Electroclinic ferroelectrics

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). 

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. 

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. 

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.

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 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. 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%.

In conclusion, we can state that the field behavior of polymer ferroelectric liquid crystals is very similar to that of the low molecular mass compounds. It can be interpreted in the framework of the theory discussed above (Sections 7.1, 7.2) with the high viscosity coefficients 7, (and others) taken into account. The other physical parameters (spontaneous polarization, the electroclinic coefficient, and elastic moduli) are of the same order of magnitude as for their low molecular counterparts. 

Table 8.7 shows that the parameters of the prototype light valve (CdS-nematic) are much worse than that of the a Si-FLC device. The operation speed of the latter comes closer to the solid electrooptical crystal modulator (PROM), but with a considerably higher resolution. Liquid crystal light valves on a Si-FLC operate using the Clark-Lagerwall mode [21], the electroclinic eflFect [22], or the deformed helix ferroelectric effect [24]. The operation speed in the two mentioned cases could be 10-100 times faster than mentioned in Table 8.7. 

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... 

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... 

Kapcanaum N, Walba DM, KorblovaE, Zhu CH, Jones C, Shen YQ, Clark NA, Giesselmann F (2009) On the origin of the "giant" electroclinic effect in a "de Vries"-type ferroelectric liquid crystal material for chirality scaising applications. ChemPhysChem 10(6) 890-892. doi 10.1002/cphc.200900065... 

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... 

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. 

Relation Between Ferroelectric and Electroclinic Properties and Molecular Chirality Chiral-Racemic Studies... 

---