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Image: Top: One photonic gel developed at Rice University was put through a series of color changes when repeatedly was- hed and exposed to new compounds. The gels show potential for inexpensive sensors and filters. © Thomas Lab/Rice
In the liquid solution, the polymer molecules are diffused, but when the liquid is applied to a surface and the solvent evaporates, the block copolymer molecules self-assemble into a layered structure. The polystyrene molecules clump together to keep water molecules out, while the poly(2-vinyl pyri- dine), P2VP for short, forms its own layers between the polystyrene. On a substrate, the layers form into a transparent stack of alternating "nano-pancakes." "The beauty of self-assembly is that it’s simultaneous, all the layers forming at once," Thomas said. The researchers exposed their films to various solutions and found different colors depending on how much solvent was taken up by the P2VP layers. For example with a chlorine/oxide/iron solution that is not readily absorbed by the P2VP, the film is transparent, Thomas said. "When we take that out, wash the film and bring in a new solution with a different ion, the color changes."
The researchers progressively turned a clear film to blue (with thiocyanate), to green (iodine), to yellow (nitrate), to orange (bromine) and finally to red (chlorine). In each case, the changes were reversible. Thomas explained that the direct exchange of counterions from the solution to the P2VP expands those layers and creates a photonic band gap – the light equivalent of a semiconducting band gap – that allows color in a specific wavelength to be reflected. "The wavelengths in that photonic band gap are forbidden to propagate," he said, which allows the gels to be tuned to react in specific ways. “Imagine a solid in which you create a band gap everywhere but along a 3-D path, and let’s say that path is a narrowly defined region you can fabricate within this otherwise photonic material. Once you put light in that path, it is forbidden to leave because it can’t enter the material, due to the band gap. This is called molding the flow of light,” he said.
Cover Image: A photonic gel developed at Rice University and the Massachusetts Institute of Technology self-assembles from long polymer molecules. Polystyrene and poly(2-vinyl pyridine) are mixed in a solution that, when evaporated, allows the polymers to quickly form into nanosized layers. The layers can be tuned to reflect specific colors when exposed to particular chemicals. © Joseph Walish/MIT
Ho Sun Lim, Jae-Hwang Lee, Joseph J. Walish, and Edwin L. Thomas: Dynamic Swelling of Tunable Full-Color Block Copolymer Photonic Gels via Counterion Exchange, In: ACS Nano ASAP, September 28, 2012, DOI: 10.1021/ nn302949n: