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Register a new userThe role of intermolecular interactions in stabilizing the structure of the nematic twist-bend phase
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Katarzyna Merkel Prof
Publication date
2020
fields
Physics
and research's language is
English
Authors
Katarzyna Merkel
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The relationship between the molecular structure and the formation of the NTB phase is still at an early stage of development. This is mainly related to molecular geometry, while the correlation between the NTB phase and the electronic structure is ambiguous. To explore the electronic effect on properties and stabilization of the NTB phase we investigated 2,3-difluoro-4,4-dipentyl-p-terphenyl dimers (DTC5Cn). We used IR polarized spectroscopy, which can at least in principle, bring information about the ordering in NTB phase. All dimers show a significant drop of the average value of the transition dipole moment d{mu}/dQ for parallel dipoles at the transition to the NTB phase, and an increase for perpendicular dipoles, despite its remaining unchanged for the monomer. These results coincide well with DFT simulations of vibrational dipole derivatives for molecules assembled in pseudo-layers of the NTB phase. The DFT calculations were used to determine the geometric and electronic properties of the hydrogen bonded complexes. We have provided experimental and theoretical evidence of stabilization of the NTB phase by arrays of multiple hydrogen bonds (XF...HX, X-benzene ring).
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We report a dynamic light scattering study of the fluctuation modes in a thermotropic liquid crystalline mixture of monomer and dimer compounds that exhibits the twist-bend nematic ($mathrm{N_{TB}}$) phase. The results reveal a spectrum of overdamped fluctuations that includes two nonhydrodynamic and one hydrodynamic mode in the $mathrm{N_{TB}}$ phase, and a single nonhydrodynamic plus two hydrodynamic modes (the usual nematic optic axis or director fluctuations) in the higher temperature, uniaxial nematic phase. The properties of these fluctuations and the conditions for their observation are comprehensively explained by a Landau-deGennes expansion of the free energy density in terms of heliconical director and helical polarization fields that characterize the $mathrm{N_{TB}}$ structure, with the latter serving as the primary order parameter. A coarse-graining approximation simplifies the theoretical analysis, and enables us to demonstrate quantitative agreement between the calculated and experimentally determined temperature dependence of the mode relaxation rates.
We study the flow behaviour of a twist-bend nematic $(N_{TB})$ liquid crystal. It shows three distinct shear stress ($sigma$) responses in a certain range of temperatures and shear rates ($dot{gamma}$). In Region-I, $sigmasimsqrt{dot{gamma}}$, in region-II, the stress shows a plateau, characterised by a power law $sigmasim{dot{gamma}}^{alpha}$, where $alphasim0.1-0.4$ and in region-III, $sigmasimdot{gamma}$. With increasing shear rate, $sigma$ changes continuously from region-I to II, whereas it changes discontinuously with a hysteresis from region-II to III. In the plateau (region-II), we observe a dynamic stress fluctuations, exhibiting regular, periodic and quasiperiodic oscillations under the application of steady shear. The observed spatiotemporal dynamics in our experiments are close to those were predicted theoretically in sheared nematogenic fluids.
Recent work indicates that twist-bend coupling plays an important role in DNA micromechanics. Here we investigate its effect on bent DNA. We provide an analytical solution of the minimum-energy shape of circular DNA, showing that twist-bend coupling induces sinusoidal twist waves. This solution is in excellent agreement with both coarse-grained simulations of minicircles and nucleosomal DNA data, which is bent and wrapped around histone proteins in a superhelical conformation. Our analysis shows that the observed twist oscillation in nucleosomal DNA, so far attributed to the interaction with the histone proteins, is an intrinsic feature of free bent DNA, and should be observable in other protein-DNA complexes.
The nanostructure of two novel sulfur containing dimer materials has been investigated experimentally by hard and by resonant tender X-ray scattering techniques. On cooling the dimers through the nematic to twist-bend nematic (N-NTB) phase transition, the correlation length associated with short-range positional order drops, while the heliconical orientational order becomes more correlated. The heliconical pitch shows a stronger temperature dependence near the N-NTB transition than observed in previously studied dimers, such as the CBnCB series of compounds. We explain both this strong variation and the dependence of the heliconical pitch on the length of the spacer connecting the monomer units by taking into account a temperature dependent molecular bend and intermolecular overlap. and. The heliconical structure is observed even in the upper 3-4{deg}C range of the smectic phase that forms just below the NTB state. The coexistence of smectic layering and the heliconical order indicates a SmCTB -type phase where the rigid units of the dimers are tilted with respect to the layer normal in order to accommodate the bent conformation of the dimers, but the tilt direction rotates along the heliconical axis. This is potentially similar to the SmCTB phase reported by Abberley et al (Nat. Commun. 2018, 9, 228) below a SmA phase.
The twist-bend nematic (Ntb) phase is a recent addition to the nematic (N) phases of liquid crystals (LCs). A net polar order in the Ntb phase under an external electric field was predicted in several recent theoretical studies but yet to be experimentally realized. We investigated the polar nature, dielectric properties, electro-optical switching and optical transmission properties of a bent LC dimer CB7CB. The LC showed a relatively high-temperature nematic (N) phase and a lower-temperature nematic (NX) phase (also called Ntb in literature). A threshold-dependent polarization current response with large polarization values was obtained in the entire mesophase range. The associated switching times were found in sub-millisecond region. This ferroelectric-like polarization resulted from collective reorientation of polar cybotactic clusters. In NX phase, electric field-induced deformation of twisted helical structures also contributed to net polarization. Dielectric measurements confirmed the presence of cybotactic clusters via relaxation processes with large activation energies. Deformation of the NX helical structure under external electric field was corroborated by polarized optical microscopy and optical transmission studies. The field-induced deformations, net polar order and fast switching will contribute towards greater understanding of the NX (or Ntb) phase dynamics. It may also find applications in next-generation electro-optic devices.
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