Anisotropic properties probe method

The special combination of orientational order and mobility possessed by liquid crystals, the wide variation in these properties in different liquid crystalline types, and the fact that bulk orientation can be easily controlled by a number of different methods have led to numerous applications in which liquid crystals are employed as anisotropic solvents for the study of various physicochemical properties of molecules. This chapter deals with the use of liquid crystals as solvents or supports in spectroscopic applications, in gas chromatography, and for chemical reaction. In each case, the emphasis is on studies in which the solute is of primary interest, and in which calamitic liquid crystals are employed as solvents to control orientation or mobility. Spectroscopic studies in which some property of the mesophase is of primary interest, whether they involve a study of probe molecules or the liquid crystal itself, are treated in earlier chapters and will not be dealt with here. In many cases, studies of this type are intimately re-... 

In another type of application a low molecular fluorescent probe is added to a system containing macromolecules. As would be expected, the rotation of a small species is insensitive to the molecular weight of high polymers, but depends on the "microscopic viscosity" which is a function of free volume. For instance, Nishijima has shown that the microscopic viscosity of liquid paraffin hydrocarbons levels off for molecular weights above 1000 and that the microscopic viscosity of polystyrene containing 10 volume"/ benzene is only 200 times as high as that of benzene (15). Nishijima also showed that the emission anisotropy is a useful index of molecular orientation. Since both the excitation and the emission are anisotropic, the method yields the fourth moment of the distribution function of orientations, while other optical properties (dichroism, birefringence) depend on the second moment (15). 

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