共查询到20条相似文献,搜索用时 0 毫秒
1.
R. Somashekar H. Somashekarappa A. P. Divya D. Revannasiddaiah M. S. Madhava 《Pramana》1999,52(1):67-74
The existing methods for the determination of the orientational distribution functionf(β) in the nematic liquid crystals using X-rays have been reviewed. A simple Fourier method which givesf(β) in terms of the measured intensity is analysed. Using this distribution function, the accuracy with which the order parameters
could be evaluated is discussed and the results show the elegance of the Fourier method used here. 相似文献
2.
Gun Yeup Kim 《Optics Communications》2011,284(21):5157-5163
The reorientational order parameter dependent complex linear susceptibilities are derived based on the angular averaged linear response theory of the molecular polarizability, which includes the Vuks type equations of anisotropic refractive indices and absorbances. The polar angular absorbance at 633 nm wavelength is measured as a function of the concentration of fullerene C60 in E7 nematic liquid crystals (NLCs), from which the order parameter is deduced. It is shown that the order parameter has a linear dependence of dopant concentration. We also present a simple optical transmission method by adopting the closed-aperture Z-scan geometry under the influence of switching on and off the dc electric field to determine the optical relaxation time and the rotational viscosity coefficient of C60 doped E7 NLC. The experimental results are pretty well explained by using the Osipov-Terentjev theory of the rotational viscosity with the measured order parameter. 相似文献
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M.B. Lacerda Santos M.A. Amato 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,7(3):393-400
Using a light-beating technique we have measured the damping time of thermal fluctuations of the nematic director for the
so called cylindrical or calamitic nematic (NC) phase of the lyotropic system K-laurate/decanol/. By varying the scattering angle in suitable geometries, we have been able to estimate the orientational diffusivities associated
to the three pure deformations of splay, twist and bend. A former measurement made in the disk-like ND phase of the same system yielded a large deviation between the splay and twist diffusivities. The effect was then attributed
to induced flows, or backflow, which could be responsible for the reduction of the splay viscosity. In fact, this is the analogous
effect, for disks, to the one recognized since long time ago arriving for rod-like molecules in a classical nematic, though
in this case it is associated with bend deformations. The analogy comes about thanks to the interchange of the role played
by disks and cylinders for, respectively, splay and bend fluctuations.The measurements reported here provide a new test on
the applicability of the backflow model to a nematic system composed of micelles, that is, aggregates made of amphiphilic
(surfactant) molecules, in its cylindrical-like variant, i.e. the NC phase. In addition, the comparative study made here with the previous results existing in the literature for the ND phase, allows us to conjecture on structural issues concerning lyotropic nematics.
Received: 29 April 1998 / Revised: 19 August 1998 / Accepted: 31 August 1998 相似文献
5.
R. F. Rodríguez H. Híjar 《The European Physical Journal B - Condensed Matter and Complex Systems》2006,50(1-2):105-110
We use a fluctuating hydrodynamic approach to calculate the orientation
fluctuations correlation functions of a thermotropic nematic liquid crystal
in a nonequilibrium state induced by a stationary heat flux. Since in this
nonequilibrium stationary state the hydrodynamic fluctuations evolve on
three widely separated times scales, we use a time-scale perturbation
procedure in order to partially diagonalize the hydrodynamic matrix. The
wave number and frequency dependence of these orientation correlation
functions is evaluated and their explicit functional form on position is
also calculated analytically in and out of equilibrium. We show that for
both states these correlations are long-ranged. This result shows that
indeed, even in equilibrium there is long-range orientational order in the
nematic, consistently with the well known properties of these systems.We
also calculate the dynamic structure of the fluid in both states for a
geometry consistent with light scattering experiments. We find that as with
isotropic simple fluids, the external temperature gradient introduces an
asymmetry in the spectrum shifting its maximum by an amount proportional to
the magnitude of the gradient. This effect may be of the order of 7 per
cent. Also, the width at half height may decrease by a factor of about 10
per cent. Since to our knowledge there are no experimental results available
in the literature to compare with, the predictions of our model calculation
remains to be assessed. 相似文献
6.
