Investigations of ferroelectric-to-paraelectric phase transition of vinylidenefluoride trifluoroethylene copolymer thin films by electromechanical interferometry

Electrostrictive and piezoelectric properties for a 1.3 μm thick film of a vinylidenefluoride/trifluoroethylene copolymer exhibiting a ferroelectric-to-paraelectric phase transition have been investigated as a function of temperature by means of electromechanical interferometry. The electrostriction remarkably increases in the vicinity of the phase transition temperature according to a sharp increase in the dielectric constant. For the unpoled sample in the ferroelectric phase with a polydomain structure in which the local spontaneous polarization is macroscopically cancelled out, the dependence of the electrostriction on the square of the applied electric field is nonlinear in the ferroelectric phase, while it is linear in the paraelectric phase. A theoretical model which takes into account the nonlinear dielectric constant can quantitatively explain the nonlinear contribution to the electrostriction in the vicinity of the ferroelectric–paraelectric phase transition, but it underestimates the contribution in the ferroelectric phase. Interactions within and/or between the ferroelectric domains in the polydomain structure are expected to contribute significantly to the nonlinear electrostriction. For poled samples a pronounced inverse-piezoelectric effect is measured. The achieved polarization and its temperature dependence are almost the same as for thicker films reported in earlier studies, obtained by different experimental techniques.