Effect of polymer charge and geometrical confinement on ion distribution and the structuring in semidilute polyelectrolyte solutions:: Comparison between AFM and SAXS

This paper focuses on the effect of geometrical confinement on the structruring of a semidilute polyelectrolyte solution in a thin film geometry. Studying poly-N-[tris(hydroxymethyl)methyl] acrylamide-co-2acrylamido-2-methylpropanesulfonate (PTRIS-co-AMPS) with different degrees of charge allows a deeper insight into the ion distribution around the polymer chains. Results from atomic force microscopy (AFM) force-distance measurements in films are compared with results from small-angle X-ray scattering (SAXS) in bulk. It is found that the characteristic lengths obtained from force oscillation measured by AFM, such as the intermediate chain distance (mesh size) and the correlation length, correlate well with those obtained from the structure peak measured by SAXS. In the direction perpendicular to the film surface, both length scales of the meshlike structure, i.e., the average chain distance and the correlation length, are not influenced by the geometrical confinement. The dependencies of force period and decay length on the polymer charge are analyzed in detail and related to the counterion distribution in the solutions. A new model for counterion condensation is proposed, in which the condensed ions are not fixed on the polyion chains and can exchange freely with the free ions. However, there is still a length scale beyond which the ion condensation theory of Manning and Oosawa is valid. Results from conductivity measurements support the proposed model.