Ferroelectriclike dielectric behavior in the piezoelectric amorphous copolymer of vinylidenecyanide and vinyl acetate

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Abstract
The alternating copolymer of vinylidenecyanide and vinyl acetate is the first amorphous polymer to show a combination of excellent properties among the several known piezoelectric polymers, e.g., high piezoelectric activity (d31=7 pC/N), high electromechanical constant (kt=0.38), and high thermal stability (glass transition temperature, Tg=170 °C). Our dielectric studies have uncovered for the first time several unusual characteristics which strongly suggest the copolymer to be a ferroelectric glass. The ferroelectric nature is manifested in the dielectric anomalies (a fourteenfold increase in the dielectric constant), the Curie–Weiss behavior and, more importantly, the positive and negative divergence of the third-order dielectric susceptibility around the transition temperature. Significantly, these transitional phenomena occur at temperatures slightly above the Tg of the copolymer. The microscopic origin of the dielectric anomalies has been attributed to the cooperative interactions of C≡N dipoles which turn active only after the main chains become mobile.