Self-organisation and alignment properties of homeotropically confined model liquid crystalline dendrimer systems

Controlling microscopic or macroscopic alignment of liquid crystalline dendrimers (LCDrs) in a porous media is an important feature for their possible potential applications. Here, we investigate structural and alignment behaviours of model LCDr system confined in a slit pore made of two parallel impenetrable walls. Isobaric–isothermal (NPT) Monte Carlo computational simulation method is used. A coarse-grained force field for inter-/intra-dendritic and LCDr-substrate interactions has been established. The inner surfaces of confining walls are made to induce homeotropic anchoring condition. According to simulation results, a variety of stable-ordered LCDr system states have been observed and analysed depending on the temperature and pressure.     

Non-symmetric liquid crystal dimers based on 1,3,4-oxadiazole derivatives: synthesis,photoluminescence and liquid crystal behaviour

A series of non-symmetric liquid crystal compounds consisting of two different semi-rigid anisometric cores, namely 1,3,4-oxidiazole and biphenyl units, and two short terminal groups, have been synthesised in good yield. It has been shown by polarising optical microscopy and differential scanning calorimetry that all these compounds display liquid crystalline behaviour, with nematic and/or smectic A mesophases. The nature of the mesophases is dependent on the electronic properties of the terminal groups. In methylene chloride solution all the compounds displayed a room temperature emission with λ_max at 358–396 nm and quantum yields of 0.29–0.56. The effect of the terminal groups on the mesomorphic and photoluminescent properties is briefly discussed in the context of their electronic characteristics.     

Non-symmetric chiral nematic liquid crystal dimers containing trifluoromethyl and 1,2-propanediol

ABSTRACT

A series of non-symmetric liquid crystal (LC) dimers with the same chiral core 1,2-propanediol (PD) have been synthesised, termed as ABBA-PD-TFBA, PBBA-PD-TFBA, ABA-PD-TFBA, PBA-PD-TFBA and AA-PD-TFBA, respectively, in which one of the two mesogenic groups, the fluorinated mesogenic unit, was kept fix and the other arm was different. The intermediate compounds and LC dimers were characterised by FTIR, ¹H NMR, differential scanning calorimetry, thermogravimetric analysis, polarised optical microscopy and X-ray diffractometer (XRD). The results of the measurements indicated that ABBA-PD-TFBA, PBBA-PD-TFBA and ABA-PD-TFBA displayed optical activity and enantiotropic chiral nematic phase, and PBA-PD-TFBA was an enantiotropic nematic LC while AA-PD-TFBA was a monotropic LC, displaying both nematic phase and smectic A phase on cooling. The results indicated that PD was able to induce the chiral nematic phase, nevertheless, the rigidity of the mesogenic arm, the flexibility of the terminal group and even the type of the terminal chemical bond played an important effect on the thermal properties of the dimers, and even on the formation of the chiral nematic phase. It is also worth noting that C=C at the terminal helped to stabilise the LC phase.     

Synthesis and mesomorphism of diacetylene-bridged triphenylene discotic liquid crystal dimers

Connecting two discotic mesogens via a spacer not only stabilizes the columnar mesophase but also leads to the formation of glass columnar phase, and therefore improves the physical properties of discotic liquid crystals as organic semiconductor. Here, we report the synthesis of eight diacetylene-bridged triphenylene discotic liquid crystal dimers, [C18H6(OCnH2n+1)4(OMe)O2C-C8H16-C≡≡ C-]2, 3(n), (n = 4-8), [C18H6(OC6H13)5O2C-C8H16-C≡≡ C-]2, 6 and [C18H6(OC6H13)5O-(CH2)m-C≡≡ C-]2, 8(m), (m = 1, 3) by Eglinto...     

Thermal and liquid crystalline properties of cellulose β-ketoesters prepared by homogeneous reaction with ketene dimers

Cellulose β-ketoesters having long alkyl or alkenyl chains were prepared by homogeneous reactions with alkyl or alkenyl ketene dimmers (OKD or AKD), and were characterized by X-ray diffractometory, differential scanning calorimetry, optical microscopy with cross polarizers and others. The Cellulose/OKD and AKD β-ketoesters with degrees of substitution (DS) of more than 1.5 were gummy solid at room temperature and had birefringence due to liquid crystalline structures in wide temperature range. The liquid crystalline/isotropic phase transition points were 150–175 °C, depending on the DS and substituents introduced into cellulose hydroxyls. X-ray diffraction patterns indicated that cellulose backbones had disordered structures at room temperature, while alkyl chains in the substituents formed ordered or crystalline domains in the cellulose β-ketoesters. Films of the cellulose/AKD β-ketoesters prepared by casting their chloroform solutions on glass plates had highly hydrophobic nature, and contact angles of water droplet on the films were more than 90°. The water-contact angles on the films decreased to some extent just after heating of the films at 105 °C, whereas they were gradually recovered to the initial values by conditioning at ambient temperature.     

