WO2002004083A2 - Surface modified membranes and methods for producing the same - Google Patents

Abstract

The present invention provides a surface modified membrane and a method for producing the same, which generally comprises: (a) placing a polymer membrane (14) in holder (18) down stream from a RF plasma generator (22, 50) in a plasma reactor (10), with an inlet section (12) and an outlet section (26) secured at o-ring joint (24); (b) generating a plasma of a surface modifying compound; and (c) providing a means to allow flow of the plasma through the interstitial surface of the polymer membrane to produce a surface modified polymer membrane, where source gases are supplied to the plasma generator via supply system (30, 40) with inlet line (42), and exhausted via line (28). The downstream exhaust region is monitored by probe (44). In one particular embodiment of the present invention, a method for producing hydrophilic polymer membranes from hydrophobic polymers is provided.

Description

SURFACE MODIFIED MEMBRANES AND METHODS FOR

PRODUCING THE SAME

FIELD OF THE INVENTION The present invention relates to a membrane which has been modified using a plasma and methods for producing the same.

BACKGROUND OF THE INVENTION In many separation processes, it is desirable to use a polymeric membrane with a hydrophilic surface. Unfortunately, most polymers are hydrophobic, which means that for many separation processes, a wetting agent must first be applied to the membrane to make it more hydrophilic. An additional problem frequently encountered with hydrophobic membranes is the lack of permanency ofthe wetting agent that is used to make them hydrophilic. This may lead to an increase in extractables or substances that may leach out of the membrane and cause contamination ofthe permeate. This problem is especially undesirable for asymmetric polysulfone membranes which are used in a variety of applications requiring repeated exposure to hot water and/or steam and in numerous ultrafiltration processes (e.g., food and dairy industries, pharmaceuticals and water treatment). While surface modification (e.g., increasing the wettability) of a polymeric membrane using a plasma is known, these plasma treatment methods typically involve placing a polymeric membrane on ^'a sample pedestal and exposing the membrane to the plasma. Unfortunately, these methods typically modify only the membrane surface which is exposed to the plasma, i.e., the surface which contacts the sample pedestal is substantially unchanged because it is shielded from being exposed to the plasma. Moreover, these methods allow only a small membrane surface depth to be modified by the exposure to plasma. Furthermore, many of these methods render the polymeric membrane hydrophilic only temporarily. Another disadvantage of current plasma treatment of polymeric membranes is that the membrane is typically placed at or near the plasma generation site which may damage the membrane. Therefore, tHer^'o "iS" need for a method for modifying the membrane surface using a plasma which renders the membrane permanently wettable. There is also a need for a method which modifies all surfaces ofthe membrane.

SUMMARY OF THE INVENTION The present invention provides a polymer membrane which has been modified by a plasma and methods for producing the same. In particular, one embodiment ofthe present invention provides a method for modifying a surface of a polymer membrane which comprises:

(i) an exterior surface; (ii) a bulk matrix; and

(iv) pores extending from said exterior surface into said bulk matrix, wherein said pores define an interstitial surface. The method generally can also include

(a) placing the polymer membrane down stream from a plasma generator, (b) generating a plasma of surface modifying compound; and

(c) providing a means to allow flow ofthe plasma through the interstitial surface ofthe polymer membrane to produce a surface modified polymer membrane. Preferably, the overall direction of plasma flow is substantially perpendicular (or orthogonal) to the exterior surface (i.e., cross section) ofthe membrane.

The present invention can be used to increase the hydrophilicity or the hydrophobicity ofthe polymeric membrane depending on the surface modifying compound used. Surface modifying compounds which increase the hydrophilicity of a polymeric membrane are well known to one of ordinary skill in the art. Particularly preferred surface modifying compounds for increasing the hydrophilicity of a membrane include oxygen, air, water, hydrogen peroxide, ammonia, helium, argon, and mixtures thereof. Similarly, surface modifying compounds which increase the hydrophobicity of a polymeric membrane are well known to one of ordinary skill in the art. Particularly preferred surface modifying compounds for increasing the hydrophobicity of a membrane include C,-C₁₀ alkane, C -C_lQ fluoroalkanes, C_rC₁₀ fluoroalkenes, fluorinated epoxides, siloxanes, and mixtures thereof.

Methods ofthe present invention are applicable to any

in particular organic polymeric membranes which are useful as a separating agent. Preferably, the membrane comprises a polymer selected from the group consisting of polysulfone, polyethersulfone, polyethylene, polystyrene, polytetrafluoroethylene, polyester, poly(methyl methacrylate), polyacrylonitrile, polyvinylidene fluoride, and mixtures thereof. More preferably, the membrane comprises a polymer selected from the group consisting of polysulfone, polyether sulfone, and mixtures thereof. While membranes may be symmetrical, asymmetric membranes are preferred.

While any method of creating a flow of plasma can be used with methods of the present invention, e.g., gravity and/or pressure differential from one side ofthe horizontal cross-section to the other side, such as high pressure plasma flow or a vacuum suction, it is preferred that the plasma flow means comprises a vacuum.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic illustration of plasma reactor for surface modifying a polymer membrane;

Figure 2 is a SEM image ofthe cross-section of a untreated asymmetric PSf membrane (BTS55);

Figure 3 is a SEM images ofthe untreated (A) open (1000X) and (B) tight (9000X) sides of a BTS55 membrane; Figure 4 is a graph showing bubble points as a function of treatment time for

BTS55 membranes treated with H₂O plasma (25 W, 50 mTorr). Dashed line indicates bubble point for untreated BTS55. Error bars represent deviations in the bubble point measured for a minimum of three plasma-treated membranes;

Figure 5 is SEM images of BTS55 membranes treated with a 25 W H₂O plasma (50 mTorr) for (A) two minutes and (B) ten minutes;

Figure 6 is a SEM image of a BTS55 membrane treated with a 100 W H₂O plasma (50 mTorr);

Figure 7 is a graph showing bubble points as a function of time after plasma treatment for BTS55. Dashed line indicates bubble point for untreated BTS55. Treated samples stored in ambient conditions; a*τg /_toure 8 is a graphic representation of contact angle for plasma-treated BTS55 as a function of heat treatment. Treated membranes were heated in a conventional oven at 130 °C for a maximum of four hours. Contact angles were measured on both the open (■) and tight (A) sides ofthe membrane after heat treatment. Dashed line indicates contact angle for untreated BTS55; Figure 9 is a graph showing contact angles for plasma-treated BTS55 as a function of heat treatment. Treated membranes were heated in a conventional oven at 130 °C (closed symbols) and 170 °C (open symbols). Data shown for membranes heated for a maximum of 15 minutes. Contact angles were measured on both the open (■) and tight (A) sides ofthe membrane after heat treatment. Dashed line indicates contact angle for untreated BTS55;

Figure 10 shows a graph of contact angles for plasma-treated BTS55 as a function of heat treatment. Samples were heated tight side up at 130 °C. Contact angles were measured on both the open (■) and tight (A) sides ofthe membrane after heat treatment. Dashed line indicates contact angle for untreated BTS55; Figure 11 A is an optical emission spectrum of a 25 W H₂O plasma (50 mTorr) without a BTS55 membrane in the reactor;

Figure 1 IB is an optical emission spectrum of a 25 W H₂O plasma (50 mTorr) with a BTS55 membrane in the reactor; and

Figure 12 is an environmental SEM image ofthe cross section of a treated PES membrane exposed to H₂O vapor in the chamber ofthe SEM. The image is oriented with the tight side at the top.

DETAILED DESCRIPTION OF THE INVENTION As used herein, the terms "membrane", "polymer membrane" and "porous polymer membrane" are used interchangeably herein and refer to a porous polymer, which are useful in filtration (e.g., of biological fluids).

The terms "asymmetric membrane" and "asymmetric polymer membrane" (e.g., asymmetric polysulfone membrane) are used interchangeably herein and refer to a membrane whose average pore size on one side ofthe surface (i.e., "rough surface side," "rough side," or "open side") is larger than the average pore size on the other side ofthe membrane surface (i.e., "smooth surface side," "smooth s fc^ ?^s tight side"). The terms "cross section" and "exterior surface" are used interchangeably herein and refer to the surface area ofthe membrane which is orthogonal to the general overall direction ofthe majority ofthe membrane's pores.

The polymer membranes ofthe present invention contain pores, which extend from the exterior surface into the bulk matrix. The polymer membrane thus has pore surfaces, which are essentially the surfaces that surround and define the pores ofthe article. The pore surfaces may sometimes be referred to as the "interstitial surface" because they surround the interstitial volume ofthe polymer membrane.

