WO2006009520A1

WO2006009520A1 - Polyimide membranes

Info

Publication number: WO2006009520A1
Application number: PCT/SG2005/000243
Authority: WO
Inventors: Tai-Shung Chung; Lu Shao
Original assignee: National University Of Singapore
Priority date: 2004-07-20
Filing date: 2005-07-20
Publication date: 2006-01-26

Abstract

A process for treating a polyimide comprising the steps of: (a) exposing the polyimide to a cross-linking agent comprising one or more amine groups, said exposing being undertaken for a period of time to form a cross-linked polyamide from at least a portion of said polyimide; and (b) maintaining the cross-linked polyamide at a selected temperature to form cross-linked polyimide from at least a portion of said cross-linked polyamide.

Description

POLYIMIDE MEMBRANES

Technical Field

The present invention relates to a process for treating polyimides, and to membranes comprising the polyimides treated by the process.

Background Polymeric membrane materials for gas separation should not only have good gas separation properties, for example high gas permeability and permselectivity, but should also maintain their intrinsic gas separation properties in complex and harsh environments. Polyimides are attractive membrane materials for gas separation because of their versatility, proccessability and good mechanical properties.

Many attempts have been made to modify the chemical structure of polyimides with the aim of obtaining both highly permeable and permselective membrane materials. However most of the polyimides produced either have relatively low selectivity or suffer severe aging and performance decay due to densification and/or plasticization. In order to overcome these problems, cross-linking modifications have been carried out.

Cross-linking modification of polyimides can be effected by several methods, such as UV light induced photochemical cross-linking reactions in benzophenone- containing polyimides, cross-linking reactions of copolyimides containing free carboxylic acid with ethylene glycol, and thermally induced cross-linking reactions. However, these cross-linking methods cannot be easily applied in practical applications, due to high cost and the functional group dependence.

There is therefore a need to provide a process for treating polyimides that overcome, or at least ameliorate, one or more of the disadvantages described above.

Summary of invention

A first aspect provides a process for treating a polyimide comprising the steps of:

(a) exposing the polyimide to a cross-linking agent comprising one or more amine groups, said exposing being undertaken under conditions so as to form a cross-linked polyamide from at least a portion of said polyimide; and

(b) maintaining the cross-linked polyamide under conditions so as to form cross-linked polyimide from at least a portion of said cross-linked polyamide. In step (a) , the conditions may comprise exposing the polyimide to the cross-linking agent at a temperature and time effective to form cross-linked polyamide from at least a portion of said polyimide.

In step (b) , the conditions may comprise maintaining the cross-linked polyamide at a temperature and time effective to form cross-linked polyimide from at least a portion of said cross-linked polyamide.

A second aspect provides a treated polyimide obtained by the process of the first aspect. A third aspect provides ^• a membrane comprising a treated polyimide obtained by the process of the first aspect. A fourth aspect provides a separation system comprising a membrane according to the third aspect.

A fifth aspect provides a gas separation module comprising a membrane according to the third aspect. A sixth aspect provides a pervaporation module comprising a membrane according to the third aspect.

A seventh aspect provides a process for separating at least one fluid or particle from a mixture comprising the steps of: (a) contacting the mixture with one side of a treated polyimide membrane obtained by the process of the first aspect; and

(b) applying a pressure to the one side of the treated polyimide membrane to cause the at least one fluid or particle to permeate said treated polyimide membrane.

An eighth aspect provides a process for treating a polyimide having a first nitrogen to fluorine ratio comprising the steps of: (a) exposing the polyimide to a cross-linking agent comprising one or more amine groups, said exposing being undertaken under conditions so as to form a cross-linked polyamide having a second nitrogen to fluorine ratio from at least a portion of said polyimide, said second nitrogen to fluorine ratio being higher than the first nitrogen to fluorine ratio; and

(b) maintaining the cross-linked polyamide under conditions so as to form cross-linked polyimide having a third nitrogen to fluorine ratio from at least a portion of said cross-linked polyamide, said third nitrogen to fluorine ratio being the same as or lower than the second nitrogen to fluorine ratio.

A ninth aspect provides a process for treating a polyimide comprising the steps of: (a) exposing the polyimide to a cross-linking agent comprising one or more amine groups, said exposing being under conditions so as to form a cross-linked polyamide from at least a portion of said polyimide; and

(b) maintaining the cross-linked polyamide under conditions so as to form cross-linked polyimide having charge transfer complexes from at least a portion of said cross-linked polyamide, said cross-linked polyimide having higher resistance to plasticization than the polyimide and the cross-linked polyamide. A tenth aspect provides a process for treating a polyimide having a first critical plasticization pressure comprising the steps of:

(a) exposing the polyimide to a cross-linking agent comprising one or more amine groups, said exposing being under conditions so as to form a cross-linked polyamide having a second critical plasticization pressure from at least a portion of said polyimide; and

(b) maintaining the cross-linked polyamide under conditions so as to form cross-linked polyimide having a third critical plasticization pressure from at least a portion of said cross-linked polyamide, said third critical plasticization pressure being higher than the first and second critical plasticization pressures.

An eleventh aspect provides a process for separating carbon dioxide from a gas mixture comprising carbon dioxide and at least one of methane and hydrogen, the process comprising the steps of:

(a) contacting the gas mixture with one side of a treated polyimide membrane made in a process according to the first aspect; and

(b) applying a pressure to the gas mixture in contact with said treated polyimide membrane to cause at least a portion of said carbon dioxide present in said gas mixture to permeate said treated polyimide membrane. It will be appreciated that the treated polyimide membrane is particularly suitable for use in removing carbon dioxide or hydrogen sulfide from natural gas, which comprises methane and hydrogen gas. Advantageously, the treated polyimide membrane can be used to separate carbon dioxide and hydrogen mixtures.

Definitions

The following words and terms used herein shall have the meaning indicated: The term "inherent viscosity" as used herein refers to ratio of the natural logarithm of the relative viscosity to the concentration of the polymer in grams per 100 ml of solvent.

Unless specified otherwise, the terms "comprising" and "comprise", and grammatical variants thereof, are intended to represent "open" or "inclusive" language such that they include recited elements but also permit inclusion of additional, unrecited elements.

The term "nitrogen to fluorine ratio" as used herein refers to ratio of the number of moles of nitrogen to the number of moles of fluorine atoms in a membrane The term "critical plasticization pressure " as used herein refers to pressure at which permeability of a membrane with respect to carbon dioxide gas or heavier hydrocarbons starts to increase as applied pressure increases.

