Nematic texture

When the isotropic liquid is cooled, the mesophase may appear at the clearing point in the form of separate small, round objects called droplets 

In most cases the textures exhibited by polymers in the nematic state are similar to those of LMMLCs. The Schlieren, threaded and marbled textures are the ones most often cited in the literature. Examples of typical polymeric textures are shown in Figs 2.15 and 2.16.  

From the observation of singularities with S= 1/2, the mesophase can be identified unambiguously as a nematic phase since these singularities occur nowhere else. Smectic C phases, which can exhibit the Schlieren texture, only form singularities with four brushes corresponding to S=+l. 

In some preparations, the disclination lines do not lie perpendicularly, but more or less horizontally, their ends being attached to the glass surfaces and the other parts floating freely in the LC. The term threads is usually used to describe these disclination lines and the corresponding texture is the threaded texture. 

Some optical observations of defects in polymeric nematic phases exhibiting threaded and Schlieren textures showed that a massive organization of macromolecules occurs upon extended annealing or as the temperature is raised. Immediately after melting of the specimen, a large number of disclination lines are observed. However, the 

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Disclinations in tire nematic phase produce tire characteristic Schlieren texture, observed under tire microscope using crossed polars for samples between glass plates when tire director takes nonunifonn orientations parallel to tire plates. In thicker films of nematics, textures of dark flexible filaments are observed, whetlier in polarized light or not. This texture, in fact, gave rise to tire tenn nematic (from tire Greek for tliread ) [40]. The director fields... 

The polyamides are soluble in high strength sulfuric acid or in mixtures of hexamethylphosphoramide, /V, /V- dim ethyl acetam i de and LiCl. In the latter, compHcated relationships exist between solvent composition and the temperature at which the Hquid crystal phase forms. The polyamide solutions show an abmpt decrease in viscosity which is characteristic of mesophase formation when a critical volume fraction of polymer ( ) is exceeded. The viscosity may decrease, however, in the Hquid crystal phase if the molecular ordering allows the rod-shaped entities to gHde past one another more easily despite the higher concentration. The Hquid crystal phase is optically anisotropic and the texture is nematic. The nematic texture can be transformed to a chiral nematic texture by adding chiral species as a dopant or incorporating a chiral unit in the main chain as a copolymer (30). 

Liquid Crystalline Structures. In certain ceUular organeUes, deoxyribonucleic acid (DNA) occurs in a concentrated form. Striking similarities between the optical properties derived from the underlying supramolecular organization of the concentrated DNA phases and those observed in chiral nematic textures have been described (36). Concentrated aqueous solutions of nucleic acids exhibit a chiral nematic texture in vitro (29,37). 

Liquid crystalline behavior occurs in the exocuticle of certain classes of beetles. The bright iridescent colors that are reflected from the surface of Scarabaeid beetles originates from a petrified chiral nematic stmctural arrangement of chitin crystaUites in the exocuticle (38). It is suggested that this chiral nematic texture forms spontaneously in a mobile, Hquid crystal phase that is present during the initial stages of the exocuticle growth cycle. 

The mesophases of LC diols la-lg were also observed directly in polarized optical micrographs taken immediately after melting the sample. Textures were identified only by comparison with published micrographs (2 ), and are therefore tentative. A nematic texture is observed for If (Fig. 2), while more highly ordered smectic textures are observed for Ib-le and Ig (Fig. 3). 

Figure 3. a) Disclinations in an otherwise smooth nematic texture of PBT in MSA b) Ring images observed on warming a nematic PBT solution see text. The large grid spacing is 80/im. 

It was first reported in the early 1970s that these melt processible polymers could best be described as thermotropic systems which usually display an nematic texture in the melt phase [5]. Subsequently, a number of additional phases have been reported ranging from discotic structures to highly ordered smectic E G systems with three dimensional order. In the last several years an IUPAC sponsored study on nomenclature on thermotropic LPCs has been underway. A more complete set of definitions will be available shortly as a result of Recommendation No. 199 IUPAC [6]. 

With respect to the higher temperature transition at 445 °C, there are two conflicting views of this transition, namely that the phase above 445 °C is a smectic C and the other that it is nematic. Based on high temperature X-ray diffraction studies, Yoon et al. have concluded that it is a smectic C (see Fig. 4) [28], Thus, in Fig. 4, the disappearance of the 211 peak indicates that the nematic E structure is converting to a nematic C. In our work, using polarizing optical microscopy, we have observed a nematic texture for high molar mass specimens heated rapidly to 480 °C, sheared, and then quenched. In the case of a... 

