Liquid Crystal Compounds Research Articles

Investigation on thermal behaviour and optical properties of non-symmetric cholesterol-based twin liquid crystals with thioester linkages

ABSTRACT Herein, a series of novel asymmetric cholesterol-based liquid crystal compounds containing thioester linkage and an analogue containing carboxylate linking unit have been synthesised and characterised. Thermal analyses demonstrated that all desired target compounds possess broad mesophase temperature ranges (ΔT, 91–142.5°C) and high thermal stability (clearing points, 214.9–259.4°C). Due to the increasing chain length of the inner flexible spacer, clearing points and entropy changes (ΔS/R) of the COS-containing dimers showed pronounced parity effects. All dimers exhibited enantiotropic mesophase behaviour with an SmC* phase and a typical cholesteric phase (N*), which were validated by XRD and POM. The effect of structural changes of COO and COS unit in the linking group on the mesomorphic properties was analysed. Comparing C6SAR with C6OAR, it was found that the thioester-containing dimer exhibited more stable mesophase, higher phase transition temperatures (about 20°C higher) and enthalpy values (ΔH). All final products exhibited selective reflection to visible light, produced multiple colours. The higher temperatures led to the shorter pitch, which can further result in shorter reflection wavelengths, and the ‘blue shift’ occurred.

Preparation of liquid crystal nanocapsules by polymerization of oil-in-water emulsion monomer droplets

Liquid crystal nanocapsules (LC-Nanocapsules) were prepared by miniemulsion polymerization of the oil-in-water emulsion monomer droplets dissolving the liquid crystal (LC) compounds. In order to establish the preparation conditions of LC-Nanocapsules exhibiting the liquid crystallinity, the effects of the capsule wall-forming monomers and the crosslinking agent concentration on the capsule structure were investigated in detail. The monodisperse colloidal products covered with the robust polymer shell wall was successfully prepared by the polymerization of the emulsion monomer droplets obtained through the phase inversion temperature emulsification technique using the amphiphilic block copolymer as an emulsifier. The endothermic peak was observed at the nematic-isotropic phase transition temperature (TNI) of the LC in the differential scanning calorimetry diagram of LC-Nanocapsules. The bright- and dark-field images of the dried thin films of LC-Nanocapsules spread on a glass substrate were found to appear repeatedly by the temperature change below and above TNI by polarized optical microscopic analysis. These results revealed that the LC-Nanocapsules with a complete engulfing morphology were successfully formed by the spontaneous coacervation phenomena between the crosslinked polymer and the LC with a progression of the polymerization, as theoretically predicted from the viewpoint of the spreading coefficients.

Reliable optoelectronic switchable device implementation by CdS nanowires conjugated bent-core liquid crystal matrix

