Smectic Phase Research Articles

Deep learning techniques for the localization and classification of liquid crystal phase transitions

Deep Learning techniques such as supervised learning with convolutional neural networks and inception models were applied to phase transitions of liquid crystals to identify transition temperatures and the respective phases involved. In this context achiral as well as chiral systems were studied involving the isotropic liquid, the nematic phase of solely orientational order, fluid smectic phases with one-dimensional positional order and hexatic phases with local two-dimensional positional, so-called bond-orientational order. Discontinuous phase transition of 1st order as well as continuous 2nd order transitions were investigated. It is demonstrated that simpler transitions, namely Iso-N, Iso-N*, and N-SmA can accurately be identified for all unseen test movies studied. For more subtle transitions, such as SmA*-SmC*, SmC*-SmI*, and SmI*-SmF*, proof-of-principle evidence is provided, demonstrating the capability of deep learning techniques to identify even those transitions, despite some incorrectly characterized test movies. Overall, we demonstrate that with the provision of a substantial and varied dataset of textures there is no principal reason why one could not develop generalizable deep learning techniques to automate the identification of liquid crystal phase sequences of novel compounds.

Diastereomeric Effect on Bulk Photovoltaic Property and Polarized Electroluminescence in Ferroelectric Liquid Crystals Containing an Extended π-Conjugated Unit

Abstract We synthesized two diastereomers comprising the same π-conjugated unit. One diastereomer exhibited a smectic crystal phase in which the chromophores tilted 45 degrees from the layer normal and macroscopic polarization was induced by a DC bias application in a cooling process from the high temperature phase to the smectic crystal phase. The other diastereomer exhibited a smectic crystal phase in which the chromophores were parallel to the layer normal and macroscopic polarization was not induced. The bulk photovoltaic effect and polarization-induced electroluminescence were observed only in the polarized smectic crystal phase in which the chromophores tilted from the layer normal. In the bulk photovoltaic effect in the tilted smectic crystal phase doped with fullerene derivative, the open circuit voltage and short circuit current were 1.03 V and 100 µAcm−2 for white light illumination (20 mWcm−2), respectively. In the polarization-induced electroluminescence in the tilted smectic crystal phase, linearly polarized emission with the dichroic ratio exceeding 10 was obtained and the axis of the linearly polarized emission could be rotated 90 degrees by an inversion of a DC bias of poling treatment.

Enhancement of the photoinduced birefringence and inverse relaxation of a liquid crystal azopolymer by doping with carbon nanostructures

• Carbon nanotubes can enhance the photoinduced birefringence of azopolymers. • The in situ doping method provides a good polymer-nanotube interaction. • The occurrence of the inverse relaxation depends on the doping method. In this work, the effect of carbon nanotubes (CNTs) and carbon nanofibers (CNFs) on the photoinduced behavior of one side-chain liquid crystal azopolymer is studied. Two distinct methods of doping the azopolymer with CNTs or CNFs are followed: during (in situ method) and after (mixing method) polymerization. Thermotropic studies show that dopants do not have a significant effect on the liquid-crystalline properties of the azopolymer, which develops a smectic A-type phase whose lamellar structure is preserved after cooling to room temperature. On the other hand, the light-induced birefringence of the azopolymer doped by the in situ method (∼ 0.095) is higher than that reached with the corresponding one doped by the mixing method (∼ 0.05). In switch-on/switch-off experiments, both the typical and the inverse relaxation processes of the photoinduced orientation are observed. The inverse relaxation occurs only in the azopolymer doped by the in situ method, which seems to provide a good interaction between the azobenzene units and the nanostructures. In addition, it was found that no minimal threshold value of irradiation dose is required to make the inverse relaxation happen. A model based on non-covalent interactions is proposed to explain the inverse relaxation promoted by nanostructures introduced by the in situ method.

Reentrant 2D Nanostructured Liquid Crystals by Competition between Molecular Packing and Conformation: Potential Design for Multistep Switching of Ionic Conductivity.

