Quantum Chemical Investigations of Structural Parameters of PVDF-based Organic Ferroelectric Materials

Abstract

Poly(vinylidene fluoride)-based polymers have been the research focus of many groups due to the discovery of piezoelectricity in this material, by Kawai et al in 1969. PVDF is constituted by sequential [-(CH2-CF2)n-] chains and known as the first crystalline organic ferroelectric material, which makes it critically important, since there exist only few classes of materials with ferroelectric properties, such as perovskite-type ABO3 metal oxides (BaTiO3, PbTiO3, or KNbO3) (Sakashita et al, 1987). Drawbacks of the latter material are related to its heavy weight, brittleness and substantial costs of device manufacturing. Thus, the organic ferroelectrics materials arise as suitable alternative to metal oxides because of better prospective technological applications. PVDF-based polymers have polymorph properties and at least five experimentally characterized crystal phases, namely, the all-trans (Tp) planar zigzag β-phase, TGa and TGp, (where G denotes the gauche form), the αand δ-phases, and the T3GT3G (T3Ga, T3Gp) γand e-phases (Lovinger, 1982; Broahurst, 1978). Nevertheless, among these phases, only the all-trans conformation (or β-phase) exhibits ferroelectric behavior. The Tp conformation of the all-trans phase, has a highly polarized backbone with the highest spontaneous polarization in a unit crystal cell (Jungnickel, 1999). This spontaneous polarization gives the special ferroelectric properties, leading to a broad potential of application in several newtechnology electronics, such as in sensors, transducers, energy storage devices, communications and microphones (Kawai, 1969; Lovinger, 1983; Jungnickel, 1999; Lando et al., 1966; Farmer et al., 1972; Hasegawa et al., 1972; Karasawa & Goddard, 1992; Tashiro et al., 1995; Nalwa, 1995; Lang, 2006; Wang et al., 1988). A drawback, however, of the synthesized β-PVDF pure material, is that the maximum crystallinity reached is 50%, which results in lower polar properties than those of pure Tp phase, if formed. Some applications currently under development include artificial muscles and harnessing energy from sea waves. The ferroelectric properties of PVDF can be enhanced by the introduction of trifluoroethylene, TrFE, as comonomer. P(VDF-TrFE) exhibits ferroelectric