JPH0456656B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Semipermeable membranes widely used industrially, including in applications involving reverse osmosis, gas separation, and ultrafiltration, are prepared from cellulose acetate by the casting process. be. While such membranes are very highly permeable to water, they are less so to sodium chloride, which reduces their performance due to the inherent sensitivity of cellose acetate to hydrolysis. . As a result, the ability to remove solutes such as sodium chloride decreases over time and the lifetime of the membrane is thus limited. Because of these hydrolytic tendencies, cellulose acetate membranes
It can only be considered for specific applications where conditions conducive to hydrolysis are not present. Therefore, chemically more stable polymers have begun to replace cellulose acetate as membrane materials.

The basic idea about semipermeable membranes useful in ultrafiltration, reverse osmosis, gas separation, etc. is that they generally exist in one of two forms: (1) (2) a dense, substantially non-porous film and (2) a porous, asymmetric film. In many cases, dense polymer films can be uniaxially or biaxially oriented, which increases their mechanical strength and selectivity as membranes.
Unfortunately, such films generally have too much resistance to desired permeate flow and have been replaced in most applications by solvent cast porous membranes. These porous membranes have a porous structure (a porous structure).
It is specifically manufactured to have a very thin, dense "active" surface supported by a substructure. There is often a gradual change in pore size throughout the thickness of the porous structure, with smaller pores being closer to the dense "active" surface. It is precisely this "active" dense surface that excludes some solute components and allows the passage of others. This selectivity is independent of the thickness of the active surface as long as the active surface does not have any pinholes or cracks. The porous substructure constitutes a mechanical support pair for the thin active surface and must exhibit as little resistance as possible to the desired permeate flow. This means that the pores are open, interconnected, and preferably perpendicular to the active surface of the membrane.
This is best achieved when the shape is vertical. Such membranes are known in the art and are described, for example, in U.S. Pat.
Obtained by careful solvent casting methods as described in 3567810, 3615024 and 3884801.

Prior art methods for producing such membranes are complex, slow, and require the use of difficult operating conditions. As a result, such procedures are limited to a limited number of polymers. This limitation makes it difficult to produce membranes at industrially viable production rates in a continuous process.
This arises from the fact that the selection of casting solvents, additives, temperature and environmental conditions of the casting solution is difficult. Even at manufacturing linear speeds of only 1 m/min, membrane flux and porosity are difficult to control with conventional methods, and mechanical properties are suboptimal due to the lack of provision for molecular orientation. not be converted into

It is inherent in all solvent casting methods that the resulting dense active skin surface of the casted film is isotropic. This applies whether the casting process is carried out in a one-step reaction that simultaneously provides a thin skin and a porous structure, or in a two-step process consisting of applying a thin skin to a preassembled porous structure. , is true. Uniaxial or biaxial stretching of the casted film during the stages of a slow and complex solvent casting process while maintaining the integrity of the primary asymmetric membrane film structure is useful. ,
There's only a small chance.

Previous attempts have been made to provide semipermeable membranes made of polyacrylonitrile and other types of polymers that are superior to cellulose acetate in chemical, mechanical, and thermal properties as well as water permeability. . Such attempts were aimed at simultaneously providing a membrane with a skin layer and a support layer using a casting method. Often, the resulting membranes did not exhibit satisfactory performance because proper casting conditions were difficult to maintain under industrial operating conditions. Producing membranes with satisfactory skin layers with such polymer types is difficult when using the casting method.
It is generally recognized that this method is tedious and difficult to implement.

A recent patent, U.S. Pat. No. 4,364,759, attempts to solve some of the problems associated with casting porous membranes by producing a hollow fiber precursor and solidifying the precursor. . The resulting membrane, however, because it is not biaxially oriented, lacks the cleavage strength and separation selectivity required for many useful applications where strength and selectivity are prerequisites. I don't have it.

What is desirable, therefore, is a method for producing porous asymmetric film membranes that is not only faster to carry out than solvent casting methods, but also capable of being applied on top of a supporting porous substrate. It can directly provide a dense active skin surface with molecular orientation. Providing such a method and resultant product fills a long felt need and represents a significant advance in the field.

"Steam conditions" as used herein and/or
or by terms such as "steaming," it is meant herein the use of mechanisms involving gases, vapors, mist, aerosols, etc., to create porosity within one surface of the membrane. shall be taken as a thing.

