Electrostriction as the Origin of Piezoelectricity in Ferroelectric Polymers

Electric-field-induced changes in thickness have been measured interferometrically for thin films of ferroelectric VDF/TrFE copolymers. For sinusoidal fields high enough to induce ferroelectric switching, the strain x in the thickness direction draws a hysteresis loop of butterfly shape while the electric displacement D draws a conventional square D–E hysteresis loop. The x is shown to be proportional to D². Their ratio gives the electrostriction constant κ₃₃ ranging from -2.1 to -2.5 m⁴/C². Low-field measurements for poled samples with remnant polarization P_r yield a linear relationship between x and D. The piezoelectric constant given by the ratio x/D coincides with 2κ₃₃P_r, implying that the piezoelectric activity of VDF copolymers originates from electrostrictive coupling as does that of traditional ferroelectrics. The role of the dimensional effect in both piezoelectricity and electrostriction is discussed.