Cholesterics and Smectics

Liquid Crystallinity. The Hquid crystalline state is characterized by orientationaHy ordered molecules. The molecules are characteristically rod-or lathe-shaped and can exist in three principal stmctural arrangements nematic, cholesteric, and smectic (see Liquid crystalline materials). 

The systematic synthesis of non amphiphilic l.c.-side chain polymers and detailed physico-chemical investigations are discussed. The phase behavior and structure ofnematic, cholesteric and smectic polymers are described. Their optical properties and the state of order of cholesteric and nematic polymers are analysed in comparison to conventional low molar mass liquid crystals. The phase transition into the glassy state and optical characterization of the anisotropic glasses having liquid crystalline structures are examined. 

Fig. 33. The temperature dependence of the orientational order parameter S of the copolymer No. 4 of Table 8 in the cholesteric and smectic state. (T = T /Tm T = clearing temperature, Tm = measuring temperature)...

As already mentioned in Chap. 2.2. one of the most obvious features of the l.c. side chain polymers is their ability to become glassy. The glass transition can be observed by cooling nematic, cholesteric and smectic polymers depending on the chemical constitution of the system and is indicated e.g. by a bend in the V(T) curves as schematically shown in Fig. 8. Two questions are of interest which will be discussed in this chapter ... 

Because of the additional translational order, the dislocations can exist in the cholesteric and smectic liquid crystals, which makes the texture of these liquid crystals even more complicated. Each liquid crystal phase shows characteristic textures and thus the optical texture becomes an important means to differentiate the phase of the liquid crystals. Liquid crystalline polymers have the same topologically stable defects as small molecular mass liquid crystals do, but the textures may be different due to the difference in the energetic stability of the same topological defects in both low molecular mass and polymeric liquid crystals (Kleman, 1991). In Chapter 3 we will discuss the textures in detail. 

Similarly, homotopy theory can be applied to other liquid crystal phases, such as the cholesteric and smectic liquid crystals (Mermin, 1979 Wang, 1986). 

Three main kinds of liquid crystal states nematic, cholesteric and smectic can occur in polymer systems. However, we will restrict the discussion to nematic polymers below, since it may be too early to adequately elaborate on the other two phases. 

On the other hand, liquid crystalline polymers applied to optical information storage has attracted great attention. The liquid crystalline polymer is applied mainly in terms of the thermo-optical effect. The backbone of liquid crystalline polymer can be polysiloxane, polyacrylate, or polyesters. In order to enhance the absorption coefficient for the writing laser beam, the dyes may be either dissolved into the liquid crystalline polymer in the guest-host model or attached to the backbone of the liquid crystalline polymer to form a copolymer. The nematic, cholesteric and smectic liquid crystalline polymers are all be able to be utilized in optical information storage. 

The liquid crystalline polymer has since developed far beyond imagination that a decade ago. The liquid crystalline polymer family has so far included the main chain-, side chain-, and crosslinked- (i.e. network or elastomer) types, and their solutions and gels. The liquid crystal phases cover nematic, cholesteric and smectics. Although the science of the liquid crystalline polymer is not fully mature, it has attracted significant research interests and has already made tremendous progress. As investments and human resources continue, the liquid crystalline polymer is expected to have an even brighter future. 

Liquid crystals are broadly classified as nematic, cholesteric and smectic (I)- There are at least nine distinct smectic polytypes bearing the rather mundane labels smectic A, B, C,... I, by the chronological order of their discovery. Some of the smectics are actually three-dimensional solids and not distinct liquid-crystal phases at all. There are three t s of liquid crystals. Thermotropic liquid-crystal phases are those observed in pure compounds or homogeneous mixtures as the temperature is changed they are conventionally classified into nematic, cholesteric, and smectic phases in Fig.2. Lyotropic liquid-crystal phases are observed when amphiphilic molecules, such as soaps, are dissolved in a suitable solvent, usually water. Solutions of polymers also exhibit liquid-crystalline order, the polymeric phases. Most of our knowledge about liquid crystals is based on the thermotropic phases and much of this understanding can be transferred to elucidate polymeric and lyotropic phases. 

In nematic liquid crystals, the molecular axes are ligned up in the same direction without any 2 dimensional ordering. Cholesteric and smectic liquid crystals are composed of layers of molecules with long molecular axes parallel and perpendicular to the planes, respectively. 

Lyotropic liquid crystals occur abundantly in nature, being ubiquitous in living systems.Their structures are quite complex and are only just beginning to be elucidated. However, in this monograph we shall be confining our attention mainly to the physics of low molecular weight thermotropic liquid crystals and do not propose to discuss polymer and lyotropic systems in any further detail. In chapters 2-5, we deal with the nematic, cholesteric and smectic mesophases of rod-like molecules and in chapter 6 discotic systems. 

The force g normal to the layers will be associated with permeation effects. The idea of permeation was put forward originally by Helfrich to explain the very high viscosity coefficients of cholesteric and smectic liquid crystals at low shear rates (see figs. 4.5.1 and 5.3.7). In cholesterics, permeation falls conceptually within the framework of the Ericksen-Leslie theory > (see 4.5.1), but in the case of smectics, it invokes an entirely new mechanism reminiscent of the drift of charge carriers in the hopping model for electrical conduction (fig. 5.3.8). 

