Glassy gel is hard as plastic and stretches 7 times its length

A material made of liquid salt mixed with polymers is extremely stretchy but still as strong as the plastics used to make water bottles

By Karmela Padavic-Callaghan
19 June 2024

When you think of gel, you might imagine goo – but a new gel-like material has been engineered to be soft enough to stretch to almost seven times its original length while still being strong and clear, like glass.

Michael Dickey at North Carolina State University says his team discovered these “glassy gels” when his student, Meixiang Wang, was experimenting with ionic liquids and kept finding unexpected mechanical properties. The materials they devised are more than 50 per cent liquid, but as strong as the plastics used for water bottles, while also being very stretchy and sticky. “There are a bunch of cool things about them,” he says.

Each glassy gel consists of long molecules called polymers mixed with an ionic liquid, a fluid that is essentially a salt in liquid form. The gel is a transparent solid that can withstand up to 400 times atmospheric pressure, but also stretch very easily up to 670 per cent. Dickey says that this could make it well-suited for building soft robotic grippers or 3D printing deformable materials.

He and his colleagues made glassy gels from several different mixtures of polymers and liquid salts and found that their strength and stretch depended on the precise ratio used.

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Article
Published: 19 June 2024

Glassy gels toughened by solvent

Meixiang Wang, Xun Xiao, Salma Siddika, Mohammad Shamsi, Ethan Frey, Wen Qian, Wubin Bai, Brendan T. O’Connor & Michael D. Dickey
Nature (2024) Cite this article

Abstract
Glassy polymers are generally stiff and strong yet have limited extensibility1. By swelling with solvent, glassy polymers can become gels that are soft and weak yet have enhanced extensibility1,2,3. 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, ionogels4,5) 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)6, at room temperature. Despite being more than 54 wt% liquid, the glassy gels exhibit enormous fracture strength (42 MPa), 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.

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