Water sorption and transport in blends of poly(vinyl pyrrolidone) and polysulfone

Water sorption and transport properties for a series of miscible blends of hydrophobic bisphenol A polysulfone and hydrophilic poly(vinyl pyrrolidone) are reported. Study was restricted to blends that remained homogeneous after exposure to liquid water. The solubility of water in the blend films increased with increasing hydrophilic polymer content. Equilibrium sorption isotherms show dual-mode behavior at low activities and swelling behavior at high activities. The sorption kinetics are generally Fickian for blends containing 20% poly(vinyl pyrrolidone) or less, but exhibit two-stage behavior in blends containing 40% poly(vinyl pyrrolidone). Diffusion coefficients extrapolated to zero concentration decrease with increasing poly(vinyl pyrrolidone) content, owing to a decrease in the fractional free volume. However, the diffusion coefficient becomes a greater function of activity as the composition of hydrophilic polymer in the blend is increased, due to plasticization of the material by large levels of sorbed water. Permeability coefficients generally decrease with increasing poly(vinyl pyrrolidone) content for blends containing 20% poly(vinyl pyrrolidone) or less because the decrease in the diffusion coefficient is greater than the increase in the solubility coefficient. Blends containing 40% poly(vinyl pyrrolidone) have permeability coefficients greater than those of polysulfone due to high water solubility. The permeability coefficients depend on water concentration in approximately the same way for all blends. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys, 35: 655–674, 1997     

Transport and mechanical properties of blends of poly(ϵ‐caprolactone) and a modified montmorillonite‐ poly(ϵ‐caprolactone) nanocomposite

The structural characterization and transport properties of blends of a commercial high molecular weight poly(?‐caprolactone) with different amounts of a montmorillonite‐poly(?‐caprolactone) nanocomposite containing 30 wt % clay were studied. Two different vapors were used for the sorption and diffusion analysis—water as a hydrophilic permeant and dichloromethane as anorganic permeant—in the range of vapor activity between 0.2 and 0.8. The blends showed improved mechanical properties in terms of flexibility and drawability as compared with the starting nanocomposites. The permeability (P), calculated as the product of the sorption (S) and the zero‐concentration diffusion coefficient (D₀), showed a strong dependence on the clay content in the blends. It greatly decreased on increasing the montmorillonite content for both vapors. This behavior was largely dominated by the diffusion parameters. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1118–1124, 2002     

Water vapor sorption and diffusion in glassy polymers

By establishing relationships between polymer structure and gas permeation behavior, significant advances have been made in designing materials for membrane separation of gas mixtures. However, the situation is not so well understood when water vapor is one of the components since water molecules may interact with the polymer (plasticization) or each other (clustering) in ways that complicate the structure-property relationships. In addition, accurate measurement of water sorption, diffusion, and permeation is more complicated than for gases because of the unique hydrogen bonding capability of water, e.g., its tendency to strongly adsorb on high-energy surfaces and high heat of vaporization. A progress report on a broad program to understand water sorption and diffusion in glassy polymers that may be of interest for membrane applications is outlined; specific strategies include studies of structurally related polymers and miscible blends of hydrophobic/hydrophilic polymer pairs.     

Water vapor transport in a series of polyarylates

Water vapor sorption and transport properties are presented for a series of polyarylates. The effect of making various structural changes in the polymer backbone has been studied, by comparing the transport properties to those of the base polymer bisphenol A isophthalate. All the polymers exhibit an upturn in the sorption isotherm at moderate to high activities, which in most cases is caused by plasticization of the polymer by water vapor. The response of water vapor to the structural changes is compared to that of the permanent gases reported previously for these materials. A good correlation is established between the diffusion and permeability coefficients and the fractional free volume. A similar behavior is seen in gases. However, the solubility coefficient, which decreases with increase in the fractional free volume, shows an opposite effect. Water vapor has the ability to hydrogen bond with itself and interact strongly with the polymer. In the case of solubility, the water vapor–polymer interaction plays an important role and overrides the effect of the increase in fractional free volume. The structural changes described here have similar effects on the permeation of water in a series of polysulfones reported earlier, as seen here for the polyarylates.     

