Phase separation in carboxylated polysulfone/solvent/water systems

The cloud point curves for the ternary systems carboxylated polysulfones/solvent/water were determined by a titration method. Membrane-forming solvents used were NMP, DMAc, DMF, DMSO, TMU, and DMPU. Four modified polysulfones containing 0.43, 0.93, 1.38, and 1.93 carboxyl groups per repeat unit were synthesized for this study. Water/solvent theta-compositions and solubility parameters for these polymers were also estimated.     

Liquid-liquid phase separation during polysulfone membrane preparation

The cloud point curves for a ternary system of PSf/NMP/water were determined by a titration method at 15 ‡C, 30 ‡C, 45 ‡C and 60 ‡C. A small amount of water (3–10 wt% water) was needed to achieve liquid-liquid phase separation and the temperature effect was not significant. The vitrification composition for the ternary system of PSf/NMP/water was determined by DSC measurements for various compositions. Cross-sectional membrane morphologies were examined by SEM (Scanning Electron Microscope) and surface morphologies by AFM (Atomic Force Microscope) varying the parameters including polymer concentration and bath compositions. It was observed that there were two different modes of phase separation during the formation of polysulfone membrane when the coagulation conditions were varied. This paper was presented at The 5th International Symposium on Separation Technology-Korea and Japan held at Seoul between August 19 and 21, 1999     

Phase separation phenomena of polysulfone/solvent/organic nonsolvent and polyethersulfone/solvent/organic nonsolvent systems

The precipitation values (PVs) of several organic nonsolvents in polysulfone (PSf)/solvent and polyethersulfone (PESf)/solvent systems were measured in temperatures ranging from 10 to 80°C by the direct titration method and compared with those of water in the same systems. The solvents used were N-methyl-2-pyrrolidone (NMP) and N,N-dimethylacetamide (DMAC); the organic nonsolvents employed were methanol, ethanol, 1-propanol, 1-butanol, 1-pentanol, ethylene glycol, and diethylene glycol as well as acetic acid and propionic acid. The compositions of nonsolvent, polymer, and solvent at the precipitation points for different polymer concentrations up to 10 wt % were also determined at 30°C with respect to both the polymers and six nonsolvents presented. These results were used to obtain the polymer precipitation curves in the polymer-solvent-nonsolvent triangular phase diagrams and to determine the theta composition of solvent-nonsolvent for a polymer. The results show that the precipitation value of nonsolvent in polymer/solvent systems depends on both the nature of polymer, solvent, and nonsolvent used and the temperature. The effect of temperature on the precipitation value was observed to be dramatically different for different polymer/solvent/nonsolvent systems. These results were explained on the basis of polar and nonpolar interactions of the polymer, solvent, and nonsolvent system. The results indicate that the precipitation values of the type presented in this paper not only give a relative measure of the nonsolvent tolerance of the polymer/solvent system involved and the strength of solvent and nonsolvent for the polymer, but also determine the relative location of the polymer precipitation curve in the triangular phase diagram. © 1993 John Wiley & Sons, Inc.     

Liquid-liquid phase separation in multicomponent polymer systems. XIII quasi-binary phase diagrams with three-liquid-phase separation

Using the Gibbs free energy of mixing relation of Flory and Huggins with a concentration-dependent interaction parameter one can calculate quasi-binary phase diagrams for solvent-polymer systems. If the polymer has a very asymmetric molecular weight distribution, separation into three-liquid-phases occurs which may result in a cloud-point curve without stable critical point. In the latter case the spinodal does not touch the stable branches of the cloud-point curve and the coexistence curves show marked bends and kinks. These phenomena could be qualitatively verified by experiments on the system cyclohexane-polystyrene. The phase-volume ratio method for the determination of consolute states itself clearly indicates when situations prevail precluding its applicability.     

Liquid-liquid phase transition in flow systems

Liquid-liquid phase transition is occurring in many chemical engineering processes either as a desired phenomenon or as an undesired side effect. Typically, the phase split is modeled by neglecting all non-equilibrium effects. Here, a simple non-equilibrium situation is considered. Convective flow of a liquid along a decreasing temperature profile in a cooled channel is studied analytically. Three different scenarios for the transition process with two typical phase diagrams for binary mixtures are examined. For a phase diagram with critical concentration, phase segregation occurs via spinodal decomposition as a convective instability. For a cigar-shaped phase diagram the phase transformation is shown to evolve in analogy to directional solidification. Finger-like structures may be established under certain circumstances.     

