Homeotropic phase

Note 2 A cubic mesophase is optically isotropic it may be distinguished from an isotropic liquid or a homeotropic phase by the fact that the optically-black isotropic phase or homeotropic phase nucleates in the bireffingent SmC phase in straight-edged squares, rhombi, hexagons and rectangles. 

If one makes use of the rather limited information available and given above one may infer that a tilt of between 20° and 30° is normal for straight chain azobenzene derivatives when deposited as LB films, even when a homeotropic phase exists. Such a structure can only be produced in a rather loosely packed film. At the moment it is an open question whether monolayers of these materials exist in the hexatic phase as is the case for fatty acids or whether the structure more nearly corresponds to the smectic-A phase. In the case of the birefringent phase described by Jones et al. [151] it was shown that, once this phase was established, further layers deposited by the LB technique go down in an epitaxial manner. 

Figure 4.10. Interference figures of a uniaxial liquid crystal a, b, c for the homeotropic phase with increased thickness from a to c d shows vibration directions for different parts of these figures, e represents interference figure of the homogeneous phase.

Stelzer et al. [109] have studied the case of a nematic phase in the vicinity of a smooth solid wall. A distance-dependent potential was applied to favour alignment along the surface normal near the interface that is, a homeotropic anchoring force was applied. The liquid crystal was modelled with the GB(3.0, 5.0, 2, 1) potential and the simulations were run at temperatures and densities corresponding to the nematic phase. Away from the walls the molecules behave just as in the bulk. However, as the wall is approached, oscillations appear in the density profile indicating that a layered structure is induced by the interface, as we can see from the snapshot in Fig. 19. These layers are... 

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... 

The textures in homeotropic lamellar phases of lecithin are studied in lecithin-water phases by polarizing microscopy and in dried phases by electron microscopy. In the former, we observe the La phase (the chains are liquid, the polar heads disordered)—the texture displays classical FriedeVs oily streaks, which we interpret as clusters of parallel dislocations whose core is split in two disclinations of opposite sign, with a transversal instability of the confocal domain type. In the latter case, the nature of the lamellar phase is less understood. However, the elementary defects (negative staining) are quenched from the La phase they are dislocations or Grandjean terraces, where the same transversal instability can occur. We also observed dislocations with an extended core these defects seem typical of the phase in the electron microscope. 

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. 

Fig. 8. Liquid-crystalline textures of didodecyl-PPE 12c. Bars represent 20 microns. These PPEs form homeotropic smectic phases the textures are typical for an edge-on view of lamellae that show apparent nematic ordering in this projection [46],...

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. 

Self-standing nanostructured two-dimensional polymer films were prepared by in situ photopolymerization of ionic liquid crystalline monomer 11 that forms homeotropic monodomains of the smectic A phase on a glass plate (Figure 25.4). The film of 12 has a macroscopically oriented layered nanostructure as presented in Figure 25.5. 

---