Abstract:
The invention is a semi-permeable membrane which comprises a polymeric matrix with two porous surfaces and a region which functions to separate one or more gases from one or more other gases. 
     The membranes of this invention exhibit excellent separation factors and flux. Such membranes are less prone to being damaged due to handling and exposure to contaminants due to the internal region which affects the separation, as the porous surfaces function to protect such region.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to novel semi-permeable membranes useful for separating one or more gases from one or more other gases. 
     In various industries, it is necessary or highly desirable to separate one component from another in a gaseous stream. Processes used to perform such separations include pressure swing absorption, cryogenics, and membrane separations. In a membrane separation, a gaseous stream containing the components to be separated is contacted with a membrane, wherein the membrane separates two regions in a manner such that only those materials which permeate through the membrane can communicate from one region to the other. Such membranes are semi-permeable, in that one or more component of the gaseous mixture selectively permeates through the membrane at a rate much higher than one or more of the components in the gaseous stream. Among such separations are the separation of oxygen from nitrogen, and carbon dioxide from methane. The gaseous mixture is contacted with the membrane in a manner such that the selectively permeable species is preferentially transported through the membrane to the other region. The component which is non-selectively permeable may permeate through the membrane but at a much slower rate than the selectively permeable species. It is this difference in rates of permeation which is used to separate the gaseous species or reduce the concentration of the less selectively permeated species in the region to which the permeating gases permeate, or decrease the concentration of the more selectively permeated gas in the region from which the permeating gases permeate. 
     In such separations, the relative rate of permeation, that is, the difference in rate of permeation between the selectively permeating gas and the non-selectively permeating gas, is a major factor in the separation achieved. The higher the ratio of permeation of the selectively permeable gas over the non-selectively permeable gas, the better the membrane will perform. Therefore, it is desirable to have as high a ratio as possible. 
     Another important property of membranes is the permeability of the gases through the membrane. If the permeability is too low, the membrane may not provide enough flow through the membrane to be economical for separations. Some potential candidates for membrane separations provide good separation factors but low permeabilities for dense membranes. Flux is the volumetric flow of gas through a particular membrane for a unit area and time, and indicates the productivity of the membrane. The separation factor is the ratio of the permeabilities of the selectively permeating species over the non-selectively permeating species. One technique used to improve the flow of the permeating gases through the membrane is to form asymmetric membranes from such polymers. Asymmetric membranes comprise a membrane with a thin dense region wherein the separation is effected, and a larger region which is porous through which gases pass with little resistance which provides support for the thin dense layer. The discriminating region is much thinner than a solid or homogeneous membrane can be, as the porous layer provides the structural integrity of the membrane and supports the thin dense, layer. This thin, dense layer is located on one surface of the membrane. The formation of an asymmetric membrane with good separation factors and permeabilities is a difficult chemistry and engineering problem. As the thin, dense layer is on one of the surfaces of the membrane, this thin, dense layer is subject to being damaged by handling or exposure to contaminants. This damage can result in leaks in the membrane and render the membrane less effective in separating gases. 
     Presently, membranes derived from acetate esters, for example cellulose diacetate, and cellulose triacetate, polyamides, polyimides, and olefins, for example polyethylene, polypropylene, poly-4-methylpentene-1, are used for gas separations. Recently it has been discovered that bisphenol based polycarbonates, and polyestercarbonates wherein at least 25 percent by weight of the bisphenol moieties are tetrahalogenated, wherein the halogen is Cl or Br, exhibit excellent separation factors for the separation of oxygen from nitrogen, but exhibit low flux in the dense form. 
     What are needed are membranes with regions capable of separating one or more gases from one or more other gases which have both good separation factors and flux. What are further needed are membranes which have such regions which are not subject to damage due to handling or exposure to contaminants. What are further needed are membranes which exhibit good physical properties. 
     SUMMARY OF THE INVENTION 
     The invention is a semi-permeable membrane which comprises a polymeric matrix with two porous surfaces and a region which functions to separate one or more gases from one or more other gases. 
     The membranes of this invention exhibit excellent separation factors and flux. Such membranes are less prone to being damaged due to handling and exposure to contaminants due to the internal region which affects the separation, as the porous surfaces function to protect such region. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The membranes of this invention have two porous surfaces. More particularly, the membranes have two porous regions which start at the surface of the membrane and continue for some distance into the membrane. Such porous regions are capable of passing the desired gases to separate through such regions without much resistance. The pores on the surfaces are large enough such that gases freely pass through them without any resistance. Preferably, the pores on the surfaces are between about 250 and 10,000Å. In the embodiment wherein the membrane is a hollow fiber, the inner surfaces preferably have pores of from about 250 to about 10,000Å, and the outer surfaces preferably have pores of from 250 to 3,000Å. 
     The membranes of this invention comprise a porous layer on both sides of this membrane, i.e., both the exterior and the interior of a hollow fiber, with an interior region which is discriminating or functions as if it were dense, that is a permeant cannot cross from one surface of the membrane to the other without permeating into and through a non-porous or dense region of the membrane. Such a discriminating region may be a region of non-continuous porosity. In one embodiment of the hollow fiber form of this membrane the region of non-continuous porosity is located near the lumen of the fiber. 
     The critical feature of the invention is that such membranes function to separate one or more gases from one or more other gases. Preferably such membranes have an internal region which functions to separate one or more of the gases contacted with the membranes from one or more of the other gases contacted with the membranes. This region may be a dense region, a region of non-continuous porosity, or a region which resembles a closed cell foam. 
     The membranes of this invention may be prepared from any polymeric material which has inherent properties which pass one or more gases through its bulk phase at a faster rate than one or more other gases. Those skilled in the art would recognize which polymeric materials would be suitable. Preferable polymeric materials comprise polyimides, polycarbonates, polyesters, polyestercarbonates, polysulphones, polyethersulphones, polyamides, polyphenylene oxides, and polyolefins. More preferred polymeric materials comprise polyesters, polycarbonates, and polyestercarbonates. Even more preferred polymeric materials comprise polycarbonates. More preferred polycarbonates are those derived from a bisphenol wherein at least 25 percent of the bisphenol moieties in the backbone of the polymer are tetrahalogenated wherein the halogen is chlorine or bromine. The polymers useful in this invention should be polymerized to the extent that the polymers will form a membrane with sufficient mechanical strength to withstand use conditions. 
     The membranes may be fabricated in any useful membrane form, for example flat sheet, hollow fiber, or hollow tube form. The preferred form is the hollow fiber form. These membranes may be used in any form of membrane device, for example hollow fiber devices, hollow tube devices, spiral wound devices, and plate and frame devices. 
     The membranes of this invention may be used to separate components in a gaseous stream capable of being separated; such separations are well known to those skilled in the art. Preferred separations include the separation of oxygen from nitrogen, nitrogen from methane, carbon dioxide from light hydrocarbons especially methane, and helium and hydrogen from other gases such as light hydrocarbons. The preferred separation is the separation of oxygen from nitrogen. 
     The membranes of this invention may be prepared by the following process. A mixture of the polymeric material, a solvent for the polymeric material, and a non-solvent for the polymeric material is prepared. Such mixture is preferably homogeneous at extrusion temperatures. The mixture should be sufficiently viscous to retain its integrity until the membrane is formed. It is preferable that the mixture is close to the phase boundary between a one- phase mixture and a two-phase mixture, so the concentrations of the components should be chosen such that the mixture is near the boundary. If the non-solvent concentration is too low, the discriminating region will form on one surface of the membrane. If the non-solvent concentration is too high, the mixture will not be homogeneous and the membrane may have pores which communicate through the membrane. The polymer concentration should be high enough such that the mixture is sufficiently viscous to extrude and retain its shape at under extrusion conditions. If the polymer concentration is too high, the discriminating region will be too thick and the pores will be too small, thus reducing the flux through the formed membrane. The mixture is heated to, or above, the extrusion temperature. The temperature for the extrusion is that temperature such that the mixture has sufficient viscosity for extrusion, and which facilitates phase inversion of the material when exposed to the conditions of the quench zone or zones. The polymer mixture is extruded through a die of the desired shape into and through one or more quench zones, wherein one of the quench zones comprises a liquid which is a solvent for the solvent and non-solvent, and which has very low solubility in the polymeric material. The process is performed under conditions such that the polymer mixture undergoes phase inversion in one or more of the quench zones, wherein a phase rich in the polymeric material and a phase rich in the solvent and the non-solvent are formed, and the solvent and non-solvent is removed from both phases. 
     The preferred polycarbonates useful in this invention are derived from bisphenols wherein a significant portion of the bisphenols used to prepare the polycarbonates are tetrahalosubstituted; more preferably the tetrahalo substituents are found in the 3,5-positions on the aromatic or phenolic rings. The presence of a significant portion of the residue of tetrahalo bisphenols enhance the properties of membranes that are prepared therefrom. More particularly, such membranes have enhanced separation factors with respect to oxygen/nitrogen, hydrogen/methane, and carbon dioxide/methane separations. 
     More preferably the polycarbonates useful in this invention comprise polymers with backbone units which correspond to the formula ##STR1## wherein R at each occurrence is independently H, Cl, Br, or C 1  -C 4  alkyl; and, R 1  is carbonyl, --S--, --SO 2  --, --O--, a c 1  -C 6  divalent hydrocarbon, a C 1  -C 6  divalent halocarbon radical, or an inertly substituted C 1  -C 6  hydrocarbon radical, with the proviso that at least 25 weight percent of the bisphenol moieties in Formula I bear R groups which are exclusively Br, Cl, or mixtures thereof. 
     Preferably, at least 35 weight percent of the bisphenol moieties in the polycarbonate backbone bear R groups which are exclusively bromine, chlorine, or mixtures thereof. More preferably, at least 50 weight percent of the bisphenol moieties in the backbone bear R groups which are exclusively bromine, chlorine, or mixtures thereof. Even more preferably, at least 75 weight percent of the bisphenol moieties in the polycarbonate backbone bear R groups which are exclusively bromine, chlorine, or mixtures thereof. Even more preferably, the polycarbonate is derived from bisphenols where R is exclusively bromine, chlorine, or mixtures thereof. In the embodiment wherein the polycarbonate is prepared from tetrachlorobisphenols, it is preferable that the polycarbonate backbone contain about 90 percent by weight or greater units derived from tetrachlorobisphenols, more preferably 95 percent by weight, and more preferably 100 percent by weight. Bromine is the preferred halogen herein. Examples of preferred bisphenols which bear R groups which are exclusively Br or Cl are 2,2-bis(3,5-bromo-4-hydroxy-phenyl)propane and 2,2-bis(3,5-chloro-4-hydroxy-phenyl)propane with 2,2-bis(3,5-bromo-4-hydroxy-phenyl)propane being most preferred. Preferably those R groups which are not halogen are methyl or hydrogen, and most preferably hydrogen. 
     In the hereinbefore presented formulas, R is preferably chlorine, bromine, hydrogen or C 1-4  alkyl, more preferably chlorine, bromine, hydrogen, or methyl, even more preferably chlorine and bromine, and most preferably bromine. R 1  is preferably a C 1-6  divalent hydrocarbon, more preferably a C 1-6  alkylidene moiety, even more preferably an isopropylidene moiety. 
     The polycarbonates useful in this invention can be prepared by any process known in the art which prepares polycarbonates with suitable properties for membrane formation. See Encyclopedia of Polymer Science &amp; Technology, Editor Mark et al, Interscience Division of John Wiley &amp; Sons, N.Y., N.Y., 1969, Vol. 10, pages 714-725 (relevant portions incorporated herein by reference). The polymers useful in this invention should be polymerized to the extent that the polymers will form a membrane with sufficient mechanical strength to withstand use conditions. 
     In one preferred embodiment the halogenated bisphenol based polycarbonate membranes are prepared by the process which comprises: 
     (A) forming a mixture comprising 
     (i) a bisphenol polycarbonate wherein at least 25 percent by weight of the bisphenol moieties are tetra halogenated wherein the halogen is chlorine or bromine; 
     (ii) a solvent for the polycarbonate which comprises a glycol ether which corresponds to the formula R 3  O--(CH 2  CH 2  O) r  --R 3  wherein R 3  is methyl or ethyl, and r is an integer of between about 1 and 20 ; a dialkyl ketone wherein the alkyl groups independently are methyl or ethyl; morpholine substituted on the nitrogen atom with an alkyl, formyl or alkanoyl moiety; pyrrolidinone or N--C 1-4  alkyl, N--C 5-6  cycloalkyl, or N--C 6-10  aryl or alkaryl substituted pyrrolidinone; C 1-4  alkoxycarbonyl, formyl, nitro, or halo substituted benzene; tetrahydrofuran; dimethyl formamide, cyclohexanone; N,N-dimethyl acetamide; acetophenone; caprolactone; methylene chloride; sulfolane; cyclohexyl acetate; 1,1,3,3,-tetramethylurea; isophorone; 1-formyl-piperidine; methyl salicylate; hexamethylphosphoramide; phenyl ether; or bromonaphthalene; and, 
     (iii) a non-solvent for the polycarbonate which comprises a glycol or glycol ether which corresponds to the formula R 4  O--(CH 2  CH 2  O) q  --R 4  wherein R 4  independently in each occurrence is hydrogen or C 1-4  alkyl, and q is an integer of about 1 to about 250; an ester corresponding to the formula R 5  COOR 6  wherein R 5  is hydrogen or C 1-19  alkyl, and R 6  is C 1-10  alkyl; a C 1-10  alkanol; cyclo-hexane, unsubstituted or substituted with an alkyl, cycloalkyl, or perfluoroalkyl moiety; a C 5-20  alkane; a dialkyl ketone wherein at least one of the alkyl moieties is C 3  or greater; an amide corresponding to the formula R 7  CONHR 8  where in R 7  is hydrogen or C 1-10  alkyl and R 8  is C 1-10  alkyl; an acetyl or C 1-10  alkyl nitrile; acetone; a C 1-10  alkyl aldehyde; a trialkyl amine; nitromethane; trialkyl orthoformate; diacetone alcohol; dimethyl malonate; decahydronaphthalene; tetrahydronaphthalene; malononitrile; dicyclophexyl; ethylene carbonate; sulfolane; alkyl or cycloalkyl substituted benzene; or water; 
     (B) heating the mixture to a temperature at which the mixture forms a homogeneous fluid and is extrudable; 
     (C) extruding the heated mixture into a shape suitable for membrane use; and 
     (D) passing the formed membrane through one or more quench zones, wherein the mixture phase separates, and the major portion of the solvent and non-solvent are removed from the formed membrane wherein one of such quench zones comprises a liquid which has a very low solubility in the polycarbonate,; 
     wherein the membrane formed has a porous outer and inner surface with a discriminating region capable of separating oxygen from nitrogen. 
     The polycarbonate mixture may be extruded into any shape which is useful as a membrane. Such shapes include flat sheets, hollow tubes, and hollow fibers. The most preferred shape is the hollow fiber shape. The process for preparing this preferred shape may be described as follows. The following description of the process with respect to the formation of hollow fiber membranes refers to one fiber, but the process may be performed on one fiber at a time or a multitude of fibers simultaneously. In fact, most hollow fiber preparation processes involve forming several fibers and processing them simultaneously. The description shall be understood to include forming and processing one fiber or a multitude of fibers simultaneously. 
     A process for preparing a hollow fiber comprising a tetrahalogenated bisphenol polycarbonate which comprises: 
     (A) forming a mixture comprising 
     (i) a bisphenol polycarbonate wherein at least 25 percent by weight of the bisphenol moieties are tetrahalogenated wherein the halogen is chlorine or bromine; 
     (ii) a solvent for the polycarbonate as described hereinbefore; and, 
     (iii) a non-solvent for the polycarbonate as described hereinbefore; 
     wherein the mixture has a sufficient viscosity to allow extrusion at temperatures at which the mixture is homogeneous; 
     (B) heating the mixture to a temperature at which the mixture forms a homogeneous fluid and is extrudable; 
     (C) extruding the heated mixture into a hollow fiber form; 
     (D) passing the formed fiber through one or more quench zones wherein the mixture phase separates, and the major portion of the solvent and non-solvent are removed from the formed fiber, while a core fluid is passed down the hollow core of the fiber under conditions sufficient to prevent the fiber from collapsing, wherein one of the quench zones comprises a liquid which has low solubility in the polycarbonate; and, 
     wherein the fiber formed has a porous inner and outer surface and the fiber is capable of separating oxygen from nitrogen. 
     Preferably, the polymer solvent non-solvent mixture has a viscosity at extrusion temperatures of about 10,000 to about 200,000 poise in the embodiment wherein the core fluid is a gas, and more preferably between about 30,000 and 45,000 poise. The viscosities described herein are based upon rheometric measurements taken at 82° C. at a frequency of 1 radian per second. Preferably, the polymer used to prepare the membranes has a molecular weight (M w ) of 100,000 or greater, more preferably between 100,000 and 300,000. 
     Discriminating region refers to a region which functions to separate one or more gases from one or more other gases, and may be a non-porous region or the equivalent of a non-porous region, for example, a region on non-continuous porosity. &#34;Homogeneous fluid&#34; as used herein refers to a fluid which is a mixture of components and which is in one phase. Extrusion refers herein to passing a fluid of the polymer mixture through a die to form the fluid into the desired shape. &#34;Extrudable&#34; as used herein refers to a material which is capable of extrusion to form a desired shape, wherein the material formed to such shape once formed retains such shape. &#34;Quench&#34; as used herein refers to exposing the polymer mixture to conditions such that the polymer mixture partially or completely undergoes a phase separation. &#34;Phase separation&#34; refers herein to the phenomena wherein the polymer mixture undergoes separation into a polymer rich phase and a solvent-non-solvent rich phase. &#34;Leaching&#34; as used herein refers to the phenomena wherein entrained solvent and non-solvent liquids are removed from the polymer rich phase. 
     The polymer mixture, which is extruded to form the membranes of this invention, comprises the polycarbonate described hereinbefore, a solvent for the polycarbonate, and a non-solvent for the polycarbonate. The solvent functions to dissolve the polymer and the non-solvent into a homogeneous solution at the temperatures used for extrusion so that the mixture may be extruded. The non-solvent functions to aid in the formation of pores in the polymer when it undergoes phase separation in the quench zone. 
     An optional fourth component, a dissolving medium, may be added to the polymer mixture to aid in the formation of a homogeneous mixture. The dissolving medium is used to enhance the dissolution of the polymer into the solvent non-solvent mixture. Usually the dissolving medium is removed from the mixture prior to the extrusion, usually by flashing it off. 
     The solvent may be any solvent for the polymer from which the membranes are to be formed, which dissolves enough of the polymer so as to form a solution viscous enough to be extrudable at the extrusion temperatures. The amount of solvent used depends upon the polymer used, the non-solvent used, the desired properties of the membrane, and the method of quenching the fiber. 
     In the embodiment wherein the polymer is a tetrahalosubstituted bisphenol based polycarbonate, the following solvents are preferred. The glycol ethers useful as a solvent for the polycarbonate corresponds to the formula R 3  O--(CH 2  CH 2  O) r  R 3  wherein R 3  is methyl or ethyl, and r is an integer of between about 1 and 20. Preferably, r is an integer of between about 1 and about 10 , and even more preferably between about 1 and about 4, most preferably when R 3  is methyl r is between about 1 and about 4, and when R 3  is ethyl r is between about 2 and about 4. Examples of such glycol ethers include ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, and bis(2-methoxy-ethyl ether). Preferred dialkyl ketones useful as solvents for the polycarbonates include dimethyl ketone, diethyl ketone, and methyl ethyl ketone. Preferred substituted morpholines are those with a C 1-10  alkyl, formyl or C 1-10  alkanoyl moiety substituted on the nitrogen atom; more preferred are those with a C 1-4  alkyl, formyl or C 1-4  alkanoyl moiety substituted on the nitrogen atom. Examples of substituted morpholines include N-formylmorpholine and N-ethylmorpholine. Preferred pyrrolidinones useful as solvents include pyrrolidinone, N-methyl pyrrolidinone, N-ethyl pyrrolidinone, N-cyclophexyl pyrrolidinone, N-benzyl pyrrolidinone, and N-phenyl pyrrolidinone; with N-methyl pyrrolidinone and N-ethyl pyrrolidinone more preferred; and N-methyl pyrrolidinone most preferred. The term pyrrolidinone as used herein refers to compounds named as pyrrolidinones and pyrrolidones. Preferred substituted benzenes useful as solvents for the polycarbonates correspond to the formula: ##STR2## wherein R 9  is C 1-4  alkoxycarbonyl, nitro, halo or a formyl moiety; and b is an integer of about 1 to about 6, with the proviso that wherein R 9  is alkoxycarbonyl b is 1 . The preferred halogens are chlorine and bromine, with chlorine most preferred. Preferably, b is between about 1 and 3. Examples of substituted benzenes useful as solvents include chlorobenzene, dichlorobenzene, benzaldehyde, nitrobenzene, ethyl benzoate, methyl benzoate, and 1,2,4-trichlorobenzene. 
     Preferred solvents comprise N-methylpyrrolidinone, tetrahydrofuran, ethylene glycol dimethylether, diethylketone, N-ethylmorpholine, dimethylformamide, cyclohexanone, bis(2-methoxyethylether), N,N-dimethylacetamide, acetophenone, methylene chloride, or sulfolane. More preferred solvents include N-methylpyrrolidinone, ethylene glycol dimethylether, tetrahydrofuran, diethylene glycol dimethylether, acetophenone, methylene chloride, or cyclohexanone. The most preferred solvent is N-methylpyrrolidinone. 
