Anisotropic polyimide aerogels fabricated by directional freezing

In this study, we adapted a simple, low-cost, and environmentally friendly method to fabricate anisotropic polyimide (PI) aerogels. During directional freezing of an aqueous poly(amic acid) ammonium salt solution from the profile to the center axis of the cylindrical mold, the ice crystals preferred to grow along the radial direction of the cylindrical mold. After the ice crystals were sublimated by freeze drying, an anisotropic pore structure was formed in the aerogels. The prepared PI aerogels had lower densities (0.04-0.22 g/cm(3)) and higher porosities (84-97%) and exhibited anisotropy in both their pore structures and properties. Their compressive modulus and strength in the horizontal direction were both higher than those in the vertical direction, and they also had good compression recovery in the vertical direction. Moreover, their heat-transfer performance also exhibited anisotropy. The heat transfer in the horizontal direction of the aerogels was much faster than that in the vertical direction. (c) 2018 Wiley Periodicals, Inc.