Differential permeation. Part II: Behaviors of polyvinylalocohol films in transient permeation

The variations of the permeation rate of ethanol in ethanol-water mixtures through poly(vinylalcohol) membranes of different crystallinities were studied in the transient regime. We observed an anomalous two-wave kinetics, which was accounted for in terms of a model postulating two diffusion-pathways for ethanol molecules in the polymer. One wave corresponds to the permeation of ethanol through the amorphous polymer matrix, which occurs whatever the water content in the mixture. The other wave appears only when the water content in the mixture exceeds a threshold. The higher the crystallinity, the higher the value of this threshold. This wave is attributed to the diffusion of ethanol molecules through clusters formed with the water molecules sorbed on the hydroxyl sites.     

Solvent-polymer interaction: I. Molecular transport of some selected organic liquids in polymer membrane

Methods and microtechniques for determining solubility, diffusivity, thermodynamic properties, and kinetic parameters of 12 selected organic liquid solvents in polyurethane membrane by thermogravimetry (TG) are described. TG provides a simple, sensitive, rapid, and accurate microtechnique for measuring a minute change in weight (or mass) of a substance as a function of time at isothermal condition or as a function of temperature at dynamic manner. Thus from a single isothermal TG-desorption experiment, solubility and diffusivity of solvent molecules in polyurethane membrane were obtained simultaneously. Furthermore, by a dynamic TG-desorption run, kinetic parameter such as activation energy of desorption of solvent molecules from polyurethane membrane was determined. In addition, much other useful information such as equilibrium sorption constant, the changes in standard enthalpy and standard entropy of sorption, permeability, the activation energy of diffusion and so forth for solvents in polyurethane membrane are also evaluated and discussed. Finally, the correlation between the microscopic molecular structure and macroscopic properties of solvent molecules in polymer membrane is interpreted in terms of linear free energy relationships.     

Molecular Filtration of Hydrocarbon Isomers Through Polymer/[Liquid Crystal] Composite Membranes

Ultrathin membranes of a polymer/(liquid crystal) mixture were prepared by spreading a single drop of a casting solution on the water surface. The thickness and the aggregation state of the water-cast membrane can be controlled by the kind of solvent and the concentration of the solution. In the case of a liquid crystalline state above the crystal-nematic phase transition temperature, T _KN, the polymer (liquid crystal) composite membrane follows Henry's law for the sorption isotherm of hydrocarbon gases and, also, Fickian sorption for the sorption-desorption kinetics. These results indicate that hydrocarbon gases permeate through a homogeneous medium composed of liquid crystalline molecules. Therefore, the permeability coefficients of hydrocarbon gases can be controlled by the dimensions of the channels through which the gas molecules diffuse. The channel for diffusion is generated by thermal or fluctuating molecular motion which opens up the intermolecular distance between liquid crystalline molecules. In the case of a self-supported liquid crystalline membrane, the channel dimension can be controlled in the range of several Å by both the intermolecular distance and the degree of thermal molecular motion of the liquid crystalline molecules. Separation of hydrocarbon isomers was investigated by use of composite membranes composed of a polymer matrix and self-supported liquid crystalline molecules.     

Permeation,sorption, and diffusion of water in ethyl cellulose

The permeability of ethyl cellulose to water vapor and liquid water was measured as a function of temperature. A change of slope was found in the Arrhenius plots at about 50°C., close to the glass transition. The sorption isotherms showed essentially zero heat of mixing in agreement with other workers. The diffusion constants were measured in four ways, viz., sorption, desorption, time lag, and by dividing the permeability constants by the equilibrium solubility coefficients. The time lag method gave diffusion constants which were independent of concentration, whereas the other three methods led to diffusion constants which steadily decreased with concentration. All the methods, however, extrapolated to about the same value at zero concentration. The decreasing diffusivities are believed to be due to the clustering of water molecules in the polymer. However, no clustering appeared to take place under the conditions of the time lag measurements.     

