Wolfgang Stark

Infrared spectrometric study of poly(vinylidene fluoride) films in cyclic electric fields

Abstract 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.

Direct correlations between mechanical-relaxation measurements. Differential-scanning-calorimetry data, and thermally stimulated charge-decay results from Teflon-FEP electrets

Commercial Teflon-FEP films from DuPont were studied by means of several different techniques: The complex shear modulus was measured with a torsion pendulum at 1 Hz between -180 and +260/spl deg/C; the complex dielectric permittivity was studied between 100 Hz and 1 MHz from -150 to +300/spl deg/C; differential scanning calorimetry (DSC) was used to determine the phase transitions in the temperature range from -150 to +300/spl deg/C. The results are compared to reported thermally stimulated discharge (TSD) curves for the same polymer. It is found that the known TSD peaks around -80/spl deg/C, around +80/spl deg/C, and above +200/spl deg/C are closely matched by corresponding features of the mechanical, dielectric, and calorimetric measurements which are ascribed to relaxations at the /spl beta/ or glass II transition, at the /spl alpha/ or glass I transition, and in the premelting region, respectively. While the origin of these TSD peaks from movements of the polymer molecules is quite certain, the TSD peak around +155/spl deg/C found on negatively surface-charged samples requires apparently a more complex explanation. Since the TSD peaks of positively and negatively charged electrets from the other Teflons (PTFE, PFA, and AT) are often found at the same temperatures, their respective discharge-current peaks may have the same physical origins.

Charge stability of teflon-FEP electrets charged at melting temperatures

To reach as high charge stability as possible we tried to charge FEP at temperatures in the melting region. Therefore the foil was heat sealed on a metal plate. After charging the samples were also thermally aged at melting temperature in some experiments. Charge stability was investigated by thermally stimulated charge decay (TSCD) and thermally stimulated current in open circuit (TSC-O).

Influence and implications of surface contaminations on the charge decay in electrets

An investigation of the charge decay of Teflon FEP electrets at elevated temperatures showed considerable differences from sample to sample. Stronger charge decay for open stored samples suggest that surface contamination must have a significant influence on decay rate. For cleaning, distilled acetone was used. Cleaned sample had good reproducibility and a lower charge decay rate. Thermally stimulated charge decay measurements showed that cleaning shifts the charge decay into higher temperature ranges.<<ETX>>

Frequency shifts of IR bands sensitive to dipole orientation in PVDF during poling

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.<<ETX>>

Time dependence of electret potential during current limited charging on a small air gap and corona charging

Charge build-up of electret foils (fluoroethylene propylene) at current-limited charging in a small parallel air gap and at corona charging was measured. Calculation of charge build-up is impossible because the effective charging resistance of the air gap depends on the actual electret charge itself. Therefore the effective charging resistance for both charging methods was determined experimentally. The results of these measurements were used to calculate charging time functions. A comparison with the measured results gives an indication of the influence of leakage currents through the foil. It is shown that increasing electret potential reduces the air gap potential and leads to an increasing charging resistance.<<ETX>>