Electroclinic smectics

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. 5.10.5. 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. >)...

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... 

While the linearity is thus not a problem, the fact that the periodic pitch changes during switching is a problem. At high tilt this will lead to the appearance of ripples in the homogeneous structure. This is very similar to the instability appearing in the electroclinic switching in the smectic A ... 

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. 

We see, that the phase transitimi temperature increases, because the dipole-dipole interactions (f -term) stabilise the smectic C phase. Note that the held induced tilt angle (or electroclinic coefficient Cc) diverges at a temperature T. This means, that at even inhnitesimally low held would create a finite tilt. This is the soft mode of the director motion any small force (not necessary electric) would cause the tilt of the director, because, at the transition, the medium becomes soft with respect to the tilt. The corresponding dielectric susceptibility shows the Curie-Weiss law ... 

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]. 

Chiellini, E., Galli, G., Lagerwall, S. T., and Komitov, L., Electroclinic properties of chiral smectic A liquid crystalline polymers and block copolymers, Macromoi. Svmp., 96, 79-94 (1995). 

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.

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. 

Electroclinic Effect near the Smectic A C Phase Transition... 

The electroclinic effect or the field induced tilt angle in the smectic A phase near the smectic A-C phase transition was discovered by Garoff and Meyer [19]. Later on, several authors investigated the electrooptical characteristics of the effect [20, 36, 105-106] and developed novel materials for its application [12, 107, 108]. 

The electroclinic or soft mode effect could be explained within the framework of the Landau theory of phase transitions [4]. We can write the expansion of the free energy density g of smectic A up to the 0 term as follows ... 

The combination of a computer and an electronically addressable two-dimensional spatial light modulator is a good device in performing highspeed computations. The modulators could be used for basic logic operations while a computer could be used for programming and memory [57]. The operation speed ranges from 10 ms (pixel for nematics) to 0.5 /iS (pixel in electroclinic effect in smectic A liquid crystals), see Chapter 7. 

Shashidhar R, Naciri J, Ratna BR (2000) Large electroclinic effect and associated properties of chiral smectic a liquid crystals. In Prigogine I, Rice SA and Vij JK (eds) Advances in chemical physics, vol 113, John Wiley Sons, Inc., Hoboken, NJ, USA. pp 51-76. doi 10.1002/9780470141724.ch2... 

Kohler R, Stannarius R, Tolksdorf C, Zentel R (2005) Electroclinic effect in fi ee-standing smectic elastomer films. Appl Phys A-Mater Sci Process 80(2) 381-388. doi 10.1007/ S00339-003-2267-5... 

Electric field is also expected as an effective external field to drive finite and fast deformation in LCEs, because, as is well known for low molecular mass LCs (LMM-LCs), an electric field is capable of inducing fast rotation of the director toward the field direction [6]. This electrically driven director rotation results in a large and fast change in optical birefringence that is called the electro-optical (EO) effect. The EO effect is a key principle of LC displays. Electrically induced deformation of LCEs is also attractive when they are used for soft actuators a fast actuation is expected, and electric field is an easily controlled external variable. However, in general, it is difficult for LCEs in the neat state to exhibit finite deformation in response to the modest electric fields accessible in laboratories. Some chiral smectic elastomers in the neat state show finite deformation stemming from electroclinic effects [7,8], but that is beyond the scope of this article we focus on deformation by director rotation. 

This phenomenon can be explained by the surface electroclinic effect of the smectic A phase as shown in Fig. 6.1.20. Nakagawa et al. have suggested the existence of a local electric field at the boundary or surface field, which in turn is explained by the contact between the two materials, liquid crystal and aligning film. The induction of a molecular tilt by the electroclinic effect [39] can be expected under exposure to such a surface field. Since FLC molecules are aligned with the direction of rubbing, the layer is formed with a layer deviation angle 6. 

Similar effects can be described for other phases. The smectic A phase of chiral compounds may show electroclinic effect (Effect (a)) or can be transferred to the TGBa phase (Effect (c)). The smectic C phase of chiral compounds may have hehcal ordering (Effect (b)), but can have polar ferroelectric properties (Effect (a)) even without helical structures. Further, a TGBc phase or a Sd phase might be induced (Effect (c)). 

Smectic Liquid Crystals Ferroelectric Properties and Electroclinic Effect... 

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