Structure-dielectric property correlations in polysiloxane- and DDSQ-based polyimides measured at high frequencies

The development of polymer dielectrics with extremely low dissipation factors (Df) in the GHz range is essential for next-generation high-frequency communication and advanced semiconductor technologies. In this study, silicon-containing polyimides (PIs) were systematically synthesized through the incorporation of polysiloxane and double-decker silsesquioxane (DDSQ) units, aiming to elucidate the correlation between molecular structures, higher-order structures, and dielectric properties. Poly(amic acid) precursors were prepared using various tetracarboxylic dianhydrides and silicon-containing diamines, followed by thermal imidization to obtain PI films. Wide-angle X-ray diffraction analysis revealed the formation of periodic layered structures in polysiloxane-containing PIs and the preservation of the cage-like architecture in DDSQ-containing PIs. Dielectric measurements at 10 and 20 GHz demonstrated that the introduction of silicon-containing units reduced the dielectric constant (Dk) to less than 3.0 because of increased free volume. Notably, the Df values of DDSQ-containing PIs were markedly lower ( ~ 0.002–0.003) than those of polysiloxane-based PIs because of the effective suppression of dipolar orientational polarization originating from the rigid and highly symmetric DDSQ framework. These findings provide an important molecular design strategy for low-loss polymer dielectrics and highlight the potential of DDSQ-based PIs as promising candidates for interlayer insulating materials in high-frequency electronic devices. This study clarifies how molecular structure governs GHz dielectric behavior in silicon-containing polyimides. Flexible polysiloxane units increase dipolar orientational polarization (Pd) and raise the dissipation factor (Df), whereas rigid DDSQ cages suppress molecular relaxation and yield ultra-low loss. The strong linear Pd–Df correlation demonstrates that orientational polarization is the primary factor controlling dielectric dissipation in hybrid polyimides.