The role of transient hexagonal phase in crystallization of polymers

Abstract

Crystallization studies of polyethylene (PE) under hydrostatic pressure substantiate the earlier findings by Bassett and Turner [1, 2] that crystallization occurs initially in the hexagonal (h) phase, even within the thermodynamically stable orthorhombic (o) region. Present studies demonstrate that this is, in fact, a necessary condition for crystallization, at least in the temperature and pressure range in question; in fact, once a crystal transforms from h to o phase, further growth of crystal stops. Morphological features show that crystal growth includes both lateral and thickening growth; in the latter case the chains thicken continuously from folded to extended form and beyond. It is the thickness in a crystal which governs the phase inversion from h to o phase. Thickness dependence on phase inversion is explicitly shown in the polymer trans 1,4-polybutadience, where the mobile h phase, similar to PE, can exist, even at atmospheric pressure.