The first examples of room temperature liquid crystal dimers based on cholesterol and pentaalkynylbenzene

In this article, we report the synthesis of the first examples of room temperature liquid crystal dimers based on cholesterol and pentaalkynylbenzene linked via flexible alkyl spacers. The thermal behaviour and mesomorphic properties of the newly synthesised compounds were investigated by differential scanning calorimetry, polarised optical microscopy and X-ray diffraction studies. All the synthesised dimers showed nematic (N*) phase at room temperature and exhibited excellent fluorescent emission properties. The steady-state anisotropy and fluorescence lifetime were measured in order to gain further insights into the supramolecular arrangement of these dimers.     

Monte Carlo simulations of a liquid crystal copolymer in the solid state

The condensed phase of the alternating copolyester of p-hydroxybenzoic acid (HBA) and 2-hydroxy-6-naphthoic acid (HNA) is investigated by studying the room temperature packing arrangement of the copolymer chains. A molecular modeling methodology is employed with a Monte Carlo sampling of the configurational phase space. Realistic poly(HBA-alt-HNA) polymer chains are represented by an explicit atom representation of the HBA/HNA dimers. States are sampled from the NVT ensemble using a sampling scheme consisting of (1) valence and torsional variations, (2) rigid body rotations of the chain about the chain axis, and (3) rigid body translations of the chain. The effect of chain packing on the conformation of chains, as well as the relative intra- and intermolecular orientations of aromatic rings, is investigated. Correlation of chain positioning along the chain axis is dominated by aromatic rings maintaining a center-to-center plane of registry. These layers of aromatic units pack with a preference for edge-to-face orientations in a herringbone-type pattern and have an intermolecular ring angle between the pairs of aromatic rings in the unit cell that is ca. 68°. The aromatic rings, on average, are rotated 38° out from the b–c plane. The phenylene rings of these copolyesters are less restricted in their relative orientation in comparison to the naphthalene rings. Intramolecular orientational probability density distributions indicate a preference for staggering the successive aromatic rings along the chain, with a staggering angle of ca. 66°. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 727–741, 1998     

Star-branched polymers in an adsorbing slit: a Monte Carlo study

A coarse-grained model of star-branched polymer chains confined in a slit was studied. The slit was formed by two parallel impenetrable surfaces, which were attractive for polymer beads. The polymer chains were flexible homopolymers built of identical united atoms whose positions in space were restricted to the vertices of a simple cubic lattice. The chains were regular star polymers consisted of f = 3 branches of equal length. The chains were modeled in good solvent conditions and, thus, there were no long-range specific interactions between the polymer beads-only the excluded volume was present. Monte Carlo simulations were carried out using the algorithm based on a chain's local changes of conformation. The influence of the chain length, the distances between the confining surfaces, and the strength of the adsorption on the properties of the star-branched polymers was studied. It was shown that the universal behavior found previously for the dimension of chains was not valid for some dynamic properties. The strongly adsorbed chains can change their position so that they swap between both surfaces with frequency depending on the size of the slit and on the temperature only.     

New symmetric azomethinic dimer: the influence of structural heterogeneity on the liquid crystalline behaviour

The paper deals with the study of the thermotropic liquid crystalline properties of a new azomethinic symmetric dimer and some of its structural heterogeneous mixtures with one of its reagents. Their thermotropic behaviour was monitored by polarised light microscopy, differential scanning microscopy and variable temperature X-ray diffraction measurements. The influence of the structural heterogeneity on the mesomorphic behaviour was established and some similarities with the mesomorphic behaviour of the polydisperse polymers were underlined.     

Dielectric relaxation study of a H shaped liquid crystal dimer

The present article reports dielectric relaxation study of an unusual shaped liquid crystal dimer (H shape) in the temperature range 40–85°C. The study of this liquid crystal dimer is important due to the presence of azo central linkage, which is photosensitive. The dielectric relaxation study indicates coupling between the electric field and the liquid crystal molecule of H shaped dimer. Various dielectric properties for this liquid crystal dimer are quite different from the conventional liquid crystals. The dielectric relaxation observed in the smectic phase is due to the reorientation of molecule along the director in the presence of an applied electric field. The temperature dependence of the dielectric parameters, such as relaxation frequency, relaxation strength and distribution parameter have been evaluated for this liquid crystal dimer for both planar as well as homeotropically aligned cell and then compared. The Cole–Cole equation has been used to evaluate the aforementioned parameters.     

