The Lyotropic SmC Analog Phase

In this thesis a lyotropic analog of the thermotropic chiral smectic C (SmC ) phase is presented for the first time. So far, only very scarce examples of the achiral variant of this phase have been known in lyotropic liquid crystals and no comprehensive studies have been performed on them. Thus, the focus of the present thesis is on the proof of existence and characterization of this novel phase. Furthermore, a tentative model of the lyotropic SmC analog phase is introduced. Thereby, this thesis contributes to the unification of the often separately treated fields of lyotropic and thermotropic liquid crystals. 

Fig. 1.6 Phase diagram of 5-[4-(5- -heptylpyrimidine-2-yl)phenyloxy]-pentane-l,2-diol and water (phase diagram redrawn after [22]). It was shown in later work, that the lyotropic SmC analog phase is indeed a columnar phase [23, 24], The isotropic phase is denoted with the abbreviation Tso and the two crystalline phases with Crj or CT2 , respectively. For an explanation of the occurring mesophases and their abbreviations see Chap. 3...

Fig. 1.7 Phase diagram of 1,4-phenylene bis(4-((2,5,8,ll,14,17-hexaoxanonadecan-19-yl)oxy) benzoate) and water (redrawn after [25]). The abbreviation D stands for dystetic, Iso for isotropic and Cr for crystalline. The inset shows a two-dimensional X-ray diffraction image of an aligned sample of the lyotropic SmC analog phase. The direction of an applied magnetic field H is indicated (adapted from [25], Copyright 1988 Taylor Francis, www.tandfonline.com)...

Fig. 1.8 Phase diagram of l-(2-hydroxyethyl)-l-(2-((2-hydroxyethyl)(2-((2-hydroxyethyl)(12-(4-((4-nitro-phenyl)diazenyl)phenoxy)dodecyl)amino)ethyl)amino)ethyl)aziridin-l-ium bromide and water (redrawn after [26]). The inset shows the texture between crossed polarizers of the lyotropic SmC analog phase (adapted from [26] with permission of the Royal Society of Chemistry)...

The second example is a system composed of water and an ionic amphiphile which incorporates several ethylene imine units and hydroxyl groups [26]. The phase diagram is shown in Fig. 1.8. The lyotropic SmC analog phase is stabilized over a quite broad concentration range. To prove the correct phase assignment of the lyotropic SmC analog phase, the authors provided X-ray dififaction data as well as texture images, which exhibit the characteristic schlieren texture known from thermotropic SmC phases cf. inset of Fig. 1.8). 

Detailed investigation of structural and physical properties of the lyotropic SmC analog phase by means of X-ray diffraction, tilt angle measurements and differential scanning calorimetry. The impact of changes in temperature and solvent concentration on the structure of the lyotropic SmC analog phase shall be analyzed. 

Design of a first structural model of the lyotropic SmC analog phase. 

In the following subsection detailed phase diagrams of three solvent/surfactant mixtures will be presented. In two of those phase diagrams, i.e. the C50/water and the C50/formamide system, the lyotropic SmC analog phase can be found. Furthermore, as a counterexample for a C50 system in which the lyo-SmC phase does not occur, the C50/iV-methylformamide system was investigated. Characteristic textures of the individual phases will be displayed to document the correct assignment of the phases. Additionally, the other liquid crystalline phases which appear in the phase diagrams will be characterized briefly. 

Phase Diagrams of C50/Solvent Systems Exhibiting the Lyotropic SmC Analog Phase... 

The phase diagram of C50 and formamide is displayed in Fig. 5.15. The sequence of the phases looks quite similar to the one of the phase diagram presented in Fig. 5.14. The most significant difference is that the lamellar La phase is much more stabilized in mixtures with formamide than with water and thus, the other phases appear at lower solvent concentrations. As a result, the lyotropic SmC analog phase only occurs between 7 and 30 wt% of formamide. Moreover,... 

Coherently, such zigzag defect lines can also be observed in surface-stabilized samples of the lyotropic SmC analog phase as shown in Fig. 5.16e. 

