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

ABSTRACT 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.