Liquid Crystal Phase Research Articles

Frequency suppression of director oscillations in AC-driven liquid-crystal-based terahertz phase shifters.

Frequency-induced instability is widely present in nematic liquid crystals (LCs), which poses a problem in improving liquid-crystal-based phase-shift devices driven by alternating currents. Herein, the Fréedericksz transition of thick nematic LCs was investigated under alternating electric fields to reveal the suppression of frequency-induced instability in the low frequency range. By extending the Frank-Leslie equation to the AC-driven case, the response of the LC was numerically calculated, and the frequency threshold for suppressing the driven instability was estimated in conjunction with the perturbation method. Experimentally, the frequency suppression of LC fluctuations was verified by using applied electric fields. In addition, the root-mean-square-error of the refractive index was measured to be less than 2 × 10-5, which excludes the convective instability-generating domains in devices. It was revealed that the fabricated thick LC phase shifters provided a phase shift of more than 360° at 2 THz and can be used in the terahertz band. It was observed that the electrically driven phase-shift characteristics were in accordance with the theoretical results as the threshold frequency condition was satisfied. This work provides an experimental and theoretical reference for improving modulation performance and enhancing the characterization of AC-driven LC-based phase-shift devices.

Ultrathin, Large‐Area, and Multifunctional Polarizer Based on Highly Ordered Carbon Nanotubes Produced by Simple Shear Flow

AbstractAligning carbon nanotubes (CNTs) with high orientational ordering in a 2D array is essential for realizing optical polarizers with high polarization efficiency over a wide spectral range. Two methods are reported: dispersing CNTs in a polymer matrix followed by stretching, and mechanical stretching of a vertically grown CNT forest and then transferring it onto a substrate. However, neither can realize nanometer‐thin or large polarizers. Herein, a novel approach is demonstrated to construct a large and thin (150 nm) conductive CNT polarizer by achieving liquid crystal (LC) phase of size‐controlled CNTs in chlorosulfonic acid, unidirectional shearing, and then drying. The polarizer exhibits an excellent polarization efficiency of 98.4% for ultraviolet (365 nm) and 96.0% for visible light (550 nm). This performance is maintained even at 400 °C, while the conventional iodine‐type polarizer starts to lose efficiency over 100 °C. The CNT polarizer with high polarization efficiency for UV can replace the costly nano‐patterned wire‐grid polarizers currently used for photoaligning LCs and also its multifunction playing role of polarizer, electrode, and LC alignment is confirmed with switching of LC device. This study provides a new paradigm for realizing ultrathin and large CNT polarizers for broadband applications with outstanding heat resistance.

Stimuli-responsive grating with a tunable period and a diffraction order.

The next-generation diffraction grating not only requires a nonmechanical control and a remote control to generate a diffraction pattern but also requires a tunable period. Here, we propose a stimuli-responsive liquid crystal (LC) phase grating with a tunable period and a diffraction order. The stimuli-responsive LC diffraction grating is composed of periodically arranged electrodes on a double-sided glass substrate. By adjusting the driving scheme, the pitch and diffraction order of the LC grating can be switched between three different modes. The experimental results show that the LC grating has a lower driving voltage (∼5 V). In addition, the tunable LC grating can achieve more diffraction orders, which can be applied to a holographic display to achieve a wide-viewing angle and an enlarged size.

Clustering-Triggered Emission Liquid Crystalline Polymer Bearing Cholesterol: Tunable Circularly Polarized Luminescence and Room-Temperature Phosphorescence.

Circularly polarized luminescence (CPL) materials with clustering-triggered emission (CTE) characteristic have gradually attracted attention for their unique photophysical properties. However, the majority of reported clusteroluminogens lack chirality and exhibit heterogeneity, making it challenging to achieve a well-defined helical structure necessary for efficient CPL with high dissymmetry factor (glum ). In this paper, chiral liquid crystals are constructed to obtain CTE-based CPL materials with high glum values. Side chain liquid crystal polymer PM6Chol bearing cholesterol clusteroluminogens are designed and synthesized. PM6Chol-coated film and PM6Chol thermal-treated film are also successfully prepared by different film-forming methods. Both the films inherit the CTE characteristic of cholesterol and show excitation wavelength-dependent luminescent behavior. However, the two polymer films exhibit different liquid crystal phase structures, with PM6Chol-coated film being a chiral bilayer smectic C phase and PM6Chol thermal-treated film being an achiral bilayer smectic A phase. Attributed to helical arrangement of cholesterol, PM6Chol-coated film emits efficient CPL with glum values up to 1.0 × 10-1 . For PM6Chol thermal-treated film, no CPL signal is detected due to the absence of helical structure. However, it shows obvious room-temperature phosphorescence with 2.0 s afterglow and 23.9ms lifetime.

