Liquid crystals Essay

Introduction to Liquid Crystals

A liquid crystal is a thermodynamic stable stage characterized by anisotropy of belongingss Without the being of a 3-dimensional crystal lattice, by and large lying in the temperature Range between the solid and isotropous liquid stage, therefore the term mesophase. Liquid crystal stuffs are alone in their belongingss and utilizations. As research into this field Continues and as new applications are developed, liquid crystals will play an of import function in Modern engineering. This tutorial provides an debut to the scientific discipline and applications of these stuffs.

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The term liquid crystal signifies a province of collection that is intermediate between the crystalline solid and the formless liquid. As a regulation a substance in this province is strongly anisotropic some of its belongingss and yet exhibits a certain grade of fluidness, which in some instance may be comparable to that of an ordinary liquid. The first observations of liquid crystalline or muscular behaviors were made towards the terminal of the last century by reinitzer and lehmann.

What are Liquid Crystals?

Liquid crystal stuffs by and large have several common features. Among these are a rod-shaped molecular construction, rigidity of the long axis, and strong dipole and/or easy polarizable substituents. A dipole is present when we have two equal electric or magnetic charges of opposite mark, separated by a little distance. In the electric instance, the dipole minute is given by the merchandise of one charge and the distance of separation. Applies to bear down and current distributions as good. In the electric instance, a supplanting of charge distribution produces a dipole minute, as in a molecule.

The separating feature of the liquid crystalline province is the inclination of the molecules ( mesogens ) to indicate along a common axis, called the manager ( the molecular way of preferable orientation in liquid crystalline mesophases ) . This is in contrast to molecules in the liquid stage, which have no intrinsic order. In the solid province, molecules are extremely ordered and have small translational freedom. The characteristic orientational order of the liquid crystal province is between the traditional solid and liquid stages and this is the beginning of the term mesogenic province, used synonymously with liquid crystal province. Note the mean alliance of the molecules for each stage in the undermentioned diagram.

A mesogen is stiff rodlike or disclike molecules which are constituents of liquid crystalline stuffs. It is sometimes hard to find whether a stuff is in a crystal or liquid crystal province. Crystalline stuffs demonstrate long scope periodic order in three dimensions. By definition, an isotropic ( Having belongingss that are the same regardless of the way of measuring. In the isotropic province, all waies are identical from each other ) liquid has no orientational order. Substances that are n’t every bit ordered as a solid, yet have some grade of alliance are decently called liquid crystals.

Liquid Crystal Phases

Liquid crystal stages are formed by a broad assortment of molecules.

They can be divided into two categories, thermo tropic and allotropic. Passages to thermotropic stages are initiated by alterations in temperature, while those to lyotropic stages can besides be initiated by alterations in concentration.

Thermotropic Phases

Thermotropic liquid crystals can by and large be formed by prolate ( calamitic ) molecules or oblate ( discotic ) molecules.
Liquid crystal stages formed by calamitic molecules fall into three different classs: nematic, chiral nematic, and smectic.

Nematic Liquid Crystal Phase

The simplest liquid crystal stage is called the nematic stage ( N ) . It is characterized by a high grade of long scope orientational order but no translational order. Molecules in a nematic stage spontaneously order with their ( for calamitic molecules ) long axes approximately parallel.

Conventional diagram of a nematic liquid crystal

A uniformly aligned nematic has a preferable way, frequently described in footings of a unit vector called the manager. More by and large a majority nematic will incorporate spheres. The orientation of the manager is changeless in each sphere but is different in different spheres. Viewed under a polarizing microscope the defect parts associating these spheres appear as dark togss

Chiral Nematic Liquid Crystal Phase

Chiral molecules can besides organize nematic stages called chiral nematic ( or cholesteric ) phases ( N* ) . This stage shows nematic telling but the preferable way rotates throughout the sample. The axis of this rotary motion is normal to the manager.
An illustration of this is shown in Fig ( B ) . The distance over which the manager rotates by 360 is called the chiral pitch and is by and large of the order of 100s of nanometers, the wavelength of seeable visible radiation. A non-chiral nematic stage can be thought of as a chiral nematic with an infinite pitch. Fig ( B ) .

Smectic Liquid Crystal Phases

Smectic stages have farther grades of order compared to the nematic stage. In the simplest smectic stage, the smectic-A ( SmA ) stage, the molecules order into beds, with the bed normal analogue to the manager. Within the beds, liquid like construction remains, as shown in Fig. 1.3.

Closely related to the SmA stage is the smectic-C ( SmC ) stage. Here the molecules form a bed construction but the long axes of the molecules, and therefore the manager, lies at an angle to the bed normal, as shown in Fig. 1.4. There are many other smectic stages which have long scope order within the beds Smectic stages can besides be formed by chiral molecules, taking to chiral smectic stages.

