Cross-shaped liquid crystals

Fig. 9. Smectic fan-shaped texture observed between crossed polarizers. Courtesy of G. H. Brown, Liquid Crystal Institute, Kent State University.

Note 3 A MCPLC with cross-shaped mesogenic groups (b or g) is known as a cruciform (or star) polymer liquid crystal. 

T he behavior of liquid crystals in applied electric fields has been the object of several recent studies (1, 3, 4)- The materials primarily used here were those in which the dipole moment of the molecule was not in the same direction as the molecular axis. When an electric field is applied to such a system by transparent electrodes, the characteristic cigar-shaped domains shown in Figure 1 for p-azoxyanisole are readily observed, using relatively low magnification with or without polarized light. The optical behavior of such domains between crossed polarizers indicates that their optic axis is essentially parallel to the electrode surface and essentially perpendicular to the direction of the applied field. 

The macroscopic theory of elasticity can explain why longitudinal ridges and troughs on the surface of a glass are conducive to the planar homogeneous alignment of nematic liquid crystals [23]. For simplicity, a sinusoidal shape is chosen for the cross-section of a surface relief with the wavevector q directed along x, see Fig. 10.21a ... 

Another type of liquid crystalline polymers are those having rod-shaped side chains. TTius, the backbone may be a random coil, but the side chains are organized into one- or two-dimensional liquid crystals. The stiff backbone types make very high modulus fibers and high temperature plastics. The side chain types are useful for their action in magnetic and electrical fields. (Note the behavior of battery-driven liquid crystalline watches, for example. The liquid crystalline material is held between crossed Nicols the orientation of the molecules is controlled by an electric field.) In the following, the terms mesophase and liquid crystal phase are used interchangably. 

Discotic liquid crystals are a prototypical self-assembled columnar system [9-13]. This class of liquid-crystalline compounds is relatively new, discovered in 1977 by Chandrasekar and coworkers [14,15]. The assembly motif, shown in Figure 2, for this class of compounds has an aromatic core that is surrounded by hydrocarbon chains. These disk-shaped molecules then stack to form columns. These one-dimensional stacks aggregate to form arrayed columns. When the columns have a circular cross-section they typically stack into a hexagonal arrangement as shown in Figure 2(a). Some of the original discotics were hexa-substituted phthalocyanines (1), benzenes (2), and triphenylenes (3), shown in Figme 2(b). The self-assembly of classical discotics will not be presented in depth here because it has been a focus of several comprehensive reviews [9-15]. 

Figure 5.2. Monolayers of the amphiphile 1-monopalmitoyl-( )-glycerol at the air-water interface assemble in domains in which the molecular tilt azimuth is organized in star-shaped patterns. It is possible to preserve this order during the transfer on to a solid support. LB monolayers of this material have been utilized for the anchoring of nematic liquid crystals. The order within the monolayer determines the order within the bulk phase of the nematic liquid crystal (LC). The image here shows the LC cell between crossed polarizers. (From J. Fang, U. Gehlert, R. Shashidar and C. Knobler, Langmuir (1999), 15, 297)...

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