CA1127472A

CA1127472A - Gas separation membranes and process for the preparation thereof

Info

Publication number: CA1127472A
Application number: CA316,540A
Authority: CA
Inventors: Robert L. Riley; Richard L. Grabowsky
Original assignee: UOP LLC
Current assignee: Honeywell UOP LLC
Priority date: 1977-11-25
Filing date: 1978-11-17
Publication date: 1982-07-13
Also published as: FR2409781B1; IL56015A; DE2850998A1; GB2011804A; AU4187178A; DK525378A; JPS5482380A; NZ188992A; MX150671A; IL56015A0; JPS593201B2; AU529088B2; FR2409781A1; GB2011804B

Abstract

GAS SEPARATION MEMBRANES AND PROCESS
FOR THE PREPARATION THEREOF

ABSTRACT

Membranes for the separation of gases comprise a thin film of a semipermeable material composited on a porous support member. The membranes as exemplified by a thin film of polymerized dimethyl silicone on a cellulose nitrate/
cellulose acetate member are prepared by passing the support member through a solution of a chlorinated hydrocarbon solvent containing a semipermeable membrane forming monomer and a cross-linking agent followed by cross-linkage of the monomer effected by treatment at an elevated temperature.

Description

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GAS SEPARATION MEMBRANES AND PROCESS
FOR THE PREPARi~TION THEREOF

SPEC IF I CATI ON

The use of semipermeable membranes for reverse osmosis or ultrafiltration processes is known. For example, in a reverse osmosis process high pressure saline water may be placed in contact with a semipermeable membrane which is permeable to water but relatively impermeable to salt. The brine which is collected or concentrated is separated from the water which may then be utilized for personal use such as drinking, use in cooking, etc. It has now been discovered that certain membranes may be utilized for the separation of various gases. The separation of gas utilizing a membrane is effected by passing a feed stream conslstlng of a mixture of gases across the surface of the membrane. Inasmuch as the - feed stream is at an elevated pressure the most permeable com-ponent of the mixture will pass~through the membrane at a more rapid rate than will the least permeable component. There-fore, the permeate stream which is passed through the membrane is enriched by contain~ng the most permeable component while, conversely, the resldue stream is enriched-~in the least per~
meable component of the feed.
This abllity to separate gases from a mlxture stream will find many applications in commercial~ uses. For example, gas separation systems may be used for oxygen enrichment of air for improved combustion efficiencies and conservation of energy resources. ~ikewise, nitrogen e~richment of air may be applicable where inert atmospheres are required.
Other applications of gas separation would include helium recovery from natural gas, hydrogen enrichment in industrial

-2-- , , process applications, and scrubbiny o~ acid gases~ Specific uses for oxygen enrichment of air would be breathing systems for submarines and other underwater stations, improved heart-lung machines, and other lung assist devices. ~nother specific application of the gas separation systems would be for use in aircraft where the system would provide an oxygen enrich-- ~ . . ~- ................... . ,- ~
ment for life-support systems and~nitrogen enrichment for providing an inert atmosphere for fuel systems. In addition, the gas separation system may be used for environmental im-provement whereby methane can be separated from carbon di-oxide in waste gases for sewage treatment processes, as well as producing oxygen enriched air to enhance sewage digestion.
The present invention relates to membrane for the separation of gases. More specifically, the invention is concerned with membranes which are applicable for the sepa-ration of gases from a mixture thereof and to a process for preparing gas separation membranes. ~
As hereinbefore set forth the separation of various gases from a mixture thereof may constitute an important ad-vance in commercial applications. This is becoming increas- , ingly important in view of the necessity to conserve ener,gy. ~ -~
A particular application would relate'to increasing the efficiency of combustion processes when utilizing fossil fuels in commercial combustion applications. For example, the effective increase in the heating value of the fuel re-sults from a direct increase in the energy density of the reacting gases. By utilizing a gas separation membranè in coal gasification, lt may be possible to provide an oxygen . ''` ..
- _3_ ' enrichment of air for the production of low and medium British therma unit (stu3 product gases as well as an oxygen enrichment of air for the combustion of these gases. For example by placing a gas membrane separation system in close proximity , 5 to both gas production and gas combustlon facillties, it wouldallow a site-located oxygen enrichment plant to supply both processes without the additional expense of transporting the gas or duplicating enrichment facilities.
It is therefore an object of this invention to pro-.:
vide a membrane for the separation of various gas components contained in a mixture thereof.
A further objectof this invention is to provide a membrane for the separation of gases as well as a process for preparing these membranes.
In one aspect an embodiment of this invention resides in a membrane for the separation of gases which com-prises direct application of a thin film of a semipermeable material an a porous~support member.
A further embodiment of this invention is found in a process for producing a membrane ~or the separation of gases which comprises forming a thin film of a semipermeable membrane directly on the surface of a porous support member by passing said porous support member through a solution of a chlorinated hydrocarbon solvent containing a semipermeable - membrane forming monomer and a cross-linking agent, there-after cross-linking said monomer by treatment at an elevated~
temperature, and recovering the resultant membrane.

