Demixing of Oxides under a Temperature Gradient

The microscopic mechanism of a demixing of binary oxide solid solution in an oxidizing atmosphere under a temperature gradient is investigated by the path probability method of irreversibly statistical mechanics. The problem is reduced to a steady-state metal ion transport under the coexistence of an oxygen partial pressure gradient and a temperature gradient. Calculations are performed under two different conditions: (i) demixing in a steady-state diffusion process under the two driving forces and (ii) a redistribution of constituent metal ions in a closed system under a given temperature gradient (Soret effect). The results can be interpreted as the superposition of the contributions of two driving forces plus a cross term which tends to zero as the driving forces become small. The difference between these two cases is discussed in detail and some controversies in the past work with respect to the difference are pointed out.     

Mixing of viscous liquid mixtures

When a quiescent binary mixture is instantaneously brought from two- to one-phase region of its phase diagram, the resulting mixing process is driven both by diffusion and by convection. Applying the diffuse interface model, here we show by numerical simulation that the bulk motion that is induced during phase transition effectively slows down the mixing process. In fact, in our model convection is induced by a non-equilibrium body force that is proportional to the chemical potential gradients, expressing the tendency of the demixing system to minimize its free energy. In liquid systems, as this driving force induces a material flux which is much larger than that due to pure molecular diffusion, drops tend to coalesce and form larger domains which eventually must dissolve by diffusion. Therefore, in the absence of any external agitation, mixing is slower in mixtures with larger viscosities, contrary to common thinking. In addition, as expected, the mixing rate decreases as the Margules parameter Ψ increases, where Ψ describes the relative weight of enthalpic versus entropic forces.     

Time Evolution of Demixing in Oxides under an Oxygen Potential Gradient

Time evolution of concentration profiles leading to steady-state demixing in oxides under oxygen partial pressure gradient is treated based on a microscopic model by applying the path probability method of irreversible statistical mechanics. The redistribution of vacancies and cations toward the steady-state distribution has essentially been reduced to the diffusion of vacancies and the interdiffusion of cations along the specimen. Atomistic features of the problem, such as a vast difference in the relaxation times for the redistribution of vacancies and cations, are emphasized. Time required for the system to reach the steady state is then estimated in terms of fundamental material properties.     

In situ analysis of solvent/nonsolvent exchange and phase separation processes during the membrane formation of polylactides

Membrane formation of polylactides has been studied using in situ analysis techniques. An experimental method based on the use of dark ground optics and reflected light illumination is used to monitor the mass transfer and phase separation dynamics during for mation. Additionally, the phase separation and structure formation has been studied using optical microscopy. The results of the dark ground optics technique for the polymer/solvent/nonsolvent systems poly-L-lactide/chloroform/methanol and poly-DL-lactide/chloroform/methanol showed that the diffusion kinetics were similar for the semicrystalline poly-L-lactide (PLLA) and the amorphous poly-DL-lactide. The influence of the molecular weight of the polymers on the diffusion kinetics was found to be negligible. Increasing the polymer concentration of the casting solution decreased the rate of diffusion. The phase separation of poly-DL-lactide was studied with optical microscopy and found to proceed via liquid-liquid demixing. For poly-L-lactide solutions of relatively low concentration (5–6% w/w), phase separation proceeded via liquid-liquid demixing followed by crystallization. For more concentrated PLLA solutions, phase separation proceeded directly via solid-liquid demixing processes. Additionally, for 6% w/w solutions of poly-L-lactide in dioxane immersed in methanol, precipitation also occurred solely via solid-liquid demixing. © 1996 John Wiley & Sons, Inc.     

Influence of Coherency Stress on Kinetic Demixing

The influence of coherency stress, arising from differences in the partial molar volumes of the cations or an imposed displacement boundary condition, on composition profiles during kinetic demixing in ternary oxides subjected to a gradient in oxygen chemical potential is investigated. Coherency stresses can either enhance or diminish the magnitude of the kinetic demixing and shifts in interfacial composition owing to coherency stress on the order of several atomic percent, measured with respect to the case where stress is neglected, can be expected. For (Co, Mg)O subjected to displacement boundary conditions, steady-state and time-dependent demixing profiles are calculated and compared with experimental measurements. Although the composition profiles in the demixed solids are modified because of coherency stress, the main features of kinetic demixing remain unchanged.     

