smectic-A Phase Research Articles

Dielectric studies and critical behaviour in the vicinity of nematic–isotropic phase transition of a smectogenic binary mixture showing induced nematic phase

The phase diagram of a binary mixture composed of compounds, one having NCS terminal group (4DBT, showing smectic A1 phase) and the other with CN terminal group (11OCB, showing smectic Ad phase), exhibiting induced nematic phase in a certain concentration range (0.100 < x4DBT < 0.951) is reported here. Results of the static dielectric parameters measurement on this binary system within the entire mesomorphic range are presented. Evidence of strong pretransitional behaviour near the nematic–isotropic (N–I) phase transition, indicating the influence of tricritical behaviour, is observed. Precise determination of discontinuity (ΔT) and the critical exponent (α) of N–I phase transition have been carried out. Moreover, the order parameter critical exponent β is correctly predicted by the tricritical hypothesis through the dielectric anisotropy data for all the investigated mixtures.

Synthesis and Mesomorphic Behavior of Calamitic Liquid Crystals with a Biphenyl Moiety

A new mesogenic homologous series having a biphenyl moiety has been synthesized by condensing 4-hydroxy-4′-nitrobiphenyl- and methoxy-substituted 4-n-alkoxy cinnamoyl chlorides, and their liquid crystalline properties have been studied. All the members of the series are enantiotropic liquid crystals. The methyl to n-pentyl derivatives exhibit both Smectic A (SmA) and Nematic (N) phases; the higher members, starting with the n-hexyl derivative show only a SmA phase. The plot of transition temperatures versus number of carbon atoms in the alkoxy chain exhibits zig-zag pattern for Sm-N and N-Isotropic (Iso) transition temperature curves. The average thermal stability is compared with other related homologous series. The introduction of polar nitro group increases significant intermolecular force of attraction which stabilizes the molecular orientation. This results into the increase in the thermal stability of the system.

Induced Thermotropic Smectic Phases in Binary/Ternary Mixture of Nonmesogenic Compounds

We report the results of our studies on the optical and thermal properties of the mixture of Sodium Lauryl Sulfate (SLS) and Dodecyl trymethyl ammonium bromide + Ethylene glycol (DTAB + EG) exhibits very interesting induced thermotropic smectic phases such as smectic-A, smectic-D, smectic-B, and smectic-E phases, sequentially when the specimen is cooled from its isotropic liquid phase. Different liquid crystalline phases observed in the mixture were studied using DSC, X-ray, and Optical microscopic techniques. The temperature variation of optical anisotropy has also been discussed. Helfrich potential and elastic moduli have been estimated in the smectic phase using Helfrich model.

New features of a smectic-A-crystal-B phase transition in a homologous series of liquid crystals

The components Lj of the Lorentz tensor and the polarizability density of molecules G in the smectic-A and crystalline-B phases have been determined for homologues of the series of alkyl-p-(4-alkoxybenzylideneamino-)cinnamates. The quantity Lj (G) in both phases is a linear (quadratic) function of the orientational order parameter of molecules S, which is invariant (noninvariant) with respect to the A-B transition, which is manifested in the form of jumps δLj and δG and enhancement of the G(S) dependence. An increase in the length of terminal molecular chains and weakening of interlayer correlation of molecules are accompanied by strengthening of the A-B transition of the first order and G(S) dependences in both phases together with an increase in δLj and δG. Change δG and dependence G(S) in the B phase are related to change in the conformation (flattening) of aromatic molecular cores.

Origin of weak layer contraction in de Vries smectic liquid crystals.

