Charge effects on the formation of multilayers containing strong polyelectrolytes
Abstract
The effect of polymer charge density, polyelectrolyte concentration, and ionic strength on the formation of multilayer films by sequential adsorption of a polyanion (poly(styrene sulfonate), PSS) and a cationic statistical copolymer poly(diallyl-dimethyl-ammoniumchloride-stat-N-methyl-N-vinylacetamide), P(DADMAC-stat-NMVA) is investigated. The degree of charge (DC) of the polycation is controlled through the ratio of cationic DADMAC and neutral NMVA within the copolymer. The resulting polymer films are characterized by ellipsometry and by scanning force microcopy (SFM). For the present system, a minimum DC of about 68% is needed for the formation of stable films. It is proposed that this abrupt change in adsorption behavior at a DC around 68% is related to a threshold of charge reversal, which is necessary for the formation of a multilayer system. Above a DC of 75% the film thickness decreases slightly because of changes in polymer conformation. This results in a thickness maximum the position of which is shifted with varying polymer concentration. The ionic strength has a pronounced effect on the thickness above the charge reversal threshold, that is, where multilayers are formed. With increasing ionic strength, the chains undergo a transition from an extended to a coiled conformation, which leads to an increase in film thickness. Such conformational changes of the polyelectrolytes are indicated by changes in roughness as derived from SFM images. It is concluded that multilayer formation in the present system is electrostatically driven.