Liquid-liquid phase separation in multicomponenent polymer systems: XV thermodynamic aspects of polymer compatibility

Cloud-point curves in mixtures of short-chain polymers often have irregular shapes, showing shoulders or two maxima. Limits of the thermodynamic stability as measured by Pulse Induced Critical Scattering also appear to be bimodal in such cases. A possible explanation of this phenomenon might be that the constituent molecules influence each other's chain flexibility, the more flexible chains becoming less flexible upon addition of stiffer ones, and vice versa. Such a feature is incorporated i.a. in Huggins' new theory in which orientational entropy correction terms relate the average randomness of orientation of a segment with respect to the preceding one in the chain to the surroundings of the segment.     

Structure development via reaction-induced phase separation in tetrafunctional epoxy/polysulfone blends

For the cure process of tetrafunctional epoxy resin/polysulfone(EP/PSF) blends, we investigated the effect of cure temperature and blend composition on the phase separation behavior by light scattering and the structure development during cure by an optical microscope. The EP/PSF blend without the curing agent was shown to exhibit an LCST-type phase behavior (LCST = 241°C). At the early stage of curing, the EP/PSF blend was homogeneous at the cure temperature. As the cure reaction proceeded, the blend was thrust into a two-phase regime by the LCST depression caused by the increase in a molecular weight of the epoxy-rich phase, and the phase separation took place via a spinodal decomposition (SD) or nucleation and growth (NG) mode, depending on the blend composition and the cure temperature. When cured isothermally at 220°C, the blend exhibited a sea-island morphology formed via the NG mode below 5 wt % PSF content, while the SD mode prevailed above 20 wt % PSF content. At the intermediate composition range, combined morphology with both sea-island and cocontinuous structure was observed. On the other hand, by lowering the cure temperature and/or increasing the content of PSF component, a two-phase structure with a shorter periodic distance was obtained. It seems that the rate of the phase separation is considerable reduced, while that of the cure reaction is not as much. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 2233–2242, 1997     

Effects of molecular weight of polysulfone on phase separation behavior for cyanate ester/polysulfone blends

The effects of molecular weight of polysulfone (PSF) on the morphology of bisphenol‐A dicyanate (BADCy)/PSF blends were studied. Because the viscosity of the blend increased and the miscibility between BADCy and PSF decreased with the increase of PSF molecular weight, these two competing effects on the phase‐separation were investigated. It was observed that the effect of viscosity was predominant: the viscosity of the blends at the onset point of phase separation increased with the increase of PSF molecular weight. The phase separation mechanism depends on the viscosity of the blends at the onset point of phase separation and determines the morphology of the blends. Because the increasing viscosity with increasing the molecular weight of PSF suppressed the nucleation and growth even with 10 phr of PSF content, phase separation occurred through spinodal decomposition to form the combined morphology having both PSF particle structure and BADCy particle structure. The combined morphology and the BADCy particle structure were obtained with a smaller amount of high molecular weight PSF content. This indicates that the viscosity of the blends at the onset point of phase separation is the critical parameter that determines the morphology of the blends. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 921–927, 2000     

Layered structure formation in the reaction-induced phase separation of epoxy/polysulfone blends

In an epoxy/polysulfone blend, the reaction-induced phase separation behavior and the final morphology were investigated. Three distinct morphological structures were obtained. Sea-island and nodular structures were observed at lower and higher polysulfone contents, respectively. A three-layered structure was obtained in the middle polysulfone concentration range. In order to understand the formation of three-layered structure, phase separation process was studied using time-resolved light scattering, phase-contrast optical microscope and scanning electron microscope. Bicontinuous structure formed uniformly in the whole sample at the beginning of phase separation. After the phase structure grew for a certain time, large domains formed and developed. Then, the large epoxy-rich domains gradually flew to the outer space of the sample film. This process assisted the formation of the three-layered structure. The mechanism of the formation of the three-layered structure was discussed based on the different viscoelastic properties of the components.     

Morphology profiles obtained by reaction‐induced phase separation in epoxy/polysulfone/poly(ether imide) systems

The reaction‐induced phase separation in epoxy/aromatic diamine formulations simultaneously modified with two immiscible thermoplastics (TPs), poly(ether imide) (PEI) and polysulfone (PSF), has been studied. The epoxy monomer was based on the diglycidyl ether of bisphenol A (DGEBA) and the aromatic diamine was 4,4′‐methylenebis(3‐chloro 2,6‐diethylaniline) (MCDEA). Phase‐separation conversions are reported for various PSF/PEI proportions for blends containing 10 wt% total TP. On the basis of phase‐separation results, a conversion–composition phase diagram at 200 °C was compiled. This diagram was used to design particular cure cycles in order to generate different morphologies during the phase‐separation process. It was found that, depending on the PSF/PEI ratio employed, a particulate or a morphology characterized by a distribution of irregular PEI‐rich domains dispersed in an epoxy‐rich phase was obtained for initially miscible blends. Scanning electron microscopy (SEM) characterization revealed that the PEI‐rich phase exhibits a phase‐inverted structure and the epoxy‐rich matrix presents a bimodal size distribution of TP‐rich particles. For PSF/PEI ratios near the miscibility limit, slight temperature change result in morphology profiles. Copyright © 2005 Society of Chemical Industry     

