Smectic A-nematic Phase Transition Research Articles

Investigation on the critical behaviour in the vicinity of the smectic-A to re-entrant nematic-phase transition

ABSTRACT The study focused on the binary system 6OCB and 8OCB, which is well-suited for investigating stable re-entrant nematic-phase transitions. High-resolution measurements of heat capacity and birefringence are used to analyse critical behaviours near the nematic – smectic-A (N-SmA) and the smectic-A – re-entrant nematic (SmA-) phase transitions. The critical behaviour of these transitions has been explored with the aid of a differential quotient extracted from the birefringencevalues. Simultaneously, critical exponents extracted from the specific heat capacity anomaly are also used to analyse the critical behaviour of the phase transition. The results indicate that the SmA to phase transition exhibits non-universal behaviour, with effective exponents falling between the tricritical (TCP) and 3D-XY values, similar to the N-SmA phase transition. The study reveals the existence of two tricritical points: one associated with the N-SmA transition and the other corresponding to the SmA- transition. This is supported by plotting the McMillan ratio against the critical exponents obtained from both birefringence and specific heat capacity anomaly. Again, with an increase in the SmA range, the SmA- transition tends towards the tricritical point (TCP), suggesting a relationship between the SmA range and the transitional behaviour.

Nematic to smectic-A phase transition in mixture of liquid crystal and nonmesogenic impurities: existence of tricritical point

ABSTRACTExperiments on the mixture of liquid crystals and nonmesogenic impurities showed the significant role of nonmesogenic impurities on the nematic–smectic-A phase transition. Using both Flory–Huggins theory of isotropic mixing and Landau–de Gennes theory, we present a phenomenological theory that discusses the role of such impurities on the nematic–smectic-A phase transition in a mixture of smectic liquid crystal and nonmesogenic impurities. We discuss the impact of nonmesogenic impurities on the order parameters, Frank elastic constants (splay and bend) and transition temperature of the nematic–smectic-A phase transition. Our theoretical results show that there exists a tricritical point for which the second-order nematic–smectic-A phase transition becomes first order at a tricritical point by increasing the concentration of nonmesogenic solute. We find a remarkable agreement between theoretical and experimental results.

PAb03 スメクティックA-ネマティック相転移点近傍での粘性と電気粘性効果

PAb03 スメクティックA-ネマティック相転移点近傍での粘性と電気粘性効果

Measurement of splay elastic constant and rotational viscosity of an induced nematic binary system

ABSTRACTWe investigated the temperature dependence of the splay elastic constant (K11) and rotational viscosity (γ1) of a binary liquid crystal system comprising 5-trans-n-butyl-2-(4-isothiocyanatophenyl)-1,3-dioxane (4DBT) and 4-cyano-4′-n-undecyloxy-biphenyl (11OCB), exhibiting induced nematic phase. Both the splay elastic constant and rotational viscosity increased following lowering of the temperature. In the vicinity of smectic A-nematic (SmA-N) transition, both the relaxation time and rotational viscosity exhibited a strong pretransitional behaviour. The critical exponent (ν) presented here led to valuable qualitative information about the pre-transitional behaviour of the rotational viscosity data near the SmA-N phase transition. The extracted ν values ranging between 0.336 and 0.352 are in fair agreement with those predicted in the de Gennes model as compared to that by the mean-field model.

Effect of the hockey-shaped mesogen as an additive on the critical behaviour at the smectic A to nematic phase transition

We present systematic experimental data on high resolution measurements of temperature dependence of optical birefringence (Δn) in a binary liquid crystalline system comprising calamitic 4-cyano-4′-octyloxybiphenyl (8OCB) and a hockey stick-shaped compound, 4-(3-decyloxyphenyliminomethyl) phenyl 4-decyloxycinnamate (SF7). The hockey stick-shaped mesogen shows a smectic A (SmA) phase in addition to two polymorphic tilted smectic phases. From an analysis of the Δn data, a quotient Q(T) has been derived which exhibits power law divergences both above and below the smectic A-nematic (SmA-N) phase transition. A noticeable shrinkage in width of the nematic range has been observed for a small increase in concentration of the angular mesogenic dopant. In the vicinity of the SmA-N phase transition, the values of effective critical exponent (α′), characterizing the critical fluctuation at this transition, have appeared to be in between those predicted by the 3D-XY and tricritical hypothesis. Furthermore, with increasing hockey stick-shaped molecular concentration, the SmA-N transition shows a tendency to be driven towards a first order nature.

On the phase transitions of 8CB/Sn2P2S6 liquid crystal nanocolloids.

Using differential scanning calorimetry measurements, the influence of Sn2P2S6 ferroelectric nanoparticles on the phase transition temperatures of the 8CB liquid crystal is studied. The spontaneous polarization, ionic and anchoring effects are discussed. For low concentration of dopant, the global effect leads to a decrease and an increase of the nematic-isotropic and the smectic A-nematic phase transition temperatures, respectively. For high concentrations, due to aggregates formation, the predominant anchoring effect induces a decrease of the both phase transition temperatures.

