Smectic phases double layer

The so ealled a phase of the fatty alcohols is a thermotropic type smectic B liquid crystal with a hexagonal arrangement of the moleeules within the double layers. It is initially formed from the melt during the manufaeturing proeess and normally transformed into a erystalline modifleation on eooling. However, the erystallization of the gel matrix ean be avoided if the a phase ean be kept stable as it eools to room temperature. This ean be achieved by eombining appropriate surfaetants, such as myristyl or lauryl alcohol and cholesterol, a mixture of whieh forms a lamellar liquid... 

Rod-Like Liquid Crystals with One (Semi)Perfluorinated Chain Double Layer Smectic Phases... 

So far we have discussed 2D density modulated phases that are formed by deformation or breaking of the layers. However, there are also 2D phases with more subtle electron density modulations. In some cases additional peaks observed in the XRD pattern (Fig. 10) are related to a double layer periodicity in the structure. As double layer periodicity was observed in the bent-core liquid crystals formed by the asymmetric as well as symmetric molecules [22-25] it should be assumed that the mechanism leading to bilayers must be different from that of the pairing of longitudinal dipole moments of molecules from the neighboring layers, which is valid for smectic antiphases made by asymmetric rod-like molecules. 

We have also shown the existence of 2D phases due to more subtle electron density changes. In some cases additional peaks are observed in the XRD pattern, signifying a double layer periodicity in the system, which can be accounted for if a general orientation of the polar director is allowed. If the polar director is not perpendicular to the tilt plane there exists a component of polarization in the direction of the smectic layer normal (longitudinal polarization). By double layer periodicity the system escapes from the polar structure and in addition achieves better packing of the molecular cores and molecular tails. 

Most mesogenic salts derived from aliphatic acids, (R-COO) M (R = alkyl (normal and branched) or alkenyl M=Li, Na, K, Rb, Cs, NH4, Tl, Pb or other metal) form layered structures (lamellar phases, neat phases) that are similar to smectic A phases. However, mesogenic salts form double layers and are not miscible with smectic A phases [276]. Some of the materials show very complicated polymorphism with a large number of mesophases [276-279]. In general, the transition temperatures of the salts are quite high compared with those of nonpolar liquid crystals. Most of the salts can also form lyotropic liquid crystals. 

Figure 21. Schematic representation of singular lines in a smectic C phase, (a, b) The distribution of the directors in four-branch nuclei, the layers being seen from the top open circles indicate the singularity position. (c, d) Cross-sections of singular lines parallel to layers, (e) Superimposed layers of a smectic C phase can be devoid of defects, whereas an edge dislocation is present for the rotation periodicity (left-handed helix). The tilted molecules show a-Ak rotation on the left-hand side of the diagam and only a -2it one on the right-hand side. This situation involves the presence of a line similar to (d) in the defect core. The double lines indicate the locus of directors parallel to the plane of the page.

The simultaneous occurrence of nematic, cubic, layer and columnar structures in pure mesogens is of great importance to bridging the gap between calamitic and discotic molecules and the corresponding mesophases. Smectic, cubic and columnar phases have also been described in polycatenar compounds, and thus double-swallow-tailed mesogens can also be considered to be a special case of tetracatenar compounds (see Chap. XII of this volume). 

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