Liquid Crystals: Introduction. Classification and chemical properties of liquid crystals. Applications of liquid crystals

Some polymers have tendency to align their chains parallel over a long distance in the direction of melt flow during melt processing (drawing/extrusion). These chains arrange without entanglements, prior to the crystallization.

The polymers which have tendency to align their chains parallel over a long distance, prior to the crystallization from their melt or solution, are known as liquid-crystal polymers.

The distinguishing characteristic of the liquid crystalline state is :> the tendency of the molecules (Mesogen- is the fundamental unit of a liquid crystal that induces structural order in the crystals) to point along a common axis, called the director. This is in contrast to molecules in the liquid phase, which have no intrinsic order. In the solid state, molecules are highly ordered and have little translational freedom.

The characteristic orientational order of the liquid crystal state is between the traditional solid and liquid phases and this is the origin of the term mesogenic state, used synonymously with liquid crystal state.

If a polymer satisfies any one of the below structural requirements, then it can show liquid-crystal polymer behavior, either in melt phase or in solution phase. (i) Organic polymer molecules should have highly polarised chain structure. (ii) The polymer molecules should have aromatic rings in chain so that their structure is like strip. (iii) The polymer molecules should be like disc.

The LCP has long range and oriented molecules. Therefore it shows optical property useful in making optical fibres. High mechanical strength, very high tensile strength and high thermal resistance.

Classification of Liquid crystals; Liquid crystals may be divided into two broad categories, according to the principle means of breaking down the complete order of the solid state.

Thermotropic liquid crystals are established solely by the adjustment of temperature.

Those polymers which have tendency to form parallel aligned chains over a long distance before crystallization from their melt during melt processing, are thermotropic LCP.

Thermotropic liquid crystals are of three types, namely Nematic Cholesteric and smectic. Schematic of ordering in chiral liquid crystal phases. The chiral nematic phase (left), also called the cholesteric phase, and the smectic C* phase (right).

The three major classes of liquid crystals (nematic, cholesteric and smectic) are distinguished by the different kinds of molecular order they exhibit

(i) Nematic structure: (State of parallel arrangement but not well defined)

(i) Nematic structure: The molecules in the nematic structure maintain a parallel or nearly parallel arrangement to each other along the long molecular axes. They are mobile in three directions and can rotate about one axis.

(i) Nematic structure:

(i) Nematic structure: ‘Energy required to deform a nematic liquid crystal is so small that even the slightest perturbation caused by a dust particle can distort the structure considerably

(ii) Cholesteric structure Another type of nematic liquid-crystal modification is the cholesteric sructure, so-named because many compounds that form this mesophase are derivatives of cholesterol The structure of the cholesteric phase is shown in Fig.

(ii) Cholesteric structure The cholesteric liquid crystal phase is typically composed of nematic mesogenic molecules containing a chiral centre which produces intermolecular forces that favour alignment between molecules at a slight angle to one another.

(ii) Cholesteric structure The cholesteric structure looks like a HELICAL

(iii) Smectic structure: The word “smectic” is derived from the Greek word for soap. It is the thick, slippery substance often found at the bottom of a soap dish is actually a type of smectic liquid crystal.

Molecules in this phase show a degree of translational order not present in the nematic. In the smectic state, the molecules maintain the general orientational order of nematics, but also tend to align themselves in layers or planes. Motion is restricted to within these planes

Isotropic: Same distance but different directions

Lyotropic LCP Lyotropic LCP : Those liquid-crystal polymers which have tendency to form parallel alligned chains over a long distance before crystallization from their solution, during wet processing, are lyotropic LCP.

They occur in concentrated solutions of rod like molecules in a isotropic solvent (usually water) The stability of the mesophases is as readily influenced by concentration of solute as by temperature.

Examples are sodium laurate in water and phosphatidyl choline in water. Lyotropic mesophases are important in soaps, gels and colloids, and are of great interest in biology Soap films are multicomponent lyotropic systems A structural formula of alpha-cephalin (phosphatidylethanolamine) is an example

Smectic arrangements in lyotropic mesomorphism of amphiphilic molecules is illustrated in Fig.

Properties of Liquid Crystal Polymers

CHEMICAL PROPERTIES OF.LIQUID CRYSTALS Liquid crystals can be classified into two main categories thermotropic liquid crystals, and lyotropic liquid crystals.

Thermotropic liquid crystals

Thermotropic transactions occur in most liquid crystals, and they are defined by the fact that transition to the liquid crystalline state are induced thermally. That is, one can arrive at the liquid crystalline state by raising the temperature of a solid and/or lowering the temperature of a liquid.

Thermotropic liquid crystals Thermotropic liquid crystals can be classified into two types Enantiotropic liquid crystals, which can be changed into the liquid crystal state from either lowering the temperature of a liquid or raising of the temperature of a solid

Monotropic liquid crystals, which can only be changed into the liquid crystal state from either an increase in the temperature of a solid or a decrease in the temperature of a liquid, but not both.

Lyotropic liquid crystal

Lyotropic liquid crystal transitions occur with the influence of solvents, not by a change in temperature. Lyotropic mesophases occur as a result of solvent-induced aggregation of the constituent mesogens(molecules) into micellar structure

Lyotropic mesogens are typically amphiphilic, meaning that they are composed of both Iyophilic (solvent-attracting) and lyophobic (solvent-repelling) parts. This causes them to form into micellar structures in the presence of a solvent

As the concentration of the solution is increased and the solution is cooled, the micelles increase in size and eventually coalesce(Coming together to form one mass). This separates the newly formed liquid crystalline state from the solvent.

There are two types of thermotropic liquid crystals Discotics and rod-shaped molecules. Discotics are flat disc-like molecules consisting of a core of adjacent aromatic rings. Rod-shaped molecules have an elongated, anisotropic geometry, which allows for preferential alignment along one spatial direction.

Properties: i) They have higher crystallinity in solid state, with extended chain crystalline order. ii) Their fibers have very high tensile strength. iii) They are very tough.

iv) They have higher Tm (and Tg) but still they are easy to process from melt. v) Because of fibrous nature of the extended chain crystals, these plastics behave as “self-reinforced plastics” with excellent mechanical properties, especially in chain direction.

vi) Because of high distortion temperatures, they can be used at higher continuous working temperature vii) The LCP does not require reinforcement by glass fibers or carbon fibers but if done so, a very high strength and very high thermal stability material results.

viii) The LCP has property of multiple internal reflection of light (character for optical fiber)

Applications 1. Optical fibers making, telecommunication use. 2. Electrical and electronics applications.

3. Transport, automotive, military applications. 4 3. Transport, automotive, military applications. 4. Aircraft and aerospace applications. (Lenses) 5. Chemical and consumer appliances. ( optical electrodes, sun glasses etc) 6. Reinforcement of plastics for obtaining fiber plastics which are very strong (as fillers and and for composites)

They are used as displays in digital wrist-watches, calculators, panel meters, and industrial products. They can be used to record, store, and display images which can be projected onto a large-screen. They also have potential use as television displays.

1. Displays : The two features which make them more desirable for displays than other materials are lower power consumption and the clarity of display in the presence of bright light.

The power requirements are often so low that a digital display on a wristwatch requires about the same power as does the mechanism which runs the watch. The two modes most widely used in liquid crystal displays are dynamic-scattering and field-effect.

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