This invention relates to semi-permeable membranes, and more particularly to novel processes and compositions useful for the preparation of anisotropic membranes.

Semi-porous polymer membranes are useful in ultrafiltration and reverse osmosis for such applications as the desalination of water, and in industrial liquid and gas separations processes. Membranes are commercially available in various geometric configurations, most prominently thin sheets and annular fibers.

Semipermeable membranes are commonly produced by a phase separation process in which a solid cellular polymer structure is formed from a homogeneous liquid mixture of thermoplastic polymer and a vehicle or diluent, or from a dispersion of the polymer in the diluent. Phase separation processes used in the preparation of semi-permeable membranes include vapor-induced phase separation, liquid- induced phase separation, and thermally-induced phase separation. Vapor-induced separation is sometimes referred to as"dry casting"or"air casting."In conventional vapor- induced separation processes, a low rate of evaporation is ordinarily maintained, resulting in protracted processing.

Liquid-induced phase separation, sometimes referred to as "immersion casting"or"wet casting,"can be operated at significantly higher mass transfer rates and productivity than conventional air casting.

Illustrative of the thermally-induced, or"melt casting,"processes are those described in Castro U. S.

Patent No. 4,247,298 and Shipman U. S. Patent No. 4,539,256.

In Castro a homogeneous liquid mixture of polymer and a diluent is prepared at an elevated temperature above the binodal decomposition line for a system consisting of the components of the mixture. The mixture is cooled under non- equilibrium conditions to a temperature below the binodal curve to cause a spinodal liquid/liquid phase separation, forming a continuous polymer-rich phase having droplets of

diluent liquid distributed therein. The resulting liquid dispersion is further cooled to produce a continuous solid crystalline or amorphous polymer membrane structure having pores initially filled by droplets of the diluent. The diluent is thereafter displaced by forcing a relatively volatile liquid through the solid porous membrane. Once the diluent has been displaced the volatile liquid is removed by heating or passing a gaseous fluid through the membrane, which then includes a void fraction comprising pores initially formed in the spaces occupied by droplets of diluent. Adjacent cells are said to be interconnected by smaller pores or passageways.

Matsuyama et al.,"Effects of Thermal History on Anisotropic and Asymmetric Membranes Formed by Thermally Induced Phase Separation,"J. Membrane Science 142 (1998), pp. 27-42, and Matsuyama et al.,"Formation of Anisotropic Membranes via Thermally Induced Phase Separation,"Polymer 40 (1999) pp. 2289 to 2301 describe formation of asymmetric membranes by a combination of evaporation and thermally induced phase separation. A nascent membrane is formed from a single phase liquid mixture comprising a thermoplastic and a solvent therefor, solvent is evaporated from a surface of the membrane to create a polymer concentration gradient extending inwardly from the surface of the membrane, and the nascent membrane is then quenched to cause formation of a solid polymer membrane. In the bulk of the membrane, the polymer phase separates from the solvent phase by spinodal decomposition, while in the outer margin from which evaporation has occurred, the polymer apparently precipitates as a solid.

Macheras U. S. Patent No. 5,871,680 describes a process for the preparation of anisotropic hollow fibers useful as permselective separation membranes. A nascent liquid phase fiber is formed in a spinneret from a polymer solution or dope, and the nascent fiber is thereafter passed through a plurality of abutting coagulation media.

Solidification/precipitation of a solid polymer membrane is

obtained as a result of a phase inversion process, i. e., by diffusion induced phase separation. Anisotropic character is achieved by controlled evaporation from a nascent membrane before immersion in the coagulation media. The spinning solution or dope is prepared by dissolving one or more polymers in a system containing at least one additive such as a nonsolvent, a pore-forming agent, or a surfactant.

The reference further states that, in some instances, it may be beneficial to have a mixture of high boiling and low boiling solvents as part of the spinning solution.

Evaporation of solvent and coagulation are all conducted at temperatures that are apparently well below the crystallization or glass transition temperature of the polymer component of the dope.

Shipman U. S. Patent No. 4,539,256 describes a process in which a melt blend of a crystallizable thermoplastic polymer and a diluent is formed into a shaped article. The shaped article is cooled to a temperature at which the thermoplastic polymer crystallizes to cause phase separation to occur and produce a dispersion of the thermoplastic polymer in the diluent. An article is produced comprising an aggregate of a first phase comprising particles of the polymer in a second phase comprising the diluent, with adjacent thermoplastic polymer particles being distinct but connected by plural zones of continuity. The article so produced is then oriented in at least one direction to separate adjacent particles of crystallized thermoplastic from one another and provide a network of interconnected micropores, and to permanently attenuate the thermoplastic polymeric material in the zones of continuity to form fibrils.

Mechanical integrity of the semi-permeable membranes and exigencies of the processes for their preparation require that the membranes have a certain minimum thickness.

