smectic-A Transition Research Articles

Effects of quantum dots on the nematic to smectic-A phase transition

ABSTRACT Experiments on the composites of liquid crystals and quantum dots showed the significant role of quantum dots on the properties of the nematic–smectic-A phase transition. The effects of quantum dots on the nematic-smectic-A phase transition have been discussed using Flory- Huggins theory of isotropic mixing and Landau-de Gennes theory. The impact of quantum dots on the smectic-A order parameters, nematic–smectic-A transition temperature, splay and bend elastic constants has been discussed. The smectic-A order parameters, transition temperature and elastic constants decrease with the increase of volume fraction of quantum dots.

Landau-de Gennes theory of the nematic to smectic—A phase transition under confinement conditions

We study the nature of the nematic to smectic-A phase transition under confinement conditions in the context of Landau-de Gennes theory. The effect of quenched disorder on the nematic to smectic-A transition temperature, order parameter, enthalpy, and thermal hysteresis are discussed followed by phase diagrams. The tricritical behavior of the nematic to smectic-A phase under confinement conditions is predicted.

Twist-bend nematics and heliconical cholesterics: a physico-chemical analysis of phase transitions and related specific properties

ABSTRACT Certain nematic liquid crystals, for example, those formed by banana-shaped molecules, can exhibit a low-temperature twist-bend nematic (Ntb) phase. Upon addition of chiral dopants, a chiral version of the twist-bend phase (N*tb) can be observed below the conventional chiral nematic (N*) phase, while under electric field the N* phase is transformed into a specific state known as ‘oblique helicoidal cholesteric’. In this work, we have studied the well-known Ntb-forming system CB7CB:CB6OCB with addition of 5CB and chiral dopants (ChDs) using optical microscopy, differential scanning calorimetry (DSC), and measurements of temperature-dependent optical transmission using optical microscopy. Effects of 5CB and different chiral dopants (of steroid or non-steroid nature) upon thermal stability of Ntb phase were analysed, and selective reflection in the visible range could be observed in the N* phase with a sufficiently high ChD content. As a peculiar feature, sharp unwinding of the cholesteric helix was observed at lower temperatures when approaching the N*tb phase, which appears to be similar to the helix unwinding close to the smectic-A transition.

Pyridine-Based Three-Ring Bent-Shape Supramolecular Hydrogen Bond-Induced Liquid Crystalline Complexes: Preparation and Density Functional Theory Investigation

A series of new supramolecular three-ring bent-shape Schiff base liquid crystal (LC) complexes were prepared and studied. On one side, two alkoxy chain lengths of the carboxylic acids were used, namely eight and sixteen carbons. Moreover, on the other side, terminal small compact groups, which substituted aniline, with different polarities were utilized. Furthermore, the hydrogen-bonding interactions in the formed complexes were elucidated by Fourier-transform infrared (FT–IR) spectroscopy. The mesomorphic thermal and optical characteristics of the samples were determined by differential thermal analysis (DSC) and polarized optical microscopy (POM). The complexes exhibited enantiotropic and dimorphic mesophase behaviors. The results indicate that the polarity of the compact groups and the lengths of the alkoxy chains greatly impacted the mesomorphic characteristics and thermal stabilities of the mesophases. The observed values of the enthalpy changes (ΔH) associated with the crystalline smectic-A (TCr-SmA) transitions were extremely small compared with the conventional values that characterize supramolecular hydrogen-bonded liquid crystalline complexes. ΔH, which corresponded to the nematic isotropic transitions (TN-I), varied from 0.13 to 9.54 kJ/mol depending mainly on the polarity of the groups that were para-attached to the aniline moiety. Finally, the theoretical results obtained by density functional theory (DFT) calculations were discussed. The DFT geometrical structures showed non-coplanar structures. The mesomorphic range was correlated with the calculated dipole moment, polarizability and the aspect ratios of the investigated compounds.

Effect of cross-link concentration on the isotropic to smectic-A phase transition in smectic elastomers

We describe the isotropic to smectic-A phase transition in smectic elastomers within Landau phenomenological theory. We analyze the effects of external mechanical stress and cross-link concentration on the isotropic to smectic-A phase transition. Both the orientational and translational order parameters decrease with the increase of cross-link concentration. The first order isotropic to smectic-A transition in low-molecular-weight materials, can become weak first order transition in smectic elastomers. The effect of cross-link concentration on the strain birefringence and correlation length in the isotropic phase of the transition is discussed.

