Temperature-concentration Plane Research Articles

Thermoreversible Gel-Dispersed Liquid Crystals.

A simple model is introduced to describe phase behaviours of binary mixtures of a thermoreversible gel and a low-molecular-weight liquid crystal (LC). We predict novel phase diagrams on the temperature-concentration plane, including sol-gel transition, nematic-isotropic phase transition, and phase separation. At high temperatures, the phase separation between the isotropic sol and gel phases appears. As the temperature decreases, we have the phase separation between nematic sol and isotropic gel phases, in which the nematic domains are dispersed in the isotropic gel phase. We suggest that thermoreversible gelation of reactive molecules mixed with LCs will become one of the new classes of polymer-dispersed liquid crystals.

Uniaxial and biaxial nematic phases in binary mixtures of banana-shaped molecules

We introduce a mean-field theory to describe uniaxial and biaxial nematic phases in binary mixtures of banana-shaped molecules. We extend our previous theory for pure banana-shaped molecules to the binary mixtures and calculate phase diagrams on the temperature-concentration plane. We predict uniaxial-biaxial nematic phase transitions and Landau points in the binary mixtures. The biaxial nematic phases of banana-shaped molecules are promoted by mixing the two components with different bend angles.

Theory of nematic ordering driven by hydrogen bonding between rods and solvent molecules

ABSTRACT A mean-field theory is introduced to describe nematic () ordering and phase separations in aqueous low-molecular-weight liquid crystalline molecules (or short rodlike molecules). We consider a physical bonding (or a hydrogen bonding) between the rod and solvent (water) molecules. Our theory predicts that the hydrogen bonds between the unlike molecules stabilise the phase and take place novel phase separations on the temperature-concentration plane. Upon increasing the number of functional groups on the rods, capable of forming the physical bond with the solvent molecule, the stable phase region is broadened on the phase diagram. We also find a miscibility window of the phase and closed-loop coexistence curves with upper and lower critical solution temperatures near the phase.

Helical inversions and phase separations in binary mixtures of cholesteric liquid crystalline molecules

ABSTRACTA mean field theory is introduced to describe a helical inversion in nematic-cholesteric mixtures and binary cholesteric mixtures. Taking into account a chiral coupling between unlike molecules, the helical pitch is derived as a function of orientational order parameters and concentration. We find the conditions of helical inversions for the mixtures and derive the concentration of the helical inversions, depending on the chiral interaction. The numerical results are in agreement with the experimental results. We also examine phase separations on the temperature-concentration plane, including the helical inversions. A large variety of phase separations, such as the two-phase coexistence between a right-handed rich and a left-handed rich phase, etc., are predicted.

Twist-bend nematic phases in binary mixtures of banana-shaped liquid crystalline molecules

ABSTRACTWe present a mean field theory to describe twist-bend nematic () phases in binary mixtures of banana shaped, such as bent-core or dimer, liquid crystalline (LC) molecules with , where is a twist and is a bend elastic constant. We introduce the free energy for the mixtures by taking into account the director-pitch (DP) coupling of the consisting LC molecules ) and calculate the phase diagrams on the temperature-concentration plane. Our theory demonstrates that the isotropic (I), or para-nematic (), nematic (), and phases appear, depending on the strength of the DP coupling parameter and predicts various phase transitions: the first-order and transitions, and the second-order transition, etc., on the temperature-concentration plane. Our theory has a good agreement with the experimental phase diagrams of the mixtures.

2D-3D transition in Cu-TiS2 system.

The phase diagram of the quasi-binary Cu-TiS2 system has been studied in situ in the temperature-concentration plane for the first time. Phase transitions between 2D (layered) and 3D (cubic) phases in CuxTiS2 (x = 0-0.5) intercalation compounds have been studied by the X-ray diffraction technique in the temperature range 20-1000 °C. It has been found that the stability of the layered phase is determined by the distribution of copper atoms between the octahedral and tetrahedral crystallographic sites. The occupation of octahedral sites dominates at low temperatures. Upon heating, tetrahedral site occupation is limited due to elastic lattice distortion and the layered phase becomes unstable and transforms to the spinel. Further heating allows the distribution of copper between octahedral and tetrahedral sites; the layered phase becomes stable again.

