Competition between α and γ 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. Detailed 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.%). Differential scanning calorimetry shows a double melting peak. Nucleating agents of different types are found to enhance gamma phase crystallization, even in high molecular weight homopolymers. In any of the materials studied the amount of gamma phase decreases with increasing cooling rate, going to zero at a cooling rate of about 10 degreesC s(-1). We interpret thr observations in terms of the kinetics of growth and the phase stability, which shows that: (a) there is a region in which the gamma phase has a lower fret: energy than the alpha phase; and (b) this region reaches lower temperatures with increasing comonomer content. (C) 2000 Elsevier Science Ltd. All rights reserved.