A new approach for self-reporting materials

NCCR Bio-Inspired Materials researchers led by Principal Investigator Professor Christoph Weder (Adolphe Merkle Institute) continue to investigate polymers that change their color as the result of damage or stress, and which can thereby indicate potential damages inflicted upon them.

Polymers that display this so-called mechanochromic behavior could be used to help prevent catastrophic failure in structures where they play a role, as they provide readily perceptible optical cues. Previous research has focused largely on applications that can be detected on the macroscopic scale. However, scientists are now investigating applications to sense microscopic defects, aging, wear, and fatigue. Besides limiting the resistance of structural components, this type of damage can eventually lead to larger defects, and potentially to disaster. Mechanochromic polymers could therefore have a significant impact as self-reporting materials in industries such as aviation, car manufacturing, construction, and healthcare. This built-in monitoring technology could also allow maintenance to be carried out on a need basis, rather than having to adhere to strict timetables.

One strategy to induce mechanochromic behavior in polymers has been to incorporate motifs known as mechanophores, which usually contain weak bonds that dissociate when activated by mechanical forces, leading to color changes.   

Another approach to design these color-changing polymers is to incorporate microcapsules containing a dye. When they are damaged, the capsules release the dye, which is then activated by a physical or chemical change. This technique has been used, for example, for carbonless copy paper and in pressure-monitoring films.  In a recent study, the NCCR researchers for the first time used this approach in connection with a “solvatochromic” dye, i.e., a compound whose color changes with the nature of its environment.  

The NCCR researchers simply incorporated the solvatochromic dye and the solvent in microcapsules, and embedded the latter in several matrix polymers. When these composites are exposed to damage or excessive stress, the dye solution escapes the capsule, the solvent evaporates, and the dye molecules find themselves surrounded by polymer instead of solvent, which triggers a pronounced and permanent color change.

“These systems are extremely versatile,” says NCCR alum Dr. Céline Calvino. “If you change the polymer matrix, or switch the type of solvent used, you obtain a variety of different color responses.” In addition, the approach allows for a quantitative evaluation of the damage inflicted upon a polymer matrix. The capsules that escape unscathed can serve as a reference and allow one to determine the level of response.

According to Calvino, the next steps would involve the development of fabrication techniques to generate capsules with different dimensions and structures. This would notably lead to new properties, and allow the encapsulation of different core materials. “All of these factors could be tuned depending on the area of application,” she adds. “But to achieve this, and improve our capability to rationally design efficient self-reporting materials, further investigations are needed in which the influence of the capsule material, the shell thickness, and the size of the capsules on the deformation behavior in different polymers are studied.”

Reference: Calvino, C.; Weder, C. Microcapsule-containing Self-Reporting Polymers, Small, 2018, 14, 1802489.