Properties Of Liquid Crystals Research Articles

The relaxation dynamics and dielectric properties of cyanobiphenyl-based nematic tripod liquid crystals

We present a detailed investigation of the phase behaviour, molecular relaxation dynamics, rheology and dielectric properties of two cyanobiphenyl-based liquid crystal (LC) tripods, differing only in the length of their spacer units (6 or 9 carbons). These LC tripods combine properties of low molecular weight LCs and LC polymers, resulting in a range of advantageous properties including a wide nematic range (ΔT>90 K) and a large birefringence (0.3 at T−TNI=-80 K for the 6 spacer tripod). Using broadband dielectric relaxation spectroscopy, calorimetry, oscillatory and steady state shear rheology, we identified four molecular relaxation processes: the structural (α) relaxation (defining the glass transition temperature, Tg); the δ relaxation (reorientation of the mesogen unit around its short axis); the β relaxation (reorientation around its long axis); and the γ relaxation (internal tripod arm fluctuations). The β and γ relaxations follow Arrhenius temperature dependencies in the glass, whereas the α and δ relaxations merge above, but near Tg, where they follow a non-Arrhenius VFT behaviour. For higher temperatures, the two relaxations separate and for T>T⁎, where T⁎ marks a dynamic crossover, the α relaxation transitions from VFT to Arrhenius behaviour. For the two tripods, ratios of T⁎/Tg=1.14 and T⁎/Tg=1.13 were observed respectively, consistent with the ratio observed for many side-chain LC polymers (and other LC systems), and consistent with the ratio where a dynamic crossover is typically observed also for non-LC glass-formers. However, for non-LC systems, the transition to Arrhenius behaviour happens at significantly higher temperatures relative to Tg and the dynamic crossover at T⁎ is typically observed as a transition between two different VFT behaviours. We argue that for the tripods these differences arise from differences in the molecular relaxation mechanisms induced by the LC order. Finally, for temperatures where δ and α relaxations are separated, we find that ion conductivity decouples from the α relaxation, instead following the δ relaxation; this demonstrates that the ion transport properties for the tripods can be tuned by the design of the tripod mesogen arm.

Synthesis and characterization of biowaste-based porous carbon nanoparticle-polymer dispersed ferroelectric liquid crystal composites

Herein, porous carbon nanoparticles (PCNPs) were synthesized using magnolia champaca seed pods and studied their doping effect on the polymer-dispersed ferroelectric liquid crystal (PDFLC) properties. The effect of PCNPs concentration (≤0.75 wt.%) on the morphology of PDFLC, polarization, and permittivity are investigated in thin sample cells. Field emission scanning electron microscope image results indicate the spherical shape of PCNPs of particle size ∼27 nm diameter. Temperature-dependent electro-optic and dielectric properties are also investigated in the wide SmC* phase and near transition temperature of SmC*-SmA*. Polarising optical microscope textures confirm the non-homogeneity of FLC molecules in the polymer matrix as a function of PCNPs concentration. Thespontaneous polarization and anchoring energy coefficients increase with increasing the doping amount of PCNPs.The phase transition temperature is found unaffected by PCNP doping in all prepared samples. In PDFLC and PCNPs doped PDFLC composites, usualbehaviourof permittivity as a function of temperature is observed. Fluorescence spectra show an enhanced two-fold increase in emission intensity peak at 412 nm wavelength for 0.5 wt.% PCNPs doped PDFLC while concentration-dependent quenching and slight redshift have been observed for the 0.75 wt.% PCNPs doped PDFLC. The enhanced electro-optic and dielectric properties observed in the composites suggest potential applications in displays, sensors, and optical devices. The findings open doors for further exploration and utilization of these functional materials in advanced electronic and photonic technologies.

