[0001] This application claims the benefit of provisional application No. 61/974,067, filed April 2, 2014, entitled Filter Membrane, the entire teachings are incorporated herein by reference.

FIELD OF THE INVENTION

[0002] The invention includes embodiments that relate to membranes. Particularly, the invention includes embodiments that are directed to thermally cross-linked polyamide-imide membranes, method of making such, and devices using such.

BACKGROUND

[0003] Some poly(amide-imide) (PAI) membranes and methods of making them are known. However, there is a continuing need for new porous membranes with enhanced chemical resistance, wettability, low flow loss, and improved particle retention.

SUMMARY

[0004] The purpose and advantages of embodiments of the invention will be set forth and apparent from the description that follows, as well as will be learned by practice of the embodiments of the invention. Additional advantages will be realized and attained by the methods and systems particularly pointed out in the written description and claims hereof, as well as from the appended drawings.

[0005] An embodiment of the invention provides a thermally cross-linked microporous membrane. The thermally cross-linked microporous membrane includes a thermally cross- linked polyamide-imide polymer. The thermally cross-linked microporous membrane has an HFE bubble point from about 25 psi to about 200 psi and an IPA flow-time from about 400 seconds to about 40,000 seconds.

[0006] A second embodiment of the invention provides another thermally cross-linked poly(amide-imide) membrane. The thermally cross-linked membrane has anHFE bubble point from about 25 psi to about 200 psi. The thermally cross-linked membrane is microporous; asymmetric and has a tight layer with a thickness of less than or equal to 10 microns. [0007] A third embodiment of the invention provides a microporous membrane. The microporous membrane includes a chemically resistant polyamide-imide polymer. The microporous membrane has an HFE bubble point from about 25 psi to about 200 psi and an IPA flow-time from about 400 seconds to about 40,000 seconds.

[0008] A fourth embodiment of the invention provides another membrane. The membrane comprises a chemically resistant polyamide-imide polymer. The membrane has an HFE bubble point from about 25 psi to about 200 psi. The membrane is microporous;

asymmetric and has a tight layer with a thickness of less than or equal to 10 microns.

[0009] A fifth embodiment of the invention provides a filtration device. The filtration device includes a filter incorporating a thermally cross-linked microporous membrane. The thermally cross-linked microporous membrane includes a thermally cross-linked polyamide- imide polymer, wherein the thermally cross-linked microporous membrane has a bubble point and an IPA flow-time. The thermally cross-linked microporous membrane has an HFE bubble point from about 25 psi to about 200 psi and an IPA flow-time from about 400 seconds to about 40,000 seconds.

[0010] A sixth embodiment of the invention provides another filtration device. The filtration device includes a filter incorporating a microporous membrane. The microporous membrane includes a chemically resistant polyamide-imide polymer, wherein the microporous membrane has a bubble point and an IPA flow-time. The microporous membrane has an HFE bubble point from about 25 psi to about 200 psi and an IPA flow-time from about 400 seconds to about 40,000 seconds.

[0011] The accompanying figures, which are incorporated in and constitute part of this specification, are included to illustrate and provide a further understanding of the method and system of the invention. Together with the description, the drawings serve to explain the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] The drawings accompanying and forming part of this specification are included to depict certain aspects of the invention. A clearer impression of the invention, and of the components and operation of systems provided with the invention, will become more readily apparent by referring to the exemplary, and therefore non-limiting, embodiments illustrated in the drawings, wherein identical reference numerals designate the same components. Note that the features illustrated in the drawings are not necessarily drawn to scale. [0013] FIG. 1 is a schematic representation of a membrane in accordance with an embodiment of the invention.

[0014] FIG. 2A is an SEM image of a Torlon® membrane that can be employed in manufacturing a cross-linked membrane described herein.

[0015] FIG. 2B is an SEM image of a cross-linked Torlon® membrane described herein.

[0016] FIG. 3 is a flow chart of a method of making a membrane in accordance with embodiments of the invention.

[0017] FIGs. 4A and 4B are bar plots showing tensile strength and tensile stress, respectively, of the non-cross-linked poly(amide-imide) (Torlon®) membranes following exposure to 10% aqueous hydrochloric acid.

[0018] FIGs. 5A and 5B are bar plots showing tensile strength and tensile stress, respectively, of the thermally cross-linked poly(amide-imide) (Torlon®) membranes following exposure to 10% aqueous hydrochloric acid.

