A high-pressure phase of polyethylene and chain-extended growth

Optical and x-ray observations of polyethylene have been made at high pressures and temperatures using a gasketed diamond-anvil cell. The experiments confirm the existence of the high-pressure phase previously postulated by Bassett and Turner. The new phase is hexagonal, with orthohexagonal lattice parameters of a = 8.46 Å and b = 4.88 Å. Comparison with the previously measured volume change indicates that there is a decrease in the c dimension to 2.45 Å per ethylene unit in transforming from orthorhombic to hexagonal structures. The likely implication is that the molecules in the hexagonal phase do not have an all-trans conformation. Chain-extended growth is the result of crystallization from the melt into the hexagonal phase, whereas chain-folded growth is the familiar process of melt crystallization.into the orthorhombic phase. Chain-extended lamellae are observed to grow outwards behind a growing edge with a permanent narrowed profile, showing that the lamellar thickness is determined in a region extending several microns behind the growth front.