title: Twist, tilt, and orientational order at the nematic to twist-bend nematic phase transition of 1″,9″-bis(4-cyanobiphenyl-4'-yl) nonane: A dielectric, (2)H NMR, and calorimetric study.
abstract: The nature of the nematic-nematic phase transition in the liquid crystal dimer 1″,9″-bis(4-cyanobiphenyl-4'-yl) nonane (CB9CB) has been investigated using techniques of calorimetry, dynamic dielectric response measurements, and (2)H NMR spectroscopy. The experimental results for CB9CB show that, like the shorter homologue CB7CB, the studied material exhibits a normal nematic phase, which on cooling undergoes a transition to the twist-bend nematic phase (N(TB)), a uniaxial nematic phase, promoted by the average bent molecular shape, in which the director tilts and precesses describing a conical helix. Modulated differential scanning calorimetry has been used to analyze the nature of the N(TB)-N phase transition, which is found to be weakly first order, but close to tricritical. Additionally broadband dielectric spectroscopy and (2)H magnetic resonance studies have revealed information on the structural characteristics of the recently discovered twist-bend nematic phase. Analysis of the dynamic dielectric response in both nematic phases has provided an estimate of the conical angle of the heliconical structure for the N(TB) phase. Capacitance measurements of the electric-field realignment of the director in initially planar aligned cells have yielded values for the splay and bend elastic constants in the high temperature nematic phase. The bend elastic constant is small and decreases with decreasing temperature as the twist-bend phase is approached. This behavior is expected theoretically and has been observed in materials that form the twist-bend nematic phase. (2)H NMR measurements characterize the chiral helical twist identified in the twist-bend nematic phase and also allow the determination of the temperature dependence of the conical angle and the orientational order parameter with respect to the director.
authors.affiliation: Departamento de Física Aplicada II, Facultad de Ciencia y Tecnología, Universidad del País Vasco, Apartado 644, E-48080 Bilbao, Spain.
authors.forename: Beatriz
authors.initials: B
authors.lastname: Robles-Hernández
authors.affiliation: Departamento de Física Aplicada II, Facultad de Ciencia y Tecnología, Universidad del País Vasco, Apartado 644, E-48080 Bilbao, Spain.|Otto-von-Guericke Universitat Magdeburg, Institute for Experimental Physics, ANP, 39106 Magdeburg, Germany.
authors.forename: Nerea
authors.initials: N
authors.lastname: Sebastián
authors.affiliation: Departamento de Física Aplicada II, Facultad de Ciencia y Tecnología, Universidad del País Vasco, Apartado 644, E-48080 Bilbao, Spain.
authors.forename: M Rosario
authors.initials: MR
authors.lastname: de la Fuente
authors.affiliation: Grup de Propietas Físiques dels Materials (GRPFM), Departament de Física i Enginyeria Nuclear, E.T.S.E.I.B. Universitat Politècnica de Catalunya, Diagonal 647, E- 08028 Barcelona, Spain.
authors.forename: David O
authors.initials: DO
authors.lastname: López
authors.affiliation: Grup de Propietas Físiques dels Materials (GRPFM), Departament de Física i Enginyeria Nuclear, E.T.S.E.I.B. Universitat Politècnica de Catalunya, Diagonal 647, E- 08028 Barcelona, Spain.
authors.forename: Sergio
authors.initials: S
authors.lastname: Diez-Berart
authors.affiliation: Grup de Propietas Físiques dels Materials (GRPFM), Departament de Física i Enginyeria Nuclear, E.T.S.E.I.B. Universitat Politècnica de Catalunya, Diagonal 647, E- 08028 Barcelona, Spain.
authors.forename: Josep
authors.initials: J
authors.lastname: Salud
authors.affiliation: Departamento de Química Orgánica, Facultad de Ciencias-Instituto de Ciencia de Materiales de Aragón, Universidad de Zaragoza-CSIC, E-50009 Zaragoza, Spain.
authors.forename: M Blanca
authors.initials: MB
authors.lastname: Ros
authors.affiliation: Chemistry, University of Southampton, Highfield, Southampton SO17 1BJ, United Kingdom.
authors.forename: David A
authors.initials: DA
authors.lastname: Dunmur
authors.affiliation: Chemistry, University of Southampton, Highfield, Southampton SO17 1BJ, United Kingdom.
authors.forename: Geoffrey R
authors.initials: GR
authors.lastname: Luckhurst
authors.affiliation: Chemistry, University of Southampton, Highfield, Southampton SO17 1BJ, United Kingdom.
authors.forename: Bakir A
authors.initials: BA
authors.lastname: Timimi