Nematic viscosities measurements

Generally large yield stress effects were dominant in the nematic melts, but they were strongly pre-history dependent. A three region flow curve for 15 mol % modified poly(pheny1-1,4-phenylene terephthalate) was probably due to a not completely molten system. Dynamic viscosity measurements showed strong pseudoplastic behaviour. Strain and time dependence phenomena were not observed. 

An anomalous increase of A, has been observe but the principal experimental difficulty in determining the critical exponent accurately is that the normal nematic viscosity (in the absence of correlations) is itself strongly temperature dependent and the anomalous part forms only a relatively small contribution. However, careful measurements have indicated a mean field behaviour. 

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. 

Sometimes the volume viscosity, t/v, of a nematic liquid crystal is measured, which is close to the Leslie viscosities combination t]2 [14]. There is no theoretical explanation of the viscosity behavior of different liquid crystal substances and their mixtures. Also, there exist only a few works where the viscosity measurements are related to the corresponding molecular structure [28]. However, new liquid crystal, low-viscosity, materials are being successfully developed. To make these materials, the following phenomenological rules should be remembered [14] ... 

Figure 12. Apparent viscosity measured in a capillary as a function of temperature for 4-ethoxybenzylidene-4 -n-butylaniline (20 -4) and the homologous pen-toxy compound 50 4 in the nematic and isotropic phases. T, Clearing point temperature.

There are other reports on the study of pretransitional dynamics in polymeric and lyotropic nematics. Quantitative measurements of ratios of Frank elastic constants and Leslie viscosities in the pretransitional range of poly-y-benzyl-glutamate polymeric nematic are reported by Taratuta et al. [85]. McClymer and Keyes [86-88] report light scattering studies of pretransitional dynamics of potassium laurate-decanol-D20 system. An interesting study of a magnetic-field induced I N phase transition in a colloidal suspension is reported by Tang and Fraden [89]. 

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.

The flow properties of cholesteric liquid crystals are surprisingly different from those of the nematics. The most important difference is that, in some directions (along the helical axis), the viscosity measured in Poiseuille flow geometries (see Appendix B) is about six orders of magnitude larger than in the isotropic phase, or in the cholesteric phase when the flow direction is perpendicular to the helix axis. In this latter case, the viscosity is similar to that of nematics, although the behavior is somewhat non-Newtonian above a pitch-dependent threshold shear rate. It was found that the shear rate above which the fluid becomes non-Newtonian is inversely proportional to the square of the pitch. The apparent viscosities as the function of shear rate of materials with different pitch values are shown in Figure 4.6. 

The first viscosity measurements of nematic liquid crystals were performed with classical shear flow viscometers flow induces a change in orientation of the director, see EQN (4), so the effective viscosity measured is r o [54,55]. Since r o can be considered a relatively good approximation to the Miesowicz r i, conventional viscometers are still in use in conjunction with yi measurements [44,56,82,86]. However, for the purpose of measuring the viscosity anisotropy oldo instruments had to be modified and new ones developed. 

Characterization439 Inherent viscosity before and after solid-sate polymerization is 0.46 and 3.20 dL/g, respectively (0.5 g/dL in pentafluorophenol at 25°C). DSC Tg = 135°C, Tm = 317°C. A copolyester of similar composition440 exhibited a liquid crystalline behavior with crystal-nematic and nematic-isotropic transition temperatures at 307 and 410°C, respectively (measured by DSC and hot-stage polarizing microscopy). The high-resolution solid-state 13C NMR study of a copolyester with a composition corresponding to z2/zi = 1-35 has been reported.441... 

To extract concrete predictions for experimental parameters from our calculations is a non-trivial task, because neither the energetic constant B nor the rotational viscosity yi are used for the hydrodynamic description of the smectic A phase (but play an important role in our model). Therefore, we rely here on measurements in the vicinity of the nematic-smectic A phase transition. Measurements on LMW liquid crystals made by Litster [33] in the vicinity of the nematic-smectic A transition indicate that B is approximately one order of magnitude less than Bo. As for j we could not find any measurements which would allow an estimate of its value in the smectic A phase. In the nematic phase y increases drastically towards the nematic-smectic A transition (see, e.g., [51]). Numerical simulations on a molecular scale are also a promising approach to determine these constants [52],... 