Reentrant phenomena in soft matter and biosystems have attracted considerable attention because their properties are closely related to high functionality. Here, we report a combined experimental and computational study on the self-assembly and reentrant behavior of a single-component thermotropic smectic liquid crystal toward the realization of dynamically functional materials. We have designed and synthesized a mesogenic molecule consisting of an alicyclic trans,trans-bicyclohexyl mesogen and a polar cyclic carbonate group connected by a flexible tetra(oxyethylene) spacer. The molecule exhibits an unprecedented sequence of layered smectic phases, in the order: smectic A-smectic B-reentrant smectic A. Electron density profiles and large-scale molecular dynamics simulations indicate that competition between the stacking of bicyclohexyl mesogens and the conformational flexibility of tetra(oxyethylene) chains induces this unusual reentrant behavior. Ion-conductive reentrant liquid-crystalline materials have been developed, which undergo the multistep conductivity changes in response to temperature. The reentrant liquid crystals have potential as new mesogenic materials exhibiting switching functions.

Synthesis of Liquid crystals with terminal polar groups and study their mesomorphic and spectroscopic properties

Synthesis new Schiffs bases substituted with polar groups CI . Br , F , N02, CH3 , OCH3 , COCH3 in para position or aniline moiety, and derived Cu ( II ) complex from the Schiff base which substituted with COCH3. The Schiffs bases substituent with CH3 , OCH3 exhibited Nematic phase but the other Schiff bases and Cu ( II ) complex exhibited Smectic phase ( Sc ) only. Molecular complexes of Cu (II) complex
 with various nitrogen bases were studied spectrophotometrically in benzene solvent at
 different temperatures. These entire complex show a new broad charge transfer band in the visible region of the electromagnetic spectrum.
 All the donors are known to form I: I complex with the acceptor and their
 stoichiometry was unaffected by the variation of temperature over a small interval. The thermodynamic and spectroscopic parameters were evaluated. The order of donor strength followed the sequence: 
 Butyl amine> 4 - methyl pyridine > N,N –
 dimethyl aniline.

Statement of Retraction: Phase Transition Behavior of Smectic Phases in Binary Mixture of Liquid Crystals

This article refers to:Phase Transition Behavior of Smectic Phases in Binary Mixture of Liquid Crystals

Statement of Retraction: Induced Polymorphism of Smectic Phases in Binary Mixture of Non-Mesogenic Compounds

Statement of Retraction: Induced Polymorphism of Smectic Phases in Binary Mixture of Non-Mesogenic Compounds

Statement of Retraction: Phase Diagram Involving Induced Smectic Phases in Binary Mixture of Two Liquid Crystals

This article refers to:Phase Diagram Involving Induced Smectic Phases in Binary Mixture of Two Liquid Crystals

Statement of Retraction: Induced Chiral Smectic Phase in Mixtures of Mesogenic and Non-Mesogenic Compounds

Statement of Retraction: Induced Chiral Smectic Phase in Mixtures of Mesogenic and Non-Mesogenic Compounds

Statement of Retraction: Induced Thermotropic Smectic Phases in Binary/Ternary Mixture of Nonmesogenic Compounds

Statement of Retraction: Induced Thermotropic Smectic Phases in Binary/Ternary Mixture of Nonmesogenic Compounds

Graphene Oxide Induced Sub-structures of Bi-tilted Smectic CG in Dimer Liquid Crystals

A nanocomposite built of the hydrogen-bonded (HB) dimer liquid crystal 4-heptyloxybenzoic acid (7OBA) and Graphene oxide (GO) exhibiting a cascade of phase transitions and phases not typical for pristine 7OBA, was investigated by optical polarization microtexture analysis and Raman spectroscopy. We use the 7OBA/GO nanocomposite beacause of the effective functionalizing properties of GO to study the generation and development of the ferroelectric smectic CG phase in its bi-tilted configuration. For the appearance of the CG phase with its substructures we propose an explanation based on the HB formation between 7OBA dimers in closed and open conformations and the GO sheets, aided by the π–π interaction with graphene hexagons which are free from functional groups. We propose molecular and macroscopic models for the substructures CGcl and CGln and indicate their triclinic C1 symmetry, characterized by a polar vector directed oblique to the layers. This confirms that the bi-tilted CG and its substructures are ferroelectric in the bulk similarly to solid ferroelectrics, thus showing potential as material for controllable photonic functional devices.

