Mechanical Properties of a Metal-Organic Framework formed by

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Mechanical Properties of a Metal-Organic Framework formed by Covalent Crosslinking of Metal-Organic Polyhedra Garima Lal, Maziar Derakhshandeh, Farid Akhtar, Denis Spasyuk, Milana Trifkovic, and George K. H. Shimizu J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 24 Dec 2018 Downloaded from http://pubs.acs.org on December 24, 2018

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Journal of the American Chemical Society

Mechanical Properties of a Metal-Organic Framework formed by Covalent Crosslinking of Metal-Organic Polyhedra Garima Lal,† Maziar Derakhshandeh,‡ Farid Akhtar,§ Denis M. Spasyuk, Jian-Bin Lin,† Milana Trifkovic,‡* and George K. H. Shimizu †* † Department

of Chemistry, University of Calgary, Calgary, Alberta, T2N1N4, Canada Department of Chemical and Petroleum Engineering, University of Calgary, Calgary, Alberta, T2N1N4, Canada § Department of Engineering Sciences and Mathematics, Luleå University of Technology, 97187 Luleå, Sweden Canadian Light Source, 44 Innovation Blvd., Saskatoon, Saskatchewan, S7N2V3, Canada ‡

Supporting Information Placeholder

ABSTRACT: Overcoming the brittleness of metal organic frameworks (MOFs) is a challenge for industrial applications. To increase the mechanical strength, MOFs have been blended with polymers to form composites. However, this also brings challenges such as, integration and integrity of MOF in the composite which can hamper the selectivity of gas separations. In this report, an “all MOF” material with mechanical flexibility has been prepared by covalent crosslinking of metal-organic polyhedra (MOPs). The ubiquitous Cu24 isophthalate MOP has been decorated with long alkyl chain having terminal alkene functionalities so that MOPs can be crosslinked via olefin metathesis using Grubbs 2nd generation catalyst. Different degrees of crosslinked MOP materials have been obtained by varying the amount of catalyst in the reaction. Rheology of these structures with varying number of crosslinks was performed to assess the crosslink density and its homogeneity throughout the sample. The mechanical properties were further investigated by the nanoindentation method which showed increasing hardness with higher crosslink density. Thus, this strategy of crosslinking MOPs with covalent flexible units allows us to create MOFs of increasing mechanical strength while retaining the MOP cavities.

the field of MOF polymer composites has become increasingly active.5,6 Polymer composite materials of porous inorganic solids have been extensively studied.5 In any composite, ideally one can merge the desirable properties of the individual materials while circumventing any shortcomings. For example, one may wish to marry the selective permeability of an inorganic porous solid with the mechanical strength of an organic polymer.6 Such combinations of quite chemically different materials bring numerous challenges regardless of the specific material pairing. For instance, it is desirable to have a uniformly well-dispersed loaded phase.6 The loaded phase should not be entombed by the polymer so as to compromise function (e.g. porosity). The loaded phase should adhere well to the support as detachment would create non-selective pathways through the composite. Owing to the chemical differences at surfaces, this particle adhesion is a ubiquitous obstacle when trying to merge different materials whether metal oxides and polymers or even MOFs and polymers.5 Here, we present the formation of a macroscopically flexible MOF structure by the chemical cross-linking of alkyl chain-functionalized metal-organic polyhedra (MOPs).7,8 MOPs are discrete structures composed of metal ion and organic linkers enclosing a 3-D cavity. ACS Paragon Plus Environment

INTRODUCTION Metal-Organic Frameworks (MOFs) are typically composed of metal ion or metal ion cluster vertices bridged by organic linkers. They are touted for their porosity and systematically tunable structures enabling design of new materials.1 In the vast majority of MOFs, the organic linkers are rigid aryl units to sustain the structure by framing pores. The use of non-rigid linkers can lead to networks without open pores or it could lead to sponge-like networks capable of flexing their structure.2 When flexible linkers have been used for MOF materials, the emphasis has been on any MOF properties that may be switched or gated by the molecular level breathing or swelling of the structure.3 An aspect that is much less studied is the macroscopic flexibility of the framework. Many potential MOF application require thin film or membrane manifestations that would benefit from some structural pliancy.4 In this light, it is relevant to point out that, when flexible linkers have been studied in MOFs, the structural pliancy is derived from free rotation about an alkyl chain of 2-6 carbon atoms.3b It is a considerable challenge to engineer macroscopic flexibility into materials that have an intrinsic salt-like quality and so

