CRYSTAL-STRUCTURE AND MORPHOLOGY OF MELT-CRYSTALLIZED POLY(PROPYLENE-STAT-ETHYLENE)FRACTIONS

Crystallinity, crystal structure and lamellar thickness in melt-crysta llized samples of poly(propylene-stat-ethylene) fractions with 2.7-11. 0 mol% ethylene comonomer and of approximately constant tacticity were assessed by wide- and small-angle X-ray scattering, differential scan ning calorimetry and infra-red spectroscopy. Most of the samples were crystallized under isothermal conditions at 373 K. In comparison with an isotactic homopolymer of polypropylene, the copolymers showed lower crystallinity, melting enthalpy and average length of 3/1 helices, a slightly larger unit cell, a longer long period and an invariant lamel lar thickness. The X-ray crystallinity of the copolymers remained appr oximately constant with increasing ethylene content, whereas the gamma -crystallinity increased and the heat of fusion decreased moderately. It is suggested that the ethylene units are partially included in the crystals, and that this causes the invariance in crystallinity and cry stal thickness. The observed gradual decrease in average 3/1 helix len gth with increasing ethylene content as assessed by infra-red spectros copy is in accordance with this suggestion. Copyright (C) 1996 Elsevie r Science Ltd.