Effects of Attractive Interactions on Nanoparticle Diffusion in Entangled Polymer Melts

Developing a complete picture for the mechanism of nanoparticle diffusion in model polymer nanocomposites remains a great challenge, especially experimentally. Using Rutherford backscattering spectroscopy, we have measured the translational diffusion coefficient of spherical nanoparticles (diameter $=$ 20 nm) infiltrated into poly(2-vinylpyridine) melts across a range of molecular weights (35-300 kg/mol). Our results reveal that the diffusion coefficient of nanoparticles in attractive nanocomposites is several times slower than what is predicted from the melt viscosity according to the Stokes-Einstein (SE) relation. This runs contrary to recent theoretical studies of non-attractive systems, where it is predicted that nanoparticle diffusion can be many orders of magnitude faster than SE predictions. Potential explanations for this unusual slowing of nanoparticle diffusion are discussed.