Dynamics of Star Polyelectrolytes in Multilayer Assemblies

We report on the effect of the molecular architecture of poly(acrylic acid) (PAA) on its diffusion within electrostatic layer-by-layer (LbL) films with poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) at low pH. Ellipsometry measurements with dry films and films exposed to aqueous solutions both indicated faster mobility of star PAA. Specifically, in-situ ellipsometry revealed that the diffusion coefficient of an 8-arm star polyacid during penetration within wet pre-assembled LbL films was 4-fold higher than that of its linear counterpart. The difference in mobility of star and linear PAA was also evident in neutron reflectometry experiments, which showed fast intermixing of polymer layers in star PAA-containing films upon exposure to salt solutions. We suggest that higher mobility of star polymers results from a smaller number of contacts between the star and linear polymers, and support this conclusion by FTIR measurements of a lower ionization degree of assembled star PAA, and by isothermal titration calorimetry (ITC) studies of interpolymer binding in the solution that showed lower PDMAEMA-to-PAA ratio and higher dissociation constants for star PAA/PDMAEMA complexes, which indicated weaker binding of star PAA with the polycation.