Ionic liquids boosted rigid polyimide foaming: Heterogeneous porous structure and efficient microwave absorption

The supercritical CO 2 (scCO 2 ) foaming method shows potential for fabricating porous polyimide (PI) materials with small pore size and high void fraction. However, conventional PI polymers suffer from low gas solubility and a rigid molecular backbone, resulting in poor foaming dynamics and high foaming resistance. In this work, we synthesized a thermally stable PI polymer with a rigid backbone, into which highly compatible ionic liquids (ILs) molecules were incorporated. The introduced ILs acted as molecular lubricants, simultaneously enhancing foaming dynamics and reducing foaming resistance. The resulting ILs/PI foam materials shows small pore size (9–12 μm), hydrophobic property (water contact angle 126.5 o ), flame retard property, low temperature toughness (−196 °C) and high thermal resistance property (406 °C). Moreover, a heterogeneous layered porous PI structure was fabricated by modulating the ILs content across different layers, creating alternating porous and solid regions. Leveraging this platform, CNT@Fe₃O₄ nanofillers, providing both dielectric and magnetic loss, were introduced into the porous layer, while conductive CNT was incorporated into the solid layer. The resulting heterogeneous porous PI nanocomposite materials exhibited ultra-low reflectivity electromagnetic interference (EMI) shielding performance (average A/R coefficient ratio of 67.1).