Self-healing materials have long been the subject of study among many research groups, with scientists seeking inspiration from biological organisms that have evolved the remarkable ability to initiate self-healing and self-repair after sustaining damage. Such materials have many potential applications—for example, in clothes, artificial bones and cartilage, electronic circuit boards and spacecrafts. The material made by Philippe Cordier and his colleagues at the Industrial Physics and Chemistry Higher Educational Institution in Paris used simple ingredients like fatty acids (present in vegetable oils) and urea (a waste compound present in urine that can be made synthetically). The rubber-like material synthesized is an example of a ‘supramolecular assembly,’ where non-covalent interactions hold the material together. The self-repairing properties of the material, its relatively simple synthesis procedure, and the low cost of starting ingredients make it promising for future applications. In addition, this material is environmentally friendly, as it can be easily processed, reused and recycled.
Conventional rubbers that can be stretched to several times their resting length, and that recover their original shape and size when released, are made of long chains of cross-linked polymers. Such chains are held together by strong associations called covalent bonds, which do not readily re-form when broken. Thus the macroscopic rubber does not self-repair when these microscopic covalent bonds break during a fracture.
In contrast, Cordier and his colleagues achieved the elastic properties of rubber using a mixture of small ditopic molecules (that can associate with two other molecules covalently) and multitopic molecules (that can associate with more than two molecules covalently). The resulting short covalent network of molecules then undergoes partial cross-linking by relatively weak hydrogen bonds to make an extended network of molecules that
give rise to a material with elastic non-crystalline properties. (Hydrogen bonds are weak associations that, for example, are responsible for many of water’s unique physical properties like its high surface tension and nearly universal solvent properties).
When the rubber-like material is fractured, it is the active ends of the hydrogen bond network, rather than that of the covalent bonds, that are exposed. These broken ends can simply be recombined at room temperature by bringing them together, which allows the hydrogen bonds to reform and the material to heal. Thus breakages in the macroscopic rubber are automatically repaired by re-formation of the broken microscopic hydrogen bonds. As the time for which the broken ends are kept together is increased, more associations form, resulting in more complete recovery. Typically the ends must be kept together for at least 15 minutes for healing to occur.
An important factor determining the extent of recovery after fracture of the material is the amount of time that elapses between severing and bringing the ends together, because gradually the molecules at the ends increasingly associate with their neighbors, thereby deactivating the ends. At room temperature, effective healing can be achieved as long as the ends are reunited within about 18 hours, with the material losing entirely its ability to self-repair in about a week; at 120 degrees Celsius the material loses its healing properties in only five minutes.
In an accompanying news article in Nature, Justin Mynar and Takuzo Aida from the University of Tokyo, who describe the material as having “a touch of magic about it,” acknowledged the importance of the findings, noting, “Other chemists will soon be searching for, and designing and studying, a variety of small molecules to obtain specific mechanical properties in new self-healing materials...Synthesis of a rubber-like material that can be recycled might not seem exciting. But one that can also repeatedly repair itself at room temperature, without adhesives, really stretches the imagination.”