Liquid Crystals

Liquid CrystalsLiquid CrystalsChapter 8, Box 8.1Chapter 8, Box 8.1

MesophaseMesophase

Solid Mesophase Liquid

Common Shapes: Common Shapes: AnisotropicAnisotropic

Thin, Long Disk-shaped

Liquid CrystalLiquid Crystal TransitionTransition

– Temperature (Temperature (thermotropicthermotropic))– Pressure (Pressure (lyotropiclyotropic))

Mesophase, Liquid Crystal

Solid Liquid

Types of long range Types of long range orderorder

Nematic phaseNematic phase– Parallel alignmentParallel alignment– Lack layer Lack layer

arrangementarrangement

Smectic phaseSmectic phase– Parallel alignmentParallel alignment– LayeredLayered

Cholesteric phaseCholesteric phase– Molecules lie in sheets Molecules lie in sheets

at different angles at different angles (helical structure). (helical structure).

Importance of Cholesteric Importance of Cholesteric phasephase

– Form Helical Form Helical structurestructure(dependent on (dependent on temp.)temp.)

Diffract lightDiffract light Colors Colors

LCD LayersLCD Layers

Twisted nematic LCDTwisted nematic LCD

•Twisted: rotates polarized light 90o

•Supertwisted: rotates polarized light 270o

Nematic Crystals•Anisotropic optical properties•Response to electric fields•Used in Liquid Crystal Display

Optical ResponseOptical Response

•Applied voltage changes tilt angle of nematic layers•Tilt Angle changes light transmission

Super-Twisted Nematic Super-Twisted Nematic LCDLCD

270o 90o

•Abrupt tilt angle change with 270o twist•On/Off response faster•210o used for grayscale

Obtaining a Useful Obtaining a Useful Temperature RangeTemperature Range

•Nematic exists over larger range than either separate solution does

ConclusionsConclusions Mesophase valuable toolMesophase valuable tool LCD one example of applicationLCD one example of application

ReferencesReferences1.1. Atkins, P. and Julio de Paula. Atkins, P. and Julio de Paula. Physical ChemistryPhysical Chemistry. 7. 7thth ed. ed.

W.H. Freeman and Co. New York: 2002; 209-210.W.H. Freeman and Co. New York: 2002; 209-210.2.2. ““Chemical of the Week: Liquid Crystals.” University of Chemical of the Week: Liquid Crystals.” University of

Wisconsin-Madison. Wisconsin-Madison. http://scifun.chem.wisc.edu/chemweek/LiqXtal/LiqXtal.htmhttp://scifun.chem.wisc.edu/chemweek/LiqXtal/LiqXtal.htmll

3.3. ““Introduction to Liquid Crystal Displays.” Introduction to Liquid Crystal Displays.” http://plc.cwru.edu/tutorial/enhanced/files/lcd/tn/tn.htmhttp://plc.cwru.edu/tutorial/enhanced/files/lcd/tn/tn.htm