Preparation and Characterization of Cellulose Nanofibers/Polybenzoxazine–Poly(vinyl alcohol) Double Network Foam Based on Multiple Hydrogen Bond Structure

Utilizing a multiple hydrogen bond structure, a dual-network hybrid foam of cellulose nanofibers/polybenzoxazine–poly(vinyl alcohol) (CNFs/PBZ–PVA) was successfully synthesized for the first time through a one-pot method. By combining the rigidity of the CNFs–PVA network and the elasticity of the PBZ–PVA network, a dual-network foam with balanced mechanical properties and resilience was achieved. SEM results revealed that CNFs/PBZ–PVA possesses a regular honeycomb network structure, ensuring uniform pore distribution (10–40 μm) and a porosity of over 97%. Chemical analysis indicated that through the introduction of long-chain polybenzoxazine molecules and their connection through the intermediate carrier of poly(vinyl alcohol), the CNFs–PVA and PBZ–PVA networks were physically entangled, forming a dense, uniformly porous, and heat-resistant hybrid foam structure with multiple hydrogen bonds. At room temperature, the foam exhibited outstanding resilience, quickly recovering after 60% compression deformation and displaying fatigue resistance (a recovery rate of more than 95% can be maintained after 30 compression cycles). Moreover, its resilience remained unchanged after treatment with liquid nitrogen (−196 °C), overcoming the shortcomings of conventional nanocellulose-based foams in terms of poor resilience and fragility. Additionally, under extremely acidic or alkaline conditions, the hybrid foam demonstrated excellent chemical stability, expanding its potential applications in extreme conditions. Finally, the CNFs foam exhibited remarkable thermal insulation (thermal conductivity is low to 0.057 W/(m·K)) and self-extinguishing properties, making it a promising material for certain special environments.