Spin Probe ESR Study of Cation Effects on Methanol and DMMP Solvation in Sulfonated Poly(styrene-isobutylene-styrene) Triblock Copolymers at High Ion-Exchange Capacities

摘要

The solvent uptake properties of sulfonated poly(styrene-isobutylene-styrene) (sSIBS)triblock copolymers exchanged with Mg2+, Al3+, Ba2+, and Cs+ ions are investigated using the electron spin resonance (ESR) spin probe method with the nitroxide probes 2,2,6,6-tetramethyl-4-piperidone N-oxide (TEMPONE) and 4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPOL). sSIBS membranes were equilibrated with varying amounts of methanol, dimethyl methyl phosphonate (DMMP), both as pure solvents and as solvent-water mixtures. As a standard for comparison, Li+ ion exchanged Nafion 117 membranes were also characterized using the same spin probes and solvent combinations. Solvent uptake trends were measured for each of the membrane-solvent combinations. Some membranes, particularly those exchanged with Cs, preferentially uptake water in the presence of organic solvent. sSIBS membranes containing pure methanol orDMMPexhibit two phases which were assigned to the sulfonated styrene block of the polymer, and to the hydrophobic isobutylene block. Whereas methanol enhances the phase separation,DMMPpenetrates the less mobile hydrophobic phase at a concentration that depends upon the identity of the exchange ion, which increases in the order Cs+>Ba2 Al3+>Mg2+ Thus, cations appear to disrupt the lamellar structure of the polymer, making the nonpolar phase more susceptible to permeation by the solvent. This disruption correlates with the free volume contribution to the rotational diffusion of the probe in both phases, suggesting that the cations affect the solvation of the different phases of sSIBS mainly through their influence on the ordering of the solvent. No clear correlation of this effect with either ionic radius or charge could be identified.