Structural investigations of the de Vries smectic-A (SmA) and smectic-C (SmC) phases of four mesogens containing a trisiloxane end segment reveal a linear molecular conformation in the SmA phase and a bent conformation resembling a hockey stick in the SmC phase. The siloxane and the hydrocarbon parts of the molecule tilt at different angles relative to the smectic layer normal and are oriented along different directions. For the compounds investigated, the shape of orientational distribution function (ODF) is found to be sugarloaf shaped and not the widely expected volcano like with positive orientational order parameters: ⟨P2⟩ = 0.53-0.78, ⟨P4⟩ = 0.14-0.45, and ⟨P6⟩∼0.10. The increase in the effective molecular length, and consequently in the smectic layer spacing caused by reduced fluctuations and the corresponding narrowing of the ODF, counteracts the effect of molecular tilt and significantly reduces the SmC layer contraction. Maximum tilt of the hydrocarbon part of the molecule lies between approximately 18° and 25° and between 6° and 12° for the siloxane part. The critical exponent of the tilt order parameter, β∼0.25, is in agreement with tricritical behavior at the SmA-SmC transition for two compounds and has lower value for first-order transition in the other compounds with finite enthalpy of transition.

Measurements of the dielectric and viscoelastic constants in mixtures of 4,4'-n-octyl-cyanobiphenyl and biphenyl.

We performed measurements of the dielectric constants, splay elastic constant, and rotational viscosity of the nematic phase of mixtures of 4,4'-n-octyl-cyanobiphenyl (8CB) and biphenyl (BP). In contrast with previous results of DasGupta etal. [Phys. Rev. E 63, 041703 (2001); Phys. Lett. A 288, 323 (2001)], we do not find any anomaly of these constants when the smectic-A phase is approached at all concentrations of BP. These results are compatible with recent calorimetric measurements of Denolf etal. [Phys. Rev. Lett. 97, 107801 (2006); Phys. Rev. E 76, 051702 (2007)] and the absence of a tricritical point in the phase diagram. The origin of the anomalies observed by DasGupta etal. at large concentration of BP is also briefly discussed and a likely explanation in terms of biphenyl evaporation is proposed.

Mesomorphic, micro-Raman and DFT studies of new calamitic liquid crystals; methyl 4-[4-(4-alkoxy benzoyloxy)benzylideneamino]benzoates

The mesomorphic properties of newly synthesized homologous series of calamitic liquid crystals; methyl 4-[4-(4-alkoxy benzoyloxy)benzylideneamino]benzoates, H2n+1CnOC6H4COOC6H4C(H)N C6H4COOCH3; n=6, 8, 10, 12, 14, 16 (MABBAB-n) containing ester and Schiff base groups as linker have been studied by temperature dependent micro-Raman study, differential scanning calorimetry (DSC) and polarizing optical microscopy (POM). All members of this series exhibit enantiotropic smectic A (SmA) mesophase with oily streak and focal conic textures. Analyses of Raman marker bands of phenyl rings, Schiff base and ester groups of MABBAB-10 confirm the phase transitions. The Raman study also gives an evidence of breaking of weak intermolecular hydrogen bonds associated with ester groups and formation of new hydrogen bonds through CN bond at Cr→SmA phase transition. The monomer and dimer were optimized and vibrational assignment of MABBAB-10 was also done with density functional theoretical (DFT) technique to understand the experimental results.

Smectic-phase-induced interdigitated orientation of polythiophenes bearing phenylnaphthalene mesogen

Polythiophenes bearing mesogenic phenylnaphthalene side chains at 3-position, poly(3-{10-[6-(4-octylphenyl)naphthalene-2-yloxy]decyl}thiophene-2,5-diyl) and poly(3-{10-[4-(6-octyl-2-naphthyl)phenyloxy]decyl}thiophene-2,5-diyl), in which each mesogenic core linked reversely, and their model liquid crystals corresponding to each phenylnaphthalene side chains were synthesised and compared each other to investigate liquid crystalline property and molecular arrangement. Yamamoto polymerisation of corresponding 2,5-dibromothiophene monomers afforded soluble polythiophenes. They had similar number average molecular weights about 10 kDa and exhibited similar photophysical properties, i.e, UV–vis absorption λmax due to polythiophene backbones around 390 nm and fluorescence λmax around 540 nm in solution (560 nm in film state) with energy transfer from an excited phenylnaphthalene side group to the backbone. The respective model liquid crystals showed different behaviours of phase transition temperatures, enthalpies, and reversibility, whilst both polymers had similar phase transition behaviours. A well-developed fan-shaped texture in the temperature range between isotropic liquid and glass phases was observed for the polymers by polarising optical microscopy. It is suggested that both polymers had an almost identical smectic A1 phase, which led to glass state by retaining the interdigitated molecular arrangement through a crystal E phase on cooling.

