Novel membranes and process for making them

A process for preparing a microporous membrane, which comprises casting the membrane from a solution comprising a mixture of at least one pore-forming polymer and at least one film-forming polymer, and a solvent for the mixture of polymers, wherein the at least one pore-forming polymer is one which if cast alone would contract to form either large pores or a non-uniform distribution of material.

FIELD OF THE INVENTION 
The present invention relates to novel microporous membranes and a method 
for making them. 
BACKGROUND OF THE INVENTION 
Microporous membranes with pore size varying from 0.01 to 10 microns are 
known in the literature, but are generally easily fouled and/or relatively 
hydrophobic, or weak and hydrophilic, compacting at high pressures and 
difficult to produce with high porosity and/or well defined surface pores. 
Prior art method of microporous membrane production are numerous. Some 
examples will be given in the description which follows. 
(1) A solution of polymer is cast with subsequent solvent removal under 
carefully controlled conditions. Such removal must be carried out very 
slowly, which results in an expensive process of poor reproducibility, 
with a wide variation of pore size for a given membrane. Carefully 
controlled conditions of temperature, humidity and air circulation must be 
maintained. 
(2) A well-defined narrow range of pore sizes may be obtained by a nuclear 
penetration and etching technique. The pore volume is very low however 
(less than 15%), in order to minimise degradation. In addition, many 
polymers are unsuitable for use in this process, because of poor etching 
characteristics. 
(3) Ultraviolet and electron beams may be used to polymerize monomers into 
a three dimensional cross-linked microporous membrane (U.S. Pat. No. 
4,466,931). The formation of pores occurs during the polymerization 
process, whereby the polymer precipitates from the solution of monomer. 
While this process may achieve rapid production rates, the membranes are 
isotropic and more easily fouled than asymmetric structures. In addition, 
filters of 1 to 10.mu. have not been reproducibly obtained, and there is a 
problem of leachable materials. 
(4) A solution of a membrane forming polymer may be quenched in a polymer 
nonsolvent, e.g. water (Marinaccio et al U.S. Pat. No. 3,876,738 and Pall, 
EP No. 0 005 536). Many different materials such as cellulose derivatives, 
polyamides, polysulfones and polycarbonates have been used, and these 
membranes are generally characterized by asymmetric structures. Control of 
pore size is generally a function of careful formulation of the polymer 
solution, which is brought to the point of incipient precipitation and/or 
the use of a gelatin bath containing a high percentage of polymer solvent. 
Careful stirring, temperature and rates of quenching are mandatory for a 
uniform product. It is difficult, however, via this method to achieve 
uniform microfilters with pores sizes reliably in the range of 1 to 
10.mu.. It is also a costly process to carry out, with a large expenditure 
for solvents. 
(5) Sintering of particles of organic materials such as polyolefins (e.g. 
high density polyethylene), polytetrafluoroethylene, polyvinylidene 
fluoride, and inorganics such as alumina, silica, zirconia, graphite and 
other forms of carbon have produced useful microlfilters. In the case of 
organic polymers, it has been difficult to achieve uniform and narrow pore 
size distribution and the membranes have been isotropic rather than 
asymmetric. Inorganic membranes have achieved asymmetry by sintering two 
or three layers of different particle sizes. These membranes foul 
relatively easily and are considerably more expensive than organic 
polymers. 
(6) Microporous membranes have also been made by a microcracking process 
(Ind. Eng., Prod. Res. Develop., Vol. 13, No. 1, 1974). For example, 
deformation of annealed polypropylene parallel to the direction of 
extrusion and followed by high temperatures for stabilization of the 
elongated films, gives rectangular pores (for example 2000 A and 4000 A by 
200 and 400 A, respectively). However, in order to maintain mechanical 
strength, these membranes do not have a high density of pores. In 
addition, these membranes are isotropic and demonstrate relatively low 
flux. 
