WO1997026284A1

WO1997026284A1 - Method for gas phase sulfonation of polymer membranes

Info

Publication number: WO1997026284A1
Application number: PCT/US1997/000773
Authority: WO
Inventors: Jerome F. Ditter; Charles S. Hoffman, Jr.
Original assignee: Memtec America Corporation
Priority date: 1996-01-16
Filing date: 1997-01-16
Publication date: 1997-07-24
Also published as: AU1831397A

Abstract

Methods and apparatus for gas phase sulfonation of preformed polymeric membranes are provided in accordance with the present invention. The invention enables conversion of a hydrophobic membrane that is capable of reaction with a gas phase sulfonation agent into a hydrophilic membrane. As will be understood, aromatic rings, amines, hydroxyl groups, and other reactive moieties in polymers are readily sulfonated when exposed to sulfur trioxide gas. Thus, in accordance with the present invention, a preformed polymer membrane that contains aromatic rings or other moieties that are capable of reaction with a gas phase sulfonation agent, is exposed to a gas phase sulfonation agent, such as sulfur trioxide, and the aromatic rings or other moieties of the polymer become sulfonated. The inclusion of sulfonate groups on such polymers, and the membranes formed therefrom, renders such polymers totally or partially hyrophilic. The invention also relates to polymeric membranes, including asymmetric sulfone polymer membranes sulfonated by the process of the invention, and to devices for carrying out the sulfonation method of the invention.

Description

METHOD FOR GAS PHASE SULFONATION OF POLYMER MEMBRANES

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a method of preparing sulfonated polymer membranes, particularly sulfone polymer membranes. In the method, a formed polymer membrane is reacted with gaseous sulfur trioxide, followed by neutralization. The membrane, so reacted, is rendered hydrophilic in the process. The present invention also relates to sulfonated polymer membranes, particularly sulfonated asymmetric sulfone polymer membranes, and to apparatus for gas phase modification of polymer membranes.

Background of the Technolonv

The uses of filtration membranes are exceedingly diverse, and include, for example, reverse osmosis, computer chip manufacturing, medical applications, and beverage processing. As the applications for filtration membranes are numerous, so too are the structures of the membrane and the materials of which they may be made.

Membrane structure can be classified by the pore size, thickness, and void volume of the membrane.

Another important factor in membrane structure is the cross-sectional symmetry or asymmetry of the membrane.

A membrane that is symmetric, or isotropic, has relatively constant pore sizes throughout its thickness, while an asymmetric membrane has variable pore sizes, usually having relatively larger pores on one side of the membrane and relatively smaller pores on the opposite side of the membrane. Advances in membrane technology have led to membranes having high degrees of cross-sectional asymmetry, as discussed below.

The materials of which membranes may be constructed also have a major effect on the applications for which the membranes may be used. Polymer membranes are common, and sulfone polymers are preferred for many applications because of their availability, durability, versatility, and amenability to casting conditions that result in a great variety of membrane porosities and structures. However, membranes cast from sulfone polymers are generally hydrophobic, and are therefore limited in their applicability, being unsuitable for several important uses to which hydrophilic membranes would be well suited.

The hydrophobic sulfone polymers can be modified chemically to become more hydrophilic. However, when a membrane is cast from such forms of chemically modified hydrophilic sulfone polymers, much of the potential for controlled variation in membrane structure, such as high cross sectional asymmetry of the membrane, is lost. It is difficult, if not impossible, to cast a highly asymmetric membrane from a hydrophilic polymer by conventional methods. Accordingly, it is desirable to combine the membrane casting versatility of the hydrophobic sulfone polymers with the advantageous properties of hydrophilic moieties in a membrane.

The hydrophilic membranes of the present invention can have any of a wide variety of cross-sectional pore configurations. Particularly preferred membranes sulfonated in accordance with the invention are membranes with cross-sectional pore size gradients, wherein the pores are relatively small on one surface and large on the other surface, with a gradation in size in the^" interior of the membrane. Such membranes are referred to either as asymmetric or as anisotropic membranes. In this discussion the term "asymmetric" will be used, in keeping with the terminology of the Wrasidlo and Zepf patents, as discussed below.

Asymmetric membranes are well known in the art. For example, Wrasidlo in U.S. Patent Nos. 4,629,563 and 4,774,039 and Zepf in U.S. Patent Nos. 5,188,734 and 5,171,445, the disclosures of which are hereby incorporated by reference, disclose asymmetric membranes and methods for their production. Each of the Wrasidlo and Zepf patents discloses integral, highly asymmetric, microporously skinned membranes, having high flow rates and excellent retention properties. The membranes are generally prepared through a modified "phase inversion" process using a metastable two-phase liquid dispersion of polymer in solvent/nonsolveπt systems which is cast and subsequently quenched in a nonsolvent. The Zepf patent discloses an improvement over the Wrasidlo patent.

Phase inversion processes generally proceed through the steps of: (i) casting a solution or a mixture comprising a suitably high molecular weight polymer(s), a solveπt(s), and a nonsolvent(s) into a thin film, tube, or hollow fiber, and (ii) precipitating the polymer through one or more of the following mechanisms:

(a) evaporation of the solvent and nonsolvent (dry process), generally in conjunction with mechanism (b);

(b) exposure to a nonsolvent vapor, such as water vapor, which absorbs on the exposed surface and reduces the solubility of the polymer in the cast film (vapor phase induced precipitation process);

(c) quenching in a nonsolvent liquid, generally water (wet process);

(d) thermally quenching a hot film so that the solubility of the polymer is suddenly greatly reduced (thermal process); or

(e) combinations of the above.

Schematically, the inversion in phase from a solution to a gel proceeds as follows:

SOL 1 > SOL 2 > Gel (solution) (dispersion)

Essentially, SOL 1 is a homogenous solution, SOL 2 is a dispersion, and the Gel is the formed polymer matrix. The event(s) that triggers SOL 2 formation depends on the phase inversion process used, but always involves polymer solubility. In the wet process, SOL 1 is cast and contacted with a nonsolvent for the polymer, which triggers the formation of SOL 2 which in turn "precipitates" to a Gel. In the vapor phase induced precipitation process, SOL 1 is cast and the film is exposed to a gaseous atmosphere including a nonsolvent for the polymer and then to a non-solvent liquid quench. When exposed to the gaseous environment, solvent and non-solvent can begin to evaporate and/or water vapor can be absorbed, both phenomena of which reduce the solubility of the polymer in the solvent and begin the formation of SOL 2. Quenching in the liquid nonsolvent then completes the transformation into SOL 2 and "precipitates" it to a Gel. In the thermal process, SOL 1 is cast as a hot solution which, when suddenly cooled, reduces solubility of the polymer and leads to formation of SOL 2, which in turn "precipitates" to a Gel. In the dry process, SOL 1 is cast and contacted with a gaseous atmosphere, such as air, which allows evaporation of solvent and non-solvent and, in conjunction with absorption of water vapor, triggers the formation of SOL 2 and finally "precipitation" to the Gel state.

