Molecular Liquid Crystals Research Articles

Multifaceted regulation of chiroptical properties and self‐assembly behaviors of chiral fluorescent polymers

AbstractThe multifaceted regulation of the chiroptical properties and self‐assembly behaviors of chiral fluorescent polymers is of great significance yet remains challenging to achieve. Herein, a series of novel salen‐based chiral fluorescent polymers with aggregation‐induced emission and varied substitution manners were facilely and efficiently synthesized. Multiple factors were systematically investigated on the chiroptical properties and self‐assembly performance of these polymers, which include molecular structures, solvent environments, metal coordination, and liquid crystal (LC) assemblies. Sutle change in the solvent composition can lead to diverse assembly morphologies of all these chiral polymers, from single‐handed helical fibers, helical toroids or loops, to spherical structures, consequently leading to an aggregation‐reduced circular dichroism (CD) phenomenon. The polymers bearing salen units show highly selective and reversible coordination with Zn2+ and can also induce multiple responses in the absorption, luminescence, CD, and circularly polarized luminescence (CPL) of these chiral fluorescent polymers via a coordination‐ and dissociation‐initiated self‐assembly tuning. Furthermore, a small amount of the chiral fluorescent polymer in can induce achiral nematic 4‐cyano‐4′‐n‐pentylbiphenyl (5CB) to form ordered chiral nematic LC phase with significant improvement in their CD and CPL signal. The absolute absorption and luminescent dissymmetry factors of the resulting supramolecular assemblies can reach the order of 10−1.

Preparation and properties of phosphorus-containing thermotropic liquid crystal copolyester fibers with excellent flame retardance and mechanical property

In order to acquire polymeric fibers have both excellent flame retardance and mechanical properties, high molecular weight phosphorus-containing liquid crystal copolyesters derived from 6-hydroxy-2-naphthenic acid (HNA), terephthalic acid (TA) and 10-(2,5-dihydroxypheny)-10-hydro-9-oxa-10-phospha phenanthrene-10-oxide (DOPO-HQ) were synthesized via “one-pot” melt polymerization, and then the fibers were obtained by melt spinning method. The structure and property of the copolyesters were studied elaborately by experimental characterization technique and molecular simulation method. Shear shinning behavior was observed from rheology measurement, indicating that the copolyesters had excellent melt fluidity and spinnability. The rigidity of molecular chain was efficiently improved by the phosphorus-containing monomer DOPO-HQ and naphthalene HNA units which could be reflected by the molecular simulation results as well as high Tg values. The flame retardant mechanism of the phosphorus-containing flame retardant copolyester was systematically studied via limiting oxygen index (LOI), vertical burning (UL-94) test, cone calorimetry, scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The results showed that the copolyesters exhibit synergistic flame retardant mechanism of gas phase and condensed phase. Meanwhile, mechanical property of the as-spun fibers could be significantly improved by increasing the spinneret draw ratio (SDR) attributed to high molecular chain orientation. The maximum tensile strength of the as-spun fiber was 0.77 GPa observed for P-HNA60P20 at a relatively low SDR of 29. It was shown the unique combination of good thermal stability, spinnability as well as mechanical and flame retardant properties of the copolyesters would make them could be used as high-performance flame retardant materials.

Layer topology of smectic grain boundaries

ABSTRACT Grain boundaries in extremely confined colloidal smectics possess a topological fine structure with coexisting nematic and tetratic symmetry of the director field. An alternative way to approach the problem of smectic topology is via the layer structure, which is typically more accessible in experiments on molecular liquid crystals. Here, we consider exemplary density functional theory results for two-dimensional hard rods to illustrate how the concept of endpoint defects, which appear as tetratic disclinations of quarter charge in director topology, translates to smectic layer topology built around the networks formed by density maxima and minima. By doing so, we elaborate on further advantages of a topological concept evolving around the layer structure rather than the director field, such as providing insight in the structure of edge dislocations or virtual defects at the confining walls.