The terms "membrane surface modifying compound" and "surface modifying compound" are used interchangeably herein and refer to a compound which is used to generate a plasma that modifies the surface property of a membrane such that the hydrophilicity or hydrophobicity ofthe membrane is increased. Exemplary membrane surface modifying compounds include those described in the Summary ofthe Invention section above. The term "permanent" when referring to the surface modification ofthe membrane refers to a time period for which the surface modification properties (or characteristics) last for at least about 1 month, preferably at least about 2 months, more preferably at least about 3 months, still more preferably at least about 4 months, and most preferably at least about 1 year. The present invention will now be described in detail in reference to a method for producing a hydrophilic membrane, which is particularly preferred in many separation processes. However, it should be appreciated that methods ofthe present invention can be used to produce a hydrophobic membrane (i.e., the resulting membrane is more hydrophobic than the untreated membrane) by selecting an appropriate surface modifying compound. In many separation processes, it is desirable to use a polymeric membrane with a hydrophilic surface. Unfortunately, most polymers are hydrophobic; therefore, a separation of an aqueous sample typically requires a wetting agent to be applied to the membrane to make it more hydrophilic. An additional problem frequently encountered with hydrophobic membranes is the lack of permanency ofthe wetting agent that is used to make them hydrophilic. This may lead to an increase in extractables or substances that may leach out ofthe membrane and cause contamination ofthe permeate. This is particularl^^g '*- problematic for asymmetric polysulfone membranes which are widely used in a variety of applications requiring repeated exposures to hot water and/or steam and in numerous ultrafiltration processes, e.g., food and dairy industries, pharmaceuticals, and water treatment. Past difficulties with rendering these highly hydrophobic membranes hydrophilic include wetting agent treatments that are not permanent, that alter the bulk properties ofthe membrane, and/or that can not withstand steam exposure.

While there has been some work on a plasma treatment of membrane to increase its hydrophilicity, it is believed that these methods generally involve placing a membrane on a sample "pedestal" and generating the plasma for exposure. Unfortunately, the amount of plasma exposure ofthe membrane surface which is in contact with the pedestal is virtually non-existent and minimal at best, i.e., typically only one horizontal cross sectional surface ofthe membrane is exposed to (i.e., treated by) the plasma. Thus, most plasma treated membranes are hydrophilic in only selected areas ofthe membrane.

By using methods ofthe present invention, it has been discovered that substantially the entire cross section ofthe membrane can be treated with a plasma.

Moreover, it has been discovered that methods ofthe present invention allow substantially a complete penetration ofthe plasma through the membrane, i.e., preferably the entire thickness ofthe membrane is exposed to the plasma. Furthermore, it has been discovered that methods ofthe present invention render the membrane permanently hydrophilic. For the sake of brevity, convenience and illustration, this detailed description ofthe invention will now be illustrated in reference to Figure 1. It is to be understood, however, that the invention as a whole is not intended to be so limiting, and that one skilled in the art will recognize that the concept ofthe present invention will be applicable by the use of other appropriate apparatus which may be used to modify the surface of a membrane in accordance with the techniques discussed herein. Such apparatuses suitable for use in the instant invention will be readily apparent to those skilled in the art.

In one particular embodiment ofthe present invention, a reactor for producing an inductively-coupled, low density radio-frequency (rf) H₂O plasma is the tubular glass reactor shown in Figure 1. Further description of this reactor appears in K. H. A. Bogart et al. "Plasma enhanced chemical vapor deposition of SiO₂ using novel alkoxysilane precursors", J. Vac. Sci. Technol. A 13(2), 476-480 (1995). It should be appreciated that the reactor need not be a tubular glass reactor, for example, the cross-section ofthe reactor can be in any shape including a circle, square, rectangle, rhombus, trapezoid, elipse and the like. Furthermore, the reactor may be made of any of a variety of materials such as stainless steel, glass, plastics such as polycarbonates, and other materials known to those skilled in the art. When operated as described herein, this reactor is used to achieve flowing glow discharge to modify the surfaces of polymer membranes. In a preferred embodiment, the reactor includes at least one cylindrical, holder (hereinafter "membrane holder," which can be made of glass) to orient the polymer membrane perpendicular to the flowing discharge is illustrated in Figure 1 set forth herein, and may be operated as described below. As shown in Figure 1, the present invention comprises a longitudinal oriented plasma reactor 10 formed from two sections, namely an inlet section 12 and an exhaust section 26. These sections are secured together at an O-ring joint 24 (e.g., 50 mm O-ring joint) using a conventional stainless steel clamp, allowing easy access to the interior ofthe plasma chamber. Glass is the preferred material from which to construct the plasma reactor. Reactor 10 also comprises a separate cylindrical membrane holder (e.g., about 90 mm in length) 18, also preferably constructed from glass. One end ofthe membrane holder is threaded with a screw top (e.g., 30 mm dia.) and the opposite end is open to allow flow ofthe plasma through the membrane. The membrane to be treated is placed in the membrane holder and substantially oriented perpendicular to the inductor coil (hereinafter "discharge region"). When operated as described herein, the membrane holder was situated in the exhaust section ofthe plasma reactor downstream from the discharge region (e.g., about 9 cm) to minimize plasma-induced damage to tne polymer membrane. This perpendicular placement ofthe membrane cross-section to the plasma flow allows maximal exposure and penetration ofthe plasma through the thickness ofthe membrane. In addition, this arrangement allows chemical modification ofthe entire membrane cross section.

The exhaust section ofthe plasma rector is in fluid communication with a vacuum pump via line 28 positioned therebetween. Optionally, a liquid N₂ cold trap may be positioned between the plasma reactor and the vacuum pump to avoid undesirable source gases or byproducts from entering the vacuum pump. The exhaust section may also, or alternatively, be fitted with a replaceable fused silica window, allowing coaxial observation of emission from the plasma using an optical fiber, and may- s sS ted as described below. Plasma emission may be imaged onto the 10 mm entrance slit of, for example, an Ocean Optics S2000 triple spectrometer equipped with three 1800 grooves/nm holographic gratings and three 2048 element linear charge coupled device-array detectors.

Optionally, a Langmuir probe may also be attached to the exhaust section to measure electron and ion energies in the discharge region. The length and geometry ofthe exhaust region may also be modified to accommodate "downstream" and "remote" treatments. Optionally, an exhaust section may be lengthened to 36 cm to perform treatments at increasing distances from the discharge region. The longitudinal exhaust (or inlet) section can also be replaced with an L-shaped exhaust (or inlet) section to perform remote plasma treatments, in which the porous article is not directly exposed to the glow discharge.

Optionally, the longitudinal exhaust section can also be replaced with a T-shaped exhaust section to perform remote plasma treatments, in which the porous article is not directly exposed to the glow discharge.

The reactor 10 further comprises an inlet line 42 through which the precursor to the reactive gas-phase radical (also termed "source gas" or "surface modifying agent") may be introduced to the plasma reactor. The source gas may be gas or liquid vapor. A more preferred embodiment ofthe source gas is H₂O vapor, which when operated as described herein may be introduced from a 100 mL Pyrex glass sidearm vacuum flask, which is operatively interconnected to the plasma reactor near the inlet section. Distilled water may be subjected to several freeze-pump-thaw cycles to remove dissolved gases prior to use. Water is then introduced into the plasma reactor through a Teflon stopcock 40, using a vacuum pump that draws water vapor through the plasma reactor. The lines through which the water is admitted to the plasma reactor may or may not be heated to achieve the operating vapor pressure. When operated as described herein, the vapor pressure is controlled using a Nupro bellows-sealed metering valve. The pressure ofthe H₂O vapor is allowed to stabilize prior to generating the glow discharge or to the addition of diluent inert gas (e.g. Ar) through an MKS mass flow controller. The total pressure in the chamber is monitored with an MKS Baratron capacitance manometer which is insensitive to differing gas compositions and is stabilized to the desired value prior to generating the glow discharge. The discharge generating apparatus (or "plasma generator") comprises a power supply and an

the source gas to a discharge. Suitable power supplies include any radio-frequency (rf), microwave or direct current (DC) power supplies. A suitable rf power supply may be obtained from, for example, Advanced Energy, and identified under their trade designation as an RFX-600 power supply. When operated as described herein, 13.56 MHz rf power from the RFX-600 may be inductively-coupled to the source gas in the plasma reactor by an eight turn nickel plated copper coil and tuned with a Jennings 100 pF variable capacitor (also collectively termed "rf matching network"). This configuration may be used with any source gas ofthe present invention. A pulsed rf power supply from, for example, RF Power Products (a subsidiary of Advanced Energy) and identified under their trade designation as an RF5S may also be used for any source gas ofthe invention, especially those that may polymerize in the plasma (also termed "polymer-forming plasmas"). Further description of pulsed plasma systems and their operation appears in N. M. Mackie et al. "Characterization of pulsed-plasma-polymerized aromatic films" Langmuir 14, 1227-1235 (1998), which is incorporated herein by reference in its entirety.