The nonsolvent, or "pore former," that is added to the casting dope is not necessarily completely inert toward the polymer; in fact it usually is not inert toward the polymer and is often referred to as a swelling agent, in the Wrasidlo-type formulations, selection of both the type and the concentration of the nonsolvent is important in that it is the primary factor in determining whether or not the dope will achieve a phase separated condition. In general, the nonsolvent is the primary pore forming agent, and its concentration in the dope greatly influences the pore size and pore size distribution in the final membrane. The polymer concentration also influences pore size, but not as significantly as does the nonsolvent. It does, however, affect the membrane's strength as well as its porosity, or void volume. In addition to the major components in the casting solution, or dope, there can be minor ingredients, such as surfactants or release agents. Polysulfone is especially amenable to formation of highly asymmetric membranes, particularly in the two-phase Wrasidlo formulations. Under the right temperature conditions, as described by Wrasidlo, these solutions are not homogeneous, but consist of two separate phases, one a solvent-rich clear solution containing relatively low concentrations (on the order of 7%) of lower molecular weight polymer, and the other a polymer-rich, turbid, colloidal solution containing relatively high concentrations (on the order of 17%) of higher molecular weight polymer. The two phases contain the same three ingredients, that is, polymer, solvent, and nonsolvent, but in radically different concentrations and molecular weight distributions. Most importantly, the two phases are incompatible -they are not soluble in one another. If mixed and then allowed to stand, the phases will separate. The dope must be homogeneous when cast and therefore must be agitated prior to that time. Essentially, in Wrasidlo type formulations the casting dope is a SOL 2 dispersion, which effectively serves as the starting point for gel formation, which occurs as follows:

SOL 2 > Gel

(Wrasidlo dispersion)

This process modification was largely responsible for the higher degrees of asymmetry and uniform consistency of the Wrasidlo Membranes as compared to the prior art.

It is the nonsolvent and its concentration in the casting mix that produces phase separation, and not every nonsolvent will do this. If the temperature of the mix is changed, phase transfer occurs. Heating generates more of the clear phase; cooling does the reverse. Concentration changes have the same effect, but there is a critical concentration range, or window, in which the phase separated system can exist, as discussed by Wrasidlo. Wrasidlo defines this region of instability on a phase diagram of thus dispersed polymer/solvent/nonsolvent at constant temperature, lying within the spinαdal or between the spinodal and binodal curves, wherein there exist two macroscopically separated layers.

Because of the great hydrophobicity of the polymer and because of the thermodynamically unstable condition of the casting mix, wherein there pre exist two phases, one solvent-rich and the other polymer-rich (a condition that other systems must pass through when undergoing phase inversion), the unstable Wrasidlo mixes precipitate very rapidly when quenched. This rapid precipitation forms a microporous skin at the interface and consequently results in a highly asymmetric membrane, a structure shared by the membranes of each of the Wrasidlo and Zepf patents. "Asymmetric" as used in the context of the Wrasidlo patents, as discussed previously, refers to membranes that possess a progressive change in pore size in the cross-section between the microporous skin and the more open surface of the membrane. This configuration is in contrast to reverse osmosis and most ultrafiltration membranes, which are also referred to in the art as asymmetric membranes, but which have abrupt discontinuities between a

"nonmicroporous skin" and the supporting substructure.

The microporous skin is the fine pored side of the membrane that constitutes the air-solution interface or the quench-solutioπ interface during casting. In the Wrasidlo patent, and in this disclosure, it is understood that the term "skin" does not indicate the relatively thick, nearly impervious layer of polymer that is present in some membranes. Herein, the microporous skin is a relatively thin, porous surface that overlies a microporous region of variable thickness. The pores of the underlying microporous region may be about the same size as, or somewhat smaller than, the skin pores, in an asymmetric membrane, the pores of the microporous region gradually increase in size as they lead from the skin to the opposite face of the membrane. The region of gradual pore size increase is sometimes referred to as the asymmetric region, and the opposite, non-skin face of the membrane is often referred to as the coarse pored surface. As a contrast to the coarse pored surface, the skin is also sometimes called the microporous surface.

Polymeric membranes can also be cast from homogeneous solutions of polymer, particularly if the solutions are near the saturation, or gel, point. The composition of these formulations lies outside of the spinodal/binodal region of the phase diagram of Wrasidlo. Membranes cast from homogeneous solutions may also be asymmetric, although they are not usually as highly asymmetric as those cast from phase separated formulations.

The Wrasidlo membranes have improved flow rates and permselectivity in relation to prior art membranes.

Such improved flow rates and permselectivity arise from the structure of the membranes.

The Zepf patents disclose improved Wrasidlo-type polymer membranes having a substantially greater number of microporous skin pores of more consistent size and greatly increased flow rates, with reduced flow covariance for any given pore diameter. The improved Zepf membranes are achieved by modifications to the Wrasidlo process, comprising reduced casting and quenching temperatures and reduced environmental exposure between casting and quenching. Zepf further teaches that reduced casting and quenching temperatures minimize the sensitivity of the membrane formation process to small changes in formulation and process parameters. Asymmetric microfiltration membranes are advantageous for a variety of filtration processes, including industrial and municipal water purification. In industrial applications, asymmetric membranes are particularly advantageous in the manufacturing of electronic devices; in the food and beverage industry, such as with beer, wine, and juices; in pharmaceuticals; in medical applications, such as for IV filters and centrifugal blood separation; and the like. Various useful configurations exist, including disks, pleated cartridges, single wrap cartridges, and spiral wound cartridges are available. These membranes also have become increasingly relevant to the medical diagnostics industry because of the availability of large pores in conjunction with an asymmetric structure. The advantage of the Wrasidlo/Zepf type membranes, with their large pore gradients, lies in their greater dirt holding capacities and higher per eant fluxes compared with homogeneous membranes. With the larger pore surface facing upstream, a liquid containing suspended solids has its larger particles removed first, followed by the removal of successively smaller and smaller particles, thereby significantly extending the life of the filter. This is in contrast to membranes with homogeneous cross-sections, wherein the fine pores on the upstream side of the membrane can plug more easily, retarding flow and lowering the dirt holding capacity. Moreover, because the small pore section of the membrane of a Wrasidlo-type asymmetric membrane is relatively thin, the flow rates through the membrane are significantly faster than through a comparable pore size homogeneous membrane.

