TITLE: Porous membrane formed from an interpenetrating polymer network having
hydrophilic surface
European Patent EP0430082
 B1

ABSTRACT:
Abstract of EP0430082
A porous membrane is formed from an interpenetrating polymer network of
a hydrophobic polymer and a polymerized and crosslinked hydrophilic
monomeric composition. A solution of the polymer and monomeric
composition is cast, exposed to ultraviolet radiation, coagulated and
dried. The resulting dried membrane is annealed in order to render its
surface hydrophilic.

INVENTORS:
Allegrezza, Anthony E. (254 Congress Street, Milford, Massachusetts,
01757, US)
Bellantoni, Ellen C. (608 Windsor Drive, Framingham, Massachusetts,
01701, US)

APPLICATION NUMBER: EP19900122338
PUBLICATION DATE: 06/19/1996
FILING DATE: 11/22/1990

ASSIGNEE: MILLIPORE CORPORATION (80 Ashby Road, Bedford, Massachusetts, 01730,
US)
INTERNATIONAL CLASSES: B01D71/32; B01D67/00; B01D69/02; B01D69/14; B01D71/34; B01D71/40;
(IPC1-7): B01D71/34; B01D67/00; B01D69/02; B01D71/40; B01D71/76
EUROPEAN CLASSES: B01D67/00K12; B01D67/00K14B; B01D67/00R10; B01D69/14B; B01D71/34

DOMESTIC PATENT REFERENCES:
EP0054354 Membrane comprising an intimate blend of polymers.
EP0186758 Porous membrane having hydrophilic surface and process of its
 manufacture.
EP0261734 A process for the preparation of hydrophilic membranes and
 such membranes.

FOREIGN REFERENCES:
4012324 Crosslinked, interpolymer fixed-charge membranes
4119581 Membrane consisting of polyquaternary amine ion exchange
  polymer network interpenetrating the chains of thermoplastic matrix
  polymer
4708981 Water insoluble proton conducting polymers
4752624 Process for the preparation of multi-component polymer membrane
4787976 Non-adsorptive, semipermeable filtration membrane
0276993

OTHER REFERENCES:
Encyclopedia of Polymeric Science and Engineering, Vol.8, 2nd edition
(1987), J.Wiley&Sons, N.Y., USA; pp.279-284 "Interpenetrating polymer
networks"
Attorney, Agent or Firm:
Henkel, Feiler, Hänzel & Partner (Möhlstrasse 37, München, 81675, DE)

