Influence of Adsorption Conditions on the Structure of Polyelectrolyte Multilayers Karsten Bu ¨ scher, †,‡ Karlheinz Graf, § Heiko Ahrens, † and Christiane A. Helm* ,† Angewandte Physik, Universita ¨ t Greifswald, Friedrich-Ludwig-Jahn-Str. 16, D-17487 Greifswald, Germany, Institut fu ¨ r Physikalische Chemie, Universita ¨ t Mainz, Jakob-Welder Weg 11, D-55099 Mainz, Germany, and Institut fu ¨ r Physikalische Chemie II, Universita ¨ t Siegen, Adolf-Reichwein-Str., D-57068 Siegen, Germany Received November 16, 2001. In Final Form: January 30, 2002 Polyelectrolyte adsorption onto an oppositely charged interface is determined by electrostatic and secondary interactions. Since polyelectrolytes precipitate at elevated temperatures, the secondary interactions are presumably temperature dependent. This idea is tested for poly(allylamine) hydrochloride/ polystyrene sulfonate (PAH/PSS) films adsorbed from aqueous KCl solution (high salt conditions) at temperatures between 5 and 40 °C. KCl was chosen because the films were thicker than those obtained from NaCl or CsCl solutions indicating strong specific binding between K and PSS. The film thickness increases continuously with the adsorption temperature; the changes amount to 20-40%, depending on salt conditions. Furthermore, the roughness is increased, up to a factor of 5. The latter is attributed to the decreased percentage of strong electrostatic bonds within the polyelectrolyte multilayer. Another path to increased roughening is using low-weight polymers with a contour length similar to the thickness of a polycation/polyanion pair. Introduction Polyelectrolyte multilayers 1 formed by sequential ad- sorption of alternating charged polyelectrolytes have been investigated a lot in recent years. 2 With this technique, layered polymeric multicomposites with nanometer con- trol can be built very easily, even on rough or bent sur- faces. 3,4 The buildup principle is based on electrostatic interaction; during each adsorption step the surface charge is reversed, 5-7 and thus the adsorption of an oppositely charged polyion is possible again. Yet, the actual surface coverage is also determined by secondary interactions 8 and steric repulsion. 9 The range of possible applications of polyelectrolyte multilayers increased even further when it was realized that not only long-chain molecules but any adsorbed multi-ion such as nanocolloids 10,11 or proteins 12 can be incorporated in a polyelectrolyte multilayer. However, the adsorption properties of polyelectrolytes are only qualitatively understood. Experimental find- ings 13,14 and theoretical predictions 15,16 obtained so far may be summarized as follows: Polyelectrolyte adsorption in general is driven by either electrostatic attraction between the polyelectrolyte segments and the surface carrying opposite charges or short-range attractive forces (van der Waals, hydrophobic, hydrogen bonding, or specific forces), 8 leading in most cases to charge reversal. Entropic effects such as counterion release from both the surface and the polyelectrolyte chains 14 also promote polyelectrolyte ad- sorption. Besides those short-ranged attractive forces, also repulsive forces determine the polyelectrolyte conforma- tion: a multi-ion approaching a surface of the same charge experiences a strong and long-ranged electrostatic repul- sion. The polyelectrolyte conformation resulting from this short-ranged attraction and long-ranged repulsion is very flat, corresponding to one or two chain diameters. 14,16 When the substrate with the adsorbed film is removed from the solution, the adsorbed layer remains on the surface because of the pronounced adsorption/desorption hyster- esis that is typical for multisegment adsorption. 17 For polyelectrolyte multilayers, the flat, homogeneous ad- sorption implies a small roughness of the film/air interface, which is very convenient for a controlled polyelectrolyte multilayer architecture. The flat conformation of adsorbed polyelectrolytes is very different from the coiled conformation (approaching a few radii of gyration) of neutral polymers. 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