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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, 2H NMR, and calorimetric study

AutorRobles-Hernández, Beatriz; Sebastián, Nerea; Fuente, M. Rosario de la; López, David O.; Diez-Berart, Sergio; Ros, M. Blanca; Dunmur, David A.
Fecha de publicación2015
EditorAmerican Physical Society
CitaciónPhysical Review E 92(6): 062505 (2015)
ResumenThe 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 H2 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 (NTB), 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 NTB-N phase transition, which is found to be weakly first order, but close to tricritical. Additionally broadband dielectric spectroscopy and H2 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 NTB 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. H2 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.
DescripciónUnder the terms of the Creative Commons Attribution license.-- et al.
Versión del editorhttp://dx.doi.org/10.1103/PhysRevE.92.062505
URIhttp://hdl.handle.net/10261/147971
DOI10.1103/PhysRevE.92.062505
Identificadoresdoi: 10.1103/PhysRevE.92.062505
issn: 2470-0045
e-issn: 2470-0053
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