Nematic matrix

Dichroic dyes of positive and negative dichroism are required for various configurations of the Heilmeier and Zanoni as well as the White and Taylor GH-LCDs with positive and negative contrast. They should be chemically, photochemically and electrochemically stable, as well as exhibit a high order parameter, a high dichroic ratio and good solubility in the host nematic matrix. 

Chart 1.9 Chemical structures of compounds A and B, forming a chiral nematic matrix, and of an oligomeric luminophore. 

This discrepancy clearly indicates that the BC nematics characterized by the presence of polar clusters cannot be regarded - at least from the point of view of the flexoelectric response - as a homogeneous continuum instead the behaviour of the clusters in and their interaction with the surrounding nematic matrix should be handled separately. This challenging problem still waits for a solution. 

Figure 3 shows the temperature dependence of the orientational order parameter of dissolved dye in the nematic matrix of polymer la. 

Of course, in each case a particular form of the potential curve depends on chemical stmcture of constituting molecules. For instance, in nemato-cholesteric mixtures, y((j)) depends on the structure of both a nematic matrix and a chiral dopant. 

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]. 

For some applications, nematic mixtures have to be doped with chiral compounds, such dopants must not deteriorate the chemical stability and thermodynamic parameters of the mixtures. To this end, compounds with a high twisting power and good solubility are used. In special cases a dopant can be chosen which can compensate for the temperature drift of some important physical parameters of the nematic matrix. Compounds (l.xxxia) and (l.xxxib) are examples of the left- and right-handed chiral dopants, respectively. 

In this case it was assumed that the factors of the internal field F and h are the same for both compounds. The former discussed problem of different degrees of order for the two components was considered. N means the number of particles per unit volume. Equation (4) can be simplified if component B is not oriented in the nematic matrix or exhibit a very small degree of order 5b(x, T). By this approximation one should expect a change of Ae(x, T) with the molar fraction x and with the temperature. 

When a small concentration of neat chiral nematic is added to an achiral nematic matrix, then a large pitch (p) mixture is... 

Figure 38. Schematic operation of the White-Taylor dye guest-host chiral nematic electrooptic cell. In (a) for zero applied field the axis of each focal-conic domain is random in the x, y plane, as therefore is the dye, using homeotropic surface alignment. In (b) the texture is planar for the zero field state and therefore the dye spirals around the z direction. In (c) the focal conic (a) or planar (b) transition to homeotropic nematic has taken place above the threshold voltage V,], (WT). The black ellipses represent the dyes in the chiral nematic matrix.

The structure of the cholesteric mesophase of copolymers is essentially dependent on the nature of the nematic matrix and the concentration of chole-... 

The helix induction force for the series of copolymers examined coincides with the corresponding value for a mixture of low-molecular-weight liquid crystals—p-butoxybenzylidene-p -butylaniline with cholesteryl pnq)ionate—where there are no specific interactions in the nematic matrix, and the chirality is caused by the cholesterol fragment... 

The helix induction force also changes in substitution of the nematic matrix. The helix induction force is lower in matrices which exhibit smectogenic properties the copolymers in series 10 and 11 with phenylbenzoate groups and... 

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