Smectic applications

In this section we will discuss in some detail the application of X-ray diffraction and IR dichroism for the structure determination and identification of diverse LC phases. The general feature, revealed by X-ray diffraction (XRD), of all smectic phases is the set of sharp (OOn) Bragg peaks due to the periodicity of the layers [43]. The in-plane order is determined from the half-width of the inplane (hkO) peaks and varies from 2 to 3 intermolecular distances in smectics A and C to 6-30 intermolecular distances in the hexatic phase, which is characterized by six-fold symmetry in location of the in-plane diffuse maxima. The lamellar crystalline phases (smectics B, E, G, I) possess sharp in-plane diffraction peaks, indicating long-range periodicity within the layers. 

The most successful continuum description of membrane elasticity, dynamics, and thermodynamics is based on the smectic bilayer model (for examples of different versions and applications of this approach see Ref. 76-82 and references therein). We introduce this model in conjunction with the question of membrane undulations. 

Since P must remain normal to z and n, the polarization vector forms a helix, where P is everywhere normal to the helix axis. While locally a macroscopic dipole is present, globally this polarization averages to zero due to the presence of the SmC helix. Such a structure is sometimes termed a helical antiferroelectric. But, even with a helix of infinite pitch (i.e., no helix), which can happen in the SmC phase, bulk samples of SmC material still are not ferroelectric. A ferroelectric material must possess at least two degenerate states, or orientations of the polarization, which exist in distinct free-energy wells, and which can be interconverted by application of an electric field. In the case of a bulk SmC material with infinite pitch, all orientations of the director on the tilt cone are degenerate. In this case the polarization would simply line up parallel to an applied field oriented along any axis in the smectic layer plane, with no wells or barriers (and no hysteresis) associated with the reorientation of the polarization. While interesting, such behavior is not that of a true ferroelectric. 

Figure 8.13 Hypothetical smectic mesogen with hinge in center of core is illustrated. Such material could in principal switch to ferroelectric state, which we term the SmAPp, upon application of electric field in plane of layers. If this state exists in well on configurational hypersurface, then ground-state structure is antiferroelectric, denoted SmAPA.

Using this method, the M6R8/PM6R8 blend showed precisely the behavior expected for the achiral SmAPA structure. Specifically, the optical properties of the films were consistent with a biaxial smectic structure (i.e., two different refractive indices in the layer plane). The thickness of the films was quantized in units of one bilayer. Upon application of an electric field, it was seen that films with an even number of bilayers behaved in a nonpolar way, while films with an odd number of bilayers responded strongly to the field, showing that they must possess net spontaneous polarization. Note that the electric fields in this experiment are not strong enough to switch an antiferroelectric to a ferroelectric state. Reorientation of the polarization field (and director structure) of the polar film in the presence of a field can easily be seen, however. 

Note 7 When the tilt direction alternates from layer to layer, the smectic mesophase is antiferroelectric such mesophases do not possess spontaneous polarization. They can be turned into ferroelectric structures through the application of an electric field. 

It may be asserted that the fundamental reason arises from the fact that, while parallel arrangements of anisotropic objects lead to a decrease in orientational entropy, there is an increase in positional entropy. Thus, in some cases, greater positional order will be entropically favorable. This theory therefore predicts that a solution of rod-shaped objects will undergo a phase transition at sufficient concentration into a nematic phase. Recently, this theory has been used to observe the phase transition between nematic and smectic-A at very high concentration (Hanif et al.). Although this model is conceptually helpful, its mathematical formulation makes several assumptions that limit its applicability to real systems. 

We briefly discussed the origin and structure of liquid crystals in Section 4.13. The last decade has witnessed a surge of interest in liquid crystals because of their applications in display devices (devices that convert an electrical signal into visual information). The design of liquid crystal (LC) devices relies on the relation between the molecular structure and the phase behaviour (relative smectic-nematic tendency, NI etc.) as well as the physical properties of the liquid crystals (Chandrasekhar, 1994). 

It can be safely predicted that applications of liquid crystals will expand in the future to more and more sophisticated areas of electronics. Potential applications of ferroelectric liquid crystals (e.g. fast shutters, complex multiplexed displays) are particularly exciting. The only LC that can show ferroelectric property is the chiral smectic C. Viable ferroelectric displays have however not yet materialized. Antifer-roelectric phases may also have good potential in display applications. Supertwisted nematic displays of twist artgles of around 240° and materials with low viscosity which respond relatively fast, have found considerable application. Another development is the polymer dispersed liquid crystal display in which small nematic droplets ( 2 gm in diameter) are formed in a polymer matrix. Liquid crystalline elastomers with novel physical properties would have many applications. 

A good deal in synthesis effort has been devoted to chiral liquid crystals, especially those w ith chiral smectic C phases. The chiral smectic C phase is ferroelectric. w hich gives it properties quite useful lor applications. Perhaps the most important properly of these phases is that a lateral dipnle can produce a spontaneous polarization... 

Polarized light is the must powerful tool for investigating liquid crystals, all of which exhibit characteristic optical properties. A smectic liquid crystal transmits light more slow ly perpendicular to the layers than parallel to them. Such substances are said to be optically positive. Nematic liquid crystals are also optically positive, bui their action is less definite than that of smectic liquid crystals. However, the application of a magnetic field to nematic liquid crystals lines up their molecules, changing their optical properties and even their viscosity. 

Useful applications have been found lor the varied effects of these crystal changes. One of the first came from the properly of selectively reflecting visible light because this is lempcraiurv-dependent. the property can be used as a temperature detector, and in gel lurm liquid crystals have been used lor the early detection of those cancers which cause hot spots in the body. Applications of the smectic modifications arise from their ferroelectric properties this phase can function as a fast-switching light-valve device with memory. This kind of application requires some... 

Recently, fluorinated chiral LC with highly tilted smectic C phases and opposite tilt direction in adjacent layers (anticlinic tilt, SmCA ) became of significant interest, as Lagerwall et al. proposed an application of 90° tilted anticlinic and... 

The use of a smectic A LC instead of a nematic LC allows for memory-type H-PDLCs, as shown by Date et al. [25], At an appropriate temperature for the LC used, the grating could be switched off with a 10 ms electric field pulse of 30 V/p,m. In contrast, nematic H-PDLCs require a continuous application of the field to maintain the off state. The smectic H-PDLCs were turned back on by warming them above a critical temperature. 

The following table lists the liquid crystalline materials that are useful as gas chromatographic stationary phases in both packed and open tubular column applications. In each case, the name, structure, and transition temperatures are provided (where available), along with a description of the separations that have been done using these materials. The table has been divided into two sections. The first section contains information on phases that have either smectic or nematic phases or both, while the second section contains mesogens that have a cholesteric phase. It should be noted that each material may be used for separations other than those listed, but the listing contains the applications reported in the literature. 

In an interesting application, the long-chain substituted 4- -hexoxybenzylidene-4 -iodoaniline (HBIA), 5 has been incorporated in the liquid-crystal phase of 4- -hexoxy-benzylidene-4 -propylaniline (HBPA, 6)13. The intensity pattern of the Mossbauer spectrum allowed the orientation of the molecule to be estimated, showing that the smectic tilt angle was 50°. 

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