Influence of the order of polymer melt on the crystallization behavior: I.Double melting endotherms of isotactic polypropylene

The influence of the order of polymer melt on the subsequent crystallizatio n and melting has been carefully studied. The experimental data show that t he order of isotactic polypropylene (iPP) melt decreases with increases in the fusion temperature. For an IPP sample isothermally crystallized at 130 degreesC for half an hour, the degree of order of melt is higher when the f usion temperature is lower than about 170.5 degreesC, hence the lamellae fo rmed in a rapid cooling process are perfect. If the fusion temperature is n ot higher than 167 degreesC, some thicker lamellae can exist in the melt. T he melting of these unmelted lamellae and those lamellae recrystallized in the cooling process result in double endotherms. On the other hand, when th e fusion temperature is higher than 170.5 degreesC, the order of the iPP me lt decreases greatly; thus, the lamellae formed in the following cooling pr ocess are imperfect. At a lower heating rate, the recrystallization or reor ganization of these imperfect lamellae also leads to double melting endothe rms.