The role of molecular defects on the structure and phase transitions of poly(vinylidene fluoride)

Abstract

Having prepared poly(vinylidene fluoride) (PVF₂) with 0.2–23.5 mol% head-head/tail-tail (HHTT) defects (by comparison with the commonly obtainable 3.5–6.0 mol%), we examined the influence of such regiodefects on crystal structure, polymorphism and Curie transitions. The structure at ambient temperature changes from α to β as the HHTT content is increased beyond ca. 11.4 mol%. At that composition, either of these two phases may predominate, depending upon temperature of crystallization (higher ones favouring β). For defect contents over 15.5 mol%, the polymer chains show progressive intramolecular disorder, yet are packed with increasing intermolecular order in a pseudohexagonal lattice. When PVF₂ containing 13.5–15.5 mol% regiodefects is heated, clear ferroelectric-paraelectric phase transformations are obtained; these are reversible (with thermal hysteresis) upon subsequent cooling. This confirms our previous findings of Curie transitions in PVF₂ copolymers with tri-or tetra-fluoroethylene, and shows that such Curie behaviour is an inherent property of PVF₂ itself. No solid-state transformations were observed for polymers with over 15.5 mol% defects, their structure remaining akin to that of the paraelectric phase at all temperatures between −10°C and the melting point. Within the same temperature range, the essentially isoregic PVF₂ (0.2 mol% defects) remains predominately in the α-phase. On the other hand, the polymer having 11.4 mol% defects shows a remarkable transformation during heating, from the anti-polar α-phase to the paraelectric phase.