Browsing by Author "Bruckner, Johanna Ricarda"

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A first example of a lyotropic smectic C* analog phase : design, properties and chirality effects
(2015) Bruckner, Johanna Ricarda; Gießelmann, Frank (Prof. Dr. rer. nat.)
The subject of this thesis is the discovery and characterization of a novel lyotropic liquid crystalline phase. It is the lyotropic counterpart of the thermotropic SmC* phase, which became famous as the only ferroelectric material in nature which is fluid. In addition to providing evidence for the existence of this lyotropic SmC* analog phase, the main focus of the work presented is on the investigation of its structural properties and chirality effects. The schematic phase diagrams of various surfactant / solvent systems were studied systematically. The results show that certain structural elements are required for the formation of the lyotropic SmC* analog phase. In addition to a SmC*-promoting aromatic core and a hydrogen-bonding head group, the presence of an ethylene glycol unit linking those two elements seems to be crucial for the surfactant molecule. The solvent molecules should be highly polar, possess a small molecular volume and be able to form multiple hydrogen bonds leading to a dense three-dimensional hydrogen bond network. Of the systems investigated, only one surfactant molecule formed the lyotropic SmC* analog phase with the two solvents water or formamide. The detailed phase diagrams of these surfactant / solvent systems exhibit a variety of lyotropic liquid crystalline phases. Remarkably, the lyotropic SmC* analog phase occurs only at elevated solvent concentrations, which shows that this is a true lyotropic phase. The SmC* analog nature of the lyotropic phase was demonstrated by the observation of characteristic textures such as broken fan-shaped textures, schlieren textures, zigzag defects, spontaneous tilt domains in the surface-stabilized state and pitch lines, as well as one- and two-dimensional X-ray experiments. Furthermore, temperature and concentration dependent measurements of the layer spacing as well as of the tilt angle were performed. The latter showed that the tilt angle decreases with increasing solvent concentration and that the lamellar Lα to SmC* analog phase transition is driven from first to second order. A significant result of this thesis is that the lyotropic SmC* analog phase also exhibits analog chirality effects as known from its thermotropic counterpart, even though the surfactant bilayers are separated by substantial layers of achiral solvent molecules. The investigation of the helical pitch showed that it is in the order of several micrometers for both solvents. The temperature dependence of the pitch is comparable to the thermotropic case. The concentration dependence, in contrast, is counterintuitive, the helical twist increasing with increasing formamide concentration even though the number density of chiral molecules in the mixtures decreases. The second chirality effect investigated was the polar electro-optical switching between two surface-stabilized states. The polar nature of the observed effect indicates that the lyotropic SmC* analog phase possesses a spontaneous electrical polarization similar to its thermotropic analog. Based on the results obtained in this thesis, a first model of the lyotropic analog of the SmC* phase was developed. The model suggests that the correlation of the director tilt as well as its helical precession takes place via a strong, three-dimensional hydrogen bond network formed by the solvent molecules. When the solvent concentration is increased, the solvent layers become thicker and their rigidity is reduced, which explains the effects observed. In addition to the stated aims of this thesis, the phase diagram of the selected surfactant using N-methylformamide as solvent was investigated. It showed that no lyotropic SmC* analog phase occurs with a solvent unable to form a three-dimensional hydrogen bond network. However, two other interesting phases appear by adding this solvent. The first phase is a rare example of a re entrant cholesteric phase and the second is a solvent-induced twist grain boundary phase, which is the first observation of this phase in a lyotropic liquid crystal. In conclusion, this work shows that a lamellar, tilted, fluid phase exists in lyotropic liquid crystals and that it exhibits characteristic chirality effects, namely helicity and spontaneous electrical polarization, known from the thermotropic ferroelectric SmC* phase. These results contribute significantly to a better understanding of lyotropic liquid crystals and bridge a substantial gap between the two fields of liquid crystalline research.