Microscale molecular engineering of intrinsically flame-retardant polyurea with balanced mechanical performance.

Polyurea (PUA) has emerged as a premier material for advanced protective coatings in civil and defense infrastructure owing to its exceptional damage tolerance and robust physical/chemical properties. However, its application to scenarios with flame safety requirements remains limited. Traditional additive flame retardants often compromise mechanical properties owing to inadequate interface compatibility. Herein, we report a reactive chain-extension strategy that circumvents this trade-off by covalently incorporating a novel amino cyclotriphosphazene derivative (DACP) as a multifunctional hard segment building block into the PUA network. This approach enables homogeneous dispersion and interface-free architecture at the molecular level, ensuring structural integrity and permanent flame retardancy. When the DACP content reaches 35 wt%, the resulting composite achieves a limiting oxygen index of 27.3%. Meanwhile, the peak heat release rate decreases by 75.4%, and the total heat release drops by 35.6%. Remarkably, the material retains 95% of its original tensile strength along with a modest (8%) increase in fracture elongation. Microscale structural analyses reveal that DACP simultaneously preserves toughness and promotes the continuous formation of a graphitized coke layer to achieve condensed-phase shielding, while producing a synergistic effect with gaseous radical quenching. Overall, this design strategy offers a new paradigm for advanced polymer composites, where fire safety and mechanical robustness are integrated at the microstructural level.