Elucidating the Variable-Temperature Mechanical Properties of a Negative Thermal Expansion Metal Organic Framework

We report the first experimental study into the thermomechanical and viscoelastic properties of a metal organic framework (MOF) material. Nanoindentations show a decrease in the Young's modulus, consistent with classical molecular dynamics simulations, and hardness of HKUST-1 with increasing temperature over the 25-100 degrees C range. Variable-temperature dynamic mechanical analysis reveals significant creep behavior, with a reduction of 56% and 88% of the hardness over 10 min at 25 and 100 degrees C, respectively. This result suggests that, despite the increased density that results from increasing temperature in the negative thermal expansion MOF, the thermally induced softening due to vibrational and entropic contributions plays a more dominant role in dictating the material's temperature-dependent mechanical behavior.