Thermotropic liquid crystals characteristics

These structures are extensively described in the current literature (Fanum, 2008 Friberg, 1976 Birdi, 2002 Holmberg, 2004 Somasundaran, 2006). Even within the same phases, their self-assembled structures are tunable by the concentration for example, in lamellar phases, the layer distances increase with the solvent volume. Lamellar structures are found in systems such as the common hand soap, which consists of ca. 0% soap + 20% water. The layers of soap molecules are separated by a region of water (including, salts etc.) as a kind of sandwich. The x-ray diffraction analysis shows this structure very clearly. Since lyotropic liquid crystals rely on a subtle balance of intermolecular interactions, it is more difficult to analyze their structures and properties than those of thermotropic liquid crystals. Similar phases and characteristics can be observed in immiscible diblock copolymers. 

Liquid crystal polymers (LCP) are polymers that exhibit liquid crystal characteristics either in solution (lyotropic liquid crystal) or in the melt (thermotropic liquid crystal) [Ballauf, 1989 Finkelmann, 1987 Morgan et al., 1987]. We need to define the liquid crystal state before proceeding. Crystalline solids have three-dimensional, long-range ordering of molecules. The molecules are said to be ordered or oriented with respect to their centers of mass and their molecular axes. The physical properties (e.g., refractive index, electrical conductivity, coefficient of thermal expansion) of a wide variety of crystalline substances vary in different directions. Such substances are referred to as anisotropic substances. Substances that have the same properties in all directions are referred to as isotropic substances. For example, liquids that possess no long-range molecular order in any dimension are described as isotropic. 

Interest in thermotropic liquid crystals has focussed mainly on macroscopic properties studies relating these properties to the microscopic molecular order are new. Lyotropic liquid crystals, e.g. lipid-water systems, however, are better known from a microscopic point of view. We detail the descriptions of chain flexibility that were obtained from recent DMR experiments on deuterated soap molecules. Models were developed, and most chain deformations appear to result from intramolecular isomeric rotations that are compatible with intermodular steric hindrance. The characteristic times of chain motions can be estimated from earlier proton resonance experiments. There is a possibility of collective motions in the bilayer. The biological relevance of these findings is considered briefly. Recent similar DMR studies of thermotropic liquid crystals also suggest some molecular flexibility. 

It is observed from experiments that HPC exhibits thermotropic cholesteric liquid crystal characteristics as well (Tsvetkov, 1989). 

Fig. 10 shows the spectra of the LC2 sample. The only spectral differences between the phases are small bands (side peaks) near 1360 and 1440 cm-l and a small change of bandwidth of some of the main bands. These are characteristics of thermotropic liquid crystals (5). 

In conclusion, this work shows that a lamellar, lilted, fluid phase exists in lyotropic liquid crystals and that it exhibits characteristic chirality effects, namely helicity and spontaneous electrical polarization, known Irom the thermotropic ferroelectric SmC phase. These results contribute significantly to a better understanding of lyotropic liquid crystals and bridge a substantial gap between the two fields of liquid crystal research. In accordance with the established nomenclature of lyotropic and thermotropic liquid crystals, the novel phase is suggested to be denoted as the lamellar L, phase, where the index a denotes a tilted fluid phase and the superscript indicates that molecules are chiral. 

The characteristic feature of thermotropic liquid crystals is the preferred orientation of molecules along a certain direction which is described by a... 

This chapter is organized as follows. The various types of liquid crystals are introduced in Section 5.2. Some important characteristics of liquid crystalline materials that result from the anisometry of liquid crystal molecules are discussed in Section 5.3. Then, in Section 5.4, the identification of liquid crystal phases is considered. Orientational order is a defining characteristic of thermotropic liquid crystals, and Section 5.5 is devoted to it. Section... 

Because of the existence of the quasi-hex-agonal lattice in the tilted hexatic phase, there are three possible molecular tilt directions, The molecular tilt points towards the nearest neighbor for the SmI phase, towards the next nearest neighbor for the SmF phase, and towards an intermediate site for the novel SmL phase (see Fig. 1). Thus the former two have a higher symmetry than the SmL phase, which was first discovered in a lyotropic liquid crystal system [87]. The most common tilted hexatic phases found in thermotropic liquid crystal compounds are SmI and SmF. As mentioned previously, the pseudo-hexagonal molecular arrangement which is the characteristic feature of the hexatic phase, was first identified for the SmI phase. 

They are thermotropic liquid crystals obtained from aromatic homo- or copolyesters, which exhibit particularly high thermomechanical characteristics while preserving excellent impact strength up to very low temperatures. Taking into account their high cost, they are utilized only for high-valued applications, particularly in electronics industry. 

An accepted experimental proof for the existence of a biaxial nematic phase in a thermotropic liquid crystal remained missing for a very long time. However, in recent years, biaxial nematic phases have been found in liquid crystalline polymers as well as in liquid crystals made of rod-disc mesogens, banana-shaped (bent-core) molecules, and organo-siloxane tetrapodes. Here, some characteristics of these systems and the corresponding experimental procedure for the investigation of phase biaxiality will be introduced. Further details for the individual systems can be found in the cited literature. 

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). 

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