WO2010111755A2

WO2010111755A2 - Improved method for making cross-linked polyimide membranes

Info

Publication number: WO2010111755A2
Application number: PCT/BE2010/000028
Authority: WO
Inventors: Angels Cano Odena; Guy Koeckelberghs; Katrien Vanherck; Ivo Vankelecom
Original assignee: Katholieke Universiteit Leuven - K.U.Leuven R & D
Priority date: 2009-04-01
Filing date: 2010-03-31
Publication date: 2010-10-07
Also published as: WO2010111755A3

Abstract

Improved methods for the preparation of cross-linked polyimide membranes are disclosed. The process comprises preparing a polyimide polymer cast solution, casting said polyimide polymer solution onto a suitable substrate and solidifying said film, whereby cross-linking occur before the membrane is fully solidified. More in particular, cross-linking agents are either added to the polyimide polymer cast solution or added to the phase inversion coagulation medium.

Description

IMPROVED METHOD FOR MAKING CROSS-LINKED POLYIMIDE MEMBRANES

FIELD OF THE INVENTION

The present invention relates to improved methods for the preparation of cross-linked polyimide membranes. The present invention provides a modification of the generally applied process of contacting ready-made, solidified polyimide membranes with a solution of primary or secondary di-, tri- or polyamines. More specifically, the present invention relates to methods for the preparation of cross-linked polyimide membranes wherein a multifunctional nucleophile, such as multifunctional amines, diols or thiols, is contacted with the polyimide polymers prior to or during the membrane solidification process. More specifically, said multifunctional nucleophile is introduced in the polyimide casting solution or in the coagulation or phase inversion bath.

BACKGROUND OF THE INVENTION

Membrane separation processes are an increasingly important field in the art of separation science. They can be applied in the separation of components in the gas phase or liquid phase. (Basic Principles of Membrane Technology, Second Edition, M. Mulder, Kluwer Academic Press, Dordrecht. 564p).

Polyimides form a class of polymers characterized by the imide bond within the repeating unit. They are well known for their excellent thermal and oxidative stability as well as their excellent mechanical properties [1]. Polyimide membranes have thus been widely synthesized for use in gas separation [2], pervaporation [3-6] and liquid filtration [7-9]. The polyimide membranes are usually prepared via solvent evaporation until a solid film is obtained (resulting in dense, symmetric membranes) or via a phase inversion process (resulting in integrally skinned asymmetric or porous symmetric or asymmetric membranes, the latter having a thin, dense toplayer with pores typically in the range below 5 nm diameter resting on a more porous support). Asymmetric polyimide membranes have been prepared as flat sheet membranes and hollow fibre membranes.

Polyimide membranes applied in gas separation are discussed in US patents 4,705,540; 4,838,900 and 4,474,858. A major problem with polyimide membranes used in gas separation, is their plasticization in complex and harsh environments, resulting in a loss of separation performance [10]. In liquid filtration, polyimide membranes have been applied for the separation of aromatic from non-aromatic hydrocarbons (US patent 6,180,008), for the separation of lube oil from organic solvents such as toluene and hexane (US patents 5,264,166; 5,360,530; 5,494,566 and 5,651,877), for the recovery of organic solvents and valuable components [11] and so on. In certain classes of solvents, such as chlorinated solvents and aprotic solvents, polyimide membranes become unstable, resulting in swelling or dissolution of the polymer. In general, cross-linking prevents the material from swelling in the presence of plasticizing agents and promotes chemical and thermal stability [10]. It provides better long-term stability.

MEMBRANE CROSS-LINKING

Several membrane cross-linking methods have been proposed as a solution for these problems. Cross-linking of polyimide can be carried out in hot air (JP09324049) or ambient environment [12], by UV-irradiation [12] or by chemical reactions.

Various cross-linking methods by chemical modification of polyimide are described in literature, for instance the preparation of cross-linked polyimide from cross-linked polyamides (WO2004087793) or the introduction of cross-linkable groups during the polymer synthesis [1 , 13, JP2001323067, WO2003053548]. In US Patent 6,755,900, a diol is added to a polymer solution containing a polyimide synthesized to contain carboxylic groups. The diol will react with the carboxylic functionality to form a monoester and after the membrane is formed, cross-linking can be induced by thermal treatment. US Pat. No. 3,533,997 discloses aromatic polyimides which incorporate a pendant carboxylic acid function and the cross- linking of such materials through interaction of the pendant carboxylic acid functions and di- to tetra-amine radicals. US Pat. No. 4,734,464 discloses a solvent resistant composition comprising a siloxane-containing polyimide and an aromatic compound containing at least two reactive groups such as amino groups which are heated to at least 150⁰C. These methods are usually quite complicated and demand a lot of organic synthesis effort.

The chemical reaction of polyimide membranes after their formation ("post-synthesis cross- linking" or also referred to as "POST") with primary or secondary di-, tri- or polyamines such as ethylenediamine and p-xylenediamine was originally described as a cross-linking method for gas separation membranes to avoid their plasticization and improve permeabilities [14, 15, 16]. It was later investigated as a method to improve solvent stability of nanofiltration membranes in chlorinated and aprotic solvents [17, 18]. Furthermore, US Pat. No. 4,981 ,497 discloses membranes consisting of aromatic polyimides that are cross-linked after their formation by chemical reaction with primary or secondary mono-, di-, tri- or polyamines, by immersing the already cast and solidified membrane for a certain time in a solution containing said amine-compound, in order to obtain improved gas separation characteristics and improved environmental resistance. WO 2007/125367 discloses asymmetric polyimide nanofiltration membranes that are cross-linked after their formation by chemical reaction with a mono-, di-, tri- and/or polyamine and are impregnated with a conditioning, aiming at improved stability in organic solvents. WO2008/138078 discloses a modification of ultra- and nanofiltration polyimide membranes after their formation by immersion in a solution of a mono-, di-, tri- or polyamine, this to enhance their solvent stability, more specifically their resistance to aprotic solvents. US Pat. No. 7,169,885 discloses a process to cross-link a polyimide membrane by exposing the membrane after its formation to a cross-linking agent selected from a group of dendrimers and hyperbranched polymers containing at least two primary amine groups.

However, several problems remain when preparing cross-linked polyimide membranes according to the methods of the above prior art documents. Methods comprising the introduction of cross-linkable groups in the polymers, as in e.g. US Patent 6,755,900 or US Pat. No. 3,533,997 are usually quite complicated and demand a lot of organic synthesis effort. Often a high temperature is needed to complete cross-linking. Alternatively, cross- linking of the membrane after membrane formation via reaction with an amine involves an extra time-consuming, thus uneconomic, step in the membrane synthesis process, which is also hard to upscale. Indeed, following casting and solidification of the polyimide membrane, cross-linking occurs by exposing the cast and solidified membrane to a solution comprising amines. Also, the amine-crosslinking method requires the swelling of the polymer chains in the membrane to make them more accessible for reaction with the amine and ensuring a complete crosslinking of the entire bulk of the membrane. Such membrane swelling requires that the crosslinking agent is dissolved in a suitable (organic) solvent, such as methanol. Indeed, in a study by Yang et al. (20) an aqueous solution of a diamine was used to primarily crosslink the pore walls and active surface of a polyimide membrane. The post-synthesis crosslinking thus also involves a (organic) solvent consuming, and consequently environmentally unfriendly, step.

