Introduction to Liquid Crystals
A liquid crystal is a thermodynamic stable phase characterized by anisotropy of properties With no existence of an three-dimensional crystal lattice, generally lying down in the temperature Range between the solid and isotropic liquid stage, hence the term mesophase. Liquid crystal materials are unique in their properties and uses. As research into this field Continues so when new applications are developed, liquid crystals will play an important role in Modern technology. This tutorial provides an release to the technology and applications of these materials.
The term liquid crystal signifies a state of aggregation that is intermediate between your crystalline sturdy and the amorphous water. Generally a material in this talk about is highly anisotropic some of its properties yet exhibits a certain degree of fluidity, which in a few circumstance may be comparable to that of a typical water. The first observations of liquid crystalline or mesomorphic behavior were made towards the end of the last century by reinitzer and lehmann.
What are Liquid Crystals?
Liquid crystal materials generally have several common characteristics. Among they are a rodlike molecular composition, rigidness of the long axis, and strong dipole and/or easily polarizable substituents. A dipole exists whenever we have two identical electric or magnetic charges of complete opposite sign, separated by a small distance. Within the electric circumstance, the dipole minute is distributed by the product of 1 charge and the distance of separation. Pertains to demand and current distributions as well. Within the electric circumstance, a displacement of fee circulation produces a dipole point in time, as in a molecule.
The distinguishing attribute of the liquid crystalline express is the tendency of the substances (mesogens) to point along a standard axis, called the director (the molecular way of preferred orientation in liquid crystalline mesophases). That is in contrast to molecules in the liquid phase, which have no intrinsic order. In the solid state, molecules are highly ordered and also have little translational independence. The quality orientational order of the water crystal talk about is between your traditional sound and liquid phases and this is the origin of the word mesogenic state, used synonymously with liquid crystal state. Please note the average positioning of the substances for each stage in the following diagram.
A mesogen is rigid rodlike or disclike molecules which are components of liquid crystalline materials. It really is sometimes difficult to determine whether a material is a crystal or water crystal express. Crystalline materials demonstrate long range periodic order in three measurements. By definition, an isotropic (Having properties that will be the same whatever the direction of measurement. In the isotropic state, all directions are indistinguishable from each other)liquid does not have any orientational order. Chemicals that aren't as bought as a solid, yet involve some degree of position are properly called liquid crystals.
Liquid Crystal Phases
- Liquid crystal phases are developed by a wide variety of molecules.
- They can be divided into two classes, thermo tropic and allotropic. Transitions to thermotropic stages are initiated by changes in temps, while those to lyotropic phases can be initiated by changes in attention.
Thermotropic Phases
- Thermotropic liquid crystals can generally be shaped by prolate (calamitic) substances or oblate (discotic) molecules.
- Liquid crystal stages shaped by calamitic substances belong to three different categories: nematic, chiral nematic, and smectic.
Nematic Water Crystal Phase
- The simplest liquid crystal stage is named the nematic period (N). It really is characterized by a high degree of long range orientational order but no translational order. Substances in a nematic period spontaneously order using their (for calamitic substances) long axes approximately parallel. Schematic diagram of a nematic water crystal
- A uniformly aligned nematic has a preferred direction, often referred to in terms of your device vector called the director. More generally a bulk nematic will contain domains. The orientation of the director is regular in each area but is different in various domains. Viewed under a polarizing microscope the defect areas linking these domains appear as dark threads
Chiral Nematic Liquid Crystal Phase
- Chiral substances can also form nematic phases called chiral nematic (or cholesteric) stages (N*). This period shows nematic ordering but the preferred route rotates throughout the sample. The axis of the rotation is normal to the director.
- An exemplory case of this is shown in Fig (b). The length over which the director rotates by 360 is called the chiral pitch and is generally of the order of hundreds of nanometres, the wavelength of obvious light. A non-chiral nematic stage can be thought of as a chiral nematic with an infinite pitch. Fig (b).
Smectic Liquid Crystal Phases
Smectic phases have further levels of order set alongside the nematic phase. In the simplest smectic phase, the smectic-A (SmA) phase, the molecules order into layers, with the part normal parallel to the director. Inside the layers, liquid like composition remains, as shown in Fig. 1. 3.
Closely related to the SmA stage is the smectic-C (SmC) period. Here the molecules form a coating structure but the long axes of the substances, and therefore the director, lies at an position to the layer normal, as shown in Fig. 1. 4. There are a great many other smectic phases that have long range order within the layers Smectic phases can even be formed by chiral molecules, leading to chiral smectic phases.
