Liquid-crystalline phases, classification

Over the past two decades, liquid crystal polymers (LCP s) have received a considerable amount of attention in both academic and industrial laboratories. Often termed mesomorphic (meaning having "middle form"), liquid crystalline phases have a degree of order between that of the zero ordered liquid and that of the three dimensional crystal lattice. Recent reviews of liquid crystal polymers have provided a fundamental understanding of the synthesis, classification, morphology, and rheology of this unique class of materials (52-541. 

Two other structures, which together with the lamellar phase are the most important liquid-crystalline phases in lipid-water systems, are shown in Fig. 8.9. It should be pointed out that the classification of lipids into polar and non-polar is best defined from their interaction with water. Lipids which do not give lipid-water phases are thus non-polar whereas those forming aqueous phases are classified as polar lipids. 

Since the first discovery of the liquid crystalline phase over one hundred years ago, the classification of the distinct liquid crystalline phases in small-molecule liquid crystals has been well established (7,2). As shown in Figure 1, the least ordered liquid crystalline phase is the nematic phase that only possesses molecular orientational order due to the anisotropy of the molecular geometric shape. The next ordering level introduced is the layer structure in addition to the molecular orientation to lorm a smectic A (S/J or a smectic C (Sc) phase. Following the phase the hexatic B (Ho), smectic crystm B (So) and smectic crystal E (S ) phases are observed. In this series the long axis of the molecules is oriented perpendicular to the layer surface while order is increasingly developed from positional order normal to the layer in bond... 

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. 

Identification and systematic classification of scientific phenomena is vital in any area of research. Liquid crystals are no exception and many different liquid crystalline phases and other mesophases have been identified and classified according to their distinct phase... 

Liquid crystal materials can be grouped into two main classifications, thermotropics and lyotropics. A thermotropic phase is one that can form by heating or cooling a material. Just as we see a phase transition between solid and liquid as we heat and melt ice, in thermotropic liquid crystals, additional melting points can be observed in between the solid and the liquid phases. These are the thermotropic liquid crystalline phases. A lyotropic liquid crystal phase is formed by molecules dissolved in a solvent, and phases form at certain concentrations in that solvent. In this chapter, we focus on descriptions of the thermotropic liquid crystal phases. Lyotropics, although also liquid crystals, are described in detail in Chapter 3 on surfactants. 

Liquid crystals can be divided into two main classes those forming liquid crystalline phases in the melt are called thermotropic and those forming liquid crystalline phase in solution are referred to as lyotropic. Depending on the type of molecular order in the mesophase these classification can be broken down further into three main categories smectic, nematic, and cholesteric [4—7]. 

Substances that show a liquid crystalline phase, or mesophase, are called mesogens. Several thousands of compounds, both with low molecular mass and polymeric, are now known to form mesophases. They are mainly highly geometrically anisotropic in shape, rodlike or disclike (hence the terms calamitic and discotic liquid crystals), or they are anisotropic in solubility properties, like amphiphilic molecules and, depending on their detailed molecular structure, they can exhibit one or more mesophases between the crystalline solid and the isotropic liquid. Transitions to these intermediate states may be induced by purely thermal processes (thermotropic liquid crystals) or by the action of solvents (lyotropic liquid crystals). Each of these two categories can be further divided according to the structure of the mesophases and/or molecules Scheme 1 shows the classification of thermotropic mesophases. 

Friedel proposed the first classification scheme of LCPs (Felter and Dourson 1997) dividing them into three different t)q)es of mesogens (materials able to sustain mesophases), based upon the level of order the molecules possessed in the bulk material. Three major mesophases have been identified to describe the liquid crystalline phase nematic, cholestoric and smectic (Singh 2000). In nematic mesophase the molecules have no positional order but have a long range orientational order. But in cholestoric phase, molecules possess different positional... 

Fig. 3.2 Ternary phase diagram (schematic) of water-oil-surfactant mixtures showing Winsor classification and probable internal structures Lj, one-phase region of normal micelles or oil-in-water (o/w) microemulsion L2. reverse micelles or water-in-oil (w/o) microemulsions D, anisotropic lamellar liquid crystalline phase. Other symbols fie microemulsion O oil W water...

After perusing the classification in the previous section, one can ask It looks fine, but why do we need such a classification Actually, I started the development of this classification at about the same time I started to work on PLCs, simply so as not to get lost in all these structures. Analyzing the structures, I discovered another good reason molecular structures of PLCs determine their phase structures and properties. This important fact is also illustrated throughout this book hence there are separate chapters on longitudinal and on comb PLCs. Other authors have reached similar conclusions from their particular results. For instance, Ebert et aO say that In most liquid-crystalline systems it is predominantly the molecular shape which determines which kind of liquid-crystalline phase is formed. ... 

Differential scanning calorimetry (DSC) is a common technique for the classification of individual phase transitions in liquid-crystalline materials and has been applied for the phase characterization of alkyl-modified chromatographic surfaces. Hansen and Callis [187] applied DSC to investigate phase changes in Cig and C22... 

Fig. 1. Classification of H-bonded liquid-crystalline materials exhibiting homogeneous phases Type A-I association of identical molecules Type A-II association of different molecules...

Typical phase morphologies of block copolymers are illustrated in Figs. 5.38 0 and 5.79. In the classification of phases, the phase-separated block copolymers are considered to be amphiphilic liquid crystals despite the fact that inside the phase areas the typical liquid-crystalline order is missing (see Sect. 5.5). In this section the question will be addressed what happens when the usually micro- or nano-phase separated block copolymers show solubility, i.e., when the amphiphilic liquid crystal becomes thermotropic, i.e., dissolve at a given temperature. 

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