Nematic solutions, molecular orientation

J. G., De Lange, G. A. Molecular solutes in nematic liquid crystals orientational order and electric field gradients. Chem. Phys. Lett. 1983, 99, 271-274. 

From slow-shear-rate solutions of the Smoluchowski equation, Eq. (11-3), with the Onsager potential, Semenov (1987) and Kuzuu and Doi (1983, 1984) computed the theoretical Leslie-Ericksen viscosities. They predicted that ai/a2 < 0 (i.e., tumbling behavior) for all concentrations in the nematic state. The ratio jai is directly related to the tumbling parameter X by X = (1 -h a3/a2)/(l — aj/aa). Note the tumbling parameter X is not to be confused with the persistence length Xp.) Thus, X < I whenever ai/a2 < 0. As discussed in Section 10.2.4.1, an approximate solution of Eq. (11-3) predicts that for long, thin, stiff molecules, X is related to the second and fourth moments Sa and S4 of the molecular orientational distribution function (Stepanov 1983 Kroger and Sellers 1995 Archer and Larson 1995) ... 

For an elongated molecule such as acetonitrile one can assume with certainty that in a nematic solution the long molecular axis will be oriented preferably parallel to the optical axis, and therefore c3 is positive. The... 

As mentioned in introduction, the competition of molecular motion and deformation applied by the solvent evaporation determines the molecular orientation in a solution-cast film. In other words, CTA chains orient in the film plane by applied uniaxial compression deformation due to the solvent evaporation. At the same time, TCP molecules orient accompanied with the CTA chains. Therefore, the orientation relaxation of TCP molecules will be affected strongly by the existence of a solvent, although the relaxation time is reduced for both CTA and TCP. The experimental results indicate that the nematic interaction, i.e., orientation of TCP molecules, occurs only at the final stage of evaporation. Therefore, a solvent that retards the evaporation rate at the final stage to obtain smooth surface will have a strong influence on the orientation of additives. 

The overall order parameter, (Pa), for an oriented nematic solution may be expressed in terms of the order parameter of the director field of the domains. Pa, and of the molecular order parameter, (Pa), characterizing the degree of order... 

On the other hand, literature data show [16] that different cellulose derivatives which form liquid crystalline solutions in organic solvents may also form cholesteric thermotropic phases in the absence of a solvent—with spontaneous molecular orientation and cholesteric reflection, such as 2-acetoxypropyl cellulose, 2-hydroxypropyl cellulose, the trifluoroacetate ester of hydroxypropyl cellulose, the propanoate ester of hydroxypropyl cellulose, the benzoate ester of hydroxypropyl cellulose, 2-ethoxypropyl cellulose, acetoacetoxypropyl cellulose, trifluoroacetoxypropyl cellulose, the phenylac-etate and 3-phenylpropionate of hydroxypropyl cellulose, phenylacetoxy, 4-methoxy-phenylacetoxy, p-tolylacetoxy cellulose, trimethylsilyl cellulose, trialkyl cellulose, cellulose trialkanoate, the trialkyl ester of (tri-o-carboxymethyl) cellulose, 6-o-a-(l-methylnaphthalene)-2,3-o-pentyl cellulose, etc. Moreover, the suspensions of cellulose crystallites spontaneously form the chiral nematic phase. The formation of mesophase suspension of cellulose crystalHtes varies from one type of cellulose to another, being influenced, in the formation of the chiral nematic phase, by the mineral acid selected... 

In this context, literature [90] states that at room temperature, acetoxypropyl cellulose exhibits both chiral nematic phases—the lyotropic and the termotropic one. When subjected to specific conditions of shear flow, the cellulose derivative cholesteric liquid crystal suffers transformations, such as cholesteric helix and cholesteric-to-nematic transition. The films prepared from anisotropic solutions of termotropic acetoxypropyl cellulose in an isotropic solvent exhibit anisotropic mechanical properties, generated by the molecular orientation of the solution under shear stress. Thus, liquid crystalline solutions give rise to films with anisotropic mechanical properties the films are brittle when stretched parallel to the shear direction and ductile when stretched perpendicular to it. 

Polarized absorption (or fluorescence) spectra are taken in uniformly aligned nematic phases. The orientation of the optical transition moment in the molecular fixed coordinate system of the solute can be determined from the change in the optical density of the solute produced by the partial solute alignment [7]. 

Nematic solutions present an anisotropic scatterii medium, with fluctuations in molecular orientation resulting in appreciable depolarized scatterii Thus, in this case, it is the horizontally polarized component the scattering with vertically polarized incident light that is of most interest [134, 135]. The scattering from concentration fluctuations is much smaller than that from orientation fluctuations. The anisotropy of a nematic mesophase of rodlike chains is measured by an order parameter S, given by [119,134, 135]... 

Due to the anisotropy in the molecular magnetic and electrical properties of liquid crystals they can, when in their nematic mesophase, be orientated by the application of external magnetic and electric fields. In the context of NMR this enables liquid crystals containing low concentrations of dissolved solutes to... 

When limiting our attention to low-molecular-weight nematics, we may expect that, in general, flow has the following effects (1) it alters the distribution of molecular orientations about the nematic axis (director) and (2) it affects the director itself. In other words, the velocity v(r) and the director n(r) are coupled under flow of nematic solutions. Next, we first present the expressions for stress, then discuss some important features of the Ericksen-Leslie theory, and finally show relationships existing between the six Leslie coefficients and three molecular parameters appearing in the Doi theory. The presentation of the entire Ericksen-Leslie theory (Ericksen 1960 Leslie 1966, 1968, 1979) is beyond the scope of this chapter. 

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