Liquid Crystal Phase Research Articles

Phase transition and gelation in cellulose nanocrystal-based aqueous suspensions studied by SANS

HypothesisAqueous suspensions of cellulose nanocrystals (CNC) form a re-entrant liquid crystal (LC) phase with increasing salinity. Phase separation occurs in this LC state leading to a biphasic gel with a flow programmable structure that can be used to form anisotropic soft materials. We term this state a Liquid Crystal Hydroglass (LCH). Defining the mechanisms by which the LCH forms requires detailed structural analysis at the mesoscopic length scale. ExperimentsBy utilising Small Angle Neutron Scattering (SANS), we investigated the microstructure transitions in CNC suspensions, with a particular focus on the unique LC re-entrancy and gelation into the biphasic LCH. FindingsScattering from LCH gels comprises contributions from a dispersed liquid state and static heterogeneity, characterised using a Lorentzian-Gaussian model of inhomogeneity. This conceptually supports a gelation mechanism (spinodal decomposition) in CNC suspensions towards a biphasic structure of the LCH. It also demonstrates that, with increasing salinity, the non-monotonic variation in effective volume fraction of CNC rods fundamentally causes the LC re-entrancy. This work provides the first experimental characterisation of the LC-re-entrancy and formation of an anisotropic LCH gel. The proposed mechanism can be extended to understanding the general behaviour of anisotropic colloids.

Shear-induced flow and dynamics of the nematic phase of Sunset Yellow lyotropic chromonic liquid crystal

ABSTRACT Fundamental understanding of rheological properties of lyotropic chromonic liquid crystals is inadequate compared to their thermotropic counterparts in spite of the growing interest in these materials. Here, we investigate the rheological properties of a lyotropic chromonic nematic liquid crystal (LCLC), prepared by dissolving 32 wt% of Sunset Yellow (SSY) in water. The flow curves (stress-strain rate, σ − γ ˙ ) at different temperatures exhibit three flow regimes R1, R2 and R3, respectively. At room temperature, in R1 ( γ ˙ < γ ˙ cl ) and R3 ( γ ˙ > γ ˙ cu ) the sample exhibits Newtonian flow behaviour ( σ ∝ γ ˙ ), whereas, in R2 ( γ ˙ cl < γ ˙ < γ ˙ cu ), it shows a power-law dependence ( σ ∼ γ ˙ β ). With increasing shear rate the change of stress from R1 to R2 is continuous, whereas the change is discontinuous from R2 to R3. We observe stress fluctuations under steady shear in regime R2 similar to those reported in worm-like micellar systems and in thermotropic twist-bend nematic liquid crystals. The non-monotonic flow behaviour and the stress dynamics are corroborated using in situ rheo-microscopy.

Optical Phased Array-Based Laser Beam Array Subdivide Pixel Method for Improving Three-Dimensional Imaging Resolution

The small number of pixels in the current linear mode avalanche photodiode (LM-APD) array limits its three-dimensional (3D) imaging resolution. We use an optical phased array-based beam array subdivided pixel method to improve the 3D imaging resolution, using an optical phased array to generate a beam array with the same number of pixels as the LM-APD array and matching positions and controlling each sub-beam in the beam array to scan in the field of view of the corresponding pixel. The sub-beam divergence angle in the beam array is smaller than the instantaneous field of view angle of a single pixel in the LM-APD array. The sub-beam scanning in a single pixel’s field of view realizes the multiple acquisition of the target 3D information by the LM-APD array, thus improving the resolution of the LM-APD array. The distribution of the beam array in the far field is simulated, and the main performance parameters of 3D imaging are analyzed. Finally, a liquid crystal phase modulator is used as an optical phased array device to conduct experiments on a target 20 m away, and the results prove that our method can improve the resolution from 4 × 4 to 8 × 8, which can be improved at least four times.