     The non-solvent may be any compound which does not substantially dissolve the polymer from which the membrane is to be prepared at extrusion temperatures, is soluble in the solvent, and which aids in the formation of pores in the polymer rich phase when the spin mixture is extruded into a quench zone. The amount of non-solvent used depends upon the polymer used, the solvent used, the desired properties of the membrane, and the method of quenching the fiber. The particular non-solvents useful for each polymer are well known to those skilled in the art or readily ascertainable by those skilled in the art. 
     The following non-solvents are preferred for the polycarbonates. The glycols and glycol ethers useful as non-solvents for the polycarbonate correspond to the formula R 4  O-(CH 2  CH 2  O) q  -R 4  wherein R 4  is independently in each occurrence hydrogen or C 1-4  alkyl, and q is an integer or about 1 to about 250. Preferably R 4  is hydrogen. Preferably q is an integer of about 2 to about 100, more preferably of about 3 to about 60, and most preferably about 3 to about 15. Examples of preferred glycols and glycols ethers include 2-ethoxyethanol, polyethylene glycols with molecular weights of up to about 1450, triethylene glycol, diethylene glycol, diethylene glycol dibutylether. Esters useful as non-solvents correspond to the formula R 5  COOR 6  wherein R 5  is hydrogen or C 1-19  alkyl, and R 6  is C 1-10  alkyl. Preferably R 5  is hydrogen or C 1-4  alkyl, and R 6  is C 1-4  alkyl. Most preferably, R 6  is ethyl or methyl. Examples of preferred esters include methyl formate, ethyl formate, methyl acetate, n-octyl acetate, methyl laurate, methyl myristate, butyl stearate, and methyl stearate. Preferred alkanols useful as non-solvents include methanol, ethanol, 2-propanol, and 1-hexanol. Preferred cyclophexanes useful as non-solvents include those which are unsubstituted or substituted with a C 1-10  alkyl, C 3-10  cycloalkyl or C 1-4  perfluoroalkyl moiety, More preferred cyclohexanes useful as non-solvents include those which are unsubstituted or substituted with a C 1-4  alkyl, C 5-6  cycloalkyl or trifluoromethyl moiety. Examples of such cyclohexanes include cyclohexane, methylcyclohexane, isopropylcyclophexane, t-butyl-cyclohexane and dicyclohexyl. Preferred C 5-20  alkanes useful as non-solvents include hexane, dodecane, and hexadecane. Preferred dialkyl ketones useful as non-solvents for the polycarbonates include those wherein one of the alkyl moieties is C 3-10  and the other C 1-10 . Examples of preferred dialkyl ketones useful for non-solvents include methyl isobutyl ketone, and diisopropyl ketone. Preferred amides useful as non-solvents include those amides corresponding to the formula R 7  CONHR 8  wherein R 7  is preferably hydrogen or C 1-3  alkyl, and R 8  is preferably C 1-4  alkyl. Examples of preferred amides include N-methyl formamide, and N-methyl acetamide. Preferable nitriles include acetyl and C 1-3  alkyl nitriles. Examples of preferred nitriles include acetonitrile, and propionitrile. Preferred aldehydes are C 1-4  alkyl aldehydes, with butyraldehyde most preferred. Preferred substituted benzenes include formyl, alkyl, and cycloalkyl substituted benzenes which correspond to the formula ##STR3## wherein R 10  is C 1-10  alkyl, C 3-10  cycloalkyl, or formyl, and b is as defined hereinbefore. Preferably, R 10  is C 1-4  alkyl, C 5-6  cycloalkyl, or formyl. 
     Preferred non-solvents for the polycarbonates include triethylene glycol, 2-ethoxyethanol, diethylene glycol dibutyl ether, polyethylene glycols with molecular weights of up to about 1450, diethylene glycol, dodecane, hexadecane, cyclohexane, methylcyclohexane, perchloroethylene, diisopropylketone, isopropylketone, isopropylcyclohexane, t-butylcyclohexane, N-methylformamide, decylene, N-methylacetamide, tetralin, dicyclohexyl, cyclophexyl benzene, diethylene glycol dibutylether, carbon tetrachloride, or water. More preferred non-solvents for the polycarbonates include water, diisopropylketone, tetraethylene glycol dimethylether, diethylene glycol dibutyl ether, hexadecane, diethylene glycol, triethylene glycol, polyethylene glycol with molecular weights of up to about 1450, 2-ethoxyethanol, carbon tetrachloride, or dodecane. The most preferred non-solvents for the polycarbonates are triethylene glycol, and polyethylene glycols with molecular weights of up to about 400. 
     Some compounds may be both a solvent and a non-solvent, wherein its function is dictated by the temperature at which the membrane is formed. 
     In some embodiments, a solubilizing agent is used to aid in preparing a homogeneous polymer mixture. The solubilizing agent may be any solvent which aids in preparing a homogeneous polymer mixture. The solubilizing agent is preferably a solvent which has a boiling point lower than the extrusion temperature and the boiling points of the solvent and non-solvent. The polymer mixture may be formed at temperatures below the extrusion temperature and the solubilizing agent aids in forming a homogeneous mixture at such temperatures. Preferably the solubilizing agent flashes off, or is removed, prior to extrusion. Preferred solubilizing agents for the polycarbonate based mixtures include halogenated hydrocarbons, cyclic and non-cyclic ethers, and alkyl ketones. More preferred solubilizing agents include methylene chloride, tetrahydrofuran, methyl ethyl ketone, methyl iodide, and chloroform. A most preferred solubilizing agent is methylene chloride. 
     The solubilizing agent is not used in embodiments where the mixture is mixed at elevated temperatures under high shear or with good mixing, where the mixture is to be extruded shortly after formation of the homogeneous mixture. 
     Certain solvents and non-solvents may cause degradation of the polymer if the polymer mixture is maintained at elevated temperatures for extended periods of time. The solvent and non-solvent should be chosen to be compatible; in particular the non-solvent must be soluble in the solvent, and the non-solvent must be capable of forming pores in the quenched polymer in the presence of the solvent. Skilled artisans often describe the solvent and non-solvent as a solvent non-solvent pair. Preferred solvent non-solvent pairs for the polycarbonates include N-methylpyrrolidinone and triethylene glycol, N-methylpyrrolidinone and a polyethylene glycol with a molecular weight of up to about 1450, ethylene glycol dimethylether and water, tetrahydrofuran and water, ethylene glycol dimethylether and diisopropylketone, tetrahydrofuran and diisopropylketone, diethylene glycol dimethylether and water, diethylene glycol dimethylether and tetralin, tetraetylene glycol dimethylether and N-methyl-acetamide, acetophenone and diethylene glycol dibutylether, methylene chloride and carbon tetrachloride, cyclohexanone and dodecane, and acetophenone and hexadecane. More preferred solvent non-solvent pairs for the polycarbonates are N-methylpyrrolidinone and triethylene glycol, cyclohexanone and dodecane, N-methylpyrrolidinone and a polyethylene glycol with a molecular weight of up to about 400, and acetophenone and hexadecane. The most preferred solvent non-solvent pairs for the polycarbonates are N-methylpyrrolidinone and triethylene glycol, and N-methylpyrrolidinone and a polyethylene glycol with a molecular weight of up to about 400. 
     The polymer mixture should comprise appropriate amounts of the polymer, solvent, and non-solvent to be extrudable at the extrusion temperatures and to form the membranes of this invention. In particular, the solution should have an acceptable viscosity for such extrusion at extrusion temperatures. The upper limit on the viscosity is that viscosity at which the solution is too viscous to extrude. The lower limit on the viscosity is that viscosity at which the fiber loses its integrity after leaving the vicinity of the extrusion die. 
     Preferably, the spin composition comprises between 30 and 60 percent by weight of the polymer, and 40 and 70 percent by weight of the combined solvent and non-solvent. More preferably, the spin composition comprises between 40 and 60 percent by weight of the polymer, and 40 and 60 percent by weight of the combined solvent and non-solvent. In the embodiment wherein the polymer is a tetrahalogenated bisphenol based polycarbonate the spin composition comprises, even more preferably, between about 44 and 56 percent by weight of polymer and between about 44 and 56 percent by weight of a mixture of solvent and non-solvent, and most preferably, the spin composition comprises between 50 and 55 percent by weight of polymer and between about 45 and 50 percent by weight of the combined solvent and non-solvent. The ratio of the solvent to the non-solvent is dependent upon the polymer, the solvent and the non-solvent used and the relative solubilities with respect to one another. The solvent and non-solvent are present in a ratio of between about 0.9 and 5.1, more preferably between about 1.8 and 2.7, and most preferably between about 2.0 and 2.4. 
     Prior to extrusion, the mixture is heated to a temperature at which the mixture is homogeneous and has an appropriate viscosity for extrusion. The upper limit on the pre-extrusion temperature is that temperature at which the polymer undergoes detrimental degradation in the presence of the particular solvent and non-solvent. Detrimental degradation means herein that the polymer degrades sufficiently that the viscosity of the polymer mixture is significantly lowered below that viscosity at which acceptable membranes can be formed, or the polymer mixture cannot form a membrane of this invention which is capable of separating oxygen from nitrogen. In the embodiment wherein the membrane is a hollow fiber and the core fluid is a gas, this is indicated where the hollow fiber collapses in the quench zone. In the embodiment wherein the polymer is a tetrahalosubstituted polycarbonate the preferred upper pre-extrusion temperatures are about 200° C. or below, more preferred upper pre-extrusion temperatures are about 130° C. or below. This upper limit is significantly affected by the kind of extrusion apparatus that is used. Generally there is a tradeoff between the temperature to which you can raise the composition and the residence time in the heating area. With lower residence times, the polymer mixture can be heated to higher temperatures. The lower limit on the pre-extrusion temperature is that temperature at which the viscosity of the spin solution is sufficiently low enough to allow extrusion. In the embodiment wherein the polymer is a tetrahalosubstituted polycarbonate the preferred lower temperatures are 50° C. or above, more preferred lower temperatures are 90° C. or above. Generally, the spin solution is extruded at the temperatures described hereinbefore with respect to the pre-extrusion heating. Provided the temperature of the polymer mixture during extrusion is within the functional limits described hereinbefore, the actual temperature of extrusion can be significantly lower than the pre-extrusion temperature, for example, as much as 20° C. lower. 
     The polymer is extruded into one or more quench zones. Such zones function to facilitate phase separation of the polymer mixture, removal of a large portion of the solvent and the non-solvent, and to provide a zone where the fiber may be drawn to its final diameter. The quench zone may comprise one or more zones. At least one of such zones must be a liquid zone which comprises a liquid which has a low solubility in the polymer from which the membrane is to be formed. Such zones may further comprise air quench zones, liquid quench zones, or a combination thereof. In one embodiment, the extruded polymer mixture may be extruded into an air quench zone, and the extruded polymer mixture is thereafter passed into one or more liquid quench zones. In another embodiment, the polymer mixture may be extruded directly into a liquid quench zone, and thereafter may be passed into one or more additional liquid quench zones. 
     In a preferred embodiment the polymer mixture is extruded into an air quench zone, and thereafter passed into one or more liquid quench zones. In this embodiment the polymer mixture begins to lose a portion of the solvent and non-solvent due to volatilization and a significant amount of the draw down of the fiber occurs in the air quench zone. Further, the phase separation of the polymer mixture may begin in this zone. The temperature and residence time in the air quench zone should be sufficient such that there is partial removal of the solvent in the zone, the polymer mixture does not undergo complete phase separation in said zone, and the fiber undergoes significant draw down during the process. If the polymer mixture completely phase separates prior to exiting the air quench zone, a significant amount of solvent and non-solvent may be entrained in the polymer mixture such that it cannot easily be removed from the polymer. The upper limit on the temperature on the air zone is that temperature below which the polymer mixture has sufficient viscosity to retain its shape and integrity. Preferred upper temperatures are about 90° C. or below, with more preferred upper temperatures being about 40° C. or below, and the most preferred upper temperatures being about 25° C. or below. The lower temperature of the air quench zone is that temperature above which the polymer mixture undergoes substantially complete phase separation while in the air quench zone. Preferred lower temperatures are about 0° C. or above, with more preferred lower temperatures being about 10° C. or above, and the most preferred lower temperatures being about 20° C. or above. As noted hereinbefore, the temperatures and the residence time are interdependent variables; at cooler temperatures the residence time is shorter while at higher temperatures the residence time is longer so as to achieve the desired results in the air quench zone. 
     The preferred upper limit of the residence time in the air quench zone is about 10 seconds or less, more preferably 6 seconds or less, and most preferably 1 second or less. The lower residence time is preferably 0.1 seconds or greater and most preferably 0.25 seconds or greater. If the environment in the air quench zone is too humid, damage to the extruded shape may occur. Preferably, the humidity at about 24° C. is 60 percent or less. A more preferred humidity range is between about 30 and 60 percent at 24° C. It may be desirable to place a shroud around the air quench zone so as to cut down variability in the shapes due to undesired air currents. In some embodiments, it may be desirable to cool the shroud so as to allow better control of the quenching in the air draw zone. In one embodiment it may be preferred to pass a gentle stream of air in a counter current fashion along the membrane as it is being extruded. 
     The speed of extrusion is not critical to the process, provided an acceptable residence time in the quench zones is achieved. Therefore, the line speed may be as fast as the equipment, including the size of the quench zones, allows and which results in acceptable properties in the membranes. Preferably, lower line speeds are 50 feet per minute or above, with about 150 feet per minute or above preferred. Preferably, upper line speeds are 1000 feet per minute or below, with 500 feet per minute or below preferred. 
     In that embodiment where hollow fiber membranes are being formed, a core fluid is preferably passed down the core of the hollow fiber to prevent the fiber from collapsing. The core fluid may be any fluid which prevents the fiber from collapsing and which does not deleteriously affect the membrane properties. The core fluid may be a gas or a liquid, preferably a gas. Preferred core gases may include air, nitrogen, argon, or a gas which enhances the properties of the membrane. The core fluid pressure may be any pressure which prevents the fiber from collapsing and which does not deleteriously affect the membrane properties of the fiber, and is preferably between about 0.1 and 0.5 inches of water, more preferably 0.25 to 0.4 inches of water. 
     From the air quench zone, the membrane is passed into one or more liquid quench baths. In the liquid quench baths, the membrane completes phase separation and a major portion of the remaining solvent and non-solvent are removed. The liquid quench baths can comprise any liquid which dissolves both the solvent and non-solvent and which does not deleteriously affect the membrane properties. Furthermore, the liquid used in the quench zones should have a very low solubility in the polymer; preferably the solubility is about 5.0 percent by weight or lower. More preferably, the quench liquid has a solubility in the polymer of about 3.0 percent by weight or lower, even more preferably of about 1.0 percent by weight or lower, and most preferably of about 0.5 percent by weight or lower. Examples of preferred quench liquids include lower alcohols, water, fluorocarbons, lower aliphatic hydrocarbons, or mixtures thereof. The preferred quench bath liquid for the tetrahalosubstututed bisphenol based polycarbonates is water. 
     Optionally, after leaving the first liquid quench bath, the membrane may be passed through or contacted with other liquid baths. The conditions of each bath is dependent upon the number of baths used and the conditions of the other baths. The conditions of the first liquid quench bath is dependent upon whether other liquid quench baths are used. If only one bath is used, the conditions should be such that the fiber completes its phase separation, and the majority of the solvent and non-solvent are removed from the fiber in the bath. Under such circumstances, a preferred upper temperature is 90° C. or below and 30° C. or below being most preferred. The preferred lower temperature is 0° C. or above, with 20° C. or above being more preferred. The residence time under this condition should be sufficient to allow completion of the phase separation of the fiber and to allow removal of a significant portion of the remaining solvent and non-solvent. The residence time in the single bath can be as long as the economics of the process allow. Such residence times may be as long as no deleterious effects result from such residence times, for example damage from bacterial growth. Residence times of up to several days may be used. Preferably, the upper limit on the residence time is about 30 minutes or lower, more preferably 10 minutes or lower. Preferably, the lower residence time is 2 minutes or greater, more preferably 5 minutes or greater. 
     In a preferred embodiment, two liquid quench baths are used. In this embodiment, the quench bath temperature and residence time in the first quench bath should be sufficient to result in significant phase separation of the polymer mixture in said zone, and to allow some of the solvent and non-solvent to be removed from the fiber. The lower bath temperature may be the temperature above the freezing point of the bath. Generally, the lower the bath temperature the better the bath functions. Where the bath comprises water the preferred lower temperature is 0° C. or greater. The preferred upper temperature is 30° C. or less, more preferably 20° C. or less, and most preferably 10° C. or less. The lower limit on the residence time is preferably 0.1 seconds or greater, and more preferably 1.0 second or greater. The upper residence time is preferably 600 seconds or less, more preferably 300 seconds or less, even more preferably 20 seconds or less, the most preferably 2 seconds or less. The second liquid quench bath functions to remove most of the remaining solvent and non-solvent. The conditions of the second liquid quench bath should be such that most of the solvent and non-solvent are removed during its presence in the bath. The temperature of the second liquid quench bath is that temperature which facilitates the removal of the solvent and non-solvent from the membrane. The upper temperature is that temperature at which either the bath remains in the liquid form, or the fiber properties are deleteriously affected. The lower limit on temperature is that temperature below which the solvent and non-solvent are no longer removed from the polymer mixture at an acceptable rate. In the most preferred embodiment wherein the bath comprises water, the preferred lower temperatures are 70° C. or above with a more preferred lower temperature of 80° C. or above. In this embodiment, preferred upper temperatures are 100° C. or below and more preferred upper temperatures are 90° C. Generally, as the temperature is lowered, the residence time required to achieve the same removal of solvent and non-solvent becomes longer. After the fiber is removed from the one or more quench baths, the fiber preferably contains 1.2 percent or less of the solvent and non-solvent, and more preferably less than 0.5 percent by weight of the solvent and non-solvent. 
     In the embodiments wherein one or more liquid quench baths are used, after removal from the first liquid quench bath, the fibers are passed over a set of godets and either passed into another bath or taken up. After completion of the processing, the fibers may be stored in a liquid which does not deleteriously affect the properties of the fibers. The most preferred liquid is water. 
     In the embodiment wherein the membrane is a hollow fiber, the fiber size is that fiber size which allows good separation of oxygen from nitrogen with reasonable flux, and acceptable pressure drops across the fiber. Preferably, the fiber size is between about 175 ×117 (outside diameter OD × inside diameter ID) microns to about 100 ×65 microns and has an OD/ID ratio of about 1.5. In the embodiment wherein the membrane is prepared from a tetrahalosubstituted bisphenol based polycarbonate, the membrane preferably has a separation factor for oxygen and nitrogen of 6.0 and greater, more preferably 6.5 or greater, and most preferably 6.8 or greater. Preferably, such membrane has a flux of 3.0 ×10 -6  scc/cm 2  cmHg sec. or greater, more preferably 6.0 ×10 -6  scc/cm 2  cmHg sec. or greater, and most preferably 8.0 ×10 -6  scc/cm 2  cmHg sec. or greater. 
     Before fabrication of a module, the membrane is preferably dried. The membrane may be dried by exposing it to the flow of air or an inert gas. Such exposure preferably takes place at a temperature of between about 20° C. and about 80° C. Such gas flow may be a gentle flow either substantially perpendicular to the longitudinal direction of the membrane or along the longitudinal direction of the membrane. In another embodiment wherein the membrane is in the hollow fiber form, the gas may be blown down the core during winding of the module. The gas used to dry the membrane may be any gas which is sufficiently dry to aid in the removal of liquid from the pores and lumen of the membrane. Such gases include nitrogen, argon, and air. 
     As used herein, the term semi-permeable membrane refers to a membrane which displays different permeabilities for different species of molecules, and therefore may be used in the separation of ions and molecules having different permeabilities across the membrane. Permeate as used herein refers to those species which permeate through the membrane at a much faster rate than other species. Non-permeate refers herein to those species which permeate at a much slower rate than the other species present. 
     Preferably, the membrane exhibits permeability properties similar to a dense membrane with an effective thickness of about 10μ or less, more preferably of about 1.5μ or less and most preferably of about 0.5μor less. Effective thickness means herein that the membrane functions as if it is a homogeneous flat membrane of such thickness. 
     The membrane of this invention may be used for separating oxygen from nitrogen by contacting a gaseous stream containing oxygen and nitrogen with the membrane of this invention under conditions such that oxygen selectively permeates through the membrane, in comparison to nitrogen. Preferably, the membrane is sealingly engaged to a vessel which defines a space communicating with only one side of the membrane, such that the permeable oxygen contacting the other side of the membrane can permeate through the membrane to the non-communicating space, at a significantly faster rate than the nitrogen communicates or permeates through the membrane. Preferably, the oxygen and nitrogen are a part of an air stream. Preferably, the pressure on the communicating side of the membrane is between about 40 psia (about 276 kPa) and about 1000 psia (about 6900 kPa), more preferably between about 80 (about 551 kPa) and about 160 psia (about 1002 kPa). The temperature at which the mixed oxygen and nitrogen stream is contacted with the membrane is preferably between about -10 and 80° C., most preferably between about 0 and 45° C. The pressure differential across the membrane is preferably between about 40 psia (about 276 kPa) and about 1000 psia (about 6900 kPa), and more preferably between about 95 (about 65 kPa) and about 160 psia (about 1002 kPa). 
     In one preferred embodiment, the membrane is in a hollow fiber form. In the embodiment wherein the membrane is in hollow fiber form, it is preferable to contact the mixed nitrogen and oxygen stream with a membrane on the inside of the hollow fiber under conditions such that the oxygen selectively permeates out of the hollow fibers and a stream which is rich in oxygen is taken off of the shell side of the membrane. This oxygen enriched stream can be further oxygen enriched by contacting with one or more membranes in succession. Alternatively, the non-permeating oxygen depleted nitrogen stream may be further depleted of oxygen by contacting the stream with one or more further membranes in succession. 
    
