The Liquid-crystalline State - Molecular Shape and Structure and Liquid Crystals - Thermotropic Liquid Crystals - Lyotropic Liquid Crystals - Plastic Crystals - Structures of Thermotropic Liquid Crystals - Structures of Smectic Liquid Crystals
Thermotropic Liquid Crystals
In the classical melting of a solid to a liquid, the organized molecular array of the solid collapses to give the disorganized liquid where the molecules tumble and rotate freely. At the melt point the molecules undergo large and rapid simultaneous changes in rotational, positional, and orientational order. However, in melting processes mediated by liquid-crystalline behavior there is a stepwise breakdown in this order. The incremental steps of this break-down occur with increasing temperature, thus producing a variety of thermodynamically stable, intermediary states between the solid and the liquid. This collection of structurally unique phases constitutes the thermotropic liquid-crystalline, mesomorphic state.
As with other states of matter, these mesophases are indefinitely stable at defined temperatures and pressures. For example, a compound such as water can exhibit three states; ice (solid), water (liquid), and steam (gas), and concomitantly a mesomorphic material exhibits four; solid, liquid-crystal, liquid and gas. Furthermore; it is also possible to observe a liquid-crystalline phase at room temperature in certain materials such as 4-n-pentyl-4'-cyanobiphenyl (5CB) which exists in a nematic phase between 22 and 35 °C.
The liquid crystal - liquid crystal transitions and the liquid crystal to liquid transition (clearing or isotropization point) of a mesogenic material are essentially reversible and occur with little hysteresis in temperature. The melting point of a material is usually a constant, but the recrystallization process can be subject to supercooling. Thus, mesophases which are formed on the heating cycle are thermodynamically stable, and are called enantiotropic phases, whereas phases that are formed below the melt point on cooling cycles, and are revealed because of the supercooling of the material, are metastable and are called monotropic phases.
Thermotropic liquid crystal phases are principally divided into three groups as follows. In disordered or anisotropic plastic crystals (soft crystals) the molecules have long range positional order, but also exhibit rapid dynamic motion. In smectic and columnar discotic mesophases the molecules do not possess long range translational order, yet they still retain layer ordering (in the case of smectics) or columnar ordering (in the case of discotics), again the molecules are in dynamic motion. In nematic phases the molecules are only orientationally ordered and exhibit rapid and diffuse molecular motion.
The Liquid-crystalline State - Molecular Shape and Structure and Liquid Crystals - Thermotropic Liquid Crystals - Lyotropic Liquid Crystals - Plastic Crystals - Structures of Thermotropic Liquid Crystals - Structures of Smectic Liquid Crystals