Smectic State Research Articles

Effects of structural imperfections on the dynamic mechanical response of main‐chain smectic elastomers

AbstractWe examine the influence of structural imperfections on mechanical damping in polydomain smectic main‐chain liquid crystalline elastomers (MCLCE) subjected to small strain oscillatory shear. The mechanical loss factor tan δ = G″(ω)/G′(ω) exhibits a strong maximum (tan δ ≈ 1.0) near the smectic‐isotropic (clearing) transition. “Optimal” elastomers that exhibit minimal equilibrium swelling in a good solvent are compared with highly swelling “imperfect elastomers” that contain higher concentrations of structural imperfections such as pendant chains. For the imperfect elastomers, tan δ is markedly enhanced in the isotropic state because of relaxation of pendant chains and other imperfections. However, within the smectic state, the magnitude of tan δ and its temperature dependence are similar for optimal and imperfect elastomers at ω = 1 Hz. The prominent loss peak near the clearing transition arises from segment‐level relaxations that are insensitive to the details of chain connectivity. Smectic MCLCE can be tailored for applications as vibration‐damping materials by manipulating the clearing transition temperature through the backbone structure or by deliberate introduction of structural imperfections such as pendant chains. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 3267–3276, 2007

Structure of Macroscopic Monodomains and Its Soft Confinements of Chiral Smectic Phases on Crystallization in a Main-Chain Nonracemic Liquid Crystalline Polyester

A main-chain nonracemic chiral liquid crystalline (LC) polyester was synthesized using an A−B type condensation reaction of a chiral monomer, R-(−)4‘-{ω-[2-(p-hydroxy-o-nitrophenyloxy)-1-propyloxy]-1-heptyloxy}-4-biphenylcarboxylic acid, and was abbreviated as PET(R*-7). Wide-angle X-ray diffraction (WAXD) experiments combined with differential scanning calorimetry measurements revealed multiple phase transitions including a chiral twist grain boundary A (TGBA*) phase, a chiral smectic A (SmA*) phase, a chiral smectic C (SmC*) phase, and a highly ordered monoclinic crystalline (KM) phase with decreasing temperature. Flat-elongated and right-handed double-twisted helical single crystals were grown thermotropically in the smectic LC states. Both crystal types possessed the identical monoclinic unit cell: a = 1.04 nm, b = 0.46 nm, c = 5.59 nm, α = β = 90°, and γ = 84.2° based on the WAXD and selected area electron diffraction results. Uniaxial elongation of centimeter-sized films (along the xz-plane in a th...

Influence of preparation conditions and heat treatment on the properties of supercooled smectic cholesteryl myristate nanoparticles

Colloidal dispersions of cholesterol esters in the supercooled smectic state are under investigation as a novel drug carrier system in particular with respect to parenteral application. In the present study, suitable conditions for the homogenization of cholesteryl myristate dispersions stabilized with a phospholipid/bile salt blend were evaluated. For effective particle size reduction homogenization with high pressure and at temperatures above the melting temperature of the cholesterol ester (isotropic melt) is necessary. Homogenization at lower temperature where the matrix lipid is in the smectic state is less effective even when applying the highest homogenization pressure possible but still leads to dispersions with particles in the colloidal size range. Since sterility is required for parenteral medications and is usually achieved by autoclaving for aqueous systems, the physical and chemical stability of cholesteryl myristate nanoparticles stabilized with different surface active agents during heat treatment was investigated as well. The dispersions were characterized by particle size and zeta potential measurements, differential scanning calorimetry (DSC) and high performance thin layer chromatography (HPTLC). The results indicate that cholesteryl myristate nanoparticles stabilized with phospholipid/sodium glycocholate, polyvinyl alcohol, poloxamer and poloxamine can be sterilized by autoclaving. Compared to cholesterol ester free dispersions of phospholipids, the phospholipid seems to be more stable against hydrolysis during prolonged heat treatment in the phospholipid/bile salt containing cholesteryl myristate dispersions.

Self-assembly of carbazole-containing gelators: alignment of the chromophore in fibrous aggregates

Carbazole-containing gelators derived from l-isoleucine have been developed. They form elongated self-assembled fibers in common organic solvents and in liquid crystals, leading to the efficient gelation of these solvents. Spectroscopic studies indicate that the carbazolyl moieties are one-dimensionally stacked in the fibers. The stacking of the carbazolyl moieties is reversible by the association and dissociation of the hydrogen bonding. Moreover, anisotropically aligned fibers have been obtained in a homogeneously oriented smectic state of liquid crystals. This template behavior would serve as a versatile approach to the functionalization of self-assembled fibers.