    
     SPECIFIC EMBODIMENTS 
     The following examples are included for illustrative purposes only and do not limit the scope of the claims or the invention. Unless otherwise stated, all parts and percentages are by weight. In each of the examples at least four samples are tested for permeation properties. The flux and separation factor data are reported as an average of all of the samples with a standard deviation. Molecular weights as used herein are weight average molecular weights measured using narrow molecular weight range polystyrene standards. 
     Examples 1-14 
     PREFERRED SPIN CONDITIONS FOR TETRABROMOBISPHENOL A POLYCARBONATE HOLLOW FIBER MEMBRANES 
     In order to form a basis from which comparisons can be made, a set of standard spinning and processing conditions are developed. The selection of this standard set of conditions is based on data from early attempts to prepare fibers and corresponds to fiber that is easily formed and gives consistent results from one spin run to another. A series of fourteen experiments using the standard set of conditions are performed to prepare fibers and the fibers are tested for oxygen and nitrogen permeation. These permeation and separation factors for these fourteen experiments are averaged to give a performance standard to measure all other experiments against. 
     a composition of 52 weight percent tetrabromobisphenol A polycarbonate, 32.5 weight percent N-Methyl pyrrolidone (solvent), and 15.5 weight percent of triethylene glycol (non-solvent), (solvent to non-solvent ratio of 2.1) is fed into the melt pot of a melt pot extruder. Methylene chloride in an amount equal to about 30 weight percent of the total composition is added to the vessel. The mixture is heated to 95° C. and held until the mixture is a homogeneous solution. Most of the methylene chloride flashes during this heating step. A nitrogen purge is passed into the melt pot at 500 cc per minute and nitrogen containing volatilized methylene chloride is withdrawn from a port in the melt pot. From the melt pot the composition is passed to a transfer line and pumped to the spinnerette at a flow rate of 15 g/min. The transfer line and spinnerette face are held at a temperature of 75° C. The composition is extruded into a hollow fiber shape through an annulus of 254 microns (0.01 inch) with an outside diameter of 1727 microns (0.068 inch) with a core gas pin feeding a core gas of nitrogen down the bore at a rate of 8.8 standard cubic centimeters a minute. The line speed is 100 ft per minute. The fiber is extruded into an air quench zone of a length of 1 foot at ambient temperature. The fiber is passed into a quench bath of water at 4° C. with a residence time of 1.7 seconds. The fiber is taken up and thereafter placed into a bath of water at 90° C. for ten minutes. The fibers are hung vertically and dried by passing air over the fibers at a flow of 100 ft/min over the fibers for about two hours. The fibers prepared have a size of 140 ×94 microns (OD ×ID). The membranes prepared in Examples 1 and 14 are examined by photomicrographs and such membranes have a porous outer surface, a porous inner surface, and have a region which separates oxygen from nitrogen as demonstrated by the separation factors stated. 
     PERMEABILITY TESTING PROCEDURE 
     After the fiber is dried the fibers are tested for permeation properties. The test device is a pressure vessel with four ports, two tubesheet ports, one feed port through which the compressed gas enters the vessel, and an exit or purge port through which the compressed gas can be purged from the vessel. Two hundred ten (210) fibers are passed into one of the tubesheet ports and out the other allowing for a 31.5 cm length of the fibers to be contained within the test device. Epoxy tubesheets are formed in the two tubesheet ports to give a leak-tight bond between the fiber and the two ports. Test units are then pressurized with nitrogen at 50 psig at allowing compressed nitrogen to enter the test device through the feed port while leaving the exit port closed. The exit port is then opened for two minutes to purge the vessel of air and then closed with pure nitrogen left in the vessel. With the exit port closed and the feed port opened, the gas contained within the test device, by means of a pressure driving force, permeates through the walls of the hollow fibers and passes through the lumen of the fibers and out through the tubesheet ports where the flow rate is measured either by means of bubble or mass flow meters. There is negligible back pressure on the gas exiting the tubesheet. After testing with nitrogen the feed gas is changed to oxygen and the vessel is purged for about two minutes to give pure oxygen at 50 psig in the test device. The amount of oxygen permeating through the fiber walls is measured by combining the outputs from the two tubesheet ports. From these flow measurements, the gas permeation rates and separation factor can be calculated by use of the following equations. ##EQU1## The units are scc/cm 2  cmHg sec. 
     Measured flow = standard cubic centimeters / minute. 
     Surface area of fibers = 3.14 × OD (outside diameter, cm) × length × the number of fibers. 
     Pressure (cm Hg) = psi × 76 /14.7. 
     The results are compiled in Table 1. 
     Separation factor is defined as the Oxygen flux divided by the Nitrogen flux. 
     
