Liquid crystals p-azoxyanisole

This paper is concerned with the liquid crystals, p-azoxyanisole and anisal-p-aminoazobenzene. The structural formulas are ... 

McColl, J. R., Effect of pressure on order in the nematic liquid crystal p-azoxyanisol, Phys. Lett., 55-51 912). 

In these terms C. Mauguin reported in 1913 the first observation of the orienting action of freshly cleaved surfaces of the muscovite mica on a nematic liquid crystal p-azoxyanisole ... 

Before going on to discuss briefly other basic liquid crystal phases, it is worth recording some points about the development of nematic liquid crystals, since a substantial part of this book concerns these substances. The first nematic liquid crystal, p-azoxyanisole (PAA), was synthesised by Gattermann and Ritschke [102], who reported their results in 1890 it was the first liquid crystal not based on a naturally occurring substance and went on to be extensively investigated by Lehmann and others. The first relatively stable room temperature nematic liquid crystal, 4-methoxybenzylidene-4 -butylaniline (MBBA), was synthesised by Kelker and Scheurle [142] in 1969, but, for various reasons, this substance was not considered suitable for some applications. The nematic liquid crystal 4-pentyl-4 -cyanobiphenyl (5CB) is an example of a stable room temperature material designed for use in twisted nematic displays (discussed in Section 3.7) and was synthesised by Gray and co-workers in the early 1970 some details can be found in articles [117, 118). 

A typical liquid-crystal molecule, such as p-azoxyanisole, is long and rodlike (14). Their rodlike shape enables the molecules to stack together like dry, uncooked spaghetti they lie parallel to one another but are free to slide past one another along their long axes. Liquid crystals are anisotropic because of this ordering. Anisotropic materials have properties that depend on the direction of measurement. The viscosity of liquid crystals is least in the direction parallel to the long... 

A typical liquid crystal molecule, such as p-azoxyanisole, is long and rodlike (14). 

Grushka, E. and Solsky, J.F., p-Azoxyanisole liquid crystal as a stationary phase for capillary column gas chromatography, Anal. Chem., 45, 1836, 1973. 

In the nematic liquid crystalline state, groups of molecules orientate spontaneously with their long axes parallel, but they are not ordered into layers. Because the molecules have freedom of rotation about their long axis, the nematic liquid crystals are quite mobile and are readily orientated by electric or magnetic fields. Nematic liquid crystals are formed, for example, when p-azoxyanisole is heated. 

Like ebulliometry, the cryoscopic method is also limited to relatively low-molecular-weight polymers with up to 50,000. A newer variation of this method involves the solute-induced depression of the nematic-isotropic transition temperature for a liquid crystal solvent such as p-azoxyanisole [2]. This technique appears to be particularly convenient for high-molecular-weight polymers and may be applicable for M values as high as 10 . 

T he behavior of liquid crystals in applied electric fields has been the object of several recent studies (1, 3, 4)- The materials primarily used here were those in which the dipole moment of the molecule was not in the same direction as the molecular axis. When an electric field is applied to such a system by transparent electrodes, the characteristic cigar-shaped domains shown in Figure 1 for p-azoxyanisole are readily observed, using relatively low magnification with or without polarized light. The optical behavior of such domains between crossed polarizers indicates that their optic axis is essentially parallel to the electrode surface and essentially perpendicular to the direction of the applied field. 

We have studied the field- and temperature-dependence of the susceptibility of the liquid crystal phase of p-azoxyanisole for fields from 201 to 2550 gauss. The results at the highest fields agree with those of Foex at 8000 gauss, but at lower fields the susceptibility becomes field-dependent. The experimental results may be adequately represented by a simple theoretical expression obtained from statistical considerations. The only adjustable parameter in the theoretical formula is an effective mass which arises from interactions between the molecules and amounts to 2 X 10 n gram. 

Although for p-azoxyanisole (PAA) a number of measurements in electric fields have been reported (I, 5), only Foex ( ) reported measurements in a high magnetic field. In this paper we report the results of magnetic susceptibility measurements on PAA, which have been obtained by using the Faraday method with a sensitive balance described by Poulis et al. (6). The sample consisted of 0.2841 gram of PAA held in an evacuated spherical quartz container. Measurements were made at fields of 2550, 1050, 275, and 200 oersteds at a number of temperatures in the liquid crystal and liquid phase. The absolute accuracy of the susceptibility measurements was about 3%, determined by the irreproducibility in the adjustment of... 

Fig. 3.10.2. Electrohydrodynamic alignment patterns in nematic liquid crystals, (a) Williams domains in a 38 //m thick sample of p-azoxyanisole. 7.8 V, 100 Hz. (Penz. ) (Z>) Chevron pattern of oscillating domains in MBBA. Sample thickness 100 fjm. Distance between bright lines 5 /im. 260 V, 120 Hz. (Orsay Liquid...

Liquid crystals is one of the names by which certain mesomorphic phases or mesophases are known, and as the name implies refers to materials which show a high degree of structural order, yet are liquid. The topic has been reviewed recently. " Those most used in NMR work have been the thermotropic variety which have a definite meso-phase range between the melting point and a second transition producing the isotropic liquid. Both pure substances (e.g. p-azoxyanisole) and mixtures (e.g. butyl-/7-(p-ethoxybenzoyl)phenyl carbonate and p-(/7-ethoxyphenylazo)phenyl heptanoate) which provide a mesophase at room temperature, have been used. 

The aromatic azoxy compounds were among the first nematic liquid crystals to be studied and p-azoxyanisole (1) was the most widely used compound. 

For example, p-azoxyanisole (see Table 1) is a typical LC compound with a nearly linear stmcture. From a steric point of view, this is a rigid rod of length 20 A. The two benzoate tings are nearly coplanar. The majority of rod-shaped or board (lath)-shaped liquid crystals have one, two, and even three benzene tings chemically linked directly to each other or via linking units, such as —CH=CH—, —CH=N—, —NH—CO—, and —COO—, placed in the core stmcture. 

We mentioned already cholesteryl benzoate as one of the first discovered liquid crystals, although the chemical structure of cholesterol was unknown at the time. From this compound the name cholesteric phase was derived. p-Azoxyanisole (PAA), which belongs to the nOAOB homologous series (cf. Table I), was one of the first synthesized liquid crystals with known chemical structure. It is a typical representative of a rodlike compound. The first six members of this series form the nematic phase,... 

In order to avoid disturbing solvent lines a solvent with a large number of protons should be used which yields a uniform background of overlapping lines onto which the sharp lines of the guest are superimposed. P-azoxyanisole, for instance, does produce a discreet background [40]. A number of liquid crystals used for nmr spectroscopy have been summarized in the review by Diehl and Khetrapal [43]. 

Spherulites showing concentric layers present a disclination radius or diameter, but this structure is due to a topological constraint and does not seem to be linked to liquid crystal growth. Very rapid growth of cholesteric phases often generates screw dislocations of the two types shown in Fig. 24i and j, and this has been filmed by Rault in p-azoxyanisol added to cholesterol benzoate [98, 99]. Slow growth does not result in the production of these defects. 

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