(7) Yet another popular technique is the phase separation of a polymer 
solution or dispersion. In effect, a polymer is brought into solution or 
dispersed at elevated temperature and then solidified by cooling and 
removing the liquid or solvent (U.S. Pat. No. 3,812,224). In one approach 
(Castro, U.S. Pat. No. 4,247,498) any synthetic thermoplastic polymer may 
be dissolved in a compatible liquid to form a solution. The plastic is 
then rendered microporous by cooling at a rate fast enough to prevent 
liquid-liquid phase separation. The cooling rate criteria must be 
carefully adjusted to form cellular microstructures of spherical shapes 
and pores or passageways interconnecting adjacent cells. The basic 
structure is relatively homogeneous and isotropic. Though this method 
results in highly porous membranes, the isotropic structure is susceptible 
to fouling and plugging, and compaction at elevated operating pressures. 
Materials commonly used for making microfilters are polycarbonates, 
polyamides (nylon 6, nylon 6,6, nylon 610, nylon 13), polysulfones, 
cellulose derivatives (for example, cellulose, cellulose discetate, 
cellulose triacetate, cellulose nitrate), polyacrylonitrile and 
copolymers, polypropylene, polytetrafluoroethylene, alumina, silica, 
carbon, polyvinylidene fluoride, high and low density polyethylene, 
polypropylene, polystyrene, polyvinyl chloride, 
acrylonitrile-butadiene-styrene terpolymers, styrene-acrylonitrile and 
styrene-butadiene copolymers, polyvinylacetate, polyvinylidene chloride, 
ethylenevinylacetate copolymers, ethylene-acrylic copolymers, 
polymethylacrylates, and oxidation polymers such as polyphonylencoxide. 
The present invention provides microporous membranes and a procedure for 
making them, which overcomes these shortcomings of the current state of 
art. 
SUMMARY OF THE INVENTION 
There is accordingly provided, in accordance with the present invention, a 
process for preparing a microporous membrane, which comprises casting the 
membrane from a solution comprising a mixture of at least one pore-forming 
polymer and at least one film-forming polymer, and a solvent for the 
mixture of polymers, wherein the at least one pore-forming polymer is one 
which if cast alone would contract to form either large pores or a 
non-uniform distribution of material. 
The solution preferably comprises a water-miscible solvent and the membrane 
is gelled in water, or in an aqueous solution or mixture. Additionally, 
the solution may comprise a liquid which is non-solvent for the mixture of 
polymers, and/or a salt. The membrane may be cast onto a discrete support 
and may optionally be subsequently removed therefrom, or it may be 
extended, e.g. in the form of a hollow fiber or tubelet. 
In a particular embodiment of the process of the invention, at least one 
polymer used in forming the membrane comprises reactive functions such as 
e.g. those selected from the group consisting of halogens, glycidyl, 
isocyanato, isothiocyanato, amino, hydroxyl and sulfhydryl, which are 
incorporated in the membrane structure. Subsequently to membrane 
formation, the reactive halogens, glycidyl, isocyanato or isothiocyanato, 
for example, are reacted with a momomeric or polymeric substance 
containing one or more reactive groups selected from reactive primary 
amino, secondary amino, tertiary amino, OH and SH groups. Such a substance 
may be, for example, a tertiary amine, in which case the reaction product 
will be a membrane containing quaternary ammonium groups. Alternatively, 
the substance may be at least one polymer containing --NH--, OH or SH 
groups, when the reaction product will be a cross-linked membrane; in this 
context, it is to be understood that in such --NH-- groups, one free 
valency is attached to the at least one polymer, and the other free 
valency is attached to a hydrogen atom, a substituent group, or the at 
least one polymer. The substance containing one or more reactive groups 
may be synthetic or natural and may be biologically active or inactive. 
Where the substance is biologically active it may be monomeric, oligomeric 
or polymeric. As examples of biologically active substances, there may be 
mentioned enzymes, hormones, antigens and antibodies. If the membrane 
contains primary amino, secondary amino, tertiary amino, hydroxyl or 
sulfhydryl groups, then it may be reacted with monomers or polymers 
containing e.g. functions selected from the group consisting of reactive 
halogens, glycidyl, isocyanato and isothiocyanato. These monomers or 
polymers may also contain anionic or cationic groups, or ether groups. The 
resultant membrane may contain hydrophilic and/or anionic and/or cationic 
groups on its external and internal surfaces. 
