On the Role of the Hexagonal Phase in the Crystallization of Polyethylene

Abstract

Polyethylene forms a two-dimensional hexagonal phase, stable at ≥ ∼ 3 GPadepending on molecular length, which in recent years has been claimed to intervene in crystallizationprior to the formation of the usual orthorhombic phase even at atmospheric pressure. This claim isevaluated and shown to be without substance. There is very little evidence that the theoretical possibilityof thin lamellae being more stable in the hexagonal phase than the orthorhombic at atmospheric pressure,if the former has sufficiently low fold surface free energy, does occur in practice. But the existenceof single crystals of the orthorhombic phase unambiguously shows that they did not have a hexagonalprecursor; that would have made them threefold twins. The overwhelming mass of evidence is that orthorhombicand hexagonal phases crystallize independently in accordance with the phase diagram and kinetic competitionduring growth, as has been understood since the hexagonal phase was discovered.