Polymorphism of racemic poly(L-lactide)/poly(D-lactide) blend: effect of melt and cold crystallization.

The crystallization and melting behaviors and crystalline structure of melt and cold crystallized poly(L-lactide)/poly(D-lactide) (PLLA/PDLA) blend were investigated by differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD), respectively. The isothermal crystallization kinetics during the melt and cold crystallization process were analyzed using the Avrami equation. The overall crystallization rate constant (k) of cold crystallization is much higher than that of melt crystallization. Moreover, k as a function of crystallization temperature shows different trends in melt and cold crystallization, indicating different crystallization mechanisms in the melt and cold crystallization. The polymorphic crystallization of homocrystallites (the transition crystallization temperature from δ to α form) is not altered by either the equimolar blending of PLLA and PDLA or the type of crystallization procedures, while the crystallization window for exclusive stereocomplex crystallites is widened from 170 °C for melt crystallization to 170-200 °C for cold crystallization. The stereocomplex crystallites are hard to form in both melt and cold crystallization at crystallization temperatures of 90 and 100 °C, and the crystallinity of stereocomplex crystallites for cold crystallization is higher than that of melt crystallization at temperatures above 110 °C. Especially, a pure and significantly higher crystallinity of stereocomplex crystallites can be achieved at 170-200 °C by cold crystallization. The results provide a huge possibility to control stereocomplex crystallization to enlarge its applications.