Highly Negative Poisson’s Ratio in Thermally Conductive Covalent Organic Frameworks

Abstract

The prospect of combining two-dimensional materials in vertical stacks has created a new paradigm for materials scientists and engineers. Herein, we show that stacks of two-dimensional covalent organic frameworks are endowed with a host of unique physical properties that combine low densities, high thermal conductivities, and highly negative Poisson’s ratios. Our systematic atomistic simulations demonstrate that the tunable mechanical and thermal properties arise from their singular layered architecture comprising strongly bonded light atoms and periodic laminar pores. For example, the negative Poisson’s ratio arises from the weak van der Waals interactions between the two-dimensional layers along with the strong covalent bonds that act as hinges along the layers, which facilitate the twisting and swiveling motion of the phenyl rings relative to the tensile plane. The mechanical and thermal properties of two-dimensional covalent organic frameworks can be tailored through structural modularities such as control over the pore size and/or interlayer separation. We reveal that these materials mark a regime of materials design that combines low densities with high thermal conductivities arising from their nanoporous yet covalently interconnected structure.

Pore Size Dictates Anisotropic Thermal Conductivity of Two-Dimensional Covalent Organic Frameworks with Adsorbed Gases

“…The possibility of merging two-dimensional (2D) layered materials into vertical stacks has opened new frontiers in materials science. − One such class of emerging materials is 2D covalent organic frameworks (COFs) that combine covalently bonded light atoms forming 2D sheets with one-dimensional (1D) open channels. Their unique and tunable microstructure endows them with several exceptional physical attributes, − the most notable of which has been their unrivaled potential for gas separation, adsorption, and storage applications, which mainly derives from their large surface areas and high porosities. − Moreover, their 1D open nanopores make them ideal candidates for directional transport of ions and gases, setting them up as attractive candidates for drug delivery applications and as solid-state electrolytes for next-generation energy storage devices. − …”

Nanoarchitectured Conjugated Microporous Polymers: State of the Art Synthetic Strategies and Opportunities for Adsorption Science

“…With the continuous exploration of new materials, two-dimensional (2D) and three-dimensional (3D) porous organic polymers (POPs) as a new kind of porous materials have attracted tremendous attention by researchers due to their characteristics of large specific surface area, high porosity, and adjustable pore size. − Since POPs have good thermal and chemical stability, large specific surface area, and designable structure and function, they have been widely used in the fields such as energy storage, hydrogen storage, catalysis, and adsorption . According to their synthetic reactions and chemical structures, POPs can be subdivided into covalent organic frameworks (COFs), − hyper-cross-linked polymers (HCPs), − polymers of intrinsic micropores (PIMs), − conjugated microporous polymers (CMPs), − covalent triazine frameworks (CTFs), − porous aromatic frameworks (PAFs), − etc. Among these POPs, CMPs are a class of amorphous materials that permit the linking of building blocks in a π-conjugated fashion and possess 2D or 3D frameworks .…”

Thermally Conductive Self-Healing Nanoporous Materials Based on Hydrogen-Bonded Organic Frameworks

“…Recently, we have shown that covalent organic frameworks (COFs) mark a new regime of materials design that combine ultralow mass densities and remarkably high thermal conductivities in porous crystals (Figure 1). 3,11 Herein, we will show that another class of porous crystals based on hydrogen-bonded organic frameworks (HOFs) also possess high thermal conductivities with low mass densities (as shown in Figure 1), all the while demonstrating a unique set of mechanical properties such as higher flexibilities and the ability to "self-heal" after a catastrophic breakdown of the porous framework.…”

“…For MOF-5, we observe a monotonically increasing stress response due to the application of strain. Similarly, we also include the stress–strain curves for COF-5 as calculated in our previous work . Although COF-5 displays an overall better flexibility as compared to the MOF-5 structure, for both types of frameworks there is a considerable buildup of stress due to the application of a uniaxial strain, which is in contrast to the response of our HOF structure.…”

“…(a) Comparison of the stress–strain relationships for COF-5 (taken from ref ) under uniaxial tension in the zigzag and arm-chair directions, MOF-5 (from additional simulations performed with the model as described in ref ), and our ABTPA HOF under uniaxial tension in the x -direction. Snapshots of the computational domains at uniaxial strain levels of (b) 20% and (c) 40%.…”