Theoretical phase diagram calculation and membrane morphology evaluation for water/solvent/polyethersulfone systems

Theoretical ternary phase diagrams with very good agreement with experimental cloud point data were constructed for water/N,N-dimethylacetamide (DMAc)/polyethersulfone (PES) and water/N-methyl-2-pyrrolidone (NMP)/polyethersulfone systems. Theoretical phase diagrams were determined based on the extended Flory-Huggins theory of polymer solutions. To construct the theoretical phase diagrams, all binary interaction parameters were determined accurately and thoroughly revisited. Also, the structures of membranes prepared of these systems by phase separation process were investigated. The morphological studies showed that in spite of better miscibility between water and DMAc compared to water and NMP, channel-like structures were observed in membranes prepared of water/NMP/PES systems but tear-like structures with more spongy areas were observed in membranes prepared of water/DMAc/PES system. According to the constructed theoretical ternary phase diagrams of these systems, these unexpected observations were attributed to the higher concentration of polymer in the polymer-rich phase of water/DMAc/PES system, which causes an early vitrification in this system which suppresses the growth of macrovoids.     

Phase separation in polyetherimide/solvent/nonsolvent systems and membrane formation

Phase separation phenomena of polyetherimide (PEI)/solvent/nonsolvent systems were investigated by measuring their precipitation values over the temperature range from 20 to 50°C. The solvents used are N‐methyl‐2‐pyrrolidone (NMP), dimethylacetamide (DMAC), and dimethylformamide (DMF). Nine nonsolvents were employed including water, methanol, ethanol, 1‐propanol, 2‐propanol, acetic acid, propionic acid, ethylene glycol, and diethylene glycol. Based on the measured precipitation values, critical solubility parameters for PEI were calculated, and the partial solubility boundary for PEI was obtained in a two‐dimensional solubility parameter coordinate graph. The relationship between solvent strength and membrane structure was examined using PEI hollow‐fiber membranes prepared from binary polymer solutions containing NMP, DMAC, and DMF as solvents. Water was used both as internal and external coagulants. The cross‐sectional structure and gas permeation properties of these hollow fibers were examined. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 1789–1796, 1999     

Phase behavior and morphological studies of polyimide/PVP/solvent/water systems by phase inversion

The solubility gaps for poly(vinyl pyrrolidone) (PVP) in four polyimide solutions (NMP, DMF, GBL, DMSO) were determined by cloud point measurement and correlated with χ_PI/Solvent and Δδ_PVP/Solvent. Membranes prepared with NMP and DMF systems showed a tendency of suppressing fingerlike structure with addition of PVP. On the other hand, membranes prepared with GBL and DMSO systems showed an inclination toward inducing macrovoid formation. These effects of PVP on the membrane morphology were explained by means of miscibility gap, viscosity of the polymer solution, polymer–polymer phase separation, and overall porosity. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 3481–3488, 2001     

Specific interactions in ternary system quaternized polysulfone/mixed solvent

Theoretical and experimental aspects on the specific interactions developed via electrostatic interactions and hydrogen‐bonding in a ternary system formed of a proton‐donor solvent (N,N‐dimethylformamide or methanol), a proton‐acceptor solvent (water), and a quaternized polysulfone with various contents of ionic chlorine, which indicates a proton‐acceptor character, are investigated. Thus, the interactions of the ternary systems are corrected on the basis of the association phenomena defined through association constants. Numerical values for these constants were evaluated as a function of the system composition, by mathematical simulations for an accurate adjustment of preferential adsorption, determined by the Flory–Huggins–Pouchly theoretical approach applied to the experimental data. POLYM. ENG. SCI., 2009. © 2008 Society of Plastics Engineers     

Liquid-liquid phase separation in a polyethylene blend monitored by crystallization kinetics and crystal-decorated phase morphologies