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They are stable, soluble entities and can be functionalized to offer a variety of applications.9 In this work, the previously reported cuboctahedral Cu24 isophthalate MOP10 was modified in the 5-position of the isophthalate linkers with long alkyl chains, half of which had terminal alkenes (Figure 1). These alkenyl MOPs were then linked using olefin metathesis reactions.11 Transmission electron microscopy clearly shows retention of the MOP structures in the crosslinked materials (Figure 4). The degree of crosslinking was varied synthetically and the crosslinked MOP structures were studied by nanoindentation and rheology to gain insight into their mechanical properties and crosslinking efficiency.12 In this system, the MOPs are ultimately bridged by -OC18H34O- units (Scheme 1) between the isophthalate ligands and so this is actually not a MOF composite but, by the IUPAC definition of metal-organic framework, still simply a MOF, albeit one with very long and unevenly distributed linkers.13 Scheme 1. Schematic representation of MOP 1 undergoing inter-MOP metathesis to form -OC18H34Olinkages with an internal alkene unit as demarcated in the ring. MOP 1 further crosslinks in different directions to give the crosslinked product i.e. a MOF.

RESULTS AND DISCUSSION In our work, the Cu24 isophthalate MOP has been chosen due to its solution stability and easy derivatization at the 5-position of the isophthalate linker.9,10 To make a mechanically flexible structure, crosslinking Cu24 MOPs with long alkyl chains was envisaged. 5-(Dec-9-en-1yloxy) isophthalic acid (L1) (see ESI, 3a) with terminal alkene functionality was chosen as one of the linkers as it has a long alkyl chain and a crosslinkable functional group. With 24 isophthalate groups per MOP representing functionalizable units, to moderate the extent of crosslinking the alkenyl isophthalate was diluted with another linker, 5-(octyloxy)isophthalic acid

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(L2) (see ESI, 3b) having no alkene functionality. Thus, the 1:1 L1: L2 MOP was synthesized by mixing Cu(NO3)2. 2.5 H2O (2 mmol) with equimolar amounts of both the linkers L1, L2 (each 1 mmol) in methanol. To this solution, a base 2,6-Lutidine was added to generate blue precipitates which on further processing gave the required 1:1 L1: L2 MOPs as blue powder, EA verified (see detailed procedure in ESI, section 4). The presence of 50% of each linker on average in the solid was verified by digesting bulk MOPs in conc. HCl (see Figure S3) and quantifying each linker by 1H-NMR in DMSO-d6. Peaks at ~4.9 and ~5.8 ppm are characteristic of the terminal alkene of L1 and the peak at ~0.86 ppm is characteristic to the terminal methyl of L2; their respective areas permit the amount of each ligand in the total sample to be quantified (see Figure S2). The ratio of linkers in an individual MOP can vary and it is unlikely that each MOP will have precisely 12 L1 and 12 L2. These MOPs having long alkyl chain appendages are soluble in common organic solvents such as dichloromethane (DCM), chloroform, ethyl acetate and THF.14 The monomeric 1:1 MOP blue powder was recrystallized by dissolving the MOPs in hexane using 2,3 drops of 1-octanol and layering this solution over DMF solvent.14 The solution was left for a week and produced small blue needle-like crystals of 1:1 MOP, 1 [C282 Cu24 N10 O150] (see Figure S1). Crystallography was performed using synchrotron X-rays15 (details in ESI, Section 11), where the crystal was solved in Pnnm space group and the ~2 nm Cu24 cuboctahedral core was confirmed. The alkyl chains about the periphery were highly disordered and could not be completely resolved though their presence is confirmed by NMR spectroscopy. With the disorder, only the connectivity map of the MOP core could be obtained (see Figure 1) but this is sufficient to confirm the MOP structure.9,10 The powder x-ray diffraction (PXRD) pattern simulated from the crystal structure (see Figure 1b) shows a prominent low angle peak at 2Ɵ= 3.570 corresponding to the [101] plane and d-spacing of 2.47 nm. This peak for the ground crystals is offset by 0.5° from the simulated structure due to the solvent loss during sample preparation. The as-synthesized MOP powder is polycrystalline in nature and thus, exhibits a broader peak. In the crystal structure, the MOPs pack in a bodycentered orthorhombic structure with a nearest interMOP distance of ~2.8 nm between MOPs at the body center and the corners of the unit cell. The MOPs parallel to the a-axis have the center to center distance equal to the unit cell edge length of 4.16 nm while those parallel to the b and c-axes lie at 2.37 nm and 3.07 nm, respectively (see Figure 1c). Furukawa et al.14a and Perry et al.14b reported similar long alkyl chain decorated MOPs (e.g. 5-dodecyloxy Cu24 MOP). Similar to those structures, the alkyl chains here entangle or interdigitate between the neighbouring MOPs as shown by the distance between the cores is less than the end to end distance of two extended decyloxy chains, 2.39 nm