As used herein, the term "about", in the context of concentrations of components of the formulations, typically means +/- 5% of the stated value, more typically +/- 4% of the stated value, more typically +/- 3% of the stated value, more typically, +/- 2% of the stated value, even more typically +/- 1% of the stated value, and even more typically +/- 0.5% of the stated value.

Throughout this disclosure, certain embodiments may be disclosed in a range format. It should be understood that the description in range format is merely for convenience and brevity and should not be construed as an inflexible limitation on the scope of the disclosed ranges. Accordingly, the description of a range should be considered to have specifically disclosed all the possible sub-ranges as well as individual numerical values within that range. For example, description of a range such as from 1 to 6 should be considered to have specifically disclosed sub-ranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6 etc., as well as individual numbers within that range, for example, 1, 2, 3, 4, 5, and 6. This applies regardless of the breadth of the range. Detailed Disclosure of Embodiments

Exemplary, non-limiting embodiments of a process for treating polyimides, and membranes comprising the polyimides treated by the process, will now be disclosed. The present invention provides a process for treating polyimide comprising the steps of:

(a) exposing the polyimide to a cross-linking agent comprising one or more amine groups, said exposing being undertaken for a period of time to form a cross-linked polyamide from at least a portion of said polyimide; and

(b) maintaining the cross-linked polyamide at a selected temperature to form cross-linked polyimide from at least a portion of said cross-linked polyamide.

The exposing step (a) may comprise the step of: (al) dissolving the cross-linking agent in a solvent to form a cross-linking solution. The solvent can be an organic solvent such as ketone, for example ether, or an alcohol, for example methanol, ethanol or propanol, or -Θ3? any solvent that is suitable for dissolving the cross- linking agent. The amount of cross-linking agent in the solution can be selected from the group consisting of: about 1 wt % to about 50 wt %; about 1 wt % to about 40 wt %; about 1 wt % to about 30 wt %; about 1 wt % to about 20 wt %; about 1 wt % to about 10 wt %; about 10 wt % to about 50 wt %; about 20 wt % to about 50 wt %; about 30 wt % to about 50 wt %; and about 40 wt % to about 50 wt %.

The exposing step is preferably undertaken at a temperature below the boiling point of the solvent. The exposing step may undertaken at a temperature selected from the group consisting of: about 0⁰C to about 150⁰C, about 10°C to about 150⁰C, about 20⁰C to about 150°C, about 30°C to about 150⁰C, about 40°C to about 150⁰C, about 50°C to about 150⁰C, 0⁰C to about 120⁰C, 0⁰C to about 100⁰C, 0⁰C to about 75°C, 0⁰C to about 50⁰C, 0⁰C to about 30⁰C, and 0⁰C to about 30⁰C.

The time undertaken for the exposing step (a) will be dependent on the concentration of the cross-linking agent in solution and the temperature of the solution. The exposing step (a) may be undertaken for a time period selected from the group consisting of: about 3 seconds to about 24 hours; about 60 seconds to about 24 hours; about 5 minutes to about 24 hours; about 30 minutes to about 24 hours; about 45 minutes to about 24 hours; about 60 minutes to about 24 hours; about 2 hours to about 24 hours; about 6 hours to about 24 hours; about 12 hours to about 24 hours; and about 18 hours to about 24 hours.

The process may comprise, before step (b) , the step of:

(c) removing said exposed polyimide from said cross- linking agent.

The removing step (c) may further comprise the step of:

(cl) washing the polyimide with an inert solvent.

The inert solvent in the washing step (cl) may be water or an alcohol such as methanol.

The removing step (c) may be by filtration.

The removing step (c) may further comprise the step of:

(c2) drying the polyimide after step (cl) . The polyimide may be dried in a vacuum or in the presence of an inert gases. The maintaining step (b) may comprise the step of: (bl) heating the cross-linked polyamide to the selected temperature.

The maintaining step (b) may comprise the step of: (b2) annealing the cross-linked polyamide to the selected temperature.

The heating step (bl) or the annealing step (b2) may be carried out in an oven. The polyimide can be heated in a vacuum or in the presence of inert gases. The process may further comprise the step of:

(c) cooling the polyimide after step (b) . In one embodiment, the cooling comprises exposing the heated cross-linked polyimide to ambient temperature conditions. In other embodiments, the rate of cooling may be controlled. Advantageously, the cooling will be undertaken at a rate whereby damage is prevented from occurring for the cross-linked polyimide.

The selected temperature of step (b) may be selected from the group consisting of: about 50⁰C to about 400⁰C; about 50⁰C to about 350⁰C; about 80⁰C to about 350⁰C; about 80⁰C to about 300C; about 80⁰C to about 250⁰C; about

80⁰C to about 200⁰C; about 80⁰C to about 150⁰C; about 80⁰C to about 100⁰C; about 100⁰C to about 350⁰C; about 150⁰C to about 350⁰C; about 200⁰C to about 350⁰C; and about 250⁰C to about 350⁰C.

Step (b) may be undertaken for a time period selected from the group consisting of: about 0.1 hours to about 24 hours or more, about 0.1 hours to about 12 hours, about 0.1 hours to about 6 hours, about 0.1 hours to about 3 hours, about 0.5 hours to about 24 hours, about 1 hour to about 24 hours, about 2 hours to about 24 hours, about 3 hours to about 24 hours, about 6 hours to about 24 hours, and about 12 hours to about 24 hours.

The present invention also provides a process for treating a polyimide having a first nitrogen to fluorine ratio comprising the steps of:

(a) exposing the polyimide to a cross-linking agent comprising one or more amine groups, said exposing being undertaken under conditions so as to form a cross-linked polyamide having a second nitrogen to fluorine ratio from at least a portion of said polyimide, said second nitrogen to fluorine ratio being higher than the first nitrogen to fluorine ratio; and

It will be appreciated that the fluorine content remains constant in the polyimides during cross-linking and thermal annealing. The nitrogen content, on the other hand, increases as the imide groups are converted to amide groups during cross-linking, and decreases as the amide groups are converted into imide groups during thermal annealing. Accordingly, the second nitrogen to fluorine ratio is higher than the first nitrogen to fluorine ratio, and the third nitrogen to fluorine ratio is lower than the second nitrogen to fluorine ratio as defined above. It will be appreciated that the first, second and third nitrogen to fluorine ratios are dependent on the type of polyimide membrane used. The first nitrogen to fluorine ratio may be selected from the group consisting of: about 0.05 to about 0.4; about 0.1 to about 0.4; about 0.2 to about 0.4; about 0.3 to about 0.4; about 0.05 to about 0.35; about 0.05 to about 0.3; about 0.05 to about 0.2; and about 0.3 to about 0.4. The second nitrogen to fluorine ratio may be selected from the group consisting of: about 0.5 to about 0.8; about 0.5 to about 0.7; about 0.5 to about 0.6; about 0.6 to about 0.8;.and about 0.7 to about 0.8. The third nitrogen to fluorine ratio may be selected from the group consisting of: about 0.5 to about 0.7; about 0.5 to about 0.6; and about 0.4 to about 0.5.

