U.S. patent application number 10/780542 was filed with the patent office on 2005-03-03 for graft polymeric membranes and ion-exchange membranes formed therefrom.
This patent application is currently assigned to Ballard Power Systems Inc.. Invention is credited to Choudhury, Biswajit, Steck, Alfred E., Stone, Charles.
Application Number | 20050049319 10/780542 |
Document ID | / |
Family ID | 34222403 |
Filed Date | 2005-03-03 |
United States Patent
Application |
20050049319 |
Kind Code |
A1 |
Stone, Charles ; et
al. |
March 3, 2005 |
Graft polymeric membranes and ion-exchange membranes formed
therefrom
Abstract
Graft polymeric membranes and methods for making graft polymeric
membranes have one or more trifluorovinyl aromatic monomers that
are radiation graft polymerized to a polymeric base film. The
membranes comprise a polymeric base film to which has been graft
polymerized substituted .alpha.,.alpha.,.beta.-trifluorostyrene
and/or .alpha.,.alpha.,.beta.-tri- fluorovinylnaphthylene monomers,
which are activated towards graft polymerization. As ion-exchange
membranes, the membranes are suitable for use in electrode
apparatus, including membrane electrode assemblies in, for example,
fuel cells. The membranes can also be crosslinked.
Inventors: |
Stone, Charles; (West
Vancouver, CA) ; Steck, Alfred E.; (West Vancouver,
CA) ; Choudhury, Biswajit; (Kingston, CA) |
Correspondence
Address: |
SEED INTELLECTUAL PROPERTY LAW GROUP PLLC
701 FIFTH AVE
SUITE 6300
SEATTLE
WA
98104-7092
US
|
Assignee: |
Ballard Power Systems Inc.
Burnaby
CA
|
Family ID: |
34222403 |
Appl. No.: |
10/780542 |
Filed: |
February 17, 2004 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
10780542 |
Feb 17, 2004 |
|
|
|
09934176 |
Aug 21, 2001 |
|
|
|
6723758 |
|
|
|
|
09934176 |
Aug 21, 2001 |
|
|
|
09503760 |
Feb 14, 2000 |
|
|
|
09503760 |
Feb 14, 2000 |
|
|
|
08967960 |
Nov 12, 1997 |
|
|
|
6359019 |
|
|
|
|
Current U.S.
Class: |
521/27 ; 521/31;
521/32; 521/33; 525/359.1; 525/416 |
Current CPC
Class: |
C08F 255/023 20130101;
Y02E 60/50 20130101; H01M 8/1088 20130101; B01D 71/82 20130101;
B01D 71/36 20130101; B01D 2323/38 20130101; B01D 71/32 20130101;
Y02P 70/50 20151101; B01D 67/0093 20130101; B01D 71/28 20130101;
C08F 255/02 20130101; C08F 291/18 20130101; C08J 7/16 20130101;
C08J 5/225 20130101; H01M 8/1039 20130101; H01M 8/1023 20130101;
H01M 8/1072 20130101; C08F 291/00 20130101; C08F 259/08 20130101;
C08J 2327/12 20130101 |
Class at
Publication: |
521/027 ;
521/031; 521/032; 521/033; 525/359.1; 525/416 |
International
Class: |
C08J 005/20 |
Claims
1. A membrane comprising a polymeric base film to which has been
graft polymerized a monomer selected from the group consisting of
monomers of formula (I) 6and formula (II) 7where A.sub.1, A.sub.2,
and B.sub.1, B.sub.2 are independently selected from the group of
consisting of: hydrogen, lower alkyl, lower fluoroalkyl, cyclic
alkyl, cyclic amine, cyclic ether, cyclic thioether, Ar, wherein Ar
is other than Ph when one of A.sub.1 and A.sub.2 is hydrogen,
CH(X)Ph, where X is selected from the group consisting of hydrogen,
fluorine, lower alkyl, lower fluoroalkyl and Ph, PRR' and
P(OR)(OR'), where R and R' are independently selected from the
group consisting of lower alkyl, cyclic alkyl and Ph, and wherein
at least one of substituents A.sub.1, A.sub.2, B.sub.1 and B.sub.2
is other than hydrogen.
2-36. (Cancelled).
37. A membrane comprising a polymeric base film with grafted chains
comprising monomer units selected from the group consisting of
monomer units of formula (IV) 8and formula (V) 9where A.sub.1,
A.sub.2, and B.sub.1, B.sub.2 are independently selected from the
group consisting of: hydrogen, lower alkyl, lower fluoroalkyl,
cyclic alkyl, cyclic amine, cyclic ether, cyclic thioether, Ar,
wherein Ar is other than Ph when one of A.sub.1 and A.sub.2 is
hydrogen, CH(X)Ph, where X is selected from the group consisting of
hydrogen, fluorine, lower alkyl, lower fluoroalkyl and Ph, PRR' and
P(OR)(OR'), where R and R' are independently selected from the
group consisting of lower alkyl, cyclic alkyl and Ph, and wherein
at least one of substitutents A.sub.1, A.sub.2, B.sub.1 and B.sub.2
is other than hydrogen.
38-68. (Cancelled).
69. A method of preparing a membrane, the method comprising graft
polymerizing to a polymeric base film a monomer selected from the
group consisting of monomers of formula (I) 10and formula (II)
11wherein, in the selected monomer, at least one of substitutents
A.sub.1, A.sub.2, and B.sub.1, B.sub.2 is a non-hydrogen
substituent that activates said monomer with respect to said graft
polymerization, and said method further comprises: introducing a
sulfonate group into at least a portion of said graft polymerized
monomer units; and converting at least a portion of said
non-hydrogen substituents to substituents that are deactivating
with respect to desulfonation.
