Rotational viscosity mixtures

Fig. 13.7 Experimental temperature dependence of the soft-mode relaxation time (main plot), and demonstration of the Curie type behaviour of the inverse relaxation time on both sides of the phase transition (inset) in accordance with Eqs. (13.18) and (13.19) depicted by solid lines. Experimental parameters chiral mixture with Ps 2 mC/m, a = 5-10 J m Tch = 49°C, cell thickness 10 pm, the rotational viscosity found is = 0.36 Pa s or 3.6 P...

The coefficient y is rotational viscosity of the director similar to coefficient yi for nematics. In fact, it does not include a factor of sin cp and, in the same temperature range, can be considerably larger than the viscosity ytp for the Gold-stone mode. This may be illustrated by Fig. 13.10 the temperature dependence of viscosities y and have been measured for a chiral mixture that shows the nematic, smectic A and smectic C phases [15]. The pyroelectric and electrooptic techniques were the most appropriate, respectively, for the measurements of ya and ytp describing the viscous relaxation of the amplitude and phase of the SmC order parameter. The result of measurements clearly shows that y is much larger than y and, in fact, corresponds to nematic viscosity yj. 

Figure 8.19 shows the voltage-dependent optical transmittance of a V A cell with JAn = 350 nm between crossed polarizers. For computer calculations, a single-domain VA cell employing Merck high resistivity MLC-6608 LC mixture is considered. Some physical properties of MLC-6608 are summarized as follows n = 1.558, = 1.476 (at 2 = 589nm and T= 20°Q clearing temperature 7]. = 90 °C dielectric anisotropy Ac = -4.2, and rotational viscosity = 186 mPas at 20 °C. In principle, to obtain 100% transmittance for a transmissive VA... 

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 some cases, magnetically induced transient twist distortions have been observed in both thermotropic (MBBA [89]) and lyotropic (PBG [90]) systems. In this case, backflow effects are allowed only in a nonlinear regime, for strong distortions. The physical origin of this phenomenon could be the faster response times of modulated structures, as compared with uniform ones. When the equilibrium director distribution is approached, i.e. a relaxation process is over, the transient structures disappear. The emergence and subsequent evolution of the spatial periodicity of the transient structures have been considered theoretically [89,90]. In addition, the pattern kinetics have been studied in detail experimentally [91] on a mixture of a polymer compound with a low-molecular-mass matrix. The polymer considerably increases the rotational viscosity of the substance and reduces the threshold for pattern formation. This indicates the possibility of recording the pattern using a video camera. A typical transient pattern is shown in Fig. 14 [91]. 

It is known that the temperature dependence of the viscosity of polymeric materials can be described as a universal function of the difference T-Tg. Schad and Zeller have shown that the rotational viscosity for a large number of liquid crystal mixtures used for displays can also be described by such a universal function if the influence of the order parameter in the neighbourhood of the clearing point is not taken into account. 

Low rotational viscosities, which are necessary for outdoor or video applications of liquid crystal displays, are obtained in mixtures with unpolar substances having short side chains such as, for example, dialkylcy-clohexylphenyls or the corresponding alkenyl compounds [83, 84]. Lateral substitUT ents lead to a viscosity increase as well as the substitution of hydrogen atoms at the benzene ring by halogens. The effect is small for fluorine atoms and increases in the sequence Flateral cyano group is more pronounced. 

For liquid crystal displays, mixtures of liquid crystals are always used. Therefore, there is an essential interest in models that predict the rotational viscosity of mixtures from the rotational viscosities of the pure components. With the exception of mixtures of very similar compounds, the dependence of the shear viscosity of isotropic liquids on the mixture composition is normally complex. Due to the additional dependence on the order parameter, one cannot expect a simple concentration dependence for the rotational viscosity of liquid crystals. Figure 20 shows the rotational viscosities of a series of mixtures between the ester LCl... 

Figure 20. Rotational viscosity of mixtures of the esters LCl and LC2 (see text) as a function of temperature. The concentrations of the mixtures differ by 10 mol% for each curve.

