Responsive Properties of Bis-urea-based Supramolecular Hydrogels

Abstract

Physical hydrogels exhibit sol-gel reversibility, which is affected by external stimuli such as variations in pH and temperature. One class of hydrogelators, known as the low-molecular- weight gelators, can form fibrillar networks through noncovalent self-assembly, resulting in the formation of physical gels. This study aims to investigate the pH-responsive properties of bis-urea-based supramolecular hydrogelators. A series of bis-urea compounds containing an aromatic linker and various end groups were synthesized via a one-pot condensation reaction. Gelation of the compounds in water was conducted at pH 2–13, whereby the inversion technique was used to confirm the hydrogel formation. The compound bearing an m-picolyl end group effectively forms gels within the pH 3–12 range. The gelator also exhibits decreasing critical gel concentration with increasing pH, presumably due to the ionizability of the pyridyl groups. Rheological analysis of the gels determined the storage modulus (G’) to be greater than the loss modulus (G”), which indicated the expected solid-like behavior of the hydrogel at low strain. The morphology of the gel obtained by scanning electron microscopy revealed a fibrous aggregated structure. Results obtained from this study may suggest the applicability of bis-urea compounds as pH-sensitive physical hydrogelators.