Bridging organic, molecular, macromolecular, supramolecular and biological sciences to create functions via fluorine chemistry and fluorinated reagents

After a brief introduction highlighting the challenges of fluorine chemistry and the latest developments in the field, this Perspective will discuss how a combination of fluorine and fluorous chemistry together with fluorinated reagents helped to bridge between organic, molecular, macromolecular, supramolecular and biological sciences to create functions in the laboratory of the corresponding author. The reactivity of fluoride as a leaving group is best illustrated by SNAr reactions when it helped to demonstrate single electron transfer-mediated side reactions and through molecular design replaced activated aryl fluorides with aryl chlorides in the synthesis of poly(etherketone)s. Subsequently it was demonstrated how Ni(II) sigma complexes provided an orthogonal approach to the Suzuki-type cross-coupling of arylfluorides, other halides and all aryl C–O based electrophiles. Fluorinated reagents facilitated cylotrimetrization vs cyclotetramerization of bis(methoxy)benzyl chloride and alcohol and the synthesis of the simplest molecular liquid crystals. Triflic acid, methyl triflate facilitated the most tolerant living polymerizations including of cyclic siloxanes, functional vinyl ethers and oxazolines to generate self-organizable dendronized polymers while fluorine, trifluoromethyl and trifluoromethoxy groups facilitated disassembly and reassembly of liquid crystal polyethers and poly(p-phenylenes). Fluorinated stereocenters accessed the first heterochiral recognition in side-chain liquid crystal poly(vinyl ether)s and their model compounds. Alkali metal triflates mediated self-organization of supramolecular nonfluorinated and fluorinated self-assembling minidendrons, dendrons, dendrimers and self-organizable dendronized polymers. The role of fluorinated alkyl groups and of alkali metal triflates in the self-assembly, disassembly and isomorphic replacement analysis, of supramolecular helical columns, of the assembly of helical cogwheel coat and of spherical supramolecular dendrimers forming Frank-Kasper periodic and quasiperiodic arrays was highlighted. A brief discussion of fluorinated amino acids, peptides and peptoids and their potential role in the self-assembly and functions resulted from dendritic dipeptides followed by a discussion of semifluorinated amphiphilic Janus dendrimers as models of biological membranes, including for cell fusion and fission, concludes this Perspective.

Eliminating Ambiguities in Electrical Measurements of Advanced Liquid Crystal Materials

Existing and future display and non-display applications of thermotropic liquid crystals rely on the development of new mesogenic materials. Electrical measurements of such materials determine their suitability for a specific application. In the case of molecular liquid crystals, their direct current (DC) electrical conductivity is caused by inorganic and/or organic ions typically present in small quantities even in highly purified materials. Important information about ions in liquid crystals can be obtained by measuring their DC electrical conductivity. Available experimental reports indicate that evaluation of the DC electrical conductivity of liquid crystals is a very non-trivial task as there are many ambiguities. In this paper, we discuss how to eliminate ambiguities in electrical measurements of liquid crystals by considering interactions between ions and substrates of a liquid crystal cell. In addition, we analyze factors affecting a proper evaluation of DC electrical conductivity of advanced multifunctional materials composed of liquid crystals and nanoparticles.

Machine learning for soft and liquid molecular materials

This review discusses three types of soft matter and liquid molecular materials, namely hydrogels, liquid crystals and gas bubbles in liquids, which are explored with an emergent machine learning approach.

Deformable Photonic Crystals Based on Chiral Liquid Crystals with Thermal-Mediative Shape Memory Effect

We propose a deformable photonic crystal that exhibits the thermal-mediative shape memory effect. The chiral liquid crystalline polymeric scaffold, which produces the structural colors from a helical twist of the liquid crystal director, is prepared through phase-stabilization of a reactive mesogen in a small molecular chiral liquid crystal (CLC), polymerization, and removal of the CLC. The prepolymer of polyurethane acrylate (PUA) is then infiltrated in the prepared scaffold and subsequently photo-polymerized to form a CLC-PUA composite film. Upon compression, this film shows the blue shift of the structural color and retains this color-shift as released from compression. As the temperature increases, the color is recovered to a pristine state. The concept proposed in this study will be useful for designing mechanochromic soft materials.

A perspective on the Langmuir adsorption model applied to molecular liquid crystals containing ions and nanoparticles

Electrical properties of molecular liquid crystals doped with nanomaterials have been extensively studied over the past two decades. In general, nanoparticles in liquid crystals can play a dual role acting as either ion capturing agents or ion generating objects. An interplay between ions and nanomaterials in liquid crystals can result in a great variety of nontrivial ionic effects. This perspective discusses the Langmuir adsorption model applied to describe ionic phenomena in molecular liquid crystals doped with nanoparticles. The applicability and limitations of this model are emphasized. Within a range of its applicability, the Langmuir adsorption model can be used to obtain a wealth of information about nanoparticle-induced ionic effects in molecular liquid crystals. In addition, this model also offers important improvements to standard experimental procedures for evaluation of electrical properties of advanced liquid crystal materials.

