Sustainable and tough polyurethane films with self-healability and flame retardance enabled by reversible chemistry and cyclotriphosphazene

Broadly applied polyurethane (PU) films are required to be self-healing, flame-retardant, recyclable, mechanically reinforced, etc., but integrating these ideal properties into a single system is a huge challenge. On the basis of reversible Diels-Alder (DA) reactions and cyclotriphosphazene, a series of novel self-healing, flame-retardant, recyclable, and mechanically robust polyurethane films (PU-DA-x) was prepared. Owing to the linear polyurethane chains containing pendant furan being effectively cross-linked by tri-maleimide end-capped cyclotriphosphazene through DA covalent bonds, the tensile stress at break and Young's modulus increased by 293% and 732%, respectively. Based on the reversible DA reactions, the excellent self-healing capability of the cross-linked polyurethane films was studied qualitatively and quantitatively. In addition, due to the introduction of cyclotriphosphazene, the synthetic polyurethane elastomers exhibited extraordinary flame retardance. In addition, the PU-DA-x films were endowed with good recyclability, relying on the retro-DA/DA process.