The effect of the molecular structure on flexoelectric coupling in the chiral nematic phase
In order to understand the correlations between molecular structure and flexoelectric properties inherent in the electro-optic switching behaviour of chiral nematic liquid crystals, we have examined the properties of monomesogenic and bimesogenic achiral nematic materials doped with a low concentration (3–6% by weight) of a chiral additive. On comparing the properties of host nematogenic materials, specifically the monomesogens 7OCB and 7CB and the bimesogen CBO8OCB, we find that the cyanobiphenyl (CB) and oxycyanobiphenyl (OCB) mesogenic units undergo particularly strong flexoelectric coupling. In contrast with the behaviour of achiral nematic materials, however, we find that the inclusion of a significant transverse molecular dipole (7OCB) affects neither the sign of the flexoelectric coefficient nor its magnitude when compared with a similar material bearing a smaller transverse dipole (7CB). The values of the ratio of flexoelectric coefficient to elastic constant ē/K are large and of similar magnitude in all three materials (0.4 ≤ ē/K ≤ 0.6 C N−1 m−1). The bimesogenic mixture has the separate advantage of possessing a low dielectric anisotropy, which allows this material to exhibit a complete (2ϕ) optic axis rotation of 45° in a bipolar field of amplitude 6 V µm−1. Additionally, we note that the occurrence of blue phases can be exploited to simplify the procedure required to align the chiral nematic phase in the uniaxial lying helix configuration for the flexoelectric electro-optic effect.