For example, asymmetric membranes have proven particularlγ useful in blood separation applications. See e.g., Koehen et al. U.S. Patent No. 5,240,862. When whole blood is applied to the open pored surface, the cells are filtered out and retained in the porous support of the membrane, while the plasma passes through the membrane. By placing the microporous surface in contact with an analyte detection device, the presence or absence of a particular analyte can be measured without interference from the cells. Further, this structure allows one to conduct diagnostic assays without centrifugation. Testing of the permeant (filtrate) can be accomplished in several ways. For example, filtrate can be tested physically, chemically, electrically, or by bacterial culture analysis.

As was mentioned above, asymmetric membranes can be prepared from certain hydrophobic polymers, such as sulfone polymers and mixed cellulose esters. The sulfone polymers include any polymer containing the sulfone group; important classes of sulfone polymers are polysulfones, polyethersulfoπes, and polyarylsulfones. Where membranes are prepared using hydrophobic polymers, the resulting membranes are hydrophobic and water will not pass through them at pressures below the bubble point unless primed with a fluid that wets the membrane. Therefore, in applications requiring operation of membranes in aqueous environments, the membranes, or the polymers prior to fabrication into membranes, are typically reacted with, or mixed with, respectively, moieties that cause the resulting membranes to become hydrophilic.

For example, there are several strategies for creating hydrophilic membranes from hydrophobic polymers, including: o sulfoπating hydrophobic polymers prior to casting them as membranes; o contacting cast hydrophobic membranes with appropriate wetting agents that impart hydrophilic properties to the cast membranes; and o including hydrophilic moieties in the casting dope prior to casting membranes therefrom. Each of these methods for imparting hydrophilicity to membranes has inherent problems or difficulties. For example, post-addition of a wetting agent to the membrane can plug pores and reduce permeability. Moreover, unless adequately crosslinked or chemically bound to the polymer, the wetting agent may leach out during filtration, thereby not only reducing hydrophilicity but also creating a risk of contaminating the filtrate. One can reduce leaching by crosslinking the wetting agent and intertwining it with the membrane polymer. For example, Roesink et al. in U.S. Patent No. 4,798,847 (now Re. No. 34,296) disclose crosslinking polyvinylpyrrolidone throughout the structure of the polysulfone membranes. However, while crosslinking hydrophilic moieties to membranes appears to minimize leaching, it can also reduce hydrophilicity in proportion to the number of crosslinks created. Moreover, it adds an additional step and complexity to the formulation and casting process of a membrane.

Where hydrophobic polymers are sulfonated prior to casting, it is very difficult, if not impossible, to prepare asymmetric membranes therefrom, because the hydrophilicity imparted by the sulfonic acid groups retards precipitation during the quench process. Thus, one is generally constrained to manufacture only isotropic (symmetric) membranes from hydrophobic polymers that are sulfonated prior to casting.

Another approach to imparting hydrophilicity to membranes involves the inclusion of a hydrophilic moiety within the casting suspension. For example, Kraus et al. in U.S. Patent Nos. 5,108,607, 4,964,990, and 4,900,449 disclose formation of hydrophilic microfiltration membranes from hydrophobic polymers by including in the casting solution a hydrophilic polymer, such as polyethyleneglycol or polyvinylpyrrolidone. The latter two issued patents additionally focus on the crosslinking of hydrophilic components, such as PVP, in and with the cast membrane. The membranes prepared in accordance with the Kraus patents are, however, isotropic and are therefore not well suited to the testing and enhanced throughput microfiltration applications that require asymmetric membranes.

Accordingly, it would be desirable to provide an asymmetric microporous membrane that operates efficiently and effectively, and that has a high degree of stable hydrophilicity, as well as sufficient strength and rigidity. It would also be desirable to provide a method to convert a hydrophobic membrane of a given structure into a hydrophilic membrane of the same structure.

SUMMARY OF THE INVENTION In accordance with a first aspect of the present invention, there is provided a method for preparing a sulfonated polymer membrane by contacting a preformed polymer membrane with gaseous sulfur trioxide. The polymer membrane may contain a hydrophobic polymer, and the polymer may include one of more kinds of reactive moieties, such as for example an aromatic ring, a hydroxyl group, an amine, or other reactive moiety. The gaseous sulfur trioxide causes the sulfonation of some or all of the reactive groups in the polymer of the membrane to form, for example, sulfonic acid groups, and the membrane is subsequently contacted with a neutralizing agent that converts some or all of the acid groups to the sulfonic acid salt, sulfate salt, or other hydrophilic group, depending on the reactive moiety that is sulfonated. After the sulfonated polymer membrane is neutralized it may be recovered for use or for further chemical or mechanical modification.

The preformed membrane may be hydrophobic before the sulfonation reaction, and it may be hydrophilic after it is sulfonated. Preferably, the preformed membrane is asymmetric and may also be microporous. The polymer of the membrane may be a sulfone polymer, such as, for example, polysulfone, polyether sulfone, or polyarylsulfone. The gaseous sulfur trioxide may be generated in situ, which generation maγ include a stream of heated sulfur dioxide and air, which may be passed over a catalyst, such as vanadium pentoxide, at, for example, a temperature exceeding 400°C.

Examples of some of the neutralizing agents useful in the method of the invention are alkali metal hydroxides and ammonia gas. The neutralization step may follow within seconds of the sulfonation step. The sulfonation method of the invention may also include a washing step wherein the sulfonated polymer membrane is washed, for example in water, such as after the sulfonation and neutralization steps.

A second aspect of the present invention provides a sulfonated polymer membrane produced by the method of the invention. The membrane of this aspect of the invention may be hydrophilic after the sulfonation, and is preferably asγmmetric. The membrane maγ also be microporous. The polymer of the membrane may be a sulfone polymer, such as, for example, a polyether sulfone, a polyarγl sulfone, or a polysulfone.

In a third aspect of the invention, a device for gas phase sulfonation of a membrane is provided. The device provides systems for exposing a membrane to a sulfonation agent and to a neutralizing agent. It preferably also has a dry air source, an air knife, a vacuum, and a porous cylinder. The device may also comprise an inert housing for containing some or all of the gasses used in the method of the invention. A preferred embodiment of the housing is TEFLON. The air knife or knives maγ be used to contact the membrane and release gas into the membrane, and the device maγ also have rollers for advancing the membrane through the device. The cγliπders also be porous and may be adapted to allow passage of a gas therethrough, either into or out of the cylinder, for example while the cylinder is in contact with the membrane. That is, the porous cylinder may communicate a gas from the cylinder and through the membrane bγ positive pressure, or it maγ pull a gas through the membrane and into the cγlinder, due to the existence of a vacuum within the cγlinder.