The present invention further provides a process for treating a polyimide comprising the steps of:

(a) exposing the polyimide to a cross-linking agent comprising one or more amine groups, said exposing being under conditions so as to form a cross-linked polyamide from at least a portion of said polyimide; and (b) maintaining the cross-linked polyamide under conditions so as to form cross-linked polyimide having charge transfer complexes from at least a portion of said cross-linked polyamide, said cross-linked polyimide having higher resistance to plasticization than the polyimide and the cross-linked polyamide.

It will be appreciated that the resistance to plasticization of a membrane is such that the higher the amount of charge transfer complexes formed, the higher the resistance of the membrane to undergo plasticization. Accordingly, the cross-linked polyimide has a higher resistance to plasticization than the polyimide and the cross-linked polyamide as defined above. The formation of charge transfer complexes result in the change in colour of the membrane to yellow. The higher the quantity of charge transfer complexes formed, the darker the yellow colour of the membrane

The present invention also provides a process for treating a polyimide having a first critical plasticization pressure comprising the steps of:

(b) maintaining the cross-linked polyamide under conditions so as to form cross-linked polyimide having a third critical plasticization pressure from at least a portion of said cross-linked polyamide, said third critical plasticization pressure being higher than the first and second critical plasticization pressures. Operation of membranes with carbon dioxide or hydrocarbons can result in failure of the membranes due to plasticization. This failure occurs as a consequence of chemical interaction between the carbon dioxide or heavier hydrocarbons with the membrane material thereby resulting in an increase in permeability of these gases through the membrane. The pressure at which plasticization occurs is known as the critical plasticization pressure, and accordingly, the critical plasticization pressure is a pressure at which permeability of the membrane starts to increase as applied pressure increases. The higher the magnitude of the critical plasticization pressure the better the resistance to plasticization. Accordingly, the third critical plasticization pressure is higher than the first and second critical plasticization pressures as defined above.

It will be appreciated that the first, second and third critical plasticization pressures are dependent on the type of membrane, the type of gases to which the membrane was exposed, and the type of cross-linking agent. The first critical plasticization pressure may be selected from the group consisting of: about 100 psia to about 400 psia; about 200 psia to about 400 psia; about 300 psia to about 400 psia; about 100 psia to about 300 psia; and about 100 psia to about 200 psia. The second critical plasticization pressure may be selected from the group consisting of: about 100 psia to about 300 psia; about 200 psia to about 300 psia; and about 100 psia to about 200 psia. The third critical plasticization pressure may be selected from the group consisting of: about 500 psia to about 1500 psia;. about 500 psia to about 1200 psia; about 500 psia to about 1000 psia; about 500 psia to about 800 psia; about 800 psia to about 1500 psia; about 1000 psia to about 1500 psia; and about 1200 psia to about 1500 psia.

Cross-Linking Agents

The cross-linking agent has one or more amine groups, i.e., 1, 2 , 3, 4, 5, 6, 7, 8, 9, 10 or more amine groups. The cross-linking agent can be a diamino cross-linking agent. The diamino cross-linking agent may have the general Formula 1:

wherein R is a hydrocarbon. The hydrocarbon may be a saturated or unsaturated, branched or straight chain aliphatic or an aliphatic ring hydrocarbon.

The saturated or unsaturated branched or straight chain aliphatic hydrocarbon may have a number of carbon atoms selected from the group consisting of: 0 to about 18, 1 to about 12, 1 to about 8, 1 to about 6, 1 to about 4, about 2 to about 18, about 6 to about 18, about 8 to about 18 and about 12 to about 18.

The aliphatic hydrocarbon may have a number of carbon atoms selected from the group consisting of: 3 to about 18, 3 to about 12, 3 to about 8, 3 to about 6, 3 to about 4, about 4 to about 18, about 6 to about 18, about 8 to about 18 and about 12 to about 18. Exemplary aliphatic hydrocarbons include alkyls such as methyl, ethyl, propyl, isopropyl, butyl and tert-butyl, pentyl, hexyl, heptyl, octyl, akenyls such as ethenyl, propenyl, isopropenyl, and butenyl, alkynyls such as ethynyl, propynyl, isopropynyl, and butynyl, cycloalkyls such as cyclopentyl, cyclobutyl and cyclohexyl, cycloalkenyls such as cyclopentenyl, cyclohexenyl and cycloheptenyl, heterocycloalkyls such as oxiranyl, and tetrahydropyranyl, and heterocycloalkenyls.

Exemplary diamino reagents include ethylenediamine, propylenediamine, trimethylenediamine, diethylenetriamine, triethylenetertramine, hexamethylenediamine, heptamethylenediamine, octamethylenediamine, nonamethylenediamine, decamethylenediamine, 1, 12-dodecanediamine, 1,18- octadecanediamine, 3-methylheptamethylenediamine, 4,4- dimethylheptamethylenediamine, 4- methylnonamethylenediamine, 5-methylnonamethylenediamine, 2, 5-dimethylhexamethylenediamine, 2,5- dimethylheptamethylenediamine, 2, 2- dimethylpropylenediamine, N-methyl-bis (3-aminopropyl) amine, 3-methoxyhexamethylenediamine, l,2-bis(3- aminopropoxy) ethane, bis (3-aminopropyl) sulfide, 1,4- cyclohexanediamine, bis- (4-aminocyclohexyl) methane, m- phenylenediamine, p-phenylenediamine, 2, 4-diaminotoluene, 2, β-diaminotoluene, m-xylylenediamine, p-xylylenediamine, 2-methyl-4, 6-diethyl- 1, 3-phenylene-diamine, 5-methyl- 4, 6-diethyl-l, 3-phenylene-diamine, benzidine, 3,3'- dimethylbenzidine, 3,3' -dimethoxybenzidine, 1,5- diaminonaphthalene, bis (4-aminophenyl) methane, bis (2- chloro-4-amino-3, 5-diethylphenyl) methane, bis (4- aminophenyl) propane, 2, 4-bis (b-amino-t-butyl) toluene, bis (p-b-amino-t-butylphenyl) ether, bis (p-b-methyl-o- aminophenyl) benzene, bis (p-b-methyl-o-aminopentyl) benzene, 1, 3-diamino-4-isopropylbe- nzene, bis (4- aminophenyl) sulfide, bis (4-aminophenyl) sulfone, bis(4- aminophenyl) ether and 1, 3-bis (3-aminopropyl) tetramethyldisiloxane and mixtures thereof. Suitable amine functional compositions for the present invention incorporate primary amines. Specific examples of aliphatic amines include methylamine, ethylamine, propylamine, isopropylamine, butylamine, isobutylamine, cyclohexylamine, cyclohexanebis (methylamine) , dimethylamine, diethylamine, dipropylamine, diisopropylamine, ethylene diamine, N,N'- dimethylethylene diamine, N,N'-diethylethylenediamine, diethylenetriamine, triethylenetetraamine, tetraethylene pentaamine, pentaethylenehexamine, 3- aminopropyldimethylethoxysilane, 3- aminopropyldiethoxysilane, N- methylaminopropyltrimethoxysilane, 3- aminopropyltriethoxysilane, N- inethylarαinopropyltriinethoxysilane, 3-aminopropyl terminated polydimethylsiloxanes, and the like. Specific examples of aliphatic aromatic amines include raeta- xylylenediamine, para-xylylenediamine and the like.

