Free volume regulated crosslinked polyimide foams with enhanced toughness and thermal resistance

Polyimide foams (PIFs) are attractive lightweight materials owing to their high mechanical strength, excellent thermal resistance, thermal insulation, and inherent flame retardancy. Nevertheless, the incorporation of crosslinked architectures often leads to a pronounced trade-off between heat resistance and toughness. To address this long-standing limitation, a tetra-functional spirocyclic anhydride is rationally designed and employed as a multifunctional crosslinker to fabricate crosslinked PIFs. The combined effects of spirocycle-induced free volume and crosslinked network result in a refined and homogeneous cellular architecture, with the average cell size reduced from 320 ± 100 μm to 146 ± 49 μm. Benefiting from the synergistic effects of cellular refinement and crosslinking reinforcement, the PIFs exhibit markedly improved mechanical performance, the specific strength, specific modulus and toughness increased to 194 %, 175 % and 245 % of the initial value respectively, together with enhanced fatigue resistance (93.44 % strength retention after 30 cycles). Notably, these performance metrics exceed those of many previously reported PIFs with comparable or even higher densities. Moreover, the introduction of spirocyclic crosslinked structures significantly improved thermal resistance, leading to an increase in glass transition temperature by 62 °C. This work demonstrates an effective dual-enhancement strategy that improves mechanical robustness, toughness and heat resistance of crosslinked PIFs, offering a viable design paradigm for advanced polymer foams operating under extreme conditions.