Effects of molecular weight on stereocomplex and crystallization of PLLA/PDLA blends

The chain dynamics affects the chain motions and the crystallization behaviors of polymers, and the crystallization ability of poly(lactic acid) (PLA) homopolymers can be substantially enhanced by incorporating stereocomplex-PLA (s-PLA). The effects of s-PLA molecular weight on the enhanced crystallization of the homopolymer were characterized and analyzed in this study. The crystallization behaviors, morphologies and structures of the homopolymer in PLA/s-PLA blends were investigated using DSC, POM, WAXD and FTIR during or after isothermal and non-isothermal crystallization. The blends compose homopolymer and the same content of the s-PLAs with three different molecular weights. The DSC results showed that the homopolymer in the blends containing the higher molecular weight of the s-PLA crystallizes at higher crystallization temperature than that containing the lower molecular weight of the s-PLA during the non-isothermal crystallization. The crystallization half-time of the homopolymer blending with the higher molecular weight s-PLA is shorter than that blending with the lower molecular weight of the s-PLA during the isothermal crystallization. POM images showed that the spherulite density of the homopolymer blending with the higher molecular weight of the s-PLA is significantly higher than that with the lower molecular weight of the s-PLA. The Mpemba effect was first experimentally observed in the melting crystallization of polymer blends containing PDLA and s-PLA. A possible explanation is proposed and discussed to elucidate the impact of s-PLA molecular weight on the crystallization behaviors of PLA homopolymer in the blends.