Poly±mide Membranes

In one embodiment, a polyimide membrane comprises the polyimide. The membrane comprising the treated polyimide can be formed from film casting, extrusion or melt blowing. The polyimide membrane may be in the form of a dense film, asymmetric film, asymmetric hollow fiber, dual layer hollow fiber, composite membrane of polyimides, or any form suitable for use in fluid separation systems or fluid/particle separation systems. Exemplary separation systems include filtration, gas separation, pervaporation, micro-filtration, nano- filtration, and reverse osmosis. The polyimide may be an aromatic polyimide. The polyimide may comprise one or more ketone groups.

In one embodiment, the polyimide is represented by the general formula 2:

wherein

Ari is a quadrivalent organic group,

Ar₂ is an aromatic diamine moiety, and n is the number of monomer units in the polyimide such that the polyimide has an inherent viscosity of at least 0.3 as measured at 25°C on a 0.5% by weight solution in N-methylpyrrolidinone.

The quadrivalent organic group Ar₁ can be selected from the group consisting of:

Λ

The aromatic diamine moiety Ar₂ can be selected from the group consisting of:

Z can be selected from the group consisting of:

CF₃

-C— I —o— CF₃

O CH₃ Il

-S Il -c— -CH₂ O I CH-,

-O -÷o-

X, Xi, X₂ and X₃ can be hydrogen, Ci to C₅ alkyl groups, Ci to C₅ alkoxy groups, phenyl or phenoxy groups.

The polyimide can also be a polyimide having a similar structure as that of ULTEM® (polyetherimide) , MATRIMID®, P84® (BTDA-TDI/MDI, copolyimide of 3, 3' 4,4'- benzophenone tetracarboxylic dianhydride and 80% methylphenylene-diamine + 20% methylene diamine) or similar materials and blends. In one embodiment, the treated polyimide is used to as a membrane. The membrane comprising the treated polyimide can be used in a separation process to separate one or more solids and/or one or more fluids from a mixture. The separation process may be selected from the group consisting of pervaporation, ultrafiltration, nano- filtration, reverse osmosis, water treatment, gas separation and micro-filtration.

The polyimide membrane may be suitable for use in membrane-based separation techniques, for example gas separation, filtration, micro-filtration, ultrafiltration, reverse osmosis or pervaporation. The polyimide membrane may, for example, be suitable for separation of fluid mixtures such as a mixture of CO₂ and CH₄ gases, a mixture if H₂ and N₂ gases, a mixture of H₂ and CO₂ gases, a mixture of He and N₂ gases or a mixture of C₂-C₄ hydrocarbons. The polyimide membrane may also be suitable for separating particles from fluids.

Accordingly, disclosed embodiments provides a process for separating at least one fluid or particle from a mixture comprising the steps of:

(a) contacting the mixture of fluids with one side of a treated polyimide membrane obtained by the process as described above; and (b) applying a pressure to the one side of the polyimide membrane to cause the at least one fluid or particle to permeate said treated polyimide membrane.

The process for separating the at least one fluid or particle from the mixture can be carried out in a separation system which comprises the membrane obtained by the process as described above. Brief Description Of Drawings

The accompanying drawings illustrate a disclosed embodiment and serves to explain the principles of the disclosed embodiment. It is to be understood, however, that the drawings are designed for purposes of illustration only, and not as a definition of the limits of the invention.

FIG. 1 shows a FTIR-ATR spectra of 6FDA-durene before and after cross-linking FIG. 2 shows a FTIR-ATR spectra of CHBA cross-linked 6FDA-durene before and after thermal annealing.

FIG. 3 shows a graph comparing the effects of applied pressure on relative CO₂ permeability of CHBA cross-linked and thermal annealed 6FDA-durene. FIG. 4 shows a graph comparing the effects of applied pressure on relative CO₂ permeability of EA cross-linked and thermal annealed 6FDA-durene.

Best Mode

Non-limiting examples of a process for treating polyimide, including the best mode, will be further described in greater detail by reference to specific

Examples, which should not be construed as in any way limiting the scope of the invention. Treatment of poly±mide ±n accordance with a preferred estπbodiment

A polyimide material, 6FDA-Durene polyimide, having a chemical structure below:

was formed into a polyimide film membrane. The polyimide film membrane was prepared by dissolving 2 wt % of the polyimide material in dichloromethane to form a polymer solution, filtering the solution using Whatman' s filter

(lμm) to remove undissolved polyimide material, and casting a film of the polyimide solution onto a silicon wafer at ambient temperature. The film was left alone to slowly evaporate the dichloromethane. Thereafter, the film was dried in a vacuum oven at 250⁰C for 48 hours to remove any residual dichloromethane to thereby form the polyimide film membrane.

The cross-linking agents used were 1_/3- cyclohexanebis (methylamine) (CHBA) and ethylene diamine

(EA) . These can be purchased from Sigma-Aldrich. Each of the cross linking agents were prepared into solutions by dissolving 10 wt % of the cross-linking agent in methanol. Two polyimide film membranes were prepared, such that one was immersed in the CHBA cross-linking solution and the other was immersed in the cross-linking EA solution. The polyimide film membranes were, immersed in the cross-linking solutions for 30 min (for the CHBA cross-linking solution) and 5 min (for the EA cross- linking solution) for cross-linking reaction to occur between the membranes and the cross-linking agents. The cross-linked polyimide film membranes were washed with fresh methanol immediately after removal from the cross- linking solutions to wash away residual cross-linking agents. The cross-linked polyimide film membranes were dried at room temperature for about 1 day to ensure complete removal of the methanol.

