Peter N. Pintauro

Sulfonated polyphosphazene ion-exchange membranes

Abstract Four phosphazene polymers: poly[(3-methylphenoxy)(phenoxy)phosphazene], poly[(4-methylphenoxy)(phenoxy)phosphazene], poly[(3-ethylphenoxy)(phenoxy)phosphazene] and poly[(4-ethylphenoxy)(phenoxy)phosphazene] were sulfonated in solution with SO3 and cast into membranes from N,N-dimethylacetamide or 1-methyl-2-pyrrolidinone solvents at a temperature of 80°C. Methylphenoxy polymers were resistant to degradation and the sulfonation degree was easily controlled. The ethylphenoxy polymers underwent severe degradation during sulfonation and were unusable as membranes. Depending on the molar ratio of SO3 to the polymer mer, water insoluble membranes from the poly[(methylphenoxy)(phenoxy)phosphazenes] had an ion-exchange capacity ranging from near 0 to 2.3 mmol/g, an ac impedance in 0.1 N NaCl between 48 kohm m and 0.04 ohm m, and swelling in water (SO3H-form) from 0.1 to 0.9 g/g. Poly[(3-methylphenoxy)(phenoxy)phosphazene] was found to be the best starting material, in terms of the ease in controlling the degree of sulfonation and the highest polymer ion-exchange capacity for a water insoluble membrane.

Asymmetric PVDF hollow-fiber membranes for organic/water pervaporation separations

Abstract Asymmetric PVDF hollow-fiber pervaporation membranes, with an inner diameter of 0.05–0.06 cm, an outer diameter of 0.07–0.08 cm, and a dense layer (≈ 3 μm in thickness) on the inner fiber wall, have been fabricated and tested for the removal of ppm concentrations of organics from water. Membranes were made by air-drying the outside of the fibers for ca. 20 s and passing a fluid through the fiber bore. The set of casting conditions that produced the best hollow fiber, with a benzene separation factor of 1834 (for a 120 ppm benzene-in-water feed solution at 25°C and a downstream pressure of 0.025 atm) and a tensile strength 26.8 MPa, was a spinning solution of 25 wt% PVDF/30 wt% dimethylacetamide/45 wt% acetone and a bore fluid of 70 vol% water/25 vol% acetone/5 vol% dimethylacetamide. These membranes also effectively separated toluene, chloroform, and styrene from water. A small module containing 6–30 PVDF hollow fibers performed equally well for organic extraction from water with either a bore-side or shell-side feed when the feed-flow rate was sufficiently high to eliminate concentration polarization. Changes in organic flux and separation factor for variations in the organic feed concentration, downstream pressure, and temperature were qualitatively similar to those observed with asymmetric flat sheet PVDF pervaporation membranes.

Separation of dilute organic/water mixtures with asymmetric poly(vinylidene fluoride) membranes

Abstract An in-depth investigation of integral asymmetric poly(vinylidene fluoride) (PVDF) membranes has been carried out for the extraction of polar and non-polar organic compounds from dilute organic-in-water feed solutions. Membrane performance for low and high-boiling non-polar organic feed components was excellent, with separation factors as high as 4900 and high organic transmembrane fluxes. Polar organic feed components such as ethanol and acetone were also separated effectively from water but the separation factors were lower than non-polar organics. There was no change in membrane performance when either the dense or porous face of a PVDF membrane contacted the feed solution as long as the feed solution flow rate was sufficiently high. The effect of membrane preparation conditions, such as casting solution composition, air humidity and temperature during film drying, and the molecular weight of PVDF, on membrane performance was quantified. Although the water flux through the resulting films changed significantly, the organic (benzene) flux was essentially independent of the fabrication method. Variations in casting conditions also changed the mean diameter of a small number of pores in the dense layer of PVDF membranes. Transmembrane water fluxes during benzene/water separations correlated with increasing pore size, indicating that such pores were providing pathways for water movement across the hydrophobic PVDF dense layer. Based on benzene swelling and diffusivity measurements in homogeneous PVDF films, pores in the dense layer of an asymmetric membrane control permeate enrichment by either a membrane distillation, pore flow, or capillary condensation mechanism. The total time for asymmetric membrane casting was reduced from 69–72 min to 7 min, while maintaining a high organic (benzene) separation factor with only a small drop in transmembrane benzene flux.

