Liquid Crystals (Topics in Physical Chemistry)

Liquid Crystal
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The order parameters of the uniaxial and the biaxial nematic phases are discussed, followed by the smectic A and the smectic C phases. Anisotropic interactions between mesogenic molecules are introduced. The molecular statistical theory of the nematic phase leads to the Maier—Saupe theory and the temperature dependence of the scalar order parameter.

Liquid Crystals - an overview | ScienceDirect Topics

Further aspects, like excluded volume effects, packing entropy and the density functional approach, are also discussed, together with a molecular theory of biaxial nematic liquid crystals and their phase diagram. The following chapters are then mainly related to the chemical aspects of liquid crystals. The methodologies discussed are retrosynthetic analysis, functional group addition and interconversion, and protecting groups. Examples for thermotropic liquid crystals are given for calamitic rod-shaped mesogens, bent-core systems, dimers and discotics.

In Chapter 7, Edward Davis and John Goodby summarise Symmetry and Chirality in Liquid Crystals with the introduction of enantiomers, diastereoisomers and dissymmetric molecules, as well as helical structures. The discussion of the frustrated phases sees the introduction of the structural features of Blue phases, Twist Grain Boundary phases and smectic Blue phases. Molecular topology, building blocks, electrostatic interactions, lateral substitution, steric shape and excluded volume are all factors that are identified to contribute to the properties phases and transition temperatures of mesogenic systems.

Part III provides the reader with an overview of characterisation techniques for liquid crystals. Boundary conditions are discussed, along with the well-known schlieren defect textures of the nematic and the smectic C phase, providing schematic images of the director fields of various point defects.

Continuing, the focal conic texture is shown, edge and screw dislocations and Dupin cyclides are introduced to discuss the smectic layer structure and director field. This is followed by the polygonal textures of the smectic A and tilt domains of the smectic C phase and the parabolic defects. The Volterra process is introduced for a number of situations, discussing different signs and strength of defects and defect lines.

The frustrated phases, such as Blue phase and TGB phase, are shown by their standard textures and schematic structures. Columnar and lyotropic phases are also discussed.

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This is a nice introductory chapter to the textures of liquid crystals; it is just a pity that most of the textures are not shown in colour photographs. A second basic technique employed for the characterisation of liquid crystals is X-ray Scattering , which is introduced in Chapter 10 by Dena Agra-Kooijman and Satyendra Kumar.

Again, a very useful overview of the technique and the diffraction patterns observed for nematic, fluid smectic and hexatic phases. Further, analysis techniques are discussed for the determination of the order parameters and the tilt angle. These techniques are then applied to the SmA—SmC transition, deVries phases, cybotactic groups, bent-core liquid crystals and biaxial nematics, as well as chromonic liquid crystals.

These are confocal microscopy, fluorescent microscopy and the combination of both, fluorescent confocal polarising microscopy. Peter Raynes adds Chapter 12 on Mixed Systems, Phase Diagrams, and Eutectic Mixtures , which are important for the development of materials that can be used in applications. The fundamentals of the physical chemistry of mixtures are provided, ideal and non-ideal behaviour discussed, along with ideal mixtures, semi-ideal mixtures and the solubility equation. Chapter 13 by Claudia Schmidt and Hans Wolfgang Spiess introduces Magnetic Resonance as a tool for the characterisation of liquid crystals.

The basic concepts, together with line shapes and spin interactions, are provided, as is a discussion of multidimensional spectroscopy. The applications of NMR in liquid crystal research are discussed in detail, from phase behaviour, molecular orientation and molecular dynamics to LC polymers, liquid crystals in confined geometries and viscoelastic properties. Neutron Scattering , as summarised by Robert Richardson in Chapter 14, is another of the fundamental techniques to characterise liquid crystals.

The fundamentals of neutron scattering, also with respect to X-ray scattering, are outlined and applied to determine the orientational and positional order.

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Side-group and main-chain liquid crystalline polymers are discussed. Further, the dynamics of liquid crystals are pointed out.

Progress in liquid crystal chemistry Sabine Laschat. Sabine Laschat Guest Editor.

Bonding , 95, — References 1,2. Go to references 1,2. References All Thematic Issues All volumes Article is part of the thematic issue Progress in liquid crystal chemistry. Functional properties of metallomesogens modulated by molecular and supramolecular exotic arrangements. Novel banana-discotic hybrid architectures. Asymmetric synthesis of propargylamines as amino acid surrogates in peptidomimetics.

From one point in empty space, the view is the same regardless of which direction one looks. There is continuous rotational symmetry—namely, the symmetry of a perfect sphere. In the crystal shown in Figure 1A , however, the distance to the nearest molecule from any given molecule depends on the direction taken.

Furthermore, the molecules themselves may have shapes that are less symmetric than a sphere.

A crystal possesses a certain discrete set of angles of rotation that leave the appearance unchanged. The continuous rotational symmetry of empty space is broken, and only a discrete symmetry exists. Broken rotational symmetry influences many important properties of crystals. Their resistance to compression, for example, may vary according to the direction along which one squeezes the crystal.

Transparent crystals, such as quartz, may exhibit an optical property known as birefringence. When a light ray passes through a birefringent crystal, it is bent, or refracted, at an angle depending on the direction of the light and also its polarization, so that the single ray is broken up into two polarized rays. This is why one sees a double image when looking through such crystals. In a liquid such as the one shown in Figure 1D , all the molecules sit in random positions with random orientations.

This does not mean that there is less symmetry than in the crystal, however.


All positions are actually equivalent to one another, and likewise all orientations are equivalent, because in a liquid the molecules are in constant motion. At one instant the molecules in the liquid may occupy the positions and orientations shown in Figure 1D , but a moment later the molecules will move to previously empty points in space. Likewise, at one instant a molecule points in one direction, and the next instant it points in another. Liquids share the homogeneity and isotropy of empty space; they have continuous translational and rotational symmetries. No form of matter has greater symmetry.

As a general rule, molecules solidify into crystal lattices with low symmetry at low temperatures. Both translational and rotational symmetries are discrete.

Liquid Crystals (Topics in Physical Chemistry)

At high temperatures, after melting , liquids have high symmetry. Translational and rotational symmetries are continuous. High temperatures provide molecules with the energy needed for motion. The mobility disorders the crystal and raises its symmetry. Low temperatures limit motion and the possible molecular arrangements. As a result, molecules remain relatively immobile in low-energy, low-symmetry configurations. Liquid crystals, sometimes called mesophases, occupy the middle ground between crystalline solids and ordinary liquids with regard to symmetry, energy, and properties.

Not all molecules have liquid crystal phases.

Liquid Crystals, Laptops and Life

Water molecules, for example, melt directly from solid crystalline ice into liquid water. The most widely studied liquid-crystal-forming molecules are elongated, rodlike molecules, rather like grains of rice in shape but far smaller in size.

click here A popular example is the molecule p -azoxyanisole PAA :. Typical liquid crystal structures include the smectic shown in Figure 1B and the nematic in Figure 1C this nomenclature , invented in the s by the French scientist Georges Friedel , will be explained below. The smectic phase differs from the solid phase in that translational symmetry is discrete in one direction—the vertical in Figure 1B —and continuous in the remaining two. The continuous translational symmetry is horizontal in the figure, because molecule positions are disordered and mobile in this direction.

The remaining direction with continuous translational symmetry is not visible, because this figure is only two-dimensional. To envision its three-dimensional structure, imagine the figure extending out of the page.