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We have reported recently ESR spectra of perfluorinated ionomers ("Nafion") neutralized by Li+ and doped with doxylstearic acids and esters as spin probes. The dominant, slow-motional, ESR signal detected in aqueous solutions and in swollen membranes has been assigned to spin probes associated with aggregated polymer chains; an additional spectral component is motionally narrowed. In this study we present details on the self-assembling process of ionomer chains by modeling the dynamics of three spin probes that differ in their hydrophobicity and position of the nitroxide probe relative to the polar head. An axially symmetric rotational diffusion defined by the components of the rotational tensor and RI, and by the diffusion tilt angle 21, (the angle between the rotational diffusion symmetry axis and the direction of the nitrogen 2p, atomic orbital) as a model for the dynamics of the spin probes was assumed. Good agreement with experimental spectra was obtained by choosing a "multisite" model, which assumes a distribution of orientations for the rotational diffusion symmetry axis of the spin probes. The jump diffusion model with RLJZ = 1 was adopted. A distribution of orientations for the diffusion symmetry axis and a high anisotropy ratio N = RI& of the spin probe rotational diffusion (exceeding 100 in most cases) are the most important conclusions deduced from the simulations. High N values have been taken as an indication for the immobilization of the spin probes in large polymeric aggregates. The differences between the dynamics of the probes at a given temperature and between the behavior of the probes in solutions and in the membranes are interpreted in terms of different locations of the nitroxide group of the probes in the aggregates. This study suggests that doxylstearic type spin probes are excellent indicators for the dynamics in perfluorinated amphiphilic systems.

Steady-state and time-resolved fluorescence measurements were performed on aqueous solutions of poly(ethylene-co-methacrylic acid) (EMAA) ionomers using pyrene (P) and 1,3-bis(1-pyrenyl) propane (P3P) as the luminophores and the nitroxide radical 5-doxylstearic acid methyl ester (5DSE) as the fluorescence quencher. The ionomers contained 7.5 mol % methacrylic acid and were neutralized (90%) with KOH. The fluorescence spectra of P and P3P together with the electron spin resonance (ESR) spectra of 5DSE indicate that all probes are located in an environment of low polarity and high viscosity that was identified as the hydrophobic micellar core of EMAA aggregates. The local polarity was estimated from the intensity ratio of the third to the first vibronic peaks (I III /I I) in the fluorescence spectrum of P. The critical micelle concentration (CMC) of the ionomer, CMC = 0.02% (w/w) EMAA, was deduced from the dependence of I III /I I on the ionomer concentration. The spectroscopic data (ESR and fluorescence) point to the existence, below the CMC, of intramolecular (unimeric) micelles, which are in equilibrium with large aggregates above the CMC. The microviscosity of the micellar core, η, was estimated to be ≥230 cP at ambient temperature on the basis of the fluorescence spectra of P3P, from a calibration curve of the intensity ratio of the excimer to monomer emissions, I E /I M, vs viscosity in 14 nonaqueous solvents of known viscosities. The fluorescence decay of P in EMAA in the presence of 5DSE as the quencher was analyzed with two kinetic models, the Infelta−Tachiya model based on a Poisson distribution of the quencher and the luminophore in the micelles and the general approach of dispersive kinetics that introduced the time-dependent rate coefficient, k qm, for intramicellar quenching. The second model gives a better fit to the experimental data, and the low value of the dispersive parameter, α = 0.30 (compared to unity for classical kinetics), indicates a broad distribution of the quenching rate constants. The results obtained in this study add important structural details to the recent models for self-assembly of EMAA ionomers, which were deduced from spin probe ESR spectroscopy and small-angle neutron scattering.