We show that the thermomechanical properties of polymer nanocomposites are critically affected by polymer-particle wetting behavior. Silica nanoparticles grafted with dense polystyrene brushes of degree of polymerization 1050 are blended with polystyrene melts to form nanocomposites. It was found that low molecular weight (MW) polystyrene melts with lengths < 880 wet these particles. Concurrently, the glass transition temperature (T-g) of the nanocomposite increases. At higher MW, the matrix does not wet the particles and the T-g decreases. (c) 2006 Wiley Periodicals, Inc.