A rubber that can repair itself spontaneously in ambient conditions: This is the amazing material that a CNRS/ESPCI team led by Ludwik Leibler has invented. The potential applications are countless.
Rubber is expandable because it is composed of flexible macromolecular networks, made up of a series of molecules strongly fastened together in chains. By identifying a certain mixture of ditopic and tritopic molecules that are able to connect to two or three others at a time through hydrogen linkages, scientists at the MMC 1
lab managed to increase molecular interactions–yielding enough flexibility to make the material mend itself on its own. 2
The result is a rubber-like material that remains solid, and yet can use these molecular linkages to repair itself spontaneously if it breaks. Two pieces that have been cut apart will re-connect on their own within hours as long as they are placed in contact, regardless of any specific temperature or pressure applied. Once repaired, the material can again undergo significant distortions (up to 400% of its size) without breaking. It can also be torn apart and mended repeatedly. “Rubber elasticity combined with this self-mending property is something that we had never seen before,” reports François Tournilhac, who worked on the project.
Researchers used a mixture of fatty acid molecules from various heteroatomic groups (amide, urea, N-carbamyl, or imidazolidone, instead of carbon or hydrogen), which latch together due to their di- and tri-functional properties. At room temperature, the material built from this mixture will self-repair. At high temperatures (130-150°C), it can be reshaped.
One of the goals in this project was to use low-cost raw materials that are readily available, for smooth technology transfer to industrial applications. Raw materials of plant origin, such as pine, sunflower, corn, or rapeseed, are best, since they are non-toxic and renewable. It turns out that certain derivatives of vegetable oil fatty acids happen to contain tri-functional precursors, which in this case is advantageous.
Leibler’s laboratory is working hard with the chemicals manufacturer Arkema to design products based on this invention. The rubber will be especially useful in its compressed form. “In cases where the rubber needs to be extended, one must wait a while before two pieces can mend. But in products that use compressed rubber –even something as basic as shoe soles–the process occurs much more rapidly and doesn’t need such low temperatures, making it more useful in industrial applications,” explains Tournilhac.
This new kind of material is not completely self-adhesive: only freshly cut or broken parts are able to self-repair. To the touch, the surface feels like a rubber band or a polyethylene bag. Arkema is using the findings to create adhesive materials, asphalt additives, protective coatings, and other polymers. Market avenues are numerous, ranging from construction and cosmetics to ink jet printers and electronics.
1. Laboratoire Matière molle et chimie (CNRS / Ecole supérieure de physique et de chimie industrielles de Paris).
2. P. Cordier, et al., “Self-healing and thermoreversible rubber from supramolecular assembly.” Nature, 2008. 451: 977 - 80.