CA1321687C - Process and dope for forming asymmetric gas separation membranes having graded density skins - Google Patents

Abstract

36-21(5441) PROCESS AND DOPE FOR FORMING ASYMMETRIC GAS SEPARATION MEMBRANES HAVING GRADED DENSITY SKINS Abstract of the Disclosure A process and dope are disclosed for forming high free volume, asymmetric gas separation membranes having graded density skins and macrovoid-free mor-phology from a solvent system comprising a Lewis acid, Lewis base, and a Lewis acid:base complex which dissolve hydrophobic polymers. The dopes have high total solids, high viscosity and low coagulation (g) values which enhance rapid gelation without the formation of macrovoids and which minimize densifi-cation. The dope solvent system complexes are readily disassociated by the coagulation medium and the component molecules are miscible in the medium which provides for rapid desolvation of the formed membrane thus providing a membrane having low residual solvent retention.

Description

~321687 -1- 36-21(5441)A
PROCESS AND DOPE FOR FORMING ASYMMETRIC GAS
SEPARATION MEMBRANES HAVING GRADED DENSITY SKINS

This invention relates to a process for forming asymmetric membranes having graded density skins for separating gases. In another aspect, the invention relates to a membrane dope comprising a solvent system of Lewis acids and Lewis bases and Lewis acid:base complexes which dissolves high weight percent total solids of membrane forming polymers. In yet another aspect, the invention relates to a process for forming gas separation membranes from high viscos-ity dopes having low coagulation (g) values which enhance rapid gelation and desolvation resulting in macrovoid-free membranes having high free volume.
The viability of the use of membranes for gas separations as compared to other separation procedures such as adsorption, absorption, and lique-faction often depends on the cost of the apparatus and its operation, including energy consumption, degree of selectivity of separation and rate of separation.
Other important considerations are the total pressure loss caused by the apparatus conducting the separation process, the useful life for such apparatus and the size and the ease of use of such apparatus. Thus, membranes are sought which provide desired selectivi-ties of separation, fluxes and strength. Moreover, inorder to be commercially attractive on an economic basis, the membranes are preferably capable of being manufactured in large quantities while achieving a reliable product quality and being readily and rela-tively inexpensively assembled into a permeator.Particularly advantageous membranes are unitary, asymmetric, hollow fiber membranes which have a relatively thin separating layer, barrier layer, or active layer, integral with a porous substrate which , ''~, ; ;.

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provides support to the separating layer and offers little, if any, resistance to passage of gases. In order to prepare these asymmetric membranes, a unitary membrane structure must be formed which possesses diametrically opposed structures. The separating layer must be formed such that it is thin and possess-es few, if any, pores or other defects. On the other hand, the conditions of formation of the asymmetric membrane, must also result in a support structure which is highly open such that it offers low resis-tance to gas flow. Numerous proposals have been made pertaining to the preparation of asymmetric membranes prepared in film and in hollow fiber form. In gener-al, asymmetric membranes are prepared in film form by casting a solution of the polymer onto a surface, e.g., polished glass. The cast polymer solution may be allowed to coagulate at least partially, in air or a gaseous or vaporous environment and then immersed in a liquid coagulant. Considerable flexibility exists in preparing film membranes. For instance, since the polymer solution is placed on a support, the membrane precursor structure need not be self-supporting at least until after coagulation is completed. Similar-ly, since one surface of the cast membrane is in contact with the support, each side of the membrane may be subjected to different coagulation conditions thereby permitting substantially different structures to be achieved at each surface of the membrane.
Accordingly, membranes having a relatively thin layer with essentially an absence of pores may be achieved at one surface of the film membrane, while the remain-der of the membrane may be relatively porous. More-over, since the film membrane precursor is supported, the coagulation conditions, including coagulation times, can vary widely to achieve the desired film membrane structure. In some instances, however, film 132~87 -3- 36-21(5441)A

membranes may not be as attractive as hollow fiber membranes due to the need for film membranes to be supported to withstand operating conditions and the overall complexity of apparatus containing film membranes. Membranes in the configuration of hollow fibers may overcome some of the deficiencies of film membranes for many separation operations. Hollow fibers are generally self supporting even under severe operating conditions and can provide a greater amount of membrane surface area per unit volume of separation apparatus than that which can be provided by film membranes. Thus, a separation apparatus containing hollow fibers can be attractive from a standpoint of convenience, size and reduced complexity of design.
Many different process considerations are involved in making a hollow fiber membrane than are involved in making a film membrane. For instance, no solid support, or interface can be provided in a process for spinning hollow fiber membranes. More-over, in spinning procedures, the polymer solution must be of sufficient viscosity to provide a self-sup-porting extrudate prior to and during gelation, and the gelation must be quickly effected after extrusion such that the hollow fiber membrane is not adversely affected.
Processes for the formation of asymmetric membranes must not only meet the criteria of forming the membranes, but also must be compatible with hollow fiber spinning capabilities. Hence, many constraints are placed upon the techniques available to produce asymmetric hollow fiber membranes. Commonly, in hollow fiber membrane spinning procedures, a solution of the polymer, i.e., a dope to form the hollow fiber membrane in a solvent system, is extruded from a spinnerette suitable for forming a hollow fiber configuration. A gas or liquid is injected into the . ~
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bore of the hollow fiber extrudate such that the hollow fiber configuration can be maintained. The hollow fiber extrudate must quickly be coagulated, e.g., by contact with a non-solvent for the polymer, such that the hollow fiber configuration can be maintained. The hollow fiber spinning process con-tains many variables which may affect the structure or morphology of the hollow fiber membrane such as the conditions of the polymer solution when extruded from the spinnerette, the nature of the fluid maintained in the bore of the hollow fiber membrane extrudate, the environment to which the exterior of the hollow fiber extrudate is subjected, the rapidity of coagulation of the polymer and the hollow fiber extrudate and the like. Since the hollow fiber forming process is one of coagulation of the polymer from a polymer solution, the nature of the solvent system for the polymer is highly influential in determining the morphology of the hollow fiber membrane and its gas separation properties.
The solvent system must possess numerous properties in order to be suitable for forming asym-metric membranes. For example, the solvent system must be capable of dissolving the polymer for forming the hollow fiber membrane, but yet permit the polymer to readily coagulate to form the asymmetric structure.
Furthermore, when hollow fiber membranes are desired, the solvent system should enable the polymer solution to be prepared having suitable viscosities for hollow fiber membrane formation.
A hollow fiber spinning dope should be comprised of a solvent system which is miscible with the non-solvent used to assist in coagulation of the polymer and should be capable of being removed, e.g., by washing from the coagulated structure, such that the membrane is not unduly plasticized or densified , :

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and thereby made undesirable for gas separation.
Moreover, the solvent system should not exhibit excessive heats of dilution in the non-solvent used to assist in coagulating the polymer.
More recently, asymmetric gas separation membranes have been spun from solvent systems which include the non-solvent in the solvent system with the polymer which comprises the spinning dope. In this way greater control of the gelation is achieved and simpler coagulation procedures are possible. All components of the solvent system, the spinning dope, the coagulation medium, and the washing fluids should work in cooperation in order to avoid undesired morphology in separating skin densities, membrane strengths and the like. Moreover, in the quantities required to effect, e.g., coagulation, washing, etc., the expense of certain nonaqueous materials could be a factor in the economics of the spinning process.
Accordingly, it is desired to use highly safe, readily available materials, such as water, whenever possible in the spinning process, especially as a coagulation medium and in washing to remove solvent from the hollow fiber membrane. The ability to use water, depends to a large extent, upon the properties of the solvent system with respect to water and the polymer's physical properties contained in the spinning dopes as well as the impact of water upon removing solvents and non-solvents from the formed membrane.
Loeb and Sourirajan, U.S. Patent Number 3,133,132, demonstrated for the first time that it was possible through the preparation of asymmetric mem-branes to decouple the so called intrinsic permeabili-ties from intrinsic separation factors which result from the use of dense membranes. Asymmetric membranes taught by Loeb and Sourirajan and multicomponent membranes taught by Henis and Tripodi, U.S. Patent ~ 3 2 ~
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4,230,463, even though presenting higher permeabilities than dense membranes, still couple these permeabilities with separation factors in an interdependency. The high permeability (P/~) values at constant or improved, and in some cases at essentially intrinsic separation factor presented by the present invention, amounts to a second decoupling which is achieved by the modifica-tion of the membrane skin structure. As demonstrated by the elevated first heat Tgs of the membranes according to the invention, the origin of increased permeability is related to increased free volume.
Selectivity is maintained constant or improved because the membranes are asymmetric and macrovoid-free and lS have graded density skin.
Loeb and Sourirajan disclosed a method for preparing a modified cellulose acetate membrane for desalination of water by first casting a solution of cellulose acetate as a thin layer, and then forming a dense membrane skin on the thin layer through various techniques such as solvent evaporation followed by quenching in cold water. The formation of these dense-skinned membranes generally involved a final annealing treatment in hot water. The membranes prepared by the Loeb-Sourirajan method are composed of two distinct regions made of the same cellulose acetate material, a thin, dense, semipermeable skin and a less dense, void-containing, nonselective support region. Since the membranes are not of ; 30 essentially the same density throughout their struc-ture, they fall within the definition of anisotropic membranes. Because of these distinct regions and the difference in membrane properties which can be ob-served depending on which surface of the membrane faces a fluid feed mixture, the Loeb-Sourirajan membranes can be described as being asymmetric.

