Reed-inspired dual-siloxane engineering of cellulose nanofibril/alginate aerogels for wet-dry-tolerant thermal insulation.

Cellulose-based aerogels are promising sustainable thermal insulators, but their hydrophilic skeletons absorb moisture, causing capillary-shrinkage-induced collapse and insulation loss. Meanwhile, rigid hydrogen-bonded or ionically cross-linked networks fatigue under cyclic deformation. Inspired by reed structures, which integrate a water-repellent exterior with a lightweight porous interior, a two-step methyltrimethoxysilane (MTMS)/polydimethylsiloxane (PDMS) dual-siloxane strategy was developed to regulate wettability and junction flexibility in cellulose nanofibril/sodium alginate aerogels, where MTMS forms a water-repellent external interface and PDMS improves the flexibility and cyclic stability of the interconnected porous framework. Chemical vapor deposition of MTMS created a methyl-rich, low-surface-energy layer, and subsequent PDMS vacuum impregnation and thermal curing formed flexible coatings and bridges at fibrillar junctions. The optimized aerogel showed a water contact angle of 139.1° and retained 92.18% and approximately 68% of peak stress after 10 and 100 compression cycles at 50% strain, respectively. Its thermal conductivity changed from 48 mW m-1 K-1 in the dry state to 72 mW m-1 K-1 under near-saturated wet conditions and recovered to 56 mW m-1 K-1 after re-drying following 72 h wet-dry cycling. Although its dry-state thermal conductivity is higher than that of many conventional commercial insulation materials, the aerogel exhibited more stable thermal-insulation performance and partial recovery after re-drying following the 72 h wet-dry cycling protocol. Roof-model tests confirmed more stable insulation under simulated rainfall and weathering. This work provides a structure-oriented strategy for biomass-derived aerogels in wet-dry environments.