Comparison of the Structure of Polyelectrolyte Multilayer Films Exhibiting a Linear and an Exponential Growth Regime: An in Situ Atomic Force Microscopy Study Ph. Lavalle,* ,† C. Gergely, † F. J. G. Cuisinier, † G. Decher, ‡,§ P. Schaaf, ‡,⊥ J. C. Voegel, † and C. Picart †,⊥ INSERM Unite ´ 424, Fe ´ de ´ ration de Recherches “Odontologie”, Universite ´ Louis Pasteur, 11 rue Humann, F-67085 Strasbourg Cedex, France; Institut Charles Sadron (CNRS UPR 22), 6 rue Boussingault, F-67083 Strasbourg Cedex, France; Universite ´ Louis Pasteur, Faculte ´ de Chimie, 1 rue Blaise Pascal, F-67008 Strasbourg Cedex, France; and Ecole Europe ´ enne de Chimie, Polyme ` res et Mate ´ riaux de Strasbourg, 25, rue Becquerel, F-67087 Strasbourg Cedex 2, France Received November 13, 2001; Revised Manuscript Received March 13, 2002 ABSTRACT: We report here on the structural characterization of polyelectrolytes multilayer films formed by poly(L-glutamic acid) and poly(L-lysine) (PGA/PLL). The growth of this system is compared to that of poly(styrenesulfonate)/poly(allylamine hydrochloride) (PSS/PAH) multilayers by means of in situ atomic force microscopy (AFM) and by optical waveguide lightmode spectroscopy (OWLS). In contrary to the (PSS/PAH) i films that are growing linearly with the number of deposited layer pairs i, optical data evidenced that the (PGA/PLL)i films are characterized by an exponential growth. The analysis of the structure of the (PSS/PAH)i films reveals a smooth featureless surface covered by small globules. On the other hand, (PGA/PLL)i films form extended structures that appear with a vermiculate pattern. We propose a new growth mechanism based on polyelectrolyte diffusion in and out of the film coupled to the formation of polyanion/polycation complexes at the surface of the film in order to explain the whole results. I. Introduction The alternate immersion of a charged surface in a polyanion and a polycation solution leads usually to the formation of films known as polyelectrolyte multilay- ers. 1,2 These polyanion/polycation structures are not neutral, but a charge overcompensation appears on the surface; this constitutes the buildup motor of the poly- electrolyte multilayer films. 3 The first films that were explored were built with highly charged polyelectrolytes, such as poly(styrenesulfonate) and poly(allylamine hy- drochloride), afterward denoted (PSS/PAH). 4 These films exhibited remarkable and fascinating properties: the film thickness and the deposited amount of poly- electrolytes increased typically linearly with the number of deposited layer pairs. 5-7 They present a layered structure, each polyelectrolyte layer interpenetrating with some of the closest neighboring ones. 2,8,9 However, recent experimental results seem to indicate that the ideal picture of a linearly growing stratified smooth film does not constitute the only possible buildup mecha- nism. Parameters such as the chemical nature of the polymer pair, the nature and concentration of salt added to solutions used for deposition and/or rinsing, and others have a strong influence on the nature of the polyion complex formed. Experimental evidence sup- ports the idea that, by increasing the salt concentration for a given system, the film thickness and the adsorbed amount of polyelectrolytes can increase more rapidly than linearly with the number of deposited layer pairs. 10-13 This change from a linear to a superlinear growth regime was suggested for the PSS/PAH 10 and for the poly(diallyldimethylammonium chloride)/poly- (styrenesulfonate) (PDDA/PSS) 12 film architectures. Other systems composed of glycosaminoglycans and poly(L-lysine) also exhibit such a superlinear growth regime. 14-17 Recently, McAloney et al. 12 followed the buildup process of PDDA/PSS multilayers by AFM and studied the film structure at different ionic strength of the building polyelectrolyte solutions. Their images revealed that the film was featureless and slightly granular at low ionic strength when the film growth can be de- scribed as being linear. For salt concentration higher than 0.3 M the film growth changes to overshooting mode, and a vermiculate surface pattern associated with an increase of the film roughness was observed. It is thus tempting to associate a linear growth regime to a flat smooth film and a superlinear regime to a more rough and “heterogeneous” film topography. Unfortu- nately, all these studies were performed by rinsing the films in pure water after each polyelectrolyte deposition step during their buildup process and by imaging the film after the multilayer had been dried. It is known that pH and ionic strength jumps applied during or after the film construction can anneal surface roughness 18 but also lead to more dramatic structural rearrangements such as inducing porosity in the multilayer struc- ture. 19,20 Moreover, there exist indications that the film structure is also altered by the drying process. 21 The goal of the present article is to provide further information on the mechanisms that lead either to a linear or to a superlinear growth regime and in par- ticular to verify whether this superlinear growth regime is always associate with increased surface roughness. Therefore, we investigate the buildup of poly(L-glutamic acid)/poly(L-lysine) (PGA/PLL) multilayer films grown under superlinear conditions by means of in situ AFM and by optical waveguide lightmode spectroscopy † INSERM. ‡ Institut Charles Sadron. § Universite ´ Louis Pasteur. ⊥ Polyme `res et Mate ´riaux de Strasbourg. * Corresponding author. 10.1021/ma0119833 CCC: $22.00 © xxxx American Chemical Society PAGE EST: 7.3 Published on Web 00/00/0000