Grafting Kinetics and Compatibility Simulation in Surface Modification of Cellulose Nanocrystals with Poly(lactic acid) for Composites

Mechanical enhancement of cellulose nanocrystals (CNCs), natural biomass-based nanoparticles, to polymeric composites is one of the representative applications attributed to their intrinsic property of a high specific modulus. The purpose of this work is to investigate in depth the connections between surface modification and interfacial compatibility of CNC-enhanced composites in the classic case of chemical grafting on CNCs with poly(lactic acid) (PLA) chains. According to the analysis of grafting kinetics, varied surface grafting contents or degrees of surface substitution can be achieved on the modified CNCs (CNC-g-PLA), leading to tailored surface polarities as confirmed by both experiments and theoretical prediction. From the combined analyses of mechanical enhancements of CNC and CNC-g-PLA for composites, a concept of critical surface modification is proposed, considering the reasonable degree of surface modification but not the traditional “as much as possible” modification in the study of CNC-enhanced composite systems. Molecular simulation is performed to theoretically explain the influence of surface modification on the changed nanocrystal–nanocrystal and nanocrystal–matrix interactions, which is further applied in the compatibility analysis and to provide evidence of critical surface modification in composites.