Melting behavior of the oriented β-phase polypropylene texture crystallized by the temperature slope method

Isotactic polypropylene consisting of uniaxially oriented beta-phase lamellae was crystallized in a temperature gradient. The beta --> alpha transition was investigated by simultaneous measurements with differential scanning calorimetry (DSC) and X-ray diffraction using synchrotron radiation (SR). To compare the transition mechanism, the beta-phase sample was deformed by rolling it along the direction of the crystallization. During rolling, the beta-crystal is deformed by interlamellar and interchain slip, which induces c-axis-oriented molecules along the rolling direction. The melting behavior is changed by the rolling deformation. For the as-grown beta-crystal, the DSC thermogram has three peaks: the beta-melting endotherm at 150 degrees C, an exotherm by recrystallization into the alpha-form, and the endotherm at 167 degrees C caused by melting of the recrystallized alpha-form. After the rolling deformation, the beta-endotherm is extinguished by the successive exotherm. Simultaneous X-ray measurements reveal that the beta --> alpha transition is shifted to a lower temperature and that the recrystallized alpha-form has a c-axis-orientation caused by the rolling deformation. In the process of the beta --> alpha transition, higher-order lamellar structure is developed earlier than formation of the crystalline structure. In this study, the heating phenomena, such as the beta --> alpha transition and thickening of the beta- and alpha-lamellae, are consistently explained by a mechanism involving melting and subsequent recrystallization.