Porous polymeric membrane and related filters and methods

Abstract

A porous polymer membrane comprising two opposing sides and having a variable pore structure through the thickness of the membrane; a filter component and a filter comprising this type of porous polymer membrane; a method for manufacturing the membrane, filter component and filter; and a method for using the polymer filter membrane, filter component or filter are described.

Description

Technology Field

[0001] The following description relates to a porous polymer filter membrane having two opposing faces and a variable pore structure through the thickness of the membrane; additionally, a filter component and a filter comprising this type of porous polymer filter membrane; a method for manufacturing the porous polymer filter membrane, the filter component and the filter; and a method for removing unwanted substances from a fluid by filtering a fluid, such as a liquid chemical, using the porous polymer filter membrane, the filter component or the filter. Background Technology

[0002] The primary application of filter membranes is to remove unwanted substances from the flow of useful fluids. Many gaseous and liquid fluids, including environmental air, drinking water, liquid industrial solvents and processing fluids, industrial gases used in manufacturing or processing (e.g., in semiconductor fabrication), and liquids with medical or pharmaceutical uses, are processed using filters in industry. The unwanted substances removed from fluids include impurities and contaminants, such as particles, microorganisms, and dissolved chemical species. Specific examples of impurity removal applications of filter membranes include removing particles or bacteria from therapeutic solutions in the pharmaceutical industry, for use in microelectronics technology and semiconductor processing, and for processing ultrapure aqueous solutions and organic solvent solutions for air and water purification processes.

[0003] To perform a filtration function, the filter product includes a filter membrane responsible for removing unwanted substances from a fluid. The filter membrane may be in the form of a flat sheet, such as one that can be wound (e.g., spirally) or pleated as needed. Alternatively, the filter membrane may be in the form of a hollow fiber. The filter membrane may be contained within a housing that includes an inlet and an outlet, so that the fluid to be filtered enters through the inlet and passes through the filter membrane before passing through the outlet.

[0004] A filter membrane may consist of a porous polymer film having an average pore size that can be selected based on the expected use of the filter, namely the type of filtration performed using the filter. Typical pore sizes are in the micron or sub-micron range, e.g., about 0.001 microns to about 10 microns. Membranes with an average pore size of about 0.001 to about 0.05 microns are often classified as ultrafilter membranes. Membranes with a pore size of about 0.05 to 10 microns are often classified as microporous membranes.

[0005] For commercial use, the filter membrane must be of a type that can be efficiently manufactured and assembled into a filter product. The membrane must be able to be produced efficiently and must possess mechanical properties, such as strength and flexibility, that allow the filter membrane to withstand assembly into a filter cartridge or filter form. In addition to mechanical properties, the membrane must have suitable chemical functionality and microstructure for high-performance filtration.

[0006] Various techniques for forming porous filter membranes are known. Exemplary techniques include, among others, melt-extrusion (e.g., melt-casting) techniques and immersion casting (phase inversion) techniques. Different techniques for forming porous materials can produce different porous membrane structures in terms of the size and distribution of pores formed within the membrane; that is, different techniques produce different pore sizes and membrane structures, which are often referred to as morphology, meaning the uniformity, shape, and distribution of pores within the membrane.

[0007] Examples of membrane morphologies include homogeneous (isotropic) and asymmetric (anisotropic). Membranes having pores of substantially uniform size distributed evenly across the entire membrane are often referred to as isotropic or "homogeneous." Anisotropic (also known as "asymmetric") membranes can be considered to have a morphology in which a pore size gradient exists across the membrane; for example, the membrane may have a porous structure in which relatively larger pores are found on one membrane surface and relatively smaller pores on another, with the pore structure varying along the membrane thickness. The term "asymmetric" is often used interchangeably with the term "anisotropic." Often, the part of the membrane with relatively smaller pores (compared to other regions) is referred to as the "dense" region. The part of the membrane with larger pores is often called the "open" region.

[0008] Industries utilizing filters, including the manufacturing and processing industries of semiconductor materials and microelectronic devices, maintain a continuous interest in discovering improved filter membranes and filters, including new filter products with improved performance such as enhanced retention rates. These new filters must be able to be produced efficiently in a cost-effective, for example, profitable manner.

[0009] summation

[0010] The field of microelectronic device fabrication (e.g., microelectronic and semiconductor device fabrication) requires continuous improvement of fabrication materials and methods to maintain parallel and continuous improvement in the performance (e.g., speed and reliability) of microelectronic devices. Opportunities to improve microelectronic device fabrication exist in all aspects of the manufacturing process, including methods and systems for filtering liquid materials used during fabrication.

[0011] Two important aspects of commercial filter membrane manufacturing are efficient and cost-effective manufacturing, and high performance in terms of retention rate, which is related to the amount of particles the filter membrane can remove from a fluid stream.

[0012] A technique that can be effective in improving retention rates is to use multiple membranes in series, for example, by bringing the filtration surfaces of two membranes into contact in a laminate and passing the fluid through the two membranes in series. This can often be referred to as "layering" or "stacking" of filter membranes to form a multilayer membrane. When layering techniques are performed using asymmetric membranes, each having a "dense" region and an "open" region, the fluid passing through the two membranes must pass through the two "dense" regions, and the membrane retention rate can be increased.

[0013] Although layering technology is feasible and potentially effective, using two layered membranes doubles the cost of the membrane material. Therefore, there is interest in mimicking the structure of two membranes, i.e., two-layer stacked membranes, into a single monolithic membrane manufactured by a single membrane formation step, that is, fabricating a single monolithic membrane containing two dense layers using a single manufacturing step, for example, a "double-density" membrane. The two dense layers of the membrane are preferably manufactured using separate steps and are not combined; for example, the membrane is not manufactured by combining two separate membranes into a multilayer membrane.

[0014] Accordingly, the present description relates to a membrane formed integrally to include two dense regions and one or more open regions in a single forming step, a method for manufacturing the membrane, as well as a method for using the membrane as a filter membrane and a filter product including the membrane. Such a porous membrane can be manufactured by a novel and original method in which a polymer solution is extruded through a die in the form of a polymer solution film, and then the polymer within the film is solidified on both sides of the film, particularly by exposing both opposing sides of the film to conditions that produce a dense morphology on both sides of the solidified film.

