Smectic Structure Research Articles

Ferroelectric Smectic C Liquid Crystal Phase with Spontaneous Polarization in the Direction of the Director.

In the previous study, the existence of an unidentified ferroelectric smectic phase is demonstrated in the low-temperature region of the ferroelectric smectic A phase, where the layer spacing decreases with decreasing temperature. In the present study, the phase is identified by taking 2D X-ray diffraction images of a magnetically oriented sample while allowing it to rotate and constructed a 3D reciprocal space with the sample rotation angle as the third axis for the whole picture of the reciprocal lattice vectors originating from the smectic structure. Consequently, circular diffraction images are obtained when the reciprocal lattice vectors are evenly distributed on the conical surface at a certain inclination angle in the reciprocal space. This result provides clear evidence that the phase in question is smectic C. The polarization properties also showed that the observed smectic C phase has spontaneous polarization in the direction parallel to the director and is identified as ferroelectric smectic C. These results provide a new type of classification for liquid crystalline phases that has been established over many years and is a significant contribution to the basic science of soft matter research.

Domain reorientation due to smectic layer instability in high tilt angle-based surface stabilized ferroelectric liquid crystal cell

High tilt angle (45°) ferroelectric liquid crystal (FLC) in surface stabilized geometry, having no chiral smectic A (SmA∗) phase, has been studied for the reorientation of the smectic layers near the transition temperature (Tc). The electro-optical studies have shown the stripe domain formation in which the liquid crystal molecules are aligned along the rubbing direction, but the smectic layers are tilted away from the rubbing direction at room temperature. In such high tilt angle FLCs, the molecular alignment and smectic layer formation start from bottom and top rubbed grooves at Tc. The domain formation of the FLC takes place in the middle of the top and bottom surfaces due to the frustration of the dipolar interaction. The smectic layer switching is observed by optical microscopy and confirmed by the dielectric spectroscopy method near Tc of SmC∗ and chiral nematic phases. Domain switching has shown a larger switching angle than the molecular tilt angle within the smectic layer. These studies are expected to be significant for understanding the smectic layer structure and the domain switching process, which may pave the way for large optical switching devices.

Structure Investigation of Mesogenic Homo- and Copolymers Based on Chiral and Achiral Acrylates

The X-ray diffraction method was used to study the structure of mesogenic homo- and copolymers based on chiral and achiral acrylates (disubstituted biphenyl and phenylbenzoate) under the orienting influence of magnetic and (in some cases) electric fields. The analysis of diffraction patterns was carried out in terms of the Hosemann model of a paracrystal using structural modeling and diffraction calculations on models. It was established that chiral and achiral homopolymers form smectic bilayer polar structures, but the structure of the achiral homopolymer possesses weak polarization due to small value of total dipole moment of C=O groups in the side chains. The copolymers (as the chiral homopolymer) form smectic bilayer polar helicoidal structures. The distribution of chiral and achiral components in the layers of the common bilayer depends on their ratios in the copolymer. In turn, this distribution affects both the step of helicoidal structure and the nature of structural-phase transformations in the copolymers. A comparative evaluation of the orientational effects of magnetic and electric fields on the structure of the chiral homopolymer and the copolymers has been carried out. The analysis of the temperature dependences of diffraction and structural parameters of the investigated polymers allowed us to explain some features of their behaviour within seemingly similar (by the type of X-ray patterns) smectic phases.