Biaxial mesophase behavior of amphiphilic anisometric colloids: a simulation study.

The phase behavior of amphiphilic anisometric particles is explored using Monte Carlo simulations. The particles are composed of two incompatible laterally attached units: a spherocylinder and a spheroplatelet. A liquid crystalline phase polymorphism is obtained including biaxial nematic, (quasi long range biaxial) calamitic smectic-A, biaxial lamellar and columnar phases. The simulation results demonstrate intriguing phase transitions such as nematic-nematic, discotic nematic to (quasi long range biaxial) calamitic smectic-A, biaxial nematic to uniaxial calamitic smectic-A, and isotropic or discotic nematic to biaxial lamellar phases that possess nematic ordering within the layers. These findings are rationalized in terms of molecular geometry and amphiphilicity of different molecular units. The molecular model can be used as a tool for the prediction of the complex phase behavior that is relevant to liquid crystalline colloids.

Critical behavior of dielectric permittivity in the vicinity of nematic–isotropic and smectic–nematic phase transitions in smectogenic binary mixtures

We report the phase diagram of a binary mixture composed of compounds, one having NCS terminal group (5DBT, showing smectic A1 phase) and the other with CN terminal group (10OCB, showing smectic Ad phase), exhibiting induced nematic phase in a certain concentration range (0.05<x5DBT<0.95). Measurement of static dielectric parameters on this binary system within the whole mesomorphic range is being reported. Pretransitional behavior at the nematic–isotropic and the smectic A–nematic phase transitions has been studied by measuring the static dielectric parameters in the entire mesomorphic range. Precise determination of discontinuity (ΔT) and the critical exponent (α) has been carried out for the nematic–isotropic phase transition. Effective molecular dipole moment and the angle of inclination with respect to the molecular long axis have been determined. High resolution density measurement is also carried out for one concentration x5DBT=0.301 and from the molar volume so obtained, the discontinuity (ΔT) of the transition and the critical exponent are measured. Additionally, the nature of the N–SmA phase transition has been studied for a few mixtures from dielectric anisotropy measurements to explore the critical behavior of the N–SmA phase transition.

Rare Earth Metal Complexes of a Mesogenic Schiff-Base, N,N′-di-(4-decyloxysalicylidene)-1′, 4′-diaminobutane: Synthesis and Characterization

A mesogenic Schiff-base, N,N′-di-(4-decyloxysalicylidene)-1′,4′-diaminobutane, (H2L) that exhibits smectic-B and nematic mesophases, was synthesized and its structure studied by elemental analysis, mass spectrometry, NMR & IR techniques. H2L, upon condensation with hydrated lanthanide(III) nitrates yields LnIII complexes of the composition [Ln2(LH2)3(NO3)4](NO3)2, where LnIII = La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, and Ho. The PrIII complex shows smectic-A and nematic phases. The IR and NMR spectral data imply a bidentate bonding of H2L in zwitterionic form (as LH2) to the LnIII ions through two phenolate oxygen atoms, rendering the overall geometry of the complexes possibly to distorted mono-capped octahedra.

