Researchers introduce a groundbreaking polymer technique that changes a long-standing engineering principle, allowing for materials that combine both flexibility and rigidity.
Researchers at the University of Virginia’s School of Engineering and Applied Science have created a novel polymer design that challenges established beliefs in polymer engineering. It is no longer a given that a stiffer polymer must compromise its stretchability.
“We are tackling a core issue that has been deemed impossible to resolve since the creation of vulcanized rubber in 1839,” shared Liheng Cai, an assistant professor of materials science, as well as chemical engineering.
This innovation emerged when Charles Goodyear stumbled upon the method of heating natural rubber with sulfur, forming chemical links between the rubber molecules. This process of crosslinking results in a polymer network, transforming initially pliable rubber into a robust, elastic material.
For over a century, the belief persisted that enhancing the stiffness of a polymer means sacrificing some level of stretchability.
However, Cai’s group, led by Ph.D. student Baiqiang Huang, has shown a different approach with their innovative “foldable bottlebrush polymer networks.” Their findings, supported by Cai’s National Science Foundation CAREER Award, have been featured on the cover of the Nov. 27 edition of Science Advances.
‘Separation’ of Stiffness and Flexibility
“This limitation has hindered the development of materials capable of being both stretchable and stiff, compelling engineers to choose one attribute at the expense of another,” Huang explained. “Picture a heart implant that flexes with every heartbeat while maintaining durability over years.”
Huang is the primary author of the paper, working alongside postdoctoral researcher Shifeng Nian and Cai.
Crosslinked polymers are integral to many consumer products, ranging from car tires to household items, and are increasingly embraced in the fields of biomaterials and healthcare technology.
The team envisions various applications for their creation, including prosthetics, medical implants, advanced wearable tech, and “muscles” for flexible robotic systems that require continuous bending and stretching.
The relationship between stiffness and extensibility—how much a material can stretch without breaking—emanates from the same foundational element: polymer strands interlinked by crosslinks. Historically, increasing the number of crosslinks has been the standard method to enhance a polymer’s stiffness.
While this does stiffen the material, it fails to address the compromise between stiffness and stretchability. Polymer networks with a higher crosslink density become stiffer, yet they lose their ability to deform, leading to breakage under tension.
“Our team discovered that by crafting foldable bottlebrush polymers capable of storing extra length within their structure, we could ‘decouple’ stiffness and stretchability—allowing for increased stretch without reducing stiffness,” Cai explained. “Our method represents a shift in focus, emphasizing the molecular design of the network strands rather than just the crosslinks.”
Mechanics of the Foldable Design
Unlike typical linear polymer strands, Cai’s design resembles a bottlebrush, featuring multiple flexible side chains extending from a central core.
A critical aspect is that the backbone can compress and expand similarly to an accordion, unfolding as it stretches. When the material is tugged, concealed length within the polymer unravels, enabling it to extend up to 40 times more than conventional polymers without any loss of strength.
Simultaneously, the side chains dictate the stiffness, which means that stiffness and stretchability can finally be altered independently.
This represents a “universal” strategy for polymer networks, as the elements that constitute the foldable bottlebrush polymer are not limited to specific chemical types.
As an illustration, one of their designs employs a polymer for the side chains that remains flexible even in low temperatures. Alternatively, utilizing a different synthetic polymer commonly found in biomaterial engineering for the side chains can yield a gel that simulates living tissue.
Like many of the innovative materials produced in Cai’s lab, the foldable bottlebrush polymer is designed to be compatible with 3D printing. This holds true even when incorporated with inorganic nanoparticles, which can be engineered to demonstrate complex electric, magnetic, or optical characteristics.
For instance, they can integrate conductive nanoparticles like silver or gold nanorods, which are essential for stretchable and wearable electronics.
“These components provide us with limitless possibilities for fabricating materials that balance durability and flexibility while utilizing the properties of inorganic nanoparticles as per specific needs,” added Cai.
