Nematic field

Here R and R are the rms (root mean square) end-to-end distances of the polymer chain parallel and perpendicular to the director, respectively, xr is the conformational relaxation time of the polymer, c the polymer concentration, N is the degree of polymerization, and T the temperature (K). These results are interesting, because they predict that if the dissolved polymer stretches along the director, because of an interaction with the nematic field, the increment in r]c will be much larger than that in T]b. Specifically, from Eqs. (1.82) and (1.83), the ratio of the two scales as... 

Photinos, D. J., Samulski, E. T., andToriumi, H., Alkyl chains in a nematic field 1. A treatment of conformer shape, J. Phys. Chem., 94,4688 694 (1990a). 

The nature of the relative dispositions of the backbone and the mesogenic unit are controlled by the strengths of the interactions between the mesogenic units and polymer backbones. The latter term may be resolved into two components one component favors parallel alignment between the mesogenic units and the polymer chains simply because of the nematic field, and another component... 

It was found that cooperative alignment of certain nematic compounds with strong dipole moments along their long axes could be used for the twisted-nematic field effect. The class of Schiff base compounds of structure (9) was found quite suitable for this effect and a number of derivatives and mixtures were prepared.Several mixtures were found to be nematic from below room temperature to above 90 C. 

After the discovery of the liquid crystal behavior of HPC in solution, a multitude of studies have reported the ordering phenomena of different cellulose derivatives, according to the examples given below [90]. Figure 14.6 presents the optical microscopy of an aqueous suspension of microcrystalline cellulose at a concentration of 16 wt%, where the nematic field and a chiral nematic phase typical to a fingerprint texture maybe observed 10 minutes and respectively 1 day after the preparation of the suspension [116]. 

As mentioned in the previous section on confined systems, viscoelastic properties of liquid crystals (see Sec. 1 of Chap. Ill and Sec. 8 of Chap. VII of this Volume) can be determined from experiments in which the nematic field is distorted by surface forces. Similarly, the influence of an external field, namely, the magnetic field, on the director orientation, is the basis of several magnetic resonance experiments which can be used to determine viscoelastic parameters of nematic phases. 

We have seen by Eqs. [38] and [39] that the normal waves are linearly polarized in the local frame of reference for 0. As the local frame rotates d = 2ttzIP), so do the polarization vectors of the normal waves. This is the waveguide regime discussed by de Gennes and is also the regime in which twisted nematic field-effect devices operate (values typical of such devices are P 50 Aim, 1.65 and X 0.5 A m, leading to a value of X 0.006). 

The twisted nematic field effect is probably the most important of the field effects because of its combined properties of very low voltage threshold, low resistive power dissipation, and relatively wide viewing angle in the reflective mode. 

The optical properties of the twisted nematic field effect are par-... 

The characteristic features of the nematic ensemble elucidated above are put together in a simple illustration depicted in Fig. 6, which shows nematic arrangements of mainchain LCs in contrast to those of the adjacent isotropic and crystalline phases. In the nematic field, both spacer and mesogenic units at the terminals tend to align along the domain axis. Consequently, the individual mesogenic cores inevitably incline to some extent with respect to the direction of the molecular... 

Until the mid-1990s and after 20 years of intense research on nematic field-effect LCDs it was still uncertain whether LCDs and LC materials could indeed meet the short response time requirements and the optical quality required for LCD television. Therefore, parallel to nematic LCD research, strong efforts were made to find effects based on the inherently faster responding ferroelectric liquid crystals (FLCs). Unfortunately, FLCs proved to be difficult to surface-align, rendering them up to now commercially applicable only for niche products such as electronic eye shutters or time sequential LCD projection. FLC examples are the surface-stabilized ferroelectric (SSF)-LCD of Clark and Lagerwall [40] which initiated FLC-LCD development and the deformed helix ferroelectric (DHF)-LCD of Beresnev et al. [41], In 1995 a TFT-addressed black-white DHF-LCD television prototype with 20 ps response time and broad field of view was developed by the author and coworkers in collaboration with Philips [42] (Fig. 6.5a). 

Polymer entropy is antagonistic to nematic (or smectic) order. A flexible polymer is expelled from a nematic solvent, since a chain stretched out to be parallel to a nematic field loses a lot of entropy equally the nematic field, distorting to be locally parallel to a random polymer, would develop a large nematic elastic energy. 

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