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

Abstract

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 ∼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.

Understanding the Anisotropic Elastic Properties of Metal–Organic Frameworks at the Nanoscale: The Instructive Example of MOF-74

“…Thus, for the projection onto the xy -plane, the maxima are essentially oriented along the zigzag directions of the honeycomb pattern, while the minima of E align with the armchair directions (see Figure b). This is in conceptual agreement with recent findings for a hexagonal covalent organic framework . The minor deviation from a perfect alignment is attributed to the small inclinations of the axes of the linkers relative to those walls, as shown in Figure d.…”

“…This is in conceptual agreement with recent findings for a hexagonal covalent organic framework. 77 The minor deviation from a perfect alignment is attributed to the small inclinations of the axes of the linkers relative to those walls, as shown in Figure 3d.…”

Covalent Organic Framework as an Efficient Protection Layer for a Stable Lithium‐Metal Anode

“…Second, the Young's modulus of COF-5 typically ranges from 15-20 GPa which is higher than that of metallic Li ( � 5 GPa). [11] This makes the COF layer strong enough to prevent dendrite growth. [12] The photograph of the COF-Li in Figure 1d indicates that such a thin coating layer on the Li only slightly influences its visual observation, and the metallic appearance can be seen.…”

Covalent Organic Framework (COF)‐Based Hybrids for Electrocatalysis: Recent Advances and Perspectives

“…However, in some cases, the formation of defects is unfavorable, which could be responsible for the limited crystallinity of COF materials, [ 198 ] and pose threats to the mechanical stability of COFs. [ 199 ] It was proposed that the rapid assembly mode of COF layers under typical synthesis conditions could lead to a plethora of defects that are difficult to anneal to form lone range order crystals. [ 198 ]…”

“…However, in some cases, the formation of defects is unfavorable, which could be responsible for the limited crystallinity of COF materials, [198] and pose threats to the mechanical stability of COFs. [199] It was proposed that the rapid assembly mode of COF layers under typical synthesis conditions could lead to a plethora of defects that are difficult to anneal to form lone range order crystals. [198] Incorporation of other active metallic components to COFs and regulation of the electronic structures to achieve better reactant-adsorption properties will be a meaningful topic for developing effective COF-based electrocatalysts.…”