Fig. 5a-b. Nematic texture of the homopolymer PHBA a DP 15, specimen quenched from 440 °C, no external shear applied b DP > 100, specimen quenched from above 480 °C, external shear was applied to increase the rate at which the fluid specimen sheared between the glass surface above... 

The polyamides are soluble in high sirength sulfuric acid or in mixtures of hexamelhylphosphoiamide. AMV-diinethylaeciamidc. and l.ifT. The liquid-crystal phase is optically anisotropic and the texture is nematic. The nematic texture can he transformed lo a chiral nematic texture by adding chiral species us a dopant or incorporating a chiral unit in the main chain as a copolymer. 

Figure 2. DSC trace of methyl-substituted poly(ester) with pentamethylene spacer, which displayed both smectic and nematic textures.

Liquid crystalline polyguanidines 25 are obtained from the carbodiimide 24 also display a lyotropic nematic texture caused by strong dipolar-dipolar interaction between the main chain and side chains. ... 

Except for the structure containing 100% of ferrocene unit, which decomposed before melting, all the organometallic copolymers exhibited birefringent melts. Nematic textures were identified by means of polarized optical microscopy and, in one case, by X-ray diffraction studies. For comparison purposes, a polymer without ferrocene unit was prepared, but showed no mesomorphism. The authors deduced that the ferrocene framework was contributing to the liquid crystallinity of the ferrocene-containing polymers. 

An earlier study of the molecularly doped polymer liquid crystalline system shown in Figure 6.9 revealed more complex behavior (27, 32). In this study, the amount of induced nonlinearity was path dependent, with the largest observed value occurring when poling was done near Tg ( 25 C). The host polymer exhibits a nematic-to-isotropic transition at = 100 C and a nematic texture between Tg and. Poling at 50 C produces a decrease in approximately 1 order of magnitude. The l/T dependence... 

The aspect ratio of the p-methoxycinnamate moiety was extended by synthesizing the phenyl ester of p-methoxycinnamate (see Scheme IV). Microscopy studies did not reveal any l.c. mesophase in monomer 9. However, the resulting homopolymer IX possessed l.c. order. Microscopy observations on polymer IX revealed a very stable homogeneous nematic texture. The DSC results showed a Tg at 21°C and a nematic to isotropic transition at 168°C. 

The reformation of chiral nematic phase from nematic phase sometimes goes through the transient state of band texture. The formation of band texture is also a relaxation phenomenon. For HPC/H2O solutions, the band texture is only observed when the molecules have been well orientated in the shear direction.A critical lower shear rate limit exists because of the stability of chiral nematic textures. 

Senthil and Kannan prepared ferrocene-based liquid crystalline polymers containing phosphate groups in the backbone.191,192 The majority of these polymers showed grainy nematic textures, whereas polymers with 8 and 10 methylene groups in the backbone displayed clear nematic textures. Liquid crystalline behavior was not observed for the polymer incorporating both ethylene spacers and a pendent phenyl group. 

Fig. 13. A Batonet texture of the smectic-A phase of PEI 82i upon cooling from the isotropic melt. B Smectic texture of 83a. C Nematic texture of co PEI 83g...

Figure 4.15. A nematic texture showing singularities of high strength. The sample is a nematic polymer with two-dimensional type mesogenic units.

When an isotropic texture appears on cooling from an isotropic melt, it is formed from the coalescence of droplets that separate from the liquid. This mechanism is indicative of a nematic phase, and one of three possible characteristic textures may result, depending on the way the droplets form larger domains. These are the Schlieren, the threaded, or the marbled nematic textures. An example of Schlieren texture is shown in Figure II. 12. 

Optical observations of mesophase textures have been previously discussed in detail It is to be stressed here that they are generally in excellent agreement with thermal analysis data and indicate the existence of smectic and nematic phases in samples 42 (w=2) through i (DP =4.1). On the contrary, polymer 43 7 CBPj =8.7) displayed a weakly birefringent, possibly nematic, texture. 

Fig. 4.3 a, b Homeotropic and schlieren textures of the nematic phase of 8CB containing 5 wt% 1 coated GNPs. c Sheared nematic texture near to the edge of the cover slip, d Focal-conic defect texture of the smectic A phase. Magnification xlOO [46], Copyright from Wiley-VCH 2011... 

The turbidity of the nematic states of low molecular weight compounds is due primarily to fluctuations in the orientation of the director over distances on the order of the wave length of visible light (12). The optical clarity of PCBAS indicates that the regions of nematic organization are much smaller in size. Light is not appreciably scattered, nor are the typical nematic textures resolvable in the polarizing microscope. 

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