CLAIMS:
1. A porous membrane having a water wettable surface which membrane
comprises a porous semi-interpenetrating polymer network of a
hydrophobic fluorinated hydrocarbon polymer and a hydrophilic
cross-linked polymer, the interpenetrating polymer network being formed
by synthesising and crosslinking the hydrophilic polymer in the
immediate presence of the hydrophobic polymer, the entire surface of
the membrane including the pore surface being formed from said
hydrophobic polymer in semi-crystalline form and said hydrophilic
polymer.
2. The porous membrane of claim 1 wherein said membrane has a maximum
pore size between about 0.01 and 0.1 µm (microns).
3. The porous membrane of claim 2 wherein said membrane has an
isopropyl alcohol bubble point of between about 275 kPa (40 psi) and
about 963 kPa (140 psi), a water flux of above about 1426 hl/m²/d (3500
gallons per square foot per day) at 172 kPa (25 psi) for a membrane
having a bubble point of about 275 kPa (40 psi), a water flux of above
about 407.5 hl/m²/d (1000 gallons per square foot per day) at 172 kPa
(25 psi) for a membrane having a bubble point of about 550 kPA (80
psi), and a water flux above about 407.5 hl/m²/d (1000 gallons per
square foot per day) at 172 kPA (25 psi) for a membrane having a bubble
point of about 963 kPa (140 psi).
4. The membrane of any one of claims 1, 2, or 3 wherein the hydrophobic
polymer is polyvinylidene fluoride.
5. The porous membrane of any one of claims 1, 2 or 3 wherein said
hydrophilic polymer is formed from an hydroxyalkyl acrylate.
6. A process for forming a porous membrane having an entirely water
wettable surface including the pore surface, said membrane being formed
of a semi-interpenetrating polymer network of a hydrophobic polymer and
a cross-linked hydrophilic second polymer, the entire surface of the
membrane including the pore surface being formed from said hydrophobic
polymer in semi-crystalline form and said hydrophilic polymer, which
comprises: (a) forming a solution of said hydrophobic polymer, a
monomeric composition being a precursor of said hydrophilic polymer, a
crosslinking agent for said monomeric composition and a photoinitiator
for said monomer composition, (b) casting said solution on a substrate,
(c) exposing said cast solution to ultraviolet radiation to polymerize
and cross-link said monomeric composition, thereby effecting formation
of an semi-interpenetrating polymeric network in solution, (d)
coagulating said cast solution, (e) removing solvent from said
coagulated solution to form a dried porous membrane, and (f) annealing
said dried porous membrane by a heat treatment in order to increase
crystallinity of the hydrophobic polymer and to thereby increase the
water wettability of the entire surface of said membrane, including the
pore surface.
7. The process of claim 6 wherein said dried porous membrane is
restrained to prevent shrinkage during annealing.
8. The process of any one of claims 6 or 7 wherein said annealing step
is conducted at a temperature between about 130°C and about 160°C for a
time between about 20 minutes and 60 minutes.
9. The process of any one of claims 6 or 7 wherein said hydrophobic
polymer is a fluorinated hydrocarbon polymer.
10. The process of any one of claims 6 or 7 wherein said hydrophobic
polymer is a polyvinylidene fluoride.
11. The process of any one of claims 6 or 7 wherein said monomeric
composition is an hydroxyalkyl acrylate.
12. The process of any one of claims 6 or 7 wherein said porous
membrane has a maximum pore size between about 0.01 and 0.1 µm (micron)
and has an isopropyl alcohol bubble point of between about 275 kPa (40
psi) and 963 kPa (140 psi), a water flux of above about 1426 hl/m²/d
(3500 gallons per square foot per day) at 172 kPa (25 psi) for a
membrane having a bubble point of about 275 kPa (40 psi), a water flux
of above about 407.5 hl/m²/d (1000 galons per square foot per day) at
172 kPa (25 psi) for a membrane having a bubble point of about 550 kPa
(80 psi) and a water flux of above about 407.5 hl/m²/d (1000 gallons
per square foot per day) at 172 kPa (25 psi) for a membrane having a
bubble point of about 963 kPa (140 psi).

