Clearing point, liquid crystals

If the temperature continues to increase, the molecular structure of the helix disbands and the molecules are uniformly distributed like in an isotropic liquid. Above this so-called clearing point, the crystals are optical inactive again. Exceeding this temperature might lead to a permanent damage of the TLCs, depending on time and extent of the overheating. 

Most solid materials produce isotropic liquids directly upon melting. However, in some cases one or more intermediate phases are formed (called mesophases), where the material retains some ordered structure but already shows the mobility characteristic of a liquid. These materials are liquid crystal (LCs)(or mesogens) of the thermotropic type, and can display several transitions between phases at different temperatures crystal-crystal transition (between solid phases), melting point (solid to first mesophase transition), mesophase-mesophase transition (when several mesophases exist), and clearing point (last mesophase to isotropic liquid transition) [1]. Often the transitions are observed both upon heating and on cooling (enantiotropic transitions), but sometimes they appear only upon cooling (monotropic transitions). 

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. 

Examination of the thermal behaviour showed that with three exceptions, all complexes showed a monotropic SmA phase with in almost all cases, melting being observed between 88 and 99 °C, with clearing between 82 and 89 °C. Of the three exceptions, 15-6,8 and 15-8,10 showed no liquid crystal phase at all, while 15-12,6 showed an additional monotropic nematic phase. A curious feature of these complexes is the apparent insensitivity of the melting and clearing points to both n and m. 

C (point a), a single phase between 28°C and 55°C (points b and c) and again two phases above 55 C (point d). At high X gp the situation becomes very complex. Only one case is shown for = 0.70. At Xj gp 0.755 a clear solution is observed below 20 C (point e , a cloudy mixture appears above 20 C but does not unmix (point g), and two distinct phases are present above 35 C (point k). We are possibly in a region where liquid crystals or inverse micelles are formed. 

Furancarboxylic acid (6, 44) is most satisfactorily purified by recrystallizing from carbon tetrachloride. A few cc. of water should be added to coagulate and float the dark impurities. The clear subjacent liquid is separated and deposits colorless crystals of correct melting point. 

Depending on temperature, transitions between distinct types of LC phases can occur.3 All transitions between various liquid crystal phases with 0D, ID, or 2D periodicity (nematic, smectic, and columnar phases) and between these liquid crystal phases and the isotropic liquid state are reversible with nearly no hysteresis. However, due to the kinetic nature of crystallization, strong hysteresis can occur for the transition to solid crystalline phases (overcooling), which allows liquid crystal phases to be observed below the melting point, and these phases are termed monotropic (monotropic phases are shown in parenthesis). Some overcooling could also be found for mesophases with 3D order, namely cubic phases. The order-disorder transition from the liquid crystalline phases to the isotropic liquid state (assigned as clearing temperature) is used as a measure of the stability of the LC phase considered.4... 

After DMAP-catalyzed esterification of 42a with tri-benzoate esters, liquid crystals 55, 56 (Scheme 32) were obtained, which both displayed narrow nematic phases as identified by their typical Schlieren texture. The melting and clearing points are high and, in the case of 55, accompanied by decomposition. Compounds with shorter arms were also synthesized but found to be crystalline. 

Figure 2.8 The dependence of the refractive indices, n and ng, of the ordinary and extraordinary rays, respectively, on the temperature, T.for a typical nematic liquid crystal. Above the clearing point, Tg, there is no birefringence and only one refractive index,is observed. ...

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