We describe here the design and synthesis of the polyhedral oligomeric silsesquioxane (POSS)-based dual-functional molecular fillers for simultaneously lowering refractive indices and improving thermomechanical properties of conventional polymers. We prepared the composite films with poly(methyl methacrylate) and polystyrene containing the series of the POSS derivatives with the single functional unit for interacting with polymer chains and heptacyclopentyl substituents for creating exclusive volumes around the POSS core. From the measurements of refractive indices of polymer composites, it was revealed that all POSS fillers can lower the refractive index of the films. In addition, thermal stabilities and mechanical properties were enhanced by adding POSS fillers. The filler effect on the thermal properties can be explained by the structural features of POSS: The highly symmetrical structure of the silica cube should suppress thermal motions, resulting in the large enhancement of thermomechanical properties of polymer matrices. (c) 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 3583-3589