Development and characterization of cellulose acetate benzoate flat osmotic membranes

Different parameters of casting solutions and casting conditions were studied for the development of cellulose acetate benzoate flat osmotic membranes. Casting solutions were prepared with different concentrations of the polymer, the additive, and the solvent; viscosity of the casting solution; and the thickness of the membrane developed. The membranes were given different evaporation periods and annealing temperatures under different RH. Different annealing baths were also used. Based on these, conditions were optimized for the development of cellulose acetate benzoate flat osmotic membranes. These membranes were characterized with respect to bound water content, specific water content, transport properties by direct osmosis, salt intake by direct immersion, water permeability coefficient of the dense membrane, diffusion coefficient, salt permeability, and salt distribution by electrical conductivity. Also, cellulose acetate benzoate membranes were compared with conventionally used cellulose acetate membranes. © 1994 John Wiley & Sons, Inc.     

Membrane potential measurements on cellulose acetate membranes

Membrane potentials and apparent transport numbers of the cation are reported for cured cellulose acetate membranes bounded by HCl, NaCl, KCl and MgCl₂ solutions, using Ag/AgCl electrodes and a flow-cell method. Membranes cured at 70°, 80° and 90° are used. Bounding solution concentrations vary from 0.005 to 0.05 M at the high concentration side (bounding the dense side of the membrane), and are kept constant at 0.002 M for the low concentration solution. In the KCl 90° membrane case the low concentration solution is varied as well, from 0.0001 to 0.002 M. Results show that cured cellulose acetate membranes are permselective towards univalent cations. This is interpreted as resulting from a low cation-exchange capacity of the dense layer of the cured membrane. The permselectivity increases with increased curing temperature. Addition of a non-electrolyte to the low concentration side reverses the osmotic flow and leads to higher apparent transport numbers of the cation. It is concluded that diffusion in small pores contributes significantly to the transport of ionic solutes through uncompacted membranes.     

Permeation of water and sodium chloride through cellulose acetate

The factors contributing to the selective permeation of water and sodium chloride through cellulose acetate membranes have been examined by the use of radioactive tracers. With decreasing acetyl content both the partition coefficient (solubility) and diffusion coefficient of water increased, the latter the more sharply. The effect was even more pronounced for salt, indicating that increasing selectivity with acetyl content stems mainly from increasingly preferential restrictions on salt mobility. Trends identical with those mentioned for decreasing acetyl content were found for increasing amounts of cellulose acetate solvents that had been extracted with water to yield more highly swollen membranes. A free-volume treatment for diffusion of small molecules below the glass transition temperature with the aid of subgroup motion in the polymer is used for both components. The water content of the membrane at (or near) saturation emerged as the predominant factor in the permeation behavior. In view of the similarity in the activation energies of water and salt diffusion the far steeper dependence of the salt diffusion coefficient on water content could not be accounted for by size differences between the diffusing species and has been attributed to confinement of salt ions to locations at which multiple water contacts are feasible.     

Production and characterization of membranes of recycled waste materials: Cellulose acetate,obtained from sugarcane bagasse with polystyrene from plastics cups

Membranes of cellulose acetate from sugarcane bagasse (CA), as well as blends of this cellulose acetate and polystyrene from plastic cups (CA/PS) were produced by casting utilizing dichloromethane as solvent at the concentration 12% w/w. The membranes were characterized regarding ion diffusion by dialysis and properties of pure water permeation rate, PEG rejection (utilizing an aqueous solution 1% w/v of polyethylene glycol (PEG), 45 and 80 kDa). Thermal characterization by thermogravimetric analysis and differential scanning calorimetry were also performed. The morphology of the membranes' cross‐sections was evaluated by scanning electron microscopy. The experiment of ion diffusion by dialysis showed that the ion diffusion coefficient of CA membrane is comparable to that found in the literature for membranes of commercial cellulose triacetate, 8.47 × 10^?8 cm² s^?1, while the ion diffusion coefficient of blends decreased as PS was added to the system. Regarding transport driven by pressure, CA membrane presented low rejection of PEG 80 kDa. These results showed that CA membrane could be used in a range of application comprehending the process of ultrafiltration or microfiltration. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers.     

