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In this work, ZIF-8 microcrystals were synthesized according to previous reports and drop- casted on gold-coated Pyrex glass (Fig. 8.2 and 8.3).19 We have observed the morphological changes and fragmentation of ZIF-8 microcrystals after low shock loading pressure, and the amorphization and structural collapse after high shock loading pressure. Furthermore, we determined that flyer plate shock compression is much more devastating compared to static and quasistatic compression with comparable loading pressure but with a much different exposure time (nanoseconds vs. minutes).

The optical microscopic images taken before and after flyer plate shock experiments show the formation of well-defined points of impact (Fig. 8.4 and 8.2). From the experimentally measured impact velocity, we can calculate the effective maximum compression pressure using the Rankine-Hugoniot equation; for example, with a flyer plate velocity of 0.8 km/s the resulting compression pressure is ≈2.5 GPa (Fig. 8.9). At 0.8 km s-1, the coated thin gold layer is not destroyed and the glass substrate keeps its integrity; at higher velocities (over 1.3 km s-1), however, the gold layer and glass substrate are damaged and an obvious crater is created at the site of flyer plate impact.

Scanning electron microscopy (SEM) was used to examine the effects of these shock impacts on the ZIF-8 layer and indicate that an irreversible morphological change was induced by the flyer plate shock compression (Fig. 8.4e-h). The initial ZIF-8 crystals have rhombic dodecahedral morphology with average size around 1.2 µm. After dynamic compression, the ZIF-8 crystals were crushed and fragmented into small fragments at a flyer plate velocity of 0.8 km/s, and subsequently agglomerated at higher velocities up to 1.6 km s-1. These morphological changes in the ZIF-8 induced by the flyer plate shock compression are irreversible.

To better understand these morphological changes, we examined the powder X-ray diffraction (PXRD) of the ZIF-8 samples after shock impact. After shock compression at 0.8 km s- 1

, the same major diffraction peaks as the initial ZIF-8 crystals are observed, but with significant broadening and loss of intensity (Fig. 8.6). This suggests that the long term order of ZIF-8 structure was mostly maintained, but broadened peaks indicate that both the ZIF-8 crystal particle size and crystallinity were substantially decreased, which is consistent with SEM images.

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If we compare the PXRD spectra of the flyer plate shocked ZIF-8 sample (at 0.8 km s-1, i.e. ≈2.5 GPa) to a sample exposed to 2 GPa static compression generated by a hydraulic piston pelletizer,20 much more damage and much greater amorphization has been observed after static compression (after static compression, the PXRD shows nearly complete amorphization with only one highly broadened and weakened peak at 6.6o corresponding to Zn–2-MeIm–Zn unit20). The lessened effect of dynamic compression may be the result of the short time duration of the shock compared to the hydraulic piston (nanoseconds vs. minutes). Complete amorphization is still achievable by dynamic compression, however, at higher impact velocities. For example, after dynamic compression at a flyer plate velocity of 1.3 km s-1 (≈5 GPa), the diffraction pattern lacked any sharp features (Fig. 8.6c), demonstrating complete amorphization of the ZIF-8 crystals.

The IR spectra offer more information about structural changes in ZIF-8 after shock compression.28,29 The spectrum of initially desolvated ZIF-8 is consistent with results previously reported in the literature (Fig. 8.7a). The major IR absorption bands (1800 – 650 cm-1) are associated with the vibrations of ligand’s methyl group and the imidazole ring.28,30 The absorption band at 1580 cm-1 have been ascribed to the conjugated double bond (C=N or C=C) ring stretching, and the complicated bands in the range of 1350-1500 cm-1 are assigned to other ring stretches. The bands between the region of 900-1350 cm-1 and below 800 cm-1 have been associated to the in-plane and out-of-plane bending of the imidazole rings, respectively.

After shock compression at low flyer plate velocity (0.8 km s-1, Fig. 8.7b), the FT-IR spectrum shows only minor differences compared to the initial ZIF-8 spectrum, which suggests that the ZIF-8 structure was maintained after lesser shock compression, consistent with the PXRD (Fig. 8.6b). With the increased flyer plate velocity (1.3 km s-1, Fig. 8.7c), broadened peaks are observed which indicate that site-to-site variation among the 2-methylimidazolates in the solid have been generated, presumably due to the distortion of local structure and breakage of long term order. Even so, at the 1.3 km s-1 flyer plate velocity, ZIF-8 maintained most of its local chemical coordination environment, even when its long term order had been completely lost (as indicated by the PXRD, Fig. 8.6c).

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After the highest velocity flyer plate impact (1.6 km s-1, Fig. 8.7d), significant changes are observed, including the appearance of several new absorption bands (1608, 1510, 1257, 1040 and 830 cm-1), some of which may represent splitting of bands from unshocked ZIF-8 (Fig. 8.7d). ZIF-8 crystallizes in the highest symmetry space group (cubic, I-43m); the PXRD shows complete amorphization of ZIF-8 after strong shock compression (Fig 8.6c). The onset of new peaks in the IR indicates that the chemical environment of the ligands has changed and the higher symmetry of ZIF-8 has been at least partially lost. For example, the split peak around 1600 cm-1 (peak i, Fig. 8.7d) suggests a lowered symmetry of the vibrational modes of imidazole rings conjugated double bonds; as another example, the new band at 1510 cm-1 (peak ii, Fig. 8.7d) reflects a lowered symmetry and coupling of the imidazole ring stretching modes coupled to ring-CH and CH3 deformations.31 The new band at 1257 cm-1 (peak iii, Fig. 8.7d) is consistent with the ring- methyl stretch not normally seen in high symmetry 2-MeIm ligation, but is observed with lower symmetry 4-methyimidazolate ligation, which is again consistent with shock compression breakage of the high symmetry structure of ZIF-8.31 In addition, the other two new bands at 1040 and 830 cm-1 (peaks iv and v, Fig. 8.7d) are associated with lowered symmetry of imidazole rings.31

One may also make an interesting comparison of the IR spectra of highly symmetric bridging 2-methylimidazolate ligands vs. lowered symmetry environments. The FT-IR spectra of uncompressed ZIF-8 and one-dimensional Ag (I)-2-MeIm coordination polymer (Fig. 8b vs. 8c) are extremely similar.32 In contrast, the increased complexity of the shock-compressed ZIF-8 IR spectrum compares to the lowered symmetry of the non-ligated 2-methylimidazole (2-MeImH), as seen in Fig. 8.8a vs. 8.8d.

To compare the effects of static vs. dynamic compression, a diamond anvil cell (DAC) was utilized to generate ≈7 GPa static compression on ZIF-8 crystals (Fig. 8.9). Under the static compression, all the absorbance bands are blue-shifted around 30 cm-1 and pressure- broadened (Fig. 8.9). Upon release of the static compression, the IR spectrum reverts mostly back to the initial ZIF-8 spectrum (Fig. 8.9), which suggests little permanent structural change occurs under these static compression conditions at 7 GPa. This contrasts with the effects of shock compressed ZIF-8 with a flyer plate velocity of 1.6 km s-1 that corresponds to a maximum