DESCRIPTION:
BACKGROUND OF THE INVENTION
This invention relates to a microporous or ultrafiltration membrane
having a hydrophilic surface and formed from an semi-interpenetrating
network of a hydrophobic polymer and a hydrophilic cross linked
monomeric composition and to the process for forming the membrane.
According to Encyc. of Polym. Sci. and Technology, Vol. 8, 2nd ed., J.
Wiley & Sons, N.Y., U.S.A., pp. 279-284, an interpenetrating polymer
network (IPN) is an intimate combination of two polymers both in
network form, at least one of which is synthesized or crosslinked in
the intermediate presence of the other (see page 279). A semi-IPN is
formed, if one of the two polymers is in network form (crosslinked) and
the other a linear polymer, i.e. not crosslinked (see page 283).
In many applications of filtration technology, it is desirable to
utilize a membrane filter which is mechanically strong, is thermally
stable, is relatively inert chemically and is insoluble in most organic
solvents. Often, it is desirable that the membrane has surface
properties which are radically different from, and sometimes
incompatible with, the bulk properties of the membrane. Desirable
surface properties include wettability, low protein adsorbing tendency,
thromboresistivity, controlled ion exchange capacity and controlled
surface chemical reactivity.
Conventional methodology presently used to achieve the duality of
function of bulk properties which differ from the surface properties is
to coat a preformed membrane having the desired bulk properties with an
oligomer or polymer having the desired surface properties. Typical
coating materials include surfactants and water soluble polymers such
as polyvinylpyrrolidone. This approach has disadvantages, particularly
because the coating reduces flux by reducing pore size; flux reduction
is especially severe for small (<0.1µm(.lu)) pore membranes.
It also has been proposed to utilize graft polymerization techniques to
modify the surface characteristics of a polymeric substrate. Typical
examples of graft polymerization are shown for example in
US-A-3,253,057; US-A-4,151,225; US-A-4,278,777 and US-A-4,311,573. It
is difficult to utilize presently available graft polymerization
techniques to modify the surface properties of the porous membrane.
This is because it is difficult to modify the entire surface of the
membrane including the surfaces within the pores while avoiding
significant pore blockage and while retaining membrane porosity.
It has been proposed in US-A-4,618,533 to form a porous membrane having
a porous membrane substrate to which is directly coated a cross-linked
polymer formed from a monomer polymerized with a free radical initiator
in situ on the substrate. The resulting composite membrane has
essentially the same porous configuration as the porous substrate. It
is disclosed that the presence of a polymerization initiator and a
cross-linking agent are necessary in order to effect the desired
polymerization and cross-linking in situ and thereby to obtain the
desired porous configuration of the membrane product, i.e., little or
no blockage of the pores, because the pores are large.
US-A-4,119,581 discloses a method for producing an ion-exchange
non-porous membrane from a solution of a thermoplastic polymer (fluoro
carbons are mentioned) and monomers which is cast in sheet form and,
when exposed to heat or gamma radiation, becomes polymerized to form an
interpenetrating network of polymer chains. The monomers used to form
the ion exchange capacity of the membrane are a diamine and a dihalide.
They undergo the Menshutkin reaction to produce inherently ionic
polymers.
US-A-4,302,334 discloses a process for making a microporous membrane
from a casting solution of a hydrophobic polyvinylidene fluoride
preferably of high crystallinity and a vinyl acetate polymer. The
resulting polymer blend solution is cast on a substrate in the usual
way and coagulated to form a porous membrane. The polyvinyl acetate is
then hydrolyzed to polyvinyl alcohol, a hydrophilic polymer.
US-A 4,012,324 discloses a method for making porous membranes from a
solution of a polyelectrolyte, a matrix polymeric mixture and a
cross-linking agent, e.g., an epoxy. Cross-linking is effected by
heating.
The crosslinking groups can be a part of either the matrix polymer or
the polyelectrolyte reacting with one another on the same chain.
Further, in some formulations a chemical crosslinking reaction can take
place between the crosslinking agent and either the matrix polymer or
the polyelectrolyte. However, in contrast to the present invention to
be described below according to US-A-4 012 324 the hydrophilic polymer
is polymerized beforehand and not in the casting solution and the
crosslinking step only is carried out after the drying of the cast
membrane film.
EP-A-0 261 734 discloses a process for the preparation of hydrophilic
membranes by coagulation of a solution of at least one hydrophobic
polymer and at least one hydrophilic polymer in a suitable solvent
medium on a coagulation medium. A mixed solution of the two polymers is
coagulated, dried at higher temperatures and the hydrophilic polymer
component is thereafter thermally or chemically crosslinked.
US-A-4 752 624 discloses a process for the preparation of a non-porous
interpenetrating polymer network membrane, composed of a hydrophilic
and a hydrophobic component.
US-A-4 787 976 discloses a protein non-adsorptive semipermeable
filtration membrane comprising an interpenetrating polymer network. The
membrane is prepared from a casting solution comprising a hydrophilic
urethane prepolymer polymerizable upon contact with a coagulating
liquid, a polymer that will not react readily with the prepolymer and
that is substantially insoluble in said coagulating bath, and solvent.
The solution is cast as a film on a support, the film then being
immersed in a coagulation bath which effects polymerisation of the
prepolymer and the formation of the pores. No annealing treatment is
mentioned.
The prior art discloses a variety of ways to produce porous hydrophilic
membranes from materials that are primarily hydrophobic. None discuss a
two step process for producing hydrophilic porous membranes by
polymerizing a hydrophilic monomer in a solution of a hydrophobic
polymer, subsequently casting a membrane from the solution; and then
annealing to produce a hydrophilic membrane.

