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 µ${\mathrm{m}}^{-1}$ 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 µ${\mathrm{m}}^{-1}$ interval, going from 19 to 33 ${\mathrm{m}}^4{\mathrm{C}}^{-1}$. This corresponds to a 1% strain, i.e., 25% of the total deformation measured at 100 V µ${\mathrm{m}}^{-1}$. 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.

Figure 1

Sketch of the heterogeneous cantilever (PEN substrate/PEDOT:PSS/terpolymer/PEDOT:PSS) and the measurement setup allowing for collecting polarization versus electric field (Radiant tester) and mechanical deflection versus electric field.

Figure 2

(a) Polarization cycle of the terpolymer at 1 Hz and 20 V µm⁻¹ and the linear fit corresponding to ${ϵ}_r=29.5$. (b) Simultaneous strain measurement, fitted with Eq. (2). (c) Strain from (b) plotted against squared polarization and fitted with Eq. (1).

Figure 3

(a) Polarization cycle of the terpolymer at 1 Hz and 120 V µm⁻¹ (b) Simultaneous strain measurement. (c) Unipolar strain lower branch in (b) plotted against squared polarization and fitted with Eq. (1). The colored background is a visual aid for the discussion below.

Figure 4

Raw spectra of the terpolymer under electric field. The data are separated into three panels based on the range of electric field applied. (a) 0–17 V µm⁻¹. (b) 23−71 V µm⁻¹. (c) 95−166 V µm⁻¹. (d)–(f) fit with the same two pseudo-Voigt functions.

Figure 5

(a) Normalized integrated intensity and full width at half maximum of the full asymmetric peak, (b) d spacing of the two fitting Bragg peaks, (c) normalized integrated intensity of the (200) and (110) fitting peaks, and (d) a and b cell parameters deduced from the fitting, as a function of electric field. (e) Schematic representation of polymer chain along the c axis. (f) Representation of the crystal unit cell.

Figure 6

Schematic representation of the unit-cell parameters as a function of the applied electric field. The left panel represents the high-temperature state without field, where the lattice symmetry is hexagonal. With the application of an electric field the lattice is further distorted and ends up orthorhombic, as represented on the middle-right and right panels. The color coding for the three regimes is the same as in Figs. 3 and 5.