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The order parameter S zz and the molecular biaxiality parameter D = S xx - S yy have been evaluated for various nematic liquid crystal materials employing infrared dichroism measurements. The influence of the terminal group, the ring systems and an ester linkage group on S zz and D has been studied for various terminally polar and terminally non-polar liquid crystal materials. Our experimental results for the cyanobiphenyl CB5 are in agreement with NMR-studies, if we allow for a small local field correction which is nearly within our experimental measuring accuracy. For the first time, the order parameters S xx, S yy and S zz of heptyl-cyanophenyl-cyclohexan (PCH7) are determined independently by analyzing 3 different benzene ring skeleton vibrations. The existence of a weakly pronounced molecular biaxiality is demonstrated for most of the liquid crystal materials investigated. Moreover the variation of D as a function of S zz is found to be approximately of the form predicted by the molecular field theory.
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Kiefer et al. (1989) studied this question.
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