Self-Healing Rubber

Henry Fountain wrote about it in yesterday's New York Times Science section, as follows (note that the video on the Times website is much better than that above).

Researchers Develop a Type of Rubber That Can Repair Itself

Flubber was pretty good stuff. The super-rubber of a 1961 Disney film starring Fred MacMurray (and of a 1997 remake starring Robin Williams) could literally make people jump through basketball hoops.

But could Flubber heal itself?

A new type of rubber developed by researchers in Paris can. When a rubberband made from the material is cut and the two ends are pressed together, they quickly form a strong new bond without glue or heat.

Ludwik Leibler, Philippe Cordier and colleagues at the Higher School of Industrial Physics and Chemistry, part of the National Center for Scientific Research, describe the new material in the journal Nature. It is made from readily available ingredients — vegetable oils and urea — and can be repaired and reprocessed many times without losing its elasticity.

Dr. Leibler said conventional rubber consists of huge molecules with strong covalent bonds holding them together. Their idea was to use smaller molecules and link them using hydrogen bonds. “Imagine that the interactions are strong enough so the whole thing holds together,” he said.

The processes involved — studied in a field known as supramolecular chemistry — are complex. The material can easily crystallize or be too fluid. “At first we obtained things that were like chewing gum,” Dr. Leibler said. “Not quite what we wanted.”

Eventually they came up with a material comparable to conventional rubber in elasticity and in how well it recovers its original size after stretching. By tweaking the ingredients, Dr. Leibler said, it should be possible to produce a range of materials with different properties. Already a chemical company, Arkema, is working on commercializing the process.

The hydrogen bonds that hold the small molecules together are also key to the healing process. When two cut ends are brought together, molecules on each side reform bonds with those on the other. Time is of the essence: if the ends are not brought together quickly enough, the molecules will form bonds with other molecules on their own side, making a repair impossible.

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Here's a link to the editor's summary of the Nature paper; the summary itself follows.

Self-Mending Rubber

When a rubber-band breaks, that's it: time to get another one. But a remarkable new material described in this issue behaves rather differently. Consisting of molecules containing three different functional groups that form multiple hydrogen bonds, the molecules associate to form a 'supramolecular rubber' containing both chains and cross-links. The system shows rubber-like behaviour, that is, recoverable extensibility when stretched to several times its original length. In contrast to conventional rubbers made of macromolecules, these systems when broken or cut can self-heal when the fractured surfaces are brought together at room temperature. The new material can be synthesized from simple ingredients — fatty acids and urea — and once synthesized it is readily reprocessed. In its current form supramolecular rubber has slow strain recovery and it 'creeps' under stress, but by adjusting the starting ingredients, a spectrum of properties is attainable.

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Here's a link to the Nature paper; the first paragraph follows.

Self-healing and thermoreversible rubber from supramolecular assembly

Rubbers exhibit enormous extensibility up to several hundred per cent, compared with a few per cent for ordinary solids, and have the ability to recover their original shape and dimensions on release of stress. Rubber elasticity is a property of macromolecules that are either covalently cross-linked or connected in a network by physical associations such as small glassy or crystalline domains, ionic aggregates or multiple hydrogen bonds. Covalent cross-links or strong physical associations prevent flow and creep. Here we design and synthesize molecules that associate together to form both chains and cross-links via hydrogen bonds. The system shows recoverable extensibility up to several hundred per cent and little creep under load. In striking contrast to conventional cross-linked or thermoreversible rubbers made of macromolecules, these systems, when broken or cut, can be simply repaired by bringing together fractured surfaces to self-heal at room temperature. Repaired samples recuperate their enormous extensibility. The process of breaking and healing can be repeated many times. These materials can be easily processed, re-used and recycled. Their unique self-repairing properties, the simplicity of their synthesis, their availability from renewable resources and the low cost of raw ingredients (fatty acids and urea) bode well for future applications.