POROUS POLYBENZOXAZOLE AND POLYBENZOTHIAZOLE MEMBRANES

Abstract: Porous membranes containing polybenzoxazole, polybenzothiazole and/or copolymers thereof and processes for making and using them are disclosed.

Description: POROUS POLYBENZOXAZOLE AND POLYBENZOTHIAZOLE MEMBRANES The present invention relates to the field of porous membranes and supports. Porous separation membranes comprise a solid polymer matrix having well-defined pores through which fluids and small solids may pass. The pores are inter¬ connected, and transport occurs within the pores. The porous membrane separates a mixture containing two or more components by sieving. Pore size is governed by the use intended for the membrane. "Ultra-filtration" membranes typically have average pore sizes of 20 to 500 angstroms. "Microfiltration" membranes may have average pore sizes between 0.05 to 5 μm. Those limitations are not hard and fast but are simply numbers of convenience, and membranes having other pore sizes may also be useful. Membranes having larger pores are frequently used as a supporting layer in a composite membrane, providing support for a thinner dense or porous layer but not contributing substantially to the separation otherwise. Likewise, a single asymmetric membrane may contain a layer having larger pores supporting one or more discriminating layers of dense polymer or porous polymer having smaller pores. The separation performance of porous membranes is typically judged on the basis of flux and retention. Flux is a measure of the rate of flow of permeate across a given area of the membrane. Flux is typically mea¬ sured in units of: (ml atSTP permeant crossing membrane) (rπ2 surface of membrane)(hour)(cm Hg pressure across membrane) Another unit is gallons/ft^-day. Retention is a measure of percentage of molecules of a particular size that the membrane will allow to pass. Rejection is frequently described in terms of molecular weight cut¬ off. T e synthesis and use of porous membranes is described in numerous general references, such as 15 Kirk-Othmer Ency. Chem Tech. , Membrane Technology, at 92 (J. Wiley & Sons 1981); 23 Kirk-Othmer Ency. Chem Tech., Ultrafiltration, at 439 (J. Wiley & Sons 1981); Strathman, "Synthetic Membranes and Their Preparation", Synthetic Membranes; Science Engineering and Applica¬ tions 1-37 (D. Reidel publ., 1986); and Kesting, Syn¬ thetic Polymeric Membranes (2nd Ed.) 1-21, 153-185 and 234-285 (J. Wiley & Son) * (1985). Certain properties are desirable for effective use of membranes having good flux and retention proper¬ ties. The membrane should be strong enough to withstand the pressure necessary to force the permeate through the membrane at desired flux rates. The membrane should be insoluble with respect to all components of the mixture which it is intended to separate. If elevated use tem¬ peratures are desirable, then the membrane must maintain its integrity at those temperatures for a reasonable period of time. What is needed are new porous membranes offering one or more advantage in good flux, good reten¬ tion and/or molecular weight cut off, good strength, good solvent resistance and good high-temperature stability. One aspect of the present invention is a mem¬ brane separation apparatus containing: (1) a high pressure zone, (2) a low pressure zone, and (3) a porous membrane, said elements being arranged such that a fluid passes from the high pressure zone to the low pressure zone by permeation through the porous membrane, characterized in that the porous membrane contains a polybenzoxazole polymer or copolymer or a polybenzothiazole polymer or copolymer. A second aspect of the present invention is a process for separating a mixture that contains at least one fluid component and at least one separable component, wherein the mixture is contacted with one face of a porous membrane, which has pores of a size suitable to reject at least part of the separable component, under a pressure sufficient to cause the fluid to permeate through the membrane, characterized in that the porous membrane contains a polybenzoxazole polymer or copolymer or a polybenzothiazole polymer or copolymer. A third aspect of the present invention is a porous hollow fiber membrane, characterized in that the membrane contains a polybenzoxazole polymer or copolymer or a polybenzothiazole polymer or copolymer. A fourth aspect of the present invention is a membrane comprising a polybenzoxazole or polybenzo- 0 thiazole polymer or copolymer, characterized in that the membrane is no more than 500 microns thick and has an open cell structure with an average cell diameter between 1 micron and 200 microns. 5 A fifth aspect of the present invention is a process for making a porous membrane comprising the steps of: Q ( 1 ) forming a homogeneous solution containing a solvent acid, a PB0 or PBT polymer, and a pore-forming compound which is soluble in solvent acid and in organic solvent but is insoluble in water; 5 (2) contacting the homogeneous solution of step (1) with an aqueous diluent under conditions such that the PB0 or PBT polymer and the pore-forming compound coprecipitate to form a film or membrane containing dis¬ 0 crete zones of PB0 or PBT and discrete zones of pore-forming compounds; and (3) contacting the film or membrane of step (2) with an organic solvent such that the pore-forming compound is dissolved from the film or membrane resulting in a porous structure. PBO and PBT polymers and copolymers frequently have very high solvent resistance for most organic solvents, have a high tensile strength and modulus, and have a high continuous use temperature. Depending upon pore size, the membrane may be used either as a discriminating layer to separate macromolecules from smaller fluid molecules or as a supporting layer for a different discriminating layer in a composite membrane. The membrane may also be an asymmetric membrane, having at least two layers, at least one of which is porous. Definition The following terms are used repeatedly throughout this application, and have the meaning and preferred embodiments defined herein unless otherwise specified. AA/BB-Polybenzazole (AA/BB-PBZ) - a polybenz- azole polymer characterized by mer units having a first aromatic group (Ar1), a first and a second azole ring fused with said first aromatic group, and a divalent organic moiety (DM) bonded by a single bond to the 2-carbon of the second azole ring. The divalent organic moiety (DM) is inert under conditions suitable to syn¬ thesize PBZ polymers; it is preferably a second aromatic group (Ar2). Mer units are preferably linked by a bond from the divalent organic group (DM) to the 2-carbon of the first azole ring in an adjacent mer unit. Mer units suitable for AA/BB-PBZ polymers are preferably repre¬ sented by Formula 1 : wherein Z is as defined for azole rings subsequently and all other characters have the meaning and preferred embodiments previously given. AB-Polybenzazole (AB-PBZ) - a polybenzazole polymer characterized by mer units having a first aro¬ matic group and a single azole ring fused with said first aromatic group. The units are linked by a bond from the 2-carbon of the azole ring in one mer unit to the aromatic group of an adjacent mer unit. Mer units suitable for AB-PBZ polymers are preferably represented by Formula 2: wherein Z is as defined for azole rings subsequently and all other characters have the meaning and preferred embodiments previously given. o-Amino-basic moiety - a moiety, which is bonded to an aromatic group, comprising and preferably consisting essentially of, ( 1 ) a primary amine group bonded to the aromatic group and (2) a hydroxy, thiol or primary or secondary amine group bonded to the aro¬ matic group ortho to said primary amine group. O-amino-basic moieties in monomers useful to make polymers of the present invention