A Bright Way To Detect Mechanical Stress

A team of Swiss and Japanese scientist has developed a polymer that switches on and off its fluorescence in response to mechanical stress.

AsianScientist (May 10, 2019) – Molecules that fluoresce under mechanical stress may one day be used to assess the structural integrity of materials, according to research by an international team of scientists. The research is published in ACS Central Science.

Predicting when a material will fail under mechanical stress is tricky business. Hence, scientists at the University of Fribourg, Switzerland, and Japan’s Hokkaido University sought to develop stress-indicating molecules that can be integrated into polymers and signal damages or excessive mechanical loads optically.

To develop stress-responsive polymers that change their color or fluorescence under stress, researchers tend to design 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 pre-defined responses.

A fundamental limitation of this methodology, however, is that the weak bonds can also break upon exposure to light or heat. This lack of specificity reduces the practical usefulness of stress-indicating polymers. It normally also makes the effect irreversible.

Hence, the researchers devised a new type of sensor molecule that can only be activated by mechanical force. Unlike in previous force-transducing molecules, no chemical bond breaking takes place. Instead, the new sensor molecules consist of two parts that mechanically interlocked.

This interconnection prevents the separation of the two parts, while still allowing them to be pushed together or pulled away from each other. Such molecular pushing and pulling causes the molecule’s fluorescence to change from off to on.

“We chose to demonstrate this by tackling materials that display white fluorescence when stretched,” said Assistant Professor Dr. Yoshimitsu Sagara of Hokkaido University. “Mechanoresponsive white fluorescence is in general difficult to achieve. It requires the combination of three sensor molecules with pre-defined emission colors: blue, green and red (or orange). In addition, the sensor molecules need to exhibit a similar response to mechanical stress to achieve on/off switching of white emission when they are blended.”

Because no chemical bonds are broken, the process is also fully reversible. Thus, when the new sensor molecules were incorporated in an elastic polymer, the fluorescence was turned on when the material was stretched, then turned off when the force was removed and the material contracted. Moreover, the fluorescence intensity, or brightness, was shown to correlate with the extent of deformation.

Potential applications for such materials include built-in monitors that send visual warning signs before a part fails, or that enable engineers to map stresses in parts under load. The sensor molecules also promise to be useful for fundamental, molecular-level investigations of stress-transfer mechanisms in synthetic materials as well as in biological systems.

The article can be found at: Sagara et al. (2019) Rotaxane-Based Mechanophores Enable Polymers with Mechanically Switchable White Photoluminescence.


Source: Hokkaido University. Photo credit: Hokkaido University.
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