Polymorphic Crystalline Structure and Crystal Morphology of Enantiomeric Poly(lactic acid) Blends Tailored by a Self-Assemblable Aryl Amide Nucleator

Abstract

Stereocomplex (SC) crystallization has been an effective method to improve the heat resistance of poly(lactic acid) (PLA). However, the preparation of SC-type PLA material is still a challenge because SC crystallization is much less prevailing than homocrystallization in the high-molecular-weight (HMW) poly(l-lactic acid)/poly(d-lactic acid) (PLLA/PDLA) racemic blends. In this study, we have successfully promoted SC formation and controlled crystal morphology of a HMW PLLA/PDLA blend by using a self-assemblable aryl amide nucleator, N,N′,N″-tricyclohexyl-1,3,5-benzenetricarboxylamide (BTCA). Crystallization kinetics, polymorphic crystalline structure, crystal morphology and superstructure of BTCA-nucleated PLLA/PDLA blends were investigated. During the nonisothermal melt crystallization and isothermal crystallization at different temperatures (80–170 °C), the crystallization rate of PLLA/PDLA blend is significantly promoted and the fraction of SCs is enhanced with incorporating small amount of BTCA. SCs are exclusively formed in the BTCA-nucleated PLLA/PDLA blends at a high crystallization temperature (e.g., 170 °C). Because of the diversified self-assembled structure of BTCA in polymer melts, SCs with short, long shish-kebab-like, and granular structures are attained with varying the BTCA concentration. BTCA-promoted SC formation of PLLA/PDLA blend is ascribed to the hydrogen bonding interactions between BTCA and PLLA, PDLA chains.