Study Of Phase Separation Research Articles

Grand canonical Monte Carlo simulation of a model colloid-polymer mixture: coexistence line, critical behavior, and interfacial tension.

Grand canonical Monte Carlo simulations are used to study phase separation in a simple colloid-polymer model, the so-called Asakura-Oosawa model. To overcome the problem of small acceptance rates of the grand-canonical moves, cluster moves are introduced. Successive umbrella sampling, recently introduced by Virnau and Muller [J. Chem. Phys. 120, 10925 (2004)], is used to access the phase-separated regime. The unmixing binodal and the interfacial tension are measured and compared to theoretical predictions. By means of finite-size scaling, the behavior close to the critical point is also investigated. Close to criticality, we observe substantial deviations from mean-field behavior.

Two-dimensional model for mixtures of enantiomers, bent hard needles: a Monte Carlo simulation

We study the statistical mechanical conditions under which segregation of racemic mixtures of chiral molecules is possible in a two-dimensional fluid model. Motivated by experimental evidence indicating that chiral hydrophilic heads of amphiphilic molecules lying in a monolayer can crystallize undergoing a chiral phase separation, we propose a two-dimensional system to model the projection of the chiral head of amphiphilic molecules in a monolayer. The molecules of our model are infinitely hard and infinitely thin. We consider interactions with only a repulsive contribution where molecules have no effective area (two-dimensional volume). As a consequence all effects found are due to excluded area. The Monte Carlo Gibbs ensemble is used to study phase separation whereas constant pressure simulations are performed to obtain equations of state of pure and racemic systems. We find that for this simple model segregation is generally possible in the very high density regime.

Using Kinetic Monte Carlo simulations to study phase separation in Alloys

We review recent extensions of the kinetic Ising model used to investigate phase separation in binary alloys. Firstly, vacancies are included to model the diffusion of the atoms on the microscopic scale more realistically. These can change the coarsening rate and the coarsening mechanism. Secondly, the lattice is allowed to deform owing to the different sizes of the atoms and the resulting misfit between precipitates and matrix. The deformability of the lattice induces long-range elastic interactions between the atoms. These change the shape, orientation, and arrangement of the precipitates. The growth of the precipitates need not follow the law.

Phase Separation in Polyfluorene-Based Conjugated Polymer Blends: Lateral and Vertical Analysis of Blend Spin-Cast Thin Films

We have studied phase separation in polyfluorene-based electroluminescent polymer blends consisting of poly(2,7-(9,9-di-n-octylfluorene-alt-benzothiadiazole) (F8BT) and poly(2,7-(9,9-di-n-octylfluorene)-alt-(1,4-phenylene-((4-sec-butylphenyl)imino)-1,4-phenylene)) (TFB). We present detailed studies on spin-cast thin-film morphology of the blends, using micro-Raman spectroscopy and X-ray photoelectron spectroscopy. Micron-scale lateral phase separation is observed in these blend thin films. However, these phase-separated domains are not pure at the submicron length scale, and a nanoscale vertical phase segregation occurs with enrichment of the lower surface energy component (TFB) at both air and substrate interfaces. Imaging of the spatial uniformity of electroluminescence emission on the microscopic scale indicates spatially localised charge-carrier recombination in light-emitting diodes (LEDs) fabricated with these blends. On the basis of these studies, we propose a model for the development of thin-film...

Precipitation of oppositely charged polyelectrolytes in salt solutions.

We study phase separation in symmetric solutions of weakly charged flexible chains of opposite sign. Precipitation is caused by effective attractions due to charge fluctuations and by short-range attractions between monomers. The contribution from charge fluctuations is computed within the random phase approximation (RPA), which takes into account the connectivity of charges in the polyions. The impenetrability of the ions is accounted for by using a modified Coulomb potential in the RPA. In good solvent conditions the precipitate monotonically swells and eventually dissolves upon addition of salt. However, near the theta-solvent condition, but still in the good solvent, the precipitate can be stable at any salt concentration. Moreover, the density of the precipitate after initial decrease can increase with addition of salt. This effect is a result of redistribution of salt between the precipitate and the supernatant, which is due to an interplay of electrostatic and hardcore interactions. For not too weakly charged polyions the precipitate properties become strongly dependent on temperature even in good solvent conditions.

Phase separation study near the tricritical point in [formula omitted]– [formula omitted] mixtures

We are studying phase separation in 3 He – 4 He mixtures using inter-digital capacitor (IDC) sensors located on the top of our cell. The IDCs measure the phase separation by probing the local concentration near each sensor. The IDCs were developed as part of our effort to eventually perform similar measurements on the International Space Station, in the absence of the stabilizing influence of gravity. The IDCs appear to be well suited for making measurements of the phase separation on both sides of the tricritical point, including on the 3 He rich side of the phase diagram where they observe the minority 4 He rich phase forming a macroscopic thick film at phase separation.

