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

The crystal growth rate of the alpha(1), alpha(2) and beta and 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 a, form showed a bell-shaped Curve with the maximum growth rate (G(max)) at 70 degrees C. The crystal growth rate of the form induced by calcium pimelate also showed a bell-shaped curve with G(max) at 79 degrees C. Two crossover points on the growth rate curve from the at form to the beta form or vice versa were observed at 90 degrees C and 133 degrees 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 a2 form for thin films crystallized at 140 degrees C. The alpha(2) fraction became detectable in X-ray diffraction pattern for temperatures above 110 degrees C. The alpha(2) fraction increased with crystallization temperature and saturated to 100% at about 140 degrees C. In the temperature region from 110 degrees C to 140 degrees 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 alpha(1) and alpha(2) forms. Above 140 degrees C, the growth rate variations disappeared, since the fraction of the a, reached to 100%. The extrapolated G(max) of the alpha(2) showed at 72 degrees C.