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

Daniel A Paterson, Min Gao, Young-Ki Kim, Afsoon Jamali, Kirsten Finley, Beatriz Robles-Hernandez, Sergio Diez-Berat, Josep Salud, M. Rosario de la Fuente, Bakir A. Timimi, Herbert Zimmermann, Cristina Greco, Alberta Ferrarini, John M. D. Storey, David Orencio Lopez, Oleg D. Lavrentovich, Geoffrey R. Luckhurst, Corrie T Imrie

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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 (NTB) phase transition is observed at 109˚C and a nematic-isotropic phase transition at 153˚C. The NTB 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 NTB and N phases indicates a locally intercalated arrangement of the molecules, and the helicoidal pitch length in the NTB 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 NTB-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, TNTBN/TNI, the order parameter of the mesogenic arms in the N phase close to the NTB-N transition, and the order parameter with respect to the helix axis which is related to the conical angle for the NTB phase.
Original languageEnglish
Pages (from-to)6827-6840
Number of pages14
JournalSoft matter
Volume12
Issue number32
Early online date11 Jul 2016
DOIs
Publication statusPublished - 2016

ASJC Scopus subject areas

  • Chemistry(all)
  • Condensed Matter Physics

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    Paterson, D. A., Gao, M., Kim, Y-K., Jamali, A., Finley, K., Robles-Hernandez, B., Diez-Berat, S., Salud, J., de la Fuente, M. R., Timimi, B. A., Zimmermann, H., Greco, C., Ferrarini, A., Storey, J. M. D., Lopez, D. O., Lavrentovich, O. D., Luckhurst, G. R., & Imrie, C. T. (2016). 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. Soft matter, 12(32), 6827-6840. https://doi.org/10.1039/C6SM00537C