Researchers Create “Glassy Gels”, A New Class of Materials

Researchers at North Carolina State University have created a new class of materials that blends the hardness of glass with the stretchability of gels. The new materials – which they have termed “glassy gels” – can stretch up to five times their original length without breaking, while still being extremely tough and fracture-resistant.

The easy-to-make materials could have applications in 3D printing, batteries and soft robotics, the researchers say. Their research is published in Nature.

What is a glassy gel?

Polymer gels and glassy polymers are both types of polymer material, with radically different properties.

A plastic chair would be an example of a glassy polymer – it is stiff and strong because the polymer chains it is made up of are all interacting with each other, keeping them held together in a very rigid shape.

These interactions can be lessened by adding liquid to the polymer, which will turn it into a gel. A good example of a polymer gel is a contact lens, which contains a large proportion of water to make it pliable and comfortable on the eye. Anyone who has ever fallen asleep with their contact lenses in can tell you that this is not the case for a dry contact lens.

These polymer gels and glasses are extremely useful materials, with almost completely opposite properties. Glassy polymers are stiff and strong, but they are also brittle; gels are stretchier, but they are not very strong.

But what if there was a way to combine the most favorable properties of both materials? Enter the glassy gels.

“We’ve created a class of materials that we’ve termed glassy gels, which are as hard as glassy polymers, but – if you apply enough force – can stretch up to five times their original length, rather than breaking,” said study author Michael Dickey, a professor of chemical and biomolecular engineering at North Carolina State University (NC State). “What’s more, once the material has been stretched, you can get it to return to its original shape by applying heat. In addition, the surface of the glassy gels is highly adhesive, which is unusual for hard materials.”

Making a glassy gel

Just like a regular polymer gel, glassy gels are made by adding liquid to a precursor glassy polymer. This high liquid content is also responsible for one of the material’s most interesting properties – its ability to conduct electricity despite being a tough plastic.

“A key thing that distinguishes glassy gels is that they are more than 50% liquid, which makes them more efficient conductors of electricity than common plastics that have comparable physical characteristics,” said Meixiang Wang, a 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.”

So how do you make a glassy gel and not just a regular gel? The trick is in what type of liquid is added. To make their glassy gels, the NC State researchers added an ionic liquid to a mix of glassy polymer precursors. This mixture is then poured into a mold, cured with ultraviolet light and demolded to reveal the finished glassy gel material.

“The ionic liquid is a solvent, like water, but is made entirely of ions,” explained 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, which allows it to be stretchable like a gel,” he continued. “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 is what makes it glassy. The end result is that the material is hard due to the attractive forces, but is still capable of stretching due to the extra spacing.”

While not every class of polymer that the researchers tested was able to form a glassy gel, they found that a significant number of different polymer types were compatible with this straightforward synthesis process.

“Polymers that are charged or polar hold promise for glassy gels, because they’re attracted to the ionic liquid,” Dickey said.

Future applications

The researchers report that their glassy gels exhibit “enormous” fracture strength, toughness, yield strength and a high Young’s modulus (a measure of stiffness) comparable to that of strong thermoplastics such as polyethylene. But the novelty of this new material class is that, unlike thermoplastics, they can be stretched up to five times their original length without breaking.

The gels also have shape memory, which can be programmed by deforming the material as it is heated and cooled to “fix” it in a certain shape, which is reset easily through further heating. Heat can also be used to self-heal the glassy polymer or join two fragments of the polymer together.

Additionally, despite consisting of between 50-60% liquid, the new glassy gels did not appear to naturally evaporate and dry out as seen with traditional polymer gels.

“Maybe the most intriguing characteristic of the glassy gels is how adhesive they are,” Dickey added. “Because while we understand what makes them hard and stretchable, we can only speculate about what makes them so sticky.”

While this suite of material properties certainly makes for some interesting potential applications in robotics or 3D printing, the researchers believe that their ease to make could be the thing to set these materials apart as an option for further development.

“Creating glassy gels is a simple process that can be done by curing it in any type of mold or by 3D printing it,” said 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,” he added.

Reference: Wang M, Xiao X, Siddika S, et al. Glassy gels toughened by solvent. Nature. 2024. doi:10.1038/s41586-024-07564-0

This article is a rework of a press release issued by North Carolina State University. Material has been edited for length and content.