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

upstart writes:

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

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.

[...] "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."

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

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