Solubility,diffusivity, and mobility of n-pentane and ethanol in poly(1-trimethylsilyl-1-propyne)

The diffusion coefficient of ethanol and of n-pentane in PTMSP, at 27°C, was measured as a function of concentration up to a penetrant content of about 12% by weight, for polymer samples obtained through different processes; differential sorptions and desorptions with vapor phases were considered. In the case of ethanol a nonmonotonous behavior was observed for the diffusivity, while in the case of n-pentane the same property was found to monotonously decrease with increasing the penetrant content. The sorption isotherms were also reported, indicating that n-pentane exhibits a typical dual mode behavior, while ethanol follows an unusual s-shape curve. The chemical potential of the dissolved penetrants, calculated directly from the isotherms, shows the very different importance of the energetic interactions of the two penetrants with the polymer units. In spite of the remarkably different concentration dependencies observed for both solubility and diffusivity of the two penetrants, the mobility factors are in both cases monotonously decreasing with the penetrant concentration, and follow very similar trends. The significant differences observed for the concentration dependence of the diffusion coefficients are, thus, associated to the thermodynamic contributions, which are very different for n-pentane and ethanol. Different polymeric films, obtained through different solvent evaporation processes, show quite different solubility, diffusivity and mobility for both ethanol and n-pentane. On the other hand, the ratio between the mobility of the two penetrants as well as the slope of mobility as function of the concentration remains the same for all the different samples inspected. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35 : 2245–2258, 1997     

Effect of blend composition on diffusivity and solubility of small molecules in polystyrene/poly(vinyl methyl ether) blends

The capillary column inverse gas chromatography technique was used to determine diffusivity and solubility data for several solvents in polymer blends composed of polystyrene and poly(vinyl methyl ether) (PVME). Diffusivity behaved as expected, increasing as the concentration of PVME increased in the blend. Knowing only the free‐volume parameters for the pure polymers, the free‐volume theory was successfully applied to predict the dependence of the diffusion coefficients on the blend composition. Transport in blends above the glass transition temperature is controlled by free volume, and the effect of concentration fluctuations is minimal at the temperatures studied. Experimental data show an increase in the partition coefficient of some solvents in the blends with respect to the pure polymers. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2071–2082, 2007     

Sorption of methanol/MTBE and diffusion of methanol in 6FDA–ODA polyimide

This article discusses the diffusion and solubility behavior of methanol/methyl tert‐butyl ether (MTBE) in glassy 6FDA–ODA polyimide prepared from hexafluoroisopropylidene 2,2‐bis(phthalic anhydride) (6FDA) and oxydianiline (ODA). The diffusion coefficients and sorption isotherm of methanol vapor in 6FDA–ODA polyimide at various pressures and film thicknesses were obtained with a McBain‐type vapor sorption apparatus. Methanol/MTBE mixed‐liquid sorption isotherms were obtained by head‐space chromatography and compared with a pure methanol sorption isotherm obtained with a quartz spring balance. Methanol sorption isotherms obtained with the two methods were almost identical. Both methanol sorption isotherms obeyed the dual‐mode model at a lower activity, which is typical for glassy polymer behavior. The MTBE was readily sorbed into the polymer in the presence of methanol, but the MTBE sorption isotherm exhibited a highly nonideal behavior. The MTBE sorption levels were a strong function of the methanol sorption level. Methanol diffusion in the polymer was analyzed in terms of the partial immobilization model with model parameters obtained from average diffusion coefficients and the dual‐mode sorption parameters. Simple average diffusion coefficients were obtained from sorption kinetics experiments, whereas the dual‐mode sorption parameters were obtained from equilibrium methanol sorption experiments. An analysis of the mobility and solubility data for methanol indicated that methanol tends to form clusters at higher sorption levels. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 2254–2267, 2000     