Cellulose acetate and sulfonated polysulfone blend ultrafiltration membranes. I. Preparation and characterization

Modification of polymeric membrane materials by incorporation of hydrophilicity results in membranes with low fouling behavior and high flux. Hence, Polysulfone was functionalized by sulfonation and ultrafiltration membranes were prepared based on sulfonated polysulfone and cellulose acetate in various blend compositions. Polyethyleneglycol 600 was employed as a nonsolvent additive in various concentrations to the casting solution to improve the ultrafiltration performance of the resulting membranes. The total polymer concentration, cellulose acetate, and sulfonated polysulfone polymer blend composition, additive concentration, and its compatibility with polymer blends were optimized. The membranes prepared were characterized in terms of compaction, pure water flux, membrane resistance, and water content. The compaction takes place within 3–4 h for all the membranes. The pure water flux is determined largely by the composition of sulfonated polysulfone and concentration of additive. Membrane resistance is inversely proportional to pure water flux, and water content is proportional to pure water flux for all the membranes. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 1749–1761, 2002     

Concentration and pressure dependence of rate of membrane permeation

The permeation of water and sodium chloride in cellulose acetate membranes has been examined over a wide range of concentration and pressure. The results obtained from reverse osmosis experiments have been used to evaluate relations derived on the assumption that permeation of both solution components takes place by diffusion down a concentration gradient in the membrane. With the aid of equilibrium and raidoactive tracer measurements, most of the deviations could be attributed to nonconstancy of the diffusion, and, especially, the distribution coefficients of water. A comparison of the net flux in reverse osmosis with the rate of tracer permeation in the same membrane provided positive evidence to show that hydrodynamic flow under pressure cannot account for the water flux through the membrane. Differences in the shape of the distribution isotherms for salt and water between solution and membrane provide an explanation for the high selectivity of cellulose acetate membranes in favor of water.     

Flow method for study of gas transport in polymers. Application to the study of oxygen,nitrogen, and carbon dioxide permeation through cellulose acetate membranes

Transient permeation of oxygen, nitrogen, and carbon dioxide through certain cellulose acetate (CA) dense membrances made from solutions in dioxane and dioxane-added ethanol were studied by the flow method. In order to explain the overshoots for transient permeation rates in the latter case, a new model is proposed. It postulates a time-dependence solubility coefficient, in the form of a series of exponential terms, which is combined in Fick's second law and solved. The model fits well the obtained permeation kinetics, leading to values of the diffusion, and permeation coefficients, and the structure change-related time. The overshoots are explained by the formation of unstable structure from dioxane-ethanol solvent mixtures, a memory effect of aggregated chains, which undergoes consolidation upon gas penetration. The stable state which results from the consolidation process is the same as that of membrances obtained from pure dioxane solutions. © 1994 John Wiley & Sons, Inc.     

Sorption and diffusion of water in glass ribbon-reinforced composites

The processes of sorption and diffusion of water in anisotropic glass ribbon-reinforced composite films of controlled structural and physical characteristics were investigated in terms of the film properties, e.g., geometry, orientation, and volume fraction of the filler; molding characteristics of the composite film; and the adhesion between the glass ribbon and continuous cellulose acetate matrix. While the diffusion of water through unfilled cellulose acetate film was found to be a simple activated process with very little concentration dependence, the diffusion behavior of glass ribbon-reinforced cellulose acetate was found to be anomalous and concentration dependent.     

Diffusion of solvent from a cast cellulose acetate solution during the formation of skinned membranes

The transport of solvent out of a cast cellulose acetate (CA) solution into the coagulation bath during membrane formation is treated as a diffusion process. From the increase of solvent concentration in the bath with time (solvent leaching experiments) an overall solvent diffusion coefficient has been calculated. In size these coefficients compare well to mutual pseudo-binary solvent-non-solvent diffusion coefficients determined by means of a classical boundary broadening method applied to ternary solutions with fixed CA concentration, but with a gradient in solvent-nonsolvent composition. Since binary polymer-solvent interdiffusion coefficients are at least one order of magnitude lower, it is concluded that the diffusion of solvent into the coagulation bath is essentially a pseudo-binary solvent-non-solvent diffusion process. Combination of experimental results with model calculations for the effect of a thin dense skin on the diffusion of solvent out of the sublayer shows that the casting-leaching diffusion coefficient can be used to describe the out-diffusion of solvent from the layer under the skin provided that the relative skin resistance is not too high, or that the skin thickness is small.     