[0015] As an example of a method useful for manufacturing a film as described having the morphology as described, the film may be formed by forming an extruded liquid film of a liquid polymer solution containing a non-coagulated or partially coagulated polymer dissolved or suspended in a solvent, and simultaneously exposing both opposing surfaces of the film to conditions (e.g., temperature, non-solvent, humidity, evaporation) that induce coagulation of the dissolved polymer on both sides of the film in substantially the same manner, thereby causing the polymer to coagulate within the film. As one example, the technique for inducing coagulation is by non-solvent-induced phase separation (NIPS) technique. As another example, the coagulation of the polymer may be induced by heat-induced phase separation (TIPS). As yet another example, the coagulation of the polymer may be induced by exposing the extruded film to humidity. As yet another example, the coagulation of the polymer may be induced by the evaporation of the solvent from the extruded film.

[0016] In one aspect, the extruded porous polymer sheet membrane comprises a first surface, a second surface, and a thickness between the first surface and the second surface, a first thickness region including a portion of the first surface and the membrane in the thickness direction, a second thickness region including a portion of the second surface and the membrane in the thickness direction, and a third thickness region extending in the thickness direction between the first thickness region and the second thickness region. The average pore size of the first thickness region and the average pore size of the second thickness region are both smaller than the average pore size of the third region.

[0017] In another aspect, a method for manufacturing an extruded porous polymer sheet membrane having opposing first and second surfaces, a thickness between opposing surfaces, and pores having a non-uniform pore size comprises the steps of: forming a polymer-containing liquid containing a polymer in a solvent; passing the polymer-containing liquid through an extrusion die to form an extruded film of the polymer-containing liquid; and exposing both sides of the extruded film to conditions that induce coagulation of the polymer on both sides of the film. Brief explanation of the drawing

[0018] Figures 1a and 1b are cross-sectional photographs of the membrane described herein. Figures 2a, 2b, 3a, and 3b are illustrations of the exemplary method and system of the present description (schematic and not necessarily consistent with scale). Figure 4 shows performance data of an exemplary filter membrane of the description. Specific details for implementing the invention

[0019] The following description relates to an extruded porous polymer membrane that may be effective as a filter membrane, a filter product comprising the filter membrane, and a method for manufacturing and using the porous membrane.

[0020] A porous polymer membrane has two opposing surfaces (or opposing "faces") and a thickness between the two opposing surfaces. The pores of the porous membrane are located across the thickness of the membrane, allowing fluid flow from one side of the membrane through the thickness to the opposite side and through the opposite side while removing particles or contaminants from the fluid. Therefore, the membrane is permeable to fluids such as liquids. This type of membrane is often referred to as an "open pore" membrane in contrast to a "closed pore" membrane. An open pore membrane may take the form of a thin film or sheet of an extruded porous polymer material having an open pore porous structure comprising a polymer matrix that defines a number of open "cells"—which are relatively uniform thicknesses and three-dimensional pore structures or pores. Open cells may be referred to as openings, pores, channels, or passages that are mostly interconnected between adjacent cells, allowing a fluid, such as a liquid, to flow from one side of the membrane to the other through the thickness of the membrane.

[0021] The porous membrane as described comprises pores of different pore sizes, and the size of the pores within the membrane varies with the thickness of the membrane. As used herein, “pore size” refers to a region (or “part”) of the membrane, for example, a region of thickness located between two different depths of the membrane, or the size of the pores at a specific depth of the membrane defined for the thickness of the membrane (e.g., average size). For example, a region of the membrane may be a part of the membrane between the surface and a specified depth of the membrane, for example, a depth of 10 microns below the surface. As another example, a region of the membrane may be a part located between two different depths, for example, a part of the membrane located in the middle one-third of the thickness of the membrane.

[0022] The porous membrane described has a morphology with non-uniform (non-isotropic, non-homogeneous) pore sizes across the thickness of the membrane. The non-uniform morphology may be referred to as a "variable" morphology having multiple (e.g., at least three) regions of different pore sizes at different locations along the thickness of the membrane. Within the context of this description, a membrane having a "variable pore structure" comprises pores of different sizes across the thickness of the membrane between two opposing surfaces of the membrane. The pore sizes may vary gradually or non-gradually across the thickness of the membrane (based on depth). (See, for example, FIGS. 2a, 2b, 3a, and 3b.)

[0023] The membrane of this description comprises, more specifically, two “dense” regions and at least one “open” region located between the two “dense” regions; these membranes are referred herein as “double-dense” membranes, or membranes having a “double-dense” morphology. One region of the membrane may be referred to as a “dense region,” which means a region having relatively smaller pores over the membrane thickness. A second region of the membrane is also a second dense region having relatively smaller pores over the membrane thickness. Inside (or in the “middle”) of the membrane there is an “open region” located between two opposing dense regions. The “open” region is a portion of the thickness containing pores that are relatively larger than the pores of both dense regions. A membrane as described, having such a configuration—namely, two dense regions and an open region inside the membrane between the two dense regions—may be referred to as a “double-dense” porous membrane having a “variable” pore size.

[0024] A porous membrane having two dense regions and an open internal region is considered to be "integral." Specifically, the entire thickness of the membrane and both opposing surfaces are structurally formed and constructed together as a single and continuous membrane by a single forming step, for example, a single step involving the extrusion of the film and the solidification of the polymer of the film (including co-extrusion using a multi-slot die).

[0025] In contrast to the described monolithic porous membrane, other types of known porous membranes may be non-monolithic. These include membranes often referred to as multilayer or “composite” membranes, which are manufactured by combining or arranging two distinct membrane layers, each having a different morphology or chemical composition, on opposing surfaces and optionally attaching or bonding them together. Such composite membranes may include a multilayer structure formed by combining a first porous membrane layer, which is formed in one forming step and has a first (e.g., larger pore size) morphology, with a second porous membrane layer formed in a second forming step different from the step of forming the first porous layer. Such a type of multilayer or “composite” membrane structure formed from multiple distinct films manufactured by multiple steps is not considered an “monolithic” membrane.

[0026] Another example of a membrane not considered integral according to this description is a porous membrane comprising a base membrane prepared during a membrane-forming step and one or more coatings (e.g., one or more polymer coatings) applied to one or more surfaces of the base membrane in a subsequent coating step, which are coated on one or both surfaces. An exemplary membrane may be a homogeneous (or non-homogeneous) base membrane coated with a polymer on one or both surfaces after being formed, having a relatively uniform (or non-uniform) pore size throughout. The polymer coating may be the same or different from the polymer of the base membrane. The polymer coating may reduce the pore size on one or two coated surfaces.

[0027] The film of this description may have any useful thickness, for example, in the range of 10 to 300 microns, for example, 25 or 40 microns, and up to a thickness in the range of 250 or 200 microns.

