The polymer reversibly shines white when it is stretched

Researchers from the Adolphe Merkle Institute (AMI) of the University of Friborg and the University of Hokkaido in Japan have developed a method to customize the properties of the indicator molecules of Constraints that can be incorporated into polymers and report excessive damage or mechanical loads with an optical signal.

As part of their research activities at the National Competence Center for Research on Bio-inspired Materials, Professor Christoph Weder, Chairman of the Board of Polymers and Materials Chemistry at the MAI, and his team are studying polymers that change color or fluorescence characteristics when they are placed under mechanical control. charge. The predominant approach to achieving this function is based on specially designed sensor molecules that contain weak chemical bonds that break when the applied mechanical force exceeds a certain threshold. This effect can cause a color change or other predefined answers. A fundamental limitation of this approach, however, is that weak bonds can also break when exposed to light or heat. This lack of specificity reduces the practical utility of stress indicating polymers. This also makes the effect irreversible.

Attending this problem, Weder and Dr. Yoshimitsu Sagara – a Japanese researcher who spent two years in Weder's group at AMI before joining Hokkaido University as an assistant professor – have designed a new type of sensor molecule that can only be activated by mechanical force. Unlike previous force transduction molecules, no chemical bond break occurs. Instead, the new sensor molecules consist of two parts that fit together mechanically. This interconnection prevents the separation of the two parts, while allowing them to be moved together or away from each other. Such molecular movements cause a change in the fluorescence of the molecule.

In a new publication in the open access journal ACS Central ScienceWeder, Sagara and their colleagues say this new concept is robust and versatile. "The design approach makes it possible to adapt the properties of such sensor molecules because their behavior is fairly predictable," says Weder. "We chose to demonstrate this by tackling materials that exhibit white fluorescence when they are stretched," adds Sagara. "Mechano-reactive white fluorescence is usually difficult to obtain and requires the combination of three sensor molecules with predefined emission colors: blue, green and red (or orange). In addition, the sensor molecules must also exhibit a similar response to mechanical stresses to achieve activation / deactivation of the white emission when they are mixed.

As expected, the polymers containing the new motifs do not produce fluorescence in the absence of mechanical strength, but they become highly fluorescent – red, green or blue, when only one type of molecule sensor is used, the white when they are combined – when they are stretched. Since no chemical bond is broken, the process is also totally reversible. Thus, when the new sensor molecules were incorporated into an elastic polymer, the fluorescence was activated when the material was stretched and deactivated when the force was removed and the material contracted. In addition, the intensity of fluorescence, or brightness, has been shown to correlate with the extent of deformation.

Potential applications of such materials include built-in monitors that send visual warning signals prior to the failure of a part or that allow engineers to map the stresses in the underloaded parts and help them better understand the parts. design. Sensor molecules also promise to be useful for fundamental studies, at the molecular level, of stress transfer mechanisms in synthetic materials as well as in biological systems.

The Swiss-Japanese team is currently collaborating to further simplify the design to extend the concept to materials that change color instead of fluorescence. The response of such patterns could be inspected without any auxiliary means and would therefore be more useful for practical applications.