Discotic Water Crystal Phases
- Liquid crystal stages formed by discotic molecules belong to three different categories: discotic nematic, discotic chiral nematic, and columnar.
- The discotic nematic is comparable in composition to the calamitic nematic, although in cases like this the brief axes of the molecules tend to lay parallel. A similar holds for the discotic chiral nematic phases.
- Columnar phases will be the discotic exact carbon copy of the smectic phase. Here the substances form columns. In the easiest case the short axes of the substances lay parallel to the axis of the column and the columns are arbitrarily distributed in space. More difficult discotic phases are present, where the short molecular axes rest at an viewpoint to the column and translational order prevails between your columns, analogous to the more complicated smectic phases.
Other Thermotropic Phases
Most of the phases exhibited by low molecular mass water crystals are identified above. Recently however there has been much affinity for the so-called `banana' phases produced by bent-core molecules
- Some of the phases are chiral even though the molecules building them are achiral.
- Some high molecular mass polymers, liquid crystalline polymers (LCP), can also form liquid crystal phases.
- These get into two categories depending on where in fact the mesogenic area of the molecule is located. In the event the mesogenic unit is contained within the key polymer chain then it is termed a main chain water crystal polymer (MCLCP).
- These fall under two categories depending on where the mesogenic area of the molecule is situated. In the event the mesogenic unit is comprised within the primary polymer chain then it is termed a main chain liquid crystal polymer (MCLCP).
Lyotropic Liquid Crystal Phases
- Lyotropic water crystal stages are created by amphiphilic substances.
- These often contain a polar brain group mounted on a number of non-polar chains and tend to be known as surfactants (surface productive brokers).
- A schematic is shown in Fig. 1. 5. When these are dissolved in an appropriate solvent they self-assemble therefore the polar (hydrophilic) minds protect the non-polar (hydrophobic) tails. These set ups are known as micelles.
- At low surfactant concentrations they are about spherical, as shown in Fig. 1. 6. As the surfactant Concentration boosts then other phases are created.
- These include the hexagonal phase where the amphiphiles form cylinders that load up in a hexagonal array and the lamellar stage where in fact the amphiphiles form a bilayer structure.
Structure-Property Connections in Liquid
Crystals
- Despite this there exists only an unhealthy understanding of how changes in molecular structure affect material properties. For liquid crystals this is complicated by several factors.
- Firstly, liquid crystal stages are developed by materials from the whole spectrum of chemical substance classes: organic and natural, organometallic, and biological substances can all form liquid crystal phases.
- Secondly, mesogenic molecules are usually quite large. A typical low mass mesogen will have from about 40-100 atoms. For liquid crystalline polymers and dendrimers this can be nearer to a few thousand atoms. Liquid crystals have a tendency to be adaptable. This inhibits crystallization, avoiding the direct transition from an isotropic water to a crystalline sturdy. It also triggers a large amount of conformational independence, so properties are generally dependant on more than simply the equilibrium structure.
- Finally liquid crystal molecules often contain disparate parts (such as alkyl and perfluoroalkyl chains) that can have a huge influence on the phase behaviour.
- Investigation of these factors by experimental or theoretical means should ideally lead to a better knowledge of structure-property relationships in liquid crystals.
- In principle the partnership between molecular structure and macroscopic properties can be looked into through the synthesis of series of similar mesogenic chemical substances. This however can be time consuming and may require many difficult and expensive syntheses. Thus, the capability to determine the properties of any molecular structure before synthesis would be suitable. It really is here that simulations can play an important role. Simulations on basic molecular models can be used to find features that can lead to a specific property or period.
- Atomistic simulations can be used to determine material properties of a particular molecular framework.
Chemical Properties of Liquid Crystals
Liquid crystals can be classified into two main categories: thermotropic liquid crystals, And lyotropic liquid crystals. Both of these types of liquid crystals are recognized by the mechanisms that drive their self-organization, but they are also similar in lots of ways. Thermotropic transactions occur generally in most liquid crystals, and they are defined by the fact that the transitions to the water crystalline condition are induced thermally. That's, one can reach the Water crystalline talk about by nurturing the heat range of a good and/or minimizing the temperature of the Liquid.