Solvent sorption measurements in polymeric membranes with ATR-IR spectroscopy

Long-term stability and performance of polymeric membranes in solvent and mixed solvent media can be reduced due to sorption and swelling of the membrane matrix. For this reason quantification of sorption and swelling is of major importance for the development of future applications of membrane processes in solvent and mixed solvent media. In this work a method is discussed, based on attenuated total reflectance infrared spectroscopy (ATR-IR), to establish sorption and sorption selectivity of a cellulose acetate (CA) membrane in water/methanol and water/ethanol mixtures. By analysis of specific peaks from the ATR-IR spectra of the solvents, the preferential sorption of water in CA membranes can be quantified. In the presence of methanol, the selectivity for water ranges from 2.5 to 3.5 between 52 and 90% of methanol. For ethanol, the selectivity for water ranges from about 1 (30% ethanol) to 2 (90% ethanol). From the work it follows that ATR-IR provides an easy and non-destructive method to study the sorption behavior of the polymeric membrane separation layer.     

Structure and dynamics of halogenoethanol-water mixtures studied by large-angle X-ray scattering, small-angle neutron scattering, and NMR relaxation

To clarify the structure of solvent clusters formed in halogenoethanol-water mixtures at the molecular level, large-angle X-ray scattering (LAXS) measurements have been made at 298 K on 2,2,2-trifluoroethanol (TFE), 2,2,2-trichloroethanol (TCE), and their aqueous mixtures in the TFE and TCE mole fraction ranges of 0.002 < or = x(TFE) < or = 0.9 and 0.5 < or = x(TCE) < or = 0.9, respectively. The radial distribution functions (RDFs) for TFE-water mixtures have shown that the structural transition from inherent TFE structure to the tetrahedral-like structure of water takes place at x(TFE) approximately 0.2. In the TCE-water mixtures inherent TCE structure remains in the range of 0.5 < or = x(TCE) < or = 1. Small-angle neutron scattering (SANS) experiments have been performed on CF(3)CH(2)OD- (TFE-d(1)-) D(2)O and CF(3)CD(2)OH- (TFE-d(2)-) H(2)O mixtures in the TFE mole fraction range of 0.05 < or = x(TFE) < or = 0.8. The SANS results in terms of the Ornstein-Zernike correlation length have revealed that TFE and water molecules are most heterogeneously mixed with each other in the TFE-water mixture at x(TFE) approximately 0.15, i.e., both TFE clusters and water clusters are most enhanced in the mixture. To evaluate the dynamics of TFE and ethanol (EtOH) molecules in TFE-water and ethanol-water mixtures, respectively, (1)H NMR relaxation rates for the methylene group within alcohol molecules have been measured by using an inversion-recovery method. The alcohol concentration dependence of the relaxation rates for the TFE-water and ethanol-water mixtures has shown a break point at x(TFE) approximately 0.15 and x(EtOH) approximately 0.2, respectively, where the structural transition from alcohol clusters to the tetrahedral-like structure of water takes place. On the basis of the present results, the most likely structure models of solvent clusters predominantly formed in TFE-water and TCE-water mixtures are proposed. In addition, effects of halogenation of the hydrophobic groups on clustering of alcohol molecules are discussed from the present results, together with the previous ones for ethanol-water and 1,1,1,3,3,3-hexafluoro-2-propanol- (HFIP-) water mixtures.     

Equilibrium desorption isotherms of water,ethanol, ethyl acetate,and toluene on a sodium smectite clay

Desorption isotherms for water, ethanol, ethyl acetate and toluene from a sodium smectite clay have been determined by both dynamic vapor sorption (DVS) measurements and Knudsen thermogravimetry (KTGA), at the exception of toluene that was measured only by the DVS method. The results obtained using these two methods were in satisfactory agreement, providing reliable insight into the desorption process, with certain lack of precision for ethyl acetate. The observed desorption behaviour suggests a liquid like phase at high volatile load, and a sorbed state in which molecules interact with the counter ions, at low volatile contents. However, the isotherms for water determined at various temperatures nearly superposed when plotted as a function of water activity, indicating the strength of the interactions in the clay–water system to remain of the same order of magnitude as that in bulk water, consistent with previous ab initio calculations.     

