Perfluorinated Ionomer Membranes Research Articles

Atomistic simulation and molecular dynamics of model systems for perfluorinated ionomer membranes

An atomistic model for perfluorinated ionomer membranes (PIMs), in particular Nafion materials, is presented and used in conjunction with NVT molecular dynamics simulations to investigate the dynamic and configurational properties of these polymers. It is found that the electrostatic term in the force field is responsible for the formation of an apparently phase separated morphology which is selectively conductive, favouring the passage of cations. Specifically, the mobility of H3O+ ions is found to be ∽3.2 times greater than that of OH- ions, under the application of an external electric field. This phenomenon is shown to be consistent with a jump diffusion model of ion transport in PIMs. There is also evidence for the existence of water in two distinct environments in the simulations: both tightly bound to ion exchange groups, and more loosely associated with the fluorocarbon matrix.

Organic-inorganic hybrid materials: perfluorinated ionomers as sol-gel polymerization templates for inorganic alkoxides

Mauritz et al. exploited the polar/nonpolar nanophase-separated morphologies of Nafion® perfluorinated ionomer membranes, as well as a Nafion® ionomeric precursor film, as interactive templates that are capable of directing the condensation polymerizations of inorganic alkoxides and organoalkoxysilanes as well as the ultimate geometrical distribution of the inorganic oxide, or organically modified silicon oxide phases that result upon drying. This paper is a review of our extensive studies involving the in situ sol-gel reactions of the alkoxides of silicon, titanium, aluminum, zirconium and organoalkoxysilanes, as well as their mixtures and two step reactions involving these monomers. Throughout this presentation, we demonstrate how various spectroscopic (FTIR, 29Si solid state NMR, dielectric relaxation, pyrene (Py) fluorescence probe emission + UV absorption) microscopic, X-ray scattering, thermal (DSC, TGA, TGA-FTIR, DMA), mechanical tensile, and gas permeation tools were employed to interrogate the structures and properties of these heterogeneous materials over a range of dimensional scales ranging from the molecular to the macroscopic. Importantly, we established that these organic/inorganic materials are indeed structured on the scale of nanometers. Methods of tailoring the energetic environment, or polarity, within the cluster domains by the insertion of inorganic oxide or organically modified silicate nanostructures are presented. Finally, we discuss the potential for these nanocomposite membranes within a number of areas including gas and liquid separations technology as well as proton exchange membranes for fuel cell technology.

Steady-state and time-resolved fluorescence spectroscopic probing of microstructures and properties of perfluorinated polyelectrolyte membranes

A systematic luminescence spectroscopic study of the microstructures and properties of Nafion®, the Dow Chemical films, and the newly synthesized sulfonimide ionomer membrane is reported. The strongly environment-sensitive luminescent probes 6-propionyl-2-(N,N-dimethylamino)naphthalene, 4-(N,N-dimethylamino)benzonitrile, pyrene, and ethidium bromide were used. The results suggest that the different perfluorinated ionomer membranes have generally similar properties with respect to the luminescent molecular probes and that the membrane structures consist of a substantial region that may be characterized as a heterogeneous mixture of water molecules and branches of the perfluorinated polymers. In addition, the effects of thermally stressing the ionomer membranes in an aqueous solution at temperatures up to 250°C on the properties of the membranes are evaluated and discussed.

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

The fluorescent probes pyrene and Ru(bpy)32+ were used to study the relative locations of cations and anions in microphase separated perfluorinated ionomers. After determining that these probes are located in the interfacial regions that separate the fluorocarbon phase from the ion and water-rich phase, cationic and anionic quencher ions were introduced into the system and changes in fluorescence monitored. Results show that Cs+ is located on the aqueous side of the interfacial region, in close proximity to pyrene, while I- is located deeper into the interfacial region and farther away from the aqueous phase. These differences are greater for carboxylate ionomers than for sulfonate ionomers, in close parallel to their relative abilities to discriminate against anion transport in practical membrane-based applications. These results show that the phenomenon of superselectivity is based on the differences in the average locations and therefore the transport pathways of cations and anions in perfluorinated io...

