Investigation of multiaxis molecular motion by off-magic angle spinning deuteron NMR

The relatively new deuteron NMR method of off-axis-magic angle spinning (OMAS) has been extended and used to investigate multiaxis rotational jump motion. Floquet theory is developed for simulating deuteron OMAS spectra with multisite jumps at different rates about noncoincident axes, and efficient procedures are presented for computing the sideband line shapes. It is demonstrated experimentally that reproducible adjustment of the angle between the rotor axis and the static magnetic field is feasible with precision approaching +/- 0.01 degrees. This leads to the reintroduction of a scaled, first-order quadrupole coupling that defines a new kinetic window and makes deuteron OMAS much more sensitive than ordinary magic angle spinning to motion on the kilohertz time scale. Temperature-dependent deuteron OMAS line shapes of octanoic acid/urea-d4 inclusion compound have been recorded and fitted, using least-squares procedures, to provide rates of rotation about both CN and CO bonds. The Arrhenius activation parameters for rotation about CN bonds, Ea = 60.4+/-2.4 kJ/mol and ln(A) = 24.9+/-0.3, agree well with previous values determined by selective inversion experiments. However, OMAS yields Ea = 26.3+/-0.4 kJ/mole and ln(A) = 24.9+/-0.3 for whole-body rotation about the CO bond axis in contrast to previous analysis of static quadrupole echo (QE) line shapes which gave Ea = 22.3+/-0.3 kJ/mole and ln(A) = 24.8+/-0.6 for the same sample. The underlying homogeneous linewidths of OMAS spectra are much smaller than those of QE spectra, and this provides higher precision and less systematic error in the determination of rates.     

Landau model of the direct isotropic to smectic-C*A phase transition in antiferroelectric liquid crystals

An orientational order parameter is proposed for the isotropic to smectic C*A phase transition in antiferroelectric liquid crystals. A phenomenological theory is developed to describe the direct isotropic to smectic-C*A phase transition on the basis of a free energy expansion. The free energy is written in terms of the coupled order parameters including the antipolar correlations. We present a detailed analysis of the different phases that can occur and analyze the question: under which conditions a direct isotropic to smectic-C*A phase transition is possible when compared to other phase transitions? The theoretical results are compared with experimental results.     

Kerr effect of low melting nematic liquid crystals with negative dielectric anisotropy

The two low melting nematic liquid crystals, 2-chloro-4-heptylphenyl 4-pentylbicyclo[2,2,2]octane-1-carboxylate (7CP5BOC) and 2-chloro-4-heptylphenyl 4-heptylbicyclo[2,2,2]octane-1-carboxylate (7CP7BOC) have been investigated to determine their electro-optical behaviour and third order non-linearity by the static Kerr effect method. Both liquid crystals are laterally substituted by a single chlorine atom located close to the ester linking group. The temperature dependence of the electric Kerr constant in the isotropic phase and the pretransitional behaviour have been investigated for these low birefringence nematic liquid crystals in the isotropic phase. Both the compounds, with negative dielectric anisotropy, have a positive Kerr constant. The Landau-de Gennes model was obeyed for these compounds.     

Stiffening of Semirigid Polymer Chains Before the Isotropic-Aniostropic Transition

Introduction　　 The conformation of the synthetic polypeptide such as poly(γ-benzyl L-glutamate)(PBLG) or poly(β-phenethyl L-aspartate) (PPLA) is maintained in theα-helix in numerousorganic solvents that supports the intramolecular hydrogen bonding.Above a critical volumefraction of a polymer,the randomness of the orientation along the long axes of the macro-molecule is lost and a lyotropic liquid crystal is formed[1 ] .　　 If the polypeptide is dissolved in a binary solvent mixture c…     

Influence of the order parameter and of the director orientation on the surface tension of free nematic films

A radial hydrodynamic flow in the nematic phase of free, suspended cylindrical films of 4-n-heptyl- and decyloxybenzoic acid and in 4,4'-di-n-heptyl-oxyazoxybenzene has been observed. The flow starts about 7°C before the phase transition into the smectic C phase. Under the same experimental conditions such a hydrodynamic flow is not established in free nematic films of 4-n-heptyl-and octyloxy-4'-cyanobiphenyl, 4,4'-dimethoxyazoxybenzene and N-(4-ethoxybenzylidene)-4'-n-butylaniline after the completion of the transition from the isotropic liquid to the nematic phase. The observed hydrodynamic flow is explained by a non-linear temperature dependence of the surface tension.     

