Plasticity of Metal-Organic Framework Crystals: Thermally Activated Collapse of Nanopores

Unrecoverable deformation or plasticity can be generated in crystalline metal-organic frameworks (MOFs) by compressive loading with different rates in various applications. Herein, plastic behaviors of MOF HKUST-1 crystals are investigated by a series of in situ strain-rate-dependent compression tests. The yield strength is found to significantly increase with increasing strain rate, following a logarithmic dependence. Our reactive molecular dynamics simulations illustrate that the yielding of crystalline HKUST-1 is induced by the irreversible collapse of its nanopores, which can be accelerated by thermal activation at finite temperatures. Based on this mechanism together with the reaction rate theory, we derive an analytical expression relating the yield strength of MOFs and strain rate, which fits experimental findings well. Overall, this work can expand our current understanding of MOF plasticity, which is of importance for the mechanical shaping and various applications of MOF crystals.

Functions and applications of emerging metal-organic-framework liquids and glasses

Traditional metal-organic-frameworks (MOFs) have been extensively studied and applied in various fields across chemistry, biology and engineering in the past decades. Recently, a family of emerging MOF liquids and glasses have gained ever-growing research interests owing to their fascinating phase transitions and unique functions. To date, a growing number of MOF crystals have been found to be capable of transforming into liquid and glassy states under external stimuli, which overcomes the limitations of MOF crystals by introducing functional disorder in a controlled manner and offering some desirable properties. This review is dedicated to compiling recent advances in the fundamental understanding of the phase and structure evolution during crystal melting and glass formation in order to give insights into the underlying conversion mechanism. Benefiting from the disordered metal-ligand arrangement and free grain boundaries, various functional properties of liquid and glassy MOFs including porosity, ionic conductivity, and optical/mechanical properties are summarized and evaluated in detail, accompanied by the structure-property correlation. At the same time, their potential applications are further assessed from a developmental perspective according to their unique functions. Finally, we summarize the current progress in the development of liquid/glassy MOFs and point out the serious challenges as well as the potential solutions. This work provides perspectives on the functional applications of liquid/glassy MOFs and highlights the future research directions for the advancement of MOF liquids and glasses.

Advancing the Mechanical Performance of Glasses: Perspectives and Challenges

Glasses are materials that lack a crystalline microstructure and long-range atomic order. Instead, they feature heterogeneity and disorder on superstructural scales, which have profound consequences for their elastic response, material strength, fracture toughness, and the characteristics of dynamic fracture. These structure-property relations present a rich field of study in fundamental glass physics and are also becoming increasingly important in the design of modern materials with improved mechanical performance. A first step in this direction involves glass-like materials that retain optical transparency and the haptics of classical glass products, while overcoming the limitations of brittleness. Among these, novel types of oxide glasses, hybrid glasses, phase-separated glasses, and bioinspired glass-polymer composites hold significant promise. Such materials are designed from the bottom-up, building on structure-property relations, modeling of stresses and strains at relevant length scales, and machine learning predictions. Their fabrication requires a more scientifically driven approach to materials design and processing, building on the physics of structural disorder and its consequences for structural rearrangements, defect initiation, and dynamic fracture in response to mechanical load. In this article, a perspective is provided on this highly interdisciplinary field of research in terms of its most recent challenges and opportunities.

Plasticity of Metal-Organic Framework Glasses

Metal-organic framework (MOF) glasses provide new perspectives on many material properties due to their unique chemical and structural nature. Their mechanical properties are of particular interest because glasses are inherently brittle, which limits their applications as structural materials. Here we perform strain-rate-dependent uniaxial micropillar compression experiments on a_gZIF-62, a_gZIF-UC-5, and a_gTIF-4, a series of MOF glasses with different substituting linker molecules, and find that these glasses show substantial plasticity, at least on the micrometer scale. At a quasi-static strain rate of 0.001 s^-1, the micropillars yielded at approximately 0.32 GPa and subsequently deformed plastically up to 35% strain, irrespective of the type of substituting linker. With increasing strain rate, the yield strength of a_gZIF-62 evolved with the strain-rate sensitivity m = 0.024 to reach a yield strength of 0.44 GPa at a strain rate of 510 s^-1. On the basis of this relatively low strain-rate sensitivity and the absence of serrated flow, we conclude that structural densification is the predominant mechanism that accommodates such extensive plasticity.

Mechanics, Ionics, and Optics of Metal-Organic Framework and Coordination Polymer Glasses

Melt and glassy states of coordination polymers (CPs)/metal-organic frameworks (MOFs) have gained attention as a new class of amorphous materials. Many bridging ligands such as azolate, nitrile, thiocyanide, thiolate, pyridine, sulfonate, and amide are available to construct crystals with melting temperatures in the range of 60-593 °C. Here, we discuss the mechanism of crystal melting, glass structures, and mechanical properties by considering both experimental and theoretical studies. High and exclusive H⁺ or Li⁺ conductivities in moldable CP glasses have been proven in the all-solid-state devices such as fuel cells or secondary batteries. Transparent glasses with wide composition and available dopants are also attractive for nonlinear optics, photoconductivity, emission, and light-harvesting. The ongoing challenge in the field is to develop the design principles of CP/MOF melts and glasses, corresponding functions of mass (ion, electron, photon, phonon, and so forth). transport and conversion, and the integration of devices with the use of their tunable mechanical properties.