On the orthorhombic to hexagonal phase transformation in polyethylene single crystals

The orthorhombic to hexagonal transformation in polyethylene single crystals was examined at room temperature and 110°C using electron diffraction. The transformation involves an anisotropic lattice expansion and the formation of a projected 6‐fold symmetric atomic potential on a plane normal to the chain axis. The continuous transformation to the hexagonal phase at low temperatures was shown to occur primarily by crosslinks formed between chains during irradiation. At high temperatures, following nucleation of the hexagonal phase by an initial electron dose, further growth of the hexagonal phase without irradiation is believed to occur by the introduction of chain ends (which provides for easy chain rotation) into the crystal interior during annealing.