Electric field and porosity dependent permittivity in electron beam irradiated PMMA

The effect of electron beam radiation on permittivity of polymethyl methacrylate (PMMA) samples is investigated. Experimental results show that the variance of permittivity in irradiated PMMA is mainly determined by two factors. One is porosity of the material. The irradiating process increases the porosity due to the escape of the small molecule (e.g. CO, CO2, CH4 etc.) produced by material degradation. The other is the distribution of injected charge. If electric field generated by injected charge is not inverse symmetrical, the polar PMMA sample should be subjected to extra polarization by the field. Combining Wakino’s model of permittivity of mixture materials, Clausius-Mosotti equation is utilized to analyze the variation of permittivity in the irradiated PMMA samples.