Polymer Dispersed Cholesteric Liquid Crystal Mixtures for Optical Time–Temperature Integrators

AbstractLow molecular weight cholesteric liquid crystals (CLCs) can be trapped as droplets or particles inside a polymer binder, forming polymer dispersed cholesteric liquid crystal (PDCLC) systems, which are typically used for smart windows, displays, and optical sensors. While dispersing a single CLC mixture via emulsification, rendering reflective PDCLC films upon drying with a stable optical response, is well studied, the incorporation of distinct microdroplets inside a PDCLC coating is barely explored. Here, structural colored PDCLC coatings are prepared by mixing distinctive thermosensitive photonic microdroplets, featuring an optical time–temperature response that is visualized as a reflective color loss or shift dependent on the composition of the CLC mixtures. The presented optical time–temperature response is induced by diffusion of CLCs between adjacent liquid crystal domains via interconnecting passageways, causing an irreversible optical response until homogeneous mixing is attained. Hence, blending distinct CLC microdroplets in a polymer matrix provides a facile and scalable method for fabricating optical time–temperature integrators; moreover, alternative optical time–temperature responses can be realized when adjusting the composition of the photonic mixtures, demonstrating the potential of this method to acquire novel photonic applications based on low molecular weight CLCs.

Chiral self-assembly of cellulose nanocrystals is driven by crystallite bundles

The transfer of chirality across length-scales is an intriguing and universal natural phenomenon. However, connecting the properties of individual building blocks to the emergent features of their resulting large-scale structure remains a challenge. In this work, we investigate the origins of mesophase chirality in cellulose nanocrystal suspensions, whose self-assembly into chiral photonic films has attracted significant interest. By correlating the ensemble behaviour in suspensions and films with a quantitative morphological analysis of the individual nanoparticles, we reveal an inverse relationship between the cholesteric pitch and the abundance of laterally-bound composite particles. These ‘bundles’ thus act as colloidal chiral dopants, analogous to those used in molecular liquid crystals, providing the missing link in the hierarchical transfer of chirality from the molecular to the colloidal scale.

Polymer-Surfactant Driven Interactions and the Resultant Microstructure in Protein-Containing Liquid Crystal Droplets.

Integration of molecular liquid crystals (LCs) with functional proteins can provide new class of materials for potential applications in optical biosensing. However, hydrophobic nematic LCs (length ∼ 1-2 nm) and hydrophilic proteins, size ∼ O (nm), do not intermix without chemical modification of at least one of them. Bioconjugation of proteins with a polyethylene glycol-based polymeric surfactant (PS) can provide a core-shell system that is sequestered within nonaqueous LC (4-cyano-4'-pentylbiphenyl) microdroplets. However, the nature of interactions between the components and detailed understanding of the resultant hybrid microstructure remains unclear. Here, using a combination of isothermal titration calorimetry (ITC), fluorescence microscopy, and infrared-imaging spectroscopy, we show that strong hydrophobic interactions between the LC and PS drives the sequestration of a myoglobin-PS (Mb-PS; dispersed in the aqueous phase) into the LC spherical microdroplets or even into a bulk LC phase. The average values of both, the binding constant and the standard molar enthalpy change, are increased by approximately a factor of 2.5 times when the unmodified Mb is conjugated to the PS. Small-angle X-ray scattering studies reveal that LC molecules act as a solvent for the Mb-PS conjugate; furthermore, the LC long-range order is disturbed due to mixing, as exemplified by the change in its coherence length from 8.9 to 5.7 nm. Detailed all-atomistic molecular dynamic simulations for a three-component PS-water-LC system show a change in interaction energy of -144 kJ mol-1 PS-1 upon the contact of PS chains (initially dispersed in water) with LC and agree with the ITC experiments.