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A specific embodiment of this invention resides in the membrane for the separation of gas which comprises direct application of a thin film of dimethyl silicone on a porous cellulose nitrate/cellulose acetate support member. ` ~-Another specific embodiment of this invention re-sides in a process for producing a membrane for the separation ? o gasès which comprises pass;ng a cellulose nitrate/cellulose i acetate support membrane through a solution of chloroform containing dimethyl silicone and a cross-linking agent, there-after cross-linking the dimethyl silicone monomer by trèat~
ment at a temperature in the range of from 50 to 150C., and recovering the resultant membrane.
The present invention is concerned with a thin film membrane which may be utilized for the separation of gases. The membrane will consist of a thin semipermeable barrier composited on a finely porous support member. By utilizing a thin imperfection-free semipermeable barrler ;`
which has been prepared from a polymeric material hereinafter set forth in greater detail, the selected gas or gases wlll pass through the barrier with little hindrance whi1e the other gases which are not desired will be less able to pene-trate the barrier. In the preferred embodiment of the in-vention the thin~ilm,~semipermeable barrier will~possess a thickness ranging from 250 to 10,000 Angstroms, the pre-ferred thickness being from 250 to 500 Angstroms. The thick-ness of the film may be controlled by the concentration of the polymeric forming material in the solution as well as the rate of withdrawal of the porous support-member from :

the solution. E~amples o~ semipermeable membxane forming monomers which may be employed to form the thin film barrier of the present invention will include silicone containing compounds such as dimethyl silicone, silicone-carbonate co-polymersand fluorinated silicones. Other semipermeable membrane forming monomers which may be employed include polystyrene,` polycarbonates, polyphenylene oxi'des, polyure~
thanes, styrene butadiene copolymers, polyarylethers, ethylene vinyl acetate copolymer's, vinyl polymers and copolymers, epoxides, ethyl cellulose, cellulose acetate, mixed cellu-lose esters, cellulose nitrate, ABS (acrylonitrile butadiene-styrene), melamine formaldehyde and acrylics.
The aforementioned semipermeable membrane forming monomer is composited on a finely porous support membrane such as polysulfone or cellulose nitrate/cellulose acetate, which may, if so desired, be impregnated on a fabric such as Dacron~
The finely porous support member or membrane will possess a `' thickness ranging from 50 to 200 microns.
Examples of gas separation membranes of the present invention will include dimethyl silicone composited on a cellulose nitrate/cellulose acetate'support, sil`icone-car-~onate copolymer composited on a cellulose nitrate/cellulose acetate support, polystyrene composited on a cellulose ni- -trate/cellulose acetate support,'polycarbona~e composited on a cellulose nitrate~cellulose acetate support, cellulose ~ ' acetate composited on a cellulose nitrate/cellulose acetate support, polyphenylene oxide composited on a cellulose ni-tra~-e/cellulose acetate support, ethyl cellulose composited - ~ .
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on a cellulose nitrate/cellulose acetate support, polyamide composited on a cellulose nitrate/cellulose acetate support, cellulose acetate butyrate composited on a cellulose nitrate/
cellulose acetate support, dimethyl silicone aomposlted on a polysulfone support, silicone-carbonate composited on a ~ "
polysulfone support~ polystyrene composited on a polysulfone support, polycarbonate composited on a polysulfone support, `
cellulose acetate composited on a polysulfone support, poly-phenylene oxide oomposited on a polysulfone support~ ethyl ~ `
cellulose composited~on a polysulfone support, polyamide composited on a polysulfone support and cellulose acetate butyrate composited on a polysulfone support. It is to be understood that the aforementioned list of semipermeable membrane forming monomers, finely porous support membranes and gas membranes are only representative of he types of compounds which may be used and the membranes formed in the present invention, and that~said inventlon~is not necessarily ` ``
limited thereto.
The membranes for the separation of gases of the~
present invention are prepared by forming the very thin layer of polymer directly on the finely porous surface of the sup-porting member or membrane by passing the latter through a solution which contains the semipermeable membrane forming monomer. The thickness of the film, which is preferably in a range of from 250 to 500 Angstroms is controlled by the concentration of the polymer forming monomer in the solution as well as the rate of withdrawal from the solution. By utilizing this method of asymmetric membrane preparation it ~L~Lf~9L7'~

