L.M. Torell

Ion pairing effects in poly(propylene glycol)–salt complexes as a function of molecular weight and temperature: A Raman scattering study using NaCF3SO3 and LiClO4

The symmetric stretch modes for the anions in poly(propylene glycol) (PPG) complexed with NaCF3SO3 and LiClO4 at an ether oxygen of alkali metal cation ratio of 16:1 have been studied as a function of the molecular weight of PPG and temperature using Raman spectroscopy. The splitting of these modes has been analyzed in terms of ‘‘free’’ anions and ion pairs. We have observed that the number of ion pairs increases with increasing molecular weight and increasing temperature. The effect is greater in the PPG–NaCF3SO3 complex than in the PPG–LiClO4 complex. We have found a simple exponential relationship for the number of free anions in a master‐plot representation using a shift factor T* which increases with increasing molecular weight. These results are discussed in terms of the entropy and energetics of polymer–salt systems.

Raman spectroscopic study of ion–ion interaction and its temperature dependence in a poly(propylene‐oxide)‐based NaCF3SO3 –polymer electrolyte

Raman scattering measurements have been carried out on poly(propylene oxide) complexed with NaCF3SO3 salt of concentration O:M=30:1 (where O:M is the PO:Na ratio) over a temperature range of 186–360 K in order to study ion–ion associations of the dopant salt and their temperature dependence. Splitting of the symmetric stretching mode of the CF3SO−3 anion into a double band was observed and attributed to the existence of different environments of the anions. A two‐component band analysis led to the identification of coexisting dissociated free ions and ion pairs, suggested to be in contact. Below the glass transition temperature, Tg, the intensity of the mode corresponding to the free ions was more or less constant with temperature; the amount of free ions in the glassy state was found to be about 84% of the total salt concentration. Above Tg the amount of dissociated free ions decreased rapidly with temperature in an Arrhenius‐type behavior. The resulting reduction of the number of charge carriers has lit...

Short time structural relaxation processes in liquids: Comparison of experimental and computer simulation glass transitions on picosecond time scales

Recent Brillouin scattering studies of relaxation in the simple ion system Ca++–K+/NO−3 (two particles of argon structure and a small anion) provide the experimental basis for a direct comparison of glass transformation phenomenology in computer simulated LJ argon ‘‘glass,’’ with that in laboratory substances studied on the same time scale. Most of the attenuated glass transition characteristics observed for LJ argon are found in the ionic system, and the same relation of the dispersion midpoint to the so‐called ‘‘ideal’’ glass transition temperature is observed. Analysis of the real and imaginary parts of the complex longitudinal modulus shows that at high temperatures the relaxation function for Ca++–K+–NO−3, in strong contrast with that at normal low temperature behavior, closely approaches a simple exponential decay with Arrhenius form for the relaxation time. Furthermore, the high temperature Arrhenius plot extrapolates naturally to the reciprocal quasilattice vibration frequency determined by far infrared absorption studies on the glassy solid. Because of the low activation energy in the high temperature regime the glass ‘‘transition’’ observed with decreasing temperature is smeared out almost beyond recognition. It is argued that this will be a rather general phenomenon for hyperquenched glasses and that for such cases, the fictive temperature concept which associates the glass structure with an internally equilibrated liquid structure (that at Tfictive) must collapse. Parallels between LJ argon and Ca+–K+–NO3–glasses are discussed.Recent Brillouin scattering studies of relaxation in the simple ion system Ca++–K+/NO−3 (two particles of argon structure and a small anion) provide the experimental basis for a direct comparison of glass transformation phenomenology in computer simulated LJ argon ‘‘glass,’’ with that in laboratory substances studied on the same time scale. Most of the attenuated glass transition characteristics observed for LJ argon are found in the ionic system, and the same relation of the dispersion midpoint to the so‐called ‘‘ideal’’ glass transition temperature is observed. Analysis of the real and imaginary parts of the complex longitudinal modulus shows that at high temperatures the relaxation function for Ca++–K+–NO−3, in strong contrast with that at normal low temperature behavior, closely approaches a simple exponential decay with Arrhenius form for the relaxation time. Furthermore, the high temperature Arrhenius plot extrapolates naturally to the reciprocal quasilattice vibration frequency determined by far in...

Raman and Brillouin scattering of LiClO4 complexed in poly(propylene‐glycol)

Raman spectra of LiClO4 complexed in poly(propylene‐glycol) (PPG) have been obtained for concentrations of the monomer to salt ratio (ether oxygen):Li in the range 30:1–5:1. Splitting of the symmetric stretching mode of the ClO4− anion was observed with an intensity profile that varied with salt concentration. This phenomenon indicates a changing environment about the anion. A two‐component band analysis leads to the identification of dissociated ions on one hand and solvent‐separated ion pairs on the other. The concentration of ion pairs is relatively low compared to that of the dissociated ions, which are predominant for all concentrations. Despite the observed increase in the absolute number of dissociated ions at higher salt concentration, the electrical conductivity is reported to decrease in the same range. This indicates that the number of ‘‘free’’ charge carriers is of less importance for the conductivity than the mobility, which is damped in this concentration range. Frequency shifts of the disor...

