Nematic liquid crystal phase characteristics

Finally, dispersions of MWCNT in chiral nematic liquid crystals were studied as well. These experiments suggested no change in the helical twisting characteristics of the chiral nematic phase. However, the MWCNTs were thought to disrupt the translational order in the SmA phase (decrease of the SmA-N phase transition) yet follow the twist of the nematic director in the chiral nematic phase [498]. 

The liquid crystal melt, which comes into being at the glass-rubber transition or at the crystal-melt transition, may have several phase states (Mesophases) one or more smectic melt phases, a nematic phase and sometimes a chiral or cholesteric phase the final phase will be the isotropic liquid phase, if no previous decomposition takes place. All mesophase transitions are thermodynamically real first order effects, in contradistinction to the glass-rubber transition. A schematic representation of some characteristic liquid crystal phase structures is shown in Fig. 6.13, where also so-called columnar phases formed from disclike molecules is given. 

FIG. 6.13 Schematic representation of some characteristic liquid crystal phase structures. Nematic, smectic and cholesteric phases formed from rod-like molecules columnar phases formed from disc-like molecules (from Jansen, 1996). 

The use of birefringence to determine the behavior of 5( 7) is a natural choice since the principal characteristic of the nematic phase is optical birefringence i.e., the refractive index differs for light polarized parallel (/ n) or perpendicular (%) to the axis of molecular alignment. Eor a nematic liquid crystal, the director n specifies this optical z axis and / n = and = Dg are called the extraordinary and ordinary refractive indices, respectively. In general, rig > rig and the difference is the refractive index anisotropy (birefringence)... 

Blue phases are liquid crystal phases that appear between chiral nematic phases with relatively short helical pitches and isotropic phases. Generally, blue phases have the following characteristics ... 

Abe, A., lizumi, E., and Sasanuma, Y, Phase behavior and ordering characteristics of some chain molecules dissolved in a nematic liquid crystal, 4 -methoxybenzylidene-4-n-butylaniline, Polym. J., 25,1087-1098 (1993). 

The NMR technique has provided useful information regarding the orientational characteristics of nematic liquid crystals [96]. In many examples, the order parameters of the mesogenic core comprising a linear array of aromatic nuclei have been accurately determined from the observed dipolar (Dhd) and quadrupolar (Av) splittings by using deuterium-substituted samples. An attempt was made to elucidate the nematic conformation of the polymethylene-type spacer involved in dimer compounds, o, ct)-bis(4-cyanobiphenyl-4 -yloxy)alkanes (CBA- , with = 9,10), by the combined use of NMR and RIS analysis. The chemical structures and the phase transition data obtained by differential scanning calorimetry (DSC) are shown in... 

In the X-ray experiments on nematic 8CB, the smectic ordering was observed at the free surface (air-nematic interface). The same phenomenon has also been observed at the solid-nematic interface by the X-ray, an electrooptical technique and molecular force measurements. The principle of the latter is shown in Fig. 10.7. For two mica cylinders submerged in nematic liquid crystal, their interaction force measured with a balance oscillates with a distance between the cylinders and the period of oscillations was found to be equal to molecular length 1. This clearly shows the periodicity in density characteristic of a smectic phase [8]. 

Fig. 9.0. (a) Thin layer of nematic liquid crystal observed with a polarising microscope, (b) Model of a nematic liquid crystal made up of long molecules, (c) Micrograph of a smectic phase. Note the characteristic geometric shapes, called focal conics, formed from ellipses and hyperbolas, (d) Layered structure of a smectic liquid crystal... 

Whilst cholesteric liquid crystals, as the name implies, were originally based on cholesterol structures, it is useful to consider the structural features of other molecules that exhibit these phase characteristics. In general, cholestric mesogens are twisted nematic phases, but the feature that separates them from other similar molecules is that they possess an asymmetric chiral centre. The structural and geometric factors which influence the temperature of the N I transition are exactly those which influence the other nematic liquid crystals. It is usually found that the twisting power of the system diminishes as the chiral centre is moved away from the core structure, i.e. as n increases in the structure shown below ... 

