Polar anchoring strengths of nematic liquid crystal on high-density polymer brush surfaces

ABSTRACT

Herein, the polar anchoring energy coefficient (A_θ) of nematic liquid crystal (NLC) was examined for high-density polymer brushes via capacitance measurements. The A_θ is 10^?4 J m^?2 for the brushes of poly(methyl methacrylate), poly(ethyl methacrylate) and poly(styrene). The value decreases to 10^?5 J m^?2 for poly(n-butyl methacrylate) and poly(hexyl methacrylate) with lower glass transition temperatures. However, each polymer brush displays a constant A_θ value over a temperature range of ?15°C to 90°C, which is hardly affected by the graft density and brush thickness. At 25°C, A_θ is 10 times greater than the corresponding azimuthal anchoring energy coefficient (A_φ); therefore, NLCs on polymer brushes can be preferentially aligned along the in-plane component of the applied field.     

Creation of a nematic phase in mixtures of smectic A1 phases

Abstract

Phase diagrams were determined for binary mixtures consisting of two 5-n-alkyl-2-(4'-isothiocyanatophenyl)-1,3-dioxane compounds (n-DBT) or 4'-isothio-cyanatophenyl 4-(trans-4'-n-decylcyclohexyl)benzoate and n-DBT. All compounds investigated have monolayer smectic A phases. A nematic phase in the upper temperature range and a nematic gap between two smectic regions also were observed, with the smectic layer spacing ratio, d/d', of 1.23 and 1.87 respectively. The variation of the enthalpy of transition with mixture composition in relation to changes of layer spacing ratio are also discussed for these systems.     

Creation of a nematic phase in mixtures of smectic A1 phases

Phase diagrams were determined for binary mixtures consisting of two 5-n-alkyl-2-(4'-isothiocyanatophenyl)-1,3-dioxane compounds (n-DBT) or 4'-isothio-cyanatophenyl 4-(trans-4'-n-decylcyclohexyl)benzoate and n-DBT. All compounds investigated have monolayer smectic A phases. A nematic phase in the upper temperature range and a nematic gap between two smectic regions also were observed, with the smectic layer spacing ratio, d/d', of 1.23 and 1.87 respectively. The variation of the enthalpy of transition with mixture composition in relation to changes of layer spacing ratio are also discussed for these systems.     

Recent advances in nanostructured composite solid electrolyte

Solid-state lithium batteries (SSLBs) potentially offer safer and higher energy density batteries than traditional Li-ion batteries, but many challenges remain in the development of high-performance SSLBs. For example, solid-state electrolytes with high ionic conductivity are still critically needed. Composite solid electrolytes (CSEs), which are constituted of ceramic fillers dispersed in polymer matrices, may potentially combine the advantages of ceramic and polymer electrolytes and thus have been intensively investigated. Recent works have found that the size, geometry, and dispersion of ceramic ?llers strongly influence the conductivity of CSEs. This review aims at giving a summary of the recent progresses in CSE including the developments in materials as well as mechanistic characterizations. We believe the latest scientific insights will help the researchers in the field to better design CSEs toward the development of high-performance SSLBs.     

Recent advances in radical-mediated [2+2+m] annulation of 1,n-enynes

1,n-Enynes are a versatile class of unsaturated hydrocarbons that found broad applications in the synthetic community of natural products, biologically active structures and functional materials. Typical methods for the transformations of 1,n-enynes include the radical-mediated [2+2+m] annulation reaction as they are particularly efficient accesses to functionalized polycyclic compounds. We herein highlighted recent process in the radical-mediated [2+2+m] annulation of 1,n-enynes, including [2+2+1] and[2+2+2] modes, for the construction of five-to six-membered-ring-fused polycyclic scaffolds. Meanwhile, the mechanisms for these transformations were described.     

Control of anchoring of nematic fluids at polymer surfaces created by in situ photopolymerization

In situ photopolymerization of alkyl acrylate monomers in the presence of a nematic fluid provides a cellular matrix of liquid crystalline droplets in which the chemical structure of the encapsulating polymer exerts control over the alignment (anchoring) of the liquid crystalline molecules. Control is obtained by variation of the alkyl side chains and through copolymerization of two dissimilar monofunctional acrylates. For example, among a series of poly(methylheptyl acrylate)s, the 1-methylheptyl analogue prefers planar anchoring of a nematic (TL205) over the temperature range studied. However, the polymers of other methylheptyl side chains display a homeotropic-to-planar anchoring thermal transition temperature similar to that of the n-heptyl analogue. Copolymerization of two monofunctional acrylates with opposing tendencies of aligning liquid crystal leads to tunability of anchoring behavior over a wide temperature range. The broad anchoring transitions we observed provide a way of achieving highly tilted anchoring.     

