  Understanding of the complex mechanical behavior of metal-organic frameworks
(MOF) beyond their elastic limit will allow the design of real-world
applications in chemical engineering, optoelectronics, energy conversion
apparatus, and sensing devices. Through in situ compression of micropillars,
the uniaxial stress-strain curves of a copper paddlewheel MOF (HKUST-1) were
determined along two unique crystallographic directions, namely the (100) and
(111) facets. We show strongly anisotropic elastic response where the ratio of
the Young's moduli are E(111) ~ 3.6 x E(100), followed by extensive plastic
flows. Likewise, the yield strengths are considerably different, in which
Y(111) ~ 2 x Y(100) because of the underlying framework anisotropy. We measure
the fracture toughness using micropillar splitting. While in situ tests
revealed differential cracking behavior, the resultant toughness values of the
two facets are comparable, yielding Kc ~ 0.5 MPa m^1/2. This work provides new
insights of porous framework ductility at the micron scale and failure by bonds
breakage.