Thermomechanical Coupling in Polydomain Liquid Crystal Elastomers

Abstract Liquid crystal elastomers (LCEs) are made of liquid crystal molecules integrated with rubber-like polymer networks. An LCE exhibits both the thermotropic property of liquid crystals and the large deformation of elastomers. It can be monodomain or polydomain in the nematic phase and transforms to an isotropic phase at elevated temperature. These features have enabled various new applications of LCEs in robotics and other fields. However, despite substantial research and development in recent years, thermomechanical coupling in polydomain LCEs remains poorly studied, such as their temperature-dependent mechanical response and stretch-influenced isotropic-nematic phase transition. This knowledge gap severely limits the fundamental understanding of the structure-property relationship, as well as future developments of LCEs with precisely controlled material behaviors. Here, we construct a theoretical model to investigate the thermomechanical coupling in polydomain LCEs. The model includes a quasi-convex elastic energy of the polymer network and a free energy of mesogens. We study the working conditions where a polydomain LCE is subjected to various prescribed planar stretches and temperatures. The quasi-convex elastic energy enables a “mechanical phase diagram” that describes the macroscopic effective mechanical response of the material, and the free energy of mesogens governs their first-order nematic-isotropic phase transition. The evolution of the mechanical phase diagram and the order parameter with temperature is predicted and discussed. Unique temperature-dependent mechanical behaviors of the polydomain LCE that have never been reported before are shown in their stress-stretch curves. These results are hoped to motivate future fundamental studies and new applications of thermomechanical LCEs.

Composite stability and electro-optical properties of carbon nanotubes/5CB-based nematic hybrid liquid crystals

ABSTRACT This study examined the impact of different concentrations of carbon nanotubes (CNTs) on five nematic hybrid liquid crystals (LCs) based on 5CB. Electro-optical properties of the composite were tested, and the interaction between CNTs and LCs was theoretically analysed by first principles. Experimental results showed the effect of CNTs on electro-optical properties of hybrid LCs depended on the groups contained in monomer LCs. LCs with polar groups at the end increased the dielectric anisotropy of the composite, while the impact of CNTs on hybrid LCs containing non-polar terminal monomers was the opposite. The same variation was reflected in splay elastic constants and clearing points. Meanwhile, dielectric anisotropy, splay elastic constant, and rotational viscosity coefficients all changed with CNTs doping concentrations, reflecting the order of binding stability between representative groups and CNTs, and the variation of threshold voltage and response time reflected the order of binding stability between LC molecules and CNTs, consistent with theoretical calculations. Conductivity tests showed the additive of CNTs increased the conductivity of the composite and revealed the impact of CNTs on threshold voltage. Dielectric tests showed the incorporation of CNTs increased dielectric real and imaginary parts in low-frequency regions and increased relaxation frequency in high-frequency regions.

Novel star-shaped liquid crystal macromolecules based on conjugated structures of azo and Schiff bases: synthesis and properties

ABSTRACT In this paper, a series of novel liquid crystal units (ASE-n, n = 4, 6, 8) of azo Schiff bases with sulphonic acid (-SO3H) as terminal substituents and different alkyl chain lengths were designed and synthesised. Nine star-shaped liquid crystal macromolecules with different central groups (ASE-P-n, ASE-T-n, ASE-G-n, n = 4, 6, 8) were synthesised by esterification of three novel azo Schiff base liquid crystal units (ASE-n, n = 4, 6, 8) with phloroglucinol, pentaerythritol and glucose. The structure of the synthesised compounds were analysed by infrared spectroscopy and 1H NMR. The results showed that all the compounds were consistent with the molecular structure design. The properties of liquid crystal such as phase transition temperature and liquid crystal mesophase range were characterised by POM and DSC. The effects of alkyl chain length and branched degree on the properties of liquid crystal were studied. The maximum absorption spectrum of the compound was determined by UV-Vis spectrophotometry. The photoisomerisation reaction of the compound was also studied. Under ultraviolet light (λ = 365 nm) irradiation, the effect of different duration on the cis-absorption percentage of azo (-N=N-) bond in the compound was studied.

Thermoresponsive cholesteric liquid–crystal systems doped with terpenoids as drug delivery systems for skin applications

