Wedge shaped amphiphilic sulfonate molecules : phase behavior, ion conductivity and membrane preparation

This dissertation describes the synthesis, phase behavior and ion-conductive properties of various wedge-shaped amphiphilic sulfonate molecules including lithium/sodium/potassium/cesium 2,3,4-tris(11'-acryloyloxyundecyl-1'-oxy)benzene-sulfonates (A-Li, A-Na, A-K and A-Cs), and sodium 4'-[3'',4'',5''-tris(11'''-methacryloyloxyundecyl-1'''-oxy)benzoyloxy]azobenzene-4-sulfonate (Azo-Na). It is pursued that via supramolecular self-assembly these molecules are able to create ionic channels of different geometry depending on the molecular geometry and environmental conditions. The introduction of polymerizable groups is intended to allow in the second step the covalent incorporation of these channels into a polymer matrix by means of polymerization reaction. In Chapter 1, the structure and properties of a perfluorinated sulfonic acid ionomer membrane: Nafion is described. In Chapter 2, the synthesis of A-Li, A-Na, A-K and A-Cs is reported. In Chapter 3, the relation between the structure and conducting properties of A-Na is studied as a function of relative humidity (RH). This compound forms a disordered columnar (Colhd) structure with an ionic channel along the axis of the cylinder. Upon increase of RH up to 55%, the columnar structure remains but a minor water uptake and a slight swelling of the cylinders are observed, which results, however, in a significant enhancement of the ion conductivity by 4 orders of magnitude. Further water uptake by increasing humidity leads to a dramatic change of the mesophase structure whereby bicontinuous cubic phases (Cubbi) are formed. The Colhd to Cubbi transition is accompanied by an additional substantial increase of conductivity due to the formation of an interconnected network of ionic channels. Chapter 4 reports the study on the orientation of the Colhd phase formed by A-Na. The columns of A-Na are oriented preferentially in plane (homogeneous or planar orientation) on both hydrophilic and hydrophobic substrates. Anodized aluminium oxide (AAO) porous substrate is used to achieve a homeotropic orientation of the columnar phase. It is further demonstrated that the oriented sample has a higher conductivity than that of a non-oriented one. In Chapter 5, membranes are prepared via photo-polymerization of A-Na under different relative humidity. It is found that the polymerization does not change significantly the phase structure of the material at different humidity conditions. Interestingly, in comparison with the monomer sample, the membranes prepared by polymerization at RH=100% condition exhibits a higher ion conductivity, but the membranes polymerized at RH≤55% conditions are much less conductive. In Chapter 6, the four salts with different counterions, namely A-Li, A-Na, A-K, and A-Cs are studied regarding their phase behavior and ion conductivity. The real-time X-ray measurements are carried out to reveal the detailed phase transitions. A-Li and A-Cs are isotropic at room temperature, meanwhile A-Na and A-K form a disordered hexagonal columnar mesophase. By cooling, A-Li and A-Cs undergo a transition from isotropic to lamellar phase. And A-Na and A-K are transformed from the hexagonal columnar to a centered rectangular columnar phase. Furthermore, it is found that the salts, which form a columnar phase, show much higher water uptake ability, which results also in much higher ion conductivity. In Chapter 7, the synthesis, phase behavior and ion conductivity of a new methacrylated wedge-shaped sulfonate molecule, sodium 4'-[3'',4'',5''-tris(11'''-meth-acryloylundecyl-1'''-oxy) benzoyl]azobenzene-4-sulfonate (Azo-Na), are described. It is found that Azo-Na forms an orthorhombic crystalline phase at low temperature, and by heating it is transformed into a disordered hexagonal columnar (Colhd) phase. The formation of the Colhd mesophase is also observed by water uptake of Azo-Na at RH=100%. In this case, according to the X-ray scattering data, very thick supramolecular columns with a diameter of 12.5 nm are formed. According to gravimetry analysis, at RH=100%, Azo-Na uptakes 15.3 water molecules per sulfonate group. At 25 °C, the dry sample shows a very low conductivity of 7.32×10-6 μS/cm, but at RH=100%, the conductivity increases to 102.9 μS/cm. In Chapter 8, two partially deuterated wedge-shaped amphiphilc sulfonate molecules, namely sodium and potassium 2,3,4-tris(n-butyl-d9-octyloxy)benzene-sulfonates, are synthesized. The whole synthesis procedure of these compounds consists of 8 steps. All intermediate and final products are characterized by means of 1H NMR and 13C NMR spectroscopy. These compounds will be studied in future by solid state deuterium NMR to elucidate the packing of alkyl groups in different mesophases