[0028] The membrane comprises two opposing, relatively parallel surfaces extending in both the longitudinal and transverse directions, and a thickness extending in a third direction and located between the two opposing surfaces. The intermediate position between the two surfaces is considered to be a virtual "centerline" that bisects the thickness of the membrane, that is, extending in the longitudinal and transverse directions at equal distances from each of the two opposing surfaces.

[0029] A “thickness region” of a film is a portion of the film that extends along the length and width dimensions of the film over a certain portion of the film’s thickness. For the purposes of this description and claims, the film may be considered to comprise at least three thickness regions: a first dense region, one open region, and a second dense region. The open region is located between the two dense regions. One or both of the dense regions may comprise the surface of the film, but the dense region is not required to comprise the surface.

[0030] In a specific example, the membrane may include a first thickness region that includes a first surface of the membrane and extends to a depth below the first surface but does not extend to the midline of the membrane; a second thickness region that includes a second surface of the membrane and extends to a depth below the second surface but does not extend to the midline of the membrane; and a third thickness region that extends across the remainder of the thickness of the membrane between the first thickness region and the second thickness region and optionally and typically includes the midline. Examples of useful depths below the first surface of the membrane that define the size (thickness) of the first thickness region relative to the total thickness of the membrane include depths equal to 2, 5, 10, 20, 25, 30, or 33 percent of the total thickness of the membrane. Likewise, examples of useful depths below the second surface of the membrane that define the size (thickness) of the second thickness region relative to the total thickness of the membrane include depths equal to 2, 5, 10, 20, 25, 30, or 33 percent of the total thickness of the membrane.

[0031] According to an exemplary membrane, the double close-pack morphology can be defined in terms of the relatively small average pore size of each of the two close-pack regions compared to the relatively larger pore size of the open region. The average pore size of the close-pack region may be any average pore size of the close portion of the useful filter membrane, including but not limited to average pore sizes in the range of 1 nanometer to 10 microns, for example, 1 nanometer to 5 microns or 10 nanometers to 1 or 2 microns. The first close-pack region and the second close-pack region may each independently have the same or different average pore sizes, and the average pore sizes of both are within one of the mentioned ranges. The average pore size of the open region may be any average pore size of the open portion of the useful filter membrane, including but not limited to an average pore size greater than 10 microns, for example, from 5 nanometers to 50 microns, for example, from 20 nanometers to 10 microns, or in the range of 50 nanometers to 1, 2, or 5 microns. Alternatively or additionally, the average pore size of the open region may also be larger by at least 10, 20, 50, or 100% or more than the average pore size of the dense region of one or both; that is, the average pore size of the open region may be at least 10, 20, 50, or 100% larger than the average pore size of the dense region of both.

[0032] When considering the size of the thickness region according to the thickness of the film, the present invention does not require specific sizes of each region or relative sizes of regions other than that the average pore size of the region is as described, the first thickness region includes a depth position of the first minimum pore size on one side of the centerline, and the second thickness region includes a depth position of the second minimum pore size on the second side of the centerline; and the minimum pore size of the third region is greater than the first minimum pore size and greater than the second minimum pore size.

[0033] More specifically, the double close morphology may be defined, alternatively or additionally, in terms of the existence of two different “minimum” pore sizes (smallest measured pore sizes) located at a specific depth of the membrane and on the opposite side of the membrane's centerline, respectively, independently. The “minimum” pore size is the smallest measured pore size on one side of the centerline, measured over at least a portion of the membrane's area at a uniform single depth below the membrane's surface. According to the membranes described here, the membrane has a first minimum pore size at a specific depth on one side of the centerline and a second minimum pore size at a specific depth on the second side of the centerline. The depth of the first minimum pore size from the first surface may be the same as or different from the depth of the second minimum pore size from the second surface. The first minimum pore size will be the minimum pore size existing at a specific depth between the centerline and the first surface. The second minimum pore size will be the minimum pore size existing at a specific depth between the centerline and the second surface. These two minimum pore sizes are the two smallest pore sizes of the membrane, and the membrane does not have any other depth where a lower average pore size exists.

[0034] According to certain exemplary embodiments, the minimum pore size (first minimum or second minimum pore size) may be located on the surface (first surface or second surface) (where the depth of the location of the minimum pore size is 0) or near the surface, for example, within 2, 5, 10, or 20% of the film thickness from the surface.

[0035] The respective locations of the first minimum pore size and the second minimum pore size may be independently located on the surface or at a depth from the surface. For example, the location of the first minimum pore size may be on the first surface (at a depth of 0) while the location of the second minimum pore size may be on the second surface; the location of the first minimum pore size may be below the first surface while the location of the second minimum pore size may be below the second surface; the location of the first minimum pore size may be on the first surface while the location of the second minimum pore size may be below the second surface; or the location of the first minimum pore size may be below the first surface while the location of the second minimum pore size may be on the second surface.

[0036] The average pore size in a region of the membrane, or at a specific depth of the membrane, can be measured by any useful technique, including manual or electronic visual examination of images produced by a scanning electron microscope (SEM). As an example of these methods, computer software programs can be used to evaluate the pore size within a region of the membrane or at a depth. As another example, the pore size in a region of the membrane or at a depth can be measured by visually inspecting a hard copy of the SEM micrograph.

[0037] An exemplary membrane can be described in terms of asymmetry, which is the relationship between the measured pore size of the open region and the measured pore size of the dense region. The asymmetry (D) of the membrane described can be defined as follows:

[0038] D = (Average pore size of the third thickness region) / A

[0039] A can be defined as one of the average pore size of the first region; the average pore size of the second region; the first minimum pore size or the second minimum pore size.

[0040] The useful and desirable membrane of this description may have an asymmetry (D) of 5 or more, e.g. 10, 20, or 30 or more, when calculated using one or more of the different possible pore sizes of group A.

[0041] Referring to FIG. 1a, a cross-section of an exemplary porous membrane (10) having a surface (12), a second surface (14), and a thickness between these two surfaces is shown. A centerline (50) is located midway between the surface (12) and the surface (14). The first thickness region (20) is between the surface (12) and the first depth (D 1 It is located in the thickness region between the second surface (14) and the second depth (D 2 It is located in the thickness region between the first thickness region (20) and the second thickness region (40). The third thickness region (30) is located in the thickness region between the first thickness region (20) and the second thickness region (40). The average pore size of each of the first thickness region (20), the second thickness region (40), and the third thickness region (50) can be measured over the three-dimensional thickness of each region. According to the present description, the average pore size of the first thickness region is substantially (e.g., measurably) smaller than the average pore size of the third thickness region, and the average pore size of the second thickness region is substantially smaller than the average pore size of the third thickness region.

