Phase birefringent

Appearance, 500x clear a few spots microscopic birefringement— phases ... 

If one makes use of the rather limited information available and given above one may infer that a tilt of between 20° and 30° is normal for straight chain azobenzene derivatives when deposited as LB films, even when a homeotropic phase exists. Such a structure can only be produced in a rather loosely packed film. At the moment it is an open question whether monolayers of these materials exist in the hexatic phase as is the case for fatty acids or whether the structure more nearly corresponds to the smectic-A phase. In the case of the birefringent phase described by Jones et al. [151] it was shown that, once this phase was established, further layers deposited by the LB technique go down in an epitaxial manner. 

Refractive index data are very useful for the quantitation of isotropic (liquid and cubic liquid crystal) phases, and for the calibration of cell thickness and nonflatness. Hovever, the analysis of birefringent phases using refractive index data has been found to be unreliable (9). A problem arises from the fact that the orientation of such phases relative to the direction of the light path, as veil as the system variables, influence refractive indices. In order to use refractive index data for quantitation, a phase must spontaneously orient in a reproducible fashion. Such orientation does occur in the case of fluid lamellar phases (as in short chain polyoxyethylene nonionic systems (7)), but viscous lamellar phases, hexagonal phases, and crystal phases do not orient to a sufficient degree. 

Figure 4.36. Birefringence phase shift for different values of the internal magnetic anisotropy of the particles. The set of material parameters and meanings of the thin lines are the same as those for Figure 4.34.

The dark regions correspond to liquid birefringent phases. 

Figure 3. W/S pseudoternary diagrams at T = 21.5 °C ( in volume) of the water-dodecane-SDS-pentanol system. Top W/S = 1 bottom W/S = 1.4. L, Lg, and L designate respectively an isotropic phase, a birefringent phase and a flow birefringent liquid phase.

Table I shows that at least one of the phases is birefringent between 0.8 and 2.0 gm/dl NaCl. Three phases exist for certain compositions with a lamellar phase at the top, a streaming birefringent phase in the middle and an isotropic phase at the bottom. Such is the case when the salinity is 1.4 or 1.8 gm/dl and the polymer concentration is 750 ppm Xanthan (Figure 2(b)). With or without polymer, two phases are present at 2.2 gm/dl NaCl. The added polymer appears to remain mostly in the lower phase and consequently increases the viscosity. When no salt is present, the aqueous solutions containing polymer are isotropic phases up to a polymer concentration of 1000 ppm. The aqueous solutions at 1500 ppm and higher concentrations of polymer show streaming birefringence.

Polarizing microscopy identifies birefringent phases, for example liquid crystalline phases (17,18), and reveals dispersed particles, provided they are larger than about 1y (ultimate sensitivity is about 0.3y). 

The contribution of dissolved surfactant, whose concentration was only 0.001M, compared to 7M of decane, to the observed Class I peaks must have been negligible. Class II peaks were not observed in Spectrum 13 of the birefringent phase, and Class I peaks were broadened (linewidth about 30 Hz) compared to the peaks in Spectrum 12 (linewidth less than 5 Hz). Therefore it seems quite possible that the dispersed birefringent phase did give Class I peaks in Spectrum 12, but that these peaks, due to either the surfactant or to absorbed decane or to both, merged with those of the decane in the isotropic phase. 

The peaks observed in Spectrum 13 indicate hindered molecular motion in the birefringent phase. Further work is needed to resolve which molecules gave the observed signal. That the peaks appear to be superpositions of a narrow peak and a much broader one is tantalizing. 

In solutions of normally encountered randomly coiled macromolecules the formation of spontaneously birefringent phases is not expected. (Phase separation does occur when the interaction between solute molecules is strong.) Here, we have investigated solutions of the polyelectrolyte sodium carboxymethylcellulose in water, which are rubberlike at high concentrations 12). (Materials that form such solutions are commonly called gums.)... 

In order to improve the understanding of these systems, Kunieda and coworkers examined the thermotropic behavior of poly(oxyethylene) cholesteryl ethers with different chain lengths, ChFOn, mixed with water at a fixed concentration ( 25 wt%) [32]. This study focused on the different fusion mechanisms that were involved in the solid-liquid phase transition. The soUd-Uquid transition temperature for ChFOn as a function of n is shown in Figure 4.3 (for comparison, the transition temperature for polyethylene glycol is also shown). In both cases, the transition temperature decreased when the chain length was diminished. However, for the cholesterol surfactant, when n < 10, a birefringent phase appeared between... 

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