Classification and Examples of Liquid Crystals
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Classification
Liquid crystals can be first organized into thermotropic (temperature dependent) and lyotropic (concentration dependent). Within thermotropic liquid crystals there are high molar mass materials (suitable for polymer) and low molar mass materials. Within the polymeric kind there are main chain polymers where the chains align in a direction, or a chain where there is comb-like structure with side chains that hang off the polymer. The director is oriented either along the main chain or parallel to the side chains.
In the low molar mass materials there are disc like molecules (discotic) and rod like molecules (calamitic liquid crystals). Within the rod-like LC are nematic (N) with ordinary Nematics or Twisted Nematics in which the director changes across the material, and smectic (S) (also known as bookshelf liquid crystals) which tend to line up perpendicularly (Sa) or tilted on the long axis (Sc)
Calamitic vs Discotic
Remember molecules are not liquid crystals--phases are!
The cyanobiphenyls are used routinely in displays. These calamitic liquid crystals have a rigid polarizable core and have a flexible aliphatic tail on one of the sides. The cores makes them want to line up and be more crystalline, while the flexible tail lowers the degree of order so they can flow. The may have polar groups such the cyano group along the main axis, a fluorine group off the main axis, or a dipole moment due to nitrogens in one of the rings along the axis. This will be important when we attempt to orient the materials in an applied field.
The discotics such as triphenylenes can stack up but have long tails (akyl or alkoxy with C12 groups) that allow the discs to remain flexible so they do not become fully crystalline but has a discotic liquid crystal phase.
Classification of Liquid Crystals According to Degree of Order
Nematics (from the Greek: nematon- thread-like) are uniaxial liquid crystals in which the average direction of the long axes of the molecules defines the direction N.
Nematic liquid crystals have one axis in which the polarizability, and the dielectric constant and the index of refraction are different from the other two axes (the illustration is exaggerated for how aligned the molecules would be) Note the lack of positional order along the axis of the director. The spacing between molecules in all directions is not uniform. They have characteristic textures when viewed under cross polarized light microscope. These materials are birefringent and allowing light with different polarizations to go through at different speeds.
Two examples are cyanobiphenyls and another with rigid and mantle group in the system.
It is important to be able to control the temperature range in which liquid crystals operate. Most liquid displays are not made up out of single molecules. It is made up with a combination of molecules that gives the desirable transition temperatures on the low end and high end of operating temperatures.
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