Switchable adhesion
Smart coatings, whose properties can be changed easily and reliably, have a wide range of potential uses. Here we look at how suitably chosen surfaces stick to each other under some circumstances, but not under others. This smart adhesion property might be used to temporarily grip and manipulate objects, or allow components to be easily assembled, then disassembled and recycled at the end of their working lives.
Polyelectrolytes are a simple group of 'smart' materials, whose properties can be tuned by environmental factors. Above their pKa poly(carboxylic acids) are ionized, so the polymer molecule carries a negative charge. By contrast poly(amines) accept protons to become is positively charged in neutral and acidic solutions, as shown in the diagram below:
Above: At pH7 (neutral pH) the polyacid and polybase are oppositely charged. In highly acidic or basic solutions, however, only one of the polymers is charged.
In this study a cross linked poly(methacrylic acid) gel was prepared. This did not stick to the test silicon surface at any pH. If the test surface is modified by attaching a poly-basic brush formed from poly[2-(dimethyl amino)ethyl methacrylate], however, then strong adhesion is found - but only at neutral pH's.
If the pH is reduced, the two surfaces can be separated without damaging either surface, as shown in the animation below:
Above: At pH7 the polyacid gel blob adheres strongly to the polybase brush coating the silicon slide. As the pH falls to 1, however, the adhesive strength fails, and the gel detatches. (Requires Flash).
References
"Controlling Network-Brush Interactions to Achieve Switchable Adhesion" Rita La Spina, Michael R Tomlinson, Lorena Ruiz-P.rez, Arnaud Chiche, Sean Langridge and Mark Geoghegan Angew. Chem. Int. Ed. 2007, 46, 6460-6463.
A movie showing this separation is available on Mark Geoghegan's website: Switchable adhesion between oppositely charged polyelectrolytes
"Switchable adhesion between oppositely charged polyelectrolytes" Rita La Spina, Arnaud Chiche, Michael R. Tomlinson and Mark Geoghegan European Coatings Conference and Journal, 2010.
Acknowledgements
The author would like to thank Prof Mark Geoghegan of the University of Sheffield for assistance in preparing this feature.