Water vapor sorption and transport in dense polyimide membranes

The sorption and transport of water vapor in five dense polyimide membranes were studied by thermogravimetry. The sorption isotherms of water vapor in the polyimides could be successfully interpreted by both the dual‐mode sorption model and the Guggenheim–Anderson–de Boer equation. The water vapor diffusion behavior was found to be nearly Fickian at higher water vapor activities, whereas non‐Fickian diffusion was observed at lower water activities. The phenomena could be well described by the mechanism of combined Fickian and time‐dependent diffusion. The diffusion coefficient and water vapor uptake in the polyimides were strongly dependent on the polymer molecular structure. Except for the polyimide prepared from 3,3′,4,4′‐diphenylsulfone tetracarboxylic dianhydride and 1,3‐bis(4‐aminophenoxy) benzene, the permeability of water vapor in the dense polyimide membranes predicted from the sorption measurement at 30°C corresponded well with the water vapor permeability measured at 85°C. Among the polyimides studied, pyromellitic dianhydride–4,4′‐diaminophenylsulfone (50 mol%)/4,4′‐oxydianiline (50 mol%) showed both high water sorption and diffusion and, therefore, high water vapor permeability, which for vapor permeation membranes is necessary for the separation of water vapor from gas streams. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 87: 2306–2317, 2003     

Modification of cellulose acetate: Its characterization and application as an ultrafiltration membrane

The development of cellulose acetate blend membranes using a commercial grade Mycell cellulose acetate and cellulose diacetate with suitable pore structure is discussed. These membranes were characterized in terms of resistance of the membrane, pure water flux, the molecular weight cutoff, water content, pore size, and porosity. The removal of copper metal ions by this blend membrane using polyethyleneimine as a chelating agent was studied. The effects of copper ion concentration and casting solution composition on separation are also discussed. A possible correlation between feed and permeate concentration of copper ion is evaluated. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 67:1939–1946, 1998     

Development of a new asymmetric alloy membrane for water desalination

The preparation of a novel asymmetric membrane cast from a poly (4-vinylpyridine) and cellulose acetate homogeneous mixture is discussed. The two polymers were found to be miscible in concentrated solutions. Asymmetric membranes were cast from a sexted dope mixture containing: chloroform, methanol, acetone and formamide. The as-cast solution, when coagulated in water at ambient temperature, yields a highly plasticized asymmetric matrix which, upon slow diffusion of the chloroform into the water, solidifies into a glassy state. The membranes following this stage, do not require annealing and yield high fluxes and salt rejection when tested in reverse osmosis. The asymmetric morphology was confirmed by scanning electron microscopy studies which revealed a dense skin resting upon a highly porous, open-celled, foam-like structure. This structure retains its original wet dimensions upon drying. Brief attempts to quaternize the matrix are also reported.     

The permeation behavior of metal complex solutions through cellulose acetate membranes

Transport phenomena of several kinds of metal complexes were investigated with cellulose acetate membranes annealed at 65°–76°C. In reverse osmosis experiments, the rejections of metal complexes involving organic sequestering agents such as EDTA or citric acid were much higher than those of the corresponding metal ions. While, in the case of metal complexes involving small inorganic ligands such as NH₃ or SCN^-, their rejections did not necessarily increase with the increase in the coordination numbers of the metal ions. To more precisely understand such transport behaviors, the distribution and the diffusion coefficients of metal complexes were obtained by desorption-rate measurements with dense cellulose acetate membranes. The results revealed that the distribution of a metal ion to the membrane was largely depended on the coexisting ligands. Attempts were also made to explain the distribution coefficient from the microscopic point of view by using Glueckauf's equation.     

亚砜基改性纤维素膜的SO2气体渗透性能研究

分别使用二甲基亚砜浸泡的物理方法及苯基乙烯基亚砜加成的化学方法对纤维素膜进行了改性,并对改性膜的性能进行了测试.结果表明改性后纤维素膜仍为致密结构,SO2的渗透性能及其对N2的分离性能明显提高,其中改性液中添加二甲基亚砜的均相化学加成反应所得的改性膜具有较好的SO2渗透稳定性.     

Selective molecular oxygen transport through a cellulose acetate membrane containing an electron-poor iron(II) porphyrin complexes

Molecular oxygen diffusion in the cellulose acetate membranes containing the 5,10,15, 20-tetrakis(pentafluorophenyl)-21H,23H-porphine iron(II) was studied. Both the permeability coefficient and the separation factor for oxygen in the membrane containing the iron(II) porphyrin complex were increased with decreasing the upstream gas pressure which correspond to a dual-mode oxygen transport. The effects of the axial ligands of the iron(II) porphyrin on oxygen permeation was also examined in the same cellulose acetate membrane. The fluoride and 2-methyl imidazole ligands coordination to the iron(II) porphyrin induce to increase the oxygen permeability coefficient and the value of ideal separation factor.     

