Interaction of ionomers and polyelectrolytes with divalent transition metal cations (Cu2+ and VO2+): A study by electron spin resonance (ESR) spectroscopy and viscosimetry

The interactions in aqueous media between ion-containing polymers (ionomers and polyelectrolytes) and the divalent cations Cu2+ and VO2+ were studied by electron spin resonance (ESR) at X and L bands and by viscometry. The po lymeric systems studied were poly(ethylene-co-methaclylic acid) (EMAA) iono mer, perfluorinated ionomer (Nafion), poly(acrylic acid) (PAA), and poly(st yrene sulfonic acid) (PSSA). ESR spectra indicate immobilization of both ca tions in EMAA solutions, due to cation ligation to two carboxylic groups fr om a multichain micelle. In PAA, the attachment of the cations to the polym er chain depends on the pH: no attachment at low pH (1.5), and progressive bonding as the pH is increased. Two types Of CU2+/PAA complexes were detect ed and were assigned respectively to ligation of the cation to one and to t wo carboxylic groups in the PAA chain. Only one type of complex with PAA wa s detected for VO2+ cations in the pH range up to 8.5. No evidence for comp lexation was detected for both cations in the Nafion and PSSA systems. Visc osity measurements indicate that the micellar dimensions in EMAA were reduc ed upon progressive addition of Cu2+ cations. We have also detected a time dependence of the reduced viscosity after dilution of the EMAA solutions, p ossibly 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 m ultiplets, and a "polyelectrolyte" regime in polar solvents such as water, where the ion pairs are dissociated. The present study indicates that the i nteractions responsible for ionomer or polyelectrolyte regimes depend not o nly on the solvent but also on the type of ionic groups in the polymer, the type of counterions, and the pH.