Physical liquid crystals

P.J, Collings and Michael Hird, Introduction to Liquid Crystals Chemistry and Physics. Liquid Crystals Book Series. Boca Raton, FL CRC Press (1997),... 

D. O. Shah and W. C. Hsieh, Microemulsions, Liquid Crystals and Enhanced Oil Recovery, in Theory, Practice, and Process Principles for Physical Separations, Engineering Foundation, New York, 1977. 

Luckliurst G R and Gray G W (eds) 1979 The Molecular Physics of Liquid Crystals (London Academic) de Gennes P G and Prost J 1993 The Physics of Liquid Crystals 2nd edn (Oxford Oxford University Press)... 

DIFLUOROBENZENES Interest in the commercialization of difluoroaromatics in crop protection chemicals and dmgs (Table 5) continues to be strong. Numerous liquid crystals containing the 1,2-difluorobenzene moiety have been synthesized. Table 6 lists physical properties of commercially significant intermediates such as o-, m-, and -difluorobenzene, 2,4-difluoroaniline and 2,6-difluorobenzonitrile. The LD q values for the three isomeric difluorobenzenes are identical 55 g/m for 2 h (inhalation, mouse) (127). 

Liquid crystals represent a state of matter with physical properties normally associated with both soHds and Hquids. Liquid crystals are fluid in that the molecules are free to diffuse about, endowing the substance with the flow properties of a fluid. As the molecules diffuse, however, a small degree of long-range orientational and sometimes positional order is maintained, causing the substance to be anisotropic as is typical of soflds. Therefore, Hquid crystals are anisotropic fluids and thus a fourth phase of matter. There are many Hquid crystal phases, each exhibiting different forms of orientational and positional order, but in most cases these phases are thermodynamically stable for temperature ranges between the soHd and isotropic Hquid phases. Liquid crystallinity is also referred to as mesomorphism. 

One is inclined to think of materials as being solids when editing an encyclopedia of materials some years ago, I found it required an effort of imagination to include articles on various aspects of water, and on inks. Yet one of the most important families of materials in the general area of consumer electronics are liquid crystals, used in inexpensive displays, for instance in digital watches and calculators. They have a fascinating history as well as deep physics. 

De Gennes, P.G. and Prost, J. (1993) The Physics of Liquid Crystals, 2nd edition (Clarendon Press, Oxford). 

Hilsum, C. and Raynes, E.P. (editors) (1983) Liquid Crystals Their Physics. Chemistry and Applications (The Royal Soeiety, London). 

Kahn, F. The Molecular Physics of Liquid-Crystal Devices. Physics Today (May 1982), pp, 66-74... 

Polyelectrolytes such as the ion exchange plastics form an interesting group of materials because of their ability to interact with water solutions. They have been used in medical applications involving the removal of heavy metal ions from the human body. They can be used to interact with external electric fields and change their physical properties drastically as is illustrated by the fact that some electrically active liquid crystals are polyelectrolytes of low molecular weight. 

A distinction between a solid and liquid is often made in terms of the presence of a crystalline or noncrystalline state. Crystals have definite lines of cleavage and an orderly geometric structure. Thus, diamond is crystalline and solid, while glass is not. The hardness of the substance does not determine the physical state. Soft crystals such as sodium metal, naphthalene, and ice are solid while supercooled glycerine or supercooled quartz are not crystalline and are better considered to be supercooled liquids. Intermediate between the solid and liquid are liquid crystals, which have orderly structures in one or two dimensions,4 but not all three. These demonstrate that science is never as simple as we try to make it through our classification schemes. We will see that thermodynamics handles such exceptions with ease. 

General reviews of the structure and properties of liquid crystals can be found in the following G. H. Brown, J. W. Doane, and V. D. Neff. "A Review of the Structure and Physical Properties of Liquid Crystals." CRC Press, Cleveland, Ohio, 1971 P. J. Collings and M. Hind, Introduction to Liquid Crystals. Nature s Delicate Phase of Matter," Taylor and Francis, Inc., Bristol. Pennsylvania, 1997 P. J. Collins, "Liquid Crystals. Nature s Delicate Phase of Matter," Princeton University Press. Princeton. New Jersey, 1990. A thermodynamic description of the phase properties of liquid crystals can be found in S. Kumar, editor, "Liquid Crystals in the Nineties and Beyond, World Scientific, Riven Edge, New Jersey, 1995. 

The properties of materials are consequences of their structures at the molecular level. Solids are the mainstays of technology, and it is hardly surprising that so much effort has gone into the development and understanding of their properties. We dealt with their electrical properties in Sections 3.13 and 3.14. Here we explore some of their other physical properties as well as the properties of the much softer materials known as liquid crystals. 

DeGennes PG, Prost J (1993) The physics of liquid crystals. Oxford University Press, Oxford... 

Gray GW (1979) Liquid crystals and molecular structure. In Molecular physics of liquid crystals. Academic Press, New York... 

The frustration effects are implicit in many physical systems, as different as spin glass magnets, adsorbed monomolecular films and liquid crystals [32, 54, 55], In the case of polar mesogens the dipolar frustrations may be modelled by a spin system on a triangular lattice (Fig, 5), The corresponding Hamiltonian consists of a two particle dipolar potential that has competing parallel dipole and antiparallel dipole interactions [321, The system is analyzed in terms of dimers and trimers of dipoles. When the dipolar forces between two of them cancel, the third dipole experiences no overall interaction. It is free to permeate out of the layer, thus frustrating smectic order. 

See, for example, (a) Leadbetter AJ (1979) In Luckhurst GR, Gray GW (eds) The molecular physics of hquid crystals. Academic Press, London, chap 13 (b) Pershan PS (1988) Structure of hquid crystaUine phases. World Scientific, Singapore (c) Ostrovskii Bl (1989) Sov Sci Rev Sec A 12(2) 86 (d) Shashidhar R (1992) In Martelucci S, Chester AN (eds) Phase transitions in liquid crystals. Plenum Press, New York, chaps 15, 16 Blinov LM, Lobko TA, Ostrovskii Bl, Sulianov SN, Tournilchac FG (1993) J Phys 11 3 1121... 

Liquid crystals (LCs) are organic liquids with long-range ordered structures. They have anisotropic optical and physical behaviors and are similar to crystal in electric field. They can be characterized by the long-range order of their molecular orientation. According to the shape and molecular direction, LCs can be sorted as four types nematic LC, smectic LC, cholesteric LC, and discotic LC, and their ideal models are shown in Fig. 23 [52,55]. 

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