Vibrational Spectroscopy for the Determination of Ionizable Group Content in Ionomer Materials.

An approach based on vibrational spectral measurements is described for determining the ionizable group content of ion conducting polymer membrane materials. Aimed at supporting the assessment of membrane stability and wear characteristics, performance is evaluated for attenuated total reflection Fourier transform infrared (ATR FT-IR) spectroscopy, confocal Raman microscopy, and ATR FT-IR microscopy using perfluorinated ionomer membrane standards. One set of ionomer standards contained a sulfonic acid ionizable group and the other a sulfonyl imide group. The average number of backbone tetrafluoroethylene (TFE) units separating the ionizable-group containing side chains was in the range of 7.2-2.1 (sulfonic acid set) and 10.5-4.6 (sulfonyl imide set). A poly(tetrafluoroethylene) (PTFE) sample was included as a blank, representing the limit of zero ionizable group (and maximum TFE) content. Calibration relationships were derived from area-normalized vibrational spectra. For all three methods, calibration models applied to independent spectral measurements of samples predicted the ratio of backbone TFE groups to ionizable groups in the repeat unit ( m) with a standard error of ≤ ±0.3. The confocal Raman and ATR FT-IR microscopy techniques achieved ideal blank responses and the lowest prediction errors, down to m ± 0.1 at the 90% confidence level. With its relative simplicity, low sample size requirements, and potential for quantitative micron-scale spatial mapping of the ionizable group content within a membrane, the approach has application to advancing materials development, including exploratory synthesis, durability and wear assessment, and in situ studies of membrane process.

The effect of low-molecular-weight poly(ethylene glycol) (PEG) plasticizers on the transport properties of lithium fluorosulfonimide ionic melt electrolytes.

The influence of low-molecular-weight poly(ethylene glycol) (PEG, Mw ≈ 550 Da) plasticizers on the rheology and ion-transport properties of fluorosulfonimide-based polyether ionic melt (IM) electrolytes has been investigated experimentally and via molecular dynamics (MD) simulations. Addition of PEG plasticizer to samples of IM electrolytes caused a decrease in electrolyte viscosity coupled to an increase in ionic conductivity. MD simulations revealed that addition of plasticizer increased self-diffusion coefficients for both cations and anions with the plasticizer being the fastest diffusing species. Application of a VTF model to fit variable-temperature conductivity and fluidity data shows that plasticization decreases the apparent activation energy (Ea) and pre-exponential factor A for ion transport and also for viscous flow. Increased ionic conductivity with plasticization is thought to reflect a combination of factors including lower viscosity and faster polyether chain segmental dynamics in the electrolyte, coupled with a change in the ion transport mechanism to favor ion solvation and transport by polyethers derived from the plasticizer. Current interrupt experiments with Li/electrolyte/Li cells revealed evidence for salt concentration polarization in electrolytes containing large amounts of plasticizer but not in electrolytes without added plasticizer.

Trifluoromethylation of perfluorinated diacylfluorides: synthesis of the diketone CF3C(O)CF2C(O)CF3 and of new perfluorinated diol (CF3)2C(OH)CF2C(OH)(CF3)2.

Perfluoromalonyl difluoride reacts with TMS-CF3 (1:1) in the presence of KF to give the new diketone CF2(C(O)CF3)2. A large excess (5:1) of TMS-CF3 results in the presumed potassium dialkoxide [(CF3)2COK]2CF2 which yields the 1,3-ditertiarydiol [(CF3)2C(OH)]2CF2 on reaction with H2SO4.

Thermal processing as a means to prepare durable, submicron thickness ionomer films for study by transmission infrared spectroscopy.

A high temperature solution processing method was adapted to prepare durable, freestanding, submicrometer thickness films for transmission infrared spectroscopy studies of ionomer membrane. The materials retain structural integrity following cleaning and ion-exchange steps in boiling solutions, similar to a commercial fuel cell membrane. Unlike commercial membrane, which typically has thicknesses of >25 µm, the structural properties of the submicrometer thickness materials can be probed in mid-infrared spectral measurements with the use of transmission sampling. Relative to the infrared attenuated total reflection (ATR) technique, transmission measurements can sample ionomer membrane materials more uniformly and suffer less distortion from optical effects. Spectra are reported for thermally processed Nafion and related perfluoroalkyl ionomer materials containing phosphonate and phosphinate moieties substituted for the sulfonate end group on the side chain. Band assignments for complex or unexpected features are aided by density functional theory (DFT) calculations.

Syntheses of a perfluoroethanesulfonyl fluoride vinyl ether.

The target monomer, perfluoroethanesulfonyl fluoride vinyl ether 8, was obtained from the starting material, iodoperfluoroethanesulfonyl fluoride 1, through the intermediate, acyl fluoride 4. Two different synthetic routes were utilized in the conversion of 4 to 8.

Effect of perfluoroalkyl chain length on proton conduction in fluoroalkylated phosphonic, phosphinic, and sulfonic acids.