However, for gas separations and reverse osmosis applications as desalination of water, the membranes should present a barrier having a very fine average pore size.

These requirements tends to conflict because a polymer membrane which meets both the thickness required for mechanical integrity and pore size required for effective separation may exhibit a flow resistance which is excessive for many if not most commercial and industrial applications.

The flow rate of a fluid through a homogeneous porous structure is inversely proportional to the thickness of the structure. The flow rate through a laminate is determined by the sum of the resistances to flow of the individual layers in the laminate. The resistance to flow of each layer in the laminate is directly proportional to its thickness, and inversely proportional to its porosity and pore size. Thus, to maximize flow under constant pressure difference through a laminate of two layers of which one possesses smaller size pores, the small pore size layer thickness should be minimized. Accordingly, ultrafiltration and reverse osmosis membranes have conventionally possessed a highly asymmetric"skinned"structure, wherein the skin has the smallest pores in the structure, while the rest of the structure, of much larger pore size, serves to support the skin where the beneficial separation takes place.

It has long been known that anisotropic membranes can be produced from such materials as cellulose acetate. For example, when a solution comprising cellulose acetate in acetone and formamide is prepared by air casting followed by immersion in water, the solid membrane obtained has an anisotropic structure including a skin structure of very low porosity over a bulk structure of much greater porosity.

Such membranes have been demonstrated to be useful in applications such as desalination. As long as the skin preserves its integrity, the membrane provides efficient separation with modest pressure drop. However, the skin is extremely thin and is readily damaged by scratching or abrasion, resulting in loss of integrity of the membrane and by-pass of brackish water through the damaged areas. In other applications, macromolecules or other undesired components may flow through such damaged areas.

Summary of the Invention The present invention provides novel methods for preparing anisotropic polymer membranes having a relatively low permeability outer stratum or"skin structure"which affords efficient separations. Although the outer stratum or skin is thin enough to allow effective separation without excessive pressure drop, it has sufficient thickness and mechanical strength to maintain its structural integrity and ultrafiltration function in carrying out commercial and industrial separations. The process is reliable and subject to effective control. Conditions of the process may be adjusted to modify the characteristics of the marginal outer layer and, if desired to provide successive outer strata of progressively varying liquid permeability and bubble point.

Membranes produced in accordance with the invention are useful in a multitude of applications, prominently including such diverse applications as separation of proteins from body fluids, separation of viruses from biological specimens, desalination of water, and separation of industrial gases. The membranes are particularly suited for use in hemodialysis.

The present invention further provides novel compositions which are useful in the process of the invention for the preparation of semi-permeable polymer membranes having the characteristics indicated above.

Briefly, therefore, the present invention is directed to a process for the preparation of an anisotropic porous polymer membrane. A single phase liquid mixture is prepared comprising a polymer and a vaporizable liquid diluent at a temperature at which the diluent is fully miscible with the polymer in the relative proportions of diluent and polymer contained in the liquid mixture. The diluent is at least partially immiscible that the polymer in such proportions at a temperature below the mixing temperature. The vapor pressure of the diluent is sufficient for evaporation thereof from the mixture within a temperature range between the mixing temperature and a temperature at which spinodal

decomposition of the single phase liquid mixture may be initiated on cooling thereof. A nascent membrane is formed comprising a single phase liquid mixture, and diluent is evaporated from an outer liquid stratum of the nascent membrane extending inwardly from the surface thereof, thereby increasing the concentration of polymer in the outer liquid stratum to a concentration higher than the concentration of polymer in the bulk of the nascent membrane structure. The nascent membrane is quenched to cause spinodal decomposition of the single phase liquid mixture in the outer liquid stratum and in the interior of the nascent membrane into separate liquid phases comprising a continuous liquid phase comprising the polymer. The nascent membrane is further cooled to solidify the continuous phase comprising the polymer, thereby forming a solid polymer membrane comprising a porous outer stratum having an average pore size smaller than the average pore size in another stratum in the interior of the solid polymer membrane.

The invention is particularly directed to such a process in which evaporation of diluent from the outer liquid stratum is effected under vacuum.

The invention is further directed to a process for preparation of an anisotropic membrane in the manner described above and wherein a surface of the nascent membrane is cooled by evaporation of diluent from the outer liquid stratum thereof.

The invention is further directed to a composition useful for the preparation of a semi-porous membrane. The composition comprises a film forming thermoplastic polymer and at least two diluents which together form a single phase liquid mixture with the polymer at temperatures above the melting point of the polymer. A phase diagram for compositions consisting of the components of the composition comprises a critical point and a eutectic at which the concentration of the polymer is higher than at the critical point. The concentration of the polymer in the composition is greater than the concentration at the critical point. At

least one of the diluents is vaporizable for concentrating the single phase liquid mixture.

