Nematic-smectic Phase Transition Research Articles

Critical linear thermal expansion in the smectic-Aphase near the nematic-smectic phase transition

Recent high-resolution x-ray investigations of the smectic- A (SmA) phase near the nematic-to-SmA transition provide information about the critical behavior of the linear thermal expansion coefficient alpha// parallel to the director. Combining such data with available volume thermal expansion alpha(V) data yields the in-plane linear expansion coefficient alpha(perpendicular) . The critical behaviors of alpha// and alpha(perpendicular) are the same as those for alpha(V) and the heat capacity Cp. However, for any given liquid crystal, alpha//(crit) and alpha(perpendicular)(crit) differ in sign. Furthermore, the quantity alpha// (crit) is positive for SmAd partial bilayer smectics, while it is negative for nonpolar SmAm monomeric smectics. This feature is discussed in terms of the molecular structural aspects of these smectic phases.

Making the core of a mesogenic particle bulkier could enhance mesophase stability

The liquid crystal phase behaviour of an idealized model, consisting of a linear array of nine soft repulsive spheres, has been studied by Onsager theory with Parsons scaling. Phase transitions have been calculated as a function of the contact distance of the central sphere, σ c, with respect to σ, the contact distance of the other beads. For both the isotropic–nematic and nematic–smectic phase transitions, it exists a σ* such that, for σ < σ c ⩽ σ*, the phase transition, at a fixed pressure, occurs at a higher temperature than in the case with σ c = σ; the opposite happens for σ c > σ*. These results could be of interest in the field of fullerene containing liquid crystals.

IONIC CONDUCTION IN NEMATIC AND SMECTIC A LIQUID CRYSTALS

Permittivity dispersion measurements in a very low frequency range were made on n-pentyl (5CB) and n-octyl 8CB cyano biphenyls, providing characterization of mobile ions by the diffusion coefficient, or the mobility, and their temperature dependence in the nematic phase and/or the smectic A phase. Ionic conduction was confirmed in the nematic and the smectic A phases from the continuity of these temperature-dependent properties in the isotropic, the nematic, and the smectic A phases with reasonable changes at the transition temperatures. The Walden rule was found to be applicable to the ionic conduction in the nematic phase of 5CB without a pre-transitional phenomemon near the nematic-smectic phase transition, when appropriate mean values are taken for both the anisotropic mobility of ions and the anisotropic viscosity of the liquid crystal substance. It was also pointed out that care should be taken for the applicability of the Walden rule to liquid crystals in the case when the anisotropy of ionic conduction is argued, especially for liquid crystal materials containing mobile ions with a far less anisotropic shape than the liquid crystal molecules.

Contact Angle between Smectic Film and Its Meniscus

We apply de Gennes model (Langmuir, 1990, 6, 1448−1450) for presmectic ordering to a freely suspended smectic film coupled to the meniscus. We show that the quartic term in the expansion of the free energy in power of the smectic order parameter cannot be neglected in the model in order to explain the full behavior of the contact angle, θm, between the film and the meniscus. As shown previously (Phys. Rev. E 2001, 63, 021705-1−021705-9), this contact angle is nonzero due to the disjoining pressure caused by the enhanced smectic ordering at the film surfaces. The temperature dependence of θm with a maximum above the bulk temperature of the nematic-smectic phase transition, TNA, is qualitatively reproduced by the model for all film thicknesses h = Nd, where N is the number of layers and d the smectic period, although some discrepancy between theory and experiment for the temperature at the maximum still remains. The apparent change of slope of the contact angle at TNA observed experimentally in thin films cannot be reproduced by the mean-field treatment of the model and presumably critical fluctuations in the meniscus and their influence on the behavior of the surface tension close to TNA have to be taken into account.