Novel method for order parameter of ferroelectric liquid crystals by image analysis

A novel method for temperature dependence of order parameter calculations at different wavelengths through birefringence measurements for ferroelectric liquid crystals (S)-(-)-2-methylbutyl 4′-(4″-n-alkanoyloxy benzoyloxy) biphenyl-4-carboxylates (where n?=?16 and 18) has been proposed using image analysis and linear regression method of Kuczynski equation. Measurements are made for its liquid crystal phases (SmecticC*, SmecticA) to the isotropic phase of the sample. The birefringence values of the samples at different wavelengths are obtained from the textural analysis of liquid crystal as a function of temperature using MATLAB software.     

Simulation of the rheological properties of liquid media containing solid anisometric particles

Theoretical simulation of the rheological properties of liquid media containing solid anisometric particles is performed. Procedures for the calculation of the degree of ordering in the nematic systems under equilibrium conditions and their rheological characteristics are proposed. Earlier developed notions of the calculation procedures for the properties of systems considered are analyzed and corrections to the theory are substantiated. Based on developed model notions, equilibrium and transport equations are derived and a program for their numerical solution is proposed. Model calculations of the lines of phase transition and the ordering of systems containing isotropic and nematic phases, as well as anisotropic (in orienting field) and effective dynamic viscosities under the conditions of free flow of such systems, are carried out at various concentrations and geometry of anisometric particles. A comparison of calculated and experimental data for the solutions of polymer-salt compositions with induced chain rigidity demonstrates the adequacy of the model proposed.     

Two-dimensional double-quantum 2H NMR spectroscopy in the solid state under OMAS conditions: correlating 2H chemical shifts with quasistatic line shapes.

Solid-state 2H NMR spectroscopy is a well-established and versatile method to study molecular orientation and dynamics in selectively deuterated samples. Herein, we introduce a 2D 2H double-quantum (DQ) NMR experiment performed under fast magic-angle spinning with a slight offset of the magic angle (OMAS). The experiment combines 2H chemical-shift resolution with DQ-filtered quasistatic 2H line shapes. In this way, it is possible to separate 2H resonances and to independently determine 2H quadrupole couplings at multiple sites. While 2H chemical shifts are resolved in the 2H DQ dimension, the quadrupole parameters can be obtained from characteristic line shapes which are reintroduced in the second dimension by the magic-angle offset. The 2D 2H DQ OMAS experiment is demonstrated on L-histidine which was deuterated at multiple sites by recrystallisation from D2O.     

Polymerization of vinylpyridinium salts. III. Further studies on the interfacial and isotropic polymerization of 4-vinyl-N-methylpyridinium methylsulfate

In the present study the polymerization of 4-vinyl-N-methylpyridinium methylsulfate at liquid–liquid interfaces is investigated. Various experimental procedures that may favor the interfacial mode of polymerization over the isotropic one are discussed. The effectiveness of the methods was evaluated by physical characterization studies and particularly polymer tacticity. This last property is directly associated with the achieved organization of the monomers at interfaces. ¹³C-NMR technique was used for tacticity determination. According to tacticity, random polymers are primarily obtained that result from an isotropic polymerization in the aqueous phase, where the monomer is extremely soluble. The small predominance of the syndiotactic triad over the isotactic one is explained by the occurrence of an interfacial polymerization induced by the orientation of the monomers. A polymerization model is proposed that justifies the stereospecificity modification of polymerization.     

Optical activity and light scattering in the isotropic and smectic A* phases of a highly chiral smectic liquid crystal

Abstract

The results of optical activity measurements on the smectic A* phase of 1-methylheptyl 4′-[(4-n-tetradecyloxyphenyl)proprioloyloxy]biphenyl-4-carboxylate (14P1M7) and the chiral nematic phase of a chiral–racemic mixture of S-4-(2-methylbutyl)phenyl 4-decyloxybenzoate (CE6) are shown to be extremely similar. This is in full agreement with the proposed model of the A* phase as a twistgrain-boundary (TGB) phase. In addition, new light scattering measurements using circularly polarized light in a back-scattering geometry yield information on the fluctuations in the isotropic phase. Unlike in chiral nematics where only one structural mode is affected, the data show a strong deviation from the normal temperature dependence near the isotropic–smectic A* transition for two structural modes. Possible reasons for this behaviour in highly chiral smectic liquid crystals are discussed.     