comprise a hydroxy or thiol moiety, and most preferably comprise a hydroxy moiety. Aromatic group (Ar) - any aromatic ring or ring system. Size is not critical as long as the aromatic group is not so big that it interferes with the synthe¬ sis, fabrication or use of the polymer which contains the aromatic group. Each aromatic group independently preferably comprises no more than about 18 carbon atoms, more preferably no more than about 12 carbon atoms and most preferably no more than about 6 carbon atoms. Each may comprise a nitrogen-containing heterocyclic ring, but is preferably carbocyclic and more preferably hydrocarbyl. Unless otherwise specified, each aromatic group may comprise a single aromatic ring, a fused ring system or an unfused ring system containing two or more aromat¬ ic rings joined by bonds or by divalent moieties that are inert with respect to PBZ polymerizing reagents under polymerization conditions. Suitable divalent moieties comprise, for example, a carbonyl group, a sulfonyl group, an oxygen atom, a sulfur atom, an alkyl group and/or a perfluorinated alkyl group. Each aro¬ matic group is preferably a single six-membered ring. Each aromatic group may contain substituents which are stable in solvent acid and do not interfere with the synthesis, fabrication and/or use of the poly- benzazole polymer or copolymer. Examples of preferred substituents include halogens, alkoxy moieties, aryloxy moieties or alkyl groups. More preferred substituents are either an alkyl group having no more than about 6 carbon atoms or a halogen. Most preferably, each aro¬ matic group contains only those substituents specifi¬ cally called for hereinafter. Azole ring - an oxazole, thiazole or imidazole ring. The carbon atom bonded to both the nitrogen atom and the oxygen, sulfur or second nitrogen atom is the 2-carbon, as depicted in Formula 3 wherein Z is -0-, -S- or -NR-; and R is hydrogen, an aromatic or an aliphatic group, preferably hydrogen or an alkyl group, and most preferably hydrogen. In poly¬ mers of the present invention, each azole ring is inde¬ pendently oxazole or thiazole and is preferably oxazole. In PBZ polymers, the 4- and 5-carbon of each azole ring is ordinarily fused with an aromatic group. Electron-deficient carbon group (Q) - any group containing a carbon atom which can react in solvent acid with an o-amino-basic moiety to form an azole ring, such as the groups listed in column 24, lines 59-66 of the 4,533 > 693 patent. Preferred electron-deficient carbon groups are carboxylic acids, acid halides, metal carboxylate salts, cyano groups and trihalomethyl groups. Halogens in electron-deficient carbon groups are preferably chlorine, bromine or iodine and more preferably chlorine. Polybenzazole (PBZ) polymer - A polymer from the group of polybenzoxazoles and polybenzobisoxazoles (PBO), polybenzothiazoles and polybenzobisthiazoles (PBT) and polybenzimidazoles or polybenzobisimidazoles (PBI). For the purposes of this application, the term "polybenzoxazole (PBO)" refers broadly to polymers in which each mer unit contains an oxazole ring bonded to an aromatic group, which need not necessarily be a ben- zene ring. The term "polybenzoxazole (PBO)" also refers broadly to poly(phenylene-benzo-bis-oxazole)s and other polymers wherein each mer unit comprises a plurality of oxazole rings fused to an aromatic group. The same understandings shall apply to the terms polybenzothia- zole (PBT) and polybenzimidazole (PBI). Polybenzazole polymers used in membranes of the present invention are PBO and/or PBT polymers and/or copolymers. Solvent acid - any non-oxidizing liquid acid capable of dissolving PBZ polymers, such as sulfuric acid, methanesulfonic acid, trifluoromethylsulfonic acid, polyphosphoric acid and mixtures thereof. It must be sufficiently non-oxidizing that it does not substan¬ tially oxidize AB- and BB-PBZ monomers which are dis- solved therein. Solvent acids are preferably dehydrat¬ ing acids, such as polyphosphoric acid or a mixture of methanesulfonic acid and phosphorus pentoxide. Pre¬ ferred concentrations of P2O5 -*- n the methanesulfonic acid are described in U.S. Patents 4,847,350 and 4,722,678. Concentrations of P2O5 i n the polyphosphoric acids are described in U.S. Patents 4,533,693 and 4,722,678. It preferably contains at least about 80 weight percent P2O5 a t the commencement of the reaction, more preferably at least about 85 weight percent and most preferably 87 weight percent and preferably has a P2O5 content of at most about 90 percent, more preferably at most about 88 percent. AA-monomers contain two electron-deficient car¬ bon groups linked by a divalent organic moiety does not interfere with synthesis, fabrication or use of the polymer.. Electron-deficient carbon groups conform to the definitions and preferred embodiments previously set out. The divalent organic moiety preferably comprises a saturated aliphatic moiety or an aromatic group, more preferably comprises an aromatic moiety, and most pref¬ erably comprises an aromatic moiety having the electron- -deficient carbon groups in para positions with respect to each other. Examples of suitable AA-monomers are set out in U.S. Patent 4,533,693, columns 25-32, Tables 4-6. Preferred examples of AA-monomers include terephthalic acid, isophthalic acid, bis-(4-benzoic) acid and oxy- -bis-(4-benzoic acid) and acid halides thereof. Other examples of AA-monomers include bis-(4-benzoic acid) sulfone, bis-(4-benzoic acid) ketone, bis-(4-benzoic acid)isopropane, 1,1,1,3,3,3-hexafluoro-2,2-bis- -(4-benzoic acid)-propane, 1 ,4-bis-(oxy-4-benzoic acid)perfluorocyclobutane and acid halides thereof. AA- -monomers preferably conform with formula 4: 4 Q-DM-Q ,- wherein DM is the divalent organic moiety and each Q is an electron-deficient carbon group. BB-monomers comprise: 10 1. an aromatic group 2. a first o-amino-basic moiety bonded to said aromatic group; and 3. a second o-amino-basic moiety bonded to said aromatic group. 15 The aromatic group and o-amino-basic moieties have the meaning and preferred embodiments previously defined. The BB-monomer preferably complies with Formula 5: 5 wherein: Ar ' is an aromatic moiety as previ¬ ously described; and each Z conforms to the definition and 0 preferred embodiments previously given in describing azole rings. The aromatic group may comprise a plurality of fused and unfused rings, such as a tetravalent naphthyl, biphenyl, diphenyl ether or diphenyl sulfone moiety, but prefer¬ ably is a six-membered ring, such as a tetravalent phe- nylene or pyridinylene ring. Suitable examples of BB- -monomers are described in U.S. Patent 4,533,693, col¬ umns 19-24, Tables 1-3. Examples of preferred BB-mono- c - mers include 4,6-diaminoresorcinol, 2,5-diaminohydro- quinone, 1 ,4-dithio-2,5-diaminobenzene, bis-(3-amino- -4-hydroxy-phenyl) ether, bis-(3-amino-4-hydroxy-phenyl) sulfone, bis-(3-amino-4-hydroxy-phenyl) ketone, bis- -(3-amino-4-hydroxy-phenyl) isopropane, 1 , 1, 1 ,3,3,3-hex- 10 afluoro-2,2-bis-(3-amino-4-hydroxy-phenyl)propane or 3,3'-diamino-4,4'dihydroxy-biphenyl. BB-monomers are typically stored as salts of non-oxidizing acids, such as hydrogen chloride or phosphoric acid. 15 AB-monomers preferably comprise: 1. an aromatic group 2. a first o-amino-basic moiety 2 Q bonded to said aromatic group; and 3. an electron-deficient carbon group linked to said aromatic group. The electron-deficient carbon group may be bonded 25 directly to the aromatic group in the monomer or may be linked to the aromatic group by a moiety such as an ali¬ phatic or aromatic moiety which is inert with respect to all reagents under reaction conditions. AB-monomers preferably conform with Formula 6(a): 30 NH- 6(a) Q-RB- ■ Ar ZH and more preferably conform with Formula 6(b): wherein RB is either a bond or an aliphatic or aromatic moiety which is inert