CRITICAL FLUCTUATIONS AND QUENCHED DISORDERED TWO-DIMENSIONAL CHARGE STRIPES IN LA5/3SR1/3NIO4

Using high-resolution x-ray scattering, we have demonstrated the existance of quenched disordered charge stripes in a single crystal of La 5/3 Sr 1/3 NiO 4 at low temperatures. Above the second-order transition critical scattering was observed due to fluctuations into the charge stripe phase. The charge stripes are shown to be two dimensional in nature both by measurements of their correlation lengths (ξ a ≈185 Å, ξ b ≈400 Å, ξ c ≈25 Å.) and by the critical exponents of the charge stripe transition. The charge stripes are disordered and that the length scale of the disorder is quenched. Detailed measurements of the correlation lengths at higher temperatures, just below the charge stripe melting temperature, display an anisotropic broadening indicative of an order-disorder transition. We ascribe these results to a smectic-nematic phase transition at approximately 225 K. Such electronic quantum liquid-crystal phase transitions have been theoretically predicted before, but these results provide the first experimental evidence for them in charge stripe nickelates.

Avoidance model for soft particles. II. Positional ordering of charged rods

The phase diagram of parallel, charged spherocylinders is computed. The topology of the diagram is found to be similar to the uncharged one, but there are several qualitative changes. Regions of phase coexistence are significantly narrower and positional ordering is stabilized by the electrostatic repulsions. The nematic phase occupies a very narrow zone. We suggest that soft repulsions between surfactant micelles may be responsible for the absence of a nematic phase in most surfactant systems. We also present comparisons with the observed nematic-smectic phase transition for fd and tobacco mosaic virus particles.

Smectic–nematic phase transition as wrinkling transition in a stack of membranes

We point out that the smectic–nematic phase transition may considered as a transition of a stack of membranes in 2+ ϵ dimensions, in which the layers become so wrinkled that they interpenetrate each other are no longer distinguishable.

Disorder effects in low concentration dispersions of small quartz spheres in cyanobiphenyl liquid crystals

A photopyroelectric study of the specific heat and the thermal conductivity of small quartz sphere (aerosil) dispersions in heptylcyanobiphenyl (7CB) and octylcyanobiphenyl (8CB) liquid crystals with a concentration rho(s)=0.005 g cm(-3) is reported. The thermal conductivity data show that, even at this low concentration, the elastic strain introduced in the liquid crystal by the aerosil particle is not completely annealed at the nematic-isotropic (NI) phase transition. In 8CB, annealing has been found at the smectic-nematic phase transition, while in the case of 7CB it occurs in the nematic phase also. Moreover, the depression of the NI transition temperature has been found to be much smaller than the one predicted by a random field model in which quenched disorder simply affects the average order of the sample and no effects of the elastic strain are considered.

Monte Carlo simulation of smectic liquid crystals and the electroclinic effect: The role of molecular shape

Using Monte Carlo simulation methods, we explore the role of molecular shape in the phase behavior of liquid crystals and the electroclinic effect. We study a "bent-rod" mesogen shaped like the letter Z, composed of seven soft spheres bonded rigidly together with no intramolecular degrees of freedom. For strongly angled molecules, we find that steric repulsion alone provides the driving force for a smectic-C phase, even without intermolecular dipole-dipole interactions. For weakly angled (nearly rodlike) molecules, we find a stable smectic-A (SmA) phase and a strong electroclinic effect with a saturation tilt angle of about 19 degrees. In the SmA phase we find evidence of vortexlike point defects. We also observe a field-induced nematic-smectic phase transition.

Nonequilibrium-molecular-dynamics investigation of the presmectic behavior of the viscosities of a Gay-Berne nematic liquid crystal.

Using the method of nonequilibrium molecular dynamics, the behavior of the Miesowicz, Helfrich, and Leslie viscosities as functions of temperature and density are investigated. In particular, attention is focused on the region immediately preceding the nematic-smectic phase transition. The Miesowicz viscosity eta1 for the orientation parallel to the direction of flow and the Helfrich viscosity eta12 are both found to increase rapidly as the phase transition is approached. In most of the cases investigated, a critical exponent nu=1 / 3 is found. Values closer to nu=1 / 4 are also found, but in these cases, the errors are sufficiently large for nu=1 / 3 to be possible. No nonregular behavior of the viscosity coefficients eta2, eta3, gamma1, and gamma2 in the presmectic region was detected.

