Lightweight and Ultrastrong Polymer Foams with Unusually Superior Flame Retardancy

High-performance flame-retardant materials are urgently needed to address outstanding issues that pertain to safety. Traditional flame retardants are toxic to the environment and/or lack the physical properties required for use in many contemporary applications. Here, we show that isocyanate-based polyimide (PI) foam, a flammable material, can exhibit unusually superior flame retardancy as well as other excellent properties, such as being lightweight and displaying high mechanical strength, by incorporating red phosphorus (RP)-hybridized graphene. The covalent bonds formed between the graphene platelets and the PI matrix provide the resultant PI foam with a specific Young's modulus (83 kNm kg^-1) that is comparable to or even higher than those displayed by state-of-the-art foams, including silica aerogels, polystyrene foams, and polyurethane foams. In addition, even a low content of the RP-hybridized graphene (2.2 wt %) results in an exceptionally higher limiting oxygen index (39.4) than those of traditional flame-retardant polymer-based materials (typically 20-30). The resultant PI foam also exhibits thermal insulation properties that are similar to that of air. Moreover, the RP-hybridized graphene is prepared using a one-step ball milling process in 100% yield, and does not require solvent or produce waste. The preparation of the flame-retardant PI foams can be scaled as the starting materials are commercially available and the techniques employed are industrially compatible.