Effects of dye doping on electro-optical, thermo-electro-optical and dielectric properties of polymer dispersed liquid crystal films

In this study, polymer dispersed liquid crystal (PDLC) films comprising a phase separated low molecular weight thermotropic liquid crystal (LC) and high molecular weight polymer were, prepared using the polymer–induced phase separation method. In order to enhance the optical efficiency of the PDLC films, LC was doped with dichroic azo dye. Various characterization techniques were employed to determine the properties of the components and, the operating principles of the dye–doped PDLC (DPDLC) films. Morphological studies using polarizing optical microscopy and scanning electron microscopy indicated that the LC droplet configuration changed with various dye concentrations and voltages. Electro-optical analysis showed that the performance of the DPDLC film with a low dye concentration (0.007%) was excellent, with a high contrast ratio (241) and transmittance difference (ΔT = 98.57%), low threshold (VTH = 0.85 V/μm) and saturation voltages (VSAT = 1.45 V/μm), and a minimum hysteresis effect. The effects of temperature on the electro-optical properties of the DPDLC films were studied. Absorbance analyses were conducted using ultraviolet–visible spectrophotometry to complement the transmittance analyses. Dielectric relaxation spectroscopy was employed to measure relaxation frequency/time of DPDLC films. Debye and Cole – Cole modeling were conducted to understand nature of the relaxation process. The zero values of the distribution parameter (α) for all of the DPDLC composite films confirmed their Debye type relaxation process.

A novel small molecular liquid crystal catalytic amplification-nanogold SPR aptamer absorption assay for trace oxytetracycline

Liquid crystals (LCs) have been applied for a long time in the field of analytical chemistry. To date, there are no reports about utilization of LCs as the catalyst to amplification analytical signal. In this article, three small molecules LCs in water aqueous solutions were characterized using molecular spectra and particle size analysis. The characterization indicated that there are nanoparticles in the system. Among the them, 4-heptylbenzoic acid (HPB) exhibits the most sensitive performance in the analytical system based on the reduction of HAuCl4 to gold nanoparticles (AuNPs) by NaH2PO2 by the spectrophotometric slope evaluation procedure. As the concentration of LCs catalyst increases, the AuNPs surface plasmon resonance (SPR) absorption peak at 550 nm increases linearly, that can be utilized to amply the absorption signal. Based on the LCs catalytic amplification reaction and immunoreaction, a new SPR spectrophotometric analysis method was developed for the label-free detection of oxytetracycline, with a detection limit of 0.50 ng/mL. The method was also successfully applied for the detection of oxytetracycline-spiked environmental water samples to demonstrate its practical usefulness.

Multifunctionality by dispersion of magnetic nanoparticles in anisotropic matrices

Abstract Interactions between magnetic nanoparticles and an anisotropic environment give rise to a variety of new magneto-optical, rheological and mechanical phenomena. This opens new avenues for developing novel multifunctional materials. In the course of this project, we investigated three types of anisotropic systems: dispersions of shape-anisotropic nanocrystals, magnetically doped molecular and colloidal liquid crystals, and organoferrogels. They were investigated by means of magneto-optical observations and by a magneto-mechanical torsion pendulum method.

Multivariate Design of 3D Printed Immediate-Release Tablets with Liquid Crystal-Forming Drug-Itraconazole.

The simplicity of object shape and composition modification make additive manufacturing a great option for customized dosage form production. To achieve this goal, the correlation between structural and functional attributes of the printed objects needs to be analyzed. So far, it has not been deeply investigated in 3D printing-related papers. The aim of our study was to modify the functionalities of printed tablets containing liquid crystal-forming drug itraconazole by introducing polyvinylpyrrolidone-based polymers into the filament-forming matrices composed predominantly of poly(vinyl alcohol). The effect of the molecular reorganization of the drug and improved tablets’ disintegration was analyzed in terms of itraconazole dissolution. Micro-computed tomography was applied to analyze how the design of a printed object (in this case, a degree of an infill) affects its reproducibility during printing. It was also used to analyze the structure of the printed dosage forms. The results indicated that the improved disintegration obtained due to the use of Kollidon®CL-M was more beneficial for the dissolution of itraconazole than the molecular rearrangement and liquid crystal phase transitions. The lower infill density favored faster dissolution of the drug from printed tablets. However, it negatively affected the reproducibility of the 3D printed object.