Therefore, there clearly remains a need for improved methods for the preparation of cross- linked polyimide membranes. SUMMARY OF THE INVENTION

The present invention provides improved and faster preparation methods to obtain cross- linked polyimide membranes without the need for pendant carboxylic acid functions or siloxane in the polyimide polymers or membranes and without the need of an extra solvent- based post-membrane solidification crosslinking step. In the new approach disclosed in this invention, a reactive compound with at least two nucleophile functional groups in its structure ("cross-linking agent"), preferably a multifunctional amine, is added before or during membrane solidification, such as by introducing said multifunctional nucleophile in the polyimide casting solution and/or the phase inversion or coagulation bath. Thus, the present invention relates to a process for obtaining a cross-linked polyimide membrane, wherein at least a fraction of the polyimide groups is cross-linked, said process comprising the steps of (i) preparing a polyimide casting solution comprising a polyimide polymer and a solvent system for said polymer; (ii) casting a film of said casting solution onto a supporting substrate; and (iii) solidifying the cast film; wherein the polyimide polymers are contacted with a multifunctional, nucleophile cross-linking reagent, preferably a multifunctional amine, in any of the steps (i), (ii) or (iii) before the polyimide membrane is formed or solidified.

A first preferred embodiment of the present invention comprises the addition of a suitable amount of a reactive compound with at least two nucleophile functional groups, such as amine groups, in its structure ("cross-linking agent") to the polyimide comprising casting solution. The solution is allowed to react for a certain period of time before casting the membrane. This method allows the synthesis of dense or porous cross-linked polyimide membranes. Particularly, said method comprises the steps of: (a) Preparing a polyimide casting solution comprising a polyimide polymer, additives comprising at least one cross-linking agent, and a solvent system for said polymer and additives; (b) Casting a film of said casting solution onto a supporting substrate, which can optionally be followed by a short evaporation stap; (c) Solidifying the membrane Preferably, said membrane solidification occurs by a phase inversion step, either via contact with a vapour or a liquid non-solvent. Preferably, said method further comprises the step of (d) treating the membrane with wash baths and/or conditioning baths. Optionally, the membranes may be dried as a further step (e) following step (d).

A second preferred embodiment of the invention is specific for the synthesis of polyimide membranes formed by phase inversion via immersion precipitation. Said method according to the second embodiment of the present invention comprises the addition of a suitable amount of a reactive compound with at least two nucleophile functional groups in its structure, such as a di-, tri- or polyamine, to the phase inversion coagulation bath. Any non- solvent for polyimide that is a solvent for the cross-linking agent, in particular an amine- compound, may be used in this process to induce the phase inversion. Particularly, said method comprises the steps of:

(a) Preparing a polyimide casting solution comprising a polyimide polymer and a solvent system for said polymer; (b) Casting a film of said casting solution onto a supporting substrate, which can optionally be followed by a short evaporation step; (c) Immersing the film cast on the substrate into a coagulation medium comprising at least one cross-linking agent. Preferably, said method further comprises the step of (d) treating the membrane with wash baths and/or conditioning baths. Optionally, the membranes may be dried as a further step (e) following step (d).

The resulting cross-linked membranes obtained by either embodiments of the invention may be applied in membrane processes such as gas separation, microfiltration, ultrafiltration, nanofiltration, pervaporation, fuel cells and so on. They are resistant to plasticisation and to organic solvents, more specifically to the aprotic solvents (dimethylformamide, dimethylacetamide, N-methylpyrrolidon, dimethylsulfoxide, tetrahydrofuran, γ-butyrolacton) and chlorinated solvents.

DETAILED DESCRIPTION

Legends to the figures

Figure 1 shows the chemical structure of the commercial polyimide Matrimid® (Huntsman).

Figure 2 shows the chemical structure of the commercially available polyetherimide (Sigma Aldrich 700193).

Figure 3 shows the structural formula I representing a part of the imide linkages of Lenzing P84 polyimide.

Figure 4 shows the structural formula Il representing a part of the imide linkages of Lenzing P84 polyimide.

Figure 5 shows the chemical structure of the commercial polyimide Torlon ® (Solvay).

Figure 6 represents the stability after 2 weeks soaking in DMF of crosslinked membranes prepared from casting solution L1 (see Table 5.1) and crosslinked during coagulation (SIM) with different crosslinking times and concentrations. Degree of stability of the membranes is indicated by letters 'C for cross-linked, 'S' for swollen or incompletely cross-linked and 'D' for dissolved or uncross-linked. Cross-linkers are p-xylylenediamine (XDA), ethylenediamine (EDA), hexanediamine (HDA), polyethyleneimine (PEI), diethyltriamine (DETA) and dimethylethyldiamine (DMEDA).

Figure 7 represents the stability after 2 weeks soaking in DMF of membranes prepared from casting solution L1 (see Table 5.1) and crosslinked post-membrane-synthesis (POST) with different crosslinking times and concentrations. Degree of stability of the membranes is indicated by letters 'C for cross-linked, 'S' for swollen or incompletely cross-linked and 'D' for dissolved or uncross-linked. Cross-linkers are p-xylylenediamine (XDA), ethylenediamine (EDA), hexanediamine (HDA), polyethyleneimine (PEI), diethyltriamine (DETA) and dimethylethyldiamine (DMEDA).

Description

In the generally applied methods for cross-linking polyimide membranes, cross-linking is performed post-membrane synthesis, i.e. after formation of the membrane, in a separate, time-consuming, hard to upscale step wherein the formed membrane is incubated in a solution comprising said cross-linking agent (within the context of the present invention also referred to as the POST crosslinking method). In said prior art methods, the crosslinking reaction rate depends on a lot of parameters, including reactivity, hydrophobicity and steric hindrance of polyimide and crosslinker, making it hard to predict and control the reaction rate. Furthermore, contacting a reactive compound with the polyimide polymers prior to membrane formation can be expected to negatively affect membrane formation and, hence, the structure, the mechanically stability and the performance of the resulting membrane. Within this context, the inventors surprisingly found that cross-linked polyimide membranes, particularly suitable for solvent resistant ultra- and nanofiltration, can be successfully prepared when the polyimide polymer is contacted with a cross-linking agent, such as a multifunctional amine or another multifunctional nucleophile molecule, in an early stage of the membrane synthesis process, i.e. before the membrane is fully formed, such as before the casting process (e.g. by inclusion of the nucleophile in the polyimide solution) or during the immersion of the cast polyimide solution in the phase inversion bath. This way, the need for an extra cross-linking step after formation and solidification of the membrane, wherein the membrane is contacted with the cross-linking agent in a solvent suitable for membrane swelling, is eliminated and there is also no need for the polyimide comprised in the membrane to comprise pendant carboxylic acid functions or siloxane.

As used herein, the term "phase inversion" refers to the controlled transformation of a thermodynamically stable polyimide solution to a solid phase (membrane) by liquid-liquid- demixing. It can be carried out by immersion of the cast membrane solution in a bath ("coagulation bath") comprising a non-solvent for the polymer, possibly following a (short) solvent evaporation step during which a certain phase inversion can take place already or not (immersion precipitation); or by contacting the cast membrane with a vapour phase comprising a non-solvent for the polymer; or by thermal precipitation.

