Liquid Crystal Phase Transition Research Articles

Pattern-Directed Phase Transitions and VOC Sensing of Liquid Crystal Films

Solvent vapor exposure could transform a crystalline or smectic liquid crystal (LC) film into nematic and isotropic phases under ambient conditions. The average time for such phase transitions is found to linearly reduce with an increase in vapor pressure and reduction in the molecular weight of solvents. Such responses of solvent vapor-annealed phase transitions of a nanoparticle-loaded LC droplet were then converted into an electrical signal, wherein the electrical resistance reduced (increased) with time upon destruction (restoration) of the orientational order of the LC matrix. Variation in the electrical response was used to identify the volatile organic vapors, phase transition of LCs, rate of diffusion–absorption of solvent into LCs, and rate of desorption–evaporation of solvent from LCs. Pattern-directed phase transitions on physically heterogeneous surfaces showed a faster (slower) kinetics on thinner (thicker) patterns. However, for chemically heterogeneous surfaces, weaker (stronger) anchoring of LCs on hydrophobic (hydrophilic) patches ensured a faster (slower) transition.

Phase transitions in nematics: textures with tactoids and disclinations

We demonstrate that a first order isotropic-to-nematic phase transition in liquid crystals can be succesfully modeled within the generalized Landau-de Gennes theory by selecting an appropriate combination of elastic constants. The numerical simulations of the model established in this paper qualitatively reproduce the experimentally observed configurations that include interfaces and topological defects in the nematic phase.

Large Graphene Oxide Flakes: From Isotopic to Nematic Liquid Crystal Phase Transition

Herein, graphene oxide (GO) with large flakes of average size 38 μm is studied, following the isotropic–nematic phase transition over time and determining the critical concentration, i.e., the minimum value for the formation of the pure liquid crystal (LC) phase. The GO flakes in aqueous suspensions can form LC phases, which are attractive for various applications because of their exceptional properties. Stability over time is very important for their applications, especially, if the GO suspension is in the biphase concentration region in which a crucial change can occur—the isotropic LC phase separation. GO LCs have a wide biphase region, and even very low concentration suspensions appear purely nematic when prepared. However, over time, they appear to be biphasic and higher concentrations are needed for pure LC phase. The changes in the critical concentration were followed for over 4 months, and the findings indicate that it increases with time until it reaches a stable value that is, nevertheless, very low in concentration, 0.7 mg mL−1 (0.035 vol%). From the stable threshold volume fraction, the effective flake thickness, relevant in the LC phase formation behavior, is estimated. This value is almost double than what is measured by atomic force microscopy.

Analysis of UV Assisted Phase Transitions in Mixtures of Liquid Crystals with Photochromic Compounds by Photopyroelectric Calorimetry

In this work, an upgraded photopyroelectric setup has been used to investigate phase transitions in liquid crystals dispersed with photochromic molecules. In particular, we have studied the thermal diffusivity before, during, and after the photo-induced conformational changes of the dispersed photochromic molecules. Upon sample irradiation, an opposite shift of the phase transition temperatures were observed in the investigated 8CB–NP and 8CB–7AB mixtures due to the different changes in the shape undergone by the dispersed active molecules.

The effect of CoPt-coated reduced-graphene oxide nanosheets upon the Smectic-A to Smectic-C* phase transition of a chiral liquid crystal

ABSTRACTWe report on high-resolution calorimetry and small-angle X-ray scattering measurements along the Smectic-A to chiral Smectic-C* phase transition of the liquid crystal 4-(2-methyl butyl) phenyl 4-n-octylbiphenyl-4-carboxylate with dispersed, CoPt nanoparticle-coated reduced-graphene oxide nanosheets. The temperature dependence of heat capacity and smectic layer spacing are obtained in the vicinity of Smectic-A to chiral Smectic-C* phase transition. Though no remarkable pretransitional effects are present, the critical fits show a crossover from mean-field near a tricritical point to classical mean-field compared to pure liquid crystal. The X-ray data yield a dilation of smectic layer thickness, indicating the assembly of graphene oxide nanosheets between the smectic layers.

A Bright Future for Liquid Functional Materials?

Liquid metals were merely sci-fi fantasy a few decades ago, playing a prominent role in the Terminator film franchise. Current research is leading to an array of emerging functional liquids. Here, we discuss the exciting possibilities of such fluid materials.