Water transport properties of thermoplastic polyurethane films

Two linear segmented polyurethanes, based on poly(oxyethylene) (POE) as a soft segment and 4,4′‐diphenylmethane diisocyanate and 1,4‐butanediol as hard segments and differing in their soft segment length, have been studied from a water vapor transport point of view. For both polyurethanes, the water sorption is governed by a Fickian process, and the thermoplastic polyurethane with the longer POE segments displays the higher water diffusion rate. The water sorption isotherms are Brunauer Emmet Teller (BET) type III for both thermoplastic polyurethanes, and the water uptakes are directly related to the polymer POE content. The Flory–Huggins theory cannot correctly describe the sorption isotherms. More sophisticated approaches (Koningsveld–Kleinjtens or Guggenheim‐Anderson‐de Boer (GAB) models) are needed to fit the experimental water uptakes. The positive deviation from Henry's law and the decrease in the apparent diffusion coefficient observed at a high activity have been particularly studied. In this activity range, an isotherm analysis based on the cluster integral of Zimm and Lundberg suggests some clustering phenomenon, which seems consistent with the diffusion coefficient variation. In agreement with the sorption results, the water permeability coefficients are small at low activities, and they increase greatly with the relative pressure of water. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 473–492, 2004     

Experimental and theoretical investigation of anomalous sorption kinetics in the methanol vapor–poly(methyl methacrylate) system at 35 °C

Non‐Fickian sorption kinetics of methanol vapor in a poly(methyl methacrylate) film of 8 μm, at 35 °C, are presented. The behavior of the system was studied in series of interval absorption runs. The relevant diffusion and viscous relaxation processes were studied by kinetic analysis of the sorption kinetic curves, using the relaxation‐dependent solubility model. The sorption isotherm concaves upward at high activities, typical to Florry–Huggins behavior, while it exhibits a convex‐upward curvature at low methanol vapor activities, indicating sorption in the excess free volume of the polymer matrix. Thermodynamic diffusivity presents a complex functional dependence on the concentration, while relaxation rate is found to be a function of concentration as well as of concentration interval. Relaxation rate becomes increasingly concentration‐dependent as the effective glass transition of the system is approached. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 3173–3184, 2006     

Development of novel elastomeric blends derived from chlorinated nitrile rubber and chlorinated ethylene propylene diene rubber

Chlorinated nitrile rubber (Cl-NBR) has been blended with chlorinated ethylene propylene diene rubber (Cl-EPDM) in different ratios by a conventional mill mixing method. The effect of the blend ratio on processing characteristics, mechanical properties (such as tensile and tear strength, elongation at break, hardness, abrasion resistance, heat build-up and resilience), structure, morphology, glass transition temperature (Tg), thermal stability, flame retardancy, oil resistance, AC conductivity, dielectric properties and transport behavior of petrol, diesel and kerosene were investigated. The shift in absorption bands of blends studied from FTIR spectra, single Tg from DSC analysis and decrease in amorphous nature from XRD showed the molecular miscibility in Cl-NBR/Cl-EPDM blends. SEM images showed the uniform mixing of both Cl-NBR and Cl-EPDM in a 50/50 blend ratio. The TGA curves indicated the better thermal stability of the polymer blend. The elongation at break, heat build-up, resilience and hardness of the polymer blend decreases with an increase in Cl-NBR content in the blend whereas the flame and oil resistance were increased with increase in Cl-NBR content. Among the polymer blends, the maximum torque, tensile strength, tear and abrasion resistance was obtained for the 50/50 blend ratio because of the effective interfacial interactions between the blend components. AC conductivity and dielectric properties of polymer blend increased with increase in the ratio of Cl-NBR in the blend. Different transport properties such as diffusion, permeation and sorption coefficient were measured with respect to nature of solvent and different blend ratios. Temperature dependence of diffusion was used to estimate the activation parameters and the mechanism of transport found to be anomalous.     

Synthesis,physical characterization,and acetone sorption kinetics in random copolymers of poly(ethylene terephthalate) and poly(ethylene 2,6-naphthalate)

Random copolymers of poly(ethylene terephthalate) (PET) and poly(ethylene 2,6-naphthalate) (PEN) were synthesized by melt condensation. In a series of thin, solvent cast films of varying PEN content, acetone diffusivity and solubility were determined at 35°C and an acetone pressure of 5.4 cm Hg. The kinetics of acetone sorption in the copolymer films are well described by a Fickian model. Both solubility and diffusivity decrease with increasing PEN content. The acetone diffusion coefficient decreases 93% from PET to PET/85PEN, a copolymer in which 85 weight percent of the dimethyl terephthalate in PET has been replace by dimethyl naphthalate 2,6-dicarboxylate. The acetone solubility coefficient in the amorphous regions of the polymer decreases by approximately a factor of two over the same composition range. The glass/rubber transition temperatures of these materials rise monotonically with increasing PEN content. Copolymers containing 20 to 80 wt % PEN are amorphous. Samples with <20% or >80% PEN contain measurable levels of crystallinity. Estimated fractional free volume in the amorphous regions of these samples is lower in the copolymers than in either of the homopolymers. Relative free volume as probed by positron annihilation lifetime spectroscopy (PALS) decreases systematically with increasing PEN content. Acetone diffusion coefficients correlate well with PALS results. Infrared spectroscopy suggests an increase in the fraction of ethylene glycol units in the trans conformation in the amorphous phase as the concentration of PEN in the copolymer increases. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 2981–3000, 1998     

