Structural studies on thin organic coatings built by repeated adsorption of polyelectrolytes

Polyelectrolyte multilayers are obtained by repeated physisorption of oppositely charged polyions. We report on the effect of the linear charge density of a series of aromatic ionenes on the growth of stable ionene/poly(vinyl sulfate) multilayers. Stable multilayers could not be grown in water below a critical linear charge density, corresponding to a charge parameter of about 0.7 (in water at 20 degrees C). We then present two potential ways to overcome the critical charge density limitation. The first one relies on the existence of a specific matching between a polycation and a polyanion, partly based on non-electrostatic intramolecular interactions, giving rise to a surface-constrained complexation leading to multilayers of extremely high supramolecular order. The growth mechanism of the multilayers is different from what is the usual case for stronger polyelectrolytes, since it occurs in a loose unstructured boundary layer at the sample surface, and involves organization processes similar to those occurring upon formation of insoluble structured polyelectrolyte complexes. The relationship between thickness and number of deposition cycles (d vs, n) could be described by an n(1.36) scaling law. The second way is to use a chemical reactivation of the outermost polyelectrolyte layer after its deposition by electrostatic adsorption, in order to regenerate a net charge at the interface allowing further adsorption. This technique is larger in scope, since it may lead to interesting properties such as non-centrosymmetry of the multilayers. (C) 1998 Elsevier Science S.A. All rights reserved