The present invention thus relates to improved methods for obtaining cross-linked polyimide membranes, wherein at least a fraction of the polyimide groups is cross-linked, wherein a multifunctional, nucleophile cross-linking agent is introduced early in the membrane preparation process, either before or during the formation of the membrane. In particular, said methods according to the present invention comprises the steps of: (a) preparing a polyimide casting solution comprising a polyimide polymer and a solvent system for said polymer; (b) casting a film of said casting solution onto a supporting substrate; (c) solidifying the cast film, such as by evaporating the solvent and/or by a phase inversion step, either via contact with a vapour or liquid non-solvent, and (d) treating the membrane with one or more wash baths and or conditioning baths, wherein said cross-linking agent is not included in said wash baths or conditioning baths. Preferably, the polyimide is in contact, preferably allowed to react, with said multifunctional, nucleophile cross-linking agent in one or more of the steps (a), (b) or (c). Optionally, the membranes may be dried as a further step (e) following step (el)-

The invention provides methods for obtaining cross-linked polyimide membranes, comprising the synthesis of membranes wherein solidification occurs preferably via a phase inversion process (resulting in integrally skinned asymmetric or porous symmetric or asymmetric membranes), said solidification process known to one skilled in the art. The invention may thus be applied to obtain integrally skinned asymmetric polyimide membranes. Asymmetric membranes will be familiar to one of skill in this art and include an entity composed of a dense ultra-thin top "skin" layer over a thicker more porous substructure of the same material, i.e. as being integrally skinned. Typically, the asymmetric membrane is supported on a suitable porous backing or support material. In addition, the invention also relates to methods to obtain dense, symmetric polyimide membranes or to prepare porous, symmetric membranes. At least a fraction of the imide groups of the resulting membranes will be cross- linked.

A preferred embodiment of the present invention ("Method 1") involves adding one or more reactive compounds with at least two nucleophile functional groups in its structure, i.e. a cross-linking agent, to a polymer casting solution comprising a polyimide polymer and a solvent system for this polymer. This solution is mixed, and possibly also allowed to react already, for a certain period of time before casting the solution. Pre-cast gellification should be avoided. This method allows the synthesis of both dense and porous cross-linked polyimide membranes. Thus, this embodiment of the present invention relates to improved methods for obtaining cross-linked polyimide membranes characterized in that said method comprises the steps of (a) preparing a polyimide casting solution comprising a polyimide polymer, additives comprising at least one cross-linking agent, and a solvent system for said polymer; (b) casting a film of said casting solution onto a supporting substrate; and (c) solidifying the cast film, preferably by a phase inversion step either via contact with a vapour or a liquid non-solvent.

Preferably, said method further comprises (d) treating the membrane with wash baths and/or conditioning baths before drying. It may be advantageous to perform step (c) after a partial solvent evaporation step in order to obtain integrally skinned asymmetric cross-linked polyimide membranes.

Another preferred embodiment of the present invention ("Method 2", within the context of the present invention also referred to as the SIM crosslinking method) involves adding a certain amount of a cross-linking agent to the phase inversion coagulation bath. This is specific for the synthesis of polyimide membranes formed by phase inversion via immersion precipitation. Any non-solvent for polyimide that is a solvent for the cross-linking agent may be used in this process. Thus, this embodiment of the present invention relates to improved methods for obtaining cross-linked polyimide membranes characterized in that said method comprises the steps of (a) preparing a polyimide casting solution comprising a polyimide polymer and a solvent system for said polymer that is preferably water miscible; (b) casting a film of said casting solution onto a supporting substrate; and (c) immersing the film cast on the substrate into a coagulation medium, preferably an aqueous medium, comprising at least one cross-linking agent.

In the methods according to the present invention crosslinking already takes place before and during the membrane solidification, rather than after, as in the post-synthesis crosslinking methods of the prior art. Without being bound by theory, it seems that at least some crosslinker will be in contact with the polymer chains before solidification and that membrane swelling is thus not required to allow contact between the polyimide chains and the crosslinker. CROSS-LINKING AGENT

As used herein, the term "cross-linking agent" or "cross-linker" refers to a reactive compound with at least two nucleophile functional groups in its structure. The nucleophile functional groups in the chemical structure of the reactive cross-linking agent can be any nucleophile functional groups, preferably but not limited to amine, alcohol, or thiol groups.

The cross-linking agents applied in the invention may be multifunctional amines, alcohols, thiols or other chemical compounds comprising at least two nucleophile functional groups. Amines will generally react faster with the imide functionalities than thiols or alcohols. The reaction of an imide group with an amine will result in an amide bond, the reaction with an alcohol or thiol will result respectively in an ester or thioester bond. As a consequence, a polyimide cross-linked by multifunctional alcohols or thiols will be unstable if the membrane is contacted with alcoholic solvents. It should however remain stable in contact with aliphatics, aromatics, ketones and aldehydes

Suitable amino-cross-linkers or amino compounds which can modify the resulting polyimide membrane include primary and/or secondary di-, tri- or polyamines. More specifically, suitable compounds for polyimide crosslinking or modification include hydrazine, aliphatic amines, aliphatic-aromatic amines and aromatic amines. Aliphatic amines include 1 ,2- diaminoethane, 1 ,3-diaminopropane, diaminobutane, diaminopentane, diaminohexane, diaminoheptane, diamino-octane, diaminononane, diaminodecane, methylamine, ethylamine, propylamine, isopropylamine, butylamine, isobutylamine, pentaamine, cyclohexylamine, dimethylamine, diethylamine, dipropylamine, diisopropylamine, N.N'-dimethylethylene diamine, N,N'-diethylethylenediamine, diethylenetriamine, triethylenetetraamine, tetraethylene pentaamine, pentaethylenehexamine, tris(2-aminoethyl)amine, 3- aminopropylmethyldiethoxysilane, 3-aminopropyldimethylethoxysilane, 3- aminopropyldiethoxysilane, N-methylaminopropyl-trimethoxysilane, 3- aminopropyltriethoxysilane, N-methylaminopropyltrimethoxysilane, 3-aminopropyl terminated polydimethylsiloxanes, polyethyleneimine, polyallylamine, polyvinylamine, polyether diamines based predominantly on a polyethylene oxide backbone with a molecular weight of 50 to 20,000, trimethoxysilylpropyl-substituted polyethyleneamine having a molecular weight of 200 to 200,000, polyethyleneamine having a molecular weight of 1 ,000 to 200,000, aqueous ammonium hydroxide, and the like. Preferred examples of aliphatic aromatic amines are m- xylylendiamine, p-xylylenediamine, and the like. The group of aromatic amines includes aniline, aniline derivatives, phenylene diamines, methylene dianiline, oxydianiline and the like. These should not be taken as a limitation. Virtually all multifunctional primary and secondary amino compositions are suitable for cross-linking.

Suitable alcoholic cross-linking or modifying agents are ethylene glycol, propylene glycol, 1 ,3-propanediol, 1 ,4-butanediol, 1 ,2-butanediol, 1 ,3-butanediol, pentanediol, hexanediol, heptanediol, octanediol, nonanediol, decanediol, glycerol, glucose, and the like. These should not be taken as a limitation. Virtually all multifunctional alcohol compositions are suitable for cross-linking.

CASTING SOLUTION

It is understood by a person skilled in the art that different polyimide polymer sources, that are presented in the prior art, can be used in the different embodiments of the invention. These include US Patent Nos 4,981 ,497; 4,931 ,182; 6,660,062; 5,246,166; 6,180,008; 4,307,135; 3,708,458; 3,789,079; 3,546,175; 3,179,632; 3,179,633; 3,925,211 ; 4,113,628; 3,816,303; 4,240,914; 3,822,202; 3,853,754; 1 ,434,629. Preferably, the polyimides used in the present invention do not comprise pendant carboxylic acid functions or do not incorporate siloxane-comprising polyimides. Preferred polyimides are the commercially available Matrimid® 5218 (Huntsman Corporation, Germany), a commercially available polyetherimide (Sigma Aldrich) and Lenzing P84 polyimide.

