Bidirectional anisotropic polyimide/bacterial cellulose aerogels by freeze-drying for super-thermal insulation

There has been a growing interest in developing novel thermal insulation materials since the traditional insulation material is unable to manage heat intelligently. Herein, bidirectional anisotropic polyimide/bacterial cellulose (b-PI/BC) aerogels with good structural formability, high mechanical strength, and excellent thermal insulation properties have been prepared via bidirectional freezing technique. Polyimide makes the composite aerogels robust in mechanics, while uniform dispersion of bacterial cellulose in the aerogel can inhibit the shrinkage and retain the structural integrity, leading to higher porosity and lower density, thus reducing heat conduction throughout the whole aerogel. Due to its well-aligned lamellar structure obtained by bidirectional freezing technique, b-PI/BC aerogel exhibits distinct anisotropic thermal insulation behaviors with ultra-low thermal conductivity of 23 mW m−1 K−1 in the radial direction (perpendicular to the lamella) and nearly twice (44 mW m−1 K−1) in the axial direction (parallel to the lamella). The anisotropy of the thermal conductivity can substantially reduce the heat transfer in the radial direction, while helps heat diffusion in-plane to avoid heat localization, which possess remarkable advantages over the random and unidirectional counterparts. Therefore, this anisotropic PI/BC aerogel can serve as a promising heat management material for practical and complex thermal insulation applications.