The invention is further directed to a nascent membrane structure comprising a film forming thermoplastic polymer and at least two diluents which together form a single phase liquid mixture with the polymer at temperatures above the melting point of the polymer. At least one of the diluents is vaporizable for concentrating the single phase liquid mixture to a temperature near the binodal phase separation line for compositions consisting of components of the aforesaid composition.

The invention is further directed to a process for the preparation of an anisotropic porous polymer membrane. In the process, a single phase liquid mixture comprising the polymer and a vaporizable liquid diluent is prepared as described above, and a nascent membrane is formed which comprises the single phase liquid mixture. Diluent is removed from an outer liquid stratum of the nascent membrane extending inwardly of the surface thereby increasing the concentration of the polymer in the outer liquid stratum to a concentration higher than the concentration of polymer in the bulk of the nascent membrane structure. The nascent membrane is quenched to cause spinodal decomposition of the single phase liquid mixture in the outer liquid stratum and in the interior of the nascent membrane, thereby forming within the nascent membrane a continuous liquid phase comprising the polymer and a separate continuous liquid phase comprising the diluent, the separate liquid phases being interpenetratingly oriented within the nascent membrane. The nascent membrane is further cooled to solidify the continuous phase comprising the polymer, thereby forming a solid polymer membrane comprising an outer stratum having an average pore size smaller than the average pore size in another stratum in the interior of the solid polymer membrane.

The invention is further directed to a process for the preparation of an anisotropic porous polymer membrane in

which a single phase liquid mixture is prepared comprising a polymer, a vaporizable liquid diluent, and another diluent of lower volatility than the vaporizable diluent. The mixture is prepared at a temperature at which the diluents are fully miscible with a polymer in the relative proportions of diluents and polymer contained in the liquid mixture. The diluents are at least partially immiscible with the polymer in such proportions at a temperature below the mixing temperature. The vapor pressure of at least one of the diluents is sufficient for evaporation thereof from the mixture within a temperature range between the mixing temperature and the temperature at which spinodal decomposition of the single phase liquid mixture may be initiated on cooling thereof. A nascent membrane is formed comprising a single phase liquid mixture and volatilizable diluent is evaporated from the outer liquid stratum of the nascent membrane extending inwardly from a surface thereof, thereby increasing the concentration of polymer in the outer liquid stratum to a concentration higher than the concentration of polymer in the bulk of the nascent membrane structure. The nascent membrane is quenched to cause spinodal decomposition of the single phase liquid mixture into separate liquid phases comprising a continuous liquid phase comprising a polymer. The nascent membrane is further cooled to solidify the continuous phase comprising the polymer thereby forming a solid polymer membrane comprising a porous outer stratum having an average pore size smaller than the average pore size in another stratum in the interior of the solid polymer membrane.

Other objects and features will be in part apparent and in part pointed out hereinafter.

Brief Description of the Drawings Fig. 1 is an illustrative phase diagram for a system comprising a polymer and a vaporizable liquid diluent with which the polymer is fully miscible at a temperature above the melt temperature of the polymer;

Fig. 2 is a plot of temperature vs. time in an outer stratum of a film from which an anisotropic semi-permeable polymer membrane is produced in accordance with the process of the invention; and Fig. 3 is a schematic illustration of an apparatus useful in carrying out the process of the invention.

Corresponding reference characters indicate corresponding parts throughout the drawings.

Description of the Preferred Embodiment Illustrated in Fig. 1 is a phase diagram typical of compositions useful in the process of the invention. Such compositions comprise a thermoplastic polymer P, and a diluent Di with which the polymer is at least partially immiscible at ambient temperature but fully miscible at elevated temperatures, typically above the melting point or glass transition temperature of the polymer. The coordinates for point C on the diagram are the critical composition for solutions of polymer P, in diluent Dl, i. e., the composition of P, and D, which exhibits the highest spinodal decomposition temperature, and the critical temperature, i. e., the spinodal decomposition temperature for the critical composition. The coordinates for point E are the eutectic composition and the melting point of the eutectic. Plotted to the left of the eutectic are the spinodal and binodal phase separation curves, and, below these, the polymer solidification line. Plotted to the right of the eutectic is the freezing point (or glass transition temperature) depression curve for the polymer in the Pl/Dl system. As illustrated in the drawing, for compositions useful in the process of the invention, the phase diagram comprises a critical point C that is preferably joined to the eutectic by a binodal phase separation line without intervening nodes or inflections.

In accordance with the process, a single phase liquid mixture is prepared having the composition and temperature of point A, i. e., having a P, content Ca above the critical

composition but below the eutectic, and a temperature Ta at which Pi and D, are fully miscible. The diluent has sufficient volatility to be vaporizable from compositions ranging from Ca to the eutectic composition Ce or higher, at temperatures between Ta and a temperature significantly lower than Ta, e. g., Tb, the temperature at point B, or below.