Dynamic diselenide bond‐enabled liquid crystal elastomer‐based two‐way shape memory aerogels with weldability and closed‐loop recyclability

AbstractTwo‐way shape memory polymeric aerogels (2W‐SMPAs), with the ability to undergo reversible shape deformation in response to external stimuli, have extensive application in diverse fields such as actuators, sensors, robotics, and other relevant domains. In this study, we introduce a novel approach for fabricating a 2W‐SMPA material based on liquid crystal elastomers (LCEs) incorporating dynamic diselenide bonds. The aerogel exhibits liquid crystal phases, excellent compressibility and shape stability, and the mesogens are uniaxial‐oriented along the stretching direction. By capitalizing on the dynamic diselenide bonds, the LCE‐based aerogel demonstrated remarkable reprogrammability, weldability, and recyclability through thermal reorganization. The shape‐programmed aerogel sample exhibits reversible shrinking deformation during the heating and cooling cycles, ultimately achieving a maximum shrinkage ratio of 26.1%. Moreover, the LCE‐based aerogel's porous structure and monodomain orientation effectively enable the adsorption of the photothermal dye DR1 and facilitated the reversible photothermal‐induced shape deformation when exposed to 520 nm light irradiation. These findings reveal the potential application of this innovative LCE‐based aerogel material, enabled by dynamic diselenide bonds, in various areas including control devices, soft actuators, and other diverse fields.

TWINS: improved spatial and angular phase calibration for holography.

We present TWINS (TWo INclining Slits), a method for characterizing phase spatial light modulators (SLMs), inspired by Young interferometry. TWINS is an elegant and versatile approach, using minimal equipment and alignment. It measures phase response locally rather than globally, both horizontally and vertically, with high resolution and at wide angles. It can also measure beam intensity profiles as directly seen by the SLM. TWINS characterizes the anisotropic aberrations in the mainstream models of liquid crystal phase SLMs, which is crucial to improve hologram quality. Compensating for anisotropic aberrations measured by TWINS improved the image quality of planar holograms by 10dB.

A Novel Method for Quadrature Signal Construction in a Semiconductor Self-Mixing Interferometry System Using a Liquid Crystal Phase Shifter

We have established a novel method for quadrature signal construction in a semiconductor laser diode self-mixing interferometry system using two photodiodes and a beam splitter with a liquid crystal phase shifter (LCPS). This method entails placing an LCPS between the photodiode and the beam splitter so that another phase shift self-mixing signal can be obtained. Then, an arctangent phase algorithm can be used to demodulate the pair of quadrature signals to reconstruct the vibration information of the target object. This method simplifies the self-mixing signal demodulation process and the reconstruction of vibration information. Our experimental results demonstrate the feasibility of using self-mixing phase shifter detection for self-mixing optical measurements. The work illustrates a sort of efficient and referable novel design guidance model which supports the quadrature signals construction in a self-mixing interferometer based on a semiconductor laser diode.

Design, synthesis and properties of mesogen jacketed liquid crystalline polymers with fluorinated triphenylene pendants