70. A method of preparing a membrane, said method comprising graft
polymerizing to a polymeric base film a monomer selected from the
group consisting of monomers of formula (I) 12and formula (II)
13where A.sub.1, A.sub.2, and B.sub.1, B.sub.2 are independently
selected from the group consisting of: hydrogen, lower alkyl, lower
fluoroalkyl, cyclic alkyl, cyclic amine, cyclic ether, cyclic
thioether, Ar, wherein Ar is other than Ph when one of A.sub.1 and
A.sub.2 is hydrogen, CH(X)Ph, where X is selected from the group
consisting of hydrogen, fluorine, lower alkyl, lower fluoroalkyl
and Ph, PRR' and P(OR)(OR'), where R and R' are independently
selected from the group consisting of lower alkyl, cyclic alkyl and
Ph, and wherein at least one of substitutents A.sub.1, A.sub.2,
B.sub.1 and B.sub.2 is other than hydrogen.
71-80. (Cancelled).
81. A method of preparing a membrane comprising graft polymerizing
to a polymeric base film a monomer selected from the group
consisting of monomers of formula (I) 14and formula (II) 15where
A.sub.1 and B.sub.1 are independently selected from the group
consisting of: PRR', P(OR)(OR'), and SR, where R and R' are
independently selected from the group consisting of lower alkyl,
cyclic alkyl and Ph, and A.sub.2 is selected from the group
consisting of A.sub.1 and hydrogen, and B.sub.2 is selected from
the group consisting of B.sub.1 and hydrogen, the method further
comprising subjecting at least a portion of one of the PRR', the
P(OR)(OR') and the SR groups to oxidation.
82-85. (Cancelled).
Description
CROSS-REFERENCE TO RELATED APPLICATION(S)
[0001] This application is a continuation of U.S. application Ser.
No. 09/934,176 filed Aug. 21, 2001 (allowed); which is a
continuation-in-part of U.S. application Ser. No. 09/503,760 filed
Feb. 14, 2000 (abandoned); which is a continuation-in-part of U.S.
application Ser. No. 08/967,960 filed Nov. 12, 1997 (U.S. Pat. No.
6,359,019), all of which are incorporated herein by reference in
their entireties. The '760 and '960 applications describe polymeric
compositions comprising a polymeric base film to which has been
radiation grafted one or more of a variety of substituted
trifluorovinyl aromatic monomers. These compositions are suitable
use as membranes, particular as ion-exchange membranes.
FIELD OF THE INVENTION
[0002] The present invention relates to graft polymeric membranes
in which one or more trifluorovinyl aromatic monomers are radiation
graft polymerized to a polymeric base film, and methods for making
same wherein the grafted polymeric chains are modified to
incorporate ion-exchange groups. The resultant membranes are useful
in dialysis applications, and particularly in electrochemical
applications, for example as membrane electrolytes in
electrochemical fuel cells and electrolyzers.
BACKGROUND OF THE INVENTION
[0003] The preparation of graft polymeric membranes by radiation
grafting of a monomer to a polymeric base film has been
demonstrated for various combinations of monomers and base films.
The grafting of styrene to a polymeric base film, and subsequent
sulfonation of the grafted polystyrene chains has been employd to
prepare ion-exchange membranes.
[0004] U.S. Pat. No. 4,012,303, reports the radiation grafting of
.alpha.,.beta.,.beta.-trifluorostyrene (TFS) to polymeric base
films using gamma ray co-irradiation, followed by the introduction
of various ion-exchange substituents to the pendant aromatic rings
of the grafted chains. With co-irradiation, since the TFS monomer
is simultaneously irradiated, undesirable processes such as monomer
dimerization and/or independent homopolymerization of the monomer
may occur in competition with the desired graft polymerization
reaction.
[0005] U.S. Pat. No. 4,012,303 also reports that the TFS monomer
may be first sulfonated and then grafted to the base film. Thus,
the introduction of ion-exchange groups into the membrane can be
done as part of the grafting process, or in a second step.
[0006] More recently, the grafting of TFS to pre-irradiated
polymeric base films, followed by the introduction of various
substituents to the pendant aromatic rings of the grafted chain has
been reported in U.S. Pat. No. 4,605,685. Solid or porous polymeric
base films, such as for example polyethylene and
polytetrafluoroethylene, are pre-irradiated and then contacted with
TFS neat or in solution. Pre-irradiation is reportedly a more
economic and efficient grafting technique, reportedly giving a
percentage graft of 10-50% in reaction times of 1-50 hours.
Aromatic sulfonation, haloalkylation, amination, hydroxylation,
carboxylation, phosphonation and phosphorylation are among the
reactions subsequently employd to introduce ion-exchange groups
into the grafted polymeric chains. Levels of post-sulfonation from
40% to 100% are reported.
[0007] In either case the prior art TFS-based grafted membranes
incorporate statistically a maximum of one functional group per
monomer unit in the grafted chain. Further, they typically
incorporate only one type of functional group as substituents on
the pendant aromatic rings in the grafted chains.
[0008] In the present membranes, one or more types of substituted
TFS monomers and/or substituted
.alpha.,.beta.,.beta.-trifluorovinylnaphthyle- ne (TFN) monomers
are grafted to polymeric base films, the substituents being
selected to offer particular advantages, for example:
[0009] (a) Substituted TFS and/or TFN monomers that are activated
have increased reactivity in the grafting reaction facilitating
graft polymerization. By "activated" it is meant that either the
percentage graft yield of the graft polymerization reaction is
increased, or that the rate of the reaction is increased, in
reactions employing the substituted monomers relative to reactions
employing unsubstituted monomers.
