Mixtures of Smectics

The polymorphism of mixtures of smectogenic substances is extremely rich and we shall limit ourselves to only two examples. The first of them demonstrates how the miscibility rule helps to identify smectic phases in a new compound [42]. The compound 4(4-n-hexyloxy-benzylidenamino)- 

Another example shows the smectic A polymorphism. Fig. 1.17 [43]. In mixtures of 80CB (l.xv) and 4-n-heptyloxyphenyl-4 -cyanobenzoyloxyben-zoate (DB70CN), in addition to the bilayer 5a,2 and the partially bilayer 5a,d phases, a novel incommensurate phase 5a,ic appears. Its X-ray diffraction pattern consists of both 5a,u and 5a,2 periodicities which are incommensurate. The SA2 SAd transition line (for x 0.24) corresponds to a continuous transition between the two phases. Other examples of the incommensurate and modulated smectic A phases are given in [4, 44]. 

FIGURE 1.16. The isobaric phase diagram of mixtures HOPDOB-HBAB (a simplified version of [42]). Dashed lines correspond to monotropic transitions. 

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Isozaki, T., Fujikawa, T., Takezoe, H., Fukuda, A., Hagiwara, T., Suzuki, Y., Kawamura, I. Competition between ferroelectric and antiferroelectric interactions stabilizing varieties of phases in binary mixtures of smectic liquid crystals. Jpn. J. Appl. Phys. 31, L1435-L1438 (1992)... 

Detailed x-ray diffraction studies on polar liquid crystals have demonstrated tire existence of multiple smectic A and smectic C phases [M, 15 and 16]. The first evidence for a smectic A-smectic A phase transition was provided by tire optical microscopy observations of Sigaud etal [17] on binary mixtures of two smectogens. Different stmctures exist due to tire competing effects of dipolar interactions (which can lead to alternating head-tail or interdigitated stmctures) and steric effects (which lead to a layer period equal to tire molecular lengtli). These... 

Tbe purpose of tbe bydroxyl group is to acbieve some hydrogen bonding with the nearby carbonyl group and therefore hinder the motion of the chiral center. Another way to achieve the chiral smectic Cphase is to add a chiral dopant to a smectic Chquid crystal. In order to achieve a material with fast switching times, a chiral compound with high spontaneous polarization is sometimes added to a mixture of low viscosity achiral smectic C compounds. These dopants sometimes possess Hquid crystal phases in pure form and sometimes do not. 

Both compound I and a mixture of the two derivatives M70 (70% 1 30% II) display the usual monolayer smectic Aj phase on cooling down from the isotropic liquid. At lower temperatures the particular smectic phases (X and X ) showing polar behaviour have been observed [78]. The smectic X phase in... 

Fig. 13. X-ray intensity profile with diffi action vector along qx (in the plane of layers) in the smectic X phase for the mixture of polyphilic compounds M70. The solid line through the data points is the smn of three Lorentzian peaks which are shown hy broken curves (Blinov et al. [44])...

Ballesteros, B., Coco, S. and Espinet, P. (2004) Mesomorphic Mixtures of Metal Isocyanide Complexes, Induding Smectic C Mesophases at room temperature and Liquid Crystalline Molecular Alloys . Chemistry of Materials, 16, 2062-2067. 

The amorphous diacrylate monomers chosen for study were two commercially available monomers, p-phenylene diacrylate (PPDA) and 1,6-hexanediol diacrylate (HDDA) (Polysciences, Inc., Warrington, PA). The liquid crystalline diacrylate studied was 1,4-di-(4-(6-acryloyloxyhexyloxy)benzoyloxy)-2-methylbenzene (C6M) (13). Chemical structures of these monomers as well as pertinent physical and LC properties are given in Figure 1. All monomers were used without further purification. The ferroelectric liquid crystal mixture consisted of a 1 1 mixture of W7 and W82 (1) (Displaytech, Boulder, CO). This mixture exhibits isotropic (I), smectic A... 

The so ealled a phase of the fatty alcohols is a thermotropic type smectic B liquid crystal with a hexagonal arrangement of the moleeules within the double layers. It is initially formed from the melt during the manufaeturing proeess and normally transformed into a erystalline modifleation on eooling. However, the erystallization of the gel matrix ean be avoided if the a phase ean be kept stable as it eools to room temperature. This ean be achieved by eombining appropriate surfaetants, such as myristyl or lauryl alcohol and cholesterol, a mixture of whieh forms a lamellar liquid... 

In order to understand the basic principles of operation of the many different kinds of LCDs being developed and/or manufactured at the present time, it is necessary to briefly describe the liquid crystalline state and then define the physical properties of direct relevance to LCDs. First, the nematic, smectic and columnar liquid crystalline states will be described briefly. However, the rest of the monograph dealing with liquid crystals will concentrate on nematic liquid crystals and their physical properties, since the vast majority of LCDs manufactured operate using mixtures of thermotropic, non-amphiphilic rodlike organic compounds in the nematic state. 

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. 

Bipyridinium cation 9 [58], phosphonium [59-62], guanidinium [63], and vina-midinium [64] cations were used to form thermotropic ionic liquid crystals. For example, the mixture of diheptyl and dioctyl viologens 9 (20 80 by wt%) with TFSI anion forms a smectic A phase between 22° and 132°C. It was reported that the viologens show fluorescence in both non-polar and polar organic solvents, which can be useful for the development of biological and chemical sensors [65]. 

