Flame‐Retarded Poly(Urethane‐Imide) Films Prepared via a Processable Intermediate Route With Enhanced Thermal Stability

ABSTRACT Poly(urethane‐imide)s (PUIs) combine the flexibility of polyurethanes with the thermal stability of polyimides; however, their inherent flammability and limited processability restrict wider application in fire‐safety‐critical environments. In this work, these limitations were addressed by integrating a reactive phosphorus‐containing polyol into the polymer backbone together with a processable intermediate strategy enabling film fabrication prior to imidization. An isocyanate‐terminated prepolymer derived from methylene diphenyl diisocyanate (MDI) and a phosphorus‐containing polyol (OP‐550) was reacted with pyromellitic dianhydride (PMDA) under mild conditions to form a soluble intermediate. The intermediate allowed solution casting of uniform films, which were subsequently converted into poly(urethane‐imide) structures through thermal imidization. FTIR spectroscopy confirmed formation of urethane and imide linkages. Thermogravimetric analysis revealed multi‐step degradation behavior, with increasing PMDA content promoting enhanced char formation and improved thermal stability. All compositions achieved UL‐94 VTM‐0 classification, exhibiting rapid self‐extinguishing behavior and suppression of melt dripping. SEM analysis of the residual char showed a cellular char morphology indicative of condensed‐phase flame‐retardant action. Mechanical testing demonstrated increased rigidity with higher imide incorporation, highlighting a balance between thermal stability and flexibility. The proposed strategy provides an effective route for producing thermally stable, self‐extinguishing PUI films for applications requiring enhanced flame resistance.