Researchers are creating a new class of materials called ‘glassy gels’

Researchers have created a new class of materials called ‘glassy gels’ that are very hard and difficult to break despite containing more than 50% liquid. Combined with the fact that glassy gels are easy to produce, the material shows promise for a variety of applications.

An article describing this work, entitled “Glassy Gels Toughened by Solvent,” appears in the journal Nature.

Gels and glassy polymers are classes of materials that have historically been considered distinct from each other. Glassy polymers are hard, stiff and often brittle. They are used to make things like water bottles or airplane windows. Gels, like contact lenses, contain liquid and are soft and stretchy.

“We have created a class of materials that we have called glassy gels, which are as hard as glassy polymers, but which – if you apply enough force – can stretch up to five times their original length, rather than breaking,” says Michael. Dickey, corresponding author on a paper on the work and the Camille and Henry Dreyfus Professor of Chemical and Biomolecular Engineering at North Carolina State University. “Additionally, once the material has been stretched, you can use heat to return the material to its original shape. In addition, the surface of the glassy gels is very adhesive, which is unusual for hard materials.”

“One key thing that sets glassy gels apart is that they are more than 50% liquid, making them more efficient conductors of electricity than regular plastics with similar physical characteristics,” said Meixiang Wang, co-lead author of the paper and postdoctoral researcher. researcher at NC State. “Given the number of unique properties they possess, we are optimistic that these materials will be useful.”

Glassy gels, as the name suggests, are essentially a material that combines some of the most attractive properties of both glassy polymers and gels. To make them, the 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 “hardens” the material. The mold is then removed, leaving the vitreous gel behind.

“The ionic liquid is a solvent, just like water, but composed entirely of ions,” says Dickey. “Normally, when you add a solvent to a polymer, the solvent pushes the polymer chains apart, making the polymer soft and stretchy. That’s why a wet contact lens is pliable, and a dry contact lens is not. In glassy gels, the solvent pushes the molecular chains in the polymer separate, making it stretchable like a gel.

“However, the ions in the solvent are strongly attracted to the polymer, which prevents the polymer chains from moving. The inability of chains to move makes it glassy. The end result is that the material hardens due to the attractive forces, but can still stretch due to the extra distance.”

The researchers found that glassy gels can be made with a variety of different polymers and ionic liquids, although not all classes of polymers can be used to make glassy gels.

“Polymers that are charged or polar promise to form glassy gels because they are attracted to the ionic liquid,” says Dickey.

During tests, the researchers found that the glassy gels do not evaporate or dry out, even though they are 50-60% liquid.

“Perhaps the most intriguing feature of the glassy gels is how adhesive they are,” says Dickey. “Because while we understand what makes them hard and stretchy, we can only speculate about what makes them so sticky.”

The researchers also think that glassy gels hold promise for practical applications because they are easy to make.

“Making glass-like gels is a simple process that can be accomplished by curing it in any type of mold or by 3D printing it,” says Dickey. “Most plastics with similar mechanical properties require manufacturers to make polymer as a raw material and then transport that polymer to another facility where the polymer is melted and formed into the final product.

“We are excited to see how glassy gels can be used and are open to working with collaborators in identifying applications for these materials.”

Co-lead author of the paper is Xun Xiao of the University of North Carolina at Chapel Hill. The article 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 student at NC State; Brendan O’Connor, professor of mechanical and aerospace engineering at NC State; Wubin Bai, professor of applied sciences at UNC; and Wen Qian, associate professor of mechanical engineering and materials science at the University of Nebraska-Lincoln.