Jörgen Tegenfeldt

Crystallinity and morphology of PVdF-HFP based gel electrolytes.

Abstract The structural behaviour of a random copolymer consisting of polyvinylidene fluoride (PVdF) and hexafluoropropylene (HFP), when mixed with organic solvents and a lithium salt, has been investigated. In its neat form, the copolymer exhibits a semicrystalline matrix, where the PVdF constituent crystallises partially. The crystalline phase appears mostly as the non-polar (trans–gauche–trans–gauche′) phase II. The relative crystallinity of the PVdF domains remains constant on addition of a plasticiser, propylene carbonate (PC) or triethylene glycol dimethyl ether (TG), but is greatly decreased on addition of the salts lithium bis(trifluoromethane sulfone) imide, Li(TFSI) or lithium tetrafluoroborate, LiBF4. This is shown by the results from both wide-angle (WAXS) and small-angle (SAXS) X-ray scattering and FTIR-spectroscopy. Surprisingly, the addition of such a polar medium as the salt, and to a lesser extent the plasticisers, also causes a change in the structure of the crystalline phase to a more polar phase III (intermediate polar conformation). The thermal stability of the gels and the large influence the salt addition has on the material is also shown by the results from differential scanning calorimetry (DSC) measurements.

FTIR study of water in cast Nafion films

The state of water in cast perfluorosulphonic Nafion films has been investigated using Fourier transform infrared spectroscopy (FTIR). FTIR spectra of cast Nafion films equilibrated in different relative humidities (RH) in a non flowing vapour environment, and in 100% RH at different temperatures have been made. In contrast to previously published results on thicker membranes the FTIR experiments indicate that there is already a full proton transfer from the sulphonic acid group in Nafion when there is one water molecule per sulphonate group.

Proton spin diffusion for spatial heterogeneity and morphology investigations of polymers

The diffusion of Zeeman spin order in the laboratory frame for a system with microscopic spatial heterogeneity, due to either molecular mobility or chemical heterogeneity, has been analysed by solving the diffusion equation in the limit of two different models. By simulation of proton spin diffusion nuclear magnetic resonance (NMR) spectral areas for poly(ethylene oxide) we have demonstrated, for both models, the sensitivity of these observables to phase domain size and to the dimensionality of the diffusion process. The possibility to obtain information about the morphology is analysed. Results from calculations based on the two models are compared with experimental spin diffusion results from 1H NMR spectra of poly(ethylene oxide). Two methods for filtering the z-magnetization from the different phase domains in a solid are discussed. The efficiency of a magnetization dipolar filter based on magic and polarization echoes is presented.

On the correlation between deuteron quadrupole coupling constants, O-H and O-D stretching frequencies and hydrogen-bond distances in solid hydrates

Abstract Correlation curves are reported relating deuteron quadrupole coupling constants ( e 2 qQ/h ) to O-H and O-D stretching frequencies ( v OH and v OD ) for isotopically-dilute HDO molecules. Correlation curves are also given relating these quantities to hydrogen-bond distances. The best correlations are found to be between ( e 2 qQ/h ) and v OH and v OD whereas the correlations involving intermolecular geometries show considerably greater scatter. Complications caused by the effects of anharmonicity on the stretching vibrations and of vibrational averaging of the quadrupole coupling and geometrical parameters are discussed in light of the observed correlations. It is concluded that, although these effects are large, their magnitude is also strongly correlated with the stretching frequencies, quadrupole couplings and geometrical parameters.

O-H and O-D stretching vibrations in isotopically dilute HDO molecules in some solid hydrates

Abstract Vibrational frequencies vOH and vOD have been measured for isotopically-dilute HDO molecules in eleven solid hydrates at 90 K. The results have been used to prepare a plot of the ratio vOH/vOD versus vOH. The ratios fall on a smooth curve and decrease with decreasing frequencies vOH. The anharmonicity constants ωexe have been estimated. They were found to increase with decreasing vOH.

Diffusion and ionic conductivity in Li(CF3SO3)PEG10 and LiN(CF3SO2)2PEG10

Self-diffusion of the cation, the anion and the polymer chain in the low-molecular-weight polymer electrolyte systems Li(CF3SO3)PEG10 and LiN(CF3SO2)2PEG10 has been studied as a function of temperature using nuclear magnetic resonance spectroscopy. The diffusion behaviour is very similar in the two systems, where the cation is diffusing attached to the polymer chain, while the anion is moving slightly decoupled. However, all species are slightly more mobile in LiN(CF3SO2)2PEG10 than in Li(CF3SO3)PEG10, due to the plasticizing effect of the N(CF3SO2)2− ion, which also explains the higher conductivity for the LiN(CF3SO2)2PEG10 system. The temperature dependence of the diffusion coefficients is of Arrhenius type while the ionic conductivity follows the Vogel-Tammann-Fulcher relation. The diffusion has been compared with the conductivity by applying the Nernst-Einstein relation: a slight discrepancy is found for Li(CF3SO3)PEG10, while the agreement is very good for LiN(CF3SO2)2PEG10. Spin-lattice relaxation experiments have shown that the polymer chains undergo local conformational transformations, providing an additional diffusion mechanism for the ions.

