Elastic polymer that is both stiff and tough, resolves long-standing quandary

One hydrogel with a high degree of entanglement (left) and one that is regular (right). Credit: Suo Lab/Harvard SEAS
The polymer science of rubber tires has enabled Teflon, Kevlar and plastic water bottles to be made, as well as nylon jackets and many other everyday features. The elastic polymers (also known as elastomers) can be stretched and released multiple times and are used in many applications, such as gloves or heart valves. They need to last a long period of time without tearing. Polymer scientists have struggled with this conundrum for years: Elastic polymers are flexible and can also be hard, but not both.

Scientists are faced with this stiffness-toughness conflict when creating polymers for applications such as tissue regeneration, bioadhesives and bioprinting.

Researchers from Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS), have developed an elastomer that is both tough and stiff in a paper published in Science today.

Zhigang, the Allen E. and Marilyn M. Puckett Professor in Mechanics and Materials and the senior author of this study, stated that scientists face an urgent problem: plastic pollution. "The discovery of biodegradable plastics has brought us back to the fundamental question why some polymers are tough and others are brittle. How can we make polymers resist breaking under repeated stretching?

Monomer building blocks are used to create polymer chains. Crosslinking polymer chains with covalent bonds makes a material elastic. Crosslinks are shorter and more rigid the longer the polymer chains.

Junsoo Kim (a graduate student at SEAS) said that as your polymer chains get shorter, the energy you are able to store in the material decreases and it becomes more brittle. He was also co-first author. "If there are only a few crosslinks in your chain, the chains will be longer. The material is tough, but not useful.

Researchers looked at physical bonds rather than chemical bonds in order to create a polymer that was both tough and stiff. Although these physical bonds are known as entanglements for nearly as long as polymer science has been around, they have been believed to only affect stiffness and not toughness.

The SEAS research team discovered that a polymer can become strong and stiff if it has enough entanglements. The researchers used a concentrated monomer precursor solution that contained 10 times more water than other polymer recipe to create highly entangled polymers.

Each polymer chain is composed of many entanglements along its length, with a cross-link at the ends. A stretched polymer (middle), showing the transmission of tension to other chains. Credit: Suo Lab/Harvard SEAS

Guogao Zhang, postdoctoral fellow at SEAS, and co-first author of the paper, said that "by combining all monomers in this solution with less liquid and then polymerizing them, we made them entangled like tangled strings" The polymers are physically intertwined, just like knitted fabrics.

There are hundreds of such entanglements. Only a few chemical crosslinks will keep the polymer stable.

"As elastomers these polymers are high toughness and strength, and fatigue resistant," stated Meixuanzi Shi (a visiting scholar at SEAS) and co-author. The polymers can be submerged in water to form hydrogels. They have low friction and wear resistance.

The polymers are resistant to fatigue and wear well, increasing their durability and longevity.

Zhang stated that "our research has shown that entanglements are more effective than crosslinks in reducing the use of certain plastics and increasing the material's durability."

Kim stated, "We hope this new understanding about polymer structure will increase opportunities for applications and pave way for more durable, long-lasting materials with exceptional mechanical properties."

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Further information: Zhigang Suo. Fracture fatigue friction of polymers in which the entanglements greatly exceed crosslinks. Science (2021). Information from Science Zhigang Suo: Fracture and fatigue of polymers in which entanglements significantly outnumber crosslinks. Journal (2021). DOI: 10.1126/science.abg6320

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