Melt–Prepolymerization–Foaming for Ultralight Phthalonitrile Aerospace Foams

Next-generation aerospace platforms demand lightweight sandwich-core foams that withstand extreme temperatures (200–400 °C) while maintaining structural integrity. However, conventional polymer foam degrades above 200 °C. Phthalonitrile (PN) resins offer exceptional thermal stability but are hindered by high melting points, narrow processing windows, and curing challenges. Here, we present a melt–prepolymerization–foaming (MPF) strategy that overcomes the incompatibility between the high melting point and high curing temperature of PN and the relatively low decomposition temperature of conventional chemical foaming agents, which otherwise leads to pore coarsening or even foaming failure. We successfully fabricate tailorable cellular Phthalonitrile foams (PNF) with ultralow density (0.056 g·cm–3) and a high expansion ratio (up to 22.82). By decoupling prepolymerization from gas release, we achieve precise control over rheology and pore morphology, enabling closed-cell structures with uniform pore size (200–800 μm) and superior thermal stability (5% weight loss temperature up to 527 °C) and up to 15.309 MPa·cm3·g–1 specific strength. The resulting foams outperform existing high-temperature foams in density-normalized strength and thermal resistance, offering a scalable solution for extreme-environment applications in aerospace and beyond.