The Electret Properties Of Cellular PP And Solid PEN Film

Cellular polypropylene (PP) ferroelectret film and solid poly(ethylene naphthalene-2, 6-dicarboxylate) (PEN) film are two of the most attractive materials developed in the latest years. The excellent electromechanical (especially piezoelectric) properties of cellular PP film and the space charge storage ability of PEN film were systematically studied in this dissertation.The piezoelectric d₃₃ coefficient was deduced by analyzing the theoretical model of cellular film. Several commercially available cellular PP films were treated by pressure expansion technique, and their piezoelectricity was more than one order of magnitude larger than that of non treated samples. Based on this, possibilities of further optimizing the piezoelectricity of pressure-expanded cellular PP film were studied: for the first time, the piezoelectricity of cellular PP films with different porosity was markedly enhanced by proper treatment and corona charging in gas with high electric breakdown strength. As an instance, through different experimental methods, the piezoelectricity of both EUH75 and SHD50 film, which have different porosity, was improved by corona charging in hexafluoride gas (SF₆). Besides, the inner charging field of cellular PP film with low porosity is improved dramatically by contact charging, which modifies the piezoelectricity of the sample. The physical mechanism and the electret behavior of above results are discussed and analyzed.By dielectric resonance technique and TSD method, influence of heat treatment temperature after the pressure expansion on the electromechanical properties of cellular PP film was systematically discussed: over the temperature range from RT to the melting temperature of PP, the thickness of the pressure expanded cellular PP film increases with the heat treatment temperature, accompanied by a decrease of the resonance frequency. Compared to that mentioned above, obviously different effect of heat treatment temperature on the electromechanical properties of cellular PP film is observed. At a...