[0042] FIG. 1a also shows a depth (22) of the first minimum pore size located at a depth on one side of the center point (50). As indicated, the depth (22) of the first minimum pore size is a depth of 0, which means that the first minimum pore size is located on the surface (12). FIG. 1a also shows a depth (42) as the second minimum depth located on the second side of the center point (50) and at a depth slightly below the second surface (14). The first minimum pore size and the second minimum pore size are located at each depth on or below each surface (12 and 14), and each depth is measured as the depth of the membrane having the smallest average pore size for all pores on the same face of the center point (50) that extend along the length and width of the membrane, namely within the membrane (10) (depth (22)) between the surface (12) and the center point (50) and within the membrane (10) (depth (42)) between the surface (14) and the center point (50). Each minimum pore size is the smallest average pore size at any specific depth on each face of the center line, and each face of the membrane relative to the center line does not have any other depth having an average pore size smaller than its minimum pore size.

[0043] Referring to FIG. 1b, a cross-section of a porous membrane (10) having a surface (12), a second surface (14), and a thickness between these two surfaces is shown. A centerline (50) is located midway between the surface (12) and the surface (14). The first thickness region (20) is between the surface (12) and the first depth (D 1 It is located in the thickness region between the second surface (14) and the second depth (D 2 It is located in the thickness region between ). The third thickness region (30) is located between the first thickness region (20) and the second thickness region (40), for example, at a depth (D 1 ) and depth (D 2It is located in the thickness region between the first thickness region (20), the second thickness region (40), and the third thickness region (50). The average pore size of the pores across each of the first thickness region (20), the second thickness region (40), and the third thickness region (50) can be measured across the three-dimensional thickness region. According to the present description, the average pore size of the pores in the first thickness region is substantially smaller than the average pore size of the pores in the third thickness region, and the average pore size of the pores in the second thickness region is substantially smaller than the average pore size of the pores in the third thickness region.

[0044] FIG. 1b also shows a depth (22) of a first minimum pore size located on the surface of the membrane (10) on one side of the center point (50) or at a depth thereto, as well as a depth (42) of a second minimum pore size located on the second surface of the membrane (10) on the second side of the center point (50) or at a depth thereto. In the membrane presented, the location of the first minimum pore size (22) and the location of the second minimum pore size (42) are respectively on or near the surface (12) and (14). Each of the two minimum pore sizes is the smallest average pore size on one side of the membrane relative to the center line, and each side of the membrane does not have any other depth having an average pore size smaller than its minimum pore size.

[0045] A porous film as described can be manufactured by a method comprising a novel and original method for forming a polymer extruded film, comprising exposing both surfaces of the film to conditions that induce the polymer of the film to coagulate, wherein the conditions are the same conditions applied in the same manner to both surfaces. Because both surfaces of the extruded film are exposed to the same conditions that induce coagulation in the same manner at the same timing, the method as described can be effective for forming a coagulated film comprising two opposing surfaces having pores formed simultaneously by the same coagulation step; the pores on each of the two opposing surfaces may be smaller than the pores in the inner region of the film, thereby producing a "double-dense" film; and the pores on the two opposing surfaces may have similar morphologies due to each surface undergoing coagulation based on substantially the same or similar coagulation-inducing conditions and timing. The resulting coagulated film comprises two opposing (dense) surface or thickness regions having similar, similar, or substantially identical morphologies, one on each side of the centerline, and also comprises an internal region having a (open) morphology different from the morphology of the two surfaces. An exemplary film may have one surface or region having a morphology having a relatively small pore size, a second (opposite) surface or region having a morphology having a relatively small pore size, and an internal portion between the two surfaces or regions having a morphology having a relatively larger pore size compared to the first and second surfaces.

[0046] As an example of a method useful for manufacturing a membrane as described having the morphology as described, the membrane may be formed by extruding a liquid polymer solution containing a non-coagulated (optional also, a certain amount of coagulated or partially coagulated) polymer dissolved (or suspended) in a solvent, and causing the polymer to coagulate within the film by exposing both of the two opposing surfaces of the film together or simultaneously to conditions (e.g., temperature, non-solvent, humidity, solvent evaporation) that induce concomitant or co-coagulation of the polymer on both of the two opposing surfaces. As one example, the technique inducing coagulation is by non-solvent-induced phase separation (NIPS) technique. As another example, the coagulation of the polymer may be induced by heat-induced phase separation (TIPS).

[0047] By conventional non-solvent-induced phase separation (NIPS), a polymer solution containing a polymer, typically dissolved or suspended in a solvent, is extruded and cast onto a support layer (e.g., a support polymer film) to form a cast film. The cast film has one surface in contact with the support layer and covered by the support layer, and another (opposite) surface that is exposed or open. Subsequently, the support layer and the extruded and cast films are immersed in a coagulation bath containing a "non-solvent," which refers to a liquid in which the polymer is not substantially soluble. When the cast film is immersed in the non-solvent, an exchange of solvent and non-solvent occurs at one exposed surface between the non-solvent bath and the solvent of the extruded film polymer solution. Along with this solvent exchange, the non-solvent is present with the polymer within the cast film, causing the polymer to precipitate from the cast polymer solution film onto the support layer. The polymer must be highly soluble in the solvent of the polymer solution and substantially insoluble in the "non-solvent" of the bath so that the polymer is efficiently precipitated or coagulated upon contact with the non-solvent of the coagulation bath (e.g., an aqueous liquid).

[0048] According to these techniques, the liquid nonsolvent comes into contact with only one surface, which is the exposed surface of the cast polymer solution film, but does not come into contact with the opposite surface of the film that is in contact with the support layer. Because the nonsolvent comes into contact with only one surface of the cast film and solvent-exchange occurs only through that one surface of the cast film, the solidified polymer film exhibits a different heterogeneity, for example, an asymmetric morphology, on one side of the film compared to the other side of the film. Typically, after solidification, the film has an asymmetric morphology comprising a first morphology on one side of the film, a substantially different second morphology on a second side of the film, and an intermediate morphology between the two sides that varies over the thickness of the film with a pore size that gradually changes over a range of pore sizes, for example, between the pore size on one surface and the pore size on the second surface. For example, one surface may have a relatively small average pore size, the opposite surface may have a relatively larger average pore size, and all locations (depths) between the two surfaces will have an average pore size between the two average pore sizes of the opposing surfaces. Optionally, one surface may be considered to exhibit a "dense" morphology, and the second surface may be considered to exhibit an "open" morphology.

