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 degrees C and a nematic-isotropic phase transition at 153 degrees C. The N-TB phase assignment has been confirmed using polarised light microscopy, freeze fracture transmission electron microscopy (FFTEM), H-2-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 H-2-NMR studies of CB6OCB-d(2), and those already known for CB7CB-d(4). 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.