Critical behaviour of a polar-polar binary system from optical birefringence measurements

ABSTRACTHigh resolution measurements of optical birefringence (Δn) have been carried out to study the critical behaviour at the nematic-isotropic (N–I) and smectic A-nematic (SmA–N) phase transitions in a binary system composed of nonyloxycyanobiphenyl (9OCB) and heptylcyanobiphenyl (7CB). From a fitting procedure compatible with the weakly first order nature of the N–I transition, the order parameter critical exponent β at the N–I transition has been found to be in well agreement with the tricritical hypothesis. The pretransitional behaviour at the SmA–N transition has been investigated with the aid of a differential parameter extracted from Δn data and the yielded critical exponent α has been found to be non-universal in nature. A weak birefringence have been found to develop on approaching the N–I transition temperature, which is tentatively attributed to surface induced ordering. In this case, a treatment identical to that for the SmA–N transition yields a tricritical nature for the N–I transition in the investigated mixtures. Such an outcome has also appeared to be in well agreement with those obtained from the direct analysis of the Δn data (i.e. in parameterisation of β).

Effect of hockey-stick-shaped molecules on the critical behavior at the nematic to isotropic and smectic-Ato nematic phase transitions in octylcyanobiphenyl

In the field of soft matter research, the characteristic behavior of both nematic-isotropic (N-I) and smectic-A nematic(Sm-A N) phase transitions has gained considerable attention due to their several attractive features. In this work, a high-resolution measurement of optical birefringence (Δn) has been performed to probe the critical behavior at the N-I and Sm-A N phase transitions in a binary system comprising the rodlike octylcyanobiphenyl and a laterally methyl substituted hockey-stick-shaped mesogen, 4-(3-n-decyloxy-2-methyl-phenyliminomethyl)phenyl 4-n-dodecyloxycinnamate. For the investigated mixtures, the critical exponent β related to the limiting behavior of the nematic order parameter close to the N-I phase transition has come out to be in good conformity with the tricritical hypothesis. Moreover, the yielded effective critical exponents (α', β', γ') characterizing the critical fluctuation near the Sm-A N phase transition have appeared to be nonuniversal in nature. With increasing hockey-stick-shaped dopant concentration, the Sm-A N phase transition demonstrates a strong tendency to be driven towards a first-order nature. Such a behavior has been accounted for by considering a modification of the effective intermolecular interactions and hence the related coupling between the nematic and smectic order parameters, caused by the introduction of the angular mesogenic molecules.

Anomaly of Pretransitional Behavior at the Nematic–Smectic-A Phase Transition of Amphiphilic Liquid Crystals with a Hydrophilic Group

In order to clarify the origin of the X-ray diffraction peak corresponding to the smectic-like layer ordering appearing even in the nematic phase over a wide temperature range above the nematic-smectic A (NA) phase transition in liquid crystal (LC) molecules with a hydroxy group, we investigated the critical behavior of bend elastic constants and the correlation length of the smectic-like ordering in the N phase. It is found that cybotactic clusters with the transient layer ordering grow up extremely even far above the NA phase transition, and the critical exponent of the correlation length of the cybotactic clusters is estimated anomalously larger than that of conventional LC materials. Furthermore, we measured diffusion constants parallel and perpendicular to the director in the N phase, and concluded that cybotactic clusters with the smectic layer ordering create a finite potential barrier to prevent diffusion of the molecules parallel to the director as well as the true smectic A layer structure.

ASYMPTOTICS OF MINIMIZERS OF VARIATIONAL PROBLEMS IN A MULTI-CONNECTED DOMAIN ASSOCIATED WITH THE THEORY OF LIQUID CRYSTALS

We consider the asymptotic behavior of minimizers of the Ericksen functional with the Dirichlet boundary condition in a general domain. We examine the asymptotic behavior of minimizers of the functional as some of the elastic coefficients tend to the infinity, according to the de Gennes predictions with respect to nematic smectic-A phase transition.

Tricritical point induced by smectic ordering of a nematic gel

The authors study volume phase transitions of a nematic gel immersed in a liquid crystal (LC) solvent, which shows a second-order nematic-smectic A phase transition (NST). Combining Flory's elastic energy [Principles of Polymer Chemistry (Cornell University Press, Ithaca, 1953)] for a swelling of the gel with the McMillan model [Phys. Rev. A 4, 1238 (1971)] for smectic ordering, the authors calculate the equilibrium swelling of the gel and smectic order parameters as a function of temperature. The authors take into account an attractive interaction parameter c between the gel and LC solvents. On increasing the value of the coupling constant c, a second-order NST of the gel is changed to a first-order one and a continuous volume phase transition of the gel is changed to a discontinuous one. The authors find a tricritical point of the gel induced by smectic ordering.

How Do Defects Transform at the Smectic A-Nematic Phase Transition?

