Ionically crosslinked poly(acrylic acid) membranes. IV. Composite reverse osmosis membranes

Composite reverse osmosis membranes of ionically crosslinked poly(acrylic acid) (PAA) have been prepared. The process consists of coating a porous polysulfone membrane with a thin layer of dilute solution of PAA, drying the thin liquid layer, and ionically crosslinking the PAA in solutions of different salts. The influence of some important preparation parameters on the properties of these membranes was investigated. It was found that all these membranes possessed ion exchange properties and could be ion exchanged; some of them have been found to be suitable for the desalination of low concentration salt solutions. By comparison to cellulose acetate (CA) membranes, higher rejection of phenols may be an interesting property of these membranes.     

Ionically crosslinked poly(acrylic acid) membranes. I. Wet technique

A study has been made of the ionic crosslinking of poly(acrylic acid) membranes for possible applications in dialysis and reverse osmosis. The technique consists of casting a film of poly(acrylic acid) neutralized with sodium hydroxide, followed by immersion in appropriate metal salts (aluminium, zinc, and chromium salts). A qualitative rate model has been developed to guide this synthesis. Since both metal cations and protons in solution compete for the carboxylic acid sites, acid–base properties of the metal and polycarboxylic acid appear to be important for successful membrane formation. The use of a nonsolvent for the polymer in the crosslinking solution was tested and found to give improved membranes under some conditions. The nonsolvents tested were methanol, acetone, benzene, and dimethylformamide, dimethylformamide being the most successful. Crosslinking agents such as the Zn²⁺ and Cr³⁺ salts were tried but were not as successful as the Al³⁺ salts. A series of membranes was synthesized in aluminium/dimethylformamide crosslinking baths under various conditions in order to determine suitable procedures for preparation of membranes.     

Preparation and properties of poly(acrylic acid) composite membranes

A new type of membrane has been prepared for hyperfiltration (reverse osmosis) desalination that is essentially a very thin polyelectrolyte membrane. It is prepared by casting an aqueous solution of a polyelectrolyte, specifically poly(acrylic acid) (PAA), directly on one surface of a finely porous support membrane. In hyperfiltration tests, these composite membranes exhibit desalination performance comparable in dilute solutions to that observed with cellulose acetate membranes of the Loeb-Sourirajan type. The water flux through these membranes is linear in the pressure up to 100 atm. Salt rejection is a function of pressure; it is also a function of the concentration of the feed solution and the charge of the counterion, in qualitative agreement with the Donnan ion-exclusion mechanism. Typical long-term results range from water fluxes of 2 × 10^?3 g/cm²-sec (50 gal/ft²-day) and 80% salt rejection to 0.2 × 10^?3 g/cm²-sec (5 gal/ft²-day) and >99.5% salt rejection at 1500 psi with 0.3 wt-% NaCl. These membranes appear to be useful for brackish water desalination.     

Grafted polypropylene membranes for purification of water containing sodium alkylbenzenesulfonate

The possibility of eliminating sodium alkylbenzenesulfonate (ABS) from water by reverse osmosis was examined by operating with polypropylene membranes grafted with poly(acrylic acid). Flux through the membranes decreases with increasing thickness. For 23μ thickness and 0.5 1./hr-m² flux, the rejection is 100% (operating pressure, 50 atm: concentration, 1 g ABS/l.). For 5μ thickness the rejection is lower and is related to ABS concentration: rejection increases with increasing concentration and reaches 60% for concentrations higher than 5 g ABS/l. (flux, 51./hr-m²; operating pressure, 50 atm). This is ascribed to the prevailing ultrafiltration as regards the reverse osmosis. ABS, at higher concentrations than the critical value, gives micelles whose dimensions might be compared to pores. The membrane behavior is influenced by previous treatments; when the membrane has been conditioned in NaCl solution, it is more selective and more permeable; the rejection reaches 90% with fluxes of 6.8 l./hr-m² for concentrations of 5 g ABS/1. (operating pressure, 50 atm). In order to relate the properties to the structure of membranes, we have examined them by electron microscope. The ion exchange which takes place between membrane and NaCl and between membrane and ABS has also been studied.     