Water sorption and transport in a series of polysulfones

Water sorption and transport properties for a series of polysulfones are presented and interpreted in terms of the changes in the structure of the repeat unit compared to that of bisphenol A polysulfone. The differences between the sorption and diffusion of water and of permanent gases in these materials are also discussed. Water has the ability to interact with the polymer and with itself through hydrogen bonding in a way that permanent gases cannot. The equilibrium solubility of water in the polymer, unlike permanent gases, does not have a simple dependence on free volume but correlates more strongly with the frequency of hydrogen bonding sites on the polymer. Analysis of the sorption isotherms using the method of Zimm and Lundberg suggests that water molecules cluster in these polysulfones to various extents. For each polysulfone except polyethersulfone, the water diffusion coefficient decreases with increasing activity, which also suggests water clustering. For most of these materials, the water diffusion coefficient is larger than that of bisphenol A polysulfone and is directly related to the polymer free volume. Water permeability in these materials broadly correlates with the polymer free volume, but a favorable water-polymer interaction can be an overriding factor. © 1996 John Wiley & Sons, Inc.     

Effect of molecular association on solubility,diffusion, and permeability in polymeric membranes

The filling‐type membrane is composed of grafted polymer and solvent‐resistant substrate; the calculation of solubility, diffusivity and swelling‐suppression effect by the substrate permits the prediction of solvent permeability. As noted in our previous article, the use of this approach, called membrane design, resulted in accurate prediction of the permeability of aromatic compounds. In this study, the influence of hydrogen bonding on solubility and diffusivity is investigated both theoretically and experimentally. The solubility of chloroform and dichloromethane in poly(acrylate)s increases, and their diffusivity decreases, compared with that estimated without considering the hydrogen‐bonding effect. Solubilities predicted by the lattice‐fluid hydrogen‐bonding (LFHB) model show good agreement with the results of vapor sorption. Comparison of diffusion coefficients measured by vapor permeation with those predicted from free volume theory reveals that the decrease of solvent diffusion coefficient is approximately proportional to the fraction of associated molecules. Fluxes of chloroform and dichloromethane were measured by vapor permeation experiments through filling‐type acrylate membranes, and predictions agree well with experiments. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 171–181, 2000     

Characterization of water vapor transport in glassy polyacrylonitrile by combined permeation and sorption techniques

Diffusion time lags, steady state permeabilities and sorption/desorption kinetics are reported for water vapor in biaxially oriented, solvent cast polyacrylonitrile (PAN) films. A wide range of vapor activities was studied at 15°C, 30°C, and 45°C. The transient and steady state permeation behavior at low and intermediate upstream vapor activities suggests that Fickian transport occurs under most of the conditions studied. Specifically, time lags predicted by Fick's law using the concentration-dependent diffusion coefficient derived from steady state permeation measurements agree reasonably well with experimentally measured values in most cases. p]Integral sorption/desorption kinetics at low and intermediate vapor activities also appear to be Fickian with a concentration-dependent diffusion coefficient. The form of the concentration dependency, evaluated from the “long time” solution of the diffusion equation for sorption experiments, is consistent with the form established for the diffusion coefficient from the steady-state permeation data. The diffusion coefficient exhibits a maximum near the concentration at which clustering is initiated. Presumably, the effective diffusion coefficient of water increases initially due to plasticizing or dual mode sorption effects associated with gap filling in the glassy matrix. As clustering becomes significant, the effective mobility of water is substantially reduced; therefore, the diffusion coefficient decreases at higher activities as clustering becomes the dominant mode of sorption. p]A tendency of the “early time” sorption/desorption kinetic data to exhibit concavity to the square root time axis at high activities suggests that time-dependent reductions in the diffusion coefficient may be occurring. Such reductions could be related to the kinetics of cluster formation at the higher vapor activities during sorption and to slow polymer consolidation during desorption. Any such non-Fickian effects, related to chain segment relaxations occurring over time scales similar to those of a diffusional jump, appear to be of importance only at short times. The short time nature of any such processes is suggested by the fact that diffusion coefficients evaluated from the “long time” solution to the diffusion equation for sorption are consistent with coefficients evaluated from steady state permeation data, in which case all time-dependent relaxation phenomena should be absent.     

