Metal-Organic Frameworks: When Crumpling Under Stress is a Good Thing

Header reprinted (adapted) with permission from Pixabay.

Title: Shock Wave Energy Absorption in Metal–Organic Framework
Authors: Xuan Zhou, Yu-Run Miao, William L. Shaw, Kenneth S. Suslick, and Dana D. Dlott
Publication Info: Journal of the American Chemical Society, 2019, Article ASAP DOI: 10.1021/jacs.8b12905

What happens if an airplane flies through a hailstorm? Or a construction worker is hit by a piece of falling debris? An airplane wing or a helmet could crumple under a large impact – or it could absorb the energy of the impact and dissipate it safely. Shock-absorbing materials can save lives. That’s why researchers are constantly searching for new materials for everything from body armor to airplane wings.

Metal-organic frameworks, or MOFs, are one type of material that might offer better protection in the future for those with dangerous jobs. Researchers have now demonstrated that the MOF studied in this paper can absorb 3 – 7 times more energy than a polymer material of a similar weight, PMMA – the industry standard for shock absorption.

Just as the name implies, MOFs are composed of lace-like structures of metal atoms connected by chains of carbon and other “organic” atoms (Figure 1). The MOF studied in this paper is called ZIF-8; it contains zinc, carbon, and nitrogen.

However, what makes a MOF special is not the atoms it contains, but what it doesn’t contain. Like a piece of swiss cheese, MOFs have many large, empty spaces, called pores (Figure 1). Pores are created by the specific bonding patterns between the metal atoms and the connecting chains of carbon: different connecting chains can make the pores larger or smaller, allowing researchers to change the properties of the MOF.

Figure 1. Left: Diagram of a MOF framework showing the pores (empty spaces) created by the metal-organic
framework (blue pyramids and yellow sphere). Right: MOF framework; blue dots = Zn atoms, green = C, black = N. Reprinted (adapted) with permission from Zhou, X. et al. J. Am. Chem. Soc., Article ASAP 2019. Copyright (2019) American Chemical Society.

These pores are also essential to the MOF’s ability to absorb impacts. Researchers identified three reasons why this MOF, ZIF-8, performs so much better than the polymer standard, PMMA.

First, the MOF studied here was in powder form. Like punching a sandbag, when the powder was subjected to an impact, it compressed. The MOF itself was not destroyed, but some of the energy of the impact was dissipated when the powder compressed.

Figure 2. The apparatus used to measure the effect of an impact on the MOF. An aluminum plate is launched by a laser pulse at different impact speeds. Below the plate, a powder of the MOF is spread on top of a microscope. The energy of the shock enters the microscope lens, which vibrates with a certain frequency that can be measured using the fiber optic and converted to determine the energy of the impact.

The second reason the MOF performed so well is due to the collapse of its unique pore structure. When the impact occurs, the tiny voids in each MOF crystal can absorb the energy, collapsing the MOF and dissipating the energy of the impact. The researchers were able to confirm this by looking at Raman spectra (which looks at vibrational frequencies of bonds in a molecule) of the MOF before and after impact (Figure 3). Broadening of certain transitions (corresponding to different chemical bonds in the MOF) told the researchers that the structure of the MOF was changing, probably due to pore collapse.

Examination of the Raman spectra post-impact also revealed that while most of the MOF bonds remained intact, some metal-organic bonds were broken (indicated by a disappearing transition in the spectrum). This breaking of certain bonds in the MOF is the third method of energy dissipation in the material. Bond breaking only occurred at very high impact strengths, and while the researchers didn’t test the samples multiple times, it implies that the material would be fundamentally damaged after a very strong impact.

Figure 3. Left: the MOF structure collapses (from bottom to top) with higher and higher impact strengths. Right: Raman spectra show the disappearance of peaks associated with chemical bonds as impact strength increases. Reprinted (adapted) with permission from Zhou, X. et al. J. Am. Chem. Soc., Article ASAP 2019. Copyright (2019) American Chemical Society.

The multiple methods of impact dissipation discovered by the researchers highlight the importance of carefully studying new shock-absorbing materials. By understanding the properties of these materials, we can design even more efficient kinds of protective equipment. Novel materials like MOFs could one day show up in everything from body armor to your bicycle helmet.


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