Equation (52) is normally used for the estimation of the rotational viscosity of mixtures that have to be optimized for display applications. As the mixture must fulfil several demands it consists of a large number of compounds (even including non-meso-genic compounds) with strongly varying properties. Therefore, direct use of Eq. (52) is impossible. A common procedure is to determine an effective rotational viscosity, from the effect of the component i on the rotational viscosity of a standard liquid crys-... 

Figure 21. Dependence of the rotational viscosity, 7i, of a liquid crystal mixture on the operation temperature, T.

LC mixture parameters such as clearing point, birefringence, rotational viscosity, dielectric anisotropy and elastic constants can be optimized for a special applications without affecting the compensation effect. 

Compared with the corresponding neutral structures the rotational viscosity, /j, of polar substituted LC - structures is increased (compare Structures 2.2Z7.4, 3.3/S.3 and 3.5/S.6). As given in Eqs. (19) and (20) this leads to an unwanted increase of the switching time. Therefore it is practice [89] in the design of mixtures for TN cells to include compounds with both large and small positive Ae. The use of polar three ring materi-... 

Table 13. Helical twisting power (HTP) (measured just above SmA -N ), rotational viscosity, response time and spontaneous polarization measured at 25 °C for mixtures of 10 mol% of 5-([cis- (25,3S)-epoxy]oc-tanoyloxy)phenyI-2-octyloxyprimidine or 5-([trans -(2/ ,3S)-epoxy]octanoyIoxy)phenyl-2-octyloxyprim-idine and a host mixture (C-SmC 10 °C SmC-SmA 84.5 °C SmA-N 93.5 °C N-I 105 °C) [27].

Figure 78 shows a plot of the soft mode and Goldstone mode rotational viscosities measured on either side of the phase transition between the smectic A and SmC. It can be seen that, except in the vicinity of the phase transition, the viscosity seems to connect fairly well between the two phases. The activation energies of these two processes are, however, different. This result may be compared to results obtained by Pozhidayev et al. [148], referred to in Fig. 67. They performed measurements of y beginning in the chiral nematic phase of a liquid crystal mixture with corresponding measurements in the SmC phase, and have shown the viscosity values on an Arrhenius plot for the N and SmC phases. Despite missing data of y in the smectic A phase they extrapolate the N values of y down to the smectic C phase and get a reasonably smooth fit. Their measurements also show that y is larger than y, and this is universally the case.

Figure 78. The soft mode and Goldstone mode rotational viscosities as a function of temperature. The material is mixture KU-lOO synthesized at Seoul University, Korea (courtessy of Prof. Kim Yong Bai) (from Buivydas [155]).

The electron acceptors discussed so far resemble these terminal polar compounds in their structure, or are even identical. Cladis, for example, has studied the phase behavior of mixtures of butyloxybenzylidene octyl-aniline with cyanooctyloxybiphenyl showing stabilized smectic phases of the A- and B-type, as well as an induced SmE phase the results are summarized in a Landau description [23 g]. However, the phase behavior of this particular system, strongly related to that studied by Park et al. [8], is discussed in terms of dipolar pair formation. Furthermore, the phase behavior and macroscopic properties, e.g., densities and rotational viscosities, in mixtures of polar 4-cyano derivatives of biphenyl with apolar azoxy compounds, were found to differ significantly from those comprising the relat-... 

In applications where response speed is critical (i.e., STN displays) the measurement of r is not sufficiently discriminating when comparing compounds and mixtures, and therefore the rotational viscosity (yj) is measured. This viscosity is conqilicated to measure but is more related to the liquid crystal director movement in a display cell. It has values of between 0.02 and 0.5 Pa s. 

Fig. 1.6 Reduction of the viscoelastic constant ratio Xi/k of positive dielectric commercial LC mixtures developed from 1970 to the 1990s by Roche, BDH, and Merck. RO = Roche E-7 = BDH and ZLI = Merck yi = rotational viscosity,...

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... 

Since conoscopic experiments require macroscopic uniformly oriented samples, mixtures of the polymer with 30%, 40% and 50 % of a cyanophenylester, a low molar mass liquid crystal, were investigated. The addition of a low molar mass liquid crystal serves to lower the glass transition temperature and consequently the rotational viscosity. These mixtures could therefore be oriented using a procedure that is depicted in Figure 5-7. 

---