[0010] (b) Substituted TFS and/or TFN monomers in which the
substituents are activating with respect to the grafting reaction,
but which can be converted so as to be de-activating with respect
to subsequent reactions to introduce, for example, ion-exchange
functionality into the grafted chains, and thereby permit the
introduction of ion-exchange groups that are more stable under
certain conditions.
[0011] (c) Substituted TFS and/or TFN monomers in which the
substituents are activating with respect to the grafting reaction,
but which can be converted so as to be de-activating after
introduction of ion-exchange functionality into the grafted
chains.
[0012] (d) Grafted chains comprising monomer units with more than
one aromatic ring permit the introduction of more than one
ion-exchange group per grafted monomer unit, enabling the
achievement of higher ion-exchange capacities at lower percentage
grafts than in prior art grafted polymeric membranes.
[0013] (e) Substituted TFS and/or TFN monomers in which the
substituents are precursors to ion-exchange groups may be
transformed to ion-exchange groups after the grafting reaction, and
can facilitate the introduction of more than one type of
ion-exchange group into the grafted chains, for example, so that
both cation and anion-exchange groups may be incorporated in a
membrane.
[0014] (f) Substituted TFS and/or TFN monomers in which the
substituents contain functionality that can be further reacted to
allow for the preparation of crosslinked graft polymeric membranes
that may display, for example, greater dimensional stability under
certain conditions than similar graft polymeric membranes that are
not crosslinked.
SUMMARY OF THE INVENTION
[0015] A graft polymeric membrane is provided in which one or more
types of trifluorovinyl aromatic monomers are graft polymerized to
a polymeric base film. In one embodiment, the membrane comprises a
polymeric base film to which has been graft polymerized a monomer
(meaning at least one type of monomer) selected from the group
consisting of monomers of the following formulae (I) and (II):
1
[0016] where A.sub.1, A.sub.2, and B.sub.1, B.sub.2 are
independently selected from the group consisting of hydrogen, lower
alkyl, lower fluoroalkyl, cyclic alkyl, cyclic amine, cyclic ether,
cyclic thioether, Ar (with the proviso that where one of A.sub.1
and A.sub.2 is hydrogen, Ar is other than Ph), CH(X)Ph, where X is
selected from the group consisting of hydrogen, fluorine, lower
alkyl, lower fluoroalkyl and Ph, PRR' and P(OR) (OR'), where R and
R' are independently selected from the group consisting of lower
alkyl, cyclic alkyl and Ph, and where R and R' can be the same or
different); and, wherein A.sub.1, A.sub.2, B.sub.1, and B.sub.2 can
be the same or different, provided that in the selected monomer at
least one of the substituents A.sub.1, A.sub.2, B.sub.1, B.sub.2 is
other than hydrogen. In other words there is at least one of the
foregoing substituted monomers employd in the graft polymerization
reaction. The selected substituted monomer(s) may have one or two
non-hydrogen substituents.
[0017] Of the listed alkyl substituents, lower alkyl and cyclic
alkyl are generally preferred, with methyl (Me) being most
preferred. Thus, membranes where one or both substituents on the
selected monomer of formula (I) or (II) are Me are particularly
preferred, with para-Me being the most desirable substitution
position in formula (I)). In these embodiments the base film
preferably comprises poly(ethylene-co-tetrafluo- roethylene).
[0018] In embodiments in which a polymeric base film has been graft
polymerized with a monomer of formula (I) in which A.sub.1 is Ar
and A.sub.2 is hydrogen, Ar is preferably a fused polycyclic
aromatic with two fused rings, biphenyl, or a heteroaromatic group
with at least one heteroatom that is preferably nitrogen, oxygen or
sulfur. If the heteroaromatic group contains more than one
heteroatom, the heteroatoms may be the same or different. If one of
the heteroatoms is nitrogen it may be advantageously N-alkylated or
N-benzylated for certain membrane applications. Monocyclic
heteroaromatics are generally preferred over polycyclic
heteroaromatics.
[0019] The above graft polymeric membrane may comprise a single
monomer, whereby the grafted chains are homopolymeric, or may
comprise more than one monomer such that the grafted chains are
copolymeric. For example, the graft polymeric membrane may comprise
more than one monomer of formula (I) having different A.sub.1
and/or A.sub.2 substituents, more than one monomer of formula (II)
having different B.sub.1 and/or B.sub.2 substituents, more than one
monomer of either formula (I) or formula (II) having the same
substituents located at different positions, or monomers of both
formula (I) and (II), such that the grafted chains are
copolymeric.
[0020] In another embodiment of the present graft polymeric
membrane, the membrane comprises a polymeric base film to which has
been graft polymerized, with the foregoing monomers, a monomer of
the following formula (III): 2
[0021] where D is selected from the group consisting of hydrogen,
fluorine, CF.sub.3, CF.sub.2H, CF.dbd.CF.sub.2, SO.sub.2F and
SO.sub.3.sup.-M.sup.+.
