Highly dispersible ternary composites with high transparency and ultra low dielectric constants based on hyperbranched polyimide with organosilane termini and cross-linked polyimide with silica

Abstract

Flexible insulating materials that are both thermally and mechanically stable, highly transparent, and have low dielectric constants are highly desirable for electronic applications. With these technical needs, a highly disperse inorganic matrix is the most important factor in polyimide–inorganic composites. We report an optimised method for the preparation of a hyperbranched polyimide using HBPI_{BPADA-TAP(Si)}. This method involves modifying the polymer termini by coupling (3-isocyanatopropyl)triethoxysilane to HBPI_{BPADA-TAP(OH)} via the hydroxyl (–OH) groups at peripheral positions of the polymer chain. Then, based on the HBPI_{BPADA-TAP(Si)} with silane-modified termini, linear PI_6FDA-APB(Si) and tetraethoxysilane cross-linking agent were used to prepare ternary composites, PI_6FDA-APB(Si)_HBPI_{BPADA-TAP(Si)}_SiO₂, by sol–gel cross-linking reaction. The dielectric constant (D_k) of PI_6FDA-APB(Si)_HBPI_{BPADA-TAP(Si)}-30%_SiO₂-20% was very low, 2.04, and the optical transparencies of the ternary hybrid composite films also improved over those of similar composites due to the synergistic interactions between HBPI_{BPADA-TAP(Si)} and PI_6FDA-APB(Si) that improves phase dispersion. The highest transparency, 95% at 450 nm, was obtained for PI_6FDA-APB(Si)_HBPI_{BPADA-TAP(Si)}-30%_SiO₂-20%, a significant improvement from that (87%) of the binary composite of PI_6FDA-APB(Si)_SiO₂-20%. The glass transition temperature (T_g) of PI_6FDA-APB(Si)_HBPI_{BPADA-TAP(Si)}-30%_SiO₂-20% is 212.6 °C, which is the highest in the ternary composite series. PI_6FDA-APB(Si)_HBPI_{BPADA-TAP(Si)}-40%_SiO₂-20% has the largest storage modulus, 2952.0 MPa at 180 °C. The tan δ values of the composite films decreased from 0.96 to 0.73 with increasing HBPI_{BPADA-TAP(Si)} content. The ternary hybrid composites with densely cross-linked SiO₂ covalent networks developed in this study have improved dielectric, optical, thermal, and mechanical properties. Our fabrication method paves the way to the facile production of high-performance flexible and transparent electronic circuits that could be used in a broad range of applications in future electronics.