Jun 22 2011
To describe them as microscopically small would be an understatement, yet they could play a big role in future technology: Polymer chains of approximately 100 nanometers in length could operate as tiny switches in advanced applications.
In the past, the reaction time of such nanostructures was thought to be too slow, but a research group at the University of Duisburg-Essen (UDE) under the direction of Dr. Nils Hartmann at the Center for Nanointegration (CeNIDE) has disproven this theory, and their results have been published in the international journal "Angewandte Chemie" (Vol. 123, No. 19).
The polymer Poly(N-Isopropylacrylamide) can also be found in a modified form in diapers. This material keeps the baby's bottom dry by wicking away the moisture-for that reason it is classified as a "hydrogel". Prof. Dr. Mathias Ulbricht, who holds a Chair in Technical Chemistry at CeNIDE, established routines which allow one to firmly attach such polymer chains to a surface. At temperatures below 32°C the layers absorb water and the structure resembles a brush.
When the temperature rises above the critical point, the tiny chains collapse and form dense layers. Depending on the structure, the thickness of the layers decreases by at least one half. In this way, the material could be used to regulate valves in small openings and channels, such as those used in membrane technology or microfluidics. It could be used to measure temperature or moisture, control the release of drugs within the human body, or work as a miniaturized switch for many other processes. With the aid of nanopolymers, small structures could also be created that would react much more quickly than their macroscopic counterparts. According to theory, the water needed for the structural changes would have to travel a shorter diffusion path; however, in many applications speed proved to remain a problem. Repeated tests showed reaction times on the order of seconds-much too slow for high speed applications.
Dr. Nils Hartmann is group leader at the Chair of Physical Chemistry and a member of CeNIDE. He recognized the primary hurdle in previous experiments: In order to measure the rate of the process, a characterization method is required that is faster than the process itself as well as a manipulation tool that will immediately cause the reaction of the polymer. Hartmann explains that, "The overall technique must be well adapted to the switching process, otherwise itself will cause a delayed reaction of the material." At least one of these two issues have not been recognized in previous research.
Hartmann's team developed a new stroboscopic method whereby the researchers heat the substrate/polymer interface with a laser. When the laser is on, the polymer becomes hot instantaneously; when it is off, the heat dissipates immediately. In order to observe the switching process, an optical microscope with a CCD camera was used. During stroboscopic measurements each frame captured a slightly delayed time interval of the heating and cooling phases. Within 16 seconds this method was able to completely measure the temperature-dependent kinetics. The results revealed that the hydrogel reacts within micro- or milliseconds to the laser-induced change of temperature. Hartmann happily remarked, "This alone is a completely new discovery, but we can also demonstrate that the polymer is not damaged, even after thousands of repetitions, so it is suitable for long-term use."
Other experts also regard these discoveries as extremely significant, which is demonstrated by its publication as a "VIP Paper" in the esteemed journal of "Angewandte Chemie."