Static and Time-Dependent Plasticity of Monolithic Metal–Organic Frameworks

Mechanical properties of monolithic metal–organic frameworks (MOFs), especially static and time-dependent plasticity, are essential to the mechanical stability of their device applications. In this study, the hardness and elastic moduli of monoZIF-8 and monoUiO-66 MOFs were compared and analyzed in terms of formation enthalpy, topological structure, and crystal defects formed during sol–gel synthesis. For the first time, the creep behaviors of monoUiO-66 and monoZIF-8 were investigated using nanoindentation. The stress exponent n values for creep were calculated to be about 9 and 40 for monoUiO-66 and monoZIF-8, respectively, implying grain boundary sliding (GBS, n = 2) between nanocrystals is unlikely to occur. The suspected absence of GBS was also supported by the identical creep behavior of monoZIF-8 and a ZIF-8 single crystal. Small activation volumes V* determined for monoUiO-66 (0.721–0.839 nm3) and monoZIF-8 (1.29–2.54 nm3) correspond to only 2–3 intrinsic pores in the MOF frameworks, indicating that pore collapse is probably the predominant creep mechanism.