Viscosity ferroelectrics

It can be safely predicted that applications of liquid crystals will expand in the future to more and more sophisticated areas of electronics. Potential applications of ferroelectric liquid crystals (e.g. fast shutters, complex multiplexed displays) are particularly exciting. The only LC that can show ferroelectric property is the chiral smectic C. Viable ferroelectric displays have however not yet materialized. Antifer-roelectric phases may also have good potential in display applications. Supertwisted nematic displays of twist artgles of around 240° and materials with low viscosity which respond relatively fast, have found considerable application. Another development is the polymer dispersed liquid crystal display in which small nematic droplets ( 2 gm in diameter) are formed in a polymer matrix. Liquid crystalline elastomers with novel physical properties would have many applications. 

The first report of a ferroelectric effect by a metallomesogen was for an open-book palladium(II) complexes (Figure 44) of type discussed above, where the chirality was introduced in the bridging carboxylate [14a]. The H NMR spectra identified that the complex was a mix of isomers trans-AR,R (34%), trans-AR,R (34%) and cw-R,R (32%). Ferroelectric switching was demonstrated, although at a much slower rate ( 1 s) than in organic liquid crystal systems, due to the high viscosity of the material. 

Measuring r, Ps and the applied electric field E, the apparent rotational viscosity of ferroelectric liquid crystals is obtained. 

Figure 6.42. The switch time r (a) and the rotational viscosity q (b) vs. temperature for small molecular mass (L1-L3) and polymeric (P1-P3) ferroelectric liquid crystals. (Modified from Sekiya et al., 1993. Reproduced by permission of Taylor Francis, )...

For the small molecular mass ferroelectric liquid crystal when reversing the polarity of the applied electric voltage the molecules rotate locally while their molecular mass centers don t necessarily move accordingly. But for side chain ferroelectric liquid crystalline polymers, as one of the side group ends is confined to backbone, shown in Figure 6.43, the polarity reversion must be accompanied by the movement of their mass centers, which causes a backflow in order to re-distribute the mass centers. Moreover, the side groups may collide with each other. The effect results in the displacement of the backbone. The above effects increase the difficulty of re-orientation and hence increase the viscosity. 

The molecule contains Si-0 bonds. FTIR suggested that the Si-0 bonds move when the side groups move. Hence, ferroelectric liquid crystalline polymers have higher rotational viscosities than small molecular mass ferroelectric liquid crystals. In Figure 6.44 the relation of rotational viscosity r/ and molecular weight Mw at 600 °C is plotted, rj increases as Mw increases and the quadratic law is observed. 

In principle, liquid crystalline polymers can be applied in displays. Unfortunately, the response of them to the external fields isn t satisfactory because their viscosity is greater than the small molecular mass liquid crystals by a few orders of magnitude. In fact, only when the temperature is near the glass transition temperature, can the response be measured in seconds. Apparently, this is far from the real requirement. One may mix the liquid crystalline polymer with small molecular mass liquid crystal for such a purpose, but the mixture doesn t show an advantage over the small molecular mass liquid crystal displays. The ferroelectric liquid crystalline polymer is an exception. It works with a very fast effect and can achieve a display with a response time of a few milliseconds or a fewr tens of milliseconds. 

Ferroelectric switching is the most likely process encountered in FLCPs. A solution of the equation of motion of the SmC director, when the dielectric anisotropy is supposed to be negligible, leads to the introduction of a switching time r which is related to the rotational viscosity y and the spontaneous polarization by the following rela-... 

As there is no appropriate method to measure directly the rotational viscosity of ferroelectric liquid crystals, y is generally deduced from the electrooptic response time measurements [ 12,18,44]. The relationship between fio 9o and t is not straightforward and requires the use of a theoretical model for the optical transmission based on the bookshelf geometry briefly summarized in the following. 

Freely-suspended Films of Polymeric Liquid Crystals. The stabilization of freely-suspended films by using polymeric liquid crystals is obviously interesting and has been attempted previously. Unfortunately it seems to be extremely difficult to polymerize films of liquid crystalline monomers as these films were reported to always break during polymerization. It seems to be equally difficult to fabricate FS-films of polymeric liquid crystals in their smectic A and smectic C phases, most likely due to their enhanced viscosities. However, if one heats slightly into the isotropic phase it is possible to spread a film across an aperture which thins out to form a truly freely-suspended liquid crystal film after cooling into the smectic phases (57). Films of this type are homeotropic in the smectic A phase and show birefringence when cooled to the ferroelectric smectic C ... 

H. Endo, S. Hachiya, T. Sekiya, K. Kawasaki, Rotational viscosity of ferroelectric liquid-crystalline polysiloxanes. Liq. Cryst. 12, 147-155 (1992)... 

At present, eight different phases are known in banana compounds dependent on particular in-plane packing symmetry and they usually labelled as Bi, B2, -Bg, etc., counted from the isotropic phase [44]. Among them the B2 phase is especially interesting, because it has low viscosity and can easily be switched by an electric field with rather short switching times [45]. In fact, the B2 phase is basically a conglomerate of chiral and achiral antiferroelectric structures SmCAPA and SmCAPA mixed with some percentage of the two ferroelectric structures. 

Using constraint director dynamics, McWhirter and Patey [206] also determine the shear and twist viscosities describing the coupling between the pressure and shear rate tensors and the Miesowicz viscosities (linear combinations of the former) and show that the latter are qualitatively similar to those of a ferroelectric tetragonal 1 lattice in accord with the fact that the short-range spatial correlations in the ferroelectric liquid state are similar to those of the tetragonal lattice structure [102]. 

However, LC conjugated polymers have an inevitable problem that is frequently encountered during macroscopic alignment, namely, it takes a long time for the LC conjugated polymers to be aligned completely because of the high viscosity inherent to polymeric materials. It is well known that ferroelectric... 

The temperature dependence of the dielectric constant of P2 is shown in Figures 12.9 and 12.10. The dielectric constant (e) suddenly increased in the temperature region of the SmC phase. This is the typical ferroelectric behavior of FLC, because the dielectric constant of the ferroelectric material is inversely proportional to the absolute temperature according to the Curie-Weiss law, i.e. e = C(T — Tq), where C is the Curie constant and Tq is the Curie temperature. At the same time, the dielectric constant increased with decrease in frequency. This tendency is due to the high viscosity of the LC conjugated polymer. 

Gouda, F., Skarp, K., Andersson, G., Lagerwall, S. T., Stebler. B., and Helgee, B., Soft mode response and rotational viscosity of a ferroelectric liquid crystal polymer, Jpn. J. Appl. Phys., 34, 5653-5657 (1995). 

Chiral smectic C materials have mostly been designed to have a high Pg and a low viscosity to enable their use in ferroelectric devices. The stracture of chiral smectic C materials is sirmmarised by a template structure in Figure 6.4 which also shows the types of moieties and their combination. The first S material used to establish the viability of a device was DOBAMBC (7a) in 1974. The use of ester and ether moieties provides the necessary lateral dipoles for molecular tilting and the use of the conjugated cinnamate... 

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