MIT researchers have developed gels which can change colors almost instantaneously when they're exposed to a variety of stimuli, such as temperature, pressure or humidity. These gels could be used to design inexpensive sensors. For example, these gel-based sensors could be useful in a food processing plant to 'indicate whether food that must remain dry has been overly exposed to humidity.' Now the researchers are working on a gel which would change color in response of electricity.
The photography above, taken by Donna Coveney, describes how "photonic gel crystals demonstrate the 'tunability' of materials made from alternating layers of hard and soft polymers. The soft polymers are easily swollen with liquid or vapor causing the materials to reflect different colors of light based on the way their molecules are chemically 'tuned.' (Credit: MIT) Here is a link to a larger version of this picture. And here is another link to a short video, "Structured gel turns colors based on stimuli," available in several formats, which shows the versatility of these gels.
This structured gel has been developed under the supervision of Edwin Thomas, head of MIT's Department of Materials Science and Engineering (DMSE). This research project has been conducted by graduate students from his research group, Youngjong Kang, now a professor at Hanyang University in Seoul, Korea, Joseph Walish and Taras Gorishnyy.
How did they create this gel changing color on demand? "They started with a self-assembling block copolymer thin film made of alternating layers of two materials, polystyrene and poly-2-vinyl-pyridine. The thickness of those layers and their refractive indices determine what color light will be reflected by the resulting gel. By keeping the thickness of the polystyrene layer constant and altering the thickness of the poly-2-vinyl-pyridine layer with external stimuli such as pH and salt concentration, the researchers were able to change the gel's color in fractions of a second."
And how did they alter the thickness of the poly-2-vinyl-pyridine layer? "The key to manipulating the thickness of the poly-2-vinyl-pyridine (2VP) layer is to give the nitrogens on each segment of the 2VP block a positive charge, yielding a polyelectrolyte chain that can swell to more than 1,000 percent its volume in water."
So what's next? As "the new gels are also sensitive to changes in pressure, humidity and temperature, 'You can use mechanical or chemical forces to get really big responses, going through the entire range of light from ultraviolet (300 nanometers) to infrared' (1600 nm), Thomas said. The research team is also working on a gel that changes color in response to applied voltages."
For more information, this research work will be published in Nature Materials and is already available as an 'advance online publication' under the name "Broad-wavelength-range chemically tunable block-copolymer photonic gels" (October 21, 2007). Here is the beginning of the abstract. "Responsive photonic crystals have been developed for chemical sensing using the variation of optical properties due to interaction with their environment. Photonic crystals with tunability in the visible or near-infrared region are of interest for controlling and processing light for active components of display, sensory or telecommunication devices. Here, we report a hydrophobic block–hydrophilic polyelectrolyte block polymer that forms a simple one-dimensional periodic lamellar structure. This results in a responsive photonic crystal that can be tuned via swelling of the hydrophilic layers by contact with a fluid reservoir."
Sources: Anne Trafton, MIT News Office, October 21, 2007; and various websites
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