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

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

A multifunctional shape-morphing elastomer with liquid metal inclusions

Natural soft tissue achieves a rich variety of functionality through a hierarchy of molecular, microscale, and mesoscale structures and ordering. Inspired by such architectures, we introduce a soft, multifunctional composite capable of a unique combination of sensing, mechanically robust electronic connectivity, and active shape morphing. The material is composed of a compliant and deformable liquid crystal elastomer (LCE) matrix that can achieve macroscopic shape change through a liquid crystal phase transition. The matrix is dispersed with liquid metal (LM) microparticles that are used to tailor the thermal and electrical conductivity of the LCE without detrimentally altering its mechanical or shape-morphing properties. Demonstrations of this composite for sensing, actuation, circuitry, and soft robot locomotion suggest the potential for versatile, tissue-like multifunctionality.

On the Disappearance of the Crystal—Liquid Phase Transition as the Number of Atoms in the System Decreases

An expression for the Helmholtz free energy of a nanocrystal that contains vacancies in lattice and the delocalized (diffusional) atoms is obtained on the basis of the previously proposed three-phase model of a simple matter and the RP(vac)-model of a nanocrystal. A model of the Gibbs surface, on which a portion of cells are vacant and a part of atoms are in the delocalized state, is proposed. The fact that a proportion of delocalized atoms are delocalized in a “bulk” way and a proportion of them are delocalized in a “surface way” is taken into account. The equation of state is calculated for argon, whose atoms interact via the Mie–Lennard-Jones pairwise potential. Calculations for the macrosystem were show that, at average temperatures, the equation of state has two S loops on isotherms corresponding to the crystal—liquid (C—L) and liquid—gas (L—G) phase transitions (PTs). At high temperatures, the S loop of the L—G PT contracts to the critical point. At low temperatures, two S loops of the C—L and L—G PTs merge into one large S loop corresponding to the C—G PT. As the number of atoms (N) in the nanosystem decreases, the S loops of both phase transitions in the isotherm decrease, and the C—L S loop disappears at a certain value of the number of atoms (N0). It is shown that N0 increases as the temperature of the isotherm increases and the nanosystem shape deviates from the most energetically optimal one (it is cubic for the RP(vac)-model). The C—L PT disappears in a cluster consisting of N < N0 atoms. Such a cluster gradually transforms into the liquid phase in the case of an isothermal increase in the specific volume.

Toluene-induced phase transitions in blue phase liquid crystals

ABSTRACTWe report phase transitions in blue phase-forming liquid crystals (LCs) that are triggered by exposure to toluene vapours. Specifically, we reveal that room-temperature cholesteric phase mixtures of MLC-2142 and S-811 form blue phases (BP I, II and III) with increasing vapour pressure of toluene. To probe the mechanism underlying this observation, we investigated the phase behaviour of mixtures of BP-forming LCs containing a range of non-volatile aromatic compounds (e.g. pyrene). We interpret our observations to indicate that the principal effect of small aromatic compounds is to decrease the energy penalty associated with the formation of disclination lines in BPs. We also conclude that the absorption of toluene into the BP-forming LCs lowers the energy required for the formation of disclination cores in the BP phase, thus allowing the elastically favoured double-twist cylinders to form at lower temperatures. We demonstrate that BP-forming LCs containing pyrene can be used to detect toluene at concentrations below 200 ppm at room temperature. Overall, these results guide the design of LC-based materials that respond to VOCs at concentrations relevant to occupational settings.

Temperature- and pressure-induced phase transitions of choline chloride-urea deep eutectic solvent

FT-IR spectroscopy has been used to decipher structural rearrangements in a choline chloride-urea (ChCl-Urea) deep eutectic solvent, as a function of temperature (−60–20 °C) at atmospheric pressure and as a function of pressure (0.1 MPa to 5.2 GPa) at room temperature. Under atmospheric pressure, the ChCl-Urea deep eutectic solvent (DES) underwent glass-to-crystal and crystal-to-liquid phase transitions during the heating process. However, at room temperature, with pressure increasing, the ChCl-Urea DES experienced liquid-liquid and liquid-crystal phase transitions at approximately 1.6 and 2.6 GPa. In addition, the partial breakage of intermolecular hydrogen bonds between ChCl and urea was observed during the isothermal crystallization process; however, intermolecular hydrogen bonding between two urea molecules was enhanced during the pressure-induced crystallization process. Consequently, urea could act as a “spacer” in the regular ChCl packing of ChCl-Urea DES cocrystals, and could regulate the intermolecular hydrogen bondings according to the competition among the complexed cations and anions in different crystallization environments.

