Liquid Crystals PDF

Liquid Crystal In a crystalline state – ❖ Regular arrangement in repeated manner ❖ A rigid arrangement, immobile ❖ Tend to orient in a preferred direction - the molecules have a positional and orientational order. In liquid state - ❖ Random arrangement. ❖ Molecules neither occupy specific posit...

Liquid Crystal In a crystalline state – ❖ Regular arrangement in repeated manner ❖ A rigid arrangement, immobile ❖ Tend to orient in a preferred direction - the molecules have a positional and orientational order. In liquid state - ❖ Random arrangement. ❖ Molecules neither occupy specific positions nor remain oriented ❖ The molecules are somewhat free to move  Liquid Crystal The discovery of LCs in 1888 is commonly attributed to the Austrian botanist Friedrich. Reinitzer. He observed a “double melting” behavior of cholesteryl benzoate. The crystals of this material melted at 145.5oC into a cloudy fluid, which upon further heating to 178.5oC became clear. This discovery represented the first recorded documentation of the LC phase. 1888 by Prof. Reinitzer, a botanist, Germany ❖ A liquid crystal (LC) is a state of matter that exists between solids and liquids with both the properties. ❖ Normally when a low molar mass solid melts, it forms an ordinary liquid and is isotropic. ❖ They undergo more than a single transition in passing from solid to liquid through different intermediate states on heating. ❖ These intermediate states with different molecular ordering are also known as mesophases.  Average direction of the long molecular axes of all molecules in the liquid crystal- Director. The direction of preferred orientation in a liquid crystal is called the director and maybe imagined to be directed towards the top or bottom of the page Since the molecules are in constant motion, in LC phase they spend more time pointing along the director than along any other direction The extent of orientational order can be described by taking an average. An average of 0° indicates perfect orientation (solids). An average of greater than 45° indicates no orientational order (liquids). However, in LC’s a smaller average angle with the director is observed which indicates orientational order. 7 LCs must satisfy the following characteristic properties 1) Liquid crystalline nature should be a) at room temperature and b) the entire temperature range of the device operation 2) Chemically, electrochemically, photochemically and thermally stable 3) Permanent electric dipole, 4) Should possess easily polarizable substituents Molecule must have some rigidity in its central region The ends of the molecule are somewhat flexible  Classification 1) Based on the response to temperature/Solvent ❖Thermotropic ❖Lyotropic Thermotropic LC Based on the mesophases Based on the shape of ❖Nematics constituting molecule ❖Smectics ❖Calamitic ❖Columnar ❖Discotic ❖Cubic ❖Polycatenar ❖Bent (Banana) A) Thermotropic - which undergo transitions by variation in temperature and exhibit Liquid Crystalline behavior. Mesophase formation is temperature dependent Solid Liquid Crystal Liquid Cholesteryl 145.5oC 178.5oC Benzoate p-azoxyphenetole 137oC 167oC P-azoxyanisole 116oC 135oC B) Lyotropic - which undergo transitions by the influence of solvents Mesophase formation is solvent dependent Example: Sodium stearate/soap, phospolipids The amphiphilic compounds are characterized by two distinct moieties, a hydrophilic polar “head” and a hydrophobic “tail”. Organic molecules, which are able to form Thermotropic as well as Lyotropic mesophases, are termed as Amphotropic LCs. Based on the phase structure Nematic Cubic Liquid Smectics Crystals Columnar Nematic phase: The least ordered mesophase (the closest to the isotropic liquid state) is the nematic (N) phase, where the molecules have only an orientational order but not positional order Smectic phases In smectic (S) phase, in addition to the orientational order the molecules possess less or more positional order, such that the molecules organize in layered structures. Smectic A Smectic B ✓ That is, 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, ✓ The increased order means that the smectic state is more "solid-like" than the nematic. Columnar Phases Columnar (Col) phases are more ordered. Here the disc- shaped cores have a tendency to stack one on the top of another, forming columns. Arrangement of these columns into different lattice patterns gives rise to a number of columnar mesophases, namely columnar rectangular (Colr) and columnar hexagonal (Colh). The general structure of Columnar phases, where the molecules are aligned in the same orientation and form columns Cubic Phase of LCs This phase is exhibited by lyotropic LC Structures with micellar lattice units Based on the shape Thermotropic LC The essential requirement for a molecule to be a thermotropic LC is a structure consisting of a central rigid core (often aromatic) and a flexible peripheral moiety (generally aliphatic groups). Calamitic Thermtropic Discotic Polycatenar Bent (Banana) Calamitic LCs Calamitic or rod-like LCs are those mesomorphic compounds that possess an elongated shape. Molecular length (l) being significantly greater than the molecular breadth (b).  