Synthesis and thermal properties of side-chain liquid crystalline polyethers with racemic and chiral backbone

Racemic and chiral [(4-cyano-4′-biphenyl)oxy] and [(4-methoxy-4′-biphenyl)oxy]methyloxiranes were prepared from racemic epichlorohydrin or racemic and chiral glycidols and polymerized in dimethylsulfoxide (DMSO) with Bu^tOK as the initiator system. Initial phase identifications were made by differential scanning calorimetry (DSC) and optical microscopy techniques and confirmed by X-ray diffraction measurements. Upon heating, all the monomers show only a crystal–isotropic phase transition. The racemic and chiral [(4-cyano-4′-biphenyl)oxy]methyloxiranes exhibit a nematic and a cholesteric monotropic phase, respectively. Methoxybiphenyl substituted polyethers are crystalline and insoluble in virtually all common solvents. Cyanobiphenyl substituted polyethers are soluble under the same experimental conditions and show enantiotropic liquid crystalline properties. The racemic polymer exhibits a nematic phase, while the optically active polymer forms a cholesteric phase.     

Unusual dynamic behavior in the isotropic phase of banana mesogens detected by 2H NMR line width and T2 measurements

In this work the first experimental observation of a peculiar behavior in the isotropic phase of liquid crystals by means of 2H NMR is reported. In particular, two five-ring banana-shaped mesogens, the 1,3-phenylenebis{4,4'-(11-undecenyloxy)benzoyloxy}benzoate (Pbis11BB) and its 4-chloro homologue (ClPbis11BB), selectively deuterium labeled on their central rings, are the subject of our investigation. The dynamic behavior of the two liquid crystals was studied in their isotropic phases and in the nematic phase of ClPbis11BB by means of 2H NMR line width and spin-spin relaxation time (T2) analysis. The results obtained reveal that the unusual line broadening observed in the 2H NMR spectra in the isotropic phase, even far above the isotropic phase-mesophase transition, has a homogeneous nature, thus indicating the presence of reorientational motions much slower than in conventional isotropic liquid-crystalline phases.     

低相变温度卟啉液晶的合成与表征

合成了4个系列的8种对脂肪酰氧基苯基卟啉化合物。通过紫外-可见光谱、红外光声光谱、元素分析、核磁共振氢谱和质谱等分析手段进行了表征,并用DSC和偏光显微镜研究了它们的液晶行为。     

A mode coupling theory description of the short- and long-time dynamics of nematogens in the isotropic phase

Optical heterodyne-detected optical Kerr effect (OHD-OKE) experimental data are pre-sented on nematogens 4-(trans-4-n-octylcyclohexyl)isothiocyanatobenzene (8-CHBT), and 4-(4-pentyl-cyclohexyl)-benzonitrile (5-PCH) in the isotropic phase. The 8-CHBT and 5-PCH data and previously published data on 4-pentyl-4-biphenylcarbonitrile (5-CB) are analyzed using a modification of a schematic mode coupling theory (MCT) that has been successful in describing the dynamics of supercooled liquids. At long time, the OHD-OKE data (orientational relaxation) are well described with the standard Landau-de Gennes (LdG) theory. The data decay as a single exponential. The decay time diverges as the isotropic to nematic phase transition is approached from above. Previously there has been no theory that can describe the complex dynamics that occur at times short compared to the LdG exponential decay. Earlier, it has been noted that the short-time nematogen dynamics, which consist of several power laws, have a functional form identical to that observed for the short time behavior of the orientational relaxation of supercooled liquids. The temperature-dependent orientational dynamics of supercooled liquids have recently been successfully described using a schematic mode coupling theory. The schematic MCT theory that fits the supercooled liquid data does not reproduce the nematogen data within experimental error. The similarities of the nematogen data to the supercooled liquid data are the motivation for applying a modification of the successful MCT theory to nematogen dynamics in the isotropic phase. The results presented below show that the new schematic MCT theory does an excellent job of reproducing the nematogen isotropic phase OHD-OKE data on all time scales and at all temperatures.     

