Smectic phase reorientation

The non-collective motions include the rotational and translational self-diffusion of molecules as in normal liquids. Molecular reorientations under the influence of a potential of mean torque set up by the neighbours have been described by the small step rotational diffusion model.118 124 The roto-translational diffusion of molecules in uniaxial smectic phases has also been theoretically treated.125,126 This theory has only been tested by a spin relaxation study of a solute in a smectic phase.127 Translational self-diffusion (TD)29 is an intermolecular relaxation mechanism, and is important when proton is used to probe spin relaxation in LC. TD also enters indirectly in the treatment of spin relaxation by DF. Theories for TD in isotropic liquids and cubic solids128 130 have been extended to LC in the nematic (N),131 smectic A (SmA),132 and smectic B (SmB)133 phases. In addition to the overall motion of the molecule, internal bond rotations within the flexible chain(s) of a meso-genic molecule can also cause spin relaxation. The conformational transitions in the side chain are usually much faster than the rotational diffusive motion of the molecular core. 

A variety of effects can occur in the TGB phases due to the influence of an electric field. The coupling between the director and the field may be due to the dielectric anisotropy Sa, or due to the dependence of the smectic tilt angle on the electric field (electroclinic effect), or due to the spontaneous polarization. In contrast to the typical behavior of smectic phases, a small electric field cannot only result in a reorientation of the director, but also in a reorientation of the smectic layers [138], Higher fields can cause a reorientation of the pitch axis, helical unwinding [139], [140], a shift of the wavelength of selective refiection [141], or field-induced phase transitions [103], [141]. 

Whilst tilted smectic phases of chiral materials are in general electro-optically switchable, many chiral tilted columnar systems show no optical effect whatsoever when an applied voltage is reversed (Figure 11.12). This resilience to the reorientation of dipoles by an electrical field may in some cases be due to the more crystalline than liquid crystalline nature of the systems, but even in the quite conventional high-temperature tilted phase of the first chiral material (triphenylene-2,3,6,7,10,ll-hexayl ( S )-3-... 

Figure 27 presents the lnT (p) dependencies obtained for 8CB. The activation enthalpies A f/ and activation volumes V for the nematic and smectic phases are presented in Figure 28, both for 8CB and 80CB. The activation enthalpies obtained for the smectic phases increase for 8CB but decrease for 80CB with rising pressure, whereas it always decreases in the nematic phase. Also the plots for the activation volumes (Fig. 28b) exhibit similar trends. The opposite pressure dependencies observed for the nematic and smectic A phase of 8CB have been discussed in terms of a peculiar pressure influence on the molecular associations. The existence of dimers in the smectic layers enlarges the free volume that facilitates the molecular reorientations thus the activation enthalpy for the smectic phase is reduced, which has also been noted for other liquid crystals. ... 

The ordering of the transverse molecular axes, which occurs in certain low-temperature smectic phases, has been studied by NMR and NQR methods [7.40]. These measurements show that the uniaxial reorientation of the molecular cores around their long axes are strongly biased. It is generally assumed that in nematic and smectic A phases, the uniaxial rotation (7-motion) is not biased. However, recent neutron quasielastic scattering experiments [7.41] in the nematic phase of MBBA seem to support the notion that the rigid benzylideneaniline core is restricted to a uniaxial rotational diffusion of finite angular excursion. Restricted libration within 7 =z 00/2 for internal motions in macromolecules has been considered by London and Avitabile [7.42], and Wittebort and Szabo [7.43]. 

The direct NMR method for determining translational difiFusion constants in liquid crystals was described previously. The indirect NMR methods involve measurements of spin-lattice relaxation times (Ti,Ti ),Tip) [7.45]. Prom their temperature and frequency dependences, it is hoped to gain information on the self-diflPusion. In favorable cases, where detailed theories of spin relaxation exist, difiFusion constants may be calculated. Such theories, in principle, can be developed [7.16] for translational difiFusion. However, until recently, only a relaxation theory of translational difiFusion in isotropic hquids or cubic solids was available [7.66-7.68]. This has been used to obtain the difiFusion correlation times in nematic and smectic phases [7.69-7.71]. Further, an average translational difiFusion constant may be estimated if the mean square displacement is known. However, accurate determination of the difiFusion correlation times is possible in liquid crystals provided that a proper theory of translational difiFusion is available for liquid crystals, and the contribution of this difiFusion to the overall relaxation rate is known. In practice, all of the other relaxation mechanisms must first be identified and their contributions subtracted from the observed spin relaxation rate so as to isolate the contribution from translational difiFusion. This often requires careful measurements of proton Ti over a very wide frequency range [7.72]. For spin - nuclei, dipolar interactions may be modulated by intramolecular (e.g., collective motion, reorientation) and/or intermolecular (e.g., self-diffusion) processes. Because the intramolecular (Ti ) and intermolecular... 

In the same way that the molecules of N phases can be electrically reoriented, the molecules of the smectic phase can be dielectrically reoriented by electric fields, albeit at a higher voltage. However, unlike in the N phase, when the field is removed, the bulk viscosity of the smectic phase inhibits relaxation and bistability is favored. This can be an advantage unless the procedure has to be reversed, because this cannot be achieved so easily. Reversal is accomplished by heating to either the less viscous N or isotropic liquid phases (as used in the laser and thermally addressed devices) or by causing electrohydrodynamic scattering to occur (see Sec. 3.6). In this section we shall specifically consider the dielectric reorientation effect. In itself it may not be particularly useful, but when combined with other techniques, it can lead to interesting devices. 

Approximately 20 different smectic phases have been identified up to now [3]. Eight among them consist of so-called tilted phases, Le.. the long axes of the moleciiles are tilted with respect to the layer normal (, Sb Sr. So. Sh, Si, Sk. and Sm). If these latter mesophases consist of chiral molecules, they in principle match the requiremeni for intrinsic ferroelectric polarization. In six of these tilled chiral phases (denoted herein by an asterisk), spontaneous polarization has been measured (Sr. Sr. , So St. Sy ). For technological application in electrooptical devices, the chiral smectic C phase Sc is prominent due to its lowest ordering a hence highest fluidity, making reorientation processes caused by electric fields very find. 

The possibility of laser-induced director axis reorientations in the smectic phase was first theoretically studied in 1981. For smectic-A and smectic-B, one can see that a reorientation of the director axis will involve a change in the layer spacing. As... 

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. 

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