Nematic modifications

Transitions with the participation of liquid crystals sometimes show characteristic phenomena. If a nematic modification turns to a smectic A or smectic C phase, transient stripes in the form of a myelinic texture (also called chevron texture or striated texture) are often visible. Typically for the polyester prepared from di-w-propyl-/ -terphenyl-4,4" carboxylate and tetramethylene glycol, the nematic phase separates from the isotropic liquid on cooling in droplets which coalesce and form large domains. Cooling of the threaded-schlieren texture produces a transition to the smectic A phase this change is characterized by transition phenomena, mostly stripes, which broaden into larger areas ( transition bars ). 

The cholesteric phase maybe considered a modification of the nematic phase since its molecular stmcture is similar. The cholesteric phase is characterized by a continuous change in the direction of the long axes of the molecules in adjacent layers within the sample. This leads to a twist about an axis perpendicular to the long axes of the molecules. If the pitch of the heHcal stmcture is the same as a wavelength of visible light, selective reflection of monochromatic light can be observed in the form of iridescent colors. 

Other more exotic types of calamitic liquid crystal molecules include those having chiral components. This molecular modification leads to the formation of chiral nematic phases in which the director adopts a natural helical twist which may range from sub-micron to macroscopic length scales. Chirality coupled with smectic ordering may also lead to the formation of ferroelectric phases [20]. 

In 1933, Bernal and Crowfoot [1] reported on the solid state polymorphism of p-azoxyanisole. They found two crystalline modifications of this compound, a stable yellow form and an unstable white polymorph. Krigbaum et al. [31 reexamined the crystal structure of the stable yellow form. The compound shows an imbricated structure which is the basic packing required for nematic behaviour according to Gray [132]. 

Crystalline forms presenting large amounts of disorder of the kind (ii) or (iii) are generally called mesomorphic modifications (Section 3.6), in analogy with the ordered liquids (smectic and nematic). In these cases the lack of periodicities in one or two dimensions (e.g., along the chain axes or along the directions normal to the chain axes) prevents the definition of a unit cell. Typical features in the X-ray diffraction patterns of mesomorphic forms are diffuse halos on the equator or on the layer lines depending on the kind of disorder present. 

Discotic liquid crystals arise from disk-shaped molecules as nematic or cholesteric mesophases. Their structural characteristics are similar to the respective ealamitie mesophases, that is, the normals of the disks are oriented parallel. Instead of the smectic mesophases, diseotie columnar liquid crystals arise from eonnecting the disks to each other. The columns of the discotic columnar mesophase form a two-dimensional lattice whieh is in a hexagonal or rectangular modification. In addition, the columns may be tilted (Fig. 2f,g). 

A purely organic chiral nitroxide which shows liquid crystalline behaviour as well as intriguing magnetic properties and a dependence on the enantiomeric nature has been reported [180]. The reason for studying the compounds was to increase the sensitivity of mesophases to magnetic and electric fields. The racemic modification of the radical, which displays a nematic phase, proved to be more sensitive to alignment than the cholesteric phase with the enantiomers present. It was proposed that the compounds may also be used to study the dynamic nature of mesophases by electron paramagnetic resonance spectroscopy. 

Cell Construction. The optical interference technique to be used here is a modification of that described by Gramsbergen and de Jeu. This method uses a thin paraUel-plate cell containing a planar aligned nematic liquid crystal sample. The sample cell consists of two parallel 15 X 50 mm microscope slides separated by a gap D in the range 50-100 /zm, which is determined by spacer shims. A description of its construction is given below. 

The smectic state, apart from exhibiting a rich variety of modifications, is also thermodynamically ordered. Thus, there is a well-defined sequence of phases formed on cooling from the isotropic liquid. For instance, the following transition sequence would be observed for a ferrocene liquid crystal exhibiting enantiotropic smectic C, smectic A and nematic phases ... 