The effects of increasing perfluoroalkyl chain length on the molecular properties of viscosity, diffusivity, and ionic conductivity of a series of acid model compounds analogous to comb-branch perfluorinated ionomers functionalized with phosphonic, phosphinic, and sulfonic protogenic groups are reported. Anhydrous proton transport by a Grotthuss-like hopping mechanism was observed to occur efficiently in phosphorus-based fluoroalkylated model acids but only when there is a relatively low perfluoroalkyl content. The decrease in degree of dissociation of the protogenic groups follows the order phosphonic > phosphinic > sulfonic, and the degree of dissociation and the magnitude of ion-ion correlations are approximately independent of chain length.

Perfluoroalkyl phosphonic and phosphinic acids as proton conductors for anhydrous proton-exchange membranes.

A study of proton-transport rates and mechanisms under anhydrous conditions using a series of acid model compounds, analogous to comb-branch perfluorinated ionomers functionalized with phosphonic, phosphinic, sulfonic, and carboxylic acid protogenic groups, is reported. Model compounds are characterized with respect to proton conductivity, viscosity, proton, and anion (conjugate base) self-diffusion coefficients, and Hammett acidity. The highest conductivities, and also the highest viscosities, are observed for the phosphonic and phosphinic acid model compounds. Arrhenius analysis of conductivity and viscosity for these two acids reveals much lower activation energies for ion transport than for viscous flow. Additionally, the proton self-diffusion coefficients are much higher than the conjugate-base self-diffusion coefficients for these two acids. Taken together, these data suggest that anhydrous proton transport in the phosphonic and phosphinic acid model compounds occurs primarily by a structure-diffusion, hopping-based mechanism rather than a vehicle mechanism. Further analysis of ionic conductivity and ion self-diffusion rates by using the Nernst-Einstein equation reveals that the phosphonic and phosphinic acid model compounds are relatively highly dissociated even under anhydrous conditions. In contrast, sulfonic and carboxylic acid-based systems exhibit relatively low degrees of dissociation under anhydrous conditions. These findings suggest that fluoroalkyl phosphonic and phosphinic acids are good candidates for further development as anhydrous, high-temperature proton conductors.

Infrared spectroscopy of bis[(perfluoroalkyl)sulfonyl] imide ionomer membrane materials.

Structural properties of the proton-exchanged forms of bis[(perfluoroalkyl)sulfonyl] imide (PFSI) ionomer materials were investigated. The hydration and dehydration of samples prepared as thin films and freestanding membrane were probed by applying transmission infrared spectroscopy. Spectral bands were assigned and effects of water incorporation into membrane pores and channels were understood by drawing upon results from related measurements performed on the structurally similar, perfluorosulfonic acid ionomer, Nafion. Both PFSI and Nafion membrane materials display a prominent infrared absorbance band near 1060 cm(-1) that arises from a vibrational mode of the ionizable group present on the side chains that extend from the poly(tetrafluoroethylene) backbone on the polymers. The mode can be traced to symmetric stretching of the -SO(3)(-) (sulfonate) group in Nafion and to antisymmetric S-N-S stretching within the sulfonyl imide end group (-SO(2)(N(-))SO(2)CF(3)) in the PFSI materials. For Nafion samples, the position and width of the band near 1060 cm(-1) are strongly sensitive to membrane hydration, whereas the band position and shape change only slightly during hydration and dehydration of PFSI materials. The possibility for greater charge delocalization over the sulfonyl imide moiety and shielding of hydrophilic species by the terminal -CF(3) group are suggested to explain the differences. These effects also likely influence the stretching modes of the side chain C-O-C groups. A pair of bands, sensitive to hydration and traceable to different C-O-C groups in a side chain, is present in the 970-990 cm(-1) region of Nafion. However, the two features are not well resolved and are less sensitive to hydration in spectra of PFSI samples. The most intense ionomer spectral bands arise from modes involving C-F stretching motion and appear between 1150 and 1250 cm(-1). Toward the high energy side of the envelope, there is substantial overlap with features of sulfonate group antisymmetric SO stretching modes in Nafion, but SO stretching modes of the sulfonyl imide moiety are higher in energy and better resolved in spectra of the PFSIs. During water uptake from a dry state into PFSI materials, a progression of features characteristic of solvated H(3)O(+) species appears across the water O-H stretching (2800-3800 cm(-1)) and H-O-H bending (1500-2000 cm(-1)) regions, similar to responses observed for water inside proton-exchanged Nafion.

Fluoroalkylation of imidazoles by hypervalent iodonium salts.

N-1H,1H-perfluoroalkylation and regioselective C-perfluoroalkylation of imidazoles were obtained using the corresponding hypervalent iodonium salts.

Construction of N-1H,1H-perfluoroalkylated peptide bonds.

The preparation of a variety of optically pure peptides containing an N-1H,1H-perfluoroalkyl label on a selected backbone amide bond is now possible.

Transport properties of solid polymer electrolytes prepared from oligomeric fluorosulfonimide lithium salts dissolved in high molecular weight poly(ethylene oxide).

