Membrane Casting Solution Research Articles

Application of surface modifying macromolecules in polyethersulfone membranes: Influence on PES surface chemistry and physical properties

Novel surface modifying macromolecules (SMMs) were developed for incorporation into polyethersulfone (PES) membranes, intended for pervaporation applications. These materials were synthesized with a diisocyanate, polypropylene oxide (PPO), and a fluoro-alcohol, and characterized for elemental analysis, molecular weight, and glass transition temperatures. PES/SMM blends with eight types of SMMs were characterized for surface and physical properties and compared with PES. Water droplet contact angle measurements and X-ray photoelectron spectroscopy data showed that the SMMs migrated to the surface and rendered the PES material more hydrophobic. While advancing contact angle data were equivalent to those of pure Teflon™, the highest average values of receding angles of these systems were less than those of commercial Teflon™. The opaqueness of PES/SMMs films and data from differential scanning calorimetry experiments showed that the SMMs were either immiscible or only partially miscible with PES. It was also observed, for a fixed PES concentration of 25 wt %, that increases in the molecular weight of the SMMs and the weight fraction of PPO in the SMMs led to phase separation in the ternary PES/SMMs/dimethylacetamide (i.e., membrane casting solution) system. On the other hand, in the binary PES/SMMs system (i.e., cast membrane film), an increasing weight fraction of fluorine in the SMMs contributed to an increase in the phase separation. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 1363–1378, 1999

Sulfonated and crosslinked polyphosphazene-based proton-exchange membranes

Proton-exchange membranes, for possible use in H 2/O 2 and direct methanol fuel cells have been fabricated from poly[bis(3-methylphenoxy)phosphazene] by first sulfonating the base polymer with SO 3 and then solution-casting thin films. The ion-exchange capacity of the membrane was 1.4 mmol/g. Polymer crosslinking was carried out by dissolving benzophenone photoinitiator in the membrane casting solution and then exposing the resulting films after solvent evaporation to UV light. The crosslinked membranes look particularly promising for possible proton exchange membrane (PEM) fuel cell applications. A sulfonated and crosslinked polyphosphazene membrane swelled less than Nafion 117 in both water and methanol. Proton conductivities in crosslinked and non-crosslinked 200 μm thick water-equilibrated polyphosphazene films at temperatures between 25°C and 65°C were essentially the same and only 30% lower than those for Nafion 117. Additionally, water and methanol diffusivities in the crosslinked polyphosphazene membrane were very low (≤1.2×10 −7 cm 2/s). Sulfonated/crosslinked polyphosphazene films showed no signs of mechanical failure (softening) up to 173°C and a pressure of 800 kPa and did not degrade chemically when soaked in a hot hydrogen peroxide/ferrous ion solution.

Organotin — environmental fate and effects

Novel polysulfone (PSF) ultrafiltration membranes with pH and thermo sensitivity were prepared using the copolymer poly (N-vinylcaprolactam-co-acrylic acid) (poly (VCL-co-AA)) as an additive. The copolymer was synthesized by free radical solution polymerization, and directly blended in membrane casting solution to prepare the asymmetric membranes by phase inversion method. The presence of copolymer in blended membranes was confirmed by comparing IR spectra of plain PSF membrane and blended PSF membrane. The morphology and structure of the resulting membranes were observed by scanning electron microscope (SEM) and field emission scanning electron microscope (FESEM). The membranes were characterized in terms of pure water flux (PWF), hydraulic permeability and hydrophilicity. PWF increased from 1 to 18.8 L/m2h, when wt.% of copolymer increases from 0 to 2.5. Similarly, water contact angle decreased from 76° to 58°. The blended membranes showed evident of pH and thermo sensitivity. The stimuli responsive behaviour of membranes was also investigated in terms of pH and temperature. In addition, solution of bovine serum albumin (BSA) was used to study the permeation and antifouling performance of prepared membranes. A significant increase in BSA flux (from 0.061 L/m2h for plain membrane to 3.37 L/m2h for membrane with 6 wt.% of copolymer) and decrease in membrane fouling was observed.

