Rubber that pulls itself together
Polymers
March 20, 2008
French researchers have created a material with the elasticity of rubber that will repeatedly repair itself at room temperature [Cordier et al., Nature (2008) 451, 977]. The reusable polymer can be synthesized simply using low-cost raw ingredients that are available from renewable resources.
The reversible stretchiness of most conventional rubbers is produced by long chains of cross-linked polymers. Philippe Cordier and colleagues from the Ecole Supérieure de Physique et Chimie Industrielles (ESPCI/CNRS) in Paris, France instead used a supramolecular network to engineer the desired rubber-like elasticity.
The researchers built their assembly from fatty diacids and triacids from vegetable oils. The starting mixture is condensed with diethylene triamine and then reacted with urea to produce a mixture of oligomers with complementary hydrogen bonding groups. Hydrogen bonding between these ditopic and multitopic molecules produces a network with partial cross-linking, effectively inhibiting crystallization and promoting elasticity.
Rubber-like behavior was confirmed in a series of deformation experiments. The material recovers its shape after being subject to strains of above 100% and exhibits little creep under load.
More remarkably, if the material is cut or torn, it can be repaired by simply bringing the severed ends back together. The healing process is achieved quickly (in a few minutes) and without the need for external heat.
Broken samples were successfully mended at room temperature up to a week after fracture, although when the parts had been separated for this length of time, they had to be held together longer (at least 15 mins) before repairing themselves.
The novel self-healing property is also a result of the hydrogen bonding, Cordier and colleagues explain. Fracture exposes many nonassociated groups in the hydrogen-bonded network because the strength of supramolecular associations is lower than that of covalent bonds. These groups are ‘eager’ to link and will efficiently bridge the interface when brought back into contact.
“This is a very significant piece of research that beautifully and very powerfully brings together supramolecular design and a deep understanding of polymer physics to achieve a combination of rubber-like behavior and self-healing properties,” comments polymer physicist Franco Cacialli of University College London, UK.
Bert W. Meijer, professor of organic chemistry at Eindhoven University of Technology in the Netherlands, predicts that the work will give a “new boost” to the development of supramolecular materials.
Potential applications for the new material include self-repairing clothes, long-lasting coatings and paints for houses and cars, self-healing bone and cartilage biomaterials, and robust semiconducting polymers.
Paula Gould