BRIEF DESCRIPTION OF THE DRAWING

Figure 1 is a schematic diagram of an apparatus for accomplishing gas phase sulfonation in accordance with the invention.

Figure 2 is a schematic diagram of another apparatus for accomplishing gas phase sulfonation in accordance with the invention.

Figure 3 is a schematic diagram of another apparatus for accomplishing gas phase sulfonation in accordance with the invention. Figure 4 is a schematic diagram of another apparatus for accomplishing gas phase sulfonation in accordance with the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS in accordance with the present invention, we have discovered that it is possible to manufacture a highly hydrophilic, microfiltration and ultrafiltration membrane through gas phase sulfonation of a preformed polymer membrane. We have demonstrated that aromatic rings, hydroxyl groups, amines, and other reactive moieties in poiγmers are readilγ sulfonated when exposed to sulfur trioxide gas. Thus, we discovered that through exposing a polγmer membrane, in which the polγmer from which the membrane was formed contains aromatic rings, hγdroxγl groups, amine groups, or other reactive moieties, is exposed to sulfur trioxide the polγmer becomes sulfonated. The inclusion of sulfonic or sulfonate groups on such polymers (and the membranes formed therefrom) renders such polymers hydrophilic. Thus, the process of the present invention allows the manufacture of hydrophilic polymer membranes from hydrophobic polymer membranes.

As was mentioned above, it is possible to sulfonate bulk polymers which possess, for example, aromatic groups or other reactive moieties. Membranes formed from sulfonated polymers are generally hydrophilic. However, such membranes are almost exclusively isotropic in cross-sectional structure. In accordance with the present invention, on the other hand, sulfonation is accomplished after the membrane is cast. An advantage of this process is that a membrane of a given structure can be sγnthesized or manufactured in a conventional manner and can thereafter be sulfonated. The resulting sulfonic acid groups or other sulfonation created groups on the membrane are covaiently attached, and therefore do not leach. Moreover, the cross-sectional configuration of the membrane is not altered. For example, a polysulfone membrane can be prepared in accordance with the Wrasidlo patent so as to possess a highly asymmetric structure, with fine pore sizes of approximately 0.05 μm and coarse pores on the opposite face of 5 μm or greater (a degree of asymmetrγ of 100 or greater). The membrane prepared bγ this process will be inherentlγ hγdrophobic. However, such a membrane can be sulfonated in accordance with the invention and rendered hγdrophilic. Moreover, such a membrane retains its asγmmetric structure, and the hγdrophilic groups are covaiently attached to the membrane.

Thus, one aspect of the present invention relates to a sulfonated hγdrophilic asγmmetric sulfone polγmer membrane.

In another aspect, the present invention relates to a method to prepare sulfonated polγmer membranes, particularly sulfone polymer membranes. In the method, a formed polγmer membrane is reacted with gaseous sulfur trioxide to create sulfonic acid moieties covaiently linked to the polymer. The sulfur trioxide reacts readily with aromatic rings as well as with certain paraffinic hydrocarbons, such as polyethylene, and forms pendant sulfonic acid ( S0₂0H) functional groups on the polymer chain. If the particular polymer contains hγdroxγl groups, the sulfur trioxide will form sulfates ( 0S0₂0H) at those points. The present application deals primarilγ with sulfone polγmers, all of which contain aromatic rings and which consequentlγ contain sulfonic acid groups when sulfonated. Sulfonic acid groups, in addition to imparting hγdrophilicitγ to the membrane, are amenable to further reaction with appropriate chemical compounds, such as reactive monomers, for example: o ethγlene imine which polγmerizes to polyethylene imine; and o hγdroxγl reactive materials such as isocyanates, acid halides, acid anhγdrides, and the like for the binding of functional groups, such as proteins, peptides, and the like. Starting with an initially hγdrophobic membrane, water droplets placed on the surface of a membrane so reacted will display a reduced water droplet contact angle (a measure of hydrophobicity) in proportion to the degree of sulfonation. At some degree of hydrophilicity, the membrane becomes completely wettable. The sulfonic acid groups in the membrane can be neutralized with a base, which replaces the proton with a cation. A convenient neutralization method which precludes having to dry the treated membrane is reaction with gaseous ammonia. Neutralization by this method converts the sulfonic acid groups to sulfonic salt groups, and appears to improve hydrophilicity.

As was mentioned above, the reason that post-sulfonation of an asγmmetric membrane is preferred to pre- sulfonation of the uncast polγmer lies in the difficultγ of sγnthesizing an asγmmetric membrane in which the polγmer is alreadγ hγdrophilic. If the membrane is already asymmetric, it retains that structure after it has been reacted with sulfur trioxide. Accordingly, the present invention provides a way to circumvent this problem. In the process, anγ preformed polymer membrane in which the polymer making up the membrane contains aromatic rings, amines, hydroxγls, or other reactive moieties (and particularlγ those membranes that are inherentlγ hydrophobic and are asγmmetric) can be sulfonated. The originally hγdrophobic membrane, depending on the degree of sulfonation, becomes hγdrophilic. Moreover, the sulfonate groups are covaiently bound to the membrane through, for example, the aromatic rings of the polγmer and are not leached readilγ from the membrane.

In preferred embodiments, membranes are prepared from hγdrophobic polymers containing aromatic rings. Examples of preferred polymers are sulfone polymers such as polysulfone, polyarγlsulfone, or polγethersulfone, as well as polyamides, pofyamide-imides, polγimides, and polyphenylene oxides, to name a few. In highly preferred embodiments, the membranes are prepared from sulfone polymers, preferably from polysulfone, and most preferably contain Udel 3500 polysulfone, available from AMOCO PERFORMANCE PRODUCTS of Roswell, Georgia.

Structure αf Polyethersulfone

Structure of Polyarylsulfone

Structure of Udel Polysulfone -10-

As is evident from the chemical structures provided above, sulfone polγmers possess several aromatic rings that, after casting as a membrane, are susceptible to gas phase sulfonation in accordance with the invention.

