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Relationship between Molecular Association and Re-entrant Phenomena in Polar Calamitic Liquid Crystals
The
relationship between molecular association and re-entrant phase
behavior in polar calamitic liquid crystals has been explored in two
families of materials: the 4′-alkoxy-4-cyanobiphenyls
(6OCB and 8OCB) and the 4′-alkoxy-4-nitrobiphenyls. Although
re-entrant nematic phase behavior has previously been observed in
the phase diagram of 6OCB/8OCB, this is not observed in mixtures of
the analogous nitro materials. As there is no stabilization of the
smectic A phase in mixture studies, it was conjectured that the degree
of association for the nitro systems is greater than that for the
cyano analogues. This hypothesis was tested by using measured dielectric
anisotropies and computed molecular properties to obtain a value of
the Kirkwood factor, <i>g</i>, which describes the degree
of association of dipoles in a liquid. These computed values of <i>g</i> confirm that the degree of association for nitro materials
is greater than that for cyano and offer a useful method for quantifying
molecular association in systems exhibiting a re-entrant polymorphism