  We investigate the dynamics of nanoparticles in semidilute polymer solutions
when the nanoparticles are comparably sized to the polymer coils using
explicit- and implicit-solvent simulation methods. The nanoparticle dynamics
are subdiffusive on short time scales before transitioning to diffusive motion
on long time scales. The long-time diffusivities scale according to theoretical
predictions based on full dynamic coupling to the polymer segmental
relaxations. In agreement with our recent experiments, however, we observe that
the nanoparticle subdiffusive exponents are significantly larger than predicted
by the coupling theory over a broad range of polymer concentrations. We
attribute this discrepancy in the subdiffusive regime to the presence of an
additional coupling mechanism between the nanoparticle dynamics and the polymer
center-of-mass motion, which differs from the polymer relaxations that control
the long-time diffusion. This coupling is retained even in the absence of
many-body hydrodynamic interactions when the long-time dynamics of the colloids
and polymers are matched.
