Dual chemical crosslinking strategy to fabricate lightweight, flame-retardant, high-modulus and hydrophobic cellulose cryogel

Cellulose cryogel shows great application potential as a thermal insulation material because of its eco-friendliness, lightweight, high porosity and highly-efficient thermal insulation property. However, the high flammability and hydrophilicity have become bottlenecks to restrict its application in the thermal insulation field. Herein, we synthesized an amylose derivative with ammonium phosphate groups (AM), and reported a dual chemical crosslinking strategy to fabricate lightweight, flame-retardant, high-modulus and hydrophobic cellulose cryogel with AM and methyltrimethoxysilane (MTMS) via baking-crosslinking and chemical vapor deposition techniques. The dual crosslinking structure endowed the composite cryogel (AM30Si) with a high specific modulus of 47.0 MPa/(g/cm³), which enabled it to sustain 12,500 times its own weight. The thermal conductivity of AM30Si was only 28.7 mW/(m·K), which benefited from its anfractuous three-dimensional porous network structure. The P/N/Si synergy enhanced the flame retardancy of AM30Si, and its UL-94 rating and LOI value reached V-0 and 39.2%, respectively. Moreover, AM30Si possessed satisfactory hydrophobicity, oil absorption and continuous oil-water separation ability. This study provides not only an insight into the syntheses of reactive polysaccharide derivatives with high flame-retardant activity, but also an innovative solution to simultaneously address the inflammability, hydrophilicity and inadequate strength of cellulose cryogel while largely maintaining its lightweight feature.