In another aspect, there is provided in accordance with the present 
invention the membrane per se, which is preparable by the above-described 
process of the invention. The membrane is composed of at least two 
different polymers, at least one of which is a film former, and at least 
another of which is a pore former which if cast alone would form large 
pores or a non-uniform distribution of material; and as to structure, the 
membrane comprises a microporous surface of well defined pore shapes, 
which surface is integrally and continuously connected to a pore support 
of the same material as the surface, in which pore support there exists a 
non-exact distribution of materials. It is moreover preferred that the 
pores occupy not less than about 10%, and preferably not less than about 
50%, of the area of the microporous surface. The membrane may possess a 
structure which approximates to that of an array of sintered particles; 
the pores may be of relatively larger cross-sectional area on the 
microporous surface and taper to a relatively smaller cross-section 
towards the interior of the membrane. 
In the membrane of the present invention, at least the majority of the 
pores may be characterized by one or more of the following 
characteristics, namely: 
relatively larger cross-sectional area on the microporous surface and 
tapering to a relatively smaller cross-section towards the interior of the 
membrane; and/or 
a cross-sectional shape at said surface approximating to circles or 
ellipses, or to segments, cresents or annuli derived from such circles or 
ellipses; and/or 
diameters which lie within the range of from 0.01 to 20 microns, preferably 
from 0.01 to 10 microns. 
The invention also extends to the membrane of the invention, mounted on a 
discrete support layer.

DETAILED DESCRIPTION OF THE INVENTION 
The process of the invention involves the mixing of 2 types of polymer in a 
water miscible solvent(s); the polymer solvent solution is preferably cast 
on a support and immersed into an aqueous bath. The resulting membrane 
contains a majority of micropores which may vary between 0.01 and 10.mu., 
as a function of the parameters described below. One essential component 
in the mixture is one or more hydrophobic pore-forming polymers. The 
suitability of this component to function as described herein is, that if 
cast alone, it will contract on the support to form large pores (greater 
than 1 micron in its largest dimension, preferably greater than 10 microns 
and most preferably greater than 50 microns), or it will give rise to a 
non-uniform distribution of material, portions of which are isolated on 
the support. The function of the other essential polymer component within 
the membrane, is to form a stronger film and to retain and control the 
pore extension properties of the pore-forming polymer. 
Examples of such pore-forming polymers are halomethylated 
polyphenyleneoxide (for example, bromomethylated, 2,6 dimethyl 
polyphenylene oxide), brominated polyphenylene oxide, polystyrene 
derivatives, halomethylated polystyrene, nitropolystyrene, perfluoro 
polymers, silicone polymers (for example, polydimethylsiloxane, polymethyl 
ethylsiloxane), nitrated polyphenyleneoxide, nitrated polysulfones, 
polyisobutylenes, polyisoprenes, and halogenated polysulfones. 
Examples of the film-forming polymers are addition, condensation and 
oxidation polymers such as polysulfones, sulfonated polysulfones, 
chloromethylated polysulfones, polyvinyl chloride, polyvinylidene 
chloride, polytetrafluoroethylene based copolymers, 
polychlorotrifluoroethylene, polyvinylidene fluoride and its copolymers, 
with hexafluoropropylene or chlorotrifluoroethylene, polyacrylonitrile 
(and copolymers of acrylonitrile with vinyl acetate, acrylic and 
methacrylic acid), nylon 6, nylon 6,6, polyether sulfones, polysulfones 
made with Bisphenol A, polyimideamides, polyamides, 
polyetherphenylketones, polyimides, polyepoxy compounds, polycarbonates 
and oxidation polymers, e.g. polyphenylene oxides. 
Important parameters of casting are viscosity, as controlled by polymer 
concentration, polymer molecular weight, solvent, temperature and 
additives such as nonsolvents and salts. As a general rule, high viscosity 
solutions produce small pore sizes, as they decrease flow or movement of 
the pore-forming polymer. Viscosities in the range of 50 to 50,000 cps may 
be used, preferably 100 to 30,000 and most preferred 200 to 20,000 cps. 
