Crystallization behavior of asymmetric PLLA/PDLA blends.

The effects of the addition of poly(D-lactide) (PDLA) on the crystallization behavior of poly(L-lactide)(PLLA) were investigated by means of differential scanning calorimetry (DSC) and temperature-dependent X-ray diffraction(XRD). When the blends were cooled from different temperatures (250, 240, and 190 °C) at the rate of cooling of 5 °C/min, stereocomplex (sc) crystallites could stay at diverse states. Accordingly, the stereocomplexes acted as a nucleation agent exerting distinct effects on PLLA crystallization. The speculated mechanisms of the stereocomplex formation and the effectiveness as a nucleating agent are schematically described. Moreover, temperature-dependent XRD was carried out to further investigate the melt-crystallization behavior of PLLA/PDLA blends in real time. Temperature-dependent XRD results indicated that even at 240 °C the stereocomplex crystallites in all blend samples existed clearly, which could not be detected by DSC. These XRD results further suggest that the onset T(c) values for the PLLA α-form crystals formation were 160, 120, 140, and 160 °C, respectively, for neat PLLA, PLLA/PDLA 95/5, 90/10, and 80/20 as well as 70/30 samples.