[0019] FIG. 6 is a plot showing the percent of retention of 25 nm polystryrene (G25) nanoparticles as a function of monolayer concentration by the cross-linked PAI membrane.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

[0020] Reference will now be made in detail to exemplary embodiments of the invention, which are illustrated in the accompanying figures and examples. Referring to the drawings in general, it will be understood that the illustrations are for the purpose of describing a particular embodiment of the invention and are not intended to limit the invention thereto.

[0021] Whenever a particular embodiment of the invention is said to comprise or consist of at least one element of a group and combinations thereof, it is understood that the embodiment may comprise or consist of any of the elements of the group, either individually or in combination with any of the other elements of that group. Furthermore, when any variable occurs more than one time in any constituent or in formula, its definition on each occurrence is independent of its definition at every other occurrence. Also, combinations of substituents and/or variables are permissible only if such combinations result in stable compounds.

Definitions of Terms

[0022] It must also be noted that as used herein and in the appended claims, the singular forms "a", "an", and "the" include plural reference unless the context clearly dictates otherwise. Thus, for example, reference to a "non-solvent" is a reference to one or more non- solvents and equivalents thereof known to those skilled in the art, and so forth. Unless defined otherwise, all technical and scientific terms used herein have the same meanings as commonly understood by one of ordinary skill in the art. Methods and materials similar or equivalent to those described herein can be used in the practice or testing of embodiments of the present invention. All publications mentioned herein are incorporated by reference in their entirety. "Optional" or "optionally" means that the subsequently described event or circumstance may or may not occur, and that the description includes instances where the event occurs and instances where it does not. All numeric values herein can be modified by the term "about," whether or not explicitly indicated.

[0023] All numeric values herein can be modified by the term "about," whether or not explicitly indicated. The term "about" generally refers to a range of numbers that one of skill in the art would consider equivalent to the recited value (i.e., having the same function or result). In some embodiments the term "about" refers to ±10% of the stated value, in other embodiments the term "about" refers to ±2% of the stated value.

[0024] The term "alkyl", as used herein, unless otherwise indicated, means straight or branched saturated monovalent hydrocarbon radicals of formula C _n H _2n+ i. In some embodiments, n is from 1 to 18. In other embodiments, n is from 1 to 12. Preferably, n is from 1 to 6. In some embodiments, n is 1-1000, for example, n is 1 -100. Alkyl can optionally be substituted with -OH, -SH, halogen, amino, cyano, nitro, a C - C ₁₂ alkyl, Ci- C ₁₂ haloalkyl, Ci- C ₁₂ alkoxy, Q- Q ₂ haloalkoxy or C C ₁₂ alkyl sulfanyl. In some embodiments, alkyl can optionally be substituted with one or more halogen, hydroxyl, Ci- Q ₂ alkyl, C ₂ - Ci ₂ alkenyl or C ₂ - C ₁₂ alkynyl group, Q- Ci ₂ alkoxy, or C C ₁₂ haloalkyl. The term alkyl can also refer to cycloalkyl.

[0025] As used herein, an "alkenyl group", alone or as a part of a larger moiety (e.g., cycloalkene oxide), is preferably a straight chained or branched aliphatic group having one or more double bonds with 2 to about 12 carbon atoms, e.g., ethenyl, 1-propenyl, 1-butenyl, 2-butenyl, 2 -methyl- 1-propenyl, pentenyl, hexenyl, heptenyl or octenyl, or a cycloaliphatic group having one or more double bonds with 3 to about 12 carbon atoms. As used herein, an "alkynyl" group, alone or as a part of a larger moiety, is preferably a straight chained or branched aliphatic group having one or more triple bonds with 2 to about 12 carbon atoms, e.g., ethynyl, 1-propynyl, 1-butynyl, 3-methyl-l-butynyl, 3, 3 -dimethyl- 1-butynyl, pentynyl, hexynyl, heptynyl or octynyl, or a cycloaliphatic group having one or more triple bonds with 3 to about 12 carbon atoms. [0026] The term "cycloalkyl", as used herein, means saturated cyclic hydrocarbons, i.e. compounds where all ring atoms are carbons. In some embodiments, a cycloalkyl comprises from 3 to 18 carbons. Preferably, a cycloalkyl comprises from 3 to 6 carbons. Examples of cycloalkyl include, but are not limited to, cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, and cycloheptyl. In some embodiments, cycloalkyl can optionally be substituted with one or more halogen, hydroxyl, Ci- C ₁₂ alkyl, C ₂ - C ₁₂ alkenyl or C ₂ - C ₁₂ alkynyl group, Ci- C ₁₂ alkoxy, or Ci- C ₁₂ haloalkyl.