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(see Figure 2). The integrity of the MOP structure remains intact when dissolved or suspended in common organic solvents, however, the copper paddlewheel cleaves when introduced to water converting polyhedra into a polymeric structure.9e,f As MOP 1 is hydrophobic due to the peripheral alkyl chains, the transformation is slow but eventual degradation to the polymeric form is seen when exposed to water for a month (see Figure S9). Olefin metathesis via Grubbs’ 2nd generation catalyst (G2)16 was conducted by solubilising the MOPs in DCM17 under inert atmosphere18 (detailed procedure in ESI, section 5). Here, three versions of crosslinked MOP 1 (i.e. ~20%, ~40% and ~80% crosslinking, Scheme 1) were obtained by varying the amount of the catalyst. For metathesis, 5 mol% of the catalyst G2 was used per alkene site, as utilized by Chatterjee et al.19a To achieve the desired degree of crosslinking of 1, the mass of G2 used is calculated (see Table S1) assuming the ideal amount of 12 L1 in each MOP. The metathesized product with ~20% (MOPx20), ~40% (MOPx40) and ~80% (MOPx80) crosslinking were obtained where the percentages (+/- 3%) were calculated from the 1H-NMR integrations (see Figure 2) in DMSO-d6 after digesting the crosslinked product in conc. HCl. During

Figure 1. (a) SC-XRD of MOP 1, showing the core connectivity only as alkyl chains are disordered, (b) PXRD pattern as depicted, (c) Packed crystal structure of MOP 1. metathesis, an internal alkene L3 is formed whose equivalent “c” hydrogens (see Figure 2 scheme) show a characteristic peak at ~5.35 ppm; whereas hydrogens in the precursor terminal alkene L1, “a” and “b” show characteristic peaks at 5.8 and 4.9 ppm, respectively.20 The crosslink density of the metathesized product was calculated from 1H-NMR integrations of theses peaks (see details in ESI, section 5). It must be noted that



these integrations do not distinguish inter-MOP versus intra-MOP alkene metathesis that does not form true crosslinks. Metathesized products, MOPx40 and MOPx80 precipitated out of the reaction solution and were completely insoluble in common organic solvents such as DCM, chloroform, ethyl acetate, THF and DMF, unlike the precursor MOP 1. Thus, these products were washed with DCM and methanol to remove unreacted G2 and other impurities. MOPx20 was partially soluble in DCM and was washed with methanol only. The fact that MOPx40 and MOPx80 are insoluble indicated that significant inter-MOP crosslinking had occurred, leading to formation of a networked MOP solid. Had there been exclusively intra-MOP crosslinking, the metathesized MOP would still exist as a soluble discrete unit. In case of MOPx20, the partial solubility indicated that crosslinking did not proceed sufficiently, and this may have been because of intra-MOP crosslinks, inhomogeneous localized crosslinking or a combination of the two. The crosslinked products MOPx20, MOPx40 and MOPx80 were further characterised by FT-IR spectroscopy which indicated the bridging coordination behavior of the carboxylate (see Figure S6, ESI section 7) in the complex and retention of the paddlewheel motif.21 Despite the crosslinked MOPs having different solubilities, their PXRD patterns (see Figure 3) as well as transmission electron microscopy (TEM) images (see

