Cholesteric pitch temperature

Thermotropic cholesterics have several practical applications, some of which are very widespread. Most of the liquid crystal displays produced use either the twisted nematic (see Figure 7.3) or the supertwisted nematic electrooptical effects.6 The liquid crystal materials used in these cells contain a chiral component (effectively a cholesteric phase) which determines the twisting direction. Cholesteric LCs can also be used for storage displays utilizing the dynamic scattering mode.7 Short-pitch cholesterics with temperature-dependent selective reflection in the visible region show different colors at different temperatures and are used for popular digital thermometers.8... 

In addition to the rigidity, steric effects and flexibility of side groups seems to Influence the formation and properties of cellulosic mesophases they allow (or not) the existence of a mesophase before crystallization, influence the temperature for onset of a mesophase, and contribute to the value of the cholesteric pitch. 

The recent studies on the structure and properties of polypeptide liquid crystals, which are formed in solution as well as in the solid state, are reviewed in this article. Especially the cholesteric pitch and the cholesteric sense (right-handed or left-handed), which are characteristic factors of cholesteric liquid crystals, are discussed in detail in relation to the effects of temperature, concentration, and solvent. Further cholesteric liquid crystalline structure retained in cast fdms and thermotropic mesomorphic state in some copolypeptides are also discussed. 

The temperature dependence of the cholesteric pitch in the polypeptide liquid crystals has been investigated in various solvents. The pitch P is related to the twisting angle (p between neighboring molecules separated by a distance d along the axis of torsion as follows. 

Fig. 26a and b. Temperature dependence of CD spectra of a colored PBLG film a cholesteric pitch bands around 500 nm b LCICD of the side-chain phenyl groups... 

We have discussed in detail the cholesteric pitch and the cholesteric sense in polypeptide liquid crystals, centering around our own work, which had not been investigated much since Robinson had presented excellent works about a quarter of a century ago. We have show that compensation of the cholesteric sense caused by temperature and solvent can be consistently explained by the theory of Kimura et al., in which the intermolecular force is assumed as the sum of a repulsion of a hardcore with shape of a twisted rod and of dispersion forces of Maier-Saupe-Goossens type. 

Recently Blumstein and coworkers reported on the thermotropic properties of a series of main chain copolyesters with different azoxybenzene mesogenic units and flexible spacers consisting of varying ratios of (-I-) 3-methyl adipic acid and 1,12-dodecanedioic acid. Melting temperature of the copolyesters showed minimum values for either the 50/50 compositions or the 25/75 combination of the two spacer components, depending on the nature of the mesogenic units. However, Tj tended to decrease linearly with the content of the 1,12-dodecanedioic acid unit, except in one case. They also observed that the cholesteric pitch of the copolyesters seemed to increase as the concentration of the achiral unit, dodecanedioic acid, increased. Similar observations were reported earlier by Strzelecki s group... 

Dependence of cholesteric pitch on temperature and external field... 

Then the liquid crystalline monomers were polymerized under an ultraviolet irradiation. The system became a cholesteric gel. After heating the sample and removing the chiral molecules CB15, the helical structure remained. In Table 6.10, the cholesteric pitch P and reflective peak wavelength Amax are listed as a function of temperature at which polymerization occurs. The CB15 weighed 28% in the original mixture before the removal. 

Fig. 2 POM observations of a concentrated PCIBLA solution (25 wt%) in TCE. The birefringent cholesteric texture changes its sign from (a) left (90°C) to (c) right (102°C) with increasing temperature, (b) The cholesteric pitch diverges in the transition region (97°C). The screw sense of the polypeptide backbone transforms from right to left, accordingly. Reprinted with permission from [76]. Copyright 2005 Wiley-VCH...

LCs were the earliest studied structures, in which polypeptide homopolymer rods pack in an ordered manner to form smectic, nematic, and cholesteric phases. The smectic LCs are mainly formed by polypeptide homopolymers with identical polymer length. The cholesteric phase can be prepared by synthetic polypeptides with polydisperse chain length. The nematic phase can be regarded as a special example of the cholesteric phase with an infinite cholesteric pitch. The cholesteric pitch and chirahty in the polypeptide LCs are dependent on many factors, such as temperature, polymer concentration, solvent nature, and polypeptide cOTiformation. Deep understanding of such phenomena is necessary for preparation of ordered polypeptide assembles with delicate stmctures. The addition of denaturing solvent to polypeptide solution can lead to an anisotropic-isotropic reentrant transition at low temperatures where the intramolecular helix-coil transformation occurs. However, the helical structure is more stable in LC phase than in dilute solution due to the conformational ordering effect. 