                       TABLE 1______________________________________                   Oxygen/Nitrogen                   SeparationExample      Oxygen Flux.sup.2                   Factor______________________________________ 1           4.8 ± .5                   6.8 ± .1 2           7.4 ± .4                   6.4 ± .1 3           6.2 ± .1                   6.4 ± .3 4           7.6 ± .3                   6.7 ± .1 5           7.0 ± .1                   5.9 ± .1 6           5.8 ± .2                   6.8 ± .2 7           9.0 ± .2                   6.8 ± .2 8           8.3 ± .1                   6.7 ± .1 9           7.2 ± .1                   6.0 ± .2.sup. 10.sup.1        4.4 ± .4                   6.0 ± .211           6.5 ± .3                   6.0 ± .512           6.1 ± .1                   6.2 ± .113           7.4 ± .1                   6.5 ± .114           8.1 ± .1                   6.6 ± .2AVERAGE       7.0 ± 1.1                   6.4 ± .3______________________________________ .sup.1 Not included in average, suspect blend composition .sup.2 Units (1 × 10.sup.-6) scc/cm.sup.2 · cmHg .multidot sec 
    
     Examples 15-36 
     Hollow fibers are prepared using the standard conditions described hereinbefore using several spin compositions. The hollow fibers prepared are tested for oxygen and nitrogen permeability using the procedure described hereinbefore. The various spin compositions and results are compiled in Table 2. The spin compositions have about 1 to about 6 percent residual methylene chloride therein. 
     Example 35 is performed using some different conditions than the other examples. During the blend formation stage the blend is heated to 120° C. The spinnerette temperature is controlled at 70° C. There is a 9 inch air quench zone. The fiber is passed from the quench bath to a leach bath at ambient temperatures. The fiber is exposed to a further bath of water for 10 minutes at 80° C. Thereafter the fiber is extracted with a mixture of 50/50 mixture of isooctane and isopropanol for one hour. The fibers are examined by photomicrograph. The membrane has a dense region of the outer surface and a porous region below the dense region. Therefore the membrane of Example 35 is not an example of the invention. 
     
