The surface structure and local piezoelectric response of film samples of P(VDF-TrFE) copolymer and composites based on it were studied by atomic and piezoelectric force microscopy. Lamellar crystals 2.0 × 0.6 μm in size were detected in nominally pure P(VDF-TrFE) samples. Introduction of crystalline ferroelectrics increases the crystalline phase fraction in the polymer matrix and changes the shape and sizes of lamellar crystallites. Hysteresis loops of the local piezoelectric response for composite regions corresponding to the polymer matrix and crystalline ferroelectric inclusions were obtained.

The temperature dependences of the real and imaginary parts of the complex permittivity of vinylidene fluoride-trifluoroethylene copolymer films at temperatures ranging from −40 to 140°C are determined in the frequency range 10 −1 –10 8 Hz. An analysis of the experimental data has demonstrated that the behavior of the dielectric characteristics of the copolymer in the course of heating to temperatures above the ferroelectric phase transition point and during subsequent cooling differs substantially. In the latter case, the high-frequency dielectric response exhibits properties characteristic of relaxor ferroelectrics. The observed features are discussed in terms of the competition of two mechanisms associated with the short-range and long-range forces.

Thin films of poly(vinylidene fluoride-hexafluoropropylene) P(VDF-HFP) show significant electroactive properties, such as piezoelectricity, pyroelectricity and electrostriction. Suitable polar P(VDF-HFP) copolymer films can be prepared by melt-pressing or solution-casting. Dipolar orientation causes the macroscopic polarization and thus also the symmetry breaking necessary for electroactive properties.We discuss the polarization build-up in thin, stretched and non-stretched, films of P(VDF-HFP) copolymer with a HFP content of 15%. Poling currents measured in-situ during electric poling are analyzed and the polarization is calculated. Suitable electric poling leads to hysteresis phenomena of the polarization as a function of the electric field as well as to significant polarization during switching experiments. Our results indicate dipolar orientation also in non-stretched P(VDF-HFP) films.

Because of their unusual mechanical properties and their excellent charge-storage capabilities, porous polytetrafluoroethylene (PTFE) films are attractive for electret applications. The direct application of electrodes is facilitated and the charge stability of the open-porous films is improved if the porous PTFE films are coated with layers of other suitable polymers. Here, we report about the preparation of coated porous PTFE films and their investigation by means of scanning electron microscopy, corona charging, and surface-potential measurements. For coating, non-porous polymers like polystyrene (PS), polyethylene terephthalate (PETP) and Teflon (R) AF were employed. After optimization, such layered films may be suitable for a range of electret-transducer applications under various environmental conditions.

Films of poly(vinylidene fluoride-hexafluoropropylene) copolymer [P(VDF-HFP)] were cast from a dimethylsulfoxide (DMSO)/acetone solution of Solef®85-15 P(VDF-HFP) copolymer powder grade 21508. Undrawn and uniaxially drawn cast copolymer films were investigated with respect to their piezo- and pyroelectric properties. Quasistatic charge integration was employed for the determination of the poling-field dependence of the piezoelectric d31 and pyroelectric p3 coefficients. In addition, the thermal stability of the pyroelectric effect was studied with a combination of thermally stimulated discharge (TSD) and temperature-modulation techniques. Cast copolymer films could withstand electrical poling fields of up to 400 MV/m. The maximum values of d31=30 pC/N and p3=49 μC/(m2K) for uniaxially drawn samples are similar to those found on commercial PVDF films and much higher than those on pure PVDF films cast from solution. Samples kept for 5 min at 150 °C still exhibit 30–40% of the initial pyroelectric effect [up to around 20 μC/(m2K)]. After this annealing step, no further decay of the pyroelectric coefficient could be observed during storage at 120 °C for several hours.

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 d/sub 33/ coefficients of up to approximately 600 pC/N or 600 pm/V. 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.

Porous PTFE films were corona charged with high voltages at room and elevated temperatures and their direct and inverse piezoelectric coefficients were determined. In both cases, piezoelectric d(33) coefficients of up to 600 pC/N were found. These values are more than one order of magnitude higher than those of conventional piezoelectric polymers such as poly(vinylidene fluoride). They are comparable to the coefficients of inorganic piezoelectrics and of the recently developed porous polypropylene electrets, but have the added advantage of a much better thermal stability.

After suitable charging at room or elevated temperatures, porous films of polytetrafluoroethylene (PTFE) exhibit direct as well as inverse piezoelectricity. For the present study, the inverse piezoelectric responses of charged porous PTFE films are measured by means of an interferometric technique as well as an acoustic method. In addition to materials issues such as sample and electrode preparation as well as electret charging, the two methods are briefly described and the respective results obtained with them are discussed in comparison to each other

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.

Single-film bipolar electrets of porous polytetrafluoroethylene are generated by means of a two-step corona-charging process at elevated temperatures. Quasi-static direct piezoelectric coefficients of up to 0.15 nC/N have been observed on these films. In addition, multiple-layer stacks of porous and non-porous polytetrafluoroethylene films with monopolar charge were also investigated. While the piezoelectric responses of the stacks were not as high as those of the single films, the multiple-film arrangements may have other advantages such as better electrical shielding or tunable mechanical properties and adjustable acoustical impedance; Our new results are discussed in the context of the emerging field of porous polymer electrets with many potential device applications.

