ON CIRCULAR CRYSTALS OF POLYETHYLENE

Individual lamellae of anabaric polyethylene have been studied after extraction from their matrix. Those formed as a minority component (below the triple point) at 0.3 GPa are circular discs with tapering edges and diameters of tens of micrometres. As recovered, they show an internal microstructure composed of three sets of lines at ∼60° to each other. These are a legacy of the hexagonal-to-orthorhombic transition and confirm the anabaric origin. The discovery of circular habits supersedes earlier observations of elongated crystals, then considered to have formed anabarically, although this attribution raised a problem in that they did not possess the symmetry of the high-pressure phase. In fact, these elongated lamellae are elliptic and closely resemble those formed from the melt at low supercoolings at atmospheric pressure. It is suggested that they would in practice have crystallized in the orthorhombic structure. The circular habit must make niches available at the highest growth temperatures and thereby raises problems for kinetic theories, which currently predict regime I growth (on planar facets) at low supercoolings. Whereas recent developments have given an explanation of elliptic habits in terms of differences between types of close-packed growth surfaces in the orthorhombic phase, this possibility does not exist for the two-dimensional hexagonal structure of the anabaric phase. The phenomenon seems rather to be a result of surface roughening. Following crystallization at 0.5 GPa, anabaric lamellae have only been extracted from blends. Those isolated from the interior of a 5% blend with branched polyethylene have a more feathery habit suggestive of incipient spherulitic growth but have still grown as discs of constant shape. It is, therefore, unlikely that segregation of impurities could have caused the perturbation from a circular habit. © 1990.