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Abstract
A theory of the nematic isotropic phase equilibrium in mixtures of thermotropic nematogens and flexible coils is presented. The steric part of the partition function is evaluated along the lines given earlier by Flory for phase equilibria of rods and coils in lyotropic systems. The soft anisotropic forces between the molecules of the nematogenic component are treated by an additional factor in the partition function. From these premises the respective phase diagrams may be calculated using the characteristic parameters of the pure components (nematogen: axial ratio, characteristic temperature, thermal expansion coefficient; coiled solute: contour length, thermal expansion coefficient) without resort to adjustable parameters. The biphasic region where a nematic and an isotropic phase coexist is predicted to increase markedly with increasing chain length of the coiled component; in case of a polymeric solute there is a virtually complete exclusion of the coils from the nematic phase. Experimental phase diagrams obtained on mixtures of low molecular weight nematogens and flexible solutes are in semiquantitative agreement with theory, for increasing chain length of the flexible compound nearly full agreement is achieved.
