Dry synthesis of mesoporous nanosheet assembly constructed by cyclomatrix polyphosphazene frameworks and its application in flame retardant polypropylene

Polyphosphazene is utilized in various fields due to its versatility and nontoxic nature; however, the polycondensation reaction in its preparation process relies on hazardous organic solvents and acid-binding agents. In this work, we proposed a synthetic method based on both molecule and nanostructure design to prepare cyclomatrix polyphosphazene frameworks in a green and sustainable manner without any solvent. Using block copolymer Pluronic F127 as the structural directing agent and porogen, the synthesized product was assembled by nanosheets with a mesoporous structure. The generality of the proposed strategy was also examined in depth. After being doped with cobalt, its structure-property relationship was studied in flame retarding polypropylene (PP). Polyphosphazene with a high specific surface area and unique morphology was first applied as an eco-friendly halogen-free flame retardant. With the addition of 18 wt% of the product, the flame retardant PP composite achieved a UL-94V-0 rating, with a limiting oxygen index of 26.7%. The peak of heat release rate and total smoke production were also reduced by 60.5% and 32.6%, respectively. The composite exhibited an outstanding water resistance, as evidenced by constant V-0 rating after soaking in water for 3 days. This systematic research will shed light on the advanced structural design and eco-benign synthesis strategy of polyphosphazene for more feasible applications.