Enhancing the performance of high luminescent and dielectrically capable cadmium sulfide nanowire (CdS NW) is of great importance, because of their promising ability in analyzing the dimensionality and size. The tuned physical characteristics of semiconductor CdS NWs allowed the manipulation of both electronic and optoelectronic devices at the nanoscale by dispersing a new bent core (BC) liquid crystal (LC) compound. This was derived from a 4-chlororesorcinol central core unit with two terephthalate based rod-like units carrying chiral ( S )-3, 7-dimethyloctyloxy (namely ‘CPDB’) terminal chains, which have been synthesized in a pure solvo-chemical process. The mesomorphic properties of the newly-prepared bent-core LC, exhibiting an enantiotropy ‘Sm A’ phase as a result of dispersing 0.005% of CdS NWs, were investigated by several spectroscopic investigations. In addition, the hydrothermal fabrication of CdS NWs with a high-yield was modified with a cationic agent, cetyltrimethyl ammonium bromide (CTAB), which was utilized as a compatibilizer for providing a better interaction with LC molecules and giving a homogeneous solution. This work focused on the experimental investigation and optimization, using a combinational view of bent-core liquid crystal (CPDB) compound dispersion which was achieved in a controllable manner. The product of the resulting composite matrix has a very outstanding and promising behavior, e.g. semiconductor nanostructures emission polarization that can be manipulated using an external bias modulation of the novel switchable device, which was found quite convincing in the recent trends of brand-new technologies. In particular, the electro-optic responses by POM of various mesophases were investigated from the view point of the CdS incorporated bent-core LC matrix formation and transitional phase variants of ‘ON’ and ‘OFF’ states, which were depending on the geometrical parameters of CdS NW's. Finally, the future challenges and prospects of any other nanomaterials dispersed into CPDB compound which will give rise to an increase of the mesomorphic range by preserving the mesophase type were explored in detail. • High-yield CdS nanowires (7–10 nm diameter) were successfully fabricated via hydrothermal assisted route. • Chemical systhesis of bent-core liquid crystalline compounds (CPDB). • Electro-optic responses by POM of various mesophases are investigated and analyzed. • Homogeneously diffused CdS nanowires into bent-core liquid crystalline matrix. • Various textural phase patterns reveal the modification of crystallographic configuration. • Golden opportunities of nanowires into bent-core liquid crystal mesomorphic investigations.

Bent-core liquid crystal-functionalised flexible polymer substrates for liquid crystal alignment

ABSTRACTA transparent flexible polymer film is chemically functionalised with a bent-core liquid crystal (BCLC) compound for effective alignment of the bulk BCLC sample at the substrate–LC interface. The surface attachment was achieved via a simple procedure which involved pre-treatment of the polymer film (commercial name: over head projector film) using piranha solution followed by chemically attaching the BCLC compound through silane condensation reaction. Surface characterisation of the unmodified and BC-modified flexible films was carried out through X-ray photoelectron spectroscopy (XPS), attenuated total reflectance fourier transform infrared (ATR-FTIR) spectroscopy, contact angle (CA) and atomic force microscopy (AFM) techniques. The BC-modified flexible substrates are analysed for their efficiency to orient the bulk LC sample. Remarkably, the chemically modified polymer substrates are highly efficient in vertically aligning both the BC and rod-like LC samples at the substrate–LC interface, in comparison to their unmodified counterparts. The described method is simple, reproducible, surface modified substrates are highly stable and more importantly reusable. The demonstrated method for the alignment of BCLCs advances a step forward towards the realisation of applications proposed for these fascinating compounds.

Development of nematic and orthogonal smectic phases in short-core fluorinated hockey-stick shaped liquid crystal compounds

Here we report synthesis and investigations on novel short-core hockey-stick shaped molecules bearing a 4-n-alkyloxy-2-hydroxybenzylidene moiety at one end and polar fluoro-biphenyl moiety at the other end. The polar chloro and fluoro substituent at the lateral position of the core confers nematic and orthogonal smectic mesomorphism. The mesophase morphology is characterized by polarizing optical microscopy (POM), differential scanning calorimetry (DSC), X-ray diffraction (XRD), electro-optical study and dielectric spectroscopy. The fluorinated compounds 1b (n = 16) and 1c (n = 18) have long terminal hydrocarbon tail and show orthogonal smectic phases with antiferroelectric switching behaviour. The chlorinated compound 2e (n = 8) with relatively short terminal chain length exhibits only a monotropic nematic phase on cooling. The nematic phase is comprised of cybotactic clusters as confirmed by XRD and exhibits electro-optic switching. The compound 2i (n = 12) manifests a short range enantiotropic nematic and additional orthogonal smectic phase. Dielectric spectroscopy reveals polar correlations in the nematic and smectic phases.

