Dynamics of nanoparticles in polyelectrolyte solutions*

Recent theories indicate that nanoparticle dynamics in semidilute solutions of neutral, flexible polymers couple to the segmental relaxations of polymer chains. How nanoparticle dynamics couple to relaxations of charged, semiflexible polymers remains incompletely understood. Here, we probe the dynamics of polystyrene nanoparticles in solutions of a polyelectrolyte, sodium polystyrene sulfonate, using fluorescence microscopy. When the ionic strength of the solution is high, such that the polymer adopts a Gaussian conformation, the nanoparticle dynamics are coupled to the bulk solution viscosity across the dilute and semidilute regimes of polymer concentration. In near-salt-free solutions, in which the polymer adopts an extended conformation, the nanoparticle dynamics do not follow the Stokes-Einstein prediction based on the bulk viscosity across all polymer concentrations, and furthermore do not collapse onto a master curve as a function of normalized length scales. This result suggests that the coupling of nanoparticle and polymer segmental dynamics are altered by polymer conformation.