1H NMR study of molecular order and dynamics in the liquid crystal CB-C9-CB.

Molecular order and dynamics of the CB-C9-CB liquid crystalline dimer exhibiting the nematic (N) and the twist bend nematic (Ntb) phases were investigated by proton NMR spectroscopy, using fields of 0.78 T and 7.04 T, and relaxometry. The first relaxometry experiments for a very wide Larmor frequency domain (8 kHz-300 MHz) on this system, using a combination of standard and fast field cycling NMR techniques, were performed. The spectroscopy results in the Ntb phase allowed us to probe the local molecular orientation relative to the Ntb helix axis. The relaxation data were analyzed considering order director fluctuations (ODF), molecular self-diffusion (SD) and local molecular rotations/reorientations (R) relaxation mechanisms. Global fits of theoretical relaxation models, as a function of temperature and Larmor frequency, for the phases under investigation, allowed for the determination of rotational correlation times, diffusion coefficients, viscoelastic parameters, correlation lengths and activation energies (in the case of thermally activated mechanisms). A clear difference between the structures of the N and Ntb phases was detected from the results of proton spin-lattice relaxation through distinct temperature and frequency dependencies' signatures of the collective modes. Significant pre-transitional effects were observed at the N-Ntb phase transition both from relaxometry and spectroscopy data. The experimental results correlate to data and models for comparable liquid crystalline systems.

On the structure of the Nx phase of symmetric dimers: inferences from NMR.

NMR measurements on a selectively deuterated liquid crystal dimer CB-C9-CB, exhibiting two nematic phases, show that the molecules in the lower temperature nematic phase, N(X), experience a chiral environment and are ordered about a uniformly oriented director throughout the macroscopic sample. The results are contrasted with previous interpretations that suggested a twist-bend spatial variation of the director. A structural picture is proposed wherein the molecules are packed into highly correlated chiral assemblies.

Direct observation of liquid crystals using cryo-TEM: specimen preparation and low-dose imaging.

Liquid crystals (LCs) represent a challenging group of materials for direct transmission electron microscopy (TEM) studies due to the complications in specimen preparation and the severe radiation damage. In this paper, we summarize a series of specimen preparation methods, including thin film and cryo-sectioning approaches, as a comprehensive toolset enabling high-resolution direct cryo-TEM observation of a broad range of LCs. We also present comparative analysis using cryo-TEM and replica freeze-fracture TEM on both thermotropic and lyotropic LCs. In addition to the revisits of previous practices, some new concepts are introduced, e.g., suspended thermotropic LC thin films, combined high-pressure freezing and cryo-sectioning of lyotropic LCs, and the complementary applications of direct TEM and indirect replica TEM techniques. The significance of subnanometer resolution cryo-TEM observation is demonstrated in a few important issues in LC studies, including providing direct evidences for the existence of nanoscale smectic domains in nematic bent-core thermotropic LCs, comprehensive understanding of the twist-bend nematic phase, and probing the packing of columnar aggregates in lyotropic chromonic LCs. Direct TEM observation opens ways to a variety of TEM techniques, suggesting that TEM (replica, cryo, and in situ techniques), in general, may be a promising part of the solution to the lack of effective structural probe at the molecular scale in LC studies.

Surface alignment, anchoring transitions, optical properties, and topological defects in the thermotropic nematic phase of organo-siloxane tetrapodes.

We perform optical, surface anchoring, and textural studies of an organo-siloxane "tetrapode" material in the broad temperature range of the nematic phase. The optical, structural, and topological features are compatible with the uniaxial nematic order rather than with the biaxial nematic order, in the entire nematic temperature range -25 °C < T < 46 °C studied. For homeotropic alignment, the material experiences surface anchoring transition, but the director can be realigned into an optically uniaxial texture by applying a sufficiently strong electric field. The topological features of textures in cylindrical capillaries, in spherical droplets and around colloidal inclusions are consistent with the uniaxial character of the long-range nematic order. In particular, we observe isolated surface point defects - boojums and bulk point defects - hedgehogs that can exist only in the uniaxial nematic liquid crystal.

The Design and Investigation of Nanocomposites Containing Dimeric Nematogens and Liquid Crystal Gold Nanoparticles with Plasmonic Properties Showing a Nematic-Nematic Phase Transition (Nu-Nx/Ntb).

The construction of liquid crystal compositions consisting of the dimeric liquid crystal, CB_C9_CB (cyanobiphenyl dimer = 1'',9''-bis(4-cyanobiphenyl-4'-yl)nonane), and the range of nematic systems is explored. The materials include a laterally functionalized monomer, which was used to construct a phase diagram with CB_C9_CB, as well as one laterally linked dimer liquid crystal material and two liquid crystal gold nanoparticle (LC-Au-NPs) systems. For the Au-NP-LCs, the NP diameters were varied between ~3.3 nm and 10 nm. Stable mixtures that exhibit a nematic-nematic phase transition are reported and were investigated by POM (polarizing optical microscopy), DSC (differential scanning calorimetry) and X-ray diffraction studies.

Nematic-nematic phase transition in the liquid crystal dimer CBC9CB and its mixtures with 5CB: a high-resolution adiabatic scanning calorimetric study.

The phase transition behavior of the liquid crystal dimer α,ω-bis(4,4'-cyanobiphenyl)nonane (CBC9CB), which has been reported to exhibit a nematic-nematic phase transition, has been investigated by means of high-resolution adiabatic scanning calorimetry. This nematic-nematic phase transition is weakly first-order with a latent heat of 0.24±0.01 kJ kg(-1). Mixtures up to 40 wt % with 4-pentyl-4'-cyanobiphenyl (5CB) liquid crystals have also been investigated, which also show this nematic to nematic phase transition. The transition stays weakly first-order with a decreasing latent heat with increasing concentration of 5CB. For mixtures with more than 40 wt % uniaxial nematic-unknown nematic phase transition was not observed.

Mesomorphic hydrogen-bonded complexes of complementary semiperfluorinated components.

2,4-Diamino-6-phenyl-1,3,5-triazines carrying one or two semiperfluorinated chains at the phenyl substituent have been investigated in binary mixtures with partially fluorinated benzoic acids by polarizing microscopy, differential scanning calorimetry, and X-ray diffraction. The number and positions of the fluorinated tails of the acid component were systematically modified. Equimolar mixtures of the triazines with the aromatic acids form discrete hydrogen-bonded heterodimers with an elongated central core. Mesomorphic properties are observed only if at least three terminal fluoroalkyl chains are grafted to the hydrogen-bonded rigid core. The dimeric supermolecules exhibit columnar mesophases with rectangular or oblique two-dimensional lattice symmetry or bicontinuous cubic phases. The mesophases of the dimers are ribbon phases resulting from the collapse of smectic layers. Thereby, the phase type is defined by the number and positions of the fluorinated tails of the two components. Docking of two or three equivalents of two-chain benzoic acids to the diaminotriazine core leads to the formation of cylindrical aggregates, which organize into hexagonal columnar phases, whereas 1:2 and 1:3 mixtures of the two-chain triazine with a three-chain fluorinated aromatic acid display micellar cubic phases.