Effect of Graft Development in PLLA-Functionalized Cellulose Nanocrystals on Dispersibility, Transparency, and Mechanical Reinforcement in PLA Nanocomposites

Despite their potential as reinforcing agents for poly(lactic acid) (PLA), cellulose nanocrystals (CNCs) tend to self-aggregate, limiting both optical transparency and mechanical reinforcement. In this study, incorporating poly(L-lactic acid)-grafted CNCs (LCNCs) into PLA simultaneously enhanced both optical transparency and mechanical reinforcement compared to neat CNCs. Three types of LCNCs—LCNC1 (apparent molar substitution, MSapp = 3.36), LCNC2 (MSapp = 25.6), and LCNC3 (MSapp = 56.9)—were synthesized. Fourier transform infrared (FT-IR) and 1H nuclear magnetic resonance (1H NMR) spectra revealed poly(L-lactic acid) (PLLA) grafting onto the CNC surface, and the MSapp values were estimated from the 1H NMR peak integrals. X-ray diffraction patterns (XRD) showed that the native crystalline structure of the CNCs was preserved even after surface modification. LCNC3 (MSapp = 56.9) exhibited a smaller hydrodynamic size in an organic solvent, consistent with reduced aggregation, whereas LCNC1 (MSapp = 3.36) remained severely aggregated like neat CNCs. The PLA/LCNC3 nanocomposite retained transparency comparable to neat PLA and achieved the highest tensile strength among the nanocomposites considered here. These results indicate that a higher MSapp, reflecting greater PLLA graft development on CNCs, is associated with improved dispersion, optical transparency, and mechanical reinforcement in PLA nanocomposites.