Spontaneous Self-Healing…Polymer!

HippocratesAccording to Hippocrates, “Healing is a matter of time, but it is sometimes also a matter of opportunity.” Now this reality is being steadily extended into the polymer realm, and it appears the time of opportunity has come.

Crack healing in polymers has been researched since the 1980s, but it was not until the beginning of this century that the mimicking a biological system “Autonomic Healing of Polymer Composites,” was reported in Nature. Since then the research into creating new self-healing polymers has been steadily advancing, with amazing achievements, such as artificial skin application.

Self-healing polymers, which autonomously repair themselves when damaged, are truly amazing. A variety of self-healing approaches have been researched over the past decade. First developed were polymers infused with monomer-filled micro capsules that contact the catalyst upon damage and repair the crack. More recently explored have been contemporary dynamic cross-linking methods, based on reversible covalent bond formations or supramolecular interactions, involving hydrogen bonds. Typically, external application of heat, light, pressure or pH change is needed to initiate the self-healing process and reconnect the broken chemical bonds.

A new development of Spanish scientists, published in Materials Horizons in September 2013, describes a totally novel, spontaneously self-healing thermoset elastomer. According to Alais Rekondo et.al., they have obtained:

“a permanently cross-linked material, based on a covalently cured poly(urea–urethane) elastomeric network which, after being cut in half with a razor blade, is able to self-mend by simple contact at room temperature….A 97% healing efficiency of the material, as calculated by tensile strength measurements, and its easy preparation from commercially available starting materials make this new system broadly applicable in a wide number of industrial sectors where poly(urea-uretane)s are already used.”

This new approach is based on ability of aromatic disulfides to undergo metathesis (i.e. shuffle their disulfide bonds) at room temperature. The researchers have found that neither the presence of a catalyst nor external stimuli were needed for self-healing reaction, which occurred in the solid state. The self-healing speed was remarkable: 62% after 1 hour and 80% after 2 hours, as measured by a tensile strength, with a healing efficiency of 97%. The scientists explain this impressive result by dual healing mechanism, with both aromatic disulfide exchange and formation of quadruple hydrogen bonds contributing to it. See it demonstrated here:

Following the press release of RCS in September 2013 about this work, a novel “Terminator”- type self-healing polymer has been featured in several news articles and blogs, including CBS News, RCS Chemistry World, Fox News, United Press International and others. Inaccurately labeled as the “First Self-healing Polymer” by CBS, it is, though, the first spontaneous stimuli- or catalyst-independent self-healing polymer among many self-healing polymers and composites.

The application possibilities for self-healing polymers are enormous: Just imagine all possible self-reparable plastic products and coatings around you.

Source: “Autonomic Healing of Polymer Composites” ,  White, S.R., Sottos, N.R., Geubelle, P.H., Moore, J.S., Kessler, M.R., Sriram, S.R., Brown, E.N., and Viswanathan, S., Nature, 409, 794–797 (2001)
Source: Polymers Make Sensitive “Skin” , Dale McGeehon, Polymer Solutions Newsblog, November 14, 2012
Source: “Catalyst-free room-temperature self-healing elastomers based on aromatic disulfide metathesis” , A. A Rekondo, R Martin, A Ruiz de Luzuriaga, G Cabanero, H J Grande and I. Odriozola, Materials Horizons 2014 (Advance Article, DOI: 10.1039/c3mh00061c), 13 September 2013
Source: ‘Terminator’ polymer that regenerates itself ,  RCS press release, 13 September 2013
Source: “Polymer regenerates all by itself”  by Polly Wilson, RCS Chemistry World, September 13, 2013
Source: “First self-healing polymer created in Spanish lab”  by Danielle Elliot,  CBSNews.com, September 17, 2013
Source: Watch: First self-healing polymer, September 17, 2013
Source: ‘Terminator’ polymer regenerates itself,  Foxnews.com, September 16, 2013
Source: Spanish scientists create self-healing ‘Terminator’ plastic’, United Press International (UPI.com) Sept. 13, 2013
Image from Wikimedia Commons