Field-induced oblique helicoidal cholesteric phases in mixtures of chiral and achiral liquid crystalline molecules

ABSTRACTWe present a mean field theory to describe phase transitions in mixtures of chiral and achiral liquid crystalline molecules in the presence of an external field. When a longitudinal external field is applied parallel to the pitch axis of a chiral nematic phase (N*), the director of the N* phase can lead to an oblique heliconical cholesteric () state with a twist–bend distortion, in which a bend elastic constant is smaller than a twist elastic constant . We predict that the exists between N* and nematic (N) phases and can be controlled by temperature and concentration. We find various phase transitions: the first-order (isotropic) and the second-order , transitions, on the temperature–concentration plane. We also predict the pitch and the phase diagrams, depending on temperature and concentration under the external field.

Phase Separations in Mixtures of a Flexible Polymer and a Cholesteric Liquid Crystal in the Presence of an External Field

We present a mean field theory to describe phase behaviors in mixtures of a flexible polymer and a cholesteric liquid crystal under an external magnetic or electric field. Taking into account a chiral coupling between liquid crystals under the external field, we examine twist-untwist phase transitions and phase separations in the mixtures. Depending on the strength of the external field, we predict cholesteric-paranematic (Ch + N), nematic-paranematic (N + pN), cholesteric-nematic (Ch + N) phase separations on the temperature-concentration plane. We also calculate spinodal regions, which are important in phase separation kinetics inside binodal lines.

Biaxiality of cholesteric phases in rod-like polymer solutions

A mean field theory for the biaxiality of a cholesteric phase in rod-like polymer solutions is presented. By taking into account both excluded volume and attractive interactions between rods, we calculate the biaxiality of the cholesteric phase as a function of temperature and concentration. At low concentrations of rods, the biaxiality increases with increasing temperature. At high concentrations above , the biaxiality becomes the maximum as a function of temperature. We also calculate phase diagrams on the temperature–concentration plane and study phase separations between an isotropic phase and a biaxial cholesteric phase.

Proving the contact rules for phase regions: Implications to phase diagrams of metal–hydrogen systems

The rule of triple joints has recently been proved for two-dimensional phase diagrams and two-dimensional sections of phase diagrams of a higher dimension (Antonov, 2013). The present paper is extending this rule to phase diagrams of binary metal–hydrogen systems composed of a metal immersed in a hydrogen gas. The rule is shown to be valid for every triple point in the temperature–pressure phase diagrams of these systems, including the points with the intersecting boundary lines tangent to each other. The paper also considers the sufficient conditions for the application of the rule to triple points in the projections of phase diagrams onto the temperature–concentration plane.

Phase diagrams of mixtures of a polymer and a cholesteric liquid crystal under an external field.

We present a mean field theory to describe phase behaviors in mixtures of a polymer and a cholesteric liquid crystal under an external magnetic or electric field. Taking into account a chiral coupling between a polymer and a liquid crystal under the external field, we examine twist-untwist phase transitions and phase separations in the mixtures. It is found that a cholesteric-nematic phase transition can be induced by not only the external field but also concentration and temperature. Depending on the strength of the external field, we predict cholesteric-paranematic (Ch+pN), nematic-paranematic (N+pN), cholesteric-nematic (Ch+N) phase separations, etc., on the temperature-concentration plane. We also discuss mixtures of a non-chiral nematic liquid crystal and a chiral dopant.

Magnetic phase diagram of ferromagnetic shape memory alloys [formula omitted

Permeability and differential scanning calorimetry measurements were carried out on Ni2MnGa1-xFex(0⩽x⩽0.40). On the basis of the experimental results, the phase diagram in the temperature–concentration plane was determined. The characteristics of the determined phase diagram are similar to those of the phase diagrams of Ni2+xMn1-xGa,Ni2Mn1-xCuxGa and Ni2MnGa1-xCux systems, where the magnetostructural transition between the paramagnetic austenite phase (Para-A) and ferromagnetic martensite phase (Ferro-M) occurs. Furthermore, the concentration dependence of the magnetic moment per formula unit, μs, at 5K for Ni2MnGa1-xFex(0⩽x⩽0.375) is also reported.

