Temperature Dependence of Crystal Growth Rate for α and β Forms of Isotactic Polypropylene

Abstract

The crystal growth rate of the α1, α2 and β forms of isotactic polypropylene (iPP) and their morphological changes were studied in a wide range of crystallization temperatures. The temperature dependence of the crystal growth rate of the α1 form showed a bell-shaped curve with the maximum growth rate (Gmax) at 70 °C. The crystal growth rate of the β form induced by calcium pimelate also showed a bell-shaped curve with Gmax at 79 °C. Two crossover points on the growth rate curve from the α1 form to the β form or vice versa were observed at 90 °C and 133 °C, respectively. Tear-drop shaped spherulites were observed at temperatures between the two crossover points. Electron diffraction patterns showed the high ordered structure of the α2 form for thin films crystallized at 140 °C. The α2 fraction became detectable in X-ray diffraction pattern for temperatures above 110 °C. The α2 fraction increased with crystallization temperature and saturated to 100% at about 140 °C. In the temperature region from 110 °C to 140 °C, several crystal growth rates were found at the same crystallization temperature. The growth rate variations could be associated with the co-crystallization of the α1 and α2 forms. Above 140 °C, the growth rate variations disappeared, since the fraction of the α2 reached to 100%. The extrapolated Gmax of the α2 showed at 72 °C.