Thermal depolarization of PVDF : anomaly at 180°C

PVDF samples with high β content (80%), prepolarized at room temperature, were heated to higher temperatures. The polarization was measured first under open circuit conditions at the high temperature and then after cooling down again to room temperature For elevated temperatures up to 175°C the polarization was reduced to 2% of the original value, yet recovered roughly to 8% of the original value after cooling down to room temperature In contrast to this, when heating the films to 180°C, a significantly different behavior was observed in cooling down to room temperature again the polarization returned from ~1% at 180°C to ~40% of the original prepolarized value

Thermal depolarization of PVDF : anomaly at 180°C

PVDF (polyvinylidene fluoride) samples with high β-content (80%), prepolarized at room temperature, were heated to higher temperatures. The polarization was measured first under open circuit conditions at the high temperature and then cooling down again to room temperature. For elevated temperatures up to 175°C the polarization is reduced to 2% of the original value, yet covers roughly to 8% of the original value after cooling down again to room temperature. In contrast, when heating up the films to 180°C a significantly different behavior was observed: in cooling down to room temperature the polarization returns from about 1% at 180°C to about 40% of the original prepolarized value

Polarization dynamics of VDF-TrFE copolymers

The polymer polyvinylidenefluoride (PVDF) and its copolymers with trifluoroethylene (TrFE) exhibit a strong piezoelectric effect after poling in high electric fields (100 MV/m). HV impulses of definite duration have been applied to the polymer in order to study the dynamics of the poling process. The dielectric displacement during the HV impulse was recorded. After the impulse, the remanent polarization under short-circuit conditions was also measured. It was thus possible to obtain the minimum poling time dependent on the applied field strength necessary to stabilize the remanent polarization. Comparing the time development of the dielectric displacement with the corresponding remanent polarization revealed a time delay between the orientation of the dipoles and their stabilization. It is concluded that the process of orientation of the dipoles itself is not sufficient to lead to a remanent polarization and that an additional interaction between the trapped charges and the oriented dipoles can explain the stability of the remanent polarization and the observed time delay

Influence of solvents on the polarization distribution in PVDF

The authors report on polarization reductions when poled PVDF films are immersed in polar solvents like acetone, propanol, ethanol, and methanol for a few minutes. Also, immersion in nonpolar solvents, like hexane or carbon tetrachloride for some hours results in a small reduction of polarization. The amount of the reduction depends on the dipole moment of the solvent molecules, the length of the influence of the solvent, and the poling parameters. The authors also observe an influence on the remanent polarization if the films have been immersed in a solvent for some minutes before poling. They find inhomogeneous polarization distributions in these films that are different from distributions in untreated films before poling. In the case of a polar solvent pretreatment, the development of polarization profiles with several maxima is observed

Time development of multiple polarization zones in PVDF

The polarization distribution in PVDF which causes the piezo- and pyroelectricity of this polymer depends strongly on the polarization distribution. Applying an electric field of E = 60M/m causes a ca. 10μm thick polarization zone in the middle of the sample. Applying the same field in opposite direction a bimorph structure results. If the same field is applied to a homogeneously prepolarized PVDF sample a trimorph structure is formed with three polarization zones under field. But under short circuit conditions the polarization zone in the new field direction vanishes. The polarization distribution is measured with the acoustic PPS method with a spatial resolution of about 2μm

Influence of poling conditions on the gas emission of PVDF

The influence of the poling time and the polarity of the applied voltage on the time dependence of the gas emission of PVDF is investigated. If PVDF films are poled in electric fields of about 100MV/m, charges are injected and ions are formed by electrochemical reactions. By recombination and by other reactions of these ions, HF gas is evolved, diffusing out of the sample through the porous electrode. This gas emission increases with increasing field strength. At constant field it decreases to a steady state value comparable to the space-charge-limited poling current of PVDF. Free ions are trapped in the polarization zone. Under short-circuit conditions shallow trapped ions are freed and the gas emission increases strongly. The longer the poling time, the broader the polarization distribution. If the polarization zone is located near the nonporous electrode, the recombined gas molecules need longer time to reach the porous electrode. Therefore, the gas emission under short-circuit conditions is delayed by poling with negative polarity compared to positive polarity

Field induced gas emission of polymer films

Poling polyvinylidene fluoride under vacuum conditions causes strong gas emission, which is analyzed with a quadrupole mass spectrometer. The dominant peak in the mass spectrum is HF, which is produced by electrochemical reactions at the sample surface. After the electric field is switched off, the gas emission unexpectedly increases to a value higher than under the field. This can be explained by ionic charge detrappingand charge migratio

Binding energies of trapped charges in PVDF and P(VDF/TrFE)

The remanent polarization in PVDF and P(VDF/TrFE) is stabilized by trapped charges. We measured the remanent polarization P as function of temperature during heating from room temperature to 180°C. The discharge current is then computed as the first derivative -dP/dT. Assuming a Debye relaxation and a continuous distribution of binding energies g(E) of the trapped charges we can show that the discharge current I(T) is proportional to g(E=mT), with m=const. The constant m can either be calculated numerically or by analytical approximation, With this method the distribution g(E) can be determined. These data are consistent with the extraordinary long life time of the remanent polarisation in PVDF and P(VDF/TrFE) at room temperature

Polarization distribution in PVDF obtained by poling under constant current condition

Since the resistivity of PVDE films increases when the films are poled, the voltage across the samples must be increased during the poling process in order to keep the current constant. Under these conditions, the formation of a polarization zone in the center of the film depth is observed. The thickness of the polarization-free zone close to the film surfaces is found to decrease with larger current densities. The results are consistent with a model assuming charge trapping in deep traps associated with the orientation of polar crystallites

Influence of charge injection on the formation of remanent polarization in P(VDF-TrFE) copolymers

The influence of charge injection on the formation of remanent polarization of P(VDF-TrFE) copolymers was studied by inserting an insulating barrier between the metal electrodes and the P(VDF-TrFE) copolymer during the poling process. The resulting remanent polarization is significantly smaller than that of identically poled samples for the same field strength and time duration, but with directly contacting metal electrodes. This shows that the orientation of the dipoles alone is not a sufficient condition for the formation of remanent polarization in PVDF and its copolymers. The experiments indicate that the stabilization of the polarization is mediated by charge injection and charge trapping at polarized crystallite surfaces