Sol-Gel Transitions of Comb-Like Polymethacrylate Copolymers by Mechano-Thermal Stimuli in Water

Sonogelation by supramolecular gelators is highlighted as a paradigm shift for their potential applications in material and biomedical sciences. Yet, these materials require organic solvents and low-frequency ultrasound, limiting their utility. Here, a comb-like polymethacrylate copolymer is synthesized with 2,6-bis(1-methylbenzimidazolyl)-4-oxypyridine tridentate ligand randomly distributed on poly(ethylene glycol) side chains. Upon addition of zinc perchlorate salt, this copolymer rapidly forms a hydrogel after exposure to either mechanical (i.e., ultrasound) or thermal stimuli. Spectroscopic analysis, to elucidate the mechanism, indicates metal ion coordination to ligand as well as the carbonyl and ethylene glycol units; interestingly,H-1-NMR suggests that the coordination between tridentate ligand moiety and Zn(2+)is unexpectedly weaker in water than in acetonitrile. Investigations by dynamic light scattering indicates that the copolymer forms nanodispersions in excess salt, which coalesces to form a hydrogel by either heating or high intensity focused ultrasound (HIFU). Rheological quantification of these gels suggests stronger crosslinking by HIFU compared to heating.