Superior Mechanical Properties of Multi-Layer Covalent-Organic-Frameworks Enabled by Rationally Tuning Molecular Interlayer Interactions

Two-dimensional covalent-organic-frameworks (2D COFs) is a class of porous 2D polymer with periodic structure and ordered skeleton assembled from covalent bonds. 2D COFs distinct from 2D inorganic analogue by their high designability, which make them potential platform for mechanistic studies of mechanical behavior at 2D limit. In this talk, we will present the systematical study of the structure-mechanical properties relation by designing the structure of 2D COFs. With optimized Langmuir-Blodgett method, we successfully prepared layer-controlled 2D COFs, and measured their mechanical properties by nanoindentation. Our result showed that the interlayer hydrogen bonds in COFs provide enhanced interlayer interaction, leading to layer-independent mechanical properties, in sharp contract with 2D COFs without interlayer hydrogen bonds, in which the mechanical properties significantly decreased as layer number increased. DFT calculation showed that the layer-independent mechanical properties are attributed to higher energy barriers against interlayer sliding due to the presence of interlayer hydrogen bonds. These results indicate that COFs can be a very promising platform for the preparation high-performance 2D polymeric materials by precise molecular design.