Lyotropic hquid crystals

Some molecules ia a solvent form phases with orientational and/or positional order. In these systems, the transition from one phase to another can occur due to a change of concentration, so they are given the name lyotropic Hquid crystals. Of course temperature can also cause phase transitions ia these systems, so this aspect of thermotropic Hquid crystals is shared by lyotropics. The real distinctiveness of lyotropic Hquid crystals is the fact that at least two very different species of molecules must be present for these stmctures to form. 

Amphiphilic Molecules. In just about all cases of lyotropic Hquid crystals, the important component of the system is a molecule with two very different parts, one that is hydrophobic and one that is hydrophilic. These molecules are called amphiphilic because when possible they migrate to the iaterface between a polar and nonpolar Hquid. Soaps such as sodium laurate and phosphoHpids such as a-cephalin [5681-36-7] (phosphatidylethanolamine) (2) are important examples of amphiphilic molecules which form Hquid crystal phases (see Lecithin Soap). 

Flexibihty of the moieties is another important parameter because it was recently shown that flexible objects require larger volume fractions to undergo nematic ordering. Flexibihty also reduces the nematic order parameter at the transition. Intuitively, very flexible mineral polymers should not show any orientational order at rest, but may display a strong flow birefringence. Thus, any soluble system where the structural unit in the solid state is anisotropic may not necessarily be a lyotropic Hquid crystal. For example, in solution a polymer is much less constrained than in the soHd state, and hence one must consider the elastic properties of the polymer chain and whether the anisotropic units still exist in solution. As shown recently for the case of the complex fluid with a min-... 

The liquid crystal phases discussed so far are called thermotropic liquid crystals and the transitions from one phase to another are driven by varying temperature. There is another type of liquid crystals, called lyotropic hquid crystals, exhibited by molecules when they are mixed with a solvent of some kind. The phase transitions from one phase to another phase are driven by varying the solvent concentration. Lyotropic liquid crystals usually consist of amphiphilic molecules that have a hydrophobic group at one end and a hydrophilic group at the other end and the water is the solvent. The common lyotropic liquid crystal phases are micelle phase and lamellar phase. Lyotropic liquid crystals are important in biology. They will not be discussed in this book because the scope of this book is on displays and photonic devices. 

Cubic lyotropic hquid crystal phases are not as corrrmon as the lamellar or hexagortal phases. However, crrbic lyotropic phases do occtrr in differerrt regions in phase diagrams. Accordingly, there are probably a range of diEfererrt crrbic lyotropic liquid crystal phases, the exact stmcture of which relates to their position within the phase diagram. 

M. D. Onofrei, A. M. Dobos, 1. Stoica, N. Olaru, L. Olaru, and S. loan. Lyotropic Hquid crystal phases in cellulose acetate phthalate/hydroxypropyl cellulose blends. J. Polym. Environ, doi 10.1007/sl0924-013-0618-7, onHne (2013). 

Figure 2 Schemes for the self-assemhhes of an amphiphilic diblock copolymer in the presence of the solvents water and oil selective for the two blocks (a) micellar solution, (b) micellar cubic lyotropic hquid crystal (LLC), (c) hexagonal LLC, (d) lamellar LLC, (e) reverse hexagonal LLC, (f) reverse cubic LLC, (g) reverse micellar solution. (From Ref. 8, Copyright 1997 American Chemical Society.)...

Liquid crystals may be divided into two broad categories, thermotropic and lyotropic, according to the principal means of breaking down the complete order of the soHd state. Thermotropic Hquid crystals result from the melting of mesogenic soHds due to an increase in temperature. Both pure substances and mixtures form thermotropic Hquid crystals. In order for a mixture to be a thermotropic Hquid crystal, the different components must be completely miscible. Table 1 contains a few examples of the many Hquid crystal forming compounds (2). Much more is known about calamitic (rod-Hke) Hquid crystals then discotic (disk-like) Hquid crystals, since the latter were discovered only recendy. Therefore, most of this section deals exclusively with calamities, with brief coverage of discotics at the end. 

