Origin of Extended-chain Lamellae in Polyethylene

Abstract

THE texture of crystalline polymers is typically one of thin (100 Å thick) lamellae within which the long (10³–10⁵ Å) molecules lie transversely in chain-folded configurations. In certain circumstances, however, much thicker lamellae, often as thick as the molecular length, have been observed in polyethylene^1–3. An example is shown in .Fig. 1. Because a variety of evidence indicates little or no chain-folding in these layers (that is, the molecules, which lie across them, are essentially in extended configuration) they have become known as extended-chain lamellae (ECL) in contrast to the more familiar folded -chain lamellae (FCL). The circumstances in which ECL form are: (i) growth at low supercoolings for long times¹ and (ii) by recrystallization under high pressure^2,3. The layers themselves vary in thickness, being most regular in slowly crystallized samples¹. In pressure crystallized material, thicknesses varying by a factor of two for the same lamella, and ten for different ones, have been observed³. ECL are also mechanically very different from FCL, because the toughening effect of molecules crystallizing through several lamellae must then be small. In fact ECL are no longer plastic but waxy as befits their paraffinoid structure. It is, however, the growth of ECL that has provided a long standing problem in polymer physics.