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Nonlinear Dielectric Relaxations in a Vinylidene Cyanide / Vinyl Acetate Copolymer

The third-order nonlinear permittivity has been measured as a function of the frequency for an alternate copolymer of vinylidene cyanide and vinyl acetate at temperatures above Tg∼170degC. The observed frequency spectra were found to be fitted by a function Δε3/(1+(i ωτ3)β3 )3 with negative Δε3. Its real part changes sign from positive to negative with decreasing frequency. A negative Δε3 implies that the polarization tends to be saturated as the field is increased. Combined knowledge about linear and nonlinear dielectric increments allows an estimate of the size of equivalent dipoles, which explains the unusually large permittivity of this copolymer.