A Fluorescent Probe Investigation of the Origin of Superselectivity in Perfluorinated Ionomer Membranes

Robertson, M. A. F.; Yeager, H. L.

doi:10.1021/ma951616p

Cited by 30 publications

(25 citation statements)

References 29 publications

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“…The results with pyrene, however, suggest that water is present inside the fluorinated micelles even if the probe is very hydrophobic and not solvated by water. The presence of water in the Nafion aggregates can also explain the previous results for pyrene fluorescence in Nafion membranes swollen by water, which also suggested a polar environment …”

Section: Discussionsupporting

confidence: 74%

Structure and Dynamics of Micellar Aggregates in Aqueous Nafion Solutions Reported by Electron Spin Resonance and Fluorescence Probes

et al. 1999

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Aqueous solutions of Nafion, an ionomer consisting of a perfluorinated backbone and pendant sulfonic groups, were studied by the electron spin resonance (ESR) spin probe method and by fluorescence spectroscopy. The spin probes used were 4-(N,N-dimethyl-N-alkyl)ammonium-2,2,6,6-tetramethyl-piperidine-1-oxyl cations (CATn) with n, the number of carbon atoms in the alkyl group of the probe, equal to 1 (CAT1), 8 (CAT8), and 16 (CAT16). The ESR spectra of the probes were analyzed in terms of line shapes as a reflection of the local dynamics and 14N hyperfine splittings as indicators of the local polarity. The line shapes point to slower dynamics for the larger probes, CAT8 and CAT16, suggesting that these probes are located deeper inside the aggregates compared to CAT1. The high local polarity reflected in the magnetic parameters was explained by assuming water penetration into the aggregates. The local polarity was estimated from the intensity ratio, R, of the third to the first vibronic peaks (R = I III/I I) in the fluorescence spectrum of pyrene (P) as a probe; R is the polarity index. In the Nafion micelles R = 0.64, indicating a polar local environment, in accord with the ESR results. By comparison, R is 0.53 in water, 0.75 in methanol, and 1.91 in a perfluorinated oil. The fluorescence data also suggest that the upper limit of the critical micelle concentration, cmc, for Nafion is 0.01% w/w. The local viscosity was estimated to be 74 cP at ambient temperature from the fluorescence spectrum of 1,3-bis(1-pyrene)propane (P3P), based on a previously determined calibration curve of the intensity ratio of the excimer-to-monomer emissions, I E/I M, vs viscosity for 14 nonaqueous solvents of known viscosities.

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Section: Discussionsupporting

confidence: 74%

Structure and Dynamics of Micellar Aggregates in Aqueous Nafion Solutions Reported by Electron Spin Resonance and Fluorescence Probes

et al. 1999

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“…A well known example of artificial membranes is perfluorinated ionomer membranes (Nafion ® ), which have good mechanical properties and high selectivity for separating cations [70]. These systems have how ever a number of drawbacks.…”

Section: Self Assembling Systems Based On Benzenesulphonic Acid With mentioning

confidence: 99%

Self-assembling supramolecular systems of different symmetry formed by wedged macromolecular dendrons

et al. 2012

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The main stages of the self assembling of supramolecular ensembles have been revealed by study ing different functional wedged macromolecules: polymethacrylates with tapered side chains based on gallic acid, their macromonomers, and salts of 2,3,4 and 3,4,5 tris(dodecyloxy)benzenesulphonic acid. The first stage is the formation of individual supramolecular aggregates (long cylinders or spherical micelles) due to the weak noncovalent interactions of mesogenic groups and the subsequent ordering in these aggregates, which is accompanied by a decrease in the free energy of the system. Supramolecular aggregates, in turn, form 2D or 3D lattices. The shape of supramolecular aggregates and its change with temperature are delicate func tions of the mesogen chemical structure; this circumstance makes it possible to rationally design complex self assembling systems with the ability to respond smartly to external stimuli. X ray diffraction analysis allows one to study the structure of supramolecular systems with different degrees of order, determine the type of mesophases formed by these systems, and reveal the phase behavior of the material. Particular attention has been paid to the method for reconstruction of electron density distribution from the relative reflection intensity. The application of a suite of experimental methods, including wide and small angle X ray diffrac tion, molecular modeling, differential scanning calorimetry, and polarization optical microscopy, allows one to establish the relationship between the shape of the structural unit (molecule or molecular aggregate), the nature of the interaction, and the phase behavior of the material.

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“…Bulk ionomers have a separated morphology consisting of polar and nonpolar domains; in the presence of polar solvents such as water or alcohols, solvent clusters are formed, which contain the counterions and the ionic headgroups . Fluorescent probes such as metal ions, metal complexes, and polycyclic aromatic hydrocarbons have been examined in ionomers, and the intensity, lifetime, and quenching of their emission have been interpreted in terms of the size and polarity of the solvent clusters depending on the type of solvent and counterion, and the diffusion coefficients of the counterions; the variation of the fluorescence wavelength with temperature has been linked to rearrangements of the solvent cluster. − …”

Section: Introductionmentioning

confidence: 99%

Fluorescence Studies of Self-Assembly in Aqueous Solutions of Poly(ethylene-co-methacrylic acid) Ionomers

et al. 1998

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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.

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A Fluorescent Probe Investigation of the Origin of Superselectivity in Perfluorinated Ionomer Membranes

Cited by 30 publications

References 29 publications

Structure and Dynamics of Micellar Aggregates in Aqueous Nafion Solutions Reported by Electron Spin Resonance and Fluorescence Probes

Structure and Dynamics of Micellar Aggregates in Aqueous Nafion Solutions Reported by Electron Spin Resonance and Fluorescence Probes

Self-assembling supramolecular systems of different symmetry formed by wedged macromolecular dendrons

Fluorescence Studies of Self-Assembly in Aqueous Solutions of Poly(ethylene-co-methacrylic acid) Ionomers

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