Macroscopic-Scale Preparation of Aramid Nanofiber Aerogel by Modified Freezing–Drying Method

Aerogel has been widely known as a low-density and highly porous material and is closely connected with the complex processing methods, such as freeze–drying or supercritical drying. In this work, using the polymerization-induced aramid nanofiber (PANF) as a building block, we put forward a modified freezing–drying method for the high-efficiency preparation of all-para-aromatic-amide aerogels. In the preparation process, PANF hydrogel is first frozen at −18 °C and then dried at 20–150 °C for the formation of PANF aerogel. The PANF framework formed during the freezing process is crucial for the formation of the PANF aerogel. Moreover, the space-occupying effect of ice crystals is also helpful for the formation of the macroscopic pore structure in the aerogel. Aerogels with large size or well-controlled shape could be successfully obtained by this method. Through the variation of PANF concentration in the hydrogel and drying temperature, aerogels with different densities (20–185 mg/cm3) could be achieved, and the lowest density is reached at 150 °C, with the PANF concentration of 0.7%. The low-density PANF aerogels show high specific compressive strengths and low thermal conductivities, which are comparable to those resulting from the freeze–drying or supercritical drying method. Furthermore, the shrinkage phenomenon in the drying process could be skillfully utilized for the preparation of PANF aerogel-coated objects. The PANF aerogels could be applied as a thermal insulating material or shock absorption material in practical applications.