Thermotropic liquid crystals orientation states

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. 

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. 

PDES) is a thermotropic liquid-crystal material. It exists in the mesomorphic state for a certain temperature range after melting of the crystalline phase (i5). This property is due primarily to the side chains. This material has two crystalline forms, a and P, and these crystalline forms go through isomorphic transitions from al to a2 and from pi to P2 when the temperature increases. These transitions occur because of ethyl group reorientation (i6), which has an activation energy of 9.3 kcal/mol. The previous experimental results (16) did not identify the initial and final orientations. In this study, we identified these orientations by using conformational analysis. 

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... 

Davies, G.R. and Ward, I.M. (1988) Structure and properties of oriented thermotropic liquid crystal polymers in the solid state, in High Modulus Polymers (eds A.E. Zachariades and R.S. Porter), Marcel Dekker, New York, Chap. 2 Troughton, M.J., Davies, G.R. and Ward, I.M. (1989) Dynamic mechanical properties of random copolyesters of 4-hydroxybenzoic acid and 2-hydroxy 6-naphthoic acid. Polymer, 30, 58 Green, D.I., Unwin, A.P., Davies, G.R. and Ward, I.M. (1990) An aggregate model for random liquid crystalline copolyesters. Polymer, 31, 579. 

Reinitzer discovered liquid crystallinity in 1888 the so-called fourth state of matter.4 Liquid crystalline molecules combine the properties of mobility of liquids and orientational order of crystals. This phenomenon results from the anisotropy in the molecules from which the liquid crystals are built. Different factors may govern this anisotropy, for example, the presence of polar and apolar parts in the molecule, the fact that it contains flexible and rigid parts, or often a combination of both. Liquid crystals may be thermotropic, being a state of matter in between the solid and the liquid phase, or they may be lyotropic, that is, ordering induced by the solvent. In the latter case the solvent usually solvates a certain part of the molecule while the other part of the molecule helps induce aggregation, leading to mesoscopic assemblies. The first thermotropic mesophase discovered was a chiral nematic or cholesteric phase (N )4 named after the fact that it was observed in a cholesterol derivative. In hindsight, one can conclude that this was not the simplest mesophase possible. In fact, this mesophase is chiral, since the molecules are ordered in... 

The structures of liquid crystals are intermediate between the amorphous and crystalline states. They have some short-range orientational order. Some also have positional order. Thousands of organic compounds exhibit liquid crystal structures. Most have molecules that are very long and thin, but some have molecules that are flat and pancake shaped. Many compounds may exist in more than one liquid crystalline state. Transitions from one state to another may be thermotropic (caused by temperature change) or lyotropic (caused by change of solute concentration). 

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