Similady, hquid-crystal polymers exhibit considerable order in the hquid state, either in solution (lyotropic) or melt (thermotropic). When crystallized from solution or melt, they have a high degree of extended-chain crystallinity, and thus have superior mechanical properties. Kevlar (Du Pont) is an aromatic polyamide (atamid) with the repeating unit designated as (2). It is spun into... 

Licjuid Crystals. Ferroelectric Hquid crystals have been appHed to LCD (Uquid crystal display) because of their quick response (239). Ferroelectric Hquid crystals have chiral components in their molecules, some of which are derived from amino acids (240). Concentrated solutions (10—30%) of a-helix poly(amino acid)s show a lyotropic cholesteric Hquid crystalline phase, and poly(glutamic acid ester) films display a thermotropic phase (241). Their practical appHcations have not been deterrnined. 

Since their discovery in the nineteenth century [1], hquid crystals have fascinated scientists due to their unusual properties and their wide range of potential apphcations, especially in optoelectronics. LC systems can be divided into two categories thermotropic LC phases and lyotropic LC phases. Thermotropic LC systems result from anisotropic molecules or molecular parts (so called mesogens or mesogenic moieties, respectively), and form hquid crystalline phases between the soHd state and the isotropic hquid state, where they flow like liquids but possess some of the characteristic physical properties of crys-... 

Surfactant hquid crystals are normally lyotropic. The characteristics of the system, then, are highly dependent on the nature and amount of solvent present. In a phase diagram of a specific surfactant, the LC phases may span a broad region of compositions and may, in fact, constitute by far the major fraction of all possible compositions (Fig. 15.4). With the continued addition of water or other solvent, the system will eventually pass through the regions of the various mesophases into the more famihar isotropic solution phase. The solution is the most highly random state for mixtures of condensed matter and, as a result, tends to have fewer easily detected structural features. Surfactant solutions, however, are far from devoid of structure it is only the scale of the structure that changes as dilution occurs. 

Okabe, Masato. Polarizing plate with improved polarizing performance containing tabular-type lyotropic liquid crystal dye, hquid crystal display using the same, and manufacture of the polarizing plate. Jpn. Kokai Xokkyo Koho IP 2005010768, 2005 Chem. Abstr. 2005,142,103499. 

In this book we consider soft materials under the headings of polymers (Chapter 2), colloids (Chapter 3), amphiphiles (Chapter 4), hquid crystals (Chapter 5) and biological soft materials (Chapter 6). The distinctions between these systems are often not strong. For example, amphiphiles in solution and some aspects of polymer science are often considered in books on colloid chemistry. However, here we treat them separately since they are technologically important enough to merit detailed consideration on their own. The chapter on liquid crystals is in fact focused on one class of material, thermotropic liquid crystals, where phase transitions are thermally driven. However, a different class of liquid crystal phase is formed in am-phiphile solutions, where concentration is also a relevant variable. These are termed lyotropic liquid crystal phases and are discussed in Chapter 4. 

Next, the obtained polyesters and copolyesters were dissolved in styrene or vinyl acetate forming lyotropic liquid-crystalhne thermoset (LLCT) systems, capable of thermal crosslinking by the addition of a free-radical initiator. The thermal and hquid crystal properties of the novel UPs were investigated. It was also reported that UPRs prepared from propylene glycol, maleic anhydride and isophthaUc acid were used as a matrix for nematic liquid-crystal droplets [23]. 

Figure 13.3 Typical phase diagram of polymer solutions combining isotropic solutions (Iso), crystalline phase (Cr), and the intermediate liquid crystal phase (LC). Two arrows show the dimensions for thermotropic hquid crystal and lyotropic liquid crystal, respectively. Keller [18], Reproduced with permission of lUPAC.

Lekkerkerker H, Vroege G. Lyotropic colloidal and macro-molecular hquid crystals. Philos Trans R Soc London A 1993 344 419-440. 

Two general classes of liquid crystalline structures or mesophases are encountered whether one is considering surfactants or other types of material. These classes are the thermotropic hquid crystals, in which the structure and properties are determined by the temperature of the system, and lyotropic liquid crystals in which the structure is determined by specific interactions between the surfactant molecules and the solvent. With the exception of the natural fatty acid soaps, experimental data support the view that almost aU surfactant liquid crystals are lyotropic in nature. 

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