Temperature dependence of crystalline transition of highly-oriented poly(l-lactide)/poly(d-lactide) blend: In-situ synchrotron X-ray scattering study

Formation of stereocomplex crystals (sc-crystals) of poly(l-lactide)/poly(d-lactide) (PLLA/PDLA) blend has been recognized as a unique opportunity to dramatically improve the heat-resistant of poly(lactic acid) (PLA). In this study, we investigated the dynamic formation and transition of sc-crystals in PLLA/PDLA drawn film with a combination of in-situ synchrotron wide-angle X-ray scattering (WAXS), small-angle X-ray scattering (SAXS) and differential scanning calorimetry (DSC). The correlation between sc-crystals content and the competing formation of α crystals (stable phase of PLA) in homopolymers during continuous heating and cooling processes was also studied. It was found that at room temperature, the original PLLA/PDLA drawn film consisted of only α crystals, however, with temperature increasing, two populations of sc-crystals were formed at different temperatures from the oriented amorphous region and the molten α crystals in the highly-oriented sample, respectively. Furthermore, new types of sc-crystals and α crystals with the orientation perpendicular to the original sc-crystals were formed during subsequent cooling process. On the basis of the X-ray scattering and DSC data, a schematic model for crystallization and oriented variation concerning sc-crystals and α crystals was proposed.