Complex permittivity spectra of several insulating polymers at electrical and THz frequencies

Frequency and temperature dependencies of the real part (ε_r′) and imaginary part (εr″) of relative complex permittivity were measured at electrical frequencies from 10 ^-2 to 10⁵ Hz at temperatures from -80 to 200°C for various kinds of organic insulating polymer films. Furthermore, frequency spectra were obtained at 30°C in a frequency range from 0.4 to 4.0 THz for both parts. As a result, it was found that there are significant differences in both the temperature and frequency dependencies of ε_r′ and εr″;, depending mainly on the polymer being polar or nonpolar. By comparing terahertz (THz) time-domain spectra obtained for various polymeric insulating films such as polyamide and poly(L-lactide), it was found that the polar polymers have one or two distinct dielectric loss peaks around 2.0 and 3.0 THz. More specifically, among the polymers examined, only polyamide has both two peaks, while poly(L-lactide) and polyethylene terephthalate have only one peak. Nonpolar polymers show no peaks. Furthermore, it has become clear that ε_r′ approaches almost the same value of 2.3 at 3.8 THz in most polymers including polar ones. This agrees with the fact that only electronic polarization and atomic polarization associated with very light atoms such as hydrogen can participate in polarization at such high frequencies.