Glassy gels exhibit numerous mechanical properties

SOPHIA CHEN: Welcome to MRS Bulletin’s Materials News Podcast, providing breakthrough news & interviews with researchers on hot topics in materials research. My name is Sophia Chen. Michael Dickey, a chemical engineer at North Carolina State University, shows me a strange material that his team has discovered. In some ways, it’s like a gel, which consists of polymers swollen with a solvent, like what you would find in a contact lens. But this gel differs from other gels in a key way.

MICHAEL DICKEY: Normally, when you add liquids to polymers, it makes them softer. Contact lenses, Jell-O, pretty much any kind of gel you can think of are kind of soft and squishy.

SOPHIA CHEN: But this material is rigid, like a glassy polymer.

MICHAEL DICKEY: Glassy polymers just means that the polymer chains are stuck. They can't move.

SOPHIA CHEN: Dickey’s team says that these tough gels qualify as a new class of material, which combines the properties of glassy polymers with gels. They call the material glassy gels, and they have intriguing mechanical properties.

MICHAEL DICKEY: They’ve got the best properties of glassy polymers, which I mean to say that they're stiff, and they're strong. So they feel just like a lot of plastics that we have in our day-to-day lives. Like imagine a plastic pen or a plastic cup, or plastic bottle. But they also have some properties of gels, which is they contain liquid. And you can also stretch them a lot.

SOPHIA CHEN: The glassy gels have two basic ingredients: the polymer, and the solvent. Here they used a polymer known as polyacrylic acid.

MICHAEL DICKEY: It's the material used in baby diapers to absorb the urine.

SOPHIA CHEN: For the solvent, they used a class of liquids known as ionic liquids. These are salts in liquid form. The ionic liquid they used is tributyl(methyl)phosphonium dimethyl phosphate, or PP for short. 

MICHAEL DICKEY: People have estimated there could be up to 10 to the 18th—that means one with eighteen zeroes, different ionic liquids. You can add a carbon here, add a nitrogen there, and make them chemically different.

SOPHIA CHEN: Then they cure the mixture of polymers and ionic liquid into a solid by shining UV light on it. Once cured, Dickey’s team found that the glassy gel has many interesting properties. For example, they can stretch these glassy gels by more than 6 times its original length, and then it returns to its original shape and size upon heating. 

MICHAEL DICKEY: We call it shape memory.

SOPHIA CHEN: However, they have not rigorously studied how often this material can be deformed before failure. They also found that the material has self-healing properties.

MICHAEL DICKEY: If you take this material, and you cut it in half, you can actually put it back together. And with a little bit of heat, the polymer chains will kind of go across the cut interface and will bind to each other.

SOPHIA CHEN: The material is also adhesive. 

MICHAEL DICKEY: If you take a glassy gel, and you contact it with something, metal ball or another, another solid material sticks.

SOPHIA CHEN: This differs from, say, superglue, which is no longer sticky upon curing. His team actually stumbled upon this material by accident. Dickey gives a lot of credit to his postdoc at the time, Meixiang Wang, for the discovery. Wang was working with ionic liquids to develop materials for wearable sensors. She was studying ionic liquids and making gels using ionic liquids when she created the first glassy gel.

MICHAEL DICKEY: She was exploring a parameter space of the amount of polymer and the amount of liquid again, this ionic liquid. And then she found that, that she was able to make these materials with just incredible mechanical properties.

SOPHIA CHEN: Dickey thinks these glassy gels would make good adhesives or could be used in 3D printing. Also, because it’s full of ions, the material is conducting and might be useful for batteries. But ultimately, he’s looking to the broader community to figure out how these materials could be useful.

MICHAEL DICKEY: Because we didn't set out to make a material to solve a particular problem, I view this as a little bit as like a solution, looking for the right problem.

SOPHIA CHEN: While other materials might exist with comparable toughness and stretchiness, Dickey says that glassy gels offer an advantage because of its simple curing process. 

MICHAEL DICKEY: It's very easy to make this. You just mix the materials together and shine light or apply heat and that cures it.

SOPHIA CHEN: This work was published in a recent issue of Nature. My name is Sophia Chen from the Materials Research Society. For more news, log onto the MRS Bulletin website at mrsbulletin.org and follow us on X, @MRSBulletin. Don’t miss the next episode of MRS Bulletin Materials News – subscribe now. Thank you for listening.