Temperature dependence thermotropic liquid crystals

A compound which displays liquid crystal properties is referred to as a mesogen and said to exhibit mesomorphism. Liquid crystals may be considered either as disordered solids or ordered liquids, and their properties are very dependent on temperature and the presence or absence of solvent. In thermotropic liquid crystals the phases of the liquid crystals may be observed to change as the temperature is increased. In lyotropic liquid crystals the ordered crystalline state is disrupted by the addition of a solvent, which is very commonly water. For these systems temperature changes may also be... 

Liquid crystals are mainly used for decorative purposes in cosmetics. Cholesteric liquid crystals are particularly suitable because of their iridescent color effects, and find applications in nail varnish, eye shadow, and lipsticks. The structure of these thermotropic liquid crystals changes as a result of body temperature, resulting in the desired color effect. In recent times, such thermotropic cholesteric liquid crystals have been included in body care cosmetics, where they are dispersed in a hydrogel. Depending whether this dispersion requires stirring or a special spraying process, the iridescent liquid crystalline particles are distributed statistically in the gel (Estee Lau-... 

As compared to the cholesteric LC, the lyotropic LC consists of two or more components that exhibit liquid-crystalline properties (dependent on concentration, temperature, and pressure). In the lyotropic phases, solvent molecules fill the space around the compounds (such as soaps) to provide fluidity to the system. In contrast to thermotropic liquid crystals, these lyotropics have another degree of freedom of concentration that enables them to induce a variety of different phases. A typical lyotropic liquid crystal is surfactant-water-long-chain alcohol. 

In this review we are mainly concerned with thermotropic materials, i.e. with liquid crystals and LC-glasses which do not contain a solvent. The transitions of the macro-molecular, thermotropic liquid crystals are governed then by temperature, pressure and deformation. In lyotropic liquid crystals and LC-glasses a solvent or dispersing agent is present in addition. The transitions then also become concentration dependent. 

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. 

We now discuss the orientational dynamics of mesogens. The orientational dynamics in the isotropic phase of thermotropic liquid crystals near the I-N transition have drawn much attention over the years [5-7, 33-40]. The focus was initially on the verification of the Landau-de Gennes (LdG) theory, which predicts a long-time exponential decay with a strongly temperature-dependent... 

Isocyanides are excellent ligands for Pd (II) and Pt(II), to the point that they can be used in the synthesis of thermotropic liquid crystals. Some [MX2 (isocyanide)2] complexes are liquid crystals (discotic) at room temperature. The thermal properties depend very much on the cis or trans geometry of the complex. For X = I, the trans geometry is favored, whereas for X = Cl, cis complexes are formed. 

Recently, there are several reportsA.) discussing the twist mode of the binary system of the thermotropic liquid crystals composed of a nematic solvent and a cholesteric solute. Another interest is in attempting to pursue the structural similarity between thermotropic liquid crystals and lyotropic one, especially for the dependence of cholesteric pitch on the temperature. 

The extrapolated line of log S-log C crossed each other at a critical concentration Cq at which S stays constant and independent of temperature. These results suggest that the temperature dependence of the cholesteric pitch would inflect at the concentration higher than Cq This is analogous to the behavior of thermotropic liquid crystals composed of cholesteric solute and nematic solvent, where the sign of dS/dT reverses at a critical concentration. It is understood that the behavior of both thermotropic and lyotropic liquid crystals is comparable provided that the nematic substances of the former are substituted with the solvents of the latter. The critical concentration Cq is about 0.41 vol/vol and this value is very close to the concentration at which the side chains on neighboring molecules of the polymer come to contact each other ( refer to fig.5 ). From these results, it is expected that the origin or mechanism of twist would change at this concentration Cq. The... 

Two general classes of liquid crystalline structures or mesophases are encountered depending on whether one is considering surfactants or other types of material. These are the thermotropic liquid crystals, in which the structure and properties are determined by the temperature of the system, and lyotropic liquid crystals, in which the structure is determined by specific interactions... 

