Types Of Liquid Crystalline Phases Research Articles

Hockey-Stick Polycatenars: Network formation and transition from one dimensional to three-dimensional liquid crystalline phases

Photo switchable liquid crystalline (LC) materials are of great interest for optical and photonic applications. Herein we report the design, synthesis, and molecular self-assembly of the first examples of photosensitive hockey-stick (HS) polycatenars. Therefore, two new series of HSLCs derived from 4-cyanoresorcinol bent-core unit connected to a short azobenzene-based side arm with one variable alkoxy chain and a long ester-based wing terminated with two alkoxy chains at 3 and 5 positions of the terminal benzene ring are reported. They differ from each other in the length of the terminal chains connected to the long arm. The LC self-assembly of these HSLCs was investigated by polarized optical microscopy (POM), differential scanning calorimetry (DSC), X-ray diffraction (XRD) and electro optical investigations. Depending on length of terminal chains a transition from one dimensional (1D) tilted and non-tilted smectic phases to three dimensional (3D) achiral bicontinuous cubic phases with Ia3¯d symmetry (Cubbi/Ia3¯d) upon chain elongation is observed. Moreover, achiral isotropic liquid networks were observed for medium and long chain homologues. Most of mesophases are room temperature LCs phases with wide ranges as observed in the cooling cycles, where once they are formed on heating, no sign of crystallization is detected down to ambient temperature. Finally, UV light irradiation results in fast and reversible photoinduced transformation between different types of LCs phases as well as between LC phase and isotropic liquid.

Understanding Structural and Molecular Properties of Liquid Crystal Dimers: A Density Functional Approach

Liquid crystals are molecular systems that exhibit partial ordering of molecules similar to solids while maintaining the ability to flow like liquids. Depending upon the amount of ordering in the material, there are many types of liquid crystalline phases. The nematic phase is the most common and technologically most important one due to its use in display applications. In the nematic phase, the molecules tend to have the same alignment, but their positions are not correlated. In part due to fundamental scientific interest and driven by new technological motivations apart from displays, the existence of new stable nematics has been continuously searched. The continuing search led to a recent discovery of a new type of nematic phase, 1 , 2 known as the twist-bend nematic (Ntb), in certain bent-shaped liquid crystal dimers that have been supported by various independent experimental studies. Since the Ntb phase has been discovered recently, its properties have not been fully explored and a detailed description and understanding at the molecular level are still far from complete. 3 The Ntb phase’s formation is highly sensitive to any slight changes in the molecular shape arising from the chemical makeup of the linking spacer, terminal moieties and mesogenic units. 4 , Such structural features are not accessible directly through experiments. Thus, in this work, we present a set of DFT calculations on a series of liquid crystal dimers. This work aims to probe the role of certain structural features in driving the formation of the Ntb phase. This study also reveals why this phase occurs in certain bent molecules, but not in all. Since the constituent molecules are flexible and exist in a range of conformers, comparing the conformational landscapes of the dimers-exhibiting-the-Ntb-phase against those-do-not would identify the molecular conformations promoting the formation of the Ntb phase. Overall, this study evaluates ideal molecular structural features and conformational ensembles potentially responsible for the appearance of the Ntb phase.

Thermal and Emission Properties of a Series of Lanthanides Complexes with N-Biphenyl-Alkylated-4-Pyridone Ligands: Crystal Structure of a Terbium Complex with N-Benzyl-4-Pyridone