The subject of liquid crystals has now grown to become an exciting interdisciplinary field of research with important practical applications. This book presents a systematic and self-contained treatment of the physics of the different types of thermotropic liquid crystals - the three classical types, nematic, cholesteric and smectic, composed of rod-shaped molecules, and the newly discovered discotic type composed of disc-shaped molecules. The coverage includes a description of the structures of these four main types and their polymorphic modifications, their thermodynamical, optical and mechanical properties and their behaviour under external fields. The basic principles underlying the major applications of liquid crystals in display technology (for example, the twisted and supertwisted nematic devices, the surface stabilized ferroelectric device, etc.) and in thermography are also discussed. 

The two volumes on Low Molecular Weight Liquid Crystals are divided into parts dealing with calamitic liquid crystals (containing chapters about phase structures, nematics, cholesterics, and smectics), discotic liquid crystals, and non-conventional liquid crystals. 

The other thermotropic mesophases (cholesteric and smectic) also result from an ordering of solvent molecules. The cholesteric occurring in solvents possessing a centre of asymmetry, so that the structure is characterised by a helical twist. The smectic mesophase consists of molecules parallel to each other with the extra order being in layers. They are generally viscous and are not so suitable for high resolution NMR spectroscopy. 

Ilquld crystal A substance that flows like a liquid but has some order in its arrangement of molecules. Nematic crystals have long molecules all aligned in the same direction, but otherwise randomly arranged. Cholesteric and smectic liquid crystals also have... 

Rod-shaped vimses can form hquid crystalline phases, which can be controlled by factors like virus suspension concentration, ionic strength of solution, external fields, etc. For example, Ml 3 phages are found to randomly orientated in an isotropic concentration range but transform to nematic, cholesteric, and smectic phases with... 

P) handed helical long-range orientational order of cholesteric and smectic C phase and the left (M) or right (P) handed screw axes of crystals. 

Helfrich, W. Capillary flow of cholesteric and smectic liquid crystals. Phys. Rev. Lett. 23, 372-374 (1969)... 

Finally the three remaining Chapters 10-12 are devoted to optics and electrooptics of, respectively, nematic, cholesteric and smectic (ferroelectric and antiferro-electric) phases. In contrast to my earlier book published by WUey in 1983, only the most principal effects have been considered and the discussion of the underlying principles is much more detailed. 

The Austrian botanist Reinitzer, when heating cholesteryl benzoate, observed a melting point at 145.5 °C, leading to a cloudy liquid which cleared at a temperature of 179.5 °C. He had diseovered cholesteric liquid crystals. In 1922, FriedeE described a variety of different liquid crystal phases and proposed a classification scheme consisting of the three broad classes nematic, cholesteric and smectic materials. 

In this chapter we shall only be concerned with electro-optic and thermo-optic switching effects in thermotropic side-chain polymer liquid crystals. We will consider briefly the synthesis and structure of such compounds and show how the nematic, cholesteric and smectic phases arise. Since the optical properties of each of these phases are different, and may be altered depending on alignment within the phase, each gives rise to different electro-optic effects. If these are coupled to the use of dye additives or substituents, then it will be realized that a wide range of electro-optic devices based on dichroism or fluorescence as well as birefringence or scattering power may be fabricated. These will be considered and discussed in terms of their performance and potential applications. Finally, possible uses of polymer-low molar mass liquid crystal solutions will be considered in terms of electro-optic device applications. 

In our own work, carried out under an extensive collaboration with Professor George Gray, F.R.S., and Drs David Lacey and Peter Gemmell of Hull University (UK), we have studied a number of nematic, cholesteric and smectic polymer mesophases with a view to exploiting their electro-optical properties. Some of the typical structures studied are given in Fig. 3. These are only a few of the structures now available and the reader is referred to ref. 14 for a more complete range of polysiloxane systems as well as to the recent reviews" which also cover the acrylate and methacrylate backbone systems. 

Structure of thermotropic liquid crystals is rather well understood. There are three main structural types nematic, cholesteric, and smectic. In nematic liquid crystals molecules are aligned approximately in the same direction, but positionally molecules are disordered. An axis of preferable molecular orientation is called a director. More precisely, the director is defined as a unit vector n(r) that is parallel to the molecular orientation at the point r. If we use the long axis of the molecules as a reference and denote it as k, the microscopic scalar order parameter 5 is defined [16,17] as follow ... 

Both cholesteric and smectic mesophases are layered. In the former case, the periodicity arises from a natural twist to the director field, and in the latter, from a center-of-mass correlation in one dimension. There are many types of smectic phases distinguished by their symmetry and order. The set of field-induced phenomena is quite different for these two materials, owing primarily to the very different layer compressibility. That is, the cholesteric pitch can be unwound by an external field, whereas the smectic layering is typically too strong to be altered significantly. However, because of the common layered structure, there are also strong similarities. 

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