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We report on the temperature dependence of birefringence and of the staticdielectric permittivity tensor in a series of binary mixtures between thesymmetric, bent-shaped, 1",9"-bis(4-cyanobiphenyl-4'-yl) nonane (CB9CB) dimerand the monomeric nematogen 5CB. In the studied composition range the mixturesexhibit two nematic phases with distinct birefringence and dielectric features.Birefringence measurements are used to estimate the temperature dependence ofthe tilt between the axis defining the nanoscale helical modulation of the lowtemperature nematic phase with the (local) direction of the maximal alignmentof the cyanobiphenyl units. Planar as well as magnetically and/or electricallyaligned samples are used to measure the perpendicular and parallel componentsof the dielectric permittivity in both nematic phases. A self-consistentmolecular field theory that takes into account flexibility and symmetry of theconstituent mesogens is introduced for the calculation of order parameters andintra-molecular orientational dipolar correlations of the flexible dimers as afunction of temperature/concentration. Utilising the tilt angle, as calculatedfrom the birefringence measurements, and the predictions of the moleculartheory, dielectric permittivity is modelled in the framework of the anisotropicversion of the Kirkwood-Frohlich theory. Using the inter-molecular Kirkwoodcorrelation factors as adjustable parameters, excellent agreement betweentheory and permittivity measurements across the whole temperature range andcomposition of the mixtures is obtained. The importance of the orientational,intra- and inter-molecular, dipolar correlations, their relative impact on thestatic dielectric properties, as well as their connection with the localstructure of the nematic phases of bent-shaped bimesogens, is discussed.

Dipole-dipole correlations in the nematic phases of symmetric cyanobiphenyl dimers and their binary mixtures with 5CB