Measurement of visco-elastic properties in a binary system of hockey stick-shaped and calamitic mesogens.

Temperature variation of the splay elastic modulus (K11) and rotational viscosity ([Formula: see text]) have been measured throughout the entire nematic (N phase of a binary liquid crystal system comprising of host calamitic compound, 4'-octyloxy-4-cyanobiphenyl (8OCB) and small amount of a hockey stick-shaped mesogen, 4-(3-decyloxyphenyliminomethyl) phenyl 4-decyloxycinnamate (SF7). Two different structural conformations play a significant role in the phase behavior leading to a noticeable shrinkage in the nematic width. During cooling in the nematic phase, both K11 and [Formula: see text] enhance monotonically and divulge an evidence of pretransitional fluctuation near the Smectic-A (Sm-A) transition. With increase in the dopant concentration, the slope of the temperature-dependent K11 and [Formula: see text] becomes uniformly steeper. The observed variations are explained in accordance with the formation of smectic-like clusters in the nematic phase. A representative model of the smectic-like clusters in the nematic phase is presented. However, the obtained values of [Formula: see text] have found to be between that of conventional calamitics and typical bent-core mesogens. Measurement of the temperature-dependent orientational order parameter (〈P2〉 ) and activation energy (Ea) has also been carried out for all the studied mixtures, exhibiting a small augmentation due to the addition of a hockey stick-shaped compound. This indicates an enhancement in intermolecular packing within the mixtures which further influences the molecular motions.

Critical behavior in the vicinity of nematic to smectic A (N–SmA) phase transition of two polar–polar binary liquid crystalline systems

ABSTRACTPhase diagram, critical behavior and order of the nematic (N)–smectic A (SmA) phase transition of two polar–polar binary systems (i) 4-n-heptyloxy-4′-cyanobiphenyl (7OCB) and 4-n-octyloxy-4′-cyanobiphenyl (8OCB); (ii) 4-n-octyloxy-4′-cyanobiphenyl (8OCB) and 4-n-nonyloxy-4′-cyanobiphenyl (9OCB) by means of a high-resolution temperature scanning measurement of birefringence have been reported in this work. A simple power law analysis has been adopted to extract the specific heat critical exponent (α′) at N–SmA transition from birefringence data. The α′ for N–SmA transition indicates a uniform crossover behavior and has appeared to be non-universal in nature. With increasing concentration of the higher homologues for both the binary systems, the N–SmA transition reveals a strong tendency to be driven towards the tricritical nature. The 3D-XY limit (i.e. α′ = −0.007) for N–SmA transition reaches at the concentration x8OCB = 0.28 corresponding to the McMillan ratio 0.914, whereas the tricritical point has been found to appear near x9OCB = 1.0 corresponding to McMillan ratio 0.992.

Analysis of the shape of x-ray diffraction peaks originating from the hexatic phase of liquid crystal films

ABSTRACTX-ray diffraction studies of the bond-orientational order in the hexatic-B phase of 75OBC and 3(10)OBC compounds are presented. The temperature evolution of an angular profile of a single diffraction peak is analyzed. Close to the hexatic-B–smectic-A transition these profiles can be approximated by the Gaussian function. At lower temperatures in the hexatic-B phase the profiles are better fitted by the Voigt function. Theoretical analysis of the width of diffraction peaks in three-dimentional (3D) hexatics is performed on the basis of the effective Hamiltonian introduced by Aharony and Kardar [1]. Theoretical estimations are in good agreement with the results of x-ray experiments.

Twist-grain-boundary-A* phase stabilisation in confined geometry by the interfaces

ABSTRACTWe report on the appearance and stabilisation of twist-grain-boundary-A (TGB) phase in confined geometry samples of the liquid-crystalline compound CE8, by means of polarised optical microscopy. The chiral liquid crystal (LC) compound was chosen such as to undergo a cholesteric–smectic-A transition with temperature (type-I smectic), and to be close to the triple point between the cholesteric, TGB, and smectic-A phases. We used several types of anchoring conditions and various effective anchoring strengths. The results reveal that (i) the TGB phase exists as a metastable phase with strong fluctuations in the bulk, and (ii) it can be stabilised by the sample boundaries for adequate anchoring conditions. This means that we probably observed a type-I to type-II smectic transition induced by the substrate. Some dependence of the stabilisation on the surface-anchoring conditions and on the cell thickness is reported.