The single phase composition is formed into a membranous structure comprising a nascent membrane, e. g., a film or annular (hollow) filament; and the polymer concentration in an outer liquid stratum of the nascent membrane, extending inwardly from the surface thereof, is increased by evaporation of the vaporizable diluent from the surface. The polymer concentration in the outer liquid stratum is thereby increased to a value higher than that in the bulk of the structure. Preferably, the surface and outer liquid stratum are cooled by the evaporation, driving the temperature/composition co-ordinates of the single phase liquid mixture toward the binodal and spinodal separation lines. Alternatively, evaporation can be promoted by transferring heat to the outer margin sufficient to maintain its temperature substantially constant, or in some instances to increase. In any event, evaporation conditions are controlled so that the rate of removal of vaporizable diluent is greater than the rate of diffusion of diluent from the bulk of the film or filament to the outer stratum, i. e., diluent is evaporated from the outer stratum at a rapid rate. Advantageously, the evaporation is effected at subatmospheric pressure, for example under a modest vacuum of up to about 50 mmHg, typically about 10 to about 30 mmHg.

As illustrated in both Figs. 1 and 2, evaporative cooling rapidly reduces the temperature of the outer stratum of the structure to Tb and increases the concentration therein to Cbl the coordinates of point B in Fig. 1.

Preferably, evaporation is terminated at a point such as point B, at which the temperature remains high enough so that the composition of the nascent membrane within the outer stratum remains a single phase liquid prior to

quenching of the structure. The increase in concentration by evaporation is preferably sufficient so that Cb is closer to the eutectic composition Ce than to the critical composition Cc. Although preferably close to the eutectic where the spinodal decomposition and solidification equilibrium lines are converging, point Cb is also at a composition for which the spinodal decomposition temperature still exceeds the solid/liquid equilibrium temperature.

The temperature co-ordinate of point Cb is preferably as close to the binodal equilibrium as feasible, e. g., not more than about 40°C above the binodal line, more preferably not more than about 20°C above the binodal line, most preferably not more than 5° to 10°C above the binodal line. As noted, it is preferred that concentration Cb be close to a point where the loci of binodal equilibrium, spinodal decomposition, and solidification equilibrium are converging toward the eutectic. Thus, it is preferred that the temperature at point B, i. e., at the end of the evaporative cooling step be no greater than about 30 degrees C, more preferably no more than about 20 degrees C, higher than the spinodal decomposition temperature, and that the spinodal decomposition temperature at the outer surface composition be no greater than about 50 degrees C higher, preferably no greater than about 20 degrees C higher, than the solidification temperature.

The film or filament is thereafter quenched to rapidly lower the temperature below the binodal, spinodal, and polymer solidification lines. As the mixture cools below the spinodal decomposition line, the single phase mixture separates into a liquid phase predominantly comprising diluent, typically containing less than about 0.5% by weight polymer, and a second liquid phase predominantly comprising polymer. Both phases are preferably continuous, forming interpenetrating continuous liquid phase networks within the nascent membrane. As quenching proceeds below the solidification line, the phase predominantly comprising polymer solidifies, in some instances with further expulsion

of diluent, forming a continuous crystalline or amorphous solid polymer phase extending from the surface substantially throughout the bulk of the membranous structure, including the aforesaid outer stratum. Typically, spinodal phase separation may occur within a temperature range of about 100°C to about 200°C, more typically about 150°C to about 200°C. Solidification of the polymer phase is obtained on further cooling. Diluent remaining in the structure imparts open cell porosity to the continuous polymer phase. Because the residual diluent concentration is significantly lower in the outer liquid stratum from which diluent has been evaporated, the porosity, i. e., the void fraction, in the porous outer stratum is significantly lower than the void fraction in the bulk of the solidified membrane or in other strata interior thereof. Average pore size is also typically smaller due to both the lower diluent fraction and the relatively low temperature of the spinodal phase separation in the concentrated outer liquid stratum. As a result, a sheet or hollow fiber membrane is produced having highly anisotropic, typically asymmetric, configuration and properties. A dense but porous outer stratum or skin is formed which is effective for ultrafiltration and reverse osmosis applications, the void fraction of which is typically between about 5 and about 50%, with an average pore size between about 0.5 and about 0.05y. The bulk of the membrane, and the other strata within the interior thereof, typically have a void fraction between about 50% and about 80% and an average pore size between about 0.2 and about 2y. Thus, a highly efficient ultrafiltration or reverse osmosis membrane may be produced by the method of the invention in an overall thickness of between about 50y and about 500y but with a bubble point no smaller than about 5 psi.

