Imagine an armor as light as cloth but stronger than steel, built from materials that link together like a molecular chain link. Scientists may have just taken the first step towards making this a reality.
A team of researchers led by Northwestern University scientists has developed what may be the first two-dimensional (2D) mechanically interconnected material, similar to the links in a chain chain. The material, described in detail in January 16 study published in the journal Scienceit is extremely flexible and strong, with promising applications in products such as lightweight body armor and ballistic fabrics.
The researchers built the material at the nanoscale level, meaning its individual components are measurable in nanometers. Technically it is a polymer: a substance composed of large molecules, which are themselves composed of smaller chemical units called monomers. Examples of polymers include proteins, cellulose and nucleic acids.
A 2D mechanically entangled material is a polymer structure that uses mechanical bonds—bonds with physical entanglement, as opposed to, for example, covalent bonds, which typically make up polymers and involve the sharing of electrons. The material has 100 trillion mechanical connections per 0.16 square inches (1 square centimeter), the highest density of mechanical connections ever made, according to the researchers.
“We made a completely new polymer structure,” said study co-author William Dichtel of Northwestern University at the university statement. “It’s similar to chain mail in that it doesn’t tear easily because each of the mechanical links has a little bit of freedom to slide around. If you pull it, it can disperse the applied force in multiple directions. And if you wanted to tear it apart, you’d have to break it in many, many different places. We continue to investigate its properties and will likely study it for years.”
The biggest challenge in creating mechanically interconnected molecules lies in finding a way to induce the polymer to form mechanical bonds. Madison Bardot of Northwestern University, who led the study, is credited with devising a new method to achieve this. The team arranged the letter-shaped monomers into a crystal structure (a specific ordered arrangement) and reacted the crystals with another molecule. This reaction created mechanical bonds within the crystal. The final product is 2D layers of interlocking polymer sheets made of these bonds between X-shaped monomers, the gaps of which the researchers filled with more X-shaped monomers.
“It was a high-risk, high-reward idea where we had to rethink our assumptions about what kinds of reactions are possible in molecular crystals,” Dichtel said. The resulting material is incredibly strong, but still flexible and easy to handle, because individual sheets of interconnected molecules separate from each other when the polymer is dissolved in a solvent.
“Once the polymer is formed, there’s not much that holds the structure together,” he added. “So when we put it in a solvent, the crystal dissolves, but each 2D layer is held together. We can manipulate those individual leaves.”
While previous researchers have produced mechanically linked polymers in very small quantities that would be difficult to mass-produce, the team’s new method is surprisingly adaptable. They have made more than one pound (0.5 kilograms) of material, and suggest the possibility of making more.
However, even a small percentage of the new polymer structure can improve other substances. The researchers made a material consisting of 97.5% Ultem fibers (an extremely strong material from the same family as Kevlar) and 2.5% 2D polymer, and concluded that the mixture made the former significantly stronger.
“We have to do a lot more analysis, but we can say that it improves the strength of these composite materials,” continued Dichtel. “Almost every property we measured was exceptional in some way.”
This incredibly strong and flexible material could be just the armor the future has been waiting for.