STRUCTURE AND DYNAMICS OF PERFLUORINATED IONOMERS IN AQUEOUS-SOLUTIONS AND SWOLLEN MEMBRANES BASED ON SIMULATIONS OF ESR-SPECTRA FROM SPIN PROBES

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 tenser R(parallel to) and R(perpendicular to), and by the diffusion tilt angle theta (the angle between the rotational diffusion symmetry axis and the direction of the nitrogen 2p(z) 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 R(perpendicular to parallel to)tau = 1 was adopted. A distribution of orientations for the diffusion symmetry axis and a high anisotropy ratio N = R(parallel to)/R(perpendicular to) 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.