under polymerization conditions, and all characters have the meaning and preferred em- bodiments previously given. Examples of suitable AB-monomers are shown in U.S. Patent 4,533,693, column 32-35, Tables 7-8. Preferred examples include 3-amino- -4-hydroxybenzoic acid, 3-hydroxy-4-aminobenzoic acid, 3-mercapto-4-aminobenzoic acid, and the acid halides or esters thereof. Sources and synthesis for AA-, BB- and AB-mono¬ mers are given in U.S. Patent 4,533,693, columns 19-35, Tables 1-8; in Lysenko, High Purity Process for the Preparation of 4,6-Diamino-1 ,3-Benzenediol, U.S. Patent 4,766,244 (August 23, 1988); in Lysenko, Preparation of 3-Amino-4-Hydroxybenzoic Acids, U.S. Patent 4,835,306 (May 30, 1989); and in Inbasekaran, Preparation of Diamino- and Dialkylaminobenzenediols, U.S. Patent 4,806,688 (February 21, 1989). (For the purpose of this application, when the amine groups and Z moieties of a monomer are depicted as bonded to an aromatic group without indicating their position, as in the illustrations of AB- and BB-monomers previously, it shall be understood that: (1 ) each amine group is ortho to one Z moiety; and (2) if the monomer has two o-amino-basic moieties, one primary amine group and Z moi¬ ety may be in either cis position or trans position with respect to the other amine group and Z moiety, as illustrated in For¬ mulae 7(a)-(b) and as described in 11 Ency. Poly. Sci. & Eng., supra, at 602. cis trans The same understandings apply with respect to nitrogen atoms and Z moieties in an azole ring fused to an aro¬ matic moiety, as illustrated in 11 Ency. Poly. Sci. & Eng., supra, at 602.) Polymers Useful in the Prac- tice of the Present Invention Membranes of the present invention contain a polybenzazole (PBZ) polymer chosen from the group consisting of polybenzoxazole (PBO) and/or poly¬ benzothiazole (PBT) polymers or copolymers (hereinafter referred to collectively as polybenzazole polymers). The polybenzazole polymer may consist essentially of AA/BB-PBZ mer units and/or AB-PBZ mer units, as those terms are previously defined. Alternatively, the polybenzazole polymer may be a more complex copolymer, that contains both AA/BB-PBZ mer units and/or AB-PBZ mer units and mer units of other polymers such as poly(aromatic ether) or polyamide. Polybenzazole polymers that consist essentially of AA/BB-PBZ mer units and/or AB-PBZ mer units are known and have been described in numerous publications and patents, such as Sybert et al., Liquid Crystalline Polymer Compositions, Process and Products, U.S. Patent 4,772,678 (September 20, 1988); Wolfe et al., Liquid Crystalline Polymer Compositions, Process and Products, U.S. Patent 4,703,103 (October 27, 1987); Wolfe et al., Liquid Crystalline Polymer Compositions, Process and Products, U.S. Patent 4,533,692 (August 6, 1985); Wolfe et al., Liquid Crystalline Poly(2,6-Benzothiazole) Com¬ positions, Process and Products, U.S. Patent 4,533,724 (August 6, 1985); Wolfe, Liquid Crystalline Polymer Com¬ positions, Process and Products, U.S. Patent 4,533,693 (August 6, 1985); Evers, Thermoxadatively Stable Articulated p-Benzobisoxazole and p-Benzobisthiazole Polymers, U.S. Patent 4,359,567 (November 16, 1982); Tsai et al., Method for Making Heterocyclic Block Copolymer, U.S. Patent 4,578,432 (March 25, 1986) and 11 Ency. Poly. Sci. & Eng., Polybenzothiazoles and Polybenzoxazoles, 601 (J. Wiley & Sons 1988). The polybenzazole polymers are typically divided into several categories. The polymer preferably consists essentially of AA/BB-PBZ mer units. Each mer unit is individually preferably represented by one of Formulae 1 and 2, wherein each Z is an oxygen atom or a sulfur atom. The polybenzazole polymer may also be classified as a rigid rod AA/BB-polymer, a semi-rigid rod mesogenic polymer or a flexible coil (non-mesogenic) polymer, as defined in 11 Encyclopedia Poly. Sci. & Eng., supra, at 605-07. Rigid rod AA/BB-polymers consist essentially of mer units that are essentially rectilinear, i.e., containing essentially no cantenation angles less than 150°. Typically, the first aromatic group is a 1 ,2,4,5-phenylene moiety, a 2,3,5,6-pyridinylene moiety, a 2,3,5,6-pyrimidinylene moiety or a 2,3,6,7-naphthalene moiety or substituted variation thereof. It is preferably a 1 ,2,4,5-phenylene moiety. Typically, the divalent organic moiety is a 1 ,4-phenylene moiety or a 4,4'-biphenylene moiety. It is preferably a 1 ,4-phenylene moiety. The rigid rod polybenzazole polymer preferably consists essentially of mer units represented by represented by one of Formulae 8(a)-(d) and more preferably by one of Formulae 8(a) or (b). and Semi-rigid rod AB-PBZ polymers preferably consist essentially of one or more mer units represented by one of Formulae 9(a)-(d), or a substituted variation thereof, and more preferably by Formula 9(a). and Flexible coil polybenzazole polymers may contain either AB- or AA/BB-mer units, but are characterized in that those mer units are not essentially rectilinear (i.e., they have angles of cantenation which are less than 150°C). The reference K.-U. Biihler, Spezialplaste 838-866 (Akademie-Verlag 1978), reports in Table 158 from 844-854 the structures and synthesis of many flexible coil polybenzazole poly¬ mers. A preferred flexible coil polybenzazole polymer comprises "jointed" AA/BB-mer units which are repre- sented by Formula 10(a): wherein : each Ar3 is an aromatic group; X is a bond or a first divalent linking moiety (X), which is inert with respect to PBZ monomers and polymeriza¬ tion reagents under PBZ polymerization conditions; L is either Arm 0 r -Ar-X-Ar-; and each Z is independently an oxygen or a sulfur atom. Arm is a meta-oriented aromatic group, such as an m-phenylene moiety, an m-pyridinylene moiety, a 3,4'-biphenylene moiety, a 4,3'-biphenylene moiety, a 3,3'-biphenylene moiety or a 2,6-naphthalene moiety. X and X' are preferably a sulfonyl moiety, an oxygen atom, a sulfur atom or an aliphatic group, and are more. pref¬ erably either a sulfonyl moiety or an oxygen moiety. Most preferably, the X is a sulfonyl moiety and X' is an oxygen atom. Jointed AA/BB-mer units are most preferably represented by Formula 10(b): 10(b) Flexible coil polybenzazole may also be a jointed AB-polybenzazole, which has mer units such as is illustrated in Formula 11: N. 11 -Ar-X-Ar ^ >- wherein X has the definition previously give, except that it is not a bond. Flexible coil polybenzazole may also comprise an aliphatic polybenzazole mer units, which are represented by Formula 12(a) or (b): 12 N (a) ■ Ar e % >- wherein Ara is an aromatic group appropriate for an AB-polymer; R is a divalent organic moiety which com¬ prises, and preferably consists essentially of, an ali¬ phatic group which does not interfere with the synthe¬ sis, fabrication or use of the polymer, and all other moieties have the meaning previously described. The polybenzazole polymer may contain several known variations. For instance, the polymer may contain a mixture of rigid, semi-rigid and flexible mer units. It may contain both PBO and PBT mer units. Different mer units may contain different aromatic groups and/or different divalent organic moieties. The various different mer units may be contained in the polymer in random, sequential or block arrangement. A particularly desirably copolymer system is a block copolymer containing blocks of rigid rod polybenzazole linked to blocks of flexible coil polybenzazole. The flexible coil polybenzazole is preferably a "jointed" AA/BB- -polybenzazole having the definition and preferred embodiments previously given. The polybenzazole polymers can be synthesized by known processes such as those described in the references previously listed. The synthesis ordinarily involves the condensation reaction of an AA-monomer and a BB-monomer, or the self-condensation of an AB-monomer. in a