Anisotropic, elastic cellulose nanofibril cryogel cross-linked by waterborne polyurethane with excellent thermal insulation performance

As the global environment has become increasingly threatened, nanocellulose-based lightweight porous materials have attracted more and more attention in the field of thermal insulation due to their sustainability and biocompatibility. However, nanocellulose-based lightweight porous materials have poor elasticity and the porous structure is prone to irreversible collapse during compression. In order to give nanocellulose-based lightweight porous materials wider application scenarios in thermal insulation, herein, we selected blocked waterborne polyurethane (BWPU) to enhance the flexibility of nanocellulose cryogel, and successfully prepared anisotropic, elastic cellulose nanofibril-waterborne polyurethane (CNF-WPU) cryogel by directional freeze-casting and freeze-drying combined with high temperature post-treatment. The obtained anisotropic CNF-WPU cryogel has good elasticity, demonstrated by a high reversible compression of 60%. Moreover, the anisotropic structure also leads to anisotropic thermal conductivity, an extremely low thermal conductivity of 0.02755 W m⁻¹ K⁻¹ in the radial direction (perpendicular to the lamella structure) and a thermal conductivity of 0.03924 W m⁻¹ K⁻¹ in the axial direction (parallel to the lamella structure) can be achieved. And the CNF-WPU cryogel still retain its outstanding thermal insulation performance (thermal conductivity of 0.02755 W m⁻¹ K⁻¹) after a reversible compression of 60%.