  We use molecular dynamics simulations to study a semidilute, unentangled
polymer solution containing well dispersed, weakly attractive nanoparticles
(NP) of size ($\sigma_N$) smaller than the polymer radius of gyration $R_g$. We
find that if $\sigma_N$ is larger than the monomer size the polymers swell,
while smaller NPs cause chain contraction. The diffusion coefficient of polymer
chains ($D_p$) and NPs ($D_N$) decreases if the volume fraction $\phi_N$ is
increased. The decrease of $D_p$ can be well described in terms of a
confinement parameter, while $D_N$ shows a more complex dependence on
$\sigma_N$, which results from an interplay between energetic and entropic
effects. When $\phi_N$ exceeds a $\sigma_N$-dependent value, the NPs are no
longer well dispersed and $D_N$ and $D_p$ increase if $\phi_N$ is increased.