The effect of the position of chiral (−)-menthyl on the formation of blue phase and mesophase behavior in biphenyl-benzoate liquid crystals

Eight new chiral liquid crystal compounds 4-(4-menthyloxy-n-oxoalkanoyloxy) biphenyl-4′-yl 4-butoxybenzoates MnB4OB (n = 1–8), were prepared by modifying the position of chiral (−)-menthyl in the menthol based liquid crystal compounds through gradually increasing the alkyl chain length of the dicarboxylic spacer. All compounds were characterized by FT-IR and NMR spectroscopy in order to prove their chemical structures. Differential scanning calorimetry (DSC), polarized optical microscopy (POM) and X-ray diffraction were carried out to systematically investigate their phase transition behaviors. The position of chiral (−)-menthyl in relation to the core has a great effect on the formation of BPs and mesomorphic behaviors. Only CLCs M1B4OB and M2B4OB with short spacer chains presented blue phases. Furthermore, the length and parity of the flexible spacers have a profound influence on phase structures and phase transition behaviors. An odd-even effect is observed for these chiral liquid crystal compounds.

Synthesis ,Characterization and Liquid Crystal Properties Study of NovelNematic Liquid Crystal Compound.

Synthesis ,Characterization and Liquid Crystal Properties Study of NovelNematic Liquid Crystal Compound.

Synthesis and mesocrystalline properties of liquid crystal compounds with tetrathiafulvalene and two cyanobiphenyl units

ABSTRACTA series of ethylenedithiotetrathiafulvalene (EDT-TTF) compounds with cyanobiphenyl moieties attached with different length alkyl chains has been designed and synthesized. The phase behavior of these derivatives was investigated by a combination of polarizing optical microscopy, differential scanning calorimetry, and X-ray diffraction. Among the derivatives, the derivatives with spacer group lengths of n = 6–12 (3b–3d) show a distinct glassy transition during the heating and cooling processes, and 3c (n = 8) and 3d (n = 12) show liquid crystal (LC) phases below the isotropic temperature. An electrochemical study shows that the longer the spacer group, the more readily the target compound is oxidized. Density functional theory calculations show that the LC molecules are typical donor–σ-bridge–acceptor molecules.

Realignment of Liquid Crystal Shells Driven by Temperature-Dependent Surfactant Solubility.

We investigate dynamic director field variations in shells of the nematic liquid crystal (LC) compound, 4-cyano-4′-pentylbiphenyl, suspended in and containing immiscible aqueous phases. The outer and inner shell interfaces are stabilized by the cationic surfactant, cetyl trimethyl ammonium bromide (CTAB), and by the water soluble polymer, poly(vinyl alcohol) (PVA), respectively. PVA and surfactant solutions normally promote tangential and orthogonal alignments, respectively, of the LC director. The rather high Krafft temperature of CTAB, TK ≈ 25 °C, means that its solubility in water is below the critical micelle concentration at room temperature in most labs. Here, we study the effect of cooling/heating past TK on the LC shell director configuration. Within a certain concentration range, CTAB in the outer aqueous phase (and PVA in the inner) switches the LC director field from hybrid to uniformly orthogonal upon cooling below TK. We argue that the effect is related to the migration of the surfactant through the fluid LC membrane into the initially surfactant-free aqueous PVA solution, triggered by the drastically reduced water solubility of CTAB at T < TK. The results suggest that LC shells can detect solutes in the continuous phase, provided there is sufficient probability that the solute migrates through the LC into the inner aqueous phase.

Nano-objects - sculpting and shape in molecular material design (The Pierre Gilles de Gennes ILCS prize lecture)

ABSTRACTEarly liquid crystal compounds often had relatively simple molecular structures, but with the invention of electro-optical devices, the design of materials became increasingly important. Property-structure correlations have promoted computer-aided rational design to the point where it is becoming a necessity in the selection of compounds for preparation. For simple molecular architectures, allowable electrostatic interactions in the face of steric repulsion were used in a phenomenological way to generate new mesophases, such as the hexatic phase, and to engineer materials with desirable physical properties. Materials chemists, in the creation of substances of greater complexity, have built upon these forms of research. Interestingly, the larger the system often the more simply it becomes as the molecular bulk and shape begin to dominate over the surface interactions. Nano-objects of unique shapes, such as ‘Janus’ grains, created in the process of design can kindle the formation of new mesophases, such as the twist-bend and splay phases generated through the crowding of molecular particles, and minimisation of the free volume. This article explores the development of the shapes of molecular grains and the mesophases they spawn.

