Grafting Density-Controlled Interfacial Engineering for Exclusive Stereocomplex Crystallization in Poly(Lactic Acid) Enantiomers/Cellulose Nanocrystals Nanocomposites

Tailoring interfacial interactions in nanocomposites is critical for controlling crystallization behavior and enhancing material performance. However, in poly(lactic acid) (PLA) enantiomeric blends containing polymer-grafted nanoparticles, the role of the grafted polymer remains elusive due to the competitive and distinct nucleation mechanisms of homo- (HC) and stereocomplex (SC) crystallization. In this study, a series of poly(methyl methacrylate)-grafted cellulose nanocrystals (CNCs-g-PMMA) with controlled grafting densities was synthesized to investigate their influence on the polymorphic crystallization behavior of PLA. Morphological analysis and surface energy calculations revealed that higher PMMA grafting density reduced CNC aggregation and improved interfacial compatibility. Unexpectedly, while high grafting density suppressed the inherent HC-promoting effect of CNCs, it became essential for inducing SC crystallization only above a critical threshold. This divergence originates from distinct nucleation mechanisms: HC crystallization relies on hydrogen bonding between CNC surface hydroxyls and PLA carbonyl groups, whereas SC crystallization is more sensitive to the PMMA-enriched interfacial layer, which reduces conformational entropy and facilitates nucleation. This work clarifies the pivotal role of grafting density in tailoring interfacial interactions and provides a theoretical foundation for designing biobased nanocomposites.