Effects of ionic liquids as catalysts on liquid crystal aromatic polyester based on thin‐film polymerization technique

AbstractLiquid crystal aromatic polyesters (LCAPs) have been used in wide applications, such as chip packaging and airship skin. The traditional polymerization of LCAP is catalyzed by inorganic salts, for example, sodium acetate. Herein, we reported three kinds of ionic liquids (ILs) as catalysts to promote LCAP polymerization for the first time. The effects of ILs on the polymerization behaviors are investigated by in situ observing the liquid crystal (LC) phase behaviors based on the thin‐film polymerization technique. The evolution of LC phase over time was detailly investigated by analyzing the emergence, fusion, and growth of LC phase. The temperature from 170 to 210°C and the catalyst concentration in range from 0 to 0.5 wt% are compared, showing that temperature at 210°C and catalyst concentration at 0.5 wt% are the best polymerization conditions for all ILs catalysts. In comparison with trimethylsulfonium iodide and piperidine acetate, 1‐ethyl‐3‐methylimidazolium acetate has the best catalytic effect for the polymerization of LCAP in all experimental conditions. The successful polymerization was confirmed by FT‐IR spectra, showing the increased intensity of vibration peak at 1735 cm−1 attributed to the generation of ester groups while the decreased intensities of vibration peaks at 1759 and 1370 cm−1.

Impact of Molecular-level Structural Disruption on Relaxation Dynamics of Polymers with End-on and Side-on Liquid Crystal Moieties.

In side-chain liquid crystal polymers (SCLCPs), short side chains are attached on a flexible polymer backbone, and each side chain can have a liquid crystal (LC) group attached at the final bead in either an end-on or a side-on configuration. SCLCPs with random sequences of end-on and side-on LC moieties exhibit nonmonotonic thermal behavior as a function of composition, with some mixed sequences having a lower isotropic to LC phase transition than either purely end-on or side-on configurations. The origin of this nonmonotonic thermal trend lies in the disruption of molecular-level positional ordering and alignment due to the different preferred types of ordering of the different LC attachment types. We compare coarse-grained molecular dynamics (MD) simulations and experiments on SCLCP systems with only one type of LC moiety and demonstrate qualitative agreement in the observed mesophases of end-on and side-on SCLCP systems. Specifically, end-on SCLCPs display a smectic B-like mesophase, with layers of polymer between LC layers, while side-on SCLCPs exhibit a quasi-hexagonal columnar structure of polymer and a nematic surrounding the LC mesophase. Detailed analysis of SCLCP systems with various compositions of these types of LC attachments via MD reveals structural disruption in systems with intermediate compositions. Simulation snapshots and anisotropy ratio measurements show how random SCLCP systems deviate from the expected behavior of prolate or oblate systems in terms of their conformation. This molecular disruption in random SCLCP systems, particularly with a high composition of side-on LC moieties, also significantly impacts the relaxation dynamics. Modifying the composition of the LC type of attachment (molecular structure) is a possible route to tuning both the phase behavior and mechanical response of these systems.

The Ever Elusive, Yet-to-Be-Discovered Twist-Bend Nematic Phase

The second, lower-temperature nematic phase observed in nonlinear dimer liquid crystals has properties originating from nanoscale, polar, intermolecular packing preferences. It fits the description of a new liquid crystal phase discovered by Vanakaras and Photinos, called the polar-twisted nematic. It is unrelated to Meyer’s twist-bend nematic, a meta-structure having a macroscale director topology consistent with Frank–Oseen elastic theory.

Preparation and properties of novel liquid crystal ionomers bearing benzoxazole-based mesogenic side-chains and their membranes

A new heterocyclic mesogenic compound, 6-[4-(5-chloro-2-benzoxazolyl)phenoxy]-hexanethiol (CBPHT), is designed and synthesized, which shows an enantiotropic SmC mesophase with the liquid crystal phase ranges of 5.67 °C and 22.99 °C on heating and cooling, respectively. The CBPHT-based side-chain liquid crystal polymers (LCP-x) are obtained via a nucleophilic substitution between CBPHT and polyepichlorohydrin, and also shows an enantiotropic SmC mesophase with narrow temperature range. Furthermore, novel side-chain liquid crystal ionomers (LCI-x) with enantiotropic SmC mesophase are prepared via quaternization of N-methylimidazole with LCP-x, where polyepichlorohydrin, CBPHT and methylimidazolium chloride act as polymer main-chain, mesogenic side-chain, and ionic group, respectively. In addition, LCI-x membranes can be fabricated via a solution casting method, and exhibit high thermal stability, good mechanical properties and high ion conductivity. For example, LCI-0.20 with a low ion exchange capacity of 0.12 mequiv. g−1 has a large elongation at break (385 %) and a high chloride ion conductivity (20.68 mS cm−1), indicating a high potential for application as polymer electrolyte membrane.