In the mixture with 7 wt% of formamide three liquid crystalline phases can be found, namely the lyotropic SmC analog phase, the Coli phase and the lamellar L phase. The mixtures between 12 and 22 wt% of formamide only exhibit the lyotropic SmC analog phase and the lamellar L phase. While the DSC curves of the sample with 12 wt% of formamide still show a very pronounced peak at the lyotropic SmC analog to lamellar L phase transition, the peak becomes smaller and... 

The investigated surfactant/solvent mixtures of the diol C50 and water or for-mamide, respectively, are the first lyotropic systems to form a lamellar, fluid and tilted liquid crystalline phase which contains chiral surfactant molecules. The main issue of the present chapter is thus to demonstrate whether or not the lyotropic SmC analog phase exhibits similar chirality effects as known from its thermotropic counterpart. The most outstanding manifestations of chirality in the thermotropic SmC phase are helicity, due to a chirality-induced precession of the director, and ferroelectricity, due to its polar C2-point group symmetry. Thus, the focus of this chapter is on the detection and analysis of those two macroscopic chirality effects. 

F. 5.32 Striped texture of the lyotropic SmC analog phase in a sample of C50 with a 59 wt% of water at 36 °C and b 32 wt% of formamide at 30 °C. Due to the occurrence of unwinding lines, the pitch p corresponds to the distance between two stripes instead of only one (adapted from [20], Copyright 2013 Wiley-VCH Verlag GmbH Co. KGaA, Weinheim. Reproduced with permission.)... 

In Fig. 5.32b the striped texture of the lyotropic SmC phase with formamide as solvent can be seen. The stripes appear much more visible in this sample, clearly indicating the macroscopic heUcity of the lyotropic phase. The helical pitch of p = 5.2 pm is close to the value found in the mixture with water. However, there is one significant difference between the two solvents While the sample with water had to rest for several weeks before the striped texture could be detected, the sample with formamide only took seconds after the transition into the lyotropic SmC analog phase to show the texture displayed in Fig. 5.32b. This difference in the time-based evolution of the helical director configuration is quite remarkable and implies that the solvent plays a very important part in the formation of the helix, even though it has only littie impact on the absolute value of the helical pitch. Possible explanations might be the more extended solvent layer in the case of mixtures with water or a different internal structure in the solvent layer. However, these points are only speculations and the reason for the deviating behavior still has to be understood. 

In further temperature and concentration-dependent measurements of the helical pitch, only mixtures with formamide were chosen as the lengthy evolution time necessary for mixtures with water together with the ever present threat of solvent evaporation make such investigations of mixtures with water much more complicated. In Fig. 5.33 the helical pitch p is plotted versus the reduced temperature T - Tq for a sample with 18 wt% of formamide. The pitch shows the typical temperature dependence known from thermotropic SmC phases [30]. Right after the phase transition into the lyotropic SmC analog phase, the pitch increases rapidly to a value of about 5.5 pm and decreases more slowly towards a low temperature value of about 2.5 pm. However, by repeating the measurement with other concentrations of formamide, no significant difference in the value of p could be detected. 

The reason for this behavior can be found in the measuring conditions. The temperature-dependent measurement of the helical pitch was performed with the direct method cf. Sect. 4.5.1) in a sample of 30 pm thickness. Apparently, in such rather thin samples compared to the value of the helical pitch, the formation of the helical director configuration cannot take place undisturbed, but is influenced significantly by interactions with the surfaces of the liquid crystal cell. Consequently, the observed value of p depends rather on the cell gap than on the intrinsic pitch of the lyotropic SmC analog phase. Nonetheless, the measurement in Fig. 5.33 indicate that the helical pitch of the lyotropic SmC analog phase varies with temperarnre and that the temperature dependence is comparable to the one of thermotropic SmC phases. 

The inverse of the pitch which corresponds to the helical twist of the lamellas against each other, is plotted in Fig. 5.35 for different concentrations of formamide. The values shown in the upper part of Fig. 5.35 were determined with the Cano method, while the bottom part shows the results obtained by the direct method. The two plots in Fig. 5.35 basically show the same behavior. In both plots no clear temperature dependence of the helical twist can be found. Right after the phase transition into the lyotropic SmC analog phase, the helical structure is... 

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