The application potential of metal-organic frameworks was discovered about 20 years ago, and since then, nearly 100,000 such mixed porous materials have been discovered. There is great hope in technical applications, especially flexible MOF. For example, as shock absorbers, they can respond to sudden high pressures, that is, plastic deformation, by closing pores and losing volume.

 Or they can separate chemicals from each other like sponges, absorb them into their pores, and then release them under pressure. Rochus Schmid explains: "This requires less energy than the usual distillation process." However, so far, only a few such flexible MOFs have been identified.

Pressure on MOF

In order to gain insight into the underlying mechanism in such materials, the Munich research team conducted a more detailed experimental analysis of the already widely known MOF. In order to achieve this goal, the researchers subject it to uniform pressure from all sides, while using X-ray structural analysis to observe the situation inside.

Gregor Kieslich said: "We want to know how this material behaves under pressure, and what chemical factors drive the phase change between open and closed pores." The test shows that the closed pores are unstable; under pressure Next, the system lost its crystal order, in short: it collapsed.

This is not the case for variants with the same basic structure: if the research team connects the flexible side chains of carbon atoms to the organic connectors of the MOF protruding into the pores, the material remains intact when compressed and recovers when compressed Its original shape. The carbon arm turns a non-flexible material into a flexible MOF.

The secret of phase transition

The Bochum team used computer chemistry and molecular dynamics simulation to study the basic principles. Rochus Schmid summarized: "We have proved that the secret lies in the degree of freedom of the side chain, the so-called entropy. Essentially, every system in nature is striving for the maximum entropy. In short, it is to have as many degrees of freedom as possible. Distribute the energy of the system."

Schmid continued: "The large number of possible arrangements of carbon arms in the pores ensures that the open-pore structure of the MOF is entropy-stable. This facilitates the phase transition from open-pore structure to closed-pore structure and then back, rather than without carbon. As in the case of the arms, the holes break when they are squeezed together." In order to calculate such a large system of many atoms and find many possible arm-like structures in the pores, the team developed an accurate, numerical value. The effective theoretical model is used to simulate.

The key result of this research is to determine another chemical option to control and modify the macro-response behavior of smart materials through thermodynamic factors. Gregor Kieslich concluded: "Our discovery opens up a new way to achieve structural phase transitions in porous MOFs."

Link to this article： Metal organic frameworks may become more flexible

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