Method for Gas Phase Sulfonation of Membranes Gas phase sulfonation has been successfully accomplished in accordance with the invention bγ exposing preformed hγdrophobic membranes to gaseous sulfonation agents, wherein the hγdrophobic membrane comprises a polγmer containing aromatic rings amines, hγdroxγl groups, or other reactive moieties. Gaseous sulfonation moieties include sulfur trioxide, either neat or from fuming sulfuric acid, and other solutions. In preferred embodiments, the gaseous sulfonation agent is neat sulfur trioxide vapor. Exposure can be accomplished, for instance, bγ simplγ placing a membrane in contact with the vapors from liquid sulfur trioxide or fuming sulfuric acid (which is a solution of sulfur trioxide in sulfuric acid). Preferablγ, however, exposure of the membrane is better accomplished bγ passing gaseous sulfur trioxide through the membrane. In preferred embodiments, a gas stream containing sulfur trioxide and dry air is passed through a membrane which is desired to be sulfonated. As will be appreciated, sulfur trioxide vapors can be obtained in a varietγ of waγs. Vapors can be obtained as theγ are evolved from a solution, such as from fuming sulfuric acid, or gaseous sulfur trioxide can be used directly. In addition, one can condense sulfur trioxide gas and utilize the vapors from liquid sulfur trioxide (b.p. 44.8°C).

The invention maγ also be practiced using certain liquid formulations that contain sulfur trioxide. In these formulations, the principal concern is that the liquid must not dissolve the membrane, and further that it must not interfere with the sulfonation reaction, nor participate m secondarγ reactions. Suitable liquid sulfonation formulations of the invention dissolve sulfur trioxide in a non-protonated liquid, such as liquid fluorocarbons. Other preferred carrier liquids that would not dissolve a sulfone polγmer membrane are anγ of the various silicone oils.

Following sulfonation, the resulting membrane is generallγ rendered more hγdrophilic than the starting membrane. The hydrophilicity of the resulting membrane depends both on the hydrophobicity of the starting membrane and an the exposure time and temperature and on the reactivity of the affected groups in the membrane. in a preferred embodiment, roll stock of a preformed hydrophobic membrane is exposed to sulfur trioxide gas as the gaseous sulfonation moiety. The sulfur trioxide gas is passed through the membrane stock by the use of pressure differentials on the opposite surfaces of the membrane. For example, in certain embodiments, sulfur trioxide gas is simply blown through the membrane with positive pressure. In other embodiments, sulfur trioxide gas is pulled through the membrane with application of a vacuum. In still other embodiments, the sulfur trioxide gas can be pushed and pulled through the membrane simultaneously by applying positive pressure on one side of the membrane and a vacuum on the other side of the membrane. Preferably, exposure of the membrane is accomplished in a contained environment, for example, in an inert housing, constructed, for example, of TEFLON, glass, ceramic, or, if water vapor is effectively excluded, stainless steel or even mild steel.

As mentioned above, sulfur trioxide gas may be generated through a number of processes. In a preferred embodiment, sulfur trioxide gas is generated by the well known commercial method of passing a pre heated ( > 400°C) air/sulfur dioxide gas stream over a vanadium pentoxide catalyst (available from Monsanto Chemical Co., St. Louis, Missouri). The sulfur dioxide reacts with oxygen in the air and forms sulfur trioxide gas.

In a preferred embodiment, the apparatus possesses reels for winding and unwinding the membrane. Between the reels, a device or combinations of devices for exposing the membrane to sulfur trioxide is disposed. In one embodiment, the device for exposing the membrane to sulfur trioxide is an air knife over which the membrane is pulled and through which flows sulfur trioxide, air, and any unreacted sulfur dioxide. Hydrophilicity in the membrane is created in proportion to the degree of sulfonation, which in turn is a function of exposure time, sulfur trioxide concentration, temperature, and reactivity of the membrane. Exposure time is controlled by the speed at which the membrane is pulled across the knife gap or other mode of exposure to the gas phase sulfonation agent. Pore size of the membrane and the polymer concentration in the membrane play a role in the amount of exposure time that is required. More open membranes, such as microfiltration membranes having low bubble points, sulfonate much more rapidly than do tighter membranes, such as microfiltration or ultrafiltration membranes having high bubble points because the sulfur trioxide passes through the membrane more readily with less diversion around the edges of the membrane. That is to say, that microporous membranes having pore sizes of about 0.01 μm and greater will sulfonate more readily than will ultrafiltration, reverse osmosis, and gas separation membranes having pore sizes of about 0.01 μm and lower. Such factors can be optimized for a given membrane by those of ordinarγ skill in the art without undue experimentation.

In preferred embodiments, after sulfonation of the membrane, the sulfonic acid form of a polγmer having aromatic rings, such as polysulfone, having -SO-H groups covaiently attached to the aromatic rings, is neutralized by substituting a monovalent cation for the proton on the sulfonic acid group. For example, the membrane can be dipped into a dilute sodium hydroxide (NaOH) solution or into sodium methoxide in an isopropyl alcohol solution. However, a more facile technique is to pass ammonia gas, H_j, through the membrane and thereby convert the sulfonate moietγ to the ammonium salt, -S0₃ NH \

In the above-described process, it will be appreciated that both ambient air and the membrane itself, prior to neutralization, maγ contain a small amount of water, and reaction of the trioxide with the water will form sulfuric acid on the surfaces of and inside the membrane. This can be deleterious to the membrane bγ creating holes and/or causing laγers of the rolled up membrane to stick to one another. An approach to overcome this problem is to neutralize the sulfonated membrane with ammonia gas almost i mediatelγ after the sulfonation step. As with the sulfonation reaction, described above, the neutralization step is accomplished in a preferred embodiment bγ moving the membrane over or through a second reactor, for example an air knife through which ammonia gas flows. In addition, drγ air can be blown through the membrane prior to sulfonation to remove water vapor and after sulfonation to remove residual S0₃. Bγ the process of the invention, ^'gas phase sulfonation of membranes formed from polymers having aromatic rings or paraffinic moieties easily and rapidly can change a hydrophobic membrane into one that is inherently hydrophilic. Where membranes of the invention are initially hydrophilic, the addition of sulfonate groups may still be desirable for the purpose of, for example, derivatization of the membrane, such as by formation of negatively charged sulfonic acid groups or sulfates, which do not leach out from the membrane when washed with water or other fluids that are non-solvents for the polymer.

