Applications of ferroelectric liquid crystals

Another phenomenon that has potential applications is the field-induced tilt or the electroclinic effect. Unlike the SSFLC device, this effect does not possess bistability but it has a faster (submicrosecond) response. By using the same bookshelf geometry and a suitable polarizer and retarder arrangement, the electroclinic effect can be used for modulating a light signal with a transmitted intensity linearly proportional to the applied voltage or as a tunable colour filter. 

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Liquid crystals have found widespread application in optical display devices as well as in detection of temperature uniformity and impurities. These properties are related to the orientational order of molecules in the temperature region between and the melting point. The possible applications of ferroelectric liquid crystals are promising. Superconductors (type II) can be used to create high magnetic fields at low power the ability of type I superconductors to trap magnetic flux within the domains of the normal material may also have applications. 

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. 

There are also electro-optic effects using either a different geometry of surface stabilization or a completely different mechanism In the twisted ferroelectric smectic-C cell [54] the moleeules form in the zero field state a quarter helix which is removed when a dc field of either polarity is applied the optical effect is achieved in the same way as in a twisted nematic cell. Compounds with a short chiral smectic-C pitch in a thick cell are used for the distorted helix ferroelectric (DHF) device [55] this effect uses the optical difference between the zero-field state eharacterized by a fully developed short-pitch helix, and structures with a distorted or almost unwound helix in the presence of an applied field optically addressed spatial light modulators can take advantage of the DHF effect [56]. Further applications of ferroelectric liquid crystals are switchable diffraction gratings [57]. 

It is possible to use a liquid crystal material in various ways depending on the application in flexible display. In order to stabilize the substrate spacing, polymer walls and fibers allow also the application of ferroelectric liquid crystal, whose molecular alignment structure is conventionally known to be fragile and called as smectic layers. 

Azo and azoxy series of ferroelectric liquid crystals Several of these were prepared one of them is shown in Figure 39. Unfortunately, these materials are not so stable and therefore useless for practical applications. 

The twist grain boundary smectic phase was discovered serendipitously at Bell Laboratories in 1987. Its discovery followed the back-tracking of a number of decisions made concerning the development of ferroelectric liquid crystals for display device applications. 

During the 1980s the development of ferroelectric liquid crystals continued at Bell Laboratories, and the above property-structure correlations suggested to us that, for the development of smectic C and smectic materials which would be suitable for use in applications of ferroelectric displays, it would not be wise to investigate 1-methylalkyl-substituted systems because of the... 

Lagerwall, S. T., Otterholm, B., and Skarp, K., Material properties of ferroelectric liquid crystals and their relevance for applications and devices. Mol. Cry.it. Liq. Crvst., 152, 503 (1987). 

The other structural feature of polymeric mesophases is their ability to form a glassy state with a liquid crystalline order frozen in. This phenomenon may be a basis for a set of new applications of polymeric liquid crystals, especially for ferroelectric ones. 

As seen from Table 8.8, a-Si H modulators with ferroelectric liquid crystals (FLC) nearly satisfy the requirements listed above. A special type of modulator is liquid crystal shutters, which can be used either in optical data-processing systems or in more traditional applications, such as cameras or printing equipment. Higher operation speed linear arrays of shutters could be made on the basis of ferroelectric liquid crystals. The specification of one of these linear arrays is given in Table 8.9 [27]. 

J.W. Goodby, Properties and structures of ferroelectric liquid crystals, in Ferroelectric Liquid Crystals, Principles, Properties and Applications, Ch. n, Ed. G.W. Taylor, Gordon and Breach Science Publishers, Philadelphia (1991). 

One published application of time-resolved asynchronous FT-IR spectrometry involves the reorientation dynamics of ferroelectric liquid crystals induced by the reversal of the external electric field [26,27]. For the study to be described here, the liquid crystals were held in a 2-pm CaFa cell whose windows had been coated with indium tin oxide, which served as the electrode, and with a polyimide film that had been oriented by stroking it in one direction. After the sample had been heated to 90° C to ensure that it was all in the isotropic liquid phase, it was injected into the cell, which had been heated to the same temperature. The cell was then allowed to cool to 40°C at a rate of 1°C min to obtain a homogeneously oriented liquid crystal. An IR polarizer was set at an angle of 45° to the orientation direction of the liquid crystal to obtain the maximum change in the absorption spectrum. Rectangular electric pulses with 20-V peak voltages and a 20-ps period were applied to the cell. 

Figure 12. Application of trifluoromethyloxirane synthesis of ferroelectric liquid crystals...

More modern, much faster responding liquid crystalline displays use ferroelectric liquid crystals such as p decyloxybenzylidine-//-amine-2-methylbulylcinnamale, which can be switched between two stable states by application of an external field. Such properties may also be useful in optical data storage. 

The paper is organized as follows. Sect. 2 describes the general principles and recent results for the synthesis of cyclotriveratrylenes Sect. 3 describes some applications of these compounds to host-guest chemistry. This latter section is devoted principally to the cryptophanes, and to host molecules containing one CTV unit that have recently been described. Sec. 4 presents some prospective work in the field of material sciences, i.e., ferroelectric liquid crystals and organic three-dimensional charge transfer salts. 

Applications of cyclotriveratrylenes and cryptophanes to the design of materials for optoelectronics are still in the area of prospective research. We only mention here works dealing with ferroelectric liquid crystals and three-dimensional charge transfer salts. 

Calamitic metallomesogens forming a chiral smectic C phase (SmC ) are ferroelectric materials. Due to the low symmetry of this phase when the helix is unwound (C2) the molecular dipoles are aUgned within the layers of the SmC phase, giving rise to ferroelectric order in the layers. Because the SmC phase has a helical structure, there is no net macroscopic dipole moment for the bulk phase. However, it is possible to unwind the helix by application of an external electric field or by surface anchoring in thin cells. Under such conditions, a well-aligned film of the ferroelectric liquid crystal can exhibit a net polarisation, called the spontaneous polarisation (Ps). Ferroelectric liquid crystals are of interest for display applications because the macroscopic polarisation can be switched very fast by an... 

Goodby JW (1994) In Bloor D, Brook RJ, Flemings MG, Mahajan S (eds) The Encyclopedia of Advanced Materials, Pergamon Press, Oxford, p 1325-1334 Goodby JW, Nishiyama 1, Slaney AJ, Booth CJ, Toyne KJ (1993) 14 37 Goodby JW, Blinc R, Clark NA, Lagerwall ST, Osipov MA, Pikin SA, Sakurai T, Yoshino K, Zeks B (1991) Ferroelectric Liquid Crystals Principles, Properties and Applications, Gordon and Breach, Philadelphia, p 99-123... 

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. 

Ferroelectric liquid crystals (FLC) are of great interest due to their fast electro-optical response which is about 1,000 times faster than conventional twisted nematic cells [131]. The geometry used is called a surface stabilized FLC cell which utilizes a very thin gap (=2 pm) to unwind the FLC supramolecular pitch (=1-2 pm) since the bulk FLC materials do not show macroscopic polarization. This very thin gap, however, leads to difficulties in manufacturing large panels and very poor shock resistance. Researchers have proposed the concept of microphase stabilized FLC [79,109, 130] using FLC-coil diblock copolymers for electro-optical applications as shown in Fig. 15. This concept takes advantage of ferroelectric liquid crystallinity and block copolymer microphase separation since the block... 

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