Understanding the twist-bend nematic phase: the characterisation of 1-(4-cyanobiphenyl-4′-yloxy)-6-(4-cyanobiphenyl-4′-yl)hexane (CB6OCB) and comparison with CB7CB

Abstract

The synthesis and characterisation of the nonsymmetric liquid crystal dimer, 1-(4-cyanobiphenyl-4′-yloxy)-6-(4-cyanobiphenyl-4′-yl)hexane (CB6OCB) is reported. An enantiotropic nematic (N)–twist-bend nematic (N_TB) phase transition is observed at 109 °C and a nematic–isotropic phase transition at 153 °C. The N_TB phase assignment has been confirmed using polarised light microscopy, freeze fracture transmission electron microscopy (FFTEM), ²H-NMR spectroscopy, and X-ray diffraction. The effective molecular length in both the N_TB and N phases indicates a locally intercalated arrangement of the molecules, and the helicoidal pitch length in the N_TB phase is estimated to be 8.9 nm. The surface anchoring properties of CB6OCB on a number of aligning layers is reported. A Landau model is applied to describe high-resolution heat capacity measurements in the vicinity of the N_TB–N phase transition. Both the theory and heat capacity measurements agree with a very weak first-order phase transition. A complementary extended molecular field theory was found to be in suggestive accord with the ²H-NMR studies of CB6OCB-d₂, and those already known for CB7CB-d₄. These include the reduced transition temperature, T_NTBN/T_NI, the order parameter of the mesogenic arms in the N phase close to the N_TB–N transition, and the order parameter with respect to the helix axis which is related to the conical angle for the N_TB phase.