Structure of Perfluorinated Ionomer Membranes Incorporating Organic Cations

Abstract The structure of perfluorinated ionomer membranes (Nafion®) incorporating large organic cations, tetraethylammonium (TEA+) and 1,1′-dialkyl-4,4′-bipyridinium cations (CnV2+, n = 3 and 8), is studied using small-angle X-ray scattering (SAXS). For TEA+- and C3V2+-form membranes, the SAXS maxima show the presence of the ionic clusters as large as those for Na+-form (ca. 4 nm in diameter). For C8V2+-form membranes, the intercluster distance expanded to a value larger than that for H+-form after the ion-exchange reaction for 5 days. However, the water content and cluster size decreased significantly after the ion-exchange reaction for 50 days or boiling treatment.

Effects of Incorporation of Fluorocarbon and Hydrocarbon Surfactants into Perfluorosulfonic Acid (Nafion) Membranes

Perfluorinated ionomer membranes such as Nafion have numerous uses in both industrial chemical practice and in chemical research. Applications include the chloralkali process, H[sub 2]/O[sub 2] fuel cells, biomedical sensing, and other types of chemical sensors based on modified electrodes. Significant permeability improvements can be made to perfluorinated ionomer films by incorporating sulfonated surfactants of suitable size into the membrane microstructure. A variety of 20-[mu]m composite Nafion/surfactant membranes were prepared from DMF casting solutions containing Nafion and the sodium salts of perfluoro-1-butanesulfonic acid (perf-ButSO[sub 3]Na), perfluoro-1-octanesulfonic acid (perf-OctSO[sub 3]Na) and 1-octanesulfonic acid (OctSO[sub 3]Na). The time required for 50% extraction of the surfactants from the membranes into water was 1 min for OctSO[sub 3]Na, 5 min for perf-ButSO[sub 3]Na, and approximately 3 days for perf-OctSO[sub 3]Na. Extraction of perf-OctSO[sub 3]Na into isooctane contacting solutions was not observable over periods of days. For membranes containing surfactants and exchanged with silver(I) ion, 3-fold permeability improvements can be obtained for the separation of 1,5-hexadiene from 1-hexene and n-hexane without any decreases in separation factors. Observed flux improvements are larger than the increase in ion-exchange site density and are attributed to increased mobility of olefins between carrier sites due to the presence of specificmore » surfactants. Results indicate that movement of olefins in Nafion occurs primarily through an interfacial region of the film structure. The ability of a surfactant to improve transport performance is dependent on its ability to partition into the interfacial region. 27 refs., 3 figs., 2 tabs.« less

A Nuclear Magnetic Resonance Study of Water Behavior in PTFE Ionomer Membranes

High field 1H nuclear magnetic resonance has been conducted to investigate the ionic motion in perfluorinated ionomer membranes from Dow Chemical (XUS). Two distinctive proton resonance peaks were found in the vacuum‐dried samples. The designation of the upfield peak to the 1Hs associated with the sulfonate groups was proved by the fact that the intensity of the low‐field peak increases drastically in the less‐dried specimens. An asymmetrical V‐curve was observed for the spin‐lattice relaxation time plotted against the reciprocal temperature with samples saturated with , showing a distribution of the relaxation times centered around 0.3 ns. At all concentrations the major linewidths were essentially constants above certain transition temperatures and were broadened by more than one order of magnitude at reduced temperatures, yielding a similar activation energy to that from dc conductivity measurement. Quadrupole splitting was observed in the 2H spectrum at 360 K, and it reduced to a single resonance line when the sample was cooled, showing that in XUS ionomer the averaging of the quadrupole interaction is stronger than that in NafionR.

