Oriented nematics

The classical scheme for dichroism measurements implies measuring absorbances (optical densities) for light electric vector parallel and perpendicular to the orientation of director of a planarly oriented nematic or smectic sample. This approach requires high quality polarizers and planarly oriented samples. The alternative technique [50, 53] utilizes a comparison of the absorbance in the isotropic phase (Dj) with that of a homeotropically oriented smectic phase (Dh). In this case, the apparent order parameter for each vibrational oscillator of interest S (related to a certain molecular fragment) may be calculated as S = l-(Dh/Di) (l/f), where / is the thermal correction factor. The angles of orientation of vibrational oscillators (0) with respect to the normal to the smectic layers may be determined according to the equation... 

Fig. 27a and b. Scheme of thermorecording using the film of the homeotropic-oriented nematic LC polymer (a) and thermorecording of letters on the film of polymer XII at 105,5° (film thickness 40 pm) (b) I — initial homeotropic texture II — texture with the laser thermo-recorded letters... 

One potential application of the work on oriented nematic phases of rodlike molecules is to solutions containing cylindrical micelles. Orientation could be achieved by a shear field or perhaps by an electric field. Gotz and Heckman (9) confirmed the existence of anisotropic electrical conductivity for a concentrated surfactant solution in a shear field. They used their results to show that the solution contained cylindrical rather than platelike micelles. Of course, the magnitude of the electrical conductivity in an aqueous micellar solution should be quite different from that in the nematic phase of an organic material. So the conditions for and types of electrohydrodynamic instabilities could be different as well. 

D. Baalss and S. Hess, The Viscosity Coefficients of Oriented Nematic and Nematic Discotic Liquid Crystals Affine Transformation Model, Z. Naturforsch. 43a (1988) 662. 

FIGURE 7. Linear dichroic (LD) absorption spectra of phenol partially aligned in a uniaxiaUy oriented nematic liquid crystal . The curves indicate absorption measured with the electric vector of the linearly polarized radiation parallel (A) and perpendicular (B) to the director of the liquid crystalline sample. Reprinted with permission from Reference 198. Copyright (1998) American Chemical Society... 

N-p-(methoxy-o-hydroxybenzylidene)-p-aminostyrene], in the isotropic, nematic and oriented nematic states [30]. In addition, Hsu and Blumstein observed no change in the radical polymerization mechanism of another Schiff base [N-(p-cyanobenzylidene)-p-aminostyrene]... 

The mono-substituted PEIs melt from a saniditic layered structure into a normal nematic phase whereas the disubstituted presumably yield a biaxially oriented nematic melt. The disubstituted PEIs isotropise from the melt around 200 °C lower than the monosubstituted of equal chain length. As the second substituent should increase chain stiffness rather than reduce it, Kricheldorf suggests that interactions between temporarily coplanar aromatic n systems, particularly donor-acceptor interactions, can make an efficient contribution to the stabilisation of nematic phases. If this hypothesis is correct then a greater variety of non-linear monomers may be regarded as building blocks for liquid crystal polymers. 

Figure 4.31. Schematic X-ray diffraction patterns of liquid crystalline phases (a) unoriented nematic (b) oriented nematic (c) unoriented smectic A or C (d) oriented smectic A (e) oriented smectic C (f) unoriented smectic B phase.

Fig. 18A, B. USAXS-patterns of A a shear-oriented nematic block copolymer phase B the oriented mesoporous monolith prepared therefrom. The shift of the Debye-Scherrer ring is due to removal of solvent during the sol-gel process [110]...

Changes of both the scattering contour and its intensity in the magnetic field accompanying the increase of the external field strength are quite similar to those in the electric field 38). This indicates the formation and the orientation of rod-like molecular clusters and the transition toward the oriented nematic structure as well. 

Lyotropic LCPs exhibit quite characteristic viscosity behavior in solution as the polymer concentration in solution is changed. Typically the viscosity follows the trend shown in Figure 5.6. As more and more polymer is added to the solvent, the viscosity increases while the solution remains isotropic and clear. At a critical concentration (which depends on the polymer and the solvent) the solution becomes opaque and anisotropic and there occurs a sharp fall in viscosity with further increase in the polymer concentration. This results from the formation of oriented nematic domains in which the chains are now aligned in the direction of flow, thereby reducing the frictional drag on the molecules. 

This question can be answered by considering the experiments, where liquid crystals have been spread across solid crystal surfaces [15]. Macroscopic observations under a polarizing microscope reveal that crystal surfaces do orient nematic liquid crystals, which leads to the conjecture that there may be microscopic positional and orientational order at a crystal-liquid crystal interface. Indeed, this local interfacial order can be revealed in experiments with a Scanning Tunneling Microscope (STM), first perfoi med by Foster and Frommer [16]. An example of an STM image of a crystal-nematic liquid crystal interface is shown in Fig. 1.4 for the case of 8CB on graphite. One can... 

Fig. 6.9. The origin of liquid crystal alignment on rubbed polymer surfaces (A) Liquid crystal molecules have highly anisotropic charge distributions with their tt system preferentially oriented perpendicular to their long axis, which gives an ensemble of oriented (nematic) liquid crystals an asymmetric charge distribution (B). This can be characterized by molecular orientation factors (fa,fb,fc) describing the preferential orientation of the tt system. For the experimentally observed alignment directions the anisotropic charge distribution of the liquid crystal is oriented parallel to the one of the rubbed polyimide (C) and polystyrene (D) surface, which optimizes their interaction energy [3].

These results indicate that spinodal decomposition can be induced by flow even in a homopolymer system, which results in an oriented nematic phase with long chains and an isotropic liquid with short chains. Though the above argument is based on a bimodal system, the same principle has also been applied to the polydisperse case [67]. However, this approach still does not take flow-induced conformational ordering into account, which may couple to the anisotropic interactions. 

A quite different situation is observed with the linear polyesters. From the FTIR polarized spectra of an oriented nematic copolyester II the temperature dependence of the order parameter was determined It is apparent from Fig. 25 that S decreases steadily from 0.55 to 0. 5 as the temperature is raised from 250°C ( 32 ... 

Fig. 26 - Schematic X-ray patterns of the main types of oriented nematic mesophases.

Crystallization of an oriented mesophase> The crystallization process of oriented nematic melts can lead in certain cases to highly oriented crystalline fibers while, in other cases, the crystallite orientation is totally or partly disrupted by the process of crystalliza-tion% Thus, DDA-8, MAA-8 and its copolymers MAA-DDA-8 lead to strongly oriented crystalline fibers on crystallization from nematic melts oriented in a magnetic field of 12 Tesla On the other hand, DDA-9 and its copolymers are much less oriented under the same circumstances. 

Fibers obtained by extrusion from the nematic melt form either oriented crystals (n=8,9,10,12 and 14) or supercooled oriented nematic glass fibers with a high orientation function, fc O- (n=3,5,7,9,11 and copo lymer s )... 

---