New insights into the transitional behaviour of methyl-6-O-(n-dodecanoyl)-α-D-glucopyranoside using variable temperature FTIR spectroscopy and X-ray diffraction

The liquid crystalline behaviour of methyl-6-O-(n-dodecanoyl)-α-D-glucopyranoside, 1, has been characterised using X-ray diffraction and variable temperature Fourier transform infrared (FTIR spectroscopy). 1 exhibits a monotropic interdigitated smectic A phase consisting of bilayers in which the alkyl chains are overlapped. The crystal–isotropic transition is accompanied by a pronounced decrease in the strength of the hydrogen bonding network involving the sugar groups resulting in a marked change in the environment of the alkyl chains. The isotropic phase consists of disordered smectic-like domains stabilised via hydrogen bonding between the sugar groups. At the transition to the smectic A phase, a subtle change in hydrogen bonding is observed which is manifested by a change in the temperature dependence of the OH stretching peak position in the FTIR spectrum. On crystallisation, the strong hydrogen bonding network is re-established accompanied by a change in the conformational distribution of the alkyl chains. A model is proposed in which a combination of hydrogen bonding (enthalpic effects) and conformational arrangements (entropic effects) promotes initially the formation of smectic-like domains in the isotropic phase and subsequently stabilises the smectic A phase by inhibiting the microphase separation leading to the crystal phase.     

Broadband dielectric relaxation study of 6CB and 6CB-aerosil dispersions in the nematic and isotropic phases

Experimental studies of dielectric relaxation in the nematic and isotropic phases of 6CB (4-hexyl-4′-cyanobiphenyl) and two mixtures of 6CB perturbed by 160 Å hydrophobic silica aerosil are presented. The measurements have been made from 1 MHz to 1 GHz in the temperature range from 19 to 40°C. For bulk 6CB, the dynamic aspect in the isotropic phase as well as the principal dielectric permittivities ε ∥ * (ω) and ε ┴ * (ω) in the nematic phase have been fully explored. For the mixtures, measurements on the isotropic phase and also on homeotropically aligned samples in the nematic phase have been made, and these results are systematically compared with those for the bulk. The possible molecular reorientational movements corresponding to the different absorption domains in the dielectric spectrum are discussed and compared with the previously proposed interpretations.     

Phase diagram and electro-optical Kerr effect of nematic mixtures containingtolane and biphenyl cores

An experimental phase diagram for two types of nematic liquid crystals and their binary mixtures was established by polarised optical microscopy and differential scanning calorimetry methods. The mixtures comprised a tolane-base liquid crystal, 4-heptyl-3-fluoro-4-isothiocyanatotolane (7TOLF), and a biphenyl-base nematogen, 4-heptyl-3-fluoro-4-isothiocyanatobiphenyl (7BF). The static Kerr effect and third-order non-linearity were investigated for 7TOLF and 7BF and their nematic mixtures within the isotropic phase. Both the compounds have a positive and large Kerr constant. The second-order phase-transition temperatures, T?, were determined for these mixtures. The linear dependence of (T - T?)^??1 on the Kerr constant is found to be in good agreement with the predications of the Landau–de Gennes model. The third-order non-linear susceptibility,χ⁽³⁾, values were determined for these mixtures.     

Radiofrequency‐induced temperature increase as a function of cross polarization contact time in 8CB

In this work we analyse the non‐negligible heating effect that can be produced by the radiofrequency during the contact time in Hartmann–Hahn cross polarization experiments in liquid crystals. We perform the analysis in the nematic phase of 4‐n‐octyl‐4′‐cyanobiphenyl (8CB). The heating effect is quantified through the chemical shifts of the carbon resonances, which move towards their respective positions in the isotropic phase when increasing the contact time. The calibration procedure involves an independent measurement of chemical shift dependence on externally controlled bath temperature. This variation was performed in the range between 300 and 313 K covering the smectic A and nematic mesophases. An overall heating rate of 0.16 K ms^?1 is observed during the contact time in a cross polarization experiment performed at ω₁/2π ? 80 kHz. This rate goes to approximately one‐half for ω₁/2π ? 60 kHz and becomes unobservable for ω₁/2π < 50 kHz under typical experimental conditions. Copyright © 2002 John Wiley & Sons, Ltd.     

Diffusion of Rod-Like Polypeptides in the Liquid Crystalline and Isotropic Phases as Studied by High Field-Gradient NMR Spectroscopy

In this lecture the measurements and analyses of the isotropic and anisotropic diffusion coefficients(D) of rod-like polypeptide such as poly(γ-L-glutamate)(PLG) with n-alkyl side chains, of which the main chain takes the α-helical conformation, as a function of the main chain length in the thermotropic and lyotropic liquid crystalline phases over a wide range of temperatures from 30 to 80°C by means of pulse high field-gradient spin echo ¹H NMR method have been introduced. In the anisotropic diffusion, the D_∥ value in direction parallel to the α-helical chain axis is found to be much larger than the D_⊥ value in direction perpendicular to the α-helical chain axis. The diffusion process is followed by the Kirkwood theory. Further, it is described that the diffusion in the nematic liquid crystalline phase is much slower than that in the isotropic phase.