Effect of the Structure of Epoxy Monomers and Curing Agents: Toward Making Intrinsically Highly Thermally Conductive and Low-Dielectric Epoxy Resins.

The low intrinsic thermal conduction and high dielectric properties of epoxy resins have significantly limited their applications in electrical and electronic devices with high integration, high frequency, high power, and miniaturization. Herein, a liquid crystalline epoxy (LCE) monomer with a biphenyl mesogenic unit was first synthesized through an efficient one-step reaction. Subsequently, bisphenol AF (BPAF) containing low-polarizable -CF3 groups and 4,4'-diaminodiphenylmethane (DDM) were applied to cure the LCE and commercial diglycidyl ether of bisphenol A-type epoxy (E-51), respectively, to afford four kinds of epoxy resins with various intrinsic thermal conductivity and dielectricity values. Owing to the dual effect of microscopically stacking of mesogens and the contribution of fluorine to the formation of liquid crystallinity, ordered microstructures of the nematic liquid crystal phase were formed within the cross-linking network of LCE as confirmed by polarized optical microscopy and X-ray diffraction. Consequently, phonon scattering was suppressed, and the intrinsic thermal conductivity was improved considerably to 0.38 W/(m·K), nearly twice as high as that of E-51 cured with DDM (0.20 W/(m·K)). Additionally, the ordered microstructure and ultralow polar -CF3 groups within LCE cured with BPAF enabled the epoxy resin to exhibit a remarkably lower and stable dielectric constant (ε) and dielectric loss tangent (tan δ) over both low and high frequencies compared to E-51 cured with DDM. The ε decreased from 3.40 to 2.72 while the tan δ decreased from 0.044 to 0.038 at 10 GHz. This work presents a scalable and facile strategy for breaking the bottleneck of making epoxy resins simultaneously with high inherent thermal conduction and low dielectric performance.

AN OVERVIEW ON CUBOSOMES AS REMARKABLE NANOCARRIER FOR DRUG DELIVERY

Lipids have been extensively used as main ingredients in various drug delivery systems, such as liposomes, solid lipid nanoparticles, nanostructured lipid carriers and lipid-based lyotropic liquid crystals. Over the past few years, lipid-based lyotropic, bicontinuous cubic phase liquid crystals have been investigated for their applicability to controlled delivery of active ingredients. Lipid-based lyotropic liquid crystals have highly ordered, thermodynamically stable internal nanostructure, thereby offering the potential as a sustained drug release matrix. The emulsification of cubic lipid phases in water results in the production of cubosomes that can be defined as nanoparticulate disperse systems characterised by high biocompatibility and bioadhesivity. The unique microstructure of cubosomes have the potentials to control the release of active ingredients, improve drug bioavailability and reduce toxicity, enhance the stability of drugs and to increase the penetrability of drug after topical application. This reflection will provide an overview of the lipids used to prepare cubic phase at physiological temperature, as well as the influencing factors on the phase transition of liquid crystals. In particular, the most current research progresses on cubic phase as drug delivery systems and its applications will be discussed. It might act as smart lipid nanoparticles for drug delivery.