The effect of surfaces on the structural properties of liquid crystals

The effect of the surface microrelief on the structural properties of both the nematic and smectic A (SmA) phases, and the effect of the emerging smectic structure induced by the surface at temperatures close to the second-order nematic-SmA (NA) phase transition temperature TNA, on the Freedericksz threshold voltage for homogeneously aligned polar liquid crystals, is theoretically analyzed.It is shown that the orientation distortion of the director field caused by the wave-like microrelief on solid surfaces exponentially decays in the bulk of both phases. Since in the case of the SmA phase, the characteristic depth of deformation is comparable to the length of the molecule, then when the period of undulating relief passes several micrometers, the depth of penetration of the deformation is several tens of micrometers. This is two orders of magnitude greater than the penetration length of the distortion in the case of the nematic phase.The effect of the surface-induced SmA phase on the threshold voltage, for homogeneously aligned of polar liquid crystals is discussed from the energy point of view. It is shown that the Freedericksz critical voltage, for 4-n-octyl-4′-cyanobiphenyl, in a very thin cell near the second-order nematic-SmA (NA) phase transition temperature, exhibits an anomalous increase with decreasing of temperature T towards TNA.

Vertically aligned polyimide films prepared from lyotropic liquid-crystalline smectic precursors

Vertically aligned (VA), fully aromatic polyimide (PI) thin films were successfully prepared from a lyotropic smectic liquid-crystalline precursor of poly(amic ester) (PAE) containing perfluorooctyl (Rf) terminal groups using a “push-coating” method. Because the repeating units of PAE form the smectic layer, the main chains of PAE are oriented perpendicular to the layer. A homeotropically aligned smectic structure is induced by the step-by-step stacking growth of the layer from the monolayer of the Rf groups self-organized on the hydrophobic interfaces (interfaces between the PAE solution and air or a surface-modified substrate), leading to a vertically aligned PAE. By optimizing the preparation conditions, the values of the orientational order parameter (S) of the push-coated PAE film exceeded 0.7. The homeotropically aligned PAE films were imidized by heating to 300 °C, and the high S values were maintained in the subsequent imidization to give VA PI films. Despite the large standard deviation in the temperature variation of the optical path length of the VA PI films, the small and reversible variations in S confirmed that the vertical orientation of the PI films was thermodynamically stable.

Leaning induced layer undulated tilted smectic phase of asymmetric bent-core liquid crystals.

We report the synthesis and experimental studies on the liquid crystalline phase of a homologous series of compounds comprised of strongly asymmetric bent-core banana shaped molecules. Our x-ray diffraction studies clearly indicate that the compounds exhibit a frustrated tilted smectic phase with layer undulated structure. The low value of the dielectric constant as well as switching current measurements indicate the absence of polarization in this layer undulated phase. In spite of the absence of polarization, a planar aligned sample can be transformed irreversibly to a higher birefringent texture on the application of a high electric field. The zero field texture can only be retrieved by heating the sample to the isotropic phase and cooling it subsequently to the mesophase. We propose a double tilted smectic structure having layer undulation to account for the experimental observations with the layer undulation arising due to the leaning of the molecules in the layer.

Structural and Mineralogical Characterization of the Clayey Deposits of Gbédji-Kotovi in Lama Depression, Bénin

The objective of the study was to characterize the clays of the Lama depression in Benin. Macroscopic examination of hand soundings, dynamic penetration test and stratigraphic correlation of water borehole logs, as well as scanning electron microscopy (SEM) combined with energy-dispersive X-ray microanalysis (EDX) method were made on compact or stratified clays. The large grains are embedded in a sintered matrix. The texture is heterogeneous and open, including quartz, carbonates and organic matter. The fine fraction is dominated by mineral paragenesis characterizing Kaolinite-Quartz-Calcite-Gypsum dioctahedral smectites composed of: Oxygen, Silica, Carbon, Aluminum, Iron, Zinc, Titanium and Magnesium. This smectic and regular structure includes coarse grains of the order of 800 μm. The pore diameters vary from 130 μm to 1.14 μm. The inter-particle porosity is poorly developed unlike the inter-aggregate pores larger than 0.05 μm. The texture is laminar and shows elongated turbo static particles with more or less rounded edges and honeycomb particles. Quartz and carbonates induce an increase in heterogeneities which develop mechanical sensitivity and hydraulic conductivity. High contents of silica, iron or aluminum and low contents of calcium and magnesium, as well as the presence of other trace elements suggest an alumino-ferriferous clay resulting from the hydrothermal alteration of the acid granite massif. These heterogeneities promote less tortuous pores or paths, making these clays more permeable. Finally, other stabilization studies and improvements to hydraulic products and binders should favor the use of the studied clay as improved backfill or drilling muds.