Effect of chain flexibility on the nematic-smectic transition

The theory of nematic-smectic phase transition in the system of uniform semi-flexible chains with hard-core repulsion is presented. Both the general density-functional formalism the tube-model calculation show that the flexibility of the chains results in a strong first-order transition, in contrast to the common weak-cristallization scenario of the nematic-smectic transition in rigid rods. The calculated spinodal volume fraction of the uniform nematic phase and the period of the modulation instability are consistent with recent experimental results.

Isotropic–nematic–smectic: importance of being flexible

We represent a comprehensive discussion of the effect of chain flexibility on the isotropic–nematic and nematic–smectic phase transitions, based on the notion about the individual chain conformations in the nematic phase. Our approach yields “back-of-the-envelop” explanations for several universal features of these phenomena, such as weaker IN transition and much stronger NS transition for semi-flexible chains than for rigid rods.

First-order nematic-smectic phase transition for hard spherocylinders in the limit of infinite aspect ratio

We report Monte Carlo simulations of the nematic-smectic phase transition for a system of hard spherocylinders with infinite length-to-diameter ratio. A finite-size scaling analysis suggests that this system undergoes a first-order phase transition. When combined with other simulations of the phase behavior of spherocylinders, these results suggest that the nematic-smectic phase transition is first-order for all aspect ratios. This appears to rule out the possibility of a tricritical point predicted by several density-functional theories.

Nematic Bendsplay Elastisity of Cyanophenyl Compozmds with Different Terminal Groups

The elastic properties of nematic compounds with terminal cyanoaromatic and containing cycles aliphatic groups were investigates. The terminal group structure has essential influence on the bend K3 and splay K1 elastic constants and their temperature dependences. The ninth and tenth members of 4-cyanophenyl 4′-n-alkyloxybenzoate series (nCPh) and cyanobiphenyl ether with ethylenebutylcyclohexane terminal group show pretransitional divergence of K3 in the vicinity of the nematic-smectic phase transition. Outside pretransitional temperature region the alternatation from odd to even n in nCPh series is accompanied by positive jumps of the K3/K1 and change of the character of its temperature dependence. A striking unmonotonous temperature dependence of the elastic splay constant K1 was founded for cyanobiphenyl ether with butylcyclohexane terminal group. The occurrence of frozen striped texture in a smectic phase after the nematic-smectic transition in the presence of magnetic field was found for all nematics with the temperature divergence of K3. Distance between stripes was the less, the More magnetic induction.

Nematic-Smectic Transition of Semiflexible Chains.

The theory of the nematic-smectic phase transition of uniform semiflexible chains with hard-core repulsion is presented. The flexibility of the chains is shown to result in a strong first-order transition. The calculated spinodal of the uniform nematic phase and the period of the modulation instability are consistent with recent experiments.

Percus-Yevick approximation for fluids with spontaneous partial order: Results for a separable model.

Recently we have applied the Percus-Yevick approximation to nematic fluids with partial spontaneous order using a diagrammatic implementation of a Ward identity. In this paper we apply the method to study the isotropic-nematic phase transition of a separable model, where the interparticle potential independently depends on the spatial separation and the relative orientation of the particles. This approach allows us to study the transition directly without other approximations besides the Percus-Yevick closure itself. Previous works of the integral equation method on phase transitions were based on the stability criterion or coexistence condition derived from a truncated density functional expansion. By calculating the correlation functions of the isotropic phase and applying the stability criterion, we find that within the Percus-Yevick approximation there are no numerical solutions indicating an isotropic-nematic phase transition, in agreement with the work by Perera and co-workers [Mol. Phys. 60, 77 (1987); J. Chem. Phys. 89, 6941 (1988)]. With this approach, however, we can determine the orientationally dependent probability density \ensuremath{\rho} self-consistently and we find the orientationally partially ordered nematic phase within the Percus-Yevick approximation. With a general qualitative analysis, we show that the stability limit within the Percus-Yevick approximation is highly unstable numerically, which may explain why no numerical solutions reaching the stability limit have been found in previous works for either isotropic-nematic or nematic-smectic phase transitions. We also show analytically that the stability criterion can be derived from the Ward identity. \textcopyright{} 1996 The American Physical Society.

