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1.
P. C. HEMMER 《Molecular physics》2013,111(8):1153-1157
Phase equilibria for a solution of long rigid rods of diameters Di and lengths Li (i = 1,2) are studied analytically using Gaussian trial functions. The conditions on the molecular size parameters for a nematic—nematic phase transition to be possible are determined, and found to be satisfied everywhere except in a closed domain in the diameter ratio—length ratio plane. Information on the relative ordering of the two components is also provided. 相似文献
2.
E. M. Aver’yanov 《JETP Letters》1996,63(1):33-37
The first experimental data on the orientational ordering of a diskotic reentrant nematic N
D
are presented. The data show that the phase transition N
D
-D
h
(0, d) is a strong first-order transition with a large jump ΔS≃0.2 in the orientational order parameter S of the molecules. This indicates an anomalously strong coupling between the columnar and orientational ordering of the molecules
and explains the absence of fluctuational divergence of the elastic moduli K
11
and K
22
in the nematic phase near this transition.
Pis’ma Zh. éksp. Teor. Fiz. 63, No. 1, 29–32 (10 January 1996) 相似文献
3.
We discuss the symmetry properties and the macroscopic behavior of a nematic liquid crystal phase with D2d symmetry. Such a phase is a prime candidate for nematic phases made from banana-shaped molecules where the usual quadrupolar
order coexists with octupolar (tetrahedratic) order. The resulting nematic phase is nonpolar. While this phase could resemble
the classic D
∞h nematic in the polarizing microscope, it has many static as well as reversible and irreversible properties unknown to nonpolar
nematics without octupolar order. In particular, there is a linear gradient term in the free energy that selects parity leading
to ambidextrously helical ground states when the molecules are achiral. In addition, there are static and irreversible coupling
terms of a type only met otherwise in macroscopically chiral liquid crystals, e.g. the ambidextrous analogues of Lehmann-type effects known from cholesteric liquid crystals. We also discuss the role of hydrodynamic
rotations about the nematic director. For example, we show how strong external fields could alter the D2d symmetry, and describe the non-hydrodynamic aspects of the dynamics, if the two order structures, the nematic and the tetrahedratic
one, rotate relative to each other. Finally, we discuss certain nonlinear aspects of the dynamics related to the non-commutativity
of three-dimensional finite rotations as well as other structural nonlinear hydrodynamic effects. 相似文献
4.
ABSTRACTThe simple cubic spin-1 Ising model exhibits the ferromagnetic (F)–ferromagnetic (F) phase transition in the low temperature region for the interval 1.40 < d = D/J < 1.48 at k = K/J = –0.5. The degree of the F-F phase transition determines the special point on the (kBT/J, d) phase diagram. In this paper, the critical behavior of the F-F phase transition was investigated for different heating rates using the cellular automaton heating algorithm. The universality class and the type of F-F phase transition were analyzed using the finite-size scaling theory and the power law relations. The results show that the F-F phase transition may be the second order, the first order or the weak first order depending on the heating rate in the interval 1.40 < d < 1.48 for k = –0.5. 相似文献
5.
6.
A suspension of long rodlike colloids and long stiff polymers is modelled as a mixture of hard rods. The diameter of the colloid particle is finite, while the polymer is considered in the limit of zero diameter. Two types of first-order phase transition may occur in such mixtures: an isotropic-nematic phase transition if the density (or the pressure) is high enough, and a demixing transition involving two isotropic phases. The demixing transition has a critical point, and a triple point with one nematic and two isotropic phases may also exist. Phase diagrams are calculated. For the demixing isotropic-isotropic transition to be observed the ratio between the polymer length and the colloid length must exceed 0.36. 相似文献
7.
The phase transition mechanism in squaric acid is reviewed from the viewpoint of the pressure dependence of the O—H—O bond length (2R). The two-dimensional pressure effect on Tc is extracted from the hydrostatic pressure behaviour by taking account of the uniaxial stress applied perpendicular to the layer planes. The result shows that if 2R(H2SQ) is stretched to 2R(D2SQ) the phase transition temperature 7H c of H2SQ coincides with TD c of D2SQ. This result supports the suggestion that the phase transition mechanism is of the order-disorder type. 相似文献
8.