ABSTRACT An optical microscopy investigation of the SmA phase close to the nematic phase puts into evidence the interaction between focal conic domains on one hand, screw and edge dislocations on the other hand. We report here three different situations of such interactions. Our results point towards a dislocation model of the SmA → N transition.

Distorted states of smecticAin the vicinity of the nematic–smectic-Aphase transition

We propose a description of the nematic--smectic-$A$ phase transition in the framework of the concept of locality of the transformation properties pertaining to the order parameter. It is shown that in the presence of small deformations the local Landau potential of the smectic is equivalent to the Ginzburg-Landau potential. It describes the effect of screening of the stress field via the formation of a finite density of dislocations in the A-type smectic.

Orientational Order Parameter of a Binary Liquid Crystal Mixture Showing Induced Smectic Phase

Binary mixture of 4 - pentyl 4′ - cyanobiphenyl (5CB) and 4-n-pentyl phenyl 4 - n′ - pentyloxy benzoate (ME50.5) shows the presence of an induced smectic A phase. In the present work we report the density and refractive indices on this system in the entire composition range. The density and refractive indices are used to calculate molecular polarizability values parallel and perpendicular to the molecular long axes using Neugebauer's method. Orientational order parameters (OOP's) as a function of temperature and concentration are determined. The relative change in density at the smectic A-nematic phase transition shows a maximum near equimolar concentration. The orientational order parameter values and calculated packing fraction show a minimum around X ≈ 0.4 at a temperature T=35°C. Similar behaviour is also observed at a temperature T = 0.98TC. in the nematic phase where the induced smectic A phase is the precursor of the nematic phase. Average lateral distance (D) between the molecules is measured from the analysis of x-ray diffraction data. The variation of D with molar concentration in smectic A phase shows a maximum near equimolar concentration.

Effect of the temperature range of the nematic phase on the induction of a twist grain boundary phase in non-chiral liquid crystals

Recent experimental results suggest that the twist grain boundary phase may be induced in non-chiral smectic liquid crystals by confining the material in a twist cell in which the nematic directors at the two surfaces are perpendicular to each other. However, the effect of the temperature range of the nematic phase on the induction of the twist grain boundary (TGB) phase has not been studied to date. We have performed experiments on non-chiral liquid crystals having the nematic-smectic A (N-SmA) phase transition sequence and confined in a twist cell. It is observed that materials with a second order N-SmA phase transition show characteristics of a TGB phase, but a material with a first order N-SmA transition does not.

Effects of fluctuations in the orientational order parameter in the cyanobiphenyl (nCB) homologous series

Photopyroelectric measurements of the anisotropy in the thermal conductivity Deltak vs temperature in the nCB (n=5,ellipsis,9) series are reported. The data have been used to deduce the behavior of the orientational order parameter Q close to the nematic-isotropic (N-I) and smectic A-nematic (A-N) phase transitions, respectively. It has been shown that near the N-I transition the data for 5CB and 6CB are consistent with the so-called "tricritical hypothesis," which predicts beta=0.25. This is not true for 7CB and 8CB in which the order parameter exhibits a behavior that could be caused by the presence of fluctuations that become increasingly important when the transition temperature is approached. A very simple model, which takes into account the contribution of fluctuations to the orientational order, has been developed close to the A-N transition and it has been shown that it is in good agreement with the experimental results. A semiquantitative explanation for the observed behavior in compounds with different nematic range has been also given.

INFLUENCE OF PRESSURE ON SMECTIC A-NEMATIC PHASE TRANSITION

We propose a Landau-de Gennes phenomenological model to describe the pressure induced smectic A-nematic phase transition. The influence of pressure on smectic A-nematic phase transitions are discussed for varying coupling between orientational and translational order parameter with pressure. Increasing the pressure, the first order nematic-smectic A transition becomes second order at a tricritical point which agrees fairly well with available experimental results.

Thinning transitions and fluctuations of freely suspended smectic-A films as studied by specular and diffuse X-ray scattering

The fluctuations in freely suspended smectic-A films have been studied using specular and diffuse X-ray scattering. Such films thin layer-by-layer above the bulk transition temperature of the smectic-A–nematic phase transition. The electron density profiles obtained from specular reflectivity demonstrate that thinning originates from nematic layers being expelled from the interior of the film. In addition, using diffuse scattering the spectral dependence of the displacement–displacement correlation function could be measured down to molecular dimensions, with a (2+2) scattering geometry at a synchrotron source. At long in-plane length scales the fluctuations of the smectic layers were found to be conformal; i.e. top and bottom of the film fluctuated in unison. However, at increasing wave vector transfers, conformality was progressively lost.

Effect of the nematic range on the critical behavior and anisotropy of the heat transport parameters at the smectic-A–nematic phase transition

Effect of the nematic range on the critical behavior and anisotropy of the heat transport parameters at the smectic-<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>A</mml:mi></mml:math>–nematic phase transition

Viscosity coefficients near the nematic–smectic-A phase transition

Viscosity coefficients near the nematic–smectic-A phase transition