New research published last week may lead to the development of strong, self-healing plastics, inspired by the strength and flexibility of ocean mussels. The polymers may offer a way to adhere together disparate materials, such as wood and metal.
Material scientists have traditionally depended on a handful of strategies, often using long chains of molecules capable of stretching and regaining their form.
The most prevalent approach employs chemical links, called “covalent” bonds, between each polymer chain, which can connect separate strands into a three-dimensional, mesh-like form. Polymers made using this strategy are stiff, but break relatively easily under duress.
Another common strategy uses positive and negatively electrical charges to bond separate strands together. With a looser configuration composed of “ionic” bonds, these polymers can be flexible, and since opposite charges can reattach once pulled apart, this material can actually be self-healing.
Using natural polymer networks with charged ionic bonds, in addition to covalent bonds, allows mussels to enjoy the best of both approaches. Researchers are hoping to learn from this approach. They have added negatively charged chemical groups, called catechols, to soft polymers that already employ covalent bonds. When negatively charged ions are added to this solution, iron atoms attach to several nearby catechols on different strands, fostering new connections that add strength to the gel-like substance.
So far, most of these polymers have been made in water, which causes the gels to expand. If these polymers are as expanded as possible to begin with, they care unable to stretch further, and break easily.
UC Santa Barbara materials scientist Megan Valentine and her colleagues are now attempting to use this approach in a dry polymer. First, they began with the polymer polyethylene glycol (PEG), which employs a loose set of covalent bonds. They added catechol groups to each strand. Without intervention catechols react with oxygen from air or water. To avoid this reaction, the team covered the catechols with capping groups, and added acid to tear off the caps immediately before strengthening the polymer. They then added iron atoms, which reacted with the catechols, forming another network of connections.
When the researchers dried the polymer and tested it, they found it was between 100 and 1000 times stiffer than the initial PEG, and was also flexible enough to not break easily, according to the research, published in the journal Science.
The researchers were able to transform a gel-like polymer into one with a level of strength and flexibility similar to leather. While the research did not produce the strongest polymer to date, the addition of the secondary network of bonds yielded results not usually seen from making a single change to a material.
Costantino Creton, a materials scientist at the École Supérieure de Physique et de Chimie Industrielles in Paris, who was not involved in Valentine’s research, hailed the work, saying the next step was to determine if the same process could be used to strengthen other polymers. Valentine says her team is already exploring this possibility.
“It’s remarkable to have such an improvement in stiffness,” said Creton.