Matrimid® (CAS 62929-02-6) (Figure 1) is also known as the polymer with 1(or 3)-(4- aminophenyl)-2,3-dihydro-1 ,3,3 (or 1 ,1 ,3)-trimethyl-1H-inden-5-amine and 5,5'-carbonylbis- 1 ,3-isobenzofurandione. The common name is the polymer with diaminophenylindane and benzophenone tetracorboxilic dianhydride.

Polyetherimide, available at Sigma Aldrich (700193, CAS 61128-46-9) (Figure 2) is also known as poly(bisphenol A anhydride-co-1,3-phenylenediamine), Poly(bisphenol A-co-4- nitrophthalic anhydride-co-1 ,3-phenylenediamine) or Poly-[2,2'bis(4-(3,4- dicarboxyphenoxy)phenylpropane)-1 ,3-phenylene bisimide].

The polymer Lenzing P84 is available from HP polymers Gmbh, Austria. This polyimide is a copolymer derived from the co-condensation of benzophenone 3,3',4,4'-tetracarboxylic acid dianhydride (BTDA) and a mixture of di(4-aminophenyl) methane and toluene diamine or the corresponding diisocyanates, 4,4'-methylenebis(phenyl isocyanate) and toluene diisocyanate. The resulting copolyimide has imide linkages that can be represented by two structural formulas I and Il (figures 3 and 4). The copolymer comprises 10-90% (preferably 20%) of structure I and 90-10% (preferably 80%) of structure II. Torlon ®, such as Torlon ® 4000TF, is available from Solvay, Belgium (Figure 5). Chemically, Torlon is a polyamide-imide (PAI), comprising amide linkages alternative with imide linkages, and is a reaction product of trimellitic anhydride and aromatic diamines.

The rate of reaction between an amino-cross-linker and the imide-containing polymer depends on their chemical identity and the process conditions. In certain embodiments of the present invention polyimide reactivity towards the crosslinker should be taken into account. Generally, Lenzing P84® (Figure 3 & 4) and Matrimid® (Figure 1) are considered to have a high reactivity towards crosslinking as the imide rings in both of these polymers are highly electrophilic due to the electron withdrawing effect of the keton group at position C5 in the chain. Polyetherimide will correspondingly have a lower reactivity, due to the electron donating effect of the ether functions in the chain. The electron withdrawing effect of the amide function in the chain of Torlon® is lower than that of a keton function, so the reactivity of Torlon® might be lower compared to P84® and Matrimid® but should still be higher than the reactivity of polyetherimide. However, intramolecular hydrogen bonding between the amide groups might increase the reactivity of Torlon® for the crosslinking reaction, depending on the medium. Besides electronic effects, steric effects will also have an influence on the crosslinking rate of the polymers. These steric effects will be specific for each polyimide, due to the difference in chemical structure and the different interchain space in the polymer matrix.

The polyimide polymer casting solution may be prepared by dissolving a polyimide polymer in one or a mixture of organic solvents, including the following water-miscible solvents: N- methylpyrrolidone (NMP), tetrahydrofuran (THF), N,N-dimethylformarnide (DMF), N₁N- dimethylacetamide (DMA), dimethylsulfoxide (DMSO), 1 ,4-dioxane, gamme-butyrolactone, water, alcohols, ketones and formamide. The weight percent of the polyimide polymer in the casting solution may vary as a function of its molecular weight and additives and may range from 6% to 45%, preferably from about 10% to 40%, more preferably from 15% to 30% or from 16% to 28% range, most preferably from 18% to 26%. The casting temperature may vary from about -20⁰C to about 100⁰C, preferably about 0⁰C to 60⁰C, depending on the particular polymer, its molecular weight and the cosolvents and additives in the casting solution.

Thus, the polyimide polymer or its derivatives may be dissolved in a suitable solvent or solvent mixture (e.g. NMP and THF), which may or may not include cosolvents, partial solvents, nonsolvents, salts, surfactants or electrolytes, for altering or modifying the membrane morphology and its flux and rejection properties. More in particular, the polyimide polymer casting solution may optionally comprise one or more of the following additives to influence the viscosity of the casting solution and/or the (separation) properties of the resulting membrane:

(i) viscosity enhancers, preferably present in amounts less than 5 % or 10% by weight of said casting solution, such as e.g. polyvinyl pyrrolidones, polyethylene glycols and urethanes; (ii) void suppressors, preferably present in amounts less than 5% or 10 % by weight of said casting solution, such as e.g. maleic acid;

(iii) surfactants, which influence the pore structure, preferably present in amounts less than 5% by weight of said casting solution for example Triton X-100 (available from Sigma-Aldrich UK Ltd. (octylphenoxy-polyethoxyethanol)); and/or

(iv) discrete particles of an immiscible matrix, such as organic or inorganic matrices in the form of powdered solids, preferably present in amounts less than 20% by weight of said casting solution. Carbon molecular sieve matrices can be prepared by pyrolysis of any suitable material as described in US Pat.No. 6,585,802. Zeolites as described in US Pat. No. 6,755,900 may also be used as an inorganic matrix. Metal oxides, such as titanium dioxide, zinc oxide and silicon dioxide may be used, for example the materials available from Degussa AG (Germany) under their Aerosol and AdNano trademarks. Mixed metal oxides such as mixtures of cerium, zirconium, and magnesium may be used. Preferred matrices will be particles less than 1.0 micron in diameter, preferably less than 0.1 microns in diameter, and preferably less than 0.01 microns in diameter. In some cases it may be advantageous to disperse the matrices in a separate solution from the casting solution, preferably an organic solvent solution, and then subsequently add this solution to the casting solution comprising the polyimide polymer. In a preferred embodiment crystals or nanoparticles of an inorganic matrix, for example zeolites or metal oxides, may be grown to a selected size in a separate solution from the casting solution, and this dispersion solution subsequently added to the casting solution comprising the polymer. This separate solution may comprise water or an organic solvent with nanoparticles dispersed in the continuous liquid phase. In yet a further preferred embodiment, the solvent in which the matrix is dispersed may be volatile, and it may be removed from the casting solution prior to membrane casting by evaporation.

In preferred embodiments of the present invention according to Method 1 , the polyimide casting solution further comprises a suitable cross-linking agent. Without being bound by theory, it seems a slow reaction between an amino-cross-linker and the imide-containing polymer is preferred. In the case of amine crosslinkers, a too fast cross-linking reaction will induce gellification in the polymer-amine solution before membrane casting is possible. A polyimide-amine combination should be chosen such that the reaction between both is sufficiently slow. Aliphatic amines with shorter chain lengths, such as preferably but not limited to ethylenediamine (EDA), N,N'-dimethylethylene diamine, N₁N'- diethylethylenediamine, diethylenetriamine, 1 ,3-diaminopropane, diaminobutane, are less nucleophile and thus react slowly. Amines with longer or branched chains or aromatic amines, such as preferably but not limited to polyethyleneimine (PEI), polyetheramine, diaminohexane, diaminoheptane, diamino-octane, diaminononane, diaminodecane, m- xylylendiamine, p-xylylenediamine or phenylene diamine, may also be suitable as they are bulkier and steric hindrance may slow down the reaction. Slow reaction polyimide polymers, such as polyetherimide, are preferred as polymer, as it is less reactive than Matrimid® and Lenzing P84 polyimide since the ketone function is replaced by ether functions, decreasing the electrophilicity of the imide-C. The choice of the amine compound is further restricted by its solubility in the solvent system chosen to prepare the polymer casting solution. It may further be necessary to apply certain additives to the polymer casting solution to decrease or increase the reaction rate. In the case of thiols and alcohols, reaction with the imide functionalities will in general be slower. The ratio alcohol/imide and the reaction time may be higher than for amines and pre-cast gellification is expected to be less of a problem. Preferably, the molar ratio of polyimide to cross-linking agent in the polyimide polymer casting solution is between 200:1 and 0.5:1, more preferably between 100:1 and 1 :1 , most preferably between 50:1 and 1 :1. The casting solution comprising crosslinker may be cast immediately or may be allowed to react for a certain amount of time as long as no pre-cast gellification occurs, such as less than 60 min, preferably less than 30 min or 20 min, more preferably less than 15 or 10 min. It is understood that slower reacting polyimide/crosslinker combinations are allowed to have a longer reaction time, and that faster reacting polyimide/crosslinker combinations have only a short reaction time or are cast immediately after mixing or homogenisation of the casting solution.

