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

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.