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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

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

A phase that exists between solid and liquid Discovered in 19th century when studying a cholesterol

derivative

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.

Typical Representation of a LC Molecule

Mesogens Note: these molecules possess very strong dipole moment

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

Molecules in this phase are long and rod-like in shape. They are free to move in space.

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

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

If an electric field is applied to a liquid crystal the molecules will align in the same direction as the field.

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

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