Nafion-clay nanocomposite membranes: Morphology and properties

A series of Nafion-clay nanocomposite membranes were synthesized and characterized. To minimize any adverse effects on ionic conductivity the clay nanoparticles were H⁺ exchanged prior to mixing with Nafion. Well-dispersed, mechanically robust, free-standing nanocomposite membranes were prepared by casting from a water suspension at 180 °C under pressure. SAXS profiles reveal a preferential orientation of Nafion aggregates parallel to the membrane surface, or normal plane. This preferred orientation is induced by the platy nature of the clay nanoparticles, which tend to align parallel to the surface of the membrane. The nanocomposite membranes show dramatically reduced methanol permeability, while maintaining high levels of proton conductivity. The hybrid films are much stiffer and can withstand much higher temperatures compared to pure Nafion. The superior thermomechanical, electrochemical and barrier properties of the nanocomposite membranes are of significant interest for direct methanol fuel cell applications.     

Ionomer-silica hybrids via sol-gel reaction

Ionomer-silica hybrid materials were made from polyethylene-co-acrylic acid neutralized by a zinc salt (PI) and tetraethoxy silane (TEOS) via the sol-gel reaction. The effects of various experimental parameters such as solvents, H₂O/Si ratio and the amount of TEOS in the ionomer solution on the hybrid structure and properties were examined. The spectroscopic results show that solvents do not affect the structure of the hybrids, but influence the thermal properties. The hybrids made using highly polar solvent exhibit better thermal stability and dynamic mechanical properties at high TEOS contents. The amount of water used for hydrolysis and subsequent condensation play a significant role in the network formation. The varying amount of TEOS in solutions gives rise to different silica content of the hybrid. Above 50 wt%, the sample becomes opaque due to silica aggregation. The high ratio of H₂O/Si leads to phase separation during the reaction. Transparent hybrid materials can only be obtained when the ratio of H₂O/Si is below 5.     

Hydration of freestanding Nafion membrane in proton and sodium ion exchanged forms probed by infrared spectroscopy

Transmission infrared spectroscopy was used to follow the uptake of water into Nafion 112 (∼50 μm thick) membrane under conditions that enabled detection of vibrational bands for water in different environments inside membrane pores and channels. The evolution of infrared features for interfacial and weakly hydrogen bonded water were followed upon exposure of initially vacuum dried membranes, exchanged by either Na⁺ or H⁺, to low humidity atmospheres. The rapid uptake of water into H⁺ exchanged Nafion 112 precluded time resolved spectral measurements. However, the considerably slower timeframe for water incorporation into Na⁺ exchanged membrane enabled the evolution of different environments for water to be observed. Under approximately 10% relative humidity, the time dependent increases in absorbance for a mode of interfacial water near 3674 cm⁻¹ and a mode of more bulk-like, weakly hydrogen bonded water at 3525 cm⁻¹ in Na⁺ exchanged Nafion 112 could be fit by a pore diffusion model. The results provide a foundation for the application of multivariate analysis techniques to identify different structures that develop in metal cation exchanged Nafion during changes in hydration state.     

Polymer electrolyte membranes from fluorinated polyisoprene-block-sulfonated polystyrene: Membrane structure and transport properties

With a view to optimizing morphology and ultimately properties, membranes have been cast from relatively inexpensive block copolymer ionomers of fluorinated polyisoprene-block-sulfonated polystyrene (FISS) with various sulfonation levels, in both the acid form and the cesium neutralized form. The morphology of these membranes was characterized by transmission electron microscopy and ultra-small angle X-ray scattering, as well as water uptake, proton conductivity and methanol permeability within the temperature range from 20 to 60 °C. Random phase separated morphologies were obtained for all samples except the cesium sample with 50 mol% sulfonation. The transport properties increased with increasing degree of sulfonation and temperature for all samples. The acid form samples absorbed more water than the cesium samples with a maximum swelling of 595% recorded at 60 °C for the acid sample having 50 mol% sulfonation. Methanol permeability for the latter sample was more than an order of magnitude less than for Nafion 112 but so was the proton conductivity within the plane of the membrane at 20 °C. Across the plane of the membrane this sample had half the conductivity of Nafion 112 at 60 °C.     

