Polyelectrolyte Brushes

The behavior of polyelectrolyte molecules in contact with a solid surface is fundamentally different from that of uncharged polymers. When the surface attached layers consist of polymer molecules carrying charges, structure and properties of the layers are dominated by the electrostatic interactions. Mutual repulsion between the charged polymer segments and electrostatic forces between the polyelectrolyte molecules and the surface to which the chains are attached (especially if the latter one also carries a charge), influence the strength of interaction with the substrate and the physical properties of the layers strongly.

Charged polymer brushes - polyelectrolyte brushes - are interesting molecular architectures because due to the electrostatic repulsion between the polymer segments they are strongly strechted and show physical properties that are very different from their neutral counterparts. Especially their swelling behavior exhibits some unusual features and depends on the nature of the solvent system, the pH value and the concentration and chemical nature of other ions in the solution (salt). From an synthetically oriented point of view, the preparation of such architectures is also challenging because physisorption processes will not yield high graft densities or layer thicknesses due to the electrostatic repulsion between already adsorbed polyelectrolytes and further macromolecules that approach the surface.

The polyelectrolyte layers studied in our group are generated by using self assembled monolayers of azoinitiators and radical chain polymerization in situ "from" the surface.

Besides the investigations on the synthesis of such systems we also perform studies on the physical properties of these brushes. There is a vast amount of theoretical literature that describe the swelling behavior of PEL brushes but only relatively little work was carried out experimentally to check the theories. With the new systems at hand we can now realize most of the conditions (salt, pH, graft density and layer thickness) used in theory. In order to measure the swollen layer thickness we use a special ellipsometric setup, which was developed in collaboration with → PD Dr. Diethelm Johannsmann, TU Clausthal.

References
Preparation and Characterization of Polyelectrolyte Monolayers Covalently Attached to Planar Solid Substrates M. Biesalski and J. Rühe, Macromolecules32, 2309-2316 (1999).
Swelling of Polyelectrolyte Brushes in Humid Air M. Biesalski and J. Rühe, Langmuir 16, 1943-1950 (1999).
Segment Density Profiles of Polyelectrolyte Brushes Determined by Fourier Transform Ellipsometry M. Biesalski, J. Rühe and D. Johannsmann, J. Chem. Phys. 111, 7029-7037 (1999).