In this study, we report a series of lightweight, anisotropic polyimide/cellulose nanofibril (PI/CNF) composite aerogels with outstanding thermal insulation and flame retardancy via unidirectional drying and freeze drying methods. Along the axial direction, PI/CNF composite aerogels exhibit an arranged tube-like structure in parallel to ice crystal growth, thus providing superior mechanical properties. Conversely, the composite aerogels show a honeycomb-like pore structure along the radial direction, exhibiting excellent elastic properties. More importantly, the addition of CNF endows the composite aerogels with low thermal conductivity along the radial direction. The surface temperature of the composite aerogels with a thickness of 15 mm is 70% lower than that of hot stage annealing at 180 °C for 60 min. Furthermore, the composite aerogels show flame retardant and self-extinguishing performance in the combustion experiment. Therefore, the anisotropic PI/CNF composite aerogels with thermal insulation and flame retardancy are promising for expanding potential applications in aerospace fields.

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