[0022] Embodiments of the present graft polymeric membrane comprise
a polymeric base film with grafted chains comprising monomer units
selected from the group consisting of monomer units of the
following formulae (IV) and (V), wherein at least a portion of the
monomer units further optionally comprise at least one ion-exchange
substituent, in which case the membrane is an ion-exchange
membrane: 3
[0023] where, as before, A.sub.1, A.sub.2, and B.sub.1, B.sub.2 are
independently selected from the group consisting of hydrogen, lower
alkyl, lower fluoroalkyl, cyclic alkyl, cyclic amine, cyclic ether,
cyclic thioether, Ar (with the proviso that where one of A.sub.1
and A.sub.2 is hydrogen, Ar is other than Ph), CH(X)Ph, where X is
selected from the group consisting of hydrogen, fluorine, lower
alkyl, lower fluoroalkyl and Ph, PRR' and P(OR) (OR'), where R and
R' are independently selected from the group consisting of lower
alkyl, cyclic alkyl and Ph, and where R and R' can be the same or
different); and wherein A.sub.1, A.sub.2, B.sub.1, and B.sub.2 can
be the same or different, provided that at least one of the
substituents A.sub.1, A.sub.2 is other than hydrogen. The foregoing
membranes may be formed by grafting monomers to a polymeric base
film, or by grafting to some other form of polymeric substrate and
then forming the grafted material into a membrane. In some
embodiments of the ion-exchange membranes, statistically at least
50% of the monomer units in the grafted chains have at least one
ion-exchange substituent per monomer unit. In other embodiments at
least a portion of the monomer units comprise more than one
ion-exchange substituent, and/or portion of the grafted chains may
comprise at least two different types of ion-exchange groups, which
may even include both anion and cation exchange groups. The
ion-exchange substituent most typically incorporated is a sulfonate
or sulfonic acid group.
[0024] In preferred embodiments one or both substituents of the
monomer units of formulae (IV) or (V) are CH(X)Ph, where X is
selected from the smaller group consisting of hydrogen, fluorine,
Me and Ph, or Me, with para-Me being the most desirable
substitution position for the Me group in units of formula (IV). In
these embodiments, again, the base film preferably comprises
poly(ethylene-co-tetrafluoroethylene).
[0025] The grafted chains of ion-exchange membrane may further
comprise additional monomer units, such as for example, units of
formula (VI): 4
[0026] where D is selected from the group consisting of hydrogen,
fluorine, CF.sub.3, CF.sub.2H, CF.dbd.CF.sub.2, SO.sub.2F and
SO.sub.3.sup.-M.sup.+.
[0027] The ion-exchange membrane may be substantially gas
impermeable. Such impermeable ion-exchange membranes may be
incorporated into an electrode apparatus such as, for example, a
membrane electrode assembly. Electrochemical fuel cells that
comprise such ion-exchange membranes are also provided. For fuel
cell applications, the polymeric base film of the ion-exchange
membrane is preferably less than 100 .mu.m thick.
[0028] In the present graft polymeric membranes or ion-exchange
membranes, at least a portion of the grafted chains may be
crosslinked.
[0029] Other membranes may be prepared from those membranes
described above by subjecting them to a reaction process selected
from the group consisting of, for example, halomethylation,
sulfonation, phosphonation, amination, carboxylation, hydroxylation
and nitration. Membranes so prepared may be useful ion-exchange
membranes or precursors to ion-exchange membranes. Methods of
preparing the present membranes and ion-exchange membranes are also
provided.
[0030] Ion-exchange membranes may be prepared by a method which
comprises graft polymerizing to a polymeric base film a monomer
selected from the group consisting of monomers of formulae (I) and
(II) described above, wherein in the selected monomer(s) at least
one of the substituents A.sub.1, A.sub.2, and B.sub.1, B.sub.2 is a
non-hydrogen substituent which activates the monomer with respect
to graft polymerization (relative to the corresponding
unsubstituted monomer). The method further comprises introducing a
sulfonate group (or other ion-exchange group) into at least a
portion of the graft polymerized monomer units and converting at
least a portion of the non-hydrogen substituents to substituents
which are deactivating with respect to desulfonation (relative to
the unsubstituted monomer unit). The conversion of the non-hydrogen
substituent to a deactivating group may be performed before or
after introduction of the sulfonate group into the grafted
units.
[0031] Some of the membranes described above may be prepared by a
method comprising graft polymerizing to a polymeric base film a
substituted monomer selected from the group consisting of monomers
of formulae (I) and (II) described above, wherein A.sub.1, A.sub.2,
and B.sub.1, B.sub.2 are as described above.
[0032] In preferred embodiments of this method, A.sub.1 and B.sub.1
are independently selected from the group consisting of:
[0033] Ar, where Ar is selected from the group consisting of
monocyclic heteroaromatics, fused polycyclic heteroaromatics, and
heteroaromatic ring assemblies having at least one nitrogen
atom);
[0034] cyclic amine; and
[0035] phosphines of the formula PRR' and phosphites of formula
P(OR) (OR'), where R and R' are independently selected from the
group consisting of lower alkyl, cyclic alkyl and Ph, and where R
and R' can be the same or different); and
[0036] A.sub.2 and B.sub.2 are hydrogen.
[0037] The method further comprises alkylating or benzylating at
least a portion of any of the nitrogen atoms of the Ar group, the
nitrogen atoms of the cyclic amine, or the phosphorus atoms of the
phosphine or phosphite.
[0038] In other embodiments where A.sub.1 and B.sub.1 are
independently selected from the group consisting of phosphines of
the formula PRR' and phosphites of formula P(OR) (OR'), where R and
R' are independently selected from the group consisting of lower
alkyl, cyclic alkyl and Ph, and where R and R' can be the same or
different), and A.sub.2 and B.sub.2 are hydrogen, the method may
further comprise the sequential steps of introducing a nitro group
into at least a portion of the monomer units of the membrane and
converting at least a portion of those nitro groups to quaternary
ammonium groups. This method optionally further comprises
subsequently converting said phosphine or phosphite to an
ion-exchange substituent.
