Anne-Laure Rollet

Anisotropy of structure and transport properties in sulfonated polyimide membranes

An original analysis of the structure of ionomer films along the three dimensions using micro small-angle X-ray scattering (microSAXS) technique is reported. While the in-plane structure appeared as isotropic, a significant SAXS anisotropy was observed in the transverse direction, revealing a multiscale structural anisotropy from the molecular level to several tens of nanometers. Using transmission and scanning electron microscopy (TEM and SEM, respectively), a layered structure was identified up to micrometer scales. This structural anisotropy was used to interpret the conductivity data as a function of the ion content. Moreover, the diffusion coefficients of the counterions, were determined along the planar and transverse directions using pulse field gradient spin echo-NMR (PFGSE-NMR) in order to correlate structure and transport properties.

A SANS determination of the influence of external conditions on the nanostructure of nafion membrane

We have studied the structure of nafion membranes by small-angle neutron scattering in the q range of 7 10−3 A−1 to 0.4 A−1. External conditions were varied: counterions, electrolyte concentration, temperature, and pretreatment of membranes. The nanostructure of Nafion exhibits a sensivity that depends on the parameters considered: changing counterions and concentration leads to a reversible local reorganization of the hydrophilic cavities, whereas changing temperature and pretreament leads to a irreversible reoganization on a larger scale. The mean variation of the radius of the hydrophilic cavities versus these parameters is about 10%. The interface between hydrophobic and hydrophilic phases appears to be sharp except when the temperature exceeds 60 °C.

High temperature NMR study of the local structure of molten LaF3-AF (A = Li, Na, K and Rb) mixtures

The local structures of molten lanthanum alkali fluoride binaries have been studied using HT NMR technique. The chemical shifts of (19)F, (23)Na and (139)La in solid and in liquid have been compared for AF (A = alkali) and LaF(3). In pure molten alkali fluorides, the polarisability of anion-cation pairs appears to be a key parameter to depict the observed evolution of (19)F chemical shifts. The influence of the composition has also been studied by measuring the chemical shifts in molten LaF(3)-AF as a function of LaF(3) concentration. A strong influence of the alkali influence is observed. The coordination number of lanthanum is decreased versus AF amount all the more since the alkali atomic number is high. Moreover, the more polarisable the alkali, the less bridging fluorines between the LaF(x) units.

Synthesis and structure resolution of RbLaF4.

The synthesis and structure resolution of RbLaF(4) are described. RbLaF(4) is synthesized by solid-state reaction between RbF and LaF(3) at 425 °C under a nonoxidizing atmosphere. Its crystal structure has been resolved by combining neutron and synchrotron powder diffraction data refinements (Pnma,a = 6.46281(2) Å, b = 3.86498(1) Å, c = 16.17629(4) Å, Z = 4). One-dimensional (87)Rb, (139)La, and (19)F MAS NMR spectra have been recorded and are in agreement with the proposed structural model. Assignment of the (19)F resonances is performed on the basis of both (19)F-(139)La J-coupling multiplet patterns observed in a heteronuclear DQ-filtered J-resolved spectrum and (19)F-(87)Rb HMQC MAS experiments. DFT calculations of both the (19)F isotropic chemical shieldings and the (87)Rb, (139)La electric field gradient tensors using the GIPAW and PAW methods implemented in the CASTEP code are in good agreement with the experimental values and support the proposed structural model. Finally, the conductivity of RbLaF(4) and luminescence properties of Eu-doped LaRbF(4) are investigated.

Study of self-diffusion of alkali metal cations inside a Nafion membrane

A novel radiotracer method, based on the rotating electrode technique used in electrochemistry, was used for the study of the self-diffusion properties of ions in ion-exchange membranes; the self-diffusion coefficients of Na+ and Cs+ ions were determined in a Nafion 117 membrane. The influence of electrolyte concentration and temperature was investigated and an important influence of the former was observed. The results indicate that the diffusion process inside the membrane for Na+ ion depends linearly on the water volume fraction. The results for Cs+ may be a sign of stronger interactions than for Na+ ions with the membrane.

