Regensburg 2010 – scientific programme
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CPP: Fachverband Chemische Physik und Polymerphysik
CPP 18: Interfaces and Thin Films II
CPP 18.4: Talk
Tuesday, March 23, 2010, 14:45–15:00, H39
Bundle Formation in Polyelectrolyte Brushes Caused by Divalent Ions — •Heiko Ahrens1, Jens-Uwe Günther1, Stephan Förster2, and Christiane A. Helm1 — 1Institut für Physik, Ernst-Moritz-Arndt Universität, Felix-Hausdorff-Str. 6, D-17489 Greifswald, Germany — 2Institut für Physikalische Chemie, Universität Hamburg, Grindelallee 117, D-20146 Hamburg, Germany
Monolayers from amphiphilic block copolymers consisting of a fluid hydrophobic and a polyelectrolyte part are studied with X-ray reflectivity and Grazing Incidence Diffraction. The concentration of divalent cations (Ca2+, Sr2+) is varied between 0.001 and 1 mM. At 0.001 mM the brush thickness is almost constant resembling the osmotic brush phase (brush thickness almost 50 % of contour length), at 1 mM the brush shrinks and the thickness scales with the molecular area with an exponent -0.5 (brush thickness as small as 20% of the contour length). Always, between two and four chains form a bundle, independent of salt concentration. For concentrations between 0.003 and 1 mM, the peaks are very weak, suggesting only small fractions of the chains form bundles. At 0.001 mM, the peak is stronger, similar to findings with monovalent ions in solution. We conclude that the divalent ions form links between polyelectrolyte chains, which is very different from monovalent ions. For monovalent ions, the aggregation number varies due to a balance between long-ranged electrostatic repulsion, whose range and amplitude is decreased on salt addition, and a short-ranged attraction presumably due to hydrophobic forces (J.U. Günther et al., PRL 2008, 101 (2008) No. 258303).