Composites of polystyrene and surface modified cellulose nanocrystals prepared by melt processing

Cellulose nanocrystals (CNCs) were surface modified with benzoic acid anhydride (BzAnh) at room temperature in 48-h reaction time. FTIR spectroscopy confirmed the high degree of modification of the CNCs with BzAnh, while 13C CP MAS NMR showed that the total degree of modification was 16%, indicating that the degree of surface modification of the CNCs was very high (estimated to be 50% or even higher). 2D 1H-13C CP-HETCOR MAS NMR and 13C-detected proton spin-diffusion (PSD) experiments confirmed that BzAnh reacted mainly with –CH2–OH groups in the amorphous regions of CNC nanocrystals. The crystallinity indices of unmodified and BzAnh-modified CNCs showed little difference, indicating that the modification did not destroy crystalline domains, thus confirming NMR results that the esterification reaction occurred mainly in the amorphous cellulose regions. Unmodified and BzAnh-modified CNCs were used to produce nanocomposites with polystyrene (PS) by melt processing at 200 °C. SEM microscopy confirmed the improved compatibility of BzAnh-modified CNCs with PS matrix compared to PS nanocomposites prepared with unmodified CNCs. The results of tensile tests showed that the addition of BzAnh-modified CNCs increased the Young’s modulus and tensile strength of the composites, while unmodified CNCs showed deterioration in mechanical properties. The addition of 5 wt% CNCs to the PS matrix resulted in a 30% improvement in tensile strength and a 23% increase in Young’s modulus. The increased compatibility and improved mechanical properties of BzAnh-modified CNCs were attributed to the intense entanglement of PS chains and BzAnh-modified CNCs due to π–π interactions between benzene rings.