聚乙烯—丙烯酸离聚物及其与PET共混

介绍了聚乙烯－丙烯酸的接枝共聚及其钠盐的形成,并应用红外光谱进行了表征,应用聚乙烯－丙烯酸钠与ＰＥＴ共混,研究了离聚物对ＰＥＴ光学性能和力学性能的影响,并测定了共混物的等温结晶速度。     

Novel copolyester-based ionomer for a shape-memory biodegradable material

A series of biodegradable poly(oxyethylene-b-butylene adipate) ionomers (POBAis) were synthesized by bulk polymerization of adipic acid and mixed monomers of bis(poly(oxyethylene)) sulfonated dimethyl fumarate and 1,4-butanediol. The POBAis exhibited notable recovery rate ranging from 81% to 95% at near body temperature, depending on the content of ionic group. From the dynamic mechanical analysis it was found that storage modulus above rubbery plateau region significantly increased as ionic monomer was introduced up to 2.5 mol%, indicating a good dimensional stability above glass transition region. Wide angle X-ray analysis was carried out to investigate the structural variation over shape deformation and/or recovery. WAXD pattern of POBAi-2.5 revealed that α-β transition of crystal occurred during recovery process. These results may provide crucial information of the crystallizable ionomers as a potential shape-memory polymer (SMP).     

Aqueous pore structure and proton dynamics in solvated Nafion membranes

Molecular dynamics computer simulations of water/Nafion mixtures using an all-atom model were performed as a function of temperature and humidity. The simulations are aimed at investigating processes and structures on the picosecond to nanosecond time scale in the nano-phase-separated material with its technological relevance for low temperature fuel cells. Characteristic differences in aqueous pore structure were observed for systems whose water content was varied between 5 and 10 molecules per acid group in the polymer. As expected, proton transport increases significantly with increasing humidity, its mechanism is dominated by the Grotthus structural diffusion mechanism in accordance with earlier studies in simplified model pores. On the simulated time scale no unambiguous conclusions on the role of polymer dynamics for the transport in dry membranes can be drawn.     

Formation of nanoparticles during melt mixing a thermotropic liquid crystalline polyester and sulfonated polystyrene ionomers: Morphology and origin of formation

The formation of nanoparticles and the mechanism of their formation in a blend of a thermotropic liquid crystalline polyester (LCP) and the zinc salt of a lightly sulfonated polystyrene ionomer (Zn-SPS) were investigated using Fourier transform infrared (FTIR), thermogravimetric analysis (TGA), and gas chromatography-mass spectroscopy (GC-MS). Transmission electron microscopy (TEM) and wide-angle X-ray diffraction (WAXD) were used to study the morphology of the blends and structure of nanoparticles. The origin of nanoparticle formation appeared to be related to the development of phenyl acetate chain ends on the LCP that arose due to a chemical reaction between the LCP and residual catalytic amounts of zinc acetate and/or acetic acid that were present from the neutralization step in the preparation of the ionomer. Two-dimensional X-ray diffraction patterns for the blends revealed that chain-packing within nanoparticles was different than that of the LCP or the homopolymers prepared from the LCP co-monomers. The crystals formed in the nanoparticles were also stable to much higher temperature (>350 °C) than the parent LCP crystallites that melted at ∼280 °C.     

Improved electrocatalytic performance of Pd nanoparticles with size-controlled Nafion aggregates for formic acid oxidation

This work focuses on the effect of Nafion ionomer aggregation within the Pd catalytic electrode on electrocatalytic oxidation of formic acid. By a simple heat-treatment, the particle sizes of both Nafion ionomers in Nafion solution and congeries formed between Pd nanoparticles and Nafion ionomers in the catalyst ink decrease and their size distribution becomes narrow. Heat treatment of the catalyst ink leads to a significantly enhanced catalytic activity for formic acid oxidation on the Pd catalytic electrode. Such an enhancement is ascribed to the improvement in catalyst utilization verified by CO stripping voltammograms and to the decrease in charge-transfer resistance of oxidation reaction confirmed by impedance analysis. Typical XPS analysis shows that there are at least two kinds of Pd and S surface states within the catalytic electrode with the ink pre-heated at 25 °C and only one kind of Pd and S surface state at 80 °C, indicative of a better dispersion between Pd nanoparticles and smaller Nafion ionomers at a higher heat treatment temperature. Furthermore, the decrease in congeries size within the anode catalyst ink leads to a significant decrease in Nafion loading within the catalytic layer and a remarkable improvement in direct formic acid fuel cell's performance.     

Poly(arylene-ethynylene) with tuned rigidity/flexibility as reinforcing component in polystyrene-based ionomer blends

Poly(pyridylene/phenylene-ethynylene) polymers of controlled rigidity/flexibility were synthesized and used as molecularly dispersed reinforcing components in ionomer blends with partially sulfonated polystyrene. Homogeneous ionomer blends were formed due to acid-base interactions between the two blend components. The complete protonation of the poly(pyridylene/phenylene-ethynylene) in the acid/base ionomer blends has been proven by UV-VIS absorption and emission spectroscopy. Furthermore, the complete miscibility of the blend components in the ionomer blends was revealed from DSC analysis and transmission electron microscopy. The mechanical properties of the synthesized ionomer blends were determined by stress-strain measurements. The Young modulus of the blends was found to systematically vary with the rigidity/flexibility of the reinforcing polymer i.e. the molecular conformation of the reinforcing polymers as determined by small angle X-ray scattering.     

