Liquid crystal phases, characterization

Cholesteric phase A liquid crystal phase characterized by nematic-like ordering with a helical twist. 

Since Robinson [1] discovered cholesteric liquid-crystal phases in concentrated a-helical polypeptide solutions, lyotropic liquid crystallinity has been reported for such polymers as aromatic polyamides, heterocyclic polymers, DNA, cellulose and its derivatives, and some helical polysaccharides. These polymers have a structural feature in common, which is elongated (or asymmetric) shape or chain stiffness characterized by a relatively large persistence length. The minimum persistence length required for lyotropic liquid crystallinity is several nanometers1. 

In a different approach, Bruce and coworkers 210) described the preparation of liquid-crystalline derivatives of a N,C,N-Pt (II) Imninophore (Fig. 18). Interestingly, they foimd that emission in the liquid-crystal phase is characteristic of the monomeric complex, while excimer-like emission normally characterizes nonliquid-crystalline analogues. They showed that the emission of pure films is responsive to both method of preparation and tribological stimulation so that it is possible to switch in a controllable manner between monomer- and excimer-like states. 

Even though some plasma membranes, such as nerve myelin membranes, contain a high concentration of lipids that form gel phase bilayers, the presence of cholesterol keeps these membranes in a fluid phase. However, interaction with the rigid cholesterol ring affects hydrocarbon chains of lipids in the liquid crystal phase (L ) and leads to formation of a new phase, the liquid ordered (Lq) phase (27). The phase is well characterized by a variety of physical methods and does not exist in pure lipids or their mixtures. In the liquid ordered phase, the long axis rotation and lateral diffusion rates are similar to the La phase, but the acyl chains are predominantly in an all-trans conformation and, hence, the order parameters are similar to the Lp phase (see Table 1). Recently, the cholesterol-rich Lq phase has been strongly associated with microdomains in live cells—the so-called lipid rafts. ... 

The simplest type of liquid-crystal phase is the nematic phase (see Fig. 23.9b) TBBA undergoes a transition from liquid to nematic at 237°C. In a nematic liquid crystal, the molecules display a preferred orientation in a particular direction, but their centers are distributed at random, as they would be in an ordinary liquid. Although liquid-crystal phases are characterized by a net orientation of molecules over large distances, not all the molecules point in exactly the same direction. There are fluctuations in the orientation of each molecule, and only on average do the molecules have a greater probability of pointing in a particular direction. 

Liquid crystal phases, or mesophases, are characterized by a partial order, intermediate between the full orientational and translational disorder of the isotropic liquid phase and the full orientational and translational order of the crystalline phase. Thermotropic liquid-crystal phases are obtained for a given compound (or possibly a mixture) as a function of temperature, while the so-called lyotropic liquid-crystal phases are obtained as a function of the concentration of a given solute in a solvent Typical examples of the latter systems are the various types of aggregates formed by amphiphilic molecules either in water or in organic solvents. In this chapter we will be interested only in thermotropic systems. An interesting review on lyotropic ionic liquid crystals can be found in Ref. [2],... 

In fact, with increasing temperature, these materials may not completely lose their translational order while retaining their orientational order. All liquid crystals are characterized by their orientational order, but liquid crystal phases show varying amounts of translational order with the only exception of nematics. Apart from the above basic symmetries, there is another important symmetry — bond orientation symmetry. This symmetry is important when dealing with hexatic phases. Liquid crystals are classified in terms of following criterion ... 

A complete characterization of liquid crystalline polymers should include at least two aspects the characterization of the molecular structure and that of the condensed state structure. Since the first characterization is nothing more than what is practiced for non-liquid-crystalline polymers, we will restrict the discussion to only a short introduction of methods mostly used in the characterization of the presence and the main types of polymeric liquid crystal phases. The methods include the mostly used polarizing optical microscopy (POM, Section 4.1), differential scanning calorimetry (DSC, Section 4.2) and X-ray diffraction (Section 4.3). The less frequently used methods such as miscibility studies, infrared spectroscopy and NMR spectroscopy will also be discussed briefly (Section 4.4). 

