Ferroelectric VDF/TrFE copolymers studied by nonlinear dielectric spectroscopy

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Abstract
Ferroelectric copolymers of vinylidene fluoride with trifluoroethylene have been investigated by means of nonlinear dielectric measurements conducted on both time and frequency domains. Two kinds of nonlinear responses were separately observed; one associated with a local change in orientational order of crystalline dipoles constituting remanent polarization P r and the other with ferroelectric polarization reversal from—P r to P r. The frequency-domain measurement using a relatively low sinusoidal field characterized the local dipole motion in terms of the frequency spectra of nonlinear complex permittivity ε* ′′ = ε′ ′′. It is shown that ϵ'2 is negative and ϵ'3 is positive being consistent with cooperative dipoles. The time-domain measurement using a high step-field provided dynamical information about polarization reversal in terms of the field-dependent switching time. The analysis of the switching profile revealed that a change in orientational order of crystalline dipoles precedes polarization reversal.