Crystallization kinetics and morphology of poly(propylene-stat-ethylene) fractions

The crystallization and melting behaviour and the morphology of fractions of poly(propylene-stat-ethylene) with 2.7-11.0 mol% ethylene were studied by differential scanning calorimetry, wide-angle X-ray scattering, polarized light microscopy and transmission electron microscopy, after etching with permanganic acid. The inclusion of ethylene co-repeating units in isotactic polypropylene (0-11.0mol% ethylene) caused approximately linear decreases in kinetic and equilibrium melting temperatures and in the glass transition temperature with increasing ethylene content. X-ray scattering showed that the content of the gamma form increased with increasing ethylene content, increasing crystallization temperature and decreasing cooling rate from the molten state. It was shown for one of the copolymers (8.7 mol% ethylene) that during heating approximately 50% of the gamma form was converted to the alpha form before the final melting of the sample. The rest of the gamma crystals melted without transformation to the alpha form. The multimodality of the crystal melting above the crystallization temperature in the polymers with a more uniform crystal structure was caused by recrystallization during heating, whereas polymers with appreciable contents of both alpha and gamma forms exhibited multimodal melting at all the heating rates adopted. The size of the low temperature melting peak as assessed at a heating rate of 40 K min(-1) was approximately proportional to the initial content of the gamma form. By comparison with the spherulitic structure of homopolymers, that of the copolymers was coarser with internal and peripheral pockets of molten material during spherulite growth. The crystal lamellae exhibited more curvature in the copolymer samples than in the homopolymer. Copyright (C) 1996 Elsevier Science Ltd.