Poly(acrylic acid)–poly(vinyl alcohol) semi- and interpenetrating polymer network pervaporation membranes

A series of poly(acrylic acid) (PAA)–poly(vinyl alcoho) (PVA) semiinterpenetrating (SIPN) and interpenetrating (IPN) polymer network membranes were prepared by crosslinking PVA alone or by crosslinking both PVA and PAA. Glutaraldeyde and ethylene glycol were used as crosslinking agents for the PVA and PAA networks, respectively. The presence of PAA increases the permeability of the membranes while the presence of PVA improves their mechanical and film-forming properties. The mechanical properties of the membranes were investigated via tensile testing. These hydrophilic membranes are permselective to water from ethanol–water mixture and to ethanol from ethanol–benzene mixtures. The IPN membranes were employed for the former mixtures and the SPIN membranes for the latter, because the IPN ones provided too low permeation rates. The permeation rates and seperation factors were determined as functions of the IPN or SIPN composition, feed composition, and temperature. For the azeotropic ethanol–water mixture (95 wt % ethanol), the separation factor and permeation rate at 50°C of the PAA-PVA IPN membrane, containing 50 wt % PAA, were 50 and 260 g/m²h, respectively. For the ethanol–benzene mixture, the PAA–PVA SIPN membranes had separation factors between 1.4 and 1200 and permeation rates between 6 and 550 g/m²h, respectively, depending on the feed composition and temperature. © 1996 John Wiley & Sons, Inc.     

Chlorine-resistant membrane for reverse osmosis. II. Preparation of chlorine-resistant polyamide composite membranes

Polyethylenetrimesamide (PET), poly(trimesoyl piperazine) (PTP), and poly(m-phenylenetrimesamide) (PMT) were fabricated into thin film composite to evaluate the membrane performance and resistance to oxidative chlorine. The salt rejection of these membrane increased in the following order: PTP<PET<PMT. The degree of performance deterioration of the membrane after the exposure to aq hypochlorous acid was in the following order: PMT>PET>PTP. The reaction behavior of these polyamide membranes with hypochlorous acid could be properly predicted by that of corresponding polyamide powder. Thus, polyethylenetrimesamide has proved to be one of the potential chlorine-resistant reverse osmosis membrane possessing excellent rejection to monosaccharides, amino acids, phenolics, and other solutes having a molar volume larger than 80 cm³/mol.     

Cellulose–poly(acrylamide or acrylic acid) interpenetrating polymer network membranes for the pervaporation of water–ethanol mixtures

A new type of interpenetrating polymer network (IPN) pervaporation membranes based on cellulose and synthetic polymers was developed. They were prepared by free-radical polymerization of acrylamide or acrylic acid in the presence (or absence) of the crosslinking agent (allyldextran or N,N′-methylenebisacrylamide) within cellophane films swollen in the reaction mixture. The swelling behavior of these membranes in water–ethanol solutions and their separation characteristics were investigated depending on the polyacrylamide (PAAm) or poly(acrylic acid) (PAA) content in the IPN (C_p) and for ionic cellulose–PAA membranes depending on the degree of neutralization of carboxylic groups and on the type of counterions. IPN membranes were selective over a wide range of ethanol concentration in the feed. The separation factor (α) and the permeation rate (P) significantly improved with increasing C_p in IPN membranes, especially for the cellulose–PAA(K⁺ form) membranes (for 86% EtOH feed at 50°C, and α and P values reached 1500 and 1.6 kg/m² h, respectively). The results for ionic and nonionic IPN membranes were compared. The separation characteristics of membranes were in good correlation with their swelling behavior. The α values of the membranes depended on the affinity of the IPN polymer chains functional groups for water. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 69: 761–769, 1998     

Crosslinked poly(hydroxyethyl methacrylate) membranes for desalination by reverse osmosis

A series of crosslinked hydroxyethyl methacrylate (HEMA) membranes for reverse osmosis desalination has been prepared. The crosslinkers used were trimethylol propane trimethacrylate (TPT) or ethylene glycol dimethacrylate (EGD). Membranes were synthesized by polymerizing the monomers as a thin homogeneous film. In addition to reverse osmosis tests, the membranes were also characterized by osmosis experiments and sorption measurements. The reverse osmosis water flux (1500 psi applied pressure, 4% NaCl brine, pH = 5) for these membranes decreases from 0.6 gallonmil/ft²-day (GMFD) to 0.055 GMFD and salt rejection increase from 78% to a maximum of 94% as the amount of TPT is increased from 0 to 11 mole-%. Water contents decrease from 42% to 15% over the same range of crosslinker, but the preferential sorption of water to salt does not vary. Thus, rises in reverse-osmosis semipermeability were found to result from changes in water–salt diffusivity ratios. The mechanism of permselectivity has been interpreted in terms of parallel diffusive fluxes across the membrane of primary H-bonded water and secondary water plus salt ions.     

