Molecular organization in the pseudo-hexagonal crystalline phase of ethylene-propylene copolymers

A detailed X-ray diffraction pattern of an oriented sample of the ethylene-propylene copolymer (75 mol % of ethylene) in the pseudo-hexagonal form is presented. The comparison between the experimental diffraction intensities with the calculated intensities for ordered and disordered pseudohexagonal chain aggregates has allowed us to clarify some features of the structure of the pseudo-hexagonal form of the ethylene-propylene copolymers: (i) the methyl groups of the propylene monomeric units are included in the crystalline phase; (ii) the pseudo-hexagonal packing of the nearly trans-planar copolymer chains corresponds locally to relative shifts of neighboring chains nearer to those observed in the monoclinic and in the more common orthorhombic form of polyethylene; (iii) the ratio between the integrated intensities of the main peaks on the first layer line and on the equator can be accounted for by conformational disorder, implying a waviness of the nearly trans-planar chains and departures from strictly 180 degrees of internal rotation angles in the backbone chains; (iv) some further partial disorder (intermolecular translational along c and intermolecular rotational around c) should be introduced to account for the broadness of the nonequatorial peaks. A comparison between the presently proposed structure for the pseudo-hexagonal form of the ethylene-propylene copolymer with other disordered structures of homopolymers and copolymers with a hexagonal arrangement of the chain axes is also presented.