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beta-phase poly(vinylidene fluoride-hexafluoropropylene) (P(VDF-HFP)) copolymer films were prepared by uniaxially stretching solution-cast or melt-quenched samples. Different preparation routes lead to different amounts of the crystalline alpha and beta phases in the films, as detected by means of Fourier-transform infrared spectroscopy and X-ray diffractometry. The beta phase is significantly enhanced in melt-quenched and stretched films in comparison to solution-cast and stretched films. This is particularly true for copolymer samples with higher HFP content. The beta- phase enhancement is also observed in ferroelectric-hysteresis experiments where a rather high polarization of 58 mC/ m(2) was found on melt-quenched and stretched samples after poling at electric fields of 140 MV/m. After poling at 160 MV/m, one of these samples exhibited a piezoelectric d(33) coefficient as high as 21 pC/N. An electric-field-induced partial transition from the alpha to the beta phase was also observed on the melt-quenched and stretched samples. This effect leads to a further increase in the applications-relevant dipole polarization. Uniaxially stretched ferroelectric- polymer films are highly anisotropic. Dielectric resonance spectroscopy reveals a strong increase of the transverse piezoelectric d(32) coefficient and a strong decrease of the transverse elastic modulus c(32) upon heating from 20 to 50 degrees C.
Ferroelectrets are thin films of polymer foams, exhibiting piezoelectric properties after electrical charging. Ferroelectret foams usually consist of a cellular polymer structure filled with air. Polymer-air composites are elastically soft due to their high air content as well as due to the size and shape of the polymer walls. Their elastically soft composite structure is one essential key for the working principle of ferroelectrets, besides the permanent trapping of electric charges inside the polymer voids. The elastic properties allow large deformations of the electrically charged voids. However, the composite structure can also possibly limit the stability and consequently the range of applications because of, e. g., penetration of gas and liquids accompanied by discharge phenomena or because of a mechanical pre-load which may be required during the application. Here, we discuss various stability aspects related to the piezoelectric properties of polypropylene ferroelectrets. Near and below room temperature, the piezoelectric effect and the stability of the trapped charges are practically independent from humidity during long-time storage in a humid atmosphere or water, or from operating conditions, such as continuous mechanical excitation. Thermal treatment of cellular polypropylene above -10 degrees C leads to a softening of the voided structure which is apparent from the decreasing values of the elastic modulus. This decrease results in an increase of the piezoelectric activity. Heating above 60 degrees C, however, leads to a decrease in piezoelectricity
Polyvinylidene fluoride was dissolved together with solid sodium hydroxide as catalyst in a dimethylsulfoxide/ acetone mixture and moderately dehydrofluorinated. The dehydrofluorination leads to a partial degradation of the fluorohydrocarbons, and in particular to main-chain scission and to formation of carbon double or triple bonds. This enhances the absorption at UV-vis frequencies. The degradation process also generates a large amount of excess charges in the polymer, which influence the electrical polarization behavior of the dehydrofluorinated polymer. Uniaxial stretching of moderately dehydrofluorinated polyvinylidene fluoride leads to films in a polar phase. Dipole polarization in the degraded and stretched films is demonstrated by means of switching experiments
In this paper, a measuring technique is presented for the detection of radial oscillations of tube walls excited by changes in internal air pressure. On organ pipes, the oscillations were investigated by means of piezoelectric polymer films slightly tensioned around the pipe bodies. Employing sensors with patterned electrodes, the well-known elliptical oscillation of the cross section as well as an additional monopole breathing of the organ-pipe body were detected. For the monopole breathing, a close relationship between the pressure distribution of the air-column resonances inside the pipe and the circumference variations along the pipe was observed