Molecular Liquid Crystals Research Articles

Orientation and Dynamics of ZnO Nanorod Liquid Crystals in Electric Fields

ZnO nanorod polymer hybrids (i.e., ZnO nanorods coated with a block copolymer with a short anchor block (dopamine) and a longer solubilizing block of polystyrene (PS)) form liquid crystalline (LC) phases if they are dispersed at high concentration e.g., in a PS oligomer matrix. Due to the high mobility of the low T(g) -matrix the nanorod polymer hybrids show a switching behavior under an applied AC electric field. Hence, the orientation of the nanorod mesogens can be changed from planar (parallel to the substrate) to homeotropic (perpendicular) in full analogy to the switching of low molecular liquid crystals in an electric field. Dielectric measurements show that such a switching is mainly due to the cooperative LC behavior, because the rods themselves exhibit only a very small effective dipole moment. The process can be investigated by polarizing microscopy. SEM images show the orientations of the individual nanorods, which correspond to the Fredericks transition well known for liquid crystals aligned in an electric field. This was the first time such a transition could be visualized by electron microscopy due to the large nanorod mesogens. The observation is interesting to orient nanorods perpendicular to an electrode and can help to improve optoelectronic devices.

ChemInform Abstract: 2H NMR in Liquid Crystals and Membranes

Abstract Deuterium NMR spectroscopy is widely applicable to studies of the structure and dynamics of molecular solids, liquid crystals, and thin films of membrane lipids. The properties of soft nanomaterials are also accessible on the mesoscopic length scale intermediate between the molecular and bulk dimensions. For membrane lipids in the liquid-crystalline state, rapid axial averaging occurs about the director axis (the membrane normal). One can then relate the profiles of the order parameters | S CD | of the individual C– 2 H labeled segments to average bilayer properties. These include the mean area per molecule and projected acyl chain length, the area compressibility modulus, and the radius of curvature for reverse hexagonal ( H II ) phase nanotubes. In addition, measurements of the relaxation rates for Zeeman order, R 1Z , and quadrupolar order, R 1Q , enable one to investigate the mean-squared amplitudes and time-scales of the fluctuations that underlie the thermodynamic properties. A unified interpretation is provided by a composite membrane deformation model, which fits simultaneously the frequency dependence and the angular anisotropy of the R 1Z and R 1Q relaxation rates. The results suggest the bilayer dynamics in the MHz regime can be modeled in terms of nematic-like deformations of the membrane hydrocarbon interior, together with axial rotations of the lipid acyl chains. A small contribution from internal segmental motions is found, which implies the bilayer microviscosity is comparable to that of a liquid hydrocarbon. Finally, the 2 H-NMR relaxation rates of lipid bilayers containing cholesterol in the liquid-ordered phase suggest a dynamically more rigid bilayer, involving fast axial lipid rotations together with a reduction in collective bilayer deformations. Possible future applications include studies of liquid crystals and thin films of membrane lipids and surfactants, as well as lipid-protein systems.

Prediction of the Liquid-Crystalline Property Using Different Classification Methods

This article reports a new approach to predict the liquid crystalline behavior using three classes of machine learning algorithm: eager learners (C4.5, random tree, random forest, reduced error pruning tree), lazy learners (k-nearest neighbor, non-nested generalized exemplars with prototypes), and neural networks. The performance analysis for these algorithms supposes calculating the errors both for the training and validation data, in order to determine the accuracy in building the model and its generalization capability. A large database containing 390 compounds with different units connected to the bis- and tris-phenyl aromatic, azo-aromatic, azomethinic types was used as example. The prediction of liquid crystalline property is correlated with chemical structure, type of compound, molecular weight, and a series of structural characteristics estimated by mechanical molecular simulation. Advantages and disadvantages are discussed for each method; the best results are obtained with lazy classification algorithms, especially with our original one, non-nested generalized exemplars with prototypes (NNGEP). [Supplementary materials are available for this article. Go to the publisher's online edition of Molecular Crystals and Liquid Crystals for the following free supplemental resource: Data Base]

On the Effect of a Rotating Magnetic Field or Sample on the Bend Freedericksz Critical Field in Nematic Slabs

A numerical study of the effect of a continuously rotating field or sample on the magnetic Freedericksz transition in homeotropic nematics slabs is presented. Homeotropic boundary conditions with strong anchoring are considered. The magnetic field is applied parallel to the plates, i.e., in the bend Freedericksz geometry. The behaviour of a low molecular weight liquid crystal (5CB) and of a high molecular weight polymer liquid crystal (PBLG) is compared using the Leslie-Ericksen theory of nematodynamics. The effect of the sample thickness, of the magnetic field strength and of the spinning frequency on the response of both materials is studied. Our results show that, in both cases and for both materials, the value of the Freedericksz critical field H c (ν) increases with the spinning frequency ν. The transition H c (ν) → H c (0) is found to be continuous in the rotating sample case and to be discontinuous in the rotating field case. The plot of the reduced critical field vs. the reduced spinning frequency gives universal curves for both materials in both cases.

