The ferroelectric nature of a phase transition in a copolymer of vinylidene fluoride and tetrafluoroethylene

The ferroelectric properties of poly(vinylidene fluoride) [1] have recently stimulated interest in similar properties of its copolymers with other fluorocarbons. While copolymers with trifluoroethylene have been studied extensively from a ferroelectric viewpoint, (e.g. ref. 2–5), others have not. Yet, copolymers with tetrafluoroethylene are of particular interest in relation to the homopolymer because they do not introduce new chemical species (-CHF-) or tacticity defects as does trifluoroethylene, and because they can be viewed simply as PVF2 containing an increased amount of head-to-head (-CF2-CF2-) defects. [6,7] Poling of such a vinylidene fluoride/tetrafluoroethylene copolymer (hereafter abbreviated VF2/F4E) was shown through the IR studies of Latour [8] to involve dipolar realignment which was used by Stefanou [9] to account for the observed piezoactivity. The pyroelectricity of such a copolymer was compared to that of PVF2 [10], and pyroelectric, piezoelectric, and charge-transport studies showed highly inhomogenous polarization across the thickness of such copolymer films [11]. Ferroelectric studies by Hicks et al. [12] and Tasaka and Miyata [13] revealed hysteresis loops but no Curie transition, while an increase in the dielectric constant with temperature [12,14] was related to ionic impurities. However, in a recent report [15] we provided evidence for a Curie transition in a VF2/F4E copolymer quenched to 0°C. A subsequent report [16] shows evidence for the solid state transformation using dielectric and thermal techniques as well as X-ray.