Emerald Group Publishing Limited
Copyright © 2008, Emerald Group Publishing Limited
French scientists develop novel “self-healing rubber
Article Type: Mini features From: Assembly Automation, Volume 28, Issue 3
Rubbery materials can be easily stretched but it is not easy to mend them when they break. Now, however, a French research group has created a unique rubber-like material that can “self-heal” at room temperature. If the material is snapped in half, the two pieces can be made to mend themselves simply by bringing the broken surfaces back into contact with each other. This finding was reported in the journal Nature in early 2008 (Cordier et al., 2008) and is an example of the application of supramolecular chemistry. This is a scientific field that has come to the fore in recent years, in particular due to the pioneering work of Chemistry Nobel Prize winners Jean-Marie Lehn, Donald Cram and Charles Pedersen in 1987, and involves building complex molecular assemblies linked by non-permanent or reversible bonds.
The research was conducted by Ludwik Leibler and colleagues at the Ecole Supérieure de Physique et Chimie Industrielles (ESPCI/CNRS) in Paris, in collaboration with the speciality chemicals company Arkema. The ESPCI/CNRS group has been studying supramolecular materials since 2000 by creating substances with hydrogen bonds whose particularity, unlike conventional chemical (covalent) bonds, is reversibility. The new material, as yet un-named, is produced from vegetable oil and urea and consists of fatty acids, linked together by hydrogen bonds to form a macroscopic 3D network. It behaves like an ordinary rubber in that it can extended to several times its original length but if it is cut-in-half, the broken pieces will self-heal when brought together and held in contact for a few minutes (Figure 1). The fracture mends and the material can again be stretched and pulled in all directions. When stretched, the material responds by sliding into new configurations with fewer hydrogen bonds; release the tension and it contracts back to an energetically favoured configuration with more hydrogen bonds. Conventional rubbers usually consist of long polymer chains linked together by covalent bonds but the new material consists of small molecules that can link to two or more other molecules through hydrogen bonds, which are far weaker. If the material is fractured, any “open” hydrogen bonds on the broken surfaces seek out other non-linked, open bonds, allowing the two halves to reform. According to Leibler “It is important to stress that the material is not self-adhesive. The surfaces of the material are never sticky to the touch and feel like a rubber band or a plastic bag. Self-mending is possible even 12 hours after the fracture occurred”.
This development, and the broader field of other materials based around supramolecular chemistry, could find all manner of applications. The present material could be used in self-sealing joints, self-repairing tyres and indestructible children’s toys. Critically, these materials can be produced from small molecules derived from vegetable oils and processed or applied at low temperature, with behaviour that is typical of a polymer with large molecular chains, such as high rigidity and strength. A potential application is to modify a conventional polymer by integrating hydrogen bonds in its structure, as such a material would combine excellent fluidity at low-processing temperatures with solid phase properties which would be identical to, or better than, those of a non-modified polymer. A promising application concerns the bitumens laid on roads. These are modified with polymers which make them more hard wearing, albeit to the detriment of their viscosity - they then have to be heated to a high temperature, around 180°C, for a long period during their preparation and application. These are highly energy-intensive operations but by incorporating an additive derived from supramolecular chemistry, it should possible to lower to 140°C the temperature needed for their application, whilst maintaining their mechanical strength. Christian Collette, Arkema R&D Vice President, believes that these new materials will eventually yield significant benefits in many everyday applications: one can imagine all kinds of articles that could be reused after breaking or cracking, or polymers, varnishes or adhesive formulations which will be processed or applied at relatively low temperatures, thus yielding major energy savings. Arkema is now planning to commercialise some products and materials based on supramolecular technology.