Additional objects, advantages, and novel features of this invention will become apparent to those skilled in the art upon examination ofthe following examples thereof, which are not intended to be limiting.

EXAMPLES General Procedure The plasma is generated by applying approximately 5-50 W, preferably about

25 W, of continuous wave (CW) rf, e.g., 13.56 MHz, to distilled water vapor. Pressure within the reactor is maintained from about 20 mTorr to about 100 mTorr, preferably from about 40 mTorr to about 80 mTorr, and more preferably about 50 mTorr. The treatment time is generally from about 0.25 minutes to about 10 minutes, preferably from about 1 minute to about 5 minutes, and more preferably about 2 minutes.

All membrane treatments were performed in the tubular glass inductively coupled low density rf (13.56 MHz) reactor described above. The glass cylindrical membrane holder described above was used in all experiments to orient the membrane perpendicular to the inductor coil (Figure 1). This allows maximal exposure and penetration ofthe plasma through the thickness (-125 μm) ofthe membrane. This specially designed membrane holder facilitates modification ofthe entire cross section of asymmetric membranes. In addition, the membrane holder was situated in the glass reactor -9 cm downstream from the most intense region ofthe plasma glow to minimize plasma-induced damage to membranes. Three different types of commercially available asymmetric polysulfone (i.e., PSf) membranes (US Filter) were used in these experiments, BTS55, BTS80 and UF membranes, distinguished by their nominal pore size distribution. In addition, a non-porous polysulfone resin (Westlake Plastics) was also used for comparison. In other experiments, asymmetric polyethersulfone membranes (Millipore) were used. An extensive parameter study was performed for the H₂O-based plasma systems. These parameters included 5-50 W applied rf power, 50-100 mTorr chamber pressure, and treatment times from 0.25-10 minutes. The plasma systems studied included 100% Ar, 100% He, 100% O₂, 100% H₂0, 15% H₂O₂/H₂O, and 50% Ar/H₂O. Prior to use, distilled water underwent several freeze-pump-thaw cycles to remove trapped atmospheric gases. Liquid samples were introduced into the reactor from a 100 mL Pyrex glass sidearm vacuum flask with Teflon^® stopcocks and the vapor pressure was controlled (within ±2%) using a Nupro bellows-sealed metering, valve. For the 50:50 Ar/H₂O system, the H₂O pressure was allowed to stabilize prior to the addition of Ar (4.8 grade), which was added to the existing H₂O flow through an MKS mass flow controller. Thus, the amount of H₂O admitted to the reactor was relatively constant (± 2%). The pressure in the chamber was monitored with an MKS Baratron capacitance manometer which is insensitive to differing gas compositions.

Preferred conditions for treating asymmetric PSf membranes were determined to be H₂O (no additives), 25 ^"W applied rf power, 50 mTorr pressure for 2 minutes. These conditions were used in all experiments unless otherwise noted. Bubble point analysis The nomenclature used to categorize asymmetric PSf membranes (Table I) corresponds to the nominal bubble point ofthe membrane. For example, BTS55 and BTS80 membranes have nominal bubble points of 55 psi and 80 psi, respectively. The bubble point ofthe membrane is the pressure of air needed to expel liquid (e.g., water) from the pores of the membrane. By measuring the bubble point, information about the wettability ofthe membrane surface and the average pore diameter ofthe membrane can be quantitatively •fefS liied. The relationship ofthe bubble point (P) to the average pore diameter (d) ofthe membrane, the surface energy ofthe wetting liquid (γ) and the contact angle (θ) between the wetting liquid and the membrane surface is given by equation 1.

P = (4γcosθ)/d (1)

Thus, the bubble point provides information about the wetting properties with a particular liquid for a membrane with a known average pore diameter. Likewise, the average pore diameter of a membrane can be determined by porometry using a completely wetting liquid (θ = 0). See, for example, M. Mulder, Basic Principles of Membrane Technology, 2^nd Ed., Kluwer Academic Press: New York, 1996, which is incorporated herein by reference in its entirety. This technique was used to determine the average pore size for BTS55 and BTS80 membranes after plasma treatment.

Bubble point measurements were obtained with an apparatus consisting of a sample holder (49 cm²), a pressure gauge and a compressed air cylinder (4.8 grade). Prior to obtaining a bubble point measurement, the membrane was immersed in 100 mL ofthe wetting liquid. Due to the hydrophobicity of polysulfone, the membrane was first wet with a 50:50 isopropylalcohol (IPA)/H₂O solution. The aqueous EPA solution was gradually replaced with deionized water to effectively wet the pores with water. The wet membrane sample was placed in the sample holder between two protective metal screens with the tight side ofthe membrane upstream of air flow. The delivery pressure was then slowly increased until a breakthrough pressure was observed on the pressure gauge located downstream from the sample holder. This breakthrough pressure is the bubble point ofthe membrane. Unless otherwise noted, bubble points for plasma-treated membranes were measured within 24 hours of plasma treatment. Additional Analyses

Static contact angles were measured by the sessile drop method with a contact angle goniometer (Rame Hart Model 100). Measurements were taken on both sides of water drops at ambient temperature, immediately after 1 μL drops were applied to the surface and the needle tip was removed from the surface. The hysteresis ofthe water drop was evaluated by measuring the contact angle on both sides ofthe drop. In addition, measurements were made on both sides ofthe untreated and plasma-treated membranes. For each sample, three drops were placed at different locations on the sample. Reported contact angle measurements are the average ofthe measurements on each side ofthe membrane for at least three samples. Unless otherwise noted, contact angles were measured immediately after plasma treatment.

Some plasma-treated membranes were exposed to extremes in heat and humidity to simulate "aging". To simulate hot, humid conditions, plasma-treated membranes were placed in a glass, cylindrical membrane holder modified to fit inside a conventional pressure cooker. The pressure cooker was then heated until the pressure inside reached ~15 psi above atmospheric pressure or -121 °C. Once these conditions were reached, the membrane was steam-treated for 15 minutes. Plasma-treated membranes were also exposed to high, dry temperatures by heating the membranes at 130 °C and 170 °C in a conventional oven for times ranging from minutes to hours. Scanning electron microscopy (SEM) images were obtained using a Phillips

505 microscope with an accelerating voltage of 20-30 keV and a spot size of 50 nm. Prior to SEM analysis, the membrane was affixed to a standard SEM sample stub by double-sided Cu tape (3M). The membrane was affixed to the Cu tape with either the open or tight side face up depending on which side was imaged. Cross-sectional SEM images were obtained by freeze-fracture ofthe membrane in liquid nitrogen. To prevent surface charging during SEM analysis, a thin film (20 nm thick) of Au was sputtered onto the surface of all samples prior to imaging. For the asymmetric polyethersulfone membranes, environmental SEM (ESEM) images were acquired with an Electroscan ESEM 3 (Philips Electron Optics).

XPS analysis was performed on a Surface Science Instruments S-probe spectrometer located at the University of Washington, NES AC-BIO center. This system has a monochromatic Al K_α X-ray source (hv = 1486.6 eV), hemispherical analyzer, and resistive strip multichannel detector. A lo energy (-5 eV) electron gun was used for charge neutralization on the nonconducting samples. For all samples, multiple spots were analyzed. The compositions were determined from 0-1000 eV survey scans acquired with an analyzer pass energy of 150 eV. The high-resolution C_ls, O_ls, and S_2p spectra were acquired at an analyzer pass energy of 25 eV and collected at a 55° takeoff angle, which is the angle between the surface normal and the axis ofthe analyzer lens. Quantification ofthe surface alcohol content was obtained using the trifluoroacetic anhydride (TFAA) derivatization technique. This allows differentiation between C-O-H and C-O-C. In this procedure, hydroxyl groups are converted to OC(O)CF₃ groups via an esterification process. See, for example, A. Chilkoti, B. D. Ratner, Surf. Interface Anal, 1991, 17, 563=^j $hich is incorporated herein by reference in its entirety.