Polyphosphazene membranes. IV. Polymer morphology and proton conductivity in sulfonated poly[bis(3-methylphenoxy)phosphazene] films

The microstructure of sulfonated poly[bis(3-methylphenoxy)phosphazene] was studied using wide- and small-angle X-ray diffraction. A reflection peak, attributed to the presence of ionic clusters, was observed in the small-angle X-ray diffraction patterns of hydrated and dry polymers with an ion-exchange capacity (IEC) ≥0.6 mmol/g. The Bragg spacing from the ionic cluster structure was about 30 A for the nonhydrated polymer and 50 to 90 A for fully hydrated films. The effects of IEC, cation form of the polymer, temperature, and polymer water content on the cluster structure were investigated. The specific proton conductivity of water-swollen, sulfonated poly[bis(3-methylphenoxy)phosphazene] films at 25°C increased with increasing IEC, with a maximum conductivity of 0.1 S/cm at a polymer ion-exchange capacity of 1.6 mmol/g. The water-content percolation threshold for conductivity was between 17.5 and 25 vol %, and decreased with polymer IEC. The temperature dependence of proton conductivity for 1.2 mmol/g IEC poly[bis(3-methylphenoxy)phosphazene] membranes exhibited Arrhenius behavior with an apparent activation energy of 27.8 and 36.7 kJ/mol for crosslinked and noncrosslinked polymers, respectively.

Analysis of radiation-grafted membranes for fuel cell electrolytes

One of the primary obstacles to be overcome for the development of economical fuel cells is the high cost of the membrane electrolyte. The currently favoured polymer electrolytes consist of poly(tetrafluoroethylene) backbone structures and poly(perfluorosulphonic acid) side chains. In an effort to find lower cost membranes, some radiation-grafted copolymer membranes were investigated. All the membranes contained poly(styrenesulphonic acid) side chains. Three different backbone polymer structures were studied: low-density poly(styrene), poly(tetrafluoroethylene)/poly(perfluoropropylene), and poly(tetrafluoroethylene). The results indicate that the membrane consisting of a poly(tetrafluoroethylene)/poly(styrenesulphonic acid) copolymer is a promising candidate as a fuel-cell electrolyte.

Blended Polyphosphazene/Polyacrylonitrile Membranes for Direct Methanol Fuel Cells

Direct liquid methanol fuel cell tests were performed with membrane electrode assemblies (MEAs) fabricated with polyphosphazene-based proton-exchange membranes. The membranes were prepared from sulfonated poly[bis(3-methylphenoxy)phosphazene] that was blended with polyacrylonitrile and then UV cross-linked using benzophenone as the photoinitiator. MEAs worked best when a high ion-exchange capacity (high conductivity) polyphosphazene membrane contacted the electrodes, in which case the fuel cell power output was nearly the same as that with Nafion 117 (for current densities ≤ 0.15 A/cm 2 ), but the methanol crossover was three times lower than that of Nafion. With a three-membrane composite MEA (a methanol-blocking film sandwiched between two high conductivity membranes), there was a significant decrease in crossover (ten times lower than that of Nafion 117) with a modest decrease in current-voltage behavior.

Polyphosphazene membranes. III. Solid-state characterization and properties of sulfonated poly[bis(3-methylphenoxy) phosphazene]

Poly[bis(3-methylphenoxy)phosphazene] was sulfonated in a solution with SO3 and solution-cast into 100–200-μm-thick membranes from N,N-dimethylacetamide. The degree of polymer sulfonation was easily controlled and water-insoluble membranes were fabricated with an ion-exchange capacity (IEC) as high as 2.1 mmol/g. For water-insoluble polymers, there was no evidence of polyphosphazene degradation during sulfonation. The glass transition temperature varied from −28°C for the base polymer to −10°C for a sulfonated polymer with an IEC of 2.1 mmol/g. The equilibrium water swelling of membranes at 25°C increased from near zero for a 0.04-mmol/g IEC membrane to 900 % when the IEC was 2.1 mmol/g. When the IEC was < 1.0 mmol/g, SO3 attacked the methylphenoxy side chains at the para position, whereas sulfonation occurred at all available aromatic carbons for higher ion-exchange capacities. Differential scanning calorimetry, wide-angle X-ray diffraction, and polarized microscopy showed that the base polymer, poly[bis(3-methylphenoxy)phosphazene], was semicrystalline. For sulfonated polymers with a measurable IEC, the 3-dimensional crystal structure vanished but a 2-dimensional ordered phase was retained.