This work focuses on the investigation of the liquid crystalline behavior and luminescence properties of the lanthanide complexes of Eu(III), Sm(III) and Tb(III) with N-biphenyl-alkylated-4-pyridone ligands. The organic ligands having a biphenyl group attached via a long flexible spacer with either 9 or 10 carbon atoms were synthesized by the reaction between 4-hydroxypyridine and the corresponding bromide compounds. The chemical structures of the organic and lanthanide complexes were assigned based on elemental analysis, single-crystal X-ray diffraction, 1H, 13C NMR and IR spectroscopies, and thermogravimetric analysis (TGA). The X-ray diffraction analysis of a parent compound shows that the lanthanide ions are surrounded by three monodentate pyridone ligands and three bidentate nitrate ions, giving a 9-coordinate environment. The mesogenic behavior and the type of liquid crystalline phases exhibited by the new complexes were analyzed by differential scanning calorimetry (DSC) and polarizing optical microscopy (POM), and powder X-ray diffraction (XRD) studies. Only the lanthanide complexes with longer spacer (10) display a monotropic SmA phase, typically on a short thermal range (less than 10 °C). The complexes with shorter flexible chains (9) show no liquid crystalline properties with melting temperatures lower than their analogs with longer spacers. The emission spectra recorded in solid state at room temperatures show typical emission bands for each lanthanide ion employed (Eu(III), Tb(III) and Sm(III)).

Controlling liquid and liquid crystalline network formation by core-fluorination of hydrogen bonded supramolecular polycatenars

Photo-switchable materials with liquid crystalline (LC) properties and being capable of showing mirror-symmetry breaking are of special interest from a fundamental scientific point of view as well as for the development of π-conjugated materials with tailored superstructures for optical, emissive and photonic materials. Herein we report two new series of supramolecular photosensitive polycatenar (multi-chained) mesogens formed by hydrogen bonding interaction between a taper shaped triple-chain benzoic acid as the hydrogen bond-donor and fluorinated azopyridines with only a single chain as hydrogen bond-acceptors. The self-assembly of these materials was characterized by differential scanning calorimetry (DSC), polarized optical microscopy (POM) and X-ray diffraction (XRD). Depending on the position of the fluorine atom different types of LC phases were observed, including spontaneously chiral isotropic liquid (Iso1[*]) and bicontinous cubic phases (Cubbi[⁎]/I23), as well as achiral cubic (Cubbi/Ia3̄d) and non-cubic 3D phases with tetragonal symmetry. This report provides guidelines for the application of core fluorination at different positions as a powerful tool for controlling spontaneous helical self-assembly in supramolecular liquid and liquid crystalline materials.

High-Contrast and Fast Photorheological Switching of a Twist-Bend Nematic Liquid Crystal.

Smart viscoelastic materials that respond to specific stimuli are one of the most attractive classes of materials important to future technologies, such as on-demand switchable adhesion technologies, actuators, molecular clutches, and nano-/microscopic mass transporters. Recently it was found that through a special solid-liquid transition, rheological properties can exhibit significant changes, thus providing suitable smart viscoelastic materials. However, designing materials with such a property is complex, and forward and backward switching times are usually long. Therefore, it is important to explore new working mechanisms to realize solid-liquid transitions, shorten the switching time, and enhance the contrast of rheological properties during switching. Here, a light-induced crystal-liquid phase transition is observed, which is characterized by means of polarizing light microscopy (POM), photorheometry, photo-differential scanning calorimetry (photo-DSC), and X-ray diffraction (XRD). The light-induced crystal-liquid phase transition presents key features such as (1) fast switching of crystal-liquid phases for both forward and backward reactions and (2) a high contrast ratio of viscoelasticity. In the characterization, POM is advantageous in offering information on the spatial distribution of LC molecule orientations, determining the type of liquid crystalline phases appearing in the material, and studying the orientation of LCs. Photorheometry allows measurement of a material's rheological properties under light stimuli and can reveal the photorheological switching properties of materials. Photo-DSC is a technique to investigate thermodynamic information of materials in darkness and under light irradiation. Lastly, XRD allows studying of microscopic structures of materials. The goal of this article is to clearly present how to prepare and measure the discussed properties of a photorheological material.

