The potential of covalent organic frameworks to act as mechanical metamaterials

Abstract

Covalent organic frameworks (COFs) are a class of engineered molecular crystals composed of organic building blocks linked together through covalent bonds. Depending on the composition of their building blocks, COFs have been synthesised as 1D frameworks extending in a single direction, as 2D sheets or as 3D crystal lattices. Research on the mechanical properties of COFs, revealed that in certain cases, both a negative Poisson’s ratio and a negative linear compressibility can be achieved, however, such studies remain limited. Molecular modelling of 3D COFs presents several challenges, primarily due to their large pore sizes. In view of this, a methodology was developed for simulating these materials by loading nitrogen molecules within their pores. The results indicate that geometry optimisation consistently yielded structures closely resembling experimental ones when the COFs were sufficiently loaded with nitrogen molecules. Furthermore, the potential of these systems to exhibit a negative Poisson’s ratio and/or negative linear compressibility was investigated. The employed method confirms that such frameworks can exhibit both properties, illustrating the potential of 3D COFs to function as mechanical metamaterials. This study provides valuable insights into the potential auxetic nature of existing 3D COFs, paving the way for the design of other 3D COFs with inherent auxetic properties.