Liquid crystal

A liquid crystal is referred to as a substance which is liquid on the one hand as a liquid, on the other hand also direction-dependent ( anisotropic ) has physical properties, such as a crystal. With its combination of fluidity and anisotropy liquid crystals, especially in liquid crystal displays and indicators (English liquid crystal displays, LCD). Most liquid crystals are optically birefringent. Under the polarizing microscope, these then show characteristic textures. - Liquid crystals are the " soft matter " assign.

In addition to the since the late 19th century known, low molecular weight liquid crystals, liquid crystalline polymers and elastomers were discovered in the second half of the 20th century.

Usually one thinks thermotropic liquid crystals with liquid crystal. These occur during the heating of certain substances to the melt as an intermediate phase ( mesophase ) between the solid and the liquid phase. A substance may successively form a plurality of different liquid crystalline phases with increasing temperature.

In addition, there are lyotropic liquid crystals. This form can be solved if amphiphilic substances ( surfactants) in a solvent ( eg, water). With a suitable concentration of the amphiphile leads to the formation of submicroscopic structures such as vesicles and micelles arrange themselves symmetrically in the solvent, thereby forming an anisotropic liquid.

• 4.1 Nematic lyotropic mesophase
• 4.2 Hexagonal lyotropic phase

History

The first description of a liquid crystal goes back to Friedrich Reinitzer. In 1888 he described the colorful appearance during melting and solidification of cholesteryl benzoate, benzoic acid esters of the cholesterol. He remarked that this compound was liquid at 145 ° C, the polarization microscopic birefringence or milky - cloudy appearance but persisted to 179 ° C. Only at temperatures above 179 ° C resulted in a crystal-clear, "normal" liquid. Then Otto Lehmann examined this as well other substances and spoke for the first time of " flowing crystals ". In the twenties of the last century were made first, fundamental experiments on liquid crystals by Georges Friedel and Daniel forelands. Technical interest was the liquid crystals given only by the discovery of electro-optical switchability by George H. Meier healing.

A review article on the history of LCDs wrote H. Kawamoto. A compendium of 46 as an overview annotated, classical works on physics, chemistry and application technology of "Crystals did flow " comes from Sluckin et al. (see, Section 6). The Karlsruhe Institute of Technology there before Otto Lehmann lecture hall of the Physics Institute a permanent exhibition on the liquid crystal History "From cholesterol to the flat ", and an extensive list of publications can be found under "History " on the website of the German Liquid Crystal Society ( Web Links to in section 8).

Concepts and classification

Liquid crystalline phases, also called mesophases along with the konformationsungeordneten crystals and plastic crystals, form a separate state of aggregation, called mesomorphic state. A compound exhibiting a liquid crystal phase, called mesogenic. Is this mesophase nematic phase ( from Ancient Greek νῆμα nema "thread" ), it is called nematogen the connection, it is a smectic phase, smectogen.

One can distinguish between lyotropic, thermotropic and barotropic liquid crystals. In thermotropic liquid crystals or barotropic observing the training of their mesophases as a function of temperature or pressure of the pure substance.

The formation of lyotropic mesophases requires the presence of a solvent and is also a function of its concentration.

Show Amphitrope liquid crystals lyotropic and thermotropic mesophases both.

In thermodynamically stable mesophase is called enantiotropic in metastable mesophases of monotropic mesophases (as in crystalline phases).

The baying of the phases of a liquid crystal as a function of temperature different occurring is called polymorphism.

Prerequisite for the formation of a liquid crystalline phase is the anisometry of the constituent units. By far the most studied liquid crystals ( including those in liquid crystal displays ) have a rod-like ( calamitic ) molecular shape.

But many other shapes are possible, such as: discoid ( disc-shaped ), pyramidoide ( bowl - or cone-shaped ), sanidische ( board-like ), polycatenare ( calamitic with several flexible chains at one or both ends ) or curved ( banana-shaped ) molecules.

In addition to the low molecular weight, there are also high molecular weight mesogens that form also thermotropic or lyotropic liquid crystals. The latter occur in " LC main chain polymers " on (LC for Liquid Crystals ), while " LC side chain polymers" also form thermotropic liquid crystals (see web link to T. Labude in section 8). - Highly organized molecular assemblies of biopolymers, such as the tobacco mosaic virus can be mesogenic also ( lyotropic ).

A thermotropic liquid crystalline phases

There are different thermotropic liquid crystalline phases can be clearly distinguished by their microscopic structure and its macroscopic appearance from each other, such as

• Nematic phases,
• Smectic phases,
• Columnar phases.

Nematic phases

The nematic phase of achiral mesogens is the simplest type of liquid crystalline phases. In it, the molecules have an orientational order with respect to a so-called director, the unit vector of the direction on. The resulting preferred orientation is usually constant only for small volumes. The molecular centers are randomly distributed similarly to liquids: It occurs no positional order. Most nematics are uniaxial, biaxial nematic thermotropic since 2004 phases are known. Typical textures of nematic phases are thread or Schlieren textures.

For nematic phases can be calculated in a simple way the order parameter S:

Where the angle θ the orientation of a molecule singled out for preferential orientation describes; the angle brackets denote averaging over the orientation distribution of all molecules. The order parameter can take values ​​from -0.5 to 1. S = 0 indicates a lack of preferential orientation of (corresponding to an isotropic phase ), S = 1 means a fully parallel alignment of all molecules ( an ideal situation ). S = -0.5 corresponds to an orientation distribution of the molecules similar to the bristles of a bottle brush. Negative order parameters were experimentally but not yet found. The order parameter has a strong temperature dependence. It is rapidly approaching the clearing point ( transition temperature of the mesophase to the isotropic phase ) to zero. - The molecules of a nematic phase can be easily reorient by an electric field. This is utilized in LCDs.

