On the morphology of melt-crystallized polyethylene. III. Spherulitic organization

Abstract

The lamellar microstructure of a wide range of melt-crystallized polyethylenes has been examined in the context of spherulitic organization. Radial continuity is achieved through lamellar continuity. In the simplest cases ridged or planar sheets grow outwards along b, maintaining their identities over distances as long as tens of micrometres, subject to changes in relative dominance depending on whether or not they can continue to grow in the most favourable direction. The generalized 'fibres' are to be identified with dominant lamellae which lay down the framework of a spherulite's development. Banded spherulites have S-shaped dominant sheets which are multiply-connected allowing the average molecular orientation to spiral round the radius while individual lamellae are substantially untwisted. Space within spherulites is filled by branching, for which three mechanisms have been identified, namely the nucleation of new, misaligned lamellae, the incorporation of small-angle boundaries and spiral terraces associated with screw dislocations. Contrary to prediction, the width of dominant lamellae varies much less than the Keith & Padden parameter $\delta$, but remains within a factor of 3 of 3 $\mu m$. The scale of texture revealed by the segregation of 'impurities' does not appear to be determined solely by the scale of the diffusion field. It is suggested that the misfit energy between adjacent non-parallel 'fibrils', and geometrical factors, must also be taken into account.