Effects of Free Volume on Shock-Wave Energy Absorption in A Metal-Organic Framework: A Molecular Dynamics Investigation

We use an equilibrium molecular dynamics approach for shock simulations to investigate the effects of pore volume on shock-wave energy absorption in zeolitic imidazolate framework ZIF-8. The free volume fraction was varied by introducing varying numbers and various types of alcohol molecules inside ZIF-8 nanopores. The amount of shock energy absorbed is characterized according to the flyer plate experiments. Our simulations reveal molecular mechanism for shock attenuation in ZIF-8 involving the volume-collapse transition. Under shock compression, the metal- organic framework exhibits an ability to greatly reduce its volume while the internal energy increase compensated by flexible angle distortion is relatively moderate; consequently, the transmitted pressure as well as shock energy is alleviated. With the reduction of free volume fraction by introducing more guest molecules, the amount of energy absorption decreases linearly. In addition, this results in higher pressure in the elastic precursor, and a faster propagation speed of the high-energy pore-collapse wave. Various types of alcohols occupying the same volume fraction in ZIF-8 exhibit a negligible difference in shock energy, which indicates the major role of the available free volume. Our simulation study provides a molecular-level understanding of the key characteristics for the improvement of shock-protective materials.