Tacticity Effect on Mesogen-Free Liquid Crystalline Self-assembly Induced by Strong Dipole–Dipole Interactions

Strong dipole–dipole interactions in mesogen-free aliphatic comblike polymers can induce unique liquid crystalline (LC) self-assembly. By comparing atactic and isotactic poly(oxypropylene)s with n-alkylsulfonyl side groups (aPOP-SO2Cn and iPOP-SO2Cn), the effect of main-chain tacticity on the ordered phase behavior was studied in this work. For aPOP-SO2Cn, LC self-assemblies were observed when n is ≥3. Two types of LC phases were formed depending on the side chain length n: (i) a three-dimensional (3D) high-order LC structure (possibly crystal B or E) for n = 3–5 and (ii) a smectic A (SmA) structure for n = 6–12. For iPOP-SO2Cn, crystallization was favored as a result of regular main-chain configuration. Only when n ≥ 10, the isotactic comblike polymers exhibited a monotropic phase behavior with the SmA phase appearing during cooling from the isotropic melt. Regardless of atactic or isotactic main chains, these ordered LC and crystalline phases featured a double-layer structure, which was dictated by the ...

Synthesis and thermal behavior of triblock semifluorinated n-alkanes

A series of semifluorinated long-chain triblock alkanes of the general formula: H(CH2)n(CF2)6(CH2)nH, abbreviated as HnF6Hn, where n = 12, 14, 16, 18 and 20, have been synthesized in two steps, by radical addition of 1,6-diiodoperfluorohexane I(CF2)6I to two equivalents of an appropriate terminal alkene CH2 = CH(CH2)n−2H, followed by subsequent deiodination. This novel class of molecules has been subjected for the first time for thermal characterization. Although the structure of these compounds differs from that typical for liquid crystals, the investigated herein (alkyl)-fluoroalkyl-(alkyl) triblocks were found to exhibit thermotropic liquid crystalline phases. Phase polymorphism of the synthesized compounds was investigated upon both heating and cooling, in the temperature range of 270–360 K, using differential scanning calorimetry (DSC). Additionally, polarized optical microscopy was applied in order to identify the phases and characterize the phase transitions on the base of their textures. In the investigated temperature range, the measured compounds exhibit rich phase polymorphism, including different types of liquid crystalline phases (e.g., smectic A, smectic B and smectic E). Upon analyzing DSC results, it can be concluded that for compounds with n = 16, 18 and 20 all the detected phase transitions (crystalline ↔ smectic, between smectic phases, smectic ↔ isotropic) shift toward higher temperature with increasing in the hydrocarbon chain length. The phase situation for compounds with n = 12 and 14 is slightly different. However, for all the studied compounds, it can be observed that the longer hydrocarbon chain, the higher temperature of the phase transition to the isotropic phase. No metastable phases were observed.

Influence of terminal groups on liquid-crystalline polymorphism of selected azobenzene derivatives

This article describes the synthesis and liquid-crystalline characterisation of four kinds of azobenzene derivatives: 1-[4-(hexyloxy)phenyl]-2-(4-X-phenyl)diazenes (A), 4-[(4-X-phenyl)diazenyl]phenyl heptanoates (B), 4-[(4-X-phenyl)diazenyl]phenyl 4-(dodecyloxy)benzoates (C) and 4-[(4-X-phenyl)diazenyl]phenyl 4-{[(2S)-2-chloropropanoyl]oxy}benzoates (D), where X is the alkyl, alkyloxy alkylcarbonyl and alkyloxycarbonyl substituents. Among the 32 calamitic mesogens presented (13 of them are described here for the first time), the following mesophases were observed: nematic, SmA, SmC, Bhex, BCr and G. About half of the investigated compounds exhibit more than one mesophase. We noted and compared the relationship between the type of the substituent and the type of liquid-crystalline phases and their temperature ranges.