MEMBRANE CASTING

Casting of the membrane may be performed by any number of casting procedures cited in the literature, for example U.S. Patent Nos. 3,556,305, 3,567, 810, 3,615,024, 4,029,582 and 4,188,354; GB-A-2,000,720; Office of Saline Water R & D Progress Report No. 357, October 1967; Reverse Osmosis and Synthetic Membranes, Ed. Sourirajan; Murari et al, J. Membr. Sci. 16: 121-135 and 181-193 (1983).

In the different embodiments of the present invention, once the desired polyimide polymer casting solution is prepared (i.e. polyimide polymers are dissolved in a suitable solvent system, and optionally organic or inorganic matrices are added into the casting solution so that the matrices are well dispersed) and, optionally, filtered by any of the known processes (e.g. pressure filtration through microporous filters, or by centrifugation), it is cast onto a suitable support or substrate, such as glass, metal, paper, plastic, etc., from which it may then be removed. It is preferred, however, to cast onto a porous base support from which the membrane is not removed. Such porous support can take the form of an inert porous material which does not hinder the passage of permeate through the membrane and does not react with the membrane material, the casting solution, the gelation bath solvent, or the solvents which the membrane will be permeating in use.

Such porous base supports may be non-woven, or woven, including cellulosics (paper), polyethylene, polypropylene, nylon, vinyl chloride homo-and co-polymers, polystyrene, polyesters such as polyethylene terephthalate, polyvinylidene fluoride, polytetrafluoroethylene, polysulfones, polyether sulfones, poly-ether ketones (PEEK), polyphenylene oxide, polyphenyline sulphide (PPS), Ethylene-(R) ChloroTriFluoroEthylene (Halar ® ECTFE), glass fibers, metal mesh, sintered metal, porous ceramic, sintered glass, porous carbon or carbon fibre material, graphite, inorganic membranes based on alumina and/or silica (possibly coated with zirconium and/or other oxides). The membrane may otherwise be formed as a hollow fiber or tubelet, not requiring a support for practical use; or the support may be of such shape, and the membrane is cast internally thereon. Following the casting operation, a portion of the solvent may be evaporated under conditions sufficient to produce a dense, ultra-thin, top "skin" layer on the polyimide membrane. Typical evaporation conditions adequate for this purpose include exposure to air for a duration of less than 100 seconds, preferably less than 60 or 30 seconds. In yet a further preferred embodiment, air is blown over the membrane surface at 15 ⁰C to 25 ⁰C for a duration of less than 30 seconds.

SOLIDIFICATION

The coagulation bath will comprise one or a mixture of solvents, including the following water-miscible solvents: N-methylpyrrolidone (NMP), tetrahydrofuran (THF), N₁N- dimethylformamide (DMF), N,N-dimethylacetamide (DMA), dimethylsulfoxide (DMSO), 1 ,4- dioxane, gamme-butyrolactone, water, alcohols, ketones and formamide, as well as additives such as surfactants, e.g., Triton(R) X-100 (available from Sigma-Aldrich UK Ltd (octylphenoxy-polyethoxyethanol)). The conditions for effecting coagulation are well known to those skilled in the art.

In preferred embodiments of the present invention according to Method 2 (i.e. the SIM crosslinking method), the coagulation bath further comprises a suitable cross-linking agent. The rate of reaction between an amino-cross-linker and the imide-containing polymer will vary greatly dependent on their chemical identity and the process conditions. Without being bound by theory it seems that a fast reaction is preferred as well as a good diffusion of the cross-linker into the solidifying membrane matrix. The more reactive polyimides, Matrimid® and Lenzing P84 polyimide, are preferred in this method. In the case of amine cross-linking agent, the most nucleophile amines that still have a good diffusion rate into the polymer are preferred, such as preferably but not limited to 1 ,3-diaminopropane, diaminobutane, diaminopentane, diaminohexane, ethylene diamine, m-xylylendiamine, or p-xylylenediamine. More hydrophobic amines, such as xylylenediamine, will have a high distribution coefficient in the apolar polymer matrix while leaving the aqueous phase, thus favouring a faster crosslinking reaction in aqueous coagulation media. Smaller crosslinking agents, such as ethylene diamine, or 1 ,3 diaminopropane, favour a faster reaction as well since, in the absence of solvent induced swelling of the polymer matrix, smaller crosslinking agents have a faster penetration rate. The choice is further restricted by the solubility of the amine compound in the non-solvent that is used as the coagulation medium. Any solvent solubilising the named amine compounds, not dissolving the uncross-l inked polymer, still allowing the phase inversion as well as the cross-linking reaction to take place can be used as coagulation medium. An aqueous medium is preferred as water is an efficient non-solvent for polyimides. For economic and upscaling purposes, the amine that has the highest reactivity at its lowest concentration in the coagulation bath is the preferred amine, including but not limited to ethylene diamine, p-xylylenediamine, or 1,3 diaminopropane. In the case of thiols and alcohols, the reaction will take place so slowly that a long reaction time and/or an extra thermal treatment may be necessary to complete the cross-linking. The diffusion rate of the alcohol/thiol into the solidifying polymer matrix should then also be taken into account.

In a preferred embodiment, the cross-linking agent is present in 0.1 to 20 wt% of the coagulation medium, more preferably from 0.5 - 10 wt%, even more preferably from 0.5 - 5 wt% of the coagulation medium.

In the different embodiments of the present invention, reaction times can vary, such as preferably less than 60 min, more preferably less than 30 min or 20 min, most preferably less than 15 or 10min. It is understood that reaction times and concentration ratios depend on the combination of polyimide and cross-linking agent. The reaction time may be shortened by increasing cross-linker concentration in the coagulation medium. Also, reactions may proceed faster at elevated temperatures. The reaction temperature should be less than the boiling point of the coagulation medium, such as between O⁰C and 100°C, for instance between about 10⁰C and about 70⁰C or between about 20°C and 50 or 60°C. WASHING/CONDITIONING

In the different embodiments of the present invention it is preferred that the cross-linked polyimide membrane (after membrane solidification) is thoroughly rinsed or washed e.g. to remove all unreacted cross-linking agents and the coagulation medium from the membranes. The asymmetric polyimide membranes formed can be washed according to the following techniques. Typically a water-soluble organic compound such as low molecular weight alcohols and ketones including but not limited to methanol, ethanol, isopropanol, acetone, methylethyl ketone or mixtures thereof and blends with water can be used for removing the residual casting solvent (e.g. DMF), crosslinking agent or coagulation solvent from the membrane. Alternatively, the membrane may be washed with water. Removal of the residual casting solvent may require successive wash blends in a sequential solvent exchange process. Both membrane efficiency (solute rejection) and permeate flow rate can be enhanced by the proper solvent exchange process.

