Textures chiral nematics

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 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. 

Liquid crystalline (LC) solutions of cellulose derivatives form chiral nematic (cholesteric) phases. Chiral nematic phases are formed when optically active molecules are incorporated into the nematic state. A fingerprint texture is generally observed under crossed polarizers for chiral nematic liquid crystals when the axis of the helicoidal structure is perpendicular to the incident light (Fig. 2). 

In general, chiral nematic polymer liquid crystals (LCP) cannot form monodomains in which the rodlike polymers have a spatially uniform orientation within the sample. Typically, because of the high density of orientational defects, the LCPs are textured, with a distribution of polymer orientation. Microscopically, the polymer chains have a preferred orientation with a relatively narrow distribution around the average orientation. Macroscopically, the variation in space of the orientation results in a domain structure. Defects and orientational variations give rise to the polydomain texture and the overall LCP sample may be randomly ordered (Fig. 3). 

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. 

Compound 43 was found to exhibit a chiral nematic phase (see the texture in Fig. 48), hexagonal disordered columnar (see Fig. 49) and rectangular disordered columnar phases, in the sequence g 5.4 Col d 30 Col 102.3 N 107.7 °C Iso Liq [94]. Thus the increase in the number density of the meso-gens bound to the central scaffold transforms the situation from the octamers 41 and 42, which exhibit calamitic phases, to one where the hexadecamer 43 exhibits columnar phases. However, the formation of columnar phases when the dendrimer possesses rod-like mesogenic groups is not easy to vi-... 

Fig.48 The Grandjean plane texture of the chiral nematic phase of supermolecule 43. There is a blue iridescent color which is due to the selective reflection of light from the helical macrostructure (xlOO)...

The mesophase defect textures exhibited by 47 were typical of those normally found for a chiral nematic phase, except they were only revealed upon annealing, which is probably a function of the viscosity of the material. Thus the sample was annealed just below the clearing point. After 24 h, large areas of the preparation evolved to show fingerprint defects and the Grandjean plane... 

Fig. 56 a fingerprint texture for an uncovered droplet of compound 47, and b the Grandjean texture of the chiral nematic phase... 

The texture of polymeric chiral liquid crystalline phases. The chiral liquid crystalline phases include the chiral smectics and the chiral nematic or cholesteric phase. Poly(7-benzyl-L-glutamate) and derivatives of cellulose are popular examples of polymers that form a chiral mesophase. Side-chain type copolymers of two chiral monomers with flexible spacers of different, lengths and copolymers of one chiral and the other non-chiral mesogenic monomers may also form a cholesteric phase (Finkelmann et al., 1978 1980). In addition, a polymeric nematic phase may be transformed to a cholesteric phase by dissolving in a chiral compound (Fayolle et al., 1979). The first polymer that formed a chiral smectic C phase was reported by Shibaev et al. (1984). It has the sequence of phase transition of g 20-30 Sc 73-75 Sa 83-85 I with the Sc phase at the lower temperature side of Sa- More examples of Sc polymers are given by Le Barny and Dubois (1989). 

A typical fracture pattern for the TGBA phase with a surface cut nearly parallel to the pitch direction is shown in Plate 3. The surface has a regular undulating texture similar to that observed for chiral nematic phase, which also agrees with the surface contours predicted by a simple model for the surface fracture of a chiral nematic. Measurement of the minimum undulating repeat distance gives a value of 0.25 to 0.30 pm, which corresponds to one half of the pitch. Thus, the pitch is about 0.5 to 0.6 pm, which is consistent with the value determined from the optical measurements described earlier. 

Plate 3. A typical fracture pattern for the TGBA phase with a surface cut nearly parallel to the pitch direction, the surface has a regular undulating texture similar to that observed for the chiral nematic phase... 

Recently Ribeiro et al. [65] reported on the synthesis and characterization of a variety of tolanes that had optically active sulfinate groups. Some of these compounds, see 23, were found to possess a phase that exhibited oily-streak textures typical of chiral nematic phases and also defect pattern associated with columnar phases (the earlier photomicrograph Plate 6 for 14P1M7 is similar). 

Lyotropic cellulosics mostly exhibit chiral nematic phases, although columnar phases have also been observed. The molecules in the thermotropic state also form chiral nematic order, but it is sometimes possible to align them in such a way that a helicoidal structure of a chiral nematic is excluded. Upon relaxation they show banded textures. Overviews on lyotropic LC cellulosics are... 

Only a few solvents are known to dissolve cellulose completely, and solid cellulose decomposes before melting. Therefore, it is difficult to study the mesophase behavior of cellulose. Chanzy et al. [32] reported lyotropic mesophases of cellulose in a mixture of jV-methyl-morpholine-Af-oxide and water (20-50%), but were unable to determine the nature of the mesophase. Lyotropic cholesteric mesophase formation in highly concentrated mixtures of cellulose in trifluoroa-cetic acid + chlorinated-alkane solvent [33] and in ammonia/ammonium thiocyanate solutions [34] has been studied, and although poor textures were obtained in the polarizing microscope, high optical rotatory power has been measured in an optical rotation (ORD) experiment, which could be fitted to the de Vries equation [Eq. (3)] for selective reflection beyond the visible wavelength region and was taken as proof of a lyotropic chiral nematic phase. 

The optical properties of chiral nematic phases are closely related to their supermolecular Structures, as stated by the considerations of de Vries. In particular, the planar textures exhibit beautiful colors correlated to the pitch P of the helicoidal structures by Eq. (1), if the selective reflection wavelength lies in the visible range, and many examples are shown in Fig. 2. 

Various factors such as size, shape, dispersity, charge, electrolyte, and external stimuli can affect the liquid crystallinity, pitch, domain size, ordering and other properties [9]. The sample suspension was submitted to ultrasonic treatment at 800 W for 8 min. After high power sonication, we could observe a trip-like texture [Fig. 13.6a]. And "tactoid could be observed in the following 24 h, which would aggregate with each other gradually [Figs. 13.6b-e]. Then the pitch characteristic of a chiral nematic phase is clearly observed and the domain size of the chiral nematic... 

The phototuning of BPs can also be fabricated in a pure material system [147]. Das et al. reported a light-induced stable blue phase in photoresponsive diphen-ylbutadiene based mesogen 37. This compound was found to exhibit SmA and N during heating. When the temperature was kept at 118 °C, the photoisomerization induced an isothermal phase transition from SmA to N. Photoirradiation of the SmA film held at a higher temperature (124 °C) for 100 s resulted in transition to a phase with a characteristic classical BP texture showing in Fig. 5.30. The BP was thermodynamically stable and could be maintained at this state for several hours. The characteristic sharp reflection bands compared to the rather broad reflection bands observed for the chiral nematic phase confirmed the formation of BP. The photoinduced formation of the BP exhibited a reflection centered at 510 nm. Subsequent irradiation led to the blue shift to 480 nm in the reflection band. 

Fig. 11.10 a POM image of the chiral nematic (N -LC) phase of (S)-Poly-4 in 10 wt % lyotropic LC solution in toluene showing a double-spiraled texture. Inset shows a fingerprint texture with a helical half pitch of 1.5 pm. b Schematic representation of the N -LC phase of (5)-PA17. Reprinted with permission from [18]. Copyright 2012, American Chemical Society... 

FIGURE 3.1 Chiral nematic LC induced by the addition of chiral dopant into nematic LC. Schlieren texture ( left) and fingerprint texture ( right) are observed for nematic and chiral nematic LCs, respectively, in polarized optical microscope. 

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