SUMMARY OF THE INVENTION
The present invention provides a water wettable porous membrane formed
of a porous semi-interpenetrating polymer network of a hydrophobic
fluorinated hydrocarbon polymer and a crosslinked hydrophilic polymer
which is formed from a polymerized hydrophilic monomer, the
interpenetrating polymer network being formed by synthesising and
crosslinking the hydrophilic polymer in the immediate presence of the
hydrophobic polymer, wherein the entire surface of the membrane
including the pore surface is formed from said hydrophobic polymer in
semi-crystalline form and said hydrophilic polymer. The membrane is
formed from a solution of the hydrophobic polymer, a monomeric
composition which is a precursor to the hydrophilic polymer, a
cross-linking agent for the monomeric composition and a photoinitiator.
The solution is cast on a substrate and is exposed to ultraviolet
radiation in order to polymerize and cross-link the monomeric
composition in order to form an semi-interpenetrating polymeric network
in solution. The resultant polymeric solution is coagulated. During
coagulation, pores are formed in the membrane and most of the solvent
is removed. The remaining solvent is washed out of the membrane in a
subsequent washing step. The membrane is then annealed by a heat
treatment. Upon annealing, the crystallinity of the hydrophobic polymer
is increased and thereby the water wettability of the entire surface of
said membrane, including the pore surface is increased. The membranes
of this invention can be used without prewetting in filtration
processes which process aqueous solutions, such as in the
pharmaceutical industry or the electronics industry.
DESCRIPTION OF SPECIFIC EMBODIMENTS
The present invention provides a porous membrane having a hydrophilic
surface and novel morphology according to claim 1. The membrane is
formed from a semi-interpenetrating network of a hydrophobic polymer
and a hydrophilic polymer produced by polymerizing and crosslinking a
hydrophilic monomer in the immediate presence of the hydrophobic
polymer. The surface of the membrane including the pore surface is
formed from a mixture of hydrophobic and hydrophilic polymers which
renders the overall membrane surface hydrophilic. The hydrophobic
polymer is rendered crystalline and the surface is rendered hydrophilic
as a result of the annealing step described below. In one aspect of
this invention a membrane having a hydrophilic surface and an average
pore size within the range of about 0.01 to 0.1 µm (.01 to .1 microns)
is provided.
The hydrophobic polymers useful in the present invention are those
which are rendered crystalline under the annealing conditions set forth
below. Suitable hydrophobic polymers include fluorinated polymers such
as polyvinylidene fluoride or the like.
In accordance with this invention, there is provided a porous membrane
formed of an semi-interpenetrating polymeric network having the desired
bulk properties and a hydrophilic surface. The semi-interpenetrating
network of the hydrophobic polymer and hydrophilic polymer is formed
from a solution of the hydrophobic polymer, a monomeric composition
precursor of the hydrophilic polymer, a cross-linking agent for the
monomeric composition and a photoinitiator. The solution is cast onto
an appropriate substrate, after which the monomeric composition is
polymerized and cross-linked in the cast film by exposure to
ultraviolet radiation. The irradiated film then is coagulated to a
porous membrane. During coagulation, most of the solvent leaves the
membrane; the rest is washed out afterward. The porous membrane is then
annealed to make it hydrophilic.
Many monomeric compositions can be utilized herein as long as it is
capable of being polymerized by free radical polymerization and can be
cross-linked to form a hydrophilic surface on the membrane product.
Representative suitable polymerizable monomers include hydroxylalkyl
acrylates or methacrylates including 1-hydroxyprop-2-yl acrylate and
2-hydroxyprop-1-yl acrylate, hydroxypropyl methacrylate,
2,3-dihydroxypropyl acrylate, hydroxyethylacrylate,
hydroxyethylacrylate, hydroxyethyl methacrylate, N-vinyl pyrollidone or
the like or mixtures thereof.
The particular solvent employed for the hydrophobic polymer and the
monomeric composition will depend upon the particular monomeric
composition employed and upon the particular hydrophobic polymer
utilized to form the porous membrane. All that is necessary is that the
monomer, the crosslinker, the initiator, the hydrophobic polymer and
the interpenetrating network dissolve in the solvent. Representative
suitable solvents include dimethylacetamide (DMAC) or
N-methylpyrollidone (NMP). Generally the polymerizable monomer is
present in the solution at a concentration between about 3 and about
15%, preferably between 5 and 10% based upon the weight of the
solution. The hydrophobic polymer is present in the solution at a
concentration between about 10 and about 20%, preferably between about
12 and about 18%, based upon the weight of the solution. The