                       TABLE 2______________________________________  Polymer  in Spin  Solvent                 Melt Pot  Composi- to Non-          Separa-                                   Tempera-  tion     solvent   Oxygen tion   tureExample  wgt %    ratio     Flux   Factor °C.______________________________________15       45     2.1        6 ± .5                             3 ± .4                                   8516       45     2.1       2. ± 1                             5 ± .3                                   11017       45     2.5       1.7 ± .1                             5 ± .5                                   8518       45     2.3        5 ± .9                            2.4    110                      2 ± .2                            4.019       52     2.1       8 ± 1                            6.5 ± .3                                   9820       52     2.0       10 ± 1                            6.5 ± .3                                   9821       44     1.9       4.2 ± .2                            3.5 ± .2                                   8022       44     2.1        4 ± .2                            3.9 ± .3                                   8023       44     2.3        2 ± .2                            3.8 ± .1                                   8024       50     2.1        7 ±  .1                            5.5 ± .1                                   9225       54     2.1       7.3 ± .1                             6 ± .1                                   9226       51     1.9       5.4 ± .2                            5.3 ± .2                                   9527       51     2.1        7 ± .4                            5.8 ± .2                                   9528       53     2.1       3.6 ± .3                            6.3 ± .4                                   9529       53     1.9       2.8 ± .4                            5.5 ± .1                                   9530       53     1.9       4.8 ± .2                            6.7 ± .1                                   9531       52     1.9       9.7 ± .4                            4.3 ± .5                                   9532       52     2.0       9.8 ± .1                            6.2 ± .2                                   9533       52      2.05     9.1 ± .3                            6.1 ± .1                                   9534       52     2.3       *      *      *35       50     NMP only  .021   6.4    10536       52     25        0.2 ± .05                            ***    ***______________________________________ *Fiber did not phase separate **Composition not spinnable ***Not measurable, flow rates less than 0.05 
    