Stereochemistry-dependent thermotropic liquid crystalline phases of monosaccharide-based amphiphiles.

Conformational rigidity controls the bulk self-assembly and liquid crystallinity from amphiphilic block molecules to copolymers. The effects of block stereochemistry on the self-assembly have, however, been less explored. Here, we have investigated amphiphilic block molecules involving eight open-chain monosaccharide-based polyol units possessing different stereochemistries, derived from D-glucose, D-galactose, L-arabinose, D-mannose and L-rhamnose (allylated monosaccharides t-Glc*, e-Glc*, t-Gal*, e-Gal*, t-Ara*, e-Ara*, t-Man*, and t-Rha*), end-functionalized with repulsive tetradecyl alkyl chain blocks to form well-defined amphiphiles with block molecule structures. All compounds studied showed low temperature crystalline phases due to polyol crystallization, and smectic (lamellar) and isotropic phases upon heating in bulk. Hexagonal cylindrical phase was additionally observed for the composition involving t-Man*. Cubic phases were observed for e-Glc*, e-Gal*, e-Ara*, and t-Rha* derived compounds. Therein, the rich array of WAXS-reflections suggested that the crystalline polyol domains are not ultra-confined in spheres as in classic cubic phases but instead show network-like phase continuity, which is rare in bulk liquid crystals. Importantly, the transition temperatures of the self-assemblies were observed to depend strongly on the polyol stereochemistry. The findings underpin that the stereochemistry in carbohydrate-based assemblies involves complexity, which is an important parameter to be considered in material design when developing self-assemblies for different functions.

Synthesis and mesomorphic characterization of novel amphotropic steroidal esters

Amphiphilic steroidal esters as novel mesogens displaying cholesteric, polymorphic smectic, TGB and blue phases.

Phase transition and birefringence studies on some ferroelectric liquid crystals

In the present studies the homologous series of hydroxy phenylamino methyl phenoxy benzoate liquid crystals (n = 8,9 and m = 6) are used. The temperature variation of density is measured. Using density data the phase variants in the compounds are identified. All the compounds exhibit first-order phase transitions. By Newton rings method the birefringence is measured directly in smectic and nematic phases. The advantages of the newton ring technique are presented over the conventional methods for the determination of orientational order parameter. The orientational order parameter attained by the newton rings scheme is equated through the literature data, the results are discussed.

Molecular cluster analysis using local order parameters selected by machine learning.

Accurately extracting local molecular structures is essential for understanding the mechanisms of phase and structural transitions. A promising method to characterize the local molecular structure is defining the value of the local order parameter (LOP) for each particle. This work develops the Molecular Assembly structure Learning package for Identifying Order parameters (MALIO), a machine learning package that can propose an optimal (set of) LOP(s) quickly and automatically for a huge number of LOP species and various methods of selecting neighboring particles for the calculation. We applied this package to distinguish between the nematic and smectic phases of uniaxial liquid crystal molecules, and selected candidate LOPs that could be used to precisely observe the nematic-smectic phase transition. The LOP candidates were used to observe the nucleation and subsequent percolation transition, and the effect of the choice of LOP species and neighboring particles on the statistics of local molecular structures (clusters) was examined. The procedure revealed the time evolution of the number of clusters and the dependence of the percolation curve on the number of neighboring particles for each LOP species. The LOP species with the lowest dependence on the number of neighboring particles was the best-performing LOP species in the MALIO screening strategy. These results not only show that machine learning can powerfully screen a huge number of LOP species and suggest only a few promising candidates, but also indicate that MALIO can select the best LOP species.