Magnetic phase diagram of Ni2Mn1.44−xCuxSn0.56 shape memory alloys

Magnetization measurements have been carried out on the magnetic shape memory alloys (MSMAs) Ni2Mn1.44−xCuxSn0.56 (0⩽x⩽0.09). On the basis of the experimental results, the phase diagram of Ni2Mn1.44−xCuxSn0.56 (0⩽x⩽0.09) alloys in the temperature-concentration plane has been determined. The resultant phase diagram is spanned by paramagnetic austenite (Para-A), ferromagnetic austenite (Ferro-A) and ferromagnetic martensite (Ferro-M) phases. The martensitic transition temperature TM makes a broad maximum as a function of x, whereas, in the austenite phase the Curie temperature TC decreases slightly with x. The TC of Ni2Mn1.415Cu0.025Sn0.56 was found to decrease linearly with pressure, whereas TM increased linearly with pressure. The values of dTM/dp and dTC/dp of Ni2Mn1.415Cu0.025Sn0.56 were found to be 0.88K/kbar and −0.31K/kbar, respectively.

Theory of polymer-dispersed cholesteric liquid crystals

A mean field theory is presented to describe cholesteric phases in mixtures of a polymer and a cholesteric liquid crystal. Taking into account an anisotropic coupling between a polymer and a liquid crystal, we examine the helical pitch, twist elastic constant, and phase separations. Analytical expressions of the helical pitch of a cholesteric phase and the twist elastic constant are derived as a function of the orientational order parameters of a polymer and a liquid crystal and two intermolecular interaction parameters. We also find isotropic-cholesteric, cholesteric-cholesteric phase separations, and polymer-induced cholesteric phase on the temperature-concentration plane. We demonstrate that an anisotropic coupling between a polymer and a liquid crystal can stabilize a cholesteric phase in the mixtures. Our theory can also apply to mixtures of a nematic liquid crystal and a chiral dopant. We discuss the helical twisting power, which depends on temperature, concentration, and orientational order parameters. It is shown that our theory can qualitatively explain experimental observations.

Theory of liquid crystalline micelles

A theory is introduced to describe self-assembly of liquid crystalline AB diblock copolymers, consisting of a homopolymer (A) and a side-chain liquid crystalline polymer (B). We derive the free energy of the liquid crystalline micellar solutions and examine the equilibrium solution properties: critical micelle concentration (CMC), nematic-isotropic phase transition (NIT) of the rigid side-chains inside the micelle core, and phase separations. It is shown that there is a critical micelle size below which the NIT becomes continuous due to a packing effect. We also find re-entrant micellizations near the NIT temperature. The phase diagrams, including binodal, spinodal, CMC, and NIT curves are also examined on the temperature-concentration plane.

Magnetic phase diagram of Heuselr alloys Pd2Mn1+xSn1−x

Permeability, magnetization and differential scanning calorimetry measurements are carried out on Pd2Mn1+xSn1−x (0⩽x⩽0.50). On the basis of the experimental results, the phase diagram in the temperature–concentration plane is determined. The determined phase diagram is spanned by a paramagnetic austenite phase (P–A), a paramagnetic martensite phase (P–M), a ferrimagnetic austenite phase (F–A) and a martensite phase (F–M) with a small spontaneous magnetization. It is found that Pd2Mn1+xSn1−x has the characteristics of the phase diagrams of Ni2Mn1+xZ1−x (Z=In, Sn) metamagnetic shape memory alloys.