[0027] The term "haloalkyl", as used herein, includes an alkyl substituted with one or more F, CI, Br, or I, wherein alkyl is defined above.

[0028] The terms "alkoxy," as used herein, means an "alkyl-O-" group, wherein alkyl is defined above. Examples of alkoxy group include methoxy or ethoxy groups.

[0029] The term "aryl," as used herein, refers to a carbocyclic aromatic group.

Preferably, an aryl comprises from 6 to 18 carbons. Examples of aryl groups include, but are not limited to phenyl and naphthyl. Examples of aryl groups include optionally substituted groups such as phenyl, biphenyl, naphthyl, phenanthryl, anthracenyl, pyrenyl, fluoranthyl or fluorenyl. An aryl can be optionally substituted. Examples of suitable substituents on an aryl include halogen, hydroxyl, Ci- C ₁₂ alkyl, C ₂ - C ₁₂ alkene or C ₂ - C ₁₂ alkyne, C ₃ - C ₁₂ cycloalkyl, Ci- C ₁₂ haloalkyl, C C ₁₂ alkoxy, aryloxy, arylamino or aryl group.

[0030] In some embodiments, a C ₆ -C ₁₈ aryl selected from the group consisting of phenyl, indenyl, naphthyl, azulenyl, heptalenyl, biphenyl, indacenyl, acenaphthylenyl, fluorenyl, phenalenyl, phenanthrenyl, anthracenyl, cyclopentacyclooctenyl or benzocyclooctenyl. In some embodiments, a C6-Ci4 aryl selected from the group consisting of phenyl, naphthalene, anthracene, lH-phenalene, tetracene, and pentacene.

[0031] The term "aryloxy," as used herein, means an "aryl-O-" group, wherein aryl is defined above. Examples of an aryloxy group include phenoxy or naphthoxy groups.

[0032] The term (hetero)arylamine, as used herein, means an "(hetero)aryl-NH-", an "(hetero)aryl-N(alkyl)-", or an "((hetero)aryl) ₂ -N-" groups, wherein (hetero)aryl and alkyl are defined above.

[0033] The term "heteroaryl," as used herein, refers to aromatic groups containing one or more heteroatoms (O, S, or N). A heteroaryl group can be monocyclic or polycyclic, e.g. a monocyclic heteroaryl ring fused to one or more carbocyclic aromatic groups or other monocyclic heteroaryl groups. The heteroaryl groups of this invention can also include ring systems substituted with one or more oxo moieties. Examples of heteroaryl groups include, but are not limited to, pyridinyl, pyridazinyl, imidazolyl, pyrimidinyl, pyrazolyl, triazolyl, pyrazinyl, quinolyl, isoquinolyl, tetrazolyl, furyl, thienyl, isoxazolyl, thiazolyl, oxazolyl, isothiazolyl, pyrrolyl, quinolinyl, isoquinolinyl, indolyl, benzimidazolyl, benzofuranyl, cinnolinyl, indazolyl, indolizinyl, phthalazinyl, pyridazinyl, triazinyl, isoindolyl, purinyl, oxadiazolyl, thiazolyl, thiadiazolyl, furazanyl, benzofurazanyl, benzothiophenyl,

benzotriazolyl, benzothiazolyl, benzoxazolyl, quinazolinyl, quinoxalinyl, naphthyridinyl, dihydroquinolyl, tetrahydroquinolyl, dihydroisoquinolyl, tetrahydroisoquinolyl, benzofuryl, furopyridinyl, pyrolopyrimidinyl, and azaindolyl.

[0034] In other embodiments, a 5-14-membered heteroaryl group selected from the group consisting of pyridyl, 1-oxo-pyridyl, furanyl, benzo[l,3]dioxolyl, benzo[l,4]dioxinyl, thienyl, pyrrolyl, oxazolyl, imidazolyl, thiazolyl, a isoxazolyl, quinolinyl, pyrazolyl, isothiazolyl, pyridazinyl, pyrimidinyl, pyrazinyl, a triazinyl, triazolyl, thiadiazolyl, isoquinolinyl, indazolyl, benzoxazolyl, benzofuryl, indolizinyl, imidazopyridyl, tetrazolyl, benzimidazolyl, benzothiazolyl, benzothiadiazolyl, benzoxadiazolyl, indolyl, tetrahydroindolyl, azaindolyl, imidazopyridyl, quinazolinyl, purinyl, pyrrolo[2,3]pyrimidinyl, pyrazolo[3,4]pyrimidinyl, imidazo[l,2-a]pyridyl, and benzothienyl. The foregoing heteroaryl groups may be C- attached or N-attached (where such is possible). For instance, a group derived from pyrrole may be pyrrol- 1-yl (N-attached) or pyrrol-3-yl (C-attached).

