Morphology of Solution-Grown Polypropylene Crystal Aggregates

Abstract

Polypropylene aggregates, consisting of interwoven arrays of single‐crystal lamellae, have been obtained by crystallization of samples of isotactic polypropylene from dilute solutions held at constant temperatures between 85° and 115°C. The solvent used was α‐chloronaphthalene with polymer concentrations ranging from 0.4% to 0.01% by weight. The resulting crystalline aggregates were examined by means of phase contrast and electron microscopy. Similar crystal aggregates have been studied by Khoury and found to be incipient spherulites. These aggregates tend to occur in the form of open weave structures, with characteristic cusps. Lath‐like lamellar crystals, with well‐defined faces, are found at the edges of these structures. These edge lamellae produce a single‐crystal electron‐diffraction pattern corresponding to the monoclinic structural form of isotactic polypropylene. On the basis of the experimental findings, it is suggested that folding occurs in monoclinic polypropylene only along a given set of parallel fold planes, viz. {010}. Preliminary results of a study of the fracture characteristics of the crystal lamellae tend to support the suggested hypothesis.