Dislocation loops in finite systems.

The distortions induced by dislocation loops are studied in finite systems. The influence of finite size, D, of the system, surface tension, and surface-bending elastic constant on the nematic-smectic (NA) phase transition is discussed. In thin freely suspended smectic liquid-crystal films the nematic-smectic transition temperature, ${\mathit{T}}_{\mathrm{NA}}$(D), is proportional to 1/ \ensuremath{\surd}D , providing the transition is initiated by the unbinding of the dislocation loops. The smectic phase is stabilized in films, i.e., ${\mathit{T}}_{\mathrm{NA}}$(D)g${\mathit{T}}_{\mathrm{NA}}$(\ensuremath{\infty}). The loop-unbinding mechanism for the NA transition is completely suppressed in films sandwiched between solid boundaries, because the distortion energy in this case is proportional to the area of the loop, while the loop configurational entropy is proportional to its length. For temperatures T\ensuremath{\ge}${\mathit{T}}_{\mathrm{NA}}$(\ensuremath{\infty}) the equilibrium size of the loop, ${\mathit{R}}_{\mathrm{eq}}$, is proportional to the distance between solid boundaries, D. It could grow to infinity only for D\ensuremath{\rightarrow}\ensuremath{\infty} and T\ensuremath{\ge}${\mathit{T}}_{\mathrm{NA}}$(\ensuremath{\infty}).

Dislocations in lamellar and liquid crystal films: Equilibrium location, edge profiles, and phase transitions.

A continuum theory of dislocations is applied to three different problems: edge profiles in thin diblock copolymer films deposited on a solid substrate, equilibrium location of the edge dislocation in lamellar systems near free surfaces, and nematic-smectic phase transition in thin freely suspended liquid crystal films. Because of the surface tension the edge profile at the air/diblock copolymer surface in thin films is very broad. In semi-infinite lamellar systems with small surface tension the dislocation is stabilized at a finite distance from the surface. In smectic films of thickness D the temperature of the unbinding transition for dislocation loops, ${\mathit{T}}_{\mathrm{N}\mathrm{\ensuremath{-}}\mathit{A}}$(D), is proportional to 1/ \ensuremath{\surd}D .

Difference in critical behaviour of the phase transitions nematic-smectic A1 and nematic-smectic Ad

A high resolution X-ray study is presented of the nematic to smectic Aj phase transition in BOPCBOB and of the nematic to smectic Ad Phase transition in a binary mixture of 7CB and BCB (7.6CB). The known critical exponents of the heat capacity of both these systems agree with the predictions for the 3D XY model. For BOPCBOB we find values for the susceptibility y and the parallel correlation length vi that also agree with the 3D XY model, thus confirming the predictions of de Gennes. However, the values y and vi of 7.6CB are larger than the 3D XY values. This indicates a possible difference in the critical behaviour of the nematic- smectic Ai and the nematic-smectic Ad Phase transition. In all cases vi is larger than v~, Furthermore it is shown how the values for the critical exponents depend on the specific form chosen for the structure factor,

Is the smectic-nematic transition in 8CB first or second order?

We present here measurements on the smectic-nematic phase transition in 8CB using the proton nuclear-magnetic-resonance technique. Our results show that this transition is first order rather than second order as some observations have suggested.