[0035] Suitable substituents for heteroaryl are as defined above with respect to aryl group.

[0036] Suitable substituents for an alkyl, cycloalkyl include a halogen, an alkyl, an alkenyl, a cycloalkyl, a cycloalkenyl, an aryl, a heteroaryl, a haloalkyl, cyano, nitro, haloalkoxy.

[0037] Further examples of suitable substituents for a substitutable carbon atom in an aryl, a heteroaryl, alkyl or cycloalkyl include but are not limited to -OH, halogen (-F, -CI, -Br, and -I), -R, -OR, -CH ₂ R, -CH ₂ OR, -CH ₂ CH ₂ OR,. Each R is independently an alkyl group.

[0038] In some embodiments, suitable substituents for a substitutable carbon atom in an aryl, a heteroaryl or an aryl portion of an arylalkenyl include halogen, hydroxyl, Cj- Cj ₂ alkyl, C ₂ - Cj ₂ alkenyl or C ₂ - Q ₂ alkynyl group, Cj- Ci ₂ alkoxy, aryloxy group, arylamino group and Cj- Cj ₂ haloalkyl.

[0039] In addition, the above-mentioned groups may also be substituted with =0, =S, =N-alkyl. [0040] In the context of the present invention, an amino group may be a primary (-NH ₂ ), secondary (-NHR _P ), or tertiary (-NR _p R _q ), wherein R _p and Rq may be any of the alkyl, alkenyl, alkynyl, alkoxy, cyclo alkyl, cycloalkoxy, aryl, heteroaryl, and a bicyclic carbocyclic group. A (di)alkylamino group is an instance of an amino group substituted with one or two alkyls.

Cross-linking polv(amide-imide) microporous membranes,

[0041] As used herein, the term "thermally cross-linked polyamide-imide" refers to a polyamide-imide polymer subjected to the thermal crosslinking procedure, as described herein.

[0042] It has been discovered that the chemical properties of the polyamide-imide porous membranes can be significantly improved by thermal crosslinking. For example, a thermally cross-linked polyamide-imide microporous membrane is compatible with a 10% aqueous HCl solution while a non (thermally) cross-linked polyamide-imide membrane is damaged by the same 10% aqueous HCl solution after being soaked in it for three weeks. The thermally cross linked polyamide-imide membrane has greater strength after soaking in 10% aqueous HCl for three weeks than a polyamide-imide membrane that is not thermally cross linked and also soaked in 10% aqueous HCl for three weeks. Unlike the non-thermally cross-linked polyamide-imide membrane, the thermally crossed-linked polyaminde-imide membrane is also compatible with TMAH (tetramethyl ammonium hydroxide) solvent and the strength of the thermally cross-linked polyaminde-imide membrane in TMAH is greater than the strength of a non-thermally cross-linked polyamide-imide membrane after extended soaking in TMAH. The mechanical properties of the thermally cross-linked polyamide-imide membrane can be improved compared to the non-thermally cross-linked polyaminde-imide membrane, and the thermal cross-linking also alters the membrane pore structure in a manner that the membrane IPA flow-time improves while the mean bubble point of the membrane remains almost the same.

[0043] Accordingly, in one embodiment, the present invention is a microporous membrane comprising a chemically resistant polyamide-imide polymer. As used herein, the term "chemically resistant polymer" refers to such a polymer that a numerical value corresponding to a measured mechanical property of the polymer, such as tensile strain or tensile stress, is reduced, within the error of the measurement, by less than or equal to about 50% after the polymer has been exposed to 10% aqueous HCl for three weeks. In certain example embodiments, the reduction in tensile strain or tensile stress is less than or equal to about 45%, 40%, 35%, 30%, 25%, 20%, 15%, 10%, 5%, or 0%. [0044] Without being limited to any particular theory, it is believed that chemical resistance of polyamide-imide polymers improve as a result of thermal cross-linking.

Accordingly, in various embodiments, microporous membranes comprising a chemically resistant polyamide-imide polymer possess physical and technical specifications as described herein with respect to thermally cross-linked polyamide-imide membranes.

