Synergistic Cocrosslinking Strategy toward Ultrahigh Heat-Resistant Polyimide Rigid Foams with Integrated Mechanical Strength, Flame Retardancy, and Thermal Insulation

Lightweight and high-performance polyimide rigid foams (PIRFs), exhibiting outstanding heat resistance and mechanical properties, as well as excellent flame retardancy, are increasingly required for specialized load-bearing applications. However, commercially available polyimide (PI) foams, which are mostly flexible and low-density, are thermally and mechanically inadequate for high load-bearing purposes. Herein, a scalable and high-efficiency “kill two birds with one stone” cocross-linking and foaming strategy is developed for the fabrication of highly heat-resistant, mechanically strong, and flame-retardant cocross-linked polyimide rigid foams (PIRFs) using the norbornene-end-capped cross-linkable blowing agent (NE-CBA) as both cross-linking agent and blowing agent. Benefiting from the simultaneous construction of high-density cocrosslinked networks and tailorable closed-cell structures via the reaction of maleic anhydride-end-capped PI oligomer (ME-PIO) and NE-CBA, the resultant cocrosslinked PIRFs exhibit exceptional heat resistance, with an ultrahigh glass transition temperature (T_g) of 410.6 °C and a remarkable thermal degradation temperature (T_d) of 500.3 °C. Moreover, they possess outstanding mechanical properties, with a high compressive strength of 1.9 MPa at room temperature and 1.7 MPa at as high as 200 °C, with a low mass density of 157 kg/m³. In addition, the PIRFs exhibit excellent thermal insulation properties, with low thermal conductivities (λ) of 0.041 W/(m·K) at room temperature and 0.057 W/(m·K) even at 200 °C, as well as desirable flame retardancy with a high limiting oxygen index (LOI) exceeding 40%. Owing to the dual functionality of the NE-CBA employed in this strategy and the resulting synergistic effect, the PIRFs achieve the T_g > 410 °C and compressive strength exceeding 1.9 MPa at a foam density of 157 kg/m³. This represents the successful simultaneous achievement of low density, ultrahigh heat resistance, and high mechanical strength. The highly heat-resistant and mechanically strong cocross-linked PIRFs, with superior thermal insulation and flame-retardant performance, show significant potential in fields such as aerospace engineering, shipbuilding, railway transportation, and other specialized high-temperature applications.