Permeation and sorption in inhomogeneous polymer membranes

Restricting the discussion to isothermal transport of a single penetrant in a polymer sample, we define convection and diffusion. We distinguish carefully between diffusion in homogeneous and inhomogeneous samples. In the latter case, the diffusion flux is no longer directly proportional to the gradient of the concentration, i.e., Fick's first law does not hold. We point out, unless an external force responsible for convection can be identified, that diffusion with convection in a homogeneous medium is in many instances indistinguishable mathematically from diffusion in an inhomogeneous medium with microscopic inhomogeneities. Finally, we list a number of exact results available for permeation and sorption parameters for thin slab of a dilute inhomogeneous material. These parameters all yield the same value of the diffusion coefficient if the latter is constant. On the other hand, these parameters provide different functional information about the partition coefficient and diffusion coefficient for an inhomogeneous polymer-penetrant system.     

Permeation of single and mixed gases through composite membranes prepared by plasma polymerization of fluorocarbon compounds

The permeations of simple permanent gases and their mixtures through plasma-polymerized composite membranes were studied. Composite membranes were prepared by the plasma polymerization of fluorocarbon monomers such as pentafluorotoluene (PFT) and pentafluoropyridine (PFP) onto porous Celgard. For pure gases, the permeability coefficients were mainly affected by diffusivity rather than solubility. The permeability coefficient decreased as the kinetic molecular diameter of the penetrant molecules increased, and the permeability coefficients were independent of pressure. For mixed gases, the permeability coefficient was not affected by the composition of penetrants for the whole range of composition.     

Permeation of molecules through different polymeric membranes

Permeation of molecules through the membranes plays an important role in immobilized cell systems. Permeation of molecules like glucose and bovine albumin serum was studied through chitosan, polyvinyl alcohol, and polyvinyl acetate membranes using a flow cell made of two detachable compartments. Permeation of molecules through chitosan, polyvinyl alcohol, and polyvinyl acetate membranes increases with decrease in hydrophobic characteristics of the membrane. Permeation of molecules also show its dependency on the molecular weight of the solute. Distribution coefficient of glucose and bovine serum albumin in these polymeric membranes measured by equilibration technique indicates that permeation of molecules through these membranes follows pore type mechanism. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 3469–3472, 1999     

Permeation of gases through modified polymer films. I. Polyethylene–styrene graft copolymers

The permeabilities of nitrogen, oxygen, and carbon dioxide through polyethylene–styrene graft copolymer films were measured by means of a gas permeability apparatus based on a modification of Barrer's high vacuum technique. Polyethylene–styrene grafts were prepared by mutual γ-ray irradiation of low-density polyethylene films in styrene–methanol solution. Densities and thicknesses of the graft copolymer films were determined. It was observed that the gas permeability constants decreased with increasing grafting to minimum values at 20–30% styrene grafting and increased again above 30% grafting. These results are explained in terms of a decrease in the free volume of the amorphous regions of the polyethylene by a “filling in” effect of the grafted polystyrene chains. Above 30% grafting, disruption of the crystallites may occur resulting in increased gas permeation. Activation energies for gas permeation through polyethylene–styrene graft copolymer films were calculated and found to decrease with increasing per cent styrene grafting. For nitrogen permeation, the activation energy decreased from 11.7 kcal/mole for unirradiated polyethylene to 9.5 kcal/mole for a 50.5% graft. Corresponding values for oxygen and carbon dioxide were 10.2–8.2 kcal/mole for a 48.7% graft and 8.4–6.5 kcal/mole for a 50.5% graft.     

Permeation of benzene through liquid surfactant membranes

The effect of time on the permeation of benzene through aqueous surfactant membranes formed with two non-ionic surfactants has been studied on a batch scale under two different conditions of surfactant concentration and hydrocarbon: water ratio in the emulsions. The data have been correlated by an equation based on Casamatta's model of hydrocarbon permeation from an emulsion drop. The results indicate that the correlation can be used to treat the data until the onset of membrane rupture. The membrane thickness depends on the hydrocarbon: water ratio in the emulsion, and varies from about 1.57 to 5.10 μm.     