is possible to achieve several aaaitional degrees of freedom beyond that which is possible when preparing a membrane according to more conventional methods. Some examples of these advantages will include an independent selection of ' materials from which to prepare the thin semipermeable barrier --and the finely porous supporting membrane; an independent .
` ~preparation of the:thin`film and~the''porou`s~sùpport~in'g''mè'm-~'~'````''~:~'-~i' brane whereby it is possible to optimize each component for its specific~function; a reproductive variation and control 10 ' `ovèr the thickness of the thin~$ilm or semiperméable barrier ~' ~ .
which is required to attain t~e theoretical maximum in per-formance and control over the porosity and perfection of the thin semiper~eable barrier that is necessary to attain the theoretical semipermeability of t~e material.
The finely porous support membrane which is utilized as one component of the gas-membrane of the present invention 'may be prepàred by casting the support on a castlng machine~
from a-solution~which contalns the support material such~as cellulose nitrate and cellulose acetate as well as solvents such as organic materials including ketones such as acetone, ~methyl ethyl ketone, diethyl ketone and methyl propyl ketone;
` alcohols' such as methyl alcohol, ethyl alcoholj n-propyl alcohol, isopropyl alcohol, n-butyl alcohol and glycerin;
and surfactants to increase the we~tability of the com-ponents of the solution. The solution, after blending the various components thereof,is filtered to remove any foreign material by filtration through filtered material under super-atmospheric pressure usually afforded by the presence of *~ 7~

nitrogen, and thereafter is degassed to remove any dissolved inert gas such as nitrogen. The solution is fed onto the casting belt and spread on said belt at a desired thickness by mèans for controlling the thickness such as a casting knlfe. The freshly cast solution is carried on~the~belt into'a gelation chamber which is maintained at a slightly '~
`'`èlevated tempèrà`tùre in the''range'"of"'fr`'m`'30'`~``'to 40C.'''~-' '' `' ' :
After passage through this first gelation chamber wherein , . ~ ~ , the surface por`es`~size and permeability~o`f the`me~ rane~ls '`~
' controll`ed, the belt and support membranè are passed into a second gelation chamber in which the properties'of`the mem-brane are fixed. The temperature of the second gelation chamber is higher than that of the first gelation chamber in order to promote the removal of the solvents which may be present. After passing from the secona gelation chamber the membrane is removed from the casting belt~and passed to~storage. The gas mer~ranë of thè present~invention'is then prepared by passing the support membrane through a solution which contains the semipermeable membrane forming monomer and a cross-linking agent. In the preferred em-bodiment of the present invention the monomer is dissolved in an organic solvent and pr~eferably a halogenated hydro--- carbon-compound such as chloroform, iodoform,~bromoform, `
carbon tetrachloride~, carbon tetraiodide, carbon~tetra~
' 25 bromide, tetrachloroethane and tetrabromoethane. Examples of cross-linking agents which may~be employed and whic~h may be present in the solution will include toluene~diisocyanate, isophthaloyl chloride and dibutyl`tin laureate. The semi-~ ~ .
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permeable membrane forming monomer is usually present in the solution in a range of from 0.5 wt. % to 1.0 wt. ~ of the solution, the amount of monomer present in the solution being dependent upon the desired thickness of the thin film ~- 5 to be prepared. After passage through this solution the' ~~
coated finely porous support' membrane is wlthdrawn from the ''~
solution at a'determined~rate,'said'rate`~'als'ô;'belng dependen't''- ''`"```
upon the thickness of the thin film which is desired. The withdrawal rate may range from 0.25 to 1.5 cm/sec.'and will '~
depend on the thickness of the film whlch lS desired as weli as the particular type of monomer which is used to prepare the semipermeable membrane. After withdrawal from the solu-tion the coated finely porous support material is polymerized usually by treatment with'heat at a temperature ranging from 50 to 150C. for a period of time which may range from 0.5 up to 10 hours in duration, said heat treatment rendering - the poIymerized film insoluble~`in the solvent~which is em-~`` `'` "''~
ployed.
The thus formed membrane may be used as such or, if so desired, it may be subjected to additional treatment.
The additional treatment which may be employed will consist `' in treating the film with an additional amount of halogenated hydrocarbon solvent whereby the pores of the support which may contain some unpolymerized monomer will become soluble in the solvent and removed, thus increasing the permeability of the membrane without impairing the selectivity thereof.
It is also contemplated within the scope of this inventlon that after subjecting the gas membrane to the cross-linking :

