Covalent organic frameworks (COFs) are an emerging class of porous two- or three-dimensional polymeric materials with tailorable architectures. Here, we draw inspiration from the structures of rigid one-dimensional (1D) polymers like polybenzimidazole (PBI) and polybenzoxazoles (Zylon), some of the highest strength and thermally-resistant materials to date. While there are many theoretical advantages of COFs over 1D polymers, such as the expansion of strong covalent bonding into two dimensions, COF synthesis typically yields an insoluble powder. This powder is incompatible with various applications such as mechanical testing and integration into electronic devices, which require free-standing films. Herein, we demonstrate a novel method for producing films of 2D COFs with benzimidazole and benzoxazole linkages. These films are synthesized via a simple solvothermal method, where the precursor solution is drop-casted onto glass substrates and heated to initialize the formation of dynamic bonds between monomers. Once the solvent has evaporated, a pre-polymer network is left behind in the form of a uniform film. The intermediate films are then annealed under vacuum to convert the dynamic linkages irreversibly to the final product. Through this technique, varying volume and concentration of the precursor solution allow for modulation of film thickness from 3 µm down to about 60 nm. After synthesis, films are characterized via Fourier-transform infrared spectroscopy and X-Ray diffraction. To understand the morphology, films are imaged via scanning electron microscopy and atomic force microscopy. Thus far, crystalline film samples of both the benzoxazole and benzimidazole structures have been obtained. The two-step process of dynamic bond initialization and then irreversible transformation to find benzoxazole or benzimidazole product enables highly crystalline films as the COF molecules orient during the solvothermal step due to the hydrogen bonding between intermediates and are further crystallized during annealing. Finally, mechanical testing was performed on the free-standing benzoxazole COF films, obtaining ultimate tensile stress, hardness and elastic modulus values comparable to commercial 1D polymers. Future work will focus on expanding the library of high strength linkers and improving the crystallinity of the resulting films in order to realize the intrinsic properties of these materials.

Synthesis of oriented benzimidazole- and benzoxazole-linked covalent organic framework films

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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