Mesogenic molecules

The type of phase formed in a polymer liquid crystal can also be identified often by examining the manner in which it mixes with a small molecule mesogen of known mesophase type. If these textures are the same, then a mixed liquid crystal phase is formed with no observable transition between the two types of molecule. 

A Selection of Small Molecule Mesogens and the Associated Liquid Crystalline Behavior... 

Small molecule liquid crystals suffer a lowering of the mesophase-isotropic transition temperature upon lateral substitution, i,e. replacement of hydrogen on an aromatic ring. As can be seen from the compounds below the same trend is observed for both polymer and the chemically analogous small molecule. It is known that for small molecule mesogens the transition temperature, governed by the relation Tni - A H/AS, decreases upon lateral substitution primarily due to an entropic effect. The... 

FIGURE 2 Examples of small-molecule mesogens that form nematic and smectic phases. 

Transitions involving disk-like molecules, mesogenic polymers or lyotropic micelles have certainly not received such a great deal of attention and still constitute an active field of research. 

The second generic class of hydrogen-bonded, liquid-crystalline polymers is side-chain polymers. The general design of these structures entails incorporation of hydrogen-bonding sites into the sidechains of covalent polymers and is based upon the desire to enhance the mesomorphicity of small molecule mesogens... 

Molecules tliat are capable of fonning liquid crystal phases are called mesogens and have properties tliat are mesogenic. From the same root, tire tenn mesophase can be used instead of liquid crystal phase. A substance in a liquid crystal phase is tenned a liquid crystal. These conventions follow tliose in tire Handbook of Liquid Crystals, [4, 5 and 6] tire nomenclature of which [7] for various liquid crystal phases is adopted elsewhere in tliis section. 

Thennotropic liquid crystal phases are fonned by anisotropic molecules witli long-range orientational order and in many types of stmcture witli some degree of translational order. The main types of mesogen are Arose tlrat are rodlike or calamitic and Arose Arat are disclike or discotic. 

It is, however, important to note tliat individual columns are one-dimensional stacks of molecules and long-range positional order is not possible in a one-dimensional system, due to tlieniial fluctuations and, therefore, a sliarji distinction between colj. and colj. g is not possible [20]. Phases where tlie columns have a rectangular (col. ) or oblique packing (col j of columns witli a disordered stacking of mesogens have also been observed [9, 20, 25,... 

Finkelmann et al. 256 274,2781 have also investigated the synthesis and the characteristics of siloxane based, crosslinked, liquid crystalline polymers. This new type of materials displays both liquid crystallinity and rubber elasticity. The synthesis of these networks is achieved by the hydrosilation of dimethylsiloxane-(hydrogen, methyl)siloxane copolymers and vinyl terminated mesogenic molecules in the presence of low molecular weight a,co-vinyl terminated dimethylsiloxane crosslinking agents156 ... 

Small chemical changes to the tolane family of mesogenic molecules are also known to bring about major changes in phase behaviour [22]. Two examples are shown in Fig. 5 where subtle changes in the tail can eliminate the nematic phase. 

---