[0039] In still another embodiment, the present method comprises
graft polymerizing to a polymeric base film a monomer selected from
the group consisting of monomers of the formulae (I) and (II)
described above, but where A.sub.1 and B.sub.1 are independently
selected from the group consisting of PRR', P(OR) (OR'), and SR,
where R and R' are independently selected from the group consisting
of lower alkyl, cyclic alkyl and Ph, and where R and R' can be the
same or different), and A.sub.2 and B.sub.2 are the same as A.sub.1
and B.sub.1 respectively or hydrogen. The method comprises the
steps of graft polymerizing the monomers to a polymeric base film,
and oxidizing at least a portion of the PRR', P(OR) (OR'), or SR
groups to produce phosphine oxides, phosphones, phosphonates,
sulfoxides, or sulfones. The method may further comprise
introducing ion-exchange substituents into at least a portion of
said monomer units, before or after the oxidation step. Where
A.sub.1 and B.sub.1 are independently selected from the group SR,
where R is selected from the group consisting of lower alkyl,
cyclic alkyl and Ph, and A.sub.2 and B.sub.2 are the same as
A.sub.1 and B.sub.1 respectively or hydrogen, the method optionally
further comprises converting at least a portion of the SR groups to
sulfonate or sulfonic acid groups.
[0040] In the above-described embodiments the substrate for the
graft polymerization is preferably a polymeric base film. However,
the polymeric substrate may be in other forms such as, for example,
a powder or in solution, or the substrate may be an oligomer in any
form. Where the substrate is not in the form of a film an
additional step will be required to form the grafted material into
a membrane. Where the substrate is in solution an additional
solvent removal step will be required.
BRIEF DESCRIPTION OF THE DRAWINGS
[0041] FIG. 1 is a plot of cell voltage as a function of current
density (expressed in mA/cm.sup.2) in an electrochemical fuel cell
employing a sulfonated membrane of p-Me-TFS grafted
poly(ethylene-co-tetrafluoroethyl- ene) (Tefzel.RTM.) and operating
on hydrogen-oxygen (plot A) and hydrogen-air (plot B).
[0042] FIG. 2 is a plot of cell voltage as a function of current
density (expressed in mA/cm.sup.2) in an electrochemical direct
methanol fuel cell employing a sulfonated membrane of p-Me-TFS
grafted poly(ethylene-co-tetrafluoroethylene) (Tefzel.RTM.)
operating on aqueous methanol-air.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0043] As used in this description and in the appended claims, in
relation to substituents of TFS and/or TFN monomers, lower alkyl
means straight chain or branched C.sub.1-C.sub.6 alkyl groups.
Lower fluroalkyl means fluorinated straight or branched
C.sub.1-C.sub.6 saturated chains, provided that the benzylic carbon
has no more than one fluorine atom attached thereto. In preferred
embodiments, the lower alkyl and lower fluoroalkyl are
C.sub.1-C.sub.4. Other haloalkyls of the same general description
may also be employed in the present membrane; however, fluorine is
preferred due to the relative lability of chlorine, bromine and
iodine to substitution, which may result in competition in other
reaction processes or in undesirable side reactions. Cyclic alkyl
means cyclic alkyls having C.sub.3-C.sub.7 rings. Cyclic amine
means nonaromatic heterocyclic 2.degree. or 3.degree. amines having
3-7 atoms in the ring (for example, piperidine, piperazine, and
quinuclidene). Cyclic ether means nonaromatic heterocyclic ethers
having 3-7 atoms in the ring (for example, tetrahydrofuran and
dioxane). Cyclic thioether means nonarornatic heterocyclic
thioethers having 3-7 atoms in the ring (for example,
tetrahydrothiophene and dithiane). Aryl group, unless otherwise
stated, means: monocyclic aromatic rings; fused polycyclic
hydrocarbons containing at least one aromatic ring (for example,
indan); fused polycyclic aromatic hydrocarbons (for example, indene
and naphthalene); aromatic ring assemblies (for example, biphenyl);
and heteroaromatics thereof, wherein the heteroatoms are nitrogen,
oxygen, or sulfur, and the heterocyclic may contain more than one
heteroatom, and may also contain different species of heteroatom
(for example, indoline, pyrrole, pyridine, oxathiazine, and purine
). The abbreviation Me is used to represent a methyl group, and Ar
is used to represent an aryl group. The abbreviation Ph is used to
represent a phenyl group. The formula SO.sub.3.sup.-M.sup.+
represents sulfonate salts, where M.sup.+ may be any suitable
counterion, such as, for example, metal cations and quaternary
ammonium ions.
[0044] Suitable substituents for TFS and/or TFN monomers that are
activating in graft polymerization reactions include, for example:
lower alkyls; lower fluoroalkyls; cyclic alkyls; cyclic amines;
cyclic ethers; cyclic thioethers; Ar groups; and, phosphines,
phosphites, and thioethers. Substituents may be coupled to the
aromatic rings of TFS and/or TFN monomers in any position. Meta-
and para-substituted monomers are preferred, with para-substituted
monomers being more preferred.
[0045] Any radiation capable of introducing sufficient
concentrations of free radical sites on and within the base
polymeric film may be employd in the preparation of the grafted
polymeric membranes described herein. For example, the irradiation
may be by gamma rays, X-rays, electron beam, or high-energy UV
radiation. Electron beam irradiation is generally preferable as the
process times are short and thus more suited to high volume
production processes. The decay of the source and typically longer
reactions times required with gamma-ray radiation tend to render it
less suitable for high volume manufacturing processes.
