The Electroclinic Effect

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. 

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... 

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. 

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

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. 

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 switching time of the director tilt angle in the electroclinic effect is independent of the electric field, and is defined only by the rotational viscosity je and the elastic modulus A. The corresponding switching times are derived from the Landau-Khalatnikov equation for the balance of the viscous and elastic torques... 

The geometry of the cell where the electroclinic effect is observed is shown in Fig. 7.22. In the electric field the optical axis of the cell rotates by angle 6, measured as a difference between the two directors L( ) and L(0). The transmission is calculated according to the formula [20]... 

At present, the electroclinic effect is the fastest one among the other electrooptical effects in liquid crystals. References [20, 36, 105,108] demonstrate that the response times for the effect, measured at room temperature (T = 25 C), do not exceed =600 ns for the voltages of about 10-40 V and layer thicknesses of about 2 jim. The effect can be used in a wide spectral range, including the visible and near IR region. Placing two electroclinic cells one after another can ... 

According to (7.73) the electroclinic effect is useful for the linear modulation of light with the corresponding linear grey scale. 

Alternatively, an external (electric) field can be used to change the orientation of the LC director inside the network. The network will then reorient and produce a shape change. This effect can be observed either in LC actuators made from highly swollen nematic systems [7,9,21,22,185] or in bulk LCEs with ferroelectric phases (see Sect. 3). In LCEs with ferroelectric phases, the electroclinic effect... 

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. 

Pig. 6.1.20 The mechanism for the deviation of layer normal from the rubbing direction due to the electroclinic effect [38]. 

Essentially, the structural features described above apply to both non-chiral and chiral compounds. However, the presence of chiral molecules in smectic- and C phases results in additional properties and structures not present in phases of nonchiral substances. These are the ferroelectric properties and the electroclinic effect, which will be discussed in detail in Sections 8.3 and 8.4, and the helical structure in the smectic-C phase. 

The electroclinic effect is a result of the coupling between tilt and polarization. The polarization Pe, which is induced by an external field E in the smectic- phase of chiral molecules, consists of a part Po which is present in every dielectric (orientation and electronic polarization), and a part Pg which is due to the P-6 coupling and should show a similar behavior as the induced tilt angle 9. While it is difficult to separate Pg and Pq exactly, measurements of the total polarization in the smectic- phase and around the smectic- -smectic-C transition indicate at least qualitatively that Pg and 9 show very similar behavior [65]. 

By mixing a chiral liquid-crystal compound with its optical antipode, systems possessing arbitrary values of the enantiomeric excess can be designed. If a chiral compound shows smectic-C and smectic- I phases, the racemate, i.e., the 1 1 mixture of the two antipodes, also exhibits these phases but the ferroelectric properties of the smectic-C phase and the electroclinic effect in the smectic- phase are lost. This offers the unique possibility to study a given system with and without ferroelectridty or with a variable markedness of its ferroelectric properties. 

The ferroelectric liquid-crystal compounds which have been studied in chiral-racemic systems possess large values of the spontaneous polarization Ps, i.e., these compounds show a strong bilinear coupling between tilt angle and polarization. The behavior in chiral-racemic systems of these compounds can be well described assuming a simple proportionality of the bilinear P-9 coupling constant C and the enantiomeric excess Xee- This applies also for the electroclinic effect in the smectic- phase which has been studied in [74], [77]. Figure 8.12 shows the electroclinic tilt susceptibility /51 as a function of Xee at constant temperature difference to the transition to the smectic-C phase. The observed proportionality between xe ee is vvell in... 

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