[0049] A similar effect can be achieved using conventional heat-induced phase separation (TIPS) technology. In conventional TIPS technology, a polymer-containing liquid, typically containing a non-coagulated polymer (and, optionally, a partially coagulated polymer) in a solvent, is extruded through a slot die to form an extruded film. In the conventional TIPS method, one side of the extruded film is brought to a reduced temperature, for example, by contact with a low-temperature "cooling roll," to induce coagulation of the polymer in the extruded film.

[0050] Under this technique, one surface of the extruded film is exposed to a reduced temperature. The opposite surface is not exposed directly or indirectly to the reduced temperature, causing a difference in the way the polymer solidifies on one surface of the extruded film compared to the opposite surface. Because only one side of the film is exposed to the reduced temperature, the solidified polymer film is non-homogeneous and exhibits a substantially different morphology on one side of the film compared to the other side of the film. Typically, after solidification, the film has an asymmetric morphology comprising a first morphology on one side of the film, a substantially different morphology on a second side of the film, and an intermediate morphology between the two sides. For example, one surface may have a relatively small average pore size, the opposite side may have a relatively larger average pore size, and all locations (depths) between the two surfaces will have an average pore size between the two average pore sizes of the opposing surfaces. Optionally, one surface may be considered to represent a "dense" morphology, and a second surface may be considered to represent an "open" morphology.

[0051] The exemplary method of this description differs from prior phase separation techniques used to form porous polymer membranes, at least because the exemplary method described herein exposes both surfaces of an extruded (including co-extruded) film to the same or similar conditions that induce coagulation to a relatively similar degree. The conditions may involve contact with a liquid "non-solvent" or conditions of reduced temperature, for example, a liquid bath having a lower temperature. Compared to the extruded film method described, the present method does not require, and may preferably not include, the step of casting the extruded film of the polymer solution onto a support layer (e.g., polymer film or release liner), or the step of positioning only one side of the extruded film to contact a cooled surface, such as a cooling roll (e.g., through the support layer). According to the method of the present invention, both surfaces of the extruded film are exposed simultaneously to the same conditions effective in inducing coagulation, and optionally to the same degree (e.g., depending on the identity of the polymer on each surface). By exposing both surfaces of an extruded film to the same coagulation-inducing conditions in the same or similar manner, these conditions affect both sides of the film in a similar manner to induce a coagulation effect on both sides of the film, and upon coagulation, produce a film having two opposing surfaces or faces having a dense morphology for the internal region of the film, optionally two opposing similar or identical morphologies.

[0052] According to examples and preferred embodiments for manufacturing a film as described, a polymer may be combined with a solvent (e.g., completely dissolved in the solvent) to form a polymer-containing liquid (referred herein as "polymer solution" or "polymer-containing liquid") containing a polymer in a non-coagulated or partially coagulated form, which may be extruded as a liquid in the form of a sheet and subsequently coagulated on both sides of the sheet by, for example, an immersion precipitation method, e.g., the TIPS method or the NIPS method, or by another method, e.g., exposure to humidity or solvent evaporation. A single polymer-containing liquid may be used, or a multiple polymer-containing liquid may be used in a co-extruded film.

[0053] A general method may be performed using one or more steps comprising: forming or otherwise providing one or more polymer-containing liquids containing a polymer for preparing a porous polymer membrane dissolved or suspended in a solvent; extruding (including co-extrusion) one or more polymer-containing liquids from a die to form an extruded (including co-extrusion) sheet or film containing the polymer-containing liquids; coagulating (e.g., precipitating) the polymer of the polymer-containing liquids from the polymer-containing liquids to form a coagulated porous polymer membrane ("porous polymer membrane"); and optionally, drying or otherwise further processing the formed porous polymer membrane.

[0054] Examples of such methods may include extruding one or more polymer-containing liquids as a film using an extrusion die. The extrusion die may be a single-slot die or a multi-slot die. A single-slot die may be used when only a single polymer-containing liquid is used to form the film. A multi-slot die may be used when the film is formed using two or more different polymer types or polymer-containing liquids. When a multi-slot die is used, and when two or more different polymers or polymer-containing liquids are used, the extruded film is considered to be a single extruded film at the time of extrusion, even if the composition of the polymers is chemically or physically different; and the co-extruded film is considered to be a single and integral film. A co-extruded film from a multi-slot die will be exposed to coagulation-inducing conditions on only two opposing surfaces, and the resulting solidified film membrane is considered to be formed in a single extrusion step and is considered to be an integral membrane.

[0055] Any effective extrusion conditions and processing parameters may be used, including a useful stretching rate of the extruded film, a useful flow rate of the polymer-containing liquid, an air-gap distance between the die and the coagulation bath (if present), air humidity within the air-gap region (if present), and a useful, optionally conventional type and size die. The flow rate of the polymer-containing liquid passing through the die and the stretching rate of the extruded film can be used to influence the thickness of the extruded film.

[0056] After extrusion, the film is exposed on both sides to conditions that cause the polymer contained in the extruded film to coagulate. For example, the extruded film may be brought into contact with a coagulation bath containing a liquid having a lower temperature than the extruded film or containing a "non-solvent" or both, thereby causing the polymer contained in the extruded film to coagulate. An exemplary coagulation bath for the TIPS method may be a liquid bath having a temperature that causes the (non-coagulated) polymer dissolved from the polymer-containing liquid of the extruded film to coagulate, for example, water, or a liquid having a temperature of, for example, 30, 20, or less than 10 degrees Celsius. An example of a coagulation bath for NIPS may be a bath prepared of a non-solvent, for example, water, or a blend of water and an organic liquid in which the polymer solution solvent can be dissolved.

[0057] According to a specific, more specific example of these types of film-forming techniques, the extruded film (having a non-coagulated or partially coagulated polymer) may be extruded directly into a coagulation bath, or alternatively, extruded from outside the coagulation bath (e.g., vertically above) and then entered the coagulation bath immediately after extrusion. In the latter example, the film is exposed to air after being extruded from the die and before the extruded (non-coagulated) film comes into contact with the liquid in the coagulation bath. Exposing the extruded film to air ("air gap") before it enters the coagulation bath is optional. If an air gap is present, the amount of time the extruded film is exposed to air before contacting the coagulation bath may, in some cases, be less than 2 seconds, less than 1 second, or less than 0.5 seconds. In another exemplary method, the extruded film does not come into contact with air before contacting the coagulation bath, but is extruded directly into the coagulation bath.

