Molecular Insights into Interfacial Dynamics and Toughening Mechanisms for Poly(lactic acid)-Grafted Cellulose Nanocrystal Composites

Cellulose nanocrystals (CNCs) are highly regarded as one of the most promising natural biomass-based nanofillers for enhancing poly(lactic acid) (PLA) nanocomposites due to their biocompatibility and high specific modulus. This study presents a temperature-transferable and chemistry-specific coarse-grained (CG) model of PLA developed by using the well-established energy-renormalization (ER) framework. The ER-corrected PLA CG model accurately captures the temperature-dependent dynamics, density, mechanical properties, and conformational characteristics of its all-atom (AA) counterpart. It predicts an accurate glass transition temperature (326 K) and precise tensile and shear moduli, which agree with the AA model and experimental data. Additionally, taking PLA grafted to high-aspect-ratio CNCs as a model nanocomposite, we examine the interfacial behavior of the CNC/PLA nanocomposite and explore the effect of grafted chain length (n) on its mechanical properties under the same CNC content. The results demonstrate a constant Young’s modulus but a higher toughness of CNC/PLA nanocomposites with a longer n due to sufficient chain slippage and crazing fiber formation. The structural evolution during stretching, like chain alignment, bond, and angle, follows the same trend as the stress–strain curve. Furthermore, the dynamics of PLA at a segmental scale are slowed down in proximity to the CNC interface, gradually intensifying as the depth into the matrix increases, thereby revealing a consistent interfacial thickness of 3 nm in the system without grafting, as confirmed by molecular stiffness measurements. Increasing n leads to a greater interfacial thickness. These findings offer valuable insights into the mechanical properties of CNC/PLA systems and valuable guidance for designing and characterizing high-performance CNC-reinforced polymer nanocomposites.