Isotropic unstable region

Figure 1 shows the phase diagram for a = 3.4, n, = 1.5, and Up = 40. The temperature is normalized by the NIT temperature of the pure liquid crystal. The solid (broken) lines correspond to the binodal (spinodal) lines. The dashed-and-dotted line shows the NIT line. In the region between the binodal and spinodal lines, there are two metastable regions an isotropic metastable (Im) and a nematic metastable (Nm). In the low temperature of the spinodal line, we have an isotropic unstable (lu) and a nematic unstable (Nu) regions. At lower temperatures, the biphasic region where a nematic and an isotropic phase coexist appears. The stable nematic phase appears in a dilute region of the polymer concentration. The nematic and isotropic binodal lines intersect at 0 = 0 and T = T. ... 

Quenching from the isotropic phase into the unstable region inside the coexistence curve, phase separation proceeds. Phase separation in an isotropic liquid is normally induced by concentration fluctuations. However, in addition to concentration, the orientational order should also affect the phase separation dynamics in liquid crystal systems. 

Figure 2.22 Phase diagram of a polymer/liquid crystal mixture with n = n, = 2 and a = 1.4. The arrows indicate temperature quenches from a stable isotropic phase into an unstable region.

In Fig. 7, we display Sc versus c. The gel surface is unstable in the region 8 <8C against the surface disturbances, because they decrease the free energy. Surprisingly, even in the isotropic case <5=1, the surface instability occurs at negative... 

According to the principle of mutual independence of individual types of phase equilibrium it should be expected that upon a change of thermodynamic conditions in the initial monophase solution, first the equilibrium with the separation into amorphous phases is established, this equilibrium being unstable with respect to other types of phase equilibrium, and only after that the transition to the stable equilibrium takes place. As an example, we can consider the case of a gradual transition from the unstable liquid crystalhne equilibrium to the stable equilibrium with the formation of a crystallosolvate for a PBA-sulphuric acid system, which we discussed earlier In the lower left part on Fig. 15 there are the particles of liquid crystalline phase which transforms, via the isotropic phase (dark background) into a crystallosolvate (spherulites in the upper part of the figure). The process is completed by a total disappearance of the liquid crystalline phase and by the establishment of the equilibrium between the isotropic solution and the crystallosolvate, which corresponds to the region I + CS on Fig. 12. 

In this section, we focus on the interplay between phase separations and phase ordering kinetics in mixtures of a flexible polymer and a liquid crystal (nematogen). When the system is thermally quenched from a stable isotropic phase into an unstable part of the biphasic region, the fluctuations of concentration and orientation take place and isotropic or nematic droplets appear with time [109, llOj. The instability of these systems is driven by the competition between phase separation and nematic ordering. Figure 2.22 shows the phase diagram of the polymer/liquid crystal mixture with Up = fir = 2 and a = 1.4. The solid curve refers to the binodal and the dotted line shows the first-order NIT. The dash-dotted line shows the spinodal. The arrows indicate temperature quenches from a stable isotropic phase... 

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