Tailoring the charge density of surface-attached polyelectrolyte brushes

The preparation and swelling behavior of surface-attached charged statistical copolymer brushes are described. The brushes are prepared by free radical chain polymerization of monomer mixtures of 4-vinylpyridine (VP) and dimethylacrylamide (DMAA) using a surface bound azo-initiator (surface-initiated polymerization). So-prepared neutral PVP-stat-PDMMA copolymer brushes are then transferred into polyelectrolyte brushes through a polymer-analogous quaternization reaction of the respective PVP segments. By adjusting the ratio of the monomers added to the polymerization feed, the concentration of the corresponding polymer segments generated in the final surface-attached layer can be controlled, and thus, the charge density of the resulting polyelectrolyte copolymer brush can be determined. Finally, the swelling behavior of such polyelectrolyte brushes in aqueous solutions containing low molecular weight salts is investigated by using multiple-angle null ellipsometry. Depending on the charge density of the copolymers, the brushes exhibit a swelling behavior between that of a polyelectrolyte and a neutral polymer brush. Thus, the sensitivity of the layers against salt present in the surrounding medium can be precisely controlled.