This document discusses liquid crystals and their use in liquid crystal displays (LCDs). It describes the different phases of liquid crystals including nematic, smectic, and cholesteric. Nematic liquid crystals are used in LCDs as they have anisotropic optical properties and can change the orientation of polarized light in response to electric fields. LCDs work by applying a voltage to change the tilt angle of the nematic crystals and control the transmission of light, allowing pixels to be switched on or off. Super-twisted nematic LCDs provide faster response times for grayscale images.

1. Liquid Crystals Chapter 8, Box 8.1

2. Mesophase Solid Mesophase Liquid

3. Common Shapes: Anisotropic Thin, Long Disk-shaped

4. Liquid Crystal Transition Temperature ( thermotropic ) Pressure ( lyotropic ) Mesophase, Liquid Crystal Solid Liquid

5. Types of long range order Nematic phase Parallel alignment Lack layer arrangement Smectic phase Parallel alignment Layered Cholesteric phase Molecules lie in sheets at different angles (helical structure).

6. Importance of Cholesteric phase Form Helical structure (dependent on temp.) Diffract light Colors

7. LCD Layers

8. Twisted nematic LCD Twisted: rotates polarized light 90 o Supertwisted: rotates polarized light 270 o Nematic Crystals Anisotropic optical properties Response to electric fields Used in Liquid Crystal Display

9. Optical Response Applied voltage changes tilt angle of nematic layers Tilt Angle changes light transmission

10. Super-Twisted Nematic LCD 270 o 90 o Abrupt tilt angle change with 270 o twist On/Off response faster 210 o used for grayscale

11. Obtaining a Useful Temperature Range Nematic exists over larger range than either separate solution does

12. Conclusions Mesophase valuable tool LCD one example of application References Atkins, P. and Julio de Paula. Physical Chemistry . 7 th 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.html “ Introduction to Liquid Crystal Displays.” http://plc.cwru.edu/tutorial/enhanced/files/lcd/tn/tn.htm