Estimation of thermal expansion coefficients of polymeric insulating films from temperature dependence of dielectric permittivity

Dielectric permittivity ('r ) was measured for films of low-density polyethylene (LDPE), poly(methyl pentene) (PMP), syndiotactic polystyrene (SPS), and poly(ethylene terephthalate) (PET) in the frequency range from 10%2 to 3 ' 105 Hz. It was found that, in all the polymers, " r increases with an increase in temperature at frequencies below about 1.0 Hz when the temperature exceeds a certain polymer-dependent temperature. This is due to space charge polarization. At frequencies above about 1.0 Hz, r was found to decrease with an increase in temperature for LDPE, PMP, and SPS. The reason for the decrease in r is the thermal expansion of the film. Therefore, by analyzing the temperature dependence of r the thermal expansion coefficients of the three polymer films were estimated. This method can be useful for estimating the thermal expansion coefficient of polymers, especially when the direct measurement of volume expansion is difficult.