Molecule control and thermal degradation study of phosphazene cyclomatrix nanocrystals: For simultaneous enhancement of flame retardant and mechanical properties

•Phosphazene cyclomatrix with controllable molecules were designed and studied.•Chemical structure, molecule size and thermal properties were correlated.•The macromolecules self-assemble into a unique cubic crystalline structure.•Thermal degradation mechanism of macromolecules shows self-intumescent feature.•Excellent flame retardant performance and mechanical reinforcement of the macromolecules was achieved.