Synergistic Effects of Polyethylene Glycol and Cellulose Nanofibers on the Isothermal and Non-isothermal Crystallization Behaviors of Polylactide

To significantly improve the crystallinity and crystallization rate of polylactide (PLA), plasticizer (polyethylene glycol, PEG) and nucleating agent (cellulose nanofibers, CNFs) were melt-blended with PLA to prepare PLA/PEG/CNF nanocomposites. The effects of PEG and/or CNFs and cooling rate on the crystallization kinetics of PLA were investigated by HS-POM, DSC, and WAXD. The non-isothermal crystallization kinetics of modified PLA samples were evaluated by the Jeziorny’s, Ozawa’s, and Mo’s models, while their non-isothermal crystallization activation energies were determined by Friedman’s method. The polarized optical micrographs showed that CNFs served as effective nucleating agents, increasing the nucleation density of PLA spherulites, but reducing their spherulite sizes; PEG improved the mobility of PLA chains and accelerated the growth rate of PLA spherulites, thus leading to larger spherulite sizes. The non-isothermal crystallization kinetics revealed that the crystallization temperature (T_c), crystallinity (X_C), and crystallization half-time (t_1/2) of all PLA-based samples decreased with increasing cooling rate. At the same cooling rate, the incorporation of 15 wt% PEG or 3 wt% CNFs increased T_c and X_C but decreased t_1/2 of PLA by enhancing spherulite growth rate and providing more crystal nuclei, respectively. Moreover, SEM micrographs showed that the addition of PEG improved the dispersion of CNFs within the PLA matrix, and the synergistic effect of PEG and CNFs more significantly increased T_c and X_C, but reduced t_1/2. The above results demonstrated that the combination of PEG and CNFs significantly enhanced the crystallization performance of PLA, providing insights for the design of high-performance PLA-based materials.