The chapter starts with a discussion of the basics of liquid-crystal phase behavior, the types of mesophase that are formed, and methods for their characterization those requiring more detail and/or breadth are directed to the volumes of Ref 4. This chapter does not cover liquid crystals that might be considered to be somehow inorganic in origin. 

Discotic liquid crystals came to prominence in the late 1970s when Chandrasekhar, Sadashiva, and Suresh reported the discovery of this new class of liquid-crystalline molecules, which were found to form columnar phases. The first of these, a hexaalkanoate of benzene, is shown in Figure 9. There then followed a rather unfortunate confusion of nomenclature in which the phases formed by discotic molecules were themselves referred to as discotic, carrying the abbreviation D. A liquid-crystal phase must be characterized by its symmetry and organization and not the shape of the molecules of which it is composed this is particularly important in columnar systems as many non-discotic molecules exhibit columnar phases. Indeed, columnar mesophases have been recognized for many years and studies date back to at least the 1960s with the work of Skoulios with various metal soaps. " Therefore, columnar phases take the abbreviation Col followed by some descriptor that describes the symmetry of the phase. 

If more surfactant is added above the cmc, the concentration of micelles increases (rather than the concentration of free surfactant) until the micelle concentration becomes so high that they themselves organize to form ordered arrays of lyotropic liquid-crystal phase. There are several well-characterized lyotropic liquid-crystal phases and a host of so-called intermediate phases whose characterization is not unequivocal. While cmc values are typically found in the range lO -lO moldm, formation of lyotropic mesophases typically starts at around 20wt.% of the surfactant in water. 

Probably, the single most important characteristic of liquid-crystal phases is that as fluids they are anisotropic " this means that their physical properties are likewise anisotropic, and it is this feature which is the basis for the widespread application of the materials and also, in many cases, of their characterization. 

Thermotropic polyester backbone chemistry is characterized by a high degree of aromaticity, planarity, and linearity in the chain backbone. Most common moieties are p-phenylene, 1,4-biphenyl, and 2,6-naphthalyl moieties linked by ester or amide linkages. Polymers that form liquid crystal phases in the melt are thermotropic, whereas those that form liquid crystalline phases in solution are lyotropic. The all-aromatic polyester homopolymers tend to be intractable, decomposing at temperatures well below their melting points and insoluble in most... 

Polymer stabilized liquid crystals are formed when a small amount of monomer is dissolved in the liquid crystal solvent and photopolymerized in the liquid crystal phase. The resultant polymer network exhibits order, bearing an imprint of the LC template. After photopolymerization, these networks in turn can be used to align the liquid crystals. This aligning effect is a pseudo-bulk effect which is sometimes more effective than conventional surface alignment. Several characterization techniques... 

The thermotropic group consists of a crystalline liquid formed during cooling of a dry melt (similar to the processing of thermoplastics)—the first having been the aromatic polyesters (1972). When a disordered melt cools in a region of proper transition temperature (Tj), a liquid crystal phase (mesophase) is formed. It is characterized by high order (anisotropic) that creates turbidity. 

For the purpose of this article, we focus our attention on the nematic mesophase smectic orders are more crystal-like and thus are beyond our scope. Typical nematic liquid crystals are characterized by a uniaxial order, though imperfect, along the preferred axis of the domain. No such long-range order exists in directions transverse to the domain axis. In most examples, low molar mass (monomer) liquid crystals carry flexible tails. Conformational ordering of these tails in the mesophase has been extensively studied in relation to the odd-even character of the phase behavior with the number of constituent atoms of the pendant chain. Various statistical models and theories have been presented [52-57]. In most cases, however, the ordering of the tail is relatively weak [58,59]. 

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