Observation of spherulitic covalent triazine frameworks grown from melts

Abstract

As emerging porous crystalline polymers, covalent organic frameworks (COFs) present exclusively in form of polycrystalline powders, single crystals, and thin films. Here we report a spherulitic covalent triazine framework (CTF) grown from its monomeric melt, which exhibits a typical Maltese cross pattern of light extinction. A structural evolution from small-angle branched nanofibrils to a core-shell hemispherical geometry is real-time recorded by in situ microscopies and spectroscopies. Time-resolved imaging analysis on spherulitic growth unveils a nonclassical crystallization process, which involves a secondary nucleation–growth mechanism coupled with amorphous-to-crystalline transition and self-templating polymerization. Furthermore, fluorine doping into CTF enables acceleration of core-shell growth and self-templating reaction, yielding well-developed spherulites with the average size up to ~48 µm in diameter. Our findings demonstrate an unprecedented form of COF crystallites and provide insights into polymerization and crystallization processes under solvent-free conditions via in situ characterization.