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Liquid Crystals


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Liquid Crystals

  1. 1. Liquid Crystals Chapter 8, Box 8.1
  2. 2. Mesophase Solid Mesophase Liquid
  3. 3. Common Shapes: Anisotropic Thin, Long Disk-shaped
  4. 4. Liquid Crystal <ul><li>Transition </li></ul><ul><ul><li>Temperature ( thermotropic ) </li></ul></ul><ul><ul><li>Pressure ( lyotropic ) </li></ul></ul>Mesophase, Liquid Crystal Solid Liquid
  5. 5. Types of long range order <ul><li>Nematic phase </li></ul><ul><ul><li>Parallel alignment </li></ul></ul><ul><ul><li>Lack layer arrangement </li></ul></ul><ul><li>Smectic phase </li></ul><ul><ul><li>Parallel alignment </li></ul></ul><ul><ul><li>Layered </li></ul></ul><ul><li>Cholesteric phase </li></ul><ul><ul><li>Molecules lie in sheets at different angles (helical structure). </li></ul></ul>
  6. 6. Importance of Cholesteric phase <ul><ul><li>Form Helical structure </li></ul></ul><ul><ul><li>(dependent on temp.) </li></ul></ul><ul><ul><ul><li>Diffract light </li></ul></ul></ul><ul><ul><ul><li>Colors </li></ul></ul></ul>
  7. 7. LCD Layers
  8. 8. Twisted nematic LCD <ul><li>Twisted: rotates polarized light 90 o </li></ul><ul><li>Supertwisted: rotates polarized light 270 o </li></ul>Nematic Crystals <ul><ul><li>Anisotropic optical properties </li></ul></ul><ul><ul><li>Response to electric fields </li></ul></ul><ul><ul><li>Used in Liquid Crystal Display </li></ul></ul>
  9. 9. Optical Response <ul><li>Applied voltage changes tilt angle of nematic layers </li></ul><ul><li>Tilt Angle changes light transmission </li></ul>
  10. 10. Super-Twisted Nematic LCD 270 o 90 o <ul><li>Abrupt tilt angle change with 270 o twist </li></ul><ul><li>On/Off response faster </li></ul><ul><li>210 o used for grayscale </li></ul>
  11. 11. Obtaining a Useful Temperature Range <ul><li>Nematic exists over larger range than either separate solution does </li></ul>
  12. 12. Conclusions <ul><li>Mesophase valuable tool </li></ul><ul><li>LCD one example of application </li></ul><ul><li>References </li></ul><ul><li>Atkins, P. and Julio de Paula. Physical Chemistry . 7 th ed. W.H. Freeman and Co. New York: 2002; 209-210. </li></ul><ul><li>“ Chemical of the Week: Liquid Crystals.” University of Wisconsin-Madison. </li></ul><ul><li>“ Introduction to Liquid Crystal Displays.” </li></ul>