All materials we encounter in daily life inevitably suffer from damage during their use, resulting from wear and tear, abrasion, scratching, impact or corrosion. This damage starts at the smallest scale, with a few molecules breaking locally; however, as the damage accumulates it will ultimately cause the failure of the material as a whole, for example by fracture or fatigue. In the past decades, strategies have been developed to stop material damage in its tracks by imparting self-healing capabilities to a new generation of high-tech materials. Self-healing materials spontaneously and autonomously repair internal damage, without the intervention of a user, and thus avoid the otherwise unstoppable path towards catastrophic failure.
Spontaneous repair from the inside out can proceed only if the basic building blocks in these materials, such as polymer chains in self-healing elastomers and coatings, are designed in such a way that broken molecular bonds can reform spontaneously. However, the exact mechanism by which this unique process occurs has to date remained a topic of speculation, since no techniques existed to visualize the molecular motions of repair at the relevant length and time scales.
In a recent issue of Advanced Materials, a research team led by Joris Sprakel from Wageningen University and Sybrand van der Zwaag from Delft University of Technology in the Netherlands, visualized the molecular cascade of autonomous repair in a self-healing polymer for the first time. They showcased how a quantitative implementation of a medical imaging technique, utilizing the interaction of laser light with nanoparticles embedded in the polymer, can provide detailed micromechanical images of the material as a damaged spot spontaneously heals. With this novel approach, the authors visualized, with high resolution, the molecular motions responsible for the self-healing process.
It turns out that self-healing in these materials consists of several elementary processes, which include the flow of the polymer to fill the gap created by cutting through the material with a knife, adhesion of the separated parts of the solid, and the restoration of molecular bonds that had been broken by the action of the knife.
These results not only illuminate the complex cascade of steps involved in autonomous repair, which forms a stepping stone for developing even better self-healing materials in the future, but demonstrate how this novel method can shed light on such complex mechanical phenomena. In the present paper, the authors studied how induced damage can heal spontaneously, but the technique can do more. Hanne van der Kooij, the PhD student who carried out most of the work, says: “The same approach allows us to see how damage starts in the first place. How the breaking of a single molecular bond can set in motion a self-catalytic cascade that ultimately leads to macroscopic fracture”. Research in these groups is now also directed to unravel the molecular cascade of damage accumulation at the origin of fracture, delamination and wear.
(1) Hanne M. van der Kooij, Arijana Susa, Santiago J. García, Sybrand van der Zwaag & Joris Sprakel, ‘Imaging the molecular motions of autonomous repair in a self-healing polymer’, Adv. Mater. 29, 1701017 (2017).