Remarkable Melting Behavior of PLA Stereocomplex in Linear PLLA/PDLA Blends

The linear PLLA/PDLA blends were prepared by solution mixing method, and the melting behavior and structure evolution of neat PLLA and PLLA/PDLA specimens were investigated in this study. Results indicated that PLA stereocomplex crystallites (sc) preferentially formed in all blends, and the crystal structure of PLA sc and homochiral crystallites (hc) did not vary as molecular weights. The melting temperature of PLA neat specimens (Thm) increased monotonously with molecular weights. However, significantly different from the neat samples, the melting temperature of PLA sc (Tsc) increased at first, then decreased as the molecular weight of polymers increased from 4 to 100 kg/mol. When the molecular weights of PLLA and PDLA ranged from 23 to 50 kg/mol, multimelting behaviors observed at 230 °C in the blends. After annealing at a fix temperature (Tsc – 10 °C), the highest Tsc was observed at 249.9 °C in L32/D31 specimen, which was the highest report value until now. The WAXD and SAXS results attested that not crystal structure, but the variation of the thickness of lamellar crystal was the exterior reason, and the higher optical purity of PLLA and PDLA would be the inherent cause which resulted in the superior thermal properties. This investigation provides more potential for the application of PLA sc materials at higher temperature environments.