Synthesis and mesomorphism of the liquid crystal based on diosgenyl end-capped polycarbonate

ABSTRACTTo enhance the activity of the hydroxyl group of diosgenin, the diosgenyl derivative Dios–(CH2)6–OH with an alkyl spacer and end primary hydroxyl group was synthesised via the chain extension reaction between diosgenin and 1,6-hexanediol. Then the liquid crystal (LC) compounds Dios–(CH2)6–(trimethylene carbonate [TMC])n with different TMC content were obtained via the ring-opening polymerisation of TMC with Dios–(CH2)6–OH as initiator and mesogenic unit, importantly, there was no catalyst added. The chemical structures of the Dios–(CH2)6–OH and Dios–(CH2)6–(TMC)n were characterised by Fourier transform infrared (FT-IR) spectra and 1H NMR. The mesomorphism was characterised by polarising optical microscopy and X-ray diffraction measurement, and the phase transition was investigated by differential scanning calorimetry. The results showed that Dios–(CH2)6–(TMC)n displayed an enantiotropic smectic phase, and TMC content played an important role in the LC properties of Dios–(CH2)6–(TMC)n. The higher the TMC content, the lower the phase transition temperature of Dios–(CH2)6–(TMC)n.

Synthesis and mesomorphic properties and optical behaviour analysis of homologous series azobenzene liquid crystal monomer with polar substituents

ABSTRACT Twenty novel azobenzene liquid crystal micromolecular compounds named ω-[4-(p-substituted azobenzeneoxy carbonyl]acid (X-ABCnA) have been designed and synthesised, followed by studies on the thermal performance and mesomorphic properties of the compounds. The liquid crystal compounds were divided into five homologous series based on the terminal substituents R (R = CH3O, CH3, H, Cl, NO2). In each series, the number of carbons on flexible chain was 4, 6, 8 and 10, respectively. Fourier-transform infrared, proton nuclear magnetic resonance and elementary analysis demonstrated that the structure of the synthesised azobenzene liquid crystal compounds was consistent with the molecular design. The mesomorphic properties were tested, analysed and characterised by using differential scanning calorimetry and polarised optical microscopy. The melting transition (T m) of all the compounds in homologous series with different substituents appeared to decrease with the increase of carbon numbers on flexible chains. The same held true for the temperature of isotropic-mesophase/crystalline transition. The compounds with stronger polarity of terminal substituents were more likely to form broader mesogenic ranges. The liquid crystal compounds discussed in this work can be regarded as a reference for the synthesis of mesogenic arms participating in the synthesis of novel multi-arm liquid crystalline macromolecules and polymers.

Optical, Thermal, and Detailed Statistical Parameter Study on Nanoparticles Dispersed into Liquid Crystalline Composites Using MATLAB Program

The study of statistical parameters about liquid crystal (LC) phase texture gives more valuable input to understand the utility of mesogens in different applications. The investigation on LC textures of nanoparticles dispersed into liquid crystalline composites (NDLC) is experimentally analysed and correlated with theoretical studies. The metal nanoparticles ZnO, TiO2, Fe3O4, and Fe2O3 are separately dispersed into the 4-cyano 4′-propoxy-1, 1′-biphenyl (C16H15NO) liquid crystalline compound in the ratio 1:100. The SEM and EDAX analysis studies reveal the presence of nanoparticles in the LC compound. The optical textures and its corresponding phase transition temperatures of the NDLCs are obtained by using POM and DSC. Further, microscopic textures are analysed using specially designed MATLAB software and its statistical parameters such as mean, standard deviation, entropy, energy, and Legendre moments are computed. The noteworthy observation is that the extension of statistical parameters is used to calculate birefringence of nano liquid crystalline composites.