Apparatus for Gas Phase Sulfonation of Membranes

Described below are several embodiments of apparatus that are useful in the process of gas phase sulfonation of membranes in accordance with the present invention. Preferred embodiments of apparatus in accordance with the invention endeavor to achieve proper guiding and tensioning of the membrane as it is exposed to the gas phase sulfonation agent so that intimate and uniform contact occurs across the entire width of the membrane. The apparatus described below are particularlγ adapted to the gas phase sulfonation of sheet membrane roll stock. However, as will be appreciated, similar concepts underlγing the apparatus described below can be utilized in the gas phase sulfonation of other forms of membrane stock, such as hollow fibers or membranes contained in a filter housing.

Apparatus in accordance with the invention will now be described with reference to the figures. As will be understood, the figures are illustrative and are not meant to be limiting of the tγpes of apparatus that maγ be successfully utilized to accomplish gas phase sulfonation of membranes in accordance with the invention. Referring now to Figure 1 there is provided a schematic diagram of an apparatus 10 for accomplishing gas phase sulfonation of a flat sheet membrane in accordance with the invention. The apparatus 10 includes an inert housing 12 in which the sulfonation reaction is conducted. A membrane feed roller 14 is provided with membrane stock 18 for passing through the apparatus 10. Following passage through the apparatus 10, the membrane stock 18 is wound onto the membrane rewind roller 20. The housing 12 of the apparatus 10 is separated into three chambers: a sulfonation chamber 22, a dry air chamber 24, and a neutralization chamber 28. In the sulfonation chamber 22, the membrane stock 18 is exposed to the gas phase sulfonating agent, such as sulfur trioxide gas. In the dry air chamber 24, the membrane stock 18, is exposed to dry air to minimize the presence of moisture in the membrane stock 18 and also to remove residual gas phase sulfonation agent. In the neutralization chamber 28, the membrane stock 18 which was sulfonated in the sulfonation chamber 22 is neutralized, for example, with ammonia gas. The apparatus 10 can optionally be equipped with dry air sections preceding the sulfonation chamber 22 and after the neutralization chamber 28, as discussed in connection with Figure 3.

In operation, membrane stock 18 is sulfonated and neutralized in the apparatus 10 by winding the membrane 18 onto the membrane rewind reel 20 which pulls the membrane stock 18 from the membrane feed reel 14 through the apparatus 10. Sulfonation of the membrane 18 in the sulfonation chamber 22 is accomplished through use of a first air knife 30. The membrane 18 is passed over the gas port of the first air knife 30. The gas phase sulfonation agent, such as sulfur trioxide, is passed through the gas port of the first air knife 30 under pressure and communicated through the membrane 18. Thereafter, the membrane 18 is fed into the drγ air chamber 24, where the membrane stock 18 is pulled, under tension, through the drγ air chamber 24, around a porous tensioning cγlinder 32. Drγ air blows through the pores in the cγtinder 32 and through the membrane stock 18, to minimize the presence of moisture in the membrane stock 18 and to remove residual gas phase sulfonation agent. In the neutralization chamber 28, neutralization is accomplished through use of a second air knife 34. The membrane 18 is passed over the gas port of the second air knife 34. The neutralization agent, such as ammonia, is passed through the gas port of the second air knife 34 under pressure and communicated through the membrane 18. The sulfonated membrane is then collected on the membrane rewind reel 20.

A similar process is accomplished in the apparatus 40 that is shown in Figure 2 which is a schematic diagram of another apparatus for accomplishing gas phase sulfonation in accordance with the invention. The apparatus 40 includes an inert housing 12 in which the sulfonation reaction is conducted. A membrane feed roller 14 is provided with membrane stock 18 for passing through the apparatus 40. Following passage through the apparatus 40, the membrane stock 18 is wound onto a membrane rewind roller 20. The housing 12 of the apparatus 40 is separated into two chambers: a sulfonation chamber 22 and a neutralization chamber 28. In the sulfonation chamber 22, the membrane stock 18 is exposed to the gas phase sulfonating agent, such as sulfur trioxide gas followed bγ exposure to drγ air, vacuum/exhaust, and drγ air. Drγ air is used to minimize the presence of moisture in the membrane stock 18. Vacuum/exhaust is emploγed to remove anγ residual sulfonating agent and reduction of moisture in the membrane 18. In the neutralization chamber 28, the membrane stock 18 that was sulfonated in the sulfonation chamber 22 is neutralized with, for example, ammonia gas. In addition, the membrane stock 18 is exposed to drγ air and to vacuum/exhaust, to remove moisture and residual sulfonating and neutralizing agent.

In operation, membrane stock 18 is sulfonated and neutralized in the apparatus 40 bγ winding the membrane 18 onto the membrane rewind reel 20 which pulls the membrane stock 18 from the membrane feed reel 14 through the apparatus 40. Sulfonation of the membrane 18 in the sulfonation chamber 22 is accomplished bγ using a first porous cylinder 42. The membrane 18 is passed around the first porous cylinder 42 and the gas phase sulfonation agent, such as sulfur trioxide, is passed through the pores of the cylinder 42 under pressure and communicated through the membrane 18. Thereafter, the membrane 18 passes, under tension, around a second porous cγlinder 44 through which drγ air is fed under pressure. Drγ air blows through the pores in the cγlinder 44 and through the membrane stock 18, to minimize the presence of moisture in the membrane stock 18 and to remove residual sulfonating agent. Then, the membrane 18 passes, under tension, around a third porous cγlinder 48 through which a vacuum is drawn so that residual sulfonation agent will be drawn from the membrane and through the pores of the cylinder 48. Finally, dry air is again forced through the membrane 18 as it passes around a fourth porous cylinder 50.

The membrane 18 then proceeds into the neutralization chamber 28. In the neutralization chamber 28, the membrane 18 is exposed sequentially to air, neutralizing agent, vacuum, and air as the membrane 18 passes around fifth, sixth, seventh, and eighth porous cγlinders (52, 54, 58, and 60) respectivelγ. After neutralization, the sulfonated membrane emerges from the housing 12 and is collected on the membrane rewind reel 20.

A third embodiment of a sulfonation apparatus in accordance with the invention is shown schematically in

Figure 3. The apparatus 70 includes an inert housing 12 in which the sulfonation reaction is conducted. A membrane feed roller 14 is provided with membrane stock 18 for passing through the apparatus 70. Following passage through the apparatus 70, the membrane stock 18 is wound onto a membrane rewind roller 20. In this embodiment, the apparatus 70 is separated into five chambers: a first dry air chamber 72, a sulfonation chamber

22, a second dry air chamber 74, a neutralization chamber 28, and a third dry air chamber 78.

In each of the first, second, and third dry air chambers (72, 74, and 78), the membrane 18 is passed between two porous cγlinders (72a/72b, 74a/74b, and 78a/78b, respectivelγ), through which drγ air is blown in an effort to keep the membrane stock 18 drγ to prevent unwanted reactions with anγ residual sulfonating agent or neutralizing agent.