Anisotropic membranes may be produced in accordance with the process of the invention from a wide variety of thermoplastic polymers, including by way of example: polyesters such as polyethylene terephthalate and

polybutylene terephthalate; polyamides such as Nylon 6, Nylon 11, Nylon 66, and Nylon 13; polyolefins such as polyethylene, polypropylene, polystyrene, polyvinyl chloride, polyvinylbutyral, chlorinated polyethylene, acrylonitrile/butadiene/styrene, polybutylene, styrene/butadiene, and ethylene/vinyl acetate; acrylics such as poly (methyl methacrylate), poly (methyl acrylate), poly (ethyl acrylate), ethylene/acrylic acid and ethylene acrylic ester copolymers, and ethylene/acrylic ionomers; polycarbonates; and polysulfones.

Diluent liquids used in the process should remain in the liquid state at the quenching temperature, and preferably remain liquid under ambient conditions, whatever the temperature of quenching. Suitable vaporizable diluents preferably have a vapor pressure sufficient for relatively rapid evaporation from the single phase liquid mixture at atmospheric or subatmospheric pressure and above the binodal decomposition line in the concentration region between the critical concentration and the eutectic. Such diluents may typically have an atmospheric boiling point between about 650° and about 250°C. Exemplary vaporizable diluents include glycerol esters, salicylaldehyde, benzylamine, methyl benzoate, N, N-dimethylaniline, methyl salicylate, and tolylamines.

In addition to the vaporizable diluent, the single phase liquid mixture preferably comprises another diluent which is of lower volatility than the vaporizable diluent.

Preferably, the second diluent is substantially non-volatile in the temperature range in which the evaporative cooling step of the process is conducted. By selection of a vaporizable diluent of known volatility and controlling the relative proportions of diluents with respect to each other and with respect to the proportion of polymer, the porosity and average pore size of both the porous outer stratum and bulk membrane can be controlled in a desired and predictable manner. Unlike a single diluent system, in a plural diluent system comprising a vaporizable diluent the residual diluent

in the porous outer stratum can be predetermined to a large extent, thereby providing a predictable asymmetric structure. More particularly, such mix of diluents may be selected to establish an especially sharp gradient in void fraction and average pore size transversely of the membrane.

The difference in volatility between the vaporizable and non-volatile diluent and the relative concentrations of the diluents in the single phase liquid mixture are such as to yield a substantial difference in porosity and average pore size between the"skin"or porous outer stratum and the bulk membrane, and a substantial difference between the outer stratum porosity and average pore size vs. the porosity and average pore size of any other stratum in the membrane, particularly any other stratum in the membrane interior.

Such difference affords high separation efficiency at modest pressure drop. A"pore size differential"may be defined as the difference between the average pore size of the outer margin and the bulk average pore size, or the difference between the average pore size of the skin and that of another particular stratum in the interior of the membrane.

In accordance with the process of the invention, the pore size differential of a membrane produced with a combination of volatile and non-volatile diluents is generally greater than that of a membrane produced in a conventional manner.

The pore size differential of a membrane produced with a combination of volatile and non-volatile diluent is generally also greater than that of a reference membrane obtained by processing a film or fiber containing only one of the diluents (i. e., the vaporizable diluent) under evaporative cooling and quenching conditions otherwise substantially identical to the conditions under which the membranous structure containing both said diluents is processed.

The vaporizable diluent and diluent of lower volatility are miscible with each other and with the thermoplastic polymer at temperatures above the melting point of the polymer. Preferably, the polymer is substantially insoluble

in either diluent at ambient temperature. Exemplary relatively non-vaporizable diluents include glycerol, dibutyl phthalate, diethyl phthalate, diphenyl ether, diphenylamine, palmitic acid, stearic acid, oleic acid, diphenyl ether, polyethylene glycol, dodecylamine and tallowamine.

The preferred initial composition of the single phase liquid mixture depends on the configuration of the phase diagram for a system consisting of the components of the mixture. Typically, the composition initially contains between about 15% and about 40% by weight thermoplastic polymer, between about 5% and about 15% by weight vaporizable diluent, and up to about 95%, preferably between about 80% and about 45%, by weight relatively non-volatile diluent.

The single phase liquid is prepared by heating an agitated slurry of particulate thermoplastic polymer in the liquid diluent, or liquid diluent mixture, under moderate agitation to a temperature above the binodal phase separation line at the composition Ca of the mixture. The resulting liquid mixture is then formed, conveniently by casting or extrusion, into a film or filament having the configuration of the solid membrane product to be produced.

Although films and hollow fibers are the principal forms of semi-permeable membrane, the terms"membranous structure" and"nascent membrane"are intended to encompass other membrane configurations that may exist or be devised. The nascent membrane is formed at a temperature at which the diluent and polymer are fully miscible, but preferably under conditions which prevent flashing of diluent as it is formed. More particularly, in the case of extrusion, the temperature of the structure and the ambient temperature, pressure and mass transfer conditions at the exit of the extrusion die are controlled so as to minimize flashing at the exit of the die.

