Nematic planar

Note 3 The angular distribution of the director around a defect in a nematic planar texture, in the X-Y projection, can be expressed in terms of the polar angle 0 ... 

F re 3. Schematic diagram of selective reflection from a right-handed chiral nematic planar texture for A= inside the liquid ciystal. The linearly polarized input light may be considered as counter-propagating right-handed (RH) and left-handed (LH) circular components. The RH component in which the E field matches the sense of the helicoidal structure is back reflected due to director fluctuations, whilst the LH component is almost totally transmitted. By convention, the handedness is delined in terms of the progression of the E field vector in time relative to the observer. The RH rotating wave therefore has the same spatial structure as the chiral nematic at any time. 

Ki and K2 differ from /c by terms of the order of (5 e), and hence a and b are small and of the order of (6 ). Equations (28) and (29) mathematically describe the key optical properties of a chiral nematic planar texture for normally incident light [62]. 

Figure 18. Reflection coefficient 91 as a function of wavelength for light normally incident on a chiral nematic planar film for (a) a semi-infinite and (b) a finite slab. The dots are experimental and the curves computed (see Eq. 36) (redrawn with data from [82]).

Pitches can be transformed to a mesophase state by further chemical and physical operations. Heat treatment of conventional pitches results in additional aromatic polymeriza tion and the distillation of low molecular weight components. This results in an increase in size and concentration of large planar aromatic molecular species whereupon the precursor pitch is transformed to a mesophase state exhibiting the characteristics of nematic Hquid crystals (1). Additional heat treatment converts the mesophase pitch to an infusible aromatic hydrocarbon polymer designated as coke. 

The alignment of the director at the nematic free surface of real systems is not found to exhibit universal behaviour. Depending on the mesogen, homeotro-pic, tilted and planar anchoring have been observed. Clearly, to study this interface in a simulation a potential which exhibits a nematic phase in co-... 

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... 

Since many planar metal complexes have nearly axially symmetric g and AM tensors, two-dimensional powder ENDOR spectra can easily be obtained from such compounds oriented in nematic glasses84. As mentioned, interpretation of this type of spectra will be discussed in Sect. 4.3. 

Note 3 Schlieren textures observed in nematic samples with planar alignment show defect centers with two or four emerging brushes. Schlieren textures in nematic samples with tilted alignments show centers with four brushes centers with two brushes are caused by defect walls. 

Regions in a liquid crystal having a specific cellular periodic flow-pattern in the form of long rolls induced by the application of an electric field perpendicular to a nematic layer with an initial planar alignment of the director. 

The isomers coumaranone (benzo[Z> ]f uran-2 (3//) - one) and phthalide (benzo[c]furan-1(3//)-one) have yielded H NMR results in the nematic phase and hence information on bond lengths etc. both molecules are planar and it seems that inverting the O—C=0 segment has little effect (80OMR(l3)384>. 

Fig. 3 Principle anchoring conditions of nematic liquid crystals at nanoparticle surfaces (a) planar anchoring, and (b) vertical (or homeotropic) anchoring...

Tuning of the pre-tilt angle at the interface was also demonstrated by doping commonly used polyimide alignment layers with POSS nanoparticles [339]. In addition, the fabrication of a tunable liquid crystal flat microlens was achieved by placing a drop of a nematic liquid crystal doped with POSS nanoparticles onto a substrate inducing planar alignment (local HAN mode) [340]. Simultaneously, Takatoh and co-workers extended this concept to a series of metal oxide... 

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