Effects of the crystallization temperature on the crystal structure and its melting behavior of poly (l-lactic acid) (PLLA) have been investigated by means of wide-angle (WAXS) and small-angle (SAXS) X-ray scattering, optical microscopy, and differential scanning calorimetory (DSC). PLLA was found to crystallize as the α form when the crystallization temperature T c was higher than 120 °C, while significant change in lattice parameters was seen for T c's below 120 °C. The ratio of the a- and b-axis lengths begins to decrease with T c below 120 °C and is 31/2 below 90 °C, which suggests a new crystalline form with hexagonal packing, namely, the α‘ form. The possible reason for α‘ formation is discussed. High-temperature WAXS and SAXS measurements showed that α‘ crystal transforms into ordered a form during heating. The transition takes place at 150 °C without a decrease in scattering intensity and without heating rate dependence. The mechanism for the transition is discussed.

Effects of the addition of PDLA on the crystallization behavior of PLLA was investigated by means of differential scanning calorimetry, wide-angle X-ray diffraction, melt rheology, and polarized optical microscopy. Nonisothermal and isothermal crystallization behavior of PLLA including low (l-PDLA) and high molecular weight PDLA (h-PDLA) were studied. PLLA/PDLA asymmetric blends form stereocomplex (SC) crystal and stay unmelted at 200 °C in the PLLA melt. Nonisothermal crystallization measurement from 200 °C showed monotonous rise in the crystallization temperature for PLLA/h-PDLA blend, while peculiar concentration dependence was observed for PLLA/l-PDLA blends. The acceleration effect was more pronounced in PLLA/h-PDLA, although the crystallinity of SC was lower than PLLA/l-PDLA blends, which implies the importance of higher order structure of SC for the crystallization of PLLA. From isothermal crystallization kinetics measurements, the acceleration effect in PLLA/h-and l-PDLA blends was found to enhance the nucleation of crystallization but slightly interrupts the crystallization growth. The above results were reasonably explained by the model where SC crystallites are not isolated in PLLA melt but connected like a physical gel.