Eco-design of PLA/CNC nanocomposites with a dynamic-crosslinking network: enhanced nanofiller dispersion, strain hardening and mechanical properties via reactive extrusion.

In this study, a novel concept termed "polyol coating/crosslinked" was introduced for the PLA nanocomposites with a dynamic-crosslinking network (DCN) via a two-step reactive extrusion process. In this approach, polyols were used both as crosslinking components and physically coating agents of cellulose nanocrystals (CNC) and through a freeze-drying method with water, CNC dispersion inside PLA was enhanced. Meanwhile, during the establishment of DCN inside PLA, the coated CNC were subsequently connected to the DCN to enhance reinforcement. The freeze-dried, glycerol-coated CNC exhibited significantly improved dispersion, indicated via an average distributed length of approximately 400 nm, which is much smaller than the several-micrometer length of untreated CNC particles. This enhanced dispersion and reinforcement further notably reinforced melt strength, mechanical stiffness and thermal resistance of the PLA/CNC composites. Much more pronounced strain hardening behavior, a ∼20% increase in storage modulus and 3.7 °C increase in Vicat softening temperature were observed compared with the neat PLA matrix. As a practical outcome, the CNC-reinforced DCN systems demonstrated excellent foamability, achieving a 25-fold foam expansion, attributed to CNC's role as a foaming nucleating agent. Last but not least, the entire process is recyclable, re-processable, organic-solvent-free and suitable for continuous melt extrusion. Based on these results, this method could potentially be applied to transform similar polyester-based polymer wastes into a composite DCN form, offering improved mechanical and melt-strengthening properties, without sacrificing the original processability, thus realizing upcycling of polyesters.