Temperature dependent Raman spectroscopy of a nematic liquid crystal compound 6CHBT

In this study, we have chosen a room temperature nematic liquid crystal (NLC) compound namely, 6CHBT [(trans-4′-n-hexylcyclohexyl) isothiocyanatobenzene] having clearing temperature at 42.8 °C. This particular liquid crystal (LC) exhibits a broad nematic phase from 13 °C to 42.8 °C. Among all the available spectroscopy techniques, Raman spectroscopy is one of the most convenient techniques to understand the various phase transitions of a LC compound. In order to understand the phase change and the change of the molecular orientations along with their change in the chemical bond, the temperature dependent Raman study has been conducted for the LC compound 6CHBT. The transition temperatures of various phases obtained from the linewidth peak positions of Raman spectra agrees well with the literature data. From the Raman spectra, the change in chemical bonds is observed which is due to the increase in the molecular orientation of the alkyl chain in the LC compound. The bond assignment of the experimental value agrees well with the Raman assignment obtained from literature.

Removal and recovery attempt of liquid crystal from waste LCD panels using subcritical water

With the advancement of the fourth industrial revolution, the demand for LCD has widely accelerated as monitoring screens for computers and cell phones. Consequently, old LCD panels are expected to end up as a tremendous amount of e-waste. Apart from transparent electrodes and transistor, waste LCD panel also contains hazardous liquid crystal compound that can contaminate the landfill site. Thus, removing the material from waste LCD was investigated. In this study, water at subcritical state was applied at temperatures between 100 and 360 °C. Initially, the liquid crystals were extracted using toluene and were used to compare with subcritical water. The specific compounds of the liquid crystals were not identified. The liquid crystals (12 mg/g-LCD) were entirely removed from the LCD panel when treated above 300 °C by means of extraction with the subcritical water. Although liquid crystal was successfully removed, recovery was complicated due to the degradation of liquid crystals above 250 °C. A recovery of 70% was obtained at 250 °C without deformation of the molecules. Consequently, this study has shown that although it is not practical to recover LC from LCD panel waste using subcritical water, liquid crystals can be removed efficiently. This method is auspicious in reducing hazardous liquid crystal from waste LCD panel before their disposals at landfill sites.

Non-conventional multi-arm ester liquid crystal derived from cholic acid: synthesis, thermal and optical behaviour

ABSTRACTWith cholic acid as the core, five multi-arm ester liquid crystals were synthesised with different terminal substituents. B1–B5 were mesogenic arms which were linked to multifunctional chiral core cholic acid. The effect of electron-withdrawing and electron-donating groups on the mesogenic behaviour of the compounds was discussed. All products were structurally well characterised by elemental analysis, 1HNMR, and FT-IR. The phase behaviours were investigated by means of polarised optical microscopy and differential scanning calorimetry. B2, B4, B5 and C2, C4, C5 and D2, D4, D5 exhibited typical nematic texture and belonged to thermotropic enantiotropic nematic liquid crystals. B3, C3 and D3 also displayed typical nematic texture of thermotropic monotropic nematic liquid crystal. B1, C1 and D1 did not have mesogenic performance. Wider mesogenic range for 81.6°C of C5 whose terminal substituent is nitro on heating cycle while 121.1°C on cooling cycle. The results indicated that terminal substituents have a pretty important effect on the mesogenic phase and range of multi-arm liquid crystal compounds. The mesomorphic behaviour of compounds with polar groups terminally substituted performance much better than those without polar groups. In this ester multi-arm liquid crystal system, electron-withdrawing groups terminally substituted behave better than those electron-donating groups substituted.

Cholesteryl Liquid Crystals as Oil-Based Lubricant Additives: Effect of Mesogenic Phases and Structures on Tribological Characteristics.

