Cholesteric nematics, chiral

By far the most important commercial applications of nematic liquid crystals are in the a multi-billion dollar display industry. Cholesteric, or chiral nematic liquid crystals have been used in coloured guest-host displays and in thermography/ther-mochromic applications. 

A cholesteric, or chiral nematic (N ) phase. This is a positionally disordered fluid in which the constituent molecules align on average their axes along a common direction called the nematic director. Being the DNA helices chiral, the orientational order develops an additional macro-helical superstructure with the twist axis perpendicular to the local director. The phase thus consists of local nematic layers continuously twisted with respect to each other, with periodicity p/2 (where p is the cholesteric pitch see Fig. 8a) [27,28]. For 150-bp helices, the N phase appears at a concentration around 150 mg/mL in 100 mM monovalent salt conditions. This LC phase is easily observed in polarized optical microscopy. Since the N pitch extends to tens of micrometers (that is, across... 

If a nematic liquid crystal is made of chiral molecules, i.e. the molecules differ from their mirror image, a cholesteric (or chiral nematic)... 

Liquid crystalline (LC) solutions of cellulose derivatives form chiral nematic (cholesteric) phases. Chiral nematic phases are formed when optically active molecules are incorporated into the nematic state. A fingerprint texture is generally observed under crossed polarizers for chiral nematic liquid crystals when the axis of the helicoidal structure is perpendicular to the incident light (Fig. 2). 

FIGURE 11.17 Schematic representation of the cholesteric or chiral nematic phase, where p is the helix pitch length. 

Chemical modification of the polymer structure allows the obtention of nematic and smectic phases [4, 5]. If the side group and/or the chain are chiral, then cholesteric or chiral smectic C (SmC) phases can be obtained. These can also be obtained by mixing a chiral compound with the SCLCP. SmC SCLCPs are of particular interest and their behavior is described in Sec. 2 of this Chapter. 

In contrast to the extensive experimental investigations of nematic, cholesteric and chiral SmC phases, comparatively little work has been done on tbe characterization of the physical properties of SmA and SmC elastomers. The elongation, A, has been measured as a function of temperature for constant external load for a number of different loads in SmA sidechain elastomers [6]. It was found that A increases monotonically as a function of temperature for a material, which has a SmA-I transition. In addition it was shown that the elasticity modulus, E, decreases monotonically with temperature. X-ray investigations on SmA phases in side-chain LCE have been performed [70]. It was found that for the family of compounds studied, the orientation of the mesogenic groups was always perpendicular to the direction of stretching. 

The Goldstone mode in an achiral SmC tries to restore the symmetry of the smectic A phase Cooh —> Dooh by free rotation of the director along the conical surface with the smectic layer normal as a rotation axis. Thus, like chiral molecules convert a nematic into a cholesteric, they convert an achiral SmC into chiral SmC without any phase transition. In addition, mixing left (L)- and right (R)-handed additives results in a partial or complete compensation of the helical pitch both in cholesterics and chiral smectic C. For example, the L- and R- isomers of the same molecule taken in the equal amounts would give us a racemic mixture, that is achiral SmC without helicity and polarity. 

Depending on the degree of order in the system, three general classes of liquid crystalline phases may be defined, i.e. nematic, cholesteric (or chiral nematic) and smectic. These are shown schematically in Fig. 1. 

Fig. 1. Schematic representation of (a) nematic, (b) smectic and (c) cholesteric (or chiral nematic) liquid crystalline phases. In the nematic phase only orientational correlations are present with a mean alignment in the direction of the director n. In the smectic phase there are additional layer-like correlations between the molecules in planes perpendicular to the director. The planes, drawn as broken lines, are in reality due to density variations in the direction of the director. The interplane separation then corresponds to the period of these density waves. In the cholesteric phase the molecules lie in planes (defined by broken lines) twisted with respect to each other. Since the molecules in one plane exhibit nematic-like order with a mean alignment defined by the director n, the director traces out a right- or left-handed helix on translation through the cholesteric medium in a direction perpendicular to the planes. When the period of this helix is of the order of the wavelength of light, the cholesteric phase exhibits bright Bragg-like reflections. In these illustrations the space between the molecules (drawn as ellipsoids for simplicity) will be filled with the alkyl chains, etc., to give a fairly high packing...

