Scientists rediscover self-healing silicone mechanism from the 1950s

Jan 30, 2012 by Lisa Zyga report
(A) A silicone sample started off in the shape of a dog bone. (B) The sample was cut into pieces and rearranged in a dog mold. (C) The sample was remolded into a dog, in which the fracture locations are undetectable. Image credit: Zheng, et al. ©2012 American Chemical Society

(PhysOrg.com) -- Research in self-healing organic polymers has grown recently, but one simple self-healing mechanism from more than 60 years ago has been nearly forgotten until now. Using this mechanism, which is called “siloxane equilibration,” scientists have demonstrated that silicone rubber that has been cut in half with a razor blade can completely repair itself through heat-activated reversible bonding. The self-healing mechanism could have applications in a variety of areas, such as for designing self-healing surfaces of vehicles and countertops.

The researchers, grad student Peiwen Zheng and Professor Thomas J. McCarthy from the University of Massachusetts, have published a paper on the rediscovery of siloxane equilibration in a recent issue of the .

“We have been working on materials from a couple of different perspectives,” Zheng told PhysOrg.com. “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.”

The researchers performed several experiments to test the theoretical predictions from papers published in the early 1950s, as well as to extend some of the experiments performed at that time. In one experiment, Zheng and McCarthy prepared a siloxane-based mixture containing a cross-linking agent and a catalyst. Then they poured the solution into molds of various shapes, such as cylinders, disks, and dog bones. After heating the molds at 90 °C (194 °F) for four hours, the researchers removed clear, rubbery silicone shapes from the molds. The scientists described these silicone samples as “living networks.”

(A) A silicone sample in the shape of a cylinder. (B) The sample was cut in half with a razor blade. (C) The sample completely self-healed after 24 hours at high temperatures. (D) When the healed sample was bent by hand, it broke in a different location than where it had been cut. Image credit: Zheng, et al. ©2012 American Chemical Society

“The silicone network is at a chemically anionic equilibrium,” Zheng explained of the term, “where the reactive center will cleave 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 1-cm-long cylindrical sample in half using a razor blade. Then they rejoined the two pieces by wrapping them together with Teflon plumbing tape and heating them in an oven at 90 °C (194 °F) for 24 hours. When the researchers retrieved the sample and removed the tape, they found that the silicone cylinder had completely healed. Then they bent the cylinder by hand until it broke again - significantly, it broke in a different location than where it had been cut. The scientists repeated this experiment on different shaped objects with the same results.

In another experiment, the researchers molded a silicone dog bone, which they cut into multiple pieces. Then they rearranged the pieces to fit into a mold 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.

The researchers explained that, in principle, any cross-linked dimethylsilicone elastomer (only one type was used here) can be converted into a living elastomer by the addition of basic catalysts. This possibility opens up many different routes for synthesizing a variety of self-healing silicione-based materials. While the samples used here required applied heating to self-heal, the researchers predict that samples in a sealed, high-temperature environment would self-heal “autonomically,” or automatically. Zheng explained that self-healing materials, with some improvements, could lead to a variety of applications.

“It 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 molding 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 which can chemically relax stress.”

Explore further: Experiment shows potential of X-ray laser to study complex, poorly understood materials

More information: Peiwen Zheng and Thomas J. McCarthy. “A Surprise from 1954: Siloxane Equilibration Is a Simple, Robust, and Obvious Polymer Self-Healing Mechanism.” Journal of the American Chemical Society. DOI: 10.1021/ja2113257

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JRi
5 / 5 (1) Jan 28, 2012
Silicone rubbers are practically always reinforced with either fumed silica or MQ-resins. Otherwise silicone's mechanical properties are only 1/10th of normal (=unusable). Thus, it seems odd that the authors do not tests this phenomenon on reinforced silicone. Likely it fails to work in real life scenario. Similar work with the biscyclotetrasiloxane has been published by others, using triflic acid catalyst.
d2xyz
4.5 / 5 (2) Jan 28, 2012
Seems to announce the rediscovery of hot-melt ashesives and their properties. I didn't know they were lost.
CatsAreGods
5 / 5 (2) Jan 28, 2012
I find it ironic in the extreme that the sample seems to be displayed on a common self-healing cutting mat, no doubt made with silicone.
Callippo
5 / 5 (2) Jan 28, 2012
Seems to announce the rediscovery of hot-melt ashesives and their properties. I didn't know they were lost.
Self-healing polymer is quite different stuff - it doesn't undergo phase transform during its heating.
kaasinees
3 / 5 (2) Jan 28, 2012
good stuff for breast implants.
efdouglass
1 / 5 (1) Jan 28, 2012
It is a thermoplastic rubber, cutting it breaks intermolecular bonds, and they flow back together. Silly putty does the same thing, you cut it into pieces, put it back into its egg, and it "heals itself".

I am surprised the ACS published this nonsense. This is not new. Thermoplastics when heated always "heal themselves", after being "cut".

Gene
enigma13x
1 / 5 (1) Jan 28, 2012
um... call me stupid i always thought that most things that melt when cut up into pieces and placed in a mold and exposed to heat will join back together without fracture points eg plastic, wax, rubber etc or am i just not seeing the ground breaking work done here?
Mike_Bailey
not rated yet Jan 29, 2012
I can't believe that NOBODY has thought of this material's uses in robotics, androids, or prosthetic limbs yet. I would love for my robo-jogging-buddy/chefbot/sparing-partner-bot to be able to heal itself by applying a heating compress to its "abrasions", or broken "bones" .
Callippo
not rated yet Jan 29, 2012
Thermoplastics when heated always "heal themselves", after being "cut".
This is just the point: the siloxanes aren't thermoplastics and they're used just because of this property.
d2xyz
not rated yet Jan 30, 2012
If you want to see existing thermoplastic siloxanes, I suggest you obtain a sample of Dow Corning 7-4400 medical grade adhesive and heat it. It's basically as JRi said above. In this case silicone polymer chemically bonded to Q resin. The more resin incorporated into the reaction, the higher the viscosity of the resulting polymer.