Electroclinic effect

As mentioned in the introduction, chiral compounds can exhibit chiral mesophases and these are important due to the important physical properties that they may exhibit, including thermochroism, ferroelectric and electroclinic effects [15], In 1975, Meyer predicted the existence of a spontaneous polarization (Pg) in chiral, tilted smectic phases [86], and the existence of such polar order within a liquid crystal phase has important implications both scientifically and industrially [19]. The asymmetry associated with the chirality may also produce a beneficial lowering of transition temperatures. 

The structure of the smectic A phase when it is composed of optically active material (i.e., smectic A ) remains the same as that for the achiral phase. The molecules are arranged in diffuse disordered layers, and there is no long-range periodic order. However, because of the molecular chirality, the environmental symmetry is reduced to [10]. As a consequence, when an electric field is applied to a chiral smectic A= phase there will be a coupling of the electroclinic susceptibility to the field and the long axes of the molecules will tilt with respect to the layer planes. The tilt angle, for relatively low applied fields, varies linearly with the field. This linear electrooptic phenomenon is called the electroclinic effect. 

Fig. 5.8.8 presents the experimental data for the tilt angle, specific heat, and the susceptibility (the last being measured from the magnetoclinic effect, the magnetic analogue of the electroclinic effect, see 5.10.1) along with the curves fitted to (5.8.7), (5.8.8) and (5.8.9). It is seen that the agreement is very satisfactory. 

The coupling between P and 0 manifests itself even above the C -A transition an electric field induces a tilt in the A phase as well. This is called the electroclinic effect, and was first demonstrated by Garoff and Meyer. Induced tilt angles as high as 10° have been observed in high polarization materials. Due to its submicrosecond response and its linear dependence... 

Fig. 5.10.5. The temperature variation of the field-induced tilt (or the electroclinic effect) in the smectic A phase of 4-(3-methyl-2-chlorobutanoyloxy)-4 -heptyloxy biphenyl. (After Bahr and Heppke. >)...

Another phenomenon that has potential applications is the field-induced tilt or the electroclinic effect. Unlike the SSFLC device, this effect does not possess bistability but it has a faster (submicrosecond) response. By using the same bookshelf geometry and a suitable polarizer and retarder arrangement, the electroclinic effect can be used for modulating a light signal with a transmitted intensity linearly proportional to the applied voltage or as a tunable colour filter. 

Walba DM, Yang H, Shoemaker RK, Keller P, Shao R, Coleman DA, Jones CD, Nakata M, Clark NA (2006) Main-chain chiral smectic polymers showing a large electroclinic effect in the SmA phase. Chem Mater 18 4576... 

The electroclinic effect is an induced molecular tilt observed in the chiral orthogonal smectic phases, such as the smectic A phase, when an electric field is applied along the smectic layers [76]. The induced molecular tilt 0 is a linear function of the applied field E and gives rise to an induced polarization Pj... 

In the linear regime, the electroclinic effect is characterized by a fast field-indepen-dent response time T given by... 

Figure 3.16. Electroclinic effect in ferroelectric LCEs (a) Chemical structure of sample, (b) Measurement geometry the beam in the interferometer passes twice through the film to measure the electrically induced thickness modulation, (c) The viewing angle is turned by 90° around the layer normal compared with that in (b). Source Lehmann et al., 2001.

This is a chiral smectic A with symmetry Dqo. Its properties are similar to those of the achiral SmA. However, close to the transition to the smectic C phase, the chiral smectic A phase shows interesting pretransitional phenomena in the dielectric and electrooptical effects (the so-caUed soft dielectric mode and electroclinic effect). They will be discussed in Chapter 13. 

Fig. 13.9 Electroclinic effect in SmA phase. The geometry of a bookshelf cell placed between polarizer (P) and analyser (A) 9 is field controlled tilt angle of the director (a). Typical linear field dependence of angle 9( ) and characteristic soft-mode relaxation time independent of the field (b)...

The dynamics of the electroclinic effect is, in fact, the dynamics of the elastic soft mode. From Eqs. (13.18) and (13.19) follows that the switching time of the effect is defined only by viscosity and the term a(T — T ) and is independent of any characteristic size of the cell or material. It means that the relaxation of the order parameter amplitude is not of the hydrodynamic type controlled by term Kq (K is elastic coefficient). For the same reason Xg is independent of the electric field in agreement with the experimental data, shown in Fig. 13.9b. At present, the electroclinic effect is the fastest one among the other electro-optical effects in liquid crystals. 

Garoff, S., Meyer, R.B. Electroclinic effect at the A-C phase change in a chiral liquid crystal. Phys. Rev. Lett. 38, 848-851 (1977)... 

Electroclinic effect in chiral smectic liquid crystal... 

Figure 4.11 Schematic diagram showing the electroclinic effect in the smectic-A ...

The electric field can also induce a tilt in the smectic A phase formed by chiral molecules (electroclinic effect). The tilt angle, 0, is linear with applied voltage i.e., no bistability is observed, in contrast to the smectic C phase. The electroclinic switching is remarkably faster than the ferroelectric one [6,7]. For numerous FLCPs, the electroclinic switching in the smectic A phase has been studied [51,54,62,123]. 

Fig. 15. In the Sa phase, the mesogenic parts (depicted as ellipsoids) of the elastomeric macromolecule stand upright (9 = 0°) inside the single smectic layers. By applying a lateral electric field (perpendicular to the plane of the paper), a tilt angle 6 that is proportional to the electric field E can be induced (electroclinic effect). The sign of the tilt depends on the sign of the electric field E. Hence, each smectic layer shrinks by Ak/z twice during one period of the electric field. The shrinkage Ah of the whole film is measured by the interferometer as an optical phase shift between the sample beam and the reference beam.

Fig. 16. Electrostriction of a ferroelectric LC-elastomer (43). Big diagram Thickness variation Ah as a function of the applied ac voltage (/ac- Interferometric data were obtained at the fundamental frequency of the electric field (piezoelectricity, first harmonic -t) and at twice the frequency (electrostriction, second harmonic o). Sample temperature 60°C. Inset Electrostrictive coefficient a (-I-) versus temperature. At the temperature where the non-cross-linked polymer would have its phase transition Sc -Sa (about 62.5 0, the tilt angle of 0° is unstable. That is why the electroclinic effect is most effective at this temperature. An electric field of only 1.5 MV/m is sufficient to induce lateral strains of more than 4%.

When the helical structure of the chiral nematic phase is unwound by the influence of limiting walls, we can observe a linear-in-field light modulation which is caused by a small molecular tilt [85]. The effect is analogous to the electroclinic effect observed in the smectic A phase as the pretransitional phenomenon in the vicinity of the transition. 

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