Annealing effect in porous cellulose acetate membranes

On the basis of separation curves the pore size and pore dize distribution of cellulose acetate ultrafiltration membranes are determined. The change of these parameters by annealing is discussed and from the correlation between pore size and annealing temperature up to 75°C conclusions are drawn as to the structure of salt rejecting membranes annealed at 90°C.     

Ultrafine fibrous cellulose membranes from electrospinning of cellulose acetate

Three solvents, that is, acetone, acetic acid, and dimethylacetamide (DMAc), with a range of solubility parameter δ, surface tension γ, viscosity η and boiling temperature were used to generate mixtures for electrospinning cellulose acetate (CA) (degree of substitution, DS = 2.45). Although none of these solvents alone enables continuous formation of fibers, mixing DMAc with either acetone or acetic acid produced suitable solvent systems. The 2:1 acetone:DMAc mixture is the most versatile mixture because it allows CA in the 12.5–20% concentration range to be continuously electrospun into fibrous membranes. These CA solutions have η between 1.2 and 10.2 poise and γ around 26 dyne/cm and produce smooth fibers with diameters from 100 nm to ～1 μm. Fiber sizes generally decrease with decreasing CA concentrations. The nature of the collectors affects the morphology as well as packing of fibers. Fibers collected on paper have more uniform sizes, smooth surfaces, and fewer defects, whereas fibers collected on water are more varied in size. Electrically conductive solid collectors, such as Al foil and water, favor more tightly packed and less porous membranes. Porous collectors, like paper and copper mesh, produce highly porous membranes. The pores in membranes collected on the Al foil and paper are much better interconnected in the planar directions than those in membranes collected on water. There is evidence that electrospinning induces order in the fibers. Deacetylation of CA membranes is more efficient and complete in NaOH/ethanol than in aqueous NaOH, producing DS values between 0.15 and 2.33 without altering fiber surfaces, packing, or organization. The fully regenerated cellulose membranes are similarly hydrophilic as commodity cellulose fibrous matrices but absorb nearly 10 times as much water. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 2119–2129, 2002     

Modified cellulose acetate flat membranes for desalination

In this article, the radiation grafting of acrylamide on to cellulose acetate flat membranes using UV-irradiation on the initiator is described. The modified membranes thus obtained have been characterized by IR, DSC, and TGA. Their transport properties have been studied. The modified membranes exhibit higher salt rejection with slightly reduced water flux as compared with cellulose acetate membrane. The work is further extended to study the thermal stability of these modified membranes in a dry state. These modified membranes up to 330°C are stable.     

The structure of adsorbed water in cellulose acetate membranes

 The structure of water adsorbed in cellulose acetate membranes is determined by the fundamental and overtone IR spectra. Water is weakly H-bonded to ester and ether groups of the membrane, at low water contents. With increasing water content, more and more liquid-like water is observed. In addition, a small amount of a third type of water is present. The amounts of these three species are estimated from the spectra. At high water contents, the amount of liquid-like water increases strongly. The H-bond cooperativity of such water may be the cause for this increase and for the common anomalous water adsorption isotherms. The H-bond energy of the first hydration shell is relatively small, contrary to the anomalous large adsorption heats ΔH _ad. This could be described by larger van der Waals interactions between this type of water and the membrane groups as a result of a higher coordination number compared with Z=4.4 of liquid-like water. This model is in agreement with the decrease of ΔH _ad with increasing water sorption reaching the evaporization enthalpy of pure water at high water contents. Received: 24 May 1997 Accepted: 1 July 1997     

Structures of cellulose acetate membranes for reverse osmosis

Conclusions Studies under the scanning electron microscope have shown that the cellulose acetate membranes used for reverse osmosis are high-molecular-weight condensation structures of the cellular type resulting from the dropwise separation of a new liquid phase under diffusional enrichment of the polymer solution by water, the solvent. The pore diameter, and the total pore volume, both diminish on approaching the membrane surface; the diffuse character of the active layer traces back to the concentration distribution resulting from vaporization of acetone, the volatile component, from the acetone- formamide cellulose acetate solution.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 1, pp. 111–115, January, 1977.     

Counterflow affinity isotachophoresis on cellulose acetate membranes.

Counterflow isotachophoresis on cellulose acetate membranes of human alpha-fetoprotein (AFP) was performed with concanavalin A, lentil lectin, and castor bean lectin driven by electroendosmotic counterflow. This counterflow caused a uniform stream of lectin to migrate towards the cathode against AFP with carrier ampholytes in steady-state position. Retardation of microheterogeneity forms bound to lectins was observed, giving results comparable to standard crossed affinity immunoelectrophoresis. Smaller amounts of lectins and more diluted samples of AFP could be used in the described method.     