Permeation of propane and propylene thrugh cellulosic polymer membranes

Cellulosic polymers were tested by measuring their permeabilities of propane and propylene. High permeabilities and selectivities were obtained with ethyl cellulose membranes. Hollow fibers coated with ethyl cellulose were made through a solution coating method. Microporous polypropylene hollow fibers were used as base material. A dense layer of ethyl cellulose about 6–10 μm in thickness was formed on the microporous hollow fiber. The performance of a coated membrane column was tested for the separation of propane and propylene mixture gas. It was concluded that ethyl cellulose membranes have a good potential for the separation of olefins form saturated hydrocarbons.     

Permeation of solutes through chemically modified chitosan membranes

Chemically and mechanically stable chitosan and N-acylchitosan were prepared. N-Octanoylchitosan gives the pronounced decrease of water content, whereas N-benzoylchitosan keeps a higher content of water. The equilibrium sorption of various sodium salts was determined and is explained by a dual mechanism, consisting of partition and Langmuir sorption. The characteristics of the membrances and the size of the salt anions clearly influence permeability as well as sorption. Permeation behavior was interpreted by the Teorell-Meyer-Sievers (TMS) theorem. The mobility of anions decreases with Stokes radius, decreasing in the order chitosan > N-benzoylchitosan > N-octanoylchitosan, indicating the effect of the polymer chain molecules, especially a steric obstruction effect on the ion transportation through the membranes. The introduction of the N-acylated hydrophobic group to the charged chitosan membranes appears to enhance permselectivity. © 1995 John Wiley & Sons, Inc.     

Permeation of water through some heterogeneous hydrophilic membranes

The permeation of water through glyceryl methacrylate (2,3-dihydroxypropyl methacrylate) and 2hyphen;hydroxyethyl methacrylate hydrogels was increased by the incorporation of silica fillers. Both the amount of water at equilibrium and the amount of filler in the gel contribute to increase the permeability of these membranes.     

Proton-conducting polymer electrolyte membranes based on hydrocarbon polymers

This paper presents an overview of the synthesis, chemical and electrochemical properties, and polymer electrolyte fuel cell applications of new proton-conducting polymer electrolyte membranes based on hydrocarbon polymers. Due to their chemical stability, high degree of proton conductivity, and remarkable mechanical properties, perfluorinated polymer electrolytes such as Nafion^®, Aciplex^®, Flemion^®, and Dow membranes are some of the most promising electrolyte membranes for polymer electrolyte fuel cells. A number of reviews on the synthesis, electrochemical properties, and fuel cell applications of perfluorinated polymer electrolytes have also appeared during this period. While perfluorinated polymer electrolytes have satisfactory properties for a successful fuel cell electrolyte membrane, the major drawbacks to large-scale commercial use involve cost and low proton-conductivities at high temperatures and low humidities. Presently, one of the most promising ways to obtain high performance proton-conducting polymer electrolyte membranes is the use of hydrocarbon polymers for the polymer backbone. The present review attempts for the first time to summarize the synthesis, chemical and electrochemical properties, and fuel cell applications of new proton-conducting polymer electrolytes based on hydrocarbon polymers that have been made during the past decade.     

Studies on syntheses and permeabilities of special polymer membranes: 33. Permeation behaviour of aqueous alcohol solution through cellulose membranes

The permeabilities of various aqueous alcohol solutions through cellulose membranes were investigated by changing the preparation conditions of membranes, the feed concentration, and the feed solute, etc. The permeation rates for aqueous solutions of alcohols, glycols, glycerol, trihydroxyl benzene were greater than for pure water. This permeation phenomenon could be explained by some permeation models considering water cluster, activation of water molecules (second bound water-like) attached weakly to the bound water in cellulose membrane, and the surface of cellulose membrane, and plasticization of cellulose molecules.     