Thereafter, the membrane can be subjected to a conditioning step by contacting the membrane with a conditioning agent dissolved in a solvent to impregnate the membrane. The conditioning agent is typically a low volatility organic liquid. The conditioning agent may be chosen from synthetic oils (e.g., polyolefinic oils, silicone oils, polyalphaolefinic oils, polyisobutylene oils, synthetic wax isomerate oils, ester oils and alkyl aromatic oils), mineral oils (including solvent refined oils and hydroprocessed mineral oils and petroleum wax isomerate oils), vegetable fats and oils, higher alcohols (such as decanol, dodecanol, heptadecanol), glycerols, and glycols (such as polypropylene glycols, polyethylene glycols, polyalkylene glycols). Suitable solvents for dissolving the conditioning agent include alcohols, ketones, aromatics, hydrocarbons, or mixtures thereof. The use of a conditioning agent in accordance with the invention allows a suitable pore structure to be maintained in a dry state, and produces a flat sheet membrane with improved flexibility and handling characteristics.

For instance, the membrane can be subjected to a solvent-exchange procedure in a glycerol- containing isopropanol before drying, to prevent pores from collapsing, which may have a negative effect on permeability in pressure driven filtrations. In a particular embodiment the solvent exchange procedure comprises the immersion of the membrane in an isopropanol bath for at least two hours, followed by an immersion in an isopropanol-glycerol bath (typically 60-40) for at least three days.

Following treatment with the conditioning agent, the membrane is typically dried in air at ambient conditions to remove residual solvent. The cross-linking procedure according to the method of the present invention results in ultra- and nanofiltration membranes with improved stability and high permeabilities in organic solvents (or mixtures thereof) and good rejection for low-molecular weight compounds between 200-2000 Da, for instance between 200 and 700 Da. The resulting cross-linked membranes obtained by the different embodiments of the invention may be applied in membrane processes such as gas separation, microfiltration, ultrafiltration, nanofiltration, pervaporation, fuel cells and so on and can be configured in accordance with any of the designs known to those skilled in the art, such as spiral wound, plate and frame, shell and tube, and derivative designs thereof. The improved stability of the polyimide membranes obtained according to the present invention is particularly useful when the membranes are used in separation processes in organic solvents and more particularly in aprotic solvents such as dimethylformamide (DMF), N-methylpyrrolidinone (NMP), dimethylacetamide (DMAC), tetrahydrofuran (THF), γ-butyrolacton (GBL), dimethylsulphoxide (DMSO) and chlorinated solvents.

Example 1

A 15% P84 polyimide solution containing NMP and THF in a 3:1 weight ratio was prepared. The solution was cast onto a porous non-woven PP/PE support (Novatex 2471 , Freudenberg) and immersed in a solution of 10%w/v of hexanediamine in water. The resulting membrane slabs were insoluble in THF and DMF.

Example 2

A 22% P84 polyimide solution containing NMP and THF in a 5:3 weight ratio was prepared. The solution was dipcoated onto pieces of porous non-woven PP/PE support (Novatex 2471 , Freudenberg). The pieces were immersed in a solution of 0.5% p-xylenediamine in water for 3, 9 or 12 minutes. All of the resulting membrane slabs were insoluble in DMF.

Example 3

A 24% P84 polyimide solution containing NMP and THF in a 1 :1 ratio was prepared. The solution was cast onto three pieces of porous non-woven PP/PE support (Novatex 2471 , Freudenberg). Each of the three cast membranes was solidified in a coagulation bath containing 10% of EDA and were kept in the bath for respectively 3, 9 or 12 minutes. All membranes were shown to be insoluble in DMF. Filtrations with isopropanol and bromothymolblue are summarized in table 3.1.

Table 3.1. Summary of filtration results for example 3.

IMMERSION TIME (MIN) PERMEABILITY (MVI²BAR H) REJECTION (%)

~3 O02 83^2

9 0.06 83.0

12 0.04 81.2

Example 4

Polymer casting solutions were prepared in DMF with compositions as shown in table 4.1. The solutions were cast onto a porous non-woven PP/PE support (Novatex 2471 , Freudenberg). Each of the resulting membranes was insoluble in THF.

Table 4.1. Molar ratio of polymer to cross-linker in polymer casting solutions of THF stable PEI membranes solution Pl (wt%) EDA (wt%) MoIi

A 14.25 0.20 14.5

B 14.25 0.60 4.8

C 14.25 0.75 4

D 10.80 1.50 1.5

E 11.40 0.75 3.1

Example 5

Membrane preparation with simultaneous cross-linking and coagulation (referred to as "SIM")

Casting solutions containing a polyimide polymer, THF and NMP were prepared according to Table 5.1 and stirred until homogeneous. The solutions were deposited onto an NMP saturated polypropylene/polyethylene non-woven fabric (Novatex 2471) using an automated casting knife set at 250 μm (1.2 m/min, Braive Instruments, Belgium) to prepare filtration membranes. After a short evaporation time, the films were immersed in a coagulation bath of deionized water containing 0.5-10 wt% crosslinker and were kept in the bath for 2h and in a subsequent experiment for different periods of time. The resulting solidified membranes were then rinsed with water to remove excess crosslinker. The membranes on the non-woven support were stored in isopropanol until further use.

Table 5.1 : Compositions of polyimide solutions used for membrane synthesis

Solution Polymer Polyimide (wt%) /V-Methyl- Tetrahydrofuran pyrrolidone (wt%) (wt%)

L1 Lenzing P84® 22 48.75 29.25

M1 Matrimid® 22 48.75 29.25

P1 Polyetherimide 22 48.75 29.25

T1 Torlon® 22 48.75 29.25

A lower concentration of the crosslinker in the coagulation bath can significantly decrease the total production costs of the crosslinked membrane in an upscaled process. Reduction of the crosslinker concentration in the coagulation baths from 10% to 5, 2.5, 1 and 0.5% (w/v) and crosslinking the membranes for 2 h resulted in membranes which had gel contents higher than 95% after two weeks soaking in DMF, indicating in stable, crosslinked membranes. In addition, this indicates that crosslinking time can be reduced as well.

Membrane preparation with post-synthesis cross-linking (referred to as "POST")

Casting solutions containing a polyimide polymer, THF and NMP were prepared according to Table 5.1 and stirred until homogeneous. The solutions were deposited onto an NMP saturated polypropylene/polyethylene non-woven fabric (Novatex 2471) using an automated casting knife set at 250 μm (1.2 m/min, Braive Instruments, Belgium) to prepare filtration membranes. After a short evaporation time, these polymer films were immersed for at least 1 h in a coagulation bath containing deionized water, resulting in a solid membrane. The membranes were then immersed in a solution of methanol containing 0.5-10 wt% of a crosslinker for different periods of time. The membranes were rinsed with methanol to remove excess crosslinker. The membranes on the non-woven support were stored in isopropanol until further use, which is a routine lab method.

The minimum coagulation time to obtain DMF stable membranes from solution L1 at different crosslinker concentrations was used to validate the crosslinking efficiency of both SIM and POST methods. Both SIM (Figure 6) & POST (Figure 7) membranes were tested for their stability in dimethylformamide by immersing them in this solvent for two weeks. Degree of stability of the membranes is indicated in Figures 6 and 7 by letters 'C for cross-linked, 'S' for swollen or incompletely cross-linked and 'D' for dissolved or uncross-linked. Cross-linkers are p-xylylenediamine (XDA), ethylenediamine (EDA), hexanediamine (HDA), polyethyleneimine (PEI), diethyltriamine (DETA) and dimethylethyldiamine (DMEDA). Different crosslinkers were used (EDA, HAD, XDA, PEI, DETA, DMEDA) were used to account for the influence of different chemical properties of the crosslinker on concentration and time needed to obtain crosslinked polyimide membranes.