non-oxidizing dehydrating solvent acid under inert atmosphere with vigorous agitation at a temperature which is between 50°C and 120°C at the commencement of the reaction and is increased in a stepwise manner to between 190°C and 220°C at the end of the reaction. Flexible coil polybenzazoles can also be synthesized by an aromatic nucleophilic displacement mechanism, such as is described in Hedrick et al, "Syn¬ thesis of Imide-Aryl Ether Benzoxazole Random Copoly¬ mers," 30 Polymer Preprints 265-66 (1989), and Inbase- karan et al., Nucleophilic Displacement Process for Synthesis of Non-Rigid Rod Polymers, Ser. No. 313,936 (filed February 22, 1989). The polybenzazole polymer may also be a more complex random, sequential and block copolymer, that contains AB- and/or AA/BB-mer units as previously described and also contains mer units associated with other polymers, such as polyamide, polyimide, polyquinoline, polyquinoxaline and/or preferably poly- (aromatic ether ketone or sulfone) polymers. Such co¬ polymers and processes to synthesize them are described in Dahl et al., Aromatic Poly(Ether Ketones) Having Imide, Amide, Ester, Azo, Quinoxaline, Benzimidazole, Benzoxazole or Benzothiazole Groups and Method of Prep¬ aration, PCT Publication 86/02368 (April 24, 1986); and Harris et al., Copolymers Containing Polybenzoxazole, Polybenzothiazole and Polybenzimidazole Moieties, PCT Publication No. WO 90/03995 (April 19, 1990). Preferred copolymers can be described as polybenzazole- -pol (aromatic ether ketone or sulfone) copolymers (hereinafter PEK-PBZ) because they contain both benzazole moieties and sequences of aromatic groups linked by ketone and/or sulfone and/or ether moieties. A preferred example of a PEK-PB0 copolymer is illustrated in Formula 20(a): wherein each Y is a -CO- or -S0 2 - moiety, "a" is a num¬ ber of repeating units which averages at least about 0.3 and less than 10, and B is a number of repeating units greater than 1. Block copolymers of polybenzazole and PEK-PBZ are preferably represented by Formula 20(b): 20(b) wherein: each Y is independently a sulfonyl or carbonyl moiety; each Ar is independently an aromatic group; each a is independently a number of PBZ mer units equal on average to at least about 10; each b is a number of thermoplastic mer units equal to at least 1 ; each m is a number of PBZ mer units within each thermoplastic mer unit and is equal on average to 0 to 3, inclusive; and c is a number of repeating PBZ and thermoplastic blocks equal to at least 1. Membranes and Their Synthesis The present invention uses a porous membrane. The porous membrane may be isotropic, asymmetric or composite. Isotropic membranes contain a single porous layer that functions as a discriminating layer. Asymmetric and composite membranes contain a supporting layer and a discriminating layer. The supporting layer is preferably thicker and has larger pores than the discriminating layer. In an asymmetric membrane, the supporting layer and the discriminating layer have a similar chemical composition. In a composite membrane, the supporting layer and the discriminating layer have different chemical compositions. The membrane must contain at least one polybenzoxazole or polybenzothiazole polymer or copolymer as previously described (collectively referred to hereinafter as polybenzazole polymers). If the membrane is a composite membrane, the polybenzazole polymer may be in the supporting layer or in the discriminating layer or in both. Any layer containing the polybenzazole polymer may also contain other polymers that can be dissolved and coagulated with the polymer or copolymer, such as poly(aromatic ether ketone) and poly(aromatic ether sulfone) polymers and polyamide polymers. A layer containing the polybenz¬ azole polymer preferably does not contain other polymers. The layer more preferably consists essentially of a single polybenzoxazole or polybenzothiazole polymer or copolymer. The physical shape of the membrane is not critical. For instance, the membrane may be a flat membrane or it may be a hollow fiber or tube. The thickness of the membrane is limited by practical considerations. It should be thick enough that is can be made and used without undesirable loss of retention due to tearing, pinholes, etc. It should be thin enough to provide desirable flux. A flat isotropic membrane is preferably at least about 5 μm thick, more preferably at least about 10 μm thick, and is preferably at most about 500 μm thick, more preferably at most about 300 μm thick. A flat asymmetric or composite membrane is preferably at least about 10 μm thick, more preferably at least about 15 μm thick, and is preferably at most about 500 μm thick, more preferably at most about 300 μm thick. The discriminating layer in an asymmetric membrane or composite is preferably at least about 0.1 μm thick, more preferably at least about 0.2 μm thick, and is preferably at most about 20 μm thick, more preferably at most about 1 μm thick. The inside diameter of a hollow fiber isotropic or asymmetric membrane is preferably at least about 50 μm and at most about 5000 μm, and the outside diameter is preferably at least about 60 μm and at most about 6000 μm. The membrane contains pores. The size of the pores varies depending upon the purpose for the membrane and the layer of the membrane in question. The supporting layer of an asymmetric membrane or a composite membrane has very large pores, whose average size may be, for example, 2 μm or greater. The average pore size in the discriminating layer of an ultra- filtration membrane is preferably at least about 20 angstroms, and is preferably at most about 500 angstroms and more preferably at most about 100 angstroms. The average pore size in the discriminating layer of a microfiltration membrane is preferably at least about 0.05 μm, more preferably at least about 0.1 μm, and is preferably no greater than about 2 μm, more preferably no greater than about 0.2 μm. The best process to make membranes that contain a polybenzazole polymer varies, depending upon the desired properties of the membrane. The membrane may be formed by a wet phase inversion process, such as that described in R. E. Resting, Synthetic Polymeric Membranes - 2nd Ed. 251-61 (J. Wiley & Sons 1985). A dope solution containing the polymer in a solvent acid is contacted with a non- -solvent under conditions such that a membrane is formed. The solvent acid preferably comprises, and more preferably consists essentially of polyphosphoric acid (PPA) and/or methanesulfonic acid (MSA), optionally containing a minor portion of P2θ5« It most preferably consists essentially of polyphosphoric acid. The poly- benzazole polymers previously described can be synthesized by reaction of monomers in a solvent acid. Polymer may be coagulated directly from the resulting dope to form a membrane. Alternatively, the polymer or copolymer may be precipitated from the polymerization dope and redissolved to form a second dope solution suitable for membrane synthesis. The dope may contain mixtures of different polybenzazole polymers. It may also contain polymers other than PBO and PBT which are stable in the solvent acid under conditions used to make membranes and which can coprecipitate with the PBO and/or PBT polymer. Examples of solutions containing mixtures of PBO and/or PBT polymer and other polymers are described in Uy, Spinnable Dopes and Articles Therefrom, U.S. Patent 4,810,735 (March 7, 1989). Suitable polymers may include polyamides and polyaromatic ether ketones. Most preferably, the dope contains no polymer other than the PBO and/or PBT polymer or copolymer. The concentration of polymer in the dope should be chosen such that the dope is processable to form a membrane having acceptable physical properties. At very low concentrations, the ope may not contain sufficient polymer