Stereocomplex crystallization of equimolar Poly(L-lactic acid)/Poly(D-lactic acid) blends from melt with lowered chain entanglements

Abstract

Understanding polymer crystallization at the molecular level is one of the key open questions of polymer physics. Here, high molecular weight equimolar poly(L-lactic acid)/poly(D-lactic acid) (PLLA/PDLA) blends with lowered chain entanglements are prepared by the freeze-drying method. The role of chain entanglements in stereocomplex (SC) crystallization is explored. A significant increase in the crystallization kinetics of SC is observed with decreasing the entanglement density, and thus highly crystallized SC is obtained in the PLLA/PDLA blends by a process of non-isothermal crystallization. With reducing the chain entanglements of PLLA/PDLA blends, an intriguing phenomenon that SC crystallization stops at a high temperature around 200 °C and starts again at ∼160 °C on continuous cooling is observed, verifying the new concept of “poisoning by purity”. Further studies reveal that the PLLA/PDLA blends with lowered chain entanglements are more likely to form regularly folded SC lamellae, which nucleate and grow at higher temperatures. Meanwhile, the reduced chain entanglements also reduce the activation energy for chain diffusion. These two effects may be the main reason for the significantly accelerated crystallization kinetics of SC. This study provides a new idea for producing highly crystallized SC in PLLA products to improve their thermal resistance and other properties.