Imagine armor as light as cloth but stronger than steel, made of interconnected materials like molecular chainmail. Scientists may have just taken the first step towards making it a reality.
A research team led by Northwestern University scientists has created what may be the first two-dimensional (2D) mechanically linked object, similar to the links in chainmail. The story, detailed in a January 16 study published in the journal Scienceit is incredibly flexible and durable, and is useful in products such as lightweight body armor and ballistic fabrics.
The researchers created the material at the nanoscale level, meaning that its individual components are measured in nanometers. Technically a polymer: a substance made of large molecules, which are themselves made up of smaller chemical units called monomers. Examples of polymers include proteins, cellulose, and nucleic acids.
2D mechanically interlocked material is a polymer structure that uses mechanical bonds-bonds that have physical connections, as opposed to, for example, cohesive bonds, which usually make polymers and involve the sharing of electrons. The material has 100 trillion mechanical bonds per 0.16 square inch (1 square centimeter), which is the highest density of mechanical bonds ever made, according to the researchers.
“We created a completely new polymer structure,” study co-author William Dichtel of Northwestern University said in a university statement. “It’s like chainmail because it can’t tear easily because the mechanical bonds have the freedom to slide. If you pull it, it can dissipate the energy used in many ways. And if you want to rip it, you’re going to have to rip it in many, many different places. We continue to explore its properties and will probably study it for years.”
A major challenge in creating mechanically linked molecules lies in finding a way to direct the polymers to form mechanical bonds. Madison Bardot of Northwestern University, who led the study, was credited with coming up with a new way to achieve this. The team arranged the x-shaped monomers in a crystal structure (a straight-line arrangement) and replaced the crystals with another molecule. This reaction creates mechanical bonds within the crystals. The end product is 2D layers of composite polymer sheets made of these bonds between X-shaped monomers, the spaces of which the researchers filled with more X-shaped monomers.
“It was a high-risk, high-reward idea where we had to question our assumptions about what kinds of reactions are possible in molecular crystals,” Dichtel said. The resulting material is incredibly strong, yet flexible and easy to use, because the individual sheets of interconnected molecules separate when the polymer dissolves in the solvent.
“After the polymer is formed, there’s not much holding the structure together,” he added. So, when we put it in a solvent, the crystal dissolves, but each 2D layer is stuck together. We can use those individual papers.”
While previous researchers had mechanically synthesized polymers in very small quantities that would have been difficult to mass-produce, the team’s new method is surprisingly scalable. They make more than one pound (0.5 kilograms) of material, and suggest that more may be made.
Even a small percentage of the new polymer structure, however, can improve other properties. The researchers made a material made of 97.5% Ultem fiber (a very hard material in the same family as Kevlar) and 2.5% 2D polymer, and concluded that the mixture made the former much stronger.
“We have a lot of analysis to do, but we can say that it improves the strength of these composites,” Dichtel continued. “Almost every property we measured was different in some way.”
This incredibly strong and flexible feature may be the weapon the future has been waiting for.