Lamellar lyotropic smectics

Structural forces due to long-range positional order are quite easily observed in the smectic A liquid crystals. SFA measurements have been performed on lamellar lyotropic smectics [42,43] and in thermotropic smectics [44-46]. These works extend to a nanometer scale the early studies on elasticity, viscoelastic response and layers instability of smectic A, observed in macroscopic wedge-shaped piezoelectric cells [47,48]. 

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). 

Lamellar lyotropic mesophase, 15, 16/ Lamellar phases. See Smectic phases Lanthanide alkanoates mesomorphism, 103-107 metathesis reaction, 101 phase diagram, 104/ purification, 101-102 structure, 102-103... 

P. Kekicheff, H. K. Christenson. Forces measured in a swollen smectic lyotropic lamellar mesophase confined between solid surfaces. Phys Rev Lett (55 2823-2826, 1989. 

Note 5 The lyotropic equivalent of a smectic A mesophase is known as a lamellar mesophase where layers of amphiphilic molecules are separated by layers of solvent, normally water, or by oil in an inverse lamellar mesophase. 

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). 

This study has concentrated on the defects observed in lyotropic lamellar phases, and it has put into evidence the specific character of the textures compared to classical thermotropic smectic phases. In leci-... 

Little work seems to have been done on thin oriented layers of lyotropic liquid crystals although there is one recent report of preparation of such a layer of the lecithin-water lamellar phase (JO). As indicated by Brochard and de Gennes (II), theories of the hydrodynamics of thermotropic smectic materials can be adapted to describe oriented layers of lamellar liquid crystal in lyotropic systems. 

Experiments by Muller et al. [17] on the lamellar phase of a lyotropic system (an LMW surfactant) under shear suggest that multilamellar vesicles develop via an intermediate state for which one finds a distribution of director orientations in the plane perpendicular to the flow direction. These results are compatible with an undulation instability of the type proposed here, since undulations lead to such a distribution of director orientations. Furthermore, Noirez [25] found in shear experiment on a smectic A liquid crystalline polymer in a cone-plate geometry that the layer thickness reduces slightly with increasing shear. This result is compatible with the model presented here as well. 

Evans et al. also showed that the 1 1 mixture of BAN and (3, y-distearoyl-phos-photidylcholine (DSPC) gives a smectic A texture in the temperature range of 57.3 to 100°C [21]. This is the first notice of lyotropic lamellar liquid crystals formed in the ionic medium. Additionally, Seddon et al. [28] and Neve et al. [29] have described the long-chained A-alkylpyridinium or l-methyl-3-alkylimidazolium ions to display smectic liquid-crystalline phases above their melting points, when Cl or tetrachloro-metal anions like CoCl " and CuCl " are used as the counter ions. Lin et al. have also noted the liquid crystal behavior of 1-alkylimidazolium salts and the effect on the stereoselectivity of Diels-Alder reactions [30]. However, liquid crystals are classified as ionic liquid crystals (ILCs), and they are distinguished from liquid crystals that are dispersed in ionic liquids. Although the formation of micelles and liquid crystal phases in ionic liquids have been thus reported, there has been no mention of the self-assembly of developed nano-assemblies that are stably dispersed in ionic liquids. In the next section the formation of bilayer membranes and vesicles in ionic liquids is discussed. 

Lyotropic liquid crystals are principally systems that are made up of amphiphiles and suitable solvents or liquids. In essence an amphiphilic molecule has a dichotomous structure which has two halves that have vastly different physical properties, in particular their ability to mix with various liquids. For example, a dichotomous material may be made up of a fluorinated part and a hydrocarbon part. In a fluorinated solvent environment the fluorinated part of the material will mix with the solvent whereas the hydrocarbon part will be rejected. This leads to microphase separation of the two systems, i.e., the hydrocarbon parts of the amphiphile stick together and the fluorinated parts and the fluorinated liquid stick together. The reverse is the case when mixing with a hydrocarbon solvent. When such systems have no bend or splay curvature, i.e., they have zero curvature, lamellar sheets can be formed. In the case of hydrocarbon/fluorocarbon systems, a mesophase is formed where there are sheets of fluorocarbon species separated from other such sheets by sheets of hydrocarbon. This phase is called the La phase. In the La phase the molecules are orientationally ordered but positionally disordered, and as a consequence the amphiphiles are arranged perpendicular to the lamellae. The La phase of lyotropics is therefore equivalent to the smectic A phase of thermotropic liquid crystals. 

Figure 9 Schematic drawing of the membrane structure of lyotropic lamellar phases. The lamellar phase can be swollen either with water (hydrophilic solvent) or oil (hydrophobic solvent), leading to direct or inverted bilayers. The membrane thickness is (5, d is the smectic repeat distance. (From Ref 111.)...

Smectic phases are characterized by a layered structure, in which a two-dimensional fluid order prevails. In Fig. 3.4a, a schematic picture of the skeleton structure of a smectic phase is shown. The two-dimensional fluid layers are stacked upon each other with the periodicity distance d, causing a one-dimensional positional order along the direction of the layer normal k. In the case of the lyotropic lamellar L phase one smectic layer is usually referred to as a lamella. The lamella can be separated into two parts, as shown in Fig. 3.4b. The first part is a surfactant bilayer, in which the molecules are on the average oriented perpendicular to the layer plane. For conventional lyotropic mixtures polar solvents are used, which cause the hydrophobic chains to point towards the middle of the bilayers. This arrangement can be inverted by using apolar solvents, i.e. alkyls. If the surfactant molecules are interdigitated to some degree, the term partial bilayer is used. The second part of the lamella is a layer of solvent molecules, in which the molecules are believed to solely possess a fluid-like order. The solvent layers separate the surfactant bilayers from each other and should thus inhibit the transfer of information from one surfactant layer to the next. Consequently, the lamellar L phase is the only fluid. 

In lyotropics, phases with a stmcture comparable to the ones of the thermotropic hexatic smectic phases exist, as pointed out by Smith et al. [26]. Again, the surfactant molecules show a bond-orientational order within the layers and can either be parallel (lamellar Lp) [27] or tilted (lamellar Lp-) [28] with respect to the layer normal k. These phases are called gel-like rather than liquid crystalline, due to their dramatically increased viscosity compared to the lamellar L phase. This increased... 

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