Recent advances in [2+2+2] cycloaddition reactions

The [2+2+2] cycloaddition is an elegant, atom-efficient and group tolerant process for the synthesis of carbo- and heterocycles, mostly aromatic, involving the formation of several C-C bonds in a single step. Cyclotrimerisation is catalyzed by a variety of organometallic complexes, including more than 15 different metals. The aim of this tutorial review is to point out the most recent advances in this field and to encourage the use of this reaction enroute to complex molecules. After summarizing the most common catalysts and reaction conditions generally used, we survey the mechanistic features currently accepted for this reaction. Section 4 covers the scope of the different [2+2+2] cycloaddition versions starting with the cyclotrimerisation of three triple bonds, including nitriles, with especial emphasis on asymmetric reactions that create central, axial or planar chirality. Then, reactions that use double bonds are addressed. Finally, the most outstanding examples of natural products synthesis using [2+2+2] cycloadditions as a key step reported recently are shown.     

Recent advances in enantioselective [2 + 2 + 2] cycloaddition

Enantioselective cycloaddition using chiral transition metal catalysts is an atom-economical and efficient synthetic tool for the construction of chiral carbo- and heterocyclic skeletons. This short account discloses our recent results of inter- and intramolecular enantioselective [2 + 2 + 2] cycloadditions of alkyne and/or alkene moiety(ies). Chiral iridium complexes catalyzed the alkyne trimerization for the generation of axial chirality(ies), and chiral rhodium ones catalyzed alkyne-alkyne-alkene cyclization for the generation of a quaternary carbon including spirocyclic system.     

Creation of nematic and reentrant nematic phases in bi- and multi-component smectic mixtures

Abstract

Experimental results referring to the transformation of smectic phases, mainly smectic A, into nematic and reentrant nematic phases are reviewed. A new explanation of some experimental results is proposed. Factors which are responsible for the depression of smectic phases in mixtures of polar mesogens are discussed and the possibility of forming mixtures with a broad temperature range of nematic phase from smectic compounds, which can be useful for liquid crystal displays (LCDs), is shown. A nematic gap observed in some cases between monolayer (S_A1) or monolayer and partially bilayer (S_Ad) smectics results from the differences in the organization of the molecules in the smectic layers. It is concluded that polar phase from smectic A₁ phases can be divided into two groups: (a) the first one is characteristic for compounds with the -NCS, -F, -CI, -I or ?COC _m H_{2m + 1} terminal group. The spacing of the smectic layer slowly expands with the increase in alkyl chain length and the structure of the smectic A₁ phase slowly changes to be more like the smectic A_d phase (d/1 > 1). It is proposed that such a smectic is called an enhanced monolayer smectic (S_A1e (b) the second one is typical for compounds with the -CN terminal group. This kind of smectic A₁ phase is rapidly transformed into the smectic A_d phase with increasing alkyl chain length. These latter monolayer mesogens easily form the reentrant nematic phase when they are mixed with other polar smectic mesogens.     

Recent advances in the observation of disclinations of main-chain nematic polymers

Experimental observation of disclinations of main-chain nematic polymers is reviewed. The geometry of disclination structure is discussed both from observations of schlieren texture and the theory of curvature elasticity for a two-dimensional case with equal splay and bend moduli. The effects of non-equal elastic moduli, the nature of the disclination core and the observation of inversion wails are briefly mentioned. Examples of experimental observation of disclinations of high topological strengths up to S=4 are given. The main experimental methods to map the director field around disclinations are 1) lamellar decoration, 2) band-texture decoration and 3) surface microcrack decoration. The methods are discussed and compared, and examples illustrated by electron micrographs or photomicrographs are presented.     

Recent advances in the applications of [1.1.1]propellane in organic synthesis

As a highly strained small molecule, [1.1.1]propellane has been widely used in various synthetic transformations owing to the exceptional reactivity of the central bond between the two bridgehead carbons. Utilizing strain-release approaches, the rapid development of strategies for the construction of bicyclo[1.1.1]pentane (BCP) and cyclobutane derivatives using [1.1.1]propellane as the starting material has been witnessed in the past few years. In this review, we highlight the most recent advances in this field. Accordingly, the reactivity of [1.1.1]propellane can be divided into three pathways, including radical, anionic and transition metal-catalyzed pathways under appropriate conditions.     

Recent advances in the applications of [1.1.1]propellane in organic synthesis

As a highly strained small molecule, [1.1.1]propellane has been widely used in various synthetic transformations owing to the exceptional reactivity of the central bond between the two bridgehead carbons. Utilizing strain-release approaches, the rapid development of strategies for the construction of bicyclo[1.1.1]pentane (BCP) and cyclobutane derivatives using [1.1.1]propellane as the starting material has been witnessed in the past few years. In this review, we highlight the most recent advances in this field. Accordingly, the reactivity of [1.1.1]propellane can be divided into three pathways, including radical, anionic and transition metal-catalyzed pathways under appropriate conditions.     

