Salt-Induced Diffusion of Star and Linear Polyelectrolytes within Multilayer Films

This study explores the effect of salt on the diffusivityof polyelectrolytesof varied molecular architecture in layer-by-layer (LbL) films indirections parallel and perpendicular to the substrate using fluorescencerecovery after photobleaching (FRAP) and neutron reflectivity (NR)techniques, respectively. A family of linear, 4-arm, 6-arm, and 8-armpoly(methacrylic acids) (LPMAA, 4PMAA, 6PMAA, and8PMAA, respectively) of matched molecular weights were synthesizedusing atom transfer radical polymerization and assembled with a linearpolycation, poly[2-(trimethylammonium)ethyl methacrylate chloride](QPC). NR studies involving deuterated QPC revealed & SIM;10-foldhigher polycation mobility for the 8PMAA/QPC system compared to all-linearLbL films upon exposure to 0.25 M NaCl solutions at pH 6. FRAP experimentsshowed, however, that lateral diffusion of star PMAAs was lower than LPMAA at NaCl concentrations below & SIM;0.22 M NaCl,with a crossover to higher mobility of star polymers in more concentratedsalt solutions. The stronger response of diffusion of star PMAA tosalt is discussed in the context of several theories previously suggestedfor diffusivity of polyelectrolyte chains in multilayer films andcoacervates.