Electron spin resonance measurements of Cu⁺⁺ distribution in cation exchange membranes

Abstract

SynopsisElectron spin measurement of the local Cu++ concentrations have been performed in different cation exchange membranes by comparison with reference frozen aqueous solutions of Cu++ salts.Comparison of the obtained values with the average chemical concentrations has therefore permitted direct evidence of microphase separation in these systems. Local concentrations approximately four times larger than average chemical concentrations have been found. Information about composition of this ion-containing phase has also been obtained from changes in local Cu++ concentrations versus water content. INTRODUCTIONWe are interested in the microstructure of ion-containing polymers, especially in this paper of polymeric cation exchange membranes. Ion associations will generally depend on both ion concentration and the dielectric constant of the matrix. Small ionic associations called multiplets, involving a few ion pairs and larger aggregates, have to be considered when the dielectric constant of the matrix is sma1l.l In polymeric membranes, the situation is complicated because the ions of one sign are fixed on the chain leading to an excluded volume effect. Larger ionic associations are possible which give specific mechanical and physical properties to these materials. Knowledge of the microstructures of these membranes is therefore very important to explain membrane characteristics such as permselectivity, ionic conductivity, water diffusion, etc. Different experiments can be performed involving bulk properties measurements up to very local investigation of specific nuclei.The knowledge of the local ionic concentration and its comparison with the overall chemical concentration will be an efficient way to define a possible ionic phase segregation and to give information about its composition. The aim of this paper is to estimate the local Cu++ concentrations by electron spin resonance (ESR) measurements of different Cu++ neutralized cationic membranes. Small-angle neutron scattering experiments have shown that no drastic changeJournal of Applied Polymer Science, Vol. 28,1093-1103 (1983 change in the spectra is observed when varying the concentration (Fig. 1). A few empirical parameters such as the widths at half and quarter of maximum and relative heights of various components have been used to characterize the spectra, and their change vs. Cu++ concentrations have been plotted in Figures 2 and 3.The various reference solutions obtained by changing both the nature of the salt (sulfate, nitrate, and bromure) and the cryoprotector (dimethylsulfoxide, glycerol, ethyleneglycol) have given essentially the same results for the same Cu++ concentration. The changes observed from one spectrum to another are small compared to the changes observed with a small modification of the Cu++ concentration. We can therefore assume in first approximation that an ESR spectrum is characteristic of the Cu++ concentration. Because the dipole-dipole and exchange interactions are short range, the measured concentrations are local concentrati...

Compatibilization of Polymer Blends by Complexation. 1. Spectroscopic Characterization of Ion−Amide Interactions in Ionomer/Polyamide Blends

“…EPR is sensitive to the local environment of paramagnetic cations. For Cu 2+ , different spectra are obtained depending upon whether the ions are isolated from each other or in close proximity. , In a dilute solution of a polar solvent, Cu 2+ (nuclear quantum spin number = 3 / 2 ) exhibits an EPR spectrum with four lines due to electron−nuclear spin−spin coupling . In concentrated solution, however, dipole−dipole and exchange interactions between the electron spins of neighboring Cu 2+ broaden the hyperfine structure, and in the limiting case of associated Cu 2+ ions, the hyperfine structure may coalesce into a single broad-line spectrum.…”

Determination of the distance between paramagnetic centers from ESR spectra at L, S and X bands. Copper(II) in Nafion ionomers

“…The distribution of cupric ions in Nafion as a function of cation concentration has been studied by Vasquez et al 24 The method used is based on comparing ESR spectra of Cu2+ in Nafion with those obtained from reference frozen aqueous solutions of Cu2+. It was mentioned that the results are difficult to interpret when ESR spectra of the cation in ionomers reveal two (or more) different local concentrations.…”

“…This method has been used to obtain Cu2+-Cu2+ distances in cell wall polyuronides as a function of cation concentration. 24 In the simulation of the perpendicular component, there are uncertainties, due to the fact that the value of A± cannot be resolved and accurately measured. In addition, in the perpendicular region, there are sometimes forbidden transitions and possibly a rhombic component of the hyperfine and/or of the g tensor.…”