Ferroelectric nematic liquid crystals represent not only interesting fundamental science, but they also hold promise for storage capacitors with high power density or new information display ...technology having sub-millisecond switching. In this work we describe the synthesis and measurements of the physical properties of a new highly polar ferroelectric nematic compound, 4-nitrophenyl 4-(2,4-dimethoxylbenzoyl)oxy-2-fluorobenzoate (RT11001). The dipole moment of this material (along the long molecular axis) is calculated to exceed 11.5 Debye. We employ a wide range of physical characterization methods including differential scanning calorimetry (DSC), mass density measurement, optical birefringence, polarizing optical microscopy (POM), electric current analysis, and electro-optical switching, to show that RT11001 has three distinct ferroelectric states, F1, F2 and F3. F1 is purely orientationally ordered ferroelectric nematic phase (NF), F2 has a ferroelectric nematic with possibly short-range hexagonal order normal to the director (NhF), and we conjecture that F3 has a long-range hexagonal order normal to the director (ColhF).
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.
We report results of photon correlation spectroscopy studies of layer dynamics in thermotropic smectic films, carried out with nanosecond resolution. In films of a conventional smectic-A material ...(9CB), correlograms of the dynamics due to layer fluctuations are measured over a wide optical range of the in-plane wavevector, with a signal to background ratio approaching 90tension dominating the restoring force), the predicted crossover from underdamped to overdamped behavior with increasing wavevector is observed and quantitatively compared to the limit of smectic hydrodynamics conventionally used to describe the film dynamics. We discuss aspects of the data that do not seem to fit into the simplified treatment. We also discuss potential for detection of the 'peristaltic' mode in thermotropic films, where the bulk elasticity can be probed. Finally, we present results on films of an unconventional, deVries-type smectic-A.
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The dynamics of the distorted cholesteric director is studied in a complex matrix, such as mesogenic and istotropic polymer networks, by means of dynamic light scattering. We employ ...polymer-stabilized cholesteric diffraction gratings as our system to measure thermal fluctuations of the cholesteric director in photostabilized low-molecular-weight polymer networks. The relaxation rates of fast and slow dynamical modes of the distorted cholesteric directors are measured in two scattering geometries, where the scattering vector is either parallel or perpendicular to the helical axis. The dispersion relations for fluctuation wave vectors along the helical axis are found to be fundamentally different for the two types of polymer networks. Experimental dispersion curves are in agreement with the theoretical predictions developed in the present paper. A possible understanding of dispersion in distorted cholesterics is developed and the experimental results for polymer stabilized cholesteric diffraction gratings are presented in this report. We also discuss the coupling of a slow relaxation mode of distorted cholesterics in a mesogenic versus isotropic polymer network.
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We present a comprehensive set of measurements of optical, dielectric, diamagnetic, elastic and viscous properties in the nematic (N) phase formed by a liquid crystalline dimer. The studied dimer, ...1,7-bis-4-(4-cyanobiphenyl) heptane (CB7CB), is composed of two rigid rod-like cyanobiphenyl segments connected by a flexible aliphatic link with seven methyl groups. CB7CB and other nematic dimers are of interest due to their tendency to adopt bent configurations and to form two states possessing a modulated nematic director structure, namely, the twist bend nematic, NTB, and the oblique helicoidal cholesteric, ChOH, which occurs when the achiral dimer is doped with a chiral additive and exposed to an external electric or magnetic field. We characterize the material parameters as functions of temperature in the entire temperature range of the N phase, including the pre-transitional regions near the N-NTB and N-to-isotropic (I) transitions. The splay constant K11 is determined by two direct and independent techniques, namely, detection of the Frederiks transition and measurement of director fluctuation amplitudes by dynamic light scattering (DLS). The bend K33 and twist K22 constants are measured by DLS. K33 being the smallest of the three constants, shows a strong non-monotonous temperature dependence with a negative slope in both N-I and N-NTB pretransitional regions. The measured ratio K11/K22 is larger than 2 in the entire nematic temperature range. The orientational viscosities associated with splay, twist and bend fluctuations in the N phase are comparable to those of nematics formed by rod-like molecules. All three show strong temperature dependence, increasing sharply near the N-NTB transition.
We synthesized the liquid crystal dimer and trimer members of a series of flexible linear oligomers and characterized their microscopic and nanoscopic properties using resonant soft x-ray scattering ...and a number of other experimental techniques. On the microscopic scale, the twist-bend phases of the dimer and trimer appear essentially identical. However, while the liquid crystal dimer exhibits a temperature-dependent variation of its twist-bend helical pitch varying from 100 - 170 Å on heating, the trimer exhibits an essentially temperature-independent pitch of 66 Å, significantly shorter than those reported for other twist-bend forming materials in the literature. We attribute this to a specific combination of intrinsic conformational bend of the trimer molecules and a sterically favorable intercalation of the trimers over a commensurate fraction (two-thirds) of the molecular length. We develop a geometric model of the twist-bend phase for these materials with the molecules arranging into helical chain structures, and we fully determine their respective geometric parameters.
In search for novel nematic materials, a laterally linked H-shaped liquid crystal dimer have been synthesized and characterized. The distinct feature of the material is a very broad temperature range ...(about 50 oC) of the nematic phase, which is in contrast with other reported H-dimers that show predominantly smectic phases. The material exhibits interesting textural features at the scale of nanometers (presence of smectic clusters) and at the macroscopic scales. Namely, at a certain temperature, the flat samples of the material show occurrence of domain walls. These domain walls are caused by the surface anchoring transition and separate regions with differently tilted director. Both above and below this transition temperature the material represents a uniaxial nematic, as confirmed by the studies of defects in flat samples and samples with colloidal inclusions, freely suspended drops, X-ray diffraction and transmission electron microscopy.