The present invention relates to an improved method for producing porous asymmetric film membranes useful in ultrafiltration processes. More specifically, the present invention involves extruding a polymer liquefied using at least one of a polymer solvent and a non-solvent melting aid through an annular mold to produce a tube-shaped film; Each of the two surfaces of the shaped film, the inner and outer surfaces, is expanded and stretched to provide a biaxial orientation while simultaneously being exposed to different processing conditions, and the tube is subsequently cut open to create a densified structure. and providing a porous film-like membrane having a biaxially oriented surface. Regarding such methods.

In accordance with the present invention, a method of making a porous asymmetric membrane includes extruding a liquefied film-forming polymer through an annular mold to form a tubular film having an inner surface (A) and an outer surface (B). wherein the liquefied polymer is obtained by using a polymeric solvent alone or in combination with one or more melting aids, and the film is preferably formed into a tube-like structure. The two surfaces have a biaxial orientation between
(A) and (B) are each subjected to steam treatments differing from each other in conditions of at least one of temperature and/or chemical composition to form a dense, biaxially oriented surface of one and a surface of a second surface; while exposed to steam conditions so as to provide a membrane with a non-dense surface.
A method is provided comprising orienting the film and optionally cutting open the tubular membrane to provide a tabular membrane having one surface that is denser than the other surface.

The method of the invention provides a membrane having two surfaces, one of which is a dense, biaxially oriented surface;
The other surface is generally more porous. Both surfaces can be shown to have biaxial orientation, but in general one surface will be substantially more dense and biaxially oriented than the other, with the less dense surface being It is preferably porous to allow high flux through the membrane. The fact that the present invention provides a dense surface with high flow rates while simultaneously providing biaxial orientation does not provide any specificity regarding the one-step fabrication of such membranes. From the point of view of the current state of the art, this is extremely important.
The present invention is also unique in that two surfaces of the tubular film are simultaneously exposed to different steam conditions, preferably with biaxial orientation of the film.

In a preferred embodiment, the method of the invention comprises:
It is conveniently operated using polymers in liquefied form, with solids contents ranging up to about 70-80%, while providing membranes with satisfactory pore structure and biaxial orientation. This result is consistent with prior art techniques that use low levels of polymer solids, i.e. 10-20% in flat membrane casting and hollow fiber extrusion.
From a 25-50% perspective, it's amazing.

In accordance with the present invention, a porous asymmetric polymeric membrane having two surfaces is provided, one of the surfaces is denser than the other, and the denser surface has an angle greater than about 70° in the plane of the dense surface. exhibiting a small biaxial orientation angle,
The membrane exhibits a porosity gradient across its thickness with void spaces generally decreasing in size and/or number towards the denser surface. A membrane is provided whose density is such that the permeability/impermeability properties enable the desired separation to be achieved in an ultrafiltration process. The widthwise orientation of the extruded tubular film under pressure is such that the ratio of the internal diameter of the tubular film to the diameter of the internal orifice of the annulus is typically about 3.0 or greater. This is accomplished by introducing steam into the interior of the film. The longitudinal orientation of the film is effected by the stretching force provided by the seal of the roll at the bottom of the tube below the flattening guide. Preferably, the length and breadth tensions (orientations) are equivalent, but variations of up to about 25% between each other may be tolerated.

The porous asymmetric membranes produced in accordance with the present invention generally have high angle biaxial orientation, as indicated by orientation angles in the range of about 60° to 70°; Manufactured by the method of Asymmetric membranes with such a high degree of biaxial surface orientation were hitherto unknown since they cannot be obtained by methods that do not provide for stretching of the nascent membrane. This biaxial orientation is in the plane of both surfaces and is easily determined by the method described in US Pat. No. 3,275,612. Furthermore, the membrane exhibits porosity through its thickness, with void spaces generally increasing in the direction away from the dense surface. The density of the skin surface of the membrane of the present invention is such that it provides permeable/impermeable properties that produce the desired separation in ultrafiltration, and thus the actual density obtained can vary widely. However, with proper control of processing, it is possible to provide membranes with the desired porosity for a particular application. In certain cases, such as reverse osmosis, the density of the skin surface is such that it allows the passage of water, but substantially impedes the passage of certain salts, such as sodium chloride. Porosity gradients can be observed microscopically by examining cross-sections through the thickness of the membrane, and permeability can be measured by any suitable known method.