Stimuli-responsive and tunable soft-matter systems are an advanced class of materials applicable for drug delivery. Liquid crystals (LCs) are promising candidates as multifunctional materials that can respond to temperature, light or magnetic field. Particularly, ordering and physical properties of thermoresponsive LCs depend predominantly on temperature as external trigger. The current work addresses an elegant strategy to implement the anisotropic properties of thermoresponsive LCs with a view to extending their application for drug delivery. We firstly fabricated novel compositions with a thermotropic core based on natural products – cholesteryl esters and mono-/bicyclic terpenoids. The distinctive feature of aforementioned systems is their temperature-induced switchability of drug release by transition to the LC state, depending on the skin temperature. Their mesomorphic and optical behavior was characterized via differential scanning calorimetry and polarizing optical microscopy. Furthermore, we describe the dependence of helical pitch on LC formulation for various ternary cholesteric systems doped with terpenoids, suggesting that these stimuli-responsive chiral dopants are nominally untwisting. Data from fluorescence probe technique indicate that cholesteryl esters and terpenoids as essential components of those LC systems jointly disrupt the tight structure of phospholipid bilayer packing enabling the facilitated penetration of drugs. The potential of LC formulations was explored for several model drugs with diverse physicochemical properties by in vitro and ex vivo penetration tests using artificial membranes and full human skin. Our findings confirm the potential of LC systems for various applications in skin drug delivery.

Hydrogen bond ferroelectric liquid crystal for tripartite temperature sensor: Experimental and computational (DFT) studies

The mesogenic behavior and electronic properties of hydrogen bond ferroelectric liquid crystal (HBFLC) complex are studied using experimental and theoretical techniques. HBFLC complex is synthesized from mesogenic 4-hexyloxy benzoic acid (6O BA) as the proton donor and nonmesogenic chiral DL-tartaric acid (DLTA) as the proton acceptor through intermolecular hydrogen bonding. Molecular geometry of DLTA+6O BA HBFLC complex is optimized by density functional theory (DFT) calculation. An interesting observation is that polarizing optical microscope (POM) study confirms that the HBFLC complex exhibits thermochromic behavior in chiral nematic (N[Formula: see text] along with induced nontitled smectic G* phases. Another noteworthy observation is the manifestation of tripartite temperature stages. Fourier-transform infrared spectroscopy (FTIR) and natural bond orbital (NBO) analysis elucidate the intermolecular hydrogen bond between mesogenic (6O BA) and nonmesogenic (DLTA) compounds. Further, frontier molecular orbital (FMO) study and electrostatic potential (ESP) analysis explore the reactivity sites, intermolecular charge transfer and electronic behavior of HBFLC complex.

Dirac points, new photonic band gaps, and effect of magnetically induced transparency in dichroic cholesteric liquid crystals with wavelength-dependent magneto-optical activity parameter.

We investigated the properties of cholesteric liquid crystals (CLCs) being in an external static magnetic field directed along the helix axis. We considered a dichroic CLC, that is, CLC with parameters ReΔ=Reɛ_{1}-Reɛ_{2}/2=0 and ImΔ=Imɛ_{1}-Imɛ_{2}/2≠0, where ɛ_{1,2} are the principal values of the local dielectric permittivity tensor. We have shown that in the case of the wavelength dependence of the magneto-optic activity parameter, new features appear in the optics of dichroic CLCs, in particular, in this case new Dirac points appear. Dirac points are points where there is an intersection of any two wave vector curves (they degenerate) and a linear law of the wave vector dependence on the frequency near these points. And moreover, at some Dirac points photonic band gaps (PBGs) appear; at others, lines of magnetically induced transparency (MIT), that is, a full transmission band appears, in an absorbing medium. In this case a polarization-sensitive transmission band appears too. At certain values of the helix pitch of the CLC and of the magnitude of the external magnetic field, three PBGs of different nature appear: a transmittance band, two narrow lines of MIT, and at others a broadband MIT, etc. This system is nonreciprocal, and the nonreciprocity changes over a wide range. It is observed both for reflection and transmittance and for absorption. The soft-matter nature of CLCs and their response to external influences lead to easily tunable multifunctional devices that can find a variety of applications. They can be applied as tunable narrow-band or broadband filters and mirrors, a highly tunable broad/narrow-band coherent perfect absorber, transmitter, ideal optical diode, and in other devices.

Multicomponent structural color membrane based on soft lithography array for high-sensitive Raman detection

Inspired by ordered photonic crystals and structural color materials in nature, we successfully prepared hydroxypropyl cellulose (HPC) photonic films with ordered surface arrays by double-imprint soft lithography. Then we introduced another important material of the cellulose family, cellulose nanocrystals (CNC), which has liquid crystal nature and birefringent properties of the particles, into the system to realize the single-point shrinkage of the film array and the control of structural color. Through multi-component doping and concentration control, we further optimized the multi-scale structure of the materials, and obtained HPC/CNCs composite photonic films with excellent properties in color, stability and flexibility, whose elastic modulus and tensile properties are significantly higher than those of single-component. Further loading of SiO2@PDA enhances the color saturation and realizes the in-situ reduction of metal ions on the film surface. This plasma film can track a variety of substances with high sensitivity and long-term stability, showing potential application prospects in the field of surface-enhanced Raman scattering (SERS), which provides a potential possibility for chiral structures to be used in the field of biosensor detection and circularly polarized luminescence.

