Color detection of stressed polymer films, gels before they break

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Stretch films and squishy gels help make wearable electronics, soft robotics and biocompatible fabrics a reality. But too much force can cause these polymers to break without warning. To detect stress before it’s too late, researchers report in the Journal of the American Chemical Society show that they have designed a compound with “wings” that cause these materials to change color when stretched or crushed.
Plasticky films and polymer gels – flexible 3D networks filled with liquids – can be bendable, stretchable or squeezable. And while most polymer films only separate when pulled too far, many gels aren’t very strong and crack under relatively low pressure. Yet, there is no way to predict the strength of the sponge material. In previous research, Shohei Saito and his colleagues have developed V-shaped molecules known as fluttering molecular force probes (FLAPs). FLAPs have two lateral wing-like structures that flatten under pressure, causing a color change from blue to green fluorescence. This probe worked as expected when embedded in a polyurethane film, but when added to a liquid-soaked polymer gel, the compound spontaneously turned fluorescent green without any external force. So, Saito and Takuya Yamakado set out to improve the FLAP molecule so that it could accurately detect mechanical stresses in both a polymer gel and a film. For more information, see the IDTechEx report on Flexible, Printed and Organic Electronics 2020-2030: Forecasts, Technologies, Markets.
The researchers modified their earlier version by replacing the two anthraceneimide wings with pyreneimide wings, attaching them to opposite sides of the same flexible cyclooctatetraene central joint. When they added the probe to a polymer film and stretched the material, its fluorescence changed sharply from blue to green. It also produced a color change visible to the naked eye. Next, the researchers embedded the new FLAP probe in a polyurethane gel soaked in an organic solvent, creating a yellow cylinder that fluoresced blue, and then compressed the material. The cylinder fluorescence became noticeably greener as pressure was applied. In their final test, the researchers placed metallic FLAP letters on a rectangular block of the gel. They used green-to-blue fluorescence ratio maps to calculate the pressure of each letter placed on the gel below, which ranged from 0 to 1 MPa. The researchers say this study could help them develop stronger gel materials and nanoscale voltage probes for cell membranes.

Source and top image: American Chemical Society

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