Methods for modifying proton exchange membranes using the sol-gel process

Over the past 20 years, sol-gel processing has expanded into organic-inorganic hybrid materials. This expansion has benefited from the collaborations between the polymers community and the ceramics community, and the discovery that in many instances sol-gel processing and polymer processing are compatible. An active participant in this field has been Dr James E. Mark [Mark JE. Heterog Chem Rev 1996;3:307-320], and his accomplishments deserve this tribute. One example, which derives from his work in organically-modified silicates (ORMOSILS), is hybrid membranes for fuel cells. We present some recent progress in synthesis of hybrid membranes involving Nafion. These membranes have been prepared by infiltration and recasting, and contain silicates, phosphosilicates, zirconium phosphosilicates and titanosilicates.     

Porous interpenetrating network hybrids synthesized within high internal phase emulsions

‘PolyHIPE’ are porous polymers from the polymerization of monomers and crosslinking co-monomers in the continuous phase of high internal phase emulsions (HIPE). Elastomeric polyHIPE have been reinforced through the synthesis of nanocomposites using several different routes including the addition of co-monomers such as vinyltrialkoxysilane, polyhedral oligomeric silsesquioxane (POSS) bearing vinyl groups, or vinyl silsesquioxane (VSQ). This paper describes the synthesis, structure, and properties of hybrid polyHIPE with interpenetrating polymer-inorganic networks synthesized by adding tetraethylorthosilicate (TEOS) to the monomers. The HIPE becomes unstable on addition of 14 mol% TEOS and the resulting polyHIPE contains voids hundreds of micrometers in diameter. On addition of 25 mol% TEOS the large voids become coated with a brittle Si-O shell as the TEOS migrates to the interface between the organic and aqueous phases. In general, the increases in tan δ peak temperature, tan δ peak breadth, and room temperature modulus with increasing TEOS content are similar to those observed with POSS and VSQ. Unlike the polyHIPE containing vinyltrialkoxysilane, POSS, or VSQ, pyrolysis of these hybrid polyHIPE did not yield porous inorganic monoliths.     

Theoretical and experimental infrared spectra of hydrated and dehydrated sulfonated poly(ether ether ketone)

Time-dependent FT-IR spectra of sulfonated poly(ether ether ketone) during dehydration show diminishing 1081 cm⁻¹ and 1023 cm⁻¹ band intensities concurrent with the emergence and shifting of bands at 1362 cm⁻¹ and 898 cm⁻¹. Animations of density functional theory calculated normal modes enable assignment of the 1081 cm⁻¹ and 1023 cm⁻¹ bands as group modes that include a sulfonate exchange site with C_3v local symmetry, while the 1362 cm⁻¹ and 898 cm⁻¹ bands are assigned as group modes that include an associated sulfonic acid with no local symmetry (C₁). In contrast to analogously assigned Nafion group mode bands, the SPEEK C_3v and C₁ bands coexist throughout the entire dehydration–hydration cycle, suggesting the presence of associated and dissociated exchange sites in SPEEK at all states-of-hydration. This supports a morphological model for SPEEK featuring branched hydrophilic domains and dead-end aqueous confines.     

Fracture behavior of nanocomposites based on poly(ethylene-co-methacrylic acid) ionomers

The fracture behavior of nanocomposites formed from an organoclay, based on montmorillonite (MMT), and a poly(ethylene-co-methacrylic acid) ionomer prepared by melt compounding was investigated using an instrumented impact test. The data were analyzed using the essential work of fracture (EWF) methodology. Transmission electron microscopy revealed that the clay platelets were well-exfoliated in this matrix. The fracture energy of these nanocomposites increased with organoclay addition at low concentrations but decreased with further increase in organoclay concentration with a maximum between 2 and 3 wt% MMT. The initial increase in fracture energy is a result of the higher forces during loading caused by the increase in modulus and yield stress upon addition of clay; however, the fracture energy eventually decreases with further addition of clay owing to the continuous decrease in ductility or deflection during testing. The EWF of fracture analysis showed that the energy per unit area of crack surface formed exhibits a maximum at 2-3 wt% MMT while the energy dissipated per unit volume in the surrounding process zone decreases monotonically for all clay loadings with a transition from ductile to brittle behavior occurring at 7-8 wt% MMT.     

