Effect of CO2 on the crystallization of poly(lactic acid) homo-crystallites via influencing the crystal structure of stereocomplex crystallites

Abstract

The crystallization of poly(lactic acid) (PLA) homo-crystallites (HCs) and stereocomplex crystallites (SCs) is quite complicated for the inducing and competing effects of SCs on HCs. Furthermore, carbon dioxide (CO₂) would simultaneously influence the H-bond interactions of SCs, and molecular chain mobility and supercooling of SCs and HCs, which might further affect the crystallization process of SC-induced HCs and the competing crystallization of homogeneous HCs and SCs. This manuscript revealed the effect of CO₂ on the crystallization of HCs and SCs via influencing the crystallization kinetics and spherulite morphology for the first time. The results showed that CO₂ accelerated the crystallization of SCs due to the increased chain mobility and restrained the crystallization of HCs because of the decreased supercooling. HCs could only form in the crystalline assembly of SCs under CO₂ while in air, they could be generated at the growth front of SCs. Due to the increased chain diffusion, the development of branches of SCs was more adequate under CO₂, resulting in less space on the surface of SCs for HC crystallization. Therefore, under CO₂, the heterogeneous HC crystallization was shown as “bundles” and “dendritic” crystals, which was imperfect. Homogeneous and heterogeneous HCs exhibited different pressure dependencies on crystallization behaviors.