Cubic lyotropic liquid crystal phases

Figure 7.5. Structirre of the cubic lyotropic liquid crystal phase.

Structurally, the cubic lyotropic liquid crystal phases are not as well-characterised as the lamellar or hexagonal phases. However, two types of cnbic lyotropic liquid crystal phases have been estabhshed and each can be generated in the normal manner (water continuous) or in the reversed manner (non-polar chain continnous), which makes for a total of fottr different phase types. The most well-known cnbic phase consists of a cubic arrangement of molecular aggregates. The molecttlar aggregates are similar to micelles (Ij phase) or reversed micelles (1 phase). The stractrrre of the normal (1 ) cubic... 

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. 

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. 

Three different classes of lyotropic liquid crystal phase structures are widely recognised. These are the lamellar, the hexagonal and the cubic phases, and their structures have each been classified by X-ray dififiaction techniques. 

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. 

In a solvent, block copolymer phase behavior is controlled by the interaction between the segments of the polsrmers and the solvent molecules as well as the interaction between the segments of the two blocks. If the solvent is unfavorable for one block, this can lead to micelle formation in dilute solution. The phase behavior of concentrated solutions can be mapped onto that of block copolymer melts (97). Lamellar, hexagonal-packed cylinder, micellar cubic, and bicontinu-ous cubic structures have all been observed (these are all lyotropic liquid crystal phases, similar to those observed for nonionic surfactants). This is illustrated by representative phase diagrams for Pluronic triblocks in Figure 6. 

Lyotropic liquid crystal phases has been observed when l-Alkyl-3-methylimidazolium bromide (CnmimBr) was mixed with p-xylene and water. SAXS, POM, NMR and rheology measurements were performed to investigate the lyotropic liquid crystal phases. A lyotropic bicontinuous cubic phase formed in imidazolium-type ionic liquid (IL) system was found for the first time. The strong %-% stacking of imidazolium based ILs and their 71-cation interactions with p-xylene molecules have unique effect on the structural parameters.Description of NMR of quadrupolar systems using the Holstein-Primakoff (HP) formalism and its analogy with a Bose-Einstein condensate (BEC) system has been presented. Two nuclear spin... 

The lamellar lyotropic liquid crystal phase is often formed in detergent solutions. When subjected to shear lamellae can, under certain conditions, curve into closed shell structures called vesicles (Section 4.11.4). These are used in pharmaceutical and cosmetic products to deliver molecules packed into the core. Selective solubilization in micelles finds similar applications, although micelles tend to break down more rapidly than vesicles when diluted. Applications for hexagonal and cubic structures may stem from the recent discovery that they can act as templates for inorganic materials such as silica, which can be patterned into an ordered structure with a regular... 

In concentrated solution, DNA fragments can form lyotropic liquid crystal phases. Short fragments behave like rods, and so the formation of liquid crystal phases is possible. On increasing concentration (above 160 mg/ml for 50 nm DNA in physiological salt solutions), cholesteric and hexagonal columnar phases may be observed (see Chapter 5 for a discussion of these structures) Just below the cholesteric phase, a blue phase is sometimes observed. This phase is named for the colour arising from the double twist cylinders that result from the packing of helices onto a cubic lattice. 

Figure 2a shows a schematic phase diagram for lyotropic liquid crystals. This figure shows the formation of micelles, cubic phases, bicontinuous cubic phases, and lamellar phases as the concentration of surfactant increases. Also shown in this figure is a schematic diagram of an ordered bicontinuous cubic phase (Fig. 2b). Another interesting example in...

Anderson DM, Gruner SM, Leibler S (1988) Geometrical aspects of the frustration in the cubic phases of lyotropic liquid-crystals. Proc Natl Acad Sci USA 85 5364—5368... 

In Attard s approach, tetramethylorthosilicate (TMOS) was hydrolyzed and condensed in the aqneons domain of the liqnid crystal phase at pH of abont 2, leading to mesostmctured hexagonal, cubic, or lamellar sihca. Methanol from the hydrolysis of TMOS destroys the long-range order of the liquid crystal however, upon the removal of methanol, the lyotropic liquid crystal is restored and serves as the template phase for the further condensation of silicates. The resnlting pore system replicates the shape of the lyotropic mesophase, so this process is also termed nanocasting . 

This chapter focuses on the fixation of lyotropic liquid crystalline phases by the polymerization of one (or more) component(s) following equilibration of the phase. The primary emphasis will be on the polymerization of bicontinuous cubic phases, a particular class of liquid crystals which exhibit simultaneous continuity of hydrophilic — usually aqueous — and hydrophobic — typically hydrocarbon — components, a property known as bicontinuity (1), together with cubic crystallographic symmetry (2). The potential technological impact of such a process lies in the fact that after polymerization of one component to form a continuous polymeric matrix, removal of the other component creates a microporous material with a highly-branched, monodisperse, triply-periodic porespace (3). 

If the concentration of surfactant becomes high enough, surfactant structures often develop long-range order, and hence they become liquid crystalline. They are lyotropic liquid crystals, because the transition to the liquid-crystalline state is induced by concentration changes. Surfactant solutions can form nematic and smectic-A liquid-crystalline phases analogous to those discussed in Chapter 10. In addition, hexagonal and cubic phases are common in surfactant solutions. 

As for low molecular weight surfactants, the superstructures are assumed to be formed by micellar aggregates [126], But it seems that the formation of lyotropic liquid crystals is supported by the additional presence of thermotropic mesogens [87,122-124,126], Lamellar, hexagonal, cubic and even nematic and cholesteric mesophases were reported for binary systems, the latter being exceptional. Lyotropic mesophases were also observed in non-aqueous solvents [240,400,401,405], If polymerizable surfactants are studied, not only the phase diagram but also the types of mesophases observed for the monomer and the polymer may be different. 

Mesophases can be locked into a polymer network by making use of polymerizable LCs [59]. These molecules contain moieties such as acryloyl, diacety-lenic, and diene. Self-organization and in situ photopolymerization under UV irradiation will provide ordered nanostmctured polymers maintaining the stable LC order over a wide temperature range. A number of thermotropic liquid crystalline phases, including the nematic and smectic mesophases, have been successfully applied to synthesize polymer networks. Polymerization of reactive lyotropic liquid crystals also have been employed for preparation of nanoporous polymeric materials [58, 60]. For the constmction of nanoporous membranes, lyotropics hexagonal or columnar, lamellar or smectic, and bicontinuous cubic phases have been used, polymerized, and utilized demonstrated in a variety of applications (Fig. 2.11). 

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