Glassy polymers are plastics made to have properties like glass – they're strong, hard, and stiff, but also usually brittle, fracturing if you try to bend or stretch them. Gels on the other hand are soft and flexible, but also weak. The NCSU team has now developed a new material that combines the best of both worlds.

"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 Michael Dickey, corresponding author of the study. "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."

To make the glassy gels, the team combined the liquid precursor molecules of glassy polymers with an ionic liquid. The mix is then poured into a mold and exposed to UV light to cure it, before being removed from the mold. This ionic liquid acts as a solvent, which is what grants the material the powers of both glasses and gels.

"Normally when you add a solvent to a polymer, the solvent pushes apart the polymer chains, making the polymer soft and stretchable," said Dickey. "In glassy gels, the solvent pushes the molecular chains in the polymer apart, which allows it to be 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 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."

Even though they're more than 54% liquid by weight, these glassy gels were found to have fracture strength of 42 MPa, toughness of 110 MJ m-3, yield strength of 73 MPa, and a Young's modulus of 1 GPa. These stats are similar to thermoplastics like polyethylene, the team says, but unlike those materials they can also be stretched up to five times their original length.

Other advantages of the glassy gels include their ability to self-heal, and return to their original shape by applying a bit of heat. Their high liquid content also makes them more efficient electrical conductors, and they have an adhesive surface, for reasons that aren't entirely clear to the team. Most useful of all, these glassy gels are fairly easy to make.