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 tra ces 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 t emperatures of the higher molar mass BAPC-19K and BAPC-28K samples are foun d to be independent of heating rate whether or not partial melting was carr ied out prior to recording the heating trace. These results demonstrate tha t the double melting behavior observed for high molar mass BAPC cannot be e xplained by a melting-recrystallization-remelting mechanism. In contrast, h eating traces of the lower molar mass BAPC-4K sample exhibits two melting t ransitions within the high temperature endothermic region, which change bot h in magnitude and location with scanning rate, suggesting that melting-rec rystallization-remelting can occur when chain mobility is sufficient to all ow recrystallization. However, in all cases, the low and high endothermic r egions are associated with the melting of two distinct populations of cryst als, which have different thermal stability and are both present in the as- crystallized material. Crystallization studies after partial melting indica tes that the low endotherm is;associated with secondary crystals. The obser ved linear dependence of the melting temperature of secondary crystals with the square root of heating rate is consistent with superheating of seconda ry crystals. The origin of the superheating behavior is discussed in the co ntext of conformational constraints in the residual amorphous fraction and the effect of crystallization time and molar mass on the low endotherm loca tion. (C) 2000 Elsevier Science Ltd. All rights reserved.