Diffusional Response of Assembled Polyelectrolyte Chains to Salt Annealing

We report on the effect of salt on the diffusion of polyelectrolyte (PE) chains within electrostatically assembled polyelectrolyte multilayers. Layer-by-layer (LbL) films were assembled using poly(methacrylic acid) (PMAA) as a polyanion and quaternized poly-2-(dimethylamino)ethyl methacrylate as a polycation. Fluorescence recovery after photobleaching and neutron reflectometry were used to monitor the center-of-mass diffusion of PMAA chains in directions parallel and perpendicular to the substrate (D$_{\mathrm{//}}$ and D$_{\mathrm{\bot }}$, respectively). In both directions, the diffusion coefficient was exponentially dependent on salt concentration, with significantly faster diffusion in the direction parallel to the substrate. At the same time, D$_{\mathrm{//}}$ dramatically decreased with salt annealing time as the films became increasingly intermixed, reflecting strong coupling between internal layering and PE chain dynamics within LbL films.