For a thermotropic liquid crystal, its physical properties, such as birefringence, viscosity, dielectric anisotropy, and elastic constant, are all dependent on the operation temperature -except at different rates. Polymer-stabilized BPLC is no exception [45]. Figure 14.10 shows... 

In thermotropic liquid crystals, the formation of particular mesophases depends mainly on the temperature. On cooling, the structure of the mesophases becomes more and more ordered and thus less symmetric. For thermotropic mesophases formed by calamitic mesogens a fixed sequence was found [37, 38] ... 

Liquid crystals may be classified into two broad groups depending on the way in which the liquid crystal state is achieved. The materials with which most people would identify as liquid crystals by virtue of their use in the ubiquitous liquid crystal display, are known as thermotropic liquid crystals because transitions between the solid, liquid, and liquid crystal states are driven primarily using temperature as the variable. It is these materials that will be the focus of this discussion. 

Thus, all types of liquid crystals are thermodynamically stable over a definite temperature range. In depending on the conditions of LC formation, liquid crystals are divided into two groups. An LC phase formed by heating a solid or cooling an isotropic liquid is usually called thermotropic liquid crystals. ... 

Recently it has been shown that surface-induced molecular orientation can be determined by atomic force microscopy (AFM), both in the isotropic and nematic phase of thermotropic liquid crystals [6-10]. At separations of several nanometers between the homeotropically modified glass surface and the AFM tip or homeotropically modified glass microsphere, respectively, a temperature-dependent short-range attractive (prenematic) or on average repulsive, but oscillatory, (presmectic) force was observed [6,7,9]. 

The thermotropic liquid crystal, 4,4 -diheptylazoxybenzene (HAS), exhibiting isotropic, nematic and smectic phases, has been studied through e NMR. The temperature dependence of e chemical shifts and spin-lattice relaxation times of the Xe gas dissolved in HAS showed clear signatures of the phase transitions. Theoretical models have been used to understand the influence of the different phases on the isotropic and anisotropic parts of the chemical shielding. From the studies it is also inferred that in the smectic phase, Xe atoms preferentially occupy interlayer spacings rather than the interiors. Bent-core or banana-shaped molecules display an array of novel chiral liquid crystalline phases. NMR studies on two of the banana core moieties have been analyzed using ab initio structure calculations and the steric inertial frame model. ... 

Finally, it is worth mentioning that polymers such as PTS-12 (].) and 3-BCMCJ (2) can be melted without decomposition the melts show liquid crystalline behaviour which disappears upon further increase in temperature, A sharp transition to the isotropic state has not been obseirved. It is recommended to do further research on this subject which could be relevant to the general area of polymeric thermotropic liquid crystals as well as to the further discussion of the temperature dependence of chain stiffness in PDA molecules. 

For an isolated spin-1 system, it is convenient to define sum and difference magnetizations [Eqs. (2.84)-(2.85)] in the J-B experiment. The decay of the difference (quadrupolar order) proceeds exponentially at a rate T q, while the sum (Zeeman order) recovers exponentially towards equilibrium at a different rate. The J-B experiment allows simulataneous determination of these rates from which Ji uJo) and J2 2ujo) can be separated. Table 5.1 briefly summarizes thermotropic liquid crystals in which spectral density measurements were reported. Figure 5.4 illustrates the temperature and frequency dependences of spectral densities of motion (in s by including the interaction strength Kq factor) for 5CB-di5. The result is fairly typical for rod-like thermotropic liquid crystals. The spectral densities increase with decreasing temperature in the nematic phase of 5CB. The frequency dependence of Ji uJo) and J2(2a o) indicate that molecular reorientation is likely not in the fast motion regime. However, the observed temperature dependence of the relaxation rates is opposite to what is expected for simple liquids. This must be due to the anisotropic properties (e.g., viscosity) of liquid crystals. 

Thermotropic liquid crystals. Where the mesomorphic phase change depends on temperature... 

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