[0058] The type of polymer in the polymer solution and the resulting porous polymer membrane may be any type of polymer known or known to be used to form a porous membrane by NIPS or TIPS technology. In particular, for the NIPS method, examples of currently known or preferred polymers include, among others, polyamides, polyimides, polyamide-polyimides, polysulfones such as polyethersulfone or polyphenylsulfone, and fluoropolymers such as fluorinated polyvinylidene. In particular, for the TIPS method, examples of currently known or preferred polymers include, among others, polyolefins such as polyethylene and polypropylene, and fluorinated polymers such as perfluoroalkoxy (PFA). In a method using a single-slot die, only a homopolymer and a homopolymer solution are required, but a blend of polymers may also be used. In a method using a multi-slot die, e.g., a 2- or 3-slot die, different polymer types or polymer solutions having the same polymer but different compositions may flow through different slots. As an example, a first type of polymer and a first polymer solution may be used in slots forming the surface of the film, and one or more different polymers (or polymer solutions) may be used in one or more slots forming the internal region of the film.

[0059] Now, referring to FIGS. 2a, 2b, 3a, and 3b, parts of an exemplary extrusion system effective for manufacturing an exemplary polymer membrane as described are presented. FIGS. 2a and 3a are front views of the exemplary system, and FIGS. 2b and 3b show side views of the exemplary system.

[0060] Referring to FIGS. 2a and 2b, the system (100) includes an extrusion die (120), which may be a sheet extrusion die of the type typically referred to as a coat-hanger manifold die, for example. The die (120) is a single-slot extrusion die having a slot (122) at the bottom of the die. A polymer solution (130) passes internally through the manifold of the die (120) and then through the slot (122), after which the extruded polymer solution forms an extruded film (140a), which extends vertically and moves downward without being supported below the die (120). A liquid coagulation bath (150) having an upper surface or "trough line" (152) is located below the die (120).

[0061] A source of polymer solution (130) (not shown) is supplied at a constant and regular rate by a pump or other flow device and flows continuously through the die (120). After leaving the slot (122) in a downward direction, the extruded film (140a) enters and is submerged in a liquid coagulation bath (150). The extruded film (140a) exits the die slot (122) as a thin, continuous film of the polymer solution (130) containing uncoagulated polymer present in the polymer solution. To coagulate the dissolved polymer of the polymer solution (130) in the film (140a), the film (140a) containing the uncoagulated polymer then comes into contact with the liquid of the coagulation bath (150).

[0062] The liquid in the coagulation bath (150) starts at the bath line (152) and comes into contact with both opposing surfaces of the film (140a), causing the dissolved polymer of the film (140a) to coagulate on both surfaces. The polymer of the film (140a) precipitates from the solution (e.g., coagulates) to form a porous polymer film membrane (140b) containing the polymer in a coagulated form. Because the liquid in the coagulation bath (150) comes into contact with both opposing surfaces of the film (140a) simultaneously, both surfaces are equally affected by the liquid, causing the polymer to coagulate and producing a film (140b) having two opposing surfaces with similar or identical morphologies.

[0063] Optionally, and as shown in FIGS. 2a and 2b, a film (140a) made of a non-coagulated polymer solution (130) passes through a space ("air-gap") after exiting the die slot (122) and before coming into contact with the liquid in the coagulation bath (150). Upon entering the coagulation bath (150), the liquid in the coagulation bath (150) causes the polymer solution (130) to undergo phase separation to form a filter membrane (140b). Alternatively, according to other exemplary systems and methods, for example as shown in FIGS. 3a and 3b, there is no air-gap between the die slot (122) and the liquid in the coagulation bath (150), and as the polymer solution (130) exits the die slot (122) to form the membrane (140b), the phase separation (coagulation) of the polymer in the film (140a) occurs immediately inside the coagulation bath (150).

[0064] Note that these drawings show an extruded film formed from a single polymer solution using a single-slot die. In alternative methods and systems, two or more different polymer solutions (optional one or more different types of polymers) can be flowed through different slots of a multi-slot extrusion die.

[0065] As one or more optional subsequent steps, the membrane (140b) may be further processed or treated using a washing tank, a heating step, a drying step, or a winding section. For example, after a phase separation (coagulation) step in a coagulation tank (150), the membrane (140b) may be guided through a washing tank to extract residual solvent. Subsequently, the washed (still wet) membrane (140b) may be collected on a roll before or after a post-processing drying step.

[0066] As illustrated in FIGS. 2a, 2b, 3a, and 3b, the extruded film will undergo a reduction in width, thickness, or both immediately after exiting the die. This phenomenon is often referred to as "necking," meaning that after the film exits the die and moves away from it, it "necks down" to a reduced width, thickness, or both. Alternatively, this may be referred to as the "dog bone" effect. Due to this necking effect, the thickness of the extruded film may also exhibit reduced uniformity measured across the film width and at the film edges—the film thickness may become less uniform across the width direction and at the edges, i.e., exhibit increased variability. Specific locations with reduced uniformity may occur at each edge (end) of the film, and increased thickness (like a dog bone) may be present at each end. An additional potential advantage of the system and method of the present invention is the preferably high uniformity of film thickness across the width of the film, including at the ends, i.e., the reduction of the dog bone effect.

[0067] A filter membrane as described may be useful for removing one or more contaminants from a liquid by passing the liquid through the filter membrane to produce a filtered liquid. The filtered liquid will contain one or more contaminants or particulate matter at a reduced level compared to the level of contaminants or particles present in the liquid before it passes through the filter membrane.

[0068] A polymer membrane as described may provide a useful, desirable, or advantageous combination of physical properties, including performance (filtration performance as measured by "retention rate"), pore size or bubble point (related to pore size), flow, and mechanical properties (flexibility and durability or reduced brittleness).

[0069] The level of efficiency of a filter membrane in removing unwanted substances (i.e., "contaminants") from a liquid can be measured in one way as "retention rate." Retention rate, in relation to the efficiency of a filter membrane (e.g., the filter membrane described), generally refers to the total amount of impurities removed from an impurity-containing liquid (in actual or performance testing) relative to the total amount of impurities present in the liquid when the liquid passes through the filter membrane. Accordingly, the "retention rate" value of a filter membrane is a percentage, and a filter with a higher retention rate (higher percentage) is relatively more effective at removing particles from a liquid, while a filter with a lower retention rate (lower percentage) is relatively less effective at removing particles from a liquid.

