Mechanical properties of a two-dimensional Covalent Organic Framework*

Many atomically thin, two-dimensional crystals have emerged as materials with exceptional electronic, thermal, optical, and mechanical properties. These include members of the covalent organic framework (COF) family - porous materials ingeniously constructed from organic building units. While COFs show promise for gas storage applications, their mechanical properties beyond the elastic regime remain unknown. In this work, we combine first principles and reactive molecular dynamics simulations to characterize the mechanical properties of COF-1. We find that this material exhibits an anisotropic response to strain. In addition, for both pulling directions, we observe two regimes of response to external strain. By analyzing the trajectories, we find that softer and harder responses are related to reversible changes in the COF-1 microstructure as the strain increases. We also find that this process leads to a considerable increase in the material’s surface energy density. Finally, we report, for both armchair and zigzag orientations, the computed toughness, ultimate tensile strain, and ultimate tensile stress of 2D COF-1 membranes.