Computer simulation of interactions between liquid crystal molecules and polymer surfaces I. Alignment of nematic and smectic A phases

Results are presented from computer simulations of liquid crystal molecules in contact with polymeric surfaces. These form part of a study of the complex alignment interactions which operate in liquid crystal displays. The liquid crystal molecules considered are 4-n-pentyl-4'-cyanobiphenyl (5CB) and 4-n-octyl-4'-cyanobiphenyl (8CB); the polymeric surfaces simulated were crystalline polyethylene, polypropylene, poly(vinyl alcohol) and Nylon 6. Additional simulations were performed using graphite as a substrate. Polyethylene, poly(vinyl alcohol) and Nylon 6 were all found to induce orientation of the 5CB and 8CB molecules parallel to the polymer chain axes, as would be expected from experimental studies. On the other hand, polypropylene induces many different orientations with no clear preference for either. No evidence was found for the alignment of 8CB molecules on graphite substrates, in disagreement both with experimental findings and the results from previous simulations. The nature of the alignment interactions and possible reasons for the observed discrepancies are discussed.     

Flow and anchoring effects on nematic fluctuations in confined geometry

Light scattering observation of nematic director fluctuations in confined geometries can be used to obtain interaction parameters of liquid crystals with surfaces. We present the basics of the method and some examples of the results in planar and cylindrical geometries. These results were obtained after neglecting the coupling of the director motion to flow. We give analytical and numerical results of flow effects on director fluctuations in a slab. The backflow contribution to the effective viscosity is strongly suppressed so that the results for the anchoring energy remain valid. Modal dispersion relations show an interesting behaviour of avoiding crossings.     

Recent advances in immobilization methods of antibodies on solid supports

Antibody immobilization on a solid support is an essential process for the development of most immune-based assay systems. The choice of the immobilization method greatly affects antibody-antigen interactions on the assay surface. For the past several years, numerous strategies have been reported to control antibody immobilization, mainly by directing the orientation, stability, and density of bound antibodies on different assay platforms. Here we discuss recent developments in antibody immobilization methods with a particular focus on the strengths and limitations of reported approaches, and thereby provide a useful guideline for the selection of suitable antibody coupling procedures.     

Computer simulation of interactions between liquid crystal molecules and polymer surfaces I. Alignment of nematic and smectic A phases

Results are presented from computer simulations of liquid crystal molecules in contact with polymeric surfaces. These form part of a study of the complex alignment interactions which operate in liquid crystal displays. The liquid crystal molecules considered are 4-n-pentyl-4'-cyanobiphenyl (5CB) and 4-n-octyl-4'-cyanobiphenyl (8CB); the polymeric surfaces simulated were crystalline polyethylene, polypropylene, poly(vinyl alcohol) and Nylon 6. Additional simulations were performed using graphite as a substrate. Polyethylene, poly(vinyl alcohol) and Nylon 6 were all found to induce orientation of the 5CB and 8CB molecules parallel to the polymer chain axes, as would be expected from experimental studies. On the other hand, polypropylene induces many different orientations with no clear preference for either. No evidence was found for the alignment of 8CB molecules on graphite substrates, in disagreement both with experimental findings and the results from previous simulations. The nature of the alignment interactions and possible reasons for the observed discrepancies are discussed.     

Recent advances in the chemistry of thieno[2,3-b]pyridines 1. Methods of synthesis of thieno[2,3-b]pyridines

Russian Chemical Bulletin - The recent data (2007–2018) on the synthesis of thieno[2,3-b]pyridines are summarized and systematized.     

Dielectric relaxation in coexisting nematic and smectic B phases

Mixtures of a polar solute 4-n-pentyl-4'-cyanobiphenyl in a non-polar nematic solvent exhibit two separated low frequency dielectric relaxations for concentrations of the solute between 2 mol% and 20 mol% over a limited temperature range. This behaviour is attributed to coexisting nematic and smectic B phases, in which the polar solute probe has different relaxation frequencies. The observed dielectric spectra can be accurately fitted to two Debye-like relaxations, and the strengths of the absorptions are proportional to the amounts of the coexisting phases. A microscopic determination of the phase diagram confirms the assignment of the coexisting phases, and it is concluded that there is a preference of the dipolar probe molecule for the smectic B phase, which is induced as a result of solute-solvent interactions.     

Thermotropic biaxial nematic phases with negative optical character [1]

Two disc-shaped multialkynyl arene ethers (1 and 2) with unusual thermo-mesomorphic properties are presented. Conoscopic studies show that the nematic phases of these new low molecular weight liquid crystals are biaxial and that the sign of their biaxiality is negative. The diether 2 is the first discotic twin liquid crystal which exhibits a nematic phase.     

Order parameters and densities in the smectic C, smectic A and nematic phases of a liquid crystal mixture

Measurements of the orientational and translational order parameters for the nematic, smectic A and smectic C phases of a commercial liquid crystal material are reported. The order parameters have been obtained by analysis of the angular distribution of the intensity of X-rays scattered by a sample aligned by a magnetic field. Results are presented as a function of temperature, and it is found that the apparent orientational order parameter in the smectic C phase decreases with decrease in temperature. This is explained using a model of random tilt. The experimental order parameters are compared with those calculated from a mean field model.