Impact of ring structure and conjugation on the dielectric properties of polyimides at a high frequency of 10–40 GHz

Polyimides (PIs) are widely used materials due to their high thermal stability and potentially low dielectric constant (D_k) and dissipation factor (D_f). However, D_k and D_f values tend to increase at higher frequencies. Therefore, there is a significant need to reduce the D_k and D_f values of PIs, which is the main purpose of this study. This research considers multiple factors, including the ring structures and conjugations on the conformation of the PI backbone. The effects of five-membered and six-membered PIs and the conjugated length of benzene and naphthalene structures in the repeating units on the thermomechanical and dielectric properties are systematically investigated. As a result, the PI with a six-membered imide contributes to higher thermal stability, with a glass transition temperature higher than 350 °C, a thermal decomposition point of 529 °C, and a lower D_k of 2.7–2.8. Still, it exhibits a lower molecular weight than that of a five-membered imide. On the other hand, PIs with esterified structures show an extremely low D_f value, 0.002–0.005. This situation can be attributed to the vital correlation between the D_f and the volumetric dipole moment. This study also presents the different properties arising from ring structures and conjugations. Meanwhile, the relationships between D_k and volumetric polarizability and between D_f and volumetric dipole moment are confirmed. This work provides a guideline for the structure−dielectric relationship of PI at a high frequency of 10–40 GHz.