Nonlinear Dielectric Relaxations in a Vinylidene Cyanide / Vinyl Acetate Copolymer

Abstract

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 T _g ∼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.