A.K. George Mariam Al-Hinai D.M. Potukuchi S.H. Al-Harthi C. Carboni 《Phase Transitions》2013,86(12):1037-1045
Thermal microscopy (TM) as well as low-frequency (LF) dielectric relaxation studies are carried out on a nematic liquid crystal (NLC), viz E7 (Merck Ltd, UK). The isotropic to nematic (IN) transition temperatureT IN determined by TM and LF-dielectric permittivity measurements agrees with the available data. Dielectric loss studies in the frequency region of 5–10?MHz indicate a relaxation (in the kHz region) akin to Debye type off-centered dispersion. The observed nematic relaxation is found to correspond to reorientation (about the short axis) of the nematic dipole to the external field. The temperature variation of the nematic relaxation frequencyf R is found to follow an Arrhenius shift, with an activation energy of 1.7?eV. Temperature variation of the dielectric strength (Δε = ε o ? ε∞) and the distribution parameter α in the nematic phase are discussed. The dynamic response of the nematic dipoles and growth of pre-transitional fluctuations are found to be nonlinear in the vicinity of the IN transition. The value of the exponent αeff = 0.072 indicates weak growth of transitional fluctuations across the IN transition. 相似文献
9.
Extensive molecular dynamics calculations have been used to study for the first time systematically the dependence of the self-diffusion coefficients Di (i = 1, 2, 3) in binary and ternary atomic isotopic mixtures on the particle mass ratios m*2 = m 2/m 1 and m*3 = m 3/m 1 at different reduced temperatures T* and reduced particle number densities n*, using a Lennard-Jones 12-6 potential and a hard-soft-spheres potential. In addition, the dependence of Di values of binary mixtures on the mole fraction x 1 = 1—x 2 was also investigated for some thermodynamic states. The calculated values of Di can be represented quantitatively by exponential estimates of the form Di = D* i (m*2)ex i in the case of binary mixtures and Di = D 0 i (m*2)ex i (m 3)ext i in the case of ternary mixtures. D 0 i are the self-diffusion coefficients in reference mixtures of mass ratios m*2 = m*3 = 1. The dependence of the exponents ex i (m*2, T*, n*, x 1) of binary mixtures on m*2, T*, n*, and x 1 and the dependence of the exponents ext i (m*2, m*3, n*) of equimolar ternary mixtures at T* = 1.8 on the exponents ex i of the constituent binary mixtures and on m*2, m*3, and n* are discussed. 相似文献
10.
The dependence of the coupling of orientational and translational order parameters on the nematic-smectic A tricritical point (TCP), observed through birefringence experiments in the pure compounds and in the binary mixtures in the homologous series of N(p-n-alkyloxybenzylidene)p-n-alkylanilines, results in a mean field tricritical exponent. The McMillan parameter, M = T NA/T NI (where T NA and T NI are the smectic A—nematic (AN) and the nematic—isotropic (NI) phase transition temperatures respectively), is used as an effective and relevant field variable in analysing the TCP. The observed linear dependence of the jump in the orientational order parameter with new density variables in the tricritical region is found to be consistent with the results obtained from both the Landau mean field and the decoupled mean field models. The width of the mean field tricritical region is discussed in terms of the McMillan parameter. Further, the McMillan parameter is observed to be characteristic of chemical composition at the NA tricritical point. 相似文献
11.
We numerically study one-parameter family of random single-cluster systems. A finite-concentration topological phase transition
from the net-like to the tree-like phase (the latter is without a backbone) is present in all models of the class. Correlation
radius index ν
B
of the backbone in the net-like phase; graph dimensions—d
min of the tree-like phase, and D
min of the backbone in the net-like phase appear to be universal within the accuracy of our calculations, while the backbone
fractal dimension D
B
is not universal: it depends on the parameter of a model.
