Researchers at North Carolina State University have created a new class of materials called “glassy gels” that are very hard and difficult to break despite containing more than 50% liquid. Coupled with the fact that glassy gels are simple to produce, the material holds promise for a variety of applications. A paper on the work appears in Nature.

Glassy polymers are generally stiff and strong yet have limited extensibility. By swelling with solvent, glassy polymers can become gels that are soft and weak yet have enhanced extensibility. The marked changes in properties arise from the solvent increasing free volume between chains while weakening polymer–polymer interactions.

Here we show that solvating polar polymers with ionic liquids (that is, ionogels) at appropriate concentrations can produce a unique class of materials called glassy gels with desirable properties of both glasses and gels. The ionic liquid increases free volume and therefore extensibility despite the absence of conventional solvent (for example, water). Yet, the ionic liquid forms strong and abundant non-covalent crosslinks between polymer chains to render a stiff, tough, glassy, and homogeneous network (that is, no phase separation), at room temperature.

Despite being more than 54 wt% liquid, the glassy gels exhibit enormous fracture strength (42MPa), toughness (110 MJ m⁻³), yield strength (73 MPa) and Young’s modulus (1 GPa). These values are similar to those of thermoplastics such as polyethylene, yet unlike thermoplastics, the glassy gels can be deformed up to 670% strain with full and rapid recovery on heating. These transparent materials form by a one-step polymerization and have impressive adhesive, self-healing and shape-memory properties.

—Wang et al.

Glassy gels, as the name suggests, are effectively 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 “cures” the material. The mold is then removed, leaving behind the glassy gel.

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.

In testing, the researchers found that the glassy gels don’t evaporate or dry out, even though they consist of 50-60% liquid.

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

Resources

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