Calorimetric study of phase transitions in ocylcyanobiphenyl-barium titanate nanoparticle dispersions

High-resolution ac-calorimetry is reported on the weakly first-order isotropic to nematic (I-N) and the continuous nematic to smectic-A (N-SmA) phase transitions in the liquid crystal octylcyanobiphenyl (8CB) doped with a ferroelectric nanoparticle barium titanate, BaTiO3 (BT). Measurements were performed as a function of BT concentration and over a wide temperature range well above and below the two transitions. From the thermal scans of all samples (having BT mass fraction φ(m) = 0.001 to 0.014 and pure 8CB), both the I-N and the N-SmA transitions evolve in character. Specifically, there appears an unusual change of the I-N specific heat peak shape on heating as φ(m) increases. Both the transitions shift to lower temperature at a different rate for φ(m)<φ(m)(c)=0.002 as compared to that for φ(m)>φ(m)(c). The effective transition enthalpies are essentially constant and similar to that seen in the bulk. Using a simple geometric model, the mean distance between the BT particles at the cross-over φ(m)(c) is found to be x(c)~3 μm, which is consistent with an estimated surface extrapolation length b for the nematic director. This suggests that the low φ(m) regime is dominated by an impurity/disorder effect while for φ(m)>φ(m)(c) the mean distance is small enough for the LC to mediate coupling between the BT ferroelectric nanoparticles.

Isotropic to smectic-A phase transition: A review

The present review represents an attempt towards a unified picture of the direct transition from the isotropic to smectic-A phase. The isotropic to smectic-A phase transition is described in detail, including structure and nature revealed by experiments, molecular models and Landau-de Gennes theory. The wide variety of phenomena including effects of nonmesogenic impurities and electric field, hydrodynamics, fluctuations, and elastic theory of liquid crystalline elastomers is discussed. A brief review on the general information about the isotropic to lamellar phase transition in lyotropic liquid crystals has been given. The various predictions are compared with the available experimental results. Open questions and possible approaches to finding answers are discussed.

Synthesis, optical and thermal behaviour of palladium(II) complexes with 4-(4-alkoxy-2-hydroxybenzylideneamino)azobenzene

A series of new Pd(II) complexes derived from the reaction of palladium acetate with 4-(4-alkoxy-2-hydroxybenzylideneamino)azobenzene having the flexible terminal chain of OC n H2n + 1, in which n are even numbers ranging from 8 to 16, has been successfully synthesized. The physical measurement and spectroscopic techniques (FTIR and 1H-NMR) reveal that the Pd(II) complexes possess the Pd–N and Pd–O coordination modes in which the central Pd(II) adopts square-planar geometry. The observation under the polarized light shows that all the ligands and Pd(II) complexes exhibit enantiotropic mesophases. The ligands with n-octyloxy and n-decyloxy flexible chains exhibit the nematic (N) and smectic A (SmA) phases whilst the Pd(II) complexes show exclusive SmA phase. The SmA phase observed in Pd(II) complexes can be supported by the presence of focal conic fan-shaped texture with the presence of curved lines which are prominent during the cooling process. On the other hand, the comparison studies show that Pd(II) complexes possess exceptional higher phase transition temperatures as compared to the corresponding Cu(II) and Ni(II) complexes. A new series of Pd(II) complexes with azobenzene core have been synthesized and characterized. The ligands with n-octyloxy and n-decyloxy flexible chains exhibit the nematic (N) and smectic A (SmA) phases whilst the Pd(II) complexes show exclusive SmA phase. Comparison study shows that Pd(II) complexes have exceptional higher phase transition temperatures as compared to its counterparts of Cu(II) and Ni(II).

Strain induced homeotropic alignment in the smecticA phase of liquid crystals

We present a method to align liquid crystal (LC) samples in their SmecticA phase, useful to obtain permanent homeotropic single domains even in compounds where the nematic phase is absent. It is based on the application of a strain field, within the sample volume, which tends to align the molecules all towards a common direction. The study was carried out in samples of 4-decyl-4′-cyanobiphenyl (10CB), 4-octyl-4′-cyanobiphenyl (8CB) and n-hexyl-4′-n-pentyloxy biphenyl-4-carboxylate (65OBC) LCs, and the alignment efficiency was monitored by polarisation microscopy observations and by thermal diffusivity measurements.

