Calorimetric analysis of the multiple melting behavior of melt-crystallized poly(L-lactic acid) with a low optical purity

The polymorphous crystallization and multiple melting behavior of poly(l-lactic acid) (PLLA) with an optical purity of 92 % were investigated after isothermally crystallized from the melt state by wide-angle X-ray diffraction and differential scanning calorimetry. Owing to the low optical purity, it was found that the disordered (alpha') and ordered (alpha) crystalline phases of PLLA were formed in the samples crystallized at lower (< 95 A degrees C) and higher (a parts per thousand yen95 A degrees C) temperatures, respectively. The melting behavior of PLLA is different in three regions of crystallization temperature (T (c)) divided into Region I (T (c) < 95 A degrees C), Region II (95 A degrees C a parts per thousand currency sign T (c) < 120 A degrees C), and Region III (T (c) a parts per thousand yen 120 A degrees C). In Region I, an exothermic peak was observed between the low-temperature and high-temperature endothermic peaks, which results from the solid-solid phase transition of alpha'-form crystal to alpha one. In Region II, the double-melting peaks can be mainly ascribed to the melting-recrystallization-remelting of less stable alpha crystals. In Region III, the single endotherm shows that the alpha crystals formed at higher temperatures are stable enough and melt directly without the recrystallization process during heating.