A detailed investigation of the electro-optical switching parameters of an antiferroelectric liquid crystal (S)-(+)-4-(1-methylheptyloxycarbonyl) phenyl 4′-(6-octanoyloxyhex-1-oxy) biphenyl-4-carboxylate (abbreviated as S-7H6Bi) has been carried out. S-7H6Bi has paraelectric (SmA?) and ferroelectric (SmC?) phases in addition to antiferroelectric (SmC?A) phase. Switching parameters viz. spontaneous polarization and switching time were determined by polarization reversal method. The spontaneous polarization (Ps) is found to be highly temperature dependent and decreases with temperature. The maximum value of Ps is found to be ∼90 nC/cm2 whereas the switching time (ts) is found to be of the order 1-2 ms. The temperature dependent torsional viscosity (γt) is of the order 10 Pa sec. It increases with decrease in temperature. 相似文献
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《Phase Transitions》2012,85(4):297-308
A series of new symmetrical 2,5-bis(3,4-bis(n-alkoxy)phenyl)thiazolo[5,4-d]thiazoles (TTn) prepared via condensation of dithiooxamide with six different 3,4-di-n-alkoxybenzaldehydes were characterized by 1H NMR, FTIR, UV-Vis, and photoluminescence spectroscopy. Moreover, one compound lack of aliphatic chains, i.e., 2,5-diphenylthiazolo[5,4-d]thiazole (TT) was synthesized and characterized. The investigated TTn compounds emitted blue light. The effect of length of n-alkoxy (i.e., OC n H2 n +1, n?=?8, 10, 12, 14, 16, and 18) peripheral groups on thermal and mesomorphic behavior was investigated by means of differential scanning calorimetry, polarizing optical microscopy, and X-ray diffraction. Variable heating and cooling rates were used to study the liquid-crystalline properties of TTn. All compounds exhibited liquid-crystalline properties which self-organize into one or two smectic mesophases (SmB and SmE). 相似文献
9.
In this paper we study the macroscopic behavior of nematic side-chain liquid single crystal elastomers exposed to an external electric or magnetic field. For this purpose we use the framework of a continuum model. The geometries investigated comprise the bend and the twist geometry known from the classical Frederiks transition in low molecular weight liquid crystals. For the bend geometry we find a laterally homogeneous and a two-dimensional undulatory instability, which may compete at onset. In the case of the twist geometry three instabilities can occur at onset, two of which are two dimensional and clearly show undulations. As a major result we propose how the values of the twist coefficient K(2) and the values of the material parameters D(1) and D(2) connected to relative rotations between the director field and the polymer network can be determined from experimental observations. In addition, we explain why a twist experiment is probably the most suitable set-up in order to measure the material parameter D(1). 相似文献
10.
Symmetry considerations yield the general form, up to second order
terms, for the deformation elastic energy of a nematic monolayer,
composed by symmetric achiral molecules, on a rigid planar
substrate. The deformation energy contains an elastic contribution
linear in the deformation tensor, whose elements are the spatial
derivatives of the average molecular orientation. This linear
Lifshitz-invariant-like term can be responsible for a ground state
of the nematic monolayer periodically deformed if the relevant
elastic constant is stiffer than a critical value. The wave-length
of the modulation diverges at the transition threshold. We show
that only large variations of the tilt angle form stable states.
The effect of a destabilizing electric or magnetic field on the
layer is to induce (i) the transition towards the tilt-modulated
phase, while (ii) for higher enough values of the field the
modulation is destroyed. 相似文献
11.
A. Carvalho P.J. Sebastião A. Ferraz A.C. Ribeiro H.T. Nguyen 《The European physical journal. E, Soft matter》2000,2(4):351-358
In this work we present a proton NMR comparative study of the molecular order in the smectic C2 (), smectic C^* () and anticlinic smectic C phases ( and ) of two liquid crystalline compounds. The second moments of the experimental proton NMR spectra are well explained taking
into account two different molecular orientations models. From the analysis of our experimental results it was possible to
estimate the tilt angles for all the smectic C phases studied in this work.
Received 15 September 1999 and Received in final form 18 January 2000 相似文献
12.
