Nanocomposites made of a crosslinked polydimethylsiloxane matrix filled with silica beads were prepared by solvent-casting. The nanoparticle dispersion was tuned by changing the solvent. Three types of surface properties are investigated for the nanoparticles through surface grafting. Hydrophilic (unmodified), hydrophobic or carboxylic acid functionalized (allowing particle-PDMS covalent bonding) nanoparticles were thus added to the PDMS with various degree of dispersion (isolated, aggregated or percolating). Nanocomposite microstructures were analyzed by SAXS and TEM while DMA experiments highlighted the viscoelastic behavior and the mechanical reinforcement in the rubbery state. At low silica loading (Phi(SiO2) < 10 vol.%) the mechanical properties were found to be independent on the microstructure and the interactions. At higher Phi(SiO2), two types of behavior are observed when increasing Phi(SiO2). For strong interactions, a percolation phenomenon occurred, leading to a high reinforcement level (driven by interparticle contacts and aggregate morphology) whereas a simple linear modulus augmentation is observed, independently of the aggregation state, for softer interactions. (C) 2013 Elsevier Ltd. All rights reserved.