Homeotropic

FIG. 7 Same as Fig. 5, but for a nematic Gay-Berne film confined between homeotropically anchoring substrates (from Ref. 48). 

T. Gruhn, M. Schoen. A grand canonical ensemble Monte Carlo study of confined planar and homeotropically anchored Gay-Berne films. Mol Phys 95 681-692, 1998. 

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... 

Freely suspended films provide a perfect homeotropic alignment of smectic LCs since the layers always orient parallel to the LC/air interface.33 The director structure in such films can then be determined by analyzing the optical properties of plane-polarized light reflected from the surface of the films at a slightly oblique angle.34 The technique gains additional power when electrodes are added to the setup, allowing observation of the behavior of the films in the presence of an electric field parallel to the plane of the film.35... 

Amoskov, V. M. and Birshtein, T. M. (2000). Homeotropic and planar structures in liquid-crystalline polymer brushes, Vysomolekul. Soedin. A B, 42, 612-626. 

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. 

Note 2 When the alignment of the director in a homeotropic alignment deviates from the perpendieular, the alignment is said to be a pre-tilted homeotropic alignment the pre-tilt angle is the deviation from 90 . 

Fig. 20. Representing (a) homeotropic, (b) planar, and (c) uniform planar molecular...

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. 

Price and Wendorff31 > and Jabarin and Stein 32) analyzed the solidification of cholesteryl myristate. Under equilibrium conditions it changes at 357.2 K from the isotropic to the cholesteric mesophase and at 352.9 K to the smectic mesophase (see Sect. 5.1.1). At 346.8 K the smectic liquid crystal crystallized to the fully ordered crystal. Dilatometry resulted in Avrami exponents of 2, 2, and 4 for the respective transitions. The cholesteric liquid crystal has a second transition right after the relatively quick formation of a turbid homeotropic state from the isotropic melt. It aggregates without volume change to a spherulitic texture. This process was studied by microscopy32) between 343 and 355.2 K and revealed another nucleation controlled process with an Avrami exponent of 3. 

Fig. 16a-c. Modes of alignment of I.c. a homogeneous b homeotropic c twisted nematic... 

Such polymers adopt, when affected by a mechanical field, an optically uniaxial homeotropic structure polymers B.1.2, B.1.7, B.1.8 (Table 8) have positive birefringence polymers B.1.1, B.1.8. (Table 9) have negative birefringence, which has not been reported to our knowledge, for low-molecular nematic liquid crystals. Although the authors do not comment on the cause for the observed phenomenon, the fact in itself is sufficiently uncommon. 

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... 

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