Phase Separation Driven by a Fluctuating Two-Dimensional Self-Affine Potential Field

We study phase separation in a system of hard-core particles driven by a fluctuating two-dimensional self-affine potential landscape which evolves through Kardar–Parisi–Zhang (KPZ) dynamics. We find that particles tend to cluster together on a length scale which grows in time. The final phase-separated steady state is characterized by an unusual cusp singularity in the scaled correlation function and a broad distribution for the order parameter. Unlike the one-dimensional case studied earlier, the cluster-size distribution is asymmetric between particles and holes, reflecting the broken reflection symmetry of the KPZ dynamics, and has a contribution from an infinite cluster in addition to a power law part. A study of the surface in terms of coarse-grained depth variables helps understand many of these features.

Specifics of Phase Separation in Zirconium-Bearing Glazed Glasses

The results of studying phase separation in zirconium-bearing alumoboron silicate glazed glasses at the stages of melting and thermal treatment are described. The effect of glass composition on the specifics of its phase composition and structure is considered. The possibility of producing low-melting glaze coatings with preset properties is demonstrated.

Relation between aggregation and phase separation: three-dimensional Monte Carlo simulations.

We study phase separation of particles in solution using Monte Carlo simulations of reversible aggregation on a cubic lattice. Two stages of the phase separation can be clearly distinguished: initial random aggregation and subsequent densification. Step one leads to a distribution of fractal aggregates close to the binodal and to a temporary gel for large attractive interaction. Step two leads to isolated spherical dense domains close to the binodal and branched wormlike strands for large attractive interactions. The transition between the two types of structure is gradual and there is no clear feature that shows the existence of a spinodal. The first stage of the phase separation is metastable very close to the binodal or at very large interaction energy. In the latter case, the second step can be viewed as an aging process of the gel formed in the first step.

A new experimental method for studying phase separation through neutron diffraction: the case of As-rich liquid alloys in the As–S system

Neutron scattering experiments were performed on liquid mixtures in the composition range of the miscibility gap in the As-rich side of the As–S system. For each experiment, a special device enabled us to record the diffraction spectra from the upper and the lower parts of the liquid sample. The existence of nanometric-range phase separation was confirmed and the local order of each of the two demixing phases was determined.

Effect of director distortions on morphologies of phase separation in liquid crystals.

We study phase separation from a nematic liquid crystal with spatially nonuniform director gradients. Particles of a phase-separated component, which is either an isotropic fluid (silicone oil) or a nonmesogenic photopolymer, accumulate in the regions with the strongest director distortions, thus reducing the overall energy of the system.

Imaging phase-separated domains in conducting polymer blend films with near-field scanning optical microscopy

We present high-resolution images with near-field scanning optical microscopy to study phase separation in polymer films of poly(styrene) and poly(3-octyl-thiophene). Transmission and transmitted fluorescence near-field scanning optical microscope images were taken for direct visualization of the intermediate steps of phase separation in a regime where small domain sizes prevent investigation by conventional microscopy. The interpretation of near-field data on samples with large or varying film thickness or topography are also discussed, and a method for recognizing topographically induced artifacts in a quantitative way is suggested.

Phase separation of binary homopolymer and ternary homopolymer–copolymer mixtures through Gibbs ensemble simulations

A Gibbs ensemble algorithm is implemented for mixtures of homopolymers and copolymers in a cubic lattice with coordination number z=26. We consider symmetric binary mixtures of homopolymer chains and also study the influence of the presence of symmetric diblock copolymers on these mixtures. All the chain lengths in a given mixture are identical (chains up to 128 units are included). A certain proportion of vacancies is introduced in each simulation. The phase separation curves are characterized by fitting the coexistence data to theoretical expressions. The dependence of the critical temperature on chain length, fraction of vacancies and also on the volume fraction of the copolymer in the mixture has been analyzed. Despite differences between models, the conclusions with this algorithm are similar to those obtained from previous simulations performed with techniques in which finite size interfacial effects may be important. Since the computational cost of our algorithm is reasonable, the present method can be considered an efficient way to study phase separation in different types of polymer mixtures.

Late stages of ordering of thin polymer films on chemically heterogeneous substrates: energetics and metastability

By a direct numerical integration of the dynamical evolution equations, we study phase separation of thin polymer blend films in two-dimensional models. The focus of our work is on the late-time morphology of the films. We first investigate morphological transitions in a thin film on a flat, homogeneous substrate. In particular, we probe the transition between a completely wetting and a partially wetting surface morphology. This transition is controlled by a competition between surface interactions and the interfacial energy in a thin film. This competition becomes even more pronounced when a chemically patterned substrate is used. For films cast on a patterned substrate, we consider several different substrate pattern widths and surface interaction strengths in our simulations. We also probe effects of thermal fluctuations on the late-time film morphology by carrying out simulations to many different final quench temperatures. Along with quench simulations, we further study film morphology by slow cooling the film from an initial high temperature. Comparing the morphology of films for a quench case to a slow-cooling case, we identify possible barriers to reaching equilibrium in experimental situations under a quench.