H. Akiyama, M. Momose, K. Ichimura, and S. Yamamura, Surface-selective modification of poly (vinyl alcohol) films with azobenzenes for in-plane alignment photocontrol of nematic liquid crystals, Macromolecules 28, 288-293 (1995). 

By deuterium NMR, we have ascertained that the limit of solubility of 8a in the crystal-B phase is in fact somewhat less than 1.0 mol%. Under the conditions of our flash photolysis experiments, 8a is distributed between both a ketone-rich nematic phase and a ketone-depleted smectic phase at temperatures between 53-35°C. At 35°, the ketone-rich phase is transformed into a "p-phase" in which the ketone exhibits isotropic H and C NMR behavior, and which eventually crystallizes (though not under these conditions) to form a stable binary smectic or solid modification consisting of 8-10 mol% ketone and CCH-4. Thermal microscopy experiments with mixtures containing 1-3... 

However, when a laterally appended mesogen is introduced into a system where all of the other mesogens are terminally appended, as for 51, the laterally appended mesogen acts as a disruptor to the orderly packing of the molecules together. This results in the suppression of the formation of smectic modifications and support for nematic phases. Thus compound 49, which... 

The finding that the PEIs of 27b and monosubstituted hydro quinones form broad nematic phases, but show little propensity to crystallize, has prompted various modifications of their structures and properties. In this connection it should be stated that non-crystalline LC-polymers have found little interest in the past decades, but they may be attractive for various applications provided that the Tg can be varied between 90 and 250 °C. For instance, the absence of crystallinity has the advantage that the mechanical properties do not depend on the thermal history, and thus on the processing conditions. The temperature allowing a convenient processing may be reduced below 200 °C, which is of interest for the processing of LC-polymer reinforced blends and composites. Furthermore, non-crystalline nematic FC-polyesters are a useful basis for the synthesis of cholesteric lacquers, films or pigments (Sect. 5). 

Whereas, the co PEIs 39a-c, 40a-f and 4a-c are characterized by a partial replacement of the dicarboxylic acid 27b, a series of co PEIs (42) was described in a patent [56] where the composition of the diphenols was varied. Higher contents of hydroquinone raise the melting point and the crystallinity higher contents of ferf-butylhydroquinone raise the T . These series of co PEIs which allow a broad variation of the Tg were prepared by transesterification of PET with combinations of the dicarboxylic acid 27b and acetylated methyl hydroquinone, ferf-butylhydroquinone or phenylhydroquinone (43a-c)[57]. Figure 10 displays the influence of the molar composition on the Tg and on the E-modulus of the co PEIs 43b. All these co PEIs are noncrystalline and nematic up to a molar fraction of 75-80% PET. At higher PET contents crystallization of long PET blocks takes place and the LC-phase has completely vanished. A further modification of these LC-PEIs can be achieved by cocondensation of 4-acetoxybenzoic acid or 6-ace-toxy-2-naphthoic acid [58]. 

In the latter two phases backbones have the spindle-like conformation, i.e., the prolate shape with (R%) > R p), the characteristic of main chain liquid crystalline polymers. Important means of investigating the conformations of side chain liquid crystalline polymers include small angle neutron scattering from deuterium-labeled chains (Kirst Ohm, 1985), or small angle X-ray scattering on side chain liquid crystalline polymers in a small molecular mass liquid crystal solvent (Mattossi et al., 1986), deuterium nuclear resonance (Boeffel et al., 1986), the stress- or electro-optical measurements on crosslinked side chain liquid crystalline polymers (Mitchell et al., 1992), etc. Actually, the nematic (or smectic modifications) phases of the side chain liquid crystalline polymers have been substantially observed by experiments. 

Similar structural modifications using chiral aliphatic chains (citronellyloxy and its hydrogenated analog) were performed, and these yielded complexes with a chiral nematic phase, though when both terminal chains were chiral, mesomorphism was suppressed. The phases typically occurred between 120 and 160 C. Siting the bulky rhenium fragment in one extremity of the molecule resulted in the destabilization of the mesomorphism, but not its suppression. 

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