Researchers at Sandia National Laboratories (Albuquerque, NM) and the University of New Mexico (Albuquerque) have developed intelligent nanostructures that report on their environment by changing color from blue to fluorescent red under mechanical, chemical, or thermal stress. Underlying this application is a generic, efficient solution to a problem that has long puzzled modern materials science. Namely, this involves answering how to efficiently distribute conjugated polymers--inexpensive carbon-based polymers that, due to special bonding patterns, carry electrical current and produce changes in a material's optical properties--within a hard, protective structure. The Sandia method, which involves a technique that links monomers into polymers in an orderly fashion within a nanostructure, was published in the April 19th issue of Nature.
Most immediately, the self-assembling, durable structures may lower costs by reducing the need for expensive manufactured devices like stress detectors, chemical analyzers, and thermometers. "The material can distinguish between different solvents," says Sandia senior scientist and UNM professor Jeff Brinker. "There's a high correlation of color with the polarity of the solvent." The material also can report changes in mechanical stress and temperature. When the environmental disturbance is removed, the structures change back to their original color, in some cases, making them potentially reusable.
"The material is of interest to NASA--one of the sponsors of our research--for a thin film for an inflatable structure," says Brinker. "The structure's skin would require a very thin, yet strong membrane with low permeability that could sense mechanical damage from hazards such as meteorites or sandstorms." The color change of the coating would also be sensitive to the composition of chemicals hitting the structure's skin, or to dangerous increases in temperature.
A robust architecture that is optically transparent and prevents oxidative degradation of the polymer can then be patterned on surfaces and substrates. "This is a simple means of forcing organization that should help us integrate conjugated polymers into devices," says Brinker. A still-open question, though, is how best to fashion a structure for these potentially useful but fragile extended molecules. "Traditionally, bulk conjugated polymers are like a huge bowl of entangled spaghetti," he adds. "Our method organizes this jumble by forcing them to adopt a particular conformation; that is, we organize them into nanostructures. We can force them into conformations, and so define where the polymer is and isn't. Then we can control how interactions between polymer units will affect a material's electrical and optical properties."
It takes only seconds for the Sandia/UNM method to evenly predistribute monomers--simpler precursors of polymers--within a silica matrix through self-assembly. Exposure to ultraviolet (UV) light polymerizes the monomers into conjugated polymers housed in nanoscopic channels that penetrate the matrix of the material. The result is a nanocomposite that is mechanically robust and optically transparent, while also producing telltale changes of color under changing environmental conditions.