Stress sensors are important tools when it comes to evaluating the robustness of a material exposed to strong mechanical forces. In a paper in Advanced Materials, researchers at Okinawa Institute of Science and Technology Graduate University (OIST) in Japan report a new kind of sensor molecule that brightens when the material it is incorporated into comes under heavy mechanical stress.
Such light-based sensing molecules, known as photoluminescent mechanophores, are not new, but current applications of them are single-use only. They typically involve a strong force – compressing, twisting or stretching for example – breaking a specific chemical bond between two atoms or irreversibly pulling apart two complexes in the sensing molecule. This changes the wavelength – and thus the color – of the light emitted by the mechanophore.
Once these molecules have radically changed their structure in response to this force, however, it is extremely difficult for them to return to the initial situation. So while these mechanophores are useful for understanding the mechanical properties of an item or a material, they are not well suited for investigating repeated exposure to mechanical stress.
To overcome this issue, Georgy Filonenko and Julia Khusnutdinova from OIST’s Coordination Chemistry and Catalysis Unit designed a photoluminescent mechanophore that retains its properties over time and under repeated incidences of mechanical stress. The researchers incorporated this stress-sensing molecule into polyurethane, which is widely used in everyday items such as mattresses and cushions, inflatable boats, car interiors, woodworking glue and even spandex.
The scientists then stretched the resulting material with increasing force, triggering a correspondingly brighter glow under an ultraviolet light. This reaction happens within hundreds of milliseconds, resulting in an up to two-fold increase in luminescence intensity. When the mechanical stress stops, the polymer material and the mechanophore revert to their initial position, leading to a drop in intensity. This is critical as it allows for repeated applications of mechanical force.
This new mechanophore is a photoluminescent compound from recently published work by Filonenko and Khusnutdinova. Despite its very simple structure, the molecule is extremely responsive to the physical environment, producing the rapid change in luminescence intensity. The researchers incorporated these molecules directly within the repeated patterns of the polymer material.
Filonenko and Khusnutdinova found that the high mobility of the mechanophore molecules in the polymer was key to the sensor performance. When the mechanophores could move rapidly in the relaxed polymer sample, the luminescence intensity was low due to these molecular motions preventing the mechanophore from emitting light. Subjecting the material to mechanical force slowed down the polymer chain motions, allowing the mechanophore to emit light more efficiently.
“Our material shows how a macroscopic force as basic as stretching a flexible strand of material can efficiently trigger microscopic changes all the way down to isolated molecules,” said Filonenko.