The effect of surfaces on the structural properties of liquid crystals

The effect of the surface microrelief on the structural properties of both the nematic and smectic A (SmA) phases, and the effect of the emerging smectic structure induced by the surface at temperatures close to the second-order nematic-SmA (NA) phase transition temperature TNA, on the Freedericksz threshold voltage for homogeneously aligned polar liquid crystals, is theoretically analyzed.It is shown that the orientation distortion of the director field caused by the wave-like microrelief on solid surfaces exponentially decays in the bulk of both phases. Since in the case of the SmA phase, the characteristic depth of deformation is comparable to the length of the molecule, then when the period of undulating relief passes several micrometers, the depth of penetration of the deformation is several tens of micrometers. This is two orders of magnitude greater than the penetration length of the distortion in the case of the nematic phase.The effect of the surface-induced SmA phase on the threshold voltage, for homogeneously aligned of polar liquid crystals is discussed from the energy point of view. It is shown that the Freedericksz critical voltage, for 4-n-octyl-4′-cyanobiphenyl, in a very thin cell near the second-order nematic-SmA (NA) phase transition temperature, exhibits an anomalous increase with decreasing of temperature T towards TNA.

Liquid crystal biosensor based on AuNPs signal amplification for detection of human chorionic gonadotropin

The detection of human chorionic gonadotropin (HCG) allows for the determination of pregnancy and is thus crucial during early pregnancy testing. This study introduces a novel liquid crystal (LC) biosensor that employs Au nanoparticles (AuNPs) for signal amplification, thus enabling the detection of the HCG antigen in a micro, efficient, and cost-effective manner. The sensor design capitalizes on the unique properties of LC to facilitate the detection of HCG. In this research, the surface of the base substrate was first modified with material from DMOAP and APTES, and EDC/sulfo-NHS was used to couple AuNPs and β-hCG to form an AuNP-β-hCG conjugate that improves the coupling rate. The carboxyl group of the antibody was reacted with the aldehyde group of glutaraldehyde, which helps to fix the β-hCG antibody to the surface of the substrate. The HCG sample is immobilized on the surface of the substrate via antigen-antibody immunobinding. As signal amplifiers, the AuNPs can have a significant effect on the topology of the interface and the vertical order of the LC molecules, thus reducing the limit of detection. Finally, the limit of detection was calculated using the SPSS system, and the relationship between grey values and concentrations was also obtained. The detection limit for HCG can be as low as 1.916 × 10−3 mIU·mL−1 under ideal conditions. Compared to other detection methods for HCG, this structure provides a detection pathway with excellent sensitivity, low detection limits, and better specificity, thus offering a new idea for HCG or any other target requiring trace detection.

Electronic, optical and spectroscopic properties of N-dialkyl-imidazolium hexafluorophosphate (CNMIM.PF6) ionic liquid crystal molecules investigated by computational methods.

In this present work, we calculate the electronic, spectroscopic and nonlinear optical properties (NLO) of N-dialkyl-imidazolium hexafluorophosphate (CNMIM.PF6, where N = 10, 12, 14, 16, 18, 20) ionic liquid crystal molecules under the effect of alkyl chain length variation in cation moiety [CNMIM]+ with fixed anion [PF6]-. CONTEXT: The majority of research on ionic liquid crystal to date has been focused on experiments, while theoretical studies on the optical properties of ionic liquid crystal have been extremely rare. Nonlinear phenomena in optical devices have attracted many researchers. Therefore, results of NLO properties may favor facile synthesis and fabrication of novel-type of materials as well as optoelectronic devices. Spectroscopic studies elucidate further insight into ionic liquid crystal behavior. The results demonstrate that variations in alkyl chain length have an impact on the conformers' electrical, spectroscopic, and NLO properties as well as their stability. The stability of ionic liquid crystal molecules increases with increasein the alkyl chain length and the energy band gap range is 6.64-6.29 eV. Understanding ionic liquid crystal's physical behavior requires an understanding of their dipole moments and NLO features, which are covered in this article. The results of NLO characteristics for all ionic liquid crystal molecules show that their first-order hyperpolarizabilities are higher than the reference molecule (urea). METHODS: The electronic (molecular energy band gap, electrostatic potential map, as well as HOMO-LUMO orbitals) and spectroscopic (IR-RAMAN, UV) properties were evaluated with the help of theoretical model at B3LYP/6-31G(d) while the NLO study has been performed using B3LYP and M06-2X with different basis sets 6-31G(d) and 6-311++G(d,p), as implemented in Gaussian09 software.

