Dielectric reorientation, smectic

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. 

A detailed comparative study of dielectric behaviour of smectic and nematic polymers was carried out for polymers of acrylic and methacrylic series, containing identical cyanbiphenyl groups (polymers XI and XII) 137 138>. The difference in structural organization of these polymers consists in a more perfect layer packing of smectic polymer XI (see Chaps. 4.1 and 4.2) with antiparallel orientation of CN-dipoles. This shifts the relaxation process of CN-dipole reorientation to a low frequency region compared to nematic polymer XII. Identification of Arrhenius plots for dielectric relaxation frequencies fR shows that for a smectic polymer the value of fR is a couple of orders lower than for a nematic polymer (Fig. 21). Though the values... 

The process of the director reorientation in polymer ferroelectrics, as in their low-molecular counterparts, involves changes in the tilt 0) and azimuthal (f) angles. These two modes are characterized by quite different rates. The 6 process corresponds to the soft-mode distortion, and the corresponding time To diverges at the C A phase transition point. The process means the motion of the director over the conical surface around the normal to the smectic layer (the Goldstone mode). In the helical structure the latter involves the twisting-untwisting mode, tq and differ considerably from each other, because backbones participate in those modes to a different extent. This can be seen in the dielectric spectra [172], and in the pyroelectric and electrooptical response. 

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]. 

Let us identify the three principal values of the dielectric permittivity as e, (i=l, 2, 3), where 3 corresponds to the direction along the director, corresponds to the direction perpendicular to the plane of the tilt, and 2 corresponds to the direction perpendicular to the preceding two [8, 9]. When the field is oriented along the director, a Frederiks transition is possible when 6 > 3. The director should rotate around the normal to smectic layers, not changing its angle Q. The corresponding threshold field is proportional to ( 2- 3) . A field perpendicular to the director and lying in the plane of the tilt induces the same distortion when > j. Now the threshold for reorientation... 

An alternative write/erase mechanism that may be exploited to provide a scattering display in smectic liquid crystals [60] relies on the electrothermal reorientation of a material of positive dielectric anisotropy (Fig. 14). The thermal input to the material can be provided either by ohmic heating of stripe electrodes [61] in the cell, or by scanning a visible or infrared laser spot across the device. A dye can with profit be incorporated into the liquid crystal [62] to improve the absorption of the incident light. Typically, the liquid crystal used will pos-... 

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