Competing compressible and incompressible phases in rotating atomic Bose gases at filling factorν=2

We study the groundstates of weakly interacting atomic Bose gases under conditions of rapid rotation. We present the results of large-scale exact diagonalization studies on a periodic geometry (a torus) which allows studies of compressible states with broken translational symmetry. Focusing on filling factor $\ensuremath{\nu}=2$, we show a competition between the triangular vortex lattice, a quantum smectic state, and the incompressible $k=4$ Read-Rezayi state. We discuss the corrections arising from finite size effects, and the likely behavior for large system sizes. The Read-Rezayi state is stabilized by a moderate amount of additional dipolar interactions.

Magnetic Field‐Induced Instabilities in Liquid Crystals

We use the Landau–de Gennes model to investigate the magnetic field‐induced instabilities in liquid crystals. In particular, we examine the change of weak and strong stabilities in the pure smectic states and in the pure nematic states. Motivated by de Gennes’ discovery on the analogies between liquid crystals and superconductors, we introduce critical magnetic fields $H_{\text{\rm s}}$ and $H_{\rm sh}$. The pure smectic states lose their global minimality (strong stability) at $H_{\text{\rm s}}$ and lose their local minimality (weak stability) at $H_{\rm sh}$. We also examine the change of stability in the pure nematic states. We show in the case of equal elastic coefficients that a liquid crystal in a sufficiently strong magnetic field will not be in a pure nematic state, which exhibits a significant difference between the Landau–de Gennes model for liquid crystals and the Ginzburg–Landau model for superconductivity.

A bent‐core dopant‐induced smectic A* twist state†

The phase behaviour of a commercial calamitic ferroelectric liquid crystal mixture, doped with different mesogenic and non‐mesogenic bent‐core molecules was investigated through polarising microscopy, optical measurements and quenched growth. A twisted smectic structure, similar but not equivalent to a twist grain boundary (TGB) phase, and absent in the neat FLC mixture, was verified. The twisted smectic state can only be observed on cooling and its stability depends on the rate of temperature decrease, which indicates a kinetically governed behaviour. Further, the growth dynamics of the low temperature uniform SmA* bookshelf structure is dominated by viscosity instead of free energy density, as would be expected for a true thermodynamic phase transition. The investigations signify the chiral induction capability of achiral, bent‐core dopant molecules and we believe that the observed behaviour represents the onset of TGB formation at very large pitch. It can thus give valuable information for the fundamental physical understanding of twist grain boundary phase formation. †. †This paper is dedicated to Sven T. Lagerwall on the occasion of his 70th birthday

Current Topics in Smectic Liquid Crystal Research

Interest in the smectic liquid-crystalline state of matter received a substantial boost with the discovery by Meyer in the mid-1970s that a chiral smectic C (SmC*) phase exhibits a spontaneous electric polarization, and with the subsequent demonstration by Clark and Lagerwall of the surface-stabilized SmC* ferroelectric liquid crystal at the beginning of the 1980s. Since then, chiral smectic phases and their plethora of polar effects have dominated the research in this field, which today has reached a mature state where the first commercial microdisplay applications are now shipping in millions-per-year quantities. In this Review we discuss some of the topics of highest interest in current smectic liquid crystal research, and address application-relevant research (de Vries-type tilting transitions without defect generation and high-tilt antiferroelectric liquid crystals with perfect dark state) as well as more curiosity-driven research (the nature and origin of the chiral smectic C subphases and their intermediate frustrated states between ferro- and antiferroelectricity).

Landau--de Gennes Model of Liquid Crystals with Small Ginzburg--Landau Parameter

Liquid crystals may exhibit two types of behaviors, in comparison with two types of superconductors. We wish to explore type-I behavior of liquid crystals by using the Landau--de Gennes model with small Ginzburg--Landau parameter $\kappa$. In this paper, for small $\kappa$, we give an estimate on the critical wave number $Q_{c_3}$, at which a liquid crystal undergoes a phase transition from a nematic state to a smectic state. We show that, if $\kappa$ is small, a smectic phase can nucleate only from a special class of nematic states, which depends on the geometry of the domain occupied by the liquid crystal. If the wave number is further lowered, the liquid crystal is in a uniform smectic state, which resembles the perfect superconducting state of type-I superconductors.

