Viscosity chiral nematics

The polyamides are soluble in high strength sulfuric acid or in mixtures of hexamethylphosphoramide, /V, /V- dim ethyl acetam i de and LiCl. In the latter, compHcated relationships exist between solvent composition and the temperature at which the Hquid crystal phase forms. The polyamide solutions show an abmpt decrease in viscosity which is characteristic of mesophase formation when a critical volume fraction of polymer ( ) is exceeded. The viscosity may decrease, however, in the Hquid crystal phase if the molecular ordering allows the rod-shaped entities to gHde past one another more easily despite the higher concentration. The Hquid crystal phase is optically anisotropic and the texture is nematic. The nematic texture can be transformed to a chiral nematic texture by adding chiral species as a dopant or incorporating a chiral unit in the main chain as a copolymer (30). 

The viscosity of a typical cholesteric made by doping a nematic with a modest amount of chiral nematic is much lower (around 1 P or so) than that of the typical pure eholesteric. Perhaps this is because the pitch of the doped nematic is higher than that of the typical pure cholesteric, or because the twist elastic constant of the doped nematic is much lower. 

A theory by Helfrich (1969) suggests that the viscosity of a chiral nematic phase is high because the cholesteric director is blocked-, that is, it cannot respond to the flow... 

The mesophase defect textures exhibited by 47 were typical of those normally found for a chiral nematic phase, except they were only revealed upon annealing, which is probably a function of the viscosity of the material. Thus the sample was annealed just below the clearing point. After 24 h, large areas of the preparation evolved to show fingerprint defects and the Grandjean plane... 

In addition to depressing the liquid ciystal phase stability and often reducing melting points, a branched chain that introduces chirality increases viscosity. A high viscosity is not a problem in the non-display, thermochromic applications of chiral nematic materials. The steric effect of a chiral centre is well illustrated by difluoroterphenyl 5. 

Vanin et al. studied order proflles of the perdeuterated laurate chain in chiral and achiral phases by means of deuterium NMR. The quadrupole splittings measured for the sequence of CD2 moieties are all quite similar for the nematic and chiral nematic phase the conclusion was that chiral distortions of the micellar shape must be very small. Their proposal for the mechanism behind the phase chirality is analogous to the one for polypeptides an asymmetric point charge distribution which in this case was speculated to be created by the chiral dopant on the micellar surface [19]. Radley and Tracey prepared samples from a racemic detergent and observed high fluidity, whereas those consisting of some enantiomeric excess showed a significantly increased viscosity [30]. But this is no proof for the association of micelles to necklaces. 

The static theory discussed in the previous section describes the equilibrium situation in chiral nematics very well - in general, theory and experiment are in good accord. The dynamic situation is less clear. On the molecular scale, the chiral nematic and nematic phases are identical the question then becomes, how does the macroscopic twist or helicity modify the vector stress tensor of the achiral nematic phase defined by the so-called [109] Leslie friction coefficients a -a T Experimentally, viscosity coefficients that are then related to the Leslie coefficients are measured in a way that depends specifically on the experiment being used to determine them. The starting point for discussion of dynamic properties is to use classical mechanics to describe the time dependencies of the director field n (r, t), the velocity field v (r, t), and their interdependency. Excellent reviews of this, for achiral nematics, are to be found in [59,109,... 

Figure 29. Apparent viscosity, rj pp, of the chiral nematic cholesteryl acetate as a function of temperature for different shear rates. Capillary flow (s ) 10 (V), 50(x),100(n),l000 ( ), 5000 ( ) rotational shear (s" ) lO (A), and normal liquid behavior at very high shear rate (O) (redrawn from [115]).

Figure 30. Apparent viscosity, tj pp, in Poiseuille flow as a function of the shear rate for different chiral nematic pitch lengths (in micrometers) 1.9 (O), 2.6 (x), 3 (+), 3.9 ( ), 6 (A), 9.1 ( ), and °° (V) (redrawn from [122]). The helical axis was normal to the flow.

Figure 78 shows a plot of the soft mode and Goldstone mode rotational viscosities measured on either side of the phase transition between the smectic A and SmC. It can be seen that, except in the vicinity of the phase transition, the viscosity seems to connect fairly well between the two phases. The activation energies of these two processes are, however, different. This result may be compared to results obtained by Pozhidayev et al. [148], referred to in Fig. 67. They performed measurements of y beginning in the chiral nematic phase of a liquid crystal mixture with corresponding measurements in the SmC phase, and have shown the viscosity values on an Arrhenius plot for the N and SmC phases. Despite missing data of y in the smectic A phase they extrapolate the N values of y down to the smectic C phase and get a reasonably smooth fit. Their measurements also show that y is larger than y, and this is universally the case.

C1/C2 chevrons, ferroelectric devices 644 cadmium selenium TFT address 233 Cano wedge, chiral nematics 347 f, 351 Canon technology, ferroelectric devices 648 capillaiy flow, shear viscosity 143 carbocyclic compounds, charge transfer systems 958 carbocyclic rings, smectogens 412 carbon atoms, intercalated smectics 808 carbon-carbon bonds, dimers 823 carbon-carbon double bonds, chiral smectics 498 carbonaceous phases 693 carbonyl connectors, antiferroelectrics 687 carbonyl groups... 

Molecules that self-organize in chiral nematic or smectic mesophases and comprise a permanent dipole exhibit ferroelectric properties. This means that they are spontaneously polarized and are able to orient themselves under the effect of an electric field. Polymers with chiral mesogens could be used for the manufacture of large-size video screens and more generally for displays. To this end, they require a response time (i.e., the time required for mesogens to flip from one position to another) that is comparable to that of molecular liquid crystals. This response time t depends on the electric field (E), the polarization (P), and the viscosity (t]) ... 

A further development of such complexes is the synthesis of the electron donor-acceptor twins 6 of which seven representatives with three different kinds of bridging groups XY (one is chiral, see 6c, 6d and 6c, TABLE S) have ncrt only been prepared but show desired nematic prc )arties. Their columnar nematic phases have a much higher viscosity at low tonpaatures, in smne cases consulerably below room temperature. Three of these interesting twin compounds (6c, 6d and 6e) exhibit chiral nematic phases. 

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