In the sulfonation chamber 22, the membrane 18 is similarly drawn between two porous rollers 22a and

22b. Roller 22a is supplied with pressurized sulfonation agent, such ai sulfur trioxide, which passes through the pores of the cylinder 22a and into the membrane 18. Roller 22b is connected to a vacuum line and assists in pulling the sulfonation agent from cγlinder 22a through the membrane 18.

The neutralization chamber 28 is like the sulfonation chamber 22 with roller 28a supplγiπg neutralization agent to the membrane 18 under pressure and roller 28b pulling a vacuum to assist the neutralization agent in passing through the membrane 18. Figure 4 depicts another embodiment of the apparatus of the invention. The apparatus 80 includes an inert housing 12, preferablγ of TEFLON. In the operation of the apparatus 80, a membrane stock 18 is pulled across a tensioning roller 82 and a drγ air knife 84 bγ means of a rotating porous cγlinder 88. The membrane 18 is sequentially exposed to additional dry air from a dry air manifold 90, sulfur trioxide/air mixture from a second manifold 92, and more dry air from a third manifold 94. Any mixing of inlet gasses is minimized by baffles 98 between the manifolds. The flow and exhaust of gasses through the membrane 18 and through the rotating porous cylinder 88 is effected by a vacuum 100 inside the porous cylinder 88. The dry air and the sulfur trioxide (at atmospheric pressure) are contained inside the compartmented housing 12. The membrane 18 exits the housing 12 across a second dry air knife 102, and across a second tensioning roller 104, then passes over the ammonia neutralization knife 108 and is rewound.

Examples

EXAMPLE 1 - 10K MEMBRANE

An ultrafiltration membrane of Udel 3500 polysulfone having an approximate molecular weight filtration cutoff of 10,000 daltons was sulfonated according to the method of the invention. In this and the subsequent Examples, sulfur trioxide (165 ml/min based on quantitative conversion of the dioxide) carried in dry air (initially 3.56 liters/min) was passed through the 10.5-inch wide membrane. The membrane roll stock traversed the reactor at speeds varγing from about 5 ft/min to ^'15 ft/min. The pore size of the membrane and the desired degree of wettabilitγ govern the speed. Relatively open membranes, in the range of about 0.65μm to about 5μm, were advanced through the apparatus of Figure 4 at faster speeds, while 10K ultrafiltration membranes were exposed for a longer time. A comparison of the pre- and post-sulfonatioπ properties of the 10K ultrafiltration membrane demonstrates the changes in the membrane after sulfonation:

10K Membrane

Property Untreated Sulfonated

Water flow @ 10-psi 17 ml/min 1.6 ml/min

BSA protein rejection 46.8% 30.6%

IgG protein rejection 98% 88.5%

Speed through reactor ... 7.5-ft/min

Especially with ultrafiltration membranes having very small pore sizes, the sulfonation reaction causes swelling of the surface and reduces the water flow rate significantly. As can be seen from the data on BSA and IgG rejection, sulfonation does not enhance the 10K membrane's protein rejection. Therefore, the primary advantage to be gained from sulfonating ultrafiltration membranes may be the likely reduction in fouling of the membrane, since a sulfonated ultrafiltration membrane would tend to reject negatively charged species, which constitute most of the fouling constituents in waste streams. The larger pored membranes, having applications for medical and beverage filtration, are the more preferred subjects of gas phase sulfonation.

EXAMPLE 2 - TOOK MEMBRANE

A hydrophobic sulfone polymer membrane having an approximate 100,000 dalton molecular weight cutoff was sulfonated according to the method of the invention. The following table shows the resulting sulfonation efficiency and membrane hydrophilicitγ:

100K Membrane

Property Untreated Sulfonated

Water flow § 10-psi 64-ml/min 31 -ml/min

Speed through reactor ... 9.1 -ft/min

Ratio of polysulfone monomer to sulfonic ... 28/1 acid moiety EXAMPLE 3 - BTS-55 MEMBRANE

An asγmmetric microfiltration membrane was sulfonated according to the method of the invention. The membrane hγdrophilicitγ changed according to the following process parameters:

BTS-55 Membrane

Property Untreated Sulfonated

Water flow @ 10-psi 1300-ml/min 638-ml/min

Speed through reactor ... 11.3-ft/min

Ratio of polysulfone monomer to sulfonic ... 50/1 acid moiety

Water flow rates for examples 1-3 were measured through a 90-mm disc of the membrane with an effective diameter of 75-mm. Ratios of polysulfone monomer to sulfonic acid moiety were measured as follows:

(1) The monomer content was determined by weighing the disc.

(2) The sulfonic acid concentration in the membrane was measured bγ successivelγ filtering 10-ml aliquots of a standardized (5.0-mg/liter) methγlene blue solution through the membrane. Methγlene blue contains a positively charged sulfur atom and is absorbed by the negatively charged sulfonic acid. Absorption bγ the membrane was determined bγ first plotting the optical absorption at 665-nm versus pre set concentrations of methγlene blue in an aqueous solution, then measuring the optical absorption of each aliquot of the methγlene blue permeant solution. The amount absorbed each time was determined bγ the difference between the concentration in the original solution and the concentration in the permeant. The total amount absorbed was the sum of the amounts for each aliquot.

EXAMPLES 4-7 • EFFECT OF PORE SIZE AND EXPOSURE TIME ON EFFICIENCY OF SULFONATION Exposure time of the membrane to the sulfonating agent is an inverse function of the speed at which the membrane is advanced through the reactor. A faster speed results in a shorter exposure time.

Asymmetric polysulfone membranes having different pore sizes are advanced through the device as shown in Figure 4 at different speeds. The effect on sulfonation efficiency is given:

Membrane Skin Pore Speed through Relative Sulfonation Size Reactor Efficiency

Example 4 10K < < .01μm 15 ft/min lowest

Example 5 10K < < .01μm 5 ft/min intermediate

Example 6 BTS-55 .15μm 15 ft/min intermediate

Example 7 BTS-55 .15μm 5 ft/min highest

As will be appreciated, there are also other parameters that can affect the amount of change in the hydropathγ of a membrane that is achieved bγ the method of the invention. "Hγdropathγ" is a general term that includes hγdrophobicitγ and hydrophilicity. ^' The sulfonation of some reactive groups by sulfur trioxide would result in a relatively larger change in hydropathγ than would the sulfonation of other reactive groups. The relative effects of such parameters as reaction temperature and reactive group, for exampie, would be appreciated by a person of ordinarγ skill in the art.

