Orientation homeotropic

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... 

When an alternating electric field is applied to a homeotropically oriented layer of LC melt at definite temperatures (close to Tcl), a homogeneous orientation is... 

Thus, varying the frequency of an applied electric field at a given temperature, one may pass over from homeotropic orientation of the sample (transparent film) to the mode of EHD instability and reverse. This reveals interesting perspectives for controlling the optical properties of polymeric films. 

In this paper we describe two types of observations on egg-yolk lecithin. We present the results of our study of homeotropically oriented samples of La phases by polarizing optical microscopy. This study provides evidence, amid apparently nonsimilar aspects, of the existence of an elementary typical object which we have interpreted as a dislocation. We also studied thin samples of stained lecithin in the high vacuum of the electron microscope. In addition to the defects that are typical of this type of sample, we observed the same elementary object as in La lecithin. 

We studied samples of La lecithin with 16-29% water at room temperature. The samples were prepared between two microscope glass slides that were first treated with a sulfochromic solution, then carefully rinsed with distilled water and dried. They were observed by transmission polarizing microscopy. Just after deposition between the slides, the samples present a highly disordered texture which strongly diffuses light. After compression (squeezing) and/or shearing parallel to the plates, one obtains an overall homeotropic orientation that is perturbed by defects. The thickness of the samples is in the order of 20/. ... 

Above 110 °C, this arrangement becomes mobile, and a smectic C liquid-crystalline phase is entered. Samples cooled down from the isotropic melt (140 °C) show Schlieren and banded textures when viewed under crossed polarizers (Figure 8). These textures look similar to nematic Schlieren textures, but from the X-ray diffraction data it is clear that 12c forms a homeotropically oriented smectic C phase. In a nematic phase, the small-angle diffraction peak would be absent, and a broad scattering feature, a nematic streak , would be observed. Polymer 12c was the first example of a PPE derivative for which three states of matter, i.e. crystalline, thermotropic liquid crystalline, and a highly viscous isotropic liquid, were accessible [46]. 

A suitable surface treatment results in a homeotropic orientation for a nematic mixture incorporating a dichroic dye of positive contrast and an amount of a chiral dopant insufficient to overcome the surface forces and generate a twisted structure in the nematic phase. 

The first nematic guest-host prototype nematic GH-LCD reported by Heilme-ier and Zanoni " contained methyl red (157) as the dichroic dye dissolved in 4-butoxybenzoic acid as the nematic liquid crystal host. Other hosts investigated later included 4-methoxycinnamic acid and 4-ethoxy-4-aminoben-zonitrile (28), see Table 3.4. The melting point of these three single components is very high. Therefore, prototype GH-LCDs had to be operated and evaluated at very high temperatures. Thermal decomposition of the mixtures led sequentially to lower contrast, homeotropic orientation due to decomposition products and finally device breakdown. However, these initial experiments were sufficient to demonstrate the feasibility of this display type. 

A more sophisticated retardation plate is a film consisting of the perpendicular arranged combination of two optical uniaxial materials, e g., uniaxially stretched polycarbonate foil with a homeotropic oriented LC siloxane layer [18]. The purpose of such a foil is the improvement of the optical properties of LC displays, especially the viewing angle dependence of the contrast. 

Fig. 16. Combination of two optical uniaxial materials The figure shows the optical indicatrix of an uniaxially stretched plastic foil with horizontal direction of Ue and the indicatrix of a homeotropic oriented LC siloxane with horizontal direction of n (schematically).

To demonstrate the effect of such a combination a three-layer system between crossed polarizers may be considered The first picture was calculated as model for a LC display operating by change of A by application of an electric field, the second is the combination with a color-compensating foil and the third is the additional application of a homeotropic oriented layer of a LC siloxane. 

In order to promote a perpendicular (homeotropic) orientation surface active agents are adsorbed on the solid surface which lowers its surface tension (Fig. 10c). 

The displacement is probably more often only partial, leading to a lyotropic organization of the very first layer, with alternating water and surface active compounds. We believe that this is why fatty acids, their salts, amines, lecithin and surfactants promote the homeotropic orientation of the nematic TLCs which lie above it. Although this is highly speculative, it has implications in LC lubrication and explains the sensitivity of the friction coefficient of LC to moist environment. Depending upon the water concentration, the first monolayers have various structures (Fig. 13) which will have various responses to wear. 

From the description of the LC viscosity given in Paragraph V, one would expect the friction coefficient of nematic to be lower in the parallel orientation ( b ) than in the homeotropic orientation (i ). We considered the effect of the orientation induced by the solid surface with and without a surfactant treat-... 

With a microtribometer, the friction coefficient between steel and ruby was measured both under dry and tropical conditions, the lubricant being a mixture ALl. All the parts were treated with ODS. The results shown in Fig. 24 prove to be very good under dry conditions, the lubrication coefficient being lower with the treatment, indicating again that an homeotropic orientation favours the lubrication, shear occuring between alkyl chains instead of the aromatic core in parallel orientation. 

However, in humid conditions y is higher (as with pure All), the friction coefficient remains, however, lower than for the All without treatment, particularly at low speed. The surface active agents which promote the homeotropic orientation and lower the friction coefficient of the LC mixture, did not prevent the increase in y due to moisture. 

Figure 5. Bifurcation diagram on the plane of the two control parameters p and a. The solid lines 1 and 2 mark the primary instability, where the homogeneous homeotropic orientation becomes unstable. At 1, the bifurcation is a stationary (pitchfork) bifurcation, and a Hopf one at 2. The two lines connect in the Takens-Bogdanov (TB) point. The solid lines 3 and 4 mark the first gluing bifurcation and the second gluing bifurcation respectively. The dashed lines 2b and 3b mark the lines of the primary Hopf bifurcation and the first gluing bifurcation when calculated without the inclusion of flow in the equations.

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