Ionic reverse osmosis membranes of grafted polyethylene

Membranes of graft copolymers of polyethylene with poly(sodium styrene sulfonate), poly(4-vinylpyridinium methyl bromide), and poly(sodium acrylate) were prepared by using the technique of peroxide grafting. The reverse osmosis characteristics of the membranes were examined as a function of grafting yield. In these membranes, the grafting can be considered as a process of introducing ionic sites, and it depends on the conditions of the grafting reaction, such as monomer concentration and temperature. However, the overall reverse osmosis characteristic is not only dependent on the number of ionic sites introduced but also on the swelling capability of the membrane. Consequently, the salt rejection of grafted membrane of a fixed graft yield depends on the conditions of the grafting reaction. All grafted membranes which have grafting yields above a certain value behave as normal ionic polymer membranes, and their interrelationship of salt rejection and water permeability follow the general dependence found for ionic polymer membranes.     

Pervaporation separation of pentane-alcohol mixtures through ionically crosslinked blended membranes. I. Thermal measurements,electron microscopy and tensile studies

Ionically crosslinked blended membranes were prepared from blends of nylon 6 and poly(acrylic acid) (PAA) with the proportion of PAA ranging from 25 to 45 wt.-%. The technique consists of casting a film of the blend, followed by drying and immersion in an aluminum salt crosslinking solution for a predetermined time. The glass transition temperature (T_g) of the membranes was determined by differential scanning calorimetry (DSC). All the samples exhibited a single T_g, which is higher than that of either polymer. There is also a shift in the T_g with increasing scanning time. This phenomenon is attributed to the elimination of water molecules and the formation of anhydrides in the non-crosslinked PAA portion of the membrane. The membrane morphology was studied using scanning electron microscopy (SEM) and shows a dense structure without any pores. No phase separation is observed by scanning cross-sections of the samples, indicating that nylon 6 and PAA are completely miscible in the ranges studied. The membrane material strength lies in the range of 5 to 26 MPa and varies with the amount of PAA in the membrane. Samples with higher PAA content show lower tensile strength in spite of increased crosslinking density. This is due to the inherent low strength of PAA, coupled with increasing swelling of the membrane with increasing PAA content. The latter is confirmed by the measurement of water uptake into a dry membrane which increases from 23.6% to 76.3% with the membrane PAA content increasing from 25 to 45 wt.%.     

An interpolymer anionic composite reverse osmosis membrane derived from poly(vinyl alcohol) and poly(styrene sulfonic acid)

An interpolymer anionic composite membrane for reverse osmosis was prepared from poly(vinyl alcohol) and poly(styrene sulfonic acid). The effects of composition of a casting solution, heat-curing periods, and casting thickness on the reverse osmosis performance of resulted membranes have been examined. A mixture of water and ethyl alcohol (12/7, wt %) was found to be a proper solvent for casting an interpolymer membrane on the supporter. The composite membrane was formed by casting the polymer solution in ultrathin film on a microporous polypropylene supporter, evaporating the solvent, and heat-curing at 120°C for a proper period. the optimum composition of a casting solution was as follows: wt % of poly(vinyl alcohol)/poly(styrene sulfonic acid)/solvent was 3/2/95. The membrane heat-cured at 120°C for 2 h has a good performance for reverse osmosis, viz., water flux of 9.1–28.4 L/m².h at salt rejection level of 88.1–93.4% under applied pressure of 80 kg/cm² with 0.5% NaCl aqueous solution. The formation mechanism of a water-insoluble membrane was discussed.     

Synthesis and crosslinking of partially disulfonated poly(arylene ether sulfone) random copolymers as candidates for chlorine resistant reverse osmosis membranes

Biphenol-based, partially disulfonated poly(arylene ether sulfone)s synthesized by direct copolymerization show promise as potential reverse osmosis membranes. They have excellent chlorine resistance over a wide range of pHs and good anti-protein and anti-oily water fouling behavior. Crosslinking of these copolymers that have high degrees of disulfonation may improve salt rejection of the membranes for reverse osmosis performance. A series of controlled molecular weight, phenoxide-endcapped, 50% disulfonated poly(arylene ether sulfone)s were synthesized. The copolymers were reacted with a multifunctional epoxy resin and crosslinked thermally. The effects on network properties of various factors such as crosslinking time, copolymer molecular weight and epoxy concentration were investigated. The crosslinked membranes were characterized in terms of gel fraction, water uptake, swelling and self-diffusion coefficients of water. The salt rejection of the cured membranes was significantly higher than that for the uncrosslinked copolymer precursors.     

