Porous polytetrafluoroethylene films were positively or negatively corona-charged at room or elevated temperatures and their charge-storage behaviour was investigated by means of isothermal surface-potential and thermally stimulated discharge-current measurements. In addition, electron micrographs of the sample morphology were taken and the influence of high humidities on the surface-charge decay was investigated. For comparison, nominally non-porous polytetrafluoroethylene films were studied in the same manner. It was found that porosity may lead to significantly enhanced surface-charge stability for both polarities if the relative humidity is not too high. Further investigations are under way in order to better understand this behaviour and to employ it for electret applications.

Porous polytetrafluoroethylene (PTFE) films were positively or negatively corona charged at room or elevated temperatures. Their charge storage behavior was investigated by means of isothermal surface potential measurements in direct comparison to nominally nonporous samples of the same polymer. It was found that porosity may lead to significantly enhanced surface-charge stability for both polarities. Direct piezoelectricity was studied on quadruple, double, and single layer samples by means of quasi-static measurements. For the determination of indirect piezoelectricity, frequency-dependent acoustical-transducer experiments were carried out. Both applications-relevant measurements yielded piezoelectric d33 coefficients of up to approximately 600 pClN or 600 pmlV. These values are more than one order of magnitude higher than in conventional piezoelectric polymers such as polyvinylidenefluoride (PVDF) and almost comparable to the highest known values of inorganic piezoelectrics. Consequently, the novel piezoelectric porous-fluoropolymer spacecharge electrets exhibit an outstanding potential for various device applications that are very briefly discussed.

After suitable preparation, films of poly(vinylidene fluoride-hexafluoropropylene) [P(VDF-HFP)] show strong piezo- and pyroelectric effects. We investigated the polarization buildup in stretched P(VDF-HFP) copolymer films, cast from a dimethylsulfoxide/acetone solution. The films were poled under varying or constant electric fields. We observed a hysteresis of the polarization as a function of the electric field during poling of the stretched P(VDF-HFP) copolymer films with varying electric fields. From the hysteresis curve, a polarization of 24 mC/m2, and a coercive field of 100 MV/m were determined. The polarization values are lower than those of PVDF films cast from solution and stretched under the same conditions. It seems that HFP comonomer leads to a decrease in the amount of switchable dipoles. Nevertheless, the P(VDF-HFP) copolymer is very attractive because of its high piezo- and pyroelectric coefficients as well as its better chemical and thermal stability and its predicted longer lifetime. A rather uniform polarization across the whole sample thickness was found in spatially resolved measurements of the piezo- and pyroelectric properties. For comparison with PVDF and for structural information, we also report and discuss infrared transmission as well as dielectric response.

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