Bulk-Fluorinated Organosilica Hollow Spheres Enabling Ultralow-Permittivity, Humidity-Resistant, and Mechanically Robust Polyimide Films

Abstract

Flexible polyimide (PI) dielectrics must simultaneously combine a dielectric constant (Dk) below 2.5, water uptake below 1 wt %, and sufficient mechanical strength, representing a persistent materials trilemma. Post-synthetic surface fluorination of porous fillers may leave internal polar sites insufficiently passivated, thereby limiting the concurrent regulation of permittivity and moisture affinity. Herein, we demonstrate hierarchical bulk-fluorinated organosilica hollow spheres (FxHSs) synthesized through single-pot, ultrasound-assisted Pickering emulsion polymerization, in which perfluoroalkyl segments are incorporated throughout the shell network during particle formation. The resulting particles integrate a hollow core, hierarchical porosity spanning approximately 4 nm mesopores and 60 nm macropores, and a rough, litchi-like exterior. At 5 wt % filler loading, the composite film exhibits a Dk of 2.35 at 10 MHz, corresponding to a 30% reduction relative to neat PI, together with a saturated water uptake of 0.44 wt % (45% lower), a wet-state Dk increase of only 0.4% (compared with 1.5% for neat PI), and a tensile strength of 101.5 MPa (9% higher). FTIR spectral changes indicate that the local bonding environment of the PI matrix is modified upon filler incorporation and are consistent with possible interfacial interactions between residual silanols and carbonyl-containing groups. The Bruggeman-derived virtual air-equivalent volume fraction of 31.9% is used as a phenomenological descriptor of dielectric reduction. These results support bulk fluorination of hollow-sphere shell networks as a promising strategy for balancing low dielectricity, humidity resistance, and mechanical robustness in PI dielectrics.