Liquid Crystal UNIT 1_Part A PDF

Unit 1: Liquid Crystals-Part A Introduction to liquid crystals Types of liquid crystals Applications in LCDs  Liquid Crystal Liquid crystals are substances that exhibit a phase of matter that has properties between those of a conventional liquid and those of a solid. A liquid...

Unit 1: Liquid Crystals-Part A Introduction to liquid crystals Types of liquid crystals Applications in LCDs  Liquid Crystal Liquid crystals are substances that exhibit a phase of matter that has properties between those of a conventional liquid and those of a solid. A liquid crystal may flow like a liquid , but have the molecules in liquid arranged and oriented in crystal like way. Properties of liquid crystals  The molecules are rod shaped or disc shaped.  All liquid crystals are mesogens but all mesogens are not liquid crystals.  Molecules are anisotropic in nature.  Assuming that the direction of preferred orientation in a liquid crystal (LC) is ↑, this direction can be represented by an arrow, called the director of the LC.  Each molecule is orientated at some angle to the director.  These molecules possess very strong dipole moment.  The liquid crystal molecules prefer to align parallel to each other because of the strong intermolecular attraction (π-π interaction).  A typical Liquid Crystal molecule is represented by a central rigid part, known as mesogen (generally aromatic) and the flexible ends (generally aliphatic groups).  Mesogen consists of two or more ring systems connected by a central linkage group.  Liquid-crystal phases The various liquid-crystal phases (called mesophases) can be characterized by the type of ordering. One can distinguish: Positional order (whether molecules are arranged in any sort of ordered lattice), Orientational order (whether molecules are mostly pointing in the same direction), Moreover order can be either short-range (only between molecules close to each other) or long-range (extending to larger, sometimes macroscopic, dimensions). Most thermotropic LCs will have an isotropic phase at high temperature. That is heating will eventually drive them into a conventional liquid phase characterized by random  Classification of liquid crystals: Liquid Crystals Thermotropic L.C. Lyotropic L.C. (Temperature dependent) (Solvent dependent) Calamatic L.C. Discotic L.C. Lamellar Hexagonal Cubic (Rod shaped) (Disc Shaped) Smectic Nematic L.C. L.C. Discotic Nematic Discotic Columnar Smectic A Ordinary Nematic Smectic C Nematic Cholesteric 1. Thermotropic liquid crystals: They are formed by change of temperature. They occur as liquid crystals over a certain temperature range between the solid and liquid phase. Example – LCD TV’s, alarm clocks. Examples of Thermotropic LCs: Compound Transition Melting Temperature temperature (°C) (°C) p-azoxy anisole 116 135 p-methoxy 170 186 cinnamic acid Dibenzal benzidine 235 260 Types of Thermotropic LCs:- Nematic, Sematic, Chiral.  Thermotropic liquid crystals are further classified into: a) Calamatic L.C.(Rod like or elongated molecules) Calamatic liquid crystals are elongated, rod b) Discotic shaped Liquid molecules. Crystals They (Discclassified are further shape as molecules) Nematic and Smectic liquid crystals.  Nematic liquid crystals Word Nematic derived from Greek word Nema which means "thread". No positional order, but possess orientational order. Molecule have elongated rod like shape and are thread like. Do not have layered structure. Flow like normal liquids. They have low viscosity. Formed at relatively higher temperature.  Can be aligned by the application of electric on magnetic field.  Molecules are free to move in all the directions.  Flow in all directions & not in layers.  E.g.- p-azoxy anisole (first synthetic liquid crystal to be produced).  Nematic liquid crystals can be further classified as: a) Ordinary nematic phases: These molecules possess ordinary nematic phase characteristics as discussed earlier. b) Cholesteric nematic phases: This phase is also known as chiral nematic phase. The molecules are essentially chiral and resembles nematic molecules in nature.  Cholesteric Liquid Crystals (Chiral Nematic) This phase is usually observed from cholesterol derivatives. The molecules are essentially chiral. The molecules are arranged in layers. These are formed by adding chiral twisting agent to the nematic liquid crystals.  Each layer in Cholesteric liquid crystal is tilted with respect to the other one, and hence the molecules take a one complete turn of 360 degrees to make a helix.  The distance covered by the director in making a one complete turn is known as pitch.  Cholesteric Liquid crystal reflects light approximately equal to pitch.  Pitch is inversely proportional to the temperature.  Pitch is affected by temperature, pressure as well as by electric and magnetic fields.  Liquid Crystal Phases Smectic phases There are many different smectic phases, all characterized by different types and degrees of positional and orientational order. The smectic phases, which are found at lower temperatures than the nematic, form well- defined layers that can slide over one another. Schematic of alignment The smectics are thus in the smectic phases. positionally ordered along one The smectic A phase direction. In the Smectic A (left) has molecules phase, the molecules are organized into layers. In oriented along the layer the smectic C phase (right), the molecules normal, while in the Smectic C Discotic Liquid Crystals:  Molecules are essentially disc shaped.  