Two matrix-free polystyrene-grafted silica nanocomposite samples with graft chain lengths of 35 and 112 kg/mol are characterized by calorimetry and rheometry, and results are compared to neat polystyrenes of comparable molecular weights. The glass transition temperature Tg of the nanocomposites is found to be approximately 1 to 2 K higher than that of the neat materials, whereas the absolute heat capacity is approximately 4-7% lower in the glassy and liquid states. The step change in heat capacity ΔCp at Tg is 15% lower for the nanocomposites, consistent with an immobilized glassy layer of approximately 2 nm. The linear viscoelastic behavior of the nanocomposite samples differs significantly compared to their neat analogs in several ways: first, the G′ versus ω curves shift toward lower frequencies by approximately one decade due to the increase in the glass transition temperature; second, terminal flow behavior is absent; third, the rubbery plateau moduli (GN°) decreases by 7% for the 35 kg/mol grafted particles and increases by approximately two and a half-fold for the 112 kg/mol grafted particles; and fourth, the glassy modulus increases approximately 4% consistent with hydrodynamic reinforcement. On the other hand, the magnitude of the rubbery modulus is attributed to two effects, hydrodynamic reinforcement and a change in the effective entanglement density, which is governed by corona interpenetration coupled with the silica particles acting as physical entanglement points.