Some factors influencing the pitch

In this section, we present a brief survey of experimental studies on the dependence of the pitch on temperature, composition, etc. 

Dependence of pitch on temperature applications to thermography In most pure cholesteric materials, the pitch is a decreasing function of the temperature. An elementary picture of the temperature dependence of the pitch can be given in analogy with the theory of thermal expansion in crystals. Assuming anharmonic angular oscillations of the molecules about the helical axis, the mean angle between successive layers 

The strong temperature dependence of the pitch has practical applications in thermography, as was first demonstrated by Fergason. The material has to be so chosen that the pitch is of the order of the wavelength of visible light in the temperature range of interest. This is achieved by preparing suitable mixtures. Small variations of temperature 

Pollmann and Stegemeyer investigated the effect of pressure on the pitch of cholesteryl oleyl carbonate (COC) mixed with cholesteryl chloride and found that the pitch increases very rapidly with pressure, the effect being more pronounced the greater the concentration of COC. This appears at first quite surprising, but, in fact, the explanation is straightforward. We have emphasized that the pitch diverges as the temperature 

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). 

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