Pretransition range

The temperature dependence of the density far from the consolute point can be well approximated by a linear law within the accuracy of the data a very small positive deviation is to be noticed in a pretransitional range, several degrees above the phase separation. The latter is manifested by a sudden break (increase or decrease depending on the composition of the mixture). The results are similar to those previously reported for other systems in our laboratory (Hollecker et al., 1975). Along the constant x curves, the p values can be well fitted to the linear function p - Pt t< 0.01 (see Table 2). 

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]. 

In practice there are three main difficulties in the use of this technique. The 2kF and 4kF scattering is not observed in all materials or at all temperatures. Fortunately, pretransitional effects (one-dimensional fluctuations) are often observed in a wide temperature range above Tc [114]. Second, the corresponding scattering is often very weak and requires special measurement techniques. For example, in the case of TTF-TCNQ, the intensity of 2kF satellites is 10 4 that of main reflections and the onedimensional scattering at 60 K is a further 10 2 to 10 3 weaker [133]. Third, to get p, one must decide whether the observed instability occurs at the 2kF or at the 4kF electronic wave vector. 

The thickness transition at 13°C is an interesting experimental observation. It is in contrast to the pretransition in water dispersions of DMPC, for which the interlamellar spacing decreased with the fall of temperature [443], the foam bilayer thickness increases in the range of this transition upon cooling. 

In principle, any modification of the intra- or intermolecular relationships that break the averaged crystal symmetry on a macroscopic scale corresponds to a structural phase transition. Diffraction techniques (X-ray or neutron) are thus of primary importance to characterize the different phases. From the Bragg peaks that measure the long-range order of the mean crystal structure, the space groups, atomic positions and thermal parameters can be determined in each phases (Fig. 1). Moreover, in the most favorable cases, these methods can directly measure information on the order parameter and pretransitional ordering by following the superstructure at 7< Tc,... 

Dynamic mechanical measurements of the storage modulus G and the loss modulus G" in polydomain samples of cholesteric LCE have been described [9]. It turns out that G >G" throughout the whole cholesteric range and that both G and G" increase when approaching the cholesteric-isotropic and cholesteric-chiral SmC transition, respectively, thus indicating the presence of substantial pretransitional effects at both phase transitions. 

A study of the miscibility of liquid crystals is of great importance from two points of view [31]. First, mixtures manifest a variety of phases separated by phase transition lines. Varying the composition of a mixture, we can study the interaction of different structural modes (the interaction of order parameters), investigate various pretransitional phenomena [32], etc. Mixing an unknown substance with a compound having well-defined phases we can also identify the structure of the unknown substance. On the other hand, mixtures are extremely important from the technological point of view. The best liquid crystalline materials for displays are, as a rule, multicomponent mixtures with a wide temperature range of operation. Here, the problem is to compose a thermodynamically stable eutectic mixture. 

The isotropic phase of nematogens differs from conventional isotropic liquids in two aspects. First, the pretransitional phenomena in the vicinity of the clearing point dramatically change the bulk properties (in particular, the Kerr constant) of the isotropic phase due to the short-range nematiclike order. Second, quasi-nematic surface layers form at the interface with a solid substrate. Due to their dielectric (and optical) anisotropy they can contribute to the electrooptical properties of cells filled with the isotropic phase. For example, they can be reoriented by an external field (an analogy with the Frederiks transition). We will discuss briefly both phenomena. 

In the pretransition temperature region, where the nematic phase has a certain degree of short-range smectic A order, it is easy to detect both the motion of the liquid and the formation of the domain patterns [122, 123, 126]. The latter have marked differences from the domain patterns arising in the pure nematic phases. For instance, in the temperature region where the ratio of the electrical conductivity cr /crx becomes less than 1, the Kapustin-Williams domains are not observed with an initial planar orientation. Instead, different domains are formed at low frequencies (a < ujc) of the applied field. They are arranged parallel to the initial orientation of the director and have a well-defined field strength threshold, Uth oc d [123]. 

FIG. 16 Water-hexadecane system (see Table 2). Behavior of the measured enthalpies of (O) hexadecane (AHt,) and ( ) water (A// ) as a function of increasing water concentration (C) in the interval 0.105 < C < 0.420. The linear relationship holds, as shown in Fig. 14, within the interval 0.105 < C < 0.3. Pretransitional effects occur in the range 0.316 s C 0.372 phase separation develops in the interval 0.372 < C < 0.42. In the latter range, the linear relationship still holds for the isotropic upper phase of biphasic samples until the transition occurs toward a monophasic liquid crystalline mesophase. (Redrawn from Ref. 23.)... 

A considerable amount of experimental results on the free motion of bubbles and drops in quiescent non-Newtonian media is available in the literature. In most cases, the two-parameter power-law model has been used to model the ambient liquid rheology. It is worthwhile to mention here that in the bulk of the work dealing with the free fall of liquid drops in polymer solutions, the ratio of the viscosity of the dispersed phase to that of the continuous phase is in the range lO -lO" and, hence, these drops may effectively be treated as gas bubbles. At this juncture, it is also important to recall that bubbles in the pretransition region (prior to the abrupt change) behave more like solid spheres and, hence, the drag under these conditions is approximated... 

Fig. 19 shows the temperature dependence of the retention times of p- and m-xylene in 4,4 -dihexyloxyazoxybenzene. Good separation is observed both in the nematic (125 to 80°C) and in the smectic phase (T < 80°C). At the isotropic-to-nematic transition the retention time decreases in a small temperature interval (125—120°C). Such a behavior is expected according to rule 1 and has been observed in many cases [134, 128]. It should be emphasized that the drop in Vg at the isotropic-to-nematic transition takes place over a rather broad temperature range (that is 5°C), rather than sharply. This has been attributed to pretransitional effects [134]. 

In the pretransition temperature region above the smectic A-nematic phase transition, where the nematic phase has a certain degree of the short-range smectic A order, it is easy to detect both the motion of a liquid and the formation of domain patterns. The latter show marked differences from the... 

The isotropic phases of the nCBs (alkyl-cyanobiphenyls) were also studied by Butler [19] over wide temperature ranges including both pretransitional and normal liquid regions of the fluid. No evidence of pretransitional effects was observed in the Brillouin shifts, even for 12CB which exhibits... 

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