Morphological study of chain-extended growth in polyethylene: 1. Crystallization

Polyethylene crystallizes from the melt at pressures of a few kbar as lamellar crystals whose thickness extends over at least two orders of magnitude from a few hundred of Å to μm. The average value in bulk unconstrained polymer in these experiments is ∼4000 A ̊ but this value progressively decreases to ∼200 A ̊ as thinner and thinner specimens are used. Morphological evidence shows no discontinuity of physical texture throughout this range but reveals, by the occurrence of parallel changes in the thinnest films and the bulk polymer, that the entire spectrum of lamellar thicknesses is associated with crystallization into the hexagonal phase. This implies on the one hand, that lamellar thickness is not a suitable measure to distinguish the so-called chain-extended and chain-folded modes of crystallization of polyethylene and, on the other, that the very large effect of constraints needs to be included in theories of polymeric crystallization.