[0070] Particle retention can be measured by measuring the number of test particles removed from the fluid stream by a membrane placed in the fluid stream. By one method, particle retention can be measured by passing a 0.1% Triton X-100 feed aqueous solution containing 8 ppm polystyrene particles (e.g., G25 round polystyrene particles with a nominal diameter of 5 to 15 nanometers) through a 47-millimeter diameter coupon sample of the membrane at a constant flow rate of 7 milliliters per minute, in an amount sufficient to achieve a 1% monolayer coverage rate, and collecting the permeate. The concentration of polystyrene particles in the permeate can be calculated from the absorbance of the permeate. Subsequently, particle retention is calculated using the following equation:

[0071]

[0072] As used herein, "nominal diameter" refers to the diameter of a particle determined by photon correlation spectroscopy (PCS), laser diffraction, or optical microscopy. Typically, the calculated diameter or nominal diameter is expressed as the diameter of a sphere having the same projected area as the projected image of the particle. PCS, laser diffraction, and optical microscopy techniques are widely known in the field (see, for example, the literature [Jillavenkatesa, A., et al.; "Particle Size Characterization"; NIST Recommended Practice Guide; National Institute of Standards and Technology Special Publication 960-1; January 2001]).

[0073] In a preferred embodiment of the membrane as described, the membrane may exhibit a retention rate exceeding 90% for single-layer coverage rates of 0.5%, 1.0%, 1.5%, and 2.0%, and may also exceed 95, 96, or 97% for single-layer coverage rates of 0.5%, 1.0%, 1.5%, and 2.0%. With these levels of retention, these examples of the membrane of the present invention exhibit higher retention levels compared to many currently commercial filter membranes, such as similar flat sheet and hollow fiber filter membranes made of UPE. These exemplary membranes also allow for useful, excellent, or very excellent flow rates (low flow times) and exhibit mechanical properties that allow the membrane to be manufactured and assembled into a filter cartridge or filter product.

[0074] The bubble point is an understood characteristic of porous materials, including composite filter membranes as described. The bubble point may correspond to a pore size that corresponds to filtration performance. Smaller pore sizes may correlate with a higher bubble point and possibly higher filtration performance (higher retention rate). However, generally, a higher bubble point also correlates with relatively higher flow resistance and lower flux through the porous material. According to the double-density membrane of the specific example described herein, the bubble points may be similar when measured in flow through the membrane in one direction. A bubble point measured in one direction is considered similar to a bubble point measured in the opposite direction if the higher bubble point is 20% or less larger than the lower of the two bubble points, for example, if the higher of the two bubble points is 10%, 5%, 2%, or 1% or less larger than the lower of the two bubble points. Similar bubble points in this way may indicate that the film has high symmetry with respect to the centerline of the membrane.

[0075] In one method for determining the bubble point of a porous material, a sample of the porous material is immersed in and wetted with a liquid having known surface tension, and a gas pressure is applied to one side of the sample. The gas pressure is gradually increased. The minimum pressure at which the gas flows through the sample is called the bubble point. Examples of useful bubble points of a porous filter membrane as described, measured using Novec HFE 7200, IPA, or water, compressed air, or compressed N2 gas at a temperature of 20 to 30 degrees Celsius (typically 25 degrees Celsius), may be in the range of 2 to 400 psi, for example, in the range of 135 to 185 psi.

[0076] A filter membrane as described herein, or a filter or filter component containing a filter membrane as described herein, may be useful for a method of filtering liquid chemicals to purify or remove unwanted substances from liquid chemicals, particularly for a method of producing high-purity liquid chemicals useful for industrial processes requiring the input of chemicals having a very high level of purity. Generally, the liquid chemicals may be any of various useful commercial materials and may be liquid chemicals useful or used in any application for any industrial or commercial use. Specific examples of filters as described may be used to filter liquid solvents or other process liquids used in semiconductor or microelectronic fabrication applications, such as methods of semiconductor photolithography, or cleaning methods or cleaning steps for processing semiconductor wafers or microelectronic devices, or to purify useful liquid chemicals.

[0077] Some specific, non-limiting examples of solvents (including washing solutions) that can be filtered using a filter membrane as described include n-butyl acetate (nBA), isopropyl alcohol (IPA), 2-ethoxyethyl acetate (2EEA), xylene, cyclohexanone, ethyl lactate, methyl isobutyl carbinol (MIBC), methyl isobutyl ketone (MIBK), isoamyl acetate, undecane, propylene glycol methyl ether (PGME), propylene glycol monomethyl ether acetate (PGMEA), or a mixture of any of these, e.g., a mixture of PGME and PGMEA; or concentrated or diluted ammonium hydroxide, hydrogen peroxide, hydrochloric acid, HF, sulfuric acid, another peroxide solution, or a combination thereof, e.g., a combination of ammonium hydroxide and hydrogen peroxide, or a combination of hydrochloric acid and hydrogen peroxide.

[0078] The membrane may be contained within a larger filter structure, such as a filter or filter cartridge used in a filtration system. The filtration system will place a composite filter membrane, for example as part of a filter or filter cartridge, in the flow path of a liquid chemical so that at least a portion of the flow of the liquid chemical passes through the filter layer of the composite filter membrane, thereby allowing the filter layer to remove a certain amount of impurities or contaminants from the liquid chemical. The structure of the filter or filter cartridge may include one or more various additional materials and structures that support the composite filter membrane within the filter so that the fluid passes through the composite filter membrane when passing through the filter, by allowing the fluid to flow from the filter inlet, through the membrane (including the filter layer), and through the filter outlet.

[0079] Example:

[0080] Referring to Figure 4, this shows a comparison of three filter membranes made of polyether sulfone, which are manufactured by different methods and have different structures or morphologies.

[0081] One membrane is a "single-layer PES membrane." This membrane was fabricated by a conventional phase separation method such as NIPS. The single-layer PES membrane has an asymmetric profile with one side having a dense surface and the opposite side having an open surface.

[0082] One of the membranes is a "double-layer PES membrane." This membrane is a multilayer membrane manufactured by stacking two single-layer PES membranes together and using two membranes in series.

[0083] One of the membranes is a "double dense layer PES membrane" manufactured according to this description.

[0084] As shown in FIG. 4, the double-layer PES membrane of the present invention exhibits a significantly improved retention rate compared to a membrane not of the present invention that is made of the same material but has a different morphology or structure.

[0085] In a first aspect, the integrally extruded porous polymer sheet membrane comprises a first surface, a second surface, and a thickness between the first surface and the second surface, a first thickness region including a portion of the first surface and the membrane in the thickness direction, a second thickness region including a portion of the second surface and the membrane in the thickness direction, and a third thickness region extending in the thickness direction between the first thickness region and the second thickness region, wherein the average pore size of the first thickness region and the average pore size of the second thickness region are both smaller than the average pore size of the third region.

