Effect of monomer structure on the gelation of a class of metallo-supramolecular polymers

Gelation of metallo-supramolecular polymers prepared from three ditopic ligand monomers, 1, 2 and 3, derived, respectively, from tetra-, penta-, and hexaethylene glycols, in the presence of the transition metal ion Zn ^II, as well as the properties of the resulting gels, are investigated and compared. It is found that these slight changes in the core length of the monomer lead to substantial variations in the gel characteristics, which can be attributed to solubility differences between the corresponding metallo-supramolecular polymers. Observations of gelation kinetics indicate 3:Zn has the slowest and 1:Zn the fastest rate. Rheological measurements of the gelation kinetics of 1:Zn indicate unusual behavior in that the modulus exhibits a distinct maximum at a gel time of around 15-30 min. The origin of this phenomenon is suggested to be the transient formation of a metastable colloidal structure which occurs on account of the fast phase separation kinetics of 1:Zn and subsequent particle densification via redistribution of solvent. Under equivalent conditions of solvent quality and polymer concentration, the shear modulus of 2:Zn gels is substantially higher than that of 1:Zn and 3:Zn. It appears that the lower modulus of 1:Zn is a consequence of its lower solubility, which results in smaller, more dense colloidal particles and hence a lower particle volume fraction. On the other hand, 3:Zn gels also have a lower modulus than 2:Zn on account of their higher solubility, resulting in a larger sol fraction under the equivalent conditions.