Discotic liquid crystalline polymers (DLCPs) combining the advantages of discotic liquid crystals and polymers yet have been less developed compared to the calamitic counterparts mainly due to lack of DLCP materials. Here, mesogen-jacketed liquid crystalline polymers with pendant fluorinated triphenylene were successfully synthesized via proper polymerizations of the corresponding monomers respectively. Nuclear magnetic resonance spectroscopy (both 1H NMR and 13C NMR), time-of-flight mass spectrometry (MALDI-TOF MS), Fourier transform Infrared (FT-IR) spectroscopy and other characterization methods were used to confirm the structure of the products. The mesomorphic properties of the obtained triphenylene monomers with unsaturated flexible tails and resulting polymers were investigated by polarizing optical microscopy (POM), differential scanning calorimetry (DSC), and X-ray diffractometer (XRD). The propargylene-containing monomer shows a smectic liquid crystals phase and the pentynylene monomer exhibit a highly ordered columnar helical liquid crystal phase. The three side chain polymers containing fluorinated triphenylene have good thermal stability (decomposition temperature of 350 °C or above). At the same time, the polymers prepared by the polymerization of propargylene-containing monomers have both hexagonal columnar phase (Colh) and rectangular columnar phases (Colr). The polymers with long spacer chains derived from Methyl acrylate have a hexagonal columnar structure (Colh). The fluorinated triphenylene monomers and polymers with ordered liquid crystal phases synthesized in this work provide useful inspiring for structural modifications of DLCPs and are promising for applications in organic optoelectronics.

Liquid Crystalline and Gel Properties of Luminescent Cyclometalated Palladium Complexes with Benzoylthiourea Ligands.

The design and development of new luminescent metallogels formed by cyclometalated palladium(II) complexes in protic solvents were investigated by a combination of differential scanning calorimetry (DSC), polarized optical microscopy (POM), and rheology. Cyclometalated palladium(II) complexes based on imine ligand and ancillary benzoylthiourea (BTU) ligand showed red emission in solid and gel states. The formation of a lyotropic liquid crystal phase was observed for the complex bearing shorter alkyl groups on the BTU ligand. This complex also behaved as a thermotropic liquid crystal that displays a monotropic smectic A phase (SmA). Dynamic rheology measurements (frequency sweep in the 5-90 °C range) of the 1-decanol solution of palladium(II) complexes highlighted their supramolecular self-association ability to generate 3D networks and form gels as a final result.

Ferroelectric Photovoltaic Effect in the Ordered Smectic Phases of Chiral π-Conjugated Liquid Crystals: Improved Current-Voltage Characteristics by Efficient Fixation of Polar Structure

Abstract We synthesized four liquid-crystalline (LC) compounds with extended π-conjugation, incorporating a chiral branched-alkoxy chain. These compounds displayed ferroelectric chiral smectic C (SmC*) and ordered smectic phases. The carrier transport properties in the LC phases were characterized using the time-of-flight (TOF) method. Although all four chiral phenylterthiophene derivatives exhibited nondispersive hole conduction in the SmC* phases, nondispersive transient curves in the ordered smectic phases were limited. Notably, clear nondispersive photocurrent decays were observed in the ordered smectic phase of an LC compound containing an (R)-3-octyloxy moiety as the chiral unit. This compound achieved hole mobility of 4 × 10−3 cm2 V−1 s−1 in the ordered phase at 114 °C. The polar structure induced by poling treatment in the ferroelectric SmC* phase of the chiral LC compound was successfully stabilized with a lower electric field than previously reported systems. We confirmed the presence of the ferroelectric bulk photovoltaic effect in the polarized structure of the chiral-ordered smectic phase. Furthermore, we observed an enhancement in the output performance for the bulk photovoltaic effect due to the efficient stabilization of the polar structure.

Eco-Friendly Dispersant-Free Purification Method of Boron Nitride Nanotubes through Controlling Surface Tension and Steric Repulsion with Solvents.

Boron nitride nanotubes (BNNTs) were purified without the use of a dispersant by controlling the surface tension and steric repulsion of solvent molecules. This method effectively enhanced the difference in solubilities of impurities and BNNTs. The purification process involved optimizing the alkyl-chains of alcohol solvents and adjusting the concentration of alcohol solvent in water to regulate surface tension and steric repulsion. Among the solvents tested, a 70 wt% t-butylalcohol in water mixture exhibited the highest selective isolation of BNNTs from impurities based on differences in solubilities. This favorable outcome was attributed to the surface tension matching with BNNTs, steric repulsion from bulky alkyl chain structures, and differences in interfacial energy between BNNT-liquid and impurity-liquid interfaces. Through this optimized purification process, impurities were removed to an extent of up to 93.3%. Additionally, the purified BNNTs exhibited a distinct liquid crystal phase, which was not observed in the unpurified BNNTs.

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.