在从头计算的基础上,利用两态模型对溶剂对N-{4-[(反式)-2-(4-硝基苯基)乙烯基]苯基}-N,N-二苯氨基分子双光子吸收特性的影响进行了理论研究.计算结果表明,溶剂对该分子的双光子吸收截面影响较大,双光子吸收截面随着溶剂极性的增加而增加. 相似文献
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Namık Özdemir Serkan Dayan Muharrem Dinçer Nilgün Ö. Kalaycıoğlu 《Molecular physics》2013,111(6):707-723
The Schiff base compound (E)-N-{2-[(2-hydroxybenzylidene)amino]phenyl}benzenesulfonamide has been synthesized and characterized by IR, NMR and Uv-vis spectroscopies, and single-crystal X-ray diffraction technique. In addition, quantum chemical calculations employing density functional theory (DFT) method with the 6–311++G(d,p) basis set were performed to study the molecular, spectroscopic and some electronic structure properties of the title compound, and the results were compared with the experimental findings. There exists a good correlation between experimental and theoretical data. Enol-imine/keto-amine tautomerization mechanism was investigated in the gas phase and in solution phase using the polarizable continuum model (PCM) approximation. The energetic and thermodynamic parameters of the enol-imine?→?keto-amine transfer process show that the single proton exchange is thermodynamically unfavored both in the gas phase and in solution phase. However, the reverse reaction seems to be feasible with a low barrier height and is supported by negative values in enthalpy and free energy changes both in the gas phase and in solution phase. The solvent effect is found to be sizable with increasing polarity of the solvents for the reverse reaction. The predicted nonlinear optical properties of the compound are found to be much greater than those of urea. 相似文献
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合成了水杨醛缩二乙基三胺 (配体 L,2 - [(N- Salicylideneamino) ethyl]amine)与 Cu( )形成的配合物[Cu L(H2 O) ]Cl,通过元素分析、红外光谱、X射线单晶衍射进行了结构分析。配合物为单斜晶系 ,空间群P2 (1) / c,晶体学结构数据 ,a=0 .912 3(5 ) nm,b=1.382 1(7) nm,c=1.0 82 9(5 ) nm,β=10 1.712 (7)°,V=1.336 9(11) nm3 ,Z=4 ,Dcalcd=1.6 0 6 mg/ m3 ,F(0 0 0 ) =6 6 8,R1=0 .0 377,w R2 =0 .10 18。结构分析表明 ,配合物通过氢键形成二聚结构 相似文献
15.
A method has been developed for calculating the expected fluorescence lifetime of the DPH
p
PC probe distributed between different membrane environments. We show how this method can be used to distinguish between lipid transfer and fusion between large unilamellar vesicles occurring in the presence of poly(ethylene glycol) (PEG). This application of the calculation took into consideration the heterogeneity of microenvironments experienced by the probe in a sample containing vesicle aggregates of different sizes. Assuming that the aggregate size distribution was a delta function of the aggregate size, comparison of the calculated and observed lifetimes yielded an estimate of the vesicle aggregate size. For vesicles of varying compositions in the presence of dehydrating concentrations of PEG, this method suggested that only small aggreggates formed. For vesicles that could be demonstrated by other means not to have fused, the data were consistent with lipid transfer occurring only between the outer leaflets of two to four vesicles, even at high PEG concentrations. For vesicles that could be demonstrated to fuse by contents mixing and size changes, the fluorescence lifetime data were consistent with lipid transfer between both the inner and the outer leaflets of two to four fused vesicles. At very high PEG concentrations, where extensive rupture and large, multilamellar products were previously observed, the lifetime data were consistent with much more extensive lipid transfer within larger aggregates. The agreement of predictions made on the basis of lifetime measurements with other observations attests to the validity of the fluorescence lifetime method. In addition, the model and data presented here provide evidence that fusion occurs between small numbers of PEG-aggregated vesicles before the removal of PEG. 相似文献
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The Schiff base compound 2-[(4-propylphenylimino)methyl]-4-nitrophenol has been synthesized and characterized by IR, UV–Vis, and X-ray single-crystal determination. The molecular geometry from X-ray determination of the title compound in the ground state has been compared using the Hartree–Fock (HF) and density functional theory (DFT) with the 6-31G(d) basis set. The calculated results show that the DFT and HF can well reproduce the structure of the title compound. The energetic behaviour of the title compound in solvent media was examined using the B3LYP method with the 6-31G(d) basis set by applying the Onsager and the polarizable continuum model (PCM). The results obtained with these methods reveal that the PCM method provides a more stable structure than Onsager's method. Using the TD-DFT method, electronic absorption spectra of the title compound have been predicted and good agreement with the TD-DFT method and the experimental determination was found. The predicted nonlinear optical properties of the title compound are much greater than those of urea. In addition, DFT calculations of the molecular electrostatic potential and NBO analysis of the title compound were carried out at the B3LYP/6-31G(d) level of theory. 相似文献
20.
The vibrational spectral analysis was carried out by using FT-Raman and FT-IR spectroscopy in the range 4 000~400 and 4 000~400 cm-1 respectively, for N-[4-[1-hydroxy-2-[(1-methyl ethyl) amino] ethyl] phenyl] methane sulfonamide (HPAEPMS) molecule. Theoretical calculations were performed by ab initio Density Functional Theory (DFT) method using 6-31G(d,p) basis set. The complete vibrational assignments of wavenumbers were made on the basis of potential energy distribution (PED). The results of the calculations were applied to simulated spectra of the title compound, which show excellent agreement. 相似文献