Piezoelectric Properties in Uniaxially Drawn Copolymers of Vinylidene Fluoride and Trifluoroethylene

Abstract

The piezoelectric constants d _3i and e _3i (i=1-3) have been determined as a function of the remanent polarization P _r in uniaxially drawn and poled copolymers or vinylidene fluoride and trifluoroethylene or composition 52/48, 65/35, and 73/23 mol%. The e constant is linearly proportional to P _r, while the d constant is nonlinear because of an increase in the elastic constant at high P _r. The observed values of e ₃₁, e ₃₂ and e ₃₃ are compared with prediction of the dimensional effect: e ₃₁=e ₃₂=0, e ₃₃=-P _r. It is found that the dimensional effect is dominant in the observed activity of 73/27 copolymer, while in the 52/48 copolymer there is another dominating mechanism. The electrostrictive coupling characteristic of ferroelectric materials is examined and its additional contribution is shown to explain the observed piezoelectric activities in the copolymers having relatively low Curie temperature.