Thermoreversible Supramolecular Polymer Gels via Metal-Ligand Coordination in an Ionic Liquid

Thermoreversible supramolecular polymer gels were prepared via metal ligand coordination by mixing a poly(4-vinylpyridine)-b-poly(ethyl acrylate)-b-poly(4-vinylpyridine) (P4VP-PEA-P4VP) triblock copolymer and zinc chloride (ZnCl2) in a hydrophobic ionic liquid, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)-imidide. FT-IR spectroscopy revealed metal ligand coordination between zinc in ZnCl2 and pyridine groups as ligands on P4VP blocks, even in an ionic liquid. Thermoreversible viscoelastic properties between a semisolid (gel-like) state and a liquid-like state were confirmed by temperature-ramp oscillatory shear measurements. It was also revealed that thermoreversibility of supramolecular polymer gels depended strongly on stoichiometry between ligands and metals, where the maximum of storage modulus-loss modulus crossover temperature (T-gel) as an indicator of gelation was achieved when a molar amount of available coordination sites was a certain excessive amount (coordination site/ligand ratio similar to 1.6). The molar ratio at the maximum T-gel is nearly independent of the number of ligands per triblock copolymer. On the other hand, the number of ligands per triblock copolymer affected the T-gel, where a larger number of ligands per triblock copolymer gave a higher T-gel, regardless of almost the same molecular weight of triblock copolymers.