Anisotropic and Lightweight Silica Aerogel-Doped Copolymerized Polyimide Foams Fabricated by Combining Microwave-Assisted Foaming and Freeze-Drying for High-Temperature Thermal Insulation and Hydrophobic Applications

A series of silica aerogel (SiA)-containing copolymerized polyimide composite foams were prepared by combining microwave-assisted foaming with vacuum/ultrasound-assisted impregnation and freeze-drying methods. The fluorinated polyimide foams (F-PIF) featuring "bottom-up" directional growth of pores and uniform impregnation with polyimide aerogels containing fluorine and silicon moieties (FSi-PIA) and nano SiA, exhibited enhanced mechanical and thermal insulation performance as well as hydrophobicity. The relationship and mechanism between copolymerized molecular chain structure, anisotropic pore structure, and properties of composite foams were elucidated. The F-PIF/FSi-PIA/SiA composite foams showed mechanical robustness in the vertical direction (compressive strength improvement of 157.8%) and mechanical flexibility in the horizontal direction (compression response rate up to 97.9%). In addition, the prepared composite foams showed extraordinary hydrothermal aging resistance, thermal stability and thermal mechanical properties, self-extinguishing flame retardancy, and high-temperature thermal insulation with thermal conductivity ranging from 0.0251 to 0.0555 W/(m·K) between 25 and 300 °C. Benefiting from the unique anisotropic pore structure, the impregnation of FSi-PIA as well as nano SiA endowed F-PIF/FSi-PIA/SiA composite foams with a water contact angle as high as 151.1° and water absorption rate as low as 0.31 g/g. Therefore, a facile strategy was proposed to fabricate multifunctional PI composite foams that exhibit promising applications in extreme environments.