Morphology of sulfonated polyimide ionomers from ESR spectra of paramagnetic transition metal cations and nitroxide spin probes

We present a study of sulfonated polyimide block ionomer membranes (SPIs) in the dry state, and swollen by water, methanol, ethanol, and dioxane. The ionomers are based on a naphthalenic dianhydride, and differ in the ionic exchange capacity and the type of diamine in the hydrophobic block. The ionomers were studied by electron spin resonance (ESR) spectroscopy of the paramagnetic transition metal cations Cu+ and VO2+, and of two nitroxide spin probes. The results indicated the existence of separate hydrophobic and hydrophilic domains in the dry and in the swollen membranes. Water clusters with a diameter < 40 A were detected in the water-swollen membranes, but the water has glassy properties and a much lower enthalpy of fusion compared to bulk water. No evidence for the presence of solvent clusters was obtained for the membranes swollen by methanol and ethanol. An ESR signal was observed in all neat SPI ionomers, and its intensity and line shape varied with ionomer structure and preparation method. The ESR intensity of this signal is higher in the membranes in acid form compared to membranes fully neutralized by Li+ or K+. The irreversible increase of the signal intensity upon heating of the dry membranes above 360 K suggests the formation of reactive intermediates that may be involved in ionomer degradation processes. (author)