Nematogenic compounds

The mixing of nematogenic compounds with chiral solutes has been shown to lead to cholesteric phases without any chemical interactions.147 Milhaud and Michels describe the interactions of multilamellar vesicles formed from dilauryl-phosphotidylcholine (DLPC) with chiral polyene antibiotics amphotericin B (amB) and nystatin (Ny).148 Even at low concentrations of antibiotic (molar ratio of DLPC to antibiotic >130) twisted ribbons are seen to form just as the CD signals start to strengthen. The results support the concept that chiral solutes can induce chiral order in these lyotropic liquid crystalline systems and are consistent with the observations for thermotropic liquid crystal systems. Clearly the lipid membrane can be chirally influenced by the addition of appropriate solutes. 

As an example of an application of the mean field method we shall consider the theory of the dielectric anisotropy of the nematic phase. The low frequency dielectric anisotropy of a molecule is determined by two factors (i) the polarizability anisotropy which for the elongated molecules of nematogenic compounds always makes a positive contribution (i.e., a... 

For the more nematogenic compound E60/40 with 10% crosslinks, no inflection point is observed in the curve of (T) (Fig. 17b). Moreover, the saturated value of ... 

For many nematogenic compounds, the twist/splay ratio is found to be quite constant (except for pretransitional effects in... 

Although 4-methoxybenzilidene-4 -butylaniline (MBBA) is the q>och-making pure nematogen and its heat capacity has been measured by adiabatic calorimetry [14,15], this compound is not so stable chemically. So as the representative heat capacity of a nematogenic compound, FIGURE 1... 

The first success was achieved when optically active (chiral) monomeric units were combined with a nematic LC polymer 105,123,143,144). The approach was based on the idea that a cholesteric mesophase may actually be realized as a helical nematic structure. Then by chemical binding of chiral and mesogenic units into a chain, accomplished by copolymerization or copolycondensation (in case of linear polymers) of nematogenic and optically active compounds, it was found feasible to twist a nematic mesophase and obtain copolymers of cholesteric type (Table 13). 

Fig. 3 Hekates with flexible spacers. Compounds 1, 2 Thermotropic properties as a function of spacer length. Compounds 3, 4 LC properties with unchanging spacer length and variable peripheral chains. Compounds 5, 6 Nematogens claimed to form biaxial nematic phases. Cr crystal, g glass, N nematic, N cholesteric, SmX unidentified smectic phase, Colh columnar hexagonal phase. All temperature are given in °C...

For a more definitive test of the theory, Irvine has determined the optical anisotropies of nematogens and analogous compounds from depolarized Rayleigh... 

The vast majority of nematogens are polar compounds but the absence of ferroelectricity in the nematic phase shows that there is equal probability of the dipoles pointing in either direction. Because of this it is generally assumed that the permanent dipolar contribution to the orientational order is negligibly small. However, a simple calculation shows that the interaction between neighbouring dipoles is by no means trivial compared with dispersion forces, particularly in strongly polar materials. We shall now consider a model which takes into account the influence of permanent dipoles and is at the same time consistent with the non-polar character of the medium. ... 

The cyclohexyldecalin material (compound 37) has a completely saturated core that may be expected to pack well in a lamellar fashion and indeed an SA phase is exhibited. However, the broad molecular structure reduces smectic phase stability and allows a large nematic range to be generated. When this broad core structure comprises saturated and aromatic regions (compound 38), the smectic phase is completely ehminated to give a nematogen of reduced phase stability. 

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