to form a uniform membrane. At very high con¬ centrations the dope may be too viscous to use. Optimal concentrations will vary depending upon the polymer and solvent acid in the dope, the desired membrane proper¬ ties and the method of processing. Unlike the systems described in Resting, supra, at 252, dopes containing polybenzazole can be highly viscous at relatively low polymer concentrations, so that highly porous structures are formed without the need for pore forming compounds aside from the polymer, solvent acid and diluent. The viscosity of a given dope is dependent upon many factors, such as the solvent acid chosen, the chemical make-up of the polymer, the rigid¬ ity of the polymer backbone, the degree of polymeriza¬ tion of the polymer, the concentration of the polymer, and the liquid crystallinity of the dope solution. Persons of ordinary skill in the art can adjust those factors to find the optimal conditions for their desired membrane without undue experimentation. Cast membranes are synthesized by spreading the dope on a substrate, such as a glass or a supporting membrane, leveling the dope at a desired thickness, and contacting the dope with a diluent to cause it to coagu¬ late. Cast membranes are preferably formed from more dilute dopes than are extruded membranes. The concen¬ tration of rigid rod polymer in an MSA or PPA dope is preferably at least about 0.5 weight percent and more preferably at least about 1.0 weight percent. Extruded membranes are formed by forcing dope through an extrusion die, for instance in the form of a sheet or a hollow tube, across an air gap and into a coagulation bath. Hollow fibers are spun and drawn from a hollow fiber die (spinnerette) across an air gap. The spun dope is coagulated in a coagulation bath to yield a coagulated fiber. Such processes are described in 12 Kirk-Othmer Ency. Chem. Tech. , Hollow Fiber Membranes, at 492 etseq (J. Wiley & Sons 1981). The concentration of polymer may be higher than is ordinary for cast membranes. It is frequently at least about 1 percent. The concentration of rigid rod polymer in an MSA or PPA dope is preferably no more than about 50 weight percent, more preferably no more than about 40 weight percent and most preferably no more than about 16 percent. The preferred concentrations for block copolymers of rigid and flexible polybenzazole may be lower if the block copolymer has a higher viscosity than rigid polybenzazole at equivalent concentrations. The dope film may be stretched before coagula¬ tion by known processes, such as by tentering or by forming a bubble. Film extrusion and stretching pro¬ cesses are described in Harvey et al., Biaxially Oriented Ordered Polymer Films, PCT International Pub- lication No. WO 89/12072 (December 14, 1989); Harvey et al., Molecularly Ordered Tubular Components Having a Controlled Coefficient of Thermal Expansion, PCT International Publication No. W089/12546 (December 28, 1989); Lusignea et al., Multiaxially Oriented Thermo- tropic Polymer Films, PCT International Publication No. W089/12547 (December 28, 1989); and Chenevey, Process for the Production of Biaxially-Oriented Rigid Rod Heterocyclic Liquid Crystalline Polymer Films, U.S. Patent 4,898,924 (February 6, 1990). The stretching by be uniaxial or biaxial. Stretching does not appear to substantially effect the porosity of the resulting membrane, but does appear to increase the flux coming through the membrane. The coagulation of the polymer to form the membrane is accomplished by contacting the dope with a non-solvent diluent. The diluent may be any liquid which reduces the solubility of the polymer in the solvent acid so that the polymer coagulates. It may be organic or aqueous. The aqueous diluent may contain a base, or even small quantities a weak acid, but it is conveniently water at commencement. Following coagula¬ tion, the polymer is typically washed in water to remove as much residual solvent acid as is practical. Washing may continue for 24 hours or more. The polymer as coagulated forms an open micro- porous structure that remains microporous as long as the polymer remains wet. This open structure can operate as a porous membrane. The pore size is ordinarily suitable for ultrafiltration and/or microfiltration purposes. Pore size can be varied by varying factors such as the concentration of polymer in the solvent and the choice of the coagulating bath. The average pore size of a membrane coagulated from a dope containing 14 percent rigid rod polybenzazole polymer is preferably no more than about 35 angstroms, and more preferably no more than about 30 angstroms. The average pore size is preferably no less than about 25 angstroms. The flux of a membrane coagu¬ lated from dope containing about 3 percent block copoly¬ mer of rigid rod and jointed polybenzazole preferably is preferably at least about 1 Q 0 ml /( cm 2)(hr)(cm Hg) for clear water. The average pore size is preferably no more than about 35 angstroms, and more preferably no more than about 30 angstroms. The average pore size is preferably no less than about 25 angstroms. If a membrane made by the phase inversion process is oven dried, the pores frequently constrict to an undesirably small size, from which they do not open after the membrane is rewetted. The membrane may be freeze-dried to provide an open-pored membrane which can be rewetted. However, freeze-dried membranes are very brittle and ordinarily show lower flux and/or lower rejection than the initially prepared membrane. Membranes made by phase-inversion are preferably kept wet from the time that they are coagulated through completion of their use. The conditions of coagulation can be varied to provide either an isotropic membrane or an asymmetric membrane. When the membrane is contacted with diluent while one side of the membrane is protected from contact by an impermeable layer such as glass or Teflon ™ poly¬ mer, then the membrane is ordinarily asymmetric, having larger pores throughout the bulk of the membrane and smaller pores collected at one side of the membrane. Membranes having larger pore sizes can be formed by a leaching process, as described in Resting, supra, at 303-05. In a leaching process, a homogeneous dope solution containing a solvent acid, a polybenzazole polymer, and a pore-forming compound is contacted with a diluent that coagulates both the polymer or copolymer and the pore-forming compound. Then the pore-forming compound is leached away using a solvent which does not dissolve the polybenzazole polymer. The pore-forming compound must be soluble in the solvent acid and in an organic solvent that does not dissolve the polybenzazole polymer. It should precipitate under the same conditions which cause the polymer to precipitate. It preferably does not react with the solvent acid. Suitable pore-forming compounds include, for example: benzoic acid, benzophenone, 2-phenylphenol, bisphenol A, triphenylmethanol, phenylthiazine, 2-methoxynaphthalene, ethyl-4- -hydroxybenzoate, 2,5-diphenyloxazole, or low molecular weight bisphenol A epoxy resins. The pore-forming compound tends to coprecipi- tate with the polymer in discrete phases when the acid is diluted. The size of those phases governs the aver¬ age pore size in the resulting porous article. The choice of pore-forming compound has a significant impact upon the size of pore-forming compound zones in the coagulated polymer and consequently upon the average pore size of the porous article. As a general rule, pore-forming compounds which are more compatible with the PBO or PBT polymer form smaller pores than do pore- -forming compounds which are less compatible with the PBO or PBT polymer. For instance, porous articles made using benzophenone as a pore-forming compound have larger pores than do pores formed using bisphenol A as a pore-forming compound. The dope