Molecular orientation homeotropic

The often observed textures for Sa are the focal-conic fan texture and the homeotropic texture. When the molecular orientation is homeotropic, the optic axis is perpendicular to the film surface so that the preparation appears black (pseudoisotropic) on a crossed polarizing microscope. As with the homeotropic nematics, the homeotropic Sa phase can be differentiated from the true isotropic phase by conoscopic observations. Further more, if the cover slip of the homeotropic Sa preparation is slightly moved, the orientation is disturbed resulting in oily streaks in form of bright bands. The homeotropic Sa phase may thus be distinguished from the homeotropic nematic phase. 

F. 2.18 Swelling and desweUing of thin films of a planar aligned smectic network (a) and a homeotropic smectic network (b) (scale bar corresponds to 500 pm). Swelling depends on the concentration of covalent crosslinker and on the direction with respect to the molecular orientation (c) and shows hysteresis upon cycling during the activation step and the first cycle (d) and repeated cycle (e) Reproduced from Ref. [77] by permission of The Royal Society of Chemistry... 

The molecular alignment of liquid crystals on solid surfaces is not only of fundamental interest in physics [1] but is also relevant for practical applications, for example in optoelectronic devices. In liquid crystal displays the molecules are confined between two surfaces. To minimize the number of defects, surfaces are favored that induce a high degree of orientation of the molecules. Different surface treatments are used to induce and control the orientation of the molecules. A homeotropic (perpendicular) alignment is favored on hydrophobic surfaces that are rough on a molecular level. This is observed in adsorbed monolayers of a surface-active compound such as lipids or surfactant molecules on glass, both for energetic and sterical reasons. Surface modifications can alter the positional order and molecular orientation of liquid crystalline... 

Recently it has been shown that surface-induced molecular orientation can be determined by atomic force microscopy (AFM), both in the isotropic and nematic phase of thermotropic liquid crystals [6-10]. At separations of several nanometers between the homeotropically modified glass surface and the AFM tip or homeotropically modified glass microsphere, respectively, a temperature-dependent short-range attractive (prenematic) or on average repulsive, but oscillatory, (presmectic) force was observed [6,7,9]. 

Fig. 4.18 Molecular orientation in Nisseki LC Film Series. (a) Twisted orientation, (b) hybrid orientation, (c) homeotropic orientation...

Previous studies demonstrated that the molecular alignment of Pc can be controlled by a introduction of a sacrificial layer [30], inducing a conversion of the initial molecular orientation from planar to homeotropic. Like the substrate modification, PVP was the preferred material for the fabrication of the sacrificial layer. Figure 9.18 shows the AFM image of Pc films where the initial molecular orientation was successively tuned to homeotropic. The appearance of small crystallites, as seen from the image proves that the formation of the SIP, is independent of the starting LC-orientation. 

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... 

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