We have presented a discussion of the theories and experiments on laser-induced optica nonlinearities and some recently observed wave-mixing processes in nematic liquid crystals based on the phase grating induced by two laser pulses. These studies have demonstrated again the unique and interesting physical characteristics of liquid crystals that have attracted the attention of fundamental and applied researchers alike. It is also clear that some practically useful nonlinear optical devices could be constructed. The nematic phase is but one of the several mesophases of liquid crystal that possess these interesting nonlinearities. Cholesterics and smectics [4] and other hybrid forms of nematics [6] also possess large nonlinearities. We anticipate that many more effects will be observed in the near future. 

Based on the director distribution we can derive the electrooptical response of a nematic liquid crystal cell (such as birefringence), rotation of the polarization plane of the incident light, total internal reflection, absorption, or some other important characteristics of the cell. In this chapter we will consider in detail these particular features of the electrooptical phenomena in uniform structures. Special attention will be paid to their possible applications. Electrooptics of the isotropic phase and polymer nematics, including Polymer Dispersed Liquid Crystals (PDLC), are also discussed. 

FIGURE 4.12. Variation with voltage of the optical characteristics of twist cells between parallel polaroids d = 30 /xm, a mixtiure of nematic liquid crystal with Ae = +0.22, T = 23 C) [66]. (1) Transmission with a cell conventionally arranged to the polarization of the incident ray (2) transmission when the cell is rotated around the normal through 45 from the original position (3) phase lag calculated from curve 2. Axes to the right and bottom refer to (1) and (2) and axes to the left and top refer to (3). 

This effect has been called the piezoelectric effect in many publications on liquid crystals, but there is good reason for giving it a different name. The piezoelectric effect corresponds to the occurrence of a charge on the surface of the crystal when there is a translational deformation, e.g., with compression or extension. The crystal in this case must be non-centrosymmetric. An effect of this type is also characteristic of polar liquid crystalline phases, e.g., of the chiral smectic C (Chapter 7). The effect, however, in which we are interested here is caused by flexion, a purely orientation deformation in a nematic liquid crystal. Consistent with this argument [1], we will call... 

Similar materials could be obtained by an emulsification method [253]. Nematic liquid crystal is emulsified into an aqueous dispersion of a water-insoluble polymer colloid (i.e., latex paint). An emulsion is formed which contains a droplet with a diameter of a few microns. This paint emulsion is then coated onto a conductive substrate and allowed to dry. The polymer film forms around the nematic droplets. To prepare an electrooptical cell a second electrode is laminated to the PDLC film [253]. In the phase separation and solvent-casting methods the chloroform solutions of liquid crystal and polymer are also used [254, 255]. The solution is mixed with the glass spheres of the required diameter to maintain the desired gap thickness and pipetted onto a hot (140 °C) ITO-coated glass substrate [255]. After the chloroform has completely evaporated another ITO-coated glass cover is pressed onto the mixture and then it is cooled down. Structural characteristics of the PDLC films are controlled by the type of liquid crystal and polymer used, the concentration of solution, the casting solvent, the rate of solvent evaporation, perparation temperature, etc. [254]. 

The straight short lines represent the molecules. In terms of the two characteristic properties of liquid crystals, order and mobility, the two extremes among the four phases are the crystal phase and the liquid phase. The crystal phase is the one that is well ordered and the liquid phase is the one that is disordered. The two liquid-crystal phases, also known as mesophases, are ranked between the two extremes. The smectic phase is more ordered than the nematic phase. By contrast, increasing mobility runs in the opposite direction. A crystal has no mobility whereas a liquid has high mobility. Both mesophases are mobile, with nematic more so than smectic. Figure 6.1 also illustrates that crystal and liquid phases are isotropic, meaning that no direction is involved in the order and mobility. The two mesophases are anisotropic that is, both have direction. This is particularly true in the nematic phase. The properties of the nematic phase are very sensitive to direction. 

