Physical chiral nematics

When the mesogenic compounds are chiral (or when chiral molecules are added as dopants) chiral mesophases can be produced, characterized by helical ordering of the constituent molecules in the mesophase. The chiral nematic phase is also called cholesteric, taken from its first observation in a cholesteryl derivative more than one century ago. These chiral structures have reduced symmetry, which can lead to a variety of interesting physical properties such as thermocromism, ferroelectricity, and so on. 

When the helix is completely unwound a clear state is produced as a consequence of the formation of a pseudonematic phase with no helical structure from the corresponding chiral nematic phase. The analyser then absorbs linearly polarised light. Therefore, black information can be displayed at the appropriate pixels against a white background to produce an image with positive contrast. The threshold voltage for the unwinding of the helix is dependent on a number of physical parameters ... 

Woo the temperature dependence of pitch for chiral nematic polymers does not seem to follow any particular pattern. It is believed that as temperature is increased, specific interactions, e.g., hydrogen bonding, whether inter- or intramolecular or polymer-solvent interactions are destroyed. The polymer chains become more flexible and the side groups more easily relaxed, thereby changing the physical properties of the chiral nematic structure. Similarly, an increase in concentration leads to a decrease in pitch for most lyotropic cellulosic liquid crystals with the exception of cellulose tricarbanilate (CTC) in ethyl methyl ketone, 2-penta-none, or tiiethylene glycol monoether and the chlorophenyl urethane derivative in diethylene glycol monoether. ... 

The linear electro-optic effect in a cholesteric, i.e. a hard-twisted chiral nematic (the helical pitch must be less than the wavelength of visible light) was a very original proposal for using the flexoelectric effect in a new display, shutter or modulator device. The patent application by R.B. Meyer and J.S. Patel dates from 1987 and was granted in 1990. The physics was developed in a series of papers by these authors and later elaborated by others.It is now commonly called the flexoelectro-optic effect. 

P. Rudquist, The Linear Electro-optic Effect in a Chiral Nematic Liquid Crystal, Gothenburg Institute of Physics Report, No. 314, (1993). 

The corresponding dependence of the steepness of the transmission-voltage curve (TVC) on physical parameters of the chiral nematic mixture and geometry of the supertwist display is shown in Table 4.8. The correlations, mentioned in Table 4.8 are used in the practical development of supertwist mixtures for highly informative displays [129-133]. 

In the important physical properties we have discussed so far, the molecular structure contributes to the bulk optical, electrical, and thermal properties of the chiral nemat-... 

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