Flow-visualization during macrovoid pore formation in dry-cast cellulose acetate membranes

Video-microscopy flow-visualization (VMFV) is adapted to study the development of macrovoid (MV) pores in the dry-casting of cellulose acetate (CA)/acetone/water solutions. Particle tracer velocities provide the first direct evidence for the presence of solutocapillary-driven convection that can enhance mass-transfer to a MV. Three phases of MV development are observed: fast initial growth, slow growth, and collapse. During the latter, MVs were observed on occasion to initiate far from the demixing front. These studies have led to a significantly modified hypothesis for MV development. Extremely rapid initial MV growth is thought to occur owing to coalescence of dispersed phase microdroplets. To ensure net mass-transfer to a growing MV, it is postulated that a homogeneous supersaturated solution layer must exist between the demixed fluid layer and the homogeneous stable solution layer. Fast growth also involves convective mass-transfer to the MV whose surface is initially entirely immersed in this homogeneous supersaturated solution layer. Slow growth involves net transport that results from both convective mass-transfer to the MV across the portion of its surface in contact with the homogeneous supersaturated solution layer, and convective mass-transfer from the portion of its surface that extends into the homogeneous stable solution layer. Active collapse is thought to occur owing to skin formation at the MV surface. Passive collapse occurs when the convective mass-transfer from the MV in the homogeneous stable solution layer exceeds that entering the MV in the homogeneous supersaturated solution layer.     

Relationship between structure and transport in cellulose acetate desalination membranes

Several experimental techniques, including measurements of salt rejection, water flux, birefringence, and membrane opaqueness, were used to characterize the structure and performance of typical cellulose acetate desalination membranes. The results indicate that optimum salt rejection and water flux characteristics are related to the presence of molecular orientation effectively perpendicular to the membrane surface. An alignment-type flow mechanism involving water contained within the oriented structure is consistent with the observed structural and transport phenomena. The various membrane fabrication procedures apparently have significant effects on the development of molecular orientation in the thin surface layers of the membrane.     

Preparation, characterization and effect of annealing on performance of cellulose acetate/sulfonated polysulfone and cellulose acetate/epoxy resin blend ultrafiltration membranes

Polymeric membranes based on cellulose acetate (CA)--sulfonated polysulfone blends at three different polymer compositions were prepared by solution blending and phase inversion technique, characterized and subjected to annealing at 70, 80 and 90 °C. The permeate water flux, separation of bovine serum albumin and its flux by the blend membranes before and after thermal treatment, have been compared and discussed. Similarly, CA and epoxy resin (diglycidyl ether of bisphenol-A) were blended in various compositions, in the presence and in the absence of polyethyleneglycol 600 as non-solvent additive, using N,N^′-dimethylformamide as solvent, and used for preparing ultraflltration membranes by phase inversion technique. The polymer blend composition, additive concentration, casting and gelation conditions were optimized. Blend membranes were characterized in terms of compaction, pure water flux, water content and membrane resistance. The effects of polymer blend composition and additive concentration on the above parameters were determined and the results are discussed.     

Permeation and thermal osmosis of water through cellulose acetate membranes

Isothermal permeation and thermal osmosis of pure water through cellulose acetate membranes have been studied. In all cases the results have been analyzed and compared with those of the literature. Two kinds of isothermal permeation experiments have been carried out, in order to find the influence of temperature and stirring rate on the phenomenon. The activation energy for permeation has been calculated.In the case of thermal osmosis two different kinds of experiments have been developed. In the first, thermoosmotic flows have been measured. In the second, evolution towards steady states has been obtained. In both cases the experiments have been performed by varying separately the temperature difference, the mean temperature and the stirring rate. The activation energy for thermal osmosis has been calculated. The influence of stirring rate on thermal osmosis has shown that “temperature polarization” cannot be avoided with the experimental setup, but its influence may be evaluated. A method is proposed to evaluate the differential thermoosmotic permeability. It is suggested that the results obtained from steady states are more reliable than those obtained from thermoosmotic flows.     

Adsorption and electrochemical studies on surfactant modified cellulose acetate membranes

The possibility of the modification of a cellulose acetate membrane upon adsorption of ionic surfactants, cetyl pyridinium chloride and sodium dodecyl sulphate has been investigated on the basis of membrane potential studies. For the estimation of the extent of adsorption, surface tension measurements were carried out in the presence and absence of cellulose acetate membrane using surfactant solutions of variable compositions. The results have also been used for the characterization of the thermodynamic equilibrium, which the membrane surfactant system attains upon surfactant adsorption, in terms of the immobilization constant. The text was submitted by the authors in English.     

