Nonlinear dielectric constant and ferroelectric-to-paraelectric phase transition in copolymers of vinylidene fluoride and trifluoroethylene

Second- and third-order dielectric constant ε2 and ε3 of three copolymers of vinylidene fluoride and trifluoroethylene were measured over a temperature range from 20 °C to higher than the ferroelectric-to-paraelectric phase transition temperature Tc . The thermodynamical theory predicts that ε2 is negative below Tc and vanishes at Tc , whereas, for the second-order transition, ε3 is positive below Tc but turns to negative at Tc and, for the first-order transition, ε3 does not change its sign below and above Tc . According to this criterion, it is concluded that the transition is of the second order for a copolymer with vinylidene fluoride fraction of 52 mole %, but of the first order for 65 and 73 mole % copolymers. This conclusion can explain the difference in slope of ε2 versus remanent polarization among the copolymers.