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Optimal mechanical impact absorbers are reusable and exhibit high specific energy absorption. The forced intrusion of liquid water in hydrophobic nanoporous materials, such as zeolitic imidazolate frameworks (ZIFs), presents an attractive pathway to engineer such systems. However, to harness their full potential, it is crucial to understand the underlying water intrusion and extrusion mechanisms under realistic, high-rate deformation conditions. Here, we report a critical increase of the energy absorption capacity of confined water-ZIF systems at elevated strain rates. Starting from ZIF-8 as proof-of-concept, we demonstrate that this attractive rate dependence is generally applicable to cage-type ZIFs but disappears for channel-containing zeolites. Molecular simulations reveal that this phenomenon originates from the intrinsic nanosecond timescale needed for critical-sized water clusters to nucleate inside the nanocages, expediting water transport through the framework. Harnessing this fundamental understanding, design rules are formulated to construct effective, tailorable and reusable impact energy absorbers for challenging new applications.

ABSTRACT We demonstrated the fabrication of freestanding zeolitic imidazolate framework 7 (ZIF-7) nanofiber (NF) mats by means of one-step, scalable electrospinning. The formation of ZIF-7 nanoparticles embedded in polymer fibers was unambiguously pinpointed via X-ray diffraction, transmission electron microscopy, and adsorption studies. The NF mats exhibited excellent characteristics, with an average diameter of 245 nm, in the adsorption and desorption of carbon dioxide (CO2); this makes them attractive candidates for gas separation and other selective filtration applications. This excellent property of the ZIF-7 mats was explained by the gate-opening phenomenon of ZIF-7, which yielded a stepwise increase in the overall CO2 uptake capacity. The mechanical strength of the NF mats was also obtained via large-strain uniaxial tensile deformation, which enabled preliminary assessment of the mat's suitability for textiles and membranes in targeting separation and filtration applications with large-area permeability. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 43788. [ABSTRACT FROM AUTHOR]

The comprehensive elastic properties of Zeolitic Imidazolate Frameworks (ZIF-1 to ZIF-4) have been computed using density functional theory (DFT). We employed the periodic CRYSTAL14 code to calculate the single-crystal elastic coefficients (Cij) at the B3LYP level of theory. While the chemical compositions of ZIFs-1 to -4 are the same, each structure features a distinct network topology, crystal symmetry and porosity configuration, which translate into differential structure-function mechanical correlations. We elucidate the anisotropic mechanical response with respect to the directionally dependent Young's and shear moduli properties. Our theoretical results suggest that ZIF-3 adopting a dft topology has an extremely low shear resistance (Gmin = 0.1 GPa), which is also underpinning the flexible mechanism responsible for its negative Poisson's ratio (auxetic νmin = −0.43). Interestingly, we identified that ZIF-1, ZIF-2, and ZIF-4 could exhibit a nearly zero Poisson's ratio for certain crystal orientations, which is reminescent of a rare “cork-like” phenomenon where there is practically no lateral deformation corresponding to an applied axial strain. Furthermore, we determined the bulk moduli and linear compressibilities, alongside the averaged elastic properties of the ZIF polycrystals. [ABSTRACT FROM AUTHOR]

Mixed-matrix membranes (MMMs) containing nanoporous metal-organic frameworks (MOFs) represent a rapidly expanding class of next-generation membranes, targeting CO 2 capture, gas purification, and novel electrochemical technologies. In this work, we have performed an in-depth study to elucidate the basic mechanical properties underpinning the functional performance of the prototypical ZIF-8/Matrimid ® nanocomposite membranes. By adopting the colloidal solution mixing method, we have fabricated membranes with 0–30 wt% ZIF-8 nanoparticles, whose quasi-static, temperature- and time-dependent mechanical characteristics have been established by means of nanoindentation, dynamic mechanical analysis, and large-strain uniaxial tensile measurements. We show that the inclusion of ZIF-8 nanoparticles into Matrimid (a glassy polyimide) controls many important mechanical behaviour, ranging from elastic modulus, yield strength and hardness, to ductility (stretchability), fracture strength and toughness. We identified that annealing (180 °C), despite improving the gas permeability and selectivity of Matrimid-based membranes, could substantially degrade its ductility and fracture toughness, while stabilising small-strain viscoelastic response under dynamic loading. Our results suggest that an annealed <~10 wt% ZIF-8/Matrimid MMM would be ideal for practical separation applications, where an optimal combination of mechanical resilience in conjunction with excellent gas permselectivity are the prerequisites. [ABSTRACT FROM AUTHOR]

The I2-sorption and -retention properties of several existing zeolitic imidazolate frameworks (ZIF-4, -8, -69) and a novel framework, ZIF-mnIm ([Zn(mnIm)2]; mnIm=4-methyl-5-nitroimidazolate), have been characterised using microanalysis, thermogravimetric analysis and X-ray diffraction. The topologically identical ZIF-8 ([Zn(mIm)2]; mIm=2-methylimidazolate) and ZIF-mnIm display similar sorption abilities, though strikingly different guest-retention behaviour upon heating. We discover that this guest retention is greatly enhanced upon facile amorphisation by ball milling, particularly in the case of ZIF-mnIm, for which I2 loss is retarded by as much as 200 °C. It is anticipated that this general approach should be applicable to the wide range of available metal-organic framework-type materials for the permanent storage of harmful guest species. [ABSTRACT FROM AUTHOR]

The deposition of sodalite zeolitic imidazolate framework-7 (ZIF7) films by a supersonic cold-spraying technique was successfully accomplished for the first time. The high-speed impact of supersonic cold spraying increased the monodispersity of the ZIF7 crystalline structure. However, the intensity of the structural change decreased with increasing the amount of nylon in the ZIF7 suspension. Mitigating phase changes in ZIF7 occurred by the impact dampening conferred by the polymeric nature of nylon, which preserved the original three-dimensional crystalline structure of ZIF7. The inclusion of N , N -dimethylformamide (DMF) in a nylon–ZIF7 suspension improved the dispersion of ZIF7 nanoparticles, which in turn eliminated the dampening effect from the nylon and recovered the distinctive monodispersity arising from the high-speed impact. This characteristic was observed at all impact speeds for ZIF7 suspensions containing DMF. We show that high-rate cold spraying of ZIF7 particles can be combined with nylon and other pressure-transmitting media to control the formation of three distinctive phases. The unique capability to tune the crystalline structure of ZIF7 allows customization of the film functionality for specific applications. [ABSTRACT FROM AUTHOR]

We describe the first use of high-rate supersonic spray coating to deposit thin films of ZIF-8, a zeolitic metal–organic framework (MOF), adopting a sodalite architecture. This cold-spray technique is versatile and scalable, with tunable processing parameters capable of generating either a textured crystalline film or a randomly oriented polycrystalline coating on both metallic and non-metallic substrates. We provide evidence that guest occupancy by organic solvents (dimethylformamide, dimethylacetamide, and dimethylsulfoxide) in the sodalite cage of ZIF-8 structurally stabilizes the framework against high-velocity impact, resulting in the preferred orientations observed. Moreover, we show that amorphous ZIF-8 films can be straightforwardly obtained at high air pressure exceeding 7 bars in which the particle velocity is ∼500 m/s. It is anticipated that this high-throughput approach can be adapted to fabricate microstructurally compact and strongly adhered ZIF-8 films. [ABSTRACT FROM AUTHOR]