- 标题
- 摘要
- 关键词
- 实验方案
- 产品
-
1H NMR study of molecular order and dynamics in CBC9CB Liquid Crystal
摘要： 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.
关键词： molecular dynamics,liquid crystal,nematic phase,twist bend nematic phase,NMR spectroscopy,relaxometry
更新于2025-09-19 17:15:36
-
Selectivity, Thermodynamic and Anisotropic Properties of Substituted Liquid-Crystal Cyanoazoxybenzenes as Stationary Phases for Gas Chromatography
摘要： Herein we discuss physical properties of 4-(ω-hydroxyalkoxy)-4’-cyanoazoxybenzene homologs. 1D and 2D correlation NMR spectroscopy (in particular, 1Н, 15N-HMBC experiment) have allowed elucidation of structure of the prepared rod-like supramolecular cyanoazoxybenzenes. Mesomorphic properties of the compounds have been studied by means of polarization thermomicroscopy and differential scanning calorimetry. All the studied cyanoazoxybenzenes have revealed enantiotropic nematic mesomorphism over wide temperature range. Nematic mesophase of the eighth homolog has possessed large positive dielectric anisotropy. Introduction of small amounts of the prepared cyanoazoxybenzenes as additive has stabilized the mesophase and has increased the dielectric anisotropy of 4-pentyloxy-4’-cyanobiphenyl. Gas-liquid chromatography studies have shown that sorbents based on 4-(2-hydroxyethyloxy)-4’-cyanoazoxybenzene are highly selective towards various structural isomers; that cannot be achieved using conventional nematic liquid crystals. Thermodynamic evidence of specific interactions between the mesogen and the non-mesomorphic sorbate has been discovered.
关键词： Mesogen,Dielectric anisotropy,Sorbent,Hydrogen bond,Structural selectivity,NMR spectroscopy,Nematic phase,Position isomerism,Azoxybenzene,Double refraction,Supramolecule
更新于2025-09-19 17:15:36
-
Emergent biaxiality in nematic microflows illuminated by a laser beam
摘要： Anisotropic fluids (e.g. liquid crystals) offer a remarkable promise as optofluidic materials owing to the directional, tunable, and coupled interactions between the material, flow, and the optical fields. Here we present a comprehensive in silico treatment of this anisotropic interaction by performing nonequilibrium molecular dynamics simulations. We quantify the response of a nematic liquid crystal (NLC) undergoing a Poiseuille flow in the Stokes regime, while being illuminated by a laser beam incident perpendicular to the flow direction. We adopt a minimalistic model to capture the interactions, accounting for two features: first, the laser heats up the NLC locally; and second, the laser polarises the NLC and exerts an optical torque that tends to reorient molecules of the nematic phase. Because of this reorientation the liquid crystal exhibits small regions of biaxiality, where the nematic director is one symmetry axis and the axis of rotation for the reorientation of the molecules is the other one. We find that the relative strength of the viscous and the optical torques mediates the flow-induced response of the biaxial regions, thereby tuning the emergence, shape and location of the regions of enhanced biaxiality. The mechanistic framework presented here promises experimentally tractable routes toward novel optofluidic applications based on material-flow-light interactions.
关键词： optofluidics,Liquid crystal,biaxiality,nematic phase,nonequilibrium molecular dynamics
更新于2025-09-12 10:27:22
-
Effect of Biphenyl conjugation on the photosensitive properties of liquid crystalline polymers”
摘要： A series of side-chain liquid crystalline photosensitive polymers with pendent chalcone moiety were synthesized. The structure of the thermotropic, novel liquid crystalline photosensitive monomers and polymers was confirmed by IR, 1H - and 13C- NMR and UV visible spectroscopic techniques. The molecular weight of the polymers was determined by gel permeation chromatography. The photo-crosslinking properties of these polymers were studied by ultraviolet spectroscopy. The crosslinking proceeds via 2π+2π cycloaddition reaction of the styryl unit. The rate of crosslinking increased with increase in methylene chain length of the polymer. The thermal stability of the polymers was assessed by thermogravimetric analysis (TGA). The phase behavior and mesomorphism of the polymers were investigated by differential scanning calorimetry, X-ray diffraction and polarizing optical microscopy. The polymers exhibited nematic phase in hot stage optical polarized microscope (HOPM).
关键词： Chalcone,Thermal stability,Nematic phase,Mesophase,UV irradiation,Liquid crystalline polymer
更新于2025-09-10 09:29:36
-
Optical signal and optical axes in uniaxial and biaxial nematic phases
摘要： In this work we will study the behavior of the optical signal and optical axes along the continuous transition of a lyotropic system from a nematic discotic (ND) phase to a nematic calamitic (NC) phase; we will study in detail the case in which there is a biaxial nematic (NB) phase at the interstice between these two uniaxial phases. It will be shown that the optical signal and optical axes change simultaneously in order to agree with the optical signals of the uniaxial phases at their borders: the optical signal is positive at the frontiers with the ND phase and it turns to be negative at the frontiers with the NC phase. Concomitantly, the optical axes rotates from the (cid:1)ez direction to the (cid:1)ex direction. We will illustrate these findings with experimental data that confirm our results.
关键词： Optical signal,biaxial nematic phase,refractive indices,optical axes
更新于2025-09-09 09:28:46