Induction of Highly Ordered Liquid Crystalline Phase of an Azobenzene Side Chain Polymer by Contact with 4'-Pentyl-4-cyanobiphenyl: An In Situ Study.

The orientation of liquid crystal (LC) molecules is significantly governed by solid interfaces and free surfaces, and a variety of functional materials have been developed using these properties. Although LC materials are already in industrial use, particularly for LC display panels, various studies have been conducted in recent years to better grasp the interface behavior of LC molecules. In this work, we succeeded in in situ observations of induction of higher ordered LC phases at the interface between a side-chain LC azobenzene polymer film with a thickness of ∼400 nm and a low-molecular-mass nematic LC, 4'-pentyl-4-cyanobiphenyl of 35 μm thickness, using small-angle X-ray scattering measurements and polarized optical microscopy. It is revealed that the two different mesogens cooperatively form hybrid higher ordered smectic LC phases probably through weak electron transfer immediately after interfacial contact. The induction process consists of three stages in terms of dynamic structure evolutions. Upon UV irradiation, the hybrid smectic LC structure diminished. This study provides new insights into the behavior of LC molecules near the alignment film on the solid substrate.

Ion Transport in 2D Nanostructured π-Conjugated Thieno[3,2-b]thiophene-Based Liquid Crystal.

Leveraging the self-assembling behavior of liquid crystals designed for controlling ion transport is of both fundamental and technological significance. Here, we have designed and prepared a liquid crystal that contains 2,5-bis(thien-2-yl)thieno[3,2-b]thiophene (BTTT) as mesogenic core and conjugated segment and symmetric tetra(ethylene oxide) (EO4) as polar side chains for ion-conducting regions. Driven by the crystallization of the BTTT cores, BTTT/dEO4 exhibits well-ordered smectic phases below 71.5 °C as confirmed by differential scanning calorimetry, polarized optical microscopy, temperature-dependent wide-angle X-ray scattering, and grazing incidence wide-angle X-ray scattering (GIWAXS). We adopted a combination of experimental GIWAXS and molecular dynamics (MD) simulations to better understand the molecular packing of BTTT/dEO4 films, particularly when loaded with the ion-conducting salt lithium bis(trifluoromethanesulfonyl)imide (LiTFSI). Ionic conduction of BTTT/dEO4 is realized by the addition of LiTFSI, with the material able to maintain smectic phases up to r = [Li+]/[EO] = 0.1. The highest ionic conductivity of 8 × 10-3 S/cm was attained at an intermedium salt concentration of r = 0.05. It was also found that ion conduction in BTTT/dEO4 is enhanced by forming a smectic layered structure with irregular interfaces between the BTTT and EO4 layers and by the lateral film expansion upon salt addition. This can be explained by the enhancement of the misalignment and configurational entropy of the side chains, which increase their local mobility and that of the solvated ions. Our molecular design thus illustrates how, beyond the favorable energetic interactions that drive the assembly of ion solvating domains, modulation of entropic effects can also be favorably harnessed to improve ion conduction.

Alignment Control of Smectic Layer Structures in Liquid-Crystalline Polymers by Photopolymerization with Scanned Slit Light.

Photoalignment control of hierarchical structures is a key process to enhance the properties of optical and mechanical materials. We developed an in situ molecular alignment method, where photopolymerization with the scanned slit light causes molecular flow, leading to two-dimensional precise alignment of molecules over large areas; however, the alignment control has been explored only on a molecular scale. In this study, we demonstrate this photopolymerization-induced molecular flow, enabling mesoscopic alignment of smectic layer structures composed of anisotropic molecules. Side-chain liquid-crystalline polymers were obtained from two different monomers with or without alkyl spacers by photopolymerization with one-dimensionally scanned slit light. The polymer with an alkyl spacer displayed mesogens aligned parallel to the scanning direction, while the polymer with no alkyl spacer resulted in perpendicular alignment of mesogens to the scanning direction, regulated by the alignment of the polymer main chain along the light scanning direction. Moreover, the polymerization with the scanned light aligned not only the mesogens but also mesoscopic smectic layer structures over large areas, depending on the structure and scanning pattern of light. We envision that such a simple polymerization technique could become a powerful and versatile alignment platform of anisotropic materials in a wide range of scales.