solution is coagulated and washed as previously described. Unlike membranes coagulated without a pore-forming compound, membranes which are coagulated with a pore-forming compound are preferably completely or partially dried. If the membrane is completely dried prior to the leaching step, the PBO polymer is substantially inflexible during the leaching step, the pores in the membrane do not collapse during leaching, and the resulting membrane ordinarily has a larger pore size. If the membrane is only partially dried, the PBO remains flexible and the pores can par¬ tially collapse, resulting in a smaller average pore size. The membrane is contacted with an organic sol¬ vent capable of dissolving the pore-forming compound for a period of time sufficient to leach out the pore-form¬ ing compound. The solvent should be one which does not dissolve the PBO or PBT polymer. The solvent is prefer¬ ably volatile, so that it can easily be removed from the resulting porous article. Examples of suitable solvents include acetone, methanol, and methylene chloride. The membrane synthesized by the leaching pro¬ cess may have a thickness suitable to act as a discrimi¬ nating layer or a thickness suitable to act as a supporting layer. The membrane typically has an open cellular structure. The average cell size may be as low as about 1 micron or as high as about 200 microns. The average cell size is preferably at least about 3 microns and more preferably no more than about 100 microns. Apparatus and Process for Using Membranes The membranes previously described, made by either process, may be used in known applications for porous membranes. If they are intended for use as a discriminating layer, then they may be attached to a known supporting layer to form a composite membrane, or, if they are strong enough to survive the pressure gra¬ dient across the membrane, they may be used without support. If they are intended for use as a supporting layer, then they must be attached to a discriminating layer to form a composite membrane. Typically, the finished membrane, whether isotropic, composite or asymmetric, is placed in an apparatus containing a high pressure zone and a low pressure zone divided from each other by the membrane. The zones are arranged such that materials can move from the high pressure zone to the low pressure zone by per¬ meating thorough the membrane, but by no other route, at least while the membrane is in use. The exact nature of the zone may depend upon the nature of the membrane. With a hollow fiber or tubular membrane, for instance, the high pressure zone may be inside of the fiber and the low pressure zone may be outside or υice versa . With a flat membrane, the high pressure zone is against one face of the membrane and the low pressure zone is against the other. A feed mixture containing at least one low molecular weight fluid component and at least one separable component is contacted with the membrane in the high pressure zone. The fluid component must pass through the membrane readily. It usually has a low molecular weight as compared to the separable component, and is chosen such that it does not cause the membrane to degrade rapidly. Examples of suitable fluid components include water, lower molecular weight hydrocarbons and halogenated hydrocarbons. The separable component(s) should have a molecular weight high enough or a particle size large enough that a substantial portion of the separable component does not pass through the membrane. Such criteria cannot be judged without reference to the average pore size and the uniformity of pore size in the membrane. Prefer¬ ably, however, the membrane retains at least about 50 percent of the separable component, more preferably at least about 80 percent and most preferably at least about 95 percent. The pressure across the membrane is preferably high enough to maximize flux without rupturing the mem¬ brane or unduly compromising selectivity. As such, the pressure is highly dependent upon many variables, such as the fluid and separable component, the strength and thickness of the membrane and the physical form which it is in. For example, with different membranes and apparatuses, the pressure difference between the high pressure zone and the low pressure zone (the pressure across the membrane) may be at least 1 psi (70 g/cm2) or at least 10 psi (700 g/cm.2) or at least 100 psi (7000 g/cm2) or higher. Persons of ordinary skill can determine the ideal pressure for particular membranes and apparatuses without undue experimentation. The fluid component permeates through the mem¬ brane from the high pressure zone to the low pressure zone, where it may be disposed of or recovered as an enriched stream. The separable component may be recov¬ ered in an enriched stream from the high pressure zone. The flux of the separable component through the membrane varies greatly depending upon numerous factors, such as pore size and density and the conditions under which the membrane is used. The flux of clear water through ultrafiltration membranes described previously is preferably at least about 20 m ^/(m2)(hr)(cm Hg), more preferably at least about 30 ml /( m 2) (hr)(cm Hg)' and most preferably at least about 100 ml /( cm 2) (hr)(cm Hg)* The flux of clear water through the microfiltration membranes described previously is preferably at least about 200 ml /( m 2)(hr)(cm Hg)- Many apparatuses are known to carry out such a process. For instance, the peripheral area of a flat film membrane is affixed to a framing structure which supports the outer edge of the membrane. The frame may also support a screen which supports the membrane. The membrane can be affixed to the framing structure by a clamping mechanism, adhesive, chemical bonding, or other techniques known in the art. The frame may also support a screen which supports the membrane. The membrane affixed to the frame can be sealingly engaged in the conventional manner in a vessel so that the membrane surface inside the framing support separates two other¬ wise non-communicating regions in the vessel. One skilled in the art will recognize that the structure which supports the membrane can be an internal part of the vessel or even the outer edge of the membrane. The membrane separates a higher pressure zone on one side of the membrane into which the feed mixture is introduced from a lower pressure zone on the other side of the membrane. The membrane is contacted with a feed mixture under pressure, while a pressure differential is maintained across the membrane. Suitable processes and apparatuses for use of flat, hollow tube and hollow fiber membranes are described in 12 Rirk-Othmer Ency. Chem. Tech., Hollow Fiber Membranes, at 506-08 (J. Wiley & Sons 1981) and 23 Rirk-Othmer Ency. Chem. Tech. , Ultrafiltration, at 453-58 (J. Wiley & Sons 1981), R. E. Resting, supra, at 44-64; Porter, "Microfiltration", Synthetic Membranes: Science, Engineering & Applications 225-48 (D. Reidel Publ. 1986); Aptel et al., "Ultrafiltration", Synthetic Membranes: Science, Engineering & Applications 249-306 (D. Reidel Publ. 1986); and S. T. Hwang, Membranes in Separations 471-77, 481-89. The following examples are for illustrative purposes only and are not to be taken as limiting either the specification or the claims. Unless stated other¬ wise, all parts and percentages are by weight. Example 1 - Leaching Process Porous Sheet Made Using Benzoic Acid A powdered cis-PBO having an intrinsic viscos¬ ity of 21.1 dL/g is dissolved in methanesulfonic acid with benzoic acid in the proportions shown in Table I. The solutions are mechanically shaken for 24 hours resulting in a uniform, clear, brown solution. The solutions are cast on glass plates using a 12 mil clear¬ ance casting bar. The solutions are also cast upon a 3.8-mil thick Sanko™ support obtained from AWA Company