Preferential sorption and permeation of binary liquid mixtures in poly (vinyl chloride) membrane by pervaporation

Preferential sorptions and pervaporation selectivities in poly (vinyl chloride) (PVC) membrane for various binary liquid mixtures were investigated. Methanol/n-propanol, benzene/n-hexane, and ethanol/water mixtures were selected as the binary liquid mixture. In the methanol/n-propanol mixture, methanol was preferentially sorbed in the PVC membrane and predominantly permeated. In the benzene/n-hexane mixture, benzene was incorporated and permeated preferentially. In the ethanol/water mixture, ethanol was preferentially sorbed in the PVC membrane and water was preferentially permeated. The preferential sorptions were analyzed according to Mulder's model derived from Flory-Huggins thermodynamics. The pervaporation selectivity in these systems were discussed using a sorption selectivity and diffusion selectivity. © 1995 John Wiley & Sons, Inc.     

On the mechanism of separation of ethanol/water mixtures by pervaporation II. Experimental concentration profiles

Ethanol—water concentration profiles in cellulose acetate membranes were measured under steady-state pervaporation conditions. Knowledge of these profiles leads to a better understanding of the diffusion process during pervaporation. The concentration profiles were determined by a film-stack method, using three to six layers. It is shown that permeation of ethanol—water mixtures proceeds in a coupled way and that crossterm diffusion coefficients need to be considered. Furthermore, the occurrence of sorption resistances at the feed/membrane interface can be established from these experiments.     

Surfacial,liquid sorption and monolayer-forming properties of hydrophilic and hydrophobic Stöber silica particles

The surface of silica spheres with a diameter of 500 nm was modified with ethoxysilane. Hydrophilic and partially hydrophobic silica spheres were obtained, suitable for the preparation of two-dimensional monoparticle films at the liquid-air interface. The tendency of these particles to self-assemble is basically dependent on surface hydrophobicity. Liquid sorption excess isotherms were studied in ethanol-cyclohexane and ethanol-chloroform mixtures with the aim of characterizing the adsorption capacity of the particles. Specific surface area and porosity were measured by nitrogen adsorption. The specific surface area determined by liquid sorption was considerably larger than determined by gas adsorption. This is ascribed to penetration of ethanol into the pores and the swelling of the silica particles in ethanol. Surface modification of hydrophilic particles changed the film-forming properties of the particles. The compressibility and the lift-off area of the monolayer films of hydrophobic particles on water were higher than for the films of hydrophilic particles.     

Hydration mechanism of polysaccharides: A comparative study

Water sorption was studied at 20 °C on films composed of different natural polymers. Three polysaccharides were investigated: chitosan, cellulose, and alginate. The major differences between these polymers, from a structural point of view, lay in the substitution of an OH group by an NH₂ function for chitosan and by an ionic COO^?Na⁺ group for alginate. An analysis of the experimental water sorption isotherms, expressed as the number of water molecules sorbed per repeating unit in the amorphous phase, associated with an analysis of the enthalpy profile related to the water sorption allowed us to propose a water sorption mechanism in two steps for all the polymers: water sorption on polymer‐specific sites in the first step and water clustering around the first sorbed water molecules in the second step. It was determined that two water molecules interacted with the polymer chains for cellulose and chitosan, whereas four water molecules were bonded to alginate chains. The specific sorption sites were identified as OH groups for cellulose, OH and NH₂ groups for chitosan, and ionic and OH groups for alginate. A systematic reduction of the half‐sorption time was observed in the activity range corresponding to this first sorption step, and it was explained by a water plasticization effect. On the other hand, an increase in the half‐sorption time was observed in the second sorption step, at a high activity (>0.8), for chitosan and alginate. A modelization associating the Guggenheim–Anderson–de Boer model and the clustering theory, applied to our systems, allowed us to relate the occurrence of this last phenomenon to the formation of water clusters containing more than two water molecules. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 48–58, 2005     

