Abstract
The stabilization of δ-phase of poly(vinylidene fluoride) PVDF in a 14 µm-thickness ferroelectric membrane is achieved by a simple route based on the use of a dimethylformamide (DMF)/acetone solvent, in which the application of external electric field is not required. X-ray diffraction and calorimetric experiments on heating reveal that, at 154 °C, the original mixture between ferroelectric δ-phase and paraelectric α-phase transits to a system with only this latter phase in the crystalline fraction. A gradual and slight increment of amorphous fraction up to the melting at 161 °C is also observed. The existence of δ-phase is corroborated by the occurrence of a broad maximum around 154 °C in dielectric permittivity measurements, as well as the hysteresis loops observed at room temperature. These results suggest a wide thermal window for a stable δ-phase, between room temperature and 154 °C, a subsequent transition into α-phase and the corresponding melting at 161 °C. The broad dielectric maximum observed around 154 °C in dielectric and calorimetric measurements, can be associated with a diffuse ferroelectric-paraelectric transition.
Highlights
Poly(vinylidene fluoride), PVDF (-CH2-CF2-)n, is a semicrystalline polymer, whose ferroelectric nature depends of the dipoles formed between the C-H and C-F bonds, the polymeric chain conformation and the molecular packing of the crystalline region
We report the stabilization of the polar δ-phase without the application of an electric field and the study of its ferroelectric phase transition through dielectric, structural and calorimetric measurements with temperature
Glancing X-ray Diffraction (GIXRD) patterns and FTIR spectroscopy studies discard the presence of the more commons γ and β ferroelectric phases
Summary
Poly(vinylidene fluoride), PVDF (-CH2-CF2-)n, is a semicrystalline polymer, whose ferroelectric nature depends of the dipoles formed between the C-H and C-F bonds, the polymeric chain conformation and the molecular packing of the crystalline region. The most common and easy to obtain is the non-ferroelectric α-phase, which can be obtained directly during crystallization from the melt or during polymerization[3, 6] It is characterized by a trans-gauche-trans-gauche′ (TGTG′) conformation of macromolecular chains with the dipole moments aligned in antiparallel way. The difference in the polymer chain packing, respect to α, confers to δ-phase its ferroelectric character[3, 4, 10, 11] and imply only small changes in the intensities of some diffraction planes and some FTIR absorption bands between them[10] This phase has been poorly studied and it is stabilized by applying high electric field, for short periods of time, to α-phase[4, 10, 11]
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