Influence of the relative humidity on the properties of ferroelectric poly(vinylidene fluoride-trifluoroethylene)

Ultrathin films of poly(vinylidene fluoride-trifluoroethylene) copolymers were prepared by the Langmuir-Blodgett technique. These films show a hysteresis loop of the polarization versus the applied electrical field in a dry atmosphere which can be attributed to a ferroelectric phase. By increasing the relative humidity in the surrounding atmosphere, water molecules are absorbed by the samples yielding quasi-free charge carriers. These charges can fluctuate between the electrodes of the samples and build a space charge polarization superimposed to the ferroelectric polarization. Therefore, the hysteresis loops at low frequencies, where the quasi-free charges can contribute to the total polarization, are broadened and the measured remanent polarization is increased. Also, the real and the imaginary parts of the dielectric permittivity of the samples in the low frequency range are increased. Additionally, the transient decay of the retained charge is more pronounced for higher relative humidities. Furthermore, the transient desorption of the water from the polymer is measured by detection of the rapidly changed relative humidity. However, ellipsometric measurements show only a small increase in the sample thickness and no significant change in the refractive index due to the absorption of water.