Researchers at North Carolina State University have created ionogels, new materials whose extensibility and toughness are comparable to natural rubber and cartilage.
“Materials that can be deformed but are difficult to break or tear are desirable,” said Michael Dickey, co-correspondent author of a paper on the work and the Camille & Henry Dreyfus Professor of Chemical and Biomolecular Engineering at NC State. “Nature is good at this; think of cartilage as an example. But designing synthetic materials with these properties has been difficult, which makes our work here exciting.”
The new materials are ionogels, which are polymer networks containing salts (ionic liquids) that are liquid at room temperature.
Dickey and his collaborators have created ionogels that are nearly 70 percent liquid, but are said to have remarkable mechanical properties; they dissipate a lot of energy when deformed, making them very difficult to break. They are also easy to make, easy to process and can be 3D printed.
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According to the article, the ionogels have a high fracture strength (12.6 MPa), fracture energy (~ 24 kJ m−2) and Young’s modulus (46.5 MPa), while being highly stretchable (~600 percent elongation) and having self-healing and shape memory properties.
“Hydrogels, polymer networks that contain water, are quite common,” Dickey said. “Contact lenses are hydrogels, for example. But ionogels have some advantages over hydrogels. Ionic liquids don’t evaporate like water, so you don’t have to worry about the ionogels drying out. Ionogels are also electrically and thermally stable and conduct electricity well, opening up interesting possibilities for future applications.”
To make the new ionogels, the researchers started with monomers of polyacrylic acid and polyacrylamide and copolymerized them in a solution of ionic liquid using ultraviolet light.
“The end result is significantly better than the average of the two materials,” Dickey said. “It’s like adding 1+1 and getting 10. The resulting gel has the stretchability of polyacrylic acid and is even stronger than the polyacrylamide. In terms of toughness it is better than cartilage. But the differences between ionogels and hydrogels make them advantageous for different applications.”
In addition, the ionogels created by Dickey’s team also have self-healing and shape memory properties. Two pieces of ionogel placed together and exposed to heat form a strong bond. The material can also be deformed into a temporary new shape, but will return to its original shape when exposed to heat. The amount of heat required depends on how quickly the material is needed to return to its normal shape. When exposed to a temperature of 60OC, the actions last tens of seconds.
“We’re excited to have created something with really remarkable properties that can be made very easily — just shine your light on it — with widely available polymers,” Dickey said. “And you can adjust the properties of the ionogels by controlling the ratio of ingredients during the copolymerization process.
“We are already working with one industry partner and are open to working with others to develop applications for this new breed of ionogels.”