smectic-A Phase Research Articles

Rheological properties of a reentrant nematic liquid crystal

We report experimental studies on small angle light scattering (SALS), and rheodielectric and electrorheological properties of a binary mixture of octyloxy cyanobiphenyl and hexyloxy cyanobiphenyl liquid crystals. The mixture exhibits nematic (N) to smectic-A (SmA) phase transitions, and then again to a reentrant nematic (N(R)) phase transition. Rapid shear thinning in the quenched samples in the low shear rate region in the N and SmA phases observed from SALS experiments is attributed to the realignment of the director within the domains. The domains are elongated along the shear direction at higher shear rates. The temperature variation of the effective viscosity and static dielectric constant reveals the changes in the director orientation across N-SmA-N(R) phase transitions. At a steady shear rate the effective viscosity increases with the electric field in all the phases and saturates at much higher fields. It also exhibits two anomalous peaks across N-SmA-N(R) phase transitions beyond a particular field. The shear modulus of the SmA phase in an intermediate field is significantly larger than that measured at both low and high fields. This enhanced viscoelasticity of the SmA phase is argued to originate from the increased dislocation density.

Phase transition studies in symmetric dimeric liquid crystals

As a part of the systematic studies on symmetric liquid crystal dimer homologous series, α,ω-bis-(4-n-alkylaniline benzylidene-4′-oxy) alkanes, (referred to as m.OnO.m with m = 3, 4, and 5; and n = 8, 9, and 10), we present in this article the nature of phase transitions across isotropic–nematic and nematic–smectic-A (N–SmA) phases exhibited by the just mentioned compounds. The methods employed are differential scanning calorimetry and dilatometry. The compounds studied were 3.O8O.3, 4.O8O.4, and 5.O8O.5; and 3.O9.03, 5.O9O.5, 3.O10O.3, 4.O10O.4, and 5.O10O.5. Different from the case of their corresponding monomers, all these compounds exhibit a nematic phase only with the exception of 5.O8O.5 which exhibits a SA phase in addition to the nematic phase. The phase transitions viz., isotropic–nematic transitions studied in all these compounds were confirmed to be of first-order nature, whereas the N–SmA transition exhibited by the compound 5.O8O.5 only was found to be of second-order nature. We also report in this article the calculated density jumps, thermal expansion coefficient maxima, and pressure dependence of transition temperatures which are analyzed in the light of the available literature data.

Liquid crystalline dinuclear copper(II) complexes accessed from photoluminescent tridentate [ONO]-donor Schiff base ligands

Two new liquid crystalline dinuclear copper(II) complexes of the type [Cu2L2 n], H2Ln = (E)-5-(hexadecyloxy)-2-((2-hydroxyethylimino) methyl) phenol(L) and (E)-5-(hexadecyloxy)-2-((2-hydroxypropylimino) methyl) phenol (L′), have been synthesised. The compounds were characterised by Fourier transform infrared spectroscopy, 1H and 13C nuclear magnetic resonance, ultraviolet-visible spectroscopy, elemental analyses, solution electrical conductivity measurements and fast atom bombardment mass spectrometry. The phase behaviours of the compounds were examined by differential scanning calorimetry and polarised optical microscopy. Interestingly the ligand L showed a monotropic smectic A (SmA) mesophase at ∼75°C while the ligand L′ incorporating a methyl group in the spacer lacks any mesomorphism. The complexes were all found to exhibit a thermally stable enantiotropic SmA phase in the range ∼114–118°C. The ligands are blue light emitters with broad emission maxima at ∼469 nm. The density functional theory calculations revealed a distorted square planar structure around each copper(II) centre in the dinuclear framework.

X-ray studies of the smectic B phase of the 4-bromobenzylidene-4′-alkoxyanilines

The results of X-ray studies of the nine members (4 ÷ 12) of the 4-bromobenzylidene-4′-alkyloxyaniline homologous series (nBBAA) in smectic B (SmB) and smectic A (SmA) phases are presented. Interestingly, no SmC phase was identified. The thickness of smectic layers in the SmA and SmB phases was estimated showing unexpectedly that in SmA it is bigger than in the SmB. For the SmB phase, orthorhombic unit cell parameters were determined as a function of temperature. The obtained data are discussed in relation to rotational dynamics.