Molecular Order Determines Gas Transport through Smectic Liquid Crystalline Polymer Membranes with Different Chemical Compositions

Amorphous polymers are often used for gas separation but have a trade-off between gas permeability and selectivity. Here, the effect of chemical composition and temperature on gas permeability and solubility in well-ordered LC polymer membranes is investigated. Membranes with various compositions of a monomethacrylate LC (M1) with a crown ether functionality to enhance CO2 solubility and a smectic diacrylate (M2) cross-linker were fabricated, while all having the same order (smectic C) and alignment (planar). Single gas sorption and permeation data show for the membranes with 30 wt % M1 a higher CO2 solubility coefficient compared to membranes without M1, which results in a higher CO2 permeability and selectivity. For membranes that contain more than 30 wt % M1 decreasing layer spacings lead to reduced gas solubilities that result in lower gas permeabilities without additional selectivity gain toward CO2. The effect of temperature is demonstrated by comparing single gas sorption and permeation data below and above the Tg of the membranes. The diffusion coefficient increases above the Tg of the membranes with increasing M1 content leading to higher CO2 permeabilities and selectivities. These results show that not only the chemical composition but also the layer spacing of the smectic structures determines the gas separation performance of smectic LC polymer membranes.

Chiral π-Conjugated Liquid Crystals: Impacts of Ethynyl Linker and Bilateral Symmetry on the Molecular Packing and Functions

Recently, various chiral aromatic compounds, including chiral π-conjugated liquid crystals, have been developed for their unique photofunctions. One of the typical photofunctions is the bulk photovoltaic effect of ferroelectric π-conjugated liquid crystals, which integrates a polar environment based on molecular chirality with an extended π-conjugation system. Tuning the spectral properties and molecular packing is essential for improving the optical functions of the chiral π-conjugated liquid crystals. Herein, we examined the effects of an ethynyl linker and bilateral symmetry on the liquid-crystalline (LC) properties and π-conjugated system through detailed characterization via polarizing optical microscopy, differential scanning calorimetry, and X-ray diffraction analysis. The spreading of the π-conjugated system was evaluated using UV–vis absorption and photoluminescence spectroscopy. Bilateral symmetry affects the LC and photoluminescent properties. Hetero-substitution with a sparse ethynyl linker likely allows the formation of an interdigitated smectic LC structure. Because the molecular packing and photophysical properties can affect the photo- and electrical functions, we believe this study can promote the molecular design of novel functional π-conjugated materials, such as chiral ferroelectric π-conjugated liquid crystals, exhibiting the bulk photovoltaic effect.

Correlating the drying kinetics and dried morphologies of aqueous colloidal gold droplets of different particle concentrations

