On the multiple melting behavior of bisphenol-A polycarbonate

Differential scanning calorimetry studies of the heating rate dependence of the multiple melting behavior of semicrystalline bisphenol A-polycarbonate (BAPC) are presented for different molar masses. In all cases, heating traces exhibit, in addition to thr high temperature endothermic transition, a low endotherm located slightly above the crystallization temperature. After proper correction of the thermal lag effects, the high endotherm melting temperatures of the higher molar mass BAPC-19K and BAPC-28K samples are found to be independent of heating rate whether or not partial melting was carried out prior to recording the heating trace. These results demonstrate that the double melting behavior observed for high molar mass BAPC cannot be explained by a melting-recrystallization-remelting mechanism. In contrast, heating traces of the lower molar mass BAPC-4K sample exhibits two melting transitions within the high temperature endothermic region, which change both in magnitude and location with scanning rate, suggesting that melting-recrystallization-remelting can occur when chain mobility is sufficient to allow recrystallization. However, in all cases, the low and high endothermic regions are associated with the melting of two distinct populations of crystals, which have different thermal stability and are both present in the as-crystallized material. Crystallization studies after partial melting indicates that the low endotherm is;associated with secondary crystals. The observed linear dependence of the melting temperature of secondary crystals with the square root of heating rate is consistent with superheating of secondary crystals. The origin of the superheating behavior is discussed in the context of conformational constraints in the residual amorphous fraction and the effect of crystallization time and molar mass on the low endotherm location. (C) 2000 Elsevier Science Ltd. All rights reserved.