Watanabe J, Nagase T (1988) Thermotropic polypeptides. 5. Temperature dependence of cholesteric pitches exhibiting a cholesteric sense inversion. Macromolecules 21 171-175... 

Recently, there are several reportsA.) discussing the twist mode of the binary system of the thermotropic liquid crystals composed of a nematic solvent and a cholesteric solute. Another interest is in attempting to pursue the structural similarity between thermotropic liquid crystals and lyotropic one, especially for the dependence of cholesteric pitch on the temperature. 

One of the best measures of twist is thought to be the cholesteric pitch. The temperature dependence of cholesteric pitch is then measured for poly-y-benzyl-L-, and poly-y-benzyl-D-glutamates ( PBLG, and PBDG ) and poly-y-alky1-L-glutamates, in various sol-ven-t systems. The temperature dependence of some physical properties were also measured. 

Recently DuPre et al reported that, S increases linearly with temperature rise. Qualitatively, their results are consistent with ours. However, the time required to reach the equilibrium pitch, varied with temperature, the concentration of polymer, and also the thermal history. For PBLG solution in dichloro-ethane (EDC), which concentration is 0.12 vol/vol, the variation of pitch with time was measured at a constant temperature by T-jump method. Fig. 1 shows the time dependence of cholesteric pitch by the T-jump method from -2°C to +30°C, 40°C and 50°C respectively. It is clear that the time required to arrive at the equilibrium pitch is shorter at higher temperature but is still over several hours. Therefore, the equilibrium pitch must be measured after prolonged aging at each measuring temperature. It was found that the... 

The extrapolated line of log S-log C crossed each other at a critical concentration Cq at which S stays constant and independent of temperature. These results suggest that the temperature dependence of the cholesteric pitch would inflect at the concentration higher than Cq This is analogous to the behavior of thermotropic liquid crystals composed of cholesteric solute and nematic solvent, where the sign of dS/dT reverses at a critical concentration. It is understood that the behavior of both thermotropic and lyotropic liquid crystals is comparable provided that the nematic substances of the former are substituted with the solvents of the latter. The critical concentration Cq is about 0.41 vol/vol and this value is very close to the concentration at which the side chains on neighboring molecules of the polymer come to contact each other ( refer to fig.5 ). From these results, it is expected that the origin or mechanism of twist would change at this concentration Cq. The... 

It was found that the cholesteric pitch appeares again at higher temperature, and chilarity at higher temperature is opposed to that at lower temperature region, as shown in fig.14. The sign of ORD of films cast... 

The alternatively black and white retardation lines are connected to the cholesteric pitch or more precisely to the anisotropy of polarizability of the macromolecules and to their orientation relative to li t propagation. Figure 2 illustrates how in the case of the same polymer, these finger print patterns are modified by dilution of the cholesteric polymer in a simple solvent. A quantitative measurement of the disappearance of the texture at the order-disorder transition temperature can be obtained by recording the light... 

The temperature dependence of the thermotropic cholesteric pitch was found to follow the relationship described by the empirical equation (Bhadani and Gray 1983 Rusig et al. 1994 Shimamura et al. 1981 Tseng et al. 1981) ... 

Watanabe J, Nagase T (1988) ThermoLopic polypeptides. 5. Temperature dependence of cholesteric pitches exhibiting a cholesteric sense inversion. Macromolecules 21(1) 171-175 Wenzel RN (1936) Resistance of solid surfaces to wetting by water. Ltd Eng Chem 28(8) 988-994 Werbowyj RS, Gray DG (1976) Liquid crystalline structure in aqueous hydroxypropyl cellulose solutions. Mol Cryst Liq Cryst 34(4) 97-103... 

The cholesteric helices can be right-handed or left-handed and usually have a temperature-dependent pitch also, in a few cases the sense of the helix... 

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