Optical emission spectra (OES) from 425 to 713 nm for a 25 W H₂O plasma (50 mtorr vapor pressure, with and without a PSf membrane in the reactor) were obtained from a plasma reactor modified with a replaceable fused silica window located at the downstream end ofthe reactor. The placement of this window allows coaxial observation of emission from the' plasma. Plasma emission was imaged onto the 10 mm entrance slit of an Ocean Optics S2000 triple spectrometer using three optical fibers. The spectrometer is equipped with three 1800 grooves/nm holographic gratings and three 2048 element linear charge coupled device-array detectors. Emission signals were integrated for 2 s. Asymmetric polysulfone membranes

Table I summarizes the types of asymmetric PSf membranes that are commercially available with average pore sizes ranging from 1.2 μm (BTS 5) to -0.01 μm (UF). Due to the asymmetry associated with these membranes, the actual pore size can vary widely from the average pore size ofthe membrane. For example, BTS55 has an average pore size of 0.2 μm; however, as a result ofthe asymmetry of BTS55, there is a pore gradient with gradual transition from much smaller (< 0.1 μm) to much larger pores (>10 μm) across the thickness (i.e., bulk matrix) (125 μm) ofthe membrane. This distinct pore gradient can be seen in the cross-sectional SEM image for a BTS55 membrane (Figure 2). This image also shows the tortuousity associated with the pores of asymmetric membranes contributing to the difficulties encountered in modifying the entire cross section ofthe membrane.

The asymmetry associated with these membranes is further manifested in the difference in morphology between the two sides ofthe membrane. This difference can be seen in SEM images ofthe opposing sides of BTS55 membranes (Figure 3). The small pore or tight side appears smooth whereas the large pore or open side appears rough. This difference in morphology allows one to visually differentiate the open and tight sides as the open side appears dull and the tight side appears shiny.

Table I shows the bubble points measurements for untreated asymmetric PSf membranes. The bubble points measured were typically about 10 psi higher than the nominal bubble point for the material, except for BTS55 membranes. As a result, BTS55 membranes were used in parameter space study of applied plasma power, H₂O vapor flow rate (pressure) ariS. it ferment time. Table I. Bubble points measured for untreated asymmetric PSf membranes.

Membrane" Average Pore Size (μm) Bubble Point (psi)

BTS5 1.2 14.6 ± 1.7

BTS25 0.45 40.4 ± 2.9

BTS55 0.2 57.0 ± 3.3

BTS80 0.1 92.6 ± 3.2

UF ~0.01^b N/A^c a. The numbers 5, 25, 55 and 80 refer to the nominal bubble point ofthe membrane with water. b. Molecular weight cutoff ~100 kD. c. Could not be tested as pressures higher than those used for a standard test are required.

Hydrophilic surface modification by plasma treatment

BTS55 membranes were treated with a 25 W H₂O vapor plasma (50 mTorr total pressure) for times ranging from 15 seconds to 10 minutes. Figure 4 shows there is a about 20-25 psi increase in the bubble point for all plasma treatment times examined (0.25-10 min). From equation (1), an increase in the bubble point could be a result of a decrease in the contact angle or a decrease in the overall pore diameter. Hence, the observed change in the bubble point for plasma-treated BTS55 membranes may correspond to an increase in the hydrophilicity induced by plasma treatment or to adverse physical alteration in the membrane structure as a result of plasma treatment.

To ascertain whether the plasma adversely altered the physical properties of the membrane, SEM images were obtained for plasma-treated BTS55 membranes treated for 2 minutes and 10 minutes, Figure 5. The open sides ofthe plasma-treated membranes were imaged as this side was oriented closest to the most intense region ofthe plasma glow during plasma treatment and was, therefore, more likely to be physically damaged by the plasma than the downstream side. These images were then compared to images of untreated BTS55 membranes (Figure 3). It is evident from this comparison that the prolonged exposure to H₂O plasma (i.e., >2 minutes) introduces visible pitting ofthe membrane surface, thereby increasing the porosity ofthe membrane. Membranes plasma-treated for 2 minutes, however, do not exhibit any significant structural damage. Applied plasma powers higher than 25 W also adversely affect the structural integrity ofthe membrane. For example, BTS55 membranes are damaged by a 100 W H₂O plasma treatment (Figure 6). Thus, high applied rf powers and prolonged plasma exposure were avoided and typical treatments employed relatively low plasma powers (25 W) and pressures (50 mTorr) for short times (about 2 min). In addition to SEM analysis, changes in the pore diameter of asymmetric PSf membranes were quantified by porometry. The results of porometry tests of plasma-treated BTS55 membranes are summarized in Table II. These results indicate that the average pore size or mean flow pore (MFP) of BTS55 membranes is negatively affected by plasma- treatment. Overall, the pore sizes are about 35% larger than the pore sizes ofthe untreated BTS55; however, the pore sizes ofthe plasma-treated membranes are nearly identical to the values obtained for BTS55 membranes treated with a wetting agent. Therefore, plasma treatment ofthe present invention produces comparable results to those obtained by the convention of applying a wetting agent to make hydrophobic membranes hydrophilic. Table II. Porometry data for untreated and plasma-treated asymmetric PSf membranes.

Membrane Mean Flow Pore (μm) Max. Pore (μm) Min. Pore (μm) untreated BTS55 0.165 0.212 0.133 treated BTS55 0.221 0.327 0.178 untreated BTS80 0.084 0.100 0.080 treated BTS80 0.095 0.164 0.082

Moreover, the porometry results suggest that the observed increase in the bubble point for plasma-treated BTS-55 membranes may be the result of an increase in the pore diameter. To determine if the increase in the bubble point observed for plasma-treated BTS55 membranes is also due to an increase in hydrophilicity, water contact angles were measured on both sides of plasma-treated membranes immediately after plasma treatment. Table III summarizes the results of these measurements before and after plasma treatment. As seen in Table III, the untreated membranes are hydrophobic with average contact angles of about 90° (BTS55). Despite the asymmetric structure, the average contact angle measured on the open side is similar to the average contact angle measured on the tight side for each membrane type. Table III. Contact angles" for untreated, plasma-treated and aged PSf materials.

BTS55 BTS80 UF open tight open tight open tight untreated 90.0 ± 9.0° 86.2 ± 9.3° 79.7 ± 4.7° 79.0 ± 2.0° 64.3 ± 4.7° 66.3 ± 4.7° treated wettable wettable wettable wettable wettable wettable polysulfone resin ,

Side A^b Side B^c untreated 73.0 ± 2.0° 71.0 ± 2.0° treated 14.9 ± 5.5° 44.4 ± 13.1°

Values reported are the average of a minimum of three drops on each side ofthe material. b. Side A faces the inductor coil during plasma treatment. c. Side B is downstream from the inductor coil during plasma treatment.

Contact angle measurements on BTS55 membranes treated with a H₂O plasma resulted in the water drop immediately disappearing into the membrane (i.e., contact angle of 0°). This effectively demonstrates that BTS55 membranes are rendered completely water- wettable as a result of H₂O plasma treatment. Furthermore, the open and tight sides ofthe plasma-treated membranes were both completely wettable (i.e., contact angle of 0°). This suggests that the plasma penetrates the thickness ofthe membrane, thereby equally modifying both sides ofthe membrane.

For comparison, a non-porous PSf resin was placed in the membrane holder and treated with identical plasma conditions. Table III includes the results of contact angle measurements on both sides ofthe resin before and immediately after plasma treatment. The side ofthe resin closest to the inductor coil (Side A) was more wettable after treatment than before treatment. In contrast to plasma-treated BTS55 membranes, however, the downstream side (Side B) ofthe plasma-treated resin was not as wettable as the upstream side, suggesting that the complete modification of BTS55 membranes is related to its porosity.

The effectiveness ofthe H₂O plasma treatment was tested by extending the results obtained for BTS55 membranes to asymmetric PSf membranes with smaller pore sizes. BTS80 (0.1 μm) and UF (0.01 μm) membranes (Table I) were treated with a H₂O plasma for 2 minutes. Table III lists the contact angles measured on both sides of these materials before and after plasma treatment. These results are analogous to those obtained for BTS55 membranes. Both sides of BTS80 and UF membranes were rendered completely wettable as a result of plasma treatment, demonstrating the generality of this hydrophilic membrane modification.