Observation of positive and negative magneto-LC effects in all-organic nitroxide radical liquid crystals by EPR spectroscopy

The generation of spin glass-like inhomogeneous magnetic interactions (the average spin–spin interaction constant: > 0 or < 0), which we refer to as positive or negative “magneto-LC effects”, respectively, was observed in the bulk liquid crystalline (LC) phases of chiral all-organic radical compounds 2 with a positive dielectric anisotropy (Δε > 0) under weak magnetic fields. The sign and magnitude of depended on the type of LC phase or superstructure; stronger positive interactions ( > 0) operated in the chiral smectic A (SmA*) phase of (2S,5S)-2b (89% ee) than in the chiral nematic (N*) phase of (2S,5S)-2a (96% ee), whereas weak negative interactions ( < 0) were observed in the achiral nematic (N) phase of (±)-2a. The origin of the positive magneto-LC effects operating in the SmA* and N* phases was interpreted in terms of the generation of ferromagnetic head-to-tail spin–spin dipole interactions, whereas antiferromagnetic interactions arising from the formation of the RS magnetic dipolar interaction were responsible for the negative magneto-LC effects in the N phase.

Aqueous phase behavior of ionic liquid-related gemini surfactant revealed by differential scanning calorimetry and polarized optical microscopy

An aqueous phase behavior was investigated for ionic liquid-related cationic gemini surfactant, [C12im-4-C12im][Br2], in which two dodecylimidazolium parts are linked by a spacer composed of four methylene units. Differential scanning calorimetry was used to detect phase boundaries among various phases, from which a temperature−composition (T−X) phase diagram was constructed. Lyotropic liquid-crystalline phases appearing in this mixture were characterized by means of polarized optical microscopy. It was revealed that (1) two types of liquid-crystalline phases, lamellar ( $$ {{\hbox{L}}_\alpha } $$ ) and hexagonal (H1), are formed; (2) H1 phase coexists with micellar solution (L1) over a wide concentration range from a few wt.% to ca. 80 wt.% of the surfactant; and (3) the hydrated solid with the composition of [C12im-4-C12im][Br2]·2H2O exhibits incongruent melting point at which the hydrated solid decomposes into H1 phase and unhydrated surfactant solid.

Anisotropic and Inhomogeneous Magnetic Interactions Observed in All-Organic Nitroxide Radical Liquid Crystals

An anisotropic and inhomogeneous magnetic interaction (the average spin-spin interaction constant (-)J > 0) was observed in the various liquid crystalline (LC) phases of racemic and nonracemic all-organic radical LC compounds 1a and 1b. We discussed how the LC superstructures induced the magnetic interaction to operate in the LC phases in terms of spin-spin dipole and exchange interactions by means of VT-EPR spectroscopy. The magnitude of the magnetic interaction depended on the type of LC phase, or the superstructure. Furthermore, these radical LC droplets floating on water were commonly attracted to a permanent magnet and moved freely under the influence of this magnet, whereas the crystallized particles of the same compounds never responded to the magnet. The response of the LC droplets to the magnet also varied depending on the type of LC phase, that is, the extent of the magnetic interaction.

Nanostructure in block copolymer solutions: Rheology and small-angle neutron scattering

Triblock copolymers composed of poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO) present an amphiphilic character in aqueous solutions. Since PPO is less hydrophilic than PEO and since their solubilities decrease when the temperature increases, the copolymers self-assemble spontaneously, forming micelles at moderate temperatures. For higher temperatures or concentrations, the copolymers or the micelles are ordered because of repulsive interactions and form lyotropic liquid crystalline phases. These are phases of very great viscosity with the aspect of gels, and transitions between different crystalline phases can occur at fixed concentration during an increase of temperature. We studied solutions of three different copolymers. The first two have a star structure. They are both composed of four branches (EO)x (PO)y fixed on an ethylene diamine, but differ by the values of x and y . Their commercial name is Tetronic 908 (x=114, y=21) and Tetronic 704 (x=16, y=18) . The third copolymer (EO)37(PO)56(EO)37 is linear and is known under the name of Pluronic P105 . The measurements of the shear complex elastic modulus according to the temperature is used to determine the temperatures of the different transitions. Then, small-angle neutron scattering on samples under flow and true crystallographic arguments make it possible to identify the nature of the crystalline phases. For the systems studied, we show that the branched copolymers form only one type of liquid crystalline phase, which is bcc for the T908 and lamellar for the T704 . For the linear copolymer, it is possible to identify three transitions: micellar solution to hexagonal phase, hexagonal phase to body-centered cubic phase, and finally body-centered cubic phase to lamellar phase.