Transport properties of crosslinked acrylonitrile butadiene rubber/poly(ethylene‐co‐vinyl acetate) blends

The diffusion and transport of organic solvents through crosslinked nitrile rubber/poly(ethylene‐co‐vinyl acetate) (NBR/EVA) blends have been studied. The diffusion of cyclohexanone through these blends was studied with special reference to blend composition, crosslinking systems, fillers, filler loading, and temperature. At room temperature the mechanism of diffusion was found to be Fickian for cyclohexanone–NBR/EVA blend systems. However, a deviation from the Fickian mode of diffusion is observed at higher temperature. The transport coefficients, namely, intrinsic diffusion coefficient (D*), sorption coefficient (S), and permeation coefficient (P) increase with the increase in NBR content. The sorption data have been used to estimate the activation energies for permeation and diffusion. The van't Hoff relationship was used to determine the thermodynamic parameters. The affine and phantom models for chemical crosslinks were used to predict the nature of crosslinks. The experimental results were compared with the theoretical predictions. The influence of penetrants transport was studied using dichloromethane, chloroform, and carbon tetrachloride. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1815–1831, 1999     

Opposite trends in thermodynamic compatibility between copolyamide and chitosan in their binary blend

Gibbs energy, enthalpy, and entropy of mixing in binary blends of chitosan with ter‐copolyamide 6/66/610 at ambient conditions have been determined over the entire concentration range using thermodynamic cycle based on dissolution of individual polymers and their blends of different composition in a common solvent – formic acid. Experimental procedure included stepwise equilibrium vapor sorption of glacial formic acid on the cast films and isothermal microcalorimetry of dissolution of these films in liquid glacial formic acid at 25 °C. Formic acid appeared to be a very good solvent for individual polymers and their blends. Flory‐Huggins interaction parameter determined from sorption isotherms was negative and varied from ?2.56 to ?1.79 depending upon blend composition. The enthalpies of dissolution of individual polymers and their blends were strongly exothermic and varied from ?200 to ?40 Joule/g. Independent thermodynamic cycles for Gibbs free energy and enthalpy remarkably revealed similar trends in concentration dependence of different thermodynamic functions of mixing between chitosan and copolyamide. At high chitosan content, the binary blend is characterized by large and negative values of Gibbs free energy, enthalpy, and entropy of mixing that provide high polymer compatibility. On the contrary, at high copolyamide content the blends are incompatible and are characterized by positive values of enthalpy, entropy, and Gibbs free energy of mixing. Such complicated thermodynamic behavior is the result of the superposition of strong molecular interactions (H‐bonds) between polymers in the blend and isothermal fusion of copolyamide crystallites. Thermodynamic analysis correlates well with the data obtained by polarized microscopy. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2603–2613, 2007     

Sorption and diffusion of oxygen in plasticized ethyl cellulose films of varying degrees of substitution

