Nematic Eutectics

A realistic case is shown in Fig. 1.14 [34]. The phase diagram for a mixture of MBBA (l.i) with its ethoxy homologue (EBBA) displays all the features of the general scheme just discussed. However, due to the appearance of several metastable crystalline modifications of MBBA there are two eutectic points in Fig. 1.14. 

In the simplest case of the ideal solution the position of the eutectic point can be calculated not only for binary mixtures but for multicomponent ones 

FIGURE 1.13. A generalized phase diagram for a mixture of two nematogens. 

The enthalpies AHi and melting points of all i components of a mixture are assumed to be known, and the systems of z H- 1 equations allows the eutectic temperature Teu and molar parts of the components Xi to be calculated. Simultaneously, the clearing point of the mixture is calculated 

Unfortunately, most of the liquid crystalline mixtures are far from being ideal solutions. Intermolecular interactions result in the deviation of experimental data from theoretical data. Moreover, new phases can occur in mixtures of nematogens. For example, a binary mixture may be more preferable for the formation of the smectic A phase, due to a specific packing of different-type molecules with each other [35]. In addition, many mixtures display a so-called reentrant polymorphism. 

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To fill the display cell, any commercially available nematic eutectic mixture (e.g., E9 from Merck) for twisted liquid crystal displays can be used. The liquid crystal has to be introduced slowly into the gap between the glass by means of a pipette. 

The nematic LC is the original eutectic mixture of 35% (weight %) 4 (m-octyl oxy)-4-cyanobiphenyl (80CB) and 65% 4/-(n-pentyl)-4-cyanobiphenyl (5CB). The sample is doped with perylene and 2% (mol %) of NIAC, which is acrylate monomer containing the easily reduced l,4 5,8-naphthalenediimide moiety (see Scheme 3). Finally, 0.5% (mol %) of benzoin methyl ether (BME) is added to photoinitiate polymerization of the NIAC. 

As customary, acronyms will be used for nematic solvents throughout the text 5CB, (d-w-pentyl -cyanobiphenyl EBBA, vV-(/ -ethoxybenzylidene)-//-butylanyline ZLI1132, eutectic mixture of l,4-(fra 5,-4/- -alkylcyclohexyl)-cyanobenzene and, 4- trans-Ar - -pentylcyclohexyl)-cyanobiphenyl. 

Figure 4.6-10 Infrared spectra of transmittance T, reflectance R, and optical rotation p of an induced cholesteric solution at 22 °C (Ch) and as isotropic liquid at 63 °C (Is). Solvent eutectic mixture of the isomeric N-oxides of p-methoxy-p -n-butylazobenzene, see 3 of Table 4.6-1 (Nematic Phase IV Licristal E. Merck) solute 17-3-acetoxy-5-/3-androst-l-en-3-one molar fraction x = 0.05.5 (Korte and Schrader, 1981).

Figure 4.6-14 Part of the rotatory spectra of the enantiomers of Thalidomide in Nematic Phase IV Licristal (eutectic mixture of the isomeric N-oxides of p-methoxy-p -n-butylazobenzene, see 3 of Table 4.6-1 E. Merck).

Figure 4.10 Typical phase diagram of a binary mixture of nematic liquid crystals. By use of eutectic blocks of structurally similar compounds (e.g. alkyl homologs), the nematic phase range is extended to lower temperatures [32].

Among the vast variety of LC some are (or were) widely used in experiments. Early work used nematic PAA or PAP (see code). Later MBBA, nematic at room temperature, or its eutectic mixture with EBBA (named N8 in the Soviet litterature) replaced the former. When the more stable cyanobiphenyls (CB) appeared, they tended to replace MBBA, mainly the pentyl CB, the properties of which are well known. 

Among the various cholesterol derivatives the oleyl ester COC and nonanoate CN are common. Octyloxy biphenyle (M24), which melts at 54.5°C in the smectic state, turns nematic at 67°C and isotropic at 80°C, is considered as a typical smectic compound, as well as its eutectic mixture,, with nonyloxy CB (M27). The currently used LC, constituants of the mixtures employed in the text, are listed in Table II. ... 

