These materials show large changes in their physical properties depending upon the pH of the solution they are in contact with.
SFM image of PMMA brushes showing expansion on exposure to alkali solution.

Above: Scanning force microscopy images of a polymethacrylic acid brush in water at two different values of pH are shown above [1].

Polyelectrolytes have the interesting property of changing their conformation in solution depending upon whether the solution is acid or base. Normally in water or base a polyacid will donate a proton to create a hydronium ion (H3O+); however in an acidic environment, these ions are in abundance, so the polyacid keeps its proton, which usually renders it hydrophobic. A hydrophobic polymer in aqueous solution will coil up to exclude the water. In base, the de-protonated polyacid is positively charged and will swell due to the Coulombic repulsion of the negative charges. If the acid is chemically grafted to a surface (a brush; see the figure above right) the swelling is caused by the osmotic pressure of the counterions trapped within the brush. Polyelectrolyte brushes have a unique property that might be used in various nanotechnologies: they are responsive. Swollen and collapsed brushes have very different properties: Swollen brushes can be used to help suspend inorganic nanoparticles in solutions; collapsed brushes may cause aggregation of the particles. Collapsed brushes may have particles encased within them; swollen brushes may release these particles. The tribology (wear or frictional properties) of such surfaces are very different. We are using a variety of experiments to characterise such polymers and to develop ideas for technologies where they might be used [1]. We have studied the structure of polyelectrolyte brushes [2], and have shown how they can be used for switchable adhesion [3].

More about switchable adhesion


[1] A. J. Ryan et al. Faraday Disc. 128 55-74 (2005).

[2] M. Geoghegan et al. Soft Matter 2 1076-80 (2006).

[3] R. La Spina et al. Angew. Chem. Int. Ed. 46 6460-3 (2007).

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