Dynamic Viscoelastic Properties of Polystyrene Composites Reinforced with Cellulose Nanocrystals

We investigated the reinforcement mechanisms of cellulose nanocrystals, CNCs, in polymer matrices by incorporating them into polystyrene, a widely used simple amorphous resin that lacks specific interactions, such as hydrogen bonding. Rheological measurements revealed that the addition of CNC induced significant reinforcement effects in the complex moduli, particularly in the rubbery state. These effects are best described using the modified Morse theory, which accounts for the molecular dynamics of semiflexible polymers in a viscoelastic medium, rather than the classical Halpin–Tsai theory traditionally applied to straight rods. Dynamic birefringence measurements, specifically the frequency dependence of the strain-induced birefringence under oscillatory deformation, provided detailed insights into the contributions of various components to viscoelastic relaxation in the composite materials. The analyses indicated that the reinforcement effects were primarily driven by the tension mode of CNCs, originating from their undulated structure caused by thermal motion.