Identification of liquid crystal phases

Liquid crystal phases possess characteristic textures when viewed in polarized light under a microscope. These textures, which can often be used to identify phases, result from defects in the structure. Compendia of micrographs showing typical textures exist to facilitate phase identifications (see, for example, Demus and Richter, 1978). 

As in crystals, defects in liquid crystals can be classified as point, line or wall defects. Dislocations are a feature of liquid crystal phases where there is translational order, since they are line defects in this lattice order. Unlike crystals, there is a type of line defect unique to liquid crystals, termed disclination. A disclination is a discontinuity of orientation of the director field. Point defects such as point disclinations are observed at free surfaces. 

Wall defects are found, for example, in nematics aligned by magnetic or electric fields, where walls separate domains with different orientations (in a bulk sample, the director can align parallel or perpendicular to a field, both cases being physically equivalent). 

Disclinations in the nematic phase produce the characteristic Schlieren texture (Fig. 5.10a), observed under the microscope using crossed polars for 

If the director n in the xy plane is denoted by a vector n = [cos0(r), sin0(r)], then it can be shown that 9 varies with r = (x, y) as 

---

Identification of liquid crystal phases —mesophase characterisation... 

Identification of Liquid Crystal Phases —Mesophase Characterisation 183... 

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... 

Another useful tool used in conjunction with identification of liquid crystals by microscopy is a listing of the thermodynamic ordering of mesophases. For calamitic smectic phases this ordering is ... 

The liquid ciystalline phase is a distinet phase of matter, but there are many different types of liquid ciystalline phases. The various liquid crystalline phases and other mesophases are characterised and then classified according to the molecular ordering that constitutes the phase stracture. Not surprisingly, the difference between the many different liquid ciystal phases and mesophases is generally minimal. Such minimal differences in stracture mean that the precise classification of liquid crystals often requires the use of several analytical techniques and a great deal of experience. However, in some cases, classification is relatively simple. Each individual liquid crystal phase has been characterised as a distinct phase of matter by a number of different physical techniques and new liquid crystal phases continue to be discovered as the identification techniques improve. The identification and classification of liquid ciystalline and other mesophases is of vital importance to those working in any discipline of the wide field of liquid ciystals. The techniques that are used to characterise and identify liquid crystalline phases are also very relevant to a wide range of other scientific areas. The aim of this chapter is to consider the major methods of liquid crystal phase characterisation and identification. 

Some liquid crystal phases can be identified quite simply by using just one technique. However, to be more certain of the hquid crystal phase type, several different techniques are often employed. The most widely used technique of liquid crystal phase identification is optical polarising microscopy, which reveals that each different liquid crystal phase has a distinct optical texture. However, the identification of hquid crystal phases through optical polarising microscopy is often difficult and requires a lot of experience. 

Then the substitution on the La sites led Wu et al. (8) to find another superconductor, Y-Ba-Cu-0 system, and finally to overcome a long-waited technological barrier of liquid nitrogen temperature in February 1987. Independent discoveries of the same system were also reported (9-10) in a short period. The identification of the superconducting phase as Ba2YCu30y, a new crystal structure, was performed by Siegrist et al.(ll) and further refined through neutron diffraction studies (12-13). 

Thermal polarized light microscopy of liquid crystal systems still primarily involves the identification of phase types. Recently, however, a number of novel phases with complex structures have been discovered and detailed examinations of the configurations of their defects are required in order to provide a basis for future phase classification. Thermal microscopy is also used extensively in examination of the alignment processes of liquid crystals, and, in a related context, electric-field studies on meso-phases are carried out in aligned cells. Electric-field studies are now used as adjuncts to phase classification, e.g., antiferroelectric phases are sometimes identified in the microscope with the aid of electric-field studies. 

X-ray diffractometry, polarization microscopy and mixture testingt are important in the identification of mesophases of liquid crystals while DSC and DTA are the most valuable aids in revealing and confirming phase transition. Simultaneous polarization microscopy-DSC instruments are now commercially available. 

---