Permeation of vanadium cations through anionic and cationic membranes

The permeability of four commercially available ion-exchange membranes, the cationic Nafion 125 (duPont) and Selemion CMV (Asahi Glass) membranes and the anionic Selemion AMV and DMV membranes, to vanadyl, VO²⁺, and vanadic, VO ₂ ⁺ , ions was studied. The results show that two important variables determine the usefulness of a membrane as a membrane separator in a redox cell. These are selectivity and membrane-electrolyte resistance. Only the DMV membrane was considered to meet the requirements of low permeability to vanadium cations and at the same time permitting H⁺ ions to go through the membrane, thereby providing a very low membrane-electrolyte resistance in the redox fuel cell.     

Permeation of simple gases through plasma polymerized films from fluorine-containing monomers

The permeation of several different permanent gases through plasma polymer membranes was studied. The plasma polymers were produced by a microwave discharge of various fluorine-containing monomers. Plasma polymers made from fluorine-rich monomers (F/C ≈ 1) yield films with the highest selectivities; those made from low fluorine content monomers show very little selectivity among the various gases. CO₂ solubility is very high in the fluorine-rich plasma polymers and gives rise to large CO₂ to CH₂ selectivities. © 1995 John Wiley & Sons, Inc.     

Permeation of sodium dodecyl sulfate through polyaniline-modified cellulose acetate membranes

The preparation of polyaniline (PANi)-cellulose acetate (CA) blends by casting films from a suspension, is reported. Two membranes were prepared from different solvents, one with a homogeneous and the other a heterogeneous dispersion of PANi in CA matrices. The membranes were characterized by X-ray diffraction, SEM, DSC, and FTIR, and the results were compared with those obtained for pure CA and PANi films. The transport properties of water and sodium dodecyl sulfate (SDS) in membranes of the PANi-CA blends and of CA were analysed. The transport of SDS and water depends on both the bulk/polymer density and the PANi content. In the homogeneous blend, the interaction between SDS and the polymer plays an important role in the transport mechanism. An irreversible interaction is observed, which can be monitored by UV-vis spectroscopy. The spectra of homogeneous, highly transparent PANi-CA blends show a pronounced sensitivity to SDS concentration, with detection limits [SDS]≥0.1 mM for films with a PANi concentration of 0.05% w/v.     

Permeation of organic vapors through polymer in the condensation region

Permeability coefficients have been measured for normal butane, isobutane, isobutylene, and butene-1 in Zendel Copolymer-I at temperatures between 30° and 70°C and at penetrant pressures p₁ up to 5.5 atm. The permeability coefficients of these organic vapors show behavior analogous to that observed for C₃ and C₂ hydrocarbons as detailed in our earlier work; i.e., near the condensation point of the penetrant, P increases as the temperature is decreased. Isothermal plots of log P versus p₁ in that region are generally linear and can be represented by empirical relations of the form where P₀ is a constant. The slope a is a function of temperature: where a₀ is a constant and b has the same value for the four hydrocarbons investigated.     

Solute diffusion through swollen polymer membranes

Experiments were designed to study the role of the solvent in the transport of a solute through a solvent-swollen polymer membrane. A single solute (an organic dye), a single polymer (cross-linked natural rubber), and 24 different organic solvents were used for this purpose. The solute diffusion coefficient D was calculated from the measured permeability P and distribution coefficient K, and was compared to the diffusion coefficient of the solute in the pure solvent. The main parameters of the solvent were shown to be its viscosity and the degree it swells the polymer. At high swelling, the results are in agreement with a model that pictures the resistance to solute diffusion as hydrodynamic interaction with the solvent while the polymer acts as an obstruction that increases the tortuosity of the diffusion path. At very low swelling, the diffusion coefficient approaches an asymptotic limit which is independent of solvent viscosity. However, even with as low as 10% solvent, some effects of viscosity are still seen. These results are discussed in terms of a quantitative theory for the obstruction effect proposed by Meares and compared to other literature data.     

