Similarities and differences between molecular order in the nematic and twist-bend nematic phases of a symmetric liquid crystal dimer

Abstract

The order parameter, S_zz, where z is the para axis of the difluoroterphenyl groups in DTC5C9, has been obtained from chemical shift anisotropies measured by ¹³C – {¹H} NMR experiments at temperatures throughout the nematic, N_U, and twist-bend nematic, N_TB, phases shown by this compound. The order parameter temperature profiles are unusual in having a maximum value in the N_U phase and then decreasing until the N_TB phase is reached. There is a small discontinuity (∼2%) in S_zz at T_NNTB and then a gradual decrease until a new phase appears. This behaviour is interpreted as revealing a temperature-dependent tilting of local directors in both phases away from the applied magnetic field direction. In the enantiomorphic twist-bend phase this tilt is consistent with the structure of the phase as a helical arrangement of local directors, whilst in the high-temperature non-chiral nematic the tilt must involve a non-chiral arrangement. It is proposed that in both phases the tilting of directors has a common origin in the bent shape of the molecules.