Plasticized films cast from ethyl cellulose were examined to evaluate the effect of the degree of substitution, DS, and the plasticizer content on the sorption and diffusion of oxygen. Sorption and permeation measurements were performed over a temperature range of 25–65°C on three different types of ethyl cellulose in the DS range 1.7–2.5 that had been plasticized with organic esters of comparatively low molecular weight. Sorption coefficients were determined by the pressure decay method, and permeability coefficients were measured independently according to ASTM D-1434-66. The diffusion coefficients were calculated assuming Fickian transport, and were compared to the values directly obtained from the evaluation of the sorption kinetics. The permeability coefficient indicates that there is a significant improvement of the barrier properties of the materials when the DS is reduced and when the plasticizer content is at the absolute minimum required. It was found that the variation in the magnitude of the permeability coefficient is related to the value of the diffusion coefficient, which is governed by the chemical composition of the mixtures. In contrast, the solubility of oxygen was determined by the physical state of the polymer matrix and increased rapidly at temperatures significantly below the glass transition temperature. Using an ergodic model, the diffusion coefficients obtained were related to the size distribution of microvoids in the materials and relative values for the diffusion coefficient were computed as a function of DS and temperature. The model calculates the concentration (number per volume) of voids that are large enough to be occupied by a penetrant molecule. It was assumed that the unoccupied volume fraction as a function of the cohesive energy density follows a Boltzmann distribution. The cohesive energy density and the unoccupied volume fraction of the polymer-plasticizer mixtures were calculated by fitting the Simha-Somcynsky equation of state to pressure-volume-temperature data. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys, 35: 639–653, 1997     

Polymer chain diffusion in polymer blends: A theoretical interpretation based on a memory function formalism

The diffusion of polymer chains in miscible polymer blends with large dynamic asymmetry—those where the two blend components display very different segmental mobility—is not well understood yet. In the extreme case of the blend system of poly(ethylene oxide) (PEO) and poly(methyl methacrylate)(PMMA), the diffusion coefficient of PEO chains in the blend can change by more than five orders of magnitude while the segmental time scale hardly changes with respect to that of pure PEO. This behavior is not observed in blend systems with small or moderate dynamic asymmetry as, for instance, polyisoprene/poly(vinyl ethylene) blends. These two very different behaviors can be understood and quantitatively explained in a unified way in the framework of a memory function formalism, which takes into account the effect of the collective dynamics on the chain dynamics of a tagged chain. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 1239–1245     

Fourier transform infrared study on the sorption of water to various kinds of polymer thin films

The state of sorbed water and the sorbing processes of water to various polymer thin films were studied with Fourier transform infrared (FTIR) spectroscopy. To prepare the polymer films, we used poly(ethylene glycol)s of different molecular weights and various kinds of vinyl polymers, such as poly(2‐methoxyethyl acrylate). The O? H stretching band of water sorbed in the films increased gradually on contact with water vapor at 50% relative humidity and leveled off. When O? H stretching bands of water sorbed to polymer films were compared, the peak positions and profiles of water sorbed to the polymeric materials with the same hydrogen‐bonding site were similar. A hybrid density‐functional method supported the assignment of the peaks. Furthermore, the diffusion coefficient (D) of water vapor in the polymer films was estimated by time‐resolved measurements of the sorbed water at the very initial stage (0–830 s). It was clearly shown that the D values of water vapor in the polymer materials with a strong hydrogen‐bonding site were smaller than those in hydrophobic polymers. The usefulness of the FTIR technique to investigate water sorption to polymer materials was definitely demonstrated. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2175–2182, 2001     

Miscibility of polysulfone blends with poly(1‐vinylpyrrolidone‐co‐styrene) copolymers and their interaction energies

The miscibility of polysulfone (PSf) with various hydrophilic copolymers was explored. Among these blends, PSf gave homogeneous mixtures with poly(1‐vinylpyrrolidone‐co‐styrene) [P(VP–S)] copolymers when these copolymers contained 68–88 wt % 1‐vinylpyrrolidone (VP). Miscible PSf blends with P(VP–S) copolymers underwent phase separation on heating caused by lower critical solution temperature (LCST)‐type phase behavior. The phase behavior depended on the copolymer composition. Changes in the VP content of P(VP–S) copolymers from 65 to 68 wt % shifted the phase behavior from immiscibility to miscibility and the LCST behavior. The phase‐separation temperatures of the miscible blends first increased gradually with the VP content, then went through a broad maximum centered at about 80 wt % VP, and finally decreased just before the limiting content of VP for miscibility with PSf. The interaction energies of binary pairs involved in PSf/P(VP–S) blends were evaluated from the phase‐separation temperatures of PSf/P(VP–S) blends with lattice‐fluid theory combined with a binary interaction model. The decrease in the contact angle between water and the membrane surface with increasing VP content in P(VP–S) copolymers indicated that the hydrophobic properties of PSf could be improved via blending with hydrophilic P(VP–S) copolymers. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1401–1411, 2003