Quantum Hall smectics, sliding symmetry, and the renormalization group

In this paper we discuss the implication of the existence of a sliding symmetry, equivalent to the absence of a shear modulus, on the low-energy theory of the quantum hall smectic (QHS) state. We show, through renormalization group calculations, that such a symmetry causes the naive continuum approximation in the direction perpendicular to the stripes to break down through infrared divergent contributions originating from naively irrelevant operators. In particular, we show that the correct fixed point has the form of an array of sliding Luttinger liquids which is free from superficially "irrelevant operators". Similar considerations apply to all theories with sliding symmetries.

Molecular dynamics simulation of a flexible polymer network in a liquid crystal solvent; structure and equilibrium properties

A flexible regular tetrafunctional polymer network containing a low molecular liquid crystal (LC) solvent was simulated with molecular dynamics. The LC solvent comprises of anisotropic rod-like semi-flexible linear molecules composed of beads bonded by a FENE potential. Flexibility was induced by a bending potential proportional to the cosine of the angle between neighbouring valence bonds. All interactions between non-bonded beads are described by the repulsive part of the Lennard–Jones potential. The average length of the network chain was chosen to be close to the length of a mesogen. The number of network cells was constant and the simulated systems differ from each other by the number of LC layers. The simulations of a system of flexible polymer chains in a low molecular LC solvent and a system of pure low molecular LC solvent were also carried out. Increasing the density of the composite system the LC solvent experiences the same phase transition as the pure LC: isotropic, nematic and smectic. The presence of the network shifts the isotropic–nematic transition to higher densities but does not significantly change the position of nematic–smectic transition. Transition of the LC solvent into the smectic state changes the morphology of the network. The periodicity of LC phase determines the number of network layers. The presence of linear chains in the LC solvent decreases the number of LC layers in the smectic phase. The LC order induces some stretching of the network chains along the direction of orientation and at the same time causes shrinkage in the perpendicular direction especially in the smectic phase.

Development of microcapsules for electron microscopy and their application to dynamicalobservation of liquid crystals in transmission electron microscopy

Microcapsules made of hard carbon films were fabricated for electron microscopicobservations of liquid-like samples such as biological cells/molecules in water solution andliquid crystals. 4-(1-methylheptyloxycarbonyl) phenyl--octyloxybiphenyl-4-carboxylate (MHPOBC) liquid crystals housed in the capsulewere observed by transmission electron microscopy at 120 kV accelerating voltage.From the series of electron diffraction patterns, the rate of electron irradiationdamage was estimated for the cases of changing sample temperature, beamcurrent density and accelerating voltage. Phase transitions from a crystallinephase to liquid crystalline phases such as an isotropic-A phase of a smectic state(Sm-IA) and a chiral-A phaseof a smectic state (Sm-CA*) were successfully observed and the rotation fluctuation of the molecules in the phase wasmeasured by using the electron diffraction patterns.

Dendrite-like texture growth in the nematic liquid crystal phase of 4-n-heptyl- and 4-n-octyl-oxybenzoic acids aligned by a polyimide coating

An oriented dendrite-like texture is reported, appearing at a definite temperature in the nematic phase range of 4-n-heptyl- and 4-n-octyl-oxybenzoic acids (HOBA and OOBA), aligned by rubbed polyimide and preceding the smectic C phase, on cooling. Two preferred directions with respect to the ‘easy’ axis are indicated in the dendrites grown of HOBA and OOBA. We discuss a possible mechanism, at molecular and supramolecular levels, for this dendrite growth; and assume that the building ‘blocks' of the dendrites are oligomers, or mixture of oligomers with ‘free’ closed and open dimers, constituting a detached crystalline layered state (named by us SmX, a smectic state intermediate between the N1 ordinary nematic and SmC phases). The study of the dynamics of the dendrite growth demonstrates a scaling relationship typical for non-equilibrium systems. The observed dendrites can be considered as patterns formed in complex non-linear dissipative systems, driven outside of equilibrium.

Crystal Structure and Smectic Orientation of a Mesogenic Compound

The structure of the mesogenic compound 3‐Cyanobenzyl‐2,5‐bis(4‐n‐octyloxy‐benzoyl‐oxy)benzoate (3CNBOB) was investigated by single and melt crystal methods. Using a single crystal of 3CNBOB at 20 °C, the unit cell structure was determined as triclinic with two molecules in a unit cell of dimensions: a = 8.792 Å, b = 11.760 Å, c = 19.121 Å, α = 96.61°, β = 90.56°, and γ = 90.28°. Three benzene rings of the compound exhibit π–π interactions in the crystalline form. The 3CNBOB showed a liquid crystalline state in the region of 82.9 to 101.5 °C. Melt crystallization was observed by in‐situ x‐ray diffraction using a temperature slope apparatus. A smectic phase appeared in the liquid crystalline region near the boundary to the crystalline phase. The smectic molecules showed spontaneous orientation aligned with their long axes parallel to the crystallizing plane. There were two crystalline states in the bulk material. The room‐temperature crystalline form was the same as the one observed using the single crystal. The high‐temperature crystalline state developed from the smectic state. During the smectic‐crystalline transition, the packing became denser and the spontaneous orientation decreased.

