Structure and dynamics of a polymer-nanoparticle composite: Effect of nanoparticle size and volume fraction

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.