Photoluminescent Polymer Detects Stress Quickly

A new stress-detecting polymer that shines brighter when stretched could be used to measure the performance of synthetic polymers and track deterioration in materials that are used in engineering and construction.

Scientists from the Okinawa Institute of Science and Technology Graduate University (OIST) integrated copper complexes (copper atoms linked to organic molecules) into the polymer polybutylacrylate. They found that the copper complexes glowed when exposed to ultraviolet light. When the polymer was stretched, the copper complexes emitted light at a greater intensity, leading to a brighter glow. In essence, the copper complexes were acting as mechanophores — compounds that undergo a change when triggered by a mechanical force.

The copper mechanophore links two polymer chains together. When the polymer is stretched (force = F), the mechanophore glows brighter. Courtesy of OIST.

The researchers observed that the copper complexes changed shape continuously, but as the complexes increased in size, they became less flexible and gave off a brighter glow. The team surmised that the larger, less flexible complexes were able to release light more efficiently because their motion was restricted, causing them to lose less energy than the smaller, more freely moving complexes. The researchers exploited this relationship between flexibility and brightness to create a stress-detecting polymer.

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On the left are two films of the polymer, with the copper complexes incorporated as cross-linkers. On the right are two vials of the isolated copper complexes. The polymer films and isolated copper complexes luminesce under ultraviolet light. Courtesy of OIST.

“When the copper complexes are incorporated into the polymer as cross-links, the act of stretching the polymer also reduces the flexibility of the molecules,” researcher Ayumu Karimata said. “This causes the copper complexes to luminesce more efficiently with greater intensity.”

The copper mechanophores developed by the OIST team are sensitive to smaller stresses than mechanophores made from organic compounds, which change color or emit light when mechanical stress breaks a weak chemical bond. “A relatively large force is required to break the chemical bond, so the mechanophore is not sensitive to small amounts of stress,” Karimata said. “Also, the process of breaking the bond is often irreversible and so these stress sensors can only be used once.”

The copper-based mechanophores used as cross-linkers in polybutylacrylate enabled highly sensitive detection of mechanical stress even at small strain (< 50%) and stress (< 0.1 megapascal, or MPa) values, via changes in luminescence intensity.

Professor Julia Khusnutdinova and researcher Ayumu Karimata from the Coordination Chemistry and Catalysis Unit at OIST helped develop the stress-detecting polymer. Courtesy of OIST.

Karimata hopes that the acrylic polymer can eventually be adapted to create a stress-sensing acrylic paint. This could have valuable applications as a coating for different structures, such as bridges or the frames of cars and aircraft. “As we can see even from the direct visualization of the polymer, stress is applied across a material in a nonuniform way,” he said. “A stress-sensing paint would allow hot spots of stress on a material to be detected and could help prevent a structure from failing.”

The research was published in Chemical Communications (www.doi.org/10.1039/C9CC08354E).

The scientists used a CCD camera to directly visualize the changes in brightness as the polymer was stretched and released. The false color red represents high light intensity and the false color blue represents low light intensity. Courtesy of OIST.