Molecular Weight Dependence of Polymer Chain Mobility within Multilayer Films

Fluorescence recovery after photobleaching has been applied to determine, to our knowledge for the first time, the molecular weight (<i>M</i><sub>w</sub>) dependence of lateral diffusion of polymer chains within layer-by-layer (LbL) films. As shown by neutron reflectometry, polyelectrolyte multilayers containing polymethacrylic acid (PMAA, <i>M</i><sub>w</sub>/<i>M</i><sub>n</sub> < 1.05) of various molecular weights assembled from solutions of low ionic strengths at pH 4.5, where film growth was linear, showed similar diffusion of PMAA in the direction perpendicular to the film surface. At a salt concentration sufficient for unfreezing electrostatically bonded chains, layer intermixing remained almost unaffected (changes <1.0 nm), while the lateral diffusion coefficient (<i>D</i>) scaled with the PMAA molecular weight as <i>D ∼ M</i><sub>w</sub><sup>–1±0.05</sup>.