1,2-Propanediol-linked chiral symmetric and non-symmetric liquid crystal dimers containing trifluoromethyl

Four symmetric and non-symmetric chiral liquid crystal dimers containing trifluoromethyl groups, termed as TFBA-PD-TFBA, UEBBA-PD-TFBA, UEBA-PD-TFBA and UEA-PD-TFBA, respectively, have been synthesised and characterised. UEA-PD-TFBA exhibited chiral nematic phase, whereas the other three dimers displayed chiral smectic A phase. X-ray diffraction (XRD) has revealed the structure of the smectic A phase for TFBA-PD-TFBA to be intercalated, whereas that for UEBBA-PD-TFBA and UEBA-PD-TFBA to be monolayer and interdigitated, respectively. In addition, the weaker peak corresponding to a shorter layer spacing was observed for UEBBA-PD-TFBA and UEBA-PD-TFBA. Considering the results of XRD measurements and computer simulations, the structural model corresponding to the shorter layer spacing is assigned as horseshoe-like shape. The absence of smectic behaviour for UEA-PD-TFBA reveals that the weaker aromatic–aromatic interactions cannot stabilise the smectic A phase.     

Plastic and glassy crystalline states of methane: A Monte Carlo simulation study

A Monte Carlo simulation study has been carried out on the glassy crystalline phases of methane obtained by annealing or quenching the plastic (orientationally disordered) phase. Different cooling rates lead to different states of the glass. Temperature variation of the reorientational parameter suggests the presence of a transition between the plastic and glassy crystalline phases.     

Design,synthesis, and characterisation of symmetrical bent-core liquid crystalline dimers with diacetylene spacer

Two new symmetrical bent-core liquid crystalline dimers (B-DA4 and B-DA12) bearing diacetylenes spacer and different terminal alkyl chains were successfully synthesised via Sonagashira coupling reaction. The molecular structures of these dimers were confirmed by ¹H nuclear magnetic resonance (NMR), Fourier transform-infrared spectroscopy (FT-IR), Raman spectroscopy, mass spectrometry, and elemental analysis. Their thermal stability and liquid crystalline properties were characterised by thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), polarised light microscopy (PLM), and small-angle X-ray scattering (SAXS). Results showed that the diacetylene group may be thermal polymerised at about 260°C far from thermal decomposition. The dimers exhibited monotropic phase behaviour and typical layered-structure phase with long-range order on a length scale of about 6.3 nm was observed. The results mentioned above offer a promising opportunity to design polydiacetylene nanowires by thermal, UV irradiation, or scanning tunnelling microscope (STM) tip-induced polymerisation.     

Fluorinated chiral nematic liquid crystal dimers based on (S)-1-phenylethane-1,2-diol

ABSTRACT

A series of symmetric liquid crystal (LC) dimers with the same chiral core (S)-1-phenylethane-1,2-diol ((S)-PE) have been synthesised, termed TBDA-(S)-PE, 3F3B-(S)-PE, 3F2B-(S)-PE, 1F3B-(S)-PE, 1F2B-(S)-PE, respectively. Chemical structures and LC properties of the five symmetric LC dimers were characterised by Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance hydrogen spectrometer (¹H NMR), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), X-ray diffraction (XRD) and polarised optical microscopy (POM). TBDA-(S)-PE displayed enantiotropic chiral smectic A (S_mA*) phase while 3F3B-(S)-PE, 3F2B-(S)-PE, 1F2B-(S)-PE all displayed enantiotropic chiral nematic (N*) phase and 1F3B-(S)-PE displayed monotropic N* phase. The results indicated that the removal of the flexible spacers between the rigid mesogenic arm and the chiral core facilitated the formation of the N* phase. When the rigid mesogenic units are connected to the chiral core directly, the structure of the terminal fluorine group and the rigidity of the mesogenic unit played certain influence on the thermal properties of the LC dimers, but did not change the type of mesomorphic phase. Compared to 3F3B-(S)-PE and 1F3B-(S)-PE, 3F2B-(S)-PE and 1F2B-(S)-PE displayed wider LC ranges, respectively, suggesting molecular regularity had greater influence on LC-isotropic (I) transition temperature.     

