  We examine the dynamics of silica particles grafted with high molecular
weight polystyrene suspended in semidilute solutions of chemically similar
linear polymer using x-ray photon correlation spectroscopy. The particle
dynamics decouple from the bulk viscosity despite their large hydrodynamic size
and instead experience an effective viscosity that depends on the molecular
weight of the free polymer chains. Unlike for hard sphere nanoparticles in
semidilute polymer solutions, the diffusivities of the polymer-grafted
nanoparticles do not collapse onto a master curve as a function of normalized
length scales. These results suggest that the soft interaction potential
between polymer-grafted nanoparticles and free polymer allows polymer-grafted
nanoparticles to diffuse faster than predicted based on bulk rheology and
modifies the coupling between grafted particle dynamics and the relaxations of
the surrounding free polymer.