The evaporation of a gold nanorod (Au-NR) dispersion droplet, containing residual surfactant, on a hydrophilic silicon substrate results in a coffee stain with different microstructures. The distinct regions in the coffee stain, formed during different stages of droplet drying, are presumably correlated with the dynamics of the evaporating droplet. We drop-cast 2 μL CTAB capped Au-NR dispersion droplets on a hydrophilic substrate, and study their evaporation kinetics, and dried patterns using optical, electron, and atomic force microscopy. We report that droplet retraction, pinning, and quick evaporation during the initial, intermediate, and final phases of droplet drying are clearly correlated with the different coffee stain aggregation regions for a range of Au-NR concentrations. The receding velocity of the evaporating gold colloidal droplet decreases with increasing Au-NR concentration, resulting in random to cluster-like aggregations of Au-NRs outside the coffee stain. Droplet depinning is followed by a concentration-dependent droplet pinning process which results in the formation of a coffee stain edge comprising distinct domains that consist of smectic phase-like structures of Au-NRs. The duration of droplet pinning, the widths and heights of the outer coffee stain edge increase with increasing gold nanorod concentration and are consistent with theoretical predictions. Finally, the rapid evaporation of the droplet leads to the formation of non-uniform depletion region and inner coffee stain regions. Our study demonstrates that the solute deposition pattern can be tailored by controlling the evaporating droplet kinetics. The unique assemblies of Au-NRs in the coffee stain edge are potentially excellent candidates for the development of plasmonic sensors for the selective detection of trace amounts of molecules, while the large clusters of end-to-end aligned Au-NRs outside the stain could be used for the fluorescence enhancement of low quantum yield molecules.

Recyclable Periodic Nanostructure Formed by Sublimable Liquid Crystals for Robust Cell Alignment

We demonstrate a facile method to fabricate a recyclable cell-alignment scaffold using nanogrooves based on sublimable liquid crystal (LC) material. Randomly and uniaxially arranged smectic LC structures are obtained, followed by sublimation and recondensation processes, which directly produce periodic nanogrooves with dimensions of a couple of hundreds of nanometers. After treatment with osmium tetroxide (OsO4), the nanogroove can serve as a scaffold to efficiently induce directed cell growth without causing cytotoxicity, and it can be used repeatedly. Together, various cell types are applied to the nanogroove, proving the scaffold's broad applicability. Depending on the nanotopography of the LC structures, cells exhibit different morphologies and gene expression patterns, compared to cells on standard glass substrates, according to microscopic observation and qPCR. Furthermore, cell sheets can be formed, which consist of oriented cells that can be repeatedly formed and transferred to other substrates, while maintaining its organization. We believe that our cell-aligning scaffold may pave the way for the soft material field to bioengineering, which can involve fundamentals in cell behavior and function, as well as applications for regenerative medicine.

Self-assembly of Gold Nanorods into a Highly Ordered Sheet via Electrostatic Interactions with Double-stranded DNA

Assemblies of cationic gold nanorods (AuNRs) via electrostatic interactions with double-stranded (ds) DNA were investigated in solution and after evaporation as a cast-film. Interestingly, moderately positively charged AuNRs assembled with dsDNA provided a monolayer sheet with an ordered alignment resembling a two-dimensional (2D) smectic structure during solvent evaporation.

Effect of Smectic Layers on Thermal Diffusivity of Side-Chain Polymer Liquid Crystals

The thermal conductivity (TC) of polymers is known to be effectively enhanced by liquid crystallinity. Liquid crystals (LCs) exhibit anisotropic TC with a greater magnitude in the orientation direction. However, the effects of other LC structures than the orientational order, such as smectic layer structures, on TC have not been well studied. This study investigated the correlation between LC structures and thermal diffusivity using a series of LC PMnMA polymethacrylates having azobenzene mesogens in the side chains. PMnMA formed nematic (N), smectic A (SmA), and smectic C (SmC) phases. These LCs were aligned by a magnetic field or large-amplitude oscillatory shear, yielding glassy LC films with mesogen long axes in the in-plane or out-of-plane direction. These glassy LCs increased thermal diffusivities in the LC orientation direction with the degree of LC orientation, whereas N and SmC LCs had smaller values than SmA LCs. These results demonstrated that smectic layers can enhance thermal diffusivity in their normal direction.

Polymersomes with a smectic liquid crystal structure and AIE fluorescence

Fluorescent smectic polymersomes with aggregation-induced emission are prepared from an amphiphilic block copolymer containing a liquid crystal hydrophobic block and a tetraphenylethene-bearing unit between hydrophilic and hydrophobic blocks.

Design rules for 2D field mediated assembly of different shaped colloids into diverse microstructures.

