Two-dimensional (2D) 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 covalently linked network along orthogonal directions, designable structures, and multifunctional properties for various application scenarios (Science 2005, Angewandte Chemie International Edition 2018, Science 2017, Advanced Science 2019, Chemical Society Reviews 2013). However, although the outstanding mechanical properties have been proved and although 2D COFs are envisioned to be core parts in flexible electronics (Nature communications 2016, Journal of the American Chemical Society 2018, Science Advances 2020, Matter 2021), damage mechanisms and failure have not been studied systematically yet. However, his knowledge is essential and fundamental for tailoring their properties and to improve the mechanical reliability of future electronic nanodevices.

In this study, structural damage and changes in edge morphology during patterning on an imine-based 2D polymer, induced by various patterning methods (focused electron beam, focused ion beam (FIB) and mechanical carving), are unveiled and damage mechanisms are discussed. Employing meticulously optimized transferring and patterning techniques, the kinetics of fracture of 2D polyimine are studied utilizing in-situ tensile testing within a transmission electron microscope (TEM, Libra200, Carl Zeiss). A strain up to ~6.7% is achieved in 2D polyimine, the fracture strength and Young’s modulus were determined to be 0.6\pm0.2 GPa and 10.9\pm5.2 GPa, respectively. Digital image correlation (DIC) was employed for strain evolution analysis during crack propagation. The stable crack propagation shows a unique behaviour of side crack initiation. Dissimilar paths among neighbouring layers occurred due to the weak interlayer π - π stacking interaction. The trans-granular fracture is clearly identified, and its preferential cleavage orientation is well substantiated by Density Functional based Tight Bonding (DFTB) simulation. These findings not only provide a detailed instruction on the proper patterning techniques for 2D crystalline polymers, more importantly, they contribute to the understanding of the failure mechanisms of 2D crystalline polymers and provide insights for the rational design of chemical structures in 2D polymers.