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Chemical treatment of cellular polypropylene (PP) films was carried out. Their charge storage properties were investigated using the open circuit thermally stimulated discharge (TSD) current spectra and charge TSD spectra. The charge storage thermal stability of the HF-treated samples is significantly improved, as shown by a very strong high temperature peak at about 176/spl deg/C. The charge stability of treated films is improved further by appropriately prolonging HF treatment time at RT. It was found that the reaction rate of HF with the oxidized surface layer is obviously enhanced by light radiation or treating temperature. However, charge storage stability becomes worse even than the virginal film prolonging HF-treating time under light radiation. An in situ TSD charge spectrum contains combined information on the changes of the mean charge depth and the deposited charge value. The dynamic change of the mean charge depth during heating was investigated by the in situ TSD charge spectra.

P(VDF-HFP) (poly(vinylidene fluoride-hexafluoro-propylene)) copolymer is a new member of PVDF family. In this paper, the electret properties of the two kinds of casting prepared P(VDF-HFP) copolymer films whose content of HFP is 4.7% and 5.9% respectively were discussed with the focus on the influences of mechanically thermal stretching process and of the content of HFP on electret properties of the said copolymer. Experimental results point out that the said copolymer electret film is a polar material, i.e. there exist both oriented dipoles and deposited space charges in the film after charging. The mechanically thermal stretching process not only increases the density of oriented dipoles and deposited space charges but also improves the stability of deposited space charges; however, the increase of HFP content in the copolymer has a negative impact on the density of oriented dipoles and deposited space charges. The measurement result by means of Laser Doppler Vibrometer shows the copolymer in which the HFP content is 4.7% has a notable converse coefficient as high as 130pm/V.

The estimation of trap-levels for corona charged PP cellular and the influence of pressure expanding treatment on its charge stability and detrapped charge transportation were studied by means of surface potential decay measurements, TSD current spectrum analyses and charge TSD, etc. The results point out that there are three kinds of distinct energy level traps in cellular PP films. The pressure expanding treatment changes the state of traps and also, to a certain extent, reduces the charge storage stability, but basically, has no impact on its charge transport rule, in which the slow retrapping effect is dominant.

Poly(ethylene naphthalene-2, 6-dicarboxylate) (PEN) is a aromatic polyester with molecular structure similar to that of poly(ethylene terephthalate)(PET). In this paper, the electret properties of corona charged PEN were studied by measurement of surface potential decay under different storage conditions including different temperatures and relative humidities, analyses of open circuit thermally stimulated discharge (TSD) current spectra and charge TSD curves. The comparison between the electret properties of PEN and PET was carried out. The results point out that the charge stability of PEN is markedly better than that of PET under different environmental storage conditions, and the charge stability of PEN can be improved by corona charging at elevated temperature. Further, the dynamics of charge transport of PEN was investigated.

The influences of charging parameters (such as charging voltage, charging time and charging temperature, etc.) on piezoelectricity and its thermal stability of cellular polypropylene film electrets were studied by means of contact charging, constant voltage corona charging methods and the thermal stimulated discharge technique. The reasons about the influence were discussed with the material structure and theoretic model of cellular piezoelectric film electrets. The results show that the piezoelectricity and its thermal stability of the polymer cellular film electrets would be improved by optimizing the charging parameters reasonably. The work will be helpful to broaden the application for cellular PP and offer theoretic and technical base for their industrialization.