Pyroelectricity in polyvinylidene fluoride: Influence of polarization and charge

Fractional thermally stimulated depolarization (TSD) currents and temperature-dependent pyroelectricity are experimentally studied in polyvinylidene fluoride samples poled at high direct current field at room temperature. It is demonstrated by a detailed charge balance that the generally assumed proportionality between pyrocoefficients and residual polarization cannot be observed in the low temperature range where polarization irreversibly decreases during heating, while the pyrocoefficient remains constant up to about 60 degrees C. It is assumed that a portion of the residual polarization results from alignment of the dipoles in the amorphous phase of the polymer. This portion is not related to pyroelectricity and irreversibly vanishes during heating. For lack of screening charges at the crystallite boundaries, an irreversible back switching of the polarization occurs at temperatures higher than 80 degrees C. Since the relaxation of the compensating charges and that of the ferroelectric polarization are interrelated, it is not possible to distinguish between contributions from polarization and compensation charge to the measured TSD current and to decide which component is thermally more stable. All TSD current cycles show almost the same activation energy of 2.7 +/- 0.3 eV. The total released charge during the fractional TSD current experiments amounts to about 30-40% of the residual polarization, indicating that the TSD current due to charge motion partly compensates the depolarization current caused by the dipole realignment. Therefore, the integral of the measured TSD current is always smaller than the corresponding decrement of the residual polarization.