A study of thermal depolarization of polyvinylidene fluoride using x-ray pole-figure observations

Abstract

Measurements of piezoelectric and pyroelectric activity, density, and x-ray pole figures were used to study the effect of thermal aging on the state of polarization in polyvinylidene fluoride. A rolled and poled β-phase specimen of polyvinylidene fluoride was subjected to thermal aging which consisted of temperature cycling between room temperature and successively higher maximum temperatures, TMAX, where TMAX ranged from room temperature to 164 °C. We found that the room temperature piezo and pyroelectric activity decreased linearly as a function of TMAX from 75 °C to 164 °C at which temperature the specimen had 30% of its original activity; a linear extrapolation of these data to zero activity yielded a temperature Tc=207 °C. From density measurements at room temperature, crystallinity was calculated and found to remain constant during thermal cycling. X-ray pole-figure observations of the (200) (110) composite diffraction of the β-phase crystal showed single-crystal texture of the rolled specimen and confirmed the six-site model of dipole orientation. Changes in x-ray intensity at the six sites on the pole figure, as a function of thermal aging, were associated with the depolarization process which occurs via a 60° rotation of dipoles away from the direction of primary polarization. Based on these data, we propose a model which describes the state of polarization in polyvinylidene fluoride and from which we calculate the fraction of dipoles in the crystalline state contributing to the polarization.

Phase Transitions of Polarised PVDF Films in a Standard Curing Process for Composites

“…This effect can be observed in the XRD patterns, for example, when annealing at 130 °C, as shown in this study. It supports the theory [11] that the loss of β-phase is only a part of the depolarisation and the loss of orientation is at least equally important. Moreover, it implies that stabilising the orientation of the dipoles in an already pre-stretched, polarised and β-phase rich PVDF may help to keep the piezoelectric effect while annealing close to the melting range.…”

“…Up to 155 °C, the crystallinity remains constant, and the crystalline β-phase does not change to a crystalline α-phase. The phase changes do not seem to be important, while the reorientation of the β-dipoles by 60°seems to be the relevant factor in depolarisation [11]. All these findings have been observed under laboratory conditions and not in practical applications.…”

“…Even though the α-phase grows the strongest below the melting point, such a small exchange between the phases alone cannot explain the loss of polarisation after the Curie temperature is reached. The decrease in polarisation may be linked to a loss of orientation of the dipoles in the β-phase, which rotate by 60°after annealing above the Curie temperature [11]. In the Bragg-Brentano arrangement used for the XRD measurements, only those lattice planes parallel to the surface can be detected.…”

“…This is similar to an autoclave process in that length changes are suppressed due to the hydrostatic pressure. Since the disorientation of the β-dipoles seems to be the main cause of depolarisation [11], there should be a few phase transitions. This leads to the hypotheses of our study: the combination of annealing and pressure in an autoclave stabilises the piezoelectric phase up to the high temperatures needed for curing CFRP structures.…”

Phase diagram of poly(VDF-ter-TrFE-ter-CTFE) copolymers: Relationship between crystalline structure and material properties

“…The ferroelectric properties characterized by a remnant polarization (PR) and a coercive field (EC) depend on the amount and extension of crystalline lamellae [23]. In PVDF, although the ferroelectric  phase is stable until the melting temperature of the polymer, the remnant polarization decreases above 75 °C [24]. A crystalline structure of the same symmetry, with chains in the same all-trans conformation can be obtained in copolymers of vinylidene fluoride and trifluoroethylene, poly(VDF-co-TrFE), whatever the crystallization route used, from the melt or from solution [25] [26].…”