Interface-engineered polyphosphazene microspheres for simultaneous enhancement of toughness and flame retardancy in bismaleimide resins

Bismaleimide (BMI) resins are widely used in demanding industries due to their exceptional thermal and mechanical properties. However, their inherent brittleness, low impact strength, and poor flame retardancy restrict wider application. This study presents a novel strategy to address these limitations by incorporating allyl-functionalized polyphosphazene (PPZ) microspheres into the BMI matrix. The PPZ microspheres were synthesized via ultrasound-assisted polymerization and surface-modified with eugenol (EN), enabling covalent bonding with the BMI matrix during curing. These functionalized microspheres exhibited excellent thermal stability (decomposition temperature >300 °C), uniform size, and controlled reactivity, ensuring successful integration. The incorporation of 1 wt% of the functionalized microspheres (P2E) dramatically enhanced mechanical properties, increasing flexural and impact strengths by 91.3 % and 99.9 % respectively. Dynamic mechanical analysis (DMA) confirmed that this improved toughness did not compromise thermal performance, with the glass transition temperature (T_g) remaining above 260 °C. Furthermore, adding 2 wt% P2E significantly improved flame retardancy. The peak heat release rate (PHRR) and total heat release (THR) were reduced by 32 % and 21 %, respectively, while the limiting oxygen index (LOI) increased from 32.5 to 35.1. This work provides a promising approach for simultaneously improving the toughness, flame retardancy, and thermal stability of BMI resins, making them highly suitable for advanced industrial applications.