Polyhedral oligomeric silsesquioxane (POSS) are silica based nano-fillers with tunable functionality, nanometric cage structure and are promising fillers for generating high-performance mixed matrix membranes (MMMS) for gas separation processes. Here, we report highly permeable and selective MMMs fabricated from the polymer of intrinsic microporosity (PIM-1) filled with functionalized and unfunctionalized octaphenyl substituted POSS particles. Amine and nitro functionalized POSS particles (OAPS and ONPS) showed greater dispersibility in PIM1 solution compared to unfunctionalized POSS (OPS). The OPS MMMs showed an increase in single and mixed gas (CO2: N-2 and CO2: CH4, 50: 50 vol%) permeability and a decrease in selectivity. In contrast the OAPS MMMs showed significantly enhanced selectivity for air-oxygen and hydrogen-natural gas mixed gases with minimal changes in permeability. Higher loadings led to reduced permeability but significantly enhanced selectivity (similar to 200%). The physical aging behavior of nanometric level dispersed MMMs in thick (similar to 70 mu m) and thin film configurations (similar to 2.5 mu m and similar to 200 nm), showed that the addition of POSS-based particles could delay the rate of aging. We could directly correlate the enhanced performances to both an increase in mechanical rigidity of the membrane, as well as an enhanced degree of interfacial hydrogen bonding for amine functionalized POSS fillers.