Although they are considered one of the most powerful substances in the world, using them to their full potential has proven to be a difficult task. Thinner than even the thinnest onion skin paper, 2D materials have received a lot of attention due to their excellent mechanical properties. However, these properties disappear when materials are layered, thus limiting their practical application.

“Think of a graphite pencil,” says Teng Li, a professor in the Department of Mechanical Engineering at the University of Maryland (UMD). “The core is made of graphite, and graphite consists of many layers of graphene, which is considered the strongest material in the world. But the graphite pencil is not strong at all – in fact, graphite is even used as a lubricant.”

Now, Lee and colleagues at Rice University and the University of Houston have found a way to circumvent this hurdle by carefully tuning the molecular structure of two-dimensional polymers known as covalent organic frameworks (COFs). The findings are detailed in a new published study. Proceedings of the National Academy of Sciences.

“This is a very exciting starting point,” said Jun Lu, professor of materials science and nanoengineering at Rice University, who led the Rice team.

Using modeling at the molecular level, the researchers studied different functional groups, i.e. the arrangement of molecular elements, and then designed two COFs with minimal differences in structure. They then examined how COFs behaved when they were layered. It turns out that small structural differences lead to significantly different results.

The first COF, like most 2D materials, showed only weak interactions between layers, and both strength and flexibility were lost as new layers were added. Rice University postdoctoral researcher Qiyi Fang, one of the lead authors of the PNAS paper, said another COF “which shows strong interlayer interaction and retains its good mechanical properties even after multiple layers have been added.”

According to the researchers, this phenomenon is most likely related to hydrogen bonding. “Through our simulations, we found that the strong interlayer interactions in the second type of COF are the result of the significantly enhanced hydrogen bonding between their specific functional groups,” said lead author Zhengqian Pang, a UMD postdoctoral fellow and Li’s fellow. research group.

Applying their findings, the research team created a lightweight material that is not only several times stronger than steel but also retains its 2D properties even when stacked in multiple layers.

There are many potential applications. “COFs can make excellent filtration membranes,” said Lu of Rice. “For a filtration system, the structure of the functional group in the pore will be very important. For example, since you have polluted water passing through a COF membrane, the functional group in the pore will only trap the impurities and allow the desired molecule to pass through. In this process, the mechanical integrity of this membrane will be very important. Now, “We have found a way to develop very strong, very resistant to fracture, multilayered 2D polymers that would be very good candidates for membrane filtration applications.”

“Another potential application is in battery upgrades: replacing the graphite anode with a silicon one will significantly increase the capacity of today’s lithium-ion batteries,” he said.

According to Li, the research findings could lead to advances in the development of a wide variety of materials, including ceramics and metals. For example, ceramics depend on ionic bonds formed at very high temperatures, so a broken coffee cup cannot be easily repaired. Metals also require forging at high temperatures. Thanks to the molecular tweaks that the researchers are working on, such products could probably be produced and repaired without increasing the heat.

“While the immediate context is 2D materials, overall, we are pioneering ways to reap the benefits of materials without the limitations that these materials offer,” Lee said.