Influence of Field-Induced Phase Transition on Poly(Vinylidene Fluoride-Trifluoroethylene-Chlorotrifluoroethylene) Strain

This work is focused on understanding the reasons behind the large electrostrictive strain of poly(vinylidene fluoride-trifluoroethylene-chlorotrifluoroethylene) terpolymer. Although a few explanations have been proposed in the literature, it remains largely unclear. Here, the role of an electrically induced phase transition is investigated. The strain in the crystalline part of the polymer is monitored using XRD while an electric field is applied onto the sample. Three regions of interest are clearly evidenced and, of particular interest, we observe a change in crystal symmetry located on the 30--70 V \textmu{}${\mathrm{m}}^{\ensuremath{-}1}$ range. In that region, the lattice progressively loses its hexagonal symmetry and moves toward the phase usually observed at lower temperature, with a higher polar order. In parallel, we conduct macroscopic strain measurements to compare to the XRD data. Three different regimes are also observed with a sudden increase in electrostrictive coefficient on the 30--70 V \textmu{}${\mathrm{m}}^{\ensuremath{-}1}$ interval, going from 19 to 33 ${\mathrm{m}}^{4}\phantom{\rule{0.25em}{0ex}}{\mathrm{C}}^{\ensuremath{-}1}$. This corresponds to a 1% strain, i.e., 25% of the total deformation measured at 100 V \textmu{}${\mathrm{m}}^{\ensuremath{-}1}$. By thoroughly comparing macroscopic strain and x-ray measurements, we are able to single out and quantify the impact of this field-induced phase transition in the polymer overall strain.