hydrophobic polymer provides mechanical strength to the membrane while
the hydrophilic polymer provides desirable surface characteristics to a
membrane such as protein rejection. If too little hydrophilic polymer
is utilized, membrane wettability is undesirably reduced. Excessive
hydrophilic polymer results in undesirable gelling and polymer
separation. Thus, increased amounts of hydrophilic monomer cannot be
utilized as a means to improve the wettability of the membrane surface.
Suitable initiators and cross-linking agents for the monomers set forth
above are well-known in the art. For example, when utilizing acrylates
as the polymerizable monomer, suitable polymerizable initiators include
benzoin ethers such as isopropyl benzoin ether and butyl benzoin ether;
benzophenones such as benzophenone and Michler's ketone and
acetophenones such as 2-hydroxy-2-methyl phenyl propanone, α, α
dimethoxy- α-phenyl acetophenone and α,- α-, dimethoxy α-hydroxy
acetophenone or the like. When utilizing acrylates or methacrylates as
the polymerizable monomer, suitable cross-linking agents include
difunctional acrylates, or methacrylates such as tetraethylene, glycol
diacrylate or dimethacrylate. The cross-linking agent generally is
present in an amount of between about 5% and about 30% by weight,
preferably between about 6% and about 25% by weight based on the weight
of the polymerizable monomer. Greater amounts of cross-linking agent
can be used but no significant advantage is gained thereby. The
polymerization initiator is present in an amount between 2% and 8% by
weight, preferably between 3% and 5% by weight based upon the weight of
the polymerizable monomer.
After the solution of hydrophobic polymer and monomeric composition is
formed, it is cast on a substrate such as glass, polyester or nonwoven
fabric or the like to form a film of a thickness generally between 5
and about 15 µm (microns). The film is exposed to ultraviolet radiation
of low intensity so as to avoid excessive reaction rates which produce
non-uniform polymerization. Typical radiation intensities are between
about .2 and about 2 mW/cm² at a wave length between about 350 and
about 400 nm for a period of between about 10 and about 120 seconds in
order to initiate free radical polymerization and cross-linking of the
monomeric composition thereby to effect formation of an
semi-interpenetrating polymeric network in solution. The cast film of
semi-interpenetrating polymeric network solution is coagulated by
putting it into a liquid in which the network is insoluble and with
which the solvent is miscible, such as water, alcohols, alcohol-water
mixtures, or acetone-water mixtures. Pores form in the membrane during
coagulation.
Surprisingly, it has been found that the last step of the process of
this invention, i.e., the annealing step, causes the membrane to become
easily wettable. That is, the annealed membrane is entirely wettable
with an aqueous solution within a time period of less than about 1
minute. Annealing is conducted at a temperature between about 130°C and
150°C for a time between about 20 minutes and 60 minutes. Preferably
annealing is conducted while the membrane is restrained to prevent
shrinkage. As a result of annealing, the hydrophobic polymer becomes
more crystalline. The hyrophilic polymer is excluded from either of
these crystalline regions. It covers them and renders the material
surface hydrophilic. The hydrophobic polymer is referred to herein as
"semi-crystalline". By the term "semi crystalline" as used herein is
meant polymeric solids intermediate between true crystals and amorphous
structures. Typically they give X-ray patterns with several maxima,
with lines sharper than those of liquids.
The membranes of this invention can have a very small maximum pore size
of between about 0.01 to 0.1 µm (0.01 to 0.1 microns). Generally,
membranes having a pore size this small suffer from serious flux
reductions since the ratio of the decreased flux observed between a
small pore membrane and a large pore membrane is proportional to the
fourth power of the pore size ratio. These small pore membranes of this
invention have a flux of at least about 407.5 hl/m²/d (1000
gal/ft²/day) at 172 kPa (25 psi). Despite these very small maximum pore
sizes, these membranes have excellent flux characteristics. These very
small maximum pore size membranes have isopropyl alcohol bubble
pressures or points as defined herein of between about 275 kPa (40
pounds per square inch (psi)) and about 963 kPa (140 psi). Water flux
of these membranes is above about 1426 hl/m²/d (3500 gallons per square
foot per day) at 172 kPa (25 psi) (gfd at 25 psi) for these membranes
whose bubble point is about 275 kPa (40 psi), decreasing to above about
407.5 hl/m²/d (1000 gfd) at 172 kPa (25 psi) for membranes whose bubble
point is about 550 kPa (80 psi), and also above about 407.5 hl/m²/d
(1000 gfd) at 172 kPa (25 psi) for membranes whose bubble point is
about 963 kPa (140 psi) as shown in the figure.
The following examples illustrate the present invention and are not
intended to limit the same.

