Ferroelectric liquid crystals rotational viscosities

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. 

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. 

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). 

Four series of compounds, 43, 44, and 49 (m = 0, 1) were evaluated as 10% w/w additives to a ferroelectric liquid crystal mixture. The carborane derivatives of biphenyl 43 and 44 (A = D, E) slightly increased the tilt angle 0 of the SmC material and decreased rotational viscosity y, while... 

In other words, molecular design and the synthesis were carried to express various characteristics for the required purposes that depend on the specific application of the ferroelectric liquid crystal. For example, ferroelectric liquid crystals with large spontaneous polarization, a large tilt angle with small temperature dependence, a specific viscosity, a small rotational viscosity of the molecules along their long axis, a chiral smectic C phase over a wide temperature range, a characteristic anisotropy of the dielectric constant, a suitable phase sequence, a suitable helical pitch, photochemical stability, and so on have been newly synthesized [3-5]. 

Ferroelectric liquid crystals having a heterocyclic pyrimidine, pyrazine, or pyridazine ring have been developed. Phenylpyrimidine systems show a characteristic low rotational viscosity, and fused aromatics, such as fluorene, fluorenone, or naphthalene, have been discussed from the viewpoint of dipole moment, spontaneous polarization, and orientability. 

Fig. 6.2 The variation of rotational viscosity (ye) with temperature difference (T — T ) for polymer-stabilized ferroelectric liquid crystal and pure ferroelectric liquid crystal. The units are taken arbitrarily...

Pal Majumder T, Mitra M, Roy SK (1994) Dielectric relaxation and rotational viscosity of a ferroelectric liquid crystal mixture. Phys Rev E 50(6) 4976-4800 Petit M, Daoudi A, Ismaili M, Buisine JM (2006) Electroclinic effect in a chiral smectic-A liquid crystal stabilized by an anisotropic polymer network. Phys Rev E 74 061707 Petit M, Hemine J, Daoudi A, Ismaili M, Buisine JM, Da Costa A (2009) Effect of the network density on dynamics of the soft mode and the Goldstone modes in short-pitch ferroelectric liquid crystals stabihzed by an anisotropic polymer network. Phys Rev E 79 031705 Pirs J, Blinc R, Marin B, Pirs S, Doane JW (1995) Polymer network volume stabilized ferroelectric liquid crystal displays. Mol Cryst Liq Cryst 264 155-163 Polyanin AD, Zaitsev VF (2003) Handbook of exact solutions for ordinary differential equations, 2nd edn. Chapman Hall, Boca Raton... 

C. Escher, T. Geelhaar and E. Bohm, Measurement of the rotational viscosity of ferroelectric liquid crystals based on a simple dynamical model, Liq. Cryst, 3, 469-484 (1988). 

Skarp K (1988) Rotational viscosities in ferroelectric smectic liquid crystals. Ferroelectrics 84 119-142... 

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