Microscopic Modelling of Charge Transport in Molecular Crystals, Polymers and Liquid Crystals

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Crops: A Green Approach toward Self-Assembled Soft Materials

To date, a wide range of industrial materials such as solvents, fuels, synthetic fibers, and chemical products are being manufactured from petroleum resources. However, rapid depletion of fossil and petroleum resources is encouraging current and future chemists to orient their research toward designing safer chemicals, products, and processes from renewable feedstock with an increased awareness of environmental and industrial impact. Advances in genetics, biotechnology, process chemistry, and engineering are leading to a new manufacturing concept for converting renewable biomass to valuable fuels and products, generally known as the biorefinery concept. The swift integration of crop-based materials synthesis and biorefinery manufacturing technologies offers the potential for new advances in sustainable energy alternatives and biomaterials that will lead to a new manufacturing paradigm. This Account presents a novel and emerging concept of generating various forms of soft materials from crops (an alternate feedstock). In future research, developing biobased soft materials will be a fascinating yet demanding practice, which will have direct impact on industrial applications as an economically viable alternative. Here we discuss some remarkable examples of glycolipids generated from industrial byproducts such as cashew nut shell liquid, which upon self-assembly produced soft nanoarchitectures including lipid nanotubes, twisted/helical nanofibers, low-molecular-weight gels, and liquid crystals. Synthetic methods applied to a "chiral pool" of carbohydrates using the selectivity of enzyme catalysis yield amphiphilic products derived from biobased feedstock including amygdalin, trehalose, and vitamin C. This has been achieved with a lipase-mediated regioselective synthetic procedure to obtain such amphiphiles in quantitative yields. Amygdalin amphiphiles showed unique gelation behavior in a broad range of solvents such as nonpolar hexanes to polar aqueous solutions. Importantly, an enzyme triggered drug-delivery model for hydrophobic drugs was demonstrated by using these supramolecularly assembled hydrogels. Following a similar biocatalytic approach, vitamin C amphiphiles were synthesized with different hydrocarbon chain lengths, and their ability to self-assemble into molecular gels and liquid crystals has been studied in detail. Such biobased soft materials were successfully used to develop novel organic-inorganic hybrid materials by in situ synthesis of metal nanoparticles. The self-assembled soft materials were characterized by several spectroscopic techniques, UV-visible, infrared, and fluorescence spectrophotometers, as well as microscopic methods including polarized optical, confocal, scanning, and transmission electron microscopes, and thermal analysis. The molecular packing of the hierarchically assembled bilayer membranes was fully elucidated by X-ray analysis. We envision that the results summarized in this Account will encourage interdisciplinary collaboration between scientists in the fields of organic synthesis, soft materials research, and green chemistry to develop functional materials from underutilized crop-based renewable feedstock, with innovation driven both by material needs and environmentally benign design principles.

Ionic self-assembled molecular receptor-based liquid crystals with tripeptide recognition capabilities

Noncovalent interactions have been used to modify the binding properties of two-armed diketopiperazine receptors by ionic self-assembly and induced the formation of liquid-crystalline (LC) materials. The properties of the LC material were in turn modified by binding to a selectively recognized peptidic guest.

Intermolecular Interactions in Langmuir and Langmuir-Blodgett Films of Azo Dye/Liquid Crystal Mixtures

Binary mixtures of the dichroic azo dye with the liquid crystal as Langmuir and Langmuir-Blodgett films have been studied. Four azo dyes with various molecular structure and liquid crystal materials: 4-octyl-4′-cyanobiphenyl (8CB) and trans-4-octyl(4′-cyanophenyl)-cyclohexane (PCH8) were used. The surface pressure as a function of the mean molecular area for Langmuir films were measured and the organization of molecules at the air-water interface was determined. Absorption spectra in situ for Langmuir films and absorption spectra by using both natural and linearly polarized light for Langmuir-Blodgett films were recorded and information about intermolecular interactions in the mixtures of the nonamphiphilic dye and the liquid crystal with polar terminal group was obtained. Conclusions about the formation of self-aggregates between dye molecules have been drawn.