Anomalous selective reflection in cholesteryl oleyl carbonate – nematic 5CB mixtures and effects of their doping by single-walled carbon nanotubes

Liquid crystalline (LC) mixtures of cholesteryl oleyl carbonate (COC) and 4-pentyl-4'-cyanobiphenyl (5CB) as well as dispersions of single-walled carbon nanotubes (NTs) in these mixtures were studied by means of selective reflection measurements, differential scanning calorimetry (DSC) and optical microscopy. The relative mass of COC in a mixture X was varied between 0.4 and 1.0, the temperature range of measurements was between 284 and 314 K, and concentration of NTs was fixed at 0.1%. Two important anomalies were noted: (1) the cholesteric to smectic-A transition temperature increased on dilution of COC by non-smectogenic 5CB in the concentration range 0.8 < X < 1 and (2) the reciprocal pitch versus 5CB concentration dependence was essentially linear, in contrast to behaviour commonly observed in nematic-cholesteric mixtures. A model of molecular arrangement in the mixtures, accounting for the possibility of integration of 5CB dimers and monomers between COC molecules and presumably explaining the experimental data, was proposed. The helical pitch of the cholesteric mixtures remained practically unchanged upon doping by NTs, and only slight widening of the selective reflection peaks was noted. The obtained results allow considering the COC + 5CB mixtures as promising matrices for composite materials on the basis of liquid crystals and NTs.

Towards tunable defect arrangements in smectic liquid crystal shells utilizing the nematic–smectic transition in hybrid-aligned geometries

We produce and investigate liquid crystal shells with hybrid alignment—planar at one boundary, homeotropic at the other—undergoing a transition between the nematic (N) and smectic-A (SmA) phases. The shells display a dynamic sequence of patterns, the details depending on the alignment agents and on the diameter and thickness of the shell. In shells of sufficient diameter we typically find a transient striped texture near the N–SmA transition, stabilising into a pattern of tiled, more or less regularly spaced focal conic domains in the SmA phase. The domain size and spacing decrease with reduced shell thickness. In case of strong homeotropic anchoring at one boundary and small shell size, however, the increased curvature favors homeotropic against planar alignment in the smectic phase, and the shell then tends to adapt to complete homeotropic alignment at the final stage of the transition. This is the first study of hybrid-aligned smectic shells and the results constitute a beautiful demonstration of the capacity for dynamic structure formation and reformation via self-assembly in soft matter. The new patterns extend the range of arrays of topological defects that can be realised with liquid crystals in spherical morphology and the correlation between the feature arrangements and the variable parameters of the shell and its environment opens a route towards tunability. However, the observed strong impact from increasing curvature, even for these rather large shells, indicates that the choice of alignment agents inducing planar or homeotropic alignment with varying strength will become critical when targeting the most attractive colloidal size scale of about a micron or smaller.

Rheological Properties of a Chiral Liquid Crystal Exhibiting Type-II Character

We measured rheological properties of cholesteryl nonanoate and its mixture with 4′-octyloxy-4-cyanobiphenyl. In both systems, wider temperature range of blue phase under shear with anomalous increase in viscosity associated with non- Newtonian flow behavior has been observed. Shear thinning followed by Newtonian flow is found in cholesteric phase and non-Newtonian flow behavior at high shear rates is seen in smectic-A phase. In cholesteryl nonanoate a small peak in the viscosity at cholesteric to smectic-A transition along with a shear rate dependent viscosity at higher shear rate is attributed to the type-II character of the compound. We find that the type-II character of the compound is further enhanced by mixing a small amount (20 wt%) of 4′-octyloxy-4-cyanobiphenyl in cholesteryl nonanoate. No significant change in the temperature dependent rheological behavior is observed at the twist-grain-boundary-A to smectic-A transition. It is suggested that the small anomalous peak in the viscosity just above the twist-grain-boundary-A phase is due the existence of chiral line liquid phase.

