Nematic-isotropic phase transition in lyotropic liquid crystals

We propose a Landau-de Gennes phenomenological model to describe the nematic-isotropic phase transition in lyotropic liquid crystals. The possibility of a first or second order transition is explored by means of the variation of the concentration of surfactant. We show that a Landau point on the nematic-isotropic phase transition line can be achieved under certain conditions. The theoretical predictions are found to be in good qualitative agreement with available experimental results.     

Influence of non-mesogenic impurities on a nematic to isotropic phase transition

Non-mesomorphic solutes depress the normal nematic-isotropic transition temperature in liquid crystals. When non-mesomorphic solutes are added to a nematic liquid crystal, the nematic-isotropic transition temperature is depressed and a two phase region is formed due to the presence of impurities of the solutes. The present paper explains the formation of this two phase region by the Landau-de Gennes phenomenological theory, which agrees fairly well with the experimental observations. We also note that this two phase region indicates the tricritical behaviour of the nematic-isotropic phase transition and the phase diagram near the tricritical point is also obtained.     

Influence of dimerization on the nematic-isotropic phase transition in strongly polar liquid crystals

A molecular theory of the nematic-isotropic phase transition is developed for the fluid composed of rod-like particles with large longitudinal dipoles. The equilibrium between the monomers and dimers with antiparallel dipoles is explicitly taken into account and the concentration of dimers is determined self-consistently together with the nematic order parameters. We show that for small central dipole moments the nematic-isotropic transition temperature increases with the increasing dipole. This is in accordance with the results of previous approaches. By contrast, for large values of the dipole the transition temperature decreases due to the growing concentration of dimers. This result enables one to explain the results of recent computer simulations that reveal a destabilization of the nematic phase in a system of hard rods with large central longitudinal dipoles. The temperature variation of the concentration of dimers is also analysed and the sign of the concentration discontinuity at the transition point is correlated with the qualitative influence of dimers on the transition temperature.     

Kerr effect investigations in a liquid crystal

Measurements are reported for the d.c. Kerr response and the pretransitional behaviour of a new liquid crystal, 4-cyanophenyl 4-pentylbenzoate at and above the nematic-isotropic phase transition temperature. The real parts of the third order nonlinear susceptibilities chi⁽³⁾ at and above the nematic-isotropic transition temperatures are determined for the new liquid crystal from the electro-optic Kerr effect (EOKE) experiments. The variation of the Kerr constant with temperature is discussed on the basis of the Landau-de Gennes model of the nematic-isotropic phase transition. The susceptibility values are also compared with that of the well known liquid crystal 4'-pentyl-4-cyanobiphenyl (5CB). The effect of the linking group of the liquid crystal on the suceptibility value is also discussed.     

Nematic-isotropic phase transition of binary liquid crystal mixtures

We experimentally studied the nematic-isotropic phase transition of (a) binary mixtures consisting of nematic and racemic liquid crystals and (b) binary mixtures consisting of positive and negative dielectric liquid crystals. We observed that the phase transition temperature is very sensitive to the chemical structures of the constituent components. We also used Maier-Saupe theory to calculate the transition temperature of binary mixtures. By fitting the experimental data, we obtained the interaction coupling constant between the constituent components.     

Effect of pressure on liquid crystal dimers

We report high pressure investigations on a homologous series of liquid crystalline dimeric molecules in which the terminal chain length is kept constant but the length of the methylene spacer connecting the two mesogenic units is varied. We find that unlike the nematic-isotropic transition temperature and associated entropy change, there is no alternation in the slope of the nematic-isotropic phase boundary in the pressure-temperature plane as a function of the length of the spacer group in the molecule. By applying the Clausius-Clapeyron equation we conclude that the volume change at the transition should exhibit a strong odd-even effect. Measurements on the shortest homologue of the series, which is non-mesomorphic, show the expected result that the application of pressure induces mesomorphism in non-mesomorphic compounds.     

An NMR study on the shielding of the 129Xe isotope of xenon dissolved in thermotropic liquid crystals

The variation of the nuclear shielding of the ¹²⁹Xe isotope in natural xenon dissolved in various liquid crystals and liquid crystal mixtures has been studied over the temperature range from 300 to 360 K. The temperature dependence is linear in the isotropic phase of the liquid crystals. An abrupt change in the shielding is observed when passing through the nematic-isotropic and smectic A-nematic phase transitions as well as when the liquid crystal director rotates by 90° in the so-called critical mixture of ZLI 1167 and EBBA. This is interpreted as being mainly the consequence of the shielding anisotropy of xenon arising from the deformation of its electronic distribution. The shift changes observed for 4,4'-di-n-heptylazoxybenzene at the nematic-isotropic phase transition on the one hand and at the smectic A-nematic phase transition on the other are found to be opposite in sign, reflecting the change in the liquid crystal structure.     