After the single phase liquid mixture has been formed into the shape of the desired article, evaporation,

preferably evaporative cooling, is conducted to concentrate the polymer in the outer stratum of the film or filament.

Evaporation preferably reduces the temperature of the outer stratum, typically from a temperature such as Ta of Fig. 1 to a temperature that is above but preferably near to the binodal phase separation line for the composition reached in evaporative concentration, i. e., Tb as shown in the phase diagram of Fig. 1 and the cooling curve of Fig. 2.

Evaporation of vaporizable diluent increases the concentration of polymer in the outer stratum from Ca to Cb i. e., the operating line of the evaporative cooling step extends from point A to point B along a path such as designated by the line L of Fig. 1. In the course of the cooling step, the temperature declines with time along a line such as illustrated in Fig. 2. It will be understood that, where the single phase liquid mixture comprises plural diluents, the phase diagram comprises more than two dimensions and the"curve"L may comprise more than two dimensions. However, where the diluents are miscible with the thermoplastic polymer and with each other over the concentration and temperature range of the evaporative cooling step, and especially where such relationships prevail down to the spinodal decomposition lines, the process may be effectively illustrated by use of a two dimensional phase diagram, with the diluent concentration parameter representing the combined concentrations of the plural diluents.

Evaporative cooling may be conducted by introducing the membranous structure into an oven and establishing a relative flow of stripping gas such as air over the surface of the structure. The stripping gas is at elevated temperature, typically above the spinodal and binodal phase separation lines, but below the boiling point of the single phase liquid mixture within the outer stratum at the pressure of the stripping gas. In the case of a fiber, e. g., the liquid filament comprising the membranous structure may pass through a heated pipe containing

stripping air at the appropriate temperature.

Alternatively, the surface of the film or fiber may be exposed to a rarified gas at a pressure below atmospheric but above the pressure at which the diluent flashes from the outer stratum of the structure. Vacuum concentration is conducted with the temperature of the outer liquid stratum in a range wherein the vapor pressure is a substantial fraction of the total pressure but insufficient for the diluent to flash.

In the case of an annular fiber, evaporation is ordinarily effected at the external surface of the annulus, ultimately forming a low porosity and low average pore size marginal stratum at that surface. In some instances it may be desirable and feasible to evaporate diluent from the internal surface for formation of a low porosity and low average pore size margin or skin in the stratum extending toward the interior of the membrane wall from the surface bordering the hollow core. Alternatively, if a pore size and/or porosity gradient is desired in the latter stratum, it may be provided by use of a core fluid effective for extraction of diluent therefrom.

To obtain the desired average pore size differential, the proportion of diluent removed from the outer stratum is preferably sufficient to significantly increase the concentration of thermoplastic polymer in that stratum. It is further preferred that removal of vaporizable diluent be sufficient so that the ratio of the weight % concentration of polymer in the outer stratum to the weight % concentration of polymer in the bulk of the membranous structure be at least about 1.5, typically about 1.5 to about 3.

In evaporative cooling, the requisite concentrating effect requires that the temperature of the outer stratum be lowered significantly. For example, the single phase mixture (at composition Ca) is initially heated to a temperature (Ta) that is at least about 5°C, more typically between about 10° and about 30°C, higher than the temperature

on the binodal phase separation line at the concentration (Cb). The temperature decrease obtained by evaporative cooling (Ta-Tb) may vary substantially with the shape of the spinodal decomposition curve and the position of Cbin relation to that curve; but in any case, the temperature Tb achieved by evaporative cooling is preferably close to but above the binodal phase separation line at Cb, as discussed hereinabove. Optionally, the outer stratum may be cooled evaporatively to a temperature below the binodal equilibrium line, either between the binodal and spinodal line, below the spinodal line, or even below the solidification line, but if any phase separation occurs, it is necessary to immediately quench the film or fiber to avoid formation of undesired structures in the outer stratum or bulk structure.

Quenching of the nascent membrane is preferably effected immediately following the evaporative cooling step.

