Competition between alpha and gamma phases in isotactic polypropylene: effects of ethylene content and nucleating agents at different cooling rates

The influence of ethylene content, nucleating agents and cooling rate upon the formation of gamma phase in isotactic polypropylene is investigated. De tailed analysis of wide angle X-ray diffraction shows that some gamma phase can appear even in copolymers of very low ethylene content (0.5 mol.%). Di fferential scanning calorimetry shows a double melting peak. Nucleating age nts of different types are found to enhance gamma phase crystallization, ev en in high molecular weight homopolymers. In any of the materials studied t he amount of gamma phase decreases with increasing cooling rate, going to z ero at a cooling rate of about 10 degreesC s(-1). We interpret thr observat ions in terms of the kinetics of growth and the phase stability, which show s that: (a) there is a region in which the gamma phase has a lower fret: en ergy than the alpha phase; and (b) this region reaches lower temperatures w ith increasing comonomer content. (C) 2000 Elsevier Science Ltd. All rights reserved.