Polymers are ubiquitous in modern daily life. From rubber tires to plastic water bottles, from waterproof jackets to bulletproof vests, and even non-stick ware are all possible thanks to polymer science.

Elastomers or elastic polymers are used in applications where the material must survive being stretched frequently and for a long time without developing tears. Gloves, tiny valves for the heart, and more are some of the examples.

However, elastic polymers for long while, have had a limitation. Either you can make an elastomer tough, or you can make it stiff. But you couldn’t make the elastomer have both those qualities at the same time.

This has proven challenging for scientists trying to develop elastomers for applications in bioadhesives, wearable electronics, and even tissue regeneration.

Now, scientists from the Harvard School of Engineering have developed a new type of polymer that promises to solve the long-standing stiffness-toughness conundrum. In a paper published in the journal Science, the researchers have outlined their innovation.

“In addition to developing polymers for emerging applications, scientists are facing an urgent challenge: Plastic pollution,” said Zhigang Suo, senior author of the paper, and professor of Mechanics and Materials at HSEAS. “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?”

An entangled hydrogel (left) and a regular hydrogel (right). Credit: Suo Lab/Harvard SEAS

When the building blocks of polymers, called monomers, are cross-linked together to form a chain, the resultant polymer will be elastic, however, the stiffness of the crosslinks will depend on the length of the chain – the shorter the chain, the stiffer the material.

The researchers analyzed physical bonds, called entanglement, rather than chemical bonds to develop a polymer that was both tough and stiff. Science has known about entanglements for as long as polymers have existed, but they were only thought to impact stiffness, but not toughness.

However, the researchers found that the polymer could become tough without comprising its stiffness, with enough physical entanglements.

Cover Image: Shutterstock