Highly transparent triethoxysilane-terminated copolyimide and its SiO2 composite with enhanced thermal stability and reduced thermal expansion

Abstract

A highly transparent and colorless copolyimide was synthesized to contain electron withdrawing sulfone and fluorine groups. Then, the triethoxysilane terminal groups were introduced by reacting the anhydride end groups of the polymer with (3-aminopropyl)-triethoxysilane. Based on the triethoxysilane-terminated copolyimide, a series of composites of the copolyimide (PI6FDA-TFDB-mBAPS(Si)) with inorganic SiO2 network was successfully fabricated in order to satisfy the requirements for transparent insulators with high thermal stability. Because the hybrid with inorganic SiO2 networks, the composite (PI6FDA-TFDB-mBAPS(Si)_SiO2) can complementarily improve the thermal resistant drawbacks of organic systems. As a result, the composite films exhibit small values of the coefficient of thermal expansion (CTE) with the increasing SiO2 content. The coefficient of thermal expansion (CTE) of the hybrid PI6FDA-TFDB-mBAPS(Si)_SiO2-40% film is 14.4ppm°C−1 in the temperature range from 80 to 170°C. Compared to a value of 34.7ppm°C−1 for the pristine PI, a 57% reduction is achieved for the composite. The composite possesses the glass transition temperature (Tg) of 276.0°C and shows good optical transmittance of 94% at the wavelength of 450nm. The incorporation of cross-linked SiO2 networks by using the triethoxysilane-terminated PI preserves the high transparency and enhances the thermal stability for the composite films. Due to these optimized properties, the composite can show potential applications in the micro-flexible transparent electronic devices such as transparent insulating panel of advanced transparent display devices and transparent flexible circuit board.