On the mutual diffusion properties of ethanol-water mixtures

The structural organization, the number of hydrogen bonds (H bond), and the self- and mutual diffusion coefficients of ethanol-water mixtures were studied by molecular dynamics simulation. It was found that both the numbers of H bonds per water and per ethanol decrease as the mole fraction of ethanol increases. The composition dependences and the relationships between the self- and the mutual diffusion coefficients were further discussed. The self-diffusion coefficient of water has a large drop as the concentration of ethanol increases from 0 to 0.3 and then it nearly keeps constant, while that of ethanol has a minimum around ethanol mole fraction of 0.5. The mutual diffusion coefficient could be divided into two parts, the kinematic factor and the thermodynamic factor. Both the kinematic and thermodynamic factors for ethanol-water mixtures were calculated. It was found that the change trend of mutual diffusion coefficients with the composition is mainly dependent on the thermodynamic factors.     

Mass and momentum transfer in polymers. III. Effects of liquid penetrants on tensile stress relaxation of amorphous polymers

Sorption, diffusion, swelling, and tensile stress relaxation measurements were made at room temperature (23°C) for the systems poly(n-butyl methacrylate) (PBMA) with liquid methanol and ethanol, and poly(methyl acrylate) (PMA) with liquid water. Stress relaxation curves for the fully swollen polymers could be superimposed approximately with those for the dry polymers by appropriate shifting along the long axes. For PMA–water the measured curve for stress relaxation with concurrent sorption could be predicted accurately by using a moving boundary theory with data measurements of stress relaxation of the unswollen and swollen polymer combined with sorption data. The modified moving boundary theory is generalized to include the effects of dimension changes through swelling and the larger effects of plasticization associated with sorption of liquids. This improved theory accurately predicts measured curves of stress relaxation with concurrent sorption for the PBMA–alcohol systems from individual stress relaxation, sorption, diffusion and swelling data. The general approach should be applicable to other amorphous polymer–liquid swelling agent systems. The anisotropic nature of swelling of polymer films and its effect on calculated diffusion coefficients are discussed briefly.     

沉淀聚合制备聚(季戊四醇三丙烯酸酯-苯乙烯)单分散微球及其形成机理

以季戊四醇三丙烯酸酯(PETA)作交联剂,苯乙烯作共聚单体,偶氮二异丁腈作引发剂,在乙醇或其与水的混合溶剂中沉淀聚合制备了交联聚合物微球.研究了反应时间、交联剂用量以及溶剂中水含量对聚合过程及微球的影响.结果表明当PETA用量在单体质量的5%-35%之间且反应时间不低于6h时可制得单分散聚合物微球.当PETA用量低于20%时,所得微球的粒径随PETA用量的增加逐渐减小,粒径分布逐渐变窄;此后继续提高PETA用量,微球粒径又逐渐增大,粒径分布逐渐变宽.向反应介质中加入水,可明显提高微球产率及单体转化率,但其体积分数达30%时,所得微球分散性变宽.在此基础上对微球的形成机理也进行了讨论.     

Solute-solvent friction kernels and solution properties of methyl oxazoline-phenyl oxazoline (MeOx-PhOx) copolymers in binary ethanol-water mixtures

