SELF-ASSEMBLING OF PERFLUORINATED POLYMERIC SURFACTANTS IN NONAQUEOUS SOLVENTS - ELECTRON-SPIN-RESONANCE SPECTRA OF NITROXIDE SPIN PROBES IN NAFION SOLUTIONS AND SWOLLEN MEMBRANES

Structural details on the self-assembling of perfluorinated ionomer (Nafion) chains in solutions and in swollen membranes have been obtained from ESR studies in systems containing doxylstearic acid spin probes. Results previously obtained for aqueous systems are extended in this study to formamide (FA), ethanol (EtOH), and N-methylformamide (NMF) as solvents. The slow-motional ESR component detected in swollen membranes and solutions in FA has been assigned to spin probes bound to polymer aggregates. The additional, motionally averaged, component detected in FA solutions was assigned (at least in part) to spin probes associated with single chains. These assignments are similar to those in the aqueous systems. Comparison of the corresponding order parameters suggests that self-assembling of polymer amphiphiles occurs at higher polymer concentration and leads to less ordered aggregates in FA, compared to aqueous systems. The present ESR data do not indicate aggregation in Nafion/EtOH and Nafion/NMF solutions. The ESR spectra of the spin probes in membranes swollen by EtOH are consistent with a plasticizing effect of the solvent, rather than with a phase separated morphology. These conclusions are in agreement with two types of studies of Nafion membranes and solutions: by ESR in systems containing paramagnetic VO2+ as the counterion and by F-19 NMR. Based on the results for aqueous and FA systems, we propose a mechanism for the transition between the micellar structure in solution and the reverse micellar structure in the swollen membranes, which we call the fringed rod model. The model assumes that at high polymer concentrations some chains can be incorporated in more than one rod, thus effectively providing the cross-linking necessary for complete connectivity of the polymeric material.