Degradation of a fuel cell membrane, as revealed by micro-Raman spectroscopy

We report a micro-Raman spectroscopic study of a fuel cell membrane subjected to fuel cell testing until a significant decline in efficiency was noted. The membrane, poly(vinylidene fluoride)–graft–poly(styrene sulfonic acid), PVDF–g–PSSA, was produced by electron irradiation of a PVDF film, grafting with polystyrene and subsequent sulfonation. Depth profiles show that severe degradation due to loss of polystyrene sulfonic acid has occurred during the fuel cell run. The remaining amount of polystyrene sulfonic acid is of the order of 5–10% of the initial content. Confocal Raman spectroscopy is found to be an excellent tool for investigating the changes in membrane composition after fuel cell experiments.

Structure and properties of sulfonated poly [(vinylidene fluoride)–g-styrene] norous membranes porous membranes

The possibility of developing polyfvinylidene fluoride (PVDF)-based membranes as proton conductors has been investigated. Using electron beam radiation grafting and functionalisation, the composition of the membranes can be controlled. Membranes have been prepared from porous films of PVDF, and their structure and properties have been studied with Raman spectroscopy, wide angle X-ray scattering (WAXS), small angle X-ray scattering (SAXS), swelling tests, and by impedance spectroscopy. The grafting reaction of styrene, initiated in the amorphous regions and at the surfaces of the crystallites in the partly crystalline PVDF matrix, is very efficient, and high degrees of grafting can be achieved. Grafts are formed both from C–H and C–F branch pointsl Sulfonation can be accomplished to ca. 100%, and occurs mainly at the para-position of the phenyl rings. The hydration number was found to be independent of the degree of grafting, degree of sulfonation and crystallinity. The overall crystallinity decreased in the structure with the degree of grafting and sulfonation, partly owing to the diluting effect of the grafts, partly owing to efficient penetration of the grafts in the crystallites. The conductivity increased with the content of sulfonic acid groups and, in particular, with decreasing crystallinity. In the case of fully sulfonated membranes, a levelling-out of the conductivity was found at a degree of grafting of around 200%. Conductivities up to 120 mS cm–1 at room temperature were achieved.

Ionic interactions in MCF3SO3-polyether complexes containing mono-, di- and trivalent cations

Ion association has been investigated by Raman scattering from triflate salts M (CF 3 SO 3 ) x in PEO (400) for various cationic substitutions (M=li + , Ca 2+ , Cu 2+ , and Nd 3+ ) and for OH-capped, respectively CH 3 -capped, polymer chains. The ion-ion interaction shows different temperature behavior in different systems. The observations are discussed in terms of recent theories which consider volume changes (electrostriction) in the salt dissociation process

Molecular and ionic interactions in poly(acrylonitrile)- and poly(methylmetacrylate)-based gel electrolytes

Vibrational Raman and Fourier transform IR (FTIR) spectroscopy has been used to determine molecular and ionic interactions in poly(acrylonitrile)—(PAN) and poly(methylmetacrylate)—(PMMA) based gel electrolytes, containing LiClO4 salt and a mixture of ethylene carbonate (EC) and propylene carbonate (PC) as a gelating agent. A number of model systems, representing different stages of gel preparation, as well as the pure components, have also been studied. Vibrational spectra reveal interactions between EC and PC molecules in EC–PC mixture via electric dipoles located on C=O groups. In the salt-doped EC–PC system, additional bands close to the ring breathing and bending fundamentals of the carbonate ring of EC and PC, respectively, are attributed to solvent molecules interacting with the cations via Coulombic forces, the satellite band frequencies being well correlated to the cationic potentials. In the PMMA-based gels, no spectroscopically detectable interactions between the polymer matrix and the electroly...

Brillouin scattering study of hypersonic relaxation in a Ca(NO3)2 –KNO3 mixture

Brillouin spectroscopy has been used to provide information about the hypersonic velocity and absorption of thermal waves in a glass‐forming 60% KNO3–40% Ca(NO3)2 mixture (mol %) in a temperature range of 90–380°C for gigahertzian frequencies. A relaxation process was revealed in both the longitudinal velocity and attenuation. The data are consistent with ultrasonic results for the limiting low‐frequency velocity. The limiting high‐frequency velocity is considerably larger than that reported from ultrasonic studies and the position of the maximum of the amplitude absorption coefficient is shifted to higher temperatures. The variation of the relaxation time with temperature was found to be Arrhenius with a temperature‐independent activation energy in excellent accordance with the model proposed by Weiler, Bose, and Macedo. At low temperatures, a transverse mode due to shear waves was observed close to the Rayleigh line.

The molar conductivity behavior in polymer electrolytes at low salt concentrations; A Raman study of poly(propylene glycol) complexed with LiCF3SO3

Raman scattering measurements have been carried out on poly(propylene glycol) complexed with LiCF 3 SO 3 salt over a wide salt concentration range, the ether oxygen to alkali metal cation ratios (O : M) ranging from 4820 : to 12 :1. The relative concentrations of solvated anions, anion-cation pairs and ionic aggregates have been calculated from an analysis of the symmetric anion stretch. The degree of association is found to be almost constant in the O : M range 4820 : 1-40 : 1 whereafter it increases with increasing salt concentration. The results show that the dramatic increase reported for the molar conductivity in the O : M range 1000 : 1-40 : 1 cannot be explained by the redissociation of contact ion pairs or the formation of conducting triplets. Instead the major contribution to the conductivity increase seems to be a concentration dependent enhancement of the ionic mobility. A percolation based ionic hopping process involving exchange between ions in pairs and dissolved ions, either free or coordinated to ether oxygen sites, is advanced as a possible microscopic mechanism.