Crystalline structure of propylene-ethylene copolymer fractions

Crystalline structure were studied on both block and random propylene-ethylene copolymer fractions, which were obtained by temperature rising elution fractionation. The peak characteristic of α-polypropylene (PP) was observed for all fractions, except the fraction eluted at room temperature. A characteristic peak of polyethylene crystals [i.e., (200) planes] was observed in some fractions, indicating the existence of long ethylene sequence in these fractions. This is in accordance with the analysis from Fourier transform infrared spectroscopy and 13C-NMR. The γ-form crystal of PP was observed in these copolymer fractions at atmospheric pressure. It is suggested that the insertion of comonomer into the isotactic PP chain makes the crystallizable sequences sufficiently short and produces the γ-form crystal.