Researchers have created a polymer that is able to change its appearance in response to mechanical forces, meaning it can warn of damage developing in a material before the stress causes structural failure.
The researchers have reported in ACS Central Science that the first-of-its-kind elastic polymer blend can display white fluorescence when deformed, before returned to dark after relaxing back to its original shape.
A general approach to create such stress-sensing polymers is to integrate sensor molecules that change their optical properties when activated by mechanical force.
In most previous types of these polymers, activation is irreversible because it breaks covalent bonds in the sensors.
These bonds can also be unintentionally broken by other stimuli, such as heat or light, so an optical change does not always prove the material has been damaged.
The researchers wanted to overcome this, building on previous work, in which they reported a PU containing stress-sensing rotaxane consisting of ring-shaped fluorescent molecules threaded on dumbbell-shaped molecules featuring quencher groups at their centre.
In the relaxed polymer, the quenchers were near the fluorescent rings and prevented them from glowing under ultraviolet light.
Stretching the polymer moved the quenchers and fluorescent rings apart, so the material glowed green, a process that was fully reversible and couldn’t be triggered by heat or light.
In the current study, the researchers developed three new polyurethanes that glowed blue, green or orange when stretched, demonstrating that the colour can be tailored simply by using different cyclic fluorescent rings in the rotaxanes.
The materials emitted light of greater or lesser intensity, depending on how much they were stretched.
By combining the three polymers, the researcher said they created the first white-light-emitting polymer that can instantly be switched on and off mechanically.
Applications include study of the physical forces in cells and tissues as well as visualising damage in polymeric materials.