Cyclotriphosphazene-bridged periodic mesoporous organosilica-integrated cellulose nanofiber anisotropic foam with highly flame-retardant and thermally insulating properties

•We synthesized cyclotriphosphazene-bridged periodic mesoporous organosilica.•PMOPZ was integrated with CNF to design a highly anisotropic composite foam.•The composite foam has an ultra-low thermal conductivity (27 mW m−1 K−1).•The composite foam exhibits superior flame-retardant and mechanical performance. Thermally insulating materials derived from renewable cellulose nanofibers (CNF) have attracted an increasing attention from researchers. However, CNF insulating materials exhibit high combustion characteristic, restricting the potential for constructing the energy-efficient buildings. In this study, we synthesized cyclotriphosphazene-bridged periodic mesoporous organosilica (PMOPZ) for the first time. Afterwards, PMOPZ was integrated with CNF to design an anisotropic composite foam via the unidirectional freeze-drying method. The anisotropic composite foam exhibits superior flame-retardant and thermally insulating performance with low density (16.6 kg/m3) and high porosity (99.2%). PMOPZ/CNF foam has an ultra-low thermal conductivity of 27 mW m−1 K−1 in the radial direction, approaching the value of air (25 mW m−1 K−1). Compared to CNF foam, the integration of PMOPZ results in the lowest peak heat release rate (18.2 kW/m2) and total heat release (0.53 MJ/m2), corresponding to a 72.4% and 68.5% reduction respectively. The gaseous and condensed analysis reveals that silica wrapped with intumescent char residues assembles together to restrain the permeation of exterior heat and oxygen and inhibit the effusion of pyrolytic volatiles. Moreover, the composite foam also has high mechanical strength with a specific Young’s modulus of 96 kN m kg−1. Therefore, this work provides a novel strategy to achieve cellulose-based insulating materials with high performance and fire safety.