In the quenching step, as illustrated in Fig. 2, the temperature of the membranous structure should be reduced very rapidly through the spinodal decomposition line to a temperature below the solidification line at which the polymer rapidly crystallizes or assumes a relatively rigid or at least dimensionally stable amorphous structure. By minimizing the time in which the temperature of the mixture is between the binodal and spinodal separation lines, and between the spinodal and polymer solidification lines, coarsening is avoided and a monodisperse porous structure is obtained, a structure which preferably comprises interpenetrating networks defined by a first continuous liquid phase constituted predominantly of diluent, and a second continuous liquid phase constituted predominantly of polymer. Rapid cooling through the spinodal decomposition and solidification lines further conduces to a narrow distribution of pore size within the outer marginal stratum, and within other strata, in the solid membrane. The nascent membrane is preferably quenched from the temperature at the end of the evaporative cooling to a temperature below the solidification equilibrium in the outer stratum at a rate of

at least about 10°C per minute, preferably between about 10°C and about 50°C per minute. A narrow pore size distribution can be obtained when the nascent membrane is quenched from the temperature at the end of the evaporative cooling to a temperature below the solidification equilibrium in the outer stratum at a rate of at least about 1,200°C per minute, and most preferably at least about 1,400°C per minute. Quenching can be accomplished in various ways known to the art. Advantageously, quenching can be effected simply by exposure of a film or fiber to ambient air.

Alternatively, the structure can be quenched on a chill roll or by immersion in a liquid bath, e. g., an aqueous liquid, with which the polymer is not miscible.

Referring again to Figs. 1 and 2, in a preferred embodiment of the invention the combined cooling and quenching steps of the process comprise forming a membranous structure in a desired geometrical configuration, typically by extrusion into a thin film from a single phase liquid having the composition Ca at temperature Ta, and causing the film to flow downwards into a cooling bath having a temperature F. As illustrated in Fig. 3, the single phase liquid may be extruded through a spinneret to produce a fine annular strand or filament having walls comprised of the homogeneous liquid, and drawing the filament downwardly into the cooling bath. In the system of Fig. 3, a polymer solution is prepared in or charged to a spinneret feed vessel 1 positioned within a larger chamber 3. Polymer solution is agitated with an anchor or paddle mixer 5. Heat supplied by transfer from a heating fluid passing through coils 7 maintains the solution at a select controlled temperature corresponding to point A on the phase diagram of Fig. 1. The head space of vessel 1 above the polymer solution liquid level is in gas flow communication with a source of inert gas, e. g., nitrogen, for pressurizing the vessel to cause discharge of solution from a bottom outlet 9 of the vessel through a spinneret 11 below outlet 9 where a nascent single liquid phase fiber is formed. Spinneret 11

is configured for formation of a hollow fiber structure. A gaseous or liquid core fluid is supplied via a core fluid supply line 13 to the core fluid channel at the head of the spinneret. Spinneret 11 is located near the bottom of chamber 3 above a discharge conduit 15 which extends downwardly below the surface of a quenching bath 17 contained within a quench tank 19 that may be open to, or vented to, the atmosphere. The environment in chamber 3 outside vessel 1 is controlled at a temperature and pressure suited for evaporation of diluent from the outer surface of the hollow nascent fiber exiting spinneret 11. Optionally, the pressure in the chamber may be controlled below atmospheric to promote evaporation of diluent, in which instance a barometric leg of quenching fluid rises within conduit 15. The combination of temperature and pressure in chamber 3 is controlled to avoid flashing of diluent from the nascent fiber exiting the spinneret. As the fiber passes through the air or other stripping gas atmosphere between the exit of the spinneret and the surface of the quenching liquid in conduit 15 or tank 19, diluent is lost by evaporation from the outer surface to concentrate the outer liquid stratum as described above. Preferably, evaporation causes cooling of the outer stratum to the temperature of point B as illustrated in Figs. 1 and 2.

Quenching results in spinodal decomposition and formation in the hollow fiber wall of a continuous liquid phase predominantly comprising polymer, and another continuous liquid phase predominantly comprising diluent. The two continuous liquid phases are intimately interwoven with each other within the nascent fiber wall to form a pattern which provides a fine porous uniform open cell structure when the diluent phase is later removed. The porous solid hollow fiber formed in the quenching bath is captured on a fully immersed takeup roll 21 and transferred to a partially immersed takeup roll 23, whence it may be further transferred for further processing to remove the diluent phase. Optionally, the quenching bath may comprise a

relatively volatile displacement medium which removes the diluent phase, and which can be itself removed by evaporation in a subsequent process step, as discussed hereinbelow. However, in this instance, the process becomes a hybrid comprising elements of both thermally induced and extractive phase separation, which introduces additional variables that may complicate process control and alter membrane characteristics. The fibrous membrane formed in the process has a relatively highly porous bulk structure that is formed from the composition at point A and a porous outer stratum that is much"tighter,"i. e., of lower porosity and pore size due to its formation from the high polymer content composition at point B. The porous outer margin not only has a relatively low specific pore volume, but is comprised of fine uniform pores obtained by spinodal phase separation at the relatively low spinodal decomposition temperature and correspondingly high viscosity of the composition Cb which is significantly closer to the eutectic than composition Ca at point A.

