Nematic phase stability

Liquid crystals based on aliphatic isocyanides and aromatic alkynyls (compounds 16) show enantiotropic nematic phases between 110 and 160 °C. Important reductions in the transition temperatures, mainly in clearing points (<100 °C), areobtained when a branched octyl isocyanide is used. The nematic phase stability is also reduced and the complexes are thermally more stable than derivatives of aliphatic alkynes. Other structural variations such as the introduction of a lateral chlorine atom on one ring of the phenyl benzoate moiety or the use of a branched terminal alkyl chain produce a decrease of the transition temperatures enhancing the formation of enantiotropic nematic phases without decomposition. 

The nematic phase stability is higher the greater the length and length diameter ratio of the rigid-rod unit, and the shorter the flexible spacer unit, and rises ultimately to values above the temperature of decomposition of the polymer. 

The mesomorphic properties and physical properties of nematic (and smectic) materials and nltimately their suitability for applications are all fundamentally dictated by the chemical stracture of the constituent molecules. Before progressing further, several terms and their definitions need to be clarified this will be done by using the nematic phase. The term nematic phase stability refers to the upper-temperature limit (T j) to... 

Accordingly, it was a long held theory that the nematic phase, for example, was generated because of the anisotropy of the polarisabihty restrlting from the conju ted core unit and that the higher the polarisability anisotropy the higher the nematic phase stability (Tj. j)... 

Figures 3.6, 3.7 and 3.8 concisely illustrate the effect of core changes upon the nematic phase stability (Tj p. These three figures show the transition temperatures of a wide...

Despite the high melting point, compound 13 finds use in commercial nematic mixtures as a component that extends nematic phase stability. The 2,6-disubstituted naphthalene unit has been used in the generation of the nematic phase. The use of the naphthalene unit in compounds 11 and 12 extends the molecular core length but also increases the breadth by conferring a stepped stmcture, however, the overall shape is still linear. The T j values of the phenylnaphthalene materials are intermediate in value between those of small biphenyl unit and the large terphenyl material. The linearly-extended and compact polarisability conferred by the naphthalene unit enhances the Tj j, the value of which is, to some extent, reduced by the increased molecular breadth. Where the terminal cyano substituent is attached to the compact, highly polarisable naphthalene unit (12), the T j... 

In principle, bonding in thermotropic systems such as the compound in Fig. 20B and the polymer in Fig. 2A should be enhanced by the formation of the nematic phase, just as in the case of the lyotropic systems discussed below. Bla-don and Griffin [7] have indeed theoretically shown that growth is expected for a wormlike chain forming a nematic phase stabilized by soft interactions of the Maier-Saupe type. The modest effect expected would not be easily revealed by... 

Esters are very common liquid crystal compounds, and lateral fluoro substitution has provided some interesting modifications to melting points, transition temperatures, mesophase morpholo and physical properties. Compounds such as 66 [26] allow for the gen ation of high positive dielectric anisotropy, but the fluoro substituent has caused a large reduction in the nematic phase stability when compared with compound 20, due to the reduction in antiparallel correlations. However, the fluoro substituent in compound 67 [26] is somewhat shielded by the zig-zag nature of the ester linkage hence the nematic-isotropic transition temperature is identical to that of compound 20. The lateral fluoro substituent in compound 68 [51] is not as shielded by the zig-zag structure and the nematic-isotropic transition temperature is much reduced, but not to the same extent as for compound 66. 

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