New chiral liquid crystalline monomers, polymers, and elastomers derived from menthol derivatives: synthesis and mesomorphism

To investigate liquid crystalline properties and structure relationships of chiral compounds based on menthol, a series of new chiral monomers derived from menthol derivatives, a mesogenic crosslinking agent, the corresponding side chain homopolymers with siloxane backbone, and cholesteric elastomers were synthesized. The structures and purity of these chiral compounds obtained in this study were characterized using FTIR, 1H NMR, and elemental analyses. The mesomorphism and thermal stabilities were investigated using differential scanning calorimetry, polarizing optical microscopy, thermogravimetric analysis, and X-ray diffraction. The selective reflection of light for the chiral monomers was studied with UV/visible/near IR. The effect of the crosslinking agent content on the phase transition temperatures of the elastomers was discussed. It was found that these chiral monomers seemed beneficial for the formation of the mesophases when a flexible spacer was inserted between the mesogenic core and the terminal menthyl groups. All obtained chiral monomers showed a cholesteric phase, chiral smectic C phase, and cubic blue phase. The crosslinking agent exhibited a smectic A (SA) phase and nematic phase. However, their corresponding homopolymers with siloxane chains tended to form a lower order SA phase. With an increase of the mesogenic rigidity, the melting temperatures, glass transition temperatures (T g), and isotropic temperatures (T i) of chiral monomers or homopolymers all increased. For the elastomers, general tendency was toward increased T g and T i with increasing the crosslinking agent content.

A review of mathematical analysis of nematic and smectic-A liquid crystal models

We review the mathematical analysis of some uniaxial, liquid crystal phases. Firstly, we state the models for the two different studied phases: nematic and smectic-A liquid crystals. The spatial and temporal profiles of the liquid crystal configurations will be described by means of strongly nonlinear parabolic partial differential systems, which are presented at the same time. Then we will state some results about existence, regularity, time-periodicity and stability of solutions at infinite time for both models. It is our aim to show that, although nematic and smectic-A phases have different physical properties and are modelled by different nonlinear parabolic problems, there exists a common mathematical machinery to rewrite the models and obtain analytical results.

Molecular aspect ratio and anchoring strength effects in a confined Gay–Berne liquid crystal

Phase diagrams for Gay–Berne (GB) fluids were obtained from molecular dynamics simulations for GB(2, 5, 1, 2) (i.e. short mesogens) and GB(3, 5, 1, 2) (i.e. long mesogens), which yield isotropic, nematic, and smectic-B phases. The long-mesogen fluid also yields the smectic-A phase. Ordered phases of the long-mesogen fluid form at higher temperatures and lower densities when compared to those of the short-mesogen fluid. The effect of confinement under weak and strong substrate couplings in slab geometry was investigated. Compared to the bulk, the isotropic–nematic transition does not shift in temprature significantly for the weakly coupled substrate in either mesogen fluid. However, the strongly coupled substrate shifts the transition to lower temperature. Confinement induces marked stratification in the short-mesogen fluid. This effect diminishes with distance from the substrate, yielding bulk-like behaviour in the slab central region. Fluid stratification is very weak for the long-mesogen fluid, but the strongly coupled substrate induces ‘smectisation’, an ordering effect that decays with distance. Orientation of the fluid on the substrate depends on the mesogen. There is no preferred orientation in a plane parallel to the substrate for the weakly coupled case. In the strongly coupled case, the mesogen orientation mimics that of adjacent fluid layers. Planar anchoring is observed with a broad distribution of orientations in the weakly coupled case. In the strongly coupled case, the distribution leans toward planar orientations for the short-mesogen fluid, while a marginal preference for tilting persists in the long-mesogen fluid.

