Mechanical Properties of a Metal-Organic Framework formed by Covalent Cross-Linking of Metal-Organic Polyhedra

Article abstract of DOI:10.1021/jacs.8b11527

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 cross-linking of metal-organic polyhedra (MOPs). The ubiquitous Cu₂₄ isophthalate MOP has been decorated with a long alkyl chain having terminal alkene functionalities so that MOPs can be cross-linked via olefin metathesis using Grubbs second generation catalyst. Different degrees of cross-linked MOP materials have been obtained by varying the amount of catalyst in the reaction. Rheology of these structures with varying number of cross-links was performed to assess the cross-link density and its homogeneity throughout the sample. The mechanical properties were further investigated by the nanoindentation method, which showed increasing hardness with higher cross-link density. Thus, this strategy of cross-linking MOPs with covalent flexible units allows us to create MOFs of increasing mechanical strength while retaining the MOP cavities.

Full text of DOI:10.1021/jacs.8b11527

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Article
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
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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 M. Spasyuk, Jian-Bin Lin,† Milana
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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
INTRODUCTION
increasingly active.5,6
Metal-Organic Frameworks (MOFs) are typically
Polymer composite materials of porous inorganic solids
composed of metal ion or metal ion cluster vertices
have been extensively studied.5 In any composite,
bridged by organic linkers. They are touted for their
ideally one can merge the desirable properties of the
porosity and systematically tunable structures enabling
individual materials while circumventing any
design of new materials.1 In the vast majority of MOFs,
shortcomings. For example, one may wish to marry the
the organic linkers are rigid aryl units to sustain the
selective permeability of an inorganic porous solid with
structure by framing pores. The use of non-rigid linkers
the mechanical strength of an organic polymer.6 Such
can lead to networks without open pores or it could
combinations of quite chemically different materials
lead to sponge-like networks capable of flexing their
bring numerous challenges regardless of the specific
structure.2 When flexible linkers have been used for
material pairing. For instance, it is desirable to have a
MOF materials, the emphasis has been on any MOF
uniformly well-dispersed loaded phase.6 The loaded
properties that may be switched or gated by the
phase should not be entombed by the polymer so as to
molecular level breathing or swelling of the structure.3
compromise function (e.g. porosity). The loaded phase
An aspect that is much less studied is the macroscopic
should adhere well to the support as detachment would
flexibility of the framework. Many potential MOF
create non-selective pathways through the composite.
application require thin film or membrane
Owing to the chemical differences at surfaces, this
manifestations that would benefit from some structural
particle adhesion is a ubiquitous obstacle when trying
pliancy.4 In this light, it is relevant to point out that,
to merge different materials whether metal oxides and
when flexible linkers have been studied in MOFs, the
polymers or even MOFs and polymers.5
structural pliancy is derived from free rotation about an
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.
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 (L2) (see ESI, 3b) having no alkene functionality. Thus,
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 cross-
linked 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
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 body-
centered orthorhombic structure with a nearest inter-
MOP 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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Scheme 1. Schematic representation of MOP
1
undergoing inter-MOP metathesis to form -OC18H34O-
linkages 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-1-
yloxy) 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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(see Figure 2). The integrity of the MOP structure
remains intact when dissolved or suspended in
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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).
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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
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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
1
calculated from H-NMR integrations of theses peaks
(see details in ESI, section 5). It must be noted that
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these integrations do not distinguish inter-MOP versus Figure
intra-MOP alkene metathesis that does not form true
crosslinks.
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are
4)
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3
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
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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 H-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.
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Figure 3. PXRD patterns of MOP 1 and its crosslinked
products.
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in hexane or methanol and the respective images
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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 non-
porous 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.
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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 2013, the International Union of Pure and Applied
Chemistry (IUPAC) defined the term metal-organic
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framework (MOF) as a subclass of “coordination crosslinking is increased beyond 20% a clear strain
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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.
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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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, particle-
particle 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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Figure 5. Rheology measurements showing the due to the irregular interdigitation of the long alkyl
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nonlinear portion of the data, of the samples as
indicated.
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.
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 load-
depth 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 non-
crosslinked MOP 1 which would be expected to be the
most homogeneous, shows higher error which might be
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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
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Notes
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, self-
assembly of MOPs into superstructures through non-
covalent 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.
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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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Layer-by-Layer Fabrication of Oriented Porous Thin Films Based
CONCLUSION
We have presented an approach to generate flexible
MOF structures using covalent crosslinking of well-
defined 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
1
xMOPs, UV-Vis, FT-IR, H-NMR, PXRD, TGA, Crystal
Structure Data, Gas sorption isotherms, Elemental
mapping, EDX, EELS, typical Rheological behavior
and Nanoindentation graphs.
AUTHOR INFORMATION
Corresponding Author
* gshimizu@ucalgary.ca
* mtrifkov@ucalgary.ca
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