Liquid crystals
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Transcript of Liquid crystals
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1888-1899◦1888, Austrian
Botanist Freidrich Reinitzer discovers liquid crystals.
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Milky liquid at 145oC
Clear liquid at 179 ° C
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Solid
Liquid crystal “fourth state of matter”
Liquid
Gas
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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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vs.Anisotropic
Liquids and gases(uniform properties in all
directions).
Liquid Crystals have orientational order
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A fluid phase in which a liquid crystal flows and will take the shape of its container. It differs from liquid that there are still some orientation order possessed by the molecules
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A phase that exists between solid and liquid Discovered in 19th century when studying a cholesterol
derivative
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Assuming that the direction of preferred orientation in a liquid crystal (LC) is , this direction can be represented by an arrow, called the director of the LC.
Each molecule is orientated at some angle to the director.
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Typical Representation of a LC Molecule
Mesogens Note: these molecules possess very strong dipole moment
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The liquid crystal molecules prefer to align parallel to each other because of the strong intermolecular attraction (- interaction)
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Nematic, Smectic & Cholesteric
Anisotropic substances may go through one or several Liquid Crystal Phases
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Molecules in this phase are long and rod-like in shape. They are free to move in space.
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This phase can be reached at lower temperatures than the nematic phase.
Molecules align themselves in layers.(They are restricted to their plane.)
More order and higher viscosity
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Molecules with intermolecular forces that favor alignment between molecules at a slight angle to one another
The director is not fixed in space as in a nematic phase, it rotates throughout the sample
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If an electric field is applied to a liquid crystal the molecules will align in the same direction as the field.
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Criteria for a Molecule being Liquid Crystalline
• The molecule must be elongated in shape-length should be significantly greater than its width
• Molecule must have some rigidity in its central region
• The ends of the molecule are somewhat flexible
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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’.23/5/1
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N-(4- Methoxybenzylidene)-4-butylaniline (MBBA) molecule
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Temperature sensors Medical use Electrical devices Solar cells Chromatographic separations Displays Liquid Crystal Thermometers Optical Imaging