs were first modified by polyvinyl alcohol(PVA),which is also a biodegradable material.Then,CNC/CS nanocomposites were prepared by solution mixing.Two different sequences of adding raw materials were compared,and obtained CS/CNCs composites with uniform dispersion of CNCs have good dispersibility in CS matrix.The mechanical and antibacterial properties of CS/CNCs composites were evaluated.When the amount of modified CNCs were 5%,the tensile strength of CS composites increases by about 26%,the elongation at break increases by 42%,and the Young's modulus increases by 12%.Moreover,the metabolic activity of bacteria was reduced,the antibacterial rate were increase by 40%,and the antibacterial performance of nanocomposites was significantly improved.The slow crystallization rate of PLA is one of the reasons limiting its wide application,and the nucleating agent of PLA can obviously improve its crystallization performance,so the self-assembly nucleating agent of decamethylene dicarboxylic dibenzoyl hydrazide(TMC300)was chosen to physically modify the CNCs.Because TMC300 adsorbed on the surface of CNCs shields the sulfonate on CNCs,the thermal stability of CNCs was significantly improved.Then,PLA/CNCs nanocomposites were prepared by melt blending.According to the observation and analysis of microscopic morphology,modified CNCs formed differentself-assembly morphologies in confined and unconfined spaces,which changed the original spherulite structure of the PLA nanocomposite to a string crystal structure.And the analysis of the crystallization kinetics of composite materials showed that modified CNCs can increase the crystallization speed of PLA.More importantly,the self-assembly behavior of the nucleating agent induces the shish-kebab structure of PLA generation,which enhanced the interfacial structure as mechanically interfacial lock and simultaneously improved tensile strength and toughness.The dynamic mechanical results showed that the modified CNCs increase the storage modulus of PLA,so the heat resistance of PLA nanocomposites is significantly enhanced.This physical and economical modification method of CNCs is anticipated to take full advantage of CNCs to modify CS,PLA or other semi-crystalline polymers.