Liquid Crystals

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Transcript of Liquid Crystals

  • Liquid CrystalsChapter 8, Box 8.1

  • MesophaseSolidMesophaseLiquid

  • Common Shapes: AnisotropicThin, LongDisk-shaped

  • Liquid CrystalTransition

    Temperature (thermotropic)Pressure (lyotropic)Mesophase, Liquid CrystalSolidLiquid

  • Types of long range orderNematic phaseParallel alignmentLack layer arrangement

    Smectic phaseParallel alignmentLayeredCholesteric phaseMolecules lie in sheets at different angles (helical structure).

  • Importance of Cholesteric phaseForm Helical structure(dependent on temp.)Diffract lightColors

  • LCD Layers

  • Twisted nematic LCDTwisted: rotates polarized light 90oSupertwisted: rotates polarized light 270o

    Nematic CrystalsAnisotropic optical propertiesResponse to electric fieldsUsed in Liquid Crystal Display

  • Optical ResponseApplied voltage changes tilt angle of nematic layersTilt Angle changes light transmission

  • Super-Twisted Nematic LCD270o90oAbrupt tilt angle change with 270o twistOn/Off response faster210o used for grayscale

  • Obtaining a Useful Temperature RangeNematic exists over larger range than either separate solution does

  • ConclusionsMesophase valuable toolLCD one example of application

    ReferencesAtkins, P. and Julio de Paula. Physical Chemistry. 7th ed. W.H. Freeman and Co. New York: 2002; 209-210.Chemical of the Week: Liquid Crystals. University of Wisconsin-Madison. http://scifun.chem.wisc.edu/chemweek/LiqXtal/LiqXtal.htmlIntroduction to Liquid Crystal Displays. http://plc.cwru.edu/tutorial/enhanced/files/lcd/tn/tn.htm