Recent advances in perfluorinated ionomer membranes : structure, properties and applications

This paper deals with the structure and chemistry of Nafion-H, the acidic form of Nafion, which is the trade name for a perfluorinated polymer manufactured by E. I. du Pont de Nemours & Co., Inc. Nafion-H and its salts are available from Dupont in both membrane and powder forms. Nafion composites with Teflon mesh (polytetrafluoroethylene) added to give increased mechanical strength are also available. In addition, sulfonated fluorocarbons modified with other chemically active polymers have been made by Dupont and by competing companies such as Ashai Chemical Industry Co., Ashai Glass Co., Tokuyama Soda Co., and Toya Soda Co.

Structure and chemistry of nafion-h: a fluorinated sulfonic acid polymer

Pillared smectites in which the pillars consist of iron oxide are expected to have interesting and unusual magnetic properties. Several possible routes by which such materials might be made have been investigated, namely intercalation of hydroxy-Fe(III) polycations, mixed hydroxy-Fe(III)/Al polycations, phenanthroline-Fe(II) cations, and trinuclear Fe(III) acetato cations into Na-montmorillonite. Only the last of these yielded a pillared clay (PILC) on calcination. The products have been characterized using X-ray powder diffraction and 57Fe Mossbauer spectroscopy. The precursor Fe-PILC has a d-spacing of 21 A and expands to 23 A on solvation with glycol. The calcined Fe-PILC has a d-spacing of 19 A (gallery height 9·4 A) and does not expand with glycol, confirming cross-linking of the layers. From Mossbauer spectra at 4·2 K it is estimated that there are of the order of some hundred Fe atoms per pillar.

Preparation and characterization of iron oxide pillared montmorillonite

This study presents the detailed nature of iron clusters formed on Fe3+-Nafion membranes. The catalytic nature of these clusters during immobilized Fenton processes was observed to be a function of the deposition method of Fe ions on the Nafion. The nonbiodegradable azo-dye Orange II and 2-propanol were utilized as convenient organic model compounds in photoassisted Fenton degradation processes. The highest photocatalytic activity was observed when samples were prepared by ion exchange between iron(III) aqua-complexes and H+ or Na+ as counterions of the Nafion SO3- group. Spectroscopic techniques show that iron(III) in the membrane was present mainly as a mononuclear complex of [Fe(H2O)(6)](3+) and binuclear complexes [Fe(H3O2)Fe](5+) and [Fe-O-Fe](4+). If NaOH or ammonia was added to the former samples prepared by ion exchange, Nafion-Fe membranes with low photocatalytic activity were obtained showing alpha-Fe2O3 and [Fe-O-Fe](4+). Detailed high-resolution transmission electron microscopy was carried out for the Nafion-Fe ion-exchariged and also base-treated membranes showing alpha-Fe2O3 nanocrystallites of 3.5-5 nm. Spectral bands were found for iron oxides in the Fe3+-Nafion by femtosecond laser spectroscopy. The alpha-Fe2O3 nanocrystallites in the Nafion exchanged base-treated membranes presented a relaxation dynamics for the excited states close to that observed with alpha-Fe2O3 nanocrystallite colloids taken as reference compounds. Multiexponential transient absorption decay of alpha-Fe2O3 in SO3--water clusters was observed with time constants close to 320 fs, 1.5 ps, and 31 ps after the excitation pulse. Samples of Fe3+-Nafion membranes with-high activity show different transient dynamics relative to the Fe3+-Nafion with low activity. Correlation of the photocatalytic activity of Fe3+-Nafion with UV-vis, Fourier transform infrared, Mossbauer, and X-ray photoelectron spectroscopic results suggests that the photocatalytic activity correlates with the amount of mononuclear [Fe(H2O)(6)](3+), binuclear complexes [Fe(H3O2)Fe](5+) and oxo-bridged [Fe-O-Fe](4+) found in the membranes.

Catalytic Fe3+ Clusters and Complexes in Nafion Active in Photo-Fenton Processes. High-Resolution Electron Microscopy and Femtosecond Studies

Small-angle X-ray scattering profiles of the ethylene and styrene ionomers were studied to clarify the structure of ionic aggregates as a functions of the nature of cations, acid content, and degree of neutralization. The ionic cluster peak was observed for the ionomers with a certain degree of neutralization. The scattering profile of the ionic cluster peak was analyzed by the modified hard-sphere model proposed by Yarusso and Cooper , and the size of ionic aggregates, closest approach distance between the aggregates, and average system volume per ionic aggregate were obtained. The size and distance of the ionic aggregates in the ionomers with monovalent cation, sodium and potassium cations, increased with the ion content (acid content × degree of neutralization), while those with divalent one, zinc and copper cations, and trivalent one, ferric cation, changed only slightly. The number of cations in an ionic aggregate was evaluated in an approximate manner by using the average system volume per ionic agg...

Structure of Ionic Aggregates of Ionomers. 1. Variation in the Structure of Ionic Aggregates with Different Acid Content and Degree of Neutralization of Ethylene and Styrene Ionomers

Mossbauer spectra of “Nafion” perfluoronated acid membranes exchanged with iron or europium have been studied as a function of ion concentration, water content, temperature, and applied magnetic field in order to characterize the ionic phase in these materials In every sample, the recoil-free fraction falls to zero at a temperature which decreases from 280 to 220 K with increasing water content The sudden fall in recoil-free fraction corresponds to a glass transition in the ionic phase rather than its melting point In Fe2+ Nafion, the spectrum is independent of water content above 6 wt%, and is typical of fully hydrated, isolated Fe2+(H2O)6 species The isomer shift, quadrupole splitting, and linewidth all change continuously below 6 wt % water, indicating a range of environments for the iron For Fe3+ Nafion, ions in several distinct environments can be identified on the basis of their magnetic properties Isolated Fe3+ species (Fe–Fe distances greater than 12 A) showing paramagnetic hyperfine structure in their 42-K spectra predominate in slightly neutralized samples, and in those with high water content The proportion of dimers, whose quadrupole splitting is about 17 mm/s, increases with increasing iron content or decreasing water content A third component of the spectra has a quadrupole splitting of 04 mm/s It is associated with paramagnetic iron having other iron ions in the neighborhood, and also with iron ions located in the interior of small groups or chains formed from dimers There is no iron belonging to magnetically ordered clusters involving hundreds of ferric ions of the type found in some other ionomers All the exchanged ions belong to the ionic phase, and none are associated with the polymer backbone

A. Meagher

Papers

Möussbauer spectroscopy of nafion polymer membranes exchanged with Fe2+, Fe3+, and Eu3+

Microstructure of iron(III) α,ω-dicarboxylatopolybutadiene