A new design by researchers at the University of Virginia School of Engineering and Applied Science could solve the 200-year-old challenge in polymers. It breaks the dogma that stiffer polymers have to be less elastic.
In 1839, Charles Goodyear discovered the process of vulcanization of rubber. In this process, heating natural rubber with sulfur creates chemical crosslinks between the rubber molecules. These crosslinks create a polymer network and transform rubber into a durable, elastic material.
Since then, it has been believed that the elasticity must be compromised when making a stiffer polymer.
“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.
“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. Imagine, for example, a heart implant that bends and flexes with each heartbeat but still lasts for years,” Huang said.
Crosslinked polymers are used everywhere, from home appliances to healthcare devices. The polymer strands connected by crosslinks grant the material stretchability or expandability. Adding more crosslinks is a traditional way of stiffening a polymer network.
Although this adds stiffening, it makes no progress in the stiffness-stretchability trade-off. Additionally, more crosslinks snatch the freedom of deformity, breaking it easily when stretched.
To decouple the stiffness and stretchiness, the lead author, Baiqiang Huang, proposes new “foldable bottlebrush polymer networks”. This polymer could store extra length within its own structure.
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“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.“
Unlike the linear polymer strands, the foldable bottlebrush polymer embeds multiple chains radiating from a central backbone. Imagine it as an Accordion. When the material is pulled, the hidden length inside the polymer uncoils. It can elongate 40 times the standard polymer without weakening.
Meanwhile, the side chains define stiffness. Now, researchers can independently control stiffness and stretchability. This foldable polymer structure does not limit itself to a specific chemical type and stays flexible even in cold temperatures.
More interestingly, the foldable bottlebrush polymer is 3D-printable. It can be mixed with inorganic nanoparticles to exhibit intricate electric, magnetic, or optical properties.
“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.
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Journal Reference
- Huang, B., Nian, S., & Cai, H. (2024). A universal strategy for decoupling stiffness and extensibility of polymer networks. Science Advances. DOI: 10.1126/sciadv.adq3080
