Polyelectrolyte brushes at interfaces: Properties, structure and interactions

Tethered polyelectrolytes provide a powerful way to manipulate the properties of surfaces. To understand the factors affecting the structure and interaction of tethered chains, direct force measurements were performed under a wide variety of environmental conditions.;Amphiphilic diblock copolymers which form micelles in water, were adsorbed to the hydrophobized mica surface from aqueous solution with added salt. Ellipsometry and neutron reflectivity studies showed that the adsorbed micelles reorganize and their hydrophobic cores are released and spread on the surface, and the charged blocks extend into the solution and form a brush-like structure.;The properties of such adsorbed brush layers were studied using the surface force apparatus in various conditions. The study shows that the structure and interactions of the brush can be controlled by molecular weight, ionic strength, degree of charging, tethering density and counterion valence. For monovalent counterions, the brush behaves as predicted by the mean-field scaling theory. A large fraction of counterions condense to polyelectrolyte backbones, while the rest of the counterions are trapped inside the brush. The brush height is independent of salt concentration for the osmotic brush regime and follows the -1/3 power law for the salted brush regime. A force balance model was proposed in the scope of the scaling theory to describe to the interaction of the brush.;When trivalent counterions are presented, the brush collapses from the highly stretched structure. Big attraction was measured when the two brushes were separated apart at low salt. The behavior of the brush is dominated by this attractive interaction between the polyelectrolyte segments, which is electrostatic in nature and can be screened by increasing the ionic strength. In order to develop the adhesion, the two brushes have to be brought into contact. The magnitude of adhesion scales with the degree of compression to the -0.69 power, in agreement with theoretical prediction, and this demonstrates the hypothesis that the inter-layer attraction between the two brushes results from the condensed counterions in the interpenetration zone.