Liquid crystals helices

Fig. 9.8 Three different geometries of a shear of a cholesteric liquid crystal helical axis hllx (I), hllz (II) and hlly (III)...

In addition to the dielectric relaxation modes of anisotropic 3D fluids, just as nematic liquid crystals, helical SmC liquid crystals (see Chapter 1, Figure 1.11) have interesting low-frequency dielectric modes that are related... 

Fig. 6.11 Schematics of distributed feedback system for a chiral liquid crystal laser. The uniformly aligned helical structure of a chiral nematic liquid crystal provides distributed feedback for 1-dimensional lasing. Gain is provided by the optical pumping of dye molecules doped in the liquid crystal helix, shown by shading...

As witli tlie nematic phase, a chiral version of tlie smectic C phase has been observed and is denoted SniC. In tliis phase, tlie director rotates around tlie cone generated by tlie tilt angle [9,32]. This phase is helielectric, i.e. tlie spontaneous polarization induced by dipolar ordering (transverse to tlie molecular long axis) rotates around a helix. However, if tlie helix is unwound by external forces such as surface interactions, or electric fields or by compensating tlie pitch in a mixture, so tliat it becomes infinite, tlie phase becomes ferroelectric. This is tlie basis of ferroelectric liquid crystal displays (section C2.2.4.4). If tliere is an alternation in polarization direction between layers tlie phase can be ferrielectric or antiferroelectric. A smectic A phase foniied by chiral molecules is sometimes denoted SiiiA, altliough, due to the untilted symmetry of tlie phase, it is not itself chiral. This notation is strictly incorrect because tlie asterisk should be used to indicate the chirality of tlie phase and not tliat of tlie constituent molecules. 

Only fragmentary details about the structure of the main-chain liquid crystals are known (for a review see Ref.86)). Often condis crystals are confused with liquid crystals, and in many cases lyotropic liquid crystals are not separated from thermotropic materials. The problem is complicated since flexible chains, such as for example poly(gamma-benzyl glutamate)47), can become rigid by a coil-to-helix transformation. Similarly, external stress or quenching can lead to incomplete orientation which may be described as a mesophase. 

The macromolecular nature provides an interesting feature of LC polymeric cholesterics, namely the possibility of obtaining monochromic films. Thus for polymeric liquid crystals the helix pitch is practically not altered with temperature below Tg, when a cholesteric phase is frozen in a glassy matrix (Fig. 23a). This implies that fast cooling of polymeric films from a mesomorphic state (shown with arrows) fixes their optical properties, which makes it possible to use them at ordinary temperatures as selective monochromic reflectors. On the other hand, such polymeric films display the extraordinary polarizing properties of cholesterics, i.e. the different absorption... 

Cholesteric liquid crystals Historically, the name is derived from cholesterol chiral molecules like the steroids show a certain form of the nematic phase, the cholesteric one. The rigid rods are oriented parallel within virtual layers in one preferred direction (director) the director changes from one single virtual layer to the next continuously, with a certain value creating a helix. The distance between two parallel oriented directors is called the pitch (ca 0.2 pm). 

The diamter of the PBG a-helix has been estimated variously to be between 15-25 A as noted above (25). Values of d used in the determination of persistence len s are listed in the solvent categories of Table I. Values of Vp calculated from Equation 1 using x = L/d are invariably too low for the solvents of this study. Better agreement with the lattice theory has been reported for critical volume fractions of PBG in dimethylformamide and m-cresol (32). Experimental volume fractions for liquid crystal formation of PM in dioxane are lower, however, than those calculated from Flory s theory (33). PBG is known to undergo extensive... 

In a chiral smectic (Sc ) phase, the tilt angle is the same within a layer, but the tilt direction processes and traces a helical path through a stack of layers (Figure 43). It has been demonstrated that when such a helix is completely unwound, as in a surface stabilized ferroelectric liquid crystal cell, then changing the tilt of the molecules fi om +0 to —0 by alternating the direction of an applied field results in a substantial electro-optic effect, which has the features of veiy fast switching (%1 - lOps), high contrast and bistability [87]. The smectic A phase of chiral molecules may also exhibit an electro-optic effect, this arises due to molecular tilt fluctuations which transition is approached, which are combined with a... 

Calamitic metallomesogens forming a chiral smectic C phase (SmC ) are ferroelectric materials. Due to the low symmetry of this phase when the helix is unwound (C2) the molecular dipoles are aUgned within the layers of the SmC phase, giving rise to ferroelectric order in the layers. Because the SmC phase has a helical structure, there is no net macroscopic dipole moment for the bulk phase. However, it is possible to unwind the helix by application of an external electric field or by surface anchoring in thin cells. Under such conditions, a well-aligned film of the ferroelectric liquid crystal can exhibit a net polarisation, called the spontaneous polarisation (Ps). Ferroelectric liquid crystals are of interest for display applications because the macroscopic polarisation can be switched very fast by an... 

Synthetic polypeptides consist of a repeating sequence of certain amino acids and their primary structures are not as complicated as those in proteins. The polypeptides are very important polymers in both polymer and protein science. The characteristic properties related to the structure lead to possible expansion for research in the field of polymer science, to provide very different moplecules from conventional synthetic polymers. For example, the concept of the liquid crystal is expanded by revealing the variety of structures and properties of liquid crystals. Furthermore, the polypeptides are sometimes used as biomimic materials. On the other hand, synthetic polypeptides are sometimes used as model biomolecules for proteins because they take the a-helix, /3-sheet, o)-helix structure, and so on, under appropriate conditions. From such situations, it can be said that synthetic polypeptides are interdisplinary macromolecules and are very important for research work in both polymer and protein science. 

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