The effect of molecular biaxiality on the bulk properties of some nematic liquid crystals

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Abstract
The nematic substance 5CB is known from N.M.R. studies to be slightly biaxial, not in the sense that any bulk property measured in a direction at right angles to the director is liable to vary with rotation about the director, but in the sense that there are biaxial terms in the ordering matrix that describes the alignment of individual molecules; (S xx - S yy) is non-zero as well as S zz. We show that the biaxial terms should make a significant contribution to the magnetic anisotropy Δχ(m) of 5CB, and that the magnitude and temperature dependence of this bulk property, which we have measured, can be understood if, and only if, they are taken into account. The contribution which they make to the optical birefringence term ∑ should, however, be relatively trivial. Although ∑ may in principle be affected by local field corrections of a complicated nature, which do not affect Δχ(m), a new theory presented in an Appendix to the paper suggests that these too are likely to be relatively trivial. Hence we believe that ∑ is more nearly proportional than is Δχ(m) to the principal order parameter S zz. The paper includes unpublished data for the magnetic anisotropy and/or the principal refractive indices, n e and n o, in a number of other nematics (6CB, 7CB, 8CB, 9CB, 5OCB, 6OCB, 7OCB, 8OCB, 7CCH, MBBA and PAA). Comparison between the temperature dependence of Δχ(m) and of ∑ suggests that biaxiality is present in all cyanobiphenyls, on much the same scale as in 5CB, and is not affected by the presence of an oxygen atom between the phenyl core of the molecule and its alkyl tail.