Discotic mesogens are typically composed of an aromatic core surrounded by flexible alkyl chains. The aromatic cores allow charge transfer in the stacking direction through the π conjugate systems. The charge transfer allows the discotic liquid crystals to be electrically semi conductive along the stacking direction.  They are of two types: Discotic nematic Phase and Columnar Phase. Disc-shaped molecules have a tendency to lie on top of one another forming either discotic nematic phases (with discs oriented similar to that of nematic phase i.e. not having position order but having orientation order) or columnar phases (have column-like structure). Thermotropic liquid crystals Thermotropic LCs exhibit a phase transition into the liquid-crystal phase as temperature is changed. Thermotropic phases are those that occur in a certain temperature range. WHY?? If the temperature rise is too high, thermal motion will destroy the delicate cooperative ordering of the LC phase, pushing the material into a conventional isotropic liquid phase. At too low temperature, most LC materials will form a conventional crystal. Many thermotropic LCs exhibit a variety of phases as temperature is changed. For instance, on heating a particular type of LC molecule (called mesogen) may exhibit various smectic phases followed by the nematic phase and finally the isotropic phase as temperature is increased. An example of a compound displaying thermotropic LC behavior is para- azoxyanisole. Liquid Crystal Phases  Liquid crystalline phases The intermediate phases, between the solid and the liquid states, found in LC materials are called liquid crystalline phases or mesophases. These mesophases appear when the molecules of the material are sharply anisotropic, normally like a rod (calamitic molecules) or a disc (discotic molecules). Most photonic applications use LC with rod-like molecules. These materials can exhibit a variety of mesophases as a function of the temperature. At high temperature, the molecules are disordered (isotropic liquid state). When the temperature is decreased, the molecules show an orientational order, with the long molecular axis oriented in a preferred direction, but not a positional order (nematic phase, N). If the temperature is decreased further, the molecules show a partial positional order forming a layered structure, with the long axis of the molecules in the normal direction of the layer (smectic-A phase, SmA). A further decrease of the temperature produces the tilt of the molecular director with regard to the perpendicular of the layer (smectic-C phase, SmC). At low temperature the material show both orientational and positional orders (crystalline solid). A thermotropic LC can exhibit one to several LC phases between the crystal and isotropic The liquid compounds states. nOBAbe It should (n= 8, 10,and noted that12) p,nsome only -Alkyloxybenzoic acidshow LC materials show all both nematic the and mesophases. smectic Materials phases. which exhibit two or more LC phases are said to be polymesomorphic and the 4′−n−octyl−cyanobiphenyl and 4′−n−decyl−cyanobiphenyl show Isotropic-Nematic-Smectic- process is known as polymesomorphism.  Lyotropic liquid crystal A lyotropic liquid crystal consists of two or more components that exhibit liquid-crystalline properties in certain concentration ranges. The molecules that make up lyotropic liquid crystals are surfactants consisting of two distinct parts: a polar, often ionic, head and a nonpolar, often hydrocarbon tail (not all surfactants, however, form lyotropic liquid crystals).  Following the rule of "like dissolves like," the head is attracted to water, or hydrophilic, and the tail is repelled by water, or hydrophobic. When dissolved in high enough concentrations, the molecules arrange themselves so that the polar heads are in contact with a polar solvent and/or the non-polar tails are in contact with a non-polar solvent. Soaps and detergents form lyotropic liquid crystals when they combine with water. Most importantly, biological membranes display lyotropic liquid crystalline behavior.  Lyotropic liquid crystals are two component systems, where an amphiphile is dissolved in a solvent. Thus, lyotropic mesophases are solvent and concentration dependent. The amphiphillic compounds are characterized by two different moieties, a hydrophilic polar head and a hydrophobic tail. LLC are made by adding solvent to the solid until critical micelle concentration (CMC) is reached. On further addition of solvent LLC changes into liquid phase. Examples: molecules of soaps, phospholipids (present in cell membranes), toothpaste, many proteins and cell membranes, tobacco mosaic virus.  Three types of lyotropic liquid crystals are well known. These are: lamellar, hexagonal and cubic phases. i) The simplest liquid crystalline phase that is formed by spherical micelles is the ‘micellar cubic’, denoted by the symbol I1. This is a highly viscous, optically isotropic phase in which the micelles are arranged on a cubic lattice. ii) At higher amphiphile concentrations the micelles fuse to form cylindrical aggregates of indefinite length, and these cylinders are arranged on a long-ranged hexagonal lattice. This lyotropic liquid crystalline phase is known as the ‘hexagonal phase’, and is generally denoted by the symbol HI. iii) At higher concentrations of amphiphile the ‘Lamellar Phase' is formed. This phase is denoted by the symbol Lα. This phase consists of amphiphilic molecules are arranged in bilayer sheets separated by layers of water.