Session: PMSE/POLY Poster Session:

The elastic modulus of a flexible metal-organic framework (MOF) increases at the nanoscale as measured using atomic force microscopy nanoindentation and accounts for a known shape-memory effect. Flexible MOFs consist of metal ions bridged by organic ligands that combine porosity, crystallinity, and structural mobility to allow for large volume changes and dynamic guest adsorption properties. Nano- and microdimemsional crystals of [Cu₂(bdc)₂(bpy)]_n were generated by modifying ligand coordination using weak acid and solvothermal methods, respectively.
Flexible MOFs can exhibit cooperative structural transformability, where the pores expand from a nonporous closed phase to a highly-porous open phase in response to external stimuli (e.g. temperature, pressure, guest incorporation). Shape-memory polymeric frameworks retain the metastable porous phase in the absence of guest molecules. The size-dependent mechanical rigidification of a nanocrystalline flexible MOF is experimentally observed. Additionally, the nanomechanics of [Cu₂(bdc)₂(bpy)]_n are guest-dependent.
The solid-solid phase transitions make MOFs particularly advantageous for applications in energy storage, chemical separations, catalysis, and drug delivery. The mechanical properties of MOFs (e.g. elastic modulus, shear modulus, and linear compressibility) are expected to be central to their successful transition into industrial applications that require significant mechanical stress in the form of post-synthetic processing.

Structure of [Cu<sub>2</sub>(bdc)<sub>2</sub>(bpy)]<i><sub>n</sub></i>. Solid-solid transition from nonporous closed phase to porous open phase occurs as a result of methanol adsorption to give [Cu<sub>2</sub>(bdc)<sub>2</sub>(bpy)]<sub><i>n</i> </sub>containing MeOH (methanol guests and hydrogen atoms not shown for clarity).

Structure of [Cu₂(bdc)₂(bpy)]_n. Solid-solid transition from nonporous closed phase to porous open phase occurs as a result of methanol adsorption to give [Cu₂(bdc)₂(bpy)]_ncontaining MeOH (methanol guests and hydrogen atoms not shown for clarity).

Alexei Tivanski

The University of Iowa

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