Structure and dynamics of ionic aggregates in ethylene ionomer membranes: recent electron spin resonance (ESR) studies

In this review we present recent advances in the understanding of self-assembling, structure, and dynamics in poly(ethylene-co-methacrylic acid) (EMAA) ionomers as membranes and water dispersions, with focus on the information obtained by electron spin resonance (ESR) studies of amphiphilic and hydrophobic nitroxide spin probes. The probes selected for this study were doxylstearic acids and esters, with the doxyl group attached to carbon atoms at different positions relative to the head group, and a nitroxide group attached to a nonadecane backbone. Application of this approach to the study of EMAA ionomers has demonstrated that, by a judicious choice of the polarity, size, and chemical structure of the probe, it is possible to explore specific regions of the microphase-separated system; an important advantage of the 'position-selectivity' method is the ability to bypass the often difficult synthetic step of spin labeling. Information on the local environment of the probes and on chain mobility in, and in the vicinity of, the ionic aggregates in the EMAA ionomers will be discussed, and compared with results obtained by other techniques.