Figure

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are

Figure 2. (above) Scheme of the crosslinked linker L3 (internal alkene) obtained by olefin metathesis of MOP 1 which consists of L1 (terminal alkene). Here, a, b and c represent the protons of given locations; (below) Stacked 1H-NMR spectra of MOP 1, MOPx20, MOPx40, MOPx80 showing increase in the intensity of the peak at 5.35 ppm (c) with more crosslinking of MOP 1.

Figure 3. PXRD patterns of MOP 1 and its crosslinked products.


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Figure 4. TEM image of (a) original MOP 1 showing ~2 nm nanoparticles which matches the size of the cuboctahedron core shown crystallographically; (b) crosslinked MOPx40 showing similar sized nanoparticles, supporting that the MOP structure is not destroyed during metathesis. similar and also related to that of the precursor MOP 1, affirming the integrity of the MOP core in the crosslinked product. The low angle peak in the crosslinked samples indicated domains of crystallites having d-spacing of ~2.47 nm, the peak breadth indicating a lack of long range order. The domains with this d-spacing result due to formation of the metathesized linker, L3, with length of 2.39 nm (Figure 2) when fully stretched, in the crosslinked samples. Qualitative analysis of different regions of the 2-D TEM images (Figure 4) showed that adjacent MOPs were farther when crosslinked (~6.2 nm on avg.) compared to non-crosslinked MOPs (~4.9 nm on avg.). This is in accordance to the longer alkyl chain (L3) and disfavoured interdigitation after metathesis. The TEM grids were prepared by drop-casting MOP suspensions

in hexane or methanol and the respective images showed no difference in the distance between neighbouring MOPs based on interaction with different polarity solvents.9e The presence of MOPs in the TEM images was further confirmed by elemental mapping and energy-dispersive x-ray spectroscopy (EDX) (see Figure S13-15). Gas sorption isotherms were also measured for MOP 1 and MOPx40 (homogenous crosslinking). Both samples were completely nonporous to N2 at 77K, however, with CO2 at 195K/1.2 bar (see Figure S10), MOP 1 adsorbed 1.16 mmol/g while MOPx40 adsorbed 2.18 mmol/g of CO2. This indicates that additional voids are present as a result of crosslinking in MOPx40 beyond the internal MOP cavities, leading to a significant increase in adsorption compared to the parent MOP. The increase may also be, in part, due to more accessible MOP cavities after crosslinking as interdigitation and pore blockage should be disfavored.14b Adsorption isotherms of CO2, CH4 and N2 at 298K on MOP 1 and MOPx40 were also measured. The N2 isotherms both at 77K and 298K show negligible uptake for both the materials. This is because of the well-known fact of “restricted diffusion” of N2 in materials falling in lower range of microporosity (< 2 nm).22a,b In our case, the MOP pore diameter is ~1.2 nm and voids created in the crosslinked framework might be blocked by the non-crosslinked alkyl chains leading to restricted diffusion of N2 and thus, negligible uptake. It has also been studied that CO2 is good candidate for assessment of porosity of these narrow pore materials having no polar sites.22a,b In our work, CO2 isotherm at 195 K shows high adsorption for MOP 1 and MOPx40 compared to one at 298K with uptake of 0.24 mmol/g for MOP 1 and 0.25 mmol/g for MOPx40 (see Figure S11). The higher uptake at 195K can be attributed to the stronger interactions of the gas with the pores at lower temperatures. All the isotherms are accompanied by hysteresis which might be due to pore hinderance by the non-crosslinked alkyl chains or cavitation-induced evaporation while desorption, observed in narrow micropores.22c The irregularities present in the adsorption isotherms of CH4 and N2 at 298K for both the materials is due to the extremely low uptake of ~0.05 and ~0.03 mmol/g, respectively. The selectivities for CO2/N2 and CO2/CH4 were also analyzed using the ideal adsorbed solution theory (IAST). Langmuir– Freundlich method was used to fit the isothermal parameters from the experimental pure adsorption isotherms of CO2, CH4 and N2 at 298K. For 15/85 composition of CO2/N2, the selectivity is ~22 for MOP 1 and ~31 for MOPx40 while for 15/85 composition of CO2/CH4, the selectivity is very low, ~8 for both the materials (see Figure S12). The selectivities for crosslinked MOPs is similar to the parent MOP as overall the composition for both is same and the gases don’t have strong interactions with the pores at room temperatures. In 2013, the International Union of Pure and Applied Chemistry (IUPAC) defined the term metal-organic



