Primary nucleation and spherulite growth rate in isotactic polypropylene-polystyrene blends

Abstract

The morphology, primary nucleation of spherulites and their growth rate in immiscible blends of isotactic polypropylene (iPP) and atactic polystyrene (aPS) were studied. It was found that PS inclusions are dispersed in an iPP matrix and their size depends on the mixing conditions. During crystallization of iPP aPS inclusions are engulfed by growing spherulites without any pushing or deformation before engulfing. The spherulite growth rate in the blends does not depend on either concentration of aPS in the blend or time of mixing. The spherulite number per iPP volume unit in the blend increases with increasing concentration of aPS and also with increasing mixing time. It was found that additional nuclei which appear in iPP/aPS blends are of two types: heterogeneous and self-seeded primary nuclei. It was demonstrated that the reasons for the increase in primary nucleation are the migration of impurities from aPS melt to iPP melt during mixing and the nucleation of a fraction of spherulites by the iPP-aPS interface. It is suggested that the driving force for the migration process is the interfacial free energy of an impurity with respect to the iPP melt and the analogous energy with respect to the aPS melt.