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

• Published in: Physical chemistry chemical physics : PCCP 2016-04, Vol.18 (14), p.9419-943
• Main Authors: Emsley, J. W, Lelli, M, Joy, H, Tamba, M.-G, Mehl, G. H
• Format: Article
• Language: English
• Subjects: Anisotropy; Camber; Magnetic fields; Nematic; Order parameters; Phase shift; Phases; Tilt
• ISSN: 1463-9076; 1463-9084
• DOI: 10.1039/c5cp07304a

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 13 C - { 1 H} 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 NN TB 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. Carbon-13 chemical shift anisotropies reveal an unusual temperature dependence of the order parameters, S zz , for the difluoroterphenyl groups in the normal nematic, N and the twist-bend nematic, N TB , phases.