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Professur für Polymere
und Nanostrukturen
Prof. Dr. M. Schönhoff

Corrensstr. 28/30 48149 Münster
Tel.: +49 251 83-23410
Fax: +49 251 83-29138
schoenho@
uni-muenster.de

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Polyelectrolyte multilayers



The adsorption of organic molecules to interfaces is offering a variety of possibilities for the design of novel materials with structures defined on a nanometer scale.
So-called “self-assembly”- processes, employing the self-organization of molecules at interfaces as a result of for example hydrophobic or electrostatic interactions can be employed to produce layered materials.

PLANAR COATING

In the case of polyelectrolytes the electrostatic interaction can be employed by immersions into solutions, alternating between polycations and polyanions (see Figure, red: positive, blue: negative):

When a substrate with a negative surface charge is immersed into a solution with positively charged polyelectrolyte chains, the electrostatic attraction leads to an irreversible binding of the chain to the surface, while the counterions remain in the electric double layer. Thus, a monomolecular layer with a thickness on the order of one nanometer is formed. Under suitable conditions, the net surface charge is positive, such that as a next step, a negatively charged polyion can be adsorbed by immersion into a solution of polyanions. The counterions remain in the solution due to their large entropic contributions.
Repeating this procedure of immersion into solutions of chains of alternating charge is a simple way of forming a material with a large variability concerning different molecular constituents that can be employed.

PARTICLE COATING

PEMs can even be formed on colloidal particles as templates instead of planar substrates. In this case, the excess polyelectrolyte has to be removed by centrifugation or ultrafiltration.
The development of PEM- coating procedures for particles is a major achievement, since it allows to prepare dispersions with a large surface area. In such samples, PEMs can be studied by volume techniques which require a sufficient amount of compound, such as for example NMR or DSC.

After coating of colloidal particles the template itself can be removed employing suitable chemical procedures (for example a dissolution or degradation in acidic conditions). Provided that the PEM shell is stable against that treatment, a hollow polymeric capsule is remaining. Such hollow containers are interesting objects for applications such as encapsulation and the controlled release of active compounds. Therefore, the permeability and mechanisms to control it are of great interest. 


Why are we interested in these materials?

On the one hand, multilayer-materials are of fundamental interest, since they are mono-molecularly thin, i.e. their monolayer thickness corresponds to the minimum that can be reached (~nm). The properties of multilayers are therefore not dominantly influenced by the properties of the polymeric components themselves, but rather by the multiple interactions between adjacent layers. Here, a basic principle of colloid and interface science becomes relevant: Large internal interfaces dominate the material properties.

On the other hand, a number of potential applications make PEMs very interesting. Apart from pure charged homopolymers, a variety of additional components with defined functionalities can be employed in the layer formation, such as for example proteins, enzymes, inorganic nanoparticles, or functional polmers sensitive to external stimuli (pH, temperature). This leads to new possibilities of controlled material design on the smallest possible length scale available with molecular arrangements.

A basic understanding of the principles of self-organised layer formation is however, essential for any kind of application. While structural properties, such as the thickness control or the internal interdigitation of adjacent layers were already understood about a decade ago, current studies in this field focus on the internal properties of multilayers, such as the internal interactions, or the degree of dissociation.


Projects



Further reading about Polyelectrolyte multilayers:

Schönhoff, M.
Self-Assembled Polyelectrolyte Multilayers
Current Opinion in Coll. Interf. Sci. 2003 , 8(1), 86-95.

Schönhoff, M.
Layered Polyelectrolyte Complexes: Physics of Formation and Molecular Properties
J. Phys.: Cond. Matter 2003, 15(49), R1781-R1808.

Link to the Polyelectrolyte Multilayers Homepage at Florida State University.

Imprint | © 2010
Professur für Polymere
und Nanostrukturen
Prof. Dr. M. Schönhoff

Corrensstr. 28/30
· 48149 Münster
Tel.: +49 251 83-23410 · Fax: +49 251 83-29138
E-Mail: schoenho@uni-muenster.de