Fabrication of Thermal Insulative and Hydrophobic Copolymerized Polyimide Foams with Anisotropic Performance Through Microwave-assisted Foaming

In this work, lightweight, thermal insulative and hydrophobic polyimide foams (PIFs) with anisotropic pore structure were fabricated. Polyester ammonium salt (PEAS) precursor powders were synthesized by adopting copolymerization strategy where ODA was used as diamine and different molar ratios of BTDA/BPADA was used dianhydride. Afterwards, PIFs were prepared by microwave-assisted foaming and thermal imidization treatment. The “bottom-up” directional foaming process led to the formation of regular near-spherical pores in the vertical direction and well-arranged ellipsoidal strip-like pores in horizontal direction (perpendicular to the pore growth direction), thereby endowing PIFs with anisotropic mechanical, thermal insulation and hydrophobic properties. The compressive strength in the vertical direction was 1-3 times higher than that in horizontal direction. The aligned ellipsoidal strip-like pore structure in the horizontal direction endowed PIFs with low thermal conductivity (0.0267 W/(m·K)) which is quite comparable to that of air. Moreover, the water contact angel of PIFs in the vertical direction reached as high as 128.0°, displaying exceptional hydrophobicity. This work provided a facile method to fabricate multifunctional PIFs that show a promising application in the aerospace, construction, energy and nuclear energy sectors among others.