High Pressure X-Ray Studies of Polymers. III. Phase Transformation of Polyethylene.

A systematic study was made on polyethylene at high pressures and elevated temperatures using X-ray diffraction. Polyethylene with an initial chain-folded morphology and the orthorhombic crystal structure did not transform to any other phase before melting at temperature and pressure. Crystallization, however, at high pressures producing extended-chain crystals gave rise to either a new crystal phase, probably hexagonal or the orthorhombic phase depending on the crystallization conditions. A reversible solid-state phase transformation was observed between the orthorhombic and hexagonal phases. None of the previous interpretations put forward to account for the high pressure D.T.A. and dilatometric studies of polyethylene, would appear to be appropriate in view of these findings. Author