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Solvent system dope spun, asymmetric mem-branes for gas separations were more recently advanced by Henis et al, whereby a multicomponent membrane for gas separation comprised of a coating in contact with a porous separation membrane was taught with the separation properties of the multicomponent membranes being principally determined by the material of the porous separation membrane as opposed to the material of the coating. The multicomponent membranes provided gas separations membranes which could be fabricated from a wide variety of gas separation polymeric materials and thus enable greater latitude in select-ing such a membrane material which is advantageous for a given gas separation. These multicomponent mem-branes were solvent spun from dopes of glassy, aromat-ic sulfone polymers and other polymers and were gelled in water and/or other polar media. Separation factors for at least one pair of gases were increased by at least 35 percent through utilization of the multicom-ponent membranes; however, separation factor and permeability remain coupled, that is, adjustment or increase in, for example, separation factor would be at the cost of reduced permeation rate and vice versa.
SUMMARY OF THE INVENTION
A process and spinning dope is presented for preparing asymmetric gas separation membranes having graded density skins and macrovoid-free morphology comprised of aromatic sulfone polymers wherein the membranes have increase free volume as evidenced by the membrane first heat Tg which is greater than the Tg of a bulk sample of the polymers. The resultant membranes exhibit high permeabilities and the poten-tial for intrinsic separation factors realized after any porous defects of the membrane are sealed with a highly permeable silicone polymer or similar coating.
The membranes exhibit selectivities that can approach 1321~87 -8- 36-21(5441)A

those of dense films of the polymers and permeabili--ties which are much greater than those of state-of-the-art, asymmetric membranes of the same polymer, for example, as taught by Henis et al.
The process for forming high free volume, asymmetric gas separation membranes having graded density skins results from utilizing a dope of a solvent system comprised of Lewis acids, Lewis bases and Lewis acid:base complexes which dissolve aromatic sulfone polymers. The dopes have high total solids, high viscosity and low coagulation (g) values which enhance rapid gelation and allow rapid desolvation when coagulated in water without undue densification or formation of macrovoids.
The graded density skin membranes result from, for example, spinning or casting dopes comprised of aromatic sulfone polymers in a solvent system of Lewis acids, Lewis bases, and Lewis acid:base complex-es capable of dissolving the polymer and being readily disassociated by polar coagulation media, which provides macrovoid-free, asymmetric membranes possess-ing high free volume. The invention is concerned with the preparation of efficient, asymmetric, gas separa-tion membranes from various glassy, hydrophobic polymers. The performance efficiencies of the coated graded density skin membranes are determined by the comparison of permeability and selectivity vs. temper-ature curves for state-of-the-art membranes as provid-ed commercially by Permea, Inc., under the tradename of *PRISM ~ separators based on the Henis et al teach-ings. The permeabilities of the inventive membranes are considerably greater than the Henis et al mem-branes while maintaining either constant or improved separation factor which can approach intrinsic levels.
The reason for the greater permeability in the graded density skin membranes lies in part in the increased -~ *Trade Mark : .

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free volume in their skin regions which in turn is a result both of sol structure prior to gelation and of the kinetics of gelation and desolvation of the sols from which the membranes are spun or cast. The S separation factor is maintained at least at a constant level or increased, in other words, decoupled from permeability because the skins of the inventive membrane exhibit a density gradient, becoming less dense with increasing proximity to the skin/matrix interface. The as-spun membranes have a fine and uniform structure with the potential of achieving intrinsic separation factor. This potential is realized after any porous defects have been filled with highly permeable silicone polymer or equivalent coating material. The densest portion of the graded density skin, the skin of the skin, is effectively as dense as a thick dense film and hence exhibits essen-tially the same or intrinsic separation factor as the latter once porous defects have been sealed by coating.
The membranes are a result of processes involving viscous sols which are close to the point of incipient gelation. The sols can consist of an -~
aromatic sulfone polymer, for example, polysulfone, polyphenylsulfone, polyethersulfone and the like dissolved in a solvent system consisting of a Lewis acid, Lewis base and a Lewis acid:base. The fact that the Hildebrand parameter of the solvent system is reasonably close to that of the polymer results in fine and homogeneous sol and hence gel structures which have the potential for high gas separation efficiency. High sol viscosity and low non-solvent tolerance kinetically hinder densification of the gel and prevent the formation of macrovoids. At gelation, the Lewis acid:base complexes are rapidly disassociat-ed by water or other polar medium into their smaller t .
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component molecules which, together with any original uncomplexed acid and base components, are readily extracted during gelation and washing steps. All of these factors act in concert to increase the rates of gelation and desolvation and to minimize densification with the end result that unusually high free volume and graded densities are maintained in the skin layers of the membranes.
More particularly in accordance with an aspect of o the present invention there is provided a process for forming asymmetric gas separation membranes having graded density skins and macrovoid free morphology comprised of hydrophobic polymers, the membranes having increased free volume as evidenced by the membrane first heat Tg which i5 greater than the Tg of a bulk sample of the polymers, comprising: (a) dissolving the polymers in a solvent solution of Lewis acids and Lewis bases and complexes formed of the Lewis acids and bases, wherein the Lewis acids, Lewis bases, Lewis acid:base complex solvent system has a Hildebrand parameter less than about 1.5 cal / /cm3/2 of the Hildebrand parameter of the polymer; (b) creating a dope of the solvent system and polymers which is near the gel point, the dope comprised of greater than about 30% by weight total solids sol; (c) configuratinging the dope into a form suitable for use as an asymmetric gas separation membrane; (d) coagulating the formed dope; (e) desolvating the resulting formed asymmetric gas separation membrane having graded density skin and macrovoid free morphology through rapid disassociation of solvent complex; (f) washing the membrane at ambient :
temperatures in a polar medium; and (g) drying the membrane at temperatures of from about ambient to about 20C below the glass transition temperature.
More particularly in another aspect of the present ,~
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- lOa -invention there is provided a process for forming hollow fiber asymmetric gas separation membranes having graded density skins and macrovoid-free morphology comprises of hydrophobic polymers, the membranes having increased free volume as evidenced by the membrane first heat T
which is greater than the Tg oE a bulk sample of the polymers, comprising: (a) dissolving the polymers in a solvent solution of Lewis acids and Lewis bases and complexes form of the Lewis acids and Lewis bases lo wherein the Lewis acid, Lewis base and Lewis acid:base complex solvent system has a Hildebrand parameter less than about 1.5 call/2/cm3/2 of the Hildebrand parameter of the polymer; (b) forming a spinning dope of the sol-vent system and polymers which is near the gel point, the dope comprised of greater than about 30% by weight total solids sol; (c) spinning the dope through a tube-in-orifice spinnerette; (d) injecting coagulation fluid through the orifice; (e) coagulating the spun dope in a coagulation medium and forming the membrane; (f) desol-20 vating the formed membrane through rapid disassociation of the complex; (g) washing the membrane in a polar medium; and (h) drying the membrane at temperatures of from about ambient to about 20C below the glass tran-sient temperature.
DEFINITIONS
For purposes of defining the invention, the follow-ing terms and phrases will have the meanings as set out below.
In accordance with the invention, the asymmetric gas separation membranes are comprised of a graded density skin, that is, a skin which exhibits maximum density at the surface which is farther from the porous substructure. The separation membrane is essentially the same material, throughout its structure, i.e., the asymmetric separation membrane is substantially chemi-~.. .

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1321~87 - lOb -cally homogeneous as is the graded density skin. The material of the separation membrane exhibits selective permeation for at least one gas of a gaseous mixture over that of at least one remaining gas of the mixture, hence the separation membrane is defined as a "separa-tion" membrane. By describing this separation membrane as asymmetric, it is meant that the membrane has a thin, dense skin supported by a thick, porous substrate (matrix) in which both layers are formed from a single lo sol by a phase inversion process. The phase inversion process is a general process of membrane manufacture that utilizes a sol which inverts into two interdisper-sed liquid phases, that is, polymer coated micelles of the dispersed phase in a continuous second liquid phase, prior to, or simultaneously with gelation, at which time the emulsoid nature of what once was a sol is . 20 .:,. ~ . .
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immobilized as a gel. The membranes of this invention advantageously provide the separation of at least one gas from a gaseous mixture by interaction with the materials of the separation membrane, enhanced by free volume and graded density compared to that of separa-tion membranes having denser effective separation skin thicknesses.
The asymmetric membranes having graded density skins according to the invention are further enhanced for practical utilization for separating gases when the graded density skin is coated with coatings which have a particular relationship respec-tive to the membranes. Some of these relationships can conveniently be stated in terms of relative separation factors and permeabilities with respect to a pair of gases for the separation membranes, coatings and graded density skin.
The coating may be in the form of an essen-tially non-interrupted membrane, i.e., an essentially non-porous membrane, in contact with the graded density skinned membrane, or the coating may be discontinued, or interrupted. When the coating is interrupted, it is sometimes referred to as an occlud-ing material since it may occlude channels for gas flow, i.e., pores. Preferably the coating is not so thick as to adversely affect the performance of the graded density skinned membrane, e.g., by causing an undue decrease in flux or by causing such a resistance to gas flow that separation factor of the coated membrane is essentially that of the coating. Often the coating may have an average thickness of up to about lQ micrometers.
A separation factor (~a/b) for a membrane for a given pair of gases a and b is defined as the ratio of the permeability constant (Pa) of the mem-brane for gas a to the permeability constant (Pb) of .:

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the membrane for gas b. A separation factor is also equal to the ratio of the permeability (Pa/Q) of a membrane of thickness ~ for a gas of a gaseous mixture to the permeability of the same membrane to gas b, (Pb/Q) wherein the permeability for a given gas is the volume of gas, standard temperature and pressure (STP), which passes through a membrane per square centimeter of surface area, per second, for a partial pressure drop of 1 centimeter of mercury across the membrane per unit of thickness, and is expressed a P/Q=cm3/cm2-sec-cmHg.
In practice, the separation factor with respect to a given pair of gases for a given membrane can be determined employing numerous techniques which provide sufficient information for calculation of permeability constants or permeabilities for each of the pair of gases. Several of the many techniques available for determining permeability constants, ;
permeabilities, and separation factors is disclosed by Hwang, et al., Techniques of ChemistrY, Volume VII, Membranes in Separations, John Wiley & Sons, 1975 at Chapter 12, pa~es 296 to 322.

