Liquid crystals, as their name implies, are substances that exhibit properties of both liquids and crystals. Specifically, their molecules have the high orientational order found in crystalline solids as well as the low positional order found in liquids or amorphous glasses.
Most liquid crystals are thermotropic; their degree of orientational and positional order depends on temperature and so their liquid crystalline phase occurs within a limited temperature range between the solid and liquid phase.
Liquid crystal molecules are typically ‘rod shaped’ – long and thin with a rigid centre that allows them to maintain their shape. They also have flexible ends, which means that they can still flow past each other with ease. Molecules with this shape are known as calamitic liquid crystals.
It is also possible to find liquid crystals made up of disc-shaped molecules; these are given the name discotic liquid crystals. The same rules apply here – a rigid centre is essential in order for the molecule to keep its shape and flexible edges allow ease of movement. Furthermore, polymeric liquid crystals as well as those whose behaviour depend on their concentration in solution (lyotropic liquid crystals) have also been discovered.
For the purposes of this TLP we will be concentrating on calamitic (rod-shaped) liquid crystals only, however similar principles can be applied to all types of liquid crystal mentioned above.