Divalent/monovalent cation uptake selectivity in a Nafion cation-exchange membrane : Experimental and modeling studies

Abstract The equilibrium uptake of monovalent/divalent cation salt mixtures by a Nafion perfluorosulfonic acid cation-exchange membrane has been investigated using experimental measurements and a theoretical model. Membrane concentrations were determined for Nafion equilibrated in 0.15 M solutions containing Ni 2+ , Cu 2+ , Ca 2+ , or Mg 2+ with a co-absorbed monovalent cation (Li + , K + , or Cs + ). An equilibrium ion absorption model that accounts for ion hydration effects, the dielectric saturation of water molecules in a membrane pore, and the neutralization of fixed-charges by ion pairing with divalent cations, was matched to the experimental data. When the extent of ion-pair formation was used as an adjustable parameter, the model predicted accurately both the monovalent and divalent cation concentrations (with an average error of 7.7%). Both theory and experiments showed that the monovalent cation selectivity was in the same order as observed previously during the uptake of monovalent/monovalent cation salt mixtures (i.e. the monovalent cation with the larger hard sphere radius was preferentially absorbed). The computed mobile divalent cation concentration in a Nafion pore was found to be dependent on the extent of monovalent cation absorption. The number of divalent cation/sulfonate fixed-charge-site ion pairs was found to be independent of the divalent cation type, but was controlled by the type and concentration of the co-absorbed monovalent cation. The fraction of ion-paired fixed charges was correlated with the membrane pore concentration of mobile divalent cations via a Frumkin adsorption isotherm.

The electrocatalytic hydrogenation of soybean oil

Soybean oil has been hydrogenated electrocatalytically at a moderate temperature, without an external supply of pressurized H2 gas. In the electrocatalytic reaction scheme, atomic hydrogen is produced on an active Raney nickel powder cathode surface by the electrochemical reduction of water molecules from the electrolytic solution. Adsorbed hydrogen then reacts with an oil’s triglycerides to form a hydrogenated product. Experiments were carried out at 70°C with a flow-through electrochemical reactor operating in a batch recycle mode. The reaction medium was a two-phase mixture of soybean oil in a water/t-butanol solvent containing tetraethylammoniump-toluenesulfonate as the supporting electrolyte. In all experiments the reaction was allowed to continue for sufficient time to synthesize a brush hydrogenation product. The effects of oil content, applied current, solvent composition, and supporting electrolyte concentration on the efficiency of hydrogen addition to the oil and on the chemical properties of the hydrogenated oil product were determined. The electrohydrogenated oil is characterized by a high stearic acid content and a low percentage of totaltrans isomers, as compared to that produced in a traditional hydrogenation process.

Integral asymmetric poly(vinylidene fluoride) (PVDF) pervaporation membranes

Abstract A new asymmetric pervaporation membrane, composed of the fluoropolymer poly(vinylidene fluoride) (PVDF), has been fabricated and preliminary tests have been carried out to assess membrane performance for the removal of non-polar organics from water. Formation of an asymmetric microstructure proceeds via an unusual phase inversion process where a dense polymer layer forms at the membrane/casting surface interface. Asymmetric membranes are made by either a dry cast method involving complete air-drying of a PVDF film or a wet cast method of partial air-drying followed by film immersion in a series of aqueous precipitation baths. For aromatic/water and chloroform/water separations, the PVDF films performed remarkably well. For example, a benzene separation factor of 1,530 and a benzene flux of 150 g/m2-hr has been obtained for an 890 ppm benzene in water feed solution at 25°C and 0.05 atm downstream pressure. When the feed solution contained 625 ppm chloroform in water, the performance of asymmetric PVDF films at 25°C is characterized by a high organic flux (25.3–58.7 g/m2-hr) and a high separation factor (302–823).