Engineering the Impossible: Scientists Solve 200-Year-Old Polymer Puzzle

UVa researchers have developed a polymer that defies traditional trade-offs between stiffness and stretchability, enabling new applications in technology and medicine.

A groundbreaking new polymer design developed by scientists at the University of Virginia School of Engineering and Applied Science has overturned the longstanding belief that stiffer polymeric materials must be less stretchable.

“We are addressing a fundamental challenge that has been thought to be impossible to solve since the invention of vulcanized rubber in 1839,” said Liheng Cai, an assistant professor of materials science and engineering, and chemical engineering.

That’s when Charles Goodyear accidentally discovered that heating natural rubber with sulfur created chemical crosslinks between the strand-like rubber molecules. This cross-linking process creates a polymer network, transforming the sticky rubber, which melts and flows in the heat, into a durable, elastic material.

Since then, it’s been believed that if you want to make a polymer network material stiff, you have to sacrifice some stretchability.

Now, Cai’s team, led by Ph.D. student Baiqiang Huang, has debunked this notion with their new “foldable bottlebrush polymer networks.” Their work, funded by Cai’s National Science Foundation CAREER Award, appeared on the cover of the November 27 issue of Science Advances.

A “pull test” demonstrates how quickly a conventional polymer network comes apart under tension. Credit: Liheng Cai, Baiqiang Huang/Softbiomatter Lab, University of Virginia School of Engineering and Applied Science

‘Decoupling’ Stiffness and Stretchiness

“This limitation has held back the development of materials that need to be both stretchable and stiff, forcing engineers to choose one property at the expense of the other,” said Huang, who first-authored the paper with postdoctoral researchers Shifeng Nian and Cai. “Imagine, for example, a heart implant that bends and flexes with each heartbeat but still lasts for years.”

Crosslinked polymers are everywhere in products we use, from automobile tires to home appliances — and they are increasingly used in biomaterials and health care devices.

Some applications the team envisions for their material include prosthetics and medical implants, improved wearable electronics, and “muscles” for soft robotic systems that need to flex, bend and stretch repeatedly.

Stiffness and extensibility — how far a material can stretch or expand without breaking — are linked because they originate from the same building block: the polymer strands connected by crosslinks. Traditionally, the way to stiffen a polymer network is to add more crosslinks.

This stiffens the material but doesn’t solve the stiffness-stretchability trade-off. Polymer networks with more crosslinks are stiffer, but they don’t have the same freedom to deform, and they break easily when stretched.

“Our team realized that by designing foldable bottlebrush polymers that could store extra length within their own structure, we could ‘decouple’ stiffness and extensibility — in other words, build in stretchability without sacrificing stiffness,” Cai said. “Our approach is different because it focuses on the molecular design of the network strands rather than crosslinks.”

A polymer material made using the Cai laboratory’s “foldable bottlebrush polymer networks” can stretch as much as 40 times more than conventional crosslinked polymeric materials. Credit: Liheng Cai, Baiqiang Huang/Softbiomatter Lab, University of Virginia School of Engineering and Applied Science

How the Foldable Design Works

Instead of linear polymer strands, Cai’s structure resembles a bottlebrush — many flexible side chains radiating out from a central backbone.

Critically, the backbone can collapse and expand like an accordion that unfolds as it stretches. When the material is pulled, the hidden length inside the polymer uncoils, allowing it to elongate up to 40 times more than standard polymers without weakening.

Meanwhile, the side chains determine stiffness, meaning that stiffness and stretchability can finally be controlled independently.

This is a “universal” strategy for polymer networks because the components that comprise the foldable bottlebrush polymer structure are not restricted to specific chemical types.

For example, one of their designs uses a polymer for the side chains that stays flexible even in cold temperatures. However, using a different synthetic polymer, one that is commonly used in biomaterial engineering, for the side chains can produce a gel that can mimic living tissue.

Like many of the novel materials developed in Cai’s lab, the foldable bottlebrush polymer is designed to be 3D-printable. This is true even when mixed with inorganic nanoparticles, which can be designed to exhibit intricate electric, magnetic, or optical properties.

For example, they can add conductive nanoparticles, such as silver or gold nanorods, which are critical to stretchable and wearable electronics.

“These components give us endless options for designing materials that balance strength and stretchability while harnessing the properties of inorganic nanoparticles based on specific requirements,” Cai said.

Reference: “A universal strategy for decoupling stiffness and extensibility of polymer networks” by Baiqiang Huang, Shifeng Nian and Li-Heng Cai, 27 November 2024, Science Advances.
DOI: 10.1126/sciadv.adq3080

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