Electrospun crosslinked poly(acrylic acid) fiber constructs: towards a synthetic model of the cortical layer of nerve

In an attempt to mimic properties of the polyanionic nanofibrous cortical layer (ectoplasm) of nerve, tube‐shaped poly(acrylic acid) (PAA) nanofiber constructs were prepared via electrospinning. The influence of processing parameters on the morphology of the electrospun PAA nanofibers was systematically investigated. Smooth and uniform PAA nanofibers with average fiber diameter of 820 nm were produced at a concentration of 4 wt% with a flow rate of 0.8 mL h^?1 when a high voltage of 15 kV was applied. Water‐stable PAA nanofibers were obtained by thermally crosslinking PAA with ethylene glycol. The resulting tubes were neutralized to the sodium polyacrylate form and were shown to undergo reversible and abrupt length changes upon titration with CaCl₂ followed by titration with sodium citrate. The sharpness of the length transition was found to be highly dependent upon the bathing NaCl concentration and the operation temperature. It is suggested that electrospun PAA may be a promising candidate as a key element of an abiotic macromolecular mimic of selected properties of axons. © 2014 Society of Chemical Industry     

Layer‐by‐layer assembled polyelectrolyte blend membranes and their use for ion separation and rejection

Polyelectrolyte blend films and membranes were prepared upon alternating electrostatic adsorption of polyallylamine hydrochloride (PAH) and mixtures of polystyrene sulfonate (PSS) and polyacrylic acid (PAA) in different ratio on solid supports. Infrared studies indicated that the PSS‐PAA blend composition of the films always differed from the mixture composition in the dipping solution, PSS being preferentially adsorbed. Films deposited on porous supporting membranes (polyacrylonitrile/polyethylene terephthalate) were studied on their ion permeation under diffusion dialysis conditions, and their flux and salt rejection under nanofiltration and reverse osmosis (RO) conditions. Blend membranes prepared at pH 1.7 exhibit a significantly improved anion separation and salt rejection, the ideal separation factor α(NaCl/Na₂SO₄) of a membrane prepared from a PSS:PAA mixture of 1:1 (w/w) being 197 ± 10 (pure PAH/PSS: 45). The NaCl and Na₂SO₄ rejections under RO conditions are 85 and 97% (pure PAH/PSS: 15 and 27%), respectively. POLYM. ENG. SCI., 2011. © 2011 Society of Plastics Engineers     

New polymeric materials for reverse osmosis membranes

The results obtained from investigations on new aromatic polyamides containing carboxylic groups suitable for reverse osmosis are reported. The polymers are fabricated into asymmetric membranes by the Loeb-Sourirajan technique. The effects of fabrication conditions were also investigated to yield the optimum membranes for brackish water and sea water desalination, respectively. Performance characteristics of the membranes are 0.30 m³/m²·day above 98% rejection at 70 kg/cm²-35000 ppm NaCl, and 1.0–1.15 m³/m²· day, 97–98% rejection at 40 kg/cm²-5000 ppm NaCl. They exhibited good resistance in the tests carried out in alkaline medium (pH 10) and acid medium (pH 4).     

Covalent crosslinked assembly of tubular ceramic‐based multilayer nanofiltration membranes for dye desalination

A tubular ceramic‐based multilayer composite nanofiltration membrane has been developed for dye desalination. Poly(acrylic acid)(PAA)/poly(vinyl alcohol)(PVA)/glutaraldehyde(GA) was dynamically assembled on to the inner surfaces of tubular ceramic microporous substrates which had been pretreated using dynasylan ameo silane coupling agents. Subsequently, the composite membranes were thermally crosslinked to form covalent ester bonds. Experimental results proved that the composite membrane had good nanofiltration performance for dye desalination. The (GA/PVA/PAA)₃/ceramic multilayer membrane shows over 96% retention of Congo red and less than 3% NaCl retention using a permeate flux of about 25 L/(m²·h). An investigation of membrane performance as a function of operating conditions suggested that the covalent crosslinking multilayer membrane possessed much higher stability compared to other, electrostatically assembled, multilayer membranes. © 2013 American Institute of Chemical Engineers AIChE J, 59: 3834–3842, 2013     

