Isotropic liquid state

The introduction of a second chiral atom in the system leads to a reduction in the mesogenic properties and only a monotropic chiral nematic transition is observed for compound 23. However, when this compound is cooled down from the isotropic liquid state at a cooling rate of 0.5 °Cmin , very unusual blue phases BP-III, BL-II and BP-I are observed in the range 103-88 °C. Blue phases usually require pitch values below 500 nm. Hence the pitch value of the cholesteric phase for 23 must be very short, suggesting that the packing of two chiral carbons forces a faster helical shift for successive molecules packed along the perpendicular to the director. 

As indicated above in chiral mesophases, the introduction of a functional group in mesogenic stmctures offers the opportunity to achieve functional LCs. With this aim, mesomorphic crown-ether-isocyanide-gold(I) complexes (26) have been prepared recently [38]. The derivatives with one alkoxy chain show monotropic SmC mesophases at or close to room temperature. In contrast, the complexes with three alkoxy chains behave as monotropic (n = 4) or enantiotropic (n > 4) LCs. The structure of the mesophases could not be fully eluddated because X-ray diffraction studies in the mesophase were unsuccessful and mesophase characterization was made only on the basis of polarized optical microscopy. These complexes are luminescent not only in the solid state and in solution, but also in the mesophase and in the isotropic liquid state at moderate temperatures. The emission spectra of 26a with n=12 were... 

Besides this, the remarkable properties of gold(I) compounds, which often give rise to aurophilic interactions and/or to luminescence, are of interest when these properties are transported into the liquid crystal field. Although there is much still to be studied, it is already clear that luminescence can survive in the condensed but mobile state of a mesophase, and even in the isotropic liquid state of a molten gold compound. It also seems that aurophilicity can contribute in some cases to the formation of mesophases. 

Arias, J., Bardaji, M. and Espinet, P. (2008) Luminescence and Mesogenic Properties in Crown-Ether-Isocyanide or Carbene Gold(I) Complexes Luminescence in Solution, in the Solid, in the Mesophase, and in the Isotropic Liquid State. Inorganic Chemistry, 47, 3559-3567. 

Note 2 A LC state occurs between the crystalline solid and the isotropic liquid states on varying, for example, the temperature. 

Fig. 1 Organization of rod-like molecules (top) and disc-like molecules (bottom) in LC phases (for clarity the alkyl chains are not shown in the models of the phase structures). Abbreviations Iso isotropic liquid state N nematic LC phase SmA smectic A phase, SmC smectic C phase (tilted), Col columnar phase [8]...

The nematic phase (N) is the least ordered, and hence the most fluid liquid crystal phase. The order in this type of LC phases is based on a rigid and anisometric (in most cases rod-shaped or disc-shaped) molecular architecture. Such molecules tend to minimize the excluded volume between them, and this leads to long range orientational order. For rod-like molecules the ratio between molecular length and its broadness determines the stability of the nematic phase with respect to the isotropic liquid state and the stability rises with increase of this ratio. In most cases the rigid cores are combined with flexible chains, typically alkyl chains, which hinder crystallization and in this way retain fluidity despite of the onset of order. 

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... 

A relatively recent field in polymer science and technology is that of the polymeric liquid crystals. Low molecular liquid crystals have been known for a long time already they were discovered almost simultaneously by Reinitzer (1888) and Lehmann (1889). These molecules melt in steps, the so-called mescrphases (phases between the solid crystalline and the isotropic liquid states). All these molecules possess rigid molecular segments, the "mesogenic" groups, which is the reason that these molecules may show spontaneous orientation. Thus the melt shows a pronounced anisotropy and one or more thermodynamic phase transitions of the first order. 

Figure 2.2 Schematic representation of the structures of a solid, a smectic phase (SmA), the nematic phase and the isotropic liquid state formed by calamitic organic molecules with a large length-to-breadth ratio.

A thermotropic liquid crystal (mcsogen) is a compound that, on heating the crystal or on cooling the isotropic liquid, gives rise to mesomorphism. Liquid crystallinity occurs between the crystal and isotropic liquid states. The intermediate phases, or mesophases, can be either enantiotropic, i.e., thermodynamically stable, or monotropic, i.e., thermodynamically unstable. The solid to mesophase transition is referred to as the melting point, while the mesophase to isotropic liquid transition is referred to as the clearing point. 

Liquid crystals are classified into two groups known as thermotropics and lyotropics. Thermotropics are those that are formed in the melting of crystalline solids, and they can remain in the liquid crystal mesophase without decomposition, passing to the isotropic liquid state when subsequently heated. As their mesophases are turbid, the temperature at which the transition to the isotropic liquid phase takes place is called the clearing temperature. Lyotropic LCs form mesophases in concentrated solution when the concentration exceeds a critical value. 

Figure la. Schematic plot of free energies vs. temperature for a scheme that does not show a mesophase. G., G and G, are, respectively, the free energies of the crystalline, mesomorphic (virtual) and Isotropic liquid states. T.. = T Is the crystalline melting point. Here, as In subsequent Figures lb and lc, the heaviest lines correspond to the stablest state at a given temperature. 