ER EFFECT OF LOW MOLECULAR LIQUID CRYSTAL ON ONE-SIDED PATTERNED ELECTRODES

Several kinds of ER fluids (ERF) have been developed and have been applied to some mechatronics devices and processing technologies. In many conventional applications of ERFs, these devices consist of bilateral electrodes to apply electric field in ERF. However, the electric field of several kV/mm may be necessary to generate an ER effect sufficiently for practical purposes. The gap between a pair of electrodes should be, therefore, maintained narrowly and exactly for fears of short-circuit. At the same time, this electrode system also requires an interconnection on driving parts. To improve these disadvantages, we proposed "one-sided patterned electrode" (OSPE) systems in previous works. In this report, we confirmed the flow characteristics of low molecular liquid crystal (LMLC) on OSPE. Next, we also confirmed the different characteristics depending on the pattern type. Depending on results of electro-static analysis, we conclude that such a difference may results from the directors of LC molecules derived by electric field.

Boundary conditions for fluids with internal orientational degrees of freedom: Apparent velocity slip associated with the molecular alignment

Boundary effects are investigated for fluids with internal orientational degrees of freedom such as molecular liquids, thermotropic and lyotropic liquid crystals, and polymeric fluids. The orientational degrees of freedom are described by the second rank alignment tensor which is related to the birefringence. We use a standard model to describe the orientational dynamics in the presence of flow, the momentum balance equations, and a constitutive law for the pressure tensor to describe our system. In the spirit of irreversible thermodynamics, boundary conditions are formulated for the mechanical slip velocity and the flux of the alignment. They are set up such that the entropy production at the wall inferred from the entropy flux is positive definite. Even in the absence of a true mechanical slip, the coupling between orientation and flow leads to flow profiles with an apparent slip. This has consequences for the macroscopically measurable effective velocity. In analytical investigations, we consider the simplified case of an isotropic fluid in the Newtonian and stationary flow regime. For special geometries such as plane and cylindrical Couette flow, plane Poiseuille flow, and a flow down an inclined plane, we demonstrate explicitly how the boundary conditions lead to an apparent slip. Furthermore, we discuss the dependence of the effective viscosity and of the effective slip length on the model parameters.

Monodomain liquid crystal elastomers and elastomeric gels: Improved thermomechanical responses and phase behaviour by addition of low molecular weight LCs

Large monodomain-liquid crystal elastomer films have been prepared using mechanical alignment, either as pure materials or as gels doped with low molecular weight chiral liquid crystals. The thermomechanical strain behaviour of the resulting materials has been studied over a range of temperatures. The largest thermal strains have been achieved when the mechanical alignment was carried out in the smectic A phase, just below the transition to the apparent isotropic liquid phase. The pure elastomers exhibit thermomechanical responses over broad temperature ranges (ca. 60 °C), however the elastomer gels doped with a low molecular weight LC exhibit significantly sharper thermomechanical responses over a narrower temperature range (ca. 15 °C).

Phase Separation in Binary Polymer/Liquid Crystal Mixtures: Network Breaking and Domain Growth by Coalescence-induced Coalescence

A small-angle light scattering (SALS) technique together with optical microscopy observation are used to investigate phase separation kinetics in films of low molecular weight thermotropic liquid crystal (4-cyano-4'-n-octyl-biphenyl, 8CB) with flexible polymer (polystyrene, PS). The growth of domains is studied as a function of time, film thickness, and film composition. The light scattering results are correlated with the images obtained by optical microscopy observation. In this paper, we study the breaking of a bicontinuous network of polymer in liquid crystal into droplets and their further growth via the coalescence-induced coalescence mechanism. The appearance of droplets in the system leads to a strong scattering at small wave vectors, while the bicontinuous network gives a peak at a nonzero wave vector. Superposition of these scattering intensities leads to the appearance of a second peak in the full scattering intensity signal, when the bicontinuous network starts to break up into disjointed elongated domains. Finally, both peaks merge into a single peak, which moves quickly toward zero wave vectors, indicating a complete transformation of elongated domains into spherical droplets of variable size. We found that the separation process does not depend on the size of the system. Irrespective of the sample thickness, the network breaks into fragments always at the same time after temperature quench. On the basis of morphological analysis, we found that the average size of the droplets which formed from the network grows with time, t, as t(alpha), alpha = 0.9 +/- 0.1, in the isotropic phase and in the nematic phase.