Order of phase transitions and tricriticality in mixtures of octyloxycyanobiphenyl and nonyloxycyanobiphenyl liquid crystals: A high-resolution study by adiabatic scanning calorimetry

A detailed study has been performed for mixtures of octyloxycyanobiphenyl (8OCB) and nonyloxycyanobiphenyl (9OCB) liquid crystals and nine of their mixtures by means of high-resolution adiabatic scanning calorimetry. The isotropic to nematic transitions are weakly first order with latent heat values in the range usually encountered for this transition in other liquid crystals. With the exception of pure 8OCB, for which only an upper limit of 1.8 J kg(-1) for the latent heat could be established, finite latent heats have been obtained for the nematic to smectic-A transition of all the mixtures and of pure 9OCB. The concentration dependence of their latent heats could be well fitted with a crossover function consistent with a mean-field free-energy expression that has a nonzero cubic term induced by the Halperin-Lubensky-Ma (HLM) coupling between the smectic-A order parameter and the orientational director fluctuations. Clearly first-order transitions with measurable latent heats are found for mole fractions of 9OCB in the mixtures where the effective critical exponent for the specific-heat capacity has substantially lower values than the tricritical one (0.5). This is qualitatively different from what has been observed so far in other liquid-crystal systems and yields strong experimental evidence from a calorimetric experiment for the HLM coupling between the smectic-A order parameter and the director orientation fluctuations.

Dispersions of Carbon Nanotubes in Cholesteric Liquid Crystals

Dispersions were studied of multiwall carbon nanotubes in cholesteric liquid crystals of three types: nematics with chiral dopant, mixtures of cholesterol derivatives, and similar mixtures close to smectic-A transition. In the first two cases, no appreciable effects of nanotubes upon selective reflection spectra were noted, while optical transmission increased significantly at the isotropic phase transition. Aromatic and steroid matrices differed significantly in this respect, though in both cases the nanotubes were apparently oriented along the directors of quasi-nematic layers. In the third case, nanotubes slightly but clearly affected the measured selective reflection peaks, suggesting suppression of the smectic-A phase formation.

Pressure induced tricritical point at the cholesteric-smectic-A phase transition

A phenomenological theory is presented to describe the pressure effect on the cholesteric to smectic-A phase transition. The problem of first or second order character of the cholesteric to smectic-A phase transition is explored by means of the variation of the pressure. The possibility of the tricritical point at the cholesteric to smectic-A transition line is discussed in a phenomenological way. It is shown that with a particular value of the pressure, the first order cholesteric to smectic-A transition becomes second order at a pressure induced tricritical point. The obtained topologies of the pressure-temperature phase diagram qualitatively agree with experimental results.

Critical behaviour at the isotropic–nematic and nematic–smectic A phase transitions of 4‐butyloxyphenyl 4′‐decyloxybenzoate liquid crystal from refractive index data

We present new high‐resolution experimental data on the temperature dependence of the refractive index in the isotropic (I), nematic (N) and smectic A (SmA) phases of the 4‐butyloxyphenyl 4′‐decyloxybenzoate () liquid crystal. From an analysis of the data, using both the Vuks and Neugebauer models for the internal electric field, information on the orientational order parameter S(T) was derived. From a fitting procedure consistent with mean‐field theory and the first‐order character of the N–I transition, we obtain a value of 0.246±0.06 for the exponent β describing the critical behaviour of S(T) at the N–I transition, which is in good agreement with the tricritical value β = 0.25. In a temperature range of about 4 K above and below the N–SmA transition temperature, pre‐transitional evidence for the coupling between the nematic and smectic order parameters was observed. We show, for the first time, that the temperature derivative of the S(T) curve below and above the N–SmA transition has the same power law behaviour as the specific heat capacity with an effective critical exponent of 0.23±0.01.

