Control of gel morphology and properties of a class of metallo- supramolecular polymers by good/poor solvent environments

Through the utilization of a mixed solvent system, consifsting of a good solvent (DMSO) with either a nonsolvent (water) or a poor solvent (ethylene glycol), the morphology and properties of a class of stimuli-responsive métallo-supramolecular gels can be systematically tailored. These gels are formed via selfassembly of a ditopic ligand, consisting of a 2,6-bis(1′-methylbenzimidazolyl)-4-oxypyridine moiety attached to either end of a penta(ethylene glycol) core, in the presence of a transition metal ion (Zn ^II). In each solvent system, a composition window is located in which gels, varying in turbidity from highly opaque (water- or ethylene glycol-rich) to highly transparent (DMSO-rich), are formed. Morphological observations suggest gelation occurs by a common mechanism throughout the composition range, i.e., via the flocculation of semicrystalline colloidal particles. Increase of DMSO content leads to a reduction in the particle size, accompanied by an increase in sol concentration. WAXD and SAXS analysis indicates that the degree of crystallinity of the colloidal particles is dramatically decreased compared with those formed in a previous study using acetonitrile as solvent but that a new lamellar organization develops as the DMSO content increases, evidently reflecting incorporation of the good solvent into the network structure. These structural changes are accompanied by an increase in the shear storage modulus of the gel up to a maximum value, beyond which the modulus decreases. Thus, by tailoring the solvent composition, it is possible to produce highly transparent gels that, while very strong, are highly mechanoresponsive; i.e., they exhibit pronounced yielding and thixotropic behavior.