Pressure-induced ordering in adamantane: a Monte Carlo simulation study

Isothermal-isobaric ensemble Monte Carlo simulation of adamantane has been carried out with a variable shape simulation cell. The low-temperature crystalline phase and the room-temperature plastic crystalline phases have been studied employing the modified Williams potential. We show that at room temperature, the plastic crystalline phase transforms to the crystalline phase on increase in pressure. Further, we show that this is the same phase as the low-temperature ordered tetragonal phase of adamantane. The high-pressure ordered phase appears to be characterized by a slightly larger shift of the first peak toward a lower value of r in C-C, C-H, and H-H radial distribution functions as compared to the low-temperature tetragonal phase. The coexistence curve between the crystalline and plastic crystalline phase has been obtained approximately up to a pressure of 4 GPa.     

Symmetrical and asymmetrical liquid crystal dimers: synthesis,characterisation and mesomorphic behaviour

ABSTRACT

A series of new asymmetrical liquid crystal dimers, R_nO–S2O–OR_n (n = 2–10), linked by an ethylene spacer having carbothiol – COS – and carboxyl – COO – linkages to the core centre of the molecule and bearing different lengths of terminal alkoxy chains were synthesised in order to study the effects of the length of the terminal alkoxy chains on mesomorphic properties. As well as, five symmetrical and asymmetrical dimers linked by butylene spacers bearing different linkages to the core and various terminal chains were also synthesized in order to study the effect of the nature of the spacer and terminal groups. The structures of the synthesised dimers were confirmed by physico-chemical techniques, i.e. FTIR, NMR and mass spectra. Differential scanning calorimetry and polarising optical microscopy verified the liquid crystal behaviour transition temperatures. The isotropic transition temperatures of the dimers R_nO–S2O–OR_n (n = 2–10) decreased with increasing length of alkoxy chain. Structural effects on the mesomorphic and physical properties were investigated in terms of alteration of carboxylate and thioester groups linking the spacer. The mesomorphic investigation reveals that all the dimers formed an enantiotropic Nematic phase except for dimer HO2SH which is not a liquid crystal.     

Photomechanical effects in liquid crystalline polymer networks and elastomers

Liquid crystals are widely employed as stimuli‐responsive materials. Liquid crystallinity can be retained in polymeric form. Photoinduced mechanical effects in liquid crystalline polymer networks and elastomers have been a topic of considerable recent research. This review details the historical underpinnings and recent advances in the synthesis and the corresponding photomechanical response of these materials. In nearly all cases, the conversion of light into mechanical work has employed azobenzene as either a guest additive or covalently attached to the network. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018 , 56, 695–705     

Miscibility of a polyarylate with a liquid crystalline copolyester

The miscibility has been investigated for binary blends of a polyarylate (PAr) with a liquid crystalline copolyester of p-hydroxybenzoate and ethylene terephthalate units in a 6/4 molar ratio (PET/PHB). The binary blends were prepared by solution precipitation. The transitions of the PET/PHB have been measured with a rheometrics dynamic spectrometer. The phases in blends have been studied with a differential scanning calorimeter, by ther-mogravimetry and with a polarizing optical microscope. The blends exhibit two glass transitions (T_gs) over the composition range 10–90 wt %. The amorphous PET phase from the PET-PHB is found to be partially miscible with PAr, which leads to a decrease of the PAr T_g. The amount of this partially miscible portion of PET has been estimated by the Couch-man equation. On heat treatment of the blends at 250 to 300°C, transesterficiation takes place, as judged by the shift of the higher of the two T_gs. © 1993 John Wiley & Sons, Inc.     

Reverse Monte Carlo and Voronoi-Delaunay simulation and structure analysis for liquid metals

This paper discusses the applicability of the integrated reverse Monte Carlo and Voronoi-Delaunay approach to local structure analysis and medium-scale structure determination of liquid metals. The advantages of this approach are shown: generation of 3D structural models using experimental diffraction data and subsequent structure determination by atomic coordinate analysis.Original Russian Text Copyright © 2004 by A. S. Roik, V. P. Kazimirov, and V. E. SokolskiiTranslated from Zhurnal Strukturnoi Khimii, Vol. 45, No. 4, pp. 683–691, July–August, 2004.This revised version was published online in April 2005 with a corrected cover date.     

Cationic dye dimers: a theoretical study

In this work we present a quantum-mechanical study on the structure and electronic spectra of three cationic dyes monomers and dimers: acridine orange (AO), proflavine (PF) and methylene blue (MB). The geometries were obtained from crystallographic data, the electronic properties were calculated with DFT (B3LYP functional) and the theoretical spectra were obtained with ZINDO. The solvation methodology adopted was the Integral Equation Formalism (IEF) version of the Polarizable Continuum Model (PCM). This study shows that the differences, even small, between optimized and crystal geometries are responsible for important spectral characteristics. Also, it indicates possible structures for interacting dimers.