Each of the cross-linked polyimide film membranes was heated in a Centurion™ Neytech Qex vacuum furnace. The cross-linked polyimide film membranes were heated to a temperature of 100-200 ⁰C for 24 hours. The cross- linked polyimide film membranes were cooled to 50⁰C and then removed from the furnace. The two cross-linked polyimide film membranes were characterised as follows.

Characterisation of polyimi.de film membrane cross-linked, with. 1,3-cyclohexanebis (methylamine) (CHBA)

Cross-linking reaction between the polyimide dense film membrane and the CHBA cross-linking agent

The polyimide film membrane cross linked with CHBA was analysed to verify the cross-linking reaction between the CHBA cross-linking agent and the polyimide. Fourier

Transform Infrared Spectroscopy-Attenuated Total

Reflectance (FTIR-ATR) measurements were performed using a Perkin Elmer FT-IR Spectrometer Spectrum 2000 to characterise the cross-linking reaction. FIG. 1 shows a FTIR-ATR spectra of 6FDA-durene membrane before and after CHBA cross-linking. 6FDA- durene before CHBA cross-linking was characterised by bands at around 1783 cm^"1 (attributed to C=O asymmetric stretch of imide groups) , 1718 cm^"1 (attributed to C=O symmetric stretch of imide groups) , and 1351 cm^"1

(attributed to C-N stretch of imide groups) . After CHBA cross-linking, a C=O stretch band at around 1647 cm^"1 of the amide group and a C-N stretch band of the C-N-H group at around 1521 cm^"1 were observed.

The intensities of the characteristic imide peaks

(at 1783 cm^"1 and 1718 cm^"1) decrease, and the intensities of the characteristic amide peak (at 1647 cm^"1) increased with immersion time (cross-linking time) . This suggests that the degree of cross-linking increases with the immersion time.

Thermal annealing of CHBA cross-linked polyimide film membrane The effects of thermal annealing on the CHBA cross- linked membrane was further characterised by FTIR-ATR measurements as shown in FIG. 2. The original 6FDA-durene showed imide peaks at around 1783 cm^"1, 1718 cm^"1 and 1351 cm^"1. The cross-linked polyimide film membrane without thermal annealing, as indicated by spectra CHBA-30min, showed amide peaks at around 1647cm^"1 and 1521cm^"1. The cross-linked polyimide film membrane with thermal annealing, however, showed spectra having decreased intensity of amide peaks and increased intensity of imide peaks as annealing temperature increases. This suggests that thermal annealing converts the amide groups into the imide groups. The effect is more apparent at higher wave numbers (above 3100 cm^"1) in the FTIR-ATR spectra. The broad peak at around 3500 cm^"1 - 3100 cm^"1 is attributed to the NH bond(or hydrogen bond), which arises from amide groups (-NH stretch) in the membranes. After cross- linking, the broad peak of NH/hydrogen bond was generated. The broad peak, however, decreases as' thermal annealing temperature increases, and almost disappears when the annealing temperature reaches up to 200 °C.

The FTIR-ATR results above showed that the chemical structure of the 6FDA-durene membrane film changes during the cross-linking reaction and the thermal annealing treatment. It can be concluded that amine functional groups of CHBA react with the imide ring of the 6FDA- durene membrane film to form amide groups and cross- linked networks during the cross-linking step. During thermal annealing, the CHBA was released from the cross- linked structures and the imide groups regenerated. X-ray Photoelectron Spectroscopy (XPS) was carried out on the samples before and after thermal annealing. The results were summarised in Table 1. The fluorine element content remains constant in the polyimides during cross-linking. The nitrogen element content increases as the imide groups were converted to amide groups during cross-linking, and decreases as the amide groups were converted to imide groups during thermal annealing.

Accordingly, changes in the ratio of Nitrogen to Fluorine

(N/F) can be used to study changes in chemical structure of 6FDA-durene membrane during cross linking and thermal annealing. Results showed that 30min CHBA cross-linked membranes had a higher nitrogen content (N/F=0.65) than that of the original ones (N/F=0.31) . The results also showed that thermal annealing decreases the N/F ratio, and at 30 min CHBA 200 °C thermal annealing, the N/F ratio had decreased to 0.40. This is consistent with the FTIR-ATR results and confirms the reaction mechanism.

Table 1

Plasticisation of CHBA cross-linked polyimide membrane with thermal annealing

The tendency of the membrane to undergo plasticization is indicated by the formation of charge transfer complexes (CTCs) . The higher the amount of CTCs formed, the lower the tendency of the membrane to undergo plasticisation. It has been found that the membrane changes colour from transparent to yellow as more CTCs are formed.

Table 2 shows the UV wavelength and colour of CHBA cross-linked 6FDA-durene membrane at various annealing temperatures. The UV transmittance spectra of the cross- linked 6FDA-durene membranes. The UV transmittance spectra of 6FDA-durene membranes was recorded with a UV- 3101 PC Shimadzu spectrophotometer from Shimadzu Corporation, Kyoto, Japan. The data in Table 2 showed that as the annealing temperature increases, the colour of .the membrane turns to a deeper yellow colour. The change in the colour of the membrane during the thermal treatment is due to the formation of charge transfer complexes (CTCs) . This means that thermal treatment at higher temperature facilitates the formation of CTCs, which in turn indicated improved anti-plasticisation properties of the membrane.

For comparison, data of thermally treated Matrimid studied by A Bos, I.G.M. Punt, M. Wessling, H. Strathmann, Platicisation-resistant glassy polyimide membranes for CO/CH separations, Separation and Purif. Tech. 14 (1998)27 is also included in Table 2. A. Bos et al thermally treat the Marimid at 350 ⁰C. The color deepens with the thermal treatment time tehreby indicating increase in formation for CTCs. In this study, the original 6FDA-durene membranes were pre- treated at 250 °C in vacuum and were transparent in colour. This means that thermal treatment up to 250 °C had no influence on the formation of CTCs in the non- modified 6FDA-durene membranes. The cross-linked membranes still maintained a transparent colour and only showed small (9 nm) red shifts of UV absorption band compared with that of the original one. Interestingly, the thermal treatment greatly increases the red shift, and the higher the annealing temperature the larger the red shifts. Only at the 200 ⁰C annealing, the red shifts are up to 326 nm, which is greatly larger than that of Matrimid (maximum is 21 run) even though Matrimid was treated at temperature high to 350 °C.

