Influence of vinyl polyhedral oligomeric silsesquioxane with different functionalities and cage structures on the mechanical properties and ablation resistance of EPDM composites

Four vinyl polyhedral oligomeric silsesquioxane (POSS) with different functionalities and cage structures, monovinyl-POSS (monov-POSS), divinyl-POSS (div-POSS), trivinyl-POSS (triv-POSS), and tetravinyl-POSS (tetrav-POSS) were prepared and blended into fiber-reinforced ethylene propylene diene monomer (EPDM—EPDM/AF) in the form of chemical cross-linking to obtain four modified composites with various cross-linked network structures and cage structures to investigate the influences and improvement of cross-linking network and cage structures on the mechanical, thermal stability and ablation resistance of EPDM/AF. The results showed that the POSS of multi-functionality with small steric hindrance, such as tetrafunctional tetrav-POSS, can most significantly improve mechanical properties of EPDM/AF by forming denser crosslinking network structures, and the tensile strength and elongation at break of EPDM/AF/tetrav-POSS can be increased 73.3% and 42.1% compared with EPDM/AF, respectively. POSS that has a complete cage structure and enables the modified composites to obtain a relatively dense cross-linked network structure, such as difunctional div-POSS can more substantially enhance the thermal stability and ablative resistance of EPDM/AF with the LAR and MAR of EPDM/AF/div-POSS reduced by 25.2% and 10.5% compared to EPDM/AF, respectively. High-temperature thermal transition of four modified composites at gradient temperatures was investigated to explain the relationship between the structure of POSS and ablation properties. Highlights: Four POSS with different functionalities and cage structures were designed. Mechanical and ablative properties were reinforced by four vinyl POSS. Multifunctional POSS can most significantly improve mechanical properties. Functionality and cage structure together influence ablative property. High-temperature thermal transition of modified composites was investigated.