     Examples 37-40 
     Hollow fibers are prepared from spin compositions containing polymers of two different molecular weights than the molecular weights of the polymer used to set the standard conditions in Examples 1-14. The polymer content of the spin composition and the solvent to non-solvent ratio is described in Table 3. The results are contained in Table 3. 
     
                       TABLE 3______________________________________Use of Various Polymers with Different MolecularWeights                                     Melt          %                   Separa-                                     Pot          Poly-               tion   Temper-Example  M.sub.w.sup.1          mer     S/NS.sup.2                        Flux  Factor ature______________________________________37     125,000 52      2.1   6.1   2.8    8038     191,000 52      2.10  9.5   4.0    9539     191,000 52      2.13  9.4   6.8    9540     191,000 52      2.16  7.6   6.6    98Standard  163,000 52      2.1   7.0   6.4    95Condi-tions______________________________________ .sup.1 The stated values are rounded to the nearest thousand. .sup.2 S/NS is the solvent nonsolvent ratio. 
    
     Examples 41-44 
     Hollow fibers are prepared using the standard conditions using three different line speeds, and the fibers are tested for oxygen and nitrogen permeability. 
     The size of the quench zones are adjusted to keep the residence times of the fibers in the baths constant. The conditions are the results are compiled in Table 4. 
     
                       TABLE 4______________________________________FIBER PROPERTIES AS A FUNCTION OF LINE SPEED                        Oxygen/                        Nitrogen    Line Speed          Separa-    (Feet/              tion    Fiber SizeExample  Minute)   Flux      Factor  (Microns)______________________________________41        50.sup.1 5.13 ± .28                        5.44 ± .28                                140 × 9442       100.sup.1 7.02 ± .1                        5.85 ± .10                                140 × 9443       100.sup.2 5.79 ± .18                        6.80 ± .20                                140 × 9444       150.sup.2 5.59 ± .10                        7.21 ± .20                                140 × 94______________________________________ .sup.1 Fibers in Examples 41-42 are prepared from the same melt pot run. .sup.2 Fibers in Examples 43-44 are prepared from the same melt pot run. 
    
     Examples 45-60 
     Several hollow fibers are prepared using the standard conditions with the exception that different fiber sizes are prepared. The fibers are tested for oxygen and nitrogen permeabilities. The results are compiled in Table 5. 
     
                       TABLE 5______________________________________FIBER PROPERTIES AS A FUNCTION OF FIBER SIZE    Fiber Size             SeparationExample  (Microns)    O.sub.2 Flux                           Factor______________________________________ 45*     204 × 140                 0.5 ± .08                           6.4 ± .446       140 × 94                 3.4 ± .21                           5.3 ± .547       158 × 106                 3.8 ± .5                           6.7 ± .148       140 × 94                 4.8 ± .5                           6.8 ± .149        112 × 74**                 6.8       6.150       140 × 94                 8.3 ± .1                           6.7 ± .0551       125 × 85                 7.9 ± .1                           6.5 ± .252       140 × 94                 4.4 ± .4                           6.0 ± .253       112 × 74                 7.5 ± .1                           5.6 ± 0.554       140 × 94                 7.3 ± .3                           5.8 ± .155       112 × 74                 10.8 ± 5.3 ± .956       140 × 94                 7.4 ± .1                           6.5 ± .157        112 × 74**                 11        5.258       106 × 64                 8.8 ± .3                           5.3 ± .359       103 × 68                 10.9 ± 1.3                           3.7 ± 1.160       110 × 74                 10.0 ± .8                           4.0 ± 1.3______________________________________ *The fiber is solvent dried with a 50/50 mix isooctaneisopropyl alcohol prior to testing **Result of only one out of four samples 
    
     Examples 45-46 are generated from the same melt pot run. Examples 47-49 are generated from the same melt pot run. Examples 50-51 are generated from the same melt pot run. Examples 52-53 are generated from the same melt pot run. Examples 54-55 are generated from the same melt pot run. Examples 56-60 are generated from the same melt pot run. 
     The fiber size has a significant effect on the permeability characteristics of the hollow fibers. In general, the smaller the fiber the higher the intrinsic permeation rate of oxygen, while the separation factor is fairly insensitive to fiber size. 
     Examples 61-68 
     Several hollow fibers are prepared using the standard conditions described hereinbefore, with the exception that the residence time and the temperature of the quench bath is altered. The fibers are tested for oxygen and nitrogen permeability. The results are compiled in Table 6. 
     
                       TABLE 6______________________________________FIBER PROPERTIES AS A FUNCTION OF QUENCH BATHCONDITIONS   Residence                    Separa-   Time      Temperature        tionExample (Seconds) °C. Flux    Factor______________________________________61      1.7       5° C.                        8.1 ± .1                                6.0 ± .262      1.7       22° C.                        5.4 ± .2                                6.1 ± .563      1.7       5° C.                        7.0 ± .1                                5.9 ± .164      0.6       5° C.                        7.1 ± .3                                5.4 ± .165      1.7       5° C.                        9.0 ± .2                                6.8 ± .266      0.6       5° C.                        8.4 ± .2                                6.9 ± .2 67*    1.7       2° C.                        7.8 ± .1                                6.1 ± .1 68*    1.7       6° C.                        6.1 ± .3                                6.0 ± .1______________________________________ *Fiber size held at 140 × 94 
    
     Examples 61 and 62, 63 and 64, 65 and 66, and 67and 68, respectively, are from the same melt pot runs. 
     The residence time in the first liquid quench bath, from 1.7 to 0.6 seconds, has little effect on the ultimate performance of the fiber. Temperature has an effect on the fibers gas permeation properties. As the temperature is raised from 5° to 22° C., the oxygen permeability is lowered. The selectivity of the fiber appears to be unaffected by this temperature change. 
     Examples 69-88 
     Several hollow fibers are prepared using the procedure described hereinbefore, with the exception that some of the hollow fibers are processed through a third bath of water placed between the first and second baths. The third bath is held at a temperature of about 20° C. and the residence time is about two minutes. The fibers are tested for oxygen and nitrogen permeability. The results are compiled in Table 7. The total residence time of the fiber in the baths is the same whether two or three baths are used. 
     
                       TABLE 7______________________________________GAS PROPERTlES OF FIBERS PROCESSED WITH ANDWITHOUT A THIRD LIQUID BATH                           Separa Fiber  Percent  Third           tion   SizeExample  Polymer  Bath    Flux    Factor (Microns)______________________________________69     53       Y       4.6 ± 1                           7.0 ± .2                                  140 × 9470     52       N       4.8 ± 5                           6.8 ± .1                                  140 × 9471     52       Y       4.0 ± 3                           6.7 ± .1                                  158 × 10672     52       N       3.8 ± .5                           6.7 ± .1                                  158 × 10673     52       Y       6.9 ± 4                           5.2 ± .7                                  112 × 7474     52       N        6.8    6.1    112 × 7475     50       Y       6.9 ± .1                           5.5 ± .1                                  140 × 9476     50       N       7.3 ± .6                           5.1 ± .2                                  140 × 9477     50       Y       4.6 ± .2                           5.5 ±  .2                                  158 × 10678     50       N       5.1 ± .5                           5.0 ± .2                                  158 × 10679     50       Y       12.5 ± .5                           3.4 ± .2                                  112 × 7480     50       N       14.0    3.5    112 × 7481     54       Y       5.1 ± .1                           5.8 ± .2                                  140 × 9482     54       N       7.3 ± .1                           6.0 ± .1                                  140 × 9483     54       Y       2.3 ± .1                           5.4 ± .1                                  158 × 10684     54       N       4.2 ± .4                           5.8 ± .1                                  158 × 10685     54       Y       20.2 ± 3                           1.4 ± .1                                  112 × 7486     54       N       23.3 ± 2                           1.4 ± .1                                  112 × 7487     52       Y       4.8 ± .7                           6.5 ± .1                                  140 × 9488     52       N       7.4 ± .4                           6.4 ± .1                                  140 × 94______________________________________ 
    
     The presence of a third liquid bath demonstrates its greatest effect in Examples 81-86 where the polymer weight percentage is about 54. This is exhibited primarily in the oxygen permeation rate, with little affect on the separation factor. Examples 69-74, 75-80, 81-86, and 87-88, respectively, are prepared from the same melt pot run. 
     Example 89 
     A hollow fiber is prepared using the standard procedure with the addition of the third liquid bath, and the fibers are analyzed for residual solvent, and non-solvent after each bath. The temperature and residence in the third bath is about the same as the third bath in Examples 69 to 88. The total residence time of the fiber is the three baths is the same as where two baths are used. The results are compiled in Table 8. 
     