We distinguish basically two different types of nematic phases: uniaxial and biaxial nematic phases. The term uniaxial means that there is only one optical axis in the material along the polarized light can pass through the sample without changing its polarization state. This results from the fact that the indicatrix of such phases is an ellipsoid of revolution. Indicatrix this indicates the dependence of the refractive index of the direction. Similarly, there is at biaxial nematic phases two optical axes, because the indicatrix is not an ellipsoid of revolution, but a general ellipsoid.

The cholesteric phase has a nematic order with a continuously rotating preferential orientation. This results in a long-range helical superstructure with a periodicity of typically a few 100 nanometers.

The location resulting continuously twisted optical medium acts as a one-dimensional photonic crystal with a photonic band gap for circularly polarized light with the same handedness as the helical order. Therefore cholesteric liquid crystal films exhibit selective reflection of circularly polarized light. In contrast to the conventional reflection on metallic or dielectric mirrors the handedness of the circular polarization is preserved.

Smectic phases

There are multiple smectic phases. They were in the order of their discovery with smectic A, smectic B, and so called ( abbreviated as SmA, SmB, ...). From the variety of smectic phases in previous years remained upon further examination only five left ( SmA, SmC, SmB, SmF and SmI ). The other (formerly SmE, SmG, SmH, SMJ and SmK ) were found to be soft crystals - " disordered crystals with a pronounced ductility " - out and are now referred to as crystalline phases. The smectic D phase in turn turned out to be a three-dimensional cubic mesophase with superstructure. Together with the liquid crystals they belong to the soft matter.

In the smectic phase, the molecules are arranged in layers, as to form a one or two dimensional periodic structure. One distinguishes the formation of an order within the layer in smectic phases of disordered layers ( SmA and SmC ) and hexatische phase ( SmB, SmF and SmI ) after the degree.

While the longitudinal axes of the molecules in the medium are in the SmA phase perpendicular to the layer, that are parallel to the layer normal, the average molecular axis in the SmC phase is inclined to the film normal. In these two mesophases, the molecules within the layer no positional order - one could speak of a two-dimensional liquid. The classical polarization microscopic appearance of SmA - SmC phases and is a fan or polygon texture. SmC phases often show Schlieren textures. Considering thin free-standing films, the smA phase appears black, the smC phase has Schlieren textures and defects.

In contrast, in a hexagonal -range positional and long-range order of the unit cell ( bond orientational order ) for the hexatic phases. The SmB phase is analogous to SmA phase from perpendicular to the layer molecules while they are inclined in SmI and SmF phases.

Columnar phases

The characteristic columnar phases is the formation of columns of stacked disc-shaped, wedge-shaped, or the polycatenarer a mesogens. Along the columns, a position of long-range order occur (eg in the columnar - hexagonal plastic phase ). The parallel concatenation of the columns a two-dimensional packing is effected perpendicular to the column axes. In the columnar - hexagonal plastic phase of eg Triphenylensystemen enters a three-dimensional order on ( the molecules can rotate freely only in their places ). Depending on the nature of this package, one can distinguish between oblique, rectangular or hexagonal columnar mesophases. Characteristic textures are mosaic textures or textures from circular domains.

The term discotic columnar for is outdated and should only be used for disc- shaped liquid crystals mesophases.

Lyotropic liquid crystalline phases

There are various lyotropic liquid crystalline phases can be clearly distinguished by their microscopic structure and its macroscopic appearance from each other, such as

• Discontinuous cubic phase ( micellar )
• Nematic phases,
• Hexagonal phases,
• Bicontinuous cubic phase,
• Lamellar phases,
• Inverse cubic phase.

Nematic lyotropic mesophase

Nematic lyotropic mesophases are known only since 1967. They occur only in a few lyotropic systems. Most induction of the nematic phase by the addition of co-surfactants or electrolytes is necessary. A few exceptions are known in which a binary surfactant / water mixtures have a nematic phase:

• Hexadecyltrimethylammonium bromide / water
• Cäsiumpentadecafluoroctanoat / water

Structurally similar to the lyotropic nematic phase of thermotropic nematic phase: there is a single preferred direction for each unique axis of the aggregates. The units are slices or Stäbchenmizellen.

Hexagonal lyotropic phase

Phases in surfactant / water systems, with medium mixing ratios (approximately 50 wt. % Surfactant) frequently observed with an unusually high viscosity, which is usually indicative of a hexagonal phase. The existence range extends in many cases over a wide temperature and concentration ranges. In the hexagonal position away ordered aggregates are circular or oval cylindrical rods. The positional order exists in an arrangement of the units in a hexagonal lattice, ie each unit is surrounded by six other in a hexagonal closest packing.

Use

Liquid crystals, especially the thermotropic nematic phase are used in LCD screens.

In addition, a number of uses exist in other areas to take advantage of different properties of the liquid crystals changes with physical parameters (temperature, pressure, etc.). For example, you can color changes of cholesteric phases as a function of temperature for medical or industrial purposes use ( stick-on temperature sensors show at defined temperatures reversible or irreversible color change): The so-called hot-spot analysis is used for defect search of semiconductor devices by the heat distribution for as ICs is made visible by the polarization of liquid crystals. Components and heat sinks in closed devices can be controlled with respect to their maximum temperature. The surface temperature can be monitored by sensor films bonded without the contact with a thermometer to be produced. Such sensor films are often equipped with a plurality of surfaces, which react at a distance of for example 5 in the form of a Kelvin scale with a color change.

Furthermore mesophasiger tar as a raw material for the production of carbon fibers used ( sogn. pitch fibers, pitch fibers or pitch fibers ).