An intrinsic separation factor as referred to herein is the separation factor for a material which has no channels for gas flow across the materi-al, and is the highest achievable separation factor for the material. Such a material may be referred to as being continuous or non-porous. The intrinsic separation factor of a material can be approximated by measuring the separation factor of a compact membrane of the material. However, several difficulties may exist in the determination of an intrinsic separation factor including imperfections,introduced in the preparation of the compact membrane such as the presence of pores, the presence of fine particles in .'` ~

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the compact membrane, undefined molecular order due to variations in membrane preparation, and the like.
Consequently, the determined intrinsic separation factor can be lower than the intrinsic separation factor. Accordingly, a "determined intrinsic separa-tion factor" as employed herein refers to the separa-tion factor of a dry compact membrane of the material.
Coagulation value (g) is a measure of nonsolvent tolerance, or proximity to the point of incipient gelation (PIG). It is defined as the grams of water which when added to lOOg of a sol containing 2 grams of polymer @ 30C will effect phase separa-tion, J. Tsugaya and T. Miyano, Jap. Pat. Publ. 58-24, 305 (1983). The lower the g value, the more rapidly a sol will gel when immersed in water resulting in less gel densification. Furthermore, the lower the g value, the less likely will be the formation of macrovoids.
Cohesive energy density (CED) of a liquid is a measure of the forces holding the liquid together.
Its most common unit is cal/cm3. CED can be consid-ered as an index of polarity, the higher the CED, the more polar a liquid.
Hildebrand or total solubility parameter is expressed as (~) ~ = (CED)1/ 2 having the units of cal~'2/cm3'2. The single value ~ is a single value gauge of polarity, that is, the higher ~, the higher polarity. The ~ of a polymer is taken as the ~ of the fluid which effects the greatest swelling of a lightly crosslinked, insoluble sample of the polymer. The of a polymer is typically precise to no better than about ~ 0.2 units.
Dense, as in dense film refers to the absence of voids >~5 angstroms in diameter within the typical structure. However, the thin skins of inte-grally-skinned membranes, although dense on the whole, invariably contain a few large defects which must be sealed if intrinsic ~ is to be obtained.

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An asymmetric or an integrally-skinned membrane is a two or more layered membrane of the general class first prepared by Loeb-Sourirajan consisting of a thin dense skin supported by a thick porous substructure (matrix ) in which both layers are formed from a single sol by a phase inversion process.
First heat Tg is recorded at the first excursion of a differential scanning calorimetry (DSC) sample through the Tg range and may differ from that obtained in subsequent heats. If the first heat Tg is higher than that of the bulk polymer, and no crosslink-ing, etc. has occurred, it is evidence that the sample contained a free volume higher than that of the bulk polymer. If the first heat Tg is lower, the presence of residual solvent may be indicated. All of the information relative to increased free volume in a given membrane is lost after the first heat since increased segmental mobility at the Tg leads to loss of excess free volume.
Free volume (Vf) is the volume in a dense film which is available for gas permeation; Vf = Vt -VO where Vt = total macroscopic volume and VO = volume occupied by vibrating macromolecules. The Vf fraction in a typical glassy polymer is ~ 0.025 at its Tg.
Glass transition temperature (Tg) is the temperature at which the thermal expansion coefficient changes during the course of proceeding from the glassy to the rubbery state. The viscosity at the Tg = 1013 poises. Theoretically at a value of 51.6 C
below the Tg, the viscosity becomes infinite. The first heat Tg increases with increasing Vf.
Gutmann acid (AN) and base (DN) scales provide a numerical ranking of the relative strengths of Lewis acids and bases, respectively.
Macrovoids - the large (10-100 ~m) diameter voids found with the normal sponge-like matrix which .

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itself consists of small 0.1-2 ~m diameter open cells.
Macrovoid-containing membranes such as those utilized for ultrafiltration by Tsugaya et al are unsuitable for gas separations. The asymmetric gas separation membranes having graded density skins according to the invention are macrovoid-free or essentially macrovoid-free membranes. Macrovoids originate from sols which are metastable at the sol gel transition as a result of being too fluid or too far removed from the point of incipient gelation (PIG) so that sol inhomogenei-ties can arise which are then frozen into the membrane structure after gelation as discussed by R. E. Kesting, SYnthetic PolYmeric Membranes - A Structural Perspec-tive, 2nd Edition, Wiley-Interscience, 1985.
Non-solvent as used here is a liquid which even when heated to ~100C will not dissolve more than a trace of the polymer. A weak nonsolvent is defined as having ~(~ nonsolvent ~ polymer) < ~6 cal /cm and generally speaking, nonsolvent tolerance increases as ~ decreases. A strong nonsolvent conversely is defined as having a ~ >~6 cal1'2/cm3~ 2 Permeability (P) is the rate at which a gas traverses a membrane p = vol thickness = cc(STP) cm time area pressure sec cm2 cm Hg P is also expressed as P/Q (P . thickness) to remove any uncertainties as to actual skin thickness.
Phase inversion is the general process of membrane manufacture that utilizes a sol which inverts into two interdispersed liquid phasesi polymer coated micelles of the dispersed phase in a continuous second liquid phase, prior to, or simultaneously with, gelation at which time the emulsoid nature of what once was a sol is immobilized as a gel.
Potential is the unrealized capacity for exhibiting a certain property. The potential for :: . - . . , : :

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efficient air separation means that the uncoated membrane has sufficient fineness and uniformity of structure and sufficient free volume in the skin, that after coating with a permeable polymer, defects will be sealed and the fiber will yield high permeabilities and approach intrinsic selectivity.
Skin is the thin (~2500A) dense layer ob-servable with scanning electron microscope (SEM) which is found in integrally-skinned (asymmetric) membranes.
A skin can be of uniform density in which case the skin and active separating layer are identical, or it can be of graded density in which case the active layer thickness is less than the skin thickness.
Preferred hydrophobic materials or polymers as used in this description refer to water absorption values of between 0.02% and 1.0% after immersion in water for 24 hours of the materials or polymers as determined by ASTM D-570 6.1 standards; however, it is possible that other hydrophobic materials and polymers with higher water absorption value could be formed into the membrane morphologies of this invention.
Acid gases as used herein are present in most natural gas streams and are comprised of carbon dioxide and hydrogen sulfide. Many gas streams, particularly those used in a refinery or for manufac-turing gases, may contain mercaptans, carbon disulfide and carbonyl sulfide; however, these are usually present only as byproducts of some manmade process which involves natural gases containing originally carbon dioxide and hydrogen sulfide.
Wet process is that variation of phase inversion in which gelation occurs as the result of incompatibility which is effected by the immersion of the sol in a nonsolvent gelation medium such as water.

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DESCRIPTION OF THE DRAWINGS
For better understanding of the graded density skinned, asymmetric, macrovoid-free gas separation membrane having increased free volume, according to the invention, reference is made to Figures 1, 4 and 5 include both comparative and inventive references; Figures 3, 6, 8 and 10 present membranes in accordance with the invention; and Figures 2, 7 and 9 present various prior art mem-branes. Prior art Figure 2 illustrates a typical ultrafiltration membrane as shown by the photograph taken through the use of a scanning microscope magni-fication of 160. Figure 3 is a photograph taken through the use of a scanning microscope at a magnifi-cation of 170 which depicts a membrane which is in accordance with the invention. As can be seen in comparing Figures 2 and 3, the ultrafiltration mem-brane is macrovoid-ridden whereas the gas separation membrane is macrovoid-free.
Figure 1 is a schematic representation of asymmetric membranes according to the prior art and the graded density skinned, asymmetric, macrovoid-free gas separating membrane according to the invention.
Figure 4 represents the various performances of art taught membranes, i.e., asymmetric solvent spun and asymmetric solvent/nonsolvent spun gas separation membranes for pure gas, oxygen permeabilities at various temperatures as compared to the performance of the graded density skinned, asymmetric gas separation membranes of the invention.
Figure 5 is a graphic interpretation of the data collected when comparing art taught asymmetric gas separation membranes with uniform skins and porous substructures as compared to graded density skinned, asymmetric gas separation membranes with active layer and transition layer skin zones and a porous .. . . .. : .. :~

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substructure when the membranes are oxygen plasma etched and tested for separation factor using a helium and nitrogen feed stream.
Figure 6 represents the IR spectrum of a solvent system comprised of equal molar mixtures of NMP and propionic acid showing the NMP:PA complex at approximately 1653, the NMP solvent at 1690 and the propionic acid at 1732 cm~1 vs absorbance.
Figure 7 is a photograph taken through the use of an electron microscope at a magnification of 50,000 of an asymmetric, bilayer, uniform-skinned, porous substrate gas separation membrane taught by the prior art.
Figure 8 is a photograph taken through the use of an electron microscope at a magnification of 50,000 of a membrane in accordance with the invention, i.e., a graded density skinned, asymmetric gas separa-tion membrane having an active layer, a transition layer and a porous substructure.
Figure 9 is a photograph taken through the use of a scanning electron microscope at a magnifica-tion of 50,000 of a cross section of a portion of the porous substructure of an asymmetric, bilayer gas separation membrane taught by the prior art.
Figure 10 is a photograph taken through the use of a scanning electron microscope at a magnifica-tion of 50,000 of a cross section of a portion of the porous substructure of an asymmetric, trilayer gas separation membrane in accordance with the invention.
ULTRAFILTRATION VS GAS MEMBRANES
Ultrafiltration (UF) is that portion of the spectrum of pressure-driven membrane separation process which concerns itself with the separation of macromolecules from microsolutes or less successfully with the separation of macromolecules of different sizes. The pore diameter (d) range which is ' - 1 3~1 ~g.7 -19- 36-21(5441)A

encountered in UF is lOA<d<lOOOA. The nature of the normal pores in UF membranes differs in kind from the nature of the normal pores which are encountered in gas separation membranes. In the latter the normal pores are only average interchain displacements (D) between macromolecules. This point is of importance because it must be realized that there is no necessary connection, inherency, between polymers and solutions which are useful for the preparation of the two types of membrane. Thus a list of weak nonsolvents which may prove useful in the preparation of polysulfone ~PS) membranes for UF may coincidentally contain one or more nonsolvents which may also prove useful in the preparation of PS membranes for gas separations. In the case of Tsugaya, et al., a list of nonsolvents is given, most of which are unsuitable for use in the preparation of gas separation membranes. However, by coincidence, two nonsolvents, acetic and propionic acids, are listed which are useful in the preparation of gas separation membranes. The great majority of the weak nonsolvents listed in Tsugaya, et al., result in membranes which do not have the potential for efficient separation of gases. Fundamental differ-ences exist between gas separation and UF membranes and are as follows:
1) Gas separation membranes having graded density skins have the potential for intrinsic ~s.
This potential is realized when the membranes are coated with a highly permeable polymer such as a silicone. This potential is absent in the UF mem-branes because the latter contain large pores which cannot be sealed.
2) Gas separation membranes are strong and macrovoid-free with low water permeability; while UF
membranes are fragile and macrovoid-ridden with high water permeability (Fig. 2).

