View article

Highly porous aerogels, skeleton-like or sponge-like materials have aroused considerable recent interest towards various functional properties, such as lightweight construction, thermal and acoustic insulation, membranes, separation, chemical analysis, catalyst support, sensing, energy technologies, and energy absorption.[1–32] The first aerogels were demonstrated already early [33] and an extensive literature exists based on silica gels, sol-gel materials, cross-linked polymers, regenerated cellulose, and pyrolyzed carbon-based materials, as reviewed in refs.[34, 35] Aerogels are usually prepared from solvent-swollen gel networks by removing the solvent, taken that the network collapse can be suppressed by freeze-drying or supercritical drying. The air-or gas-filled skeleton structure can allow very low densities down to ca. 10 mg/cm 3 or less, high porosities in excess of 95%, and even high surface areas. As the classic routes tend to be brittle, which can reduce application potential, there has been extensive search for ductile and flexible aerogels using nanofibers. In particular, reduced brittleness and even flexibility have been shown using native cellulose nanofiber aerogels, based on nanofibrillated cellulose (NFC), also called microfibrillated cellulose (MFC) or using bacterial cellulose.[6, 12] NFC is particularly attractive due to its enhanced mechanical properties with a modulus in the range 140 GPa [36] and as cellulose is sustainable and the most abundant polymer on Earth, as available from plant cell walls.[37] Importantly, to preserve the native crystalline form, specific processes have been developed to cleave the 3–20 nm diameter …