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Patent Picks: Self-Healing Polymers

A look at recent patenting activity in these “smart” materials, brought to you by C&EN and CAS

March 30, 2015 | A version of this story appeared in Volume 93, Issue 13


Layered Material Carries More Weight

Materials that mend themselves have increased in popularity in the past decade. SOURCE: Chemical Abstracts Service
Bar graph shows a trend of increasing numbers of patents for self-healing polymers since 2005.
Materials that mend themselves have increased in popularity in the past decade. SOURCE: Chemical Abstracts Service

Inspired by the way a small cut in a person’s skin heals itself, polymer chemists would like to engineer materials that fix themselves when damaged. Researchers have been trying various strategies to bring these materials to life: embedding microcapsules into polymers that release repair agents when cut and designing polymers that expose chemically reactive bonds when scratched, to name a few. All their efforts are reflected in the increasing number of patents on self-healing polymers being filed in Chemical Abstracts Service’s databases. A few of those are highlighted here.


Longer-Lasting Batteries

Micrograph shows a lithium-ion battery wrapped with a self-healing polymeric coating. After it cracks, it heals partially within 10 hours.
Credit: Courtesy of Zhenan Bao
A cracked electrode wrapped in a self-healing material fixes itself over a period of hours (left to right).

Because of their ability to “remember” their original shape and revert to it, shape memory materials have attracted a lot of interest from scientists. Self-healing versions of these substances are particularly sought after because of their potential as construction materials that can repair their own cracks. The healing process for these materials, however, usually requires the substances to soften temporarily, which negatively affects their load-bearing capabilities. To overcome this limitation, Patrick T. Mather and colleagues at Syracuse University developed a high-strength polymeric material capable of healing at ambient temperatures. In their patent, they describe making the shape memory substance by laminating thin layers of a charged polymer, called an ionomer, and a crystalline polymer that stiffens upon cooling (WO 2014113432). The ionomer is a polystyrene sulfonate compound that gives the overall material its ability to heal, and the crystalline polymer—an atactic polystyrene or some other blend—gives the material its stiffness and shape memory.


Highly Stretchable Film Also Heals

Credit: Supramolecular Chemistry Laboratory
After being sliced and having its ends pressed back together, a transparent film heals itself within minutes (top to bottom).
A self-healing transparent polymer film gets cut and stitched back together in these images (top to bottom).
Credit: Supramolecular Chemistry Laboratory
After being sliced and having its ends pressed back together, a transparent film heals itself within minutes (top to bottom).

One reason lithium-ion batteries fail is the constant expansion and contraction of their electrodes during electrical charging and discharging cycles. This repeated stress breaks apart the metal-based particles that make up the electrodes, causing the particles to lose contact with each other and the battery to fail. Researchers at Stanford University have figured out a way to wrap an electrode with a conductive self-healing polymer to mitigate this fracturing problem. In a 2014 patent, Zhenan Bao and her team described reacting di- and triacids with triamines to synthesize randomly branched oligomers with terminal amine groups (WO 2014116335). Treating the resulting material with urea then produces a polymer bristling with hydrogen-bonding sites, which give the overall polymer its self-healing properties. Finally, the researchers treat the material with electrically conductive additives and wrap it around an electrode. The stretchability of the polymer allows it to expand and contract during battery charging and discharging cycles, and even when compromised, it can heal itself to keep the electrode intact, extending battery life.

Patent Picks is a collaborative effort by C&EN and CAS. This feature reports on trends CAS scientists observe from patents in CAS databases. Patents now generate more than 70% of the new substances appearing in the literature.

Self-healing materials come in many flavors: Some contain microcapsules that release healing agents when damaged, and some use noncovalent bonds such as hydrogen bonds to stitch themselves back together after getting cut. The latter are especially attractive because they don’t require any additives to work. Jean-Marie Lehn, winner of the 1987 Nobel Prize in Chemistry, and his research group at the University of Strasbourg, in France, adopted this approach to design self-healing transparent and elastic polymer films (WO 2015011214). In their patent, the researchers describe polymers formed from condensation of a commercially available diisocyanate derivative with a bis-hydrazide. The resulting polymers contain networks of hydrogen-bonded chains. Unlike other self-healing polymers, these materials, when cut, heal in just a few minutes at room temperature and maintain their elasticity. One self-healing substance designed by the team is so elastic, it can elongate up to 800% without permanent deformation.


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