Strong and flaw-insensitive two-dimensional covalent organic frameworks

Two-dimensional (2D) covalent organic frameworks (COFs) are promising polymeric crystalline nanomaterials with broad applications. However, the understanding of their mechanical properties and fracture mechanisms remains elusive. Here, we report a quantitative in situ tensile study of ultrathin COF_TAPB-DHTA films. The fracture strength was measured to be 0.75 ± 0.34 GPa, and the tensile modulus was measured to be 10.38 ± 3.42 GPa, with a nominal density of 0.393 g/cc, thus having specific strength equivalent to Kevlar (2 GPa·cc/g), and specific modulus comparable with titanium alloys (23 GPa·cc/g). In addition, the fracture toughness was measured to be 0.55 ± 0.09 MPa√m, and it was found that the crack propagation could be insensitive to the pre-crack when the size of pre-crack is below a critical value, leading to intriguing flaw insensitivity in such ultrathin nanomaterials. This work provides in-depth insights into the fracture properties of 2D COF films and lays a foundation for their future applications. Progress and potential  Mechanical properties of two-dimensional (2D) covalent organic frameworks (COFs), an emerging 2D polymeric material showing great potential applications in many areas, have not been systematically studied. In this paper, tensile mechanical performances of ultrathin COF_TAPB-DHTA films (∼50 nm), including fracture strength, Young's modulus, and fracture toughness, were quantitatively studied using an in situ SEM testing technique. In addition, an interesting flaw-insensitive fracture behavior was observed and analyzed using Griffith theory and Inglis theory. This work provides an in-depth understanding on the mechanical properties of high-performance 2D porous polymeric materials. In this paper, the tensile fracture strength, Young's modulus, and toughness of ultrathin COF_TAPB-DHTA films were measured to be 0.75 GPa, 10.38 GPa, and 0.55 MPa√m, respectively, using a quantitative in situ SEM tensile mechanical testing method. In addition, a flaw-insensitive fracture behavior was observed. This work provides an in-depth understanding on the mechanical properties of 2D COF films and lays a strong foundation for their applications. © 2021 Elsevier Inc.