~, . --; . , : . , .. - .

.
~-' -20- 36-21(5441)A
(3) There are also differences between the coagulation (g) values of the sols from which the respective membranes are made. Graded density skinned membranes have g values < ~2(optimally < 1.5). The UF
sols of Tsugaya et al have g values > 3 (optimally 3 <
g < 7). Graded density skin membrane sols have high percent total solids for sulfone polymers, for exam-ple, > ~32% TS and high viscosity > ~ 50,000 cps @
70C (optimally > 105 cps @ 70C) whereas Tsugaya, et al., cite only sols with total solids < 17% and viscosities ~2000 cps @ 23C. Although Tsugaya, et al., claim sols with total solids 5% < TS < 40%, sols with concentrations substantially > those cited (~17%) will yield membranes with low permeability which are useless for UF. Likewise the concentrations of weak nonsolvents cited in Tsugaya, et al., are much lower than those cited for the graded density skinned process. In fact, Tsugaya et al explicity exclude sols with g values < 2 (polysulfone) and < 1.5 (poly-ethersulfone). Although one example (#12 Table 2) ofTsugaya et al indicates a g value of 1.7 for polysulfone, this value is actually 3.1 based on reproduction of the teachings of example 12. Several other examples of Tsugaya et al were also reproduced and found to be correct. The principal point, however, is not the g value per se which is after all "a working figure", but rather the fact that low viscosity UF sols require a higher g value in order to allow time for some densification prior to gelation to increase the strength of what would otherwise be a friable gel whereas the high viscosity gas separation sols require immediate gelation to avoid densification. Thus, the requirements for g values are opposed to one another for UF and gas separation sols. Low g values are precisely those which are of critical importance with respect to the preparation of gas separation membranes with the potential for intrinsic ~. Likewise some of the 1 3216~7 -21- 36-21(5441)A

nonsolvents cited by Tsugaya, et al., are too volatile (methyl and ethyl formates), too insoluble in either the organic solvent or water (benzoic and trimellitic acids) to be useful in the preparation of optionally useful gas separation membranes. Two of the "nonsol-vents" listed, trimethyl phosphate and triethyl phosphate are actually solvents for polysulfone.
MODERN ERA OF MEMBRANOLOGY

A good general reference for membrane technology is SYnthetic PolYmeric Membrane, A Structural PersPec tive, 2nd Edition, Wiley-I.nterscience, 1985 by R. E.
Kesting. Since the beginning of the modern era of membranology starting about 1960, a primary concern has been the necessity for decoupling permeability (P) from selectivity, i.e., separation factor (~). P and a for dense films of a given polymer and in another aspect for asymmetric films of a given polymer are usually considered coupled, that is, if P is speci-fied, ~ is determined and vice versa. This concept,at first consideration, suggests that dense films for example of glassy polymers possess "intrinsic" Ps and ~s. However, since the glassy state is a non-equili-brium condition, any intrinsic value will be strongly dependent on sample history so that the concept loses significance except within narrowly defined limits.
In the sections which will follow, both the product, the graded density s~inned membranes, and a process for preparing same, including the natures of the sols which exist prior to gelation are described.
Hereinafter, the graded density skinned membrane and processes for preparing the membrane can be referred to as "Pl". The nature of the former: high separation efficiencies, high free volumes, and skins of graded density are amenable to direct, even though difficult to measure, measurements which can be interpreted with a higher degree of confidence than those of the latter and which have been deduced from polymer solution , . ~
'., . :~ ~' ' ~" ~

13~2~

-22- 36-21(5441)A

theory. However, although the interpretations which follow are plausible and even probable, due to the complicated systems and sciences involved which we are concerned with here, it is impossible to exclude every alternative explanation. The interpretations given here are to teach the inventive findings in a reason-able and logical manner.
DETAILED DESCRIPTION OF THE INVENTION
A new class of integrally-skinned, asymmet-ric gas separation membranes has been developed.
These new asymmetric membranes which have graded density skins can be considered as trilayered mem-branes wherein the membranes offer significantly higher permeabilities with constant or improved selectivity as compared to the bilayer asymmetric membranes taught by the art, for example, Henis et al.
A bilayer is comprised of a porous substructure and a thin skin of uniform density. A graded density skin membrane possesses a porous substructure and a non-uniform, graded density skin which itself consists ofa very thin dense active or separating layer whose effective thickness varies depending upon the gas to be separated and a thin, less dense, but still essen-tially non-porous transition layer. This analogy is illustrated in Figure 1 which presents a schematic representation of bilayer asymmetric and trilayer asymmetric graded density skin membranes. Trilayer structure can be obtained by controlling the kinetics of gelation and desolvation through the use of a solvent system containir.g an equilibrium concentration of a Lewis acid and a Lewis base and a Lewis acid:base complex. The solvent system is capable of dissolving high concentrations of polymer and the complex is readily disassociated by polar medium. The resultant trilayer, although less dense than a standard bilayer, asymmetric membrane, is nevertheless sufficiently fine 132~8~
-23- 36-21(5441)A

and uniform that it exhibits the potential of attain-ing intrinsic selectivity which is realized when any defects have been sealed with a highly permeable material such as silicone.
In the case of dense films such as skins of integrally-skinned membranes, gas molecules permeate by diffusion between polymer chains. The volume occupied by these gaps is known as free volume (Vf).
The greater the free volume, the more rapidly gas molecules diffuse through the membrane, hence the higher the permeability (P). Free volume is thus a measure of the looseness of packing of the macro molecules. The Vf fraction of a glassy polymer ls ~
0.025 at the Tg as determined by S. Matsuoka, PolYmer Enqineerinq and Science , _ (4), 907 (1981).
In other words, only about 1/40 of the total volume of a typical dense glassy polymer consists of free volume. However, free volume is not limited to a precisely fixed or intrinsic value since the glassy state is a non-equilibrium condition. Therefore, since P is related to free volume, P is also variable.
It follows that there is no intrinsic P for a given glassy polymer except within narrowly defined limits.
The glassy state can be approached either by cooling a melt or a rubber or by gelling a sol.
Kinetic factors influence the magnitude of free volume. As a melt or rubber cools or a sol gels, viscosity increases until it reaches a value of ~ 101 5 cps at the Tg. Theoretically at a value of 51.6C
below the Tg, the viscosity becomes infinite and free volume is fixed at whatever value is obtained at that point. The faster the glassy state is attained, the less time the macromolecules have for rearrangement and densification and the higher will be the free volume. Quantitative determinations of free volume are accessible with difficulty since free volume is -~' ' ~;

~ 32~7 -24- 36-21(5441)A

such a small fraction of the total volume of a glassy polymer. Precise density measurements to discern the small differences between large numbers are difficult to obtain. Fortunately, differential scanning calo-rimetry (DSC) represents a straightforward method of ranking membrane free volumes since Tg increases with increasing free volume (S. Matsuoka). Because gela-tion and desolvation of the entire fiber takes place rapidly, it is assumed that the Tg of the fiber as a whole is essentially equivalent to the Tg of the skin.
It should be realized that both the high free volume and the telltale high Tg will only be present for the first heat excursion of a high free volume glass through the Tg as correlated by Matsuoka. After such a glass has passed through the Tg and cooled, some of its free volume will have vanished. If reheated, it will attain only lower Tg values characteristic of bulk polymers.
Until the present invention, only two options were available to increase the free volume in a given hydrophobic glassy polymer utilized for gas separations. First, membrane density can be decreased and free volume increased through swelling the film or membrane by exposure to C02 under pressure. Carbon dioxide under pressure acts as a swelling agent and at very high pressure, it can even act as a supercritical liquid solvent, thus the C02 lessens polymer-to-polymer interaction allowing the macromolecules to move farther apart from one another. For example, Erb and Paul, J. Membrane Sci., _, 11 (1981) illustrated that the C02 absorption of melt extruded polysulfone film is increased by exposure to CO2 to 60 atm. Although not explicitly stated by Erb et al, increased CO2 absorption was due to increased free volume. Second-ly, the polymer can be modified so as to inhibit close packing. Chern et al., :

.

