Researchers Decouple Stiffness and Extensibility in Polymer EngineeringResearchers Decouple Stiffness and Extensibility in Polymer Engineering

It has been polymer engineering dogma for a couple of hundred years that the stiffer the material the less it can be stretched. Researchers at the University of Virginia School of Engineering and Applied Science (UVA) claim to have changed the rules of the game by decoupling stiffness and stretchiness by means of “foldable bottlebrush polymer networks.” Medical implants are among the applications that could benefit: “Imagine . . . a heart implant that bends and flexes with each heartbeat but still lasts for years,” mused PhD student Baiqiang Huang, who led the research under the supervision of Liheng Cai.

A centuries-old challenge 

The breakthrough addresses a fundamental challenge since the invention of vulcanized rubber in 1839, when Charles Goodyear accidentally discovered that heating natural rubber with sulfur creates chemical crosslinks between the strand-like rubber molecules, said Cai, assistant professor of materials science and engineering and chemical engineering at UVA. This crosslinking process creates a polymer network, transforming the sticky rubber, which melts and flows when heated, into a durable, elastic material.

Ever since, it’s been believed that if you want to make a polymer network material stiff, you have to sacrifice some stretchability, writes Jennifer McManamay reporting on the research on the UVA website.

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Stiffness and extensibility originate from the same molecular building block — polymer strands connected by crosslinks. Adding more crosslinks to a polymer network stiffens the material but simultaneously restricts its freedom to deform, and it breaks 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,” said Cai. “Our approach is different because it focuses on the molecular design of the network strands rather than crosslinks.”

How the foldable design works

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

Critically, the backbone can collapse and expand like an accordion that unfolds as it stretches. When the material is pulled, 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.

The components that make up the foldable bottlebrush polymer structure are not restricted to specific chemical types. For example, one design can use a polymer for the side chains that stays flexible even in cold temperatures. But using a different synthetic polymer, one that is commonly used in biomaterial engineering, for the side chains can produce a gel that mimics living tissue.

Opportunities in 3D printing

The foldable bottlebrush polymer is compatible with 3D printing, even when mixed with inorganic nanoparticles, which can be designed to exhibit intricate electric, magnetic, or optical properties. Conductive nanoparticles, such as silver or gold nanorods that are critical to stretchable and wearable electronics, also can be added.

In addition to medical implants and prosthetic devices, potential applications include improved wearable electronics and “muscles” for soft robotic systems that need to flex, bend, and stretch repeatedly, said the researchers.

The research is published in Science Advances in a paper titled, “A universal strategy for decoupling stiffness and extensibility of polymer networks.”