Thermotropic liquid crystals formation

Due to the wide spectrum of behavior exhibited by this class of compounds, the discussion is broken up into the major areas of surfactant behavior, namely, thermotropic liquid crystal formation, microemulsions, lyotropic liquid crystalline behavior, reverse micellar systems, and dilute solution. This spectrum of available modes of self-assembly facilitates their use in an increasing range of applications. 

Equation 27 is also known to reliably predict Vf for polymers that are not true rods if, as Flory [193] suggested, the aspect ratio of the Kuhn segment is utilized [175, 194, 195]. Furthermore, Eq. 27 provides some insight into the minimum aspect ratio for thermotropic liquid crystal formation. Most of the polymers of Table 3 are known to form liquid crystalline mesophases under appropriate conditions. 

The prime requirement for the formation of a thermotropic liquid crystal is an anisotropy in the molecular shape. It is to be expected, therefore, that disc-like molecules as well as rod-like molecules should exhibit liquid crystal behaviour. Indeed this possibility was appreciated many years ago by Vorlander [56] although it was not until relatively recently that the first examples of discotic liquid crystals were reported by Chandrasekhar et al. [57]. It is now recognised that discotic molecules can form a variety of columnar mesophases as well as nematic and chiral nematic phases [58]. 

When materials susceptible to liquid crystal formation are brought into the proper regime either thermotropically or lyotropically, they will spontaneously form into their characteristic liquid crystalline phase. Blumstein discusses this capability with regard to the formation of microtubules of hemoglobin105. 

One of the most classic examples of chiral expression in thermotropic liquid crystals is that of the stereospecific formation of helical fibres by di-astereomers of tartaric acid derivatised either with uracil or 2,6-diacylamino pyridine (Fig. 9) [88]. Upon mixing the complementary components, which are not liquid crystals in their pure state, mesophases form which exist over very broad temperature ranges, whose magnitude depend on whether the tartaric acid core is either d, l or meso [89]. Electron microscopy studies of samples deposited from chloroform solutions showed that aggregates formed by combination of the meso compounds gave no discernable texture, while those formed by combinations of the d or l components produced fibres of a determined handedness [90]. The observation of these fibres and their dimensions makes it possible that the structural hypothesis drawn schematically in Fig. 9 is valid. This example shows elegantly the transfer of chirality from the molecular to the supramolecular level in the nanometer to micrometer regime. 

Both thermotropic and lyotropic liquid crystal polymers exhibit characteristic features with regard to their microstructureJ Anisometrical monomers such as rods or discs are connected to chains in an appropiate manner. These anisometrical monomers are considered to be the mesogens and may be part of main chain LCP, side chain LCP, or of both types together (Fig. 6). Between the mesogens are located flexible spacers of non-mesogenic character. Sufficient flexibility is a prerequisite for liquid crystal formation, with an increase in either temperature or solvent concentration. 

A second category of liquid crystals is the type produced when certain substances, notably the esters of cholesterol, are heated. These systems are referred to as thermotropic liquid crystals and, although not formed by surfactants, their properties will be described here for purposes of comparison. The formation of a cloudy liquid when cholesteryl benzoate is heated to temperatures between 145 and 179°C was first noted in 1888 by the Austrian botanist Reinitzer. The name liquid crystal was applied to this cloudy intermediate phase because of the presence of areas with crystal-like molecular stmcture within this solution. 

The interest of 2,6-dialkylnaphthalenes as starting materials in the production of polyester fibers and plastics with superior properties [1, 2] and of thermotropic liquid crystal polymers [3] is clearly shown by the increasing number of recent patents relevant to their preparation and separation[3.5]. However, the selective formation of 2,6-dialkylnaphthalenes is not obvious, not only with conventional Friedel-Crafts catalysts [6-8], but also over solid catalysts such as silica-alumina [9-11] or zeolites. 

Chemical compounds are said to act am-photropically if they give rise to both kinds of liquid crystal formation. Such particular compounds show thermotropic liquid crystalline behavior in their pure state on heating or the formation of lyotropic mesophas-es on the addition of a further component, mostly of an inorganic or organic solvent in certain amounts. 

J. Y. Kim and S. H. Kim. In situ fibril formation of thermotropic liquid crystal polymer in polyesters blends. J. Polym. ScL, Part B Polym. Phys., 43 3600-3610, 2005. 

The section commences with monosubstituted ferrocene derivatives, after which the influence of the substitution pattern on the formation and stability of liquid-crystalline phases for disub-stituted ferrocene derivatives will be discussed. Next, the influence of the three-dimensional structure of ferrocene on the mesomorphic properties will be highlighted, optically active ferrocene materials will be described and, finally, ferrocene-containing liquid-crystalline dendrimers will be introduced ferrocene-containing liquid-crystalline polymers will not be reported. Note that a review devoted to ferrocene-containing thermotropic liquid crystals has already been published. ... 

Thermotropic polymer n. Polymer that exhibits hquid crystal formation in melt form. In thermotropic polymers there must be a balance between having the necessary degree of molecular perfection to preserve the liquid crystal formation and the amount of imperfection to permit melting at workable temperatures. These polymers give high-modulus, highly oriented, extrusion products. 

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