[0046] The polymeric base film may be pre-irradiated prior to
bringing it into contact with the monomer or monomer mixture to be
grafted or the substrate and monomer(s) may be irradiated together
(co-irradiation).
[0047] For the preparation of membranes, grafting to a polymeric
base film is generally more efficient and cost-effective than
grafting to a substrate in some other form such as a powder and
then forming a membrane from the grafted material.
[0048] The preferred polymeric base film material is dependent on
the application in which the grafted membrane is to be employd. The
base film may be a porous or dense film. Preferred substrate
materials for electrochemical applications, for example, include
hydrocarbons such as polyolefins, especially polyethylene and
polypropylene. In some applications, a perfluorinated or partially
fluorinated polymeric base film may be employd, for example,
polytetrafluoroethylene (PTFE)
poly(tetrafluoroethylene-co-hexafluoropropylene), polyvinylidene
fluoride, and preferably poly(ethylene-co-tetrafluoroethylene).
[0049] In the grafting reaction, the polymeric base film is treated
with the monomer(s) in the liquid phase, either as a neat liquid or
in a solution. Alternatively, the polymeric base film may be
treated with a mixture of liquid and vapor phase monomer(s)
(including aerosols), or with monomer(s) in the vapor phase only.
It can be advantageous to select a solvent that will cause the
solution to penetrate the base film and cause it to swell. This
facilitates grafting of the monomer(s) throughout the membrane
thickness. Preferably the irradiation and grafting process is
carried out in an inert atmosphere.
[0050] The reaction conditions may be selected so as to introduce
crosslinking between monomer units during graft polymerization or
subsequent thereto. Crosslinking may be introduced into polymeric
membranes where it is, for example, desirable to increase
dimensional stability, reduce swelling, modify chemical and/or
mechanical properties, or enhance the ion-exchange efficiency.
Methods of preparing crosslinked graft polymeric membranes are
known in the art. For example, U.S. Pat. No. 5,656,386 describes
adding a crosslinking agent to vinyl monomers to be grafted to a
membrane film, wherein the radiation grafting and crosslinking
reactions occur simultaneously.
[0051] In the present graft polymeric membranes, the constituent
monomers may be selected so as to be capable of forming crosslinks
without requiring the addition of a separate crosslinking agent. If
crosslinking is desirable, the monomer(s) preferably contains
functionality that can be crosslinked. For example, monomers having
a t-butyl group as a substituent would be less appropriate, since
such substituents do not participate readily in crosslinking
reactions. As another example, monomers having --CHF.sub.2 or
--CH(CF.sub.3).sub.2 substituents are capable of forming very
stable crosslinks, but such monomers may be so de-activating
towards polymerization that the percentage graft or rate of
grafting may fall to an undesirable level. However, such monomers
may be suitably employed in the grafting reaction provided they are
included in the monomer mixture at a relatively low mole percentage
(for example, less than about 10 mol %).
[0052] For the preparation of grafted ion-exchange membranes from
substituted TFS and/or TFN monomers, substituents that are
activating with respect to the polymerization reaction are
typically also activating towards subsequent reactions to introduce
ion-exchange groups, such as, for example, halomethylation,
sulfonation, phosphonation, amination, carboxylation, hydroxylation
(optionally combined with subsequent phosphorylation) and
nitration. Although the presence of an activating substituent may
be beneficial in that it may facilitate the introduction of the
ion-exchange group into the monomer, where the ion-exchange group
is sulfonate, for example, there may also be a disadvantage. This
is because sulfonation is a macroscopically reversible process, so
a substituent that is activating with respect to the introduction
of a sulfonate group may also make the sulfonate group less stable
under certain conditions, thereby facilitating desulfonation of the
monomer unit.
[0053] In an embodiment of the present membranes or method, the
substituted TFS and/or TFN monomers to be grafted contain a
phosphine, phosphite, or thioether substituent. These substituents
are activating with respect to the graft polymerization reaction.
Ion-exchange groups such as, for example, sulfonate, may then be
introduced into the aromatic ring of the substituted TFS and/or TFN
monomer units after graft polymerization. Then, following graft
polymerization the phosphine, phosphite or thioether groups can be
oxidized to produce phosphine oxides, phosphones, phosphonates,
sulfoxides, or sulfones. Methods suitable for such oxidations are
well known to those skilled in the art. The resulting phosphine
oxides, phosphones, phosphonates, sulfoxides and/or sulfones are
de-activating, thus making the introduced ion-exchange groups, in
particular sulfonate groups, more stable under certain
conditions.
[0054] In addition, these substituents may allow for the
introduction of additional ion-exchange functionality into the TFS
and/or TFN monomer units. For example, oxidation of the phosphite
substituent yields a phosphonate group, which on hydrolysis will
yield a cation-exchange group. Introduction of either cation or
anion-exchange groups into the substituted TFS and/or TFN monomer
units, followed by oxidation of phosphite and subsequent hydrolysis
of the phosphonate substituent, may yield TFS and/or TFN monomer
units with more than one ion-exchange group per monomer unit, on
average. As another example, the phosphine or phosphite substituent
may be alkylated or benzylated to form an anion-exchange group.