Guanosine-based thermotropic liquid crystals with tunable phase structures and ion-responsive properties

In this work, the thermotropic liquid crystals (LCs) composed of a biological molecule, guanosine 5′-monophosphate disodium salt (GMP), with different cationic surfactants were reported. When GMP interacted with the surfactants, the length of the surfactant alkyl chains was found to be of great importance in modulating the LC phase. The smectic A (SmA) phase and columnar rectangular (Colr) phase were formed upon increasing the length of the aliphatic chains due to the enlargement of the steric hindrance. Interestingly, K+ ions were observed to cause the phase transition of LCs from the SmA or Colr mesophase to a columnar hexagonal (Colh) mesophase in the GMP-surfactant complexes, which was attributed to the formation of G-quadruplexes. Accordingly, the chirality and fluorescence properties of the complexes also exhibited K+-responsive characters as the phase transition of the LCs emerged. Compared with the reported LCs containing guanosine groups, the thermotropic LCs with high elasticity and tunable mesophase structures in this study are easy to be prepared and are hopeful to provide potential applications for constructing stimuli-responsive luminescent materials, biocatalysis and biosensor devices.

Statistical theory of magnetic field induced phase transitions in negative diamagnetic anisotropy liquid crystals doped with carbon nanotubes

A statistical mean-field theory is proposed for describing the magneto-orientational ordering of a composite material, namely a liquid crystal suspension of carbon nanotubes. A nematic liquid crystal with negative anisotropy of diamagnetic susceptibility with the molecules, which have long axes tending to be orthogonal to the applied magnetic field, has been considered as a dispersion medium. Since carbon nanotubes possess positive anisotropy of the diamagnetic susceptibility, they tend to align parallelly to the field, causing competition with the ordering of the liquid crystal matrix. Phase transitions induced by temperature and magnetic field have been studied for different values of the coupling energy of carbon nanotubes with a liquid crystal matrix. It is shown that depending on the temperature, magnetic field induces the transition from a highly ordered nematic phase to a weakly ordered paranematic phase with both the positive and the negative value of the order parameter of the liquid crystal, which respectively corresponds to the orientational anisotropy of the “easy-axis” and “easy-plane” types.

In Situ Observation of Alignment Templating by Seed Crystals in Highly Anisotropic Polymer Transistors.

Due to the highly directional nature of transport in polymer-based organic field-effect transistors (OFETs), alignment of the polymer backbone can significantly affect device performance. While many methods of alignment have been detailed, the mechanism of alignment is rarely revealed-especially in cases of flow-induced alignment. Polymer aggregates are often observed in highly aligned systems, but their role is similarly unclear. Here, we present a comprehensive characterization of blade-coated P(NDI2OD-T2) (N2200) for OFET applications, including a rigorous, multimodal characterization of its in-plane alignment. Film thickness follows the expected power-law dependence on coating speed, while bulk polymer backbone orientation transitions from perpendicular to parallel to the coating direction as speed is increased. Charge carrier mobility >2 cm2/(V s) is achieved parallel to the coating direction for aligned N2200 coated at 5 mm/s and is found to be strongly correlated with the in-plane alignment of the fibrillar morphology at the film's surface, characterized with atomic force microscopy and near-edge X-ray absorption. We develop a model of N2200 crystal anisotropy through rotational scans of grazing incidence wide-angle X-ray scattering (GIWAXS) and use it to analyze simultaneous in situ GIWAXS and UV-vis reflectance data from polymer solutions coated at 5 mm/s. A small population of crystals align early in the drying process, but bulk alignment occurs very late in the drying process, likely mediated by a lyotropic liquid crystal phase transition templated by the aligned crystals. Our characterization also suggests that the majority of material in N2200 thin films is noncrystalline at these conditions.

Differential Scanning Calorimetric Study of the Effect of Cholesterol on the Thermotropic Phase Behavior of the Phospholipid 1‐Stearoyl‐2‐Oleoyl‐sn‐Glycero‐3‐Phosphocholine

AbstractUsing the differential scanning calorimetric (DSC) technique, the thermal properties of 1‐stearoyl‐2‐oleoyl‐sn‐glycero‐3‐phosphocholine (SOPC) phospholipid and its mixture with cholesterol (Chol), in the range between 10 and 50 mol.%, were investigated. A considerable modification of the structural conformation and the biophysical properties of the bilayer of the phospholipid system after the Chol incorporation were detected. Concentrations below 30 mol.%, and especially in the range 10–20 mol.%, were found to be optimal in the effective miscibility of SOPC and Chol components. The effective miscibility completeness, mainly in the gel and liquid crystal phases, was indicated. It was discovered that Chol mixed with SOPC slightly shifts its gel (L β) to liquid crystal (L α) phase transition temperature, decreases cooperativity, expressed by the van't Hoff enthalpy, and markedly and progressively reduces the transition enthalpy to almost zero at 50 mol.%. By deep incubation of the cholesteric phospholipid mixture, it was revealed that the endothermic peak associated with laminar crystal to gel phase transition does not exist in conventional pure SOPC bilayer systems.