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~zt~qz step of the process a second thin film may be formed on the surface of the cross-linked or polymerized film by passage of the membrane through the solution containing ~he monomer for a second time followe~ by a second heat treatment'to cross-link and form a second thim film on the surface of the first . . . . . . ...
thin film. This formation of the second `film will remove or ' .,~ eliminate any imperfection which may have been present,in the ,,~

By utilizing various semipermeable membrane forming monomers as'the thin film component of'the~gas'membranè of .:'' "' the presènt invention, it is possible' to effect'various gas ~, -separations inasmuch as the various polymers which are formed as a thin film possess varying permeabilities with regard to specific gases. For example, the oxygen permeabilities in various polymers is setforth in the following table.

Permeability*
Polymer . (x 109) Dimethyl Silicone Rubber 50.0 Silicone Polycarbonate Copolymer 16.0 Polybutadiene 13.0 Natural Rubber 2.4 Polyethylene, High Density 0.1 Cellulose Acetate 0~08 Nylon 6 - 0.004 Teflon~ . 0.0004 *Permeabil.ity = [Cm~ Gas.(STP), Cm~
m , Sec, Cm-Hg From a study of the above table it is therefore apparent that when utilizing dimethyl silicone as the monomer ' ` which forms the thin film it lS possible to ~btain an oxygen ~ t - B
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enrichment of the gas whereas by utilizing nylon 6 or ~eflon as the polymer it would be possible to enrich the permeate with gases other than oxygen. A dimethyl,silicone polymer itself possesses varying permeabilities and selectivities of binary gas mixtures examples of which are given in the following table. ~ ~`~ ~ '`~-`- ' ~` ~ ~ -~..... '.``''-,''~'~'''.~'`' ~ :: TABLE~
Gas Paix Permeability ~x 109) Selectivity 2/N2 50i25 2.0 CH4/He . 80/30 ' : :` 2.7 ~ ~`
CO2/CH4 270/80 3.4 C2/~2 . 270/55 - 4.9 H2O/NH3 3000/500 6.0 Co2/CO 270/30 9.0 NO2/NO 635/50 12.7 C 2/ 2 7500/365 20.6 H2S/CO ~ 840/30 ~ `28.0~
SO2/N2 1250/25 50.0 ''' Therefore, by utilizing dimethyl silicone as the semipermeable forming monomer, it is possible to effect a selectivity for specific mixtures of gases.
The gas membranes of the present invention which' have been prepared according to the process herein described , may be used in any separation device known in the artO For example, the devices may be used in either single stage or multi-stage membrane plants having been placed in elements ' or modules. For exampIe, one type of configuration in which .

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the gas membrane may be used would comprise a spiral wound element. In this type of element or module two or more sheets of semipermeable membranes which are separated by a supporting material which both supports the mèmbrane against the supporting pressure and provides a flow path ~or the output are placed in a hollow plastic tube. The membranes are seàled around ' ,, three edges or sides to preven't condensation~of,the'`p'rodù'ct ~'~'~'~''''`, '~`~
gases while the fourth edge or side is sealed to~the plàstic `' -~
tube. The plastic tube is provided with perforations inside the edge seal area in order that the product gases can be removed from the porous ~support material. The resulting con-figuration is in the form o~ an envelope which is rolled up about the central tube in the form of a spiral along with a mesh spacer which separates the facing surface membranes.
By utilizing such a type of element it is possible to take àdvantage of a number of factors which include, among others, a large membrane surface~area per unit~volume, a~convenlent~
and simple pressure vessel design and configuration which in turn will lead to a compact module plant arrangemènt, flexibility and ease in installation and in replacement of the elements inasmuch as the elements comprise two disposable units. If so desired, two or more of such elements may be connected in series if lt is desirable to increase the'packing density and to make a more efficient use of the feed gas volume~
The following examples are given to illustrate the novel gas separation membranes of~the present invention as well as the process for producing the same. However, it is to be understood that these examples are given merely for purposes ~ ' , ,.

