Nematic Materials

Another example of the coupling between microscopic and macroscopic properties is the flexo-electric effect in liquid crystals [33] which was first predicted theoretically by Meyer [34] and later observed in MBBA [35], Here orientational deformations of the director give rise to spontaneous polarisation. In nematic materials, the induced polarisation is given by... 

Polymers formed between a and c, d and e all failed to show any liquid-crystalline behaviour. However, for all a examined (m = 2,4,6 and 8), nematic phases were observed with b-4 (all monotropic) - a further monotropic nematic material was the copolymer of a-6 and b-3. Unidentified crystal smectic mesophases were reported for a further three examples. 

Figure 5.3 Twisted nematic materials for direct drive displays.

Figure 5.4 Twisted nematic materials for multiplex addressing.

Figure 5.5 Twisted nematic materials for active matrix addressing.

Theoretical analysis indicates that occurrence of such convective instabilities depends on anisotropy of electrical conductivity and dielectric properties in the initial aligned nematic material. That is, conductivity parallel to the direction of alignment must differ from conductivity perpendicular to this direction. Calculation of the stability condition requires knowledge not only of these anisotropic electrical properties but also of anisotropic elastic and viscous properties which oppose disruption of the alignment and flow. 

Liquid crystalline materials may appear cloudy. This is because the index of refraction of polarized light depends on the angle between the angle of polarization and the director. This is represented schematically in Figure 16.11. In a nematic material, there will normally be regions in which the director is oriented differently. 

As shown in Fig. 10, evaluating this relation at the onset of the instability reduces significantly the physically reasonable range for some parameters. This restriction applies only for the nematic material parameters and, in general, nothing can be said about the corresponding smectic parameters. We will, however, take the smectic parameters in the same range as the nematic ones. If not indicated otherwise we used L(0n,s) =0.1,=0.01, s)-L n s) =0.005,M0= 10 4, =0.01, -... 

This brief account of past investigations is concerned with work on polymer liquid crystals (PLCs), where the majority of work has considered nematic materials. Among the unique rheological responses associated with PLCs are... 

The life-times of such displays depend on the type of applied potential for AC operation, values of in excess of 10000 operating hours have been reported, whereas DC potentials often lead to deposition of ions, with a consequent decrease, often dramatic, in life-time. The stability of the nematic material to electrochemical effects is also of critical importance. 

Aromatic liquid crystals, such as compounds 106-109 collated in Table 3.10, have been investigated to a much lesser extent since they were perceived to be intrinsically more viscous than the analogous cyclohexane derivatives. This is indeed often the case. It was hoped that the incorporation of a fluorine atom in a lateral position of a polar compound, such as the ester (106), would create a nematic material, such as the esters (107 and 108), of positive dielectric anisotropy and low ratio of and Ae/sj.. Although this was partially... 

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