Relationship between mass ratio of nonsolvent-additive to solvent in membrane casting solution and its coagulation value

A simple equation has been developed to determine the relationship between the mass ratio of nonsolvent-additive to solvent ( R a s ) in a membrane casting solution and its coagulation value ( G v ) in preparing asymmetric membranes by the wet or the dry/wet phase inversion process. The equation is based on the precipitation values of the nonsolvent-additive ( PV a ) and the coagulant ( PV c ) in the binary polymer/solvent system. The validity of this equation was examined with observed experimental results for PESf/NMP/alcohol, PESf/NMP/H 2O, PSf/DMAC/alcohol and PSf/DMAC/H 2O polymer casting solutions using water or ethanol as the coagulant; and also with the experimental data reported in the literature for the PSf/NMP/PA system using water as the coagulant. The results show that the developed equation describes the observed experimental data very well. This equation may be used to indicate variation of phase separation behavior of the membrane casting solution with a coagulant by the introduction of a nonsolvent-additive in the solution. It can be applied readily for the formulation of composition of the membrane casting solution to make flat-sheet or hollow fiber membranes. The equation may be useful in the study and control of membrane morphology.

Effects of Membrane-Making Conditions and Shrinkage Treatment on Morphology and Performance of Cellulose Acetate Butyrate Membranes

Abstract The functions of additives in cellulose acetate butyrate (CAB) membrane casting solution, effect of thermal shrinkage treatment on porous CAB membranes, and the changes of CAB membrane surface morphology during the solvent evaporation step have been investigated. Additives (glycerol and lactic acid) in CAB membrane casting solution function only as pore number promoting agents when used at low concentration and function both as pore number and pore size promoting agents when used at higher concentrations. Triethyl phosphate in CAB membrane casting solution functions both as a pore number promoting agent and as a secondary solvent for CAB. Three distinct phases can be observed in the solvent evaporation step in making CAB membranes. With the increase in solvent evaporation time, the number of pores in the first pore size distribution increases in the initial small pore-forming phase and decreases in the large pore-forming phase, and the number of pores in the second pore size distribution always increases with solvent evaporation time. These changes in pore numbers, pore sizes, and pore number ratio in two pore size distributions as well as the membrane skin layer thickness together govern the ultimate membrane performance and result in a maximum solute separation which, in the case of CAB/ acetone membranes, falls at 60 seconds of solvent evaporation time. Significant improvement of the performance of a porous CAB membrane can be achieved by thermal shrinkage treatment. Equally high CAB membrane performance can also be achieved by using a lower concentration of additives in the membrane casting solution.

Polyamide polymer solution behaviour under chlorination conditions

The viscosity of a solution of a polymer is a measure of its molecular weight. Changes in viscosity can indicate polymer degradation. The viscosity of a polyamide polymer membrane casting solution decreases after exposure to chlorine. The adverse effect of chlorine on the polymer structure, consequently, affects the membrane performance drastically. The measurement of solution viscosity, therefore, is a useful tool for screening polymers resistant to chlorine or other oxidants.

Polysulfone Membranes. III. Performance Evaluation of Polyethersulfone—PVP Membranes

Abstract The performance of membranes produced from casting solutions consisting of polyethersulfone (PES), poly-(N-vinyl-pyrrolidone) (PVP), and N-methyl-2-pyrrolidinone (NMP) were systematically studied. Zero-shear casting solution viscosities for these polymer solutions were determined as a function of PES and PVP concentrations. Ultrafiltration membranes were then cast using the phase inversion technique and characterized by separation experiments using polyethylene glycols of various molecular weights as test solutes. A pore flow model was fitted to the resulting separation data to provide estimates of the average pore radius and membrane porosity. These parameters were used to compare laboratory results for this membrane casting solution system with performance data for commercially available polyethersulfone membranes.

Studies On Distribution and Reverse Osmosis Properties of Cellulose Acetate Derivatives for Alcohols.

The particles of cyanoethyl cellulose acetate (CACN), hydroxypropyl cellulose acetate (CAHP) and cellulose acetate have been prepared from the solutions containing the same components as reverse osmosis (RO) membrane casting solutions. The pore sizes in particles obtained have been measured by means of gel chromatography. The distribution properties for alcohols have been studied by liquid chromatography. It was found that alcohols were attracted to the surfaces of these membrane materials. The Kd values increased with the increase of carbon numbers of n-alkyl alcohols and followed the order of n->iso->sec->tert-alcohols with different structure. The separation properties of RO membranes casted from these materials for alcohols have also been investigated. It was found that the solute rejection for n-alkyl alcohols reached a maximum when the alcohol molecule containing three carbon atoms and the order of solute rejection of tert->sec--iso-> n-alcohols with different structure existed. The experimental results from liquid chromatography and reverse osmosis have been discussed according to the polar effect (Taft number σ*), nonpolar effect (Small number s*) and steric effect (Steric parameter Es) of the substituent groups in alcohols. Some relations have been found.

