An intumescent flame retardant (DOPO-DAM) containing phosphorus and nitrogen was synthesized via a two-step process and then, it was incorporated into waterborne polyurethane to serve as a reactive flame retardant for preparing flame-retarded WPU (FR-WPU). The chemical structures of DOPO-DAM and FR-WPU were confirmed by ¹H NMR, ³¹P NMR, and FTIR studies. The thermal stability, flame retardancy and mechanical properties of FR-WPU were investigated by TGA, TGA-FIR, Py-GC-MS, limiting oxygen index (LOI) test, SEM-EDS analysis, cone calorimeter test and a universal testing machine. The results showed that the conjugation of DOPO-DAM into WPU induced a slight decline in the thermal stability of FR-WPU. However, the incorporation of DOPO-DAM into WPU significantly enhanced the flame retardancy by reducing the heat release rate (HRR), total heat release (THR), smoke production rate (SPR) and total smoke production (TSP). In addition, the morphology, elemental composition and content of the residual char of the flame-retarded WPU indicated the important function of DOPO-DAM in the condensed phase. Thus, DOPO-DAM exhibited gas-phase and condensed-phase flame retardant effects on the WPU films.

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.

A novel phase change material (PCM), namely, comb polyurethane with polyethylene oxide segments as side chains (DMPEG–PU), was synthesized through the reaction between diethanolamine-modified monomethoxy polyethylene glycol (MPEG) with isophorone diisocyanate and 1,4-butanediol. The crystalline property, phase change property, and thermal stability were characterized by differential scanning calorimetry (DSC), thermogravimetric analysis (TG), wide-angle X-ray diffraction (WAXD), scanning electronic microscopy (SEM), and polarization optical microscopy (POM). An accelerated thermal cycling test was then conducted to reveal the thermal reliability of the synthesized DMPEG–PU. The WAXD patterns and POM images showed that the synthesized DMPEG–PU possesses a completely crystalline structure and smaller spherulites compared with the pristine MPEG. The DSC results showed that DMPEG–PU (with 85% weight percentage of MPEG) is a typical solid–solid PCM with high thermal transition enthalpy of more than 120 J g⁻¹, which is higher than the thermal transition enthalpy of polyethylene glycol-based polyurethane. TG results showed that the synthesized DMPEG–PU exhibits satisfactory thermal stability.

Intrinsic healing materials based on reversible bonds possessing the unique adaptive properties of self-healing and recyclability have attracted considerable attention. One of the major challenges in intrinsic healing materials that remains is how to reinforce the mechanical property without sacrificing healing efficiency. Here, furfuryl-modified silica particles (furan@SiO₂) with high functional group grafting ratio and dispersibility, formed by sol–gel process, were introduced into the maleimide-terminated polyurethane (mPU) elastomer to establish a thermo-reversible organic–inorganic network based on Diels–Alder reaction between furan@SiO₂ and mPU. The modulus and strength of the elastomeric composites were significantly enhanced compared to neat mPU. Due to the reversible crosslinking points and organic–inorganic network, the thermo-reversible polyurethane elastomeric composites maintained favourable mechanical properties with excellent thermo-driven self-healing efficiency. Moreover, the corresponding elastomeric composites retained high repair efficiency after multiple healing processes. Furthermore, the thermo-reversible polyurethane elastomeric composites could be recycled via solution casting and hot-pressing due to the thermally reversible cross-linking point. Such excellent combination of mechanical and self-healing performance reveals the potential of this new approach in designing self-healing hybrid materials with tunable structures and mechanical properties.