Strong and flaw-insensitive two-dimensional covalent organic frameworks

Summary 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 COFTAPB-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.