Phase Transition Studies and Thermodynamic Parameters of Four Members of nO.m Series—A Dilatometric Study

Density measurements are carried out on four alkoxy benzylidene alkyl anilines, viz., N-(p-n-methoxy/ethoxybenzylidene)-p-n-dodecyl and tetradecylanilines, 1O.12, 1O.14, 2O.12, and 2O.14. The first three compounds exhibit monovariant nematic phase while the last compound exhibits nematic and smectic-A phase with variable thermal ranges. The density measurements infer the first order nature for Isotropic to nematic (Iso-N) phase transformation while the nematic to smetic-A (N-SmA) transition as weak first order. Thermal expansion coefficient α estimated from density ρ is utilized for the computation of thermodynamic parameters of above liquid crystalline compounds. The temperature variation of different thermodynamic parameters like Moelwyn-Hughes parameter, reduced volume, reduced compressibility, isothermal, isochoric, and isobaric Gruneisan parameters, etc., are estimated. The results are compared with the available data in literature of other compounds.

Effect of hydrostatic pressure on the Frank splay and bend elastic constants

We report the first systematic work on the effect of hydrostatic pressure on the splay and bend elastic constants of a nematic (N) liquid crystal. At a relative temperature, the magnitudes of both the elastic constants increase significantly with increasing pressure, with the variation for the bend parameter being slightly higher. Pressure also influences the rate at which the elastic constants grow as the smectic A (SmA) phase is approached from the N phase. The studies further show that the temperature range of the N phase may not play an important role in the increase of the splay constant near the transition to the SmA phase.

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.

Molecular theory of the tilting transition in smectic liquid crystals with weak layer contraction and diffused cone orientational distribution

A molecular field theory of the smectic-A-smectic-C transition has been developed for smectics with a diffused cone orientational distribution of molecules (volcano-like distribution function) in the smectic-A phase and anomalously weak layer contraction in the smectic-C phase. Orientational order parameters and smectic layer spacing have been calculated numerically as functions of temperature and compared with the results obtained using a model with a standard Maier-Saupe-type distribution function that has been considered before. A molecular theory of the electroclinic effect in chiral smectics has also been developed using the recently proposed simple biaxial interaction potential. A comparison has been made between the absolute values and temperature variations of the electroclinic coefficient obtained using the volcano model, the model with Maier-Saupe-type distribution, and the orthodox cone model proposed by de Vries. It has been shown that the model with a conventional "sugar loaf" type orientational distribution function in the smectic-A phase is sufficient to describe the main properties of smectics with anomalously weak layer contraction.

Synthesis and phase behaviour of a homologous series of polymethacrylate-based side-chain liquid crystal polymers

The homologous series of side chain liquid crystal polymers, the poly[ω-(4-methoxyazobenzene-4′-oxy)alkyl methacrylate]s, has been prepared in which the length of the flexible alkyl spacer has been varied from 3 to 11 methylene units. All the polymers exhibit liquid crystalline behaviour. The propyl and butyl members show exclusively nematic behaviour. The pentyl, hexyl, octyl and decyl members show a nematic and a smectic A phase while the heptyl, nonyl and undecyl homologues exhibit only a smectic A phase. The smectic A phase has been studied using X-ray diffraction and assigned as a smectic A1 phase in which the side chains are fully overlapped and the backbones are confined to lie between the smectic layers. For the nonyl member an incommensurate smectic phase is observed. The dependence of the transition temperatures on the length of the flexible spacer is understood in terms of the average shapes of the side chains.

Calorimetric Investigation of the Isotropic to Smectic-A Phase Transition of Smectic Liquid-Crystalline Elastomers

A high-resolution calorimetric study has been performed in the vicinity of the isotropic to smectic-A phase transition of smectic liquid crystalline elastomers. The transition is weakly first order for both investigated samples. Nevertheless, different features have been observed when the crosslinking density is changed, such as substantial broadening of the transition and decrease of the involved latent heat. The latter is sluggishly released in a range of few K.

Low-frequency ultrasound and smectic dynamics

It is demonstrated that possible violation of the conventional hydrodynamics of smectic liquid crystals in the kilohertz frequency range is usually not observed in experiments. In ultrasonic experiments, deviations from the Stokes quadratic frequency dependence of the absorption coefficient are determined by the process of acoustic relaxation. This dependence is neither universal nor characteristic of only the smectic-A phase. The character of the frequency dependence can vary with the wave vector orientation relative to the direction of magnetic induction.

