A New Look at Old Chemistry

Title: A Surprise from 1954: Siloxane Equilibration Is a Simple, Robust, and Obvious Polymer Self-Healing Mechanism

Authors: Peiwen Zheng and Thomas J. McCarthy

Affiliation: Polymer Science and Engineering Department, University of Massachusetts

Journal: Journal of the American Chemical Society

 

Sometimes really interesting and useful information gets lost in the sands of time.  This can happen in any field, including chemistry.  The authors of this paper go back to silicone polymerizations used during World War II that might deserve a little more attention than they have received.  The authors emphasize that these reactions are not new and their work simply extends what was reported decades ago.  With this and two other articles (this one and this one) they want to remind the chemical community of some of the very interesting properties of these silicones.

Self-healing materials are a very hot area of research now.  With applications in medical implants, protective clothing, and numerous other fields it is easy to see why so many scientists are studying them.  There are a couple of ways to create a self-healing material.  In one type, some sort of “healing agent” is imbued inside the material and when the material is fractured this agent is released thereby sealing the break.  In another type the chemical bonds in the material are reversible and upon breakage some stimulus can cause the reformation of these bonds.  Here the authors discuss the self-healing nature of silicones.

In this paper the researchers use the reaction of octamethylcyclotetrasiloxane and benzoyl peroxide to form a mixture of octamethylcyclotetrasiloxane and bis(heptamethylcyclotetrasiloxanyl)ethane.  Once the polymerization initiator (bis(tetramethylammonium)oligodimethylsiloxanedio-late) was added, the mixture was poured into a variety of molds and heated at 90° C for 4 hours.  We are able to consider this reaction a “living polymer network,” because their are still reactive end groups available that can react with the chains in the network creating new links.  It is this living nature that gives the network its self-healing characteristics.

The authors were able to take a small cylinder of the material, cut it with a razor blade, and then reheat it for 24 hours to reform the cylinder.  When they strained the reformed cylinder by hand with repeated bending it broke, but not in the same place as the original cut.  They repeated this procedure using a variety of shapes and found the same results – a reformed shape did not break at the same place as the original cut.  The researchers also used fracture toughness studies to quantify this effect and determined that the original and reformed material are equally strong.

In a fun example of the properties of this system the authors shaped the polymer into a dog bone and then cut it and reformed the material into a dog shape.  This article is a great reminder that we should take some time to look back at chemistry reported decades ago to inspire us.

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