Plasma-treated BTS80 membranes were also analyzed by porometry. The results obtained for these membranes before and after plasma treatment are similar to those obtained for BTS55 membranes (Table II). Overall, an increase in the pore sizes associated with BTS80 and BTS55 membranes was observed after plasma treatment. As noted above, this phenomenon is also observed for membranes treated with a chemical wetting agent. Permanence of hydrophilic membrane modification

The permanence of plasma treatment ofthe present invention was measured by measuring contact angles and bubble points for samples aged days, weeks, and months after plasma treatment. Figure 7 shows the effect that aging has on the average bubble point of plasma-treated BTS55 membranes. The average bubble point, analyzed immediately after plasma treatment, is about 75 ± 2.2 psi (i.e., 0 days). As previously mentioned, this is about 20 psi higher than the average bubble point for untreated BTS55 membranes (57 ± 3.3 psi). Figure 7 illustrates that the average bubble points measured for samples aged for up to one month decrease slightly from the average bubble point of samples analyzed immediately after plasma treatment, but remain at about 20 psi higher than the average bubble point of untreated BTS55 membranes. These results indicate that the change in the bubble point observed for plasma-treated membranes is not affected by aging.

The most significant evidence for the permanence ofthe hydrophilic modification is contact angle measurements made on plasma-treated samples stored under ambient laboratory conditions. Plasma-treated BTS55 membranes remain completely wettable (i.e., contact angles of 0°) for at least eighteen months after plasma treatment. In addition, BTS80 and UF membranes treated with methods ofthe present invention also remained wettable for at least five months after plasma treatment. In contrast, previous attempts to make asymmetric PSf membranes hydrophilic by plasma treatment resulted in significant contact angle changes within 24 hours of plasma treatment. These results clearly indicate that H₂O plasma treatment of both microporous and UF polysulfone membranes by methods ofthe present invention is permanent, as it withstands aging under ambient conditions. Accelerated agins studies

The permanence ofthe hydrophilic membrane modification was further tested by subjecting plasma-treated BTS55 membranes to accelerated aging. These experiments exposed the membranes to extremes in humidity and temperature. The humidity or temperature at which plasma-treated membranes revert to the same degree of hydrophobicity as the untreated membrane is correlated to the robustness ofthe modification. Treated BTS55 membranes were exposed to moist (e.g., steam) and dry heat. The permanence ofthe modification was then determined by measuring contact angles on both sides ofthe membrane after the wet or dry treatment. Bubble point measurements were also used for further comparison. Contact angle measurements for the steam-treated membranes indicated they were less wettable than plasma-treated membranes without steam treatment; however, a visible contact angle could not be measured on either side ofthe steam-treated membranes. Specifically, steam-treated membranes did not immediately absorb the water drop, but absorbed the drop in <60 seconds. Although steam-treated BTS55 membranes do not lose hydrophilicity, steam treatment affects the wettability time of plasma-treated BTS55 membranes. The average bubble point of plasma-treated BTS55 membranes analyzed after steam treatment (72 ± 1.1 psi) is, however, essentially unchanged from the average bubble point measured immediately after plasma treatment (75 ± 2.2 psi). Plasma-treated membranes were steam-treated under the same conditions three times in succession. The membranes were allowed to dry between steam cycles. Contact angles were measured on each side ofthe membrane following the last steam treatment. The results obtained for two and three steam treatments were similar to those obtained for membranes steamed only once. Overall, the steam-treated membranes did not immediately absorb the water drop but instead absorbed the drop in <90 seconds. Likewise, the average bubble points obtained for membranes steamed two and three times are identical to the average bubble point obtained for membranes steamed only once. Therefore, repeated steam treatments do not cause further losses in hydrophilicity.

As another test of extreme conditions, plasma treated BTS55 membranes were placed in a clean beaker of boiling H₂O for one hour. The membranes were then allowed to dry in air. Contact angle measurements performed after treatment showed an increase to about 65° for both sides ofthe membranes. For comparison, it should be noted that contact angles for untreated BTS55 membranes is about 90°.

The effect of hot, dry conditions on the hydrophilicity induced by H₂O plasma treatment was also examined by heating plasma-treated BTS55 membranes in an oven. Contact angles were measured on both sides ofthe membrane after heat treatment. The results of two different experiments in which plasma-treated BTS55 membranes are heated at 130 °C and 170 °C for times ranging from minutes to hours are discussed here. Contact angles measured for membranes heated at 130 °C as a function of heat treatment time are shown in Figures 8-10. For example, Figure 8 illustrates that the average contact angle for both the open and tight sides ofthe membrane does not exceed 60° even after heating for four hours at 130 °C.

There is a difference in the time required for the open and tight sides to lose plasma-induced hydrophilicity when heated at 130 °C, Figure 9. Specifically, the contact angle on the open side increases to > 60° when heated for 2.5 minutes or longer at 130 °C. Conversely, the tight side ofthe membrane remains wettable until heated for >11 minutes at 130 °C. As seen in Figure 10, this effect is not dependent on the orientation ofthe membrane in the oven during heat treatment, as the tight side remains wettable for much longer than the open side under all heating conditions.

The results for plasma-treated membranes heated at 170 °C are also shown in Figure 9. At this higher temperature, the contact angles on both the open and tight sides increase to >60° after being heated for 2.5 minutes. However, the plasma-treated membranes do not revert to the degree of hydrophobicity ofthe untreated material even after 4 hours at 170 °C. Penetration of plasma treatment The penetration ofthe plasma treatment was further investigated by treating multiple membranes simultaneously. Two membranes were stacked in the membrane holder such that both open sides ofthe membrane faced the inductor coil (i.e. upstream from the plasma glow). As a result of stacking the membranes, the sides ofthe membranes on the interior ofthe stacked assembly were not directly exposed to the plasma. Water contact angles on all four sides ofthe membranes immediately after plasma treatment were then measured.

Table IV includes the results for two different experiments in which multiple BTS55 membranes were simultaneously plasma-treated for either 2 or 3 minutes. In both experiments, the surfaces ofthe membrane closest to the inductor coil (Table IV, surfaces 1 and 2) were completely wettable after plasma treatment. Likewise, the open side ofthe second membrane (Table IV, surface 3) was also rendered completely wettable by plasma treatment. The only way the surfaces on the interior ofthe stacked assembly (Table TV, surfaces 2 and 3) could be treated by the plasma is if the plasma penetrated the thickness of the membrane closest to the inductor coil. Table IN. Results for multiple membranes experiments.

Membrane Treatment Time ___*^ H₂O Contact Angle" (min) ^"f_ Side l Side 2 Side 3 Side (open)^b (tight)^c (open)⁰ (tight)^d

BTS55 2 wettable wettable wettable 55.3 ± 3.1°

BTS55 3 wettable wettable wettable wettable

BTS80 2 wettable wettable 62.7 ± 2.7° wetted <60 s^e

BTS80 5 wettable wettable wettable wettable

UF 2 wettable wetted <90 s^e wetted <90 s^e 55.3 ± 3.1°

UF 6 wettable wettable wettable wettable a. Values reported are the average of a minimum of three drops on each side ofthe material. b. Side of membrane closest to coil. c. Sides of membrane on interior of stacked assembly. d. Side of membrane farthest downstream from coil. e. Water drop did not immediately absorb, but was completely absorbed in the time indicated.

For the 2 minute plasma treatment, the surface farthest downstream from the inductor coil (Table IV, surface 4) was less wettable than the other three surfaces ofthe membranes in the stacked assembly. Although surface 4 was not completely wettable, a decrease in the contact angle relative to the untreated BTS55 membrane was observed. It was found that by increasing the treatment time to 3 minutes, all four surfaces ofthe membranes in the stacked assembly were rendered completely wettable by plasma treatment. Therefore, increasing the treatment time achieves complete modification of both membranes. These results indicate that the depth of penetration of plasma treatment by methods ofthe present invention is at least 250 μm or the thickness of about two BTS55 membranes.

