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 (d to a) 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 8C, 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.

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Polymer crystallization is crucial to the properties of polymers, which are determined by thermal dynamics and kinetics. The formation of polymorphism has turned out to be very important for applications of poly(L-lactic acid) (PLLA), because it has evolved into a competitive commodity material over the past decade. In the in situ wide-angle X-ray scattering study, it shows the growth of α-crystal of thermally treated PLLA at T c = 80 and 90 °C, which was considered only appropriate for the crystallization of its δ-form. A part of α-embryos and the locally ordered structures developed for the α-crystal may adjust their chain conformations from getting adapted to the crystallization of the δ-crystal, while the others continue to grow in their own α-form. Two crystal forms crystallize on different dynamic pathways, and the δ-form should be an independent crystal, better identified as the "δ-" instead of the previous "α′-". KEYWORDS: δand α-crystal, dynamic pathways, independent crystal, in situ wide-angle X-ray scattering, polymorphism, poly(L-lactic acid)

The topology of macromolecules is an important concept of polymer science, and cyclic polymers are the simplest model for studying the effects of topology. To explore the topological effects on crystallization behaviors of cyclic poly­(ε-caprolactone) (PCL), the cyclic and linear PCLs with anthracene groups in the main chain were used to investigate the crystallization behavior as PCL necklace with pendant and PCL asalato. Polarizing optical microscopy showed spherulite growth, and in situ wide-angle X-ray scattering or differential scanning calorimetry characterized crystallization behaviors indicated that the PCL necklace with pendants crystallize more slowly than the PCL asalato with similar molecular weight. The zero spherulite growth temperature T zg of the PCL pendants and the asalatos is the same. It is proposed that the slower crystallization rate of the cyclic PCLs is based on the slower chain conformation adjustments to form mesophases prior to crystal formation during crystallization, and controlled by the slower chain diffusion resulting from the presence of the large anthracene groups in the molecular chains.