                       TABLE 8______________________________________PLASTICIZER CONTENT OF FIBER AT VARIOUSSTAGES OF PROCESSING                            PERCENT    PERCENT     PERCENT     METHYLENESTAGE    NMP         TEG         CHLORIDE______________________________________Pre-     32.5        15.6        3.0extrusionAfter first    15.7        7.5         0bathAfter, second     5.0        0           0bathAfter third     0.7        0           0bath______________________________________ Note: Half of the solvent and nonsolvent are removed in the air quench zone and first liquid quench (the bath temperature is 4.5° C., and the residence time is 1.7 seconds). The fiber size is 140 × 94 microns. 
    
     Examples 90-105 
     Several hollow fibers are prepared using the standard conditions described hereinbefore, with the exceptions that the residence time and temperature of the second liquid bath are altered. 
     
                       TABLE 9______________________________________FIBER PROPERTIES AS A FUNCTION OF SECONDLIQUID BATH CONDITIONS                     Percent                     Residual   Temper-           Solvent       Separa-   ature    Time     and Non-      tionExample °C.            (Minutes)                     solvent                            Flux   Factor______________________________________ 90     90       10       &lt;1     3.1 ± .1                                   6.6 ± .2 91     90        1/2     0      4.9 ± .4                                   6.4 ± .1 92     90        1       0      5.0 ± .3                                   6.4 ± .3 93     90        5       0      7.0 ± .2                                   6.5 ± .1 94     90       10       0      7.4 ± .4                                   6.4 ± .1 95     90       10       0      5.8 ± .2                                   6.8 ± .2 96     70        1       0      3.9 ± .4                                   7.0 ± .1 97     70        5       0      4.4 ± .1                                   7.3 ± .2 98     70       10       0      5.0 ± .2                                   6.8 ± .1 99     90       10       0      9.0 ± .2                                   6.8 ± .2100     90       10       *3     6.3 ± .2                                   7.0 ± .3101     70       10       0      7.1 ± .1                                   7.0 ± .2102     70       10       *3     4.2 ± .2                                   7.3 ± .2103     90       10       0      7.3 ± .3                                   5.8 ± .1104     90       10       *1     6.5 ± .3                                   5.9 ± .5105     90       10       *2     5.7 ± .2                                   6.3 ± .1______________________________________ *Liquid bath has the described solvent percentages added thereto. 
    
     Examples 91-94, 95-98, 99-102, and 103-105, respectively, are prepared from the same melt pot run. 
     The gas permeability is affected by the conditions of the second liquid bath. Higher temperature and residence time result in higher gas permeability. Higher solvent content in the bath result in lowering the gas permeability significantly while the separation factor rises slightly. 
     Examples 106-111 
     Several hollow fiber membranes are prepared wherein the time period between the first bath and the second bath is varied. The fibers are tested for oxygen and nitrogen permeability. The results are compiled in Table 10. The membrane of Example 106 is examined by photomicrograph and the membrane exhibits a porous outer and a porous inner surface. 
     
                       TABLE 10______________________________________FIBER PROPERTIES RESULTING FROM DELAYBETWEEN THE TWO BATHS  Time  Between                       Fiber  Baths                         SizeExample  (Minutes)  O.sub.2 Flux                      Selectivity                                (Microns)______________________________________106     0         8.1 ± .1                      6.0 ± .2                                140 × 94107    30         7.6 ± .3                       6.7 ± .05                                140 × 94108     0         7.5 ± .1                      5.6 ± .1                                125 × 85109    15         7.0 ± .1                      6.1 ± .1                                125 × 85110    45         8.0 ± .3                      6.7 ± .2                                125 × 85111    60         7.5 ± .1                      6.0 ± .3                                125 × 85______________________________________ 
    
     Examples 106-107 and 108-111, respectively, are from the same melt pot run. 
     Examples 106-111 demonstrate the fibers experience no deleterious effects due to longer times between the baths. In fact, the use of such a delay may be beneficial. 
     Examples 112-114 
     Two spin runs are performed wherein the fibers are dried right after removal from the second liquid bath. A third spin run is performed wherein the fibers are stored in water for 20 hours between the second bath and the drying step. The fibers are tested for oxygen and nitrogen permeability. The results are compiled in Table 11. 
     
                       TABLE 11______________________________________EFFECT OF DELAY IN DRYING AFTER SECOND BATH   Water    Percent   Storage  Solvent            Separa-   Time     (In Second         tionExample (Hours)  Bath)       Flux   Factor______________________________________112     0        0           5.1 ± .2                               6.3 ± .3113     0        2           1.3 ± .1                               6.1 ± .1114     20*      0           7.3 ± .3                               5.8 ± .1______________________________________ 
    
     Examples 115-124 
     Several hollow fibers are prepared with differing amounts of residual N-methylpyrrolidone (NMP) in the final fibers. The fibers are tested for oxygen and nitrogen permeability. The membrane of Example 115 is examined by photomicrograph and the membrane exhibits a porous outer and a porous inner surface. The results are compiled in Table 12. 
     
                       TABLE 12______________________________________EFFECT OF RESIDUAL SOLVENT IN THEFIBERS ON PERMEATION                               Fiber                       Selectiv-                               SizeExample  Percent NMP             O.sub.2 Flux                       ity     (Microns)______________________________________115    0.76       7.0 ± .3                       5.8 ± .4                               140 × 94116    3.70       1.0 ± .4                       6.3 ± .4                                204 × 147117    1.06       3.9 ± .4                       6.7 ± .1                                158 × 106118    0.41       4.7 ± .4                       6.9 ± .1                               140 × 94119    0.35       6.9 ± .4                       5.6 ± .4                               112 × 74120    1.80       4.9 ± .4                       6.4 ± .4                               140 × 94121    1.54       5.0 ± .3                       6.4 ± .3                               140 × 94122    1.20       7.0 ± .2                       6/5 ± .1                               140 × 94123    1.03       7.4 ± .4                       6.4 ± .1                               140 × 94 124*  1.14       4.8 ± .7                       6.5 ± .1                               140 × 94______________________________________ *A third liquid bath at 20° C. is used with a residence time of tw minutes. 
    
     The solvent content of the fiber prepared by the process correlates well with the intrinsic gas permeation rates of the fiber. As the residual solvent in the fiber increases, the permeation rate decreases. 
     Examples 125 
     Tetrabromobisphenol A polycarbonate is tested for solubility in several solvents and non-solvents. Weighed amounts of polymer and liquid are placed in 4 dram-capacity glass vials with polyethylene-lined caps. About 2.5 grams of liquid is usually used. Initial polymer concentration is about 5 weight percent. The vials are placed on mechanical rollers for at least 24 hours or until complete solution is affected. Additional polymer, if indicated, is added to prepare concentrations of about 10, 25, and 50 weight percent. Insoluble mixtures with liquid boiling points in excess of about 100° C. are placed in a 100° C. forced-air oven for at least 24 hours&#39; observation or until solution is completed. The polymer is arbitrarily designated as being &#34;insoluble&#34; in the liquid if 5 weight percent or less dissolved; &#34;moderately&#34; soluble if 5-25 percent dissolved; and &#34;soluble&#34; if more than 25 percent dissolved. The results are compiled in Table 13. 
     