Effects ofthe porosity of BTS80 and UF membranes on the depth of penetration of plasma treatment were also investigated. Table IV summarizes the results of multiple membrane experiments performed for BTS80 and UF membranes at different treatment times. For both sets of experiments, both surfaces ofthe membrane closest to the inductor coil (Table IV, surfaces 1 and 2) are rendered completely wettable after a 2 minute plasma treatment. In contrast to the results obtained for BTS55 membranes, both surfaces 3 and 4 ofthe BTS80 membranes were not completely wettable after a 2 minute plasma treatment. Similarly, surfaces 2, 3, and 4 in the UF experiment were not immediately completely wettable after a 2 minute plasma treatment. Without being bound by any theory, it is believed that these results indicate that the low porosity associated with these membranes, particularly UF membranes, affects the throughput ofthe plasma necessary to treat those sides not directly exposed to the plasma glow. As in the BTS55 experiment, all four surfaces ofthe BTS80 and UF membranes can be rendered completely wettable by increasing the treatment time. UF membranes require the longest treatment time to obtain wettability for all four sides. It is believed that the relatively low porosity ofthe UF materials slows, but does not prevent simultaneous plasma modification of multiple UF membranes. Chemical composition of plasma-treated membranes

The chemical changes that occur in PSf membranes as a result of H₂O plasma treatment were determined by XPS analysis. Tables V and VI summarize the results from XPS analysis for untreated and plasma-treated BTS55 membranes. Table V. XPS results for controls, untreated and plasma-treated PSf membranes (BTS55)

Material XPS Atomic Percent carbon oxygen sulfur fluorine nitrogen

PTFE 33.8 ± 0.2 66.2 ± 0.2

TFAA-polystyrene^a 99.9 ± 0.1

TFAA-polyvinylalcohol^b 44.9 ± 0.2 20.4 ± 0.2 34.6 ± 0.1

Stoichiometric PSf 84.3 ± 0.8 12.0 ± 0.5 3.7 ± 0.3 untreated PSf (open) 84.3 ± 0.8 12.0 ± 0.5 3.7 ± 0.3 untreated PSf (tight) 84.5 ± 0.2 11.8 ± 0.1 3.7 ± 0.3 treated PSf (open)⁰ 74.4 ± 0.2 22.1 ± 0.5 2.6 ± 0.1 0.7 ± 0.2 treated PSf (tight) 73.7 ± 0.6 21.5 -fc O.l 2.6 ± 0.1 0.6 ± 0.1 untreated PSf (open)- TFAA^d 82.2 ± 0.2 13.0 ± 0.2 3.5 ± 0.1 1.2 ± 0.2 untreated PSf (tight)- TFAA 80.6 ± 0.4 13.7 ± 0.2 3.8 ± 0.2 1.5 ± 0.2 treated PSf (open)- TFAA 73.1 ± 0.8 18.9 ± 0.3 2.8 ± 0.1 4.2 ± 0.2 0.6 ± 0.3 treated PSf (tight)- TFAA 72.1 ± 0.8 18.8 ± 0.1 2.4 ± 0.1 5.7 ± 0.3 a. Negative derivatization control. b. Positive derivatization control. c. Trace impurities (<1%) of atomic silicon also detected. d. Trace impurities (<1%) of atomic sodium also detected.

The elemental composition for the untreated membrane is in agreement with the structure of PSf, Table V. In addition, no significant difference in composition was detected between the open and tight sides ofthe untreated BTS55 membrane. The results in Table V for plasma- treated membranes indicate that H₂O plasma treatment increases the oxygen concentration of asymmetric PSf membranes. The oxygen content for both the open and tight sides of plasma- treated BTS55 membranes (-22%) is significantly higher than that for the untreated membrane (11.8 ± 0.5%). Specifically, the increase in the high binding region ofthe C_ls spectrum upon plasma treatment is believed to be consistent with the presence of ketone/aldehyde and carboxylic acid/ester groups. Table VI provides the detailed results of the C_ls spectrum for untreated and plasma-treated BTS55 membranes. As a result of plasma treatment, the CH_X percentage decreases relative to the untreated material. Furthermore, this decrease was counterbalanced by an increase in the percentages of C-O and C-O_x groups, which were introduced by the plasma treatment. From the high-resolution S_2p spectra, it also appears that a small amount of sulfate-like groups were also introduced by the plasma treatment. These results indicate that the change in wettability observed for H₂O plasma- treated membranes is a result of chemical modification. It is believed that this formation of covalently bound hydrophilic functional groups is responsible for the observed increase in hydrophilicity ofthe treated membranes.

Table VI. XPS C_ls Percent for controls, untreated and plasma-treated PSf membranes (BTS55).

Sample XPS C,, , Percent

CH_X C-O C-Ox^a CF₂ CF₃

PTFE 100

TFAA-polystyrene^b 100

TFAA-polyvinylalcohol⁰ 28 25 23 24 polysulfone 85 15 untreated PSf (open) 84 16 untreated PSf (tight) 83 17 treated PSf (open) 75 19 treated PSf (tight) 74 18 treated PSf (open)- TFAA 16 17 a. C-O_x is the total amount of C=O, O=C-O, etc. b. Negative derivatization control, c. Positive derivatization control.

The plasma-treated membranes were further derivatized with trifluoroacetic anhydride (TFAA) to detect the presence of OH groups introduced by plasma treatment. As shown in Table VI, a small number of OH groups (-1% as 3 F atoms are substituted for each OH group) are introduced by plasma treatment. Alternate Plasma Treatments

To determine the effects of various possible reactive species in the plasmas generated by the present invention the alternate plasma systems of 100% O₂, 100% Ar, and 100% He were examined. With the O₂ system, both sides of BTS55 membranes were rendered wettable after plasma treatment. O₂ treatment of asymmetric PSf membranes by the present method was not permanent and significant contact angle increases were observed within 3 days of plasma treatment. With both the 100% Ar and 100% He plasmas, only the membrane side facing the plasma glow became hydrophillic (contact angle of 0°). The downstream side displayed reduced contact angles of about 75°. Upon standing, these contact angles again showed significant contact angle increases in a matter of days after plasma treatment. Thus, all three of these plasma treatments do afford temporary changes in wettability, but none are permanent modifications. Moreover, the rare gas plasmas do not modify the entire cross section of these membranes. Chemical composition of H₂Q plasma

One important factor contributing to the observed differences in membrane treatments between the H₂O-based systems and the alternate plasmas discussed in the previous section is the nature ofthe chemical species present in the plasma. OES was used to determine the excited state gas-phase composition of H₂O plasma generated by the methods ofthe present invention. Figure 11 A shows an optical emission spectrum of a 25 W H₂O vapor plasma (50 mTorr). This spectrum identifies the presence of OH radicals (at 306.95 and 309.14 nm), as well as H atoms (at 486.18 and 656.45 nm) in the H₂O plasma. No emission from other species is observed.

Figure 1 IB shows the OES spectrum of a 25 W H₂O vapor plasma (50 mTorr) when a PSf membrane (BTS55) was placed horizontally directly in the coil region ofthe plasma reactor. While emission from OH and H atoms is still evident in this spectrum, there are clearly strong emission lines at 282.68, 297.02, 312.71, 329.75, 483.17, and 519.48 nm. Less intense emission lines are also observed at 348.50, 450.75 and 560.67 nm. All of these new lines can be attributed to emission from CO in the plasma. See W. R. Harshbarger, R. A. Porter, T. A. Miller, P. Norton, Appl. Spectrosc, 1977, 31, 201. This indicates that the PSf is oxidized as CO, which is a known product of polymer oxidation. See, for example, E. O. Degenkolb, C. J. Mogab, M. R. Goldrich, J. E. Griffiths, Appl. Spctrosc, 1976, 30, 520. There are several significant differences between the methods ofthe present invention and currently known plasma methods for making polymeric membranes, preferably asymmetric polysulfone or asymmetric polyethersulfone membranes, wettable. These differences include: 1) complete penetration ofthe membrane modification by methods ofthe present invention; 2) permanency of plasma-induced hydrophilicity achieved by methods of the present invention; 3) degree of chemical modification; 4) generality of methods ofthe present invention; 5) minimal structural damage by plasma treatment ofthe present invention; and 6) the underlying chemical processes ofthe present invention.

A major advantage of methods ofthe present invention is that the entire membrane cross-section is modified. Penetration ofthe membrane modification by methods ofthe present invention is complete such that all surfaces ofthe membrane are treated. This is true for both microporous and ultrafiltration asymmetric PSf membranes. Without being bound by any theory, this extensive membrane modification by plasma treatment ofthe present invention is believed to be the result of allowing the plasma to flow through the pores ofthe membrane. Moreover, it is believed that the effectiveness of the H₂O plasma treatment is not limited to microporous asymmetric PSf membranes, but is also effective for asymmetric membranes with smaller pore sizes. The results obtained for BTS80 and UF membranes show that the penetration ofthe asymmetric membrane modification is extensive regardless ofthe average pore size (i.e., porosity) ofthe membrane. Moreover, penetration is not dependent on the membrane material as modification of both asymmetric polyethersulfone and polyethylene membranes were also achieved by methods of the present invention. Direct evidence for this is shown with the ESEM results for asymmetric polyethersulfone membranes (see Figure 12).

