Two-dimensional (2D) crystalline polymers, particularly covalent organic frameworks (COFs), have been emerging as a class of promising materials in electronics/optoelectronics, energy storage/conversion, gas separation and catalyst applications. They have a highly ordered covalent network along orthogonal directions, designable structures, and multifunctional properties for various application scenarios [1-5]. Over the past decades, researchers have developed synthetic strategies and have explored their mechanical properties, particularly flexibility for potential wearable devices [6-7]. However, the fracture mechanism of 2D polymers has yet been unveiled because of the lack of a suitable observation platform at molecular level, i.e. knowledge that is crucial for tailoring properties and to improve mechanical reliability of flexible device.

Here, we demonstrate study of the fracture process on an imine-based 2D COF using in-situ tensile testing in a transmission electron microscope (TEM, Libra200, Carl Zeiss). By optimizing transferring and patterning procedures, we report that a large elastic strain up to ~6.7% is achieved in 2D polyimine, with corresponds to an elastic modulus of ~10.9 GPa. The rough edge of the propagated crack with branches and delamination are clearly visible. Furthermore, both transgranular and intergranular fracture took place, which is contrary to the expectation [8] and unexplored in previously research [9]. Different local structures, grain sizes and strain distribution may lead to the emission of edge dislocation in a certain grain during the transgranular fracture and cause the following crack deflection. The results from in-situ experiments will be furtherly elucidated by molecular dynamic simulation to provide in-depth insights into intrinsic failure mechanisms of 2D polymers and pave the way for future flexible device applications.