Solution crystallization of polyethylene at high temperatures

Abstract

The three-dimensional morphologies and melting behaviour of crystals grown from various solvents at temperatures between 93.0 and 112.8° C have been studied. These crystals cover a range of lateral habits, with axial ratios from 1.60 to 3.00 and varying degrees of prism face curvature. Electron diffraction revealed two distinct types of crystal, usually present in roughly equal proportions. Sectorization could be distinguished in all preparations, although the sectors became less well defined as the axial ratio increased with concomitant development of curving prism faces. This was confirmed by both paraffin decoration and melting behaviour. Preferential sector melting was seen in only one sample type. Crystals with the larger axial ratios showed more uniform melting behaviour and also a reduced tendency to anneal on heating. Possible explanations for the observed trends are discussed, and in particular the loss of directionality of the fold depositions with development of prism face curvature is commented on with relevance to the underlying mechanism of chain folding.