[0086] As a second aspect according to the first aspect, the membrane has a centerline between a first surface and a second surface, a first thickness region includes a depth position having a minimum pore size on the first surface of the centerline, and a second thickness region includes a depth position having a minimum pore size on the second surface of the centerline.

[0087] In the third aspect according to the second aspect, the first minimum pore size is in the range of 1 nanometer to 10 microns, and the second minimum pore size is in the range of 1 nanometer to 10 microns.

[0088] In the fourth aspect according to the second or third aspect, the first minimum pore size is located between the first surface and a depth of 1 / 3 of the thickness from the first surface, the second minimum pore size is located between the second surface and a depth of 1 / 3 of the thickness from the second surface, and the average pore size of the third thickness area is measured at the middle 1 / 3 of the thickness.

[0089] A fifth aspect according to any one of the above aspects has a degree of asymmetry of 5 or more, and the degree of asymmetry (D) is defined as D = (average pore size of the third thickness region) / A, where A is one of the average pore size of the first region; the average pore size of the second region; the first minimum pore size or the second minimum pore size.

[0090] In a sixth aspect according to any of the above aspects, the membrane comprises a polymer selected from polyether sulfone and polyamide-imide.

[0091] The seventh side according to any one of the above sides has a thickness in the range of 40 to 300 microns.

[0092] The eighth aspect according to any of the above aspects has a retention rate of 90% or more measured based on a 1% monolayer using G25 round polystyrene particles having a nominal diameter of 5 to 15 nanometers.

[0093] A ninth aspect according to any one of the above aspects is prepared by a non-solvent-induced phase separation step in which a first thickness region, a second thickness region, and a third thickness region are formed using the same phase separation step.

[0094] A tenth aspect according to any one of the above aspects essentially consists of an extruded porous polymer sheet membrane having first and second surfaces, a thickness between the surfaces, a first thickness region, a second thickness region, and a third thickness region.

[0095] The 11th aspect according to any one of the 1st to 10th aspects is composed of an extruded porous polymer sheet membrane having a first and second surface, a thickness between the surfaces, a first thickness region, a second thickness region, and a third thickness region.

[0096] In the 12th aspect, the filter includes a membrane of any one of the 1st to 11th aspects.

[0097] The 13th aspect includes a corrugated membrane contained within the housing.

[0098] In the 14th aspect, a method of filtering a fluid includes passing the fluid through a filter of the 11th aspect.

[0099] In the 15th aspect according to the 14th aspect, the fluid is a semiconductor photolithography solvent, a cleaning solution, or an etching solution.

[0100] In the 16th aspect according to the 15th aspect, the fluid is selected from propylene glycol methyl ether (PGME), propylene glycol methyl ether acetate (PGMEA), cyclohexanone, and n-butyl acetate.

[0101] In the 17th aspect according to the 15th aspect, the fluid comprises a diluted or concentrated solution comprising ammonium hydroxide, hydrogen peroxide, hydrochloric acid, HF, sulfuric acid, a peroxide solution, or a combination thereof.

[0102] In the 18th aspect, a method for manufacturing an extruded porous polymer sheet membrane having opposing first and second surfaces, a thickness between opposing surfaces, and pores having a non-uniform pore size comprises: forming a polymer-containing liquid containing a polymer in a solvent; passing the polymer-containing liquid through an extrusion die to form an extruded film of the polymer-containing liquid; and exposing both sides of the extruded film to conditions that cause solidification of the polymer on both sides of the film.

[0103] The 19th aspect according to the 18th aspect further includes a step of inducing coagulation on both sides of a film extruded by a heat-induced phase separation technique.

[0104] The 20th aspect according to the 18th aspect further includes a step of inducing coagulation on both sides of a film extruded by a nonsolvent-induced phase separation technique.

[0105] The 21st aspect according to the 18th aspect further includes a step of inducing coagulation on both sides of the extruded film by exposing both sides of the extruded film to humidity.

[0106] The 22nd aspect according to the 18th aspect further includes a step of causing solidification on both sides of the extruded film by evaporation of liquid on both sides of the extruded film.

[0107] A 23rd aspect according to the 18th aspect comprises the step of contacting the first surface and the second surface of the extruded film with a coagulation bath to coagulate the dissolved polymer of the polymer solution and to form an extruded porous polymer film comprising the coagulated polymer of the polymer solution.

[0108] A 24th aspect according to any one of the 18 to 23 aspects comprises: a first surface, a second surface, and a thickness between the first surface and the second surface, a first thickness region including a portion of the first surface and the film in the thickness direction, a second thickness region including a portion of the second surface and the film in the thickness direction, and a third thickness region extending in the thickness direction between the first thickness region and the second thickness region, wherein the average pore size of the first thickness region and the average pore size of the second thickness region are both smaller than the average pore size of the third region.

[0109] In the 25th aspect according to the 23rd or 24th aspect, the extruded film exits the die through the die opening, and the die opening is submerged in the solidification liquid.

Claims

Claim 1 A monolithic extruded porous polymer sheet membrane comprises a first surface, a second surface, and a thickness between the first surface and the second surface, a first thickness region comprising a portion of the membrane in the thickness direction between the first surface and the second thickness region, a second thickness region comprising a portion of the membrane in the thickness direction between the second surface and the second thickness region, and a third thickness region extending in the thickness direction between the first thickness region and the second thickness region, wherein the average pore size of the first thickness region and the average pore size of the second thickness region are both smaller than the average pore size of the third thickness region, the membrane has a centerline between the first surface and the second surface, the first thickness region includes a depth position having a first minimum pore size on a first plane of the centerline, and the second thickness region includes a depth position having a second minimum pore size on a second plane of the centerline, the membrane has an asymmetry (D) of 5 or more, and the asymmetry (D) is defined as follows: D = (average pore size of the third thickness region) / A, where A is the average pore size of the first region; the average pore size of the second region; the first minimum pore size or the second minimum pore size; One of an integral extruded porous polymer sheet membrane, wherein the membrane exhibits at least 90% retention rate measured based on a 1% monolayer using G25 round polystyrene particles having a nominal diameter of 5 to 15 nanometers, and the retention rate is measured by passing a feed aqueous solution of 0.1% Triton X-100 containing 8 ppm polystyrene particles through a 47 mm diameter coupon sample of the membrane at a constant flow of 7 milliliters per minute to achieve a 1% monolayer coverage rate, and collecting the permeate. Claim 2 A membrane comprising a polymer selected from polyether sulfone and polyamide-imide in claim 1. Claim 3 In claim 1, a film having a thickness in the range of 40 to 300 microns.

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