EXAMPLE I
A solution was formed from 5 grams of hydroxypropylacrylate monomer,
1.3 grams of tetraethyleneglycoldiacrylate, 4 grams of a photoinitiator
comprising Darocure 1173 ( α, α-dimethoxy-α-hydroxy acetophenone)
available from E. Merck Corporation which were added to 89.7 grams of a
14 weight % solution of poly(vinylidine fluoride) and 5 weight %
lithium chloride (LiCl) in dimethylacetamide (DMAc). The solution was
cast in a film approximately 254 µm (10 mils) thick on a glass plate
and thereafter exposed to ultraviolet radiation wavelength of maximum
emission of 365 nm at an intensity of between about 0.2 and 0.6 mW/cm²
for 80 seconds. The resultant cast film was coagulated into a membrane
by placing it in a mixture of 1 volume acetone and 3 volumes water.
This membrane was annealed at 130° for 30 min under restraint. After
annealing, its flux was 802.8 hl/m²/d (1970 gal/ft²/day) at 172 kPa (25
psi) and its bubble point was 495 kPa (72 psi). It contained 3.5% by
weight poly(hydroxypropyl acrylate-co-tetraethylene glycol diacrylate).
The membrane wet completely in less than 30 seconds and retained 90% of
0.07 µm (.07u) beads when an aqueous suspension of these beads was
filtered through it.
The flux of the membranes was measured using ultrafiltration cells,
graduate cylinders, and stopwatches. 43 mm diameter disks were cut from
the membranes, wet with water, and positioned in the cells The cells
were filled with water and pressurized to 172 kPa (25 pounds per square
inch (psi)) to drive the water across the membrane. A stopwatch was
started as the pressure was applied. A reasonable volume of water,
typically 10-30 ml, was collected; the volume in milliliters and the
time to collect it, in minutes, were recorded. Milliliters per minute
per square centimeter of membrane was converted to gallons per foot²
per day (gfd).
The bubble point of the membrane is measured from the pressure required
to displace isopropyl alcohol (IPA) from an IPA-wet membrane. A
fluid-wet membrane will allow air to pass through it when the applied
air pressure exceeds the capillary attraction of the fluid to the pore.
The relation between the size of a fluid-wet cylindrical pore and the
air pressure required to empty it (P, the bubble pressure for that
cylindrical pore) is: D = 4 γ cos Ѳ/P where D is the diameter of the
pore, Ѳ is the contact angle, and γ is the surface tension of the
wetting liquid. When measured bubble pressure can be empirically
correlated to the size of real membrane pores it provides readily
obtained estimates of the sizes of real, noncylindrical pores. One
empirical method used to correlate bubble pressure with the pore sizes
of the membranes of this invention is to determine the smallest
particles that are retained by the membranes. The membrane are
challenged with sub-micron size latex beads and the fraction of beads
retained by the membrane are measured. If substantially all (>90%) of
the beads are retained by the membrane, the largest pores are smaller
than the average diameters of the latex beads.
The bubble point of the membrane is measured using a device similar to
that described by Dadenhop et al, Membrane Science and Technology, J.E.
Flynn, Ed., Plenum Press (1970). The membrane is positioned in the
device and wet with IPA. Increasing air pressure is applied to the skin
side of the membrane rapidly enough to prevent IPA-induced changes in
morphology. Typically the applied pressure starts at 0 kPa (0 psi) and
reaches 688 kPa (100 psi) within 2 minutes of wetting with IPA. The
applied pressure sufficient to cause measurable amounts of air flow
through the membrane is taken to be the bubble pressure of bubble
point, P. In this variation of the bubble point test, P estimates the
size of the largest membrane pores.
Thus the largest pores of the membrane described in Example 1 are
smaller than about 0.07 µm (.07 u), which corresponds to a bubble
pressure (bubble point) of about 495 kPa (72 psi). As shown in the
attached Figure.