Characteristic smectic structures of main-chain liquid-crystalline polyimides driven by a microphase separation between aromatic imide mesogen and a siloxane spacer

We investigated the phase transition behavior and liquid-crystalline structure for main-chain liquid-crystalline polyimides with aromatic imide cores and alkyl and dimethyl siloxane spacers, BPDA/3SiO4-3F. They formed crystal, smectic C (SC), smectic A (SA), and isotropic phases in order of increasing temperature. The characteristic structures of the SA and SC phases were extracted with wide-angle X-ray diffraction (WAXD) and Fourier transform infrared (FTIR) spectroscopy measurements. In the SC phase, the mesogenic groups were tilted by approximately 40° to the smectic layer and the unusual quasi two-dimensional structural order exists irrespective of the liquid-like packing of aromatic cores within a layer. On heating, the SC phase transformed into the SA phase with a nature of first-order transition. The resulting SA structure was also unusual. The orientational order parameter was 0.43, which was lower than 0.71 in the SC phase. This indicates that a “de Vries”-type of smectic A is formed in which the mesogenic groups are tilted away from the optic axis, but in azimuthally random directions. The average tilt angle was estimated to be 42°. These characteristics were produced because of the strong segregation effect of the dimethyl siloxane spacer groups with a relatively larger molecular area compared to alkyl chain and aromatic imide cores.

Thermal and photoinduced liquid crystalline phase transitions with a rod–disc alternative change in the molecular shape

A triphenylene derivative which has six 4-tetradecyloxyazobenzene units connected with propylene–ester linkages (1) was synthesised to study the liquid crystalline phase transitions in both thermotropic and photo-induced manners. It was found that 1 exhibits smectic A (SmA) and rectangular columnar (Colr) mesophases in an enantiotropic way. This means that 1 can behave as both rod- and disc-like molecules depending on the thermal condition. To the best of our knowledge, this is the first case in thermotropic liquid crystals where one compound exhibits both calamitic and discotic mesomorphism with the change of molecular shape in anisotropy. Under the photo-irradiation at 365 nm of wavelength which induces a photoisomerisation of azobenzene units from trans to cis forms, the phase transitions take place in the same manner as in the thermotropic ones with a certain decrease of phase transition temperatures. It is also a new finding that at 202.5 °C at which 1 exhibits a Colr mesophase without photo-irradiation, the successive phase changes are induced by increasing the light intensity to the isotropic liquid (Iso) phase by way of a SmA phase. This means that the isothermal phase transitions among Iso, SmA and Colr phases can be controlled by light intensity. This is also the first example that the isothermal liquid crystalline phase transitions are controlled by photo-irradiation among three phases (Colr, SmA and Iso), which are also accompanied with the shape change of molecules between rod and disc.

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.

Structure–property correlation of a hockey stick-shaped compound exhibiting N-SmA-SmCa phase transitions

We report the measurement of birefringence, dielectric constant, splay, bend elastic constants and rotational viscosity in the nematic phase (N) of a hockey stick-shaped compound exhibiting smectic-A (SmA) and anticlinic smectic-C (SmCa) phase transitions at lower temperature. It is found that the physical properties are significantly different than calamitics and partially similar to the bent-core liquid crystals. It exhibits the positive dielectric and the negative elastic anisotropy. The optical and the thermal measurements show that all the transitions are first order. The rotational viscosity is comparatively higher than calamitic liquid crystals and exhibits weak pre-transitional divergence. The temperature dependent static dielectric measurements indicate antiparallel correlation of both the longitudinal and the transverse components of dipole moments in both the smectic phases. The parallel component of the dielectric constant exhibits a single Debye type relaxation in all the phases. The activation energy in the N, SmA and SmCa phases are comparatively larger than that of the respective phases of calamitic liquid crystals and can partially be attributed to the higher rotational viscosity of the hockey stick-shaped compound.

Probing the pore structure of a chiral periodic mesoporous organosilica using liquid crystals

