The 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.

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]. 

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. ... 

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. 

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). 

In order to understand the basic principles of operation of the many different kinds of LCDs being developed and/or manufactured at the present time, it is necessary to briefly describe the liquid crystalline state and then define the physical properties of direct relevance to LCDs. First, the nematic, smectic and columnar liquid crystalline states will be described briefly. However, the rest of the monograph dealing with liquid crystals will concentrate on nematic liquid crystals and their physical properties, since the vast majority of LCDs manufactured operate using mixtures of thermotropic, non-amphiphilic rodlike organic compounds in the nematic state. 

Comparisons of values quoted in the literature for the physical properties of liquid crystals are often of dubious validity due to differences in the methods of assessment often carried out at different absolute temperatures (e.g. 22°C or 25°C) or reduced temperatures (e.g. T -j— 10°C or 0.95 x 7V /). The use of extrapolated data from a wide variety of nematic mixtures of different composition and properties at various concentrations is also common. Unfortunately non-ideal behaviour is common for such mixtures and non-linear behaviour is not unusual, i.e. the values extrapolated to 100% are more often than not dependent on the matrix used and the concentration of the compound to be evaluated. However, although the absolute values of the data collated in Table 3.13, measured in the same way at the same reduced temperature (0.96 X r r-/), are lower than those reported for the same compounds in the literature, usually measured at 22°C the trends and relative values are very similar. 

Woo the temperature dependence of pitch for chiral nematic polymers does not seem to follow any particular pattern. It is believed that as temperature is increased, specific interactions, e.g., hydrogen bonding, whether inter- or intramolecular or polymer-solvent interactions are destroyed. The polymer chains become more flexible and the side groups more easily relaxed, thereby changing the physical properties of the chiral nematic structure. Similarly, an increase in concentration leads to a decrease in pitch for most lyotropic cellulosic liquid crystals with the exception of cellulose tricarbanilate (CTC) in ethyl methyl ketone, 2-penta-none, or tiiethylene glycol monoether and the chlorophenyl urethane derivative in diethylene glycol monoether. ... 

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. 

Essentially all of the techniques developed to characterize MLCs can be applied to PLCs with the realization that phenomena that are dependent on reorientation processes in the liquid crystal must be considered on considerably longer time scales in a PLC. Underlying most of the physical measurements performed on liquid crystals is the relationship between the observed anisotropic properties of the mesophase and orientational order of the mesogen. For uniaxial nematic phases and idealized low molar mass mesogens (cylindrical molecules), this relationship is embodied in the following equation ... 

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... 

The book is subdivided into three parts. The first three introductory chapters include consideration of the nature of the liquid crystalline state of matter, the physical properties of mesophases related to their electroop-tical behavior, and the surface phenomena determining the quality of liquid crystal cells giving birth to many new effects. The second part (Chapters 5-7) is devoted to various electrooptical effects in nematic, cholesteric, and smectic mesophases including ferroelectric compounds. Here major emphasis is given to explaining the physical nature of the phenomena. The last part (Chapter 8) is a rather technical one. Here recent applications of liquid crystalline materials in electrooptical devices are discussed. 

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. 

The existence or nonexistence of mirror symmetry plays an important role in nature. The lack of mirror symmetry, called chirality, can be found in systems of all length scales, from elementary particles to macroscopic systems. Due to the collective behavior of the molecules in liquid crystals, molecular chirality has a particularly remarkable influence on the macroscopic physical properties of these systems. Probably, even the flrst observations of thermotropic liquid crystals by Planer (1861) and Reinitzer (1888) were due to the conspicuous selective reflection of the helical structure that occurs in chiral liquid crystals. Many physical properties of liquid crystals depend on chirality, e.g., certain linear and nonlinear optical properties, the occurrence of ferro-, ferri-, antiferro- and piezo-electric behavior, the electroclinic effect, and even the appearance of new phases. In addition, the majority of optical applications of liquid crystals is due to chiral structures, namely the ther-mochromic effect of cholesteric liquid crystals, the rotation of the plane of polarization in twisted nematic liquid crystal displays, and the ferroelectric and antiferroelectric switching of smectic 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. 

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