EXAMPLE II
This example shows the unexpected change in the properties of the
membranes on annealing. Five grams of hydroxylpropyl acrylate (HPA), 1
gram of tetraethylene glycol deacrylate (TEGDA), and 4 grams of
Darocure 1173 were added to 91.0 grams of a solution containing 14%
poly(vinylidene fluoride) 5% LiCl in DMAc. Portions of the solution
were cast separately into films 254 µm (10 mils) thick onto a glass
plate. Each film was exposed to ultraviolet light at an intensity of
between about 0.2 and 0.6 mW/cm² (wavelength of maximum emission was
365 mn.) for 15 seconds immediately after casting. The irradiated films
were coagulated with various non-solvents, washed, dried, fixed in a
frame and annealed as summarized in Table I. Properties of these
membranes before and after annealing are also given in Table I.

EXAMPLE III
This example shows that the membranes of the invention are resistant to
protein adsorption. Ten grams of HPA, 1 gram of TEGDA, 4 grams of
Darocure 1173 initiator were added to 85.0 grams of a solution
containing 14% poly(vinylidine fluoride) 5% LiCl in DMAc. The solution
was cast in a film 254 µm (10 mils) thick onto a glass plate and
immediately exposed to ultraviolet light as in Example II for 2
minutes. The irradiated film was coagulated with a mixture of 1 part
acetone and 3 parts water, washed, dried, fixed in a frame and annealed
at 130° for 30 minutes. The membrane wet completely in less than 30
seconds. It had a flux of 1589 hl/m²/d (3900 gfd) at 172 kPa (25 psi)
and a bubble point of 454 kPa (66 psi). The protein binding
characteristics of the membrane were estimated by the following
procedure. Samples of this membrane were soaked in 1% bovine serum
albumin (BSA) overnight. Also, samples of nonadsorptive hydrophilic
Durapore (TM), and highly adsorptive cellulose esters were soaked in
BSA overnight. All unbound albumin was rinsed from the membranes. The
samples were then soaked in Ponceau S red dye for 1 hour to color and
bind protein, rinsed with 5% acetic acid to remove excess dye, rinsed
again with distilled water, and allowed to dry. The amount of protein
bound to the membranes was estimated qualitatively from the amount of
color left after rinsing and drying. The membrane prepared in this
example was visually similar to the nonadsorptive hydrophilic Durapore
(TM) membranes in that both membranes were very faintly pink. The
highly adsorptive cellulose ester membranes were much more intensely
colored.

EXAMPLE IV
Ten grams of HPA, 0.6 grams of TEGDA, and 4 grams of Darocure 1173 were
added to 84.9 grams of a solution containing 14% poly(vinylidine
fluoride) 5% LiCl in DMAc. The solution was cast in a film 254 µm (10
mils) thick onto a glass plate and immediately exposed to ultraviolet
light as in Example II for 15 seconds. The irradiated film was
coagulated with water, washed, dried, fixed in a frame and annealed at
130° for 30 minutes. The membrane wets completely in less than 30
seconds. It had a flux of 412 hl/m²/d (1010 gfd) at 172 kPa (25 psi)
and a bubble point of 605 kPa (88 psi).