of Japan using a 12 mil clearance casting bar. Each cast film is immediately immersed in 15°C water for at least 4 hours. Those films which are not cast upon a Sanko support are stripped from the glass plate and taped to a Teflon™ sheet. Each film is dried in air for 16 hours, and then the films dried on Teflon™ are cut off of the Teflon sheet™. Each film is cut into samples. Each sample is immersed for 30 minutes in methanol and then dried in air at room temperature for about 2 hours. The films cast upon Sanko support are observed using a Nikon Optiphot™ transmission light microscope. Observation shows the films to have the average cell size given in Table I. The films cast upon glass are weighed and have the loss of weight shown. Example 2 - Leaching Process Porous Sheet Made Using Benzoic Acid The procedure of Example 1 is followed casting a solution containing 0.3 part PBO, 30 parts methanesul¬ fonic acid, and 3.2 parts benzoic acid on a glass plate. Examination of the membrane under light microscope shows that it is structured as a network of interconnected open cells. Example 3 - Leaching Process Porous Sheet Made Using Benzoic Acid Three solutions containing 0.5 part PBO, 30 parts methanesulfonic acid, and 4 parts benzoic acid are cast on a 3-8-mil thick Sanko™ substrate using a 15 mil clearance casting bar according to the procedures described in Example 1. Prior to being immersed in methanol, each is dried for a period of time shown in Table II. Observation of the resulting film shows the cell size set out in Table II. Example 4 - Use of Membrane Made By Leaching Process The membranes prepared in Samples 4 and 6 of Example 1 are loaded in an Amicon Model 8050™ 5 cc stir cell. The permeability of each membrane is tested using, as the feed stream, five 0.3 percent polystyrene latex solutions containing average latex particle sizes of 0.091 μm, 0.173 μm, 0.215 μm, 0.527 μm, and 0.913 μm. Permeation is tested at room temperature under 20 psig pressure. The permeate is collected and analyzed for turbidity by UV spectroscopy to determine the concentra¬ tion of polystyrene latex. The membranes show the char¬ acteristics set out in Table III. Permeation figures are in gallons per square foot per day (GFD) at 20 psig. TABLE III Sample Retention Flux (GFD) 4 7555 0.913 μm 1.2 6 3555 0.913 μm 2.7 Example 5 - Leaching Process Porous Membrane Made Using Alternative Pore-Forming Compounds The procedure of Example 1 is repeated using benzophenone (BZPN) and bisphenol A (bis-A) in place of benzoic acid. The results are reported in Table IV. Ave. Cell Size (um) 15 20 10 The experiment of Example 4 is repeated using membranes from Samples 15 and 19. The membranes show the permeation characteristics set out in Table V. TABLE V Sample Retention Flux (GFD) 15 65% 0.913 μm 1.8 19 4055 0.913 μm 1.5 Example 6 - Phase Inversion Membrane Containing Rigid Rod Polymer Two polybenzoxazole polymer dopes are synthe¬ sized according to the following process. A mixture of 1387 g of polyphosphoric acid (PPA) containing about 76 percent P2O5, 300 g of 4,6-diaminoresorcinol bis(hydro- chloride) (DAR), 233 g of terephthalic acid (TA), 0.4 g of benzoic acid (BA), and 539 g of phosphorus pentoxide (P2O5) is added to a batch reactor outfitted with a stirrer under nitrogen atmosphere. The ratios of reagents are chosen to provide a reaction mixture in which the PPA initially contains about 83.5 weight percent P2O5, and in which the finished dope contains about 14 weight percent polymer. Each mixture is stirred with heating under nitrogen flow for 20 minutes at 8θ°C to 85°C, then the temperature is raised to 105°C to 115°C for 10 to 12 hours and raised again to 120°C to 130°C for about 2 hours. During the final two hours, the reaction is carried out under vacuum. The dope is transferred under vacuum to a piston-agitated reactor at a temperature between 150°C and 190°C, and the reaction is continued in that reactor at about 190°C under vacuum for about to 2 hours until the viscosity increase in the reactors levels out. The samples are extruded through a flat die extruder set at 0.001 inch thickness using a barrel temperature of about 190°C, a barrel pressure of about 1740 psig, a die temperature of about 170°C and a die pressure of about 1100 psig. For ease of handling the extruded film is sandwiched between two sheets of Teflon™ film having 0.002 to 0.003 inch thickness. One Teflon™ sheet is peeled from each sample, and it is hand stretched on a ring for 5 days. The other side Teflon™ sheet is peeled from each sample and the sample is immersed in water. Circular membranes (25-mm diameter) are cut from each film. Each sample is placed in an Amacon Model 8010 stirred membrane holder. Distilled water is placed in the top of the membrane holder under 10 psig nitrogen pressure, and the permeate is collected and weighed. The procedure is repeated with a 4 weight percent solution of bovine albumin in distilled water. The permeate from the albumin solution is contacted with Bromcresol Green dye to color any albumin in the permeate. The absorbance of the solution is measured by spectrophotometer at about 628 nm. The absorbance is compared with the absorbance of known standard solutions of albumin to determine the retention of the solution. The first sample exhibits a flux in clear water of 31.6 ml/( m 2)(hr)(cmHg), and a flux of about 20.6 ml/( m 2)(hr)(cmHg) in the albumin solution. The second sample exhibits a flux in clear water of 31.6 ml/( m 2)(hr)(cmHg), and a flux of about 20.6 ml /(m2)(hr)(cmHg) in water containing 4 weight percent albumin. The second sample exhibits a rejection of 97 percent. Example 7 - Phase Inversion Membrane Containing Rigid Rod-Flexible Coil Block Polymer A mixture containing: (1) 1500 g of polyphosphoric acid (PPA) containing about 76 percent P2O5' (2) 63 * 7 g of bis-(4-hydroxy-3-amino- phenyl)sulfone bis(hydrochloride) (BHAPS), (3) 48.9 g of oxy-bis-(4-benzoic acid) (OBBC), (4) 505 g of phosphorus pentoxide (P 0 5 ), and (5) 500 g of dope containing 14 weight percent rigid rod-cis-polybenzoxazole in polyphosphoric acid, said polymer being from the reaction of a 9:10 molar ratio of ter- ephthalic acid and 4,6-diaminoresorcinol bis(hydrochlo ide) is added to a batch reactor outfitted with a stirrer under nitrogen atmosphere. The ratios of reagents are chosen to provide a reaction mixture in which the PPA initially contains about 86 weight percent P2θ5 > an d i which the finished dope contains about 6 weight percent polymer. The mixture is heated with stirring at 8θ°C to 100°C for about 4 hours interrupted once and over a two- -day period. The mixture is placed under vacuum (less than 1 torr) and heating is continued 4 hours. The mixture is transferred to a piston-agitated reactor. The reaction is continued at 190°C for a total of about 11 hours interrupted 5 times over a four-day period. The resulting mixture is divided into two parts and the second part is diluted to a concentration of 3 weight percent using PPA and P2C*5 ' Films are extruded from the 6 percent sample according to the procedure described in Example 6 at a barrel and die temperature of about 190°C, a barrel pressure of about 2370 psig and a die pressure of about 650 psig. Films are extruded from the 3 percent sample according to the procedure described in Example 6 at a barrel and die temperature of about 180°C, a barrel 0 pressure of about 1500 psig and a die pressure of about 190 psig. Some of the films are hand stretched on a circular hoop, others are mechanically stretched while still between the Teflon™ sheets, and others are left unstretched. Each is coagulated in a water bath. 5 The flux of the membranes for distilled water, and the flux and rejection for bovine albumin was mea¬ sured as described in Example 6. Three unstretched Q membranes have a flux for distilled water of 217, 208 and 185 ml/( m 2)(hr)(cmHg) , respectively; and in the same order a flux for the albumin solution of 128, 75 and 76 ml/( m 2)(hr)(cmHg) , respectively. A membrane stretched to three times its original width in a 5 direction transverse to that at which it was extruded has a flux of about 100.5 ml /(m2)(hr)(cmHg) for clear water and a flux for albumin solution of about 48 ml/( m 2-) ( hr -) ( cm Hg). Each of the membranes showed a rejection for albumin in excess of 95 percent. 