Scientists have demonstrated that silicone rubber can repair itself through heat-activated reversible bonding.
Using a mechanism discovered in the 1950s, known as ‘siloxane equilibration’, the team cut a silicone rubber shape in half with a razor blade and found that it completely repaired itself when it was heated up.
Peiwen Zheng, a researcher at Massachusetts University and co-author of the paper published in the Journal of the American Chemical Society, said: ‘When we rediscovered the forgotten unusual properties of silicones and combined them with today’s research interests, we found that the silicone material with the siloxane equilibration was an obvious candidate for a self-healing material.’
According to a report in PhyOrg, the researchers performed several experiments to test the theoretical predictions from papers published more than half a century ago and to extend some of the experiments carried out at that time.
In one experiment, the team prepared a siloxane-based mixture containing a cross-linking agent and a catalyst. The scientists then poured the solution into moulds of various shapes and heated them at 90°C for four hours. After removing the clear, rubbery silicone shapes from the moulds, the scientists described the silicone samples as ‘living polymer networks’.
According to Zheng, this means the silicone network is at a chemically anionic equilibrium, where the reactive centre will split and reform a covalent siloxane bond.
These bonds are reversible, which enables the two sides of a crack to reconnect under the right conditions.
To demonstrate the self-healing ability, the researchers cut a 1cm-long cylindrical sample in half. The two halves were rejoined by wrapping them together with Teflon plumbing tape and heating them in an oven at 90°C for 24 hours.
When the researchers retrieved the sample and removed the tape, they found that the silicone cylinder had completely healed.
When they bent the cylinder by hand until it broke again, they found it broke in a different location to where it had previously been cut. The scientists repeated this experiment on different-shaped objects with the same results.
In another experiment, the researchers moulded a silicone dog bone, which they cut into multiple pieces. Then they rearranged the pieces to fit into a mould of a dog. Heating the sample resulted in a silicone dog with no visible fractures or weak spots where the pieces had been fitted together.
The researchers also quantified the strength of the healed samples in comparison with the original samples using fracture toughness measurements. The data for the two types of samples was indistinguishable, indicating exceptional self healing.
Zheng explained that self-healing materials, with some improvements, could lead to a variety of applications.
‘It [silicone rubber] can be developed into self-healing coatings on auto vehicles or countertops,‘ she said. ‘It is also a “plastic” elastomer, which can be used in moulding to form desired shapes and patterns. The concept of a self-healing silicone can be used to guide the preparation of elastomers with gradient modulus, Janus elastomers, reversible surface patterns when filled with magnetic particles and super-tough materials that can chemically relax stress.’