Further, employing the additional steps of nitration followed by
conversion of the nitro group to an amino group, and subsequently
to a quaternary ammonium salt may yield monomer units having two
different anion-exchange groups. As yet another example, the
thioether substituent may be converted to a sulfonate group by, for
example, the method described in U.S. Pat. No. 5,830,962. Again,
introduction of either cation or anion-exchange groups into the
substituted TFS and/or TFN monomer units, followed by alkylation or
benzylation of the phosphine, or conversion of thioether to
sulfonate, may yield TFS and/or TFN monomer units with more than
one ion-exchange group per monomer, on average, depending upon the
compatibility of the chemistry involved. Thus, the present method
allows for the preparation of amphoteric graft ion-exchange
membranes, or graft ion-exchange membranes having two different
ion-exchange groups, simply by choosing the appropriate
ion-exchange group to be introduced into the substituted TFS and/or
TFN monomer units.
[0055] In another embodiment of the present membranes and method,
the substituted TFS and/or TFN monomers to be grafted contain a
cyclic 2.degree. or 3.degree. amine or a heteroaromatic substituent
containing at least one nitrogen heteroatom. These substituents are
also activating with respect to the graft polymerization reaction.
Following graft polymerization, the cyclic amine or heteroaromatic
substituents can be N-alkylated or N-benzylated, forming
anion-exchange sites in the grafted chains. Optionally,
cation-exchange groups may also be introduced, either before or
preferably after N-alkylation or N-benzylation, resulting in
amphoteric ion-exchange membranes.
[0056] In any of the foregoing embodiments of the present membranes
and method, sulfonate ion-exchange groups can be introduced to the
monomer units in the grafted chains. For example, the membrane,
preferably swollen with an appropriate solvent to facilitate
sulfonation throughout its thickness, can be reacted with a
solution of sulfur trioxide, or with sulfur trioxide vapor alone
(or indeed an aerosol mist of sulfur trioxide). Other sulfonation
reagents can be employd, as will be familiar to those skilled in
the art, such as oleum and chlorosulfonic acid, for example.
[0057] While the foregoing methods have been described in relation
to substituted TFS and/or TFN monomers, it will be readily apparent
to those skilled in the art that the foregoing methods are readily
adaptable to other monomers. It is anticipated that other vinyl
monomers containing an aromatic ring may be suitably adaptable to
the disclosed methods. For example, in the preparation of graft
membranes employing styrenic monomers, it would still be
advantageous to employ substituents that are activating with
respect to the graft polymerization reaction, but which can be
converted to de-activating substituents in subsequent reactions
where it is desirable to introduce, for example, ion-exchange
groups that may, by this process, be more stable under certain
conditions. In addition to styrenic monomers, it is expected that
the foregoing methods will be adaptable to substituted and
unsubstituted monomers of the following basic structures: 5
[0058] where X can be H, F or Me and
[0059] if X=F, then Y=Z=H, or one of Y, Z is H and the other is
F,
[0060] if X=H, then Y=Z=H, or one of Y, Z is H and the other is F,
and
[0061] if X=Me, then Y=Z=H.
[0062] The following examples are for purposes of illustration and
are not intended to limit the invention.
EXAMPLE 1
Grafting of para-methyl-.alpha.,.beta.,.beta.-trifluorostyrene
(p-Me-TFS) to poly(ethylene-co-tetrafluoroethylene) (Tefzel.RTM.)
Film
[0063] A 2 mil (approx. 50 .mu.m) thick 7 inch.times.7 inch (18
cm.times.18 cm) piece of poly(ethylene-co-tetrafluoroethylene)
(Tefzel.RTM.) film was irradiated with a dose of 20 Mrad using a
high energy electron beam (60 kW) radiation source, in an inert
atmosphere. The irradiated base film was kept at -30.degree. C. in
an inert atmosphere prior to use. The irradiated membrane was then
exposed to neat, degassed p-Me-TFS in an inert atmosphere at
80.degree. C. for 24 hours. The p-Me-TFS grafted film was removed,
washed with toluene and dried at 60.degree. C. The percentage graft
was 79%.
EXAMPLE 2
Grafting of para-methyl-.alpha.,.beta.,.beta.-trifluorostyrene
(p-Me-TFS) to poly(ethylene-co-tetrafluoroethylene) (Tefzel.RTM.)
Film
[0064] A 2 mil (approx. 50 .mu.m) thick 15 inch.times.15 inch (38
cm.times.38 cm) piece of poly(ethylene-co-tetrafluoroethylene)
(Tefzel.RTM.) film was irradiated with a dose of 20 Mrad using a
high energy electron beam (60 kW) radiation source, in an inert
atmosphere. The irradiated base film was stored at -30.degree. C.
in an inert atmosphere prior to use. The irradiated membrane was
then exposed to neat, degassed p-Me-TFS in an inert atmosphere at
70.degree. C. for 3 hours. The p-Me-TFS grafted film was removed,
washed with toluene and dried at 60.degree. C. The percentage graft
was 67%.
Comparative Example 3
Grafting of para-methyl-.alpha.,.beta.,.beta.-trifluorostyrene
(p-Me-TFS) and and .alpha.,.beta.,.beta.-trifluorostyrene (TFS) to
poly(ethylene-co-tetrafluoroethylene) (Tefzel.RTM.) Film
[0065] Samples of 2 inch.times.2 inch (5 cm.times.5 cm) pieces of
poly(ethylene-co-tetrafluoroethylene). (Tefzel.RTM.) film were
irradiated using a high energy electron beam (10 MeV) radiation
source, in an inert atmosphere. The irradiated base films were kept
at -30.degree. C. in an inert atmosphere prior to use. Each
irradiated base film was then exposed to either neat, degassed,
p-Me-TFS or TFS in an inert atmosphere. The grafted films were then
removed, washed with ethanol and vacuum dried overnight. Table 1
summarizes the grafting reaction conditions employed and the
percentage graft attained for each grafted film.
