Credit: © 2007 RSC

Carbon nanotubes — with their unique electrical and mechanical properties — have been widely investigated as reinforcement fillers for the development of polymer nanocomposites. Recent studies in this area have looked at composite films that incorporate arrays of vertically aligned carbon nanotubes, which have shown potential for use in many applications, ranging from dry adhesives to biological or chemical sensors.

Now, Liming Dai and colleagues1 at the University of Dayton and The Air Force Research Laboratory, both in Ohio in the US, have developed nanocomposite films that possess both self-cleaning and controlled-release properties. Following the growth of vertically aligned multiwalled nanotubes on a silicon wafer, a small precursor compound (N-isopropylacrylamide) was added and induced to react to form polymer chains in between the nanotubes. In further processing steps, the polymer chains — which are known to alter their size in response to changes in temperature — were etched slightly to ensure that only the tips of the nanotubes were exposed at the surface of the composite film. When treated with water at 20 °C, however, the polymer chains expanded out from the gaps to cover the nanotube surface, a process that could be reversed by heating the film above 32 °C.

This reversible expansion and contraction was shown to give the film self-cleaning properties because the expanding polymers were able to physically push away gold nanoparticles adsorbed on the surface of the film. Similarly, the ability of the expanding polymers to release gold nanoparticles from the film into the solution above was also demonstrated.