132~87 -25- 36-21(5441)A

Materials Science of SYnthetic Membranes, ACS Sympo-sium Series 269, D. Lloyd, ed. 1985: p. 40, showed that whereas films prepared from polysulfone which utilizes bisphenol A exhibits a P for CO2 of 4.4 and an ~ for CO2/CH4 of 28, films prepared from polysul-fone made with tetramethyl bisphenol A have a P for Co2 of 21 with an ~ for CO2/CH4 of 68. The methyl groups in the latter polymer sterically inhibit a close approach between neighboring chains thereby increasing free volume and permeability.
Although both of the above approaches have been successfully applied to thick dense films, the practicality of their application to integrally-skinned membranes is uncertain. For example, CO2 might densify the porous matrix of a skinned membrane, thereby adding to overall resistance by increasing effective skin thickness or by densifying the entire porous substructure. On the other hand, polymer modification as in the second approach is an expensive proposition and can be accompanied by losses of physical properties. Nevertheless, this latter approach has been the one most favored by other researchers as opposed to the present invention wherein the morphology of a specific polymeric mem-brane is modified through, for example, Lewis acid:base complex solvent, dope formulation which provides changed morphology, i.e., graded density skinned, asymmetric membranes upon gelation in a polar medium.
Until now, however, attempts to increase permeability while maintaining skin integrity in asymmetric or integrally-skinned membranes of glassy, hydrophobic polymers, for example, aromatic sulfone polymers have been unsuccessful. Because glassy, hydrophobic polymers, the class most suitable for gas separation membranes, have much stronger polymer-polymer interaction than polymer-water interaction, it .
~, :

1~21~7 -26- 36-21(5441)A

is difficult to control densification when sols of such polymers are gelled in water. This leads to integrally-skinned membranes whose skins have rela-tively low free volumes and hence low permeabilities.
To retard this shrinkage and thereby to retain higher permeabilities, Cabasso et al, J. AP~l. PolYm. Sci., 20, 2377, (1986); 21, 165 (1977), immersed water-gelled polysulfone, ultrafiltration membranes in concentrated sulfuric acid. Tweddle et al, Ind. Eng. Chem., Prod. Res. Dev., 22 320 (1983) carried this approach a step further and gelled NMP
solutions of polysulfone in 25% aqueous sulfuric acid.
Because the acid neutralized the basic solvent, NMP, shrinkage was retarded and increased permeability was obtained. We realized that the interaction between sulfuric acid and the weakly basic solvent NMP would greatly decrease the solvent power of NMP. This would minimize the capacity of residual solvent for affect-ing plasticization and densification and could account for the increased permeability. To determine sulfuric acid interaction with NMP, the two were mixed in a 1:1 molar ratio. A large exotherm suggested the formation of a strong complex. The complex was light colored and appeared to be stable when kept in a closed bottle for several months. However, polysulfone was insolu-ble in the 1/1 molar mixture of sulfuric acid/NMP.
Eventually, a 37% by weight polysulfone sol was prepared in a sulfuric acid/NMP (5/95), the highest tolerable concentration of sulfuric acid in a solvent system. However, fibers spun from this sol exhibited low P/Qs and a high residual concentration of NMP/
sulfuric acid as illustrated by depressed Tg in Table The fact that permeability can be decoupled from separation factor of dense membranes by the preparatlon of an integrally-skinned membrane was 1321~7 -27- 36-21(5441)A

demonstrated for the first time by Loeb and Sourarijan.
The high P/Q values at essentially intrinsic ~ in the present invention and as illustrated in Figure 4 amounts to a second decoupling, this time by modifica-tion of the structure of the skin itself. As demon-strated by the elevated first heat Tgs of Pl membranes, the origin of the increased permeability is related to the increased free volume morphology in the membrane skin as illustrated in Table 7. Concomitant to the increase in permeability, selectivity is maintained constant, clearly demonstrating the pres-ence of a skin, the structure of which requires further elucidation.
A powerful technique to define the structure of the separation layer and the internal matrix is oxygen plasma ablation. Diatomic oxygen molecules are disassociated into nascent oxygen, a strong oxidizing agent, by a radio frequency generator. At low pres-sures, these moieties are sufficiently long-lived to react with a variety of polymers reflected by a loss of weight with time. The rate of ablation is depen-dent upon the chemical structure of the polymer as well as its radio frequency power level and the oxygen flow rate.
The technique is amenable for investigating hollow fiber membrane materials because ablation occurs only on the surface rendering it possible to sequentially remove the outer layers of a hollow fiber membrane; and oxygen plasma ablation is a low tempera-ture process allowing surface etching to proceed without altering the internal structure of the system.
Experiments were run on test cells of polysulfone spun from a 1:1 propionic acid/NMP complex with 37% by weight total solid dope and from an 87/13 formyl-piperidine/formamide (FP/FA) 37% total solids (TS) dope in a Low Temperature Asher 600 at 50 watts ;, ~ . . . .

1 321~87 -28- 36-21(5441)A

radio frequency at .9mmHg. After sets of test cells were ablated for increasing intervals of time, the samples were tested for helium and nitrogen permea-bility at 1.45 x 104 Pascals (Pa). The helium/nitro-gen separation factor plotted versus etch time inminutes is shown in Figure 5.
Figure 5 reveals that the 37% (87FP/13FA) polysulfone fiber membrane has a denser, more uniform skin than does the P1 hollow fiber membrane. This is reflected by the higher initial separation factor of the uncoated fiber and the plateau region in approxi-mately the first 30 seconds of etching. The drop in separation factor measured for the FP/FA polysulfone spun material in the .5 to 2 minute range indicates a transitional zone from a dense skin to the more porous matrix. The lower separation factors of the Pl hollow fiber membranes indicate the active (separating) layer is both thinner and more porous than the asymmetric polysulfone fiber which does not have a graded density skin. No plateau is seen with the Pl fiber which has the graded density skin. Instead a gradual decrease in separation factor with etching time is observed.
This lack of a plateau indicates the existence of a density gradient in the skin of the P1 membrane with skin density decreasing with increasing proximity to the skin/matrix interface. The densest layer of the P1 fiber skin exhibits sufficient fineness and unifor-mity and contains defects small enough in size and number that it can be said to possess the potential for intrinsic ~. At the same time the fact that P1 membranes are macrovoid-free contributes to the potential for intrinsic ~. Macrovoid-containing membranes do not possess the potential for intrinsic ~. This potential is realized after the defects have been sealed by coating with a highly permeable sili-cone polymer. Low residual solvent, as determined by : ~ :

- ~ . . .
..

1321~7 -29- 36-21(5441)A

gas chromatography and first heat Tg measurements, is essential to the maintenance of high free volume and to the characteristic of permeability low creep.
A major factor throughout the development of the P1 membranes has been the importance of acid:base, hydrogen bonded complexes in solvent mixtures employed in the dopes. In concert with studies on glassy-state structure and spinning technique development, a study has also focused on the nature and extent of complex formation processes in the solvent mixtures them-selves. Such studies included various spectroscopic and thermochemical probes aimed in part at distin-guishing the more promising P1 type solvent systems from more traditional solvent/nonsolvent systems.
Clearly, complexes of the Lewis acid and Lewis base of the appropriate strength and in sufficient concentra-tion are important. Also important are solvency characteristics relative to the given polymer.
Neither of these physical elements alone is sufficient to yield optimum membrane transport properties as found in P1. For example, solvent mixtures containing formamide with basic solvents such as NMP and FP which represent certain stages of the prior art in terms of their solvency characteristics do in fact form very weak complexes. However, the weakest complexes do not form a Pl type membrane from the coagulation spinning processes. Clearly then, the combined effect of complex strength and its solvency characteristics is dominant in this present invention.
Phase inversion essentially involves the structuring of a sol with the desired degree of swelling and order and the subsequent immobilization of the sol into a gel with a corresponding degree of swelling and order. In the present instance of high viscosity (> ~ 30%) total solids sol with low (< 2) g values, there is a very close correspondence between ..

~32~8~

-30- 36-21(5441)A

sol structure that exists immediately prior to, and the gel structure which exists immediately following, gelation. Gelation and desolvation take place very rapidly which helps to avoid sol inhomogeneities that lead to macrovoids thus minimizing densification owing to plasticization and allows skin density gradients to arise and to be maintained. Since the gel structures for this invention exhibit the potential for intrinsic ~s, it can be deduced that the sols are extremely fine and uniform so that the resultant gels also exhibit these traits, together with a minimum number of small and, therefore, sealable defects. The problem hence resolves itself into the origin of fineness and uniformity of the sol state. This is the point where deductions based on polymer solution theory come into play. A number of empirically established principles must be considered.
The stronger the polymer-solvent interaction and conversely the weaker the polymer-polymer interac-tion, the finer and more uniform will be the disper-sion of macromolecules or aggregates of macromolecules in a given sol.
Sols based solely on polymer and solvent generally gel and desolvate too slowly to produce membranes with the potential for efficient separations such as, for example, the graded density skin mem-branes according to the invention. In addition, local redissolution may occur with catastrophic consequences for gel uniformity.
The addition of nonsolvent poreformers to the sol decreases gel density which lessens the resistance to gas permeation of the resultant gels.
The stronger the nonsolvent (NS), that is, the greater difference (~) between the ~NS and the ~ polymer~ the greater the coarseness and local nonuni-formity within the sol. This trend is transferred to 1321~87 -31- 36-21(5441)A

the gel and if ~ is of sufficient magnitude can lead to the failure to achieve the potential for intrinsic ~. Although this tendency can be accommodated by minimization of nonsolvent concentration so that the ~ solvent system and the ~ 1 can be maintained within specified limits, this results in minimal sol swelling and, therefore, high gel density.
Furthermore, the limit of compatibility of strong nonsolvents in a sol is usually too low to permit an appreciable degree of sol swelling in the first place.
A strong nonsolvent is defined as one having a Q (~ _~ ) > ~ ~ 6 call'2/cm3~2. In nonsolvent polymer practice, allowable ~ s will only be positive since NS
with such a strong negative ~ will not be water misci-ble, a prerequisite for a wet process. For thisreason the concentrations of strong nonsolvents such as H20 (~ 24), formamide (~ 19) or glycerol (~ 18) usually cannot exceed 15-20% of the solvent system (Table 3). In comparison to the low tolerance of polymer solutions for strong nonsolvents, higher concentrations of weak nonsolvents can be incorporat-ed. A weak nonsolvent is defined as one with a ~< ~ ~
6 call'2/cm3~2. In this case also allowable ~s tend to be positive. The potential for low density (high permeability) gels increases when high concentrations of weak nonsolvent are utilized in the sols. However, as the ~ decreases, the nonsolvent, particularly at elevated temperatures, may assume the role of a swelling agent with adverse effects on the rates of gelation and densification. Thus the choice of the proper weak nonsolvent becomes a tradeoff between the use of nonsolvents which promote sol fineness and uniformity at the low ~ end and those which promote coarseness and rapid gelation and desolvation at the high ~ end.

