Fabrication of Polyimide Foams with Enhanced Mechanical Toughness and Heat Resistance via Tuning a Free Volume Strategy

Polyimide foams (PIFs) exhibit exceptional properties, including high-temperature resistance, radiation tolerance, and thermal insulation. However, they often face the inherent performance trade-off between mechanical toughness and heat resistance due to the much more rigid molecular frameworks, restricting their application in the field of high technology. To overcome this challenge, the strategy of regulating intermolecular free volume in this study was developed by introducing trifluoromethyl side groups into the polyimide backbone. The enlarged free volume with an enhancement of 8.62% for the fractional free volume (FFV) can promote the generation of abundant nucleation sites during the foaming process and result in a uniform and refined cellular architecture, which exhibits a decreased average pore size from 320 ± 100 to 200 ± 60 μm. Benefiting from the meticulously controlled porous structure, the optimal PIFs achieve a significant improvement of mechanical performance, with compressive strength, toughness, and modulus improving 180, 173, and 278%, respectively, which are superior to those of many other previously reported PI foams with similar or even higher densities. Specifically, the fatigue resistance of the PIFs was significantly enhanced, as evidenced by a compressive strength retention of 91.1% after 30 cyclic compression tests. Meanwhile, the PIFs also demonstrate remarkable heat resistance and thermal insulation, with the glass transition temperature increased by 22 °C and thermal conductivity ranging from 0.028 to 0.030 W·m–1·K–1 at room temperature. This strategy simultaneously improved the mechanical strength and toughness as well as heat resistance of PIFs, which is beneficial for its applications in extreme environments.