Polysulfone membranes II. Performance comparison of polysulfone-poly-(N-vinyl-pyrrolidone) membranes

The characteristic behaviour of the polysulfone (PSF), poly-(N-vinyl-pyrrolidone) (PVP), N-methyl-2-pyrrolidinone (NMP) ultrafiltration membrane casting solution system was investigated. Solutions containing different weight percentages of PSF and PVP were prepared. Zero-shear casting solution viscosities were determined. Membranes were then cast using the phase inversion technique and characterized by a series of separation experiments using polyethylene glycols of varying molecular weights as test solutes. A simple pore flow model was fitted to the resulting separation data to provide estimates of the average pore radius. The resistance to fluid transport for a given membrane pore radius was described by the ratio of the number of pores per unit membrane area to the effective pore length. These two parameters were used to compare the performance of laboratory cast polysulfone membranes with data for several commercial products.

The nucleation of organic additives in membrane casting solution (I) (the theory and direct evidence)

In order to effectively control the pore size and pore size distribution of membranes, the nucleation of an organic additive, triphenylphosphine (ATP) in concentrated polymer solutions has been studied theoretically by statistical thermodynamics and experimentally by scanning electron microscopy (SEM) and nuclear magnetic resonance (NMR). A theory to predict the nucleation of an additive in a polymeric membrane casting solution has been developed based on a lattice model. Both thermodynamically stable and dynamically stable nucleation of an additive in a membrane casting solution was predicted to be possible, which was supported by the experimental results. In order to use the nucleation of organic additives in a concentrated polymer solution for the fabrication of membranes, a double phase inversion method (DPIM) was developed.

The effect of polymer solution composition and film-forming procedure on aromatic polyamide membrane skin layer structure

Linear polymer phase inversion membrane casting solutions are composed of suspensions of relatively spherical polymer macromolecules. Estimates of macromolecule dimensions can be calculated, dependent on the type and concentration of the components in each particular solution. Membrane skin layer formation may be considered as interpenetration or partial coalescence and subsequent crystallization of solution surface macromolecules. The surface morphology of asymmetric membranes can be characterized by assuming a porous structure. With knowledge of the surface porous structure, membrane performance can be predicted for most solute—solvent combinations and operating conditions. It is assumed that all permeate transport through a membrane skin layer occurs between surface paracrystalline macromolecules. Macromolecule to pore radius comparisons can then be used to determine the effect of the casting composition and/or membrane-making procedure on the disposition and subsequent degree of interpenetration of surface macromolecules. Relationships between the casting composition/membrane-making procedure and interstitial coagulation may be used for membrane design considerations. This method of analysis is shown to be useful in examining reverse osmosis (RO) and ultrafiltration (UF) aromatic polyamide films.

The study of mechanism of organic additives action in the polysulfone membrane casting solution

The interaction between polysulfone(PS) and 71 low molecular weight organic compounds with different functional groups have been studied by the gas liquid chromatography(GLC). The free energy of solution and heat of solution of these compounds to PS polymer have been obtained, and the free energy of solution has been correlated to the selected organic additives in PS-dimethylacetamide(DMAC) casting solutions. According to the cloud point and the rheological properties of PS-DMAC-additives solutions, as well as the effect of additives on the pore size in the membrane surface, the mechanism of the action of organic compounds in PS membrane casting solution has been proposed. The model suggested may give a satisfactory explanation to the experimental results.

EFFECT OF POLYMER MOLECULAR WEIGHT, SOLVENT AND CASTING SOLUTION COMPOSITION ON THE PORE SIZE AND THE PORE SIZE DISTRIBUTION OF POLYETHERSULFONE (VICTREX) MEMBRANE

The effect of the molecular weight of polyethersulfone (Victrex) polymers, the nature of the solvent and the polymer concentration on the effective hydrodynamic radius of polymer in the membrane casting solution was investigated. The membranes were prepared from the casting solutions studied above and ultrafiltration experiments were performed with polyethylene glycol solutes. The average pore sizes and the pore size distributions on the membrane surfaces were further calculated from the ultrafiltration performance data obtained above. Strong correlations were found between the effective hydrodynamic radius of the polymer in the casting solution and the average pore size on the membrane surface. The mechanism of the pore formation was proposed on the basis of the polymer size—pore size correlation.

