The “void” (empty space) component of porous soft materials is fundamental to their properties and applications, and has been the focus of study for many years. Now, researchers from Japan have solved a long-standing puzzle of these materials that will facilitate research and development, and improve their use in practical devices.
In a study recently published in Proceedings of the National Academy of Sciencesresearchers from the Institute of Industrial Science, The University of Tokyo revealed the importance of a physical property—elastic heterogeneity—in tuning the molecular adsorption/desorption properties of a common class of soft porous materials: metal–organic frameworks (MOFs).
MOFs, a research focus since the 1990s, are sponge-like materials. They are highly tunable, meaning that their size, shape, and composition can be changed by changing the metal ions and organic linkers used to build them. This tuning can ultimately change their properties, such as mechanical flexibility. In addition, upon adsorbing molecules (known as guests), the crystal structure of the MOF (known as host) can undergo non-uniform shape changes known as elastic heterogeneity.
The resulting stimuli-responsiveness and guest adsorption selectivity of MOFs have led to many applications, such as sensors, supercapacitors, and drug delivery tools.
To improve the design of advanced MOFs, researchers have long sought to understand the link between the macroscopic property of elasticity to the corresponding microscopic host-guest interactions. Gaining that understanding through computational simulation is the problem the researchers are looking to solve.
“Our simplified statistical mechanical model reveals the role of guest adsorption and desorption in elastic heterogeneity,” explained Kota Mitsumoto, lead author of the study. “We mathematically link lattice expansion and contraction to the energetics and thermodynamics of host-guest interactions.”
The simulations focused on two types of domains, which are compositional regions within MOFs: guest-adsorbed, which is harder; and guest-desorbed, which is softer. The main result of the researchers is that the shape of these domains depends on the difference in elastic stiffness between the domains. Guest adsorption corresponds to compact domains, while guest desorption corresponds to flattened domains.
“We define the entropic and energetic contributions to the transition between guest adsorption and desorption,” said Kyohei Takae, senior author. “Thus, we provide physicochemical insight into the origin of elastic heterogeneity within MOFs and similar materials.”
This work has applications in providing targeted properties to soft porous materials. For example, compact domains facilitate the energy confinement of visitors, and thus applications such as gas storage. On the other hand, flat domains can increase the surface area of MOFs, and thus accelerate chemical reactions. Sensors and many other materials that respond to stimuli can benefit from these insights.
More information:
Kota Mitsumoto et al, Elastic heterogeneity governs asymmetric adsorption-desorption in a soft porous crystal, Proceedings of the National Academy of Sciences (2023). DOI: 10.1073/pnas.2302561120
Provided by the University of Tokyo
Citation: Insights into the design of advanced stimuli-responsive porous materials (2023, July 21) retrieved 21 July 2023 from https://phys.org/news/2023-07-insights-advanced-stimuli-responsive-porous-materials.html
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