Throwing shapes: Diffracting on memory effect

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  • Published: May 15, 2018
  • Channels: X-ray Spectrometry
thumbnail image: Throwing shapes: Diffracting on memory effect

Porous shape-shifter

Researchers documented how a porous material can change and retain its shape, even after absorbing and releasing carbon dioxide. Here, the crystal's pores remain open after releasing carbon dioxide, but can be collapsed when heated. CREDIT: KYOTO UNIVERSITY ICEMS

Shape-shifting, porous materials are one step closer thanks to work carried out at Kyoto University, Japan. X-ray crystallography studies provide the details of the function known as shape-memory effect in these substances.

There is huge potential in technology for shape-memory materials. They could be used in microelectromechanical systems (MEMS) and other devices as actuators, valves and artificial muscles as well as body implants for tissue engineering and bone regeneration in the wake of injury or disease. Indeed, many such materials are already being used in some situations. However, the most well documented of these shape-memory effect materials is among ceramics and metal alloys. It is not so common nor well understood in the rarer crystalline porous solids.

Organic adsorber

Now, Susumu Kitagawa of Kyoto University's Institute for Integrated Cell-Material Sciences and colleagues in Japan, and colleagues there and in Ireland and the USA have demonstrated a shape-memory effect in a flexible metal organic material. This is only the second example of such a material reported. They offer details of this innovative substance in a recent issue of the journal Science Advances.

The team prepared crystals from a solution of zinc nitrate hexahydrate in the organic solvent dimethylformamide. Heating at 120 degrees Celsius for 24 hours produced a solid that single-crystal X-ray diffraction revealed to comprise of a slightly distorted paddlewheel-patterned lattice. The zinc ions are central zinc to the unit and surround by organic moieties. This 'alpha phase' of the crystal had 46% porosity. In other words, it might accommodate small molecules in its pores to fill almost half of its internal volume. One such small molecule might be carbon dioxide.

Phase to phase

When the team subsequently heated their alpha crystal to 130 degrees Celsius under vacuum conditions for 12 hours, they found that it would increase in density. The lattice undergoes significant distortion and the porosity plummets to a mere 15% of the volume. This crystalline phase, the team refers to as the beta form. This is the point at which carbon dioxide once again enters the story. The team added carbon dioxide to the crystal at a temperature of -78 degrees Celsius. The carbon dioxide is adsorbed into the pores of the crystal and this process causes the crystal to change shape to a less-distorted form than that observed in the beta phase. While the porosity does not recover the percentage seen in the alpha phase, it does achieve an increase to 34%. The team found that they could cycle between adsorption and desorption over ten consecutive cycles without further distortion and so they have dubbed this the gamma phase, or more specifically, the 'shape-memory' gamma phase.

The team also tested the impact of adding nitrogen or carbon monoxide at different temperatures and was also able to induce the same transformation of the crystal from its beta to its gamma phase. They could also revert the gamma phase to the beta phase by heating the material at 130 degrees Celsius under vacuum conditions for just two hours. Moreover, converting it to the original alpha phase was possible by soaking the crystals in more dimethylformamide for a mere five minutes.

Related Links

Sci Adv 2018, online: "Readily accessible shape-memory effect in a porous interpenetrated coordination network"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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