Nature of disorder in the high-pressure phase of polyethylene

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Abstract
A structural model of the high-pressure phase of polyethylene is presented, and the nature of inter-and intramolecular disorders in this phase are clarified quantitatively on the basis of x-ray diffraction patterns taken under high pressure at high temperature. The diffraction pattern is unique, one in which Bragg reflections are seen only on the equator and distinct diffuse scattering is seen on the nonequatorial lines. Calculation of the intensity of the diffraction is done from a crystalline system which contains many local defects similar to but essentially different from the model proposed by McMahon and others. The method of calculation for a one-dimensional disordered crystal developed by Kakinoki and Komura is used for calculating the intensity of the diffraction from disordered chains, and the values of the pertinent parameters that characterize the structure of the phase are determined. For example, the average length of the local defect along the chain is about 4.4 Å, and 33% of the whole system is in a disordered conformational state. In spite of being in such a conformationally disordered state, the molecules in the high-pressure phase constitute a two-dimensional lattice system with small disorders in the lateral array of the molecules.