Assembling different shaped particles into ordered microstructures is an open challenge in creating multifunctional particle-based materials and devices. Here, we report the two-dimensional (2D) AC electric field mediated assembly of different shaped colloidal particles into amorphous, liquid crystalline, and crystalline microstructures. Particle shapes investigated include disks, ellipses, squares, and rectangles, which show how systematic variations in anisotropy and corner curvature determine the number and type of resulting microstructures. AC electric fields induce dipolar interactions to control particle positional and orientational order. Microstructural states are determined via particle tracking to compute order parameters, which agree with computer simulations and show how particle packing and dipolar interactions together produce each structure. Results demonstrate how choice of particle shape and field conditions enable kinetically viable routes to assemble nematic, tetratic, and smectic liquid crystal structures as well as crystals with stretched 4- and 6-fold symmetry. Results show it is possible to assemble all corresponding hard particle phases, but also show how dipolar interactions influence and produce additional microstructures. Our findings provide design rules for the assembly of diverse microstructures of different shaped particles in AC electric fields, which could enable scalable and reconfigurable particle-based materials, displays, and printing technologies.

Liquid Crystalline Rylenediimides with Highly Order Smectic Layer Structure as a Floating Gate for Multiband Photoresponding Photonic Transistor Memory

AbstractWith the growing demands of light fidelity technology, photonic transistor memory has gained considerable attention for next‐generation optoelectronic devices. In this work, alkylated rylenediimides of 2,9‐diphenethylanthra[2,1,9‐def:6,5,10‐d“e”f]diisoquinoline‐1,3,8,10(2H,9H)‐tetraone (C8‐PDI) and 2,7‐dioctylbenzo[lmn][3,8]phenanthroline‐1,3,6,8(2H,7H)‐tetraone (C8‐NDI), and pyromellitic diimide of 2,6‐dioctylpyrrolo[3,4‐f]isoindole‐1,3,5,7(2H,6H)‐tetraone (C8‐PMDI) have been used as floating gate electrets in the photonic field‐effect transistor‐type memory. The structure‐optics‐performance relationship of these rod‐like molecules has been systematically studied, and the memory device exhibited a decent response to photowriting and electrical erasing processes, owing to the 3D ordered smectic layer structure and brickwork stacking. Therefore, an evenly distributed photogating moiety, efficient exciton dissociation associated with decent carrier tunneling and charge trapping can be obtained. Among them, the sought‐after C8‐NDI presents favorable energy level alignment, multiband photoresponding, and an optimal block ratio. The fabricated photonic memory with C8‐NDI electret presented a remarkable memory switchability with a memory ratio of 104 and a stable memory ratio of 105 over 10,000 s. To the best of knowledge, this is the first work to utilize rylenediimides based liquid crystals as an efficient charge blocking electret, and these findings open an avenue for designing rod‐like molecules with highly ordered liquid crystalline properties in the ultrafast photomemory devices.

LC‐Polymers and Smectic Phases with Special Substructures/Nanophase Segregation

AbstractLiquid crystalline (LC) polymers find still a lot of interest, whereby the focus is—at present—on LC‐elastomers with different phases and their application for mechanical actuation. While the LC‐phases in LC‐polymers can—generally—be well described in analogy to their low molar mass counterparts, some special properties became evident for some systems with smectic phases. They are related to the modulation of the nanophase segregation within the smectic layers and include often the formation of sublayers consisting of poorly compatible parts of the polymer like bulky (=non mesogenic) units, ionic groups, or poorly miscible parts of the main chain (e.g., polysiloxanes). Such a behavior can be found for semiflexible LC‐main chain polymers and for LC‐side chain polymers. Here the authors i) describe polymers for which such a modification of the smectic structure is observed, ii) discuss the origin of the deviation, and iii) describe possible application for the design of functional materials. The formation of the substructures can allow, for example, a mixing of the LC‐polymers with other polymers, which have a good affinity to the second sublayer. It also allows it to separate, to some extent, the network structure from the packing of the mesogens in LC‐polysiloxanes.