Scaling laws for the swelling of neutral and charged polymer brushes in good solvents

We compare the swelling of surface-attached neutral poly(4-vinylpyridine) (PVP) and charged poly(N-methyl-4-vinylpyridinium iodide) (MePVP) brushes under good solvent conditions using multiple-angle null ellipsometry. The polymer layers were prepared by the "grafting from" technique, where a low molecular mass initiator is self-assembled on the substrate and the polymer brush is grown in situ by free radical chain polymerization. The positively charged MePVP brush was generated from a neutral PVP brush by a polymer-analogous quaternization reaction. As predicted by mean-field theory, the thickness of the neutral swollen brush (PVP) increases with graft density whereas the thickness of the highly charged polyelectrolyte (MePVP) brush in pure water ("salt-free") is almost independent of the graft density.