Plasmas have often been used for temporary improvements in polymer wettability. For example, the wetting properties of poly(hydroxybutyrate-co-9% hydroxyvalerate) films were studied as a function ofthe type of plasma treatment (e. g. Ar, O₂, H₂O and H₂O₂ plasmas). Of these plasma treatments, H₂O and H₂O₂ were considered milder treatments, as they resulted in less surface etching and cross-linking ofthe polymer. It is believed the degree of crosslinking influences the permanence ofthe plasma treatment as H₂O and H₂O₂ plasma-treated films lost hydrophicility upon standing sooner than films treated with Ar and O₂ plasmas. Loss of plasma-induced hydrophilicity is generally attributed to chain migration from the surface to the bulk to mimmize surface energy; hence, the degree of cross-linking affects the facility with which the chains can migrate from the surface into the bulk. In contrast, H₂O plasma-treated membranes ofthe present invention are permanently hydrophilic upon standing. Indeed, plasma-treated samples ofthe present invention remain wettable with storage in ambient conditions for at least nine months. Exposure to environmental extremes such as heating in an oven or exposure to boiling H₂O generally increases the hydrophobicity of plasma-treated membranes. In both cases, it is believed that chain motion is the cause of this loss in hydrophilicity for plasma- treated membranes exposed to high temperatures as heating polymers has been shown to increase chain motion. See, for example, T. R. Gengenbach, X. Xie, R. C. Chatelier, and H. J. Griesser, J. Adhes. Sci. Technol., 8 (1994) 305. It should be noted that this type of change in plasma modified polymers with high temperatures has been observed previously for hydrophobic treatments of polystyrene. See, for example, E. Occhiello, M. Morra, P. Cinquina, F. Garbassi, Polym. Prepr., 1990, 31, 308. Similarly, a degradation in adhesion properties of polyimide/polyimide plasma treated interfaces has been observed upon exposure to humid environments. See, for example, Y. Satsu, O. Miura, R. Watanable, K. Miyazaki, Jpn. Electron. Commun.: Part 2, 1991, 74, 489. Unlike conventional methods, the hydrophilic membrane modification ofthe present invention is permanent even upon standing for a prolonged period.

It is believed the permanency ofthe hydrophilic membrane modification ofthe present invention is related to the chemical changes in PSf as a result of plasma treatment. Incorporation of new, more hydrophilic functional groups that are covalently bound to the polymeric backbone results in permanent modification. Indeed, the oxygen concentration increases to more than 20% after plasma treatment (Table N) as new C-O_x and a small number of OH groups are introduced by plasma treatment. This increase is observed for both surfaces ofthe plasma-treated membranes, further demonstrating that both surfaces are equally modified by plasma treatment.

Asymmetric PSf membranes have also been treated with inert gas/H₂O plasmas (e.g., Ar/H₂O); however, no additional benefit in adding inert gas to the H₂O plasma was determined. It is generally known that diluents can increase the fragmentation of H₂O in the plasma (i.e., increase the amount of plasma-generated OH radicals), the addition of a diluent may also have a deleterious effect as it could lead to etching ofthe polymeric material.

All ofthe conditions employed in the present invention are relatively mild (low applied rf power and pressures as well as brief treatment times). Furthermore, the membrane is placed downstream from the inductor coil (e.g., about 9 cm), limiting exposure to energetic species. While membranes treated with methods ofthe present invention did not appear damaged by H₂O plasma treatment on the scale ofthe SEM experiment, for plasma- treated BTS55, BTS80, and UF membranes an increase in average pore size, as determined by porometry, was generally observed. The increase in pore diameter resulting from plasma treatment is, however, comparable to the increase in pore diameter observed for membranes treated with a wetting agent.

Present inventors have identified the presence of excited state OH radicals in H₂O plasma under the conditions used to process PSf membranes (Figure 11 A) ofthe present invention. Moreover, present inventors have also identified the presence of additional species generated only during plasma modification of PSf membranes. The OES spectrum in Figure 1 IB identifies the presence of excited state CO molecules, which are not observed in a 25 W H₂O vapor plasma (50 mTorr) without a PSf membrane in the reactor. Thus, present inventors were able to detect reactive excited-state plasma species as well as excited-state products ofthe reaction with PSf.

The empirical approach of previous workers lacks any information about the actual interactions of OH radicals with the surface ofthe membranes, data critical to elucidation of the mechanisms responsible for modification of the surface properties of PSf. Data obtained by the present inventors using laser-induced fluorescence (LIF) to measure gas-phase densities of ground state OH radicals also indicate that higher applied rf powers lead to higher OH concentrations in H₂O plasma.

It is believed that that PSf is more susceptible to modification by oxygen- containing plasmas. The mechanism by which OH radicals are likely produced in H₂O plasmas is given in equation (2). e^" + H₂O -> OH^* + H^* + e (2)

Plasma-generated OH radicals can combine with atoms at the surface to produce C-O bonds by equation (3). CH₃ CH₂OH

I ^→ I

-C- + OH^* -C- + H^* (3)

However, based on XPS results obtained by the present inventors, there is essentially no change in the C-O groups, as suggested in equation (3), ofthe polymer after plasma treatment (Table N). Furthermore, results obtained from TFAA derivatization indicate only a small number (-1%) of alcohol groups introduced by plasma treatment.

There is, however, a significant increase in the number of CO_x groups, such as aldehyde/ketone and carboxylic acid/ester groups. Although these groups could be introduced by oxidation of alcohol groups plasma-generated by equation (3), oxidation can occur at other sites in the polymer backbone.

Without being bound by any theory, it is believed that there are at least three different possible locations for oxidation on the polymer backbone as represented in Scheme 1.

Position 2

Scheme 1 For example, oxidation at position 2 yields an aldehyde, which can be further oxidized to yield a carboxylic acid group. Additionally, oxidation at the quaternary carbon (position 1) yields a ketone functionality. A secondary oxidation pathway is at the sulfur (position 3) resulting in sulfate-like groups as seen in the high resolution S_2p spectra. Finally, XPS results obtained by the present inventors indicate the presence of nitrogen on the plasma- treated samples suggesting the presence of amide-like groups, possibly at position (2). Therefore, the detailed chemical information obtained by the present inventors from the XPS shows concurrent reaction pathways.

It is also possible that additional oxygen may be incorporated at the surface by post-plasma treatment radical quenching with atmospheric oxygen. However, it is believed oxygen-containing radical species in the H₂O plasma are primarily responsible for the chemical modification of PSf, as the present inventors have observed a dramatic increase in oxygen concentration immediately after plasma treatment (Table V). Asymmetric Polyethersulfone Membranes

As another example, asymmetric polyethersulfone (PES) membranes were also treated with the same plasma parameters used for the polysulfone membranes described above. The untreated membranes had an average pore size of -10 μm (open side) and a contact angle of 90°. The PES membranes were treated with a 25 W H₂O vapor plasma (50 mTorr total pressure) for 2 minutes. Similar to the results for PSf membranes, contact angle measurements on the treated PES membranes were impossible to perform as the water drop immediately disappeared into the membrane (i.e., contact angle of 0°). Again, both sides of the plasma-treated membranes were completely wettable. This shows that the PES membranes were also rendered completely water-wettable as a result ofthe plasma treatment.

The chemical changes that occur in the PES membranes as a result of H₂O plasma treatment were determined by XPS analysis. Table VII summarizes the results from XPS analysis for untreated and plasma treated PES membranes. These results again demonstrate that the H₂O plasma treatment significantly increases the oxygen concentration of asymmetric PES membranes. Although the exact amounts differ slightly, these results are very similar to the results for the PSf membranes given in Table V. In addition, the amount of C-O_x groups incorporated with the PES membranes is similar to that observed with the PSf membranes, Table NIL Table NIL XPS results for untreated and plasma-treated PES membranes

Material • XPS Atomic Percent carbon oxygen sulfur fluorine untreated PES (both sides) 75.5 ± 0.9 17.1 ± 0.6 6.8 ± 0.4 treated PES (both sides) 67.9 ± 1.1 25.2 ± 0.5 4.7 ± 0.2 untreated PES (both sides)- TFAA" 75.4 ± 0.4 17.8 ± 0.4 6.6 ± 0.2 treated PES (both sides)- TFAA^b 70.1 ± 0.7 20.3 ± 0.6 5.0 ± 0.1 3.4 ± 0.5 a. Trace impurities (<1%) of atomic chlorine also detected. b. Trace impurities (<2%) of atomic silicon also detected.

