Structural changes in ferroelectric polymers under the action of strong electric fields by the example of polyvinylidene fluoride

Abstract

The regularities of solid-phase polymorphic transformations under the action of strong electric fields in polyvinylidene fluoride (PVDF) films have been analyzed. For the homopolymer films crystallized in a mixture of nonpolar α and polar β phases, two transitions are revealed: α → αp and α → β. In contrast to predictions, these transitions occur simultaneously. For the isotropic PVDF films crystallized in the α phase, the critical fields corresponding to their transition to the ferroelectric β phase may differ by almost an order of magnitude. It is suggested that nucleation of a new phase occurs through the fluctuation mechanism of formation of conformational defects (of the kink-bond type) in the initial crystals. Such processes are controlled to a great extent by the dynamics of through chains in the amorphous-phase regions adjacent to crystals when a decrease in their activation energy in the processes of micro-Brownian motion increases the probability of forming conformational defects in a crystal. If PVDF is crystallized in the ferroelectric phase, the action of strong fields leads to two consequences. First, according to the X-ray diffraction data, rotation of the polar b axes of a crystal through the mechanism of 60° reorientations is observed. Second, the degree of crystallinity can increase (partially irreversibly) through an increase in the crystal size.