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 γ phase in isotactic polypropylene is investigated. Detailed analysis of wide angle X-ray diffraction shows that some γ 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 γ phase crystallization, even in high molecular weight homopolymers. In any of the materials studied the amount of γ phase decreases with increasing cooling rate, going to zero at a cooling rate of about 10°C s−1. We interpret the observations in terms of the kinetics of growth and the phase stability, which shows that: (a) there is a region in which the γ phase has a lower free energy than the α phase; and (b) this region reaches lower temperatures with increasing comonomer content.