Liquid crystals

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Information about Liquid crystals

Published on February 3, 2010

Author: MImranaziz


Introduction to Liquid Crystals : Introduction to Liquid Crystals MOHAMMAD IMRAN AZIZ Assistant Professor PHYSICS DEPARTMENT SHIBLI NATIONAL COLLEGE, AZAMGARH (India). Contents : Contents Properties of liquid crystals Types of liquid crystals Thermotropic nematic smectic cholesteric Lyotropic structures application NIOSOMES Synergistic Effects of Surfactants : Synergistic Effects of Surfactants Observed when surfactants having relatively similar structure or ionic property are mixed Resulted in the formation of liquid crystal structures or complexes at the interface by intermolecular interactions between surfactants Examples Anionic and nonionic in synthetic latex emulsion polymerisation, Mixture of a dispersant and a hydrating agent to increase dispersion stability in agricultural chemicals STATES OF MATTER : STATES OF MATTER Common states: solid liquid gas Matter can exist in other states LIQUID CRYSTALS : LIQUID CRYSTALS A state that occurs between a solid & a liquid Possess properties characteristics of both liquids & crystalline solids Also possess properties not found in either liquids or solids May response to external perturbations & some changes colour with temperature Crystals vs Liquid Crystals : A crystal is a highly ordered structure which possesses long-range positional & orientational order For many substances these two types of order are destroyed simultaneously when the crystal melts to form a liquid For some substances, these orders are destroyed in stages. These are liquid crystals. E.g. Slide 17 Crystals vs Liquid Crystals Properties of liquid crystals : Liquid crystal can flow like a liquid, due to loss of positional order Liquid crystal is optically birefringent, due to its orientational order Transition from crystalline solids to liquid crystals caused by a change of temperature – gives rise to THERMOTROPIC liquid crystals substances that are most likely to form a liquid crystal phase at a certain temperature are molecules that are ELONGATED & have some degree of RIGIDITY. Try slide 29 Properties of liquid crystals Typical chemical structures : Typical chemical structures cholesterol ester phenyl benzoates surfactants such as polyethylene-oxides, alkali soaps, ammonium salts, lecithin paraffins glycolipids cellulose derivatives Typical applications : Typical applications LCD displays dyes (cholesterics) advanced materials (Kevlar) membranes temperature measurement (by changing colours) solvents for GC, NMR, reactions, etc. Drug delivery Types of liquid crystals : Types of liquid crystals Thermotropic Phase transition depends on temperature Nematic Smectic Cholesteric Lyotropic Phase transition depends on temperature & concentration As temperature increases….. : As temperature increases….. The first liquid crystal phase is the smectic A, where there is layer-like arrangement as well as translational and rotational motion of the molecules. A further increase in temperature leads to the nematic phase, where the molecules rapidly diffuse out of the initial lattice structure and from the layer-like arrangement as well. At the highest temperatures, the material becomes an isotropic liquid where the motion of the molecules changes yet again. Nematic : Nematic Simplest form is a nematic liquid crystal i.e. long-range orientational order but no positional order The preferred direction is known as director Nematic… : Nematic… Despite the high degree of orientational order, nematic phase as a whole is in disorder i.e. NO MACROSCOPIC ORDER (orientation within a group is similar but not from one group to another) Structure of nematic phase can be altered in a number of ways. E.g. electric or magnetic field or treatment of surfaces of the sample container Thus, possible to have microscopic order & macroscopic order Nematic liquid crystals are widely used in electro-optic display devices Cholesteric : Cholesteric The first liquid crystal that was observed through a polarising microscope is cholesteryl benzoate. Thus, CHOLESTERIC liquid crystal OR chiral nematic liquid crystal E.g. cholesteryl benzoate: LC @ 147C, isotropic @ 186C Cholesteric liquid crystals have great potential uses as sensors Thermometer fashion fabrics that change colour with temperature display devices Slide 15: In CHOLESTERIC phase, there is orientational order & no positional