Although glassy polymers have strong and sturdy characteristics, their disadvantage is limited extensibility and brittleness. A university in the United States has developed a “glass gel” using a special solvent to effectively improve the drawbacks of glassy polymers, giving them the desired properties of both glass and gel.
The mechanical properties of glassy polymers are important for many consumer products, as the strong interactions between polymer chains give them high rigidity and strength. However, solvents in gels can reduce the interactions between polymer chains, softening the polymer network and lowering its mechanical performance. For example, contact lenses are made from polymer gels that swell when exposed to water.
Scientists wanted to merge glassy polymers and gel polymers to create a new type of material that is both strong and elastic. Researchers at North Carolina State University in the United States started from the solvent and developed a “glass gel” with anti-fracture, high toughness, and high deformation properties. This material can also quickly recover through brief heating, possessing adhesive, self-healing, and shape memory properties. The results were published in the journal “Nature” on June 19.
This glass gel forms a strong non-covalent bond interaction between the “ionic liquid” and “polymer” substances, relying not on electron sharing but on electron attraction.
Researchers initially conducted a pressure test on a solvent-free 1 square centimeter piece of polyacrylic acid (PAA) to test its mechanical performance. The results showed that the solvent-free polyacrylic acid could bear a load of 4,000 grams and was very hard but prone to breaking.
Then they added an ionic liquid solvent (PP) to the 1 square centimeter polyacrylic acid to transform it into a glassy gel state before conducting the pressure test. The results showed that it could still bear a load of 4,000 grams, becoming even harder and highly extensible. On the other hand, just adding water to polyacrylic acid to form a gel would only make it soft and elastic, capable of bearing a load of only 20 grams.
To determine the optimal state of polyacrylic acid and ionic liquid solvent, they conducted trials where varying concentrations of monomer polyacrylic acid were added to an appropriate amount of ionic liquid solvent to produce glass gels. The results showed that these different concentration polyacrylic acids all had high polymerization conversion rates (greater than 94%) and resulted in glass gels with high transparency.
Furthermore, the solvent content of these glass gels reached over 54% of the total weight. However, their volatility was lower than traditional polyacrylic acid gels containing only water, making them less prone to evaporation or drying out. They found that increasing the concentration of monomer polyacrylic acid could enhance its strength and toughness, supporting a weight over 200 times its own.
In addition, researchers found that the mechanical properties of this glassy gel were comparable to polymethyl methacrylate (PMMA), polystyrene, and plasticized polyvinyl chloride (PVC), and its ability to withstand external pressure was similar to polyethylene (PE) thermoplastic plastics. This material could be quickly restored to its original shape in less than a minute when heated to 80°C.
The study also showed that glassy gels could be made from various polymers and ionic liquids, with mechanical properties similar to regular plastics. However, not all types of polymers could be used to manufacture glassy gels.
The corresponding author of the study, Professor Michael Dickey from the Department of Chemistry and Biomolecular Engineering at North Carolina State University, explained, “Polymers with charges or polarity are likely to become glassy gels because they are easily attracted by ionic liquids.”
Furthermore, the experimental team stated that besides exceptional mechanical properties, glassy gels also exhibited excellent adhesive properties, shape memory, self-healing ability, and a certain level of electrical conductivity. Hence, these materials could find application value in electronics, batteries, and sensors.
The co-first author of the paper, postdoctoral researcher Meiyang Wang from the university, expressed, “The key difference between glassy gels and regular gels is that their liquid content exceeds 50%, making their conductivity better than plastics with similar physical properties. This result makes us optimistic about the potential uses of these materials.”
They believe that glassy gels have practical application prospects as they can be solidified in any type of mold or manufactured through 3D printing.
Professor Dickey stated, “The glassy gels we created are not only as hard as glassy polymers but also can elongate five times without breaking. More importantly, this material can be heated to return to its original shape after being stretched. Additionally, the surface of glassy gels exhibits high adhesive properties, which is incredible for rigid materials.”
He added, “Perhaps the most intriguing characteristic of glassy gels is their adhesiveness, and I am eager to see how glassy gels will be used and define their applications.”
This material is expected to be used in 3D modeling and on some soft mechanical grippers, thus holding high application potential in industries such as electronics and medical equipment.