Characterization of transport across cellulose acetate membranes in the presence of strong solute–membrane interactions

Certain organic solutes, including phenol, undergo anomalous enrichment when hyperfiltered through cellulose acetate membranes: the solute concentration is higher in the permeate than in the feed solution. A number of existing theoretical approaches describing hyperfiltration phenomena are presented and their merits and limitations upon application to the transport of phenol discussed. A new two-parameter transport relationship is derived based on an extension of the solution–diffusion model. The enrichment, or negative solute rejection by the membrane, is predicted to occur whenever the pressure-induced solute permeation velocity exceeds that of water. By acknowledging and incorporating the effect of pressure on the chemical potential of the solute, the present extended solution–diffusion model relationship successfully describes hyperfiltration data of phenol in homogeneous and asymmetric cellulose acetate membranes provided the contribution of convective flow to the overall solute transport is insignificant. In addition to the transport parameters of the extended solution–diffusion model, the transport parameters of the phenomenological, Kedem–Spiegler, and combined viscous flow–frictional relationship are evaluated from hyperfiltration data obtained with 0.05 and 0.1 wt % phenol feed solutions and homogeneous cellulose acetate membranes of different acetyl content.     

The electret effect in cellulose acetate reverse osmosis membranes

The electret potentials developed by reverse osmosis electret membranes help control the undesirable deposition of charged colloidal particles on the membrane surfaces during membrane desalination. These antifouling electret membranes should help prevent the costly flux declines normally associated with deposition of colloidal iron oxides on the reverse osmosis membrane surfaces. Homocharge and heterocharge behavior of cellulose acetate membrane electrets have been studied. Asymmetric reverse osmosis membranes and dense membrane films were studied. The homocharge and heterocharge of cellulose acetate reverse osmosis electret membranes have been explained.     

Water transport mechanisms through asymmetric membranes

Permeability coefficients and activation energy values for the transport of water through asymmetric cellulose acetate membranes were determined in order to establish the mechanism of the process when different driving forces are applied. A stirred Lucite cell with controlled temperature was used to measure the membrane transport properties under hydraulic and osmotic pressure differences and also in the presence of a tracer concentration gradient across the membrane. The experimental results based on the temperature dependence of water flow show that the controlling step for water transport is diffusion with net flux in the dense zone of the membrane under hydraulic or osmotic pressure gradients. When a tracer concentration gradient is used, equimolar diffusion of water in the thicker, porous zone of the membrane is the controlling mechanism. A mass transport model based on the composed structure of the membrane is presented to provide a general framework for treating the particular cases. Finally, the difference in the controlling barriers, in agreement with a previous work by Hays,¹⁸ is shown to account for the much higher absolute values of osmotic than tracer water permeabilities determined here and frequently reported in the literature.     

Permeability of water and water vapor through cellulosic membranes

To elucidate the water transport mechanism through homogeneous membranes, water and water vapor permeation through crosslinked cellulose membranes, cellulose diacetate, and cellulose triacetate membranes are studied. It is found that the water flux increases with the degree of hydration; and as for cellulose membranes, the degree of hydration is an increasing function of the degree of crosslinking. Activation energy of hydraulic permeability (K_w) is not equal to that of purely viscous flow, and is smaller than that of the water vapor diffusion coefficient (D?) for all membranes. The free-volume concept relating the molar frictional coefficient to temperature and to degree of hydration explains reasonably the temperature dependence of hydraulic permeability and of water vapor diffusion coefficient and gives adequate values for the fractional free volume of the system. The critical volume V^*, appearing in the Cohen-Turnbull expression between friction coefficient and free volume fraction, may be considered as the size of the cluster of water molecules. The value of V^* in the case of hydraulic permeability is larger than that for water vapor diffusion by several times. Furthermore, the value V^* increases with increase of degree of hydration for water permeation and water vapor diffusion.     

Study of the structure and transport of asymmetric polyamide membranes for reverse osmosis using the electron spin resonance (ESR) method

The electron spin resonance technique (ESR) was used to study the structure and transport of asymmetric aromatic polyamide membranes. TEMPO (2,2,6,6-tetramethyl-1-piperridinyloxy free radical) was used as a spin probe that was brought into the membrane either by (a) immersion ofthe membranes in aqueous TEMPO solutions, (b) reverse osmosis (RO) experiments with feed solutions involving TEMPO or (c) blending TEMPO in casting solutions. The membranes were further tested for the separation of sodium chloride and TEMPO from water by RO. It was concluded that aromatic polyamide membranes contain water channels in the polymer matrix like cellulose acetate membranes. The presence of such water channels allows aromatic polyamide membranes to be used as RO membranes. The diffusion of organic solutes through the water channels seems much slower in aromatic polyamide membranes than in cellular acetate membranes, which probably causes a higher separation of organic solutes by aromatic polyamide membranes than cellulose acetate membranes. A comparison was made with other RO membranes (cellulose acetate, CA) and ultrafiltration membranes (polyethersulphone, PES). It was observed that the ESR technique can be used to study the structure of OF and RO membranes. The presence of water channels in the polymer matrix seems indispensable for the RO membrane.