  We perform a systematic application of the hybrid particle-field molecular
dynamics technique [Milano et al, J. Chem. Phys. 2009, 130, 214106] to study
interfacial properties and potential of mean force (PMF) for separating
nanoparticles (NPs) in a melt. Specifically, we consider Silica NPs bare or
grafted with Polystyrene chains, aiming to shed light on the interactions among
free and grafted chains affecting the dispersion of NPs in the nanocomposite.
The proposed hybrid models show good performances in catching the local
structure of the chains, and in particular their density profiles, documenting
the existence of the "wet-brush-to-dry-brush" transition. By using these
models, the PMF between pairs of ungrafted and grafted NPs in Polystyrene
matrix are calculated. Moreover, we estimate the three-particle contribution to
the total PMF and its role in regulating the phase separation on the nanometer
scale. In particular, the multi-particle contribution to the PMF is able to
give an explanation of the complex experimental morphologies observed at low
grafting densities. More in general, we propose this approach and the models
utilized here for a molecular understanding of specific systems and the impact
of the chemical nature of the systems on the composite final properties.