Singularity at the Point of Transition from Equilibrium to Metastable States of a Metallic Melt

Equilibrium and metastable states of a liquid are studied near the liquid-crystal phase transition point in a metallic melt using a time-dependent four-point correlation function. This function was proposed earlier for revealing the collective atomic motion in equilibrium liquids. The characteristic changes in the behavior of liquid when it passes to the region of metastability are found. At the quantitative level, such changes manifest themselves in the specific features of the four-point correlation function in the region of crossover from the equilibrium to metastable state arising on isochoric cooling. As specific examples, the embedded atom models for copper and nickel are considered using the method of molecular dynamics.

Textural, phase transition and electro-optic studies of polymer-stabilized blue phase liquid crystals

Blue phase liquid crystal (BPLC) material was prepared using nematic liquid crystal (BL036) and a chiral dopant (CB15) in equal wt./wt. ratio. Polymer-stabilized blue phase liquid crystals (PSBPLCs) material was also prepared by polymer stabilization in BPLC material. The phase transition temperatures (Isotropic-BP, BP-N*) and electro-optic behaviour of PSBPLC material was investigated in thin cell. A platelet type texture clearly specifies the appearance of BP in the narrow range of 3.7 °C and increases up to 10.7 °C after polymer stabilization. This increment in BP range is attributed to the localization of the polymer at the disclinations lines and hence improve the thermal stability. The electro-optic parameters, such as On state voltage and Kerr constant were found 70 V and 1.04 nm/V2 respectively.

New Promesogenic Ligands for Host Medium Microencapsulation by Quantum Dots via Liquid Crystal Phase Transition Templating

The design, synthesis, properties, and performance of a new class of promesogenic calamitic side-tethering organic ligands used to direct quantum dot nanoparticle self-assembly are described. This work was motivated by inadequate modularity, step count, and yield associated with syntheses of existing ligands. Attaching the new ligands to quantum dots and dispersing them in a liquid crystal host affords hollow micrometer-sized capsules via phase transition templating. The capsules resist thermal decomposition up to 350 °C—significantly higher than any previously reported microcapsules assembled from side-tethering calamitic ligand-functionalized nanoparticles. These novel ligands can be used for encapsulation applications where stability under high temperature is required. Evaluation of the capsules by small-angle X-ray scattering shows that interparticle spacing varies from 10 to 13 nm depending on the ligand used and is correlated to aminoalkyl chain length.

Peculiarities of Defective Formations in Heterophase Regions of the Thermotropic Phase Transitions in Liquid Crystals

AbstractA unique feature of phase transitions in liquid crystalline materials is the availability of heterophase regions. In such regions, the simultaneous coexistence of high temperature and low temperature phases takes place. Thermal mobility and temperature gradient in the heterophase regions lead to the appearance of various types of defective formations in the aligned and non‐aligned textures of liquid crystalline materials. In the present work, the morphologic, structural, and optical peculiarities of defective formations and non‐homogeneities, which appear in the heterophase regions of phase transitions, are investigated. The singular points, disclination, inversion walls, droplets, and confocal formations are objects of the investigations herein. The heterophase regions of the direct and reverse nematic ↔ isotropic liquid and smectic A ↔ isotropic liquid, smectic C ↔ isotropic liquid, and smectic C* ↔ isotropic liquid thermotropic phase transitions are created by the original method, that is, the capillary temperature wedge method. Crystallo‐optics and crystallo‐physics methods are used in this work.

Fluctuations and phase transitions of uniaxial and biaxial liquid crystals using a theoretically informed Monte Carlo and a Landau free energy density

In this work, we explore fluctuations during phase transitions of uniaxial and biaxial liquid crystals using a phenomenological free energy functional. We rely on a continuum-level description of the liquid crystal ordering with a tensorial parameter and a temperature dependent Landau polynomial expansion of the tensor’s invariants. The free energy functional, over a three-dimensional periodic domain, is integrated with a Gaussian quadrature and minimized with a theoretically informed Monte Carlo method. We reconstruct analytical phase diagrams, following Landau and Doi’s notations, to verify that the free energy relaxation reaches the global minimum. Importantly, our relaxation method is able to follow the thermodynamic behavior provided by other non-phenomenological approaches; we predict the first order character of the isotropic–nematic transition, and we identify the uniaxial–biaxial transition as second order. Finally, we use a finite-size scaling method, using the nematic susceptibility, to calculate the transition temperatures for 4-Cyano-4’-pentylbiphenyl (5CB) and N-(4-methoxybenzylidene)-4-butylaniline (MBBA). Our results show good agreement with experimental values, thereby validating our minimization method. Our approach is an alternative towards the relaxation of temperature dependent continuum-level free energy functionals, in any geometry, and can incorporate complicated elastic and surface energy densities.