Non-uniform electrets consisting of at least one "soft" porous and at least one "hard" nonporous Teflon PTFE film were investigated for their piezoelectric properties. Two, three, or four films were corona-charged and assembled in various multi-layer stacks. Quasi-static piezoelectric responses of the same order as in ferroelectric fluoropolymers could be obtained in the best cases. As expected, the observed piezoelectric coefficients scale linearly with the charge density and also strongly depend on the respective layer geometry.

Thin films of the Teflon(TM) homo- and copolymers TFE, FEP, PFA, MFA, AF, and PVDF-TFE (VF2-VF4) were treated with a microwave plasma in hydrogen or argon, and charged with a point-to-grid corona. It was found that a hydrogen-plasma treatment has a greater influence on charge stability than an argon-plasma process and that the stability of plasma-treated and negatively charged fluoropolymers is lower than that of untreated samples, while the stability of plasma-treated and positively charged fluoropolymers is higher. X-ray photoelectron-spectroscopy showed ablation of fluorine and incorporation of oxygen- and nitrogen-containing groups into the surface. Furthermore, Teflon-PFA and polyethylene terephthalate (PETP) samples were charged in a radio-frequency argon plasma by way of self-biasing. In this case, the charge level increases with increasing bias and charging time. However, enhanced sample heating must be taken into account at higher plasma power. With a plasma, PFA can be charged to higher initial values, but PETP shows better charge stability.

Various types of Teflon(TM) homo- and copolymers (TFE, FEP, PFA, and Tefzel(TM) E-TFE) were negatively or positively corona- or electrode-charged in high electric fields at temperatures of up to 200 degrees C. Charge spreading in thin films of these polymers was studied by means of Piezoelectrically generated Pressure Steps (PPS). Depending on the charging parameters, space-charge profiles ranged fi om thin charge layers near the film surface to more or less uniformly spread bulk charge in both negatively and positively charged samples. Even for very high surface potentials (+/- 5 kV) and long charging times (one hour), almost no charge spreading was observed after corona charging at room temperature. At high charging temperatures and - on negatively electrode-charged Teflon(TM) FEP - at high DC fields, uniform charge spreading is found. From the calibrated results, maximum space-charge densities of up to 2400 C/m(3) were observed on room-temperature corona-charged Teflon(TM) polymers.

Rather high space-charge densities were achieved in the bulk of several Teflon fluoropolymers by means of corona charging at an elevated temperature. The resulting electric-field distributions were probed with piezoelectrically generated pressure steps; this technique is particularly well suited for directly measuring spread space-charge distributions in the volume of thin polymer films. From the calibrated pressure-step results, it is possible to estimate space-charge and thus also filled-trap densities; maximum space-charge densities of 600C/m(3) corresponding to filled-trap densities of almost 4x10(15) cm(-3) were observed on Teflon PFA under the present charging and annealing conditions, while the maximum space-charge densities in the other two Teflon variants were smaller by a factor of up to three. Some typical recent results on positively and negatively corona-charged samples of the three Teflon polymers TFE, FEP, and PFA are presented together with results on two-layer samples consisting of one positively and one negatively charged film, which demonstrate that the charges in the volumes of the individual films are quite well preserved upon mechanical (and electrical) surface contact.

PVDF was effectively dehydrofluorinated by the reaction with hot alkali-DMSO solution. PVDF degrades to products containing c=c, c=c, and c=0 groups as revealed by the corresponding bands in the IR spectrum. Because of the formation of conjugated c=c double bonds in the polymer chain, films of degraded PVDF become darker. Films have been produced by casting onto a glass plate. These cast films were stretched and poled. The results of the measurements of the piezoelectric strain coefficient d/sub 31/ and of the pyroelectric coefficient p/sub 3/ are given in comparison with undegraded films. Moderately pyroelectric coefficients of dehydrofluorinated PVDF films and their enhanced absorbance makes them attractive for potential applications in fast pyroelectric sensors.

Infrared spectra of uniaxially drawn β-poly(vinylidene fluoride) films were recorded under the influence of an electric field. The direction and strength of the applied field were changed in up to thirteen cycles with a maximum field strength of 240 MV m−1. The field-induced changes of the absorbances of the bands at 445 cm−1 (CF2 rocking) and at 510 cm−1 (CF2 bending) were evaluated. Certain amounts of 180° switching and a structure memory effect are discussed to explain the experimental results obtained for repeated repoling processes.

Spectroscopic absorption measurements in the wavelength range from 700 to 300 nm are used to study the orientation of optically nonlinear side-chain groups in an acrylic polymer and dye guest molecules in a polymer matrix by corona poling process. The time stability and the annealing behaviour of these films are measured. The poling process produces efficient orientational order at room temperature of the acrylic side-chain polymers. With stepwise annealing up to poling conditions the orientation order was measured. The guest-host systems show a different behaviour.

On the basis of a mechanical model of a vibrating CF/sub 2/-group under the influence of an electric field it is shown that the evaluation of band frequencies makes it possible to discriminate between dipole orientation angles theta and theta + pi . The field-induced frequency changes experimentally found can be explained by the model in connection with dipole reorientation caused by the field.< >

Poly(vinylidene fluoride) (PVDF) and poly(methylmethacrylate) (PMMA) have been doped with crystal violet and disperse red. Field-induced alignment of the azo dye disperse red dissolved in PMMA results in piezo- and pyroelectric activity. piezo- Field poling of PVDF films doped with crystal violet produces and pyroelectric polymer films with high absorbance in the visible spectral range. Piezo- and pyroelectric properties of the doped films are presented.<>