X-Ray Scattering Study of New Perfluorinated Ionomers

XUS perfluorinated ionomer membrane developed by Dow Chemical was investigated. Wide angle X-ray scattering results showed that this polymer does not involve detectable crystalline states and has a similar structure to the amorphous poly(tetrafluoroethylene). By 30% elongation a new maximum at 2θ= 10° and an upturn at lower angle developed. These amorphous halos are analyzed by the Fourier transformation method and a qualitative picture resolved for the changes due to the stretching

Local and long-range structure of water in a perfluorinated ionomer membrane

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTLocal and long-range structure of water in a perfluorinated ionomer membraneE. M. Lee, R. K. Thomas, A. N. Burgess, D. J. Barnes, A. K. Soper, and A. R. RennieCite this: Macromolecules 1992, 25, 12, 3106–3109Publication Date (Print):June 1, 1992Publication History Published online1 May 2002Published inissue 1 June 1992https://doi.org/10.1021/ma00038a013RIGHTS & PERMISSIONSArticle Views151Altmetric-Citations30LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (484 KB) Get e-Alerts Get e-Alerts

Water sorption and ionic diffusion in short side chain perfluorosulfonate ionomer membranes

Several properties of short side chain perfluorosulfonate ionomer membrane films, made by the Dow Chemical Company, have been measured as a function of equivalent weight of the polymer. Water and electrolyte sorption and the self-diffusion coefficients of Na +, Cs +, Cl −, and I − have been measured. along with activation energies of diffusion. Also, the near infrared combination band for OH has been used to develop insight into the nature of sorbed water in the ion clusters of these micro-phase separated materials. Results show that these polymers have properties which are similar to Nafion ® perfluorinated ionomer membranes, with their diffusional properties being intermediate between those of the sulfonate and carboxylate polymers. Equivalent weight was found to influence the spreads in diffusion coefficients among ions, which increase with increasing equivalent weight. The sorbed water for these polymers show significantly reduced hydrogen bonding strength compared to bulk water, and this property is counter-ion dependent.

Behavior of water in perfluorinated ionomer membranes containing various monovalent cations

The phase transition behavior of water in the perfluorosulfonate ionomer membrane, Nafion, was investigated using a differential scanning calorimeter (DSC). The equivalent weight of the membrane used was 1.100 g/eq.. Three kinds of alkali ions, ammonium ion and four kinds of alkyl ammonium ions were used as the membrane counter ion. Water molecules in the membranes were classified into three types, non-freezing, freezing bound and free water. From the heat of crystallization, the amount of each type of water was determined. The number of non-freezing water and freezing bound water per ion exchange site was calculated from the equilibrium amounts of each type water. The number of non-freezing and freezing bound water molecules per ion exchange site, which increased with increasing number of methyl groups of the alkyl ammonium ion, corresponds well with the permeate flux and the selectivity increased with increasing interaction between water molecules and alkyl ammonium ions. For the alkali ion series, the opposite relationship was observed between permeate flux, selectivity and number of non-freezing water per each ion, because the ionic cluster-size swelling with water determined the permeate behavior.

Proton and deutron NMR study of PTFE ionomer membranes

Proton and deutron NMR have been conducted to investigate the ionic motion in perfluorinated ionomer membranes from Dow Chemical (XUS) and DuPont (Nafion R). Two proton relaxation peaks were found in the XUS specimen absorbed with H 2O. The major (narrow) peak presented a spin-lattice relaxation time ( T 1) of 107 ms while the minor (broader) one gave much longer T 1. While the former was attributed to the water molecules involved in restricted motion, the latter was expected to be associated with the protons located in the vicinity of the sulfonate groups. Similar to the previous results from the others, only a single peak was detected in Nafion R in 1H spectra, indicating that the protons in the different environments were engaging rapid exchange within NMR time scale. In contrast to the inverse proportion dependence of the linewidth on the water sorption in Nafion R, the major line of the XUS membrane exhibited insensitive linewidth dependence on the variation of H 2O concentration. The difference was attributed to the existence of narrow breaths of the pores in XUS sample, such that free water contribution to the enhancement of proton mobility was limited. The 2H spectra of Nafion R were found to possess a doublet, due to nuclear quadrupolar interaction. Dow (XUS) membrane treated in at 100% relative humidity (RH) D 2O presented a single peak with the linewidth insensitive to the amount of heavy water absorbed. An additional rise emerged on the “shoulder” of this single peak when treated at 33% RH. It is concluded that XUS membrane does not provide strong hydrogen bonding to eliminate the rapid motion average over the nuclear quadrupole interaction.

