We quantified the stability of polystyrene- (PS-) grafted silica nanoparticles (NPs) in PS matrices with ultrasmall angle X-ray scattering (USAXS) and transmission electron microscopy (TEM) and developed a phase diagram to predict NP dispersion based on the graft polymer density, sigma, and the graft and free polymer molecular weights, orN and P, respectively. Using controlled/living polymerizations, polymer nanocomposites were formulated with silica NPs of radius, R = 9 nm where sigma = 0.10-0.70 chains/nm(2) at an essentially constant N = 61-68 kg/mol. The matrix molecular weight was varied from P = 37-465 kg/mol permitting us to vary the swelling ratio, P/N = 0.6-7.7. Using USAXS and TEM, we determined whether the PS-grafted NPs were stable and dispersed uniformly, or were unstable and aggregated within the matrix. From these measurements we developed a phase diagram for NP miscibility with respect to sigma, P, and N to determine the allophobic and autophobic transitions that correspond to the wetting the drying of the graft polymer brush which control NP stability in polymer matrices.