[0045] The thermal crosslinking of polyamide-imide membranes is achieved by raising the polyamide-imide membrane temperature. The results show that the polyamide-imide polymer can be cross-linked by raising its temperature. The degree of cross-linking depends on the temperature. In case of a Torlon® membrane, the highest cross-linking was observed at temperatures in the range of 240 degrees centigrade to 260 degrees Centigrade. Cross- linking can be carried out without adding any cross-linking agent. Accordingly some of the porous membranes as described herein can be considered as consisting essentially of polyamide-imide polymer that is thermally cross-linked. Minor impurities that may be present in the starting polyamide-imide material may be incorporated into the porous membrane, but additional cross-linking agents are absent. Additionally, porous membranes as described herein are devoid of additives which make the membrane hydrophilic such as polyethylene glycol and others. Accordingly, some other of the porous membranes as described herein can be considered as consisting essentially of polyamide-imide polymer that is thermally cross-linked and are absent extractable additives like polyethylene glycol.

[0046] The pore size of the thermally crosslinked porous polyamide-imide membranes is in the range of between 1 nanometer and 20 nanometers, in some cases the pore size of the thermally crosslinked porous polyamide-imide membranes is in the range of between 1 nanometer and 10 nanometers. Porous membranes with small pore sizes that are stable in chemicals like 10% aqueous HC1 and TMAH used in semiconductor manufacturing are advantageous for removing very small particles that can cause defects in manufactured chips and memory devices.

[0047] Generally, the method of fabricating a cross-linked poly(amide-imide) membrane include manufacturing a poly(amide-imide) porous membrane from the polymer, as described in details below, followed by thermally curing the membrane to induce cross-linking.

[0048] Any poly(amide-imide) polymer suitable for manufacturing filter membranes can be employed to manufacture the membranes described herein. Generally, a poly(amide- imide) is a polymer having at least one repeat unit represented by the following structural formula:

[0049] In structural formula (I), n is 0 or an integer between 1 and 12; for example n is 0. In another example embodiment, n is 2. In structural formula (I), R is a moiety that includes at least one C ₆ -Ci ₆ aryl or heteroaryl, optionally substituted with one or more substituents selected from d- C _n alkyl, (C ₆ -C _{] 8} )(hetero)aryl(Ci- Ci ₂ )alkyl, (C ₆ -C _{] 8} )(hetero)aryl (C ₂ - Cj ₂ )alkene, C ₃ -C ₁₂ cycloalkyl, (C ₆ -C ₁ 8)(hetero)aryl(C ₃ -Ci ₂ ) cycloalkyl, C\- C ₁₂ haloalkyl, (C ₆ -Ci ₈ )(hetero)aryl(Ci- Cj ₂ )haloalkyl, Ci- C ₁₂ alkoxy, (C6-C _]8 )(hetero)aryl(C ₁ - C ₁₂ )alkoxy, C -C] ₈ (hetero)aryloxy, C ₆ -Ci ₈ (hetero)arylamino or a C ₆ -Cj ₈ (hetero)aryl group, wherein any of the aryl or heteroaryl group is further optionally substituted with a halogen or hydroxyl.

[0050] An example of a repeat unit of formula (I) is a repeat unit of the following structural formula:

[0051] wherein R is defined above with respect to structural formula (I).

[0052] An example of group R is a moiety of the following structural formula:

[0053] One example of a polyamide-imide material useful for making membranes and thermally cross-linking them is any of the Torlon® membranes available from various vendors, e.g. from Solvay Plastics. The structures of representative Torlon® polymers are described, for example, in U.S. 4,900,449, the entire teachings of which are incorporated herein by reference. An example chemical structure of a Torlon® membrane can be described as follows:

[0054] 95% by weight of a polymer having the repeat unit represented by structural formula (IV)

[0055] 5% by weight of a polymer having the repeat unit represented by structural formula (V):

[0056] where R ¹ is represented by structural formula (II).

[0057] The high degree of chemical resistance improvement of the thermally cross-linked polyamide-imide porous membranes in acid and base was unexpected. Also the ability to effect thermal crosslinking without the presence of additives like polyethylene glycol was also unexpected. Advantageously, the elimination of the need for cross-linking agents like polyamines or cross linking agents comprising isocyanate groups reduces the chance of adding contaminants to the final membrane product.

[0058] With reference to FIG. 1, a cross-linked microporous membrane 100 is described. The cross-linked membrane 100 includes one or more polyamide-imide polymers and, in one embodiment, has an HFE bubble point from about 25 psi to about 200 psi and an IPA flow- time from about 400 seconds to about 40,000 seconds. The cross-linked membrane 100 has an open pore 12 structure and the pores 12 are interconnected allowing liquid or gas filtration.