It is clear that the reaction rate between each crosslinker and the polyimide differs between the crosslinking methods.

In the case of SIM crosslinking, the reaction medium was water - a strong non-solvent for the polyimide polymers, hence favourable for phase inversion - and the diamine was able to penetrate into the membrane matrix before or during its solidification rather than after as in the case of POST crosslinking.

Under the conditions of the experiment, XDA was the most efficient crosslinker for SIM crosslinking. A marked improvement in crosslinking time for SIM compared to POST crosslinking was observed for XDA, where 12 min of crosslinking time at 0.5% XDA and 9 min at 1% were sufficient to create crosslinked membranes by SIM crosslinking compared to 30 min at 0.5% and 12 min at 1% for POST crosslinking. It is likely that the very hydrophobic XDA will have a high distribution coefficient in the apolar polymer matrix while leaving its aqueous environment.

EDA and HAD are both slower crosslinkers. At 2.5-5 wt%, EDA yields stable membranes slightly faster than HDA. Since the contact in this case is made in the instant before the membrane solidifies, the faster penetration rate of the smaller EDA can explain these results. Indeed, unlike in the POST method, there is no solvent induced swelling of the polymer matrix, so the smaller EDA has an advantage over HDA. Similarly, as in POST crosslinking, PEI and DETA have a poorer reactivity. For DETA and PEI, longer crosslinking times and/or higher concentrations (>5%) are necessary to obtain crosslinked membranes. In the case of PEI, this can be easily explained by the fact that the long and branched polymeric chains require a longer time to diffuse into the P84® membrane matrix. For DETA, with a secondary amine group in between two ethyleneamine groups, rigidification originating from an intramolecular H-bond might explain its poorer reactivity. It is also likely that the very hydrophilic DETA has a low affinity for the hydrophobic polyimide polymers. The secondary diamine DMEDA is a very inefficient crosslinker: no stable membranes could be obtained with DMEDA within 30 min of reaction with the SIM method and stable membranes were only obtained with the POST method at a concentration of 10% and after at least 1 h reaction time. The secondary amine groups thus react much slower than the primary amine groups. Even though the alkyl groups render thesecondary amine groups more nucleophile than a primary amine group, they also increase the steric hindrance. In a reaction with sterically demanding imide groups, as in the case for polymeric imides, this steric hindrance effect thus induces a lower crosslinking rate.

Example 6

Membranes were prepared from the solutions of Table 5.1 by the SIM method according to Example 5 using different cross-linkers in the coagulation bath and reaction times. The minimum time for each polymer/cross-linker combination to reach stability in DMF is shown in table 6.1.

Table 6.1. The shortest crosslinking time to obtain DMF-stable polyimide membranes with 5%wt solutions of crosslinkers EDA, HDA or XDA. Membranes are prepared from solutions M1 , T1 , P1 and l_1 in a coagulation medium comprising the crosslinking agents.

Diamine (5%) Shortest crosslinking time to obtain stability in DMF (min)

Matrimid® Torlon® Polyetherimide Lenzing P84® B)A >60^s 12 30 9

HDA 12 12 30 12

XDA 12 12 30 3

^a Membranes that remained unstable after 60 minutes of crosslinking

In SIM crosslinking, Matrimid ®and Torlon ©consistently react similarly with all diamines, with the exception of the reaction of Matrimid ©with EDA. In comparison, polyetherimide always reacts slower, which is consistent with the expected order of reactivity. It also becomes clear that the nucleophilicity of EDA, HDA and XDA is in fact very similar, as hardly any difference in reaction time was observed. The larger reaction time for EDA and Matrimid® might be explained by the higher viscosity and stronger hydrophobicity of Matrimid ©compared to Torlon ©and the strong hydrofilicity of EDA resulting in a low diffusion rate that may affect the reaction rate. Thus, it seems that the reactivity of the polyimides is a dominating factor for crosslinking reactions in the SIM method. The lower viscosity of a Lenzing P84 ©polymer solution and its corresponding fastest crosslinking reactions, shows the important role of viscosity in crosslinking reactions in the SIM method.

These results were confirmed by contact angle measurements for membranes crosslinked for 10 and 30 min by both methods, using a 0.5 wt% XDA solution (Table 6.2). In the case of Matrimid ®, Lenzing P84 ©and polyetherimide, the contact angle after 10 min of crosslinking is reduced faster by SIM than by POST crosslinking but the difference has disappeared after 30 min. A decrease of contact angle indicates a more hydrophilic surface and thus a more crosslinked membrane, so in these cases it can be concluded that crosslinking is happening faster for the SIM method than for the POST method.

Table 6.2. Contact angles (± standard deviation) of POST and SIM crosslinked polyimide membrane surfaces measured with water. Membranes are prepared from solutions M1, L1 , P1 and T1 (see Table 5.1) and the crosslinking solutions contained 0.5 wt% XDA.

Example 7

In Table 7.1 , the SIM crosslinking method is compared to the post-synthesis crosslinking method as described in above mentioned prior art documents. Preparation of membranes according to the POST crosslinking method included a short evaporation step after casting, with an evaporation time before immersion in water bath of 45s, and crosslinking was conducted post-synthesis by immersion in crosslinking bath (methanol + 5wt%HDA) for 15 min. Preparation of SIM crosslinked membranes included also a short evaporation step after casting, with an evaporation time before immersion in crosslinking bath (water + 5wt%HDA) of 45s, and membranes resided in the coagulation bath for 15 min.

Filtrations were carried out in a high throughput filtration module, allowing for simultaneous filtrations of 16 membranes. Membrane discs were supported by a porous stainless steel disc and sealed with solvent resistant o-rings. The active membrane surface area was 0.000452m². The feed reservoir was filled with approximately 30 ml of a feed solution (35μM Rose Bengal, 1017 Da, in DMF or NMP) and was mechanically stirred at 600 rpm. A nitrogen pressure of 10 bar was applied. Filtrations were executed at room temperature. Permeate samples were collected as a function of time and weighed to determine permeances (P, l/nri2 bar h). Retentions (R, %) were defined as

(1-C_P/Cf)^χ100, where Cf and C_P denote the solute concentrations in the initial feed and in the permeates respectively. Solute concentrations were measured on a Perkin-Elmer lambda 12 UV-Vis spectrophotometer (558 nm).

Table 7.1: Dimethylformamide (DMF) and N-Methylpyrrolidon (NMP) filtration results of crosslinked polyimide membranes prepared from 20% Lenzing P84 solutions with different ratios of NMP to Tetrahydrofuran (THF) content.

