Ethylene-1-octene copolymers at elevated pressure - temperature. 1. Order - disorder transition

Homogeneous ethylene - 1-octene copolymers of 5.0 and 8.0 mol% 1-octene content lose their crystalline order and become seemingly 'amorphous-like' during isothermal pressure increase to 370 MPa at room temperature, as investigated by in-situ wide-angle X-ray scattering. The disordering in the crystalline lattice of the orthorhombic unit cell is likely to be caused either by destruction of the initial (orthorhombic) crystalline structure or by breaking up of crystallites in very small entities, which escape detection, by WAXD. On cooling from 190°C at high pressure (approx. 370 MPa), crystallization into the orthorhombic phase starts at 163°C. On further cooling a sudden densification of the orthorhombic unit cell takes place at approximately 70°C, and simultaneously a new reflection appears which can be assigned to the hexagonal phase. This phenomenon is indicative for the occurrence of a pressure-induced sequence length separation, probably leading to two crystalline populations: orthorhombic crystals, relatively large and well developed, and hexagonal structures, possibly built up of small ethylene sequences containing hexyl branches. Further, experimental observations are that both 'amorphization' and the pressure-induced sequence length separation strongly depend on the pressure, being absent at a pressure of 195 MPa. The present high-pressure X-ray results correspond well with comparable DSC results obtained recently.