J. Am. Chem. Soc. 134, 2024–2027 (2012)

Credit: © 2012 ACS

The ability to self-heal is an attractive property in many fields, from packaging to biomedicine, and a great deal of research has recently been devoted to confer it to various materials. Mimicking self-healing biomaterials — such as skin, to mention a familiar example — is one approach. Now, while investigating silicone materials for different purposes, Peiwen Zheng and Thomas McCarthy from the University of Massachusetts have come across a simple, efficient self-healing mechanism that was apparent from 1950s publications but seems to have since been overlooked.

Zheng and McCarthy partly converted a cyclic octa-methyl-tetra-siloxane species into its dimeric form featuring an ethylene bridge. An anionic (silanolate) polymerization initiator was then added, which catalysed a siloxane equilibration mechanism to form chains. The chains contained ethylene bridges serving as crosslinks, and 'living' reactive silanolate end groups that further reacted with other existing chains. This resulted in the formation of a silicone rubber that can be regarded as a living crosslinked polymer network and exhibits self-healing behaviour.

One such silicone elastomer was cut in two with a razor. It healed so well under mild heating that the repaired sample showed similar cohesive strength and fracture toughness to that of the original silicone. The anionic polymerization, the co-polymerization of the monomeric and dimeric units, and the fact that the reaction is a living process were all described in earlier reports, along with some chemical stress relaxation studies of the silicones' ability to be re-shaped — which suggests the self-healing had been observed at the time.