Equilibrium statistics of irregularly branched and dendrimeric polymeric liquid crystals

In this paper we develop a self-consistent model for the equilibrium statistics of nematic branched polymeric liquid crystals in the mean-field approximation. We have solved the resulting system of equations numerically and find a nematic-isotropic phase transition. We find that the order-disorder transition temperature scales as a function of the bond continuation probability, or equivalently the molecular weight, with an exponent that depends on the interaction potential. These results are compared with the experimentally observed behaviour.     

Density functional theory study of the nematic-isotropic transition in an hybrid cell

We have employed the density functional theory formalism to investigate the nematic-isotropic capillary transitions of a nematogen confined by walls that favor antagonist orientations to the liquid crystal molecules (hybrid cell). We analyze the behavior of the capillary transition as a function of the fluid-substrate interactions and the pore width. In addition to the usual capillary transition between isotropiclike to nematiclike states, we find that this transition can be suppressed when one substrate is wet by the isotropic phase and the other by the nematic phase. Under this condition the system presents interfacelike states which allow us to continuously transform the nematiclike phase to the isotropiclike phase without undergoing a sharp phase transition. Two different mechanisms for the disappearance of the capillary transition are identified. When the director of the nematiclike state is homogeneously planar-anchored with respect to the substrates, the capillary transition ends up in a critical point. This scenario is analogous to the observed in Ising models when confined in slit pores with opposing surface fields which have critical wetting transitions. When the nematiclike state has a linearly distorted director field, the capillary transition continuously transforms in a transition between two nematiclike states.     

Equilibrium statistics of irregularly branched and dendrimeric polymeric liquid crystals

In this paper we develop a self-consistent model for the equilibrium statistics of nematic branched polymeric liquid crystals in the mean-field approximation. We have solved the resulting system of equations numerically and find a nematic-isotropic phase transition. We find that the order-disorder transition temperature scales as a function of the bond continuation probability, or equivalently the molecular weight, with an exponent that depends on the interaction potential. These results are compared with the experimentally observed behaviour.     

The swelling of nematic elastomers by nematogenic solvents

The swelling of a nematogenic elastomer by an isotropic solvent was earlier predicted (Macromolecules 25 , 445 (1992)) to be more complex than that of a classical elastomer. We extend here to the swelling by solvents having a nematic-isotropic transition in the pure phase in the region of that of the pure elastomer. The phase diagrams are qualitatively different from the first case, but still with the novel features of a triple point and a gel-gel biphasic gap reported in the initial study. Where the cross-coupling is stronger than the self-couplings we have an azeotropic point.     

Theory of the Nematic-Isotropic Transition in a Restricted Geometry

We discuss, using a Landaude Gennes formalism, the nematic-isotropic transition temperature for a system placed between two parallel plates, subject to identical homeotropic or homogeneous boundary conditions at each plate. The temperature at the phase transition may increase or decrease as the inverse sample thickness, D^-1, increases, depending on the nature of the boundary conditions. In all cases the transition terminates at a critical point for sufficiently large D^-1, beyond which the nematic and isotropic phases are no longer distinct. The phase transition temperature is well described by a liquid crystal analogy of the Kelvin equation which can be generalized to give an exact Clausius-Clapeyron relation. Under many circumstances the system behaves from a thermodynamic point of view as though it were in a bulk ordering field. The finite geometry restricts the growth of nematic or isotropic wetting films. We discuss the disjoining pressure experiment of Horn, Israelachvili and Perez [15]. Finally we place our work in the context of recent progress in the statistical mechanics of surfaces and systems in restricted geometries.     

Investigation of the nematic-isotropic transition of a liquid crystalline polymer and determination of molecular diffusion coefficients using gas chromatography

Inverse gas chromatography has been used to study the nematic-isotropic transition of a side chain liquid crystalline polymer (LCP). The mesogenic side groups are laterally attached to a polysiloxane backbone through a flexible spacer. The nematic-isotropic transition of this LCP coated onto a glass capillary column is detected by considering the variation with temperature of the retention volume and of the theoretical plate number for the several probes. The molecular diffusion coefficients, D, of naphthalene, fluorene, pyrene and o-terphenyl have been determined at different temperatures in the nematic phase of the LCP as well as in the isotropic melt. The values ranged between 10(-14) and 10(-12) m2 s(-1) for the polynuclear aromatic hydrocarbon probes tested.     

Characteristic behavior of short-term dynamics in reorientation for Gay-Berne particles near the nematic-isotropic phase transition temperature

A specific transition behavior was found in the tumbling motion near the nematic-isotropic phase boundary using molecular dynamics simulations of the Gay-Berne mesogenic model under isobaric conditions at a reduced pressure P* of 2.0. The relaxation time for the motion obtained from the second-rank orientational time correlation function and the rotational diffusion coefficient showed a clear jump at the nematic-isotropic phase transition temperature. Regardless of the temperature dependence of the relaxation time, the change in the rotational diffusion coefficient evaluated from the orientational order parameters and the relaxation time agreed qualitatively with that of real mesogens. The rotational viscosity coefficients gamma(1) and gamma(2) were obtained from the simulation data for the relaxation time for the short-term dynamics and for the rotational diffusion coefficients. gamma(1) was proportional to (2), where is the second-rank orientational parameter. Furthermore, the rotational behavior of the model was compared with that of the Debye approximation in the isotropic phase.     