Membrane NMP:THF Method Permeation Results

Permeance Rejection

(l/m²barh) (%)

Dimethylformamide⁰

1 3:2 POST³ 1.29 ± 0.13 92.2 ± 3.6

2 3:2 SIM^b 0.43 ± 0.03 98.6 ± 0.1

3 4:3 POST³ 0.50 ± 0.09 98.6 ± 0.1

4 4:3 SIM^b 0.55 ± 0.06 97.8 ± 0.1

N methylpyrrolidon⁰

1 3:2 POST³ 0.37 ± 0.04 80.9 ± 0.2

2 3:2 SIM^b 0.12 ± 0.01 98.1 ± 0.3

3 4:3 POST³ 0.15 ± 0.01 92.7 ± 0.1

4 4:3 SIM^b 0.19 ± 0.01 97.6 ± 0.2

³ Evaporation time before immersion in water bath was 45s, crosslinking was conducted post-synthesis by immersion in crosslinking bath (methanol + 5wt%HDA) for 15 min ^b Evaporation time before immersion in crosslinking bath (water + 5wt%HDA) was 45s, membranes resided in crosslinking bath for 15 min c Feed solutions contained 35μM Rose Bengal The results (Table 7.1) show that this new method based on SIM crosslinking can be used to obtain solvent resistant membranes in the ultra- and nanofiltration range. Furthermore, particularly for the higher NMP:THF ratio (3:2), a decrease in permeance and an increase in rejection of the dye Bengal Rose (1017 Da) can be observed for the membranes prepared by SIM crosslinking when compared to POST crosslinking. This indicates that the SIM crosslinked membranes have a denser separation layer.

However, comparison of the structure of XDA crosslinked Lenzing P84® membranes prepared by both methods by taking SEM pictures of their crosssections shows that the membranes have similar cross-sections and separating layers and show the typical nodular structure. However, the fracture of the SIM crosslinked membranes seems to be more rough.

Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.

REFERENCES

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Claims

1. A process for obtaining a cross-linked polyimide membrane, wherein at least a fraction of the polyimide groups is cross-linked, said process comprising the steps of (i) preparing a polyimide casting solution comprising a polyimide polymer and a solvent system for said polymer; (ii) casting a film of said casting solution onto a supporting substrate; and (iii) solidifying the cast film comprising a phase inversion step; characterised in that that the polyimide polymers are contacted with a multifunctional, nucleophile cross-linking reagent in any of the steps (i), (ii) or (iii) before the polyimide membrane is formed or solidified.

2. A process for obtaining a cross-linked polyimide membrane, wherein at least a fraction of the polyimide groups is cross-linked, characterised in that said process comprising the steps of (i) preparing a polyimide casting solution comprising a polyimide polymer, a solvent system for said polymer and one or more additives comprising at least one multifunctional nucleophile cross-linking agent; (ii) casting a film of said casting solution onto a supporting substrate; and (iii) solidifying the cast film comprising a phase inversion step.

3. A process for obtaining a cross-linked polyimide membrane, wherein at least a fraction of the polyimide groups is cross-linked, characterised in that said process comprising the steps of (i) preparing a polyimide casting solution comprising a polyimide polymer and a solvent system for said polymer; (ii) casting a film of said casting solution onto a supporting substrate; and (iii) solidifying the cast film by immersing said film cast on the substrate, possibly after an evaporation, into a phase inversion coagulation medium, said medium comprising at least one multifunctional, nucleophile cross-linking agent.

4. A process according to claims 1 to 3 characterised in that said polyimide polymer is present in 6-35 weight percent of the casting solution.

5. A process according to any of the claims 1 to 4 wherein said polyimide polymer has the following structure:

6. A process according to any of the claims 1 to 4 wherein said polyimide polymer has the following structure:

7. A process according to any of the claims 1 to 4 wherein the polyimide is at least one copolymer derived from (a) the co-condensation of benzophenone 3,3',4,4'-tetracarboxylic acid dianhydride and a mixture of (i) di(4-aminophenyl) methane and toluene diamine, or (ii) a mixture of 4,4'-methylenebis (phenyl isocyanate) and toluene diisocyanate; or (b) the condensation of 1H,3H-Benzo[1 ,2-c:4,5-c']difuran-1 ,3,5,7-tetrone with 5,5'-cabonylbis[1 ,3- isobenzofuranidione], 1 ,3-diisocyanato-2-methylbenzene and 2,4-diisocyanato-1- methylbenzene.

8. A process according to any of the claims 1 to 4, wherein said polyimide polymer is a polyimide copolymer comprising from 10-90% of

and from 90-10% of

9. A process according to claim 8 wherein said polyimide polymer is a polyimide copolymer comprising of about 20% of

10. A process according to any of the claims 1 to 4 wherein said polyimide polymer has the following structure:

11. A process according to any of the claims 1 to 10 wherein said cross-linking agent comprises at least two functional groups in its chemical structure chosen from primary amines, secondary amines, alcohols or thiols.

12. A process according to claim 11 wherein said cross-linking agent comprises at least two primary or secondary amine functionalities in its chemical structure.

13. A process according to claim 12 wherein said cross-linking agent is a primary and/or secondary di-, tri- or polyamine.

14. A process according to claim 13 wherein said cross-linking agent is hydrazine, an aliphatic amine, an aliphatic aromatic amine or an aromatic amine.

15. A process according to claim 14 wherein said cross-linking agent is at least one compound selected from: 1,2-diaminoethane, 1,3-diaminopropane, diaminobutane, diaminopentane, diaminohexane, diaminoheptane, diamino-octane, diaminononane, diaminodecane, ethylene diamine, N,N'-dimethylethylene diamine, N, N'- diethylethylenediamine, diethylenetriamine, triethylenetetraamine, tetraethylene pentaamine, pentaethylenehexamine, tris(2-aminoethyl)amine, polyethyleneimine, polyallylamine, polyvinylamine, polyether diamines based predominantly on a polyethylene oxide backbone with a molecular weight of 50 to 20,000, trimethoxysilylpropyl-substituted polyethyleneamine having a molecular weight of 200 to 200,000, polyethyleneamine having a molecular weight of 1 ,000 to 200,000, m-xylylendiamine, p-xylylenediamine, multifunctional aniline derivatives, phenylene diamines, methylene dianiline, oxydianiline and the like.

16. A process according to claim 11 wherein said cross-linking agent comprises at least two alcohol or thiol functionalities in its chemical structure.

17. A process according to claim 16 wherein said cross-linking agent is a di-, tri- or polyol or a di-, tri- or polythiol.

18. A process according to claim 17 wherein said cross-linking agent is an aliphatic alcohol, an aliphatic aromatic alcohol or an aromatic alcohol, an aliphatic thiol, an aliphatic aromatic thiol or an aromatic thiol.

19. A process according to claim 17 wherein the compound is selected from any of the following: ethylene glycol, propylene glycol, 1,3-propanediol, 1 ,4-butanediol, 1 ,2-butanediol, 1 ,3-butanediol, pentanediol, hexanediol, heptanediol, octanediol, nonanediol, decanediol, and the like.

20. A process according to any of the claims 1 to 19 wherein said polymer casting solution comprises any or a mixture of organic solvents chosen from N-methylpyrrolidone, tetrahydrofuran, N,N-dimethylformamide, N,N-dimethylacetamide, dimethylsulfoxide, 1 ,4- dioxane, gamma-butyrolactone, water, alcohols, ketones and formamide.

21. A process according to any of the claims 1 to 20 wherein the coagulation bath used in the phase inversion membrane solidification comprises any organic solvent or any mixture of organic solvents selected from water, N-methylpyrrolidone, tetrahydrofuran, N₁N- dimethylformamide, N,N-dimethylacetamide, dimethylsulfoxide, 1 ,4-dioxane, gamma- butyrolactone, alcohols, ketones and formamide.

22. A process according to any of the claims 1 to 21 wherein the resulting membrane is stable to organic solvents.

23. A process according to claim 22 wherein resulting membrane is stable to N- methylpyrrolidone, tetrahydrofuran, N,N-dimethylformamide, N.N-dimethylacetamide, dimethylsulfoxide, 1 ,4-dioxane, gamme-butyrolactone, alcohols, ketones and formamide