                       TABLE 13______________________________________SOLUBILITY OF TETRABROMOBISPHENOL A INVARIOUS SOLVENTS             RelativeCompound          Solubility*______________________________________poly(dimethyl-    I&lt;0.8% b fsiloxane) 50 cs.perfluoro(methyl- I&lt;1.4% bcyclohexane)hexane            I&lt;1.6% btriethylamine     I&lt;4.7% bbutyl stearate    I&lt;4.9% b fmethylcyclohexane I&lt;4.6% b fdioctyl phthalate I&lt;4.7% b fdodecane          I&lt;4.7% b fisopropylcyclo-   I&lt;4.95% b fhexanet-butylcyclohexane             I&lt;4.9% b fhexadecane        I&lt;4.8% b fdiisopropyl ketone             I&lt;4.9% b fcyclohexane       I&lt;4.8% bbis(2-methoxyethyl             S&gt;50.3% bether)ethyl benzoate    S&gt;25.1&lt;50.1% bcg@fdiethylene glycol I&lt;4.9% b fdibutyl ethertriethyl          I&lt;4.5% b forthoformatemethyl isobutyl   I&lt;4.7% b f cketonetricresyl phosphate             I&lt;5.0% b&gt;5.0% fmethyl myristate  I&lt;4.9% b ftriethylene glycol             S&gt;50.4% bdimethylethern-octyl acetate   S&gt;50.1% bdicyclohexyl      I&lt;4.8% b fmethyl laurate    I&lt;4.7% b ftetraethylene     S&gt;50.3% bglycoldimethylethercarbon            I&lt;4.7% btetrachloriden-propylbenzene   I&lt;4.9% b f cmethyl stearate   I&lt;4.7% e fpiperidine        S&gt;26.3% b f dxylene            I&lt;5.5% bc&lt;5.5% fdecahydronaphtha- I&lt;4.4% b flene (cis &amp; trans)ethylbenzene      I&lt;4.9% b f cdiethyl ketone    S&gt;50.2% btoluene           I&lt;4.5% b f cN--ethylmorpholine             S&gt;50.1% bcyclohexyl acetate             S&gt;50.5% bbutyraldehyde     I&lt;4.8% btetrahydrofuran   S&gt;51.4% bethyl acetate     I&lt;4.7% b cisophorone        S&gt;25.3&lt;50.1b&gt;50.1fcyclohexylbenzene I&lt;4.8% b ftrichloroethylene S&gt;50.2% b cdiacetone alcohol I&lt;4.9% b f1,2,4-trichloro-  S&gt;25.4&lt;50.1bc?&gt;50fbenzeneperchloroethylene I&lt;4.9% b fchloroform        S&gt;50.8% b cmethyl ethyl      S&gt;50.1% b c?ketonestyrene           I&lt;4.7% b cethyl formate     I&lt;5.0% b cbenzaldehyde      S&gt;50.1% b ftetrahydro-       I&lt;4 8% b fnaphthalenechlorobenzene     S&gt;50.4% b c g@fmethyl acetate    I&lt;4.8% b cmethylene chloride             S&gt;51.1% b cacetone           I&lt;4.6% b ccyclohexanone     S&gt;50.3% b1-cyclohexyl-2-   I&lt;4.7% b&gt;4.7% fpyrrolidinoneo-dichlorobenzene S&gt;50.1% b c g@fepsilon-          S&gt;25.3&lt;50.1b&gt;50.1fcaprolactonephenyl ether      S&gt;50.1% e f c?@bmethyl formate    I&lt;5.0% bmethyl iodide     S&gt;50.2 bcyclopentanone    S&gt;50.3 bhexamethyl-       I&lt;4.9% b&gt;4.9% fphosphoramidemethyl benzoate   S&gt;50.5% b f c?@bstyrene oxide     S&gt;50.5% b f c?@b&amp;f1-ethyl-2-        S&gt;50.1% bpyrrolidinoneacetophenone      S&gt;50.1% bmethyl salicylate S&gt;25.6%&lt;50.1b&gt;50.1f1,1,3,3-          S&gt;50.3 b c g@ftetramethylurea1-bromonaphthalene             S&gt;25.3&lt;50.0% bfc1-hexanol         I&lt;4.7% b fdimethyl phthalate             I&lt;4.9% b&gt;4.9% fpyridine          S&gt;50.1% bN,N--dimethyl-    S&gt;50.2% bacetamidepropionitrile     I&lt;4.9% b ctriethyl phosphate             I&lt;4.8% bc?d?4.8% fdimethyl          I&lt;4.8% b fmalonatepolyethylene      I&lt;2.2% b fglycol E4001-acetyl-         S&gt;50.1% b2-furaldehyde     S&gt;50.1% bN--methyl-        S&gt;50.2% bpyrrolidinone1-benzyl-2-       S&gt;25.9&lt;50.1b&gt;50.1fpyrrolidone2-propanol        I&lt;2.9% b1-formyl-         S&gt;50.1% bpiperidinediiodomethane     S&gt;25.2% b facetonitrile      I&lt;4.9% bdimethyl-         M=&lt;14.1% b f csulfoxideN,N--dimethyl-    S&gt;55.0% bformamidegamma-            S&gt;50.2% bbutyrolactoneethanol           I&lt;3.9% bnitromethane      1&lt;5.0% b fN--formyl-        S&gt;25.6&lt;50.2b&gt;50.2fmorpholinesulfolane         I&lt;4.6% e&gt;4.6% fmethanol          I&lt;1.5% bN--methyl-        I&lt;4.6% e facetamide2-pyrrolidinone   S&gt;25.8&lt;50.1b&gt;50.1fdiethyl ether     I&lt;4.6% bethylene glycol   I&lt;5.3 b cdiethyl etherethylene glycol   S&gt;51.0% bdimethyl etherethylene          I&lt;5.0% e fcarbonatemalonitrile       I&lt;4.9% e fN--methyl         I&lt;5.0% b fformamide______________________________________ *I = Insoluble: &lt;=5%; M = Moderately Soluble: 5-25%; S = soluble: &gt;25; b at room temperature; c = insoluble fraction and/or solvated polymer and/o solventinduced order; d = reacts with polymer; e = at 50° C.; f = at 100° C.; g = clear. 
    
     The behavior of about 27 compounds are marked by a &#34;c&#34;. Such behavior includes (a) partial dissolution followed by opacification and whitening of the clear swollen polymer accompanied by cessation of further dissolution; this behavior is frequently accompanied by a hazy or cloudy supernatant liquid; (b) dissolution to give a clear solution followed by precipitation at the same temperature of white and opaque solid, mushy gel-like formation, or, at the extreme, a solidification to a &#34;candle wax-like&#34; solid; and (c) dissolution at elevated temperature followed by precipitation of solid, &#34;gelation&#34;, and/or a hazy-cloudy formation in the supernatant liquid upon cooling. Seven particularly severe cases of this behavior are noted. Methylene chloride solutions containing about 51% polymer become hard candle wax-like solids after about 17 days&#39; standing at room temperature. DMSO solutions contain about 14 percent polymer are readily formed at room temperature; they change to a white opaque slush after about 36 hours. Redissolution does not occur at elevated temperature. Chloroform solutions containing about 51% polymer are clear at room temperature but changed into candle wax-like solids after about 14 days. Chlorobenzene solutions containing about 50% polymer become clear stiff gels after about 11 days at room temperature. The gels become clear at 100° C. but become cloudy when cooled. Tetramethylurea containing about 50% polymer is clear and soluble at room temperature but becomes a rigid gel after about 8 days. The gel becomes clear at 100° C.; the clear solution becomes cloudy when cooled to room temperature. A clear solution of about 50% polymer in ethyl benzoate becomes a rigid, opaque gel after 11 days at room temperature. n-Propylbenzene dissolved less than 4.9% polymer at room temperature; solubility is almost complete at 100° C. The warm solution becomes a candle-like solid when cooled to room temperature. 
     Table 13 indicates several solvents and non-solvents which are good candidates for solvent non-solvent pairs useful for spinning polycarbonate membranes by the process disclosed herein. 
     Examples 126-129 
     Fibers are prepared using the same conditions as described in Example 1, with the exception that a polyethylene glycol with a molecular weight of 400 is used as the non-solvent, and the solvent non-solvent ratio is adjusted. Fibers are produced with a porous inner surface and a porous outer surface. After the fibers are prepared, the fibers are immersed in a solution of 25 percent by volume of methanol in water for two hours. The fibers are tested as described in Example 1 both before and after the immersion in the methanol and water solution, and the results are compiled in Table 14. 
     
                       TABLE 14______________________________________Use of Polyethylene Glycol as a Non-solvent                           Oxygen Separa-  Solvent                  Flux   tion  Non-              Separa-                           After  Factor  solvent  Oxygen   tion   Immer- Immer-Example  Ration   Flux     Factor sion   sion______________________________________126    2.10     4.0      7.2    7.5    7.3127    1.95     5.5      7.1    10.0   7.4128    1.80     5.9      7.5129    1.60     7.1      2.4______________________________________ 
    
     Examples 130-145 
     Fibers are prepared using the same conditions as described in Example 1, with the exception that different nonsolvents are used, and the solvent non-solvent ratio is varied. Fibers are produced with a porous inner surface and a porous outer surface. After the fibers are prepared, the fibers prepared in Examples 135, 141, 143, and 145 are immersed in a solution of 25 percent by volume of methanol in water for two hours. The fibers are tested as described in Example 1 and the results are compiled in Table 15. Those examples where the fibers are immersed in a methanol solution are tested after the immersion in the methanol solution. Membranes of Examples 133 to 135 are examined by photomicrograph and the membranes exhibit porous outer surfaces and porous inner surfaces. 
     
         ______________________________________Use of Varied Nonsolvents            SolventEx-              Non-            Speara-am-              solvent  Oxygen tion   Fiberple  Non-solvent Ratio    Flux   Factor Size______________________________________130  polyethylene            1.8      3.31 ±                            3.42 ±                                   140 × 90glycol 1450          .15    .32131  polyethylene            1.8      2.45   5.45   162 × 104glycol 1450132  ethylene glycol            3.1      9.3    1.04   140 × 90133  ethylene glycol            3.5      .35 ±                            7.8 ±                                   140 × 90                     .08    1.6134  ethylene glycol            3.5      .10 ±                            5.1 ±                                   204 × 140                     .01    0.1135  ethylene glycol            3.5      1.5 ±                            6.7 ±                                   140 × 90                     .1     0.1136  ethylene glycol            4.5      0.04   5.8    140 × 90137  ethylene    1.5      0.074  &gt;7     140 × 90carbonate138  ethylene    1.5      0.104  8.3    161 × 104carbonate139  ethylene    1.5      0.10   6.7    125 × 80carbonate140  ethylene    1.1      0.08   &gt;6     140 × 90carbonate141  ethylene    1.1      0.16   8.7    140 × 90carbonate142  ethylene    1.1      0.05   &gt;6     161 × 104carbonate143  ethylene    1.1      0.17   &gt;6     161 × 104carbonate144  ethylene    1.1      0.68   6.15   110 × 74carbonate145  ethylene    1.1      20 ±                            1.0    110 × 74carbonate            10______________________________________ Polyethylene glycol 1450 is a polyethylene glycol with a molecular weight of about 1450. 
    
     Examples 146-148 
     Three membranes are prepared using the conditions described in Examples 1-14, and the resultant membranes are examined by photomicrograph. All of the membranes exhibit a porous outer surface and a porous inner surface. The membranes are tested for permeability properties, the results are compiled in Table 15. 
     
                       TABLE 15______________________________________         Oxygen   SeparationExample       Flux     Factor______________________________________146           5.5 ± .2                  6.6 ± .2147           8.0 ± .1                  6.0 ± .2148           9.0 ± .3                  6.8 ± .2______________________________________ 
    
     All of the membranes prepared examined by photomicrograph demonstrate a porous outer surface and a porous inner surface, except the one example noted. It is believed that all of the other membranes prepared have a similar structure. Examination of the fibers by the naked eye during fiber spinning indicates that all of the fibers not examined by photomicrograph look like those examined by photomicrograph.