Nanocomposites consist of particles in the sub100 nm size range dispersed in a polymer matrix. The properties of these materials depend strongly on the degree of particle dispersion which in turn depends on how the materials are made and the strength of polymer segment/particle attractions, epc. Determining the nature of the interactions and their effect of particle dispersion is a key step in engineering nanocomposites for useful applications. Here we explore the microstructure and mechanics of 50 nm diameter silica particles dispersed in polymer melts of different molecular weight and composition. In hydroxyl terminated polyethylene oxide which is known to have a strong affinity for silica surfaces, the particles are dispersed with an apparent adsorbed polymer layer of thickness of a few polymer radii of gyration. Accounting for the adsorbed polymer layer, the melts appear hard sphere-like up to a limit where the polymer layers overlap. Depending on the polymer molecular weight this can occur before or after the suspension has undergone a glass transition due to crowding of the effective hard spheres. In polytehrahydrofuran, the small value of epc leads to polymer slip at the particle surface and the strong depletion forces that drive aggregation at modest volume fraction. These observations are discussed in light of models developed to capture the role of polymers/particle interactions on the phase behavior and microstructure of equilibrium nanocomposites.