Mechanistic Study of Amphiphilic-Assisted Self-Assembled Cadmium Sulfide Quantum Dots into 3D Superstructures.

Self-assembling of nanoparticles into complex superstructures is very challenging, which usually depends on postorganizing techniques or pre-existing templates such as polypeptide chains or DNA or external stimulus. Such self-assembled processes typically lead to close-packed structures. Here, it has been demonstrated that under carefully template-free reaction conditions CdS quantum dots (QDs) could be synthesized and simultaneously self-assembled into complex superstructures without compromising individual QD properties. The superstructures of CdS QDs attained by the chemical-based method demonstrate Stokes-shifted photoluminescence (PL) from trap states. Remarkably, the PL decay of superstructures exhibits a single-exponential feature. This behavior is unusual for the synthesized superstructures, indicating that the trap states are restricted to a narrow range. The growth mechanism of these superstructures is explained through the formation of liquid crystal phases (LCPs) with the help of a small-angle X-ray scattering (SAXS) analysis.

Non-linear optical chiral nematic bandgap

ABSTRACTWe construct a joint Lagrangian to describe the interaction between a non-magnetic chiral or twisted nematic phase and an electromagnetic wave. Using the well-known Oseen transformation, we rewrite the Lagrangian to transform the director angle of the liquid crystal in an ignorable coordinate which allows to decouple the electric components from the director angle to get the non-linear dynamical equation governing the electromagnetic fields within the liquid crystal phase. We propose a trial solution to get an approximated solution of the fields which allow us to get an intensity dependent band structure that we analyse.

Double‐State Anticounterfeiting Labels Based on Light‐Emitting Polymer‐Stabilized Blue Phase

AbstractAnticounterfeit labels with multiple information‐carrying functions serve a critical role in boosting security and minimizing counterfeit‐related losses, whose development remains challenging. Here, a security label with the ability to encode information in reflective and fluorescent sates is fabricated from light‐emitting polymer‐stabilized blue phase (LE‐PSBP) liquid crystals (LCs). The LE‐PSBP with temperature‐dependent reflective color can be encoded with information by UV irradiation through photomasks. On the other hand, introduction of a fluorescent molecule, which can be easily photocyclized upon UV irradiation, enables the convenient encoding of fluorescent information without impacting the reflective information. As an illustration, a security label with distinct information in reflective and fluorescent states is demonstrated. Such innovative security labels have tremendous potential to provide a platform for multi‐information encoding to enhance the protection level in anticounterfeiting technology.

Thermodynamically controlled multiphase separation of heterogeneous liquid crystal colloids

Phase separation is a universal physical transition process whereby a homogeneous mixture splits into two distinct compartments that are driven by the component activity, elasticity, or compositions. In the current work, we develop a series of heterogeneous colloidal suspensions that exhibit both liquid-liquid phase separation of semiflexible binary polymers and liquid crystal phase separation of rigid, rod-like nanocellulose particles. The phase behavior of the multicomponent mixture is controlled by the trade-off between thermodynamics and kinetics during the two transition processes, displaying cholesteric self-assembly of nanocellulose within or across the compartmented aqueous phases. Upon thermodynamic control, two-, three-, and four-phase coexistence behaviors with rich liquid crystal stackings are realized. Among which, each relevant multiphase separation kinetics shows fundamentally different paths governed by nucleation and growth of polymer droplets and nanocellulose tactoids. Furthermore, a coupled multiphase transition can be realized by tuning the composition and the equilibrium temperature, which results in thermotropic behavior of polymers within a lyotropic liquid crystal matrix. Finally, upon drying, the multicomponent mixture undergoes a hierarchical self-assembly of nanocellulose and polymers into stratified cholesteric films, exhibiting compartmentalized polymer distribution and anisotropic microporous structure.

