3.012 Fundamentals of Materials Science: Bonding - Nicola Marzari (MIT, Fall 2005)3.012 Fund of Mat Sci: Structure – Lecture 24LIQUID CRYSTALSLAST LECTURE !Source: Wikipedia.3.012 Fundamentals of Materials Science: Bonding - Nicola Marzari (MIT, Fall 2005)Last Homework• Study: Chapter 4 of Allen-Thomas, up to 4.4.3 included3.012 Fundamentals of Materials Science: Bonding - Nicola Marzari (MIT, Fall 2005)Last time: 1. Glass transition, and common glasses2. Size of linear polymers: random walk model (identical to diffusion in a liquid)3. Packing fraction4. Solvent – self-avoiding random walk5. Rouse and reptation diffusion6. Network glasses: oxides and chalcogenides3.012 Fundamentals of Materials Science: Bonding - Nicola Marzari (MIT, Fall 2005)Fliessende Krystalle !3.012 Fundamentals of Materials Science: Bonding - Nicola Marzari (MIT, Fall 2005)Liquid crystals• Flow easily: “liquid-like” fluidity, as in isotropic liquids• Anisotropic material properties, as in crystalsReprinted with permission from Van Duijneveldt, J., et al. Journal of Chemical Physics 112, no. 20 (22 May, 2000). Copyright 2000, American Institute of Physics and the University of Bristol, Great Britain..Images of liquid crystal structures removed for copyright reasons.3.012 Fundamentals of Materials Science: Bonding - Nicola Marzari (MIT, Fall 2005)MovieImage removed for copyright reasons. See liquid crystal movies at http://micro.magnet.fsu.edu/movies/crystals/index.html.3.012 Fundamentals of Materials Science: Bonding - Nicola Marzari (MIT, Fall 2005)Mesogens• Molecular-shape anisotropy is key structural requirement for LCs• Molecules that induce liquid-crystallinityare called mesogens• Typical mesogens are needle-like (large length-to-diameter ratio) or disk-like3.012 Fundamentals of Materials Science: Bonding - Nicola Marzari (MIT, Fall 2005)MesogensLiquid-Crystalline PhasesAnisometric MoleculesDirectorRRRRRRRR'Figure by MIT OCW.3.012 Fundamentals of Materials Science: Bonding - Nicola Marzari (MIT, Fall 2005)Liquid Crystalline Polymers• A string of mesogens– Connect into backbone– Connect as side chainMain-chain LCPsRod mesogensDisc mesogensSide-chain LCPsSide-chain LCPsFigure by MIT OCW.Figure by MIT OCW.3.012 Fundamentals of Materials Science: Bonding - Nicola Marzari (MIT, Fall 2005)Friedel classification (1922)• Nematic• Cholesteric (i.e. twisted nematic)• Smectic• Columnar (discovered later)3.012 Fundamentals of Materials Science: Bonding - Nicola Marzari (MIT, Fall 2005)Nematic phases• Long-range orientational order (uniaxial)• Short-range translational orderFigure by MIT OCW.3.012 Fundamentals of Materials Science: Bonding - Nicola Marzari (MIT, Fall 2005)Birefringence• Different refraction index n=c/v depending on direction• Schlieren pictures between cross polarizershighlight refraction-index textureImages removed for copyright reasons.3.012 Fundamentals of Materials Science: Bonding - Nicola Marzari (MIT, Fall 2005)Twisted nematic (cholesteric)Nematicalignment with slight preference for a twistImage courtesy of Kevin Yager and the Barrett Research Group. Used with permission.3.012 Fundamentals of Materials Science: Bonding - Nicola Marzari (MIT, Fall 2005)Smectic phases• Long-range one-dimensional translational order• Long-range orientational orderSmectic A Smectic C Smectic C*http://www.barrett-group.mcgill.ca/teaching/liquid_crystal/LC01.htmImage courtesy of Kevin Yager and the Barrett Research Group. Used with permission.3.012 Fundamentals of Materials Science: Bonding - Nicola Marzari (MIT, Fall 2005)Columnar phases• Typically generated by discotic mesogensImage courtesy of Kevin Yager and the Barrett Research Group. Used with permission.3.012 Fundamentals of Materials Science: Bonding - Nicola Marzari (MIT, Fall 2005)Thermotropic liquid crystal• Hierarchy of temperature-driven phase transitions3 -D LatticeOreientationSolid1 - (2-)D LatticeOrientationFluidOrientation No OrientationFluid FluidNo Lattice No LatticeAnisotropic Anisotropic AnisotropicCrystal Liquid Crystal(mesophases) LiquidIsotropicTemperatureFigure by MIT OCW.3.012 Fundamentals of Materials Science: Bonding - Nicola Marzari (MIT, Fall 2005)Lyotropic liquid crystals• Phase transitions are mostly driven by changes in concentrationFigure by MIT OCW.Concentration of Amphiphilic MoleculesTemperatureTK3.012 Fundamentals of Materials Science: Bonding - Nicola Marzari (MIT, Fall 2005)Descriptors: orientational order• Director (“a vector with no arrow”)• Director field: P(θ,φ)• Scalar orientational order parameter:23cos 12Sθ−=3.012 Fundamentals of Materials Science: Bonding - Nicola Marzari (MIT, Fall 2005)Descriptors: translational order()/2/2/2/22cos2cos2cosaaaSMazPzdzazazdzaπππ−−==∫∑∫Image courtesy of Kevin Yager and the Barrett Research Group. Used with permission.Graph removed for copyright reasons. See: p. 231, Figure 4.15 in Allen, S. M., and E.L. Thomas.The Structure of Materials. New York, NY: J. Wiley & Sons, 1999.3.012 Fundamentals of Materials Science: Bonding - Nicola Marzari (MIT, Fall 2005)Homogeneous vs homeotropic• Surface boundary conditionsFigure by MIT OCW.3.012 Fundamentals of Materials Science: Bonding - Nicola Marzari (MIT, Fall 2005)Polarizability under an electric fieldPhoto of an LCD removed for copyright reasons.Figure by MIT OCW.3.012 Fundamentals of Materials Science: Bonding - Nicola Marzari (MIT, Fall 2005)Amphiphilic mesogens• Last (5th) class of mesogens: amphiphilic molecules (they self-assemble in micelles, columns, layers, and higher-order structures that then form mesophases).Image courtesy of Kevin Yager and the Barrett Research Group. Used with permission.Hydrophobe HydrophileAnionic (phosphates, sulfonatessulfates...)Cationic (quatemary ammonium)Amphoteric (bataines)Nonionic (Ethoxylates)Figure by MIT OCW.3.012 Fundamentals of Materials Science: Bonding - Nicola Marzari (MIT, Fall 2005)Lyotropic phase diagramMicellesHexagonalLamellarCrystal + WaterTemperatureAmphiphile concentration (%)Figure by MIT OCW. Images courtesy of Kevin Yager and the Barrett Research Group. Used with permission.3.012 Fundamentals of Materials Science: Bonding - Nicola Marzari (MIT, Fall 2005)Surfactants and phospholipidsImage courtesy of Kevin Yager and the Barrett Research Group. Used with permission.DetergencyEmulsionParticleSubstrateSurfactant GreaseSolubilisedGreaseFigure by MIT
View Full Document
Unlocking...