Repulsive magneto-structural interaction in the ferromagnetic shape memory alloys Ni2Mn1+xIn1−x

A phenomenological theory is presented to study the multiferroic system in the shape memory alloys Ni2Mn1+xIn1−x with the Heusler-type structure, where the phase transitions are characterized by the two order parameters, i.e., the martensitic distortion e3 and the magnetization M. The Landau free energy is expanded in powers of e3 and M with including the Zeeman energy and the energy increase by the uniaxial force on e3. The resultant free energy is applied to analyze the experimental results, such as the phase diagram in the temperature–concentration plane and the reentrant ferromagnetism by increasing temperature. The magnetic fields are shown to cause the ferromagnetic–ferromagnetic or metamagnetic transitions as observed. Further, it is predicted that the uniaxial force can dominate the appearance of the ferromagnetism in some alloys. It is found that these exotic behaviors of this alloy system are all ascribed to the repulsive interaction between e3 and M, which originates from their biquadratic term. Through the present analyses, it is verified that this repulsive interaction is expected in other alloy systems Ni2Mn1+xX1−x (X=Sn, Sb) and makes a contrast to the attractive interaction realized in another group of the alloy systems including Ni2+xMn1−xGa and Ni2Mn1−xCuxGa.

Phase diagrams of binary mixtures of liquid crystals and rodlike polymers in the presence of an external field

We theoretically study phase separations in mixtures of a low molecular-weight-liquid crystalline molecule (LC) and a rigid-rodlike polymer (rod) under an external field, such as magnetic or electric fields. By taking into account two orientational order parameters of the rod and the LC, we define four nematic phases (N(0), N(1), N(2), N(3)) on the temperature-concentration plane. Depending on the sign of the dielectric anisotropy Δε(i) of the rod (i = 1) and LC(i = 2), we examine the phase behavior of rod/LC mixtures in the case of Δε(1) > 0, Δε(2) > 0 (a), Δε(1) < 0, Δε(2) > 0 (b), Δε(1) > 0, Δε(2) < 0 (c), and Δε(1) < 0, Δε(2) < 0 (d). We predict a variety of phase separations induced by an external field.

Phase diagram of the ferromagnetic shape memory alloys Ni2MnGa1−xCox

Permeability, magnetization, and differential scanning calorimetry measurements are carried out on the ferromagnetic shape memory alloys Ni${}_{2}$MnGa${}_{1\ensuremath{-}x}$Co${}_{x}$ (0 \ensuremath{\le}$\phantom{\rule{0.28em}{0ex}}x$ \ensuremath{\le} 0.20). On the basis of the experimental results, the phase diagram in the temperature-concentration plane is determined. The determined phase diagram is spanned by a paramagnetic austenite (Para-A) phase, paramagnetic martensite phase, ferromagnetic austenite phase, ferromagnetic martensite (Ferro-M) phase, and a premartensite phase. The measurements show that a magnetostructural transition between the Para-A and Ferro-M phases does not appear in this alloy system. The phase diagram is discussed using phenomenological Landau theory.

Fibrillization Propensity for Short Designed Hexapeptides Predicted by Computer Simulation

Assembly of normally soluble proteins into ordered aggregates, known as amyloid fibrils, is a cause or associated symptom of numerous human disorders, including Alzheimer's and the prion diseases. Here, we test the ability of discontinuous molecular dynamics (DMD) simulations based on PRIME20, a new intermediate-resolution protein force field, to predict which designed hexapeptide sequences will form fibrils, which will not, and how this depends on temperature and concentration. Simulations were performed on 48-peptide systems containing STVIIE, STVIFE, STVIVE, STAIIE, STVIAE, STVIGE, and STVIEE starting from random-coil configurations. By the end of the simulations, STVIIE and STVIFE (which form fibrils in vitro) form fibrils over a range of temperatures, STVIEE (which does not form fibrils in vitro) does not form fibrils, and STVIVE, STAIIE, STVIAE, and STVIGE (which do not form fibrils in vitro) form fibrils at lower temperatures but stop forming fibrils at higher temperatures. At the highest temperatures simulated, the results on the fibrillization propensity of the seven short de novo designed peptides all agree with the experiments of López de la Paz and Serrano. Our results suggest that the fibrillization temperature (temperature above which fibrils cease to form) is a measure of fibril stability and that by rank ordering the fibrillization temperatures of various sequences, PRIME20/DMD simulations could be used to ascertain their relative fibrillization propensities. A phase diagram showing regions in the temperature–concentration plane where fibrils are formed in our simulations is presented.