Propagation and attenuation of elastic waves in nematic elastomer hollow cylinders

As an intelligent soft material, nematic elastomers (NEs) possess both rubber elasticity and the unique properties of liquid crystals. Such combination gives NEs unusual mechanical response. In this paper, guided waves in nematic elastomer hollow cylinders are investigated. Based on the viscoelasticity theory, the wave governing equations are derived and solved by the Legendre polynomial series approach (LPSA). This approach can directly obtain eigenvalues/eigenvectors characterizing wave propagation and attenuation or field profiles. The effects of circumferential order and radius-thickness ratio on dispersion and attenuation curves are discussed. Besides, dispersion and attenuation curves with different initial director orientations are presented. It is found that the existence of the director results in mode conversions between adjacent flexural longitudinal modes and flexural torsional modes. The results provide important guidance for dynamic design and device analysis of cylinder-shaped nematic elastomer devices.

Synthesis, mesomorphic and fluorescent properties of 1,3,4-oxadiazoles/thiadiazoles with a terminal 3-fluoro-4-cyanophenyl group

A series of new liquid crystalline 1,3,4-oxa/thiadiazoles with a terminal 3-fluoro-4-cyanophenyloxy group were synthesized, and their structures were characterized by nuclear magnetic resonance (NMR) and electrospray ionization mass spectrometry (ESI-MS). The liquid crystal properties were studied by differential scanning calorimetry (DSC) and polarizing optical microscopy (POM). All of the 1,3,4-oxadizoles exhibited only Nematic mesophase, while the 1,3,4-thiadiazole analogues displayed Smectic A (SmA) and Smectic C (SmC) over a wide temperature range. The fluorescent properties were investigated using ultraviolet–visible spectrophotometer (UV–Vis) and fluorescence spectrometer. The solutions of all final compounds in CH2Cl2 individually displayed one absorption band with λabs-max values at 319–340 nm, and exhibited an intense emission with λem-max values at 432–448 nm, and the photoluminescence quantum yields reached 0.52–0.66. The effects of the heterocyclic structural units, the terminal alkoxy chain length and the lateral fluorine atom on liquid crystalline and fluorescent properties were comparatively discussed.

Design of chemically recyclable nanocellulose chiral liquid crystal photonic elastomer vitrimer and its mechanosensitive colour-changing materials

The development of nanocellulose (CNCs) chiral liquid crystal photonic elastomeric vitrimer materials is promising for achieving needed reduction in carbon emissions (elastomer material recycling) and developing novel photonic functional materials. The primary questions discussed are about what is the basic principle of chiral liquid crystal and photonic property of CNCs, how to design vitrimer elastomer materials, and what is the general approach to designing CNC chiral liquid crystal photonic elastomer vitrimer and mechanosensitive colour-changing materials.

Spectroscopy, electrochemistry and liquid crystal properties of monosubstituted ferrocene based azobenzene derivatives with 1,2,3-triazole heterocycle

Monosubstituted ferrocene based azobenzene liquid crystals were synthesized, in which ferrocene was flexibly attached to the mesomorphic core by click reaction. The counterpart compounds without ferrocene unit were also prepared. Two series of compounds displayed liquid crystal behaviours during heating and cooling, and had wide mesophase range. In the process of cooling, the compounds underwent a transformation from nematic phase to smectic C phase. When ferrocene was introduced into the molecule, the melting point of most compounds decreased by 30–40 °C, and the mesophase range was still as wide as 71–97 °C. These compounds emitted fluorescence at 400–550 nm and had strong absorption near 280 nm and 350 nm. Under the irradiation of 365 nm ultraviolet light, these compounds transformed rapidly from E to Z isomers, while the reverse reaction occurred slowly in the dark, especially the molecules containing ferrocene. Cyclic voltammetry studies showed that ferrocene based azobenzene derivatives exhibited one electron transfer processes with low formal redox potentials, indicating that these compounds are easily oxidized.