Discotic Liquid Crystal Phases

Liquid crystal stages formed by discotic molecules fall into three different classs: discotic nematic, discotic chiral nematic, and columnar.

The discotic nematic is similar in construction to the calamitic nematic, although in this instance the short axes of the molecules tend to lie parallel. The same holds for the discotic chiral nematic stages.
Columnar stages are the discotic equivalent of the smectic stage. Here the molecules form columns. In the simplest instance the short axes of the molecules lie parallel to the axis of the column and the columns are indiscriminately distributed in infinite. More complicated discotic stages exist, where the short molecular axes lie at an angle to the column and translational order exists between the columns, correspondent to the more complicated smectic stages.

Other Thermotropic Phases

Most of the stages exhibited by low molecular mass liquid crystals are described above. Recently nevertheless there has been much involvement in the alleged `banana ‘ stages formed by bent-core molecules

Some of these stages are chiral although the molecules organizing them are achiral.
Some high molecular mass polymers, liquid crystalline polymers ( LCP ) , can besides organize liquid crystal stages.
These autumn into two classs depending on where the mesogenic portion of the molecule is located. If the mesogenic unit is contained within the chief polymer concatenation so it is termed a chief concatenation liquid crystal polymer ( MCLCP ) .
These autumn into two classs depending on where the mesogenic portion of the molecule is located. If the mesogenic unit is contained within the chief polymer concatenation so it is termed a chief concatenation liquid crystal polymer ( MCLCP ) .
Lyotropic Liquid Crystal Phases
Lyotropic liquid crystal stages are formed by amphiphilic molecules.
These frequently consist of a polar caput group attached to one or more non-polar ironss and are frequently known as wetting agents ( surface active agents ) .
A conventional is shown in Fig. 1.5. When these are dissolved in an appropriate dissolver they self-assemble so the polar ( hydrophilic ) caputs protect the non-polar ( hydrophobic ) dress suits. These constructions are known as micelles.
At low surfactant concentrations these are approximately spherical, as shown in Fig. 1.6. As the surfactant Concentration additions so other stages are formed.
These include the hexangular stage where the amphiphiles form cylinders that pack in a hexangular array and the lamellar stage where the amphiphiles form a bilayer construction.
Structure-Property Relationships in Liquid

Crystals

Despite this there exists merely a hapless apprehension of how alterations in molecular construction affect stuff belongingss. For liquid crystals this is complicated by several factors.
First, liquid crystal stages are formed by stuffs from the whole spectrum of chemical categories: organic, organometallic, and biological molecules can all organize liquid crystal stages.

Second, mesogenic molecules are by and large rather big. A typical low mass mesogen will hold from about 40-100 atoms. For liquid crystalline polymers and dendrimers this can be closer to a few 1000 atoms. Liquid crystals tend to be flexible. This inhibits crystallisation, forestalling the direct passage from an isotropic liquid to a crystalline solid. It besides leads to a big grade of conformational freedom, so belongingss are by and large determined by more than merely the equilibrium construction.

Finally liquid crystal molecules frequently contain disparate parts ( such as alkyl and perfluoroalkyl ironss ) that can hold a big consequence on the stage behavior.

Probe of these factors by experimental or theoretical agencies should hopefully take to a better apprehension of structure-property relationships in liquid crystals.

In rule the relationship between molecular construction and macroscopic belongingss can be investigated through the synthesis of series of similar mesogenic compounds. This nevertheless can be clip devouring and may affect many hard and expensive syntheses. Therefore, the ability to find the belongingss of a molecular construction before synthesis would be desirable. It is here that simulations can play an of import function. Simulations on general molecular theoretical accounts can be used to happen characteristics that can take to a peculiar belongings or stage.
Atomistic simulations can be used to find material belongingss of a peculiar molecular construction.

Chemical Properties of Liquid Crystals

Liquid crystals can be classified into two chief classs: thermotropic liquid crystals, And lyotropic liquid crystals. These two types of liquid crystals are distinguished by the mechanisms that drive their self-organisation, but they are besides similar in many ways. Thermotropic minutess occur in most liquid crystals, and they are defined by the fact that the passages to the liquid crystalline province are induced thermally. That is, one can get at the Liquid crystalline province by raising the temperature of a solid and/or take downing the temperature of a Liquid.