order, BUT, director is in HELICAL ORDER. The structure of cholesteric depends on the PITCH, the distance over which the director makes one complete turn One pitch - several hundred nanometers Pitch is affected by:- Temperature Pressure Electric & magnetic fields Smectic : Smectic SMECTIC phase occurs at temperature below nematic or cholesteric Molecules align themselves approx. parallel & tend to arrange in layers Not all positional order is destroyed when a crystal melts to form a smectic liquid crystal Chiral smectic C liquid crystals are useful in LCDS LIQUID CRYSTAL POLYMERS : LIQUID CRYSTAL POLYMERS Can form nematic, cholesteric, smectic When liquid crystal polymers solidify, the liquid crystal structure ‘freeze in’ This results in materials of high tensile strength & in some cases unusual electro-optical behaviour E.g. Kevlar aramid fibre – bullet-proof vest & airplane bodies (aromatic polyamide) Examples of phase changes : Examples of phase changes Cholesteryl myristate solid smectic A 71C cholesteric isotropic 79C 85C 4, 4’-di-heptyloxyazoxybenxene solid 74C 94C 124C smectic C nematic isotropic Thermotropic vs Lyotropic : Thermotropic vs Lyotropic THERMOTROPIC Absence of solvent Rigid organic molecules Depends on Temperature Structures: Smectic Nematic Cholesteric LYOTROPIC In solvent Surfactants Depends on Temperature, Concentration, salt, alcohol Structures: Lamellar Hexagonal etc Structure formation in surfactant solution : Structure formation in surfactant solution micelle rod hexagonal monolayer bilayer Reverse micelle Formation of MICROEMULSION REVERSE HEXAGONAL Oil/alcohol Effect of temperature and concentration on the structure of lyotropic liquid crystals : Effect of temperature and concentration on the structure of lyotropic liquid crystals SURFACTANT VESICLES : SURFACTANT VESICLES [A] Phospholipids (e.g. lecithin) + H2O ----> phospholipid vesicles or liposomes [B] Liposomes + (long chain) stearylamine -------> tve charge liposome (carriers for DNA) [C] Liposomes + dicetyl phosphate ----- - ve charge liposome Vesicles : Vesicles Bilayers that fold into a 3D structure Vesicles form because they get rid of the edges of bilayers, protecting the hydrophobic chains from the water, but they still allow for relatively small layers. Lipids found in biological membranes spontaneously form vesicles in solution. Application of Liposomes : Application of Liposomes can encapsulate: drugs, proteins, enzymes administered intravenously, orally or intramuscularly decrease toxicity increase specificity of drug uptake enable slow release Problems with phospholipids : Problems with phospholipids phospholipids undergo oxidative degradation handling & storage must be under nitrogen expensive Formation of liquid crystals using surfactants : Formation of liquid crystals using surfactants [A] Anionic e.g. alkane sulfonates [B] Cationic e.g. hexadecyl trimethyl ammonium bromide [C] Amphoteric e.g. alkyl betaines Due to toxicity of ionic surfactants, the vesicles are not used for drug delivery [D] Non-ionic e.g. alcohol ethoxylates R-O-(CH2CH2O) m H m: 2-20, R : mixed; alkyl group C8C18 Niosomes : Non-ionic + cholesterol -> NIOSOMES These vesicles prolong the circulation of entrapped drug Properties depend on Composition of bilayer Method of productione.g. cholesterol & single alkyl-chain non-ionic surfactant with a glyceril head group Niosomes Examples of niosome applications : Examples of niosome applications Ketoconazole niosomes were prepared by using surfactant (Tween 40 or 80), cholesterol and drugSatturwar PM; Fulzele SV; Nande VS; Khandare, JN Indian Journal of Pharmaceutical Sciences. 2002 Mar-Apr; 64(2): 155-8 Use for topical immunisation - Bovine serum albumin (BSA)loaded niosomes composed of sorbitan monostearate/sorbitan trioleate (Span 60/Span 85), cholesterol and stearylamine as constitutive lipidsSanyog Jain1, S. P. Vyas2, Journal of Pharmacy and Pharmacology Vol. 57, No. 9, pages 1177 (2005) The three states of mater? : The three states of mater? solid liquid gas Which state is a soap bubble in? The fourth state of mater… : The fourth state of mater… e.g.: ice -> water e.g.: cholesteryl benzoate Fredrich Reinitzer, Otto Lehmann, 1888 The new liquid state showed birefringence! Orientational order but no positional order! Is liquid crystal more like solid or liquid? : Is liquid crystal more like solid or liquid? Latent heat: the energy required to cause phase transition. H2O 80 cal/g of ice -> water, 540 cal/g water -> steam. Cholesteryl myristate: 65 cal/g of solid -> LC, 7 cal/g LC- liquid. The structure of LC : The structure of LC solid liquid liquid crystal If random, <q> = 57o Types of liquid crystals : Types of liquid crystals Discotic Chiral nematic LC in an electric field : LC in an electric field Orientation of a dipole in an electric field LC film in an electric field Light & polarization : Light & polarization Interaction of light with LC : Interaction of light with LC Birefringence LCs under a polarization microcope : LCs under a polarization microcope Aligning liquid crystals : Aligning liquid crystals “rubbing” “polishing” Liquid crystal display : Liquid crystal display The operation of a pixel in LCD : The operation of a pixel in LCD Other cool gadgets from LCs : Other cool gadgets from LCs Thermochromic Ink Polymeric LC Interactive art using iridescent coloring… Optoelectronic computers… Lyotropic LCs : Lyotropic LCs Thermotropic: pure substance melts to form LC. Lyotropic LCs: mixtures of two substances that separate into different regions in liquid. micelles vesicles Lamellar bilayers Other liquid crystals in nature… : Other liquid crystals in nature… Iridescent color Introduction : Introduction Liquid crystals are very important in the study of optics, chemistry and polymer science. The discovery of the liquid crystal happened over a century and since this time the phenomena has been applied to many products in our society. This presentation will cover the properties, different phases and uses of liquid crystals. It will also give illustrations of liquid crystals from the micro perspective. Slide 45: What are liquid crystals ? Liquid crystals form from organic compounds and is thought of as the phase of matter between the solid and liquid state of a crystal.This phenomena was discovered in 1888 by Austrian chemist Frederich Reinitzer. Phases : Phases Nematic Phase : Nematic Phase Molecules in this phase are long and rod-like in shape. They are free to move in space. Chiral Nematic : Chiral Nematic This phase is composed of nematic molecules in a helical structure about the layer normal. The distance that it takes for the molecules to complete one turn is called the pitch. λ = n*p*cosθ Smectic Phase : Smectic Phase This phase can be reached at lower temperatures than the nematic phase. Molecules align themselves in layers.(They are restricted to their plane.) More order and higher viscosity Columnar Phase : Columnar Phase Characterized by its stacked columns of molecules. Discovered in 1977 by Chandrasekhar Order Parameter : Order Parameter S = ½<(3cos²θ-1)> The order parameter in a liquid crystal range from 0.3 to 0.9 Electric field Effects : Electric field Effects If an electric field is applied to a liquid crystal the molecules will align in the same direction as the field. Twisted Nematic : Twisted Nematic This is called the twisted nematic liquid crystal and the spacing between the planes change with temperature. The spacing is associated with the wavelengths of light. Slide 54: Applications Uses of Liquid Crystals : Uses of Liquid Crystals Flat screen television Wristwatches Laptop screens Digital clocks Thermometers Switchable windows Conclusion : Conclusion We know today that many chemical compounds can exist in the liquid crystal state, such as cholesteryl benzoate. Thanks to the scientists that worked so diligently toward understanding this phenomenon, the world can focus on ways to make this product useful in society. Over the last century many applications such as the detection of hot points in microcircuits, the findings of fractures or tumors in humans and the conversion of infared images have become accessible due to the understanding of pitch in a liquid crystal. Slide 57: Liquid Crystal Materials Slide 58: Lyotropics Thermotropics amphiphilic molecules, polar and non-polar parts form liquid crystal phases over certain concentration ranges when mixed with a solvent molecules consisting of a rigid core and flexible tail(s) form liquid crystal phases over certain temperature ranges. + - hydrophilic polar head hydrophobic non-polar tail flexible tail rigid core Broad Classification Slide 59: The Lyotropic Phases micelle reverse micelle cross section cross section Slide 60: Chemist’s View Physicist’s Engineer’s View Shape Anisotropy Length > Width The molecule above (5CB) is ~2 nm × 0.5 nm The Thermotropic Liquid Crystal Molecule Slide 61: Geometrical Structures of Mesogenic Molecules Low Molecular Weight