Polyimide/Internally Methyl-Modified Porous Silica Composites With Low Dielectric Loss for High-Frequency Communications

Polyimide (PI) composites with simultaneous low dielectric constant ( D k ) and dissipation factor ( D f ) are highly desirable for high-frequency communications. Conventional porous or hollow silica (SiO 2 ) fillers effectively reduce the dielectric constant but often suffer from incomplete removal of internal silanol groups and require pore-sealing processes, which compromise dielectric performance and increase processing complexity. Herein, mesoporous foam silica (MFS) with an interconnected mesoporous structure was methyl-modified to remove the silanol groups (Si  OH) from both the outer and inner surface. The obtained internally methyl-modified porous silica (MFSM) retaining amount of air was then dispersed into a polyamic acid (PAA) solution to fabricate PI/MFSM composite films. Owing to the improved compatibility between MFSM and PI, together with reduced polar groups in MFSM, the resulting PI/MFSM composite films simultaneously exhibit a low D k of 2.324 and a low D f of 0.0097 at 10 GHz, along with experimental D k values well matched with the theoretical predictions. Besides, the PI/MFSM composites also demonstrate an outstanding thermal stability ( T d5% = 573°C, T max = 603°C), a much lower coefficient of thermal expansion (CTE = 26.9 ppm/°C) matched with copper substrates, a good tensile strength (29.5 MPa) and breakdown strength (96.7 kV/mm), and a high MFSM loadings of up to 45.31 vol%. This work provides a feasible structural design strategy for developing low-dielectric PI composites for high-frequency communication applications. Highlights Internal polar (Si  OH) groups in MFSM are efficiently removed by methylation. Air introduced by MFSM effectively lowers the D k of PI/MFSM composites. Uniform MFSM dispersion in PI preserves composite flexibility.