of illustration and ~hat the present invention is not nec-essarily limited thereto.
EXAMPLE I
A finely porous cellulose nitrate/cellulose acetate porous support membrane which is used in the p-reparation of . . . ~ . - . .
a membrane`~or the separation of`gases.was prepared from.a . ' ` càsting solu~ion containing~7.8''wt'.'''%'~'o'f 'celluiosè'`nitrate, ~`' 1_3 wt. ~ or cellulose acetate, 53.7 wt. % of acetone, 20.4 `
.. ..
wt. % of absolute` alcohol, 26.6 wt. % n-butyl alcohol~ 3.8 `~ wt. % glycerin and 0.5 wt. % of a surfactant Triton~X-100.
When preparing the solution two separate acetone solutions ' of cellulose nitrate and cellulose acetate were prepared and blended into a third solution comprising a diluent mixture consisting of 55.8 parts of ethanol, 36.2 parts of n-butyl' '.
alcohol, 3.5 parts of water, 3.5 parts of glycerin and 1.1 .
part of Triton X-100. The cellulose nitrate and cellulose `- acetate stock diluents were blended with a high speed'stirrer at which point a clear solution with incompatible polymers was formed. The diluent solution was then added, the-Brook-field viscosity of the final casting solution being 700 cps at 25C. The solution was then pressure filtered -through a 5 micron polypropylene filter at an applied nitrogen pres.~ure of 138 kPa in order to remove any foreign material and gel particles which were present. The solution was then degassed in order to remove dissolved nitrogen by applying a slight vacuum to the flask for a period of several minutes. Following thi.s the solution was placed in a separatory flask equipped with a water cool condenser and the flask was immersed in a ``

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constant temperature bath of 40C. for a period ranging from

3-4 hours.
The flask was then withdrawn from the bath and mounted above and directed in~o the'casting knife reservoir of a con-- -tinuous casting machine. The solution was then fed from the ~ .. .- ~ :: .. --~
separatory flask through a regulating stopcock at the same '`
' ` ' - rate at which`the solu~ion wàs'sprèàd`onto~a stainles`s`stèel "'`'``;`i'' `'casting belt.- The sol'ution was spread~onto'~the~belt'~`at''a ' '`'' '~ `' ~ . - -: . . .
thickness of 750 microns at a rate of 25 cm/min. The belt `
was led into the first gelation chamber in which a flow of .
. . - . ..................... . . ~ .. . . ....... . . .
humid air was màintained at'`a rate of 0.~2'm3/min., while main- ~
taining the temperàture of the chamber at about 36C., the temperature of the belt being controlled by a recirculating water bed located directly below the belt. Following this the belt was led into a second gelation chamber which is main-tained at a temperature at 45C. to promote the removal of '' the solvent.-; ~fter passag~through~the second, gelation~chamb,er~
the belt was passed over an elevated temperature zone to re-move any remaining solvent. The resulting finely porous cellu-lose nitrate/cellulose acetate support member~or membrane was removed from the casting belt and rolled upon itself on storage rollers. The membrane rolls were then sealed in polyethylene tubes and stored under'refrigeration until it was ready for use as the support member upon which a semipermeable thin film membrane is to be cast. The dry membrane, the bulk of which contained fine interconnected pores, was approximately 100 microns thick. The bulk porosity, as determined from density measurements was 7a-80%, the membrane or member being asymmetric .
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with a finely porous 400 Angstrom skin on the air driea sur-face. The membrane was analyzed and from electron micrographs it was determined that the surface was 20-25% porous and con-tained between 60-70 pores per square micron.~ The diameter ~ -of the majority of the pores being less than 430 Angstroms. ~ ~ -. .. ..
To form the final membrane for the separation of gasès the finely porous support member prepared accordlng to thè above paragraphs was treated by passage through a chloro~
form solution containing 0.5% by w,eight of dimethyl silicone . . . .................. . .: ~............ . . . . -and a trace amount of a cross-linking catalyst comprising dibutyl tin laur~ate. The porous support material was drawn -~
from the solution at a withdrawal rate of 1.0 cm/sec. After cross-linking ~he thin silicone film by ~reatment for a period of 1 hour at 100C. the final gas membrane~comprised a thin - 15 film of dimethyl silicone polymer having a thickness of 5000 Angstroms composited on the cellulose nitrate/cellulose acetate finely porous support.~
To illustrate the efficiency of a membrane useEul or the separation of gases, the thin film dimethyl silicone polymer which was composited~on the finely porous surface of the cellulose nitrate/cellulose acetate support member pre-pared according`to the above paragraphs was used in a single - stage gas separation process. A feed str~eam comprisLng air was passed over the surface of this membrane at a pressure of 1034 kPa at 25C. The membrane exhibited a higher permeabillty to oxygen than to nitrogen and the permeate stream which passed through the membrane was enriched in oxygen while the residue : ~:
stream which consisted of the remainder of the unpermeated feed L2~4~7~