High Molecular Weight (polymers) ( ) n ( ) n disk-like rod-like most practical applications Slide 62: n Temperature Crystal Nematic LC Isotropic The Liquid Crystal Phase Slide 63: The Nematic Director n n The local average axis of the long molecular axis director Slide 64: n Temperature Smectic C Smectic A Nematic n z q n Other Liquid Crystal Phases Slide 65: left-handed right-handed mirror images non-superimposable H-C-C-C-C-C C N H H H H H H H H H H H-C-C-C-C-C C N H H H CH3 H H H H H H non-chiral chiral (RH) The methyl group on the 2nd carbon atom on the alkyl chain of the molecules extends out of the plane of the paper and the hydro- gen atom extends into the plane of the paper. Therefore the 2nd carbon can be thought of as a right or left handed coordinate system Chirality Slide 66: CN pitch P Ordinary Nematic Chiral Nematic director n The Chiral Nematic Slide 67: The Chiral Smectic C: Ferroelectrics q m Eye- dipole moment m fin - chiral ferroelectric LC has a dipole moment perp- endicular to its long axis, and is chiral. Slide 68: The Chiral Smectic: TGB Twisted Grain Boundary (TGB) A twisted grain boundary smectic A phase (frustrated) - TGBA* Slide 69: O C R C O C R O O C O R O C O R O C O R O C O R Discotic Liquid Crystal example: R=OCOC11H23 Slide 70: Columnar, columns of molecules in hexagonal lattice Nematic discotic phase n Discotics Liquid Crystals n Slide 71: Polymer Liquid Crystals Combining the properties of liquid crystals and polymers Main Chain Side Chain mesogenic moieties are connected head-to-tail mesogenic moieties attached as side chains on the polymer backbone rigid semi-flexible Slide 72: Polymer Liquid Crystals forming nematic liquid crystal phases n main-chain side-chain Slide 73: O C-O-(CH2)n-O R2 C-O O Example of Side-Chain Polymer LCs -(-CH2-C-)X- R1 Too slow for display applications (switching times ~ 0.5-1 s Useful for other applications such as: Optical filters Optical memory Alignment layers for low molecular weight LCs Non-linear optic devices (optical computing) Slide 74: q n The Order Parameter n no order perfect order perfect crystal isotropic fluid Slide 75:  Interactions between individual molecules are represented by a potential of average force From Statistical Mechanics (Self Consistency) Maier-Saupe Theory - Mean Field Approach {V: minimum} when phase is ordered (-P2(cosq)) {V: V=0} when phase is disordered (<P2(cosq)>) factor for intermolecular strength ( n) b=(kT)-1 q n f y Slide 76: Maier-Saupe Theory - Mean Field Approach Temperature Nematic Liquid Crystal Isotropic Fluid -0.6 0.0 1.0 Order Parameter, S Slide 77: Landau-de Gennes Theory a=ao(T-T*), ao, b, c, T*, L are phenomenological constants G is a surface interaction strength Order Parameter, S Temperature Good near NI transition surface Predicts order near surface Slide 78: Optical Anisotropy: Birefringence ordinary ray (no, ordinary index of refraction) extraordinary ray (ne, extraordinary index of refraction) Slide 79: Optical Anisotropy: Birefringence ordinary wave q extraordinary wave For propagation along the optic axis, both modes are no optic axis Slide 80: Birefringence Example: 1/4 Wave Plate Unpolarized linear polarized circular polarized polarizer LC: Dn=0.05 d What is minimum d for liquid crystal 1/4 wave plate ? Takes greater number of e-waves than o-waves to span d, use Dn=0.05 Slide 81: Creating Deformations with a Field and Surface - Bend Deformation E or B Slide 82: Creating Deformations with a Field and Surface - Splay Deformation E or B Slide 83: Creating Deformations with a Field and Surface - Twist Deformation E or B Slide 84: Elastic Constant K22: Temperature Dependence Slide 85: x e e y E n q Effects of an Electric Field Electric Free Energy Density Electric Torque Density Using De = 5 and E=0.5 V/mm Slide 86: Freedericksz Transition - The Threshold I Ec z y E x At some critical E field, the director rotates, before Ec nothing happens q n y x n E 0 0 d Slide 87: Freedericksz Transition - The Threshold II E-field free energy total free energy Minimize free energy with ‘Euler’ Equation Slide 88: Freedericksz Transition - The Threshold III 1.0 E/Ec mid-layer tilt (deg) differential equation soln. small q threshold Slide 89: Defects s=+1 s=+1 s=+1 s=1/2 s=-1/2 s=-1 s=3/2 s=+2 The singular line (disclination) is pointing out of the page, and director orientation changes by 2ps on going around the line (s is the strength) Slide 90: Thank You

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