Current-Induced Segregation in Ionic Melts

The criteria for the electrochemically driven demixing of cations in ternary molten salts and silicates are analyzed for two cases of selective ion blocking electrodes. Mathematical expressions are derived for an anion flux case and a cation flux case which relate the concentration profiles and current densities at steady state to the differences of the electrical and chemical potentials applied at the melt surfaces. The analysis can be used to evaluate component demixing if three independent transport coefficients are known as a function of composition. Conversely, demixing experiments are proposed for use in determining such parameters as the inter diffusion coefficient and transference numbers of each ion for the composition range of segregation.     

Nonequiiibrium Surface Segregation in Aluminum-Doped TiO2 under an Oxidizing Potential: Effects on Redox Color-Boundary Migration

During the oxidation of redued single crystals of Al-doped rutile, unusual anisotropies in color-boundary migration have been observed that are opposite to those predicted from published diffusion data. Analysis of the redox kinetics and of surface segregation (using ESCA and AES) shows that rapid transport of minority Al interstitials in the c -axis direction occurs under an oxidation potential, resulting in a surface segregation layer inhibiting further reoxidation. This surface segregation is nonequilibrium in nature, is driven by oxidation, and bears similarities to the phenomena of kinetic demixing in ionic systems. The results show that minority defects can play critical roles in demixing at the local scale; in their absence this system would not be expected to demix. This thus appears to be an additional mechanism for nonequilibrium interfacial segregation in ionic systems.     

Diffusional Creep and Kinetic Demixing in Yttria-Stabilized Zirconia

The creep behavior of fine-grained yttria-stabilized zirconia with 25 mol% Y₂O₃ has been characterized as part of an investigation of kinetic demixing in solid-solution oxides which are subjected to a nonhydrostatic state of stress. At temperatures between 1400° and 1600°C, the steady-state strain rate of (Zr_0.6Y_0.4)O_1.8 samples with average grain sizes between 2.5 and 14.5 μm can be summarized by the flow law ɛ= 6.5 × 10⁻⁷σ^1.2 exp[−550 (kJ/mol)/ RT ] d ^−2.2 (s⁻¹) for stresses in the range 8 to 60 MPa, where σ is in pascals and d is in meters. This flow law indicates that deformation occurs by a Nabarro-Herring creep mechanism in which the creep rate is limited by cation lattice diffusion. Kinetic demixing was not observed in deformed polycrystalline samples even though diffusional creep was rate limited by cation lattice diffusion. This result can be explained if the cation diffusivities are approximately equal or if extensive grain rotation occurs during diffusional creep.     

Demixing kinetics of phase systems employed for liquid-liquid extraction and correlation with system properties

The kinetics of phase demixing in liquid-liquid extraction for model systems comprising PEG 4000/potassium phosphate and t-butanol/ammonium sulfate was studied. The kinetics of demixing depicts the entire demixing pattern of phases of the phase system during and after extraction and hence the study is essential prior to design of large scale gravity phase separators. With an increase in composition, both the systems showed increase in demixing rate (decrease in demixing time). At high tie line length (TLL) and phase volume ratio <1, with salt rich phase as continuous phase, both PEG 4000/potassium phosphate and t-butanol/ammonium sulfate systems showed reduction in demixing time by 59% and 50%, respectively as compared to that at low TLL. An empirical equation proposed in the literature for rate of phase demixing was used to correlate the experimental data. The agreement was better for t-butanol/ammonium sulfate system when compared to PEG 4000/potassium phosphate system.     

Multicomponent diffusion in phase-separating polymer blends with different frictional interactions: a mean-friction model

We present a compact formula for describing the mean frictional forces acting on a molecule in multicomponent systems. The friction-based diffusion theory of Zielinski and Hanley was extended to newly include the friction-average molar velocity as a reference frame. The results showed that the previous diffusion theories are unified by the friction-average concept by properly choosing the average velocity. The present model based on the diffusivity-related molar average velocity provides better predictions for the diffusive flux in a ternary miscible liquid compared to the other existing theories. The application of the model in phase-separating ternary systems revealed that the introduction of a highly diffusive third component into demixing polymer blends promotes a particular enhancement of the spinodal decomposition due to the difference in the frictional interactions between polymers.     