The viscosity is a function of the molecular weight of the different 
polymers. For a given concentration and solvent high molecular weight 
polymers will give higher viscosities. The molecular weight may be chosen, 
so that the concentration of polymer(s) for achieving the desired 
viscosity ranges is not less than 5% nor more than 60% or preferably 
between 8 to 40% or most preferred, between 15 to 35% of material. The 
temperature of the casting solution may vary between -60.degree. C. and 
200.degree. C., but is preferably within the range of between 0.degree. 
and 100.degree. C., more preferably between 4.degree. and 90.degree. C. 
According to an embodiment of the invention, the polymers are dissolved in 
a solvent which is water miscible, and which dissolves the polymer 
components completely or partially. The solubility of the polymers may be 
a function of temperature, and temperatures for dissolution and for 
maintaining solubility may vary from -60.degree. to 20.degree. C., 
preferably between 0.degree. to 95.degree. C. The solution may be cast 
hot, or alternatively it may be cooled and then cast. Preferred solvents 
for the polymers are N-methypyrrolidone, pyrrolidone, pyrrolidine, 
dimethylsulfoxide, dimethylacetamide, and dimethylformamide. 
These solvents may also contain nonsolvents such as e.g. acetone, ethanol, 
methanol, butanol, ethyl acetate, water, formamide and ethylene glycol 
and/or hydrophilic polymers and/or salts such as LiHCO.sub.3, LiCl, 
MgCl.sub.2, MgClO.sub.4, ZnCl.sub.2, ZnBr.sub.2, ZnI.sub.2 (and other 
materials well known within the state of the art). The salts and 
nonsolvents may be used to control viscosity and may effect pore size and 
distribution. 
The solutions may be cast on a porous support material woven or nonwoven 
and left on the said material for additional strength, such as polyolefin 
nonwovens, polyester nonwoven, polyfluoro fabric, cellulosics, 
polyethylene, polypropylene, nylons, vinyl chloride homo- and co-polymers, 
polystyrene, polyesters such as polyethylene terephthalate, glass fibers, 
ceramics and porous metal. 
Alternatively, the solutions may be cast as flat sheets and removed from 
the support or they may be cast as hollow fibers or tubelets without the 
need of support. When supported, the membrane may be in a tubular 
configuration or in a flat sheet plate and frame, or the flat sheet may be 
wound into a spiral wound configuration or formed into a pleated 
cartridge. The tubular configuration may be formed by winding a preformed 
flat sheet and sealing the overlap via heat, adhesives or ultrasound or it 
may be formed by casting into a continuously forming tube and gelling 
immediately, such that both tube casting and gelling are carried out in 
series and continuously. Simultaneously with the winding operation or 
subsequently, a further woven or non-woven support layer may be wound so 
as to provide the tubular configuration with additional mechanical 
strength. 
The cast solution once spread upon the support into the desired shape, may 
be exposed in an evaporation step of up to about 24 hours and then 
immersed in an aqueous bath containing only water or water with 
surfactants, and/or salts and/or polymer solvents and/or hydrophilic 
polymers. Examples of such water and aqueous combinations can be found in 
many of the patents cited herein, as well as in reference mentioned 
therein and in NL No. 8104496. It is one of the advantages of the present 
invention, that the membrane may be formed without high concentration of 
solvents in the aqueous gelling bath. However, such an approach embodying 
the use of solvents is not excluded. 
In the use of such membranes the solution to be filtered should be filtered 
by crossflow or stationary application. In cross flow, the mixture, 
dispersion or solution is passed over the surface of the membrane and a 
pressure is applied. The flow over the membranes may vary from laminar to 
turbulent and the transmembrane pressure may be from 4 psi to 300 psi, 
preferably 5 to 250 psi and most preferably 10 to 50 psi. A back pressure 
may be applied continuously or periodically as required by configuration 
and application. 