The present invention will be more fully illustrated below by specific reference to the accompanying drawings, in which FIG. using a solidification zone,
Figure 3 represents a schematic diagram of an embodiment of the invention consisting of circulating air and steam over the other side of the film.

FIG. 2 represents a schematic diagram of an embodiment of the method of the invention, in which no envelopment is used in the solidification zone, but air retained, for example, in a tube-shaped film.

FIG. 3 shows the use of e.g. air and solvent vapor in the solidification zone on one side of the tubular film and saturated steam on the other side.
Figure 3 represents a schematic representation of another preferred embodiment of the method of the invention.

In processing in accordance with the present invention, liquefied film-forming polymers are used, which may be used alone or in combination with one or more melting aids. obtained by doing. The polymer solvent is
Of course, when used in the proper proportions and under the proper conditions, it completely dissolves the polymer and provides a liquefied composition that can be extruded at normal pressures. A melting aid is either a non-solvent for the polymer or a polymer solvent used in a proportion where the temperature and pressure are insufficient to dissolve the polymer under normal conditions. , a composition. As mentioned above, melting aids and polymeric solvents may be used in combination, if desired, in accordance with the present invention.

There are several procedures by which liquefied polymers can be obtained for use according to the invention. One procedure involves simply dissolving the polymer at an appropriate temperature into a suitable polymer solvent in the appropriate proportions, selected from those that undergo phase separation during solution processing during film production. That's true. Such solvents are Applied
PolymerSymposia 6, 109 (1967), hereby incorporated by reference.

Another procedure is to provide a polymer melt at elevated temperature and pressure using a suitable combination of solvent and melting aid. The use of polymeric solvents as plasticizers and melting aids in conjunction with processing is generally desirable since such use generally produces a better pore structure in the film. In this connection, it is important to note that in order to provide a melt, the proportion of the polymer solvent or polymer solvent-melting aid mixture used must be insufficient to molecularly dissolve the polymer. , is noteworthy. Useful polymer solvents generally include organic materials that are in liquid form during use, such as dimethylformamide, propylene carbonate, salt solutions, acid solutions, and mixtures. Nonsolvents for the polymer may also be used in appropriate instances as melting aids in combination with the polymer solvent. Nonsolvents can include water, low boiling alcohols, and other organic liquids that are preferably miscible with the polymer solvent or mixture thereof.

Suitable polymers, including polymer blends thereof, include polysulfones, e.g. In the formula, n is 20-100.

polyimide, polyamide, polycarbonate, polyester, methyl cellulose,
Cellulose derivatives such as ethylcellulose, cellulose acetate, etc., as well as acrylonitrile polymers and copolymers, poly(vinyl alcohol),
Included are vinyl polymers and copolymers such as poly(vinylpyrrolidone), polyolefins, and the like.

The liquefied polymer composition may also contain various additives such as liquid and/or solid filler components, lubricants, antistatic agents, pigments, reinforcing microfibrils, etc. as separate dispersed phases. Inert gases such as air and nitrogen and reactive gases such as sulfur dioxide, carbon dioxide, ammonia, etc. may also be present.

The procedure followed when a melting aid is used in conjunction with a solvent to obtain a liquefied polymer in carrying out the process of providing a tubular film in accordance with the present invention is described by M. Zwick, US Pat. Patent No.
4301112, herein incorporated by reference except for variations as described herein. When the polymer solvent is used alone in a proportion that will dissolve the polymer at normal pressure, the procedure followed is Applied.
Polymer Symposia 6 , 109 (1967), which is also hereby incorporated by reference except for the variations described therein.