The cholesteric or chiral nematic phase may be considered as a perturbed planar nematic phase in which the director in each plane is slightly rotated with respect to that of an adjacent plane. Asymmetry in the chemical structure causes this rotation which results in the director tracing out a left- or right-handed helix. When such helix pitch... 

Cholesteric A chiral nematic with twist axis normal to the director. 

In cholesteric (or chiral nematic) liquid crystals the situation is very close to usual nematics. However, due to the chirality of the molecules, the lowest state of elastic energy in cholesterics does no longer correspond to the uniform director orientation, but to the twisted one with a pitch Pq = 27r/yo where yo is the wave vector of cholesteric. Thus for cholesterics the second term in expression (2.24) must be rewritten as... 

The effects of bistability and hysteresis in supertwisted nematic layers were first investigated in [122]. To obtain twist angles larger than 90 , nematics were doped with a small amount of an optically active material. Thus a cholesteric (or chiral nematic) with a large pitch P was created, so that the pitch value had to adjust the boundary conditions for the directors on the substrates. The corresponding texture was first discovered by Grandjean and is discussed in Chapter 6. In 1984 the display based on the Supertwist Birefringent Effect (SBE) was proposed [123]. 

Light valves were first produced on the basis of the classical semiconductors, ZnS, CdS, ZnSe, CdTe, and GaAs, in contact with nematic or chiral nematic liquid crystal [18]. The basic effects in liquid crystals included electrically controlled birefringence, dynamic scattering, and the cholesteric-nematic phase transition with the frequency response limited to a few Hertz. 

Liquid crystals were first identified in 1888 ° in a molecule extracted from carrots. The first observed phase was the cholesteric or chiral nematic phase, whose shape is the subject of this chapter. Liquid crystals are made of molecules of particular shapes which, under certain conditions, align to form the phase shown in Figure 17.7. The first synthetic liquid crystals were manufactured in 1889. ° Liquid crystalline phases have been observed in many biopolymers. °... 

Reeves et al. argued that a dispersion type of interaction between single chiral molecules in adjacent micelles is rather imlikely to be essential for the phase chirality [35], Dispersion interactions between colloidal particles, as well as between single molecules, decrease with a strong power of the interparticle distance d (by d to d ). In thermotropic cholesteric or chiral smectic phases the interparticle distance is of the order of 100 pm which is small compared to the typical values of 1 to 10 nm between micelles in lyotropic chiral nematic phases. 

Figure 7-18 Chiral cholesteric (nematic) liquid crystal structure. The dotted line shows a helical path within the medium. Pitch length p = 360 rotation.

With increasing external field a series of field-induced phase transitions is observed BP,—>cholesteric, BP,->cholesteric, and then cholesteric—>nematic [52]. This is illustrated in Fig. 8 [42] which shows a voltage-temperature phase diagram for a mixture (47-53 mol%) of chiral 15-CB with 4-n-hexyloxycyanobiphenyl (60-CB). BPj loses its stability, first transforming into the cholesteric phase, because the transition enthalpy AH, is extremely small ( 50 J mol" ) for the BPj-Ch transition. This enthalpy, normalized to a unit volume (0.2 J cm ) and compared with the difference in electrostatic energy density between the two phases Se (Se = ggp, - fch 0.2) ex-... 

Chiral nematic liquid crystals, as the name suggests, are optically active variants of nematic liquid-crystalline compounds the incorporation of a chiral centre imparts properties which are unique to the chiral nematic phase and are responsible for their utilisation in a variety of differing display technologies and other related applications. The term cholesteric liquid crystal was originally used to describe this phase, and originates from the structural nature of the earliest chiral nematic liquid crystals which were derivatives of cholesterol [1,2], Nowadays, the term chiral nematic is used primarily because the materials are clearly derived from nematic type liquid crystals [3, 4], Despite these differences in definition, the terms cholesteric and chiral nematic phase are interchangeable and it is common to find references to either term in the literature. 

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