Transparent Insulators with a Tough Nanocellulose Skeleton Formed via Freeze-Drying

It is important to decrease considerable heat loss in buildings and vehicles to establish a low-carbon society. Transparent insulators applicable to windows can effectively reduce such heat loss. Conventional options for transparent insulators are silica aerogels that provide both optical transparency and thermal insulation benefits. However, silica aerogels are prepared via an unscalable supercritical drying process and are mechanically brittle, which restricts their practical use. In this study, we developed transparent and thermally insulating "cryogels" comprising mechanically strong cellulose nanofibers (CNFs). The term cryogels refers to porous structures prepared via practical freeze-drying of wet gels. In the synthesis, the agglomeration of nanofibers in a wet CNF gel during the freezing step was suppressed by rapidly freezing tert-butyl alcohol (t-BuOH)-exchanged wet gels with a stiffened network skeleton. The visible-light transmittance values of the cryogels ranged from approximately 80-90%, and the thermal conductivity reached as low as 0.023 W/m·K. We also extended this approach to chitin nanofibers (ChNFs) to demonstrate its universality. The resulting ChNF cryogels exhibited similar high transparency (∼80%) and low conductivities (0.022 W/m·K). These cryogels, with the potential for scalable production, are suitable as interspace materials for use in double-glazed windows, offering a promising solution for sustainable transparent insulators in energy-efficient building and vehicle applications.