framework (MOF) as a subclass of “coordination network with organic ligands containing potential voids”.13 The word potential is used as many systems are dynamic and the structure can change with environmental changes. Our crosslinked material, where the structure is sustained solely by coordination bonds between polycarboxylates, satisfies this terminology and can therefore, be called a MOF. Our hypothesis in making these materials was that a mechanically flexible MOF could form akin to a MOF polymer composite but without common challenges of hybrid composites such as non-homogenous dispersion of MOF and poor adhesion of MOF to the polymer. The mechanical properties of samples with varied crosslink densities were investigated with shear rheology. All the samples, the non-crosslinked (MOP 1) and crosslinked MOPx20, MOPx40, MOPx80 in powder form (~1.5 g), were ground in a mortar and pestle and subsequently dispersed in dimethyl sulfoxide (20 mL). The suspension was sonicated, centrifuged and the supernatant decanted. The thick suspension was transferred carefully between the two parallel plates of the rheometer (see Methods in ESI for detailed procedure). Here, the bottom plate is stationary and the upper one oscillates at a certain deflection angle and rotational speed delivering the shear force to the material.23 Samples were probed using amplitude sweep experiments with a frequency of 0.1 Hz to obtain the storage and loss modulus vs shear strain of the material.24 The nonlinear region is defined at the larger levels of strain where the flow disturbs the microstructures (particles or MOPs in our case, held together by forces) and thus the differences in the microstructure can be probed (see section 16 in ESI for typical rheological behavior of different suspension systems).24 The amplitude sweep experiments tests (Figure 5a-d) show that the network of particles is disturbed at strain larger than 0.1, however, no flow in the sample is expected before the crossover strain. Here, loss modulus (G”) becomes larger than the storage modulus (G’) indicating that the viscous contribution of sample becomes dominant over its elastic nature. For our samples (MOP 1, MOPx20, MOPx40 and MOPx80), the rheological data after crossover strain is more illustrative since the influence of diverse interactions becomes more pronounced under Large Amplitude Oscillatory Shear (LAOS-defined at strains larger than the crossover strain).26 Non-crosslinked MOPs should behave like soft/hairy particles due to the presence of the long alkyl chains on their surface and thus these chains may interdigitate forming weakly assembled networks.25 However, for the crosslinked samples, various MOP interactions are possible within each sample including solvent-particle interactions, particleparticle interactions via the crosslinked path(s) and particle-particle interactions via non-crosslinked chains. As shown in Figure 5c and 5d, as the degree of

