Revealing hidden stress in plastics

The Adolphe Merkle Institute’s Mechanoresponsive Materials group is developing an innovative method to visualize mechanical stress in plastics and rubbers at the molecular level, offering insights into why these materials fail — and how to design ones that last longer, heal themselves, or safely decompose.

From car tires to food packaging, polymers are indispensable but prone to unexpected breakdowns. Tiny cracks, invisible to current imaging tools, form under stress and spread, causing failures in everything from medical devices to bridge supports. Traditional microscopes can only handle resolutions down to around a few hundred nanometers, not enough to spot the nanoscale fractures that trigger collapse. “With current microscopes, the damage that matters most is still too small to see,” explains Prof. Jessica Clough, head of the group.

The proposed advance, dubbed by the researchers super-resolution force imaging (SRFI), combines cutting-edge microscopy with custom “stress-sensing” molecules, bridging a critical gap in materials science that cannot be addressed with existing techniques.

SRFI adapts super-resolution microscopy, a Nobel Prize-winning tool in biology, to the world of synthetic materials. By tagging polymers with specially designed mechanophores, molecules that glow when mechanically stressed, the team hopes to achieve nanoscopic resolutions down to tens of nanometers, revealing previously hidden damage hotspots. Their trump card is a novel coumarin-based mechanophore developed by Clough and PhD student Yang Li, which activates a bright blue, fluorescent dye when its chemical bonds break under force. This sensor is easy to synthesize, stable under heat and light.

The team embedded their coumarin mechanophores into polymethyl acrylate, a common synthetic polymer used in adhesives and coatings. When subjected to ultrasound, exerting high stress on the polymer chains, the material lit up visibly under UV light, as confirmed by spectroscopy and other analytical methods. Another test used the mechanophores as cross-links in rubbery networks, demonstrating the activation and detection of coumarins in the solid-state for the first time. Stretching these materials triggered fluorescence where molecular bonds break, suggesting that SRFI should be able to map how damage develops under deformation.

The premature failure of polymeric parts can have severe and costly consequences. A plastic pipe suddenly breaking after many years of safe operation as a result of fatigue requires an often-expensive replacement. Failures in high-performance composites such as those used in the aviation and automotive industries can also impact safety. SRFI could help reduce the impact of these issues in a variety of ways: by enabling polymers to detect microcracks early and trigger self-healing mechanisms; and emitting visible signals when nearing breakdown. Clough and her colleagues plan to tweak the coumarin design to create a “color-coded” system. By adjusting the mechanophore’s structure, they aim to produce sensors that glow red, green, or yellow under different stress levels. This could allow, for example, engineers to visually gauge material fatigue in bridges or aircraft components.

“Ultimately, we want to move beyond improving mechanical properties alone and think about how materials can contribute to sustainability throughout their whole life cycle,” says Clough. ecompose safely thanks to pre-programmed weak points that ensure rapid disintegration post-use.

These developments come as global industries seek to meet sustainability targets. Super-resolution techniques are just starting to be taken up by the polymer science community and have the potential to inform the development of recycling approaches. [“By marrying biology’s imaging prowess with materials science, SRFI can pave the way for the development of polymers that don’t just survive but thrive under pressure, making them safer and more sustainable,” adds Clough.

Reference

Li, Y.; Clough, J. M. Optical Force Monitoring in Polymeric Materials with a Coumarin-Based Mechanophore. Angew. Chem. Int. Ed. 2025, e202513283.