Solvent role on covalent organic framework thin film formation promoted by ultrasound

The present study reports the formation of thin films from covalent organic framework (COF) via ultrasound using different solvents. The morphology of the resulting films was correlated to the rate of bubble formation and the maximum bubble size, through derivations from the Rayleigh-Plesset equation. Solvents were chosen according to their different characteristics, such as polarizability, ability to disperse solutes, and bonding hydrogen bond. A very thin and organized film was obtained after exfoliation of the COF by sonication in polar solvents, such as ethanol, while rougher three-dimensional films were obtained from hexane. Our investigation revealed that there is a strong correlation (Pearson's r > 0.9) between the bubble formation capacity of a solvent, the respective polar HSP and COF film topography formed as a result of the ultrasound processing.