Uniaxial phase

The orthogonal arrangement of the disc-like molecules in the columns of and D id phases makes these phases uniaxial, while the tilted phases (Drd and Doh.d and Dt) are optically biaxial. There are two additional columnar phases labeled as and that have not yet been classified. The columnar phases were discovered before the observation of a nematic phase for disc-like molecules. Both chiral nematic phases and the re-entrant behavior have now been observed in discotics. The phase diagram and molecular structure of a typical discotic liquid crystal are shown in Fig. 1.11. Finally, it is noted that another classification scheme for the discotic mesophases has been used [1.26], which is based on the notation used for the conventional smectics. 

Fig. 5. Stress and strain during creep phase (uniaxial)...

In the case of positive dielectric anisotropy of a nematic, even a weak field makes the isotropic phase uniaxial and the N-I phase transition disappears (see Pikin [7], Chap. 4). However, the apparent N-I phase transition temperature may change with the electric or magnetic field. For a>0 and E n, the quadratic-in-field energy terms (Eq. (la) and (lb)) reduce the free energy and stabilize the anisotropic phase. In the... 

Three-ring mesogens with two longer lateral alkyl chains (e.g. 12) do not show mesophases. However, cross-shaped compounds (30) have nematic phases (uniaxial) with clearing points above 100°C [46, 59]. That is surprising because 1,2,4-tris(4-n-alkyl-oxybenzoyloxy)benzenes (19f) are not liquid crystalline, but l,3,5-tris(4-n-alkyloxy-phenyl)benzoates exhibit nematic discotic phases [83]. Apparently, such substituted benzene derivatives are located at the boundary between calamitic and discotic compounds. However, there is no example of this type that shows both columnar and nematic or smectic phases, as occurs in poly-catenar and double-swallow tailed compounds (see Sec. 5 and Chap. XII of this volume). 

A simple derivation of tliis equation based on tire lowest-order derivative (curvature) of tire layer displacement field u(r) has been provided [87]. A similar expression can be obtained for a uniaxial columnar phase [20] (witli tire columns lying in tire z direction) ... 

Jen S, Clark N A, Pershan P S and Priestley E B 1977 Polarized Raman scattering of orientational order in uniaxial liquid crystalline phases J. Chem. Phys. 66 4635-61... 

Concerning a liquid droplet deformation and drop breakup in a two-phase model flow, in particular the Newtonian drop development in Newtonian median, results of most investigations [16,21,22] may be generalized in a plot of the Weber number W,. against the vi.scos-ity ratio 8 (Fig. 9). For a simple shear flow (rotational shear flow), a U-shaped curve with a minimum corresponding to 6 = 1 is found, and for an uniaxial exten-tional flow (irrotational shear flow), a slightly decreased curve below the U-shaped curve appears. In the following text, the U-shaped curve will be called the Taylor-limit [16]. 

Figure 2. Thermal strain vs temperature curves for VsSi measured along [001] on heating (4.2-60K) and cooling (4.2-1.5K). Curve (a) is for an uniaxial stress (s 0.03o doo)) along [001] (b) and (c) are for biaxial stress applied along [100] and [010] with 0.5o (ioo> and o (ioo>, respectively. The x-ray data of Batterman and Barrett (reference 15) are also plotted for comparison. The insets show the directions of applied stresses and [in case of the curve (a)] the martensite-phase domains. (From reference 5)...

Although the common nematic and smectic A phases have uniaxial symmetry, the molecules that form them are not cylindrically symmetric, although they... 

A lattice model of uniaxial smectics, formed by molecules with flexible tails, was recently suggested by Dowell [29]. It was shown that differences in the steric (hard-repulsive) packing of rigid cores and flexible tails - as a function of tail chain flexibility - can stabilize different types of smectic A phases. These results explain the fact that virtually all molecules that form smectic phases (with only a few exceptions [la, 4]) have one or more flexible tail chains. Furthermore, as the chain tails are shortened, the smectic phase disappears, replaced by the nematic phase (Fig. 1). 

Researchers who have focused more on understanding cause-effect relationships in solution processing have given attention to film drying and pyrolysis behavior, densification processes, and nucleation and growth into the desired crystalline state. Both thermodynamic and kinetic factors associated with the phase transformation from the amorphous state to the crystalline state have been considered.11 119 Control of these factors can lead to improvements in the ability to influence the microstructure. It is noted that in the previous sentence, influence has been carefully chosen, since the ability to manipulate the factors that govern the nature of the phase transformation to the extent that full control of the microstructure is possible remains to be demonstrated. However, trends in characteristics such as film orientation and columnar versus uniaxial grains have certainly already been achieved.120... 

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