Highly-Mobile Nanoparticles that Strongly Interact with Well-Entangled Polymer Melts Diffuse via the Vehicular Mechanism

When particles are large relative to the entanglement mesh in well-entangled polymer melts, the Stokes-Einstein (SE) relation predicts that particle diffusion scales as M^-3.4. Using Rutherford backscattering spectrometry, we measure the diffusion coefficient of very small (radius ≈ 0.9 nm) octaaminophenyl silsesquioxane nanoparticles (NPs) in well-entangled poly(2-vinylpyridine) (P2VP) melts of varying molecular weight (1 – 26 entanglements/chain). We demonstrate that these small NPs diffuse between 10–10,000X faster in P2VP melts than predicted by SE, with the diffusion coefficients scaling weakly with molecular weight M^–0.7±0.1. Furthermore, we characterize the local segmental relaxation process and chain-scale center-of-mass diffusion and find reductions relative to bulk of ~80% and ~60%, respectively, at a NP concentration of up to 25 vol%. Through the combined study of NP and polymer dynamics in this attractive nanocomposite system, we demonstrate experimentally that small and highly-mobile nanoparticles in well-entangled polymer melts diffuse via the vehicular mechanism, i.e. successive NP adsorption/desorption events that occur on Rouse length and time scales.