The strong ability to form the CTCs in the cross- linked membranes of the present invention arises from the coupling of a amino cross-linking method with thermal treatment. The amino chemical cross-linking opens the main chains in the solid state polyimides, which makes the polymer chains relatively flexible. When the thermal treatment is applied, the relatively flexible chains are thermodynamically easier to form CTCs, especially at the higher temperatures.

Table 2

* A Bos, I.G.M. Punt, M. Wessling, H. Strathmann, Platicisation-resistant glassy polyimide membranes for CO/CH separations, Separation and Purif. Tech. 14 (1998)27.

Gas transport properties measurements

Pure gas permeabilities were determined by a constant volume and variable pressure method. Literature sources describe such apparatus and measurement techniques. See Methods of Experimental Physics, Vol. 16c, Academic Press, Inc., 1980, pp. 315-377; Pye, Hoehn, and Panar, Measurement of Gas Permeability of Polymers. I. "Permeabilities in Constant Volume/Variable Pressure Apparatus", Journal of Applied Polymer Science, Vol. 20, 1976, pp. 1921-1931; Pye, Hoehn, and Panar, Measurement of Gas Permeability of Polymers. II. "Apparatus for Determination of Permeabilities of Mixed Gases and Vapors", Journal of Applied Polymer Science, Vol. 20, 1976, pp. 287-301; and ASTM-1434-75,513, "Standard Test Methods for Gas Transmission Rate of Plastic Film and Sheeting. " The permeabilities were obtained in the sequence of He, H₂, O₂, N₂, CH₄ and CO₂ at 35 °C. The upstream pressure was 3.5 atm for He and H₂ and 10 atm for other gases. The ideal separation factor of a membrane for gas A to gas B was evaluated as follows:

Table 3 summarises the permeability and permselectivity of the original and modified films. It should be noted that chemical cross-linking and thermal annealing change the gas transport properties of the modified membranes from two aspects:

1. changes in the chemical compositions and the material polarity;

2. changes in the substructures of the materials by the hole filling effect (chemical cross-linking) and the formation of CTCs (thermal annealing) .

Table 3 shows the gas transport properties of cross- linked and thermal annealed 6FDA-durene membranes. The gas permeability for the original 6FDA durene membranes follows the sequence: CH₄ (kinetic diameter: 3.80 A) < N₂ (3.64 A) < O₂ (3.46 A) < He (2.6 A) < CO₂ (3.30 A) < H₂ (2.89 A) . Except for CO₂ and H₂ gas, the different gas permeability order corresponds well with the kinetic diameter of gas molecules, and the smaller the kinetic diameter of the gas the larger the permeability. As the permeability is a product of the solubility and the diffusivity, He has poor interactions (less solubility) with the polymer. Therefore, its permeability is lower than that of H₂ for these polyimides. In the CO₂ case, the deviation may result from the higher polymer- penetrant interactions of the polar CO₂, and the possibility of differing orientations of its anisometric molecules during the diffusion comparison.

However, after cross-linking and thermal annealing, the different gas permeabilities change into the order: CH₄ (kinetic diameter: 3.80 A) < N₂ (3.64 A) < O₂ (3.46 A) < CO₂ (3.30 A) < He (2.6 A) < H₂ (2.89 A), in which the CO₂ gas has changed the order and lower than the He gas. This may be due to the effects of the chemical cross- linking agents and thermal effects on CTCs formation.

The effect of thermal annealing on the permeability is a compromise of the formation of CTCs and the regeneration of imide groups (or cross-link agent release) . The thermal annealing applied at 100 °C slightly decreases the permeability of cross-linked membranes, which may mainly due to the thermodynamical rearrangement of the cross-linking structures and the initial CTCs formation. However, after 150 °C annealing, the permeability slightly increase again. It may be due to the heavily release of the cross-linking agent and the FFV redistribution during the change of amide into imide groups, which can not fully be compensated by the densifying effect of the CTCs formation. The permeability was kept relatively constant after 200 ⁰C annealing, which is a balance between the release of the cross-linking agents and the formation of CTCs.

A comparison of selectivity showed that all modified membranes have higher H₂/CO₂, O₂/N₂ and CO₂/CH₄ selectivity than those of original 6FDA-durene membranes. The O₂/N₂ selectivity of annealed samples does not change much with the annealing temperatures. On the other hand, the H₂/CO₂ and CO₂/CH₄ selectivity of the annealed sample showed relatively larger changes.. Interestingly, the CO₂/CH₄ selectivity increases with the annealing temperatures. However, the H₂/CO₂ selectivity decreases with the annealing temperatures. This is mainly due to the changes of polymer-penetrant (CO₂) interactions during the thermal annealing. When no annealing is applied, the strongest polymer-penetrant interactions delay the CO₂ diffusion through the CHBA 30 min cross-linked membranes, and accordingly showed the lowest CO₂/CH₄ selectivity and the highest H₂/CO₂ selectivity compared with the higher temperature treated samples. When the thermal annealing was applied to the cross-linked samples, the polymer- penetrant (CO₂) interactions decrease with the conversion of some amide into imide groups. As a result, the CO₂/CH₄ selectivity increases and the H₂/CO₂ selectivity decreases with the thermal annealing temperatures. It means the thermal annealing can effectively improve the CO₂/CH₄ selectivity of cross-linked βFDA-durene membranes almost without the reduction in the permeability.

It has been surprisingly been found by the inventors that a cross-linked polyimides made in a treatment process according to an embodiment of the invention can be used in a membrane for separating one or more gasses. It has surprisingly been found that such membranes have , enhanced H2/CO2 selectivity in gas separation studies. For EA cross-linked samples, the H₂/CO₂ increases from 1.28 (Original) to 5.33 (5 min crosslinking) . Considering the difficulty in separating the similar diameter gases of H₂ and CO₂, this great increase is very encouraging and the modified membranes may be very useful for hydrogen recovery.

Table 3

a. 1 Barrer = 1 x 10^~10cm³ (STP) -cm /cm² sec cmHg

CO₂ plasticization suppression by the coupling effect of CHBA chemical cross-linking and thermal annealing

CO₂ plasticization pressure of glassy polyimides is time-dependant due to the long time relaxation process of plasticization of glassy polyimides. For consistency in evaluation of critical pressure of plasticization, the CO₂ conditioning time was set to about 10 times of time lag at every tested pressure.