Influence of Casein and Milk Phospholipid Emulsifiers on the Digestion and Self-Assembled Structures of Milk Lipids

Interfacial compositions of fat globules modulate the digestion behaviour of milk triglycerides in the gastrointestinal tract, thereby affecting lipid metabolism and delivery of nutrients. In this study, we aim to understand the impact of emulsifiers on lipid digestibility and the self-assembled liquid crystal structures formed by anhydrous milk fat (AMF) during digestion. AMF was emulsified with casein and milk phospholipids, and digestion was performed in both gastric and small intestinal conditions to account for changes at the oil/water interface following enzymatic digestion in the gastric phase. Small angle X-ray scattering was used to characterise the self-assembled structures of the digestion products, while coherent anti-Stokes Raman scattering microscopy was utilised to probe changes in lipid distribution at the single droplet level during digestion. Our findings confirmed that emulsifiers play a key role in the digestion of AMF. Milk phospholipids exhibited a protective effect on milk triglycerides against pancreatic lipase digestion by slowing digestion, but this effect was slightly negated in emulsions pre-digested under gastric conditions. The overall types of liquid crystal structures formed after digestion of casein- and milk phospholipids-emulsified AMF were comparable to commercial bovine milk irrespective of gastric pre-treatment. However, emulsification of AMF with milk phospholipids resulted in changes in the microstructures of the liquid crystal phases, suggesting potential interactions between the digested products of the fat globules and milk phospholipids. This study highlights the importance of emulsifiers in regulating lipid digestion behaviour and lipid self-assembly during digestion.

Liquid crystalline reduced graphene oxide composite fibers as artificial muscles

Soft materials are known for their compliance, adaptivity and dexterity. They have been well-pursued to create artificial muscles for robotic applications, however, at the cost of load bearing, bit rates and energy efficiency compared with their rigid body counterparts. Despite significant efforts to improve their performance, very few can match the biological muscles, achieving fast speed, high elasticity, and high power density simultaneously. Here, we self-assemble composite fibers by wet-spinning graphene oxide (GO) nanosheets in their lyotropic liquid crystal (LLC) phase mixed with conducting polymers and depleting agent, poly(ethylene glycol), followed by chemical reduction of GO. The resulting reduced GO (rGO) nanosheets are highly aligned and closely packed, which is essential to their high mechanical strength and toughness and actuation behaviors. The composite fiber exhibits fast (80 ms) and reversible bending via electrostatic repulsion between the rGO sheets. When the fibers are plied with nylon yarns, our actuators can achieve 75 J/kg work capacity and 924 W/kg power density without diminishing the bending strain and efficiency up to 10,000 cycles.

Periodic liquid crystalline waveguiding microstructures

Different methods allowing for creating optical waveguides with liquid–crystal (LC) cores, in which molecules form periodic patterns with precisely controlled periods, are reported. The first one is based on reversible photoalignment with high-resolution selective illumination and allows to control the period of LC molecules inside silica microcapillaries. The second method employs microstructures formed in PDMS, allowing to obtain both: LC-core waveguides and a set of specially designed periodic microelectrodes used for the periodic reorientation of molecules. Using both methods, we successfully controlled the period of the patterned alignment in the range from about 500 µm and scaled it down to as small as 20 µm. We performed experimental studies on waveguiding phenomenon in such structures, in view to obtain transmission spectra typical to optical fiber gratings. Since the results achieved in experimental conditions differed from those expected, the additional numerical simulations were performed to explain the observed effects. Finally, we obtained the waveguiding in a blue phase LC, characterized by naturally created three-dimensional periodicity with periods smaller than one micrometer. In such a structure, we were able to observe first-order bandgap, and moreover, we were able to tune it thermally in nearly the whole visible spectral range.

Helical polymerisation using a polymer network derived from a blue phase as a template for chirality transfer

ABSTRACT We prepared a blue phase (BP) liquid crystal mixture consisting of a T-shaped nematic liquid crystal, a chiral dopant, a reacting monomer mixture and a photo-initiator. It exhibited a phase sequence of Iso-BPIII-cubic BP-N* on cooling. We produced the polymer stabilised cubic BP (PS-cubic BP) and BPIII (PS-BPIII). After washing low-molar-mass materials from each PS-BP, we obtained the corresponding polymer networks. Marked difference exhibited in a three-dimensional structure between the BPIII and cubic BP polymer networks. Although they were prepared by the photopolymerisation of achiral monomers, they showed optical activities. Furthermore, they proceed the helical polymerisation of achiral reactive monomers to produce a chiral polymer film. The optical handedness of the polymer film is corresponded to that of the original BP. We discuss how the chirality is transferred from each BP to the corresponding polymer film.