Calculation on phase diagrams of polyetherimide/N,N-dimethylacetamide/H2O-BuOH casting system and their relevance to membrane performances

The ternary phase diagrams of polyetherimide (PEI)/N,N-dimethylacetamide (DMAc) with H₂O and BuOH as non-solvent were simulated using solubility parameter and Flory-Huggins theory. The phase diagrams show that 5.5% H₂O/BuOH system containing 5% BuOH and 0.5% H₂O, or 6.5% H₂O/BuOH system containing 6.2% BuOH and 0.3% H₂O is required to induce liquid-liquid demixing for 20 wt-% PEI/DMAc casting solution. Therefore, BuOH can enhance the phase separation of the PEI casting solution and hereby induce higher porosity of the membrane, and the diffusion of BuOH into the water coagulation bath causes larger pore size easily compared with DMAc. Our predictions that the membrane pure water flux first increases then decreases, and the rejection ratio of bovine serum albumin decreases with the increasing concentration of BuOH were validated by the experiments using the prepared membranes.     

蛋白质在离子交换吸附剂颗粒中的扩散系数

采用以化学势梯度作为扩散推动力的Maxwell-Stephan扩散方程（MS方程）,结合平行扩散模型,对阴离子吸附剂Q-SepharoseFF吸附牛血清白蛋白（BSA）的动态试验数据进行了模拟,拟舍得到了BSA的表面扩散和孔扩散系数。研究表明表面扩散系数随pH降低、盐浓度增加而增大;孔扩散系数随pH增加、盐浓度降低而增加,随蛋白质初始浓度增加而减小。在表面扩散不容忽视且为非线性吸附的条件下采用Fick定律时得到的表面扩散系数是蛋白质浓度的函数,且大于根据MS方程得到的表面扩散系数。     

Demixing of Aqueous Polymer Two-Phase Systems in Low Gravity

Abstract

When polymers such as dextran and poly(ethy1ene glycol) are mixed in aqueous solution, biphasic systems often form. On Earth the emulsion formed by mixing the phases rapidly demixes because of phase density differences. Biological materials can be purified by selcctive partitioning between the phases. In the case of cells and other particulates thc efficiency of these separations appears to be compromised by the demixing process. To modify this process and to evaluate the potential of two-phase partitioning in space, experiments on the effects of gravity on phase emulsion demixing were undertaken. The behavior of phase systems possessing phase density differences and systems with essentially identical phase densities was studied at one-g and during low-g parabolic aircraft maneuvers. Results indicate that demixing can occur rather rapidly in space, although more slowly than on Earth. We have examined the demixing process from a theoretical standpoint by applying the theory of Ostwald ripening. This theory predicts demixing rates many ordcrs of magnitude lower than observed. Other possible demixing mechanisms are considered.     

Dynamical study on the late stage of demixing in poly(methyl methacrylate) and poly(vinyl acetate) blends

A dynamical study was made on demixing of an immiscible polymer blend, whose specimens were prepared by solvent casting and had very finely phase separated structures in the initial stage of the demixing. Light scattering experiments showed the applicability of a scaling rule to the later stage of the growth of phase separation structures. The demixing can be described by a scaling theory proposed by Furukawa.     

Fabrication of membranes of polyethersulfone and poly(N‐vinyl pyrrolidone): influence of glycerol on processing and transport properties

In this study, we focus on membranes of polyethersulfone and poly(N‐vinyl pyrrolidone) and elucidate the influence of composition on the rheological, diffusion and precipitation properties of solutions which are used for membrane preparation via a non‐solvent‐induced phase separation process. The low‐molar‐mass component of the solution is a mixture of the solvent N‐methyl‐2‐pyrrolidone and the non‐solvent glycerol. Cloud point, viscosity and diffusion measurements as well as precipitation experiments were performed in order to achieve a comprehensive understanding of the time dependence of the precipitation process. The addition of glycerol yields an increase of viscosity and a stronger tendency for demixing. The enhanced tendency for demixing causes a more rapid precipitation process. The average relaxation time of the solution as a function of glycerol concentration follows a similar trend to its viscosity. The increase of viscosity is associated with the increase of the monomeric friction coefficient. Two diffusive processes with clearly separated time scales appear in dynamic light scattering experiments in the presence of glycerol. This phenomenon is discussed taking into account the phase behaviour of the solution and the quality of the solvent. The addition of glycerol yields a lower pure water permeance whereas the molecular weight cut‐off is not altered in the ultrafiltration range. © 2020 The Authors. Polymer International published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.     

