Polymorphism of a high-molecular-weight racemic poly(l-lactide)/poly(d-lactide) blend: effect of melt blending with poly(methyl methacrylate)

Abstract

The effect of melt blending with poly(methyl methacrylate) (PMMA), a miscible polymer with polylactide (PLA), on the crystallization and melting behaviors and crystalline structure of high-molecular-weight poly(L-lactide)/poly(D-lactide) (PLLA/PDLA) blend was investigated by differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD). Due to the decreased segmental mobility of PLA chains and the dilution effect of the introduced PMMA, the crystallization of PLA is hindered. The polymorphic crystallization of the PLLA/PDLA blend can be tailored by the amount of PMMA and dynamic and isothermal crystallization conditions. The formation of stereocomplex (sc) crystallites is favored for the blends containing PMMA lower than 50 wt% during cold crystallization. During isothermal melt crystallization, the formation of sc crystallites is always accompanied by the generation of homochiral (hc) crystallites at a crystallization temperature (T_c) ranging from 110 to 160 °C for neat PLLA/PDLA blend, while the crystallization window for exclusive sc crystallites is widened to 160 °C, 140–160 °C, and 110–150 °C for PLLA/PDLA blends containing 10, 25, and 50 wt% PMMA, respectively. Especially, the formation of sc crystallites can be enhanced over the entire T_c range of 110–160 °C when the PMMA content is lower than 25 wt%. The enhanced sc crystallite formation can be ascribed to the reduced crystallization competition resulting from highly restricted hc crystallization. With the PMMA content increasing to 75 wt%, the inhibition effect on the crystallization of PLA suppresses either hc or sc crystallization.