What are thermotropic liquid crystals



 

As Liquid crystal is the name given to a substance that has the properties of both a liquid and crystals. This as liquid crystalline The designated state is on the one hand more or less fluid like a liquid, on the other hand it shows properties such as birefringence, i.e., in contrast to the completely isotropic liquid and analogous to the crystal, it has direction-dependent properties (see also anisotropy). These properties indicate a certain long-range order of the molecules in the liquid-crystalline state.

In the simplest case, the detection of liquid-crystalline phases is carried out by means of polarization microscopy: due to the birefringence, liquid crystals have a specific appearance here. They appear under the polarization microscope with certain, often characteristic so-called textures.

The birefringent property has also led to its widespread use in liquid crystal displays, in which liquid crystals are arranged as a layer in an electric field between polarizing filters.

history

The first description of a liquid crystal goes back to Friedrich Reinitzer. In 1888 he described the colorful appearance when cholesteryl benzoate, the benzoic acid ester of cholesterol, melted and solidified[1]. He noticed that this compound already became liquid at 145 ° C, but the polarization microscopic birefringence or milky-cloudy appearance persisted up to 179 ° C. A crystal-clear, "normal" liquid was only created at temperatures above 179 ° C. Thereupon Otto Lehmann examined these as well as other substances and spoke of for the first time flowing crystals.[2]. In the twenties of the last century, the first fundamental research on liquid crystals was carried out by Georges Friedel[3] and Daniel Vorländer [4]. The liquid crystals only became of technical interest through the discovery of electro-optical switchability by George H. Heilmeier [5].

Terms and classification

Liquid crystalline phases, too Mesophases called, form together with the conformationally disordered crystals and the plastic crystals their own aggregate state, which is called the mesomorphic state. A compound that shows a liquid crystalline phase is called Mesogenic. If this mesophase is a nematic phase, the compound is called Nematogen, is it a smectic phase, Smectogenic.

You can choose between lyotropic, thermotropic and barotropic Differentiate between liquid crystals. In the case of thermotropic or barotropic liquid crystals, the formation of their mesophases as a function of temperature or pressure in the pure substance can be observed.
The formation of lyotropic mesophases requires the presence of a solvent and is also dependent on its concentration.
Amphitrope Liquid crystals show both lyotropic and thermotropic mesophases.
In the case of thermodynamically stable mesophases, one speaks of enantiotropic, with metastable mesophases of monotropic Mesophases.
The sequence of the various phases of a liquid crystal that occur depending on the temperature is called polymorphism.

The prerequisite for the formation of a liquid-crystalline phase is the anisometry of the structural units that form it. The vast majority of researched liquid crystals (including those in liquid crystal displays) have a rod-shaped (kalamitic) molecular shape.
But many other forms are also possible, e.g. For example: discoid (disc-shaped), pyramidoid (bowl or cone-shaped), sanidic (board-like), polycatenare (kalamitic with several flexible chains at one or both ends) or curved (banana-shaped) molecules.

Thermotropic liquid crystalline phases

There are various thermotropic liquid-crystalline phases which differ significantly from one another in terms of their microscopic structure and macroscopic appearance. B.

  • nematic phases,
  • smectic phases,
  • columnar phases.

Nematic phases

The nematic phase of achiral mesogens is the simplest type of liquid-crystalline phase. In it, the molecules have an order of orientation with respect to a so-called director, the unit vector of direction. The resulting preferred orientation is usually constant only for small volumes. The molecular centers of gravity are statistically distributed in the same way as liquids: there is no long-range position order. Most nematic phases are uniaxial; since 2004 they have also been thermotropic biaxial nematic phases known. Typical textures of nematic phases are thread or streak textures.

An order parameter can easily be used for nematic phasesS. to calculate:

,

where the angle θ describes the orientation of a singled out molecule to the preferred orientation; the angle brackets mean an averaging over the orientation distribution of all molecules. The order parameter can have values ​​between -0.5 and 1. S = 0 indicates a lack of preferred orientation (corresponding to an isotropic phase), S = 1 means a completely parallel alignment of all molecules (an ideal state). S = -0.5 corresponds to an orientation distribution of the molecules similar to the bristles of a bottle brush. However, negative order parameters have not yet been found experimentally. The order parameter shows a strong temperature dependence. When approaching the clearing point (temperature of the transition from a mesophase to the isotropic phase) it quickly approaches zero.

The molecules of a nematic phase can be easily reoriented by an electric field. This is used in LCDs (LCD: Abbreviation for L.iquid C.rystal D.isplay, English for liquid crystal display).

In principle, a distinction is made between 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 which polarized light can penetrate the sample without changing its polarization state. This results from the fact that the indicatrix of such phases represents an ellipsoid of revolution. This indicatrix indicates the dependence of the refractive index on the direction. Similarly, there are two optical axes in biaxial nematic phases, since the indicatrix is ​​not an ellipsoid of rotation, but a general ellipsoid.

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

The 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. Cholesteric liquid crystal films therefore show selective reflection of circularly polarized light. In contrast to the reflection on metallic or conventional dielectric mirrors, the handedness of the circular polarization is retained.