13216~7 -32- 36-21(5441)A

Insofar as optimum gas separation membranes are concerned we have determined empirically that the potential for high permeability (low skin density) and high selectivity (macrovoid-free matrices and graded density skins with relatively small sealable defects) is not simply a matter of choosing a nonsolvent with the appropriate ~. A preferred process for forming the Pl membranes according to the invention utilizes a solvent system which contains a high concentration of Lewis acid:base complex (Table 3).
The formation of Lewis acid: base complexes can be written:
A + B = A:B
where A is the Lewis acid, B the Lewis base, and A:B
is the complex. Most commonly B is a solvent and A is a nonsolvent, for example, N-methyl pyrrolidone (NMP) is a solvent for polysulfone (PS) and propionic acid (PA) is a nonsolvent. However this is not universally true since A and B can both be solvents as, for example, when A is 2-chloropropionic acid and B is NMP.
The efficiency of a membrane gel appears to bear a relationship to the compatibility of the polymer in the solvent system (SS) provided the solvent system contains a high concentration of A:B
complex (Table 3). Here compatibility is considered proportional to ~-1 where ~ ss ~~ polymerl instance the polymer is PS (~ 10.55). The highest efficiencies are obtained when the ~ ~ ~ 1.3 cal~2/cm3'2 and poor efficiencies are obtained for ~ > ~
1.5. The utilization of solvent systems which do not form strong A:B complexes results in less efficient membranes even when the ~ is such as to indicate a high degree of compatibility (Table 2).

l~321~8~ 36-21(5441)A

It is hypothesized that this difference between the efficacy of sols which contain A:B com-plexes and those which contain solvents and non-solvents which form only weak complexes or no complexes lies in the greater uniformity on the molecular level of the former. The A:B complexes exist in equilibrium with A and B and possibly act to compatibilize free A and B in much the same manner as a graft copolymer of polymer A and polymer B acts to compatibilize a mixture of polymers A and B. In the absence of strong interactions between nonsolvent and solvent, weak thermally labile complexes may form or perhaps oligomeric clusters of nonsolvents may persist which may lead to sol nonuniformity and ultimately to gel nonuniformity. The existence of weak complexes and/or nonsolvent clustering may have the effect of leaving solvent molecules freer to engage in polymer-solvent interaction with adverse effects on the rates of gelation and desolvation. According to this point of view the role of the A:B complex is to maximize sol compatibility and uniformity at the molecular level without at the same time permitting an increase in polymer-solvent interaction.
The properties of the A:B complexes will now be considered. The ranking of the strengths of Lewis acids and bases has been affected by V. Gutmann, et al., Monats Chem., 106 1235 (1975) and 99 335 ~1968) in his acceptor number (AN) and donor number or donicity (DN) series, respectively (Tables 4 and 5).
Both of these lists have been augmented for the present study. The strength of A:B complexes was also estimated from the magnitude of the IR frequency shifts of the C = O bands of amides or in the case of DMSO and TEP of the shifts of the S = O and P = O
bands, respectively (Table 6, Fig. 6).

~, ' 132~7 -34- 36-21(5441)A

Acids which yield useful complexes appear to fall in the range ~ 47 < AN < ~ 53 with 2-chloropro-pionic acid (AN 62.4) being an outlyer of marginal interest. However not every Lewis acid in this range forms a useful complex. Glycerol, for example, although exhibiting an AN of 49.4 has only a modest ~v of ~ 15 cm~l whereas useful acids have a ~v of ~ -25 to -38 cm~l (Table 6). All useful acids to date are not only Lewis acids (electron pair acceptors) but also Br0nsted-Lowry acids (proton donors) (Table 7).
However, even in this category many ineffective species are encountered. Too strong an acid such as H2SO4 yields an A:B complex which is not only poorly compatible, but which will not readily disassociate in water and hence will be retained by the membrane.
Inadequate water solubility (benzoic and glutaric acids) results in inefficient removal and hence in poor membrane performance. To date the only accept-able acids which have been found have 12 < ~ < 13.5 optimally 12 < ~ < 12.5. Of these propionic- and butyric->acetic acid are preferred and for aesthetic reasons propionic acid is the best overall choice.
The Lewis bases which ultimately result in efficient gas separation membranes appear to have 27 <
DN < 28 and 11 < ~ < 12 which makes the ~5 (~SS ~
~polymer) <~1.5 cal 1/2/cm3/2 (Table 3). This group includes the amide solvents NMP, DMAC, FP, (N-methyl piperidone, N-methyl caprolactam). The species in parentheses are not commercially available, but because of their close relationship to NMP, it is felt that they would also be acceptable bases. The ~s for the aromatic sulfone polymers are taken as : Udel~
polysulfone (10.55), Radel~ polyphenylsulfone (~11) and Victrex~ polyethersulfone (~11.5).
The kinetics of gelation and desolvation are an important practical consideration in the design of 1321~87 -35- 36-21(5441)A

a particular sol for use in the P1 process. This is true because the amount of free volume retained depends on how rapidly the sol viscosity reaches infinity (gelation) and how rapidly the solvent is removed from the gel, thus minimizing post gelation densification. The closer the sol is to the thermody-namic point of incipient gelation (PIG), the more rapidly it will gel and the higher will be the re-tained free volume. The PIG is a function of tempera-ture and at any given temperature of the residualsolvent power, a measure for which is non-solvent tolerance. Because it is difficult to titrate the nonsolvent tolerance of a highly viscous sol, a dilute solution of polymer in a given solvent is employed instead. A convenient test for proximity to PIG is the coagulation value (g) which is defined as the grams of water which when added to lOOg of a 2%
solution of the polymer at 30C will promote phase separation (turbidity). For UF membranes where low total solids (TS) are utilized, low viscosity and the resultant macrovoids are permissible, a g optimally between 3 and 7, is preferred. This is the case because in low TS sols, gelation is deliberately retarded so as to allow densification and strengthen-ing of what would otherwise be an extremely fragilegel. However retarded gelation and low viscosity lead to sol inhomogeneities which in turn lead to the formation of macrovoids. For gas separation mem-branes, however, where the presence of macrovoids is detrimental, g values < 2 in the case of PS in PA:NMP
are preferred (optimally 0 < g < 1.5). A lower g is required for gas separation membranes than for UF
membranes because gelation must be extremely rapid to prevent the formation of the sol inhomogeneities which can lead to the formation of macrovoids and subsequent unsealable surface defects. For the same reason, sol ':

-36- 36-21(5441)A

viscosity and total solids must be higher for gas separation membranes. All of these interrelated parameters act to minimize the formation of sol inhomogeneities and macrovoids.
The fact that a high A:B molar ratio and consequently a low nonsolvent tolerance (= low g value) must be present to ensure optimum results for gas separation membranes can be seen from Table 8.
For the case of PS sols in PA:NMP the best results, i.e., highest 02P/Qs plus highest ~O2/N2 are obtained when the ratio of acid to base lies between approxi-mately 0.75 and 1.1 with g values < 2. It should also be noted that reproducibility suffers at lower A:B
ratios. This is a result of the higher g values which lead to sol inhomogeneities during the sol gel transi-tion. However, for the case of PS and other polymers in other solvents the optimum value of A:B ratio may be lower if the solvent power of the A:B complex is insufficient to permit the preparation of high TS
sols. Sols with g values < 2 were expressly excluded from earlier claims for sols which were useful for the preparation of UF membranes. This is further evidence that there is no inherency between nonsolvents for sols which are useful for UF membranes and those which are useful for gas separation membranes.
The reinforcing roles of viscosity, TS, and high PA/NMP ratio to eliminate macrovoids are shown in Table 9. In an earlier disclosure aimed at producing UF membranes a 17% TS PA/NMP molar ratio 0.44 was used to prepare macrovoid-containing UF membranes. This same sol was spun and resulted in high population macrovoid-containing fibers which were completely unsuitable for gas separations. At this PA/NMP ratio even when the TS are increased to 32% and even 37%, ~
remains low. It follows that high TS, high viscosity and a high PA/NMP A:B ratio are all required to , . . , . ~.

-37- 36-21(5441)A

produce macrovoid-free PS fibers which are suitable for gas separations. The g value (nonsolvent toler-ance) for a PA:NMP A:B ratio of 0.44 is 4.4 which is considerably greater than the optimum 0.5 < g < 1.5 for gas separation membranes. Table 9 thus adds further support to the irrelevancy to gas separation membranes based on conclusions from formulations which have been made regarding UF membranes.
COATING CONSIDERATIONS
The typical structure of the skins of integrally-skinned gas separation membranes prepared by phase inversion process is either similar to that of thick dense films or of dense films with a density gradient. However, the skin of every such membrane contains by virtue of the fact that micellar coales-cence is never complete, a small number of defects or surface pores. It has been established empirically that the P1 membrane possesses the potential for intrinsic ~. In other words, however large or numer-ous the defects, such defects are adequately sealableby known coating technique (Henis et al).
P1 membranes can be produced from various processes and polymers; however in this work P1 membranes were produced by the Lewis acid:base complex solvent system dopes of aromatic sulfone polymers (Table 11). The first heat Tgs of membranes made from sols containing Lewis A:B complexes are substantially higher than those prepared from sols containing noncomplexing nonsolvents. The superiority of the permeation characteristics of P1 type membranes is apparent in each case.
Comparison of transport properties for various gases utilizing P1 coated membranes versus coated Henis et al type membranes are illustrated in Table 11. Two aromatic sulfone polymers (polysulfone and polyethersulfone) were tested for various gas .

l 321687 -38- 36-21(5441)A

permeabilities in the Pl and non-Pl form. Table 11 is not complete in every comparison since some data was not available under comparable test conditions.
EXPERIMENTAL DETAILS
Fiber Spinninq All of the hollow fibers recorded in the following tables were spun by a standard wet spinning process. Deaerated sol (dope) was delivered at a rate of up to 20 ml/min to a tube-in-orifice type spinner-ette. The spinnerette was maintained at a temperature between 15 - 100C by the application of sufficient heat to maintain adequate sol flow. Water was inject-ed into the fiber lumen at a rate of up to 10 ml/min to form the nascent hollow fiber which was drawn at a rate of up to 100 m/min. The fiber is drawn through water baths maintained at temperatures of up to ambient and above, but less than about 50 C. The hollow fiber is then washed with water. The fiber is then wound onto a bobbin and washed for up to 7 days in running water. Hanks of hollow fibers are then formed by skeining the fiber from the bobbin. These hanks are then hung vertically and dried rapidly at about 100C.
Hollow Fiber PermeabilitY Testinq Except where otherwise noted the permeabili-ty and selectivity values were obtained from 1" x 1' elements each containing ~1200 hollow fibers. The separators were coated from a 1% Sylgard solution in isopentane. Coated separators were then placed in a pressure vessel, and the oxygen and nitrogen flux rates were measured at 50C under 2.9 x lO~ Pa using a dry tank air feed. Permeability was calculated using equation (l) P/Q = Q x C.F.(14.7 psi/atm) n~ dt~P(76 cmHg/atm)(60 sec/min) ::: ~

: ~ .