MAGE ANALYSIS STUDIES ON SUPRAMOLECULE MA: nOBA LIQUID CRYSTALS

Ponte Academic JournalJan 2020, Volume 76, Issue 1 MAGE ANALYSIS STUDIES ON SUPRAMOLECULE MA: nOBA LIQUID CRYSTALSAuthor(s): T. Kalyani ,K. Mallika, A.V.N. Ashok Kumar, S. Sreehari SastryJ. Ponte - Jan 2020 - Volume 76 - Issue 1 doi: 10.21506/j.ponte.2020.1.9 Abstract:Phase textures and transition temperatures of supramolecule MA:nOBA liquid crystals through hydrogen bond consisting non mesongen Malonic acid (MA) and mesogen p-n alkoxy benzoic acid (nOBA) where n = 10,11,12) are investigated using image analysis technique. MA moiety plays the role of spacer moiety in these supramolecules. On cooling, the supra molecular liquid crystals exhibits Smectic C, Smectic X, re-enterant Smectic C, Smectic F and Smectic G phases. Analysis of these textural features gives the information to identify the transition temperatures of MA:nOBA. This can be done by the image analysis technique on MATLAB platform. Here, the statistical parameters - mean, standard deviation, energy, entropy, contrast, and homogeneity utilizing liquid crystal textural images, are calculated basing on gray level intensities of textures as a function temperature. The abrupt changes in the statistical parameters give the transition temperatures. The obtained results with intensity changes of the images are in good agreement with differential scanning caloriemetric technique (DSC). The results of supramolecule MA:nOBA uniquely speak out that the enhancement of thermal phase stability taken place specially tilted liquid crystal phase stability which favours in the device applications. Download full text:Check if you have access through your login credentials or your institution Username Password

Paintable Encapsulated Body-Temperature-Responsive Photonic Reflectors with Arbitrary Shapes

A temperature-responsive photonic coating on a flexible substrate was prepared by a photoinduced phase-separation process. In this coating a low molecular weight cholesteric liquid crystal (Ch-LC) mixture was encapsulated between the substrate and an in situ formed protective polymer top layer. The photonic coating showed a blue-shift of the photonic reflection band of 100 nm by heating from 22 to 23 °C due to the close proximity to the smectic to cholesteric phase transition and an overall 330 nm blue-shift while heating from 22 to 45 °C. Hence, the red coating turned green upon contact with skin within seconds. Furthermore, the coating structure and composition were investigated in detail, revealing a thick top coat. The adhesion of the coating was improved by providing trays on the substrate (by etching or 3D printing), resulting in a link between arbitrary-shaped substrates and the protective polymer top layer. These bendable coatings could be of interest for sensors, anticounterfeit labels, or custom...

Photoresponsive Chiral Dopants: Light‐Driven Helicity Manipulation in Cholesteric Liquid Crystals for Optical and Mechanical Functions

AbstractThis Review focuses on recent advancements regarding the design of photoisomerizable chiral dopants which can switch the helical orientation in cholesteric liquid crystals upon light stimuli. Chiral dopants exhibiting conformational change in response to an external stimulus can provide dynamic control of the cholesteric pitch. Development of such responsive systems has been attempted since the 1970s. However, major advances on efficient chiral dopants exhibiting either high helical twisting power or large photoswitching have been achieved only in recent years. This Review covers photoresponsive low molecular weight cholesteric liquid crystal systems based on chiral dopants capable of photochemically “switching power to twist” surrounding liquid crystalline host molecules, “inverting” their handedness or even “inducing” helical organization from the racemic nematic phase.

On the second harmonic generation activity in centrosymmetric crystals

The observation of second harmonic generation (SHG) signals, which is forbidden in centrosymmetric crystals, should necessitate clarification of their causes. The authors of a recent paper (Sivaraman et al Molecular crystals & Liquid crystals 656 (2017) 153-168) report to have grown a so-called crystal of diglycine zinc dipicrate which, in fact, is tetraaquabis(glycine)zinc(II) dipicrate dihydrate. This compound despite being centrosymmetric, was reported to exhibit a strong second harmonic response viz. 1.07 times of potassium dihydrogen phosphate. Based on a reinvestigation, we prove that the observation of SHG is due to dehydration of the crystal under laser irradiation.

Hybrid photosensitive structures based on nematic liquid crystals and lithium niobate substrates

Abstract Liquid crystal cells based on lithium niobate substrates have recently been proposed as good candidates for optofluidic devices and for light-induced controlled generation of defects in liquid crystal films. The peculiarity of these structures lies in the possibility of using the bulk photovoltaic effect of lithium niobate to obtain an optically induced dc field able to affect the molecular liquid crystal director. Reversible fragmentation and self-assembling of liquid crystal droplets driven by the lithium niobate pyroelectric properties have also been reported. We review the basic results obtained so far with the aim of making the point and seeing what else can be done in the framework of the realization of hybrid structures combining lithium niobate with the electro-optical and nonlinear optical properties of liquid crystals.