NMR of Short-Chain Hydrocarbons in Nematic and Smectic A Liquid Crystals

The proton NMR spectra of ethane, propane, and n-butane codissolved and orientationally ordered in two liquid-crystal solvents that exhibit both nematic (N) and smectic A (SmA) phases (one of which also has a reentrant nematic (RN) phase) are analyzed using CMA-ES (covariance-matrix adaptation evolution strategy). To deal with problems arising from the broad liquid-crystal background signal, a smoothed experimental spectrum is fitted to a smoothed calculated spectrum. The ethane and propane dipolar couplings and anisotropic energy parameters scale with each other, and the n-butane couplings are assumed to behave likewise. This restriction on the relative values of the n-butane energy parameters facilitates a fit to the temperature dependence of the n-butane dipolar couplings, in which the six n-butane energy parameters (three for trans and three for gauche), the isotropic trans-gauche energy difference E(tg), and its temperature coefficient E(tg)', and the methyl CCH angle decrease are obtained. Unlike earlier studies, the fit does not employ a model for the anisotropic intermolecular potential. The nematic potential in the SmA phase of the liquid-crystal mixture 6OCB/8OCB is estimated by interpolated values obtained from the ethane spectra for the N and RN regions. Analysis of results for the SmA phase involves adding a nematic-smectic coupling prefactor and a smectic prefactor. Spectra obtained in the liquid-crystal 8OCB are calculated with no adjustable parameters, scaling the potentials using the ethane values, and the agreement between experiment and calculation is outstanding. Conformer populations are affected by the environment, and the isotropic solute-solvent interactions result in increased gauche populations, whereas anisotropic interactions increase the trans probability. The trans probability in the SmA phase is slightly lower than expected from the nematic results.

Synthesis, characterisation and liquid crystalline properties of some Schiff base and cinnamate central linkages involving 1,3,5-trisubstituted pyrazolone ring system and their Cu(II) complexes

Two new mesogenic homologous series, each containing 1,3,5-trisubstituted pyrazolone derivatives, 4-n-alkoxyphenyl and Schiff base–cinnamate central linkages, have been synthesised to give 4-[(5-hydroxy-3-methyl-1-phenyl-4,5-dihydro-1H-pyrazol-4-yl) methyleneamino] phenyl 3-(4-n-alkoxyphenyl)acrylate [Series-A] and 4-[(5-hydroxy-3-methyl-1-p-tolyl-4,5-dihydro-1H-pyrazol-4-yl)methyleneamino] phenyl 3-(4-n-alkoxyphenyl)acrylate [Series-B] and their Cu(II) complexes have also been synthesised. These compounds were characterised by elemental analysis, Fourier transform infrared (FT-IR), proton nuclear magnetic resonance (1H NMR), carbon-13 NMR (13C NMR) and ultraviolet (UV)-visible and mass spectral studies. Their mesomorphic behaviour was studied by polarising optical microscope (POM) with a heating stage. POM data were compared with differential scanning calorimetry thermograms. In Series-A and -B, all compounds exhibit mesomorphism. Series-A compounds exhibit an enantiotropic nematic mesophase except propyl derivative, while a smectic A (SmA) mesophase is observed from the heptyl derivative and persists up to the last member of the homologous series. n-Heptyloxy derivative is monotropic for SmA phase. Series-B compounds also exhibit the enantiotropic nematic mesophase, while the SmA mesophase is observed from the heptyl derivative and persists up to the last member of the homologous series. n-Dodecyloxy derivative exhibits monotropic SmA and nematic mesophases. The mesomorphic properties of both series are compared with each other and the other structurally related compounds. The study reveals that cinnamate linkage containing liquid crystals have higher thermal stability compared to structurally related series containing chalcone linkage. In case of complex series, only one compound from each series gives nematic mesophase.