Modified poly(vinyl chloride) matrix membranes for ion-selective field effect transistor sensors

Various membrane compositions, in which a poly(vinyl chloride)(PVC)-poly(vinyl alcohol) copolymer (copolymer VAGH) and silicon tetrachloride were incorporated as additional components in the classical PVC matrix membrane composition for ion-selective electrodes, were studied for their adhesion characteristics to the gates of field effect transistor devices used for the construction of ion-selective field effect transistors (ISFETs). These studies were carried out on a potassium ion-sensing system based on valinomycin with dioctyl adipate as plasticising solvent mediator.The inclusion of 10–25%m/m copolymer VAGH and 1%m/m silicon tetrachloride in the membrane casting solution was found to increase the adhesion of the plastic membrane to the gate, reflected by an increase in ISFET functional lifetimes of 4–5 d over those which did not contain VAGH and by ca. 10 d over those which contained neither VAGH nor silicon tetrachloride. Leaching of membrane components is suggested as a factor limiting the ISFET lifetimes in adhesion studies.

Hydrolysis and other phenomena affecting structure and performance of polyamide 6 membrane

The rate of hydrolysis of polyamide 6 in membrane casting solution containing strong mineral acid has been studied by determining changes in molecular weight, as estimated from dilute solution viscometry measurements. Hydrolysis is shown to be first order with a long half-life of about 250 days. A two-step dissolution process for polyamide 6 is proposed. The effect of extended dope maturation time upon polyamide 6 membrane preparation and performance has been examined. At short maturation times where protonation of polymer chain is occurring, flux is relatively low. A major increase in flux occurs after about 10 h, when protonation appear complete and hydrolysis has begun. Hydrolysis reduces polymer chain entanglement, and so quite different mechanisms for membrane formation exist as dope maturation time proceeds. With chains less than the critical entanglement length, nodular top layer membranes and alveolar walls lead to high water flux. However, such membranes are quite fragile.

Prediction of membrane properties using phase diagrams in the polymer-solvent-precipitant system

The concentration at the surface of a membrane casting solution cast onto glass plates is greatly influenced by rate and time of evaporation. When the phase inversion takes place, the properties of the dense surface layer are determined by the concentration of the polymer, the solvent, and the precipitant. The change in the concentration is also determined by heat of evaporation and the ratios between the rates of evaporation of the solvent and the precipitant. These data were calculated using the equations of the phase diagram for three components. Using the evaporation ratios, the evaporation ratios were calculated from a certain point of the curve of the phase diagram. The same concentrations of the polymer, solvent and precipitant can be obtained for different evaporation rates of the cast membrane.

Chitosan Membranes for Reverse Osmosis Application

Alkali resistant reverse osmosis membranes were fabricated by spreading solutions of chitosan, a poly-N-acetyl glucosamine, in 2.0% acetic acid on a glass plate. The membrane had a flux rate of 1.67 ± 10-3 cm3/cm2/sec and a salt rejection capability of 78.8% with 0.2% NaCl at 680 psi. Addition of 40% polyethylene glycol to the membrane casting solution increased permeability, while 10% chloromethyl oxirane improved durability of the membranes.

Cellulose triacetate membranes for seawater desalination

Experimental data on reverse osmosis using a sodium chloride solution by cellulose triacetate membranes are presented. The investigation involved studies on the composition of membrane casting solutions and their effects on the performance. A higher polymer concentration (11–13%) is found suitable for production of a uniform and highly salt rejecting membrane. Salt rejection of 99.0% and 4–5 GFD product water flux were obtained at 1000 psi operating pressure using 30,000 ppm TDS seawater in the initial experiments.

On the structure of concentrated cellulose acetate solutions

AbstractThe supermolecular structure of cellulose acetate membrane casting solutions was determined by measuring their rheological behavior. The method was applied to both clear and turbid casting solutions. The resulting data on activation energy of viscous flow are shown to depend on the cellulose acetate content of the casting solution and on the nature of solvent used. The separation properties of the prepared asymmetric membranes determined by reverse osmosis testing indicate a good correlation with the data obtained by the rheological measurements of the casting solutions. The latter can, therefore, be used as a practical tool for the investigation of early stages in the asymmetric membrane formation process.

A light‐scattering and membrane formation study on concentrated cellulose acetate solutions

AbstractAn improved explanation of the mechanism of the porous membrane formation with asymmetric structure has been proposed. It emerged from the correlation of the results obtained by the light‐scattering measurements of the ternary membrane casting solutions and by the reverse osmosis and ultrafiltration testing of the asymmetric membranes prepared from these solutions. The results of anisotropic light‐scattering measurements indicated the extent of supermolecular structurization of the casting solution which affected the porous structure formation in the membrane surface region. The variation of the casting solution composition and, particularly, the role of the CA/F ratio, the formamide content, and their influence on the asymmetric membrane properties were investigated and explained using the improved concept of the membrane‐making mechanism.