1 321~87 -39- 36-21(5441)A

where Q = the apparent gas flow reading from the mass flow meter (cm3(STD)/min); CF = a conversion factor for the specific gas to change the apparent flux to the actual flux; n = the number of fibers; d =
the outer diameter of the fiber (cm); t = active fiber length; P = the permeability of the skin layer; and = the thickness of the effective layer.
DSC - First Heat Tg Duplicate samples were tested on a DuPont 1090 DSC at a 20C/min heating rate through and beyond the Tg to 250C. After the first heat, the samples were cooled and subsequently reheated. Precision of the measurements is at least ~1C.
Residual Solvent Selected samples were evaluated as to residual solvent on a Perkin Elmer Sigma 115 Gas Chromatograph. Fiber samples dissolved in CH2Cl2 are compared to calibration standards containing known amounts of the solvents being assayed.
2 Plasma Etching Test cells were prepared from hollow fiber membranes spun from 37% total solids Udel 3500 PS in 43 propionic acid:57 NMP (/OW/W) and 13 formamide/87 formylpiperidine (,~OW/W). Each cell contained ten hollow fibers approximately 15 cm in length. The test cells were divided into two sets. The first set contained sixteen test cells of each type of hollow fiber. Duplicate samples of each type were placed in a Tracerlab Low Temperature Asher cell for exposure to an oxygen plasma for 0.5, 1.0, 1.5, 2.0, 3.0, 5.0 or 10.0 min. Duplicate samples of each type remained unexposed controls. The plasma was maintained with a radio frequency of 13.6 megaherz at a power level of 50 watts. The oxygen flow rate was 20 sccm. These samples were measured for their uncoated helium and 13216~7 -40- 36-21(5441)A

nitrogen permeabilities at 1.45 x 104 Pa at room temperature.
Infrared Spectra of Solvent SYstems IR spectra of pure and mixed solvents were recorded at ambient temperature on a Nicolet MX-l Fourier Transform Spectrometer. Sample liquids were examined as the capillary films pressed between the surfaces of calcium fluoride optical flats and in some cases as dilute solutions in cyclohexane using 0.1 mm fixed path length cell with CaF2 windows. Samples were examined under N2 gas atmosphere, corrected for any residual atmospheric CO2 absorptions, and analyzed digitally using Nicolet DX-l software programs.
Resolution was better than 2 cm~'.
Acceptor Number (AN) Values Using experimental procedures analogous to those of Gutmann, U. Mayer, V. Gutmann and W. Gerger, Monats. Chemie, 106, 1235 (1975), AN values were determined from p3 1 NMR measurements of (C2H5)3PO
dissolved at low concentrations in various compounds.
Spectra were recorded on a Varian Instruments XL-200 Fourier Transform NMR Spectrometer, operating at 81 MHz for detection of phosphorus - 31. Broad band proton decoupling was used to collapse the p3 1 mul-tiplet signal of (C2Hs)3PO to a single peak, whosespectral position reflects the degree of interaction between (C2Hs)3PO and a given sample compound. Peak positions were corrected for differences in bulk diamagnetic susceptibility between solvents and were reproducible to better than 0.1 ppm.
Donor Number (DN) Values Using experimental procedures analogous to those of Gutmann, V. Gutmann and A. Scherhaufer, Monats. Chemie, 99, 335 (1968) DN values were deter-mined calorimetrically from the exothermic enthalpy(-~H) of interaction between sample and antimony : .

- ~ ~
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-41- 36-21(5441)A

pentachloride, in dilute solution in 1,2-dichloro-ethane at 25 C. Measurements were run on a Setaram Model C80 Heat Flux Calorimeter, using a two compart-ment mercury sealed cell. Thermal data were analyzed using Astra Scientific software programs for digital integration of the heat flux signals.

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1321~87 -45- 36-21(5441)A

GUTMANN ACCEPTOR NUMBERS (AN) Compound ANa,b CF3SO3H 129.1G
CH3SO3H 126.3G
CF3CO2H 105.3 formic acid 83.6 2-chloro-propionic acid 62.4 lactic acid (85%) 55 9M
water 54 8M

acetic acid 52.9 , 52.6 glycerol 49 4M
propionic acid 49.1 n-butyric acid 48.3 glutaric acid 48.2M
benzoic acid 47 1M
iso-butyric acid 47.0 methanol 41.3 formamide 39 8G
ethanol 37 1G
ethyl lactate 36.5 isopropanol 33.5 N-methyl-formamide 32.1 N-methyl-acetamide 27 3M
chloroform 23.1 ethylene carbonate 22.3M
dimethyl sulfoxide 19.3 sulfolane 19.2G
acetonitrile 18.9G
propylene carbonate 18.3 trimethyl phosphate 16.3G
dimethyl formamide 16.0G

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- - 13~1~8~

-46- 36-21(5441)A

TABLE 4 (Continued) GUTMANN ACCEPTOR NUMBERS (AN) Compound ~Na,b dimethyl acetamide 13.6 5 N-methyl-2-pyrrolidone 13.3 acetone 12.5 dioxane 10.8G
hexamethyl phosphoramide 10.6 tributyl phosphate 9.9 10 ethyl acetate 9.3G
tetrahydrofuran 8.0G
hexane 0-0 a) acceptor number AN determined from P31 NMR chemical shifts of (C2H5)3PO dissolved in the indicated compound, scaled to set AN
for hexane at zero and to set AN for antimony pentachloride at 100 .

b) G indicates AN values from V. Gutmann, et al. (1975, Monats, Chemie, 106, p. 1235) or Mayer (1979, Pure and Appl. Chem., 51, p. 1697). M indicates AN values established by experimental measurements using experimental procedures analogous to those of Gutmann.

-47- 36-21(5441)A

GUTMANN DONOR NUMBERS (DN) Compound DN ' triethylamine 61.0 5 ammonia 59~oG
hexamethyl phosphoramide 38.8 pyridine 33.1 water (liquid) ~33. G
N-formyl-morpholine 32 6M
10 diethyl acetamide 32.2 diethyl formamide 30.9 dimethyl sulfoxide 29.8 dimethyl acetamide 27.8 N-methyl-2-pyrrolidone 27.3 15 N-methyl-caprolactam 27.1G
N-formyl piperidine 27.0 dimethyl formamide 26.6G
triethyl phosphate 25 4 M c G N-methyl acetamide ' ~25 tributyl phosphate 23.7 20 trimethyl phosphate 23.0 M c N-methyl formamide ' ~23 tetrahydro furan 20.0 M,c ~ formam1de ~20 diethyl ether 19.2 methanol 19.0 water (gas) 18.0 25 ethyl acetate 17.1 acetone 17.0 methyl acetate 16.5 ethylene carbonate 16.4 propylene carbonate 15.1 30 sulfolane 14.8 dioxane 14.8 acetonitrile 14.1G
acetic anhydride 10.5 1321~87 -48- 36-21(5441)A

GUTMANN DONOR NUMBERS (DN) Compound DN ' nitrobenzene 4 4G
5 nitromethane 2.7G
benzene 0.1 1, 2-dichloroethane 0.0 carbon tetrachloride 0.0 hexane o.oG

a) Donor number DN determined from the calorimetric - ~Hrxn for antimony pentachloride with the indicated compound determined at high dilution in 1, 2-dichloroethane at 25C.

b) G indicates DN values from Guttmann, et al. (1968, Monats, Chemie, 99, p. 335) or Mayer (1979, Pure and Appl. Chem., p.
1697). R indicates DN values for l-formyl-piperidine provided by Reilly Chemical Company. M indicates DN values from Murphy, this work, using experimental procedures analogous to those of Gutmann.

c) Values estimated from trends (methyl substitutions) in other compounds.

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- 1321~87 -49- 36-21(5441)A

Infrared Shifts of Lewis Acid:Base Complexes Base Acid ~ v (C = O)( ) NMP 2-chloro-propionic acid -46 cm-NMP benzoic acid -40 NMP acetic acid ::
NMP propionic acid NMP butyric acid -36 to -38 NMP iso-butyric acid NMP glutaric acid NMP lactic acid (85%) -20 NMP phenol -20 NMP hexafluoro isopropanol -19 NMP ethylene glycol, glycerol -12 to -16 NMP ethyl lactate -15 NMP methanol -14 NMP isopropanol -10 NMP water -5 to -10 NMP formamide NMP N-methyl-formamide -5 to -13 NMP N-methyl-acetamide NMP ethylene carbonate negligible(d) NMP propylene carbonate (O to -5) Hydrogen Bonding Schematic ,CH2 .CH2 CE2 ~ = O + HX~-CH~ C = O ... H - X
CH2_N \ CH2_N\

a) ~ v is the observed shift of the C = O band of NMP observed in the IR spectra tvpically of 1:1 molar mixtures of the base NMP with various acids except in case of polyprotic acids, such as glutaric acid, ethylene glycol and glycerol, where 1:1 equivalent mixtures were also exa-mined. Reference point (zero shift) is taken to be the C = O band frequency in the IR
spectrum of pure NMP.