[0059] Another embodiment of the cross-linked membrane 100 includes one or more polyamide-imide polymers, wherein the membrane 100 has an HFE bubble point from about 25 psi to about 200 psi. The membrane is asymrnetric-and microporous and has a tight layer that has a thickness of <10 microns. In a further embodiment, the membrane 100 also has an IP A flow time in a range from about 400 seconds to about 40,000 seconds.

[0060] It should be understood that the cross-linked microporous membrane 100 is not limited by its form or shape unless expressly stated and includes membranes of varying shape, form, and morphology unless expressly limited by the specification. In an

embodiment, the membrane 100 includes pleated form. In another embodiment, the membrane 100 includes hollow fiber. In yet another embodiment, the membrane 100 is in the form of flat films. In another embodiment, the membrane 100 includes composite form.

[0061] It should also be appreciated that microporous cross-linked membranes described herein can be incorporated in filtration or purification housings. An embodiment of the invention provides a filtration device 300 as shown in FIG. 1. The filtration device includes one or more filters 200 incorporating a microporous membrane 100. The microporous membrane includes polyamide-imide polymer, has an HFE bubble point, and an IPA flow-time. The microporous membrane has an HFE bubble point from about 25 psi to about 200 psi and has an IPA flow-time from about 400 seconds to about 40,000 seconds.

[0062] The cross-linked microporous membranes 100 can be pleated and bonded, including potting, to form integral devices that permit filtration and purification of liquids and other fluids that pass through the membrane in the housing. In another embodiment, the membranes can be hollow fibers and incorporated to a housing to form integral devices that permit filtration and purification of liquids and other fluids that pass through the membrane in the housing.

Polyf amide-imide) membranes suitable for practicing the present invention

[0063] Example embodiments of the membranes that could be employed in the practice of the present invention are described in PCT/US2014/065699, filed November 14, 2014, the entire teachings of which is incorporated herein by reference. Any poly(amide-imide) membrane described in PCT/US2014/065699 can be subjected to thermal treatment, as described below, resulting in a thermally cross-linked poly( amide-imide) porous membrane.

[0064] Examples of the porous poly(amide-imide) membranes that can be subjected to thermal cross-linking are described below. All testing methods used to determine pore sizes and flow times are equally applicable to the cross-linked membranes. Test methods described herein may be used to characterize the bubble point and flow time of the membranes as well as the test conditions for these values. The bubble point used to characterize the membranes 100 refers to a mean bubble point using an air flow porisometer. ASTM F316 - 03(2011 ) Standard Test Methods for Pore Size Characteristics of Membrane Filters by Bubble Point and Mean Flow Pore Test were used to calculate bubble point. In some cases, microporous membrane bubble points refer to a mean bubble point measured by an HFE-7200 (available from 3M™, St. Paul, MN). HFE-7200 bubble points can be converted into IPA bubble point values by multiplying the HFE-7200 value measured bubble point by 1.5. HFE-7200 by 3M™ is ethoxy-nonafluorobutane and has a reported surface tension of 13.6 mN/m at 25°C.

[0065] IPA flow time is the time to flow 500 milliliters of isopropyl alcohol, at a temperature of 21°C and pressure of 97,900 Pa (about 0.1 MPa, or about 1 bar, or about 14.2 psid), through a 47 millimeter disk of the microporous membrane with an area of 12.5 cm ² .

[0066] It should be appreciated that embodiments of the inventions include microporous membranes 100 with varying HFE bubble points. In an embodiment, the HFE bubble point ranges from about 25 pounds per square inch pressure to about 150 pounds per square inch pressure. In another embodiment, the HFE bubble point ranges from about 53 pounds per square inch pressure to about 99 pounds per square inch pressure. In another embodiment, the HFE bubble point ranges from about 38 pounds per square inch pressure to about 75 pounds per square inch pressure. In yet another embodiment, the HFE bubble point ranges from about 32 pounds per square inch pressure to about 38 pounds per square inch pressure.

[0067] Embodiments of the invention include microporous membranes 100 with various ranges of IPA flow times. In an embodiment, the IPA flow time ranges from about 4225 seconds to about 7535 seconds. In another embodiment, the IPA flow time ranges from about 4090 seconds to about 5580 seconds. In yet another embodiment, the IPA flow time ranges from about 3445 seconds to about 4225 seconds. In another embodiment, the IPA flow time ranges from about 2860 seconds to about 3445 seconds.

