Humidity-Robust Semi-Aromatic Polyimides with Cyclohexyl-Substituted Double-Decker-Shaped Silsesquioxane Exhibiting Ultra-Low Dielectric Losses at 10-20 GHz

Emerging high-frequency electronics require polymer dielectrics that combine a low dielectric constant ( D _k ) with an ultra-low dissipation factor ( D _f ) under realistic humidity. Here, we report semi-aromatic polyimides (PIs) incorporating cyclohexyl-substituted double-decker-shaped silsesquioxane (C-DDSQ) units that simultaneously suppress electronic and dipolar polarizations while limiting water uptake. The best formulation shows D _k = 2.57 and D _f = 0.0019 at 10 GHz under 30% RH, and maintains minimal changes up to 60% RH ( ΔD _k ≈ 0.02, ΔD _f ≈ 0.0003). Wide-angle X-ray diffraction confirms the retention of the cage-like silsesquioxane motif, and thermogravimetric analysis shows T _d−10 ≥ 470°C, indicating robust thermal endurance. Correlating D _k with in-plane refractive index ( n _TE ) reveals that the lowered n and the rigid, hydrophobic architecture of C-DDSQ account for the low losses at 10–20 GHz. These results position C-DDSQ-incorporating semi-aromatic PIs as promising interlayer dielectrics that deliver ultra-low loss, humidity stability, and high thermal tolerance without resorting to porosity or heavy fluorination.