Aqueous spherical colloidal silica (CS) particles with a diameter of 15 +/- 5 nm were modified with three different types of monofunctional silane coupling agents to prepare functionalized colloidal silica (FCS) particles. The effects of the surface chemistry of the FCS were studied as a function of the CS/FCS loading in the poly(dimethyl siloxane) (PDMS) polymer. The prepared PDMSCS/FCS composites were investigated for their physical properties both in the cured and uncured states. The extent of fillerfiller and fillerpolymer interactions was found to vary with the type of functionalizing agent used to treat the surface of the CS. The fillerfiller interaction appeared to be predominant in the PDMSCS composites, and improved fillerpolymer interaction was indicated in the case of the PDMSFCS composites. The composites containing CS treated with methyltrimethoxysilane exhibited relatively better optical and mechanical properties compared to the other PDMSFCS composites. This study highlighted the importance of judiciously choosing functionalizing agents to achieve PDMSFCS composites with predetermined optical and mechanical properties. (c) 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012