Developed by a team at the Okinawa Institute of Science and Technology Graduate University (OIST) in Japan, the stress-detecting "smart" material could be used to measure the performance of synthetic polymers and track the wear and tear on structures such as aircraft or bridges.
The scientists developed the polymer by incorporating copper complexes – structures formed by linking copper atoms to organic (carbon-containing) molecules – into a polymer called polybutylacrylate, which is made from a chemical used to synthesize acrylic paints, adhesives and sealants.
The copper complexes which link the polybutylacrylate chains together naturally glow when exposed to ultraviolet light, a property known as photoluminescence. But when the polymer is stretched, the copper complexes emit light at a greater intensity, leading to a brighter glow. The copper complexes therefore act as mechanophores, compounds which undergo a change when triggered by a mechanical force.
Most mechanophores are made from organic compounds instead of metals such as copper. Organic mechanophores change colour or emit light when mechanical stress breaks weak chemical bonds, but they have severe limitations.
“A relatively large force is required to break the chemical bond, so the [conventional] mechanophore is not sensitive to small amounts of stress,” said first author Dr Ayumu Karimata. “Also, the process of breaking the bond is often irreversible and so these stress sensors can only be used once.”
In contrast, the new copper mechanophores are sensitive to much smaller stresses and can respond quickly and reversibly. The polymer film reportedly brightened and dimmed immediately in response to being stretched and released.
The polymer could eventually be adapted to create a stress-sensing acrylic paint. This could be used as a coating for different structures, such as bridges or the frames of cars and aircraft.
“As we can see even from the direct visualisation of the polymer, stress is applied across a material in a non-uniform way," said Karimata. “A stress-sensing paint would allow hotspots of stress on a material to be detected and could help prevent a structure from failing.”
The research was published in Chemical Communications.
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