In a serendipitous discovery, scientists have created a new class of materials called “vitreous gels” that are semi-liquid but difficult to break.
Flexible, strangely sticky and capable of “self-healing” when cut, the surprising properties of these gels potentially make them useful for a wider range of applications than commonly used plastics, which are either hard and brittle or soft and tear easily.
“We’ve created a class of materials we’ve called glassy gels, which are tough like glassy polymers, but—if you apply enough force—they can stretch up to five times their original length before breaking,” says Michael. Dickey, a materials scientist at North Carolina State University (NCSU).
But as with many serendipitous scientific discoveries, the goal was never to create an entirely new class of substances, Dickey tells ScienceAlert.
“We came across these interesting materials,” he says, when NCSU researcher Meixiang Wang experimented with ionogels, materials made from a polymer swollen with an ionic liquid that conducts electricity.
Wang sought to make stretchable, wearable devices that could be used in a pressure sensor, other medical devices or robotics. By changing the composition, Wang created a gel that initially looked like “an ordinary piece of transparent, flexible plastic,” before testing showed it to be very hard—but not brittle like other common plastics.
“Once we realized they had remarkable properties, we set out to understand them better,” says Dickey.
Glass gels are made using an ionic liquid that is similar to water but made entirely of charged particles, which allows it to conduct electricity. When mixed with a polymer precursor, the liquid pushes the polymer chains apart, making the material soft and flexible. At the same time, the ions are also strongly attracted to the polymer chains, preventing their separation.
“The end result is that the material is stiff because of the attractive forces, but still able to stretch because of the larger spacing,” explains Dickey.
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Glass gels do not dry out, even though they are 50 to 60 percent liquid, and testing has shown them to have “tremendous” fracture strength and toughness.
The material can also “self-heal”, reforming if cut, and has a kind of memory that allows the stretched gel to hold its shape, only to shrink back to its original shape when heated.
Although these regenerative properties are unusual, they are not particularly new, especially for flexible gel-like materials. Recently, scientists succeeded in the much more difficult task of making typically rigid materials such as metals, glass, solar panels, and concrete that heal when cracked. If commercialized, these materials could repair themselves when damaged, helping to reduce waste in the construction, electronics and fashion industries.
But the strange combination of the remarkable nature of glass gels is something researchers want to explore further.
“Perhaps the most interesting characteristic of glass gels is how sticky they are,” says Dickey. “We understand what makes them tough and flexible, [but] we can only speculate as to why they are so sticky.”
Of course, more testing and optimization of the “gel” is needed before these gels can be used in any practical way, but when considering potential applications, Dickey says that durable materials that conduct electricity (like gel) are useful in batteries.
Other potential uses include 3D printing plastic-like materials using simpler techniques than melt processing – a method currently used to make commercial plastics from starter resins. The process often involves transporting products to multiple facilities for each step of plastic production, while glassy gels can be injected into a mold and cured with UV light.
But before working on applications, Dickey says his team wants to better understand the fundamentals of how these materials are formed and why there seems to be a “magic ratio” of solvent to polymer that creates the gels’ unique properties.
“Given the number of unique properties they have, we are optimistic that these materials will be useful,” says Wang.
The study was published in Nature.