Researchers have developed a new class of materials called “glassy gels,” which are extremely hard and difficult to break despite containing over 50% liquid. The simplicity of producing glassy gels makes them promising for various applications. A paper titled “Glassy Gels Toughened by Solvent,” detailing this work, appears in the journal Nature.

Traditionally, gels and glassy polymers are viewed as distinct materials. Glassy polymers are hard, stiff, and often brittle, used in products like water bottles and airplane windows. Gels, such as contact lenses, contain liquid and are soft and stretchy. “We’ve created a class of materials that we’ve termed glassy gels, which are as hard as glassy polymers but can stretch up to five times their original length without breaking,” says Michael Dickey, the corresponding author of the paper and the Camille and Henry Dreyfus Professor of Chemical and Biomolecular Engineering at North Carolina State University. “What’s more, once the material has been stretched, you can return it to its original shape by applying heat. Additionally, the surface of the glassy gels is highly adhesive, which is unusual for hard materials.”

“A key thing that distinguishes glassy gels is that they are more than 50% liquid, making them more efficient conductors of electricity than common plastics with comparable physical characteristics,” says Meixiang Wang, co-lead author of the paper and a postdoctoral researcher at NC State. “Considering the number of unique properties they possess, we’re optimistic that these materials will be useful.”

To create glassy gels, researchers start with the liquid precursors of glassy polymers and mix them with an ionic liquid. This combined liquid is poured into a mold and exposed to ultraviolet light, which “cures” the material. The mold is then removed, leaving behind the glassy gel. “The ionic liquid is a solvent, like water, but is made entirely of ions,” explains Dickey. “Normally, when you add a solvent to a polymer, the solvent pushes apart the polymer chains, making the polymer soft and stretchable. That’s why a wet contact lens is pliable, and a dry contact lens isn’t. In glassy gels, the solvent pushes the molecular chains in the polymer apart, allowing it to be stretchable like a gel.”

However, the ions in the solvent are strongly attracted to the polymer, preventing the polymer chains from moving. This inability of chains to move is what makes the material glassy. The result is a material that is hard due to the attractive forces but still capable of stretching due to the extra spacing.

The researchers discovered that glassy gels could be made with a variety of different polymers and ionic liquids, though not all classes of polymers can be used to create glassy gels. “Polymers that are charged or polar hold promise for glassy gels because they’re attracted to the ionic liquid,” Dickey says.

In testing, the researchers found that the glassy gels don’t evaporate or dry out, even though they consist of 50-60% liquid. “Perhaps the most intriguing characteristic of the glassy gels is how adhesive they are,” says Dickey. “While we understand what makes them hard and stretchable, we can only speculate about what makes them so sticky.”

Glassy gels also hold promise for practical applications because they’re easy to produce. “Creating glassy gels is a simple process that can be done by curing it in any type of mold or by 3D printing it,” says Dickey. “Most plastics with similar mechanical properties require manufacturers to create polymer as a feedstock and then transport that polymer to another facility where the polymer is melted and formed into the end product. We’re excited to see how glassy gels can be used and are open to working with collaborators on identifying applications for these materials.”

Co-lead author of the paper is Xun Xiao of the University of North Carolina at Chapel Hill. The paper was co-authored by Salma Siddika, a Ph.D. student at NC State; Mohammad Shamsi, a former Ph.D. student at NC State; Ethan Frey, a former undergrad at NC State; Brendan O’Connor, a professor of mechanical and aerospace engineering at NC State; Wubin Bai, a professor of applied physical sciences at UNC; and Wen Qian, a research associate professor of mechanical and materials engineering at the University of Nebraska-Lincoln.

By Impact Lab