Interaction of ionomers and polyelectrolytes with divalent transition metal cations (Cu~(2+) and VO~(2+)): A study by electron spin resonance (ESR) spectroscopy and viscosimetry

摘要

The interactions in aqueous media between ion-containing polymers (ionomers and polyelectrolytes) and the divalent cations Cu~(2+) and VO~(2+) were studied by electron spin resonance (ESR) at X and L bands and by viscometry. The polymeric systems studied were poly(ethylene-co-methacrylic acid) (EMAA) ionomer, perfluorinated ionomer (Nafion), poly(acrylic acid) (PAA), and poly(styrene sulfonic acid) (PSSA). ESR spectra indicate immobilization of both cations in EMAA solutions, due to cation ligation to two carboxylic groups from a multichain micelle. In PAA, the attachment of the cations to the polymer chain depends on the pH: no attachment at low pH (1.5), and progressive bonding as the pH is increased. Two types of Cu~(2+)/PAA complexes were detected and were assigned respectively to ligation of the cation to one and to two carboxylic groups in the PAA chain. Only one type of complex with PAA was detected for VO~(2+) cations in the pH range up to 8.5. No evidence for complexation was detected for both cations in the Nafion and PSSA systems. Viscosity measurements indicate that the micellar dimensions in EMAA were reduced upon progressive addition of Cu~(2+) cations. We have also detected a time dependence of the reduced viscosity after dilution of the EMAA solutions, possibly due to a redistribution of micelle sizes. The results were examined in light of the models for ion-containing polymers, which have proposed an `ionomer' regime in nonpolar solvents where the ion pairs aggregate into multiplets, and a `polyelectrolyte' regime in polar solvents such as water, where the ion pairs are dissociated. The present study indicates that the interactions responsible for ionomer or polyelectrolyte regimes depend not only on the solvent but also on the type of ionic groups in the polymer, the type of counterions, and the pH.