Cholesteric structures

Ritchey et al. (113) showed the introduction of trifluoroacetate groups at the unsubstituted hydroxyls of cellulose acetate causes a reversal in handedness of the cholesteric structure. Likewise the introduction of an aceto group in acetox3rpropyl cellulose changes the twist (116). 

Identify nematic, smectic, and cholesteric structures in liqnid crystalline polymers. 

The main feature identifying a cholesteric mesophase in polymers is the presence of optical texture with selective circularly-polarized light reflection. This indicates the formation of 1-helical cholesteric structure in LC copolymers. The X-ray patterns of actually all cholesteric copolymers described (with the exclusion of polymers 3.1 and 4.1, Table 13) correspond to those of nematic and cholesteric low-molecular liquid crystals, which is manifested in a single diffuse reflex at wide scattering angles. At the same time, for copolymers 3.1 and 4.1 (Table 13) small angle reflexes were observed 123), that are usually missing in low-molecular cholesterics. 

In cholesteric structures there is also alignment, but the direction of alignment rotates on a screw axis normal to the direction of alignment (Figure 16.2). This spiraling is responsible for unique optical properties. 

Coatings derived from cholesteric liquid crystalline polymers are used commercially as reflective sheets and polarisers. The liquid crystal is cooled below the vitrification temperature resulting in a solid polymer that is amorphous but contains large regions of frozen liquid crystalline order. Such structures are also found in nature in the iridescent, almost metallic colours of beetles and other insects, which result from helical cholesteric structures in the outer layer of the carapace. 

There have been a lot of studies of cholesteric films and gels in order to exploit their potential as specific optical media and as other functional materials. Most of the preparations were achieved by modification or improvement of previous attempts to immobilize the cholesteric structure of cellulose derivatives into the bulky networks either by crosslinking of cellulosic molecules with functional side-chains in the liquid-crystalline state [203], or by polymerization of monomers as lyotropic solvents for cellulose derivatives [204-206],... 

The solution specimen is usually kept in a sample tube for a few weeks after the polypeptide is dissolved in a solvent to allow the formation of liquid crystals. In carrying out usual measurements the solution is put, for example, in a rectangular quartz cell of path length 1.00 mm usually with a quartz spacer to adjust the path length from 0.100 to 0.025 mm, under a microscopic stage no cholesteric structure is detected in solutions less than 0.250 mm thick, irrespective of solvent ured 28,... 

A solution of PBLG in certain solvents such as methylene chloride and dioxane can form the cholesteric structure and exhibits stroi form optical rotation. A theoretical description for this in terms of observable quantities is in the form (4S) ... 

Liquid crystalline solutions of KB LG (or of RBDG) and films prepared from such srdutions also ow significant CD in the wavelength range of the aromatic absorption bands 46,47) the CD for PBDG in methylene chloride is positive (46). CD bands are also induced when dye mdecules are introduced in liquid crystal films of polypeptide (PMDG). These induced CD bands are interpreted as arising from the dissymmetric field of the cholesteric structure 48). 

The initial cholesteric structure present, especially m dioxane, is considered to be separated into small fragments upon putting the solution between the cone and the plate of the rheogoniometer, and it appears unthinkable that the continuous cholesteric structure is kept during the rotation of the plate. Rheo-optical studies on the liquid crystalline solution have just started (52), and will supply extra information about the solution structure under shearing stresses. 

Experimentally, the cholesteric structure parameters, i.e., pitch and handedness, can be derived from the optical properties of the phase and very specially from its so-called selective reflection. This most striking phenomenon is the reflection of one component of circular polarized radiation in a spectral interval around that wavelength which within the medium matches the cholesteric pitch, i.e. XrIh = i when n denotes the... 

As early as in 1951 de Vries showed that a twisted stack of birefringent layers is an adequate model for a cholesteric structure in order to reproduce a principally correct spectral dependence of the optical rotation also around the selective reflection band as it was recorded in a different way for Fig. 4.6-8. Even if the layer thickness is formally reduced to zero the optical rotation and its spectral dependence is preserved. Several other approaches were reported to describe particular effects of the cholesteric structure such as the selective reflectance or the rotatory anomaly (e.g. Chandrasekhar and Prasad, 1971 Chandrasekhar and Ranganath, 1974 SchSnhofer et al., 1983 Eidner et al., 1989). 

At constant temperature, when the polymer concentration is increased, anisotropic droplets appear and coalesce. Layers of the cholesteric structure can be observed Inside the droplets. 

A lyotropic, nematic solution of cellulose was formed in a NH3/NH4SCN solvent in what are presumably good solvent compositions. Evidence strongly suggests that the twisted nematic or cholesteric structure that results when solutions of chiral cellulose chains interact may be repressed or compensated so that interactions among chiral centers are minimized. Our reasoning is based a body of experimental evidence which includes ... 

The phase behavior is similar to that of a lower critical solution temperature (LCST), hence it is different from the above systems. The HPC/water system is an interesting model system because of the rich variety of phase structure 01 the material. HPC is a semicrystalline polymer in the solid state (7), but exhibits thermotropic liquid crystalline character at elevated temperatures below the melting point (8). It shows isotropic phase in dilute solutions, but forms an ordered liquid crystalline phase with cholesteric structure in concentrated solutions (4). 

Fig. 19 illustrates the change of with the solvent composition in the TCP-m-cresol system. At low temperatures, a right left right sense inversion can be induced by increasing the TCP content. The twofold inversion disappears above 60 °C since the cholesteric structure is left-handed in both TCP and m-cresol. 

A similar approach has been used to produce materials with a chiral (cholesteric) structure by performing the experiments described above in the presence of a low molecular weight chiral liquid crystalline material (Figure 9.6). The chiral material is not covalently attached to the network and can be removed subsequently to produce an imprinted chiral structure. As before, the polymer displays a nematic mesophase between the glass transition (Tg 33°C) and the transition to an isotropic fluid (rN, 128°C). 

Keywords Liguid-crystalline, polycondensation, termotropic, terefhtaloyl-di(n-oxibenzoat) links, mesohenic groups, spacers, nematic, smectic, cholesteric structures, oligoethers, blockcopolymers... 

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