Pervaporation separation of binary organic–aqueous liquid mixtures using crosslinked PVA membranes. III. Ethanol–water mixtures

The application of the pervaporation process in biotechnology is rapidly growing. A two stage pervaporation process could be applied to the downstream processing of ethanol fermentation. In this paper, the second stage process—a water-selective process—was investigated in detail using the crosslinked poly(vinyl alcohol) membranes with the low molecular weight of poly(acrylic acid) as the crosslinking agent. The ratio of poly(vinyl alcohol) and poly(acrylic acid) in the membrane was 90/10, 85/15, and 80/20 by weight. The prepared membranes were tested to separate the various compositions of the water–ethanol mixtures, specially 50/50 solution at 60°C and 30/70, 20/80, 10/90, and 4.4/95.6 solutions at 60, 70, and 75°C. For water: ethanol = 50 : 50 solution, the separation factor α_w/e = 260 at 75°C was obtained by using a PVA/PAA = 80/20 membrane. The permeation rate and the separation factor at the azeotropic point of a water–ethanol mixture showed 30 g/m²/h and 5800 at 75°C, respectively, when a PVA/PAA = 80/20 membranes was used. © 1995 John Wiley & Sons, Inc.     

Membranes composites preparees a partir d'alcool polyvinylique et de diisocyanate de toluylene destinees a l'osmose inverse

Composite reverse osmosis membranes have been prepared by coating a Millipore filter with an ultrathin layer of polyvinyl alcohol crosslinked by toluene diisocyanate. The membranes present a good selectivity towards aqueous salt solutions (for instance water flux of 500 liters/m²-day at 28°C, salt rejection of 98% with a 3.5% NaCl solution under a pressure of 100 bars) and have better temper-ature and pressure stabilities than cellulose acetate membranes. Their resistance to acid and alkaline hydrolysis is satisfactory. The influence of different preparation factors on the osmotic properties of the membranes was examined.     

Solute separation by the amidoxime of poly(4-vinylpyridine-co-acrylonitrile)

The quantitatively amidoximated poly(4-vinylpyridine-co-acrylonitrile) (4VNX) could be cast with divinyl sulfone as a crosslinking agent to form membranes for reverse osmosis. They were stronger than a cellulose acetate membrane when dried for more then 45 min at 80°C, most balanced in their performance when 4VNX was composed of about 60 mol % acrylonitrile amidoxime, and highly rejected NaCl, CoCl₂, NiCl₂, and phenol as well at pH 12. They were superior in the performance to the membranes prepared from the amidoximes of poly(2,4-diamino-6-vinyl-s-triazine-co-acrylonitrile) and polyacrylonitrile. 4VNX membranes prepared by drying for about 30 min were capable of separating NaCl and transition metals under lower pressures because of great differences in rejections between NaCl and those metals. Potential use of 4VNX membranes in ultrafiltration was demonstrated, particularly to the effect that flux was enhanced while the rejection differences were hardly changed. It was also demonstrated that the isolation of transition metals of considerably different uptake from their mixture could be achieved by breakthrough of the column packed with divinylbenzene-crosslinked 4VNX resin.     

Preparation of reverse osmosis membranes by plasma polymerization of organic compounds

The plasma polymerization of organic compounds was used to prepare a composite reverse osmosis membrane which consists of an ultrathin semipermeable membrane formed by plasma polymerization of an organic compound or compounds and a porous substrate. Many nitrogen-containing compounds (aromatic amines, heteroaromatic compounds, aliphatic amines, and nitriles) were found to yield excellent reverse osmosis membranes by plasma polymerization directly onto porous substrates such as Millipore filters, porous polysulfone filters, and porous glass tubes. Factors involved in the preparation of reverse osmosis membranes by plasma polymerization were investigated and discussed. The plasma polymerized membranes have the following unique features: (1) very stable performance independent of salt concentration and applied pressure (practically no water flux decline was observed with many membranes): (2) salt rejection and water flux both increase with time in the initial stage of reverse osmosis (consequently, the performance of the membrane improves with time of operation); (3) very high salt rejection (over 99%) with high water flux (up to 38 gfd) can be obtained with 3.5% NaCl at 1500 psi (membranes perform equally well under conditions of sea water conversion and brakish water treatment).