Temperature chiral nematic pitches

Figure 8.14a, c shows double twist cylinders, and the bold black lines in Fig. 8.14b, d show disclinations (defect Unes). In each double twist cylinder, the molecules are radially twisted towards each other through 90°. The molecules are parallel to the cylinder axis at the cylinder center and are tilted by 45° at the outer radial periphery. In other words, the molecules twist from —45° to -t45° through the cylinder, which corresponds to a quarter pitch. The diameter of a double twist cylinder is typically about 100 nm, and a simple calculation shows that approximately 200 molecules with a diameter of 0.5 nm mildly twist against each other. The lattice constant for blue phase I corresponds to a one helical pitch, and the lattice constant for blue phase II corresponds to one half helical pitch. We generally see a very small mismatch in pitch length with that of the lower-temperature chiral nematic phase. Peculiar to soft matter, a complex hierarchical structure is formed in... 

Note 3 With chiral nematic substances forming chiral nematic mesophases of short pitch (<700 nm), up to three blue phases occur in a narrow temperature range between the chiral nematic phase and the isotropic phase. 

The electro-optical characteristics of multiplexed STN-LCDs exhibit a significant dependence on temperature. This has to be compensated in order to avoid variations of the optical performance of the display with temperatures. This can be achieved electronically. However, this problem can also be solved by the use of optically active, chiral dopants. The capacitive threshold voltage of a chiral nematic mixture depends on the pitch of the mixture ... 

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. ... 

Besides degree of substitution, nature of substituents, solvent, concentration, and temperature, other factors that change the polymer-solvent interactions can affect the pitch of lyotropic cellulosic mesophases. Doping inorganic salts " or small chiral molecules into the lyotropic mesophase changes the polymer-solvent interactions. As the results, the pitch of the chiral nematic mesophase changes accordingly. 

The pitch of the helix for compound 42 was found to be approximately 0.2-0.3 xm, thus the material selectively reflects visible light over a wide temperature range. Moreover, the pitch is relatively temperature insensitive thus the material can be used in large area non-absorbing polarizers, or in optical notch filters or reflectors. In addition, in the glassy state the helical macrostructure of the chiral nematic phases is retained, thus similar applications are possible. 

Various models proposed may not account for all these experimental facts. The Keating [20] and Bottcher [21] evaluation does not account for such a variety of behavior. Goossens [22] proposed the chiral nematic structure as the result of an anisotropic dispersion energy between chiral mesogens, and predicts for thermotropic LCs a pitch that is essentially independent of temperature. Lin-Liu et al. [23] developed a theory that accounts for all the above-stated temperature effects. The temperature dependence of the pitch is determined by the shape and position of the intermolecular potential as a function of the intermolecular twist... 

There are many parameters could affect the pitch of N -LCs, such as ionic strength, drying temperature, suspension concentration, magnetic field [22] and sonication [23, 24]. Interestingly, the chiral nematic orders can be retained after evaporation of the... 

A chiral compound, dissolved in a nematic liquid crystal phase, transforms this phase into a chiral phase that is very often a chiral nematic - cholesteric -phase. Under the same condition of concentration and temperature two enantiomers induce helical structures with the same pitch but of opposite sign. The helical pitch p is for low concentrations of the dopant a linear function of mole fraction x. The molecular measure for the chiral induction is the helical twisting power (ITTP) ... 

Figure 11.19 Variation of pitch length with temperature for a thermochromic chiral nematic liquid crystal.

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