To investigate the relationship between the chemical structures of the side chains of polyoctahedral oligomeric silsesquioxanes (POSS) fillers and the ability to decrease the refractive indices of the polymer composites we examined the influence on the degree of polymer chain packing by the existence of the octa substituted POSS derivatives The polymer composites containing methyl ethyl vinyl isobutyl octyl octadecyl cyclopentyl and phenyl substituted POSS in poly (methyl methacrylate) (PMMA) were prepared The packing coefficients of the PMMA composites containing POSS derivatives were evaluated from molar refractions and refractive indices of the films with the Lorentz-Lorenz equation We found that the ethyl group shows the lowest values of the packing coefficients and a significant effect in reducing the refractive indices of the polymer composites Finally in summary it was shown that POSS molecules can intrinsically offer to reduce a packing In addition less entanglement and smaller interactions between polymer chains and the substitution groups are favorable to release a packing resulting in decreases in refractive indices Our findings described here are the first efforts to quantitatively evaluate the ability of POSS fillers to lower refractive indices at the molecular level (c) 2010 Wiley Periodicals Inc J Polym Sci Part A Polym Chem 48 5712-5717 2010