The dispersion of hairy nanoparticles in polymer melts of chemically identical chains was investigated as a function of both molecular weight and volume fraction. Here we provide conclusive evidence that the shape of the phase diagram is determined primarily by the ratio of the chain length of the polymer melt to the chain length of the polymeric shell structure (or hair) of the core/shell nanoparticles, and that the phase behavior of different hairy particles in various polymer melts can be superimposed into one universal graph. Other factors, including the hair density and the particle diameter, are not nearly as significant as the above-noted ratio in this phase separation. In addition, we show that there is a strong connection between the rheological dynamics of the particle-filled system and the thermodynamics of the phase separation behavior. The shear-induced nonlinearity in the particle-filled system appears to display features of a singularity near the phase transition point. (c) 2008 Elsevier Ltd. All rights reserved.