Periodic mesoporous organosilicas (PMO) are prepared by the surfactant-templated condensation of bridged organosilsesquioxane monomers. By controlling the nature of the organic segment, the type of surfactant and the condensation conditions, one can control the physical and chemical properties of the resulting PMO and produce highly ordered porous structures with a periodicity on the nanometer scale. The development of chiral PMO materials has been of significant interest given their potential in heterogeneous asymmetric catalysis, chiral chromatography and non-linear optics. Characterization of the chirality of pore structures in these materials thus far has been achieved by indirect methods including polarimetry and solid-state circular dichroism. We report herein a general and convenient approach to probe directly the pore structure of chiral PMO materials based on their interactions with inexpensive liquid crystalline solvents, which result in the induction of measurable chiral properties in the nematic (N) and smectic A (SmA) phases of the liquid crystals. The templated co-condensation of a biphenylene organosilsesquioxane monomer and a chiral binaphthyl organosilsesquioxane monomer produced a new chiral PMO material that was investigated as dopant in two different liquid crystal hosts. Measurements of induced circular dichroism and helical pitch in the nematic phase of the cyanobiphenyl liquid crystal 5CB, and the measurement of an induced electroclinic effect in the SmA phase of the phenyl benzoate liquid crystal 9OO4 were carried out. The induced chiral properties measured in these experiments are consistent with chirality transfer taking place inside the pores, and suggest that the inner structure of the pores in the PMO material is indeed chiral.

Hybrid rod-like and bent-core liquid crystal dimers exhibiting biaxial smectic A and nematic phases

Liquid crystal hybrid dimers involving bent-core units and rod-like units were designed, synthesized and investigated by a combination of polarizing optical microscopy (POM), differential scanning calorimetry (DSC) and X-ray diffraction studies. All the four unsymmetrical dimers 1–4 exhibited stable nematic phases over a wide temperature range. Moreover, dimer 1 exhibited the biaxial smectic A (SmA) phase and dimer 2 exhibited a highly ordered mesophase below the nematic (N) phase. Two rod-like and bent-core nematogenic units were covalently linked in dimers 3 and 4 to furnish exclusively the nematic phase. The observation of the biaxial SmA phase underlying the nematic phase in one of the bent-core–rod dimers 1 would be a significant step forward towards achieving the biaxial nematic phase and resolving controversies related to the presence of cybotactic groups.

Trimesic Acid-Based Star Mesogens with Flexible Spacer: Synthesis and Mesophase Characterization

A series of trimesic acid-based star mesogens containing an octamethylenoxy spacer and two-ring containing side arms with terminal alkoxy groups were synthesised by a divergent approach. A structurally similar mesogen containing a three-ring side arm was also synthesised. All the mesogens and intermediates were characterised thoroughly using FT-IR, 1H NMR, 13C NMR spectroscopy, and elemental analysis techniques. The mesophase characteristics were identified by hot stage optical polarising microscopy (HOPM) and differential scanning calorimetry (DSC), while X-ray was used to confirm the existence of a smectic A (SA) phase. The HOPM and DSC results revealed the presence of rich polymesomorphism in the star mesogens. The dominance of a SA phase in the series is attributed to the steric repulsion associated with steric asymmetry. The change of the side arm from two rings to three rings was found to increase the mesophase stability.

Focal conics in a smectic-A liquid crystal in microwrinkle grooves

Focal conic domains (FCDs) in smectic-A (SmA) liquid crystals (LCs) are a broad class of curvature defects in a layered system. Here, we report a regular array of FCDs in octylcyanobiphenyl in a SmA phase confined within sinusoidal microgrooves of microwrinkles with a specific periodicity. A semi-hybrid alignment is imposed by the air–LC interface (homeotropic) and the LC–curved-microwrinkle interface (unidirectional planar). The FCDs are periodically packed along the groove direction when the wrinkle wavelength is larger than a critical value of approximately 4 μm. The periodicity of the FCDs monotonically increases with increasing the wrinkle wavelength. From our optical observation, we propose an internal layer structure of the FCDs composed of a set of Dupin cyclides with the eccentricity of the basic ellipse being approximately 0.75.

Synthesis, Characterization, and Anisotropic Properties of 5-Alkoxy-2-((4-(Phenyldiazenyl)Phenylimino)Methyl)phenol and Their Copper(II) Complexes

A new homologous series of bidentate Schiff bases 5-alkoxy-2-((4-(phenyldiazenyl) phenylimino)methyl)phenol and their Cu(II) complexes have been synthesized and characterized by spectroscopic techniques along with studies on the mesomorphic properties. Phase transitions of all the Schiff bases and Cu(II) complexes in this series are enantiotropic. The ligands with n-octyloxy and n-decyloxy flexible chains exhibit the nematic (N) and smectic A (SmA) phases, the latter of which is characterized by focal conic fan-shaped texture. In contrast, the Cu(II) complexes show exclusively the SmA phase, which is ascribed to the presence of Cu‒O and Cu‒N coordination modes. For the same reason, the thermal stabilities of the complexes have also been improved.