New insight into the mechanism of enhanced crystallization of PLA in PLLA/PDLA mixture

Poly(lactic acid) (PLA) is a bio-based and compostable polymer that has quickly developed into a competitive material, but the control of crystallinity is a bottleneck in extended utilization. The crystallization of PLA has been a rich topic because of the existence of two enantiomeric forms of poly(L-lactic acid) (PLLA) and poly(d-lactic acid) (PDLA) can form stereocomplex (SC) crystal with high melting point that can be used to control the crystallization behaviors. The SC crystal was regarded as an effective nucleating agent for promoting the crystallization rate and crystallinity of PLA. We investigated cold crystallization of PLLA/PDLA (1:1) mixture with in situ WAXS measurements and found that the homo-crystal of PLA formed earlier than the SC-crystal in the mixture within the measured temperature range, which is different from the melting crystallization. The final crystalline structures are in correspondence with the melting and cold crystallization temperature, and the transition of homo-PLA ( to ) is not altered by the crystallization procedure. The SC-crystal can be detected in both cold and melting crystallization of the mixture at the temperatures lower than 150 degrees C, which is conflict with the reported results. A new crystallization mechanism of the mixture was proposed to understand the crystallization behaviors in PLLA/PDLA mixtures. (C) 2017 Wiley Periodicals, Inc.