Microporous membranes may be used for filtration and/or clarification of 
liquids, mixtures, suspensions or dispersions. They may also be used as 
supports for biologically active molecules (e.g. enzymes). For filtration 
functions, the rejection of particles or suspended matter is a function of 
pore size and charge on the membrane and particle. Because of dielectric 
or electrostatic interactions, particles or organisms smaller than the 
steric pore size may be retained. Based on this mechanism, filters are 
available with charged groups, on the surface and within their pores 
(Knight et al, European Patent Application No. 0,050,864); charges are 
attached to the membrane by crosslinking a charged monomer or polymer to 
the membrane's internal and external surface. It is emphasized that this 
approach does not reduce flow through rate. In the present invention, 
charging may be carried out by a similar procedure, but alternatively if 
one of the polymers is reactive, e.g. if in the case of haloalkylated 
polyphenyleneoxides or polysulfones, then the halogen atom may be 
substituted with groups containing cationic, anionic or neutral 
hydrophilic functions. Examples of such groups are alkyl (1 to 8 carbons 
branched or linear, saturated and unsaturated) or aromatic radicals 
containing ionic groups, such as sulfate, sulfonic, carboxylic acids, 
ammonium groups, and also phosphonium and sulfonium groups, as well as 
primary, secondary and tertiary amines and hydroxyl groups. Ion exchange 
capacities of 0.05 to 5 eq./Kg. can be achieved. Particularly advantageous 
are quaternary ammonium and/or carboxyl and/or sulfonic groups, depending 
on the purpose for which the membrane is to be applied. The alkyl or 
aromatic radical may be bonded to the polymer by primary, secondary or 
tertiary or quaternary amine, ether (--O--), sulfhydryl (--S--), carbonyl 
(--CO--), sulfone (--SO.sub.2 --), carboxyl (--O--CO--), amido 
(--NH--CO--), and thiocarbamido (--NH--CS--). In addition the membrane may 
be modified by reacting the reactive polymer component with polymers from 
solutions into which it is immersed. 
Preferably hydrophilic polymers are used to coat the microporous membrane 
substrate. The preferred polymers are polyfunctional oligomers or polymers 
which contain active hydrogen atoms bound to nitrogen, oxygen or sulfur 
atoms. The nitrogen atoms may be present as aliphatic (acyclic or cyclic), 
aromatic, or heterocyclic amino groups which can be primary, secondary or 
tertiary. Or alternatively, but less preferred, they may be polymers 
containing hydroxyl or thio functions. Examples of such polymers are 
polyethyleneimine (M.W. 150-1,000,000), which can be partially alkylated 
or otherwise modified, polyvinylamine (M.W.1000 to 2,000,000), polyvinyl 
alcohol (M.W. of 2,000 to 200,000) or partially esterified polyvinyl 
alcohol, polyvinyl-aniline, polybenzylamines, polyvinyl mercaptan, 
polymers of hydroxyalkyl and aminoalkyl acrylates or methacrylates e.g. 
2-hydroxyethyl or 2-aminoethyl methacrtylates, polyvinylimidazoline, amine 
modified polyepihalohydrin (described in GB No. 1,558,807), 
polydiallylamine derivatives, polymers containing piperidine rings 
(described in GB No. 2,027,614), amino polysulfones, amino polyarylene 
oxides (e.g. aminomethylated polyphenylene oxide), 
polyamidepolyamine-epichlorohydrin condensation products, and hydrophilic 
amines containing polymers described in EP No. 8945, and the condensation 
products of dicyandiamide, amine salt (ammonium chloride) and formaldehyde 
(U.S. Pat. No. 3,290,310). The above polymers may be in part a copolymer 
or a polymer containing other monomeric units, block polymers or graft 
polymers. If they are copolymers, the other monomeric units may or may not 
contain ionic groups (--SO.sub.3 --COO, --NR.sub.3). 
Examples are copolymers of styrenesulfonate(sodium salt)/vinylaniline, 
2-aminoethylmethacrylate/acrylic acid, 
vinylaniline/vinylbenzyltrimethylammoniumchloride or 
vinylamine/vinylsulfonate. 