Study of low energy emulsification of alkyl ketene dimer related to the phase behavior of the system

Abstract The main objective of this work was to improve the physicochemical understanding of the emulsification of a hydrophobizing agent for paper, alkyl ketene dimer (AKD), by relating the equilibrium phase behavior to the emulsification process. Emulsions have been formed with a system containing water, a surfactant mixture of alcohol ethoxylate and calcium dodecylbenzene sulphonate, dodecanol and AKD. The phase behavior was determined at a constant temperature of 60 °C because the emulsification needs to be performed at this temperature in order for AKD to be in melted stage. The study is based on a formulation for emulsification with low energy input, which has been published earlier (J. Disper. Sci. Technol. 22 (2001) 569). The result of the low energy emulsification when adding water to a mixture of AKD and surfactant was compared with the addition of AKD to a water and surfactant mixture. Emulsions with an average droplet size of less than 300 nm were obtained by adding water to the AKD-surfactant mixture. It was found that the method of emulsification, as well as the types of liquid crystalline phases involved in the emulsification process, is important. This is in accordance with a previous work by Forgiarini et al. (Langmuir 17 (2001) 2076).

Correlated layer structures: a novel type of liquid crystalline phase with 2D-lattice.

A novel liquid crystalline quaternary five-block molecule is reported which is composed of four incompatible molecular parts, a rigid biphenyl core, two polar 2,3-dihydroxypropoxy groups in the terminal 4- and 4'-positions, and a branched semiperfluorinated chain in the lateral 3-position, consisting of a perfluorinated and a lipophilic hydrocarbon wing. The self-organization of this compound was studied by polarized light optical microscopy, differential scanning calorimetry, and X-ray diffraction of aligned samples. These investigations confirm a novel liquid crystalline phase with two-dimensional (2D) lattice (columnar mesophase), which results from the positional correlation of smectic layers. The layer structure results from the segregation of the bolaamphiphilic parts from the side chains. Within the aromatic sublayers the biphenyl cores are arranged parallel to the layer planes, and the hydrogen-bonding networks of the terminal diol groups are segregated from the biphenyl cores, forming separate columns. The correlation between adjacent layers is due to the (partial) segregation of the fluorinated and hydrogenated parts of the lateral chains in the nonpolar sublayers.

Soft elasticity and mechanical damping in liquid crystalline elastomers

The dynamic soft response of polydomain liquid crystalline elastomers to simple shear is reported. Significantly, these materials also show extremely large loss behavior with tan δ exceeding 1 or even 1.5 over very wide temperature ranges, with clear implications for damping applications. By comparing materials that exhibit different types of liquid crystalline phases, we identify the nematic state as a better damping phase than that in materials with smectic phases. Additionally, we provide experimental evidence for directions which should be explored for further improvements in the damping behavior of liquid crystalline elastomers.

