Supercritical CO₂ Foaming of Molecular Chain-Modified Polyimide for High-Performance Electromagnetic Wave Absorption

The microwave absorption performance depends on both impedance matching and electromagnetic energy dissipation. Conventional filler-functionalized polyimide (PI) composites can provide strong electromagnetic attenuation, but their limited foamability under supercritical carbon dioxide (scCO₂) restricts porous structure formation and thus impairs impedance matching. To address these limitations, this study proposes an intrinsic molecular modification strategy based on a chain segment design. Furthermore, by combining synergistic dielectric-magnetic loss from nanofillers with a double-layer heterogeneous architecture, a PI-based electromagnetic shielding nanocomposite with strong microwave absorption performance and low-reflection characteristics was constructed. Bisphenol A dianhydride (BPADA) was used as the dianhydride monomer, and a series of PI copolymers with tunable chain segment compositions were prepared through the reaction of BPADA with mixed aromatic diisocyanate (MDI) and aliphatic diisocyanate (HDI), followed by thermal imidization/cyclization. The changes in molecular chain mobility, scCO₂ foaming behavior, and cellular structure associated with the nominal HDI feed fraction were systematically investigated. The results showed that increasing the nominal HDI feed fraction was associated with an improved rigid-flexible balance of the PI backbone, enhanced segmental mobility, a broader foaming window, and the formation of PI foams with uniform and compact cellular structures. CNT@Fe₃O₄ nanofillers with both dielectric and magnetic loss capabilities were further introduced to improve the intrinsic electromagnetic loss of the composite foams. The calculated microwave absorption results showed that, after pore structure regulation, the PI/CNT@Fe₃O₄ nanocomposite foam exhibited markedly enhanced microwave absorption in the X-band. At a porosity of 46 vol %, the minimum reflection loss reached -38 dB at a thickness of 2 mm. A double-layer heterogeneous structure comprising a CNT@Fe₃O₄-containing porous absorbing layer and a CNT-containing dense reflecting layer was then constructed to exploit impedance gradient transition, dielectric-magnetic synergistic loss, and multiple reflection/reabsorption processes. The optimized sample showed an average absorption coefficient of 0.92 in the X-band and a maximum A/R ratio of 332, indicating pronounced high absorption and low reflection characteristics. This study provides a new strategy for designing and fabricating lightweight, high-performance PI-based electromagnetic protection materials with an absorption-dominated shielding behavior.