Self-healing rubber promises products that mend themselves

A smart rubber that magically binds back together when snapped, cut or punctured has been developed by scientists in Paris in collaboration with the French chemical concern Arkema. The researchers say that the material could lead to self-healing consumer products such as shoes, protective coatings, vehicle fan belts, washing-up gloves, children’s toys and more.

Conventional rubber derives its strength through cross-linking of long polymer chains, where the coupling is through a combination of covalent, ionic and hydrogen bonds between the molecules. Of these only the hydrogen bonds can be remade if the material is broken, but there are normally too few hydrogen bonds for this to be a viable self-mending mechanism.

Self-healing rubber

Ludwik Leibler and his colleagues at the Industrial Physics and Chemistry Higher Educational Institution (ESPCI) have tackled the problem by doing away with covalent and ionic bonds in their new material. The result is a translucent, yellowy-brown substance formed from a network comprising a mixture of ditopic and multitopic small molecules in which cross-linking is performed by hydrogen bonds alone.

When fractured the material self-heals at room temperature simply by bringing the cut surfaces together and applying gentle pressure. Within an hour of the join the bonds rebuild themselves so thoroughly that the material can be stretched without any sign of weakness at the site of the original cut. “You can feel the material mending itself when you hold the fractured sides together,” says Leibler. “It’s a very strange feeling.”

The only downside is that in relying on hydrogen bonds alone the material is not as strong as conventional rubber. This may limit its application, but the domestic products identified by the material’s inventors seem perfectly feasible on a technical level. It even opens up the possibility of new product concepts such as plastic bags that can be ripped open and then resealed.

The French researchers have come up with a whole new class of materials, says University of North Carolina chemist Michael Rubinstein: “The material is thermoreversible – it becomes a liquid if heated, and can be re-moulded in a rubber of any shape upon cooling. But it is different from known thermoreversible rubbers in that it self-heals, recovering its elastic properties if the broken pieces are put together at the use temperature in the rubbery state.”

Thermoreversible materials have been made before, but they partially crystallise and thus behave like plastic resins. With this new rubber a mixture of molecules with a variety of architectures and strongly associating groups is used to avoid the crystallisation. “These materials form a glass rather than a crystal upon cooling, but are rubbers at room temperature,” says Rubinstein.

A patent has been issued on the technology, and Arkema plans to commercialise a number of products and materials based on the new rubber. With some of the potential applications identified so far, the question is how the market will respond to the introduction of what are normally considered throwaway items that now last much longer than before.

“Our materials are not adapted to replace rubber in all applications,” says Leibler. “For the moment the relaxation is slow due to the glass transition temperature. This we hope we will be able to improve after screening some plasticisers and/or adapting the chemistry using the principles we now understand.”

Kilogramme quantities of the material are already being made in the ESPCI labs. The process is reported to be almost completely green, and could be made completely so with a few adjustments, say the researchers.

Further reading: “Self-healing and thermoreversible rubber from supramolecular assembly”, Cordier et al. Nature 451, 977 (2008).

Figure: A piece of rubber made from a supramolecular assembly of small molecules is cut with a razor blade. When the ends are brought together at room temperature and gentle pressure applied, the cut heals within an hour leaving no trace of the original break (source: François Tournilhac & Ludwik Leibler/ESPCI).

Article first published in Nanomaterials News.