Investigation of the Helical Pitch

In samples which are substantially thicker than the helical pitch length, a helical director configuration can develop leading to a regularly striped texture (cf. Sects. 

2 and 4.5.1). The observation of such a striped texture in planarly aUgned samples already accounts for a macroscopic chirality of the phase and efforts were made to achieve this texture in the lyotropic SmC phase. Representative examples are shown in Fig. 5.32. 

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. 

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