聚乙烯吡咯烷酮添加剂对聚醚砜制膜体系的影响

本文考察了不同ＰＶＰ／ＰＥＳ比率对ＰＥＳ／ＰＥＳ／ＰＶＰ／ＤＭＡｃ／Ｈ２Ｏ制膜体系热力学平衡关系、成膜动力学因子Ｄｅ以及膜分离性能的影响，揭示了亲水性聚合物添加剂ＰＶＰ在ＰＥＳ膜形成过程中所起的作用。结果表明：ＰＶＰ不是单纯的致孔剂，它改变了制膜液相平衡关系及浸入沉淀进非溶剂侵入速速度而延迟或促进相分离，从而影响了成膜的结构和分离性能。     

Mass Transport and Chemical Demixing of Y-Ba-Cu-O Compounds During Sintering in Temperature Gradients

A thin cylindrical powder compact (3-mm diameter by 20-mm) with a nominal composition of YBa₂Cu₃O_x was sintered in a static temperature gradient of 25°C/mm in air for 12 h. The temperature at the hot end of the sample was maintained at 940°±5°C. Considerable mass transport and chemical demixing occurred along the length of the sample. The results are consistent with the formation of a liquid phase at the hot end of the sample, transport of the liquid down the temperature gradient, and the formation of different eutectic compositions along the length of the sample. The implications for processing high-T_c superconductors in the system Y-Ba-Cu-O are discussed.     

Extraction with Spinodal Decomposition – Experiment and Simulation

Experimental results on the growth and velocity of droplets (up to 0.45 mm/s) during the temperature‐induced phase separation of the 3‐methoxypropionitrile/water solvent system as well as a variety of theoretical methods used to study the demixing dynamics are presented. Due to computational costs, the simulation with Molecular Dynamics (MD) is feasible only for very short times (ps) and miniscule boxes (Å). Dissipative Particle Dynamics (DPD), where a coarse‐grained model of the system is employed, allows nanosecond simulations in a box with more than 13 million atoms in a diffusion‐dominated regime. Finally, the influence of the diffusion/convection ratio during the separation in the μs scale is studied by use of a code based on Model H for both instantaneous and non‐instantaneous quenching cases. Structural information and macroscopic parameters, such as interfacial tension, can be derived from the molecular simulations.     

Mass transfer into a horizontal layer of stagnant liquid under the action of instabilities+

The rate of absorption into a horizontal liquid layer undergoing an assumed steady cellular convection due to a concentration gradient or to the Marangoni effect is calculated. One concludes that a roll-cell model can explain the large increase in the rate of absorption observed experimentally and that for large times a diffusional model, with a constant diffusion coefficient much larger than the molecular one, describes adequately the above situations.     

Effect of stereoisomerism of poly(lactic acid) during neural guide conduit membrane synthesis

Poly(lactic acid) membranes are being developed as biomaterials for several purposes such as artificial implants for peripheral nerve injury, also known as neural guide conduits (NGC). These membranes need to meet standards of mechanical, degradability, and permeability properties, besides dimensional and structural requirements. Among the stereoisomers of polylactides, poly(l ‐lactic acid), and poly(d ,l ‐lactic acid) are the most used as biomaterials, having significant differences in solubility, crystallinity thermal, and mechanical properties. In this work, PLLA and PDLLA were compared for hollow fiber membrane synthesis by liquid induced phase separation. PLLA samples presented 18% of crystallinity, while PDLLA is amorphous. PDLLA and PLLA polymer solutions on N‐methyl‐pyrrolidone presented values of 3428 and 320.2 cP, respectively. In immersion of PLA‐NMP solutions in water, PLLA solution presented instantaneous demixing, while PDLLA showed a 28 s delay on precipitation. The PLLA–NMP–water has a larger demixing region compared to PDLLA–NMP–water system. Hollow fibers of both polymers presented closed external surface with finger‐like macropores morphology in their cross‐sections. PDLLA presented typical liquid–liquid demixing pores while PLLA presented spherulitical crystalline solid–liquid separation structures, which deeply compromised its mechanical properties. PDLLA presented as a good candidate for hollow fiber NGC material, as presented good mechanical resistance in tensile and suture simulating tests. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46190.