We investigated the rheology of titanium dioxide (TiO(2)) nanoparticles with various surface modifications in neat and binary blends of polydimethylsiloxane (PDMS) homopolymers of different molecular weights (4 k-77 k). The dispersions for bare, octadecyl-(C(18)), and PDMS-grafted particles reflect different interaction forces. For bare particles, the relative viscosity decreases monotonically with increasing melt M(w) or increasing fraction of long chains (f), consistent with thicker adsorbed layers. The octadecyl(C(18))-grafted dispersions show no dependence on melt M(w) or f, suggesting that the alkyl groups prevent polymer adsorption or bridging. Therefore, the van der Waals attractions are cut off at a separation on the order of twice the thickness of the C(18) chains (approximate to 5 nm), regardless of melt M(w) or f. The PDMS-grafted suspensions show an increase in relative viscosity with increasing melt M(w) or f, consistent with wetted polymer brushes for P < N and dewetted layers for P > N. The power law we developed previously fits the shear-rate dependent viscosities with a structural relaxation time that scales with the magnitude of the attraction, thereby reflecting the microscale dynamics. (C) 2012 The Society of Rheology. [DOI: 10.1122/1.3669646]