Liquid crystals mesogenic compounds, phase transitions

A thermotropic liquid crystal (mesogen) is a compound that, on heating the crystal or on cooling the isotropic liquid, gives rise to mesomorphism. Liquid crystallinity occurs between the crystal and isotropic liquid states. The intermediate phases, or mesophases, can be either cnantiotropic, i.e., thermodynamically stable, or monotropic, i.e., thermodynamically unstable. The solid to mesophase transition is referred to as the melting point, while the mesophase to isotropic liquid transition is referred to as the clearing point. 

The NMR technique has provided useful information regarding the orientational characteristics of nematic liquid crystals [96]. In many examples, the order parameters of the mesogenic core comprising a linear array of aromatic nuclei have been accurately determined from the observed dipolar (Dhd) and quadrupolar (Av) splittings by using deuterium-substituted samples. An attempt was made to elucidate the nematic conformation of the polymethylene-type spacer involved in dimer compounds, o, ct)-bis(4-cyanobiphenyl-4 -yloxy)alkanes (CBA- , with = 9,10), by the combined use of NMR and RIS analysis. The chemical structures and the phase transition data obtained by differential scanning calorimetry (DSC) are shown in... 

Substances that show a liquid crystalline phase, or mesophase, are called mesogens. Several thousands of compounds, both with low molecular mass and polymeric, are now known to form mesophases. They are mainly highly geometrically anisotropic in shape, rodlike or disclike (hence the terms calamitic and discotic liquid crystals), or they are anisotropic in solubility properties, like amphiphilic molecules and, depending on their detailed molecular structure, they can exhibit one or more mesophases between the crystalline solid and the isotropic liquid. Transitions to these intermediate states may be induced by purely thermal processes (thermotropic liquid crystals) or by the action of solvents (lyotropic liquid crystals). Each of these two categories can be further divided according to the structure of the mesophases and/or molecules Scheme 1 shows the classification of thermotropic mesophases. 

---