From solvent-cast to annealed and poled poly(VDF-co-TrFE) films: New insights on the defective ferroelectric phase

Abstract

Films of ferroelectric vinylidene fluoride-trifluoroethylene copolymers, poly(VDF-co-TrFE), are investigated for future applications in organic electronics. For these developments, a composition ∼70 mol % VDF is selected, thin and isotropic films are evaporated, then annealing and poling steps are performed to increase the amount of ferroelectric phase. Over the past decades, many studies have been mostly done on stretched films for various VDF/TrFE ratios but only few on isotropic films. Here, simultaneous SAXS-WAXS experiments on isotropic films allow for a better understanding of the structural transitions in these systems. As for stretched films, a ferroelectric phase containing conformational defects is observed. However, thanks to the isotropy of the films and to a decomposition of WAXS spectra into amorphous and crystalline contributions, the crystallinity and the shape of crystalline peaks are followed upon heating and cooling. We show thereby for the first time how this Defective Ferroelectric (DFE) phase is created and how it thermally evolves. The DFE phase comes from the crystallization of chemical conformational defects in the paraelectric (PE) phase. By combining structural, thermal and spectroscopic data, a structural model for the evolution of crystalline lamellae is proposed. A structural transition from the orthorhombic DFE phase to the hexagonal PE phase is highlighted below the Curie temperature. These advancements give us keys to improve properties of poly(VDF-co-TrFE) electroactive films.