Separation of liquid mixtures by using polymer membranes. I. Permeation of binary organic liquid mixtures through polyethylene

The permeation characteristics and the separation behavior of 25 combinations of binary liquid mixtures through low-density polyethylene membrane have been investigated. The organic compounds studied were members of the homologous series of liquid aliphatic hydrocarbons between n-pentane and n-nonane as well as some aromatic and cyclic compounds. A special permeation cell was designed in order to study permeation rates at different temperatures ranging from 25 to 45°C. The rate of permeation increased with temperature, and it was found that the temperature dependence of the permeation rate for both pure compounds and mixtures could be expressed by Arrheniustype relationships. The efficiency of separation, however, decreased with increasing temperature. Activation energies of permeation rànged from 16–22 kcal/mol for pure compounds and binary mixtures of benzene, n-hexane, cyclohexane, and 2–2-dimethylbutane. The effects of chemical nature, molecular size, and molecular shape of the diffusing species on the permeation and separation were studied and qualitative guidelines were suggested. The effect of the composition of the binary mixtures on the permeation rate has been investigated for several systems. Permeation enhancement effects were observed in which the mixtures permeate considerably faster than either of the pure components. Maximum permeation rates occurred at about 50 wt-% mixtures for the systems benzene–n-hexane and benzene–cyclohexane. This phenomenon is explained in terms of a combined internal plasticizing and solubility effect.     

Permeation of oxygen through polyurethane–polyepoxide interpenetrating polymer networks

Oxygen permeation studies on polyurethane (PU)/polyepoxide (EP) interpenetrating polymer networks show that the increased crosslinking density owing to additional permanent chain entanglement (resulting from interpenetration) can decrease the coefficients of permeation, diffusion, and oxygen solubility. At 20% PU, at which the crosslinking density is maximum, these coefficients retain minimum values, while the tensile strength retains a maximum value.     

Permeation and separation of organic liquid mixtures through liquid-crystalline polymer networks

A side-chain liquid-crystalline polymer (LCP) was synthesized by the addition of the mesogenic monomer to poly(methyl siloxane) in presence of a Pt-catalyst. When an aqueous solution of 10wt% ethanol was permeated through a LCP membrane by pervaporation at various temperatures, the permeation rate increased with increasing temperature and drastically changed at glass-nematic (Tg) and nematic-isotropic (TNI) transition temperatures of the LCP membrane. The LCP membrane exhibited the waterpermselectivity in the glassy and liquid-crystalline states. The ethanol concentration in the permeate increased with increasing permeation temperature and the LCP membrane changed from the waterpermselectivity to the ethanol-permselectivity around TNI. These results suggested that the permselectivity was influenced by the change of the LCP membrane structure, that is, its state transformation. It was found that a balance of the orientation of mesogenic groups and flexibility of siloxane chains is very important for the permeability and selectivity.     

Permeation of gases through modified polymer films. V. Permeation and diffusion of helium,nitrogen, methane,ethane, and propane through γ-ray crosslinked polyethylene

The permeation and diffusion of helium, nitrogen, methane, ethane, and propane through γ-irradiated polyethylene films were investigated. These studies were carried out with two objectives in mind: (1) to determine the effect of crosslinking by γ irradiation on permeability and diffusivity using the gas molecules as molecular probes; and (2) to study the plasticizing effects of the low hydrocarbons on the polyethylene film. The γ-ray-induced crosslinking efficiency of polyethylene was investigated in the following irradiation atmospheres: vacuum, acetylene, and nitrogen–acetylene mixtures. Results showed that irradiation in acetylene decreased the crosslinking efficiency while an acetylene–nitrogen atmosphere increased the efficiency compared to irradiation in vacuum. Both the permeation constants and the diffusion coefficients were found to decrease with increasing irradiation dose while the activation energies increased. The permeation constants of the organic gases through polyethylene increased with molecular diameter while the diffusion coefficients decreased. This increase in permeability was attributed to an increase in the solubility due to solubilization of the membrane by the penetrant. For example, the molecular diameter of propane is 4.397 Å compared with 2.807 Å for methane; however, propane permeated the polyethylene film at a rate twice that of methane. Nitrogen and methane have approximately the same molecular diameters—2.7085 and 2.807 Å, respectively—but owing to the plasticizing effect of methane, it permeated the film at a rate three times greater than that of nitrogen. It is interesting to note that the stronger the plasticizing ability of the penetrant, the greater the effect of the irradiation dose. The permeability of propane decreased by 40.7%, while the permeability of helium decreased by 6.4% after an irradiation dose of 50 Mrad.