/

1 [0068] Table 1 : NMP stands for N-methyl-2-pyrrolidone, EG for ethylene glycol and TEG for tri-ethylene glycol.

[0069] Another embodiment includes a microporous membrane 100 with an HFE bubble point between 25 psi and 32 psi and an IPA flow time between 2860 seconds and 3445 seconds. Another embodiment of the microporous membrane 100 includes an HFE bubble point between 32 psi and 38 psi and an IPA flow time of between 3445 seconds and 4245 seconds. Yet another embodiment of the microporous membrane 100 includes an HFE Bubble point between 38 psi and 75 psi and an IPA flow time between 4245 seconds and 5580 seconds. Another embodiment includes a microporous membrane 100 with an HFE bubble point between 53 psi and 99 psi and an IPA flow time between 4090 seconds and 7535 seconds.

[0070] It should be appreciated that embodiments of the invention include microporous membranes 100 that are symmetric or asymmetric as (as shown in FIG. 1). Symmetric membranes refer to porous membranes where the pore 12 size and/or structure are

substantially the same throughout the thickness of the membrane. The term "asymmetric" refers to a porous membrane in which the pore 12 size and/or structure are not the same from one side of the membrane to the other side as in FIG. 1. When the microporous membrane is asymmetric, such as in FIG. 1 , the asymmetric microporous membranes 100 have a tight layer 10 (bottom) and an open layer 20 (top).

[0071] Examples la- Id of the polyamide-imide microporous membranes 100 made from the dope formulation are shown in Table 1. The polyamide-imide (PAI) polymer dope formulation includes one or more polyamide-imide (PAI) polymers, one or more solvents, and one or more non-solvents. A particular embodiment of the PAI polymer dope

formulation includes N- methylpyrrolidone (NMP) as the solvent, ethylene glycol (EG) as a non-solvent, and polyamide-imide polymer. It should be appreciated that the membrane 100 also includes the reaction product of the one or more solvents with each other, the reaction product of the one or more non-solvents with the each other, the reaction product of the one or more PAI polymers with each other, and the reaction product of the one or more solvents, one or more non-solvents, and one or more PAI polymers with each other.

[0072] Examples 2a-2d are examples with dope formulations differing from Examples 1 a-d as shown in Table 1 with increased amount of water in the coagulation bath. In

Examples 2a-d, as in Examples la-d, the HFE bubble point of the membrane 100 increased as the amount of water in the coagulation bath increased as shown in Table 1. It should be appreciated that is it within the scope of invention to make microporous membranes 100 with a combination of one or more parameters such as range of IPA flow times, HFE bubble points, as well as varying the amount of water in the coagulation bath, etc.

[0073] With reference to FIG. 3, next is described an embodiment of the invention of making the membranes 100. Step 310 includes providing polyamide-imide (PAI) polymer dope formulation. The polyamide-imide (PAI) polymer dope formulation includes one or more polyamide-imide (PAI) polymers, one or more solvents, and one or more non-solvents. In an example embodiment, the polyamide-imide (PAI) polymer dope formulation includes N- methylpyrrolidone (NMP) as the solvent, ethylene glycol (EG) as a non-solvent, and polyamide-imide polymer in a ratio range of 79(NMP)/8(EG)/13(PAI) weight% to

75(NMP)/12(EG)/13(PAI) weight%.

[0074] Although the embodiments have been described with the given dope formulations, it should be understood embodiments of the invention are not restricted to the doping formulations and include varying the dope formulations in the amount and order, either individually or in a combination of two or more thereof.

[0075] In another embodiment, the dope formulation includes a plurality of polyamide- imides which differ from each other. The plurality of polyamide-imides may have various characteristics which are similar or vary from each other. In one embodiment, the pluralities of differing polyamide-imides are in various ranges. In another embodiment, the dope formulation includes a plurality of solvents which are similar or vary from each other. In one embodiment, the pluralities of differing solvents are in various ranges. In yet another embodiment, the dope formulation includes a plurality of non-solvents which are similar or vary from each other. In one embodiment, the plurality of differing non- solvents are in various ranges.

[0076] Step 320 includes phase separating the polyamide-imide (PAI) polymer dope formulation with a coagulant. In an example embodiment, the coagulant includes NMP and water in a ratio of 80(NMP)/20(water) weight% to 65(NMP)/35(water) weight %.