Rʹ L L L Rʺ A B C D In a Liquid crystal, there should be at least one rigid core. ‘L’ is the linking group (an imine, azo, azoxy, ester, thioester, C=C, and C≡C) Rʹ or Rʺ is the end group/chain (a halo group, cyano, nitro, alkyl or alkoxy groups) Discotic LCs Discotic compounds consist of flat and disc-like rigid cores which surrounded by flexible chains such as alkyl, alkyloxy These molecules (with flat and disc-like shape) are able to pack with different structures. general shape of discotic LCs, where d>>t. Molecular structure of first series of discotic LCs discovered Ex: (i) Benzene hexa n-alkanoate derivatives ii) Phthalocyanine Polycatenar Liquid crystals Hybrid class of thermotropic LCs Described with intermediate molecular features between rod-like and disc-like mesogens. The central core of polycatenar LCs comprises a calamitic region with half-discs on the extremities It exhibit both calamitic (nematics/smectic) and discotic (columnar) depending on the specific molecular structure of the components The numbers of the flexible end chains of the core can be indicated by using the term, m,n– polycatenary mesogen. E.g., Tetracatenar mesogens  Bent shape or Banana shaped LCs Mesomorphic compounds characterised by bent molecular structures It exhibit both nematic and/or smectic phase It consist of two mesogenic groups linked through a rigid core so as to form bent shaped molecule but not linear. The central rigid core may be phenyl or biphenyl or naphthyl group The mesogenic groups attached to the central core are mostly the calamitic molecules consisting of two (or more) aromatic rings with different linking groups The angle between the two calamitic wings (bending angle) is around 120°. In case of a benzene central core, the mesogens are connected in 1 and 3 positions (i.e., meta position to each other). If the central core is a naphthyl, the two calamitic wings are connected in 2 and 7 positions.  Lyotropic LCs Lyotropic LCs are two-component systems where an amphiphile is dissolved in a solvent. Thus, lyotropic mesophases are concentration and solvent dependent. The amphiphilic compounds are characterized by two distinct moieties, a hydrophilic polar “head” and a non polar hydrophobic “tail”. Examples of these kinds of molecules are soaps and various phospholipids like those present in cell membranes Soap + polar solvent like water- micelle formation Soap + non polar solvent like hexane – inverted micelle Under certain conditions, these micelles further aggregate to form more complicated assemblies, such as cubic, lamellar and hexagonal phases, which generate lyotropic liquid crystal phases. Lamellar phases are particularly significant as they form the structural basis for biological membranes Micellar aggregates and phases formed by Lyotropic liquid crystals Application of liquid crystal Liquid crystal display – devices such as watches, calculators, mobile phones, laptops etc Liquid crystal thermometers – Chiral nematic LCs reflect light and the colour of the reflected light depends on temperature  Liquid Crystals in Display systems The electro-optic effect of liquid crystals controls the brightness/darkness of the light emerging from its elements and this is used in information displays Liquid Crystal Displays are available in many sizes and can be used to display numeric, alphanumeric and graphic images A numeric display consists of seven segments for each digit. Light from the area of each of the seven segments is controlled independently and is used to create any one of the ten digits. When an electric field is applied to a segment, the liquid crystal in that segment undergoes deformation (is activated)  Nematic comes from a Greek prefix nemato means threadlike Nematic liquid crystals are used in twisted nematic display (TND) (chiral nematic) , the most common form of liquid crystal display. nematic liquid crystals are used in alphanumeric liquid- crystal displays ( LCD s), such as those found in digital wristwatches and many consumer electronic devices. In twisted nematic displays (TND), the nematic LC is sandwiched between two ITO coated glass plates with parallel (homogeneous) alignment of its molecular director with the glass walls. However, the two glass plates are twisted by 90° relative to each other (as shown in the following figures).