The surface composition of a polymer nanocomposite (PNC) is often different from the bulk due to surface energy and thermodynamic forces. Here, we probe these two contributions in a model system of PMMA grafted silica nanoparticles (PMMA-NP) in a poly(styrene-ran-acrylonitrile) (SAN) matrix using Rutherford backscattering spectrometry (RBS), transmission electron microscopy (TEM), and atomic force microscopy (AFM) as a function of thermal annealing temperature and time. Studies are performed above and below the lower critical solution temperature (LCST) of this system to decouple the two contributions. With increasing annealing time at temperatures below the LCST, a monotonic increase in surface excess of PMMA-NPs is observed, which is attributed to the difference in surface energies between the constituents. Upon quenching the film into the two-phase region, a much larger increase in PMMA-NP surface coverage is observed, attributed to a stronger thermodynamic driving force for phase separation, namely the Flory-Huggins interaction parameter. Using the measured surface excess values of PMMA-NPs at multiple annealing times and temperatures, the diffusion coefficients of the grafted nanoparticles are extracted and compared to prevailing theoretical models.