On the phenomenology of chain-extended crystallization in polyethylene

Abstract

The melting and crystallization of polyethylene have been examined in the pressure range 0 to 6 kbar. Volumetric and thermal measurements show that fractionated polymer melts and crystallizes in two clearly resolved stages above 4 kbar. It is suggested that these are due to the formation of a new, high pressure phase of polyethylene with a triple point at ∼ 3 kbar. This phase is intermediate in specific volume and entropy between orthorhombic solid and melt, but closer to the latter, and is either liquid-crystalline or solid. It is shown further that the formation of chain-extended polyethylene correlates with the occurrence of two exothermic processes (unresolved below 4 kbar) during crystallization. This leads to the hypothesis that whereas chain folded crystallization occurs by direct transformation from melt to orthorhombic solid, chain-extended polyethylene results when the intermediate phase intervenes in crystallization. This hypothesis appears capable of accounting for most of the known facts and provides a framework within which a detailed theory of chain-extended crystallization of polyethylene can be formulated.