Solvent mixtures often alter the solubility of polymeric substances. Statistical copolymers made from 2-methyl-2-oxazoline (MeOx) and 2-phenyl-2-oxazoline (PhOx) are known for their varying solubilities in pure ethanol, pure water and in binary mixtures of ethanol-water. Constrained Molecular Dynamics (MD) simulations have been carried out with an aim to explain the varying solubilities of the statistical MeOx-PhOx copolymers. The solute-solvent dynamic friction kernels calculated through constrained MD simulations corroborate the solubility pattern in these copolymers. The solvation characteristics have been analyzed in terms of the solute-solvent radial distribution functions (RDFs). The ethanol-soluble MeOx-PhOx copolymers exhibit characteristic solute-composition dependence in the dynamic solute-solvent friction kernels, indicating the strength of the solute-solvent correlations. The aggressive solvation by the ethanol molecules in the binary solvent mixtures has been brought out by the O(solute)-H(ethanol) RDFs which exhibit a characteristic dependence on the ethanol content in the solvent composition. The corresponding O(solute)-H(water) RDFs are devoid of any such composition dependence. For all the MeOx-PhOx copolymers, the O-site solvation is strongly dominated by the water molecules and the N-sites are solvated equally by both ethanol and water molecules.     

Asymmetric membrane formation via immersion precipitation method. I. Kinetic effect

The kinetic effect of the phase inversion process on the membrane morphology is explored, with emphasis on the diffusion coefficient of the nonsolvent as a measure of the solvent/nonsolvent exchange rate. The diffusion coefficient is closely related to the nonsolvent tolerance of the polymer solution, which was estimated from a pseudo-ternary phase diagram of the following system: polymer: polysulfone; solvent system: a mixture of the solvent 1-methyl-2-pyrrolidinone and a solvent additive (formic acid, water or ethanol); and nonsolvent: ethanol. Regardless of the kind of solvent additive employed, when the diffusion coefficient of the nonsolvent is high for a given gelation medium, then the membrane consists of a smooth, defect-free surface and macrovoid-free cross section, and is highly permeable to oxygen. However, using a polymer solution with a low diffusion coefficient results in a membrane of a rather defective morphology. Therefore, it is concluded that the diffusion coefficient of the nonsolvent is a crucial parameter in controlling membrane morphology.     

Sign change of the Soret coefficient of poly(ethylene oxide) in water/ethanol mixtures observed by thermal diffusion forced Rayleigh scattering

Soret coefficients of the ternary system of poly(ethylene oxide) in mixed water/ethanol solvent were measured over a wide solvent composition range by means of thermal diffusion forced Rayleigh scattering. The Soret coefficient S(T) of the polymer was found to change sign as the water content of the solvent increases with the sign change taking place at a water mass fraction of 0.83 at a temperature of 22 degrees C. For high water concentrations, the value of S(T) of poly(ethylene oxide) is positive, i.e., the polymer migrates to the cooler regions of the fluid, as is typical for polymers in good solvents. For low water content, on the other hand, the Soret coefficient of the polymer is negative, i.e., the polymer migrates to the warmer regions of the fluid. Measurements for two different polymer concentrations showed a larger magnitude of the Soret coefficient for the smaller polymer concentration. The temperature dependence of the Soret coefficient was investigated for water-rich polymer solutions and revealed a sign change from negative to positive as the temperature is increased. Thermodiffusion experiments were also performed on the binary mixture water/ethanol. For the binary mixtures, the Soret coefficient of water was observed to change sign at a water mass fraction of 0.71. This is in agreement with experimental results from the literature. Our results show that specific interactions (hydrogen bonds) between solvent molecules and between polymer and solvent molecules play an important role in thermodiffusion for this system.     

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.     

Interaction between water and polymer chains in poly(hydroxyethyl acrylate) hydrogels

Polymer networks of different cross-linking densities were prepared by copolymerisation of hydroxyethyl acrylate and ethylene glycol dimethacrylate. The average molecular weight between cross-links as well as the polymer chain mobility were characterised by means of dynamic–mechanical spectroscopy. Equilibrium sorption isotherms and the water uptake in immersion in liquid water allowed the determination of the Flory–Huggins interaction parameter between water molecules and polymer chain segments, which decreased with the water activity in the hydrogel and increased with the cross-linking density as a consequence of the hydrophobic character of the cross-linking agent. Dynamic sorption and desorption experiments were used to determine the diffusion coefficient. Received: 11 November 1999 Accepted: 28 July 2000