Enhanced mechanical properties of a metal-organic framework by polymer insertion

Tuning the mechanical properties of sol-gel monolithic metal-organic frameworks by ligand engineering

The sol-gel monolithic MOFs has come to prominent attention for industrial application owing to the higher powder packing density, enhanced processabilities and mechanical stabilities compared to the powder counterpart. The mechanical properties are particularly important during machine shaping processing because of porous framework structure. We used ligand engineering to design and synthesize monoUiO-66-type materials modified different chemical functional groups (-NH2, -2OH, -2COOH) by sol-gel method, with the aim to assess the impact of different functional groups on the mechanical properties of these monolithic materials based on nanoindentation technology. We observe larger mass and sterically bulky functional groups (-2COOH) can significantly decrease the BET areas and pore volume of monoUiO-66 through N2 adsorption isotherms at 77 K. Hence, the two -COOH groups modified monoUiO-66 tends to exhibit the higher H of 0.589 ± 0.018 GPa and E of 15.471 ± 0.250 GPa compared with monoUiO-66 modified with -NH2 (0.334 ± 0.009 GPa/11.959 ± 0.243 GPa) and -2OH (0.331 ± 0.008 GPa/10.251 ± 0.142 GPa) groups. The creep indentation tests and the jump indentation tests further demonstrate the modification by larger functional groups -COOH on monoUiO-66 could resist irreversible plastic deformation. Furthermore, the monoUiO-66-(COOH)2 has significantly smaller the activation volume of 0.34 ∼ 0.43 nm3, highlighting the introduction of -COOH groups reduced the pore volume and restrict the number of pores involved in one collapse event. Our results demonstrate the larger mass and sterically bulky functional groups have significant influence on the mechanical properties of the monoMOFs materials.

Metal Organic Frameworks: Current State and Analysis of Their Use as Modifiers of the Vulcanization Process and Properties of Rubber

The interest in and application of metal organic frameworks (MOF) is increasing every year. These substances are widely used in many places, including the separation and storage of gases and energy, catalysis, electrochemistry, optoelectronics, and medicine. Their use in polymer technology is also increasing, focusing mainly on the synthesis of MOF-polymer hybrid compounds. Due to the presence of metal ions in their structure, they can also serve as a component of the crosslinking system used for curing elastomers. This article presents the possibility of using zeolitic imidazolate framework ZIF-8 or MOF-5 as activators for sulfur vulcanization of styrene-butadiene rubber (SBR), replacing zinc oxide in conventional (CV) or effective (EF) curing systems to different extents. Their participation in the curing process and influence on the crosslinking density and structure, as well as the mechanical and thermal properties of the rubber vulcanizates, were examined.

Versatilely Manipulating the Mechanical Properties of Polymer Nanocomposites by Incorporating Porous Fillers: A Molecular Dynamics Simulation Study

Polymer nanocomposites (PNCs) have been attracting myriad scientific and technological attention due to their promising mechanical and functional properties. However, there remains a need for an efficient method that can further strengthen the mechanical performance of PNCs. Here, we propose a strategy to design and fabricate novel PNCs by incorporating porous fillers (PFs) such as metal-organic frameworks with ultrahigh specific surface areas and tunable nanospaces to polymer matrices via coarse-grained molecular dynamics simulations. Three important parameters─the polymer chain stiffness (k), the interaction strength between the PF center and the end functional groups of polymer chains (ε_center end), and the PF weight fraction (w)─are systematically examined. First, attributed to the penetration of polymer chains into PFs at a strong ε_center end, the dimension of polymer chains such as the radius of gyration and the end-to-end distance increases greatly as a function of k compared to the case of the neat polymer system. The penetration of polymer chains is validated by characterizing the radial distribution function between end functional groups and filler centers, as well as the visualization of the snapshots. Also, the dispersion state of PFs tends to be good because of the chain penetration. Then, the glass transition temperature ratio of PNCs to that of the neat systems exhibits a maximum in the case of k = 5ε, indicating that the strongest interlocking between polymer chains and PFs occurs at intermediate chain stiffness. The polymer chain dynamics of PNCs decreases to a plateau at k = 5ε and then becomes stable, and the relative mobility to that of the neat system as well presents the same variation trend. Furthermore, the mechanical property under uniaxial deformation is thoroughly studied, and intermediates k, ε_center end, and w can bring about the best mechanical property. This is because of the robust penetration and interaction, which is confirmed by calculating the stress of every component of PNCs with and without end functional groups and PF centers as well as the nonbonded interaction energy change between different components. Finally, the optimal condition (k = 5.36ε, ε_center end = 5.29ε, and w = 6.54%) to design the PNC with superior mechanical behavior is predicted by Gaussian process regression, an active machine learning (ML) method. Overall, incorporating PFs greatly enhances the entanglements and interactions between polymer chains and nanofillers and brings effective mechanical reinforcements with lower filler weight fractions. We anticipate that this will provide new routes to the design of mechanically reinforced PNCs.

