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CTOnews.com, May 7, two-dimensional materials (2D material) are thinner than paper, but they have amazing mechanical properties and are considered to be one of the strongest materials on earth. However, it is very difficult to stack these materials together. Because when these materials are superimposed together, the interaction between them is very weak, which greatly reduces their mechanical properties and limits their practical application.

Now, American scientists have found a way to break through this barrier. They fine-tuned the molecular structure of a two-dimensional polymer, the covalent organic framework (COFs), to create a new lightweight and tough material that can retain its two-dimensional properties even if it is superimposed on multiple layers.

"this is a very exciting starting point." Jun Lou, a professor in the department of materials science and nanoengineering at Rice University, said he led the research team at Rice University.

Using molecular level simulations, the researchers studied the arrangement of different functional groups, that is, molecular elements, and designed two kinds of COFs with slightly different structures. They then studied the behavior of these COFs when superimposed into multiple layers. It was found that small structural differences led to very different results.

The first COF, like most two-dimensional materials, shows only a weak interlayer interaction, and its strength and elasticity disappear as the number of layers increases. However, the second kind of COF is different. "it shows strong interlayer interaction and maintains good mechanical properties even if multiple layers are added." Said Qiyi Fang, a doctoral student at Rice University and co-lead author of the paper.

The researchers speculate that this phenomenon is probably due to hydrogen bonding. "We found from the simulation that in the second type of COF, there is a strong interlayer interaction due to the significant enhancement of hydrogen bonding between its special functional groups." Zhengqian Pang, another co-lead author of the paper and a postdoctoral fellow in the Department of Mechanical Engineering at the University of Maryland.

On this basis, the research team created a new light and tough material, which is not only several times stronger than steel, but also retains its two-dimensional properties even if it is stacked with multiple layers. This material has many potential applications.

"COFs can be made into an excellent filter membrane." Lou of Rice University said, "for a filtration system, the structure of functional groups in the pores is very important. When you have sewage passing through the COF membrane, the functional groups in the pores capture impurities and allow only the desired molecules to pass through. In this process, the mechanical integrity of the membrane is very important. Now we have a way to design very tough, fracture-resistant multilayer two-dimensional polymers, which can be very good candidates for filter membranes. "

"another potential application is to improve battery performance: using silicon instead of graphite as an anode can greatly increase the energy storage capacity of current lithium-ion battery technology." He said.

The study may also promote the design of a range of materials, including ceramics and metals. Ceramics, for example, rely on ionic bonds formed at very high temperatures, which is why a broken coffee cup cannot be easily repaired. Similarly, metals need to be forged at high temperatures. Through the molecular fine-tuning methods explored by the researchers, similar products may be made and repaired without heating.

The study was published in the Proceedings of the National Academy of Sciences at CTOnews.com.

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