Nematic-Isotropic Phase Separations

In the mixture of a polymer and a liquid crystal, the nematic-isotropic phase separation and the isotropic-isotropic one with an upper critical solution temperature has been often observed [3-9]. Typical examples are the mixtures of (p-ethoxybenzylidene)-p- -butylaniline (EBBA), and polystylene (PS) or polyetylen oxide(PEO) [3] and the mixtures of 4-cyano-4 - -heptylbiphenyl (7CB) with polymethyl methacrylate (PMMA) or PS [7]. These solutions consist of a low molecular weight liquid crystal and a sufficiently flexible polymer. The main feature of the... 

Figure 2 shows the phase diagram for a = 8. When the nematic interaction parameter a is increased, the isotropic-isotropic phase separation disappears and we have only the nematic-isotropic phase separation. As the molecular weight of the polymer increases, the nematic phase boundaries are shifted to the lower concentration and the polymers become insoluble in the nematic phase [20]. Figure 3 shows the order parameter on the nematic phase boundaries plotted against the reduced temperature. The order parameter S jumps from zero to 0.44 at T = T%i. For large values of p, the order parameter increases with decreasing temperature. 

Figure 2.7 Schematic of (a) a stable nematic (N) phase and (b) a nematic-isotropic phase separation (N + I).

Figure 14 shows the phase diagram of a flexible-semiflexible polymer blend at a constant pressure. Theoretical calculations and experimental results show that such mixtures can exhibit an isotropic-isotropic and isotropic-nematic phase separation. Our calculations are able to capture the isotropic-isotropic phase separation and serve to show that the origin of such a transition can be purely entropic. 

In this paper we propose a molecular theory combining both the orientation-dependent attractive interactions [11-15] and the excluded volume interactions [15,16] between Hquid crystals. Our theory predicts that the attractive interaction parameter between liquid crystals controls the peculiar types of phase separations, such as isotropic isotropic phase separation and nematic-isotropic one. We also examine the phase diagrams for solutions of rigid rod-like molecules [17-19] and the volume phase transition of gels in liquid crystals. 

Fig. 2 Phase diagrams for a = 8. When the nematic interaction parameter a is increased, the isotropic-isotropic phase separation disappears...

We also find the three-phase equilibrium (triple point) where two isotropic phases and a nematic phase can simultaneously coexist. At high temperature, the solution is homogeneous since the entropy of mixing is dominant. As temperature decreases, the isotropic-isotropic phase separation takes place where the interaction parameter x between solute and solvent molecules becomes dominant. Further decrease in temperature, the attractive interaction Xa between liquid crystals dominates and so the... 

A frequent situation in nematic phases close to the isotropic transition is the presence of a thin film of an isotropic phase separating the liquid crystal from the preparation glasses. The molecules lie either horizontally at this interface, or at an angle other than 90°, with no preferred direction in the horizontal plane. Defects can be numerous, as shown in Figs. 10 and 11. 

The physics of liquid crystals [97] indicate that the viscosity passes through a maximum at the point of tire transition from the isotropic to Ae LC state as a result of ordering of the object and cooperative orientation in flow for nematic systems and a large number of cholesteric systems. The existence of a maximum is usually found in flow of a liquid crystal with an uncontrollable orientation. If the coefficients corresponding to orientation of the macromolecules parallel to the direction of the velocity gradient (%) and parallel to the velocity vector (t] ) are separated out, then a more complex picture is obtained (Fig. 9.23). The maximum in the region of the nematic-isotropic phase transition for p-azoxyanisole is observed for the dependences 11(7) and A break is... 

A nematic/isotropic phase correlated 2D NMR (SC-2D NMR) spectrum of APAPA (Figure 3-12) was obtained by using the pulse sequence shown in Figure 3-1 with T,n = 0 and a microwave pulse of 10 ms duration. The temperature before the transition was kept at 108°K, and a repetition time of 120 s was used for the system to be completely back to the initial nematic phase. The NMR spectrum in the nematic phase appeared in the col dimension and that in the isotropic phase in the 0)2 dimension. The cross sections along the o)l axis clearly showed different dipolar coupling pattern for individual resonances separated in the o)2 axis. 

One of the primary features of the Gay-Berne potential is the presence of anisotropic attractive forces which should allow the observation of thermally driven phase transitions and this has proved to be the case. Thus using the parametrisation proposed by Gay and Berne, Adams et al. [9] showed that GB(3.0, 5.0, 2, 1) exhibits both nematic and isotropic phases on varying the temperature at constant density. This was chosen to be close to the transitional density for hard ellipsoids with the same ellipticity indeed it is generally the case that to observe a nematic-isotropic transition for Gay-Berne mesogens the density should be set in this way. The long range orientational order of the phase was established from the non-zero values of the orientational correlation coefficient, G2(r), at large separations and the translational disorder was apparent from the radial distribution function. 

The SD is a phase separation process usually occurring in systems consisting of more than two components such as in solutions or blends. However, in the present case the system employed is composed of one component of pure PET. In this case, what triggers such an SD type phase separation Doi et al. [24, 25] proposed a dynamic theory for the isotropic-nematic phase transition for liquid crystalline polymers in which they showed that the orientation process... 

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