Smectic phases

There are multiple smectic phases. They were named smectic A, smectic B, etc. in the order in which they were discovered (abbreviated as SmA, SmB ...). From the large number of smectic phases of earlier years, only five remained after more detailed investigations (SmA, SmC, SmB, SmF and SmI). The others (formerly SmE, SmG, SmH, SmJ and SmK) presented themselves as soft crystals, disturbed crystals with pronounced deformability, and are now referred to as crystalline phases. The smectic D phase turned out to be a three-dimensional mesophase with a cubic superstructure.

In smectic phases, the molecules are arranged in layers in such a way that they form a one- or two-dimensional periodic structure. They are differentiated according to the degree of formation of an order within the layer smectic phases from disordered layers (SmA and SmC) and hexatic phases (SmB, SmF and SmI).

While in the SmA phase the longitudinal axes of the molecules are on average perpendicular to the layer, i.e. run parallel to the normal to the layer, the central longitudinal axis of the molecules in SmC phases is inclined to the normal to the layer. In these two mesophases, the molecules within the layer have no long-range position order - one could speak of a two-dimensional liquid. The classic polarization microscopic appearance of SmA and SmC phases is a fan or polygon texture. SmC phases often also show streak textures.

In contrast to this, the hexatic phases have a hexagonal position near order and a long-range order of the unit cell (bond orientational order). Similar to the SmA phase, the SmB phase is made up of molecules perpendicular to the layer, while these are inclined in the SmI and SmF phases.

Columnary phases

The characteristic of columnar phases is the formation of columns of stacked disc-shaped, wedge-shaped, polycatenary or similar. Mesogens. There is no long-range position order along the columns. The parallel arrangement of the columns results in a two-dimensional packing perpendicular to the longitudinal axes of the columns. Depending on the nature of this packing, a distinction can be made between oblique, right-angled or hexagonal columnar mesophases. Characteristic textures are mosaic textures or textures from circular domains.

The designation discotic for columnar is out of date or should only be used for mesophases of disc-shaped liquid crystals.

Lyotropic liquid crystalline phases

There are various lyotropic liquid-crystalline phases which differ significantly from one another in terms of their microscopic structure and macroscopic appearance, e.g. B.

  • discontinuous cubic phase (micellar),
  • nematic phases,
  • hexagonal phases,
  • bicontinuous cubic phases,
  • lamellar phases,
  • inverse cubic phase.

Nematic lyotropic mesophase

Nematic lyotropic mesophases have only been known since 1967. They only occur in a few lyotropic systems. In most cases, induction of the nematic phase by adding cosurfactants or electrolytes is necessary. A few exceptions are known where binary surfactant / water mixtures have a nematic phase:

  • Hexadecyltrimethylammonium bromide / water
  • Cesium pentadecafluoric ictanoate / water

Structurally, the lyotropic nematic phase is similar to the thermotropic nematic phase: There is a single preferred direction for the particular axis of the aggregates. The aggregates are disc or rod micelles.

Hexagonal lyotropic phase

In surfactant / water systems with medium mixing ratios (about 50% by weight surfactant), phases with an unusually high viscosity are often observed, which usually indicates a hexagonal phase. In many cases, the range of existence extends over wide temperature and concentration ranges. The hexagonal, distant-position aggregates are circular or oval-cylindrical rods. The long-range position order consists of an arrangement of the aggregates in a hexagonal grid, i. H. each aggregate is surrounded by six more in a hexagonal close packing.

use

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

In addition, there are a number of uses in other areas that take advantage of changes in various properties of the liquid crystals with physical parameters (temperature, pressure, etc.). For example, color changes of cholesteric phases depending on the temperature can be used for medical or technical purposes (adhesive temperature sensors show reversible or irreversible color changes at defined temperatures). B. of ICs is made visible through the polarization of liquid crystals. Components and heat sinks in closed devices can be checked with regard to their maximum temperature. The surface temperature can be tracked through affixed sensor foils without having to make contact with a thermometer. Such sensor foils are often equipped with several surfaces that react with a color change at a distance of e.g. 5 Kelvin in the form of a scale.

Mesophase tar is also used as a starting material for the production of carbon fibers (so-called pitch fibers, tar fibers or pitch fibers).

literature

  • Pierre-Gilles de Gennes, J. Prost: The Physics of Liquid Crystals. 2nd edition. Clarendon Press, Oxford 1993 ISBN 0-19-852024-7
  • Leopold Mathelitsch, Robert Repnik, Zlatko Bradac, Mojca Vilfan, Samo Kralj: Liquid crystals at a glance: Indispensable in nature, technology and research. In: Physics in our time 34, No. 3, 2003, pp. 134-139. ISSN 0031-9252

See also

  • Liquid crystal display
  • Adaptive optics
  • Otto Lehmann

swell

  1. F. Reinitzer: Contributions to the knowledge of cholesterol. In: monthly Chem. 9, pp. 421-441, 1888
  2. O. Lehmann: About flowing crystals. In: Z. Phys. Chem. 4, pp. 462-472, 1889
  3. G. Friedel: Les états mésomorphes de la matière. In: Ann. Physique 18, pp. 273-474, 1922
  4. D. Forelands: The exploration of the molecular shape with the help of the crystalline liquids. In: Z. Phys. Chem. 105, pp. 211-254, 1923
  5. G. H. Heilmeier, L. A. Zanoni: Guest-host interactions in nematic liquid crystals. A new electro-optic effect. In: Appl. Phys. Lett. 13, pp. 91-92, 1968

Categories: Chemical group | Soft matter | Electrotechnical material | Crystallography