Thermotropic liquid crystals can be classified into two types: enantiotropic liquid crystals, Which can be changed into the liquid crystal province from either take downing the temperature of a Liquid or raising of the temperature of a solid, and monotropic liquid crystals, which can merely be Changed into the liquid crystal province from either an addition in the temperature of a solid or a Decrease in the temperature of a liquid, but non both. In general, thermotropic mesophases occur Because of anisotropic scattering forces between the molecules and because of packing Interactions.

In contrast to thermotropic mesophases, lyotropic liquid crystal passages occur with the Influence of dissolvers, non by a alteration in temperature. Lyotropic mesophases occur as a consequence of Solvent-induced collection of the component mesogens into micellar constructions. Lyotropic mesogens are typically amphiphilic, intending that they are composed of both lyophilic ( solventattracting ) And lyophobic ( solvent-repelling ) parts.

This causes them to organize into micellar constructions in the presence of a dissolver, since the lyophobic terminals will remain together as the lyophilic ends extend outward toward the solution. As the concentration of the solution is increased and The solution is cooled, the micelles addition in size and finally blend. This separates the freshly formed liquid crystalline province from the dissolver.

A really big figure of chemical compounds are known to exhibit one or several liquid crystalline stages. Despite important differences in chemical composing, these molecules have some common characteristics in chemical and physical belongingss. There are two types of thermotropic liquid crystals: discotics and bacillar molecules. Discotics are level disc-like molecules dwelling of a nucleus of next aromatic rings. This allows for two dimensional columnar telling. Bacillar molecules have an elongated, anisotropic geometry which allows for discriminatory alliance along one spacial way.

The rod-like low molar mass ( LMM ) liquid crystals, such as 5CB shown in the undermentioned Diagram: necessitate an drawn-out conformation of the molecule which must be maintained through the rigidityand one-dimensionality of its components. That is, in order for a molecule to expose the features of a liquid crystal, it must be stiff and bacillar. This is accomplished by the interconnectedness of two stiff cyclic units. The complecting group should do the ensuing compound to hold a additive planar conformation. Associating units incorporating multiple bonds such as – ( CH=N ) – , -N=N- , – ( CH=CH ) n- , -CH=N-N=CH- , etc. are used since they restrict the freedom of rotary motion. These groups can conjugate with phenylene rings, heightening the anisotropic polarizability. This increases the molecular length and maintains the rigidness.

Applications of Liquid Crystals

Liquid crystal engineering has had a major consequence many countries of scientific discipline and technology, every bit good as device engineering. Applications for this particular sort of stuff are still being discovered and go on to supply effectual solutions to many different jobs.

Liquid Crystal Displays

The most common application of liquid crystal engineering is liquid crystal shows ( LCDs. ) This Field has grown into a multi-billion dollar industry, and many important scientific and Engineering finds have been made.

Liquid Crystal Thermometers

As demonstrated earlier, chiral nematic ( cholesteric ) liquid crystals reflect light with a wavelength equal to the pitch. Because the pitch is dependent upon temperature, the colour reflected besides is dependent upon temperature. Liquid crystals make it possible to accurately gauge temperature merely by looking at the colour of the thermometer. By blending different compounds, a device for practically any temperature scope can be built. The “ temper ring ” , a popular freshness a few old ages ago, took advantage of the alone ability of the chiral nematic liquid crystal.

More of import and practical applications have been developed in such diverse countries as medical specialty and electronics. Particular liquid crystal devices can be attached to the tegument to demo a “ map ” of temperatures. This is utile because frequently physical jobs, such as tumours, have a different temperature than the environing tissue. Liquid crystal temperature detectors can besides be used to happen bad connexions on a circuit board by observing the characteristic higher temperature

Optical Imagination

An application of liquid crystals that is merely now being explored is optical imagination and recording. In this engineering, a liquid crystal cell is placed between two beds of exposure music director. Light is applied to the photoconductor, which increases the stuff ‘s conduction. This causes an electric field to develop in the liquid crystal matching to the Intensity of the visible radiation. The electric form can be transmitted by an electrode, which enables the Image to be recorded. This engineering is still being developed and is one of the most promising Areas of liquid crystal research.

Other Liquid Crystal Applications

Liquid crystals have a battalion of other utilizations. They are used for nondestructive mechanical
Testing of stuffs under emphasis. This technique is besides used for the visual image of RF ( radio frequence ) waves in wave guides. They are used in medical applications where, for illustration, transeunt force per unit area transmitted by a walk-to pes on the land is measured. Low molar mass ( LMM ) liquid crystals have applications including effaceable optical discs, full colour “ electronic slides ” for computer-aided drawing ( CAD ) , and light modulators for colour electronic imagination. As new belongingss and types of liquid crystals are investigated and researched, these stuffs are certain to derive increasing importance in industrial and scientific applications.