On hexagonal phases of paraffins and polyethylenes

Polyethylene, when irradiated to doses exceeding ca500 Mrad forms a hexagonal structure before melting at atmospheric pressure. The phase diagrams of irradiated polyethylenes have been studied as a function of pressure to 5 kbar, and used as a basis for comparing the radiation-induced hexagonal phase with the hexagonal and pseudo-hexagonal rotator phases of n-paraffins and with the disordered hexagonal (anabaric) structure formed by linear polyethylene at high pressure. At atmospheric pressure, the radiation-induced hexagonal phase is intermediate in character between the other two. It is, nevertheless, a high entropy phase whose temperature interval of existence widens, and whose nature moves towards that of the anabaric phase, with increasing pressure.