The effects of polymer-particle and particle-particle interactions on viscoelastic properties of nanocomposite materials are investigated using narrow molecular weight distribution poly(ethylene oxide) (PEO) containing isotropic silica nanospheres. Nanocomposites are prepared using a "freeze-drying" method to guarantee homogeneous dispersion of silica. The dispersion state of silica nanospheres is characterized by atomic force microscopy phase contrast imaging, and mechanical rheometry is used to study relaxation dynamics and viscoelastic properties of these materials in the melt state. Linear viscoelastic data indicate a transition to a solidlike response at low oscillation frequencies for particle volume fractions 0 as low as 2%, dramatically lower than the theoretical percolation threshold (phi similar to 30%). Nanoparticle volume fraction, surface chemistry, and PEO molecular weight are all found to strongly influence nanocomposite structure and dynamics. A filler networking mechanism, wherein nanosized silica particles surrounded by an immobilized shell of PEO are bridged by much larger polymer molecules, is proposed to explain our observations.