Preferred polymers are polyvinylalcohols, cellulose derivatives, 
polyvinylamines, polyvinylanilines, polypiperidines, polydiallylamine 
derivatives or amine modified polymers on the basis of epoxides or 
epihalogenohydrins as well as the copolymers exemplified above. 
One especially preferred polymer comprises polyaliphatic (acyclic or 
cyclic) amines. Polyethyleneimine is an example of this group. The range 
of molecular weights may be between 150 to 2,000,000 but preferably 
between 1000 and 200,000 and most preferred between 10,000 and 70,000. Low 
molecular weight polymers or oligomers (150 to 1000) may be used, but the 
increase in solute rejection of the final membrane is not as great when 
higher molecular weight polymers are used. 
In another preferred case, water soluble amphoteric or block mosaic 
polymers containing both cationic and anionic groups, together with a 
reactive function (for example, NH.sub.2 or OH groups) for reaction with 
the polyfunctional cross-linking agents are useful for forming a mixed 
charge membrane. An example of such a coating polymer is 
poly(vinylamine-vinyl-sulfonate) or partially quaternized derivatives. 
Water is the preferred solvent for the aforementioned molecules, though 
other solvents such as low molecular weight alcohols or ketones may be 
used alone or in combination with water. The range of polymer 
concentration may be from 0.1 to 80%, but preferably between 1 and 30%, 
and most preferred between 1.0 and 15%. Liquid polymers can be used 
without solvents that are as pure (100%) agents, too. 
The concentration of polymer needed to achieve optimum rejection and flux 
characteristics is a function of polymer M.W. and molecular dimensions, 
membrane porosity and pore size, temperature, time of immersion, pH and 
subsequent washing steps. These factors (together with a rinse step after 
immersion) control the thickness of the polymer layer deposited on the 
membrane. The temperature of the polymer solution during membrane 
immersion may vary from 0.degree. to 90.degree. C. The optimum temperature 
is a function of absorption rates. The time of immersion may vary between 
1 minute to 48 hours as a function of the temperature, pH, concentration 
and the M.W. dimensions and solution properties of the coating polymer. 
For example, at a pH of 8.0 and R.T. 10% polyethyleneimine in water coats 
a polysulfone membrane in 1 to 5 minutes, adequately for the practice of 
this invention. On the other hand, poly(aminostyrene) should be used for 1 
hour in immersion to achieve optimum flux/rejection characteristics. 
The thus coated membranes may be further modified chemically to alter the 
hydrophilic/hydrophobic balance, or to crosslink the coating by one or 
more of the many different procedures known in the technical literature. 
Examples of such procedures may be found in EP No. 0,050,864 and EP No. 
0,114,286. 
Thus, according to the invention a membrane is provided with a microporous 
surface of well-defined pore shapes, which is integrally and continuously 
connected to a pore support of the same material. This support is shaped 
in a non-exact distribution of materials, but may be in the preferred case 
approach an array of sintered particles. The interchannel spaces 
(interstices) may be of the same size, or greater than the pore openings 
on the surface, but in the preferred case they would be smaller. The 
approach to a sintered particle arrangement enhances compaction stability, 
while the large pore on the surface allows for high flux. The shape of the 
pore opening may be spherical, elliptical, a concentric annulus, or may 
appear as different "moon phases". The exact shape is a function of the 
polymer components, molecular weight, temperature of the casting solution 
and gelling bath, relative humidity and evaporation time prior to 
immersion. For example, a 14.5% bromomethylated polyphenylene oxide with 
10% polysulfone (in NMP solvent) upon gelling in water gives spherical 
shaped pores with ledges (FIG. 1), while the same bromomethylated 
polyphenylene oxide in 95:5 acrylonitrile-vinyl acetate copolymer gives 
annular-like concentric shaped openings (FIG. 2). It should be appreciated 
that by this approach, many different pore shapes can be achieved. A cross 
section of the bromomethylated polyphenyleneoxide and polysulfone shows 
that the integral support for the microporous structure has many irregular 
nodules, approaching a sintered particle configuration (FIG. 3). 