The penetration ofthe plasma treatment was further investigated for these membranes by environmental SEM. In this experiment, the sample chamber ofthe ESEM was pumped down from atmosphere to a pressure of 5 Torr and then flushed with H₂O vapor to 10 Torr. This process was repeated three times to exchange the chamber atmosphere with water vapor. The chamber pressure was then set to 7 Torr. When exposed to water vapor in such a manner, the treated PES sample wet out completely (and immediately), as seen in the cross sectional image shown in Figure 12. In this image, the sample is oriented with tight side at the top ofthe frame. For comparison, the cross section ofthe untreated PES membrane is very similar to that for the PSf membrane shown in Figure 2. The results for the PES membranes provided in Table Nil and Figure 12 constitute strong evidence that the invention fully penetrates and modifies the exterior surfaces and the bulk matrix of hydrophobic polymeric membranes, creating a permanently hydrophilic material.

The foregoing discussion ofthe invention has been presented for purposes of illustration and description. The foregoing is not intended to limit the invention to the form or forms disclosed herein. Although the description ofthe invention has included description of one or more embodiments and certain variations and modifications, other variations and modifications are within the scope ofthe invention, e.g., as may be within the skill and knowledge of those in the art, after understanding the present disclosure. It is intended to obtain rights which include alternative embodiments to the extent permitted, including alternate, interchangeable and/or equivalent structures, functions, ranges or steps to those claimed, whether or not such alternate, interchangeable and/or equivalent structures, functions, ranges or steps are disclosed herein, and without intending to publicly dedicate any patentable subject matter.

Claims

WHAT IS CLAIMED IS: 1. A method for modifying a surface of a polymer membrane with plasma comprising: (a) placing a polymer membrane down stream from a plasma generator, wherein said polymer membrane comprises: (i) an exterior surface; (ii) a bulk matrix; and (iv) pores extending from said exterior surface into said bulk matrix, wherein said pores define an interstitial surface; (b) generating a plasma of surface modifying compound; and (c) providing a means to allow flow of said plasma through said interstitial surface of said polymer membrane to produce a surface modified polymer membrane.

2. The method of Claim 1, wherein the overall direction of plasma flow is substantially perpendicular to said exterior surface.

3. The method of Claim 2, wherein the overall direction of plasma flow is substantially parallel to the general direction ofthe pores.

4. The method of Claim 1 , wherein said surface modified membrane is a hydrophilic membrane.

5. The method of Claim 4, wherein said membrane surface modifying compound comprises oxygen, air, water, hydrogen peroxide, ammonia, helium, argon, or mixtures thereof.

6. The method of Claim 1 , wherein said surface modified membrane is a hydrophobic membrane.

7. The method of Claim 6, wherein said membrane surface modifying compound comprises C,-C₁₀ alkane, C,-C₁₀ fluoroalkane, C_rC_I0 fluoroalkene, fluorinated epoxide, siloxane, or mixtures thereof.

8. The method of Claim 1, wherein said polymer membrane comprises a polymer selected from the group consisting of polysulfone, polyethersulfone, polyethylene, polystyrene, polytetrafluoro-ethylene, polyester, poly(methyl methacrylate), polyacrylonitrile, polyvinylidene fluoride, and mixtures thereof.

9. The method of Claim 1 , wherein said polymer membrane is an asymmetric polysulfone membrane.

10. The method of Claim 1 , wherein said plasma flow means comprises a pressure differential from one side of said exterior surface to the other side of said exterior surface.

11. A process for producing a hydrophilic polymer membrane from a hydrophobic membrane, said method comprising: (a) placing a hydrophobic polymer membrane down stream from a plasma generator, wherein said polymer membrane comprises: (i) an exterior surface; . (ii) a bulk matrix; and (iv) pores extending from said exterior surface into said bulk matrix, wherein said pores define an interstitial surface; (b) generating a plasma of surface modifying compound, wherein said surface modifying compound is capable of increasing hydrophilicity of said hydrophobic polymer membrane; and (c) providing a means to allow flow of said plasma through said interstitial surface of said hydrophobic polymer membrane to produce a hydrophilic polymer membrane.

12. The process of Claim 11 , wherein said membrane surface modifying compound comprises oxygen, air, water, hydrogen peroxide, helium, argon, or mixtures thereof.

13. The process of Claim 12, wherein said membrane surface modifying compound comprises water.

14. The process of Claim 11, wherein said polymer membrane comprises a polymer selected from the group consisting of polysulfone, polyethersulfone, polyethylene, polystyrene, polytetrafluoro-ethylene, polyester, poly(methyl methacrylate), polyacrylonitrile, polyvinylidene fluoride, and mixtures thereof.

15. The process of Claim 11 , wherein said polymer membrane is an asymmetric polysulfone membrane.

16. The process of Claim 11 , wherein said plasma flow means comprises a pressure differential from one side ofthe membrane surface to the other side ofthe membrane surface.

17. A permanently hydrophilic asymmetric polymer membrane comprising: (a) a rough surface side; (b) a smooth surface side; (c) a bulk matrix; and (d) pores extending from said rough surface side to said smooth surface side tlirough said bulk matrix, wherein said pores define an interstitial surface; wherein said bulk matrix, said rough surface side and said smoth surface side and said interstitial surfaces comprise an organic polymer comprising carbon, hydrogen and oxygen atoms.

18. The asymmetric polymer membrane ofclaim 17, wherein sessile drop contact angle of said smooth surface side is less than about 70°.

19. The asymmetric polymer membrane of claim 18, wherein sessile drop contact angle of said smooth surface side is less than 10°.

20. The asymmetric polymer membrane of claim 17, wherein bubble point of said membrane is at least about 10 psi higher than a similar asymmetric polymer membrane having substantially no oxygen atom.

21. The asymmetric polymer membrane of claim 17, wherein bubble point of said membrane is at least about 20 psi higher than a similar asymmetric polymer membrane having substantially no oxygen atom.

22. The asymmetric polymer membrane of claim 17, wherein said polymer membrane comprises a polymer selected from the group consisting of polysulfone, polyethersulfone, polyethylene, polystyrene, polytetrafluoro-ethylene, polyester, poly(methyl methacrylate), polyacrylonitrile, polyvinylidene fluoride, and mixtures thereof.

23. The asymmetric polymer membrane ofclaim 17, wherein said polymer membrane is a polysulfone asymmetric polymer membrane.

24. The asymmetric polymer membrane of claim 23, wherein said polymer membrane comprises at least about 15% carbonyl oxygen.

25. A permanently hydrophilic asymmetric polysulfone membrane, wherein said polysulfone membrane has the sessile drop contact angle of less than about 70°.

26. The asymmetric polysulfone membrane ofclaim 25, wherein said polysulfone membrane has the sessile drop contact angle of less than 10°.

27. The asymmetric polysulfone membrane of claim 25, wherein the pore gradient of said polysulfone membrane is about 100: 1 from the open side to the tight side.

28. The asymmetric polysulfone membrane of claim 25, wherein said polysulfone membrane comprises at least about 15% carbonyl oxygen.

29. The asymmetric polysulfone membrane of claim 25, wherein the bubble point of said polysulfone membrane is at least about 15 psi higher than a similar asymmetric polysulfone membrane having substantially no carbonyl oxygen. '

30. A permanently hydrophilic asymmetric polymer membrane produced by a process comprising the steps of: (a) placing a hydrophobic asymmetric polymer membrane down stream from a plasma generator, wherein said hydrophobic asymmetric polymer membrane comprises (i) a rough surface side; (ii) a smooth surface side; (iii) a bulk matrix; and (iv) pores extending from said rough surface side to said smooth surface side through said bulk matrix, wherein said pores define an interstitial surface; (b) generating a plasma of hydrophilic membrane surface modifying compound; and (c) providing a means to allow flow of said plasma through said interstitial surface of said hydrophobic asymmetric polymer membrane to produce a hydrophilic asymmetric polymer membrane.

31. The permanently hydrophilic asymmetric polymer membrane of claim 30, wherein said hydrophilic membrane surface modifying compound comprises oxygen, air, water, hydrogen peroxide, helium, argon, or mixtures thereof.

32. The permanently hydrophilic asymmetric polymer membrane of claim 30, wherein said membrane surface modifying compound comprises water.

33. The permanently hydrophilic asymmetric polymer membrane ofclaim 30, wherein said polymer membrane comprises a polymer selected from the group consisting of polysulfone, polyethersulfone, polyethylene, polystyrene, polytetrafluoro-ethylene, polyester, poly(methyl methacrylate), polyacrylonitrile, polyvinylidene fluoride, and mixtures thereof.

34. The permanently hydrophilic asymmetric polymer membrane of claim 30, wherein said polymer membrane is an asymmetric polysulfone membrane.

35. The permanently hydrophilic asymmetric polymer membrane of claim 30, wherein said plasma flow means comprises a pressure differential from one side ofthe membrane surface to the other side ofthe membrane surface.