Polyelectrolyte multilayer films of different charge density copolymers with synergistic nonelectrostatic interactions prepared by the layer-by-layer technique

Random copolymers composed of diallyldimethylammonium chloride (DADMAC) and acrylamide with varying contents (8-100 mol %) of the cationic DADMAC component were alternated with polyanionic, fully charged poly(styrenesulfonate) to form multilayer thin films. UV-vis spectrophotometry, FTIR spectroscopy, and quartz-crystal microgravimetry (QCM) were employed to follow multilayer buildup. Atomic force microscopy was used to obtain structural information. Layer thicknesses have been determined with small-angle X-ray scattering and ellipsometry, in addition to values calculated from QCM. While in previous work,(1-6) a critical charge density limit could be observed, below which no layer growth is possible; in this system, multilayer formation takes place with copolymers with charge densities as low as 8 mol %. Instead of a continuous increase of adsorbed amounts with decreasing charge density above the critical charge density, as found in previous work,(2,3,6,7) similar layer thicknesses for films with 100 and 8 mol % charged polyelectrolytes and maximally adsorbed amounts for copolymers in an intermediate charge density region have been found. This adsorption behavior is explained in terms of synergistic nonelectrostatic interactions between the polyelectrolytes used.