Engineering the Interaction of Latex Spheres with Charged Surfaces: AFM Investigation of Spherical Polyelectrolyte Brushes on Mica Y. Mei, A. Wittemann, G. Sharma, and M. Ballauff* Polymer-Institut, Universita ¨ t Karlsruhe, 76128 Karlsruhe, Germany Th. Koch, † H. Gliemann, † J. Horbach, ‡ and Th. Schimmel* ,†,‡ Institut fu ¨ r Nanotechnologie, Forschungszentrum Karlsruhe GmbH, PO Box 3640, 76021 Karlsruhe, Germany, and Institut fu ¨ r Angewandte Physik, Universita ¨ t Karlsruhe, 76128 Karlsruhe, Germany Received November 15, 2002 Revised Manuscript Received April 7, 2003 Introduction. Colloidal particles are often stabilized by long polymeric chains grafted to their surface. 1 If two such particles are dispersed in a good solvent for the chains and these particles approach each other, a repulsive interaction results. The steric interaction thus effected has been studied for decades and is well understood by now. 2,3 It can be enhanced even more if the polymers attached to the surface carry charges. The resulting electrosteric interaction can be understood in terms of the increased osmotic pressure of the counter- ions if the polyelectrolyte chains attached to the surfaces of the particles are to share the same volume. 4,5 The great practical importance of electrosteric interaction is related to the fact that most industrial latexes are stabilized in this way. 6 Application of latexes not only requires a fundamental understanding of mutual interaction, however. Control- ling the interaction of the particles with solid shells is of comparable importance when considering latex par- ticles as the base of paints and coatings. 6 A comprehen- sive investigation of these problems requires latex particles onto which the polyelectrolyte chains are firmly attached, i.e., by a chemical bond. Otherwise, the particles would disintegrate upon strongly interacting with a solid substrate. Moreover, the polyelectrolyte chains must be densely grafted to the particles. Colloidal objects carrying only a small number of chains can approach solid substrates so closely that their strong van der Waals interaction with the substrate becomes the leading effect. 1,7 Recently, we demonstrated that colloidal particles with attached polyelectrolyte chains can conveniently be prepared by photoemulsion polymerization. 8 By this method the polyelectrolyte chains can be affixed rather densely to the surface of poly(styrene) latex particles so that the overall dimensions are much larger than their average distance on the surface of the particles. Hence, a spherical polyelectrolyte brush (SPB) results, having overall dimensions in the colloidal domain (Figure 1). The chains are affixed to the particles by a chemical bond and exhibit an excellent stability against coagula- tion in solution. 9,10 Previous studies demonstrated that the dimensions of the shell can be tuned by the salt concentration in the solution. 9,10 Moreover, direct meas- urements of the osmotic pressure of aqueous solutions of the SPB 11 demonstrated that ca. 95% of the counter- ions are confined within the brush as predicted by theory. 12,13 Rheological measurements clearly pointed to the strong mutual repulsion of the particles when dispersed in water. 14 Evidently, these SPB provide a good model system for a systematic study of the interaction of sterically stabilized particles with solid substrates. Here we present the first results of a study of SPB contacting a negatively charged mica substrate. Atomic force micros- copy (AFM) in the intermittent contact mode 15 has been used as tool to investigate the topography and the phase contrast 16 of the samples. Phase contrast results from a combination of different tip-sample interactions such as local adhesion and capillary forces as well as visco- elastic damping. Recent investigations demonstrate that AFM is a powerful method to study the local organiza- tion of latex particles on solid substrates. 17-23 Two different SPB have been studied: (1) An anionic system consisting of chains of poly(styrenesulfonic acid). This system termed LQ3 has been synthesized and characterized in detail recently (see Table 1 in ref 10). (2) The SPB termed LA2 carrying positive charges. These particles have been synthesized and characterized for the present purpose. Figure 1 shows schematically the structure of the cationic SPB LA2. As cationic polyelectrolyte we used poly((2-acryloyl)ethyl)trimethyl- ammonium chloride (poly(flocryl)), which carries quar- ternized ammonia groups. Poly(flocryl) chains hence represent a quenched polyelectrolyte in which the charges are independent of the pH in the system. The choice of this particular polyelectrolyte derives from its excellent solubility in water and use as technical floc- culation agent. 6 The polyelectrolyte chains are attached to a poly(styrene) core of 136 nm (LQ3) and 90 nm (LA2) diameter with low polydispersity. Since the chains can be cleaved off after synthesis, their molecular weight and molecular weight distribution can be determined. 8 Moreover, the total charge of the SPB brush is deter- mined by titration. Hence, all pertinent parameters of the particles are known. 8-10 The only parameter that was varied in our experiments was the charge of the SPB. Differences in the adsorption behavior can hence be explained by different interaction forces of the † Forschungszentrum Karlsruhe GmbH. ‡ Universita ¨ t Karlsruhe. * Corresponding authors. E-mail: Thomas.Schimmel@physik. uni-karlsruhe.de; Matthias.Ballauff@chemie.uni-karlsruhe.de. Figure 1. Scheme of the cationic spherical polyelectrolyte brush LA2 synthesized and used in this study. Long polyelec- trolyte chains are densely attached to solid poly(styrene) particles so that a dense shell of charged chains results on the surface of the cores. The anionic latex LQ3 used herein has the same structure but with the cationic chains being replaced by the anionic poly(styrenesulfonic acid) chains. 3452 Macromolecules 2003, 36, 3452-3456 10.1021/ma0258399 CCC: $25.00 © 2003 American Chemical Society Published on Web 04/24/2003