Figure 25. Phase diagram of H2C=CHCH20BiE07/water (reproduced from [132]). (Ni, disc nematic phase E], E2 and E3, eutectic points otherwise as Fig. 14).

In order to widen the nematic range of the monomers and to decrease their melting temperatures, we have made binary mixtures of compounds n = 0 and n = 1 (Table l). We observed that the experimental diagram did not exhibit an eutectic, as is generally observed with other liquid crystals. The two compounds are probably miscible in all proportions in their crystalline state (solid solution). The phase diagrams, experimental and calculated, are plotted on Figure 1. 

Chronologically, the works that shoul be entioned first are those of Millaud et al and Noel et al who have determined the nature of the mesophase of a polyazomethine by establishing its isomorphy with low MW nematic Schiff s bases of similar structure. The binary diagrams are similar to the typical phase diagrams encountered for mixtures of two low MW nematogens an eutectic is seen in the solid-nematic curve and there is a continuous pathway... 

The discussion of mixtures is worthy of further comment. It has been known for some time that mixtures of linear, nonmesomorphic molecules with nematic compounds are characterized by a sharp decrease in both the crystal-nematic and nematic-isotropic liquid transitions with increasing concentration of nonmesomorphic component. However, mixtures of two or more nematic compounds which possess subtle differences in molecular structure do not exhibit sharp decreases in the nematic-isotropic liquid transition temperatures with molar composition although eutectic points for the crystal-nematic transitions may be obtained. Thus, the nematic-isotropic liquid transition temperatures form a smooth curve over the entire range of molar composition. Similar smooth curve surfaces are observed in ternary mixtures. This remarkable stability of the nematic mesophase is undoubtedly due to the fact that all the molecules are oriented in the same direction resulting in the formation of a pseudo-lattice type structure. On the other hand, the molecules of a nonmesomorphic guest are randomly oriented in a nematic host and their presence results in the disruption of nematic order. 

The use of the Merck bicyclohexyl eutectic mixture, (ZLI 1167) instead of the compensated nematic mixture of cholesteryl chloride... 

Few new observations in the photocurable hyperbranched polyester/eutectic nematic liquid crystals (E7) system was reported by Kim et al. [163] Higher order mesophase is induced at some specific compositions, which is found absent in their individual components and possible escape of some ingredients observed during prolonged annealing. It is observed that in the eutectic mixture some components is expelled due to its weak affinity with the hyperbranched polyester. The induced smectic phase at 63-93 wt % E7 in the blend is influenced by the shift of eutectic nematic point. The presence of isotropic (I), isotropic + nematic (I + N), pure nematic (N), nematic +smectic A (N + SmA), and induced smectic A (SmA) phase is dependent on the composition and temperature. 

Nitrile groups in one of the p-positions increase the dielectric anisotropy. Recently a dielectric of +14 was described for a binary eutectic system with nematic meso-phase at room temperature consisting of 2/3 (c) and 1/3 (d) with a crystalline-nematic temp. —30°C, nematic-isotropic temperature 62°C [81] ... 

Figure 4. Thermobarogram on heating for the mixture MET/MNT with 11.9 wt% MET. Sample weight was 1 mg. Sequence of phase transitions eutectic melting, crystal dissolution of excess MNT, nematic/ isotropic phase transition. Reproduced from [12] by permission of Gordon and Breach Science Publishers, Inc. and OPA Ltd...

The behaviour of the coefficients rji, 1)2 d 773 will be discussed for a liquid crystal having a broad nematic range and no anomalies. A mixture of nematic liquid crystals with similar structures is well suited to this purpose, and the example used here is Nematic Phase V, a former product of Merck, which consists of a eutectic mixture of the two isomers of 4-methoxy-4 -/i-butyl-azoxybenzene (65 mol%) and the two isomers of 4-methoxy-4 -ethylazobenzene (35 mol%). The nematic phase ranges from 268 K (melting point) to 347 K (clearing point). The mixture exhibits a sharp clearing point. 

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