Applications of Liquid Crystals

Monte Carlo computer simulations of spheres sectioned into a disc show that steric interactions alone can produce a nematic phase of discotic molecules. Columnar phases are also observed. 

Here d is the thickness of the cell, K is either fCi, K2 or K3 depending on the geometry (for example K in the case of Fig. 5.23) and Ae is the anisotropy in permittivity in the nematic liquid crystal. 

The first successful commercial liquid crystal display (LCD) was the twisted nematic (TN), still widely used in watches and calculators. A TN display is sketched in Fig. 5.24. It relies on the Freedericksz transition described in the preceding section. The cell consists of two glass plates coated with rubbed polyimide to induce orientation of the director parallel to the surface. In addition, there is a thin layer of the transparent conducting material, indium tin oxide. This is used to apply an electric field across the liquid crystal 

The reverse contrast, i.e. bright characters on dark, can be achieved by orienting one polarizer parallel to the rubbing direction and the other one perpendicular to it so that the device is dark in the off state. Back-lit versions with this arrangement are used in car dashboard displays. 

Important considerations for construction of a liquid crystal display include  

FIGURE 2.29 An example of differential scanning calorimetric (DSC) data for a thermotropic liquid crystal (8CB) measured on a heating run of 207min. Two peaks can be observed. The first peak with an onset at 32.16° is the smectic-A-to-nematic phase transition, and the second peak at 39.56° is the nematic-to-isotropic liquid phase transition. Delta H Is the enthalpy of the transitions. 

The main requirements on LC materials (LCMs) for electro-optics are a high clearing temperature and 

The host-guest effect results from a reorientation of dye dopants (1 -2% in the LC matrix) in an electric field. In this case the wavelength of maximum absorption of fight is shifted and the color of the LC cell changes. 

Electric current leads to degradation of a LCM and reduces the lifetime of the display. Impurities influence the stability of the material and accelerate electrodegradation. Therefore a multistage purification, consisting for example of recrystaUization, and column chromatography, to remove conducting impurities (intermediate products, water, and CO2), is necessary. Usually the specific conductivity of a LCM is lower than 10 -10 Cm/cm and corresponds to the intrinsic conductivity. 

The elastic constants (Ki) determine the switching time of the electro-optical effects. The elastic constant Ki corresponds to the S effect, K-i to the B effect, and r2 to the TN effect here K1 corresponds to splaying K2 to twisting, and K-i to bending [1.11,12]. 

Cholesteryl compounds were the first materials that found application in thermography. 

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Crossland W A and Wilkinson T D 1998 Nondisplay applications of liquid crystals Handbook of Liquid Crystals Vol 1. Fundamentals ed D Demus, J Goodby, G W Gray, H-W Speiss and V Vill (New York Wiley-VCH)... 

Many technological applications of liquid crystals, as in electro-optic display devices, are based on multicomponent mixtures. Such systems offer a route to the desired material properties which cannot be achieved simultaneously for single component systems. Mixtures also tend to exhibit a richer phase behaviour than pure systems with features such as re-entrant nematic phases [3] and nematic-nematic transitions possible. In this section, we describe simulations which have been used to study mixtures of thermotropic calamitic mesogens. 

Hopefully with this brief introduction the reader will be able to appreciate fully the chapters which follow and which have been written by experts in the field. The complexity and beauty of the liquid crystalline phase has attracted many able scientists and the applications of liquid crystals in the electronics industry have provided a secure funding base for the subject. This is therefore still a field which is expanding rapidly and many research avenues remain to be explored by newcomers. Perhaps after reading these volumes of Structure and Bonding you will be tempted to join this exciting endeavour. 

Ericksen, J. L., and Kinderlehrer, D., Theory and Applications of Liquid Crystals, Springer-Verlag, 1987. 

Being bordered by the solid and liquid states, the liquid crystal state has some of the order of a solid, combined with the fluidity of a liquid. As such, it is an anisotropic fluid and it is this anisotropy that has led to the widespread application of liquid crystals. 

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. 

One of the most specific and unique features of low-molecular liquid crystals is their ability for orientation in external fields — mechanical, electric and magnetic. It is this property that establishes wide capabilities for technical application of liquid crystals. Today electric and magnetic optics of liquid crystals are an independent and useful for practics branch of the physics of the condensed state of matter 42 43 ... 

VI. APPLICATIONS OF LIQUID CRYSTALS A. Current Display Technology... 

However, it was not until 1967 that the technology was woken from its slumber by Americans James Fergason and George Heilmeier. These two developed the first technical applications of liquid crystals in thermography and the first liquid crystal displays Merck resumed its interest in liquid crystals, and the business began. 

Meier G, Sackmann E and Grabmaier JG, "Applications of Liquid Crystals", Springer, Berlin, 1975. 

Gough KM, Murphy WF, Stroyer-Hansen T, Svendsen EN (1987) J Chem Phys 87 3341 Grabmaier JG (1975) Medical and technical applications of liquid crystals. In Meier G, Sackmann E, Grabmaier JG (eds) Applications of liquid crystals. Springer, Berlin Heidelberg New York, p 83... 

Since the description of liquid crystallinity for cholesteryl benzoate and cholesteryl acetate at the end of the 19th century by Reinitzer [1], an intense activity has been devoted to thermotropic liquid crystals, especially since the early 1970s, owing to the fabrication and application of liquid crystal displays in electronic technology. 

By far the most important application of liquid crystals is display devices. Liquid crystal displays (LCDs) are used in watches, calculators, and laptop computer screens, and for instrumentation in cars, ships, and airplanes. Several types of LCDs exist. In general their value is due to the fact that the orientation of the molecules in a nematic phase substance can be altered by the application of an external electric field, and that liquid crystals are anisotropic fluids, that is, fluids whose physical properties depend on the direction of measurement. It is not pure liquid crystalline compounds that are used in LCDs, but liquid crystal mixtures having optimized properties. 

The applications of liquid crystals and polymeric liquid crystals are under constant... 

Application of liquid crystals as lubricants will require further study in order to assess chemical stability and compatibility relevant to tribological needs. 

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. 

This book will be of great value to advanced students and research workers in condensed matter physics, chemical physics, materials science and technology with an interest in the physics, chemistry and applications of liquid crystals. 

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