Introduction to Liquid Crystals

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Introduction to Liquid Crystals

• Chang-Kui Duan

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Contents

• 0. History of LC and LCD
• Fundamentals
• Molecular structure and Chemical Composition
• Electronic Properties
• Lyotropic, Polymeric, and Thermotropic Liquid
  Crystals
• Mixtures and Composites
• Liquid Crystal Cells and Sample Preparation

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Liquid Crystal Displays (LCDs) Innovation
Timeline

• 1888-1899
• 1888, Austrian Botanist Freidrich Reinitzer
  discovers liquid crystals
• 1897,German scientist Karl Braun invents the
  cathode ray tube (CRT)

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Liquid Crystal Displays (LCDs) Innovation Timeline

• 1900-1970
• 1958, The first paper research about LCD in the
  U.S. wrote by Dr. Glenn Brown
• 1963, Richard Williams and George Heilmeier
  suggested using liquid crystal materials for
  display

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Liquid Crystal Displays (LCDs) Innovation Timeline

• 1900-1970
• 1967, James Fergason discovered the "twisted
  nematic" LCD. He produced the first practical
  displays
• 1968, RCA group had a display based on the
  dynamic scattering mode (DSM) of liquid crystals

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Liquid Crystal Displays (LCDs) Innovation Timeline

• 1970-1980
• 1972,International Liquid Crystal Company
  (ILIXCO) produced the first modern LCD watch
  using Fergason Ideal
• 1973, ,Sharp produced the first portable
  calculator, using a DSM LCD screen
• 1979, Walter Spear and Peter LeComber made the
  first color display using lightweight thin film
  transfer (TFT) LCD

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Liquid Crystal Displays (LCDs) Innovation Timeline

• 1980- 2000
• 1985, Seiko-Epson unveiled the first commercial
  LCD color TV set, which had a 2 inch view
• 1992, Sharp developed a multimedia-compatible
  16.5 inch color TFT LCD, that was the world's
  first LCD ViewCam

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Liquid Crystal Displays (LCDs) Innovation Timeline

• 2000- Present
• 2004, Philips demonstrated a 20? 3-D LCD at CeBIT
  in Hannover
• 2005, Samsung Developed World's Largest (82")
  Full HDTV TFT-LCD

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Other LCD application LCD Projector
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Digital Light Processing (DLP) Projector
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Fundamentals

• States of Matter
• What is Liquid Crystal?
• Classification

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States of Matter
Solid Liquid crystal fourth state of
matter Liquid Gas
Images MacDonald, R. Liquid Crystals -
Fascinating State of Matter or "Soft is
beautiful". Accessed 7-2006
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What is a Liquid Crystal?
Liquid Crystal a stable phase of matter
characterized by anisotropic properties without
the existence of a 3-dimensional crystal lattice
generally lying between the solid and isotropic
(liquid) phase.
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Isotropic
Liquids and gases (uniform properties in all
directions).
vs.Anisotropic
Liquid Crystals have orientational order
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Liquid Crystal Model
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Classifications

• Lyotropic, Polymeric, and Thermotropic Liquid
  Crystals
• Nematic, cholesteric, smectic, and ferroelectric.
• Small molecular, polymer

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2. Molecular structure and Chemical Composition

• A side chain R, two or more aromatic rings A and
  A, connected by linkage groups X and Y, and at
  the other end connected to a terminal group R.

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Examples
N-(4- Methoxybenzylidene)-4-butylaniline (MBBA)
molecule
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Consequence

• All the physical and optical properties of liquid
  crystals are governed by the properties of these
  constituent groups and how they are chemically
  synthesized together Dielectric constants,
  elastic constants, viscosities, absorption
  spectra, transition temperatures, existence of
  mesophases, anisotropies, and optical
  nonlinearities
• Molecules are quite large and anisotropic,
  practically impossible to treat all the possible
  variations in the molecular architecture and the
  resulting changes in the physical properties.
• Some generally applicable observations on the
  dependence of the physical properties on the
  molecular constituents.

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Chemical stability

• Depends on the central linkage group
• Schiff-base unstable
• Ester, azo, and azoxy stable, but are also quite
  susceptible to moisture, temperature change, and
  ultraviolet (UV) radiation.
• Compounds without a central linkage group are
  among the most stable liquid crystals ever
  synthesized.
• Other compounds such as pyrimide and
  phenylcyclohexane are also quite stable.

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3. Electronic Properties

• Decided by they constituent molecules
• Energy level structures rather complex
• Theories are still not sufficiently precise in
  relating the molecular structures and the liquid
  crystal responses.
• Limit here to stating some of the
  well-established results, mainly from molecular
  theory and experimental observations.

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energy levels or orbitals of aromatic rings
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Characteristic

• Very Absorptive in UV ( lt 200 nm)
• Quite transparent in Vis and IR (0.4-5 ?m)
• Far IR (gt9 ?m), very absorptive due to
  rovibrational transitions

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4. Lyotropic, Polymeric, and Thermotropic Liquid
Crystals

• Lyotropic Liquid Crystals are obtained when an
  appropriate concentration of a material is
  dissolved in some solvent Ex Soap Are of
  interest in biological studies.
• Polymeric Liquid Crystals are basically the
  polymer versions of the monomers
• Refer to the 1.3.1 and 1.3.2

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Thermotropic Liquid Crystals

• Most widely used and extensively studied Liquid
  Crystals
• There are 3 basic phases Nematics,
  Cholesterics, and Smectics

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Liquid Crystal Phases
Nematic, Smectic Cholesteric Anisotrpic
substances may go through one or several Liquid
Crystal Phases
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Nematic and Cholesteric
Namatic Cholesteric
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Smectic-A and Smectic-C
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Smectic C (ferroelectric) and unwounded Smectic
C
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4. Mixtures and Composites

• temperature ranges for pure liquid crystals are
  quite limited.
• Industrial applications employ mostly mixtures,
  composites, or doped liquid crystals with
  tailor-made physical and optical properties.
• The optical properties, dielectric anisotropies,
  and viscosities are very different from those of
  the individual mixture constituents.
• Creating mixtures is an art, guided of course by
  some scientific principles.

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Mixtures
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Dye-Doped Liquid Crystals

• Modification of their well-known linear, and more
  recently observed nonlinear, optical properties

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Polymer-Dispersed Liquid Crystals

• See textbook 1.4.3

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5. Liquid Crystal Cells and Sample Preparation

• The alignment of the liquid crystal axis in cells
  is essentially controlled by the cell walls,
  whose surfaces are treated in a variety of ways
  to achieve various director axis alignments.
• Bulk Thin Film
• Liquid Crystal Optical Slab Waveguide, Fiber, and
  Nanostructured Photonic Crystals

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Bulk Thin Film
For procedure to prepare, see 1.5.1
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• Liquid Crystal Optical Slab Waveguide, Fiber, and
  Nanostructured Photonic Crystals
• 1.5.2 for details

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Order Parameter, Phase Transition,and Free
Energies

• The contents of next chapter!