Mechanical operation could be seriously affected by friction and controlling it by oil lubrication has been considered as an effective way. Good lubricant additives are very necessary to avoid the friction damages, and to find or design new additives is always a challenge. In this study, a systematic investigation of using cholesteryl liquid crystals (LCs) as lubricant additives to obtain exceptional tribological behaviors was performed. In total, four cholesteryl LC compounds were synthesized targetedly and their thermal and mesogenic properties were studied to see the inherent relationship between the mesogenic phases and antifriction and antiwear performance. Through a series of tribological and related tests, including the UMT TriboLab test, three-dimensional optical microscopy, oil film thickness and viscosity tests, etc., the effect of the mesogenic phases and structures of the synthesized cholesteryl LCs on their tribological properties as lubricant additives was investigated and a related mechanism was analyzed. The result showed that within and close to the mesogenic phase temperature ranges, which we called as effective temperature ranges of LC additives ( TEF), the LCs presented better tribological behaviors, meaning they could be used in special lubrication applications that need to be confined in certain temperature scopes; however, the ester groups with long alky tails could help dissolve in base oils and adsorb onto the friction pairs. Among the four LCs, LC-D with a long perfluoroalkyl tail brought widest mesogenic phase with considerably enhanced lubrication performance and increased oil film thickness, viscosity, and thermal stability, indicating that the perfluoroalkyl group could be well used in the structural modification of LC additives to give unexpected tribological performance. This study, in conjunction with our experimental data, suggested that the liquid crystals may be evaluated as potential friction modifiers for temperature-controllable lubrication and also shed a fresh light on the development of novel liquid crystal lubrication materials.

Light responsive liquid crystal soft matters: structures, properties, and applications

ABSTRACTStimuli-responsive soft matters have been recognised by many scientists and engineers as very important because of their inherent characteristics in response to environmental changes. In particular, there has been much progress over the last few years on light-responsive soft matters that can respond remotely. By controlling the packing behaviours and the physical properties of the photoresponsive liquid crystal (LC) molecules, the application field is expanding, including the use in flexible polarisers, patterned objects, logic devices, biomimetic photonic crystals and energy harvesting materials. This review discusses the general concept of azobenzene-based LC compounds from small molecules to macromolecules and their corresponding structural evolutions in solid states. Finally, we identify the challenges faced in the research of photoresponsive organic materials and present future applications.

Synthesis and mesomorphism of tetrafluoro substituted 4-cyano oligophenyls

ABSTRACTThe synthesis and characterisation of new liquid crystal (LC) compounds based on terphenyl core are described. New family of dielectrically positive compounds presents alternative molecular approach to the conventional LC design. Correlations between molecular structure and mesomorphic properties for compounds being cyano terminated analogues as well as other known from the literature have been drawn. Compounds are characterised by 1H NMR spectroscopy and mass spectrometry (electron ionisation) analysis. They show monotropic nematic behaviour in broad temperature range, confirmed by a polarising thermomicroscopy, differential scanning calorimetry. Detailed synthetic procedures are attached. Synthesised compounds stand as promising components of medium to highly birefringent liquid crystalline mixtures.

The synthesis of 1-biphenyl-4-alkyl-[1,2,3]-triazoles and their mesomorphic behaviour

ABSTRACTA series of 1-biphenyl-4-alkyl-[1,2,3]-triazole-based liquid crystal compounds were synthesised via a fast and efficient microwave mediated ‘Click Reaction’ and their phase behaviour is presented. Most of the 1-(3-fluorobiphenyl)-4-alkyl-[1,2,3]-triazole compounds exhibited a smectic A phase with relatively low transition temperatures compared to previously examined aryl-triazole-based mesogens. Small angle X-ray scattering measurement indicated the existence of deep smectic phase for some cases. Structural variations gave rise to different phase behaviour in these triazole-based compounds.