A series of polyethylene (PE) blends consisting of a linear high density polyethylene (HDPE) and a linear low density polyethylene (LLDPE) with an octane-chain branch density of 120/1000 carbon was prepared at different concentrations. The two components of this set of blends possessed isorefractive indices, thus, making it difficult to detect their liquid-liquid phase separation via scattering techniques. Above the experimentally observed melting temperature of HDPE, T_m = 133 °C, this series of blends can be considered to be in the liquid state. The LLDPE crystallization temperature was below 50 °C; therefore, above 80 °C and below the melting temperature of HDPE, a series of crystalline-amorphous PE blends could be created. A specifically designed two-step isothermal experimental procedure was utilized to monitor the liquid-liquid phase separation of this set of blends. The first step was to quench the system from temperatures of known miscibility and isothermally anneal them at a temperature higher than the equilibrium melting temperature of the HDPE for the purpose of allowing the phase morphology to develop from liquid-liquid phase separation. The second step was to quench the system to a temperature at which the HDPE could rapidly crystallize. The time for developing 50% of the total crystallinity (t_1/2) was used to monitor the crystallization kinetics. Because phase separation results in HDPE-rich domains where the crystallization rates are increased, this technique provided an experimental measure to identify the binodal curve of the liquid-liquid phase separation for the system indicated by faster t_1/2. The annealing temperature in the first step that exhibits an onset of the decrease in t_1/2 is the temperature of the binodal point for that blend composition. In addition, the HDPE-rich domains crystallized to form spherulites which decorate the phase-separated morphology. Therefore, the crystal dispersion indicates whether the phase separation followed a nucleation-and-growth process or a spinodal decomposition process. These crystal-decorated morphologies enabled the spinodal curve to be experimentally determined for the first time in this set of blends.     

Effect of solvent evaporation conditions on gas separation performance for asymmetric polysulfone membranes

Asymmetric polysulfone membranes were prepared by the phase inversion technique under different solvent evaporation conditions prior to the gelation step. The membranes were cast from the two component system of polymer and N,N‐dimethylacetamide in which the polymer concentration was changed from 25.0 to 30.0%. The solvent evaporation temperature was changed from 70 to 120°C, and the evaporation time was 0–15 min. Ethanol, water, or 2‐propanol was used as the gelation media. The membranes were characterized by the measurement of oxygen/nitrogen permeation with the lamination technique and by observation with scanning electron microscopy. With an increase in the solvent evaporation time, the oxygen permeance decreased and its selectivity over nitrogen increased; although the permeance was in the range of 1–2 GPU, the oxygen selectivity over nitrogen exceeded 8. A correlation between the permeation performance and the operational parameters involved in the solvent evaporation process was obtained. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 1367–1374, 1999     

Pervaporation separation of a water/ethanol mixture by a sodium sulfonate polysulfone membrane

A sodium sulfonate polysulfone membrane was prepared for the dehydration of a water/ethanol mixture by pervaporation. The separation performances of water and ethanol were examined by the testing of the ethanol/water mixture under operating conditions. The permselectivity of the sodium sulfonate polysulfone membrane was found to strongly depend on the sodium content in the membrane. The sodium sulfonate ratio showed a significant influence on the hydrophilicity and diffusion behavior of the polysulfone membrane. Moreover, the difference in the diffusion of the permeates played an important role in the sulfonate polysulfone membrane. It was found that a high‐performance pervaporation membrane could be achieved with a sodium sulfonate polysulfone membrane. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 3374–3383, 2003     

Liquid–liquid phase separation in polysulfone/polyethersulfone/N‐methyl‐2‐ pyrrolidone/water quaternary system

To construct a phase diagram of the polysulfone (PSF)/polyethersulfone (PES)/N‐methyl‐2‐pyrrolidone (NMP)/water quaternary system, cloud point measurements were carried out by a titration method. The miscible region in the PSF/PES/NMP/water quaternary system was narrow compared to the PSF/NMP/water and PES/NMP/water ternary systems. The binary interaction parameters between PSF and PES were estimated by water sorption experiments. The calculated phase diagram based on the Flory–Huggins theory fit the experimental cloud points well. In addition to the usual polymer–liquid phase separation, polymer–polymer phase separation, which resulted in a PSF‐rich phase and a PES‐rich phase, was observed with the addition of a small amount of nonsolvent. The boundary separating these two modes of phase separation could be well described and predicted from the calculated phase diagrams with the estimated binary interaction parameters of the components. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 2113–2123, 1999     

Vitrification phenomena in polysulfone/NMP/water system

The vitrification line for the ternary system of polysulfone (PSf)/N‐methyl‐2‐pyrrolidinone (NMP)/water was determined by differential scanning calorimeter (DSC) measurements with varying compositions. Pure PSf showed both α‐ and β‐transition temperatures (T_g,α = 187.5°C, T_g,β = −21.4°C). The T_g,α of PSf decreased with increasing solvent concentration. The T_g,α of PSf decreased linearly with the addition of NMP in the concentration range of 70–90 wt % polymer. The vitrification line was indicated in the phase diagram for the ternary system of PSf/NMP/water at 15 and 60°C. As the temperature is increased, a high polymer concentration was needed to reach the vitrification condition. The vitrification composition of the polymer in the binary system of PSf and NMP was 72.0 wt % at 15°C and 79.8 wt % at 60°C. We also found that the slope of the vitrification line changed with the temperature and that a small amount of water (10–20 wt %) can induce the vitrification of the polymer solution in the PSf/NMP/water system at 15°C. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 431–438, 1999