A highly entangled hydrogel (left) and a regular hydrogel (right). Image: Suo Lab/Harvard SEAS.
A highly entangled hydrogel (left) and a regular hydrogel (right). Image: Suo Lab/Harvard SEAS.

Elastic polymers, known as elastomers, can be stretched and released repeatedly, and are used to produce everything from gloves to heart valves, where they need to last a long time without tearing. But a conundrum has long stumped polymer scientists: elastic polymers can be stiff or they can be tough, but they can’t be both.

This stiffness-toughness conflict is a challenge for scientists developing polymers for use in applications such as tissue regeneration, bioadhesives, bioprinting, wearable electronics and soft robots.

In a paper in Science, researchers from the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) report resolving this long-standing conflict and developing an elastomer that is both stiff and tough.

“In addition to developing polymers for emerging applications, scientists are facing an urgent challenge: plastic pollution,” said Zhigang Suo, professor of mechanics and materials at Harvard SEAS and senior author of the paper. “The development of biodegradable polymers has once again brought us back to fundamental questions – why are some polymers tough, but others brittle? How do we make polymers resist tearing under repeated stretching?”

Polymer chains are made by linking together monomer building blocks. To make a material elastic, the polymer chains are crosslinked by covalent bonds. The more crosslinks, the shorter the polymer chains and the stiffer the material.

“As your polymer chains become shorter, the energy you can store in the material becomes less and the material becomes brittle,” explained Junsoo Kim, a graduate student at Harvard SEAS and co-first author of the paper. “If you have only a few crosslinks, the chains are longer, and the material is tough but it’s too squishy to be useful.”

To develop a polymer that is both stiff and tough, the researchers looked to physical, rather than chemical, bonds to link the polymer chains. These physical bonds, called entanglements, have been known in the field for almost as long as polymer science has existed, but they’ve been thought only to impact stiffness, not toughness.

The SEAS research team found that with enough entanglements, a polymer could become tough without compromising stiffness. To create highly entangled polymers, the researchers used a concentrated monomer precursor solution with 10 times less water than other polymer recipes.

“By crowding all the monomers into this solution with less water and then polymerizing it, we forced them to be entangled, like tangled strings of yarn,” said Guogao Zhang, a postdoctoral fellow at Harvard SEAS and co-first author the paper. “Just like with knitted fabrics, the polymers maintain their connection with one another by being physically intertwined.”

With hundreds of these entanglements, just a handful of chemical crosslinks are required to keep the polymer stable.

“As elastomers, these polymers have high toughness, strength and fatigue resistance,” said Meixuanzi Shi, a visiting scholar at Harvard SEAS and co-author of the paper. “When the polymers are submerged in water to become hydrogels, they have low friction and high wear resistance.” That high-fatigued resistance and high-wear resistance increases the durability and lifespan of the polymers.

“Our research shows that by using entanglements rather than crosslinks, we could decrease the consumption of some plastics by increasing the durability of the materials,” said Zhang.

“We hope that this new understanding of polymer structure will expand opportunities for applications and pave the way for more sustainable, long-lasting polymer materials with these exceptional mechanical properties,” said Kim.

Harvard’s Office of Technology Development has protected the intellectual property associated with this project and is exploring commercialization opportunities.

This story is adapted from material from Harvard SEAS, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.