Thermotropic liquid crystals can be categorised into two types: enantiotropic water crystals, Which may be became the liquid crystal talk about from either reducing the temperature of any Liquid or raising of the temp of a solid, and monotropic water crystals, which can only be Changed into the liquid crystal point out from either an increase in the heat of a good or a Decrease in the temperature of a liquid, but not both. In general, thermotropic mesophases take place Due to anisotropic dispersion causes between the substances and because of packaging Interactions.
In contrast to thermotropic mesophases, lyotropic liquid crystal transitions take place with the Influence of solvents, not by the change in heat range. Lyotropic mesophases take place therefore of Solvent-induced aggregation of the constituent mesogens into micellar constructions. Lyotropic mesogens are typically amphiphilic, meaning that they are comprised of both lyophilic (solventattracting) And lyophobic (solvent-repelling) parts. This triggers them to create into micellar structures in the existence of an solvent, because the lyophobic ends will stay together as the lyophilic ends prolong outward toward the answer. As the attention of the perfect solution is is increased and The perfect solution is is cooled, the micelles increase in size and eventually coalesce. This separates the recently created liquid crystalline status from the solvent.
A very large number of chemical compounds are recognized to show one or several liquid crystalline stages. Despite significant distinctions in chemical composition, these molecules have some common features in substance and physical properties. You will discover two types of thermotropic liquid crystals: discotics and rod-shaped molecules. Discotics are chiseled disc-like molecules consisting of a primary of adjacent aromatic rings. This allows for just two dimensional columnar buying. Rod-shaped molecules have an elongated, anisotropic geometry that allows for preferential position along one spatial way. The rod-like low molar mass (LMM) water crystals, such as 5CB shown in the following Diagram: require a protracted conformation of the molecule which must be maintained through the rigidityand linearity of its constituents. That is, for a molecule to display the characteristics of any water crystal, it must be rigid and rod-shaped. This is achieved by the interconnection of two rigid cyclic products. The interconnecting group should cause the ensuing compound to have a linear planar conformation. Linking items formulated with multiple bonds such as -(CH=N)-, -N=N-, -(CH=CH)n-, -CH=N-N=CH-, etc. are being used since they restrict the flexibility of rotation. These groupings can conjugate with phenylene jewelry, improving the anisotropic polarizability. This increases the molecular span and keeps the rigidity.
Applications of Liquid Crystals
Liquid crystal technology has already established a major impact many areas of science and engineering, as well as device technology. Applications because of this special kind of materials remain being discovered and continue steadily to provide effective answers to numerous problems.
Liquid Crystal Displays
The most frequent application of water crystal technology is liquid crystal displays (LCDs. ) This Field has grown into a multi-billion dollars industry, and many significant medical and Engineering discoveries have been made.
Liquid Crystal Thermometers
As demonstrated early on, chiral nematic (cholesteric) liquid crystals echo light with a wavelength equal to the pitch. As the pitch is dependent upon temperature, the colour reflected also is dependent upon heat range. Liquid crystals make it possible to accurately determine temperature just by looking at the colour of the thermometer. By blending different compounds, a tool for pretty much any temp range can be built. The "mood ring", a favorite novelty a few years ago, took good thing about the unique capacity of the chiral nematic liquid crystal. More important and practical applications have been developed in such diverse areas as remedies and gadgets. Special liquid crystal devices can be attached to the skin to show a "map" of temperature. That is useful because often physical problems, such as tumors, have another type of temperature than the encompassing tissue. Liquid crystal temperature receptors may also be used to find bad links over a circuit table by detecting the characteristic higher temperature
Optical Imaging
An request of liquid crystals that is only now being explored is optical imaging and saving. In this particular technology, a liquid crystal cell is placed between two layers of picture conductor. Light is applied to the photoconductor, which increases the material's conductivity. This triggers a power field to build up in the liquid crystal corresponding to the Power of the light. The electric design can be transmitted by an electrode, which permits the Image to be registered. This technology continues to be being developed which is one of the most promising Areas of liquid crystal research.
Other Liquid Crystal Applications
Liquid crystals have a multitude of other uses. They are being used for nondestructive mechanical
Testing of materials under stress. This system is also used for the visualization of RF (radio consistency) waves in waveguides. They are used in medical applications where, for example, transient pressure transmitted by the walking foot on the ground is measured. Low molar mass (LMM) water crystals have applications including erasable optical disks, full color "electronic slides" for computer-aided drawing (CAD), and light modulators for color electric imaging. As new properties and types of liquid crystals are investigated and researched, these materials will definitely gain increasing importance in commercial and methodical applications.