聚(甲基丙烯酸甲酯/丙烯酸丁酯/丙烯酸钠)离聚体胶乳的合成及其性能研究

以过硫酸钾为引发剂,十二烷基硫酸钠为乳化剂,合成了稳定的聚(甲其丙烯酸甲酯/丙烯酸丁酯/丙烯酸钠)离聚体乳液,并考察丁引发剂用量,乳化剂用量,丙烯酸钠用量,反应温度,反应时间等因素对单体转化率,离聚体力学性能的影响。     

Disruption of the multiplets in poly(styrene-co-methacrylate) ionomers by the addition of aliphatic diacid salts

Dynamic mechanical properties and morphology of poly(styrene-co-sodium methacrylate) ionomers containing disodium salts of aliphatic diacid were investigated. It was found that upon the addition of diacid salts to the ionomers, the position of the matrix loss tangent peak remained constant, but the cluster loss tangent peak shifted to lower temperatures; the size of the matrix loss tangent peak increased, but that of the cluster peak decreased. In addition, the ionomers containing diacid salts showed X-ray diffraction patterns at relatively wide angles but no DSC melting peak. Thus, it was postulated that the organic salts prohibit the ionic groups of the ionomer from forming multiplets, and that the ionic groups of the ionomer, not forming the multiplets, participate in the formation of ionic aggregates with the ionic groups of the diacid salts.     

Counterion dependent crystallization kinetics in blends of a perfluorosulfonate ionomer with poly(vinylidene fluoride)

Blends of poly(vinylidene fluoride) (PVDF) with a perfluorosulfonate ionomer, Nafion^®, have been prepared and examined in terms of the crystallization kinetics of the PVDF component. In blends of PVDF with Na⁺-form Nafion^®, the rates of bulk crystallization, as observed by DSC, and the spherulitic growth rates of the PVDF component, as observed using optical microscopy, were found to be very similar to that of pure PVDF. This behavior was attributed to the course phase separation of Na⁺-form Nafion^® from PVDF and melt incompatibility of the physically cross-linked ionomer with the crystallizable component. In this segregated state, the PVDF component of the blend is allowed to crystallize in pure phases that are isolated under the influence of Nafion^®. In contrast, when the ionomer was exchanged with more weakly interacting quaternary alkylammonium counterions, a decrease in both the rate of bulk crystallization and spherulitic growth was observed. Furthermore, the crystallization kinetics of PVDF in these blends was found to be dependent on the counterion size; as the size of counterions associated with the Nafion^® component increased, the rate of crystallization decreased. This behavior was attributed to a weakening of the electrostatic interactions in the ionomer phase and thus an increase in the extent of phase mixing with the larger ions.     

Water-polymer interfacial area in Nafion: Comparison with structural models

The water-polymer interfacial area per volume, S/V, which is reflected in the Porod region of small-angle scattering data, is an important parameter of different models of the Nafion fuel cell membrane. Therefore, we have compared published experimental S/V data of Nafion over a wide range of hydration levels with various structural models featuring stiff polymer backbones, in particular the parallel water-channel and the polymer ribbon models. The S/V curve at intermediate hydration levels typical of fuel-cell conditions (ca. 20 vol% water) matches that of the parallel water-channel model with molecular corrugation. At higher hydration levels, i.e. for membranes soaked in water or autoclaved at elevated pressures, the polymer-ribbon model matches the decreasing S/V ratio with increasing water content, while the polymer-bundle model predicts a higher surface area. However, the ribbon or bundle models cannot apply at low hydration (<3 water molecules per Nafion side group), since we show that the interfacial area in this regime must increase strongly with hydration, being determined by the available surface area of the water molecules. The pronounced asymmetry of the plot of S/V vs. water volume fraction is explained in terms of the difference in the diameters of the water molecules and the polymer aggregates.     

Facilitated oxygen transport through a Nafion membrane containing cobaltporphyrin as a fixed oxygen carrier

A Nafion membrane containing a cobaltporphyrin (CoP) complex as a fixed oxygen carrier was prepared with a view to facilitate oxygen transport through the membrane. The design concept of the CoP-loaded Nafion membrane was based on the CoP's modification to place the CoP complex in a hydrophobic domain of the microphase-separated structure, in order to facilitate the oxygen transport and to maintain proton conductivity. The oxygen permeability through the CoP-loaded Nafion membrane was higher than the nitrogen permeability, and significantly enhanced at relatively-low oxygen pressures of the upstream, indicating that the fixed CoP complex acted as an oxygen hopping site to facilitate the oxygen transport. The oxygen/nitrogen permselectivity increased with the content of CoP in the Nafion membrane. Electrochemical reduction of oxygen at a glassy carbon electrode, modified with a Pt/C catalyst and the CoP-loaded Nafion membrane, provided additional support for the facilitated oxygen transport by the membrane. Increased current for the reduction of oxygen on the modified electrode by loading CoP indicated that the CoP offered the oxygen hopping site in the Nafion membrane.