Abstract
The structure, phase diagram and compression behavior of polyethylene are studied by X-ray diffraction up to 11 kbar and 300°C. Distinct diffuse scattering on the non-equatorial line is first observed for the hexagonal phase of oriented polyethylene, revealing conformational disorder of the molecule. The phase diagram determined shows that the hexagonal phase exists in a temperature range which increases with increasing pressure up to 11 kbar. It is found that the compressibility of the hexagonal phase takes higher values on the lower pressure side, and approaches the value for the orthorhombic phase with increasing pressure. A simple relation between the lattice parameters of the orthorhombic phase, ao/bo=1.57-1.58, is found to hold at the transition temperature to the hexagonal phase irrespective of pressure.