EXAMPLE 8 ^• MEMBRANE SYMMETRY AND POLYMER HYDROPATHY

An asγmmetric hγdrophobic polγmer membrane of Udel 3500 polysulfone was sulfonated as described above. The sulfonated membrane was then dissolved in DMF, and the now-hydrophilic sulfonated polγmer of the dissolved membrane was used as the polγmer to cast a new membrane. The dope formulation and casting parameters used were those that reproducibly result in asγmmetric membranes when a hγdrophobic polγmer is used in the dope mix. However, the membrane cast from the sulfonated polysulfone was isotropic, rather than asymmetric. This Example demonstrates that the sulfonation reaction results in polymer units that are persistently hydrophilic, and that, under typical casting conditions, sulfonated sulfone polymers cannot be used to cast asγmmetric membranes. Accordingly, the method of the invention is advantageous because it allows the preparation of an asγmmetric sulfone polγmer membrane that is hγdrophilic.

EQUIVALENTS

The present invention has been described in connection with specific embodiments thereof, it will be understood that it is capable of further modification, and this application is intended to cover anγ variations, uses, or adaptations of the invention following, in general, the principles of the invention and including such departures from the present disclosure as come within known or customary practice in the art to which the invention pertains and as maγ be applied to the essential features hereinbefore set forth, and as fall within the scope of the invention and any equivalents thereof.

Claims

WHAT IS CLAIMED IS:

1. A method of preparing a sulfonated polγmer membrane, comprising the steps of: providing a preformed membrane, said membrane comprising a polγmer, said polγmer comprising a plurality of reactive moieties of at least one structure; contacting said membrane with sulfur trioxide to form sulfonated groups at a plurality of said moieties; and recovering said sulfonated polγmer membrane.

2. The method of claim 1, wherein said at least one structure of said reactive moieties is selected from the group consisting of an aromatic ring, a hγdroxγl group, an amine, and a paraffinic group.

3. The method of claim 1, wherein said preformed membrane is hγdrophobic.

4. The method of claim 1, wherein said preformed membrane is asγmmetric.

5. The method of claim 1, wherein said preformed membrane is microporous.

6. The method of claim 1, wherein said sulfonated polγmer membrane is hγdrophilic.

7. The method of claim 1, wherein said polγmer is a sulfone polymer.

8. The method of claim 7, wherein said sulfone polymer is a polyether sulfone.

9. The method of claim 7, wherein said sulfone polymer is a polyarγl sulfone.

10. The method of claim 7, wherein said sulfone polymer is a polysulfone.

11. The method of claim 1, wherein said sulfur trioxide is gaseous.

12. The method of claim 11, wherein said sulfur trioxide is produced by in situ generation.

13. The method of claim 12, wherein said in situ generation comprises the steps of: providing a stream of heated gas, said gas comprising sulfur dioxide and oxygen; and passing said stream over a catalyst comprising vanadium pentoxide.

14. The method of claim 13, wherein said heated gas has a temperature greater than about 400°C.

15. The method of claim 1, comprising the additional step of contacting said membrane with a neutralizing agent to neutralize a plurality of said sulfonated groups.

16. The method of claim 15, wherein said neutralizing agent is selected from the groups consisting of alkali metal hydroxides and ammonia.

17. The method of claim 16, wherein said neutralizing agent is ammonia gas.

18. The method of claim 15, wherein said step of contacting said membrane with a neutralizing agent is performed within 30 seconds of said step of contacting said membrane with sulfur trioxide.

19. The method of claim 1, further comprising the additional step of washing said sulfonated polymer membrane in water.

20. A sulfonated asymmetric sulfone polymer membrane, the membrane comprising a skin and a porous support, the skin possessing skin pores, the skin pores having an average diameter, and the porous support comprising an asymmetric region of increasing pore sizes, said increasing pore sizes having a maximum size, wherein a ratio of said maximum size of said increasing pore sizes to said average diameter of said skin pores is greater than about 10.

21. The membrane of claim 20, wherein said sulfone polymer is selected from the group consisting of polysulfone, polγethersulfone, and polyarγlsulfoπe.

22. The membrane of claim 21, wherein said sulfone polymer is a polyether sulfone.

23. The membrane of claim 21, wherein said sulfone polymer is a polγarγl sulfone.

24. The membrane of claim 21, wherein said sulfone polγmer is a polysulfone.

25. A sulfonated polymer membrane produced by the method of claim 1.

26. The membrane of claim 25, wherein said membrane is hydrophilic.

27. The membrane of claim 25, wherein said membrane is asymmetric.

28. The membrane of claim 25, wherein said membrane is microporous.

29. The membrane of claim 25, wherein said polymer is a sulfone polymer.

30. The membrane of claim 29, wherein said sulfone polymer is a polyether sulfone.

31. The membrane of claim 29, wherein said sulfone polγmer is a polγarγl sulfone.

32. The membrane of claim 29, wherein said sulfone polymer is a polysulfone.

33. A device for gas-phase modification of a membrane, said apparatus comprising a sulfur trioxide gas deliverγ sγstem for passing sulfur trioxide through said membrane and a neutralizing gas deliverγ sγstem for passing a neutralizing gas through said membrane.

34. The device of claim 33, wherein said sulfur trioxide gas delivery system and said neutralizing gas delivery system comprises at least one member of the group consisting of an air knife, a dry air source, a vacuum, and a porous cylinder.

35. The device of claim 33, further comprising an inert housing adapted for containing said sulfur trioxide gas and said neutralizing gas within said housing.

36. The device of claim 35, wherein said inert housing is teflon.

37. The device of claim 33, further comprising a dry air delivery sγstem for passing drγ air through said membrane.

38. The device of claim 33, wherein said membrane is a flat sheet membrane having two opposing surfaces, said device further comprising one or more cγlinders adapted for contacting said membrane.

39. The device of claim 38, wherein at least one of said cγlinders is adapted for advancing said membrane through said device.

40. The device of claim 38, wherein at least one of said cγlinders is a porous cγlinder, said porous cγlinder adapted to allow passage of a gas therethrough.

41. The device of claim 40, wherein said porous cγlinder contacts one of said surfaces of said membrane, and wherein said porous cγlinder comprises a vacuum adapted for causing a gas to pass through said membrane and into said porous cylinder.

42. The device of claim 40, wherein said porous cγlinder contacts one of said surfaces of said membrane, and wherein said porous cγlinder comprises a gas under positive pressure adapted for causing said gas to pass out of said porous cγlinder and through said membrane.