~ 32~7 -50- 36-21(5441)A

TABLE 6 (Continued) Infrared Shifts of Lewis Acid:Base Complexes Base Acid ~ v (C = O)( ) FP propionic acid -32 to -33 cm-FP water negligible(d) FP formamide (0 to -5) DMF acetic acid DMF propionic acid -25 to -26 DMF water -5 to -10 DMF formamide 0 to -5 DMAC propionic acid -31 DEAC propionic acid -35 DMSO propionic acid _37(C) TEP propionic acid -68 to - 80(C) BLO acetic acid d BLO propionic acid negligible( ) BLO water (0 to -5) BLO formamide ethyl acetate propionic acid negligible(d) acetone propionic acid (0 to -5) b) ~ v is the shift to lower frequency observed typically in 1:1 molar mixtures of base and acid.
For amides (FP, DMF, DMAC and DEAC), the esters (butyrolactone and ethyl acetate) and the ketone (acetone), the base C = O band shift is monitored.
c) For bases DMSO and TEP, IR band shifts were observed for the S = O and P = O groups, respectively.
d) Indicates IR spectra of mixture wherein bands appear to be simply super positions of spectral bands characteristic of separate pure components of the mixture. 5 -51- 36-21(5441)A

TABLE 6 (Continued) Infrared Shifts of Lewis Acid:Base Complexes * The following abbreviations are used throughout:

NMP = N-methyl-2-pyrrolidone DMF = N,N-dimethylformamide DMAC = N,N-dimethylacetamide DEAC = N,N-diethylacetamide FP = 1,formylpiperidine TEP = triethylphosphate BLO = y-butyrolactone FA = formamide PA = propionic acid DMSO = dimethylsulfoxide AA = acetic acid PS = polysulfone PES = polyethersulfone PPS = polyphenylsulfone 1321~
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1321~87 -55- 36-21 (5441)A

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1 3 ~ 7 -57- 36-21(5441) Table 12 illustrates the improvement in gas separation properties obtained for a styrene-acrylo-nitrile copolymer hollow fiber membrane fabricated from a Lewis Acid:Base complex solution over a state-of-the-art membrane prepared according to the teaching of Brooks et al in U.S.P. 4,364,759. In both cases a hollow fiber membrane was fabricated from a solution containing 36% by weight of a copolymer having 47% by weight acrylonitrile into hollow fibers as previously described under the headings Experimental Details -Fiber Spinning.
Fibers were then dried at ambient tempera-ture or slightly above (Ca 50C), and then fabricated into 1" x 1' test elements containing 600 to 1200 fiber lengths. These elements were then coated from a .5% Sylgard solution and tested as previously de-scribed. In this case, however, oxygen and nitrogen flux rates were measured at 30C and 1.4 x 104Pa ~P.

13~1687 -~8- 36-21 (5441)A

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1321~87 -59- 36-21(5441) Table 13 shows the improvement in gas separation properties obtainable through the use of the acid:base complex solvent of this invention over properties obtainable using conventional art for the fabrication of hollow fiber membranes from Ultem~
1000, (~ Registered Trademark of General Electric Company) a polyetherimide. This polymer has the chemical structure shown below.
10 ~ ~0 ~

Hollow fiber membranes were produced from 37% by weight polymer solution using techniques as previously described under the headings Experimental lS Details - Fiber Spinning and were fabricated into 1" x 1' test elements. These elements were coated with .5%
Sylgard and tested for oxygen and nitrogen permeant flux at 30C and 1.4 x 104 to 7.2 x 104 Pa aP. Insuf-ficient oxygen/nitrogen permeate gas flux was obtained at 7.2 x 104 Pa aP in the first case to obtain an accurate compositional analysis of the permeate gas.
Therefore a flux rate was obtained using single component gas feeds of helium and nitrogen. P/Q is then calculated as previously described.

- 1321681~

-60- 36-21 (~441 )A

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Claims (24)

1. A process for forming asymmetric gas separation membranes having graded density skins and macrovoid free morphology comprised of hydrophobic polymers, the membranes having increased free volume as evidenced by the membrane first heat Tg which is greater than the Tg of a bulk sample of the polymers, comprising:
(a) dissolving the polymers in a solvent solution of Lewis acids and Lewis bases and complexes formed of the Lewis acids and bases, wherein the Lewis acids, Lewis bases, Lewis acid:base complex solvent system has a HIldebrand parameter less than about 1.5 cal1/2/cm3/2 of the Hildebrand parameter of the polymer;
(b) creating a dope of the solvent system and polymers which is near the gel point, the dope comprised of greater than about 30% by weight total solids sol;
(c) configurating the dope into a form suitable for use as an asymmetric gas separation membrane;
(d) coagulating the formed dope;
(e) desolvating the resulting formed asymmetric gas separation membrane having graded density skin and macrovoid free morphology through rapid disassociation of solvent complex;
(f) washing the membrane at ambient temperatures in a polar medium; and (g) drying the membrane at temperatures of from about ambient to about 20°C below the glass transition temperature.

2. The process according to claim 1 wherein the Lewis acid, Lewis base and the Lewis acid:base complex provides a solvent system having a concentration of from about 0.75:1.0 to about 1.1:1.0 molar ratio of the Lewis acid:base, the concentration of the acid being suffic-ient to provide a spinning dope of the solvent system and the polymers which is near the gel point.

3. The process according to claim 2 wherein the ratio of the acid is sufficient to lower the coagulation value (g) to less than 2.

4. The process according to claim 1 wherein the Lewis acid, Lewis base, and the Lewis acid:base complex provides a solvent system having a concentration of from 0.75:1.0 to about 1.2:1.0 molar ratio of Lewis acid:base, the concentration on the acid being suffic-ient to lower the coagulation value (g) to less than 2.

5. The process according to claim 4 wherein the Lewis acid is comprised of monocarboxylic acids having from 2 to 4 carbon atoms per molecule.

6. The process according to claim 3 wherein the dope is comprised of greater than about 30% by weight of the polymer and the dope has a viscosity of greater than about 5x104 cps at membrane forming temperatures neces-sary to produce macrovoid free membranes.

7. The process according to claim 3 wherein the desolvating and washing of the formed membrane results in a membrane having less than about 0.5% by weight residual solvent.

8. The process according to claim 3 wherein the Lewis bases are comprised of amide solvents having DN
numbers of 27 to 28 and the Lewis acids have AN numbers of from 47 to 53.

9. The process according to claim 3 wherein the Lewis acids have Hildebrand parameters of from about 12 to about 13.5 cal1/2/cm3/2 and the Lewis bases have Hildebrand parameters of from about 11 to about 12 cal1/2/cm3/2

10. The process according to claim 1 wherein the Lewis base constitutes a solvent for the polymer and is selected from the group consisting of N-methyl-2-pyr-rolidone, N,N-dimethylacetamide, 1-formylpiperidine, N-methylpiperidone and N-methylcaprolactam.

11. A process for forming hollow fiber asymmetric gas separation membranes having graded density skins and macrovoid-free morphology comprises of hydrophobic polymers, the membranes having increased free volume as evidenced by the membrane first heat Tg which is greater than the Tg of a bulk sample of the polymers, compris-ing:
(a) dissolving the polymers in a solvent solution of Lewis acids and Lewis bases and complexes form of the Lewis acids and Lewis bases wherein the Lewis acid, Lewis base and Lewis acid:base complex solvent system has a Hildebrand parameter less than about 1.5 cal1/2/cm3/2 of the Hildebrand parameter of the polymer;
(b) forming a spinning dope of the solvent system and polymers which is near the gel point, the dope comprised of greater than about 30% by weight total solids sol;
(c) spinning the dope through a tube-in-orifice spinnerette;
(d) injecting coagulation fluid through the ori-fice;
(e) coagulating the spun dope in a coagulation medium and forming the membrane;
(f) desolvating the formed membrane through rapid disassociation of the complex;
(g) washing the membrane in a polar medium; and (h) drying the membrane at temperatures of from about ambient to about 20°C below the glass transient temperature.

12. The process according to claim 11 wherein the Lewis acid, Lewis base and the Lewis acid:base complex provides a solvent system having a concentration of from about 0.75:1.0 to about 1.1:1.0 molar ratio of the Lewis acid-base, the concentration of the acid being suffic-ient to form a spinning dope of the solvent system and the polymers which is near the gel point.

13. The process according to claim 12 wherein the ratio of the acid is sufficient to lower the coagulation value (g) to less than 2.

14. The process according to claim 11 wherein the Lewis acid, Lewis base, and the Lewis acid:base complex provides a solvent system having a concentration of from 0.7:1.0 to about 1.2:1.0 molar ratio of the Lewis acid:base, the concentration of the acid being suffic-ient to lower the coagulation value (g) to less than 2.

15. The process according to claim 13 wherein the Lewis acid is comprised of monocarboxylic acids having from 2 to 4 carbon atoms per molecule.

16. The process according to claim 13 wherein the dope is spun sequentially into a dry air gap and then into a coagulation medium.

17. The process according to claim 13 wherein the dope is spun as a wet process directly into the coagula-tion medium.

18. The process according to claim 13 wherein the injected coagulation fluid into the orifice of the spinnerette and the coagulating medium into which the dope is spun is comprised of water.

19. The process according to claim 13 wherein the spinning dope is degassed and filtered before spinning.

20. The process according to claim 13 wherein the spinning dope is comprised of greater than about 30 percent by weight of the polymer and the spinning dope has a viscosity of greater than about 5x104 cps at spinning temperatures necessary to produce macrovoid-free membranes.

21. The process according to claim 13 wherein the desolvating and washing of the formed membrane results in a membrane having less than about 0.5 percent by weight residual solvent.

22. The process according to claim 13 wherein the Lewis bases are comprised of amide solvent having DN

numbers of 27 to 28 and the Lewis acids have AN numbers of from 47 to 53.

23. The process according to claim 13 wherein the Lewis acids have Hildebrand parameters of from about 12 to about 13.5 cal1/2/cm3/2 and the Lewis bases have Hildebrand parameters of from about 11 to about 12 cal1/2/cm3/2.

24. The process according to claim 13 wherein the Lewis base constitutes a solvent for the polymer and is selected from the group consisting of N-methyl-2-pyrrolidone, N,N-dimethylacetamide, 1-formylpiperidine, N-methylpiperidone and N-methyl caprolactam.