Referring to FIG. 1, which represents a generalized embodiment of the invention, an apparatus and processing procedure according to the invention is shown. A spinning head 1 is shown through which the liquefied polymer is extruded through a filter 2 and a circular mold 4 which injects the compressed gas or It includes an inlet 3 for inlet of steam and outlets 5 and 5A for escape of gas or steam. The film 8 produced at the lip of the mold is
It is inflated by the pressure of gas or steam introduced into it through the inlet 3, causing the film to be stretched longitudinally. The nascent film is
It is contained within the confines of a solidification zone 6 pressurized with steam or gas entering from 11 , the pressure therein being controlled by a valve 12 . A vapor ring 7 is optional and may provide supplemental heating to prevent cooling of the nascent film by evaporation of liquid from the film. Inside the stretching film is a mixture of externally applied gas or vapor and vaporized solvent and other components generated from the liquefied polymer. As the film progresses downward through the solidification zone, its outer surface becomes porous due to the influence of water vapor within the solidification zone, while its inner surface becomes more dense due to the action of the vapor components within it. Become. A membrane is thus produced, which is then optionally brought into contact with a quenching section 9 and guided through a flattening guide 10 to produce a flattened tube, which expands in response to internal pressure leaks. It passes through a roll seal 13 which serves both to seal the bottom of the tube and to transfer longitudinal tension forces to the membrane. The membrane leaving the solidification zone is picked up onto a take-up roll 14. The membrane can be subjected to other processing as desired, including washing out any solvent or non-solvent that may remain therein. If it is desired to dry this membrane, it can be used in combination with nip rolls 15 and 16 and guide rolls 17, 18, 19 and 20.
can be carried out in a separate processing step consisting of feeding the membrane to a tenter machine operating under suitable conditions. A pull roll 22 moves the membrane through a tenter to a final wind-up roll 23 at a controlled wind-up speed. Provisions (not shown) are also made to cut open the flattened tube-like membrane to provide its flat version. This procedure not only provides simultaneous biaxial tension to the film, but also creates a porous, less dense exterior and a more dense interior of the membrane.

In the embodiment shown in FIG. 1, the steam conditions maintained within the solidification zone and those maintained within the expanded film differ in at least one of the following conditions: temperature and/or vapor composition. There is. Specific variations are described in the accompanying examples.

Referring to FIG. 2, which represents another specific embodiment of the invention, the apparatus of FIG. 1 is modified such that a tubular film is produced in air and the inside is inflated with stagnant air. There is. That is, the air outlets 5 and 5A are connected to the solidification frame area 6.
and steam inlet 11 have been removed. Additionally, in this example, drying and cutting of the membrane is shown as part of a continuous process. The membrane exiting the roll seal 13 passes to the slit 24 and from there to the guide rolls, nip, tenter, tension roll, and final take-up roll. The porosity of the membrane is developed through the use of hot vapor ports 25 through which hot, crack-promoting non-solvent vapors are allowed to flow while the film is further stretched. , which hits one side of the tenter before entering the tenter.

Referring to FIG. 3, which represents yet another embodiment of the method of the present invention, the solidification zone is pressurized with air and solvent vapor, the inside of the film is expanded and continuously treated with pressurized steam, and the Seal 13
The apparatus of FIG. 1 operates as described in connection with FIG. 1, except that the membrane emerging from it is cut open, bombarded with hot steam, dried, and rolled up as in FIG. is shown again. In this embodiment, the inside of the extruded tube will be the less dense and porous side of the final membrane.

When processing to provide the desired membrane as described above, for example, U.S. Pat. Same 83−
No. 3603 (1983) and No. 83-766347 (1983)
The desired permeate flux through the membrane can be achieved by various pore expansion or flux enhancement treatments, either by conventional processing or as a post-treatment, in accordance with the teachings of the prior art, as taught in 2010 (2013). Additional processing can be used to increase. Other possible treatments include U.S. Pat. No. 4,147,745;
Includes those described in No. 4272378 and No. 4283359.

Variations in the processing conditions to which the inner and outer surfaces of the tubular film are exposed, as well as the polymer composition used, influence the porosity and flux characteristics of the resulting film. Additional modification of these properties may be accomplished through certain additional processing steps and/or post-processing. That is, it is possible to provide membranes with a wide range of porosity values in order to maintain a desired flux value. The density of the denser surfaces can be similarly influenced and the permeability/impermeability properties can also be controlled to provide membranes for specific ultrafiltration processes including reverse osmosis, gas separation, etc.

The invention is more fully illustrated in the Examples that follow, in which all parts and percentages are by weight unless otherwise specified.

