Chemical Modifications on Cellulose Nanocrystals for Composites: Surface Chemistry to Tailored Compatibility and Mechanical Enhancement

How do the degrees of surface modification on nanoparticles affect their enhancement to the mechanical performance of composites? This issue is generally overlooked with a consensus that surface modification can promote the compatibility between nanoparticles and polymeric matrix, which therefore advances mechanical performances of obtained composites. In this study, we designed a composite system of castor oil (CO)-based polyurethane (PU) as the matrix enhanced by cellulose nanocrystals (CNCs) with changing surface modifications of low, moderate, and high grafting of the CO chains as nanofillers. The compatibility between modified CNCs and PU was quantified by the Hansen solubility parameters, with the gradual decrease of Ra values from 14.1 to 6.1. The interfacial compatibility between nanofillers and matrix directly determined the dispersion and microstructures of composite systems and further influenced mechanical performances of materials, exhibiting the improvement on Young's modulus, tensile strength, work of fracture, and storage modulus for the modified CNC-enhanced composites. In comparison with theoretical calculations by the classic percolating network model and Halpin–Kardos model, the effects of mechanical enhancement for PU-based composites by CNCs were simulated, revealing stronger interfacial adhesion from the entanglement of grafted CO chains and PU chains and therefore a tighter binding between nanofillers and matrix in composites.