Electro-optic phenomena

Liquid-crystal electro-optic phenomena can be divided into two categories—those caused only by dielectric forces and those induced by the combination of dielectric and conduction forces. The two conduction-induced phenomena discussed later are dynamic scattering and the storage effect. Four of the dielectric phenomena, or field effects as they are sometimes known, are discussed first (1) induced birefringence, (2) twisted nematic effect, (3) guest-host interaction, and (4) cholesteric-nematic transition. 

In all of the present theories about the excitation of nematic or cholesteric liquids by an electric field, the mesomorphic material is treated as a continuous elastic anisotropic medium. The Oseen -Frank elastic theory is used to describe the interaction between the applied field and the fluid. The application of an electric field causes the liquid crystal to deform. For a material with a positive dielectric anisotropy, Ae = 0, the director aligns in the direction of 

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In recent years, considerable effort has gone into the development of a new class of electrochemical devices called chemically modified electrodes. While conventional electrodes are typified by generally nonspecific electrochemical behavior, i.e., they serve primarily as sites for heterogeneous electron transfer, the redox (reduction-oxidation) characteristics of chemically modified electrodes may be tailored to enhance desired redox processes over others. Thus, the chemical modification of an electrode surface can lead to a wide variety of effects including the retardation or acceleration of electrochemical reaction rates, protection of electrodes, electro-optical phenomena, and enhancement of electroanalytical specificity and sensitivity. As a result of the importance of these effects, a relatively new field of research has developed in which the... 

Previous findings on the actual microemulsion are given in references (9 17) where the system s phase map vs.concentration in the temperature interval (-20°C + 80°C), viscosity measurements,dielectric analysis of liquid samples against both concentration and frequency, the thermally stimulated dielectric polarization release (TSD), electro-optical phenomena, light scattering, Raman spectroscopy and sound propagation investigations are reported. 

SOME ELECTRO-OPTICAL PHENOMENA IN COMB-LIKE LIQUID CRYSTALLINE POLYMERIC AZOMETHINES... 

ELECTRO-OPTIC PHENOMENA IN SIDE-CHAIN POLYMER LIQUID CRYSTALS... 

The initial research on electro-optic phenomena in side-chain polymer liquid crystals concentrated on systems that exhibited nematic phases so that a ready comparison could be made with low molar mass mesogens. Such measurements have established that electro-optic devices are feasible and have allowed elastic constants to be deduced from applications of the continuum theory. This theory, originally derived for low molar mass nematic liquid crystals, defines a relationship for the free energy density F in terms of the elastic constants (/ ) and the director n such that ... 

If optimum values of V, T, f and d are chosen, then it has been shown that electro-optic phenomena may be induced on the millisecond-to-second timescale in these smectic polysiloxane polymers. It is important to note, however, that these electro-optic responses were achieved in the fluid-biphasic region above T . Whilst it is possible to reduce 7 to below room temperature by modifying the mesomorphic polymer structure, to produce a room-temperature device, the form of the response time-temperature relationship and the relatively limited temperature range ( 5-10°C) over which fast responses occur do not suggest a practical device. Such an application would require temperature stabilizing or compensation in its construction which would not be advantageous in comparison with current low molar mass devices. 

In this chapter we have tried to outline the chemical structure that leads to the formation of the various mesophases exhibited by side-chain polymer liquid crystals. Electro-optic phenomena in these systems have then been considered in terms of potential device applications. Since such studies have only been carried out over the last five years or so it is evident that there will be many more applications discovered over the next decade as a better understanding of the composite polymer-mesomorphic properties is attained. 

V.G. Chigrinov, Theory of Electro-Optical Phenomena in Liquid Crystals. Doctor-habile dissertation. Institute of Crystallography, Moscow, 1988. 

The different electro-optic phenomena have been classified into those that involve only dielectric forces and those that depend upon the interaction of conduction and dielectric torques. The field-effect phenomena possess several common properties. The resistivity of the materials may be as high as chemically practical, i.e., p 10 ohm-cm. For the induced birefringence, twisted nematic, and guest-host color switching effects, the threshold voltages are less than 3 or 4... 

In materials of positive dielectric anisotropy, most electro-optic phenomena are frequency independent field effects hydrodynamic effects, occurring with certain boundary conditions result in stable (laminar) flow, and no turbulent-flow reorientation is observed. ... 

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