Based on the evaluation of the results of physical measurements on polar alkenyl compounds described above, the position and configuration of the carbon-carbon double bond found in the apolar compounds is similar to that in the polar compounds, essentially for the same reasons. However, the combination of a low melting point and a high nematic clearing point of the bicyclohexane compounds (142,143 and 145) is remarkable. The birefringence and viscosity of these compounds is remarkably low. ... 

Table 3.18 Transition temperatures (°C), elastic constants (ku, k22, kjj, 10 N), dielectric anisotropy ( e), dielectric constant measured perpendicular to the director (b2.)> birefringence ( n), refractive index measured parallel and perpendicular to the director ( ng and xto), viscosity (yi, mm s ), threshold voltage (V), rate of change of voltage with temperature (%°C ) and twist angle (°) measured at 20° C unless otherwise stated for three typical nematic mixtures for STN-LCDs ...

A binary mixture of 4-pentylphenyl 4-pentylbicyclo[2.2.2]octane and 4-heptyl-phenyl 4-pentylbicyclo[2.2.2]octane, see Table 3.8, exhibits a wide nematic phase at room temperature with a high clearing point, a low birefringence (0.08), a moderate viscosity (46 cP) and a weakly negative dielectric anisotropy (Ae —1.1) measured at 20°C. It is a surprisingly good solvent for... 

Taking R(p) 1, this leaves only one temperature-independent constant, ao/rj, left to be obtained by fitting the viscosities in the nematic state below Tni. Since ao is an orientation-independent contribution, the ratios of the Miesowicz viscosities r)c -r)b - a deviate less from unity as ao//y is increased. A value aQ/rj = 0.6 fits the Miesowicz viscosities of MBBA reasonably well the predictions of the theory with ao/// = 0.6 are given by the lines in Fig. 10-9a. Measurements of the Miesowicz viscosities for other liquid crystals are similar enough to those of MBBA that this theory with ao/rj = 0.6 is likely to work equally well... 

The viscous properties of a smectic A are characterized by the same five independent viscosities that characterize the nematic. As we shall see, however, the elastic properties of the smectic are very different from those of a nematic, and some flows permitted to the nematic are effectively blocked for the smectic. For smectic C, for which the director is tilted with respect to the layers, there are some 20 viscosities needed to characterize the viscous properties (Leslie 1993). Formulas for these, derived using a method analogous to that used for nematics by Kuzuu and Doi (1983, 1984) can be found in Osipov et al. (1995). The smectic phase for which rheological properties are most commonly measured is smectic A, however, and hereafter we will limit our discussion to it. 

The introduction of defects into a smectic sample destroys its fluidity. This contrasts markedly with nematics, for which the presence of defects hardly alters the fluid s viscosity. Of course, this is because in a smectic the direction locally perpendicular to the layers is solid-like, and when defects are present, all directions acquire some solid-like character. Horn and Kleman (1978) measured shearing stresses in defect-containing smectic samples of 8CB and found that the shear stress was given by the equation of a Bingham plastic ... 

Problem 10.8 You are measuring the elasticities and viscosities of a room-temperature nematic at reduced temperatures and you find that below about 10°C the twist and bend constants K2 and become very large, while the splay constant Ki retains a modest value. Also, the Miesowicz viscosity t], becomes enormous while r) goes up only modestly. What could explain this behavior ... 

Because of the difficulty with which polymeric nematic monodomains are prepared, there are few measurements of Leslie viscosities and Frank constants for LCPs reported in the literature. The most complete data sets are for PBG solutions, reported by Lee and Meyer (1990), who dissolved the polymer in a mixed solvent of 18% dioxane and 82% dichloromethane with a few percent added dimethylformamide. Some of these data, measured by light scattering and by the response of the nematic director to an applied magnetic field, are shown in Figs. 11-19 and 11-20 and in Table 11-1. While the twist constant has a value of around K2 0.6 x 10 dyn, which is believed to be roughly independent of concentration and molecular weight, the splay and bend constants ATj and K3 are sensitive to concentration and molecular weight. 

Few other sets of viscosities exist for polymeric nematics. Yang and Shine (1993) obtained three of the Leslie viscosities for monodomains of poly(n-hexyl isocyanate) (PHIC) from rheological measurements in the presence of an electric field, and they obtained values reasonably consistent with the predictions of the Kuzuu-Doi expressions. From monodomains of the polyion PBZT, poly(l,4-phenylene-2,6-benzobisthiazole) in methane sulfonic acid, some of the Leslie-Ericksen parameters have been extracted via light-scattering and magnetic-field-reorientation studies (Berry 198S Srinivasarao and... 

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