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[Display omitted] • Design of MOF-derived hollow hybrids covered with a polyphosphazene coating. • Cobalt-phosphorus-nitrogen flame retardant elements synergistic effect. • Mismatch structure significantly improves the dispersity of fillers in EP. • The hybrids endow EP with better flame retardancy and mechanical performance. Polyphosphazene has drawn an abundance of attention as a flame retardant due to its eco-friendliness and phosphorus-nitrogen synergistic effect. ZIF-67 and cobalt compounds derived from cobalt ions released by ZIF-67 were employed as dual templates for loading polyphosphazene. We skillfully designed a polyphosphazene hollow hybrid material via the polymerization of phosphazene accompanied by the etching of ZIF-67 and cobalt compounds by generated HCl. The hybrid material (CPPHS) was composed of cobalt-doped polyphosphazene hollow sub-micron polyhedrons (CP) and polyphosphazene hollow nanospheres (PHNS) with different dimensions and magnitudes of sizes. After adding 2.0 wt% CPPHS, the limiting oxygen index of epoxy resin increased to 27.6%, while the peak heat release rate and total smoke production decreased by 40.1% and 38.8%, respectively. The formation of a continuous and strongly extended dense char layer in the EP composites was fostered by the synergistic effect of Co-P-N flame retardant components, significantly decreasing the heat flow and combustible gas exchange rate and effectively hindering the combustion chain reaction. Furthermore, the unique dimensional mismatch structure of CPPHS enhanced its dispersion in the EP matrix, enabling the EP to have superior mechanical properties that match realistic application requirements, consequently expanding the variety of applications for fire-resistant EP composites. [ABSTRACT FROM AUTHOR]

[Display omitted] • A simple method for synthesizing nano-composite flame retardants with hollow prismatic structures. • POSS can be used as a structure-directing agent to maintain the nanostructure. • Cobalt-silicon synergy for enhanced flame retardancy and smoke suppression. • Nano-composite flame retardants achieve good dispersion in epoxy resin. Polyhedral oligomeric silsesquioxanes (POSS) containing transition metals have excellent flame retardant properties. However, the flame retardant obtained by direct coordination of transition metal cobalt (Co) with POSS tends to agglomerate, resulting in unsatisfactory flame retardant efficiency. Herein, a synthetic route was designed to use a water-soluble cobalt-polyvinylpyrrolidone (Co-PVP) complex as the template and cobalt source to first grow cobalt-metal–organic frameworks (Co-MOFs) on its surface, and then an aqueous solution of Na-POSS was used as both a structure-directing agent and an etching agent to load POSS and remove the template synchronously. Finally, a hollow prism with POSS wrapped around the Co-MOF (designated Co-MOF@POSS) was obtained. The hollow structure not only showed an optimized dispersion in the matrix but also exposed more active sites, catalyzing the formation of the carbon layer, and the silica ceramic layer formed by POSS further enhanced the strength of the carbon layer. An excellent flame retardant effect was obtained by adding Co-MOF@POSS to epoxy at 2 wt%. The heat release and smoke release of the epoxy composites were significantly reduced by the synergistic effect of cobalt and silicon, and the addition of Co-MOF@POSS improved the limiting oxygen index to 27.5% and decreased the peak of heat release rate, total heat release, and total smoke production by 34.6%, 16.6% and 23.8%, respectively. This work provides new ideas for the preparation of MOF and POSS-based flame retardants. [ABSTRACT FROM AUTHOR]