High temperature dielectric loss of poly(ethylene terephthalate) films at microwave frequency

Abstract

An apparatus developed for microwave dielectric measurements at high temperatures was used to determine the dielectric loss ε″ at 4.0 GHz between 20 and 105°C for biaxially stretched and unstretched poly(ethylene terephthalate) (PET) films that had crystallinities of 35 and 5% at room temperature, respectively. The ε″ for the former increased slightly with increasing temperature, whereas that for the latter showed a gradual increase up to 50°C followed by a sharp upswing with further increase in temperature. These temperature changes in ε″ are attributable to the shift of the dielectric β relaxation to higher frequency with increasing temperature. Time dependence studies on ε″ at 4.0 GHz and fixed high temperatures for the unstretched PET film showed a significant decrease in ε″ with time only above 75°C. This phenomenon may be ascribed to the crystallization of PET in the amorphous region. The microwave method described in this paper is useful in that it allows dielectric behaviour at high temperature to be measured quickly with a sample in the contact-free state.