Diffusional Response of Layer-by-Layer Assembled Polyelectrolyte Chains to Salt Annealing

We report on the effect of salt on 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 ∥ and D ⊥, respectively). Film intermixing and polymer chain diffusion in the direction perpendicular to the substrate exhibit an unusual dependence on the PMAA molecular weight, with dramatically stronger diffusional response to salt annealing for the films formed by PMAA with the lowest molecular weight of 7 kDa. Films assembled with this lowest molecular weight PMAA reveal that both diffusion coefficients of PMAA chains ( D ∥ and D ⊥ ) depend exponentially on the concentration of salt in the annealing solutions, with much faster diffusion in the direction parallel to the substrate.