Medium dependence of the spectroscopic and photophysical properties of Ru(bpy) 2(HAT) 2+. The effect of solvent, pH and binding to polyelectrolytes

In order to assess its usefulness as a probe for medium properties, the absorption, emission and excited state decay behaviour of Ru(bpy) 2(HAT) 2+ (bpy,2,2′-bipyridyl; HAT, 1,4,5,8,9,12-hexaazatriphenylene) were studied in various solvents, low temperature ethanol—methanol glasses, perfluorinated ionomer membranes (Nafion), in the presence of DNA or synthetic polynucleotides and in a solution of sodium polystyrenesulphonate (PSSS). The emission intensity and lifetime are quenched in acid solution and in protic solvents, increasing in the order H 2O < ethanol (EtOH) < dimethylsulphoxide) DMSO) < CH 3CN. The p K a values for the first two protonations of the HAT ligand in Ru(bpy) 2(HAT) 2+ (p K a1 = − 2.5, p K a2 = −4.3) have been determined. The spectroscopic and emission lifetime properties of the complex in Nafion membranes depend markedly on the swelling solvent and membrane hydration. The acidity of the H +-form membrane at different degrees of hydration was estimated by comparison with the absorption behaviour in acid solution. In the presence of DNA in aqueous solutions, the absorption of the complex is markedly altered, resembling that in low temperature glasses, and a 400% increase in the emission intensity and lifetime is observed. Spectroscopic changes are less marked with single-stranded than with double-stranded polynucleotides and very sensitive to the presence of salt cations. They increase in the order poly(dG).poly(dC) < poly[d(GC)] < poly(dA).poly(dT) < calf thymus DNA (CT-DNA) < poly[d(AT)] (dG, deoxyguanylic acid; dC, deoxycytidylic acid; dA, deoxyadenylic acid; dT, deoxythymidylic acid). Results are consistent with the partial intercalation of the complex into DNA, with a preference for AT stretches.

Ionic Conductivity of Bulk, Gels and Solutions of Perfluorinated Ionomer Membranes

Perfluorinated ionomer membranes either sulfonated or carboxylated have been extensively studied in order to understand the relations between their structure and properties. In addition to their uses as separator in the Cl2/NaOH industry large potential applications exist in different fields like sensors, batteries, fuel cells, electrochromic displays, etc. Some of these applications are due to the possibility of getting solutions, gels from these materials in different solvents and of reconstructing membranes of different thicknesses. The aim of this presentation is to present conductivity measurements of solutions, gels and bulk membranes, which measurements are of primary interest for potential applications in electrochemical devices. Room temperature measurements of conductivity of solutions/gels: First of all we recall the results already published which show the high conductivity of these gels associated with a transport number t+=1 since the COO− or SO3− anions are covalently bound to the rigid perfluorinated rod like structure. Low temperature measurements: Polymers having ionic conductivity usually show drastic changes upon temperature associated to transition (Tg, Tm or sub Tg relaxations). Measurements of conductivity of NAFION® 1100 solutions in propylene carbonate or dimethoxyethane over a temperature range between +20°C and −40°C have shown only small changes mainly due to viscosity modifications of the solvent as evidenced from the calculated activation energies. Conductivity/swelling measurements: We have recently developed an experiment which permits to obtain both the conductivity and the swelling of ion exchange membranes. Equilibrium experiments with solvent mixtures permit to check the conductivity and swelling changes versus different parameters (dielectric constant of solvent, ionic radius of the counterion, etc.). Diffusion coefficients are also obtained from the time lag when immersing the membrane in the solvent.

Cyclodextrin Purification with Hollow Fibers

Abstract Cyclodextrins are cyclic 1–4 linked oligomers of α-D-glucopyranose prepared from starch hydrolysis through enzymatic reactions. Mixtures of the three main cyclodextrins (CD), α-, β-, and γ-CDs, are always produced. A possible facile purification process is proposed. Permeation through hollow fibers made of a perfluorinated ionomer membrane, Nafion type, is shown to be an effective way to separate α-CD from β- and γ-CD. α-CD with 95% purity was obtained after permeation through a Nafion hollow fiber of an equimolar 0.02 M solution of the three CDs. The fiber had a 56 cm2/cm3 surface area per volume ratio. Kinetic studies and continuous extraction experiments with a 2-m coiled fiber showed that it is possible to obtain a 11.5 g/L α-CD solution with 92.4% purity or a 0.6 g/L α-CD solution with 97.2% purity, depending on the flow rate. The transport of CDs through the membrane could be due to moving water pools inside the ionomer. The small α-CD fits easily in such pools when the larger β- and γ-CDs ...