Transport properties such as ionic conductivity, lithium transference number, and apparent salt diffusion coefficient are reported for solid polymer electrolytes (SPEs) prepared using several oligomeric bis[(perfluoroalkyl)sulfonyl]imide (fluorosulfonimide) lithium salts dissolved in high molecular weight poly(ethylene oxide) (PEO). The salt series consists of polyanions in which two discrete fluorosulfonimide anions are linked together by [(perfluorobutylene)disulfonyl]imide linker chains. The restricted diffusion technique was used to measure the apparent salt diffusion coefficients in SPEs, and cationic transference numbers were determined using both potentiostatic polarization and electrochemical impedance spectroscopy methods. A general trend of diminished salt diffusion coefficient with increasing anion size was observed and is opposite to the trend observed in ionic conductivity. This unexpected finding is rationalized in terms of the cumulative effects of charge carrier concentration, anion mobility, ion pairing, host plasticization by the anions, and salt phase segregation on the conductivity.

NMR and IR studies of bis((perfluoroalkyl)sulfonyl)imides.

The influence of water on the NMR and IR spectra of bis((perfluoroalkyl)sulfonyl)imides has been investigated. The large effects of water on the observed spectra suggest that the proton is predominantly bound to the nitrogen of the perfluorosulfonimide acid in the absence of water, while it protonates water to form an onium salt in the presence of water.

Rubidium bis(trifluoromethanesulfonyl)imidate dioxane disolvate.

The bis(trifluoromethanesulfonyl)imidate anion crystallizes with Rb as the title dioxane 1:2 solvate, Rb(+).CF(3)SO(2)NSO(2)CF(3)(-).2C(4)H(8)O(2), with the anion in a transoid conformation, as opposed to the cisoid form typically seen when there are significant cation-anion interactions. The Rb(I) cation is eight-coordinate, interacting with one anion in a chelating fashion and with two other anions through the remaining sulfonyl O atoms. The latter interactions link ion pairs through the formation of Rb(2)O(2) dimers about inversion centers at (0, 1/2, 0) and (1/2, 1/2, 0), forming extended columns which run parallel to the a axis of the unit cell. Rb-dioxane bridges crosslink these salt columns in the (010), (001) and (011) directions, resulting in a three-dimensional network solid. One dioxane solvent molecule is disordered over two half-occupancy sites.

A novel 1,3,5-tris(alpha,beta,beta-trifluorovinyl)benzene monomer.

The reaction of the perfluoroalkenylzinc reagent, CF2=CFZnBr, with 1,3,5-tribromobenzene in the presence of a catalytic amount of Pd(Ph3)4 yielded a novel trifunctional monomer 1,3,5-tris(alpha,beta,beta-trifluorovinyl)benzene (1).

Direct methylation and trifluoroethylation of imidazole and pyridine derivatives.

Direct methylation or trifluoroethylation of imidazole and pyridine derivatives using N-methyl bis((perfluoroalkyl)sulfonyl)imides or trifluoroethyl phenyliodonium bis((trifluoromethyl)sulfonyl)imide affords high yields of the corresponding salts. This methodology provides a simple route to a variety of room temperature ionic liquids (RTILs).

Selective Epoxidation of Olefins by Perfluoro-cis-2,3-dialkyloxaziridines(1).

Alkyl-substituted olefins are epoxidized by perfluoro-cis-2,3-dialkyloxaziridines under particularly mild conditions. Electron deficient substrates (e.g. alpha,beta-enones) can also be epoxidized, and the more electron poor the double bond is, the more severe the reactions conditions become. The epoxidation is chemoselective (secondary alcohols and their ethers do not interfere), site selective (the monoepoxide of a diene can be obtained), and stereoselective (cis-alkenes afford cis-epoxides). Various complex and polyfunctional substrates of natural origin (monoterpenes, sesquiterpenes, steroids) have been transformed effectively.

Synthesis and Reaction Chemistry of the Disodium Alkoxide of 2,6-Bis(trifluoromethyl)-2,6-dihydroxy-3,3,4,4,5,5-hexafluorooxane.

The disodium alkoxide of 2,6-bis(trifluoromethyl)-2,6-dihydroxy-3,3,4,4,5,5-hexafluorooxane, C(7)F(12)O(3)Na(2), 1, was prepared by reaction of the corresponding diol with MeONa in MeOH. This cyclic alkoxide readily mimics the reaction chemistry of the lithium and sodium alkoxides of perfluoropinacol. The reaction of the alkoxide 1 with covalent dichlorides and metallocene dichlorides yields a series of novel bicyclic ring systems. The crystal structure of the trioxasilane, 3, formed from (CH(3))(2)SiCl(2) was determined to provide structural information about these ring systems. Crystal data: C(9)H(6)O(3)F(12)Si, fw = 418.21 u, orthorhombic space group, Pnma (No. 62), a = 15.822(3) Å, b = 12.163(3) Å, c = 7.373(2) Å, V = 1418.9(4) Å(3), and D(calc) = 1.96 g cm(-)(3) for Z = 4 (molecule possesses mirror symmetry). Least-squares refinement on 1075 observed reflections (I > 3sigma(I)) converged with R = 0.0357 and R(w) = 0.0478.