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

Abstract

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 (Tgel) as an indicator of gelation was achieved when a molar amount of available coordination sites was a certain excessive amount (coordination site/ligand ratio ∼1.6). The molar ratio at the maximum Tgel is nearly independent of the number of ligands per triblock copolymer. On the other hand, the number of ligands per triblock copolymer affected the Tgel, where a larger number of ligands per triblock copolymer gave a higher Tgel, regardless of almost the same molecular weight of triblock copolymers.