Measuring self-diffusion coefficients up to 1500 K: a powerful tool to investigate the dynamics and the local structure of inorganic melts.

Self-diffusion is a fundamental property of liquid dynamics that also provides important structural information. To explore the dynamics in inorganic melts with high liquidus temperature, we propose a new setup based on pulsed field gradient NMR combined with laser heating that makes possible in situ self-diffusion coefficient measurements up to 1500 K. Applied to several corrosive molten fluorides in a wide range of compositions and temperature, we have evidenced the different key parameters of their motion along with their structural characteristics. In alkali fluorides, the self-diffusion coefficient of fluorine depends slightly on the composition compared to the temperature, displaying these systems as an ideal bath of polarizable hard spheres. In contrast, self-diffusion in rare earth and alkali fluorides mixtures presents a complicated balance between temperature and the network-forming process of the ionic long-lived units. These results open wide perspectives in the study of high temperature liquids.

Structure and dynamics in yttrium-based molten rare earth alkali fluorides.

The transport properties of molten LiF-YF3 mixtures have been studied by pulsed field gradient nuclear magnetic resonance spectroscopy, potentiometric experiments, and molecular dynamics simulations. The calculated diffusion coefficients and electric conductivities compare very well with the measurements across a wide composition range. We then extract static (radial distribution functions, coordination numbers distributions) and dynamic (cage correlation functions) quantities from the simulations. Then, we discuss the interplay between the microscopic structure of the molten salts and their dynamic properties. It is often considered that variations in the diffusion coefficient of the anions are mainly driven by the evolution of its coordination with the metallic ion (Y(3+) here). We compare this system with fluorozirconate melts and demonstrate that the coordination number is a poor indicator of the evolution of the diffusion coefficient. Instead, we propose to use the ionic bonds lifetime. We show that the weak Y-F ionic bonds in LiF-YF3 do not induce the expected tendency of the fluoride diffusion coefficient to converge toward one of the yttrium cation when the content in YF3 increases. Implications on the validity of the Nernst-Einstein relation for estimating the electrical conductivity are discussed.

Transport-structural parameters of perfluorinated membranes Nafion-117 and MF-4SK

The concentration dependences of the electroconductivity of membranes Nafion-117 and MF-4SK are studied in salt solutions under identical experimental conditions. The results are compared with nanostructural characteristics of Nafion-117 measured independently by the AFM and quasi-elastic neutron scattering methods. The role played by internal interfaces between hydrophobic and water-cluster microphases of Nafion during the membrane transition from the Na+ form to the form of tetraalkylammonium ions is determined. It is established that the specific adsorption of organic ions makes the water clusters disintegrate and the elasticity of side segments diminish; as a result, the polymer films conductivity “switches off.” This morphological effect is used for interpreting the fact that the conducting and hydrophilic properties of perfluorinated membranes substantially depend on their conditioning prior to measurements. It is shown that expanding the membranes reduces the differences between the electroconductivity and structural organization of specimens of different brands with close values of the exchange capacity.

Studies of the local structures of molten metal halides

This review covers the local structure of molten metal halide, where a somewhat wide definition of “local structure” is employed, i.e. from the first shell of interacting neighbours up to correlations arising at the nanometer length scale. These particular liquids are indeed strongly organized at unusually long distances due to the predominance of coulombic interactions. Many of them are also characterized by the formation of intermediate range ordering. It is therefore impossible to describe the local structure of these liquids omitting this longer-ranged correlation. Finally, a deeper attention is given to the systems on which recent progresses have been made in the last decade, as for instance molten fluorides and rare earth halides.

Chapter Four - NMR Studies of Molten Salt and Room Temperature Ionic Liquids

Abstract This chapter presents the nuclear magnetic resonance (NMR) developments and studies on the high temperature inorganic molten salts and on room temperature ionic liquids. It is focused on the liquid structure investigated, thanks to the chemical shifts analysis and to magnetization transfer experiments. It deals also with the dynamics at the local scale investigated using relaxometry and at the long range using pulsed field gradient NMR.