The phase transition behavior for the mixture of 13 and 16 is shown as a function of the mole fraction of 16 in Figure 25.8 [8]. Compounds 13 and 16 are miscible up to the mole fraction of 0.7 for 16 in the mixture. These mixtures exhibit smectic phases, while compound 13 alone gives a columnar phase (Col) from 79° to... 

Figure 25.8 Phase transition behavior for a mixture of 13 and 16 on the heating runs. Col columnar Cr crystalline G glassy Iso isotropic Sa smectic A Sb smectic B Sx unidentified smectic.

Figure 25.9 Polarizing optical micrographs of the equimolar mixture of 14 and 16 in the smectic A liquid crystalline state (a) homeotropically aligned monodomain is formed on a glass substrate when the sample is cooled slowly from the isotropic state at the rate of 3° C min Inset is a conoscopic image (b) unaligned polydomains are formed on a glass substrate when the sample is cooled from the isotropic state at the rate of 20°C min ...

While you will synthesize and characterize cholesterol nonanoate, you will actually study mixtures of two liquid crystalline materials each of which is a derivative of cholesterol. The other derivative is cholesteryl chloride (ChCl) whose structure along with that of cholesterol nonanoate is shown in Figure C.4. The reason to study mixtures here is for convenience of handling. Though both pure materials exhibit liquid crystalline phases (ChNon at 74" C, solid smectic 80°C smecticicholesteric 93" C cholesteric isotropic, and ChCl about 70 "C, solid cholesteric), appropriate mixtures are liquid crystalline at much lower and more manageable temperatures, for example, room temperature. 

In general, cholesteric liquid crystals are found in optically active (chiral) mesogenic materials. Nematic liquid crystals containing optically active compounds show cholesteric liquid crystalline behavior. Mixtures of right-handed and left-handed cholesteric liquid crystals at an adequate proportion give nematic liquid crystals. From these results cholesteric liquid crystals are sometimes classified into nematic liquid crystals as twisted nematics . On the other hand, cholesteric liquid crystals form batonnet and terrace-like droplets on cooling from isotropic liquids. These behaviors are characteristic of smectic liquid crystals. Furthermore, cholesteric liquid crystals correspond to optically negative mono-axial crystals, different from nematic... 

Figure 10.35 Dependence on inverse temperature of the viscosities in each of the orientations depicted in Fig. 10-28 for a ternary mixture of 73% (by weight) 7CB, 18% 80CB, and 9% 5CT that forms nematic phases at high and low temperature and has an intermediate range of smectic A phase. Ihe dashed line is an Arrhenius temperature-dependence for (From Bhattacharya and Letcher 1980, reprinted with permission from the American Physical Society.)...

Figure 10.38 Temperature-shear-rate phase diagram for 8CB in the vicinity of the transition temperature Tan = 33.58°C from the nematic to the smectic A phase. At temperatures more than 5.5°C above Tan, 8CB is a flow-aligning nematic, and it orients close to the flow direction in orientation b. As the temperature is lowered, the nematic becomes a tumbler, preferring the a orientation, mixed with either b or c. The a orientation prevails even in the smectic-A state at temperatures within a degree of Tan, especially at high shear rates. At lower temperatures, the orientation is a mixture of a and c. The globes show the ranges of orientations obtained in each of the states ac, a, a i, and b inferred from x-ray scattering. (From Safinya et al. 1991, reprinted with permission from the American Physical Society.)...

Tosoni M, Laschat S, Baro A. Synthesis of novel chiral hquids and their phase behavior in mixtures with smectic and nematic liquid crystals. Helv. Chim. Acta. 2004. 87, 2472-2479. 

In this paper, an earlier theory (12-13) for binary mixtures of backbone LCPs (and/or nonpolymeric molecules) in the nematic (N) LC phase and the isotropic (I) liquid phase has been extended to treat binary mixtures in multiple smectic-A (SA) LC phases, the N LC phase, and the I liquid phase. Either component 1 (Cl) and/or component 2 (C2) in the mixture can be a backbone LCP, a nonpolymeric LC molecule, a polymeric non-LC molecule, or a nonpolymeric non-LC molecule. Cl can also be a side-chain LCP or a combined LCP (including a SS LCP). The new theory of this paper is the mixture analogue of an earlier theory for pure (single-component) systems derived and presented in detail in References 3 and 7-10 (see also References 14-23). When only one component is present, the new mixture theory of this paper reduces to this earlier single-component theory. Therefore, only a short summary of the new mixture theory is presented here. 

In a previous paper (1), phase segregation by spinodal decomposition in mixtures of polyethylene terephthalate and polyhydroxybenzoic acid copolymer (PET-PHB) and polycarbonate (PC) has been investigated. It was shown that thermally induced phase segregation takes place above the Tg of PC and exhibits a lower critical solution temperature (LCST). However, the phase separated domains do not grow until the temperature exceeds 255°C. Some disclinations developed within the liquid crystal rich regions. Even in the pure PET-PHB component, four dark brushes with negative sense of disclinations form around 240°C, indicating the presence of nematic liquid crystals. Paci and coworkers (2) claimed that a smectic-nematic transition exists near 270°C in this liquid crystalline copolyester. 

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