Impact of Structural Defects on the Elastic Properties of Two-Dimensional Covalent Organic Frameworks (2D COFs) under Tensile Stress

Two-dimensional covalent organic frameworks (2D COFs) represent an emerging class of permanently porous, lightweight materials. While their mechanical properties represent a fundamental intrinsic feature most relevant for their applications, they remain poorly understood. This is exemplified by the fact that there exists a large variation in previously reported Young's moduli. Also, a large number of structural defects can be present within the 2D COF films, whose impact on the mechanical properties needs to be addressed. Here, based on an efficient computational protocol to evaluate the Young's moduli and Poisson's ratios of 2D COFs from molecular dynamics simulations, we investigate the mechanical properties, under tensile stress, of representative (honeycomb-kagome) boronate ester-and imine-linked 2D COFs, i.e., COF-5 and TAPB-PDA COF. In both systems, the Young's moduli are found to be dependent on the stretching direction and range from 4 to 24 GPa. A large Poisson's ratio of 0.9-1.1 is found, which suggests that 2D COFs have a large contraction in the transverse direction when stretched. These results point to 2D COFs as anisotropic elastic materials. Importantly, the presence of structural defects is found to significantly impact the mechanical properties of 2D COFs. For instance, the presence of 3% vacancies can lead to a â 50% decrease in Young's modulus. Our work provides a comprehensive understanding of the elastic properties of representative 2D COFs, a useful stepping stone when considering these systems for a variety of applications.