Electroclinic Switching

In addition to the sensitivity to ring structure, slight alterations of the hydrocarbon chain may lead to dramatic differences in electro-optic performance in chiral compounds [43]. For example, in some electroclinic Sm-A materials it has been reported that if the chain is shortened or if a double bond is localised at the end of the hydrocarbon chain, the tilt angle, electroclinic coefficient, and switching time are significantly suppressed [43]. 

Electroclinic effect118,119 Close to the phase transition from the tilted SmC phase to the orthogonal SmA phase the tilt angle becomes soft (soft mode). Consequently, one can realize faster switching times than in SSFLC and DHF cells. 

Electroclinic behavior has been recognized in few SmA LCPs [77] and response times in the sub-millisecond range have been observed. But, surprisingly, the SmC X phase of polyacrylate V-11 exhibits an electroclinic switching process with a short response time (200 p s) that is changed into a ferroelectric one as the temperature is raised. Moreover, the electroclinic-ferroelectric transition is shifted towards lower temperatures when the voltage is increased (Fig. 20). 

Additional information on ferroelectric and electroclinic switching can be obtained with broadband dielectric spectroscopy. It appears that the molecular dynamics of FLCPs are comparable to those of low molecular weight compounds [67]. However, the experimental observations are made more difficult for FLCPs than for low molecular weight SmC liquid crystals due to the conductivity contribution which takes place at frequencies below 10" Hz and to the difficulty to get a macroscopically well-aligned sample. 

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 ... 

Time-resolved FTIR is used to study the structure and dynamics of ferroelectric liquid crystalline block copolymers. From analysis of the dynamic dichroism of the FTIR spectra, it was concluded that the components in the PS microphase are oriented randomly while the liquid aystalline groups form an ordered phase. The switching is of an electroclinic type, in which the tilt angle and the mesogenic motion increase with temperature, especially if the PS block is heated above Tg. The orientation of the liquid crystalline block after... 

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. 

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]. 

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... 

Fig. 20 Change of sample width and thickness, respectively, during electroclinic switching of a free- standing FLCE film [171]...

Poths H, Andersson G, Skarp K, Zentel R (1992) Fast electroclinic switching in a ferroelectric LC-polysiloxane. Adv Mater 4(12) 792-794. doi 10.1002/adma. 19920041204... 

In contrast to the ferroelectric switching time in the smectic-C phase, the dynamics of the electroclinic effect does not depend on the chirality. The soft mode relaxation frequency was observed to show the same linear dependence on (r — Tag) in several mixtures possessing different values of the enantiomeric excess [77]. 

The existence or nonexistence of mirror symmetry plays an important role in nature. The lack of mirror symmetry, called chirality, can be found in systems of all length scales, from elementary particles to macroscopic systems. Due to the collective behavior of the molecules in liquid crystals, molecular chirality has a particularly remarkable influence on the macroscopic physical properties of these systems. Probably, even the flrst observations of thermotropic liquid crystals by Planer (1861) and Reinitzer (1888) were due to the conspicuous selective reflection of the helical structure that occurs in chiral liquid crystals. Many physical properties of liquid crystals depend on chirality, e.g., certain linear and nonlinear optical properties, the occurrence of ferro-, ferri-, antiferro- and piezo-electric behavior, the electroclinic effect, and even the appearance of new phases. In addition, the majority of optical applications of liquid crystals is due to chiral structures, namely the ther-mochromic effect of cholesteric liquid crystals, the rotation of the plane of polarization in twisted nematic liquid crystal displays, and the ferroelectric and antiferroelectric switching of smectic liquid crystals. 

The dynamics of the electroclinic effect are, in fact, the same as the dynamics of the ferroelectric soft mode. The switching time may be derived from the equation for the balance of the viscous, elastic, and electric torques ... 

The experimental data confirm the linear increase in the tilt angle with increasing E for the electroclinic effect and independence Xq of E [190] (Fig. 25). The switching time Xq may be calculated more precisely if the fourth-order term in 0 is taken into account. In this case, a decrease in Xq for larger electric fields is observed [192]. At present, the electroclinic effect is the fastest of the known electro-optical effects in liquid crystals. 

The optical pattern may be observed for several months after switching off the dc field. The theoretical model is based on the consideration of a stabilizing role of the structural defects (dislocations) interacting with free charges in a FLC layer. The tilted layer structure responsible for such defects has been observed by direct X-ray investigation. The field-off domain structure has also been observed after the application of the electric field to the smectic A phase of a ferroelectric substance with very high spontaneous polarization [196]. The phenomenon is assumed to arise due to a break in the smectic layers (very similar to that just mentioned) induced by a strong electroclinic effect. 

The thermally excited cone motion, sometimes called the spin mode (this is very similar to the spin wave motion in ferromag-nets), or the Goldstone mode, is characteristic of the nonchiral SmC phase as well as the chiral SmC phase, but is of special interest in the latter because in the chiral case it couples to an external electric field and can therefore be excited in a controlled way. This Goldstone mode is of course the one that is used for the switching mechanism in surface-stabilized ferroelectric liquid crystal devices. The tilt mode, often, especially in the SmA phase, called the soft mode (although hard to excite in comparison with the cone mode, it may soften at a transition), is very different in character, and it is convenient to separate the two motions as essentially independent of each other. Again, this mode is present in the nonchiral SmA phase but cannot be detected there by dielectric methods, because a coupling to an electric field requires the phase to be chiral. In the SmA phase this mode appears as the electroclinic effect. 

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