Mechanical behavior of metal-organic framework monoliths with enhanced fracture toughness

A facile approach is demonstrated for synthesizing a mechanically tough ZIF-71 [Zn(dcIm) 2 ; dcIm = 4,5-dichloroimidazole] monoliths with a polymer binder, employing namely poly-methyl methacrylate (PMMA) through a sol-gel process. The addition of a slight polymer binder (∼10 wt.%) to the sol-gel mixture is shown to enhance the mechanical properties such as the elastic modulus, hardness, fracture toughness, strain hardening, and creep behavior of the composite monoliths compared to the pure ZIF-71 monoliths. Nitrogen sorption measurements revealed that the composite monoliths have a high surface area (∼350 m 2 /g) although it was reduced from ∼600 m 2 /g due to pore blockage by the polymer. Composite monoliths exhibit less surface cracks and are relatively stiffer but are notably tougher than the pure ZIF-71 monoliths as the fracture toughness tripled from around 0.15 MPa m 1/2 to around 0.45 MPa m 1/2 . Despite the incorporation of a small quantity of polymer in the monolith, this has a significant impact on increasing the mechanical stiffness, hardness and fracture resistance of the resulting monoliths. Creep response observed under a constant load revealed the presence of PMMA incorporated in the composite monolith. Nearfield infrared (IR) nanospectroscopy via nanoFTIR and pseudoheterodyne (PsHet) scattering-Scanning Nearfield Optical Microscopy (s-SNOM) IR imaging on plastically deformed and indented surfaces revealed local chemical structure-mechanical interactions. Our findings suggest that the polymer chains are trapped inside the ZIF-71 structure, thereby improving the mechanical resilience that might pave the way to future scaling up of MOF monoliths for practical applications and durable devices. • Nanoparticle packing and effect of PMMA inclusion on stiffness and sorption properties of ZIF-71 monoliths was studied. • Local scale integration of PMMA in ZIF-71 monolith was confirmed by nearfield nanospectroscopy. • Fracture toughness of ZIF-71 monolith tripled by adding ∼10 wt% PMMA, resulting in a rate-dependent fracture behavior. • Plastic deformation modes encompassing creep and strain hardening were investigated.