Example 1 The apparatus shown in FIG. 1 is used to produce a tubular film from liquefied polymer fed by a 3/4 inch diameter single screw extruder. The polymer was derived from 85% acrylonitrile and 12% methyl methacrylate grafted onto preformed poly(vinyl alcohol), which was used in an amount of 3% of the repeat units in the final composition. It shall consist of repeating units. The polymer has a weight average molecular weight of about 56,000. Extruder feed was 70% polymer, 22% propylene carbonate and 8% water.
Consists of %. The melt is extruded into a solidification zone 6 filled with saturated steam at atmospheric pressure introduced from 11. The extrusion mold used had a diameter of 1/
8 inches and mold width 15 mils. The temperature of the melt is 135° C. and the feed rate is 5 g/min.
The nascent film is pulled down at a linear speed of 4 m/min. Air is passed under pressure through the inlet 3 into the center of the tubular film at a pressure sufficient to orient the film widthwise by expanding the film's diameter by four times. to be introduced.
The interior space of the tubular film remains partially saturated with propylene carbonate and water, thereby forming a coherent dense skin on the inside of the tubular film and also forming a less dense porosity. Air is allowed to exit through outlets 5 and 5A at such a rate as to form a transparent outer surface. After the resulting film is quenched at 9 with a fine spray of cold water, it is passed through a flattening guide 10 and a pair of pinch rolls 13 which both act as roll seals and longitudinally orient the film. Thus, the tube is flattened. The membrane is stored in a moisture-proof container, awaiting optional finishing treatments. This membrane sample is then washed to remove any remaining melting aid,
Dry under air at room temperature as well as at various elevated temperatures. Microscopic examination of cross-sections of dried sections reveals a pore structure with a substantial proportion of interconnected chambers and progressively finer grains in the direction of the dense integumentary surface. A typical device structure consisting of an asymmetric, porous membrane structure is seen. This microstructure is formed as a result of asymmetric environmental conditions prevailing in the vapor phase during extrusion and stretching. In this case, a dense surface is created inside the tubular film where controlled evaporation of propylene carbonate and water takes place, while on the outside the film is forced into a rapid phase separation. The X-ray orientation angle of the dense surface is between 60° and 70° depending on the drying conditions, with low eye angles obtained at low eye temperatures.
This membrane is useful in reverse osmosis processes.

Example 2 The apparatus shown in FIG. 2 was used, and the extruder and mold were as in Example 1. Using the same polymers, the extruder feed shall consist of 72 parts polymer, 24 parts propylene carbonate, and 4 parts water. The extrusion temperature is approximately 138°C and the feed rate is 5 g/min. The nascent film is withdrawn at a speed of m/min. Air under pressure is introduced into the center of the tubular extrusion via inlet 3 at a static pressure sufficient to expand the diameter by a factor of five and thereby orient the film in the width direction. As the hot melt leaves the annular mold, small amounts of water and propylene carbonate evaporate from the outer surface, forming a thin, dense skin on the outside of the tube. 9 before being flattened in the guide 10 and removed by the pinch rolls 13.
Further surface hardening occurs when the membrane is rapidly cooled by a stream of cold air. At 24, the flattened membrane is cut into two linear strips;
These are subjected to 50% linear tension in the machine direction between rolls 19 and 22. Before entering the chamber 21, while undergoing longitudinal orientation by tension, the membrane strip is placed in the steam port 2 on the side that was originally inside the flattened tube.
5, a blanket spray of water vapor at atmospheric pressure is carried out. This results in the development of an asymmetric pore structure, which is demonstrated by microscopic observation as in Example 1. The preferential molecular orientation of the dense envelope is confirmed by observation using a polarized light microscope. This membrane is useful for gas separation.

Example 3 The procedure of Example 1 is again followed in all important details, except that the chamber into which the film is extruded is pressurized and supplied with steam at 105°C. The product is similar to that obtained in Example 1, but the production rate is increased by at least 30% compared to that of Example 1.

Example 4 Chamber 6 into which a film is extruded
Example 1 is repeated again in all important details, except that the mixture is filled with isopropanol vapor at the boiling point instead of water vapor. Compared to that of the membrane of Example 1, a somewhat finer porous structure is obtained. That is, the density of the skin is almost the same as that of the membrane obtained in Example 1, but the pore structure is more uniform and shows fewer large pores.

Example 5 The polymer melt was UDEL, a product of Union Carbide Co.
Example 1 is repeated in all important details, except that it consists of a commercially available polysulfone known as P-1800, 18% dimethylformamide and 6% formamide. The extrusion temperature was 160°C, the removal rate was 3 m/min, and the radial blowing expansion rate was
Set it to 3.0.

A sample of the resulting membrane was soaked in water for 24 hours;
This is followed by several washes with alcohol and air drying. Tests as described in Example 1 again confirm the presence of an asymmetric pore structure and the orientation of molecules in the plane of the dense surface, as obtained in Example 1. Here again, the asymmetric pore structure results from the asymmetric conditions surrounding the film during its formation and stretching, and the conditions inside the tube are those often used during dry spinning of fibers. while the external conditions are similar to shock solidification in fiber spinning, which favors the development of a porous structure.
coagulanion).

