Frequency-dependent dielectric properties of aromatic polyimides in the 25–330 GHz range

The dielectric constants (Dk) and dissipation factors (Df) of 11 types of polyimides (PIs) were systematically measured between 25 and 330 GHz using Fabry–Pérot resonators. Herein, the relationships between their frequency dependence and chemical structure are discussed. As the frequency increased, all the PIs exhibited a continuous decrease in Dk and a consistent increase in Df. Notably, no distinct peaks were observed over the entire frequency range. The decrease in Dk with frequency gradually approached the square of the refractive index (n2) in the near-infrared region. Additionally, based on the correlation between the dielectric dispersion and dielectric polarization (Pt), Dk showed a stronger correlation with the electronic polarization (Pe) at higher frequencies than with the dipolar orientational polarization (Pd). Moreover, in fluorine-containing PIs, Dk declined faster with increasing frequency as the fluorine content (F%) increased. Meanwhile, the increase in Df showed a negative correlation with the weight fraction of polar groups (polar%), and the PIs containing –CF3 groups in the diamine moieties exhibited a similar rapid increase in Df with frequency, suggesting characteristic absorptions in the terahertz (THz) domain. These data will be beneficial for developing high-performance polymer-based insulating materials for 5th/6th generation wireless communication technologies.