Lyotropic liquid crystals phase

Goltner-Spickermann C (2003) Nanocasting of Lyotropic Liquid Crystal Phases for Metals and Ceramics. 226 29-54 Gouzy M-F, see Li G (2002) 218 133-158... 

The potential for novel phase behaviour in rod-coil block copolymers is illustrated by the recent work of Thomas and co-workers on poly(hexyl iso-cyanate)(PHIC)-PS rod-coil diblock copolymers (Chen etal. 1996). PHIC, which adopts a helical conformation in the solid state, has a long persistence length (50-60 A) (Bur and Fetters 1976) and can form lyotropic liquid crystal phases in solution (Aharoni 1980). The polymer studied by Thomas and co-workers has a short PS block attached to a long PHIC block. A number of morphologies were reported—wavy lamellar, zigzag and arrowhead structures—where the rod block is tilted with respect to the layers, and there are different alternations of tilt between domains (Chen et al. 1996) (Fig. 2.37). These structures are analogous to tilted smectic thermotropic liquid crystalline phases (Chen et al. 1996). 

The association of block copolymers in a selective solvent into micelles was the subject of the previous chapter. In this chapter, ordered phases in semidilute and concentrated block copolymer solutions, which often consist of ordered arrays of micelles, are considered. In a semidilute or concentrated block copolymer solution, as the concentration is increased, chains begin to overlap, and this can lead to the formation of a liquid crystalline phase such as a cubic phase of spherical micelles, a hexagonal phase of rod-like micelles or a lamellar phase. These ordered structures are associated with gel phases. Gels do not flow under their own weight, i.e. they have a finite yield stress. This contrasts with micellar solutions (sols) (discussed in Chapter 3) which flow readily due to a liquid-like organization of micelles. The ordered phases in block copolymer solutions are lyotropic liquid crystal phases that are analogous to those formed by low-molecular-weight surfactants. 

On a side note, Ouskova and co-workers also reported that the composite of magnetic /i-FejOs nanorods in 5CB showed lower threshold voltages than pure 5CB, and that the sensitivity of the nematic liquid crystal to external magnetic fields was increased in the presence of such magnetic nanorods [451]. Finally, several groups interested in the macroscopic organization and orientation of nanorods also reported on the formation of a lyotropic liquid crystal phase induced by the self-assembly of polymer-coated semiconductor nanorods [453—457], which might be used to improve the device performance, for example, of solar cells. 

Stiff rod-like helical polymers are expected to spontaneously form a thermotropic cholesteric liquid crystalline (TChLC) phase under specific conditions as well as a lyotropic liquid crystal phase. A certain rod-like poly(f-glutamate) with long alkyl side chains was recently reported to form a TChLC phase in addition to hexagonal columnar and/or smectic phases [97,98]. These properties have already been observed in other organic polymers such as cellulose and aromatic polymers. 

Lyotropic liquid crystal phases are formed by amphiphihc molecules (surfactants, block copolymers) in solution, driven by repulsive forces between hydrophobic and hydrophihc parts. In a polar solvent, the hydrophihc parts associate with the solvent, whereas the hydrophobic parts interact to form the interiors of micelles (as in low-molecular smfactants). Micelles can be spherical, rod-like or discotic in shape. The contour of the micelle is determined by the relative sizes of the hydrophihc and hydrophobic groups. MiceUe shapes are influenced by solvent, concentration and temperatme. 

The extraordinarily good compatibility of ethylene glycol-modified silanes with lyotropic liquid-crystal phases of surfactant molecules was demonstrated by Hoffinann in 1998 [11,12]. 

Figure 8.6 Cj2E08-water lyotropic liquid-crystal phase diagram. Reproduced with permission from [63], Copyright (1983) Royal Society of Chemistry...

Other surfactants show the similar lyotropic liquid-crystal phase behavior and follow the same succession of phases, but not all of the phases are always present. Figure 8.7 shows a phase diagram for the CTAB-water binary system. CMC can also be classified CMC1 (spherical micelle) and CMC2 (rod-shaped micelle). 

Figure 8.7 The lyotropic liquid crystal phase diagram for CTAB-water system...

Mesostructure syntheses can be carried out under conditions in which the silicate alone would not condense to solid (at pH 12 14 and low silicate concentration) and the surfactant CTAB (concentration < 2%) alone would not form a lyotropic liquid-crystal phase. The rapid formation of MCM-41 when surfactant solution and silicate solution are combined indicates that there is strong interaction between the cationic surfactant and anionic silicate species in the formation of mesophases. 

SBA-1 and SBA-6 were synthesized by using different surfactants and from acidic and basic synthesis media, respectively. They have the same structure and show the similar XRD patterns. Their structure is similar with cubic Ij phase, spherical micelles packed in Pm3n symmetry, in lyotropic liquid-crystal phase diagram for surfactant-water systems. 

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],... 

IS so high that the micelles have no choice but to order themselves. This ordered of micelles is a lyotropic liquid crystal phase. 

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