ADVANTAGES OF THE INVENTION 
The membranes in accordance with the present invention possess the 
following advantages: 
(1) Well defined surface pore shapes, such as circles, ellipses, or half or 
quarter circular segments may be generated. This allows for optimal design 
of flux for a given application. 
(2) The pores are larger on the surface and decrease in size to the bottom 
side of the membrane. This allows for higher fluxes, as it produces a 
mixing flow and allows for flow around trapped particles. 
(3) Micropores through a range of pore sizes 0.01 to 20.mu., preferably 0.1 
to 10.mu., may be produced. 
(4) The membranes are modified easily to alter the hydrophobic/hydrophilic 
balance, or are readily charged either positively or negatively to 
increase hydrophobicity without reduction in flux. In addition, 
biologically active components may be crosslinked to prevent dissolution 
in organic solvents and to minimize compaction under high pressure. 
(5) Biologically active membranes of this invention may be used as 
reactors, e.g. enzyme membrane reactors, or in chromatographic separation, 
or in affinity chromatography for removing specific biological species 
from complex mixtures. 
These advantageous membrane properties are achieved in accordance with 
process of the present invention which is described herein. The invention 
will now be illustrated by the following Examples. 
EXAMPLE 1 
10 grams of 2,6-dimethyl phenylene oxide (MW.about.22,000) (PPO) is 
brominated in the methyl groups with 9 grams of N-bromosuccinimide to form 
a bromomethylated PPO of 1.4 eq. Br/Kg. A solution of N-methylpyrrolidone 
containing 14.5% of this PPO derivative and 10% of a polysulfone, made 
from Bisphenol A and dichlorodiphenyl sulfone (Udel TM 1700, Union 
Carbide), was cast (0.4 mn) on a polyester nonwoven from Kolff and 
immersed in a water bath at ambient conditions. After leaching for 24 
hours the membrane was placed in a flat pressure cell with an overhead 
stirrer and gave a water flux of 18,640 l./m..sup.2 /hr., at 2 atm. 
pressure. A scanning electron micrograph of the top surface is given in 
FIG. 1. 
EXAMPLE 2 
Example 1 is repeated with 7.5% of the bromomethylated PPO instead of 
14.5%. The flux is reduced to 4266, showing the importance of the 
proportion of PPO derivative in determining pore size and distribution, 
and thus flux, though this membrane is still included within the 
invention. 
EXAMPLE 3 
Example 1 is repeated, using 10% poly(acrylnitrile-vinylacetate) (95:5) 
instead of polysulfone. The flux of this membrane at 2 atm. was 1250 
l./m..sup.2 /hr. A SEM of the surface (FIG. 2) reveals concentric sphere 
pores on the surface. 
EXAMPLE 4 
Example 1 is repeated, but instead of polysulfone, polyvinylidene fluoride 
(PVF2) (MW 70,000) is used. The PPO derivative and PVF2 are dissolved 
separately and then mixed to give the concentrations of Example 1. The 
cloudy solution is cast and gelled in a water bath. The resultant flux is 
13,760 l./m..sup.2 /hr at 2 atm. 
EXAMPLE 5 
If Example 4 is repeated, but a sulfonated PPO derivative is used (1.2 meq. 
--SO.sub.3 H groups/g.), the resulting membrane has a flux of 36 l./m2.hr. 
This demonstrates the importance of the chemical composition of the chosen 
polymer. 
EXAMPLE 6 
Example 1 is repeated, using chloromethylated(1.4 meq./g.)-polysulfone 
instead of polysulfone. The solution is heated to solubilize the polymers 
and then cast. The resulting water flux is 15,040 l./m..sup.2 /hr. If PPO 
is used instead of bromomethylated PPO, the flux is 5920 l./m..sup.2 /hr. 
EXAMPLE 7 
Example 1 is repeated, using a Bisphenol A-epichlorohydrin condensate 
polymer (MW 20,000) instead of polysulfone. The resultant membrane had a 
water flux of 8000 l./m..sup.2 /hr. 
