MOLECULAR-ORGANIZATION IN THE PSEUDO-HEXAGONAL CRYSTALLINE PHASE OF ETHYLENE-PROPYLENE COPOLYMERS

A detailed X-ray diffraction pattern of an oriented sample of the ethy lene-propylene copolymer (75 mol % of ethylene) in the pseudo-hexagona l form is presented. The comparison between the experimental diffracti on intensities with the calculated intensities for ordered and disorde red pseudohexagonal chain aggregates has allowed us to clarify some fe atures of the structure of the pseudo-hexagonal form of the ethylene-p ropylene 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 local ly to relative shifts of neighboring chains nearer to those observed i n the monoclinic and in the more common orthorhombic form of polyethyl ene; (iii) the ratio between the integrated intensities of the main pe aks on the first layer line and on the equator can be accounted for by conformational disorder, implying a waviness of the nearly trans-plan ar chains and departures from strictly 180 degrees of internal rotatio n angles in the backbone chains; (iv) some further partial disorder (i ntermolecular translational along c and intermolecular rotational arou nd c) should be introduced to account for the broadness of the nonequa torial peaks. A comparison between the presently proposed structure fo r the pseudo-hexagonal form of the ethylene-propylene copolymer with o ther disordered structures of homopolymers and copolymers with a hexag onal arrangement of the chain axes is also presented.