Toughening and stabilizing MOF crystals via polymeric guest inclusion

Improving the mechanical properties and stability of metal-organic frameworks (MOFs) is of significant interest due to their practical applications. Herein, we tune the mechanical properties of MOFs by filling the MOF pores with polymer chains. The mechanical properties reflect the filling rate, molecular weight, and inherent flexibility of the polymeric guests, imparting MOFs with improved resilience and toughness against mechanical pressures.

Synthesis and Mechanical Properties of sub 5-µm PolyUiO-66 Thin Films on Gold Surfaces

Metal-organic framework (MOF) thin films currently lack the mechanical stability needed for electronic device applications. Polymer-based metal-organic frameworks (polyMOFs) have been suggested to provide mechanical advantages over MOFs, however, the mechanical properties of polyMOFs have not yet been characterized. In this work, we developed a method to synthesize continuous sub-5 µm polyUiO-66(Zr) films on Au substrates, which allowed us to undertake initial mechanical property investigations.  Comparisons between polyUiO-66 and UiO-66 thin films determined polyUiO-66 thin films exhibit a lower modulus but similar hardness to UiO-66 thin films. The initial mechanical characterization indicates that further development is needed to leverage the mechanical property advantages of polyMOFs over MOFs. Additionally, the demonstration in this work of a continuous surface-supported polyUiO-66 thin film also enables utilization of the emerging class of polyMOF materials in sensors and devices applications.

Probing time dependent phase transformation in a flexible metal-organic framework with nanoindentation

Phase transformation in a flexible metal-organic framework, {[Zn₄(1,4-NDC)₄(1,2-BPE)₂]·xSolvent}_n, which loses guest molecules rapidly at room temperature, leading to several phase transitions, is examined using the nanoindentation technique. Nanoindentation results revealed that the time dependent transformation of an open to a closed phase happens gradually, through multiple intermediate phases, with the mechanical properties (elastic modulus and hardness) increasing as the transformation progresses from an open to a closed phase.

Application of Metal-Organic Framework-Based Composites for Gas Sensing and Effects of Synthesis Strategies on Gas-Sensitive Performance

Gas sensing materials, such as semiconducting metal oxides (SMOx), carbon-based materials, and polymers have been studied in recent years. Among of them, SMOx-based gas sensors have higher operating temperatures; sensors crafted from carbon-based materials have poor selectivity for gases and longer response times; and polymer gas sensors have poor stability and selectivity, so it is necessary to develop high-performance gas sensors. As a porous material constructed from inorganic nodes and multidentate organic bridging linkers, the metal-organic framework (MOF) shows viable applications in gas sensors due to its inherent large specific surface area and high porosity. Thus, compounding sensor materials with MOFs can create a synergistic effect. Many studies have been conducted on composite MOFs with three materials to control the synergistic effects to improve gas sensing performance. Therefore, this review summarizes the application of MOFs in sensor materials and emphasizes the synthesis progress of MOF composites. The challenges and development prospects of MOF-based composites are also discussed.

Advances in Post-Combustion CO2 Capture by Physical Adsorption: From Materials Innovation to Separation Practice

The atmospheric CO₂ concentration continues a rapid increase to its current record high value of 416 ppm for the time being. It calls for advanced CO₂ capture technologies. One of the attractive technologies is physical adsorption-based separation, which shows easy regeneration and high cycle stability, and thus reduced energy penalties and cost. The extensive research on this topic is evidenced by the growing body of scientific and technical literature. The progress spans from the innovation of novel porous adsorbents to practical separation practices. Major CO₂ capture materials include the most widely used industrially relevant porous carbons, zeolites, activated alumina, mesoporous silica, and the newly emerging metal-organic frameworks (MOFs) and covalent-organic framework (COFs). The key intrinsic properties such as pore structure, surface chemistry, preferable adsorption sites, and other structural features that would affect CO₂ capture capacity, selectivity, and recyclability are first discussed. The industrial relevant variables such as particle size of adsorbents, the mechanical strength, adsorption heat management, and other technological advances are equally important, even more crucial when scaling up from bench and pilot-scale to demonstration and commercial scale. Therefore, we aim to bring a full picture of the adsorption-based CO₂ separation technologies, from adsorbent design, intrinsic property evaluation to performance assessment not only under ideal equilibrium conditions but also in realistic pressure swing adsorption processes.