EXAMPLE 8 
The membrane of Examples 1, 3 and 6 are immersed in a solution of 5% 
triethylamine and heated at 60.degree. C. for two days. The resultant 
membranes had a cation exchange capacity of 1.6, 1.7, 2.3 meq. cationic 
group/g., respectively. 
EXAMPLE 9 
Example 8 was repeated, using in place of triethylamine, polyethyleneimine 
with an average molecular weight of 600 daltons. The resultant membranes 
had an amine content of 1,2, 1,0, and 2,2 meq./g., respectively, following 
removal of unbound polymer by washing for 24 hours. In addition, the 
resultant membranes were no longer soluble in N-methylpyrrolidone, 
dimethylformamide or dimethylsulfoxide. 
EXAMPLE 10 
In this Example, liquid poly(chlorotrifluoroethylene) (PCFE) is used as the 
pore forming polymer. A 5.0% solution of polysulfone used in the previous 
examples but with a molecular weight of 37,000, is prepared with 15% of 
PCFE and a membrane is prepared as in Example 1. The resultant flux is 
8050 l./m..sup.2 /hr. If the concentration of polysulfone is increased to 
15%, the flux in then 2000 l./m..sup.2 /hr. 
EXAMPLE 11 
In this Example, polydimethylsiloxane liquid is used with the polysulfone 
used in Example 10. A casting solution of 25% polydimethylsiloxane and 5% 
polysulfone gives a membrane with a flux of 1750 l./m..sup.2 /hr. 
EXAMPLE 12 
Example 1 is repeated using instead of polysulfone, cellulose acetate, MW 
15,000, with an acetate content of 39.4%. The resultant membrane had a 
flux of 3400 l./m..sup.2 /hr. The membrane was subjected to pH 10 in 
aqueous medium for one week, to hydrolyze the acetate groups. The 
resultant membrane was hydrophilic and had a water flux of 7620 
l./m..sup.2 /hr. 
EXAMPLE 13 
Example 12 is repeated with polyvinyl acetate instead of cellulose acetate. 
The resultant membrane after hydrolysis had a flux of 2660 l./m..sup.2 
/hr. The hydroxyl groups resulting from the hydrolysis of the acetate were 
subsequently reacted with 
4-(4,6-dichloro-1,3,5-triazin-2-yl)aminobenzenesulfonic acid to give an 
anionic membrane. 
EXAMPLE 14 
To a membrane prepared as in Example 1, a 0.5M phosphate buffer solution 
(pH 7.0) containing 6 mg./ml. chymotrypsin and 7 mg./ml. phenyl propionate 
as inhibitor was applied with a 0.1 Atm. pressure difference. The membrane 
was washed thoroughly with water. It was then placed in a pressure cell 
and washed with water, 10.sup.-3 HCl (to break up the enzyme-inhibitor 
complex) and buffer, until no UV absorbance or enzymatic activity were 
detected in the washings. The membrane was then tested for enzymatic 
activity by driving through the membrane under various pressures, and at a 
temperature of about 26.degree. C., 0.05M solutions of the following: 
(a) N-benzoyl-L-tyrosine ethyl ester in 30% EtOH - 70% tris buffer at pH 
7.8; 
(b) N-acetyl-L-tryptophan methyl ester in tris buffer at pH 7.8, containing 
0.05M CaCl.sub.2 ; and 
(c) N-acetyl-DL-phenylalanine ethyl ester in tris buffer at pH 7.8, 
containing 0.05M CaCl.sub.2. 
In all cases, hydrolysis of the specified compounds was observed. 
EXAMPLE 15 
Example 1 is repeated, except that the PPO is nitrated with concentrated 
nitric acid, instead of being bromomethylated. The resultant membrane had 
a water flux of 5600 l./m..sup.2 /hr. 
While the invention has been particularly described with respect to certain 
illustrative embodiments, it will be apparent to those skilled in the art 
that many modifications and variations may be made. The invention is 
therefore not to be construed as limited by such embodiments, rather its 
scope will be defined only by the claims which follow.