A Porous Material That Remembers Its Shape

Scientists in Japan have created a porous crystal that can change and retain its shape depending on the prevailing conditions.

AsianScientist (May 10, 2018) – Scientists at Kyoto University, Japan have designed a porous material that can change and retain its shapes—a function known as shape-memory effect. Their findings are published in Science Advances.

Shape-memory materials have applications in many fields. For example, they could be implanted in the body and then induced to change shape for a specific function, such as serving as the scaffold for bone tissue regeneration. The shape-memory effect is well documented in some materials, including ceramics and metal alloys. However, it is rare and poorly understood in crystalline porous materials.

In this study, researchers led by Professor Susumu Kitagawa of Kyoto University’s Institute for Integrated Cell-Material Sciences in Japan have demonstrated the shape-memory effect in a flexible metal organic material. The researchers first made crystals by dissolving a mixture of chemicals and zinc nitrate hexahydrate in a common solvent called dimethylformamide at 120°C for 24 hours.

Using an X-ray technique called single-crystal X-ray diffraction, the team studied the crystals’ structure. They found that the crystals consisted of slightly distorted paddlewheel-shaped lattices, which were made of central zinc ions linked to surrounding organic molecules. This ‘alpha phase’ of the crystal had 46 percent porosity, meaning that 46 percent of its volume was available for accepting new molecules.

When the team heated the alpha crystal at 130°C in a vacuum for 12 hours, the crystal became denser and its lattices became more distorted. They called this the beta phase of the crystal. The researchers then added carbon dioxide to the crystal at a temperature of -78°C. Carbon dioxide was adsorbed into the crystal’s pores, changing their shape and increasing the volume available for accepting guest molecules by 34 percent.

When the team added and removed carbon dioxide from the crystal over ten consecutive cycles, they found that it retained its shape. They called this phase of the crystal its shape-memory gamma phase. Adding nitrogen or carbon monoxide under varying temperatures also induced the transformation of the crystal from its beta to its gamma phase.

The team was able to revert the crystal’s gamma phase back to its beta phase by heating it at 130°C in a vacuum for two hours. To revert to the alpha phase, the gamma phase of the crystal was soaked in dimethylformamide for five minutes.

The team’s analyses of the crystal allowed them to have a better understanding of how its function changes along with structure. The researchers noted that their work could provide the basis for designing more shape-memory porous materials.

The article can be found at: Shivanna et al. (2018) Readily Accessible Shape-memory Effect in a Porous Interpenetrated Coordination Network.


Source: Kyoto University; Photo: Pixabay.
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