Plasticization phenomena were observed in many glassy polymers such as PSF, PMMA, polycarbonate, and polyimide (PI) . As the condensable gases such as CO₂ and organic vapor/liquids have the strong polymer-penetrant interactions, they can swell the glassy polymer and loosen the polymer matrix when the pressure is above the critical plasticization pressure. Consequently, permeability increases and selectivity decreases with pressure when plasticization happens. Therefore, the plasticization resistance should be achieved to maintain satisfactory performance of the membranes. According to the study carried out by A Bos, I.G.M. Punt, M. Wessling, H.. Strathmann, Platicisation-resistant glassy polyimide membranes for CO/CH separations, Separation and Purif. Tech. 14 (1998)27, the formation of CTCs may be a reason for the improvement of the plasticization resistance in the thermally treated Matrimid membranes. Most recently, JJ Krol, M_r Boerrigter, G.H. Koops, Polyimide hollow fibre gas separation membranes: Preparation and suppression of plasticisation on propane/propylene environments,, J. Membr. Sci., 184 (2001)275 thermally treats the Matrimid hollow fibres and found that the formation of CTCs restricted the chain mobility and suppresses plasticization in propane/propylene environments.

FIG. 3 demonstrates the change in relative CO₂ permeability of 6FDA-Durene with applied pressures. The original durene exhibited increased relative CO₂ permeability when the pressure is above approximate 300 psia. After cross-linking, the cross-linked sample becomes more easily plasticised when compared with the original one, and plasticization occurs when the pressure is just over 110 psia. This may be due to 1) methanol swelling effects, 2) loosened polyimide main chains by the low degree of cross-linking.

The thermal annealing effectively improves the plasticization resistance of the cross-linked membranes. Even at 100 °C thermal annealing, the plasticization resistance of the cross-linked 6FDA-duren membranes is almost equal to that of the original one. The higher the thermal annealing temperature, the better the plasticization resistance. After 200 °C thermal annealing, the treated membranes do not show plasticization at 50 atiri (720 psia.), which is much higher than the plasticization pressure around 110 psia of the pure cross-linked sample and around 300 psia. of the original one.

Compared with JJ Krol, M_r Boerrigter, G.H. Koops, Polyimide hollow fibre gas separation membranes:

Preparation and suppression of plasticisation on propane/propylene environments, J. Membr. Sci . , 184

(2001)215, the method of the present invention is more powerful and effective to form CTCs because the flexible cross-linked membranes have the strong ability to thermodynamically rearrange the structures and form the CTCs during the thermal annealing. Consequently, the great improvement in the plasticiation resistance is achieved by the coupling effect of the chemical cross- linking and the following thermal annealing.

Characterisation of polyimide film membrane cross-linked with ethylene diamine (EA)

To develop a more economically effective cross- linking agent for gas separation with plasticization resistant properties, EA is used to cross-link the polyimides, followed by thermal treatment of the EA cross-linked samples.

Table 4 shows XPS results carried out on the samples before and after thermal annealing. The ratio of N/F on the surface increases with the EA cross-linking time. It means that the degree of cross-linking increases with the EA cross-linking time. A comparison of the EA 5min cross- linked sample before and after thermal annealing, showed that thermal annealing decrease the ratio of N/F, which is similar to the results of CHBA cross-linked sample. And the reaction mechanism of EA cross-linking and the following thermal annealing should be the same as that of CHBA. Besides, the color of the thermal annealed samples also becomes dark yellow. It means that CTCs were formed as a result of the coupling effect of cross-linking and thermal annealing.

Table 4

FIG. 4 illustrates the effect of thermally-treated EA cross-linked sample on plasticization resistance properties. The critical plasticization pressures for the original and EA 5 min cross-linked samples were around 300 psia and 200 psia. However after 150 °C thermal annealing, the critical plasticization pressure was increased to more than 720 psi. (50 atm. ) , with shorter cross-linking time (5min) and lower thermal treatment temperature (150 ⁰C) when compared to thermally treated CHBA cross-linked samples. This illustrates that EA cross-linking agents were better suited than CHBA for samples with relative short cross-linking time and lower temperature treatment. In addition, the EA cross-linking agent is a low in cost and therefore has more potential for practical applications.

It will be apparent that various other modifications and adaptations of the invention will be apparent to the person skilled in the art after reading the foregoing disclosure without departing from the spirit and scope of the invention and it is intended that all such modifications and adaptations come within the scope of the appended claims.

Claims

1. A process for treating a polyimide comprising the steps of: (a) exposing the polyimide to a cross-linking agent comprising one or more amine groups, said exposing being under conditions so as to form a cross-linked polyamide from at least a portion of said polyimide; and

(b) maintaining the cross-linked polyamide under conditions so as to form cross-linked polyimide from at least a portion of said cross-linked polyamide.

2. A process as claimed in claim 1, wherein the conditions in the exposing step (a) comprise exposing the polyimide to the cross-linking agent at a temperature and time effective to form cross-linked polyamide from at least a portion of said polyimide.

3. A process as claimed in claim 1, wherein the conditions in the maintaining step (b) comprise maintaining the cross-linked polyamide at a temperature and time effective to form cross-linked polyimide from at least a portion of said cross-linked polyamide.

4. A process as claimed in claim 2, wherein the conditions in the exposing step (a) comprises the step of:

(al) dissolving the cross-linking agent in a solvent to form a cross-linking solution.

5. A process as claimed in claim 4, wherein the step

(al) comprises: (al.l) selecting an amount of the cross-linking agent in the solution from the group consisting of: about 1 wt % to about 50 wt %; about 1 wt % to about 40 wt %; about 1 wt % to about 30 wt %; about 1 wt % to about 20 wt %; about 1 wt % to about 10 wt %; about 10 wt % to about 50 wt %; about 20 wt % to about 50 wt %; about 30 wt % to about 50 wt %; and about 40 wt % to about 50 wt %.

6. A process as claimed in claim 2, wherein the conditions in the exposing step (a) comprises:

(a2) maintaining the temperature below the boiling point of the solvent.

7. A process as claimed in claim 6, wherein said temperature in step (a2) is selected from the group consisting of: about 0⁰C to about 150°C, about 10°C to about 150⁰C, about 20⁰C to about 150⁰C, about 30⁰C to about 150⁰C, about 40⁰C to about 150⁰C, about 50⁰C to about 150⁰C, 0⁰C to about 120⁰C, 0⁰C to about 100⁰C, 0⁰C to about 75°C, 0⁰C to about 50⁰C, and 0⁰C to about 30⁰C.

