Identification of liquid crystal phases—mesophase characterisation

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. 

All liquid crystalline phases are of importance from a purely academic point of view but some hquid crystalline phases are of particular importance because of their technological applications (see Chapter 13). Accordingly, in the course of research and commercial manufacture, it is vitally important to be able to identify the types of liquid crystal phases that are exhibited by a compound or a mixture of compounds. 

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. 

Differenhal scanning calorimetry (DSC) is nearly always employed as a complementary tool to optical microscopy and reveals the presence of mesophases and hquid crystal phases by detecting the enthalpy change that is associated with a phase transition. However, this technique cannot identify the type of hquid crystal phase, but the level of enthalpy change does give some informahon about the degree of molecular ordering within a mesophase. 

The ultimate technique for the identification and classiflcahon of mesophases is X-ray analysis. X-ray analysis of a hquid crystal will map the positions of the molecules within the phase and hence determine the phase stracture and classiflcahon to which the particular phase belongs. However, to maximise informahon, ahgned samples are needed. 

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Identification of liquid crystal phases —mesophase characterisation... 

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

Liquid crystal polymers exhibit the same liquid crystalline phases and mesophases exhibited by low molar mass mesogens. However, the identification of the mesophases generated by polymers is usually far more difficult than for low molar mass materials (see Chapter 9). Usually, the nematic phase is readily characterised but smectic phases, especially the highly ordered analogues, are often uncharacterised and simply denoted S Many liquid crystal polymers, like conventional polymers, exhibit a glass... 

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