Salient

Self-Healing Rubber

By Buzz Spence | 17 Mar, 2008

It’s remarkable how people respond to technology. Our generation never marvelled at the technology of the previous generation. Telephones, computers, and even the internet were never really amazing to us, despite their all-encompassing impact seeming veritably magical to our parents and grandparents. Contrast this with the response to the announcement by French scientists a few weeks ago of a self-healing rubber, a rubber just like normal rubber but that spontaneously heals when cut pieces are placed touching each other.

The mainstream media lapped it up, if only for a day, and chemists everywhere were bombarded with questions about how it worked. References to Terminator 2 were made. Perhaps most succinctly, the scientists responsible noted “a touch of magic about it.” Magic it remained, however, because the announcement attracted a lot less explanation than it did attention. The BBC report was reasonable but the Reuters article was decidedly less helpful, with descriptions of molecules holding hands that were so vague as to be useless as an explanation. The Dom Post remained true to form and paraphrased the Reuters report, creating blatant errors where before there had been only vagueness.

So we’ll explain it here. Chemists, rejoice – your subject has finally merited the attentions of Salient. Everyone else, explaining self-healing could be a cool, if rather geeky party trick. Or something.

Like any plastic, natural rubber is a polymer. It is composed of long chains of carbon atoms bonded together covalently (and so, for all intents and purposes, inseparably). The chains are held together by temporary dipole bonds, comparatively weak bonds that only hold chains together strongly when they pack together well. In the case of rubber, the chains pack together quite poorly, and so the chains are held together weakly and can slip over each other when the rubber is stretched. A few long covalent bonds link the chains and return them to their original positions, making the rubber elastic. Normal synthetic rubbers work similarly.

While all well and good, when rubber is cut, the chains themselves are cut and a covalent bond broken. The chains in the separated pieces are no longer tangled together and are unable to reentangle. Nor can the cut chains join back up: a strong covalent bond, once broken, basically remains broken forever.

Self-healing materials have been made before. A selfhealing brittle plastic was invented in 2001 which contained microcapsules of a monomer that polymerised when exposed to air. When broken, the microcapsules release their monomer which fills up the gaps between separate pieces and harden, gluing the pieces together. These materials aren’t so much selfhealing as self-gluing, however, and just like when you glue something together, it is never quite the same afterwards. Plus it doesn’t work for rubber. Most importantly, it has a near-zero cool factor and could not be used to make Terminators.

The new self-healing rubber is much cooler. Instead of the long chains in natural rubber, it has a witches’ brew of different shortish chains. These chains are bent, and many are Y-shaped. At their ends they are held to each other by hydrogen bonds, the type of bonds responsible for holding water droplets together. These hydrogen bonds are weaker than the covalent bonds inside natural rubber chains but are quite a bit stronger than the temporary dipole bonds between them. The witches’ brew of Y-shaped and bent chains held together at their ends form a random network like the sort that you could make if you cut out a lot of Y’s, M’s, and N’s and put joined them together facing any direction you liked. Although each chain is short, collectively they hold each other in place quite strongly. When the rubber is stretched, the chains break apart and attach to new partners forming a more spacious network; when unstretched they all pack together again as much as possible, returning the rubber to its original size.

Most importantly, hydrogen bonds can (and do) form spontaneously whenever there are appropriate bonding partners, just as water droplets can split up and reform. Self-healing is easy: as soon as the surfaces are together, the chains form new hydrogen bonds with each other. Once the random network packs back together tightly again, the rubber is almost as good as new.

Now, there is no point justifying a scientific explanation in a general interest magazine. Either you are still reading happily, or you aren’t. But if you do, you’ll be pleased to know that selfhealing rubber probably is coming to you soon. Unlike many scientists’ playthings, the rubber is made from reasonably cheap and abundant vegetable oil precursors. It’s biodegradable and fairly non-toxic, commercial production has already begun. Predictions of self-healing car tyres may never come to pass (remember, car tyres are produced in their millions, if not billions annually, and are rather hard, whereas this self-healing rubber is soft), but a whole host of other applications (rubber seals spring to mind) exist. It may change your life: if it doesn’t, it’s cool in its own right.