Liquid Crystals for Luminescent Concentrators: A Review

Luminescent optical concentrators are thin films containing fluorescent dyes that enable light collection over a wide field of view without the need to track the path of the Sun. However, a disadvantage when using luminescent concentrators is that the performance is often impeded by surface losses through these films. Liquid-crystal (LC) hosts are attractive for luminescent concentrators, as they impart, at the very least, an orientational ordering to the transition dipole moment of the dyes dispersed within these films. This enables the directivity of both the absorption and emission and can reduce surface losses by, for example, adopting the homeotropic alignment of the LC director. This article reviews the developments and applications of LCs to luminescent optical concentrators and describes the strategies that have been introduced to further combat losses by decoupling the absorption and emission processes through Förster energy transfer, the approaches employed to enhance the chemical structures of the dyes, and the methods of using alternative LC phases and external configurations. The review presents a comprehensive summary of the material combinations and the techniques that have been considered in the development of LC-based concentrator films and concludes with a discussion about the future perspectives for these exciting optical concentrators.

On structural organisation and crystallisation/vitrification phenomena in low molecular weight glass-forming liquid crystals

ABSTRACT Results of the dielectric spectroscopy, differrential scaning calorimetry, adiabatic calorimetry, X-ray diffraction and infrared spectroscopy studies concerning the formation of glasses with some degrees of long-range order and the cold crystallisation phenomenon in about 15 liquid crystal compounds are discussed. Based on the temperature dependence of molecular dynamics, the liquid-like N, N*, SmC* and SmC*A phases have been identified as the fragile glass formers, while the solid-like SmI*A, SmBcr, SmE and SmG phases and the orientationally disordered and conformationally disordered crystals as the strong ones. Features of cold crystallisation in the fragile and strong glass-forming phases are presented. A comparison of cold and melt crystallisations is made. The role of molecular chains in the stability of liquid crystal phases is emphasised. We also discuss the quasi-binary model of the liquid crystal order in the smectics.

Synthesis and properties of long-lived room-temperature phosphorescent liquid crystal polymers based on “Jacketing” effect

Polymerization is one of the most widely used strategies to achiever RTP. To date, various polymer-based RTP materials have been reported. However, how to improve the phosphorescence lifetime of polymer-based RTP materials still needs to be further researched. In this article, we proposed a new strategy to realize long-lived room-temperature phosphorescence by utilizing the “jacketing” effect of mesogen-jacketed liquid crystal polymers. Two polymers PMJ0DFM and PMJ6DFM with different flexible spacer length were synthesized, and their liquid crystal phase structures and photo-physical properties were systematically investigated. PMJ0DFM formed columnar nematic phase while PMJ6DFM formed a more ordered hexagonal columnar phase liquid crystal. For PMJ0DFM without spacer length, due to the strong “Jacketing” effect, the thermal motion of the chromophore was suppressed, and it was also more conducive to the aggregation and planarization of the chromophores. Therefore, PMJ0DFM film showed longer phosphorescence lifetime and more red-shifted phosphorescence spectrum than PMJ6DFM film. The phosphorescence lifetimes of PMJ0DFM and PMJ6DFM were 355 ms and 247 ms, respectively. In addition, we also explored their application in linearly polarized luminescent materials. When oriented, PMJ0DFM and PMJ6DFM film could emit efficient linearly polarized room temperature phosphorescence.

Charge carrier transport in perylene-based and pyrene-based columnar liquid crystals.

Electron and hole transport characteristics were evaluated for perylene-based and pyrene-based compounds using electron-only and hole-only devices. The perylene presented a columnar hexagonal liquid crystal phase at room temperature with strong molecular π-stacking inside the columns. The pyrene crystallizes bellow 166 °C, preserving the close-packed columnar rectangular structure of the mesophase. Photophysical analysis and numerical calculations assisted the interpretation of positive and negative charge carrier mobilities obtained from fitting the space charge limited regime of current vs voltage curves. The pyrene-based material demonstrated an electron mobility two orders of magnitude higher than the perylene one, indicating the potential of this class of materials as electron transporting layer.

Liquid Crystal Electrolyte with Lamellar Ionic Channels for All-In-One Gel Flexible Supercapacitor.

