Anisotropic and flexible fluorinated polyimide foams via co-polymerization and microwave-assisted foaming for efficient thermal insulation and hydrophobicity

Abstract

This work reports the flexible, hydrophobic and thermal insulative polyimide foams (PIFs) with anisotropic pore structure for potential application in high-tech fields. Polyester ammonium salt (PEAS) precursor powders with fluorinated groups were prepared by co-polymerization process and they were foamed using microwave-assisted foaming strategy. The introduction of 6FDA increased chain rigidity and decreased the melt viscosity of PEAS, which improved the foaming behavior of PEAS and the properties of subsequent PIFs. The PIFs had low density (18.61 kg/m³), high open cell rate (99.0%) and flexibility (compression response rate>97%), excellent thermal resistance (T_g = 292.9 °C) and thermal stability (T_5% = 550.1 °C). Benefiting from the “bottom-up” directional foaming behavior, anisotropic pore structures were constructed in-situ and the fluorinated PIFs exhibited anisotropic mechanical, thermal insulation performance and hydrophobic behavior. The thermal conductivity of PIFs reached 0.0269 W/(m·K) in the horizontal direction (perpendicular to pore growth direction). The top surface temperature of PIFs was 67.8 °C after being heated on a 200 °C preheated hot plate for 30 min, exhibiting superior thermal insulation. The PIFs possessed exceptional hydrophobicity with a water contact angle as high as 131.7° and water absorption rate as low as 0.65 g/g. The flexible, hydrophobic and thermal insulative PIFs demonstrates promising application for long-term use in high-tech fields such as aerospace, transportation, construction and nuclear among others.