Amphiphile und Mesogene Kohlenhydrate. 10. Mesophasenwechsel bie amphiphilen Tetradecyl-(N-acylamino)-2-desoxy-1-thio-?-D-glucopyranosiden durch Acylkettenvariation

Amphiphilic and Mesogenic Carbohydrates. 10. Change of the Type of the Mesophase by Variation of the Acyl Chain of Amphiphilic Tetradecyl (N-acylamino)-2-deoxy-1-thio-β-D-glucopyranosides The homologous amphiphilic tetradecyl-2-(N-alkanoylamino)-2-deoxy-1-thio-β-D-glucopyranosides 5a–f were synthesized from the 2-(N-alkanoylamino)-2-deoxy-D-glucopyranoses 1a–fvia the glucosyl chlorides 2a–f, the glucosyl isothiuronium chlorides 3a–f, and the acetylated thioglucosides 4a–f. On heating the amphiphiles 5a–f form different types of liquid crystalline phases in dependence of the length of the acyl chain, i.e.5a and 5b form smectic phases, 5d–f form columnar mesophases. Polarisation microscopy, DSC-measurements and contact preparations were used to characterise the liquid crystaline phases.

Induced Cholesteric Liquid Crystalline Polymers

A new type of induced cholesteric liquid crystalline thermotropic polymer was prepared by the copolymerization of the achiral mesogenic monomer, p-cyanobiphenyl-p′-methacryloyloxybenzoate or p-cyanobiphenyl-p′-acryloyloxybenzoate with the chiral and non-mesogenic monomer, 2-methyl butyl methacrylate, 2-methyl-1-butyl acrylate, or 1-menthyl methacrylate. The mesogenic group is directly attached to the methacrylate- or acrylate-backbone chain without an alkyl spacer. Homopolymers of the achiral and mesogenic monomers exhibited an enantiotropic nematic phase; however, those of the chiral and non-mesogenic monomers themselves did not exhibit any liquid crystalline phases. Copolymers with a combination of achiral mesogenic monomers and chiral non-mesogenic monomers tended to form the so-called induced cholesteric liquid crystalline phase. The type of liquid crystalline phase and transition temperature were found to be different depending upon the fraction of the chiral non-mesogenic comonomer.

Rigid-rod polyesters with flexible side chains. 4. Thermotropic behavior and phase structures in polyesters based on 1,4-dialkyl esters of pyromellitic acid and 4,4'-biphenol

A series of rigid-rod polyesters with long alkyl side chains, denoted as B-Cn, have been prepared by condensing 1,4-diallyl esters of pyromellitic acid with biphenol. The alkyl side-chain lengths were varied from hexyl (n=6) to octedecyl groups (n=18), and the thermotropic transition behavior and mesophase structure of these material were examined by optical microscopy, DSC, and X-ray measurements. An increase in the length of the side chain decreases remarlably the melting point of the crystalline phase, giving rise to a liquid crystalline phase. Two types of liquid crystalline phases with layerlike modifications, LC-1 and LC-2, have been observed. The transition behavior of these liquid crystalline phases is described as a function of the side-chain length

Liquid Crystalline Perylene Derivatives: Orientation and Phase Variation of Discotic Dyes

AbstractDiscotic liquid‐crystalline perylene derivatives can be oriented in the magnetic field far below their clearing temperatures as shown by 2H solid‐state n.m.r. spectroscopy. A mechanical orientation of these materials was achieved by drawing fibers, which were investigated by X‐ray diffraction. In addition, it is shown that not only the melting and clearing temperatures, but also the structure of the mesophases depend sensitively on the substitution pattern. The type of liquid crystalline phase can be varied from discotic to smectic.

Formation of liquid crystals in sunflower oil in water emulsions

Sunflower oil in water emulsions containing between 10–70% (w/w) oil and 1–15% (w/w) sorbitan monopalmitate (Span 40) and polyoxyethylene (20) sorbitan monopalmitate (Tween 40) in different ratios were prepared. Their rheological behavior was investigated by continuous shear rheometry and by creep viscometry. Emulsions containing ⩾5% (w/w) of Span Tween in weight ratios of 6 4 and 7 3 exhibited viscoelasticity. This was ascribed to the presence of a lamellar type of liquid crystalline phase in the systems which was confirmed by electron microscopy.