Behaviour of the anchoring strength coefficient near a structural transition at a nematic-substrate interface

A nearly symmetric alkoxyphenylbenzoate monomer is found to exhibit a transition from perpendicular to tilted alignment at a lecithin-treated glass substrate several degrees below the nematic-isotropic phase transition. By means of the Freedericksz transition the coefficient B of the anchoring energy has been obtained. It is found that B rapidly decreases, and the tilt susceptibility increases on approaching this transition.     

刚柔相嵌液晶高分子的赝二级相变温度

讨论了刚柔相嵌液晶高分子的向列相－各向同性相转变与其分子结构的关系，给出了该一级相变的赝二级相变温度Ｔ与这类液晶高分子的液晶基元和间隔基的长度，柔顺性（相关长度）以及它们之间的相互作用的关系的解析表示式，分析了液晶基元与间隔基连接处的表观弯曲（接口效应）对Ｔ的影响，文中的结论与实验相符。     

刚柔相嵌液晶高分子的赝二级相变温度

讨论了刚柔相嵌液晶高分子的向列相一各向同性相转变与其分子结构的关系．给出了该一级相变的赝二级相变温度Ｔ＊与这类液晶高分子的液晶基元和间隔基的长度、柔顺性（相关长度）以及它们之间的相互作用的关系的解析表示式．分析了液晶基元与间隔基连接处的表观弯曲（接口效应）对Ｔ＊的影响．文中的结论与实验相符．     

Ordering of anisotropic polarizable polymer chains on the full many-body level

We study the effect of dielectric anisotropy of polymers on their equilibrium ordering within mean-field theory, but with a formalism that takes into account the full n-body nature of van der Waals (vdW) forces. Dielectric anisotropy within polymers is to be expected as the electronic properties of the polymer will typically be different along the polymer than across its cross section. It is therefore physically intuitive that larger charge fluctuations can be induced along the chain than perpendicular to it. We show that this dielectric anisotropy leads to n-body interactions which can induce an isotropic-nematic transition. The two body and three body components of the full vdW interaction are extracted and it is shown how the two body term behaves like the phenomenological self-aligning-pairwise nematic interaction. At the three body interaction level we see that the nematic phase that is energetically favorable is discotic, however, on the full n-body interaction level we find that the normal axial nematic phase is always the stable ordered phase. The n-body nature of our approach also shows that the key parameter driving the nematic-isotropic transition is the bare persistence length of the polymer chain.     

Behavior of liquid crystals confined to mesoporous materials as studied by 13C NMR spectroscopy of methyl iodide and methane as probe molecules

The behavior of thermotropic nematic liquid crystals (LCs) Merck Phase 4 and ZLI 1115 confined to mesoporous controlled pore glass materials was investigated using 13C nuclear magnetic resonance spectroscopy of probe molecules methyl iodide and methane. The average pore diameters of the materials varied from 81 to 375 A, and the temperature series measurements were performed on solid, nematic, and isotropic phases of bulk LCs. Chemical shift, intensity, and line shape of the resonance signals in the spectra contain lots of information about the effect of confinement on the state of the LCs. The line shape of the 13C resonances of the CH3I molecules in LCs confined into the pores was observed to be even more sensitive to the LC orientation distribution than, for example, that of 2H spectra of deuterated LCs or 129Xe spectra of dissolved xenon gas. The effect of the magnetic field on the orientation of LC molecules inside the pores was examined in four different magnetic fields varying from 4.70 to 11.74 T. The magnetic field was found to have significant effect on the orientation of LC molecules in the largest pores and close to the nematic-isotropic phase transition temperature. The theoretical model of shielding of noble gases dissolved in LCs based on pairwise additivity approximation was utilized in the analysis of CH4 spectra. For the first time, a first-order nematic-isotropic phase transition was detected to take place inside such restrictive hosts. In the larger pores a few degrees below the nematic-isotropic phase transition of bulk LC the 13C quartet of CH3I changes as a powder pattern. Results are compared to those derived from 129Xe NMR measurements of xenon gas in similar environments.     

N.M.R. investigations and the molecular field theory of nematic mixtures

N.M.R. measurements have shown that the mixture E5 can be described by a single order parameter. Based on molecular field theory the nematic-isotropic transition and the temperature dependence of the order parameter in the nematic phase are considered for a binary mixture of nematogens. Guided by the results of the N.M.R. measurements the binary mixture is treated as an effective medium characterized by a single order parameter. Soft attractive forces are taken into account as well as the excluded volume. The coexistence of nematic and isotropic phases in the phase transition region is discussed in detail.