Synergistic enhancement of mechanical and flame-retardant properties in PEEK via MOF-on-MOF hybrid nanofillers

Polyetheretherketone (PEEK) faces a critical trade-off between flame retardant and mechanical performance in safety-critical applications. To address this dilemma, we propose an innovative MOF-on-MOF strategy, fabricating a hierarchical NH₂-UIO-66@NH₂-MIL-125 (U/M) nanofiller via PVP-assisted solvothermal synthesis. Incorporated into PEEK, U/M simultaneously enhances mechanical properties and flame retardant/smoke suppression. At 2 wt% loading (P–U/M-2 %), tensile strength increases by 21.5 % compared to pure PEEK, while cone calorimetry reveals a 19.7 % reduction in peak heat release rate (pHRR), a 52.5 % decrease total smoke production (TSP), and 36.6 % prolonged ignition time. Synergistic mechanisms are revealed: In the gas phase, U/M quenches radicals, dilutes combustibles, and adsorbs smoke; in the condensed phase, in-situ formed ZrO₂/TiO₂ catalyzes the formation of a robust graphitized char layer. Comprehensive analyses (sXAS, TG-FTIR, Raman, XPS, SEM) and theoretical calculations confirm this dual-phase action. This work pioneers MOF-on-MOF architectures as a unified solution to break the performance trade-off in high-safety PEEK composites.