0 Example 8 - Asymmetric Film Containing a Mixture of Rigid Rod and Flexible PBO A rigid rod cis-polybenzoxazole, resulting from the reaction of 4,6-diaminoresorcinol bis(hydrochloride) with terephthalic acid and having an inherent viscosity of between 27 dL /g and 33 ^ g, is obtained. A commer¬ cially available poly(aromatic ether ketone) (PEER) polymer is obtained from ICI Corp. under the trade designation PEER 450-P. The two are added to methane¬ sulfonic acid in the proportions shown in Table VI in sufficient quantity to make a dope solution containing about 2 weight percent polymer. Certain solutions having particularly high viscosities are heated to facilitate casting. The dopes are poured on heated glass plates along the length of a casting bar set for a film thickness of 0.03 inch and are cast by moving the bar at a slow and constant velocity across the plate. Masking tape is applied around the film area to prevent diluent from reaching the dope from the plate side. Each plate is immersed at a 30° angle in a coagulant bath containing either water (H2O) or acetonitrile (MeCN) for a period of one to two minutes, until coagu- lation is complete. Then each film is removed from the plate, washed with water, and stored immersed in water. Membranes are cut from each film in a region which appears free from thin spots or other defects. Permeability and retention are measured as described in Example 5 and are illustrated in Table VI. Certain of the membranes are freeze-dried by the following technique. The wet membrane is placed between two pieces of filter paper, and all are securely mounted. A thin layer of powdered dry ice is sprinkled on the filter paper and then squirted with acetone. The application of powdered dry ice and acetone is repeated on both sides of the filter paper for a period of two minutes. The membrane and filter paper are then placed in 600 ml Labconco™ freeze-drying flask, which was pre- cooled with dry ice. The flask is connected through a condenser to a vacuum pump and placed under vacuum while cooled by dry ice overnight. Optical and electron microscopy show that each film has a very open porous structure on one side and a denser less porous structure on the other. Permeability and retention are measured as described in Example 5 and are illustrated in Table VI.

Claims: Claims: 1. A membrane separation apparatus containing: (1) a high pressure zone, (2) a low pressure zone, and (3) a porous membrane, said elements being arranged such that a fluid passes from the high pressure zone to the low pressure zone by permeation through the porous membrane, characterized in that the porous membrane contains a polybenzoxazole polymer or copolymer or a polybenzothiazole polymer or copolymer. 2. A process for separating a mixture that contains at least one low molecular weight fluid compo- nent and at least one separable component, wherein the mixture is contacted with a porous membrane, which has pores of a size suitable to reject at least part of the separable component, under a pressure sufficient to cause the fluid to permeate through the membrane, characterized in that the porous membrane contains a polybenzoxazole polymer or copolymer or a polybenzothiazole polymer or copolymer. 3. A porous hollow fiber membrane, characterized in that the membrane contains a polybenzoxazole polymer or copolymer or a polybenzothiazole polymer or copolymer. ,- 4. A membrane having a layer that comprises a polybenzoxazole or polybenzothiazole polymer or copolymer, characterized in that the layer containing polybenzoxazole or polybenzothiazole polymer or copolymer is no more than 500 microns thick and has an 10 open cell structure with an average cell diameter between 1 micron and 200 microns. 5. The invention as claimed in any one of the preceding Claims wherein the polybenzoxazole or 15 polybenzothiazole polymer or copolymer consists essentially of a repeating mer unit that is represented by any one of the Formulae: 5 0 wherein each Z is an oxygen atom or a sulfur atom, L is -Arm- or -Ar-X'-Ar-, each Ar and Ar3 is an aromatic group, Ar m i s a meta-aromatic group and X and X' are each independently a sulfonyl moiety, an oxygen atom, a sulfur atom or an aliphatic group. 6. The invention as claimed in any one of the Claims 1-4 wherein the polybenzoxazole or polybenzo¬ thiazole polymer or copolymer is a copolymer that contains at least a plurality of mer units represented by any one of the Formulae: wherein each Z is an oxygen atom or a sulfur atom, L is -Arm- or -Ar-X'-Ar-, each Ar and Ar3 is an aromatic group, Arm is a meta-aromatic group and X and X' are each independently a sulfonyl moiety, an oxygen atom, a sulfur atom or an aliphatic group. 7. The invention as claimed in any one of Claims 1-6 wherein the membrane is an isotropic membrane c - that consists essentially of a single discriminating layer. 8. The invention as claimed in any one of Claims 1-6 wherein the membrane is an asymmetric 10 membrane or a composite membrane that contains a supporting layer and a discriminating layer. 9. The invention as claimed in any one of Claims 1-8 wherein the membrane is a hollow fiber 15 membrane having an average internal diameter of 50 μm to 5000 μm and an average external diameter of 60 μm to 6000 μm. 20 10. The invention as claimed in any one of Claims 1, 2, or 4-8 wherein the membrane is a flat membrane having an average thickness of 5 μm to 500 μm. 11. The invention as claimed in any one of 25 Claims 1-3 or 5-10 wherein the membrane is an ultrafiltration membrane having an average pore size of between 20 angstroms and 500 angstroms in the discriminating layer. 30 12. The invention as claimed in any one of Claims 1-3 or 5-10 wherein the membrane is a microfiltration membrane having an average pore size of between 0.05 μm and 2 μm in the discriminating layer. 13. The invention as claimed in any one of Claims 1-3 or 5-10 wherein a layer of the membrane that contains the polybenzoxazole or polybenzothiazole polymer or copolymer has an average pore size of at least 2 μm. 14. The invention as claimed in any one of Claims 1, 2, or 4-13 wherein the pressure across the membrane at least about 70 g/cm2. 15. A process for making a porous membrane comprising the steps of: ( 1 ) forming a homogeneous solution containing a solvent acid, a PBO or PBT polymer, and a pore-forming compound which is soluble in solvent acid and in organic solvent but is insoluble in water; (2) contacting the homogeneous solution of step (1) with an aqueous diluent under conditions such that the PBO or PBT polymer and the pore-forming compound coprecipitate to form a film or membrane containing dis¬ crete zones of PBO or PBT and discrete zones of pore-forming compounds; and (3) contacting the film or membrane of step (2) with an organic solvent such that the pore-forming compound is dissolved from the film or membrane resulting in a porous structure.

Inventors: CHAU CHIEH-CHUN, HWANG WEN-FANG, MADISON NORMAN L, MAHONEY ROBERT D, PIERINI PETER E, ROITMAN DANIEL B, WESSLING RITCHIE A, CHAU, CHIEH-CHUN, HWANG, WEN-FANG, MADISON, NORMAN, L, MAHONEY, ROBERT, D., PIERINI, PETER, E., ROITMAN, DANIEL, B., WESSLING, RITCHIE, A.
Applicants: DOW CHEMICAL CO, THE DOW CHEMICAL COMPANY
Application year: 1991
Main classification: B01D61
IPC codes: B01D61, B01D71, C08G73, C08G75