Example 6 In all important details, the polymer melt contains a blend of polymers.
Repeat Example 5. The melt composition is as follows: 70% polysulfone as in Example 5, GAFCo.
Poly(vinylpyrrolidone) GAF K is a product of
-406%, dimethylformamide 22% and water 2%. Microscopic examination of the resulting membrane shows an asymmetric porous structure with a fine grained structure.

Example 7 Acrylonitrile copolymer 27 described in Example 1
Prepare a polymer solution containing %. The solvent is molten caprolactam. Degas the solution at 135 °C,
It is extruded through a circular mold as shown in FIG. 1 at 125° C. with the aid of a metering pump.
The film is oriented by injecting hot (65° C.) pressurized air into the center of the tube, thereby expanding its diameter by a factor of six. at the same time,
The film is removed into the solution in an annular extrusion orifice at 6 times the linear velocity. The chamber surrounding the extruded tubular film contains air maintained at or below 25°C so that the temperature gradient across the nascent film (inside to outside) is at least 40°C. At point 9, chilled air (0° C.) hits the outside of the tube, quenching it before it is flattened. The resulting intermediate film is rolled onto a cooled roll 14, making it ready for further processing. Membrane samples are repeatedly extracted with methanol and air dried. The test is similar to Example 1, but shows an asymmetric pore structure and significant biaxial orientation on the inner surface of the dense outer skin surface.
The asymmetric film pore structure in this case is caused by the temperature gradient that exists across the nascent film during its stretching and cooling. This forces the solidifying solvent (caprolactam freezes at about 80°C) towards the colder side, and upon phase separation and solidification of the system, solid deposits are produced in an asymmetric pattern inside the pores. .

Example 8 Solvent: 81 parts of dimethyl sulfoxide, 18 parts of urea,
and 1 part poly(ethylene glycol), Example 7 is repeated in all important details except that the polymer concentration is 27%. The extrusion temperature was 110°C and again, Example 7
Maintain a temperature gradient of at least 40°C across the inside and outside of the nascent film.

A sample of the resulting intermediate membrane is extracted with butanol, then with methanol, and finally air-dried. Microscopic examination of the membrane shows an asymmetric pore structure, with a total void volume of 50% of the membrane substrate volume.
exceeds %. Molecules within the dense envelope surface exhibit a sharp biaxial orientation in the plane of the dense envelope.

[Brief explanation of drawings]

FIG. 1 represents a schematic diagram of an apparatus suitable for carrying out one embodiment of the invention, by which membranes are produced in a controlled atmosphere. FIG. 2 shows an apparatus suitable for carrying out one embodiment of the invention, in which the membrane is produced in an ambient environment and the web is cut into two separate membranes. A similar schematic diagram is shown. FIG. 3 represents a schematic diagram similar to that of FIG. 1, in which the web membrane is again cut into two separate membranes.

Claims (1)

[Claims] 1. A method of manufacturing a porous asymmetric membrane, in which a liquefied film-forming polymer is extruded through an annular mold to form an inner surface (A) and an outer surface.
(B), the liquefied polymer is obtained by using a polymer solvent alone or in combination with one or more melting aids, and the two surfaces (A) and
(B) has one dense biaxially oriented surface and one less dense porous surface each subjected to steam treatments different from each other in at least one of temperature and/or chemical composition. A method comprising biaxially orienting said film while exposing said film to steam conditions so as to provide said film, and thereafter optionally cutting said tubular film. 2. The method of claim 1, wherein the polymer is an acrylonitrile copolymer. 3. The method of claim 1, wherein said polymer is polysulfone. 4. The method of claim 1, wherein a melting aid is also present with the polymer solvent. 5 A porous asymmetric polymeric membrane with two surfaces, one denser than the other, with the surfaces biaxially oriented and the membrane having a void space with a diameter in the direction of the denser surface. exhibiting a porosity gradient across its thickness, with at least one of A membrane consisting of something that has. 6. The membrane of claim 5, wherein said polymer is an acrylonitrile copolymer. 7. The membrane of claim 5, wherein the polymer is polysulfone. 8. The film-like membrane of claim 5, wherein the film provides separation useful in reverse osmosis. 9. The film-like membrane of claim 5, wherein the film provides separation useful in gas separation.