Cholesteric pitch composition

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... 

The sense of cholesteric twist, in the liquid crystal of PBLG in dioxane, is opposed to that in CH Cl, as pointed out by Robinson. We measured the cholesteric pitch of PBLG in various mixed solvent systems, and estimated the sense of cholesteric twist in the individual solvent. If two solvents which make the sense of cholesteric twist of PBLG opposite to each other, are mixed, the cholesteric pitch of PBLG in mixed solvent will diverge at the critical composition. It is found that the sense of cholesteric twist of PBLG in dioxane and chloroform is opposite to that in dichloromethane, dichloroethane and benzene. 

As is seen from Table 13, cholesteric copolymers display a maximum of selective light reflection ( w) in an IR- or a visible part of the spectrum. By varying the composition of a copolymer, it is possible to vary Xmax, in accordance with the stipulation max = nP, is proportional to the pitch P of the helical structure of a LC polymer (n — is the refractive index). The pitch of the helix in cholesteric copolymers is usually decreased, when the temperature is raised 105) (at temperatures above Tg), which is equally common for low-molecular cholesterics142) (Fig. 23a). The observed fact that the helix pitch for LC copolymers 2.1-2.3 (Table 13, Fig. 23b) is increased, is rather unusual but explicable within the theoretical views regarding vibrational movement of macromolecular fragments and their conformational mobility 60). 

Mixtures dependence of pitch on composition We have seen in 4.1.6 that a mixture of right- and left-handed cholesterics adopts a helical structure whose pitch is sensitive to temperature and composition. This result was first described by Friedel. For a given composition, there is an inversion of the rotatory power as the temperature is varied, indicating a change of handedness of the helix. The inverse pitch exhibits a linear dependence on temperature, passing through zero at the nematic point where there is an exact compensation of the right- and left-handed forms (fig. 4.1.16). 

Many works deal with the variation of the helicoidal pitch in cholesteric phases as a function of temperature and composition. The best way to elucidate the origin of the twist seems to be to compare the pitch variations in lyotropic and in thermotropic systems. The first accurate work in this field is due to Robinson (1958-66) who studied PBLG (polybenzyl-L-glutamate) a synthetic polypeptide in organic solvents as dioxane, ethylic alcohol, chloroform etc. and Cano (1967) who made measurements of the pitches of nematic paraazoxyphenetol with different amounts of cholesterol benzoate. 

A typical case of the nonmonotonic behavior of the pitch (in fact, the inversed pitch Pq ) on mixture composition is shown in Fig. 1.18 [45, 46]. It is worthwhile to note that all left-hand cholesterics (like cholesteryl acetate, pelargonate, and oleate) form left-hand mixtures with BBBA. On the other hand, for right-twisted cholesterics (like Ch, chloride) we can observe a change in the sign of the helix from left to right. There are also compensation points where a mixture is achiral. In this case, a nematic matrix itself, not being twisted initially behaves like a left-hand cholesteric. This problem was the subject of a hot theoretical discussion [47, 48]. 

---