Polyelectrolyte surfaces can be made to stick together or separate depending upon the pH.

Polyelectrolytes are polymers that can be charged or uncharged depending on environmental pH. Bringing a poly(methacrylic acid) gel in contact with a poly[2-(dimethyl amino)ethyl methacrylate] (a polybase) brush layer, grafted on a silicon substrate causes adhesion above pH 3. Below that pH we see no adhesion. We can turn adhesion on and off, and repeat the process. This is illustrated in the video above. The adhesion is probably due to different processes, including hydrogen bonding, interactions between positive charges on the polybase, and negative charges on the polyacid, and maybe, under some circumstances, even some interdigitation between the brush into the gel. At maximum adhesion, the strength is not far off that of an epoxy [1].

Movie transcript

Three vials are visible: The vial on the left contains a hemispherical polymethacrylic acid gel swollen in deionized water, in the centre a polybase brush-modified silicon wafer also swollen in deionized water, and on the right, water at pH 1.0.

Two cartoons show how both surfaces when brought into contact may attract to one another. The polymethacrylic acid gel is then removed from the first vial and the wafer is removed from the second. The gel is placed in contact with the brush-modified wafer and immediately both adhere to one another.

Another cartoon illustrates that if the two surfaces are immersed at pH<2 they quickly come apart. To demonstrate this illustration, we place the adhered surfaces into the third vial at pH 1.0 and after a few seconds the surfaces come apart and the gel falls to the bottom of the vial.

In the media

This work has attracted some interest in the scientific media. You can read about our switchable adhesion in Technology Review, Chemistry World, and nanotechweb.org. Journalists for those three sites interviewed me to ensure accurate presentation of the work. The work has also been described on numerous other sites, such as Welt der Physik (in German) and The Engineer. Nature Materials also picked up on the work and included it as one of their "research highlights", but you'll need a subscription to read the short article. I was interviewed in Chemistry and Industry about this work (Issue August 13, 2007). A subscription is also needed to read the news article

The mainstream media has also given our new "glue" some attention too, with the work being mentioned in large-circulation newspapers. For example, I was very pleased to discover that The Daily Telegraph included a short news article on August 13, 2007. Unfortunately, this only appeared in the print version and is not available online. The German press, not to be outdone, also reported it, unsurprisingly in German, with articles in die Berliner Zeitung and Innovate! from die Süddeutsche Zeitung. Other articles (subsequently deleted or removed) include one in The Scotsman and another in Daily India.

Funding

We acknowledge financial support from the European Community’s “Marie-Curie Actions” under contract MRTN-CT-2004-504052 [POLYFILM] and from the Engineering and Physical Sciences Research Council (GR/R74383/01).

References

[1] R. La Spina, M. R. Tomlinson, L. Ruiz-Pérez, A. Chiche, S. Langridge, and M. Geoghegan "Controlling network-brush interactions to achieve switchable adhesion" Angew. Chem. Int. Ed. 46 6460-3 (2007).

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