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crosslinking is increased beyond 20% a clear strain hardening behavior for MOPx40 and MOPx80 (both G’ and G” increases) is observed for almost a decade which is in agreement with previous studies on polymers having physical links.27,28 Any strong (physical) interactions between some segments of the complex, certainly covalent crosslinks, can resist the alignment of the MOPs in the flow direction and subsequently contribute to the strain hardening behavior observed.27,28 The small upturn in both G’ and G” of MOP 1 and MOPx20 right after the crossover strain (see Figure 5a, 5b), is regarded as strong strain overshoot28 and is generally observed in materials with weaker network formation. Here, the assembly is mainly governed by the interdigitated chains, the intermolecular interactions between the solvent and the MOP giving a more weakly networked solid than in the case of higher crosslinked samples.27 MOPx20 in spite having physical links exhibits a weak network structure unlike MOPx40 or MOPx80 indicating the existence of island-like crosslinked domains with limited linkages in between them. This is, corroborated by its partial solubility in dispersing solvents like MOP 1. In addition, for MOPx80 a single crossover strain cannot be identified, instead a range of relaxations exist which is indicative of non-homogeneity in the crosslinking within the sample. Non-homogeneity in the degree of crosslinking delays the hardening behavior as regions within the test specimen with a lower crosslink density can relax faster, thereby damping the strain hardening. In addition, it is found that the volume fraction of MOPs within the suspension decreases as the degree of crosslinking increases, since particles face a barrier exerted by the alkyl chains in between them. This indicates that less MOPs are loaded on the rheometer through suspension when there are crosslinks, thereby, decreasing the loss and storage moduli (G’ and G”) (as shown in Figure 5a-d) with increase in crosslinking.


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Figure 5. Rheology measurements showing the nonlinear portion of the data, of the samples as indicated. There have been several mechanical property studies on metal-organic frameworks12,29-32 through nanoindentation but, to our knowledge, none on discrete MOPs and their crosslinked counterparts. Here, nanoindentation was also performed on MOP 1, MOPx20, MOPx40 and MOPx80 powders (see Methods in ESI) with a maximum applied load of 20 mN at the rate of 0.7 mNs-1 and a dwell time of 10 seconds before unloading. The load-depth curves obtained from nanoindentation are shown in Figure S21 in ESI. The indentation was carried out 11-21 times in different regions to obtain a representative sampling. The curves obtained for each set of the indentations in MOP 1, MOPx40 and MOPx80 were similar in nature, however, MOPx20 showed varied curves (recall this sample was not efficiently crosslinked) with final depth of the indent being between 1000-2000 nm and exhibited a range of hardness (H) and reduced elastic modulus (Er) values.33 The Berkovich tip was calibrated as per the method of Oliver and Pharr,34 and H and Er values of the samples were calculated accordingly as given in Table 1. MOPx20 shows a large variation in hardness values ranging between 0.183 to 0.336 GPa from ~1000 to ~2000 nm depth respectively (see Figure S19b). This can be attributed to the fact that it has irregular crosslinks throughout the structure. With ~20% metathesis (bulk sample), the data support a model of domains of crosslinked islands with regions both more and less extensively crosslinked.30 The bulk porosity can be seen as a flat line in the beginning of the loaddepth curve (>2000 nm final depth) which shows dramatic increase in depth, up to 500 nm, with minimal increase in the load (Figure S21b). It can be seen for the remaining samples that with increase in crosslinking of the original MOP 1, the hardness value increases confirming that with more crosslinkings, the sample irreversibly becomes stiffer and more difficult to deform.30 The Er value increases up to ~5.864 GPa with ~80% crosslinking (see Table 1), which is higher than many MOFs such as ZIF-8, ZIF-20, and MOF-5.12 Increased crosslinks provide nodes in the structure for dissipation of applied energy and avoid permanent deformation.35 The homogeneity of the sample is also evidenced by the error analysis of Er (Table 1), the propagation of error around the mean value. It can be seen from Table 1 that MOPx20 is the most inhomogeneous while MOPx40 is the most homogeneous with consistent crosslinking throughout the sample. This is also in agreement with the rheological experiments, which indicated weak network formation for MOPx20, while MOPx40 exhibited strain hardening with a single crossover strain. The noncrosslinked MOP 1 which would be expected to be the most homogeneous, shows higher error which might be

due to the irregular interdigitation of the long alkyl chains throughout the sample. It can also be noted that G’ decreases while Er increases with increase in crosslinking, this can be attributed to the fact that G’ is a bulk behavior (depends on volume fraction of the sample) while Er is a local phenomenon. Thus, we see that this method of external crosslinking of individual MOPs help create materials with higher H and E, while retaining the structure and cavity of the MOPs.