Smectic liquid crystals in an anisotropic random environment

We report a high-resolution x-ray scattering study of the smectic liquid crystal octylcyanobiphenyl (8CB) confined to aligned colloidal aerosil gels. The aligned gels introduce orientational fields that promote long-range nematic order while imposing positional random fields that couple to the smectic density wave and disrupt the formation of an ordered smectic phase. At low densities of aerosil, the low-temperature scattering intensity is consistent with the presence of a topologically ordered XY Bragg glass phase that is predicted to form in response to such anisotropic quenched disorder. The observed features of the phase include an algebraic decay of the smectic correlations, which is truncated at large length scales due to the imperfect nematic order, and a power-law exponent that agrees closely with the universal value predicted for the XY Bragg glass. At higher aerosil densities, deviations from the XY Bragg glass form are apparent. At high temperature, the scattering intensity displays pretransitional dynamic fluctuations associated with the destroyed nematic to smectic-A transition. The fluctuations obey quasicritical behavior over an extended range of reduced temperature. The effective critical exponents for the correlation lengths and smectic susceptibility differ systematically from those of pure 8CB, indicating that coupling of the nematic order to the gel suppresses its role in the smectic critical behavior.

Verifying the symmetry of differential scanning calorimeters concerning heating and cooling using liquid crystal secondary temperature standards

The German Society for Thermal Analysis (Gesellschaft für Thermische Analyse e.V.—GEFTA) recommends verifying the symmetry of differential scanning calorimeters (DSC) between heating and cooling. For this purpose the use of liquid crystal phase transitions is suggested. Merck KGaA, Darmstadt, Germany, offers three certified secondary standards for DSC calibration in heating and cooling mode, which are recommended by ASTM too. In this paper all liquid crystal phase transitions of the three liquid crystals offered are measured using different DSC's in order to verify their applicability for checking the symmetry of the instruments and the suitability for a temperature calibration in heating and cooling mode. The materials under study are: 4-cyano-4′-octylbiphenyl (8OCB, M-24 (Merck)), 4-(4-pentyl-cyclohexyl)-benzoic acid-4-propyl-phenyl ester (HP-53 (Merck)) and 4′-ethyl-4-(4-propyl-cyclohexyl)-biphenyl (BCH-52 (Merck)). The smecticA to nematic transition of M-24 (8OCB) fulfils the requirements for these purposes best. There is no detectable discontinuity in the rate dependence of the transition temperature at zero rates and the material is sufficiently stable to allow several measurements. Because of the weakness of the transition and the neighborhoods of the much stronger crystal to smecticA transition and nematic to isotropic transition instruments with sophisticated control and data treatment algorithms partly fail to detect the rate dependence of the transition correctly. In such cases HP-53 or BCH-52 can be used alternatively. In view of a temperature calibration at different temperatures in heating and cooling mode the recommended liquid crystal phase transitions are suitable. The observed supercooling effects are nearly zero for M-24 and HP-53 and less than 0.25 K for BCH-52. The liquid crystal transitions of the standards are applicable for a calibration in heating mode too.

Calorimetric study of phase transitions in a liquid-crystal-based microemulsion

A lyotropic inverse micelle phase composed of water, thermotropic liquid-crystal octylcyanobiphenyl (8CB), and surfactant (DDAB) was studied by using high-resolution calorimetry on several mixtures with 3%, 8%, and 15% micelle concentration. Calorimetric results show strong depression of the isotropic to nematic (I-N) phase-transition temperature. Broad heat-capacity anomalies show the existence of a wide coexistence range of isotropic, nematic, and smectic-A phases, which mimics the behavior of a new nearly stable thermodynamic phase. An observation of the rather sharp almost bulklike nematic to smectic-A (N-A) transition at low-temperatures indicates that our heat capacity results are consistent with the phase separation scenario in which significant number of micelles is expelled during I-N conversion leaving almost pure nematic phase at lower temperatures. It was found that micelles get almost completely remixed on heating the mixture back to the isotropic phase.

Anomalous rotational diffusion in the vicinity of the isotropic to nematic phasetransition

Dielectric relaxation measurements are performed with very high accuracy on a liquid crystalline compoundn-octylcyanobiphenyl (8CB) in the isotropic (I), nematic (N) and smectic A(SA) phases. The data obtained display an essential difference in the rotational diffusion process inthe vicinity of the I–N phase transition in comparison to that taking place in the vicinity of theN–SA phase transition. Thus, for the I–N transition, anomalously slowdiffusion (subdiffusion), characterized by an anomalous coefficientα<1, is observed, while normal Brownian rotational diffusion with is found for the N–SA transition. It is also shown how the fractal parameterα is temperature dependent with an extremely sharp variation at the I–N transition point inthe form of a lambda-like profile.