Synthesis and mesomorphic properties of swallow-tailed liquid crystal monomers and polymers

In this article, we designed and synthesized two novel swallow-tailed liquid crystal monomers (M1 and M2) with different chain lengths and rigidities centered on 3,4,5-trihydroxybenzoic acid and two series of side-chain liquid crystals polymers PN1 and PN2. All liquid crystal monomers and polymers were conformed to the molecular designs by FT-IR and NMR. The liquid crystal properties were tested by DSC and POM, and the results showed that M1 and M2 were thermotropic enantiotropic nematic liquid crystals and the side-chain liquid crystal polymers PN1 and PN2 series were typical amorphous liquid crystal polymers. With the increase of monomer content, the glass transition temperatures and clearing points of PN1 and PN2 series enhanced. With the increase of the number of mesogens in the liquid crystal monomer, the rigidity was enhanced, and the liquid crystal performance of the polymer was better.

A Review on Physical, Chemical and Optical Properties of Liquid Crystal

The foundation of the upcoming generation of cutting-edge gadgets and digitally augmented technologies is expected to be smart soft materials. Because of their responsiveness and adaptability, liquid crystals (LCs) are promising smart soft materials. In the 20th century, LCs were crucial to changing the information display sector. However, several beyond-display uses for LCs have been proven at the turn of the twentieth century, neatly using their controlled stimuli-responsive and adaptable properties. New LC materials have been developed and engineered for such applications. The review comes close with a summary and viewpoints on the potential and problems facing LCs as smart soft materials. This review is expected to inspire a wide range of concepts for the application of nature's delicate phase of matter in the generation and beyond of smart and augmented devices.

Effect of shape of ZnO nanoparticle on electro-optic and dielectric properties of nematic liquid crystal

Properties of liquid crystal incorporated with various shapes of a nanomaterial are expected to show multifaceted behaviour as properties of nanomaterial are extremely dependent on its size and shape. In view of this, we have investigated the effect of shape of ZnO nanoparticle on electro-optic and dielectric properties of nematic liquid crystal (NLC), PCH5. Nanocomposites of 0.01 wt% ZnO nano- sphere, rods, and wires in NLC are prepared. The addition of ZnO nanoparticles to NLC leads to increase in the order parameter and reduction in threshold voltage, response time as well as ionic conductivity as compared to the pure NLC host. Dielectric permittivity measurements indicate nanoparticle induces short range order in isotropic phase. Dielectric measurements also show increase in order parameter in nanocomposites with 1D nanoparticles whereas negligible change is observed in nanocomposites with spherical nanoparticles in NLC in deep nematic range. The ATR-FTIR measurements support the enhancement in parallel correlation of dipole moments in ZnO nanocomposites as compared to pure NLC.

Interplay of reorientational and thermal solitons: unveiling the dynamic coexistence and enhanced nonlinear response in nematic liquid crystals

Spatial solitons in nematic liquid crystals (NLC) are optical beams that maintain their transverse profile due to the self-focusing effect of light. The mechanism leading to the formation of solitons can be either light-induced reorientation of the elongated molecules or thermal changes in the refractive index induced by a light beam. The specific optical properties of nematic liquid crystals cause the reorientational and thermal solitons to coexist in NLC, and their mutual competition can lead to a reduction or enhancement of the overall nonlinear response of the NLC. The study of the interplay between reorientational and thermal solitons allows a better understanding of the conditions under which the solitons can coexist, compete, or enhance each other. Here, we show that the simultaneous existence of both soliton types can lead to a more robust and versatile nonlinear response and can be used in competitive and synergistic configurations by carefully controlling the temperature, intensity, and polarization of the propagating light. As a result of our experimental findings, it may be possible to shape the nature of the interplay between reorientational and thermal solitons and better understand the behavior of reorientational solitons under varying thermal conditions. Furthermore, understanding this mutual interplay may provide insights into the potential of advanced NLC-based photonic devices that exploit the joint effect of reorientational and thermal nonlinearities.