Developed in the US, the glassy gels are simple to produce and could find numerous applications. The work is described in Nature.
“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 paper on the work 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 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.”
“A key thing that distinguishes glassy gels is that they are more than 50 per cent liquid, which makes them more efficient conductors of electricity than common plastics that have comparable physical characteristics,” said 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 make glassy gels, the researchers start with the liquid precursors of glassy polymers and mix them with an ionic liquid. This combined liquid is poured into a mould and exposed to ultraviolet light, which ‘cures’ the material.
“The ionic liquid is a solvent, like water, but is made entirely of ions,” said Dickey. “Normally when you add a solvent to a polymer, the solvent pushes apart the polymer chains, making the polymer soft and stretchable. 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.”
The researchers found 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,” said Dickey.
In testing, the researchers found that the glassy gels do not evaporate or dry out, even though they consist of 50-60 per cent liquid.
“Maybe the most intriguing characteristic of the glassy gels is how adhesive they are,” said Dickey. “Because while we understand what makes them hard and stretchable, we can only speculate about what makes them so sticky.”
The researchers also think glassy gels hold promise for practical applications because they are easy to make.
“Creating glassy gels is a simple process that can be done by curing it in any type of mould 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.”
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