Table 1. Hardness, reduced elastic modulus and error ananlysis (propagation of error around the mean value) of Er for the given samples.

As load is applied on the sample, both elastic and plastic deformation occurs. The parameters of H and Er are calculated from the nature of the unloading curve as it exhibits elastic response for the recovery of the material (plastic deformation is irreversible) after indentation.29 One can also see a kink in unloading curves for all the samples at ~2.5 mN load (see Figure S21). This kink represents a phase transformation in the structure36 with increase in volume and known for Cu(II) MOFs like HKUST-1.32 Apart from MOFs, nanoindentation studies have also been performed on MOF composites to determine strength with different MOF loading in the polymer.37 As the MOF can impart selectivity in gas separations in composites, its important to know the maximum loading amount while retaining the toughness of the composite. Mechanical studies have been performed on a variety of ZIF/polymer composites using ZIF nanocrystals to obtain a regular and a continuous composite.37 Different ZIF loadings from 5-30% in polymers37a-d were tested, and the general trend found was that with the increased loading, the hardness (H) of the composite increases; however, filler content beyond 15% leads to embrittlement when there is no increase in the elastic modulus (E) of the composite.37a-d Thus, increase in both E and H value are important for the toughness and durability of a material. In our case, both E and H values increase with MOP crosslinking and thus, this approach has potential to make all MOF membranes. This could be achieved by metathesizing



the MOPs at an interface, the studies are underway. Moreover, crosslinking MOPs is advantageous over reported, crosslinked MOF nanoparticles-polymer composites8c,39 as the size of MOF nanoparticles lie in the range of 100-500 nm while the diameter of Cu24MOP is only ~2 nm leading to better intrinsic integrity of the crosslinked material. Previously, selfassembly of MOPs into superstructures through noncovalent interactions has been studied by McManus et al.,40a Niu et al.,7d and Sanchez et al.,40b however, controlled covalent-crosslinking of MOPs into MOFs to tune their mechanical behavior40c is, to our knowledge, without precedent. A flexible MOF made in such a manner has intrinsic advantages over a composite in that the coordination “pockets” in the structure are chemically bonded to the flexible units giving defined micropores while preventing leaching. Alternatively, the tuning of mechanical behavior can also be utilized to make composites where MOFs’ H and E registers well with the polymer.

CONCLUSION We have presented an approach to generate flexible MOF structures using covalent crosslinking of welldefined metal-organic polyhedra. The parent MOP and crosslinked systems had structures and mechanical properties studied by X-ray diffraction, electron microscopy, gas sorption analyses, nanoindentation and rheology measurements. Overall, we see increase in both the reduced elastic modulus (Er) and hardness (H) with increased crosslinking of the MOPs. This is a method of introducing varying hardness to a MOF while maintaining regular cuboctahedral cavities. The ~40% crosslinked product gives a MOF with uniform crosslinking and high H and Er values. The mechanical properties with increased crosslinks were explained by rheology and nanoindentation. Mechanical stability of MOFs is important for practical applications, and this approach increases robustness, while retaining the MOP pores in the system. In contrast to a MOF polymer composite, the coordination units are chemically linked to the structure. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Data for MOP 1 (CIF) Material, Methods, Synthesis of linkers; MOP and xMOPs, UV-Vis, FT-IR, 1H-NMR, PXRD, TGA, Crystal Structure Data, Gas sorption isotherms, Elemental mapping, EDX, EELS, typical Rheological behavior and Nanoindentation graphs.

AUTHOR INFORMATION Corresponding Author * [email protected] * [email protected]

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Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENT We acknowledge the microscopy and imaging facility at University of Calgary for TEM, EDX, EELS as well as 4D Labs at Simon Fraser University. GL and GKS thank Alberta Innovates Technology Futures (AITF) and the Natural Sciences and Engineering Research Council (NSERC) of Canada for support. MT thanks NSERC and the Canadian Foundation for Innovation.

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Mechanical Properties of a Metal-Organic Framework formed by

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