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

Abstract

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/cm 3 ) 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.

Anisotropic 3D Nanofibrous Porous Material Fabrication by a Liquid Film-Assisted Gas Templating Strategy for Thermal Insulation

“…In the previously reported preparation of nanofiber-based 3D porous materials, the pores are full of liquid before drying. Therefore, a cumbersome and time-consuming drying process such as freeze-drying or supercritical extraction is required to obtain the porous monoliths. − However, in the present work, as described above, the as-formed 3D NPM has no liquid in the pores, and the capillary force is significantly decreased. Therefore, the 3D NPM can be dried under ambient conditions.…”

“…Certainly, the liquid film will collapse with excess gas, and the liquid will remain as a coating on the surface of the fibers, while the fibers will not recover to their original state due to the robust and plastic nanofiber network. There is no liquid in the chambers except for that in the walls after expansion, so the capillary force was significantly decreased, which allowed the drying of the 3D NPM under ambient conditions rather than freeze-drying − or supercritical extraction. , …”

Superinsulating BNNS/PVA Composite Aerogels with High Solar Reflectance for Energy-Efficient Buildings

“…3 h). Regardless of contact and non-contact heat sources, the composite aerogel delivered superior thermal insulation performance, presenting promising applications in thermal protection [ 65 ].…”

Recent Advances in Self-Assembly and Application of Para-Aramids

“…Although a laminated PANF aerogel has a regular structure and excellent comprehensive properties, the freeze-drying method used in the preparation process is not conducive to the large-scale production of PANF aerogel, and the directional solidification condition is a little complicated, which also hinders the batch preparation of PANF aerogels. Therefore, the authors further proposed a modified freezing-drying method, which can be used for the preparation of a PANF aerogel [ 79 ]. According to the method, a PANF hydrogel with a certain solid content was first frozen at −18 °C, and then the completely frozen PANF hydrogel was dried at room temperature to 150 °C to obtain the aerogel material finally.…”