We have recently shown that silica nanopartieles grafted with polystyrene chains behave akin to block copolymers due to the "dislike" between the nanoparticles and the grafts. These decorated nanoparticles, thus, sell-assemble into various morphologies, from well-dispersed nanoparticles to anisotropic superstructures, when they are placed in homopolystyrene matrices of different molecular masses. Here, we consider a slightly different case, where the grafted chains and the matrix (both PM MA) are strongly attracted to the silica nanoparticle surface. We then conjecture that these systems show phase mixing or demixing depending on the miscibility between the brush and matrix chains ("autophobic dewetting"). At 15 mass % particle loading, composites created using the same grafted nanoparticle, hut with two different matrices, yield well dispersed nanoparticles or nanoparticle "agglomerates", respectively. Rheoloey experiments show that the composites display solid-like behavior only when the particles are aggregated. As deduced in previous work, this difference in behavior is attributed to the presence of percolating particle clusters in the agglomerated samples which allows for stress propagation through the system. Going further, we compare the local mobility of matrix and grafted segments of both composites using quasi-elastic neutron scattering experiments. For the liquid-like system, the mean square displacements of the grafted chains and matrix chains, the particle structuring and mechanical response are all unaffected by annealing time. In contrast, in the reinforced case, only the local matrix motion is unaffected by time. Since the particle clustering and solid-like mechanical reinforcement increase with increasing time, we conclude that mechanical reinforcement in polymer nanocomposites is purely based on the nanoparticles, with essentially no "interference" from the matrix. In conjunction with other results in the literature, we then surmise that mechanical reinforcement is caused by the bridging of particles by the grafted polymer layers and not due to the formation of "glassy" polymer layers on the nanoparticles.