High-flux,porous and homogeneous PVDF/cellulose microfiltration membranes

Low hydrophilicity of membranes is probably the biggest concern in membrane filtration since it increases the costs for water treatment. Conversely, application of hydrophilic biopolymers (such as cellulose) is limited because of its complex and crystalline structure. Enabling the wide use of the most common biopolymer in nature is crucial to improve the performance of water treatment, especially in terms of membrane sustainability. Here, we study the effect of cellulose dissolution in the synthesis of homogeneous PVDF/cellulose membranes. Although only partial dissolution was achieved for studied samples, adding cellulose to the membranes greatly improved their water flux. Besides, the porous structure obtained after partial solvent removal indicates the water flux (and consequently the pore size) may be tailored according to the membrane production method. Therefore, the homogeneous cellulose microfiltration membranes studied here may have potential for water treatment considering their high-water flux and low complexity to produce.

     

NMR of dissolved water in ultrathin and thick membranes of cellulose acetate

Wide-line proton magnetic resonance spectra of thick (100 μ) and ultrathin (0.25 μ) membranes of cellulose acetate have been obtained in the “bone-dry” state and in the wet state up to 2.5% water by weight. The NMR spectra were recorded with the membrane face aligned parallel and perpendicular to the magnetic field direction. The first derivative spectra of wet thick membrane (above 0.4% water) show a sharp center peak and at least one broad outer peak, both in the parallel and perpendicular orientations. The spectra of wet ultrathin membrane, on the other hand, show a pseudodoublet in the perpendicular orientation and a broad singlet in the parallel orientation. The differences in the spectra of the thick and ultrathin membranes are discussed in terms of “free” and restricted water believed to arise from the differences in the structure of thick and ultrathin membranes.     

Frictional coefficients and activation energies for permeation through cellulose acetate membranes

Analysis of previous experimental results, referring to water permeation in cellulose acetate membranes, is presented by using the frictional model proposed by Kedem and Katchalsky. The frictional coefficients decrease with temperature. The conclusions about the transport mechanisms responsible for the flow resemble those obtained in our previous papers. The activation energies for permeation have been calculated; they are of the same order of magnitude as those reported by other authors. The relation between frictional coefficients and activation energies is considered.     

Potentiometric ion- and bio-selective electrodes based on asymmetric cellulose acetate membranes

The potentiometric response properties of ammonium-, carbonate-, and proton-selective electrodes prepared by incorporating appropriate neutral carriers within novel asymmetric cellulose acetate membranes are reported. The membranes are formed by first casting a thin layer of cellulose triacetate without carrier, hydrolyzing one side of this film with base, and then on the other side casting a second layer of cellulose triacetate containing the membrane active components. The resulting asymmetric ion-selective membranes function equivalently, in terms of selectivity and response slopes, to non-asymmetric cellulose triacetate membranes and conventional poly(vinyl chloride)-based membranes. The hydrolyzed surface of the asymmetric membranes can be activated in aqueous solution with carbonyldiimidazole for the direct immobilization of proteins on the surface of the membranes, without loss in potentiometric ion-response. As an example, the immobilization of urease on the surfaces of ammonium- and carbonate-selective membranes yields potentiometric bio-selective urea-probes with desirable dynamic response properties.     

Formation of cellulose acetate membranes using a supercritical fluid assisted process

Microporous cellulose acetate membranes have been prepared from polymer–acetone solutions using a supercritical fluid phase inversion process in which CO₂ acts as the non-solvent. Series of experiments were performed at various polymer concentrations, temperatures and pressures. The structure of the resulting membranes was analysed using scanning electron microscopy. We operated with polymer concentrations ranging between 5 and 40% (w/w) in acetone obtaining different pore dimensions and membrane structures. Increasing the percentage of polymer in the solution, the structure of the membranes changed from beads-like structure to cellular structure. Polymer concentration also influenced the mean diameter of the pores that ranged from 2 to 50 μm for polymer concentrations from 40 to 5% (w/w). We also tested membrane formation pressures between 100 and 200 bar and at temperature between 45 and 65 °C. Pressure influences the change in membrane structure from cellular to beads-like, whereas temperature has a minor influence on pore size: both the effects can be partially related to CO₂ density. Cellulose acetate membrane formation mechanisms have also been discussed.     

An improved method of high-voltage isoelectric focusing on cellulose acetate membranes

An apparatus and the procedure for cellulose acetate isoelectric focusing were refined and optimized for routine use in clinical laboratories. The space in the electrophoretic chamber was minimized to maintain high humidity during the run. The water content of the thin cellulose acetate membranes in the chamber was well controlled during isoelectric focusing and the effect of atmospheric carbon dioxide was also excluded. The temperature of the membranes was kept below 2 degrees C, even under conditions of high electric field strength for faster isoelectric focusing of high-molecular-weight proteins. No special training was required for technicians in clinical laboratories to obtain reproducible isoelectric profiles of human serum proteins.