Size and curvature of filler particles affect dynamics of polymer chains in composites. In this work, effects of filler particle size, in two scales of nano- and micro-meters, on dynamics of rubbery chains and frictional properties of composites were studied. Surface modification of nano- and micro-fumed silica by grafting low molecular weight hydroxyl-terminated polybutadiene (HTPB) guaranteed similar surface energy for fillers as measured by their surface tension. Nano- and micro-composites of styrene butadiene rubber were prepared by solvent assisted mixing and progressively increasing volume fraction of fillers. Achievement of nano and micro-composites was confirmed by the scanning electron microscopy. Effect of silica aggregate size on the dynamics of rubber chains was measured by dynamic-mechanical-thermal analyzer and compared through calculation of the activation energy for mobilization of slow-dynamic chains- in the rubbery region. It was shown that nano-silica immobilizes the rubber chains more than micro-silica even at equal total interfacial area between filler aggregates and rubber matrix, especially above the percolation limit. Similar trend was seen for the coefficient of friction of composites against rough surfaces, showing the strong effect of chain dynamics on friction properties of rubber vulcanizates. (C) 2014 Elsevier Ltd. All rights reserved.