Purification of amino acids and small peptides with hollow fibers

Permeation through hollow fibers made of a perfluorinated ionomer membrane of the Nafion type is shown to be a possible way to separate amino acids and small peptides. The fiber has a surface area to volume ratio of 56 cm 2 cm −3. Twenty-six different amino acids and small peptides with up to six amino acid units were used for permeation studies. The results show that the bulk pH is the essential parameter acting on the permeation rates and diffusion coefficients through the tubing wall. The cationic forms of the solutes, at a pH lower than their isoelectric points, were highly retained by the cation-exchange membrane. The anionic forms of the solutes, at a pH higher than the isoelectric point, were less retained. The zwitterionic and non-ionic forms had the highest permeation rates, reaching 2.2 × 10 −3 s −1. The effect of methanol addition was studied. The permeation rates increased, but the selectivity decreased

Determination of self-diffusion coefficients from conductivity measurements in perfluorinated ionomer membranes

Values of the self-diffusion coefficients for sodium and cesium ions in 1200 Equivalent Weight Nafion® membranes have been deduced from conductivity measurements, assuming the validity of the Einstein relation between self-diffusion coefficient and absolute ionic mobility. Results obtained are in good agreement with previously published data obtained from direct measurements.

Effects of water and alcohols on molecular motion of perfluorinated ionomer membranes

Effects of water and alcohols on the molecular motion of perfluorosulfonate lonomer Nafion having five kinds of inorganic and organic ions were Investigated by dynamic viscoelastic and differential scanning calorimetric measurements. lonomer showed five tan δ peaks; α, β, β', γ' and γ relaxations in dry state. Absorbed water slightly affected tan δ peaks, however, absorbed alcohol Influenced the molecular motion of ionomers. lonomers showed two glass transitions in dry state corresponding to the α and β relaxations. In the ethanol content above 0.08, samples showed one glass transition at 14θ K Irrespective of counter ion. The dynamic viscoelastic spectrum of the ethanol-dry sample was more simple than that of original sample. The molecular rearrangement occurred in the ethanol absorbed state.

Distribution of the « micelles » in hydrated perfluorinated ionomer membranes from SANS experiments

The analysis of the shape and intensity of small angle neutron scattering (SANS) curves obtained on a perfluorinated ionomer membrane with different water contents suggests a model for the distribution of the hydrated « micelles » in the perfluorinated matrix. The model describes a locally ordered structure for these micelles with four first neighbours located at a well defined distance embedded in a completely disordered gas of micelles. Only two adjustable parameters are necessary to fit the experimental scattering curves over the whole q range, based on the radial distribution function associated with this locally « diamond-like » (tetrahedral) structure. It is demonstrated that both the zero angle upturn and the ionomer peak are linked with the ionic domain existence. It is shown that there is no need for long range fluctuations or large scale defects to interpret the zero angle domain. Our model is in reasonably good agreement with all the experimental results for q (q = 4 π sin θ/λ) ranging from 0.02 to 0.26 A-1 and for water contents from 6 to 26 % (volume fraction).

Determination of the permeant concentration profile in a membrane during gas permeation

Permeation and pervaporation of water through perfluorinated ionomer membranes have been studied. The water concentration profiles through the 190 μm thick membrane have been obtained for different upstream conditions by using the changes in the small-angle neutron scattering curves obtained with the membrane during steady state conditions. Scattering curves corresponding to different water concentrations in the membrane were used as reference curves. A fit procedure permitted the experimental scattering curves obtained during permeation conditions to be matched with a calculated curve obtained by decomposing the membrane in different layers having a defined water content, and therefore corresponding to a reference scattering curve. The profiles obtained in situ explain the discrepancies observed in the transport properties during permeation and pervaporation.