Phase behaviour of mixtures of charged soft disks and spheres.

We have investigated the phase behaviour of mixtures of soft disks (Gay-Berne oblate ellipsoids, GB) and soft spheres (Lennard-Jones, LJ) with opposite charge as a model of ionic liquid crystals and colloidal suspensions. We have used constant volume Molecular Dynamics simulations and fixed the stoichiometry of the mixture in order to have electroneutrality; three systems have been selected GB : LJ = 1 : 2, GB : LJ = 1 : 1 and GB : LJ = 2 : 1. For each system we have selected three values of the scaled point charge q* of the GB particles, namely 0.5, 1.0 and 2.0 (and a corresponding negative scaled charge of the LJ particles that depends on the stoichiometric ratio). We have found a very rich mesomorphism with the formation, as a function of the scaled temperature, of the isotropic phase, the discotic nematic phase, the hexagonal columnar phase and crystal phases. While the structure of the high temperature phases was similar in all systems, the hexagonal columnar phases exhibited a highly variable morphology depending on the scaled charge and stoichiometry. On the one hand, GB : LJ = 1 : 2 systems form lamellar structures, akin to smectic phases, with an alternation of layers of disks (exhibiting an hexagonal columnar phase) and layers of LJ particles (in the isotropic phase). On the other hand, for the 2 : 1 stoichiometry we observe the formation of a frustrated hexagonal columnar phase with an alternating tilt direction of the molecular axis. We rationalize these findings based on the structure of the neutral ion pair dominating the behaviour at low temperature and high charge.

Machine learning classification of polar sub-phases in liquid crystal MHPOBC.

Experimental polarising microscopy texture images of the fluid smectic phases and sub-phases of the classic liquid crystal MHPOBC were classified as paraelectric (SmA*), ferroelectric (SmC*), ferrielectric (SmC1/3*), and antiferroelectric (SmCA*) using convolutional neural networks, CNNs. Two neural network architectures were tested, a sequential convolutional neural network with varying numbers of layers and a simplified inception model with varying number of inception blocks. Both models are successful in binary classifications between different phases as well as classification between all four phases. Optimised architectures for the multi-phase classification achieved accuracies of (84 ± 2)% and (93 ± 1)% for sequential convolutional and inception networks, respectively. The results of this study contribute to the understanding of how CNNs may be used in classifying liquid crystal phases. Especially the inception model is of sufficient accuracy to allow automated characterization of liquid crystal phase sequences and thus opens a path towards an additional method to determine the phases of novel liquid crystals for applications in electro-optics, photonics or sensors. The outlined procedure of supervised machine learning can be applied to practically all liquid crystal phases and materials, provided the infrastructure of training data and computational power is provided.

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.

Conformational Change of Alkyl Chains at Phase Transitions in Thin Films of an Asymmetric Benzothienothiophene Derivative.

Among many promising organic semiconducting materials, 2-decyl-7-phenyl[1]benzothieno[3,2-b][1]benzothiophene (Ph-BTBT-C10) shows outstanding device performances for organic field-effect transistors. This compound has a highly ordered liquid crystalline state, i.e., the smectic E (SmE) phase. Although the transition from the crystalline state to the SmE phase is believed to accompany melting of the alkyl chains, no spectroscopic evidence has been found so far. In this study, the conformational change of the decyl chains in Ph-BTBT-C10 films across the phase transition is analyzed by temperature-dependent measurements in situ using infrared spectroscopy. The spectral analysis reveals that the polycrystalline film has latent conformational disorder (the gauche conformer), the rate of which becomes more pronounced with the heat treatment. As expected, melting of the decyl chains is observed above the transition temperature to the SmE phase. This study also highlights the discovery of some key bands sensitive to the phase transitions in liquid crystalline organic semiconductors.

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.