Photoluminescent polymer gets brighter under stress

February 04, 2020 //By Julien Happich
Researchers from the Okinawa Institute of Science and Technology Graduate University (OIST) have designed a new stress-detecting polymer that shines brighter when stretched or submitted to a mechanical stress.

The new polymer detailed in a the Chemical Communications journal under the title “Highly sensitive mechano-controlled luminescence in polymer films modified by dynamic CuI-based cross-linkers” consists of polybutylacrylate integrating copper complexes (where copper atoms are linked to organic molecules). The paper reports that the copper complexes are acting as a mechanophore within the polymer. When the polymer is

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

stretched under ultraviolet light, the copper complexes glow at a greater intensity than without the mechanical stress.
What the researchers observed, is 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. They then exploited this relationship between flexibility and brightness to create a stress-detecting polymer.

“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.”

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

The copper mechanophores developed by the OIST team are sensitive to smaller stresses than mechanophores made from organic compounds, which change colour or emit light when mechanical stress breaks a weak chemical bond.

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