Nanoparticle Diffusion in Crowded Polymer Nanocomposite Melts

In dilute polymer nanocomposite melts, we have previously established experimentally that small nanoparticles diffuse faster than predicted by Stokes-Einstein, a finding that is consistent with the onset of a vehicular mechanism for nanoparticle diffusion when the nanoparticles are smaller than Rg of the matrix polymer. This study examines nanoparticle diffusion in crowded polymer nanocomposites by diffusing small Al2O3 nanoparticles (NPs) into a nanocomposite comprised of SiO2 nanoparticles in poly(2-vinyl pyridine) homopolymer. Time-of-flight secondary ion mass spectroscopy (ToF-SIMS) measures Al2O3 NP diffusion coefficients within a homogeneous background of larger, immobile SiO2 NPs. By developing a geometric model for average interparticle distance in a system with two NP sizes, we quantify nanocomposite confinement relative to the Al2O3 NP size with a bound layer. At low SiO2 concentrations, Al2O3 NP diffusion aligns with neat polymer results. In more crowded nanocomposites with higher SiO2 concentrations where the interparticle distance approaches the size of the mobile Al2O3 NP, the 6.5-nm Al2O3 NPs diffuse faster than predicted by both core-shell and vehicular diffusion models. Relative to our previous studies of NPs diffusing into polymer, these findings demonstrate that the local environment in crowded systems significantly complicates NP diffusion behavior and the bound layer lifetimes.