Liquid crystalline hydrogels with nanoscale order are an attractive soft material to transport ions or electrons with high efficiency. By employing noncovalent interactions between amphiphiles and solvents, defined anisotropic ordered structures can assemble that serve as interior transmissible channels. Herein, the phase behaviors of a polymerizable amphiphile of 1-vinyl-3-alkylimidazolium bromide (VCn IMBr, n=12, 14, 16) are investigated at different concentrations in a deep eutectic solvent. The aggregation such as micelle, hexagonal, and lamellar liquid crystal phase is created. Through in-phase polymerization, the lamellar structures within an an isotropic liquid crystal can be well solidified to obtain a conductive gel electrolyte. A sandwich-structured all-in-one gel flexible supercapacitor is then built with this specific gel electrolyte. With greatly increased adhesion and minimized interfacial resistance between electrode and electrolyte, the approach will be able to create energy-storage devices with anisotropic ionic and electronic charge transportations envisioned for various electrochemical applications.

Geometry of focal conics in sessile cholesteric droplets.

Focal conic domains are defects characteristic of layered liquid crystal phases. Their association can build flowers where petals are the ellipses of the Dupin cyclides involved in these defects. We report here the observation of focal conic flowers in cholesteric droplets sessile on a glass surface and surrounded by glycerol. The observation of the droplets in different directions helps to solve the three dimensional architecture of the flower. The effects of the droplet size and of the pitch value are also reported.

Predicting the melting point of liquid crystals with directed message passing neural networks

ABSTRACT Liquid crystal (LC) phases must exist in an appropriate temperature range for practical applications and thus the melting point (MP) is a critical design target of LCs. In this work, we have evaluated the performance of directed message passing neural networks trained on a large database of organic molecules (27,000+) in the prediction of a structurally diverse set of 780 LC and LC-like molecules including cyano-, azo-, ester, cyclohexyl- and halogen-containing compounds. Our results show that the model can provide accurate MP predictions for LC and LC-like molecules with an overall Mean Absolute Error (MAE) and Root Mean Square Error (RMSE) of 23°C and 30°C, respectively. Our model provides similar levels of performance with MAEs of 20°C, 20°C, 23°C, 25°C and 24°C for azo, cyclohexyl, ester, halogen and nitrile compounds, respectively, and is most accurate for molecules with MP between 50 and 150°C (MAE = 21°C). We have developed an online tool called LCMelt (v1.0) (lcmelt.streamlit.app) where our model can be used to predict the MP of potential LC candidates before synthesis free of charge.

Janus helices: From fully attractive to hard helices.

The phase diagram of hard helices differs from its hard rods counterpart by the presence of chiral "screw" phases stemming from the characteristic helical shape, in addition to the conventional liquid crystal phases also found for rod-like particles. Using extensive Monte Carlo and Molecular Dynamics simulations, we study the effect of the addition of a short-range attractive tail representing solvent-induced interactions to a fraction of the sites forming the hard helices, ranging from a single-site attraction to fully attractive helices for a specific helical shape. Different temperature regimes exist for different fractions of the attractive sites, as assessed in terms of the relative Boyle temperatures, that are found to be rather insensitive to the specific shape of the helical particle. The temperature range probed by the present study is well above the corresponding Boyle temperatures, with the phase behaviour still mainly entropically dominated and with the existence and location of the various liquid crystal phases only marginally affected. The pressure in the equation of state is found to decrease upon increasing the fraction of attractive beads and/or on lowering the temperature at fixed volume fraction, as expected on physical grounds. All screw phases are found to be stable within the considered range of temperatures with the smectic phase becoming more stable on lowering the temperature. By contrast, the location of the transition lines do not display a simple dependence on the fraction of attractive beads in the considered range of temperatures.

Novel nanocomposites based on Tetrazine liquid crystals for energy storage application