Magnetic resonance methods have been used extensively to probe the structure and dynamics of thermotropic nematic liquid crystals both in the bulk and in confined geometry. Soon after de Gennes [27] stressed the importance of long range collective director fluctuations in the nematic phase, a variable frequency proton spin-lattice relaxation Tx) study [32] showed that the usual BPP theory [33] developed for classical liquids does not work in the case of nematic liquid crystals. In contrast to liquids, the spectral density of the autocorrelation function is non-Lorentzian in nematics. As first predicted independently by Pincus [34] and Blinc et al. [35], collective, nematic type director fluctuations should lead to a characteristic square root type dependence of the spin-lattice relaxation rate rf(DF) on the Larmor frequency % ... 

One well-known characteristic feature of nematic liquid crystals is the thread-like texture that can be observed with a polarizing microscope. The name nematic, derived from the Greek word "thread," reflects that feature. By examining the thin and thick thread-like structures in nematic liquid crystals, Otto Lehman i and Georges FriedeF deduced that this phase involves long-range orientational order. The first step to the interpretation of the threads as disclinations of the director field has been made by Oseen. Later Frank " derived Oseen s theory of curvature elasticity on a more general basis and presented it in a simpler form (see Appendix C.1). 

Liquid crystal phases can be distinguished by examining their defect textures. What characteristic defects would indicate the presence of the nematic phase or the cholesteric phase ... 

This group has a characteristically flat disk shape, which is responsible for the discotic liquid crystal phase. The nickel bis (dithiolene) core has high thermal and photochemical stability. The substituted R groups impart the solubility in nonpolar solvents. Also, if the R groups are sufficiently long, they can form a nematic liquid crystal, as shown in Figure 5 and 6. 

Identification of nematic polymeric mesophases is a more complex problem than identification of polymer smectics. The structural data are usually limited to finding the absence of small-angle reflections in the x-rays of unoriented samples. The low enthalpy of the transition from the anisotropic to the isotropic phase (Table 6.9), close to the corresponding values characteristic of low-molecular nematic liquid crystals, and the absence of layered reflections indicate the one-dimensional type of ordering, although these data are insufficient for a complete description of the structure of nematic polymers, which can be both similar to and (Afferent from low-molecular-weight nematics. 

The N -LC is used as an asymmetric liquid reaction field and it is prepared by adding a small amount of a chiral dopant to a nematic liquid crystal (N-LC). The formation of the N -LC can be confirmed under the polarizing microscope (POM) by the change of the characteristic nematic Schlieren texture to a fingerprint-like or striated Schlieren texture of the chiral nematic phase. The distance between the fingerprint lines of the optical pattern is equivalent to half of the helical pitch of the N -LC phase. Therefore, the stronger twisting power of the dopant can be observed by the shorter helical pitch of the optical pattern under the POM (Fig. 9.33). 

Neither does the microbrownian motion of the amorphous mesh inhibit the liquid crystal phase, nor does the positional order of the molecules interfere with the elasticity. Hence, as a hybrid material that combines LC and rubber characteristics, LCEs have unique properties in which the molecular orientation of the liquid crystal is strongly correlated with the macroscopic shape (deformation) which is unparalleled to other materials. The most prominent example in the physical properties derived from this property is the huge thermal deformation. Figure 10.1 shows an example of the thermal deformation behavior of side-chain nematic elastomers (NE) [3]. When the molecules transform from the random orientation in the isotropic phase to the macroscopic planar orientation in the nematic phase, the rubber extends in the direction of the liquid crystal orientation and increases with decreasing temperature as a result of an increase in the degree of liquid crystal orientation. This thermal deformation behavior is reversible, and LCEs can be even considered as a shape-memory material. Figure 10.1 is from a report of the early research on thermal deformation of LCEs, and a strain of about 40 % was observed [3]. It is said that LCEs show the largest thermal effect of all materials, and it has been reported that the thermal deformation reaches about 400 % in a main-chain type NE [4]. 

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