Phases nematic

Example The following compound exhibits, as temperature decreases, an isotropic (I) phase, nematic (N), smectic A (SmA) re-entrant nematic (Nre), re-entrant smectic A (SmAre) mesophases and a crystalline (Cr) phase, with transitions at the specified temperatures. 

Iv) Liquid crystalline solutions (v > v ) in which the molecules spontaneously organize into an orientationally ordered phase (nematic) comprising domains in which the orientation of the molecules is along a single direction, but the centre of mass positions are random. An estimate for the critical concentration for transition to the nematic phase is V l = A15L b [13]. 

Based on microscopic structural details liquid crystals form three separate ordered phases nematic, smectic, and cholesteric. 

This can be also illustrated as follows a fatty man can be surrounded by 4 - 5 closely-packed fat men. But the same man can be surrounded even by 10 slim men. The latter can parallel the increasing dimensionality of space for the fat man. Consequently, the elongation of fluctuations due to the action of the strong electric field in the homogeneous phase of binary mixtures of limited miscibility is equivalent to increased dimensionality d = A. The uniaxial symmetry is natural also for the isotropic phase nematic liquid crystals. It may be considered for supercooled nitrobenzene due to intermolecular interactions (Fig. 5). ... 

By means of the Maxwell equal area rule, the dashed line in the figure is replaced by an equal pressure solid line. The regime of low pressure and low density is the isotropic phase while the regime of high pressure and high density corresponds to the anisotropic phase — nematic liquid crystal phase. The order parameter is determined by... 

Figure 27. Snapshots of typical instantaneous configurations (left panel) and inherent structures (right panel) for the model calamitic system GB(3, 5, 2, 1) at various mesophases. From top to bottom isotropic phase, nematic phase, and smectic phase. (Reproduced from Ref. 131.)...

Figure 22 shows the phase diagram of these suspensions. The suspensions are isotropic at low concentrations. The nematic phase is found at higher concentration and is separated from the isotropic liquid by a biphasic (N/I) region. Moreover, a sol-gel transition also takes place in the middle of the nematic phase. Nematic sols display typical Schlieren textures (Fig. 23) whereas samples of nematic gels sometimes show banded textures if they were submitted to flow on filling the optical capillary tubes. 

DLCs generally exhibit two types of phases nematic phase and columnar phase (Fig. 7.1). Discotic nematic phases are rarely observed while columnar phases are commonly found. In the nematic phase, the molecules possess only orientational ordering however in the columnar phase, the disc-like molecules spontaneously self-assemble into infinite one dimensional (ID) stacks, which in turn self-organize to various two dimensional (2D) liquid crystalline lattices. The hexagonal columnar phase is the most commonly encountered phase in DLCs. Some discotic compounds are also known to exhibit lamellar and cubic phases. 

For a uniaxial phase (nematic, discotic nematic, SmA, SmB, etc.) with the symmetry axis along z, all properties along x and y are the same and Xn= X22 X33- The corresponding matrix... 

Fig. 5.16 X-ray diffractograms of p-anisalamino-cinnamic acid in different phases, nematic, smectic A and crystalline smectic...

It should be noted that polar switching behavior was not observed in either of the LC phases (nematic and smectic) on the basis of the electro-optical and second harmonic generation (SHG) analyses, which is a distinct characteristic of banana LC phases. 

Of all liquid crystalline phases, the nematic phase is the phase with the highest symmetry, i.e. Dooh, and the least order. As shown in Fig. 3.3a, b, the mesogens solely possess orientational order. Positional order of the mass centers does not occur in this phase. Nematic phases are usually built up by either rod-like or disc-like mesogens. For thermotropic liquid crystals these mesogens are therefore calamitic or discotic molecules, respectively. In both cases the phase is simply denoted with the abbreviation N. For lyotropics, the notation typically distinguishes between nematic phases Nc, which are formed by rod-like micelles, and nematic phases Np, which are composed of disc-like micelles. 

One of the standard techniques often used for this purpose is the preparation of a perfectly aligned smectic A sample. Liquid crystal often undergoes the following phase sequence isotropic phase (nematic) smectic A => smectic C. The nematic phase (if it exists) could be oriented by traditional methods, such as the rubbing of polymer films or the evaporation of inorganic films at an oblique angle (< 10 ) [135]. The quality of orientation improves for lower values of the specific heat at the nematic smectic A transition and the minimum amount of crystallization centers. 

Three different models have been proposed for the explanation of these shear-induced phenomena. The first assumes that such phenomena are due to the formation of a nematic phase under the influence of shear. According to this theory, the systems are in the isotropic micellar phase, but close to the boundary of the nematic phase, and the application of shear leads to the transition into this phase. Nematic phases can... 

Dielectric Studies at Elevated Pressures A. Isotropic Phase Nematic Phase... 

Substance References Melting Smectic Phases Nematic-Isotropic ... 

Low molecular mass semiconductors are particularly appealing for computational scientists because the timescales of their rearrangements are relatively fast and often accessible to atomistic simulations. It is then becoming possible to reproduce both the equilibrium structure and the self-assembly process of crystalline, liquid crystalline, and amorphous organic semiconductors. Among those, thermotropic LCs are certainly the most fascinating and challenging materials from the structural point of view because of the variety of different phases (nematic, smectic, columnar, and many more [124]) they exhibit in relatively small temperature windows. 

X-ray diffraction is one of the primary methods to determine the structure of a liquid crystal phase. Nematic phases are characterized by diffuse arcs at small angles which result from local smectic-type order ( cybotactic clusters . Section 5.7.1). Smectic phases are characterized by Bra spots which result from the layer periodicity (Fig. 5.14). In oriented smectic A phases, the Bragg reflections are normal to the layers and their position is reciprocally related to the smectic layer period. If q is the wavenumber defining the peak position, then from Bragg s law (Eq. 1.26) d = Injq. The molecules and layers in a smectic C phase can adopt different mutual orientations depending on how the phase is accessed. If it is obtained by cooling from an SmA phase, the layer orientation is retained and the molecules tilt. Because the layer orientation... 

In 1975 Cladis discovered the sequence of phases nematic, smectic, and again nematic at atmospheric pressure. The lower-temperature nematic phase was designated as the re-entrant nematic phase (N ). By 1977 Cladis [68] was successful in giving evidence of a pressure-induced re-entrant nematic phase (Fig. 2). The investigated compounds were cyano Schiff bases and cyanobiphenyls with terminal n-alkyl or... 

Fluctuation effects are large in polymeric liquid crystals even far from phase transition temperatures. For example, in a novel liquid crystalline elastomer with a SmA-I transition, under an external mechanical stress, it was found in a mean field limit that, well in the isotropic phase, nematic fluctuations dominate with a cross-over temperature closer to the transition where SmA fluctuations become more important [14]. 

Friedel proposed the first classification scheme of LCPs (Felter and Dourson 1997) dividing them into three different t)q)es of mesogens (materials able to sustain mesophases), based upon the level of order the molecules possessed in the bulk material. Three major mesophases have been identified to describe the liquid crystalline phase nematic, cholestoric and smectic (Singh 2000). In nematic mesophase the molecules have no positional order but have a long range orientational order. But in cholestoric phase, molecules possess different positional... 

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