Polymethyl methacrylate (PMMA) /Tetrazine liquid crystal (LCTZ12) nanocomposites with different CNTs loadings have been prepared. The prepared nanocomposites were characterized using polarizing light microscope (POM), differential scanning calorimetry (DSC), Fourier transform Infrared Spectroscopy (FTIR) and broadband dielectric spectroscopy (BDS) in the range 10-1-106 Hz respectively. The most commonly observed texture of soft crystal phases was that of a mosaic pattern, as revealed by the POM technique. PMMA alone does not change the polarization states and appeared as isotropic material, but once LCTZ12 is added, crystal growth could be clearly noticed in the needle pattern inside the polymer matrix to compose an amazing structure of soft crystal G texture. The increase of LCTZ12 to about 20 wt% in the PMMA matrix led to more heterogeneous aggregation of liquid crystalline molecules. This ratio addition is enhancing of network connections inside the PMMA. Moreover, the dispersion of carbon nanotubes mounted on catalysts is found to accumulate in segregations in PMMA/ LCTZ12 nanocomposites. However, the location of CNTs is concentrated at bottom of polymer network due to the large difference in densities of LCs, PMMA, and CNTs. DSC is in agreement with the POM result of LCTZ12, which has multi soft crystals or highly self-assembled ordered liquid crystal phases near to room temperature. The FTIR proved that blend PMMA/ LCTZ12 (80/20) has a dramatic increase in the 1387 cm−1 peak of C = C as a response of the rise in the concentration or number of aromatic rings. Additionally, the vibrational scissoring peak located at 1477 cm−1 of methylene CH2 groups is increased significantly and C–H stretching in aliphatic regions at 2850 cm−1 and 2990 cm−1 as well. However, there is no significant change observed after CNTs addition to the PMMA/ 20 wt% LCTZ12 matrix. Regarding dielectric results, it was found that, the polymer dispersed liquid crystals PDLC system composed of (PMMA/20 wt% LCTZ12) possesses the most promising dielectric properties, that is; high dielectric permittivity with low losses compared to the other ratios. The addition of CNTs results in more localization of charge carriers along with mobile ions, causing higher ionic conductivity. The conductivity ranges altered from 3.2 × 10-10 S/cm in pure PDLC system to become 1.7 × 10-3 S/cm for PDLC doped with 10 wt% CNTs nanocomposite. The electrical energy density measurements for 3,6-bis(5-(Dodecyloxy) pyridin-2-yl)-1,2,4,5-tetrazine (LCTZ12) liquid crystals with 6 cationic centers in polymeric matrices proved to be suitable for flexible and efficient electrolytes for energy storage devices.

Ionic Liquid Crystal Thin Film as Switching Layer in Nonvolatile Resistive Memory.

In this study, we propose the use of an ionic liquid crystal (ILC) as a new resistive switching layer in nonvolatile resistive random-access memory (ReRAM) devices. The high-quality vacuum-deposited ILC films of 1-hexadecyl-3-methylimidazolium hexafluorophosphate ([C16mim][PF6]) enabled to demonstrate the first operation of ReRAM devices with a low set voltage of ∼1 V and stable switching behavior for up to ∼44 cycles. The key to the successful operation is that the ILC layer is in the liquid crystal phase (smectic A), where the electric double layers formed at the electrode-ILC interfaces play a significant role. The results of basic electrical properties and I-V curve fittings suggested the following operation principle: the formation and rupture of charge-composed filaments within the ILC film, where the current conduction is primarily governed by the trap charge limited current (TCLC) mechanism. These achievements will pave the way for advanced studies of ILC-based electronic devices.

X-ray study of structural changes in “di-(2-ethylhexyl) sulfosuccinic acid (AOT) /n-heptane-water” liquid crystalline system depending on the concentration of components

Structural changes in the “sodium salt of di-(2-ethylhexyl)sulfosuccinic acid (AOT) - n-heptane-water” liquid-crystalline system depending on the AOT concentration in the n-heptane-water mixed solvent were studied using the X-ray diffraction method. The studies were carried out in a wide range of the AOT concentrations from 27 to 92% in a mixed solvent n-heptane/water=5.8 ratio.The mesomorphism of a liquid-crystalline system is on the concentration of the system. It is confirmed that at concentrations the AOT above 50%, a liquid crystal structure with an intradomain "flat" liquid crystal layered phase takes place. A “flat” lamellar lyotropic-liquid-crystalline phase with an intradomain lamellar structure is formed in the system. The structural parameters, the compactness of the liquid crystal system, the thickness of the formed layers, the surface area per molecular head on the surface of the layers depending on the concentration and thickness of the bound water layer are revealed. The liquid crystal system under study can serve as a model for studying various problems of biological membranes.