[0077] Step 330 includes thermally treating the microporous membrane obtained in step 320 to induce cross-linking. In an example embodiment, the pre-formed membranes are placed into an oven and heated to 250 ^' C for 6 hours.

EXEMPLIFICATION

Example 1 : Fabrication and characterization of thermally cross-linked poly(amide-imide microporous membranes

Fabrication

[0078] The microporous poly(amide-imide) membranes were pre-fabricated as described above, in Table 1 , Example 2d, using Torlon® poly(amide-imide) polymer, with the following modification: the coagulant used to phase-seprate the polymer was NMP/Water (60/40 weight %). [0079] Asymmetric flat sheet poly(amide-imide) membranes with a HFE-7200 bubble point of 100 psi and Isopropyl Alcohol (IP A) flow-time of 4500 sec were placed and hold onto frames of 20cm* 15cm dimensions. The frames were placed into an oven and heated up to 250 ^' C for 6 hours. The outcome was cross-linked poly(amide-imide) membrane with slightly darker color compared with the original yellowish color of the uncross-linked membrane.

Characterization

[0080] 1. Flow-Time and Bubble point

[0081] The fabricated cross-linked membranes were tested for IPA flow-time and HFE bubble point. The following Table 2 compares the results between a cross-linked and non- cross linked poly(amide-imide) membrane:

[0082] Table 2:

[0083] It can be seen that the thermal cross-linking could lead to the deformation of membrane pores in a way that some pores deformed into a bigger size and lowered the membrane initial bubble point. However, this deformation was balanced in a way that the membrane mean bubble point remained the same. Membrane Flow- Time slightly decreased due to the formation of some larger pores.

[0084] FIG. 2A and 2B are SEM images showing the P AI (Torlon®) membrane before (FIG. 2A) and after (FIG. 2B) thermal cross-lining.

[0085] 2. Chemical Resistance

[0086] In order to investigate the effect of thermal cross-linking on the chemical properties of the polyamide-imide membranes samples of both cross-linked and non- crosslinked membranes were cut in rectangular pieces with 1" by 4.5" dimensions and soaked into an aqueous solution of hydrochloric acid (10% HC1) for three weeks. Then they were dried and used for tensile test measurements. The results are summarized in FIGs. 4 A and 4B (for non-cross-linked membranes) and FIGs. 5A and 5B (for the cross-linked membranes). The data represents an average of triplicates. The error was estimated to be 10-15% (error bars not shown.)

[0087] As shown in FIG. 4 A and FIG. 4B, the mechanical properties of the non- crosslinked membrane is significantly affected by the contact with acid solution as the membrane tensile strain and tensile stress at break were both dropped drastically. In particular, as shown in FIG. 4A, a three- week long exposure to 10% aqueous HCl reduced the tensile strain of untreated Torlon® membrane from about 0.08 to about 0.02, representing a 75%) reduction on tensile strain. As shown in FIG. 4B, the tensile stress of untreated Torlon® membrane was reduced from 8.41 MPa to 3.37 MPa, representing approximately 60% reduction.

[0088] Surprisingly, the mechanical properties of the thermally cross-linked membranes did not change significantly after the exposure to hydrochloric acid, signifying unexpected improvement in chemical resistance of the cross-linked microporous membranes.

[0089] In particular, as shown in FIG. 5 A, a three-week long exposure to 10% aqueous HCl reduced the tensile strain of a thermally cross-linked Torlon® membrane from about 0.07 to about 0.05, representing a mere 29% reduction on tensile strain. As shown in FIG. 5B, the tensile stress of a thermally cross-linked Torlon® membrane showed no significant reduction (7.79 to 8.9 MPa).

[0090] 3. Particle Retention

[0091] 90 mm coupon membranes were challenged with an aqueous solution containing 8ppb G25 PSL (25 nm polystryrene) particles with 0.1 % Triton X-100 surfactants. Samples of the permeate were collected at different time intervals corresponding to different monolayer coverage of the membrane surface. The permeate samples were analyzed afterwards to measure the concentration of the G25 particles in the permeate sample and the corresponding membrane retention.

[0092] The results are presented in FIG. 6, where the retention of the same particles by a similar non-cross-linked membrane is shown for comparison. As shown in FIG. 6, the retention properties of both cross-linked and non-crosslinked PAI membranes are similar. Hence, the thermal cross-linking does not affect the retention properties of the membranes.

[0093] While the invention has been described in detail in connection with only a limited number of aspects, it should be readily understood that the invention is not limited to such disclosed aspects. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.