POLYELECTROLYTE BRUSHES AND POLYELECTROLYTE ADSORPTION LASERS

The structure of brushes and adsorption layers of polyelectrolytes con taining strong and weak groups is analysed, using both a numerical sel f-consistent-field (SCF) model and approximate analytical expressions, The latter are based upon a local electroneutrality approximation (br ushes) or upon a mutual compensation of the adsorbed polymer charge an d the diffuse double layer charge (adsorption layers); under most cond itions excellent agreement with the SCF model is obtained. Brushes wit h strong groups are relatively compact at high ionic strength and the brush thickness increases monotonously as the salt concentration is re duced. On the other hand, in ''weak'' brushes the root-mean-square thi ckness passes through a maximum as a function of the salt concentratio n, because the weak groups will discharge themselves at low; ionic str ength. Such a charge regulation is impossible for strong groups. For a dsorption layers, the adsorbed amount is high when the polymer is weak ly charged and it decreases as the polymer charge is increased. This g eneral trend is the same for strong and weak groups. However, weak gro ups can again adjust their charge. As a result, at intermediate charge densities their degree of dissociation is much lower than in the bulk solution; consequently the adsorbed amount is much higher than for st rong groups.