Patent Picks: Self-Healing Polymers

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.

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.

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.