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  • 1. Cifelli, M.
    et al.
    Domenici, V.
    Dvinskikh, Sergey
    KTH, School of Chemical Science and Engineering (CHE), Chemistry.
    Luckhurst, G. R.
    Timimi, B. A.
    The twist-bend nematic phase: translational self-diffusion and biaxiality studied by H-1 nuclear magnetic resonance diffusometry2017In: Liquid crystals (Print), ISSN 0267-8292, E-ISSN 1366-5855, Vol. 44, no 1, p. 204-218Article in journal (Refereed)
    Abstract [en]

    Recently, there has been a surge of interest in mesogens exhibiting the twist-bend nematic (N-TB) phase that is shown to be chiral even though formed by effectively achiral molecules. Although it now seems to be clear that the N-TB phase in the bulk is formed by degenerate domains having opposite handedness, the presence of a supramolecular heliconical structure proposed in the Dozov model has been contradicted by the Hoffmann et al. model in which the heliconical arrangement is replaced by a polar nematic phase. The evidence in support of this is that the quadrupolar splitting tensor measured in various experiments is uniaxial and not biaxial as expected for the twist-bend nematic structure. In this debate, among other evidence, the molecular translational diffusion, and its magnitude with respect to that in the nematic phase above the N-TB phase, has also been invoked to eliminate or to confirm one model or the other. We attempt to resolve this issue by reporting the first measurements of the translational self-diffusion coefficients in the nematic and twist-bend nematic phases formed 1 '',7 ''-bis-4-(4'-cyanobiphenyl-4'-yl) heptane (CB7CB). Such measurements certainly appear to resolve the differences between the two models in favour of that for the classic twist-bend nematic phase. [GRAPHICS] .

  • 2.
    Dvinskikh, Sergey, V
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Chemistry. St Petersburg State Univ, Lab Biomol NMR, St Petersburg, Russia..
    Nuclear magnetic resonance studies of translational diffusion in thermotropic ionic liquid crystals2019In: Liquid crystals (Print), ISSN 0267-8292, E-ISSN 1366-5855Article in journal (Refereed)
    Abstract [en]

    The NMR methodologies employed for investigating translational diffusion in anisotropic fluids and the results of their applications to ionic liquid crystals are reviewed. Experiments on ionic liquid crystals are preferably performed using oriented samples and require magnetic field gradients in orthogonal directions. Diffusion experiments in anisotropic systems with broad NMR lines are performed using line narrowing techniques and by application of strong static or pulsed field gradients for efficient gradient encoding/decoding of the spatial locations of molecules. Self-diffusion studies on various thermotropic ion-conductive materials exhibiting smectic, cubic, and columnar phases have been reported. Diffusion rates and anisotropy characterise the translational dynamics of ions in nanostructures and reflect the molecular ordering and ion pairing/dissociation processes. Distinct diffusion behaviours were observed for cations and anions. The knowledge of molecular mobility in ionic liquid crystals is important for the understanding their dynamic properties and is, therefore, valuable for the development of anisotropic soft materials for ion transport.

  • 3. Ortegren, J.
    et al.
    Busson, P.
    Gedde, Ulf W.
    KTH, Superseded Departments, Polymer Technology.
    Hult, Anders
    KTH, Superseded Departments, Polymer Technology.
    Eriksson, A.
    Lindgren, M.
    Andersson, G.
    Cone motion viscosity and optical second harmonic generation of ferroelectric liquid crystalline dendrimers2001In: Liquid crystals (Print), ISSN 0267-8292, E-ISSN 1366-5855, Vol. 28, no 6, p. 861-868Article in journal (Refereed)
    Abstract [en]

    We report second harmonic generation in a ferroelectric liquid crystalline trimer and ferroelectric liquid crystalline dendrimers of first, second and third generation. Thin cells were filled with the compounds by capillary forces at elevated temperature, and cooled from the surface stabilized ferroelectric state to below the glass transition temperature, while kept in an electric field. The cone motion viscosity and the threshold electric field for unwinding of the helix axis of the chiral tilted smectic mesophases were studied separately at elevated temperature, and these data were used to optimize the preparation of the films. The measured response time was between 0.3 and 3 ms, which corresponds to a cone motion viscosity between 0.5 and 50 Pa s. Second harmonic generation was studied both at elevated temperature with an electric field and at room temperature with and without electric field. The first generation dendrimer exhibited a strong increase in the second order non-linear optical response with time at room temperature. The d(23)-coefficient of this dendrimer was approximately four times larger than for the other macromolecules and was 0.045 pm V-1. The relatively large d-coefficient of the first generation dendrimer is ascribed to crystallization, which improved the orientation of the molecular dipoles.

  • 4. Tornblom, M.
    et al.
    Sitnikov, R.
    Henriksson, Ulf
    KTH, Superseded Departments, Chemistry.
    A H-2 NMR study of order and dynamics in the cubic I-1 phase of the dodecyltrimethylammonium chloride/water system2000In: Liquid crystals (Print), ISSN 0267-8292, E-ISSN 1366-5855, Vol. 27, no 7, p. 943-955Article in journal (Refereed)
    Abstract [en]

    The micellar cubic phase I-1 of the dodecyltrimethylammonium chloride (C(12)TACl)/water system has been studied by H-2 NMR spectroscopy. Relaxation rates of selectively deuteriated surfactant were measured at several frequencies down to 0.5 MHz in the temperature range 25-61 degrees C. The results are interpreted within models for the reorientational dynamics in I, phases and it is concluded that the remaining order after surface diffusion and aggregate rotation is small and does not vary substantially with temperature. The results are discussed in relation to the two prevailing models for the structure of these phases as suggested (separately) by Fontell and Vargas. At 0 degrees C the 2H NMR Spectrum from perdeuteriated n-dodecane solubilized in this phase shows static quadrupole splittings with a magnitude that is in good agreement with the results obtained from the analysis of the relaxation data.

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