Controlling the alignment of 1D nanochannel arrays in oriented metal-organic framework films for host-guest materials design

Controlling the direction of molecular-scale pores enables the accommodation of guest molecular-scale species with alignment in the desired direction, allowing for the development of high-performance mechanical, thermal, electronic, photonic and biomedical organic devices (host-guest approach). Regularly ordered 1D nanochannels of metal-organic frameworks (MOFs) have been demonstrated as superior hosts for aligning functional molecules and polymers. However, controlling the orientation of MOF films with 1D nanochannels at commercially relevant scales remains a significant challenge. Here, we report the fabrication of macroscopically oriented films of Cu-based pillar-layered MOFs having regularly ordered 1D nanochannels. The direction of 1D nanochannels is controllable by optimizing the crystal growth process; 1D nanochannels align either perpendicular or parallel to substrates, offering molecular-scale pore arrays for a macroscopic alignment of functional guest molecules in the desired direction. Due to the fundamental interest and widespread technological importance of controlling the alignment of functional molecules and polymers in a particular direction, orientation-controllable MOF films will open up the possibility of realising the potential of MOFs in advanced technologies.

Mitigation of Pressure-Induced Amorphization in Metal-Organic Framework ZIF-8 upon EPR Control

Pressure-induced amorphization is one of the processes inhibiting functional properties of metal-organic frameworks (MOFs). Such amorphization often occurs when MOFs are being shaped for practical applications, as well as during certain exploitations. Typically, the porosity of MOFs, which is crucial for sorption, separation, and catalysis, suffers under external pressure. We report a new experimental approach for efficient monitoring of pressure-induced processes in MOFs that employs trace amounts of spin probes (stable nitroxide radicals) embedded in the pores of MOF and detection by electron paramagnetic resonance (EPR). EPR spectra of spin probes in MOF ZIF-8 demonstrate significant changes upon pressure-induced amorphization, whose extent can be quantitatively determined from the spectral shapes. Moreover, stabilization of ZIF-8 against amorphization via reversible adsorption of various guests was studied using this approach. Mitigation effect depends on diffusion parameters and localization of guest molecules in the cavity, and maintaining of the structure and permeability up to 80% was achieved even at 1.15 GPa applied. Therefore, the proposed methodology allows significant mitigation of MOF amorphization under external pressure and conveys further perspectives of the controlled adjustment of stabilizing agents for various MOFs and their applications.

Construction of a polyMOF using a polymer ligand bearing the benzenedicarboxylic acid moiety in the side chain

Here we report a new synthetic strategy of a polyMOF consisting of a side chain ligand polymer. The polyMOF was consisting of a crystalline MOF-like structure in the polymer despite its film form.

A new supramolecular catalytic system: the self-assembly of Rh8 cage host anthracene molecules for [4 + 4] cycloaddition induced by UV irradiation

The supramolecular assembly is significant in host–guest chemistry. In this work, a new supramolecular system assembled through a distorted cuboid was introduced. Moreover, the [4 + 4] cycloaddition reaction of the guest molecules was further studied under UV light.

Controlling the Packing of Metal-Organic Layers by Inclusion of Polymer Guests

The preparation of metal-organic structures with a controlled degree of disorder is currently one of the most promising fields of materials science. Here, we describe the effect of guest polymer chains on the transformation of a metal-organic framework (MOF). Heating a pillared MOF at a controlled temperature resulted in the exclusive removal of the pillar ligands, while the connectivity of the metal-organic square-grid layers was maintained. In the absence of a polymer, 2D-layers rearranged to form a new crystalline phase. In contrast, the presence of a polymer in the MOF inhibited totally the recrystallization, leading to a turbostratic phase with layers threaded and maintained apart by the polymer chains. This work demonstrates a new synthetic approach toward the preparation of anisotropic metal-organic materials with controlled disorder. It also reveals how guests can dramatically modify the conversion of host MOFs, even though no chemical reaction occurs between them.

Metal-Organic Frameworks in Polymer Science: Polymerization Catalysis, Polymerization Environment, and Hybrid Materials

The development of metal-organic frameworks (MOFs) has had a significant impact on various fields of chemistry and materials science. Naturally, polymer science also exploited this novel type of material for various purposes, which is due to the defined porosity, high surface area, and catalytic activity of MOFs. The present review covers various topics of MOF/polymer research beginning with MOF-based polymerization catalysis. Furthermore, polymerization inside MOF pores as well as polymerization of MOF ligands is described, which have a significant effect on polymer structures. Finally, MOF/polymer hybrid and composite materials are highlighted, encompassing a range of material classes, like bulk materials, membranes, and dispersed materials. In the course of the review, various applications of MOF/polymer combinations are discussed (e.g., adsorption, gas separation, drug delivery, catalysis, organic electronics, and stimuli-responsive materials). Finally, past research is concluded and an outlook toward future development is provided.