Effects of the Wetting Particles on Phase Separation of Binary Mixtures

We study phase separation of binary mixtures in the presence of mobile particles by the lattice Monte Carlo simulation. The presence of mobile particles changes the morphology of the domain growth, in agreement with earlier experimental result. By varying the wetting interaction strength, we can control the speed of phase separation, and find a critical wetting strength beyond which the growth of the domains slows down. We propose a novel scaling function which describes the growth of the domain size L(t) as a function of time. It suggests an applicable way to tune the speed of phase separation by the coupling between the phase decomposition and the mobile particle-wetting process.

Shear effects on phase separating polymer solutions: A molecular dynamics study

We study phase separation of quenched polymer solutions under an external shear flow by using a stochastic molecular dynamics method. We observe the formation of a stringlike pattern of polymer domains normal to the direction of the shear flow. The characteristic size of the layered domains has a power law relation with the shear rate. Shear-induced rheological behavior of the mixture is also examined in terms of the excess viscosity. We find that the maximum excess viscosity scales as the shear rate, and that the numerical value of the power-law exponent agrees well with theoretical scaling predictions.

Spinodal Decomposition in a Subsurface Layer of a Polymer Blend Film

We have studied phase separation in off-critical mixtures of poly(ethylene propylene) (hPEP) and perdeuterated poly(ethylene propylene) (dPEP) inside the metastable and unstable region of the phase diagram but under the influence of a surface that attracts the majority component (dPEP). In the adjacent depletion region nucleation barriers are eliminated, and the film starts to phase separate long before phase separation occurs in the bulk. The hPEP-rich domains are confined to grow anisotropically within the unstable region. The growth gives rise to the formation of a regular roughness pattern on the surface when the domains relax into a more circular shape driven by interfacial tension. The volume fraction versus depth profiles are obtained using time-of-flight forward recoil spectrometry (TOF-FRES) while the roughness pattern that mirrors the phase morphology in the depletion layer can be monitored by scanning force microscopy (SFM). Depending on initial dPEP volume fraction the lateral bicontinuous pha...

Multiple-histogram method for quantum Monte Carlo simulations

An extension to the multiple-histogram method (sometimes referred to as the Ferrenberg-Swendsen method) for use in quantum Monte Carlo simulations is presented. This method is shown to work well for the 2D repulsive Hubbard model, allowing measurements to be taken over a continuous region of parameters. The method also reduces the error bars over the range of parameter values due the overlapping of multiple histograms. A continuous sweep of parameters and reduced error bars allow one to make more difficult measurements, such as Maxwell constructions used to study phase separation. Possibilities also exist for this method to be used for other quantum systems.

Phase separation in solution of worm-like micelles: a dilute spin-vector model

We show how the dilute spin vector model introduced originally by Wheeler and co-workers for describing the polymerization phenomenon in solutions of liquid sulphur and of living polymers may be conveniently adapted for studying phase separation in systems containing long flexible micelles. We draw an isomorphism between the coupling constant appearing in the exchange Hamiltonian and the surfactant energies in the micellar problem. We solve this problem within the mean-field approximation and compare the main results we have obtained with respect to polymer theory and previous theories of phase separation in micellar solutions. We show that the attractive interaction term between monomers renormalizes the aggregation energy and subsequently the corresponding size distribution. Under these conditions, we observe that the general aspect of the phase diagram in the plane (where is the surfactant concentration) is different from previous results. The spinodal line shows a re-entrant behaviour and, at low concentrations, we point out the possibility of specific nucleation phenomena related to the existence of a metastable transition line between a region composed of spherical micelles and another one corresponding to a dilute solution of long flexible micelles.

Phase separation kinetics in mixtures of polymers and liquid crystals

Light scattering has been used to study phase separation kinetics in mixtures containing liquid crystals and epoxy resins. In the samples studied, phase separation was induced by the polymerization of the resins with an appropriate curing agent. Experiments were carried out at different compositions and at different temperatures. The results show that the kinetic mechanism of phase separation is composition dependent. For high liquid crystal content the data are in qualitative agreement with existing theories describing spinodal decomposition; at lower concentrations the mechanism is different. The physical properties of the resulting materials are independent of the decomposition mechanism. The data have also been analysed considering the scaling behaviour expected for late stages of phase separation in polyinduced meric mixtures. Samples obtained in a narrow concentration range, where the two kinetic mechanisms overlap, exhibit peculiar physical properties.