Flexible and Super Thermal Insulating Cellulose Nanofibril/Emulsion Composite Aerogel with Quasi-Closed Pores

Because of the prevailing environment and energy challenges, there has been a growing interest in biobased materials for thermal insulation application. Although cellulose aerogel has been considered as an excellent thermal insulating material, its thermal conductivity is generally negatively affected by the interconnected internal pores. Herein, it is demonstrated that a cellulose nanofibril (CNF)/emulsion composite aerogel with quasi-closed internal pores can be facilely fabricated by Pickering emulsion templating and solvent exchange methods. The CNF-stabilized oil-in-water Pickering emulsion (with an average diameter of 1.3 μm) can be converted into quasi-closed pores by sequential solvent exchange to acetone and tert-butanol (TBA), followed by freeze-drying from TBA to suppress the formation of large ice crystals. The presence of quasi-closed pores from emulsion templating is verified by both confocal microscopy and scanning electron microscopy images and is confirmed to reduce thermal conductivity to as low as 15.5 mW/(m K). Compared to the CNF aerogel, increasing emulsion content can lead to better volume retention with significantly reduced density (11.4 mg/cm³), increased mesoporosity, and enhanced specific modulus (18.2 kPa/(mg/cm³)) and specific yield strength (1.6 kPa/(mg/cm³)). In addition, the CNF/emulsion composite aerogel also demonstrates superb flexibility and infrared shielding performance.