Crystallinity in Cast Nafion

The performance of a polymer electrolyte fuel cell is critically dependent on the water uptake in the polymer electrolyte, usually Nafion. Nafion in solution is often painted onto the electrodes of the fuel cell. It is important that this cast Nafion film stay amorphous and not crystallize. Cast Nafion films, ca. 1 {micro}m thick, crystallized on silicon plates when kept in air at room temperature for a long time. The films contain large crystalline regions ranging from 0.5 mm to several millimeters in size. X-ray diffraction (XRD) and Fourier transform infrared (FTIR) microspectroscopy have been used to investigate the crystalline and amorphous regions. The XRD shows two sharp peaks. One of the peaks is developed before the second one appears in the diffractogram, indicating that there might be two types of crystallizing processes. FTIR spectra of the amorphous and crystalline regions in the films show important differences. In the crystalline regions, the film contains the sulfonic acid at the end of the side chains; hence, the crystalline regions contain no water molecules. In the amorphous regions there is a complete proton transfer from the acid to the water molecules, and sulfonate groups are obtained.

Multipole NMR. VII: Bromine NMR quadrupolar echoes in crystalline KBr

Abstract Multipole NMR is used to calculate the responses during various pulse sequences for a solid system of spin - 1 2 nuclei subject to a strong do magnetic field and an inhomogeneous distribution of quadrupole and dipole-dipole interactions. The following sequences were treated: 90° X −τ 1 , 90° X −τ 1 −90° X −τ 2 90° X −τ 1 −90° Y −τ 2 , 90° X −τ 1 −90° X −τ 2 −90° X −τ 3 , 90° X −τ 1 −90° Y −τ 2 −90° Y −τ 3 , and 90° X −τ 1 −45° Y −τ 2 −45° Y −τ 3 . The calculations show that all of the 15 multipole polarizations (3 alignments, 6 single-, 4 double-, and 2 triple-quantum coherences) can be created in significant amounts by appropriate sequences. The calculations for each sequence have been verified by a detailed experimental study of the sequence responses for Br in a deformed single-crystal of KBr. Seven independent relaxation times were determined. The experiments provide the first evidence for octupole alignment in spin systems.

Jump motion of deuterons along hydrogen bonds in KDCO3 a deuteron relaxation study

Abstract The orientation and temperature dependences of 1 T 1 of the deuterons in single crystals of KDCO 3 were measured. The orientation dependence data are interpreted in terms of a fluctuating quadrupole coupling tensor V fl which is compared with a model in which the jumping of the deuterons between two possible sites 1 and 2 along the hydrogen bond is considered to be the spin-relaxation determining process. From the agreement between model and experiment it is concluded that such jumps do occur in KDCO 3 and dominate the deuteron relaxation rate. The prominent feature of the temperature-dependence data is a steady trend in the plot of log T 1 −1 vs 1 T toward steeper variation at lower temperatures. This is clear evidence that the two deuteron sites are energetically inequivalent with ΔE = 4.3 kJ/mol. The barrier between the two sites is E a = 3.9 kJ/mol. The temperature dependence of the rate constants k 1 and k 2 is also inferred from the data. The combination of V fl with the motionally averaged quadrupole coupling tensor V , determined from line splittings, allowed us to calculate the quadrupole coupling tensors V 1 , and V 2 corresponding to the two possible deuteron sites in KDCO 3 . The results of a similar study on tetrafluoroterephthalic acid, C 6 F 4 (COOD) 2 , are briefly mentioned.

Gel electrolytes prepared from oligo(ethylene glycol)dimethacrylate: glass transition, conductivity and Li+-coordination

The influence of two plasticizers, propylene carbonate and dimethyl sulphoxide, as well as different salt concentrations of Li(TFSI), on properties of a polymer gel electrolyte material has been studied using differential scanning calorimetry (DSC) and ac impedance and FTIR spectroscopy. Variations of glass transition temperature and the conductivity behaviours of the systems were examined, and found to be highly dependent on the amount and type of the plasticizer used. Characteristic band-shifts in FTIR spectra, indicating coordination of lithium ions, have been found for the polymer and both the plasticizers in the corresponding binary solutions. These shifts were used to study the coordination preferences in the complete ternary electrolyte system. The combined results from the three experimental techniques have been discussed.