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• A boric acid-regulated sol–gel transition strategy is used to achieve modulation of the morphology. • Ethanol is employed as a green solvent for the preparation of boron-doped polybenzoxazine (BPBz) aerogels. • The introduction of boric acid provides multifunctional properties to BPBz aerogels. • The BPBz aerogels exhibit intrinsic and alterable wettability that correlates with their morphologies. Polybenzoxazine aerogels have gained widespread attention in recent years, but the prevalent use of hazardous solvents in the preparation process has severely impeded their further development and promotion for practical applications. Herein, using ethanol as a solvent, a strategy of boric acid-induced microstructure regulation is proposed to construct boron-doped polybenzoxazine (BPBz) aerogels, which provides a feasible approach to overcome the above situation. The introduction of boric acid enhances the backbone strength by forming intermolecular bridged structures between PBz chains and allows the use of ambient pressure drying. The resulting BPBz aerogel demonstrates high stiffness (specific modulus up to 255.97 kN·m·kg−1), low bulk density (0.203 g·cm−3), and low thermal conductivity (0.043 W·m−1·K−1), exhibiting excellent fire resistance when exposed to a 1200 °C flame (the self-extinguishing interval less than 1 s). More interestingly, the aerogel demonstrates intrinsic and tunable wettability that correlates with the morphology. Combining the environmentally friendly preparation strategy and these advantages, BPBz aerogels hold promise for thermal management and fire protection applications in energy-saving construction and other harsh conditions. [ABSTRACT FROM AUTHOR]

[Display omitted] • Lightweight polybenzoxazine (PBz) aerogels were fabricated by hexamethylene diisocyanate (HDI) modification. • The introduction of HDI formed a covalent-crosslinked structure with the chemical structure of the PBz aerogel. • The HDI-modified PBz aerogel demonstrates a self-extinguishing time of less than 1 s after removal from a 1200 °C flame. The rapid advancement of modern industries has imposed demands on thermal insulators that possess not only low thermal conductivities but also multifunctional properties. Polybenzoxazine (PBz) aerogels with several advantages have garnered considerable research interest in recent years. Nevertheless, PBz aerogels acquired from ambient pressure drying still confront the challenge of exhibiting relatively high densities and thermal conductivities. In this study, to enrich the category and propose practical ideas for overcoming the above plight, hexamethylene diisocyanate (HDI) was selected as an additive to prepare lightweight and efficient thermal-insulating PBz aerogels through chemical modification. Compared with the pristine specimens (0.268 g·cm−3, 0.0387 W·m−1·K−1), the HDI-modified PBz aerogels exhibited lower densities (0.237–0.256 g·cm−3) and thermal conductivities (0.0354–0.0370 W·m−1·K−1). Moreover, the introduction of HDI has been demonstrated to not impact their superior intrinsic flame retardance, despite an observed reduction in the residual char yields in the HDI-modified groups. Given these excellent properties, PBz-HDI aerogels are expected to be promising candidates for thermal insulation and fire resistance applications. [ABSTRACT FROM AUTHOR]