Viscoelastic properties of polymer grafted nanoparticle containing composites are known to be influenced by the mixing and demixing of graft and matrix chains. In the current study, we show that glassy brushes (with high glass transition temperature, Tg) adsorbed/tethered onto nanoparticles that are dispersed within a high–mobility, low-Tg polymer matrix significantly alter the overall viscoelastic and dynamic properties of the whole nanocomposite system. This work was inspired by recent work performed at the Akcora group at Stevens Institute of Technology showing a reversible thermal stiffening in nanocomposite polymers, in which the matrix and adsorbed polymer chains had a Tg difference of 200 degrees. We used computer simulations to investigate the atomistic/molecular mechanisms that are responsible for the experimentally observed thermal stiffening behavior. Two main (and extreme) conformational states of the high–Tg grafted chains were considered during the simulations: collapsed (grafted chains are strongly adsorbed onto the nanoparticle, and therefore, phase–separated from the matrix) and stretched (grafted chains extend into the matrix). We found that when the grafted chains were stretched out, the nanocomposite had significantly greater storage modulus compared to the system with collapsed grafted chains. Detailed analysis showed that the stretched conformational state of grafted chains reduced matrix chain mobility, which led to the thermal stiffening effect. Finally, by simulating structures with a range of grafted chain conformations bounded by fully collapsed and fully stretched states, we demonstrated that grafted chain conformation could be used to control the viscoelastic response of the whole nanocomposite system.