Salt-Controlled Vertical Segregation of Mixed Polymer Brushes

The salt-controlled vertical segregation of the mixed polymer brushes in was studied using the self-consisting field approach. The brushes contained two types chains were considered. It was assumed that the selectivity to the solvent was varied for one type of chains carrying charge (polyelectrolyte chains) while the others polymer chains in brush remained hydrophilic and neutral. Solvent selectivity (i.e., the hydrophobicity of the polyelectrolyte chains) can be controlled by changing the temperature. At low salt concentrations, the polyelectrolyte chains swell and occupy the surface of the mixed brush. At high salt concentrations, the hydrophobic polyelectrolyte chains collapse and give place to neutral chains on the surface. By changing the selectivity of the solvent and the ionic strength of the solution, the surface properties of such mixed brushes can be controlled. Based on the numerical simulations results, it is shown how the critical selectivity corresponding to the segregation transition in polyelectrolyte/neutral brushes depends on the ionic strength of the solution. It is shown that at the same ionic strength, the critical selectivity increases with increasing chain grafting density and the degree of dissociation of charged groups, as well as with increasing share of polyelectrolyte chains in the mixed brush. Within the framework of the mean field theory, a two-parameter model has been constructed that quantitatively describes these dependencies.