Effect of chirality on the structural behaviour of hydrogen-bonded n-alkylammonium pyroglutamates in the crystalline and smectic state.

A set of optically active and racemic n-alkylammonium pyroglutamates from dodecyl to octadecyl were synthesized and characterised. Their thermotropic polymorphism was investigated by polarizing optical microscopy, differential scanning calorimetry and dilatometry. Their structure in the crystalline and smectic state was analysed by X-ray diffraction. The hydrogen bonding of the molecules in the crystalline and smectic layers was examined by infrared spectroscopy. The chirality control over the supramolecular self-assembly of the molecules along with the homochiral and heterochiral architecture of the self-assembled dimers are briefly discussed.

High strength disclinations in the nematic and smectic states of 2,5-bis(4-alkoxybenzoyloxy)- p -benzoquinones

High strength disclinations were observed in both nematic and smectic phases exhibited by members of the 2,5-bis(4-alkoxybenzoyloxy)-p-benzoquinone series, beyond 4-n-pentoxy. The shorter members of the series show only a nematic mesophase, the highest members only a smectic mesophase, while intermediate members exhibit both types of mesophase. These high strength disclinations seen in a single component system are very stable due to energy dispersion and the high viscosity of the smectic state. All the high strength disclinations are negative in sign.

Disordered quantum smectics

We study the effect of disorder on the properties of the Quantum Hall smectic state arising in two dimciisional electron systems in high Landau levels. We use the replica trick and the Gauzsian Variational method to treat the disorder. We find that the quantum smectics are rather different from the usual classical smectics in that the density correlations along the direction of the stripes manifest a Bragg-Glass type behaviour whereas those in the transverse direction are infra-red divergent. This results in an amsotropic behaviour of all physical quantities. We calculate the dynamical conductivity $\sigma _{xx} ({\bf q}, \omega )$ along the stripe direction and find a $\bf q$ dependent pinning peak.

Polarized luminescence from self-assembled, aligned, and cleaved supramolecules of highly ordered rodlike polymers

A hierarchical self-assembly in comb-shaped supramolecules of conjugated rodlike polymers is reported. The supramolecules consist of poly(2,5-pyridinediyl), acid dopants, and hydrogen bonded alkyl side chains. A thermotropic smectic state with an exceptionally large coherence length is formed without additional solvent. This allows facile overall alignment resulting in high dichroism and polarized photoluminescence. Solid films are formed by cleaving side groups from the supramolecules which retain the optical anisotropy together with the high photoluminescence quantum yield of pristine polymer.

Theory of the quantum Hall Smectic Phase. I. Low-energy properties of the quantum Hall smectic fixed point

We develop an effective low-energy theory of the quantum Hall (QH) smectic or stripe phase of a two-dimensional electron gas in a large magnetic field in terms of its Goldstone modes and of the charge fluctuations on each stripe. This liquid-crystal phase corresponds to a fixed point that is explicitly demonstrated to be stable against quantum fluctuations at long wavelengths. This fixed-point theory also allows an unambiguous reconstruction of the electron operator. We find that quantum fluctuations are so severe that the electron Green function decays faster than any power law, although slower than exponentially, and that consequently there is a deep pseudo-gap in the quasiparticle spectrum. We discuss, but do not resolve, the stability of the quantum Hall smectic to crystallization. Finally, the role of Coulomb interactions and the low-temperature thermodynamics of the QH smectic state are analyzed.

In-Layer Defects in Smectic C Materials

We investigate the director structure close to the core region of in-layer defects found in smectic C thin films. These defects are similar to nematic disclination lines in appearance and we use the Landau-de Gennes formulation for the free energy of the system previously used to consider nematic defects. The minimum energy core structures are numerically calculated and two possible structures found, where either a standard smectic A or a de Vries smectic state is formed at the centre of the defect. These two states correspond to the escaped and planar configurations of nematic disclination cores. Finally, the energies of these core structures are found as functions of the dimensions of the thin film.