Quenching produces a porous polymer membrane in which the open pores of both the skin and bulk structure are substantially occupied by the remaining diluent liquid. Use of the membrane for separations requires removal of the diluent. Diluent may be removed in a conventional manner by extraction or displacement using a relatively volatile liquid such as, for example, isopropanol, methyl ethyl ketone, tetrahydrofuran, ethanol, or heptane. Evaporation of the residual solvent or displacement liquid creates void space within the pores of the membrane. Alternatively, the diluent may be displaced by a passage of a gaseous fluid through the membrane. The membrane produced typically possesses the structure and properties described hereinabove. The bulk membrane inward of the outer margin or skin structure may be substantially isotropic, or may have a degree of anisotropy resulting from diffusion of diluent during evaporative cooling.

After removal of diluent, the polymer membrane may optionally be annealed to relieve stresses in the membrane structure formed in the cooling process. For certain applications, especially biomedical application, the membrane may be sterilized before use, for example by exposure to y-radiation.

In an alternative embodiment of the invention, a membrane is produced having an intermediate stratum inward of the outer stratum that has a porosity and average pore size greater than the outer stratum but lower than the porosity and average pore size of another interior stratum inward of the intermediate stratum, and lower than the bulk porosity of the membrane. Such compound structure may be produced by preparing a single phase liquid mixture comprising a first vaporizable diluent and another vaporizable diluent having a volatility lower than the volatility of a first diluent. In still further embodiments, a plurality of intermediate strata may be produced using a plurality of additional diluents having sufficient volatility for at least partial removal during evaporative cooling. Preferably, the differences in volatility among the plurality of additional diluents is such that a plurality of intermediate strata is formed between the outer marginal stratum and the another interior stratum and/or bulk of the membrane, the porosity and average pore size of each of the intermediate strata being greater than those of the marginal stratum but lower than the another interior stratum or the bulk membrane.

Typically, in such embodiments, the porosity and average pore size of the intermediate strata progressively increases in a transverse direction away from the surface and toward the aforesaid interior stratum.

Depending on selection of polymer and diluent (s) and evaporation conditions, the polymer concentration gradient across the nascent membrane may assume different configurations, either linear or non-linear. Porosity and average pore size gradients within the solidified membrane

generally conform to the shape of the polymer concentration gradients produced during the diluent evaporation step.

Where plural volatilizable diluents of significantly differing vapor pressure are used, it may be possible to achieve substantial discontinuities in the concentration gradient across the nascent membrane and the corresponding porosity and average pore size gradients across the solidified membrane.

Within the porous outer stratum of the membranes of this invention, a highly uniform pore size distribution is obtained due to formation of the pores via spinodal decomposition and rapid solidification of the continuous polymer phase thereby produced. Narrow distribution of pore size is of critical importance in ultrafiltration operations such as, for example, the removal of viruses. Such fine particles can find a path through a membrane containing pores larger than the particle even if the mean pore size is much smaller and the fraction of larger pores is very small, e. g., beyond the 2 or 3 sigma limit. But because of the exceptionally low pore size variance achieved by the process of the invention, a porous outer stratum is produced which can positively exclude passage of such fine solid materials.

Formulations useful in forming the single phase liquid mixture and liquid membranous structure are novel compositions of matter, and the nascent membrane formed from such formulations constitute novel membranous structures.

Such compositions comprise a film-forming thermoplastic polymer and at least two diluents which together form a single phase liquid mixture with the polymer at temperatures above the melting point of said polymer. At least one of the diluents is vaporizable for cooling and concentrating such single phase liquid mixture. On a phase diagram for mixtures consisting of the components of the composition, the concentration of polymer in the composition is greater than the critical point but lower than the eutectic.

Evaporative cooling of the composition may be effective to cool a single liquid phase mixture consisting of such

composition to a temperature near the binodal phase separation line on the aforesaid phase diagram. Preferably, polymer and diluents are selected so that the spinodal decomposition temperature is above the solid/liquid equilibrium temperature over a range of compositions above the critical point. Most preferably, the locus of spinodal decomposition temperatures is higher than the locus of solidification temperatures at all concentrations between the critical concentration and the eutectic.

Thermoplastic polymers and diluents useful in the composition of the invention are as described above.

Preferably, the composition comprises a second diluent of substantially lower volatility than the vaporizable diluent.

More preferably, the second diluent is substantially non- volatile under the conditions suitable for evaporative cooling and concentration of the outer stratum of a membranous structure formed from the composition.

Preferably the composition of the invention consists of components which form compositions having a phase diagram such as illustrated in Fig. 1, wherein the critical point C is preferably joined to the eutectic by a binodal phase separation line without intervening nodes or inflections.

In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results attained.

As various changes could be made in the above compositions and processes without departing from the scope of the invention, it is intended that all matter contained in the above description and shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.