Physical properties of nematic liquid crystals

Table 4.3 Relationship between the physical properties of nematic liquid crystals and the corresponding application-relevant display properties.

L. Kramer and W. Pesch, Electrohydrodynamics in nematics. In eds. D.A. Dnmmnr, A. Fnknda and G.R. Luckhurst, Physical Properties of Nematic Liquid Crystals, Inspec, London, 2001. pp. 441-454. 

There can surely be no better way of beginning a review of the physical properties of nematic liquid crystals than by discussing the effects of changing molecular structure on the melting point, transition temperatures and mesophase morphology. Such parameters are the most fundamental physical properties of liquid crystalline materials, and yet they are frequently undervalued or simply taken for granted. 

Many early studies of the physical properties of nematic liquid crystals wo-e carried out on MBBA and PAA. Prefored values for these materials are listed in [23] and reproduced in TABLE 7. There do not appear to have been any more extensive and reliable measurements on these compounds than those reported in [23]. 

We briefly discussed the origin and structure of liquid crystals in Section 4.13. The last decade has witnessed a surge of interest in liquid crystals because of their applications in display devices (devices that convert an electrical signal into visual information). The design of liquid crystal (LC) devices relies on the relation between the molecular structure and the phase behaviour (relative smectic-nematic tendency, NI etc.) as well as the physical properties of the liquid crystals (Chandrasekhar, 1994). 

Fluctuations in the order parameter are reflected in various physical properties of a liquid crystal material. In this section we will focus on the elastic (Rayleigh) scattering of light by such fluctuations in the isotropic phase of nematic and cholesteric materials near Tc. 

XL13654, and SCE9. A well-studied ferroelectric liquid crystal is DOBAMBC its molecular structure is shown in Figure 4.1 Ic. Because of these differences in the degree of order and molecular arrangement and the presence of a permanent dipole moment, the physical properties of smectic liquid crystals are quite different from those of the nematic phase. In this and the following sections we examine the pertinent physical theories and the optical properties of three exemplary types of smectics smectic-A, smec-tic-C, and (ferroelectric) smectic-C. ... 

The static continuum theory of elasticity for nematic liquid crystals has been developed by Oseen, Ericksen, Frank and others [4]. It was Oseen who introduced the concept of the vector field of the director into the physics of liquid crystals and found that a nematic is completely described by four moduli of elasticity Kn, K22, K33, and K24 [4,5] that will be discussed below. Ericksen was the first who understood the importance of asymmetry of the stress tensor for the hydrostatics of nematic liquid crystals [6] and developed the theoretical basis for the general continuum theory of liquid crystals based on conservation equations for mass, linear and angular momentum. Later the dynamic approach was further developed by Leslie (Chapter 9) and nowadays the continuum theory of liquid crystal is called Ericksen-Leslie theory. As to Frank, he presented a very clear description of the hydrostatic part of the problem and made a great contribution to the theory of defects. In this Chapter we shall discuss elastic properties of nematics based on the most popular version of Frank [7]. 

The synthesis of a great number of materials that exhibit the nematic phase has achieved many different goals. Firstly, much knowledge has been acquired of the effect of stmctural features and various combinations of stmctiual features on melting points, mesophase morphology, and stability. Secondly, many physical properties have been evaluated for a great niunber of nematic liquid crystals and the resrrlts have been linked to the stmcture. Thirdly, mixtures of nematic materials have been formulated that have been... 

Polymer networks which can memorize the orientational order of the nematic liquid crystal environment where they are assembled [71], [72], [73], [74] are particularly attractive because of their potential for a variety of electrooptic technologies. We postpone this subject to the last section and here concentrate our attention on the ordering and structures of these composite materials. These systems have many physical properties analogous to liquid crystals confined to different submicrometer-sized cavities [75], [76] and random porous matrices [77], [78], Large surface-to-volume ratios enable a strong influence of the polymer network on nematic ordering in the liquid crystalline solvent and thus govern optical properties of the composites. 

Physical Properties and Theories of Nematic Liquid Crystals... 

The magnitude of some physical properties is dependent on the direction in which they are measured with respect to the nematic director. It is this anisotropy of physical properties that makes liquid crystals useful. 

Among the characteristic physical properties of liquid crystals, what are of critical importance to display devices (LCDs) are those of macroscopic spatiotemporal scale there, the theories of liquid crystals as continuous media play essential roles. The basis of static continuum mechanics of nematic liquid crystals was established by... 

Physical properties of liquid crystals are generally anisotropic (see, for example, du Jeu, 1980). The anisotropic physical properties that are relevant to display devices are refractive index, dielectric permittivity and orientational elasticity (Raynes, 1983). A nematic LC has two principal refractive indices, Un and measured parallel and perpendicular to the nematic director respectively. The birefringence An = ny — rij is positive, typically around 0.25. The anisotropy in the dielectric permittivity which is given by As = II — Sj is the driving force for most electrooptic effects in LCs. The electric contribution to the free energy contains a term that depends on the angle between the director n and the electric field E and is given by... 

It can be safely predicted that applications of liquid crystals will expand in the future to more and more sophisticated areas of electronics. Potential applications of ferroelectric liquid crystals (e.g. fast shutters, complex multiplexed displays) are particularly exciting. The only LC that can show ferroelectric property is the chiral smectic C. Viable ferroelectric displays have however not yet materialized. Antifer-roelectric phases may also have good potential in display applications. Supertwisted nematic displays of twist artgles of around 240° and materials with low viscosity which respond relatively fast, have found considerable application. Another development is the polymer dispersed liquid crystal display in which small nematic droplets ( 2 gm in diameter) are formed in a polymer matrix. Liquid crystalline elastomers with novel physical properties would have many applications. 

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