8. A process as claimed in claim 2, wherein said conditions in the exposing step (a) comprises: (a3) maintaining the temperature for a time period selected from the group consisting of: about 3 seconds to about 24 hours; about 60 seconds to about 24 hours; about 5 minutes to about 24 hours; about 30 minutes to about 24 hours; about 45 minutes to about 24 hours; about 60 minutes to about 24 hours; about 2 hours to about 24 hours; about 6 hours to about 24 hours; about 12 hours to about 24 hours; and about 18 hours to about 24 hours.

9. A process as claimed in claim 1, comprising before step (b) , the step of:

(c) removing said exposed polyimide from said cross- linking agent.

10. A process as claimed in claim 9, wherein said removing step (c) further comprises the step of:

(cl) washing the polyimide with an inert solvent.

11. A process as claimed in claim 9, wherein said removing step (c) further comprises the step of: (c2) filtering said exposed polyimide from said cross-linking agent.

12. A process as claimed in claim 11, Wherein said removing step (c) further comprises the step of: (c3) drying the polyimide after step (c2) .

13. A process as claimed in claim 3, wherein the conditions in the maintaining step (b) comprises the step of: (bl) heating the cross-linked polyamide to the temperature.

14. A process as claimed in claim 3, wherein the conditions in the maintaining step (b) comprises the step of: (b2) annealing the cross-linked polyamide at the temperature.

15. A process as claimed in claim 3, wherein the conditions in the maintaining step (b) is undertaken in a vacuum or in the presence of one or more inert gases.

16. A process as claimed in claim 1, further comprising the step of: (d) cooling the polyimide after step (b) .

17. A process as claimed in claim 3, wherein said temperature of said maintaining step (b) is selected from the group consisting of: about 50⁰C to about 400⁰C; about 50⁰C to about 350⁰C; about 80⁰C to about 350⁰C; about 80⁰C to about 300C; about 80⁰C to about 250⁰C; about 80⁰C to about 200⁰C; about 80⁰C to about 150⁰C; about 80⁰C to about 100⁰C; about 100⁰C to about 350⁰C; about 150⁰C to about 350⁰C; about 200⁰C to about 350⁰C; and about 250⁰C to about 350⁰C.

18. A process as claimed in claim 3, wherein the conditions in the maintaining step (b) comprises the step of maintaining the temperature for a period of time selected from the group consisting of about 0.1 hours to about 24 hours, about 0.1 hours to about 12 hours, about 0.1 hours to about 6 hours, about 0.1 hours to about 3 hours, about 0.5 hours to about 24 hours, about 1 hour to about 24 hours, about 2 hours to about 24 hours, about 3 hours to about 24 hours, about 6 hours to about 24 hours, and about 12 hours to about 24 hours.

19. A process as claimed in claim 1, comprising the step of:

(e) selecting a diamino cross-linking agent as said cross-linking agent.

20. A process as claimed in claim 19, wherein said diamino cross-linking agent has the general Formula 1:

(D wherein R is a hydrocarbon.

21. A process as claimed in claim 20, wherein R is a saturated or unsaturated, branched or straight chain aliphatic hydrocarbon or a saturated or unsaturated aliphatic ring hydrocarbon.

^'22. A process as claimed in claim 21, wherein the saturated or unsaturated, branched or straight chain aliphatic hydrocarbon has a number of carbon atoms selected from the group consisting of: 0 to about 18, 1 to about 12, 1 to about 8, 1 to about 6, 1 to about 4, about 2 to about 18, about 6 to about 18, about 8 to about 18 and about 12 to about 18.

23. A process as claimed in claim 21, wherein the saturated or unsaturated aliphatic ring hydrocarbon has a number of carbon atoms selected from the group consisting of: 3 to about 18, 3 to about 12, 3 to about 8, 3 to about 6, 3 to about 4, about 4 to about 18, about 6 to about 18, about 8 to about 18 and about 12 to about 18.

24. A process as claimed in claim 1, comprising the step of:

(f) selecting an aromatic polyimide as the polyimide.

25. A process as claimed in claim 1, wherein the polyimide is represented by the general formula 2:

(2) wherein

Ari is a quadrivalent organic group, Ar₂ is an aromatic diamine moiety, and n is the number of monomer units in the polyimide such that the polyimide has an inherent viscosity of at least 0.3 as measured at 25°C on a 0.5% by weight solution in N-methylpyrrolidinone.

26. A process as claimed in claim 25, wherein the quadrivalent organic group Ari is selected from the group consisting of:

27. A process as claimed in claim 25, wherein the aromatic diamine moiety Ar₂ is selected from the group consisting of:

wherein Z is selected from the group consisting of: CF₃ O Il

-C — — C- — C— I CF₃

wherein X, Xi, X₂ and X3 is selected from the group consisting of hydrogen, Ci to C₅ alkyl groups, Ci to C₅ alkoxy groups, phenyl or phenoxy groups.

28. A treated polyimide made in a process comprising the steps of: (a) exposing the polyimide to a cross-linking agent comprising one or more amine groups, said exposing being under conditions so as to form a cross-linked polyamide from at least a portion of said polyimide; and

(b) maintaining the cross-linked polyamide under conditions to form cross-linked polyimide from at least a portion of said cross-linked polyamide.

29. A membrane comprising a treated polyimide made in a process comprising the steps of: (a) exposing the polyimide to a cross-linking agent comprising one or more amine groups, said exposing being under conditions so as to form a cross-linked polyamide from at least a portion of said polyimide; and (b) maintaining the cross-linked polyamide under conditions so as to form cross-linked polyimide from at least a portion of said cross-linked polyamide.

30. A separation system comprising a membrane according to claim 29.

31. A gas separation module comprising a membrane according to claim 29.

32. A pervaporation module comprising a membrane according to claim 29.

33. A process for separating at least one fluid or particle from a mixture comprising the steps of:

(a) contacting the mixture with one side of a treated polyimide membrane made in a process of claim 1; and

34. A process for separating carbon dioxide from a gas mixture comprising carbon dioxide and at least one of methane and hydrogen, the process comprising the steps of:

(a) contacting the gas mixture with one side of a treated polyimide membrane made in a process according to claim 1; and (b) applying a pressure to the gas mixture in contact with said treated polyimide membrane to cause at least a portion of said carbon dioxide present in said gas mixture to permeate said treated polyimide membrane.

35. A process as claimed in claim 30, wherein said gas mixture is natural gas.