On the lamellar morphology of melt-crystallized isotactic polystyrene

Abstract

The lamellar morphology of melt-crystallized isotactic polystyrene has been investigated by extending the technique of permanganic etching for electron microscopy. This paper reports on objects grown at 220°C which are comparatively uncomplicated, being aggregates of hexagonal lamellae, with smooth facets and distinguishable internal sectors, organized into axialites. To a first approximation only, hexagonal lamellae splay apart about a common diagonal presenting three characteristic projections in orthogonal directions. These are hexagonal, sheaflike and an array of approximately parallel lamellae. In reality the term axialite is an oversimplification. Splaying is not restricted to a single axis but occurs in three dimensions making these objects incipient spherulites. Polystyrene spherulites grown as low as 180°C the temperature of maximum growth rate are also constructed on the same principles as are spherulites of polyethylene and isotactic polypropylene. These are that a framework is established by individual dominant lamellae which branch and splay apart leaving interstices to be filled by later-crystallizing subsidiary lamellae. There is evidence that subsidiary lamellae contain shorter molecules on average than do dominant lamellae. The dominant/subsidiary construction is not what had been assumed in the Keith and Padden theory of spherulitic growth nor is there evidence of cellulation or local diffusion influencing development even though crystal sizes exceed the Keith and Padden parameter · by two orders of magnitude. We conclude that polymer spherulites can form independently of the mechanism proposed by Keith and Padden 14. It is suggested that the cause of lamellae splaying apart is a mutual repulsion due to uncrystallized portions of molecules between dominant lamellae.