The Polyphosphazenes Bearing Diverse Substituents and the Functional Properties of Its EPDM Composites

With the further development of the aerospace industry, solid rocket motors urgently require materials with superior mechanical properties and ablation resistance for internal insulation layers. This study innovatively introduces newly synthesized four polyphosphazenes (PPNs) bearing diverse side-chain functionalities, including diallylamine polyphosphazene (VPN), acetylenyl aniline polyphosphazene (AYP), naphthylamine polyphosphazene (NLP), and allyl phenoxy polyphosphazene (APP), into EPDM composites as functional fillers. These resulting composites all exhibited excellent mechanical and ablation resistance properties, among which EP-NLP exhibited the most outstanding performance, showing a 1089% increase in elongation at break and a 60.74% reduction in linear ablation rate, achieving values of 918.94% and 0.042 mm/s, respectively. In-depth analysis of the morphology, composition, and structure of the char layer revealed that PPNs promote the formation of phosphate structures and SiC during ablation, which led to the formation of a denser char with higher hardness and an increased degree of graphitization. The underlying mechanisms for these improvements are systematically investigated, offering a promising design strategy for advanced thermal protection systems in aerospace and solid rocket motor applications.