was enriched in nitrogen. When utilizing this 2" diareter spiral wound element which was fabricated from the dimethyl silicone thin film compoiste membrane, it was possible to attàin oxygèn enrichments of~thë pèrmèate stréàm as high as~'''- ^`~' 35~
The efficiency of combustion processes using oxygen ` `~---enriched air can be im~roved by increas~n~ the effective heàt~
:' ~ ing value of fossil fuel. -For exàmple, às ~he~oxygen conc~èn~
- tration of air is 1ncreasèù from the normal 21% to 100~, the '`- ~`' :
energy density of the combined reactants`increases about 250%~
that is,'from 3.54 ~ /m3 to~12_41 MJ~m3.. ' '~
EXAMPLE II
In this example a cellulose nitrate/cellulose acetate finely porous support member was prepared in a manner similar ' :
to that set forth in Example I above and was passed through a 0.5 wt. % solution of cellulose acetate in chloroform. .The ~- support member was withdrawn at:a rate of l:~cm~sec~. and after ...,'~
cross-linkin~ the cellulose acetate polymer the.thickness of ':
the thin film polymer on the surface of the support membrane was found to be 250 Angstroms.
EXAMPLE III :~
In this example a different finely poroug support member was prepared by casting a solution containing 15 wt.
. of polysulfone, 12.5 wt. ~methyl~cellulose and l wt. % of 2,4-diamino-6-phenyl-s-triazine on a tightly woven dacron fabric:.
Immediately after casting, the membrane was gelled 1n distilled water, rinsed thoroughly to remove the solvent and dried. The support thus prepared was used in forming a gas membrane in ' -17-. .

~ 2 which a thin film polymer consisting of dimethyl silicone having a thickness of 5000 Angstroms was composited on the surface of said support in a manner similax to that set forth in Example I abovë.

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Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process for the production of a membrane for the separation of gases which comprises forming a thin film of a non-porous semi-permeable membrane selectively permeable to gases having a thickness in the range of from about 250 Angstroms to about 10,000 Angstroms directly on the surface of a porous support member by passing said porous support member through a solution of a halogenated hydrocarbon solvent containing a semi-permeable membrane forming monomer and a cross-linking agent, withdrawing the coated porous support member from said solution, and thereafter cross-linking said monomer by treatment at an elevated temperature to form the resultant membrane comprising said porous support member coated with a thin film of a semi-permeable membrane.

2. The process of claim 1 wherein said elevated temperature is in a range of from about 50° to about 150°C.

3. The process of claim 1 wherein said halogenated hydrocarbon solvent is a halogen-substituted methane or ethane containing at least 3 halogen atoms.

4. The process of claim 1, 2 or 3 wherein said monomer is dimethyl silicone.

5. The process of claim 1, 2 or 3 wherein said monomer is a silicone-carbonate copolymer.

6. The process of claim 1, 2 or 3 wherein said monomer is polystyrene.

7. The process of claim 1, 2 or 3 wherein said porous support member is a cellulose nitrate/cellulose acetate membrane.

8. The process of claim 1, 2 or 3 wherein said porous support member is a polysulfone membrane.

9. The process of claim 1 in which said membrane is treated with a solvent subsequent to the cross-linking step.

10. The process of claim 9 wherein said solvent is a halogen-substituted methane or ethane containing at least 3 halogen atoms.