Fig. 14 Hexagonal close-packed liquid crystalline structure of the object and its transformation in to an isotropic liquid state...

The liquid crystal state is a kind of state whose order is between the crystal solid and isotropic liquid states. In the crystal state, there is a long range order in position and orientation, while in the liquid state there are no long range ordering in either of them. Figure f. 1 is a comparison of the crystal (a), the liquid crystal (b) and the liquid (c) states. 

As early as 1850, a German scientist discovered, in fact, the possibility of the liquid crystal when he found that natural fats exhibited two melting points. With increasing temperature, the substance starts to become cloudy at around 52 °C and is completely opaque at 58 °C, and then becomes clear at 62.5 °C. This observation should be the earliest record of the discovery of the liquid crystal. At around the same time, the cholesterol derivative exhibited a striking color when it cooled from the isotropic liquid state. Unfortunately, these phenomena were not given enough attention or correctly explained. The researchers themselves did not realize that they were approaching the entrance of a new field of science. Therefore, the discovery of the liquid crystal should be attributed to Reinitzer and Lehmann. 

The nature of the two endothermal processes has been revealed by study with POM that at 434 °K is the melting of the crystals and the formation of a nematic phase, while at 467 °K is the isotropization of the mesophase. If the polymer is cooled down from its isotropic liquid state the curve (B) is obtained. In the cooling process the first exothermal peak occurs at 463 °K which is the formation of the nematic phase as revealed by POM. The second exothermal peak is centered at 387 °K corresponding to the crystallization of nematic polymer. The jump in (B) of the glass transition is not as clear as in the heating curve. This is understandable because the glass transition is not a genuine thermodynamic transition. 

K (180 C) in Fig. 18-4 the transition for the system lithium stearate-white oil is at 463 K (190 C). The sharp rise in the bleeding of oil from the lithium 12-hydroxystearate grease is due to transformations to the liquid crystal and the isotropic liquid states. 

With respect to the interfacial structure of ionic liquids with solids, the molecular layering and local order in thin liquid crystalline films of [RMIM][PFis] ionic liquids with long alkyl chains (e.g., R = dodecyl and octadecyl) on solid silicon supports have been studied using X-ray reflectivity [45]. Here, thin films of the crystalline ionic liquids with a thickness of 100-210 A were deposited on polished and cleaned silicon wafers by initial spin-coating of a solution of the ionic liquids in methanol followed by heating the samples into the isotropic liquid state and crystallization. 

In the case of thermotropic liquid crystals, a liquid-crystalline phase is obtained by heating a solid mesomorphic compound. At the melting point (Tm), the thermal motion of the molecules has increased to such an extent that the material passes from the crystalline phase to the liquid-crystalline phase. A mesomorphic compound that exists in the glass state will enter the liquid-crystalline phase at the glass-transition temperature (Tg). On further heating, the orientational order of the molecules is lost as well. The LC transforms into an isotropic, clear liquid at the clearing point (or isotropization point T. Many materials are liquid-crystalline at room temperature. Several types of liquid-crystalline phases can occur between the solid state and the isotropic liquid state. Sometimes decomposition of the material occurs before the... 

The ideal behaviour assumed in calculating the enthalpy of transition is rarely observed in gel systems. Neither the gel nor the sol state are equilibrium states and therefore d(AfZ)/dT cannot be directly correlated with AC for the transition. The sol state is not an isotropic liquid state, particularly in the... 

Fig. 14 Polarized optical photomicrographs of a mixture of Cy(AzCN)2 (3wt%) and 5CB and schematic illustrations of their structures (a) isotropic liquid state at 120 C (b) nematic gel state at room temperature before UV irradiation (c) chiral nematic LC phase (LC sol state) at room temperature after UV irradiation of the nematic gel (d) chiral nematic gel state at room temperature after maintaining a chiral nematic phase...

In the highest-temperature (cubic) polymorph, both cations and anions statistically rotate and have quasi-spherical symmetry. Increases of the crystal symmetry before fusion can be seen as a preparation of anisotropic crystals to the transition into isotropic liquid state. 

The variety of phases that may be exhibited by rod-like molecules are shown in Figure 1.2. At high temperature, the molecules are in the isotropic liquid state where they do not have either positional or orientational order. The molecules can easily move around, and the material can flow like water. The translational viscosity is comparable to that of water. Both the long and short axes of the molecules can point in any direction. 

When the temperature is decreased, the material transforms into the nematic phase, which is the most common and simplest liquid crystal phase, where the molecules have orientational order but still no positional order. The molecules can still diffuse around, and the translational viscosity does not change much from that of the isotropic liquid state. The long axis of the molecules has a preferred direction. Although the molecules still swivel due to thermal motion, the time-averaged direction of the long axis of a molecule is well defined and is the same for all the molecules at macroscopic scale. The average direction of the long molecular axis is denoted by n which is a unit vector called the hquid crystal director. The short axes of the molecules have no orientational order in a uniaxial nematic liquid crystal. 

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