Dr. Edwin L. Thomas, professor in the Department of Materials Science and Engineering, and a team of researchers from Texas A&M University and Yonsei University recently discovered a helicoidal-shaped defect in layered polymers, uncovering how solvents can diffuse through layers and produce color changes.
This research was recently published in Science Advances.
In some human-interactive electronics, such as temperature gauges or health sensors, polymers are used that are capable of changing color depending on stimuli. This phenomenon is referred to as stimuli-interactive structural colors because the material reacts and changes color due to environmental changes, such as a solvent or solution.
A material that has a one-dimensional periodic structure comprised of two (A and B) layers acts like a photonic crystal and can reflect light of a given wavelength (color) depending on the thickness of each layer. Stimuli-interactive structural color works by altering photonic crystals using external stimuli or forces. The thickness of each polymer layer affects the color of the light reflected: if all the layers in a material are of the same thickness, a single color will be reflected. If different parts of the material are composed of stacks of layers, each having a different thickness, each layer will reflect a different color and the material will appear like a normal metal material, reflecting all colors.
In some cases, a preferential solvent is used to swell one of the particular polymer layers, purposefully causing color changes. The researchers noticed that the expected layers were swelling in these materials. However, it was unclear how the solvent was seeping/crossing through layers that did not swell to those that were supposed to swell.
"Let's say we put a solvent over multiple polymer A and B layers," said Thomas. "The first A layer swells, the B layer doesn't swell, but the next layer A will. How does the second layer of solvent A get through the B layer? We realized there must be something in the overall polymer structure that allows the passage of solvent to the other layers."
To understand what was occurring within the polymers, the researchers used an electron beam to develop a tomogram -- a reconstruction technique that takes very thin, two-dimensional images of sections of 3D objects to uncover what is inside.
"Suppose you had a loaf of bread, and you wanted to know if there was a hole somewhere within the loaf," said Thomas. "If you sliced it thin, you'd eventually hit the hole. You keep slicing, and then the hole would disappear. If you looked at all the slices, you could understand exactly where the holes are. This process is similar to the idea of a tomograph."
Using this method, the researchers found that within the polymer photonic crystal material, helicoidal screw dislocations (defects) were present, allowing the solvent to easily and rapidly cross through to different layers, causing the swelling and producing the stimuli-interactive structural color changes.
Typically, defects are associated with high energy and are singular (abruptly disrupting the periodicity occurring in one location). In contrast, the helicoidal defects are nonsingular and spontaneously formed -- an advantage for the materials.
"This is a good kind of defect that helps properties and allows swift and efficient penetration into the material with solvent and rapid swelling. If these things didn't exist, the only way the layers could sweat would be from the edges," said Thomas.
Because stimuli-interactive structural color presents an excellent potential for devices such as health sensors and human-interactive electronics, controlling the lateral spacing or amount of helicoidal defects could be a critical factor in future applications.
"These defects currently produce a favorable effect, but it depends on the application," he said. "Our next challenge is deciphering how to control the spacing and amount of these defects and, in turn, having more control over the time it takes for the fluid to move through the layers. Understanding these defects is key for increasing the number of applications this technology can be used in."
The Hagler Institute Fellowship supported the research completed on this project at Texas A&M.