The article is published in the original. 相似文献
12.
E. M. Aver’yanov 《Physics of the Solid State》2004,46(8):1554-1559
The orientational order parameter S of molecules in high-temperature discotic nematic liquid-crystal phases of triphenylene derivatives is investigated as a function of the length of side flexible molecular chains at different temperatures. It is established that the orientational order parameters S in the range of the transition from the nematic phase to the isotropic liquid phase (the N D -I transition) are smaller than those predicted from the molecular-statistical theory and computer simulation. It is shown that the N D -I transition is close to both the isolated Landau point and the tricritical point (regardless of the chemical structure of the molecules and the anisotropy of dispersion intermolecular interactions). Consistent explanations are offered for a number of experimental findings, such as the anomalously small changes in the enthalpy and entropy upon the N D -I transition (as compared to those revealed upon the N-I transition in calamitic nematic liquid crystals), the anomalously strong response of the isotropic phase of discotic nematic liquid crystals to external fields (thermodynamically conjugate to the order parameter S) and the long relaxation times of this response, and the formation of cybotactic discotic molecular clusters in the isotropic phase in the vicinity of the N D -I transition. 相似文献
13.
Akhileshwar Prasad 《Phase Transitions》2013,86(12):1072-1084
Refractive index measurements have been done on a polar–polar binary system, 4-cyanophenyl [4′(4″-n-heptylphenyl)] benzoate (7CPB)?+?4-cyanophenyl-4-nonylbenzoate (9.CN), exhibiting nematic–smectic Ad–re-entrant nematic phase sequence using thin prism technique. From refractive index as well as density data, the polarizability anisotropy in the mesomorphic state has been calculated from the standard Vuks model and the orientational order parameter (?P 2?) values have been determined. By measuring the refractive indices and density in the solid phase, the validity of the Haller's extrapolation method for systems having higher order phases has been verified. The temperature variation of the density, birefringence, and ?P 2? values at the nematic–smectic Ad and smectic Ad–re-entrant nematic transitions for all the mixtures are found to be continuous. The experimental ?P 2? values have been compared with our previous X-ray data and also those calculated from the Luckhurst and Timimi model. 相似文献
14.
We have investigated the structural change of dipolar hard sphere fluid while we change the dipole from an idealised point dipole (pDHS fluid) to a physically more realistic extended dipole (eDHS fluid) by increasing the distance d of the two point charges ±q while keeping the dipole moment μ = qd fixed. We discuss our results on the basis of the first- and second-rank orientational order parameters, angular distribution functions, chain-length distributions, and snapshots. At a low density, we have found chain formation with longer chains as the distance d is increased. At a high density, we have found phase transition from an orientationally ordered ferroelectric nematic phase (at low d) into an isotropic liquid containing chains (at large d). 相似文献
15.
Connecting two columnar phase forming discotic mesogens via a short rigid spacer leads to the formation of a π-conjugated
discotic dimer snowing discotic nematic (N
D) phase. Attaching branched-alkyl chains directly to the core in hexaalkynylbenzene resulted in the stabilisation of ND phase at ambient temperature. Pentalkynylbenzene derivatives possessing a combination of normal- and branched-alkoxy chains
display a very broad ND, phase which is stable well below and above the room temperature. 相似文献
16.
The molecular dynamics (MD) simulation, based on a realistic atom-atom interaction potential, was performed on 4-n-pentyl-4'-cyanobiphenyl (5CB) in the nematic phase. The rotational viscosity coefficients (RVCs) γ
i, (i = 1, 2) and the ratio of the RVCs λ = - γ
2/γ
1 were investigated. Furthermore, static and frequency-dependent dielectric constants and ε were calculated using parameters obtained from the MD simulation. Time correlation functions were computed and used to determine
the rotational diffusion coefficient, D
⊥. The RVCs and λ were evaluated using the existing statistical-mechanical approach (SMA), based on a rotational diffusion
model. The SMA rests on a model in which it is assumed that the reorientation of an individual molecule is a stochastic Brownian
motion in a certain potential of mean torque. According to the SMA, γ
i are dependent on the orientational order and rotational diffusion coefficients. The former was characterized using: i) orientational
distribution function (ODF), and ii) a set of order parameters, both derived from analyses of the MD trajectory. A reasonable
agreement between the calculated and experimental values of γ
i and λ was obtained.