Applications of self-consistent field theory in polymer systems

The self-consistent field theory (SCFT) based upon coarse-grained model is especially suitable for investigating thermodynamic equilibrium morphology and the phase diagram of inhomogeneous polymer systems subjected to phase separation. The advantage of this model is that the details of the chain such as the architecture of the chain and the sequence of blocks can be considered. We present here an overview of SCFT approach and its applications in polymeric systems. In particular, we wish to focus on our group’s achievements in applications of SCFT in such fields: simulation of microphase separation morphologies of multiblock copolymers with a complex molecular architecture, interactions between brush-coated sheets in a polymer matrix, mixtures of flexible polymers and small molecular liquid crystals at the interface, shapes of polymer-chain-anchored fluid vesicles, self-assembled morphologies of block copolymers in dilute solution, and so on. Finally, the further developments as well as the perspective applications of SCFT are discussed.

ER Effect of Low Molecular Liquid Crystal on One-sided Patterned Electrodes

ER fluid (Electro-rheological fluid) is a kind of functional fluids whose apparent viscosity changes under an electric field. Recently many researches on this material are developed for applications with clutches, brakes, dampers and other kind of mechatronics technologies. The ER fluids are classified into two types, particle-type ER fluid and homogeneous-type ER fluid. The homogeneous-type ER fluids include low molecular liquid crystal (LC). In this report, the flow characteristics of LC on one-sided patterned electrodes were confirmed. And next it was also confirmed that such effects have different characteristics depending on pattern type. According to a result of electro-static analysis and Miesowicz viscosity model, we conclude that such difference results from a director of LC molecules derived by electric field.

Photo-alignment Properties of Main-Chain Liquid-Crystalline Polyimides Containing Chalcone Derivative

We investigated photo-alignment properties of main-chain polyimides prepared from acid anhydrides [4,4′-(hexafluoro-isopropylidene) diphthalic anhydride (6FDA) or 4,4′-biphthalic anhydride (BPDA)] and diamine derivatives containing a chalcone group. The polyimides in the film state were irradiated with linearly polarized UV (LPUV) light under different conditions. The liquid-crystalline (LC) cell was fabricated from the polyimide film irradiated with LPUV light. A mixture of a low molecular liquid crystal (4-cyano-4′-n-pentylbiphenyl: 5CB) and a dichroic dye was filled in the LC cell. 5CB in the LC cell was easily aligned perpendicular to the electric vector of the incident LPUV light.

Molecular dynamics simulation of a flexible polymer network in a liquid crystalline solvent; dynamical properties

Molecular dynamics simulation was performed for the systems consisting of a flexible regular tetrafunctional polymer network and a low molecular liquid crystal (LC) solvent. The LC solvent comprises of anisotropic rod-like semiflexible linear molecules composed of beads bonded by a FENE potential. Rigidity 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. For comparison the simulations of the system of flexible polymer chains in a low molecular LC solvent and a system of pure low molecular LC solvent were also carried out. The influence of the network and linear chain polymer on the translational and rotational mobility of low molecular LC solvent was studied. The influence of the LC solvent ordering on the local translational mobility of the polymer chains was also observed.

Correlation between photorefractivity, photoconductivity, and dark conductivity in dye-doped, low molecular mass nematic liquid crystal cells

Abstract The photoconductivity and photorefractivity of dye-doped low molecular mass nematic liquid crystal homeotropic cells exhibit a threshold at an applied voltage of the order of 1 V (in our samples). At about the same voltage, the dark conductivity of the cells exhibits a change from a cubic current/voltage characteristic to a linear one. We explain these observations by postulating that the cell's conductivity is due to ionic charge carriers generated near the electrodes by some electrochemical processes activated by the injection of electrons and holes. Below the noted threshold, the dark current is limited by a residual ionic space–charge distribution, and grows as the cube of the applied voltage. In this regime, photogenerated charge carriers are screened and are prevented from forming the static space–charge field needed to initiate an orientational photorefractive response. Above that threshold, the residual ionic space–charge disappears, the cell is a weak ohmic electrolyte, and the photogenerated charges contribute to the photoconductivity and photorefractivity of the cell.

Organization of a Polar Molecule at the Air−Water Interface

We have studied the behavior of a low molecular mass organosiloxane ferroelectric liquid crystal at the air−water interface. Data obtained with Langmuir isotherms, surface potential measurements, and Brewster angle microscopy are presented and discussed. The compound forms films on the Langmuir trough, with phase transitions to multilayers upon compression. As seen with Brewster angle microscopy, the monolayers are optically isotropic whereas striking birefringent structures develop in the multilayer films. These periodic structures result from a surface elasticity phenomenon.

Organic Photochemistry within Zeolites: Selectivity Through Confinement

AbstractFor Abstract see ChemInform Abstract in Full Text.

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.