Piezoelectric Properties in Uniaxially Drawn Copolymers of Vinylidene Fluoride and Trifluoroethylene

The piezoelectric constants d3i and e3i (i=1-3) have been determined as a function of the remanent polarization Pr 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 Pr, while the d constant is nonlinear because of an increase in the elastic constant at high Pr. The observed values of e31, e32 and e33 are compared with prediction of the dimensional effect: e31=e32=0, e33=-Pr. 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.