Received 22 March 2000 and Received in final form 8 October 2000 相似文献
17.
Einstein equations are derived for D-dimensional space-time that spontaneously compactify to the product M4 × Πi = 1α Mdi in which the metric is taken to be of the generalized Robertson-Walker form. Cosmological solutions for these equations are studied with power law, oscillatory and exponential behaviour for the D-dimensional Einstein-Maxwell, N = 2, D = 10 and N = 1, D = 11 supergravity models. In the Einstein-Maxwell case the presence of a cosmological constant forces the extra dimensions to be static. Nevertheless, it is possible to find solutions with vanishing effective 4 dimensional cosmological constant with an expanding 4-dimensional space-time. In the supergravity models the requirement of having compact extra dimensions restricts the solutions to have expansion only in the 4-dimensional space-time. Matter contribution is added to the energy-momentum tensor in an attempt to find new solutions. 相似文献
18.
Wen‐xiong Shi Yun‐yan Li Jun Xu Gui‐qiu Ma 《Journal of Macromolecular Science: Physics》2013,52(6):1115-1126
The relationship of the phase morphology of polypropylene/polyethylene‐terephthalate (PP/PET) blends and their corresponding compatibilized blends with composition was investigated using digital image analysis. A diameter, d g , was defined and calculated to discuss the phase morphology of this polymer blend system. A figure‐estimation method was introduced to determine the width of the distribution of d g . Based on the method, it is proven that the distribution of d g obeys a log‐normal distribution and consequently, the distribution width, σ was calculated. Further, a fractal dimension, D f , was introduced to describe the distribution of main sizes of the particles of the dispersed phase. The results showed that, while d g increased with the concentration of the dispersed phase, σ and D f show different dependence relations on composition;σ increases monotonously but D f shows a maximum at a PET content of 30%, indicating that, even though the whole size distribution is much broader, the distribution of the main body of size becomes more uniform when the content of PET is less than 30%. 相似文献
19.
Experimental studies of the dielectric properties in the nematic and isotropic phases of 4-cyanophenyl-4′-n-octylbenzoate
(8CPB) have been performed in the frequency range from 50 kHz to 100 MHz. The relaxation process related to the rotation around
the short molecular axis has been analysed. The results obtained have revealed that in the vicinity of the I-N phase transition,
the molecular subdiffusional rotation process occurs. It is connected with the pretransitional effects which are observed
both in the static and dynamic dielectric measurements. On the basis of the temperature dependence of the relaxation time
in the nematic and isotropic phases, the orientational order parameter 〈P2〉 has been determined. The values of 〈P2〉 obtained in this way have been compared with those evaluated from the measurements of the polarized electronic absorption
of the dichroic dye, dissolved as guest probe in the mesogenic host. 相似文献
20.
Effects of fluorination and chain length of six bi-cyclohexyl phenyl compounds (3ccp-f, 3ccp-ff, 3ccp-fff, 5ccp-f, 5ccp-ff, 5ccp-fff) on their physical properties have been investigated. All the compounds exhibit nematic phase over a wide range of temperatures. Molecular mechanics calculation reveals that dipole moments of the molecules increases (1.93D to 3.37D) with fluorination in both propyl- and pentyl-based systems; increment in f to ff derivatives is more than in ff to fff derivatives. However, no change in dipole moment is observed with increased chain length. In the nematic phase, average value of dielectric constant (?avg) is less than the extrapolated values of ?iso in isotropic phase. Increased fluorination also decreases splay elastic constant, and thus faster response is expected in triply fluorinated compounds. 相似文献