Interface-engineered Polybenzoxazine/Aramid Ⅲ nanofiber composite aerogels with ultralow thermal conductivity, exceptional mechanical robustness, and intrinsic flame retardancy

High-performance aerogels, characterized by ultra-low density and very low thermal conductivity, show great potential for applications in aerospace, energy management, and fire safety. However, conventional inorganic aerogels often exhibit high brittleness and poor toughness. Single-component aramid Ⅲ nanofiber (AⅢNF) aerogels tend to collapse structurally when exposed to high temperatures or flames, which severely restricts their long-term performance. To overcome this challenge, we propose an interface-engineered in-situ polymerization coating strategy. A robust composite aerogel framework is fabricated by in-situ polymerizing polybenzoxazine (PBa) onto the surface of AⅢNFs. The preparation of this aerogel involves four main steps: (i) deprotonation and reprotonation of AⅢNFs, (ii) acid-catalyzed polymerization of benzoxazine monomers, (Ⅲ) ice-template-assisted freeze-drying, and (iv) hydrophobic modification through chemical vapor deposition. The resulting PBa/AⅢNF composite aerogel demonstrates very low thermal conductivity (0.027–0.032 W m −1 K −1 ), high compressive strength (up to 18.92 MPa), a robust compressive modulus (11.59 MPa), and intrinsic flame retardancy with a limiting oxygen index of 41.7%. Mechanistic analysis reveals that the PBa coating strengthens interfacial bonding via hydrogen bonding and π-π interactions, while generating a dense carbon layer during combustion that effectively impedes heat and oxygen transfer. In addition, the aerogel displays superhydrophobicity (contact angle > 150°) and long-term thermal aging stability at elevated temperatures. This study proposes a universal strategy that integrates high-char-forming polymers with nanofiber frameworks, enabling synergistic optimization of aerogel lightweight design, mechanical reinforcement, and multifunctional protection.