Mesophase nematic, crystallization/melting

The liquid crystal melt, which comes into being at the glass-rubber transition or at the crystal-melt transition, may have several phase states (Mesophases) one or more smectic melt phases, a nematic phase and sometimes a chiral or cholesteric phase the final phase will be the isotropic liquid phase, if no previous decomposition takes place. All mesophase transitions are thermodynamically real first order effects, in contradistinction to the glass-rubber transition. A schematic representation of some characteristic liquid crystal phase structures is shown in Fig. 6.13, where also so-called columnar phases formed from disclike molecules is given. 

Crystallization/Melting of Chain Molecules via a Nematic Mesophase. .. 131... 

Crystallization of an oriented mesophase> The crystallization process of oriented nematic melts can lead in certain cases to highly oriented crystalline fibers while, in other cases, the crystallite orientation is totally or partly disrupted by the process of crystalliza-tion% Thus, DDA-8, MAA-8 and its copolymers MAA-DDA-8 lead to strongly oriented crystalline fibers on crystallization from nematic melts oriented in a magnetic field of 12 Tesla On the other hand, DDA-9 and its copolymers are much less oriented under the same circumstances. 

There are several different phases in thermotropic liquid crystals. The structural nature of mesophases is influenced by the molecular shape and therefore depends on whether the liquid crystal is formed by rod-like or disc-like molecules. Thermotropics of rod-like molecules may be divided into two main categories nematic and smectic phases. There exist many types of smectic phases, labeled as 5, 5b, S /. When an ordered solid of a liquid crystal melts (see Fig. 1.1), it may melt into a nematic phase or a smectic A phase. Upon further heating, it eventually turns into an isotropic liquid. First, classical thermotropic liquid crystals are described, and then a group of more exotic liquid crystals like discotic thermotropics, lyotropics, and liquid crystalline polymers. 

When they are heated, mesogenic compounds do not melt directly from the highly ordered crystalline state to an isotropic liquid. They form instead, intermediate phases in which the molecules are orientated in a parallel direction and referred to as smectic (centers of the molecules organized in layers) or nematic (centers of the molecules distributed at random). Smectic and nematic mesophases are in turn divided into a variety of subgroups of thermotropic liquid crystals which will not be dealt with in detail in the present article. 

By the same procedure are obtained the corresponding biphenyl isocyanide derivatives (Figure 7.20) [18]. Now, the free isonitriles are already liquid crystals displaying nematic and SmA phases with a short range of existence at moderate temperatures (40-85 °C), while the complexes show a marked increase in the melting points and also an expansion ofthe range of existence of the mesophase (up to 140 °C N and SmA phases). The exception is the shortest iodo-derivative, which is not a mesogen. Most of the complexes decompose into the isotropic state (above 220 °C). The biphenyl moiety increases the polarizability anisotropy compared to the phenyl and hence facilitates liquid crystal behavior. 

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... 

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. 

In the following, a description of the melting processes of liquid crystals will be given, and then the general structures of the nematic, smectic A and smectic C achiral and chiral phases that are involved in TGB phenomena will be discussed. When mesophases are formed by molecules (such as 1) that have asymmetric or dissymmetric structures, a reduction in the environmental space symmetry occurs, which in some cases can induce the creation of helical... 

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