Lamellar morphologies in melt-crystallized polyethylene

Abstract

The lamellar morphologies of linear polyethylene crystallized from the melt in vacuo have been surveyed. It is shown that spherulites form by the outward growth of dominant lamellae along b, with low angle branching, with the intervening spaces subsequently filled by subsidiary lamellae. There are three principal habits of dominant lamellae: ridged sheets of alternating {201} facets, planar {201} sheets and S- or C-shaped sheets which can be derived from {201} sheets by suitable shear. These occur in increasing order of growth rate, caused either by increasing molecular mass or decreasing crystallization temperature. The most widespread morphology is one of S-shaped dominant lamellae with narrow {201} subsidiary platelets; these are found, e.g., in banded spherulites. It can be shown, at least in certain circumstances, that longer molecules tend to form the dominant lamellae and shorter ones the subsidiary. Even in quenched samples the lamellar texture is not homogeneous. Here and elsewhere it has a lateral scale which can be measured by the widths of dominant lamellae, but this varies much less than the Keith and Padden parameter δ to which it has been thought to be related.