1 TABLE 1 Film Grafting Percentage Graft thickness Dose Temperature
Time p-Me- Sample (.mu.m) (Mrad) (.degree. C.) (h) TFS TFS 1 50 20
50 24 31.8 62.8 2 50 20 80 2 33.9 42.9 3 50 40 50 24 40.7 79.2 4 50
40 80 2 43.4 58.1 5 25 20 50 24 27.7 54.7 6 25 20 80 2 29.9 37.4 7
25 40 50 24 34.7 69.7 8 25 40 80 2 37.1 49.9
[0066] As indicated in Table 1, under identical reaction conditions
the percentage graft of the graft membrane samples incorporating
substituted .alpha.,.alpha.,.beta.-trifluorostyrene monomers
(p-Me-TFS) was significantly higher than the percentage graft of
the graft membrane samples incorporating TFS monomers. For example,
the percentage graft of the p-Me-TFS samples was 25-35% greater
than the percentage graft of the corresponding TFS samples after 2
hours and 95-100% greater after 24 hours.
EXAMPLE 4
Grafting of para-methyl-.alpha.,.beta.,.beta.-trifluorostyrene
(p-Me-TFS) to poly(ethylene-co-tetrafluoroethylene) (Tefzel.RTM.)
Film and Sulfonation of the Grafted Membrane
[0067] (a) A 2 mil (approx. 50 .mu.m) thick, 7 inch.times.7 inch
(18 cm.times.18 cm) piece of poly(ethylene-co-tetrafluoroethylene)
(Tefzel.RTM.) film was irradiated with a dose of 10 Mrad using a
high energy electron beam (60 kW) radiation source, in an inert
atmosphere. The irradiated base film was kept at -30.degree. C. in
an inert atmosphere prior to use. It was then exposed to neat,
degassed, p-Me-TFS in an inert atmosphere at 50.degree. C for 60
hours. The p-Me-TFS grafted film was removed, washed with toluene
and dried at 60.degree. C. The percentage graft was 49%.
[0068] (b) A sulfonating solution was prepared by careful addition
of 30 g of liquid sulfur trioxide to 70 g of
1,1,2,2-tetrachloroethane. The grafted membrane was sulfonated by
immersion in the above-mentioned sulfonating solution for 2 hours
at 70.degree. C. The resultant ion-exchange membrane was washed
with water and dried at 60.degree. C. The equivalent weight of the
sulfonated membrane was 660 g/mol, with a water content of 26% at
room temperature.
EXAMPLE 5
Grafting of para-methyl-.alpha.,.beta.,.beta.-trifluorostyrene
(p-Me-TFS) to poly(ethylene-co-tetrafluoroethylene) (Tefzel.RTM.)
Film and Sulfonation of the Grafted Membrane
[0069] (a) A 2 mil (approx. 50 .mu.m) thick, 7 inch.times.7 inch
(18 cm.times.18 cm) piece of poly(ethylene-co-tetrafluoroethylene)
(Tefzel.RTM.) film was grafted with
para-methyl-.alpha.,.beta.,.beta.-tri- fluorostyrene similarly as
in Example 4, using a 5 Mrad irradiation dose. The percentage-graft
was 35%.
[0070] (b) The grafted film was sulfonated according to the
procedure described in step (b) of Example 4. The equivalent weight
of the sulfonated membrane was 821 g/mol, with a water content of
18% at room temperature.
EXAMPLE 6
Use of Sulfonated p-Me-TFS grafted
poly(ethylene-co-tetrafluoroethylene) (Tefzel.RTM.) Membrane as
Ion-exchange Membrane in a Fuel Cell
[0071] The membrane prepared as described in Example 4 was bonded
to two catalyzed carbon fiber paper electrodes to form a membrane
electrode assembly having a total platinum catalyst loading of 1
mg/cm.sup.2. The membrane electrode assembly was tested in a
Ballard Mark IV single cell fuel cell. The following operating
conditions were employd:
[0072] Temperature: 80.degree. C.
[0073] Reactant inlet pressure:
[0074] 3.02 bara for oxidant and fuel
[0075] Reactant stoichiometries:
[0076] 2.0 oxidant and 1.5 hydrogen.
[0077] FIG. 1 shows polarization plots of voltage as a function of
current density for the sulfonated grafted membrane employed in a
membrane electrode assembly in the electrochemical fuel cell
operating on hydrogen-oxygen (plot A) and hydrogen-air (plot
B).
EXAMPLE 7
Use of Sulfonated p-Me-TFS grafted
poly(ethylene-co-tetrafluoroethylene) (Tefzel.RTM.) Membrane as an
Ion-exchange Membrane in a Fuel Cell
[0078] The membrane prepared as described in Example 5 was bonded
to two catalyzed carbon fiber paper electrodes to form a membrane
electrode assembly having a total platinum catalyst loading of 8
mg/cm.sup.2. The membrane electrode assembly was tested in a
Ballard Mark IV single cell direct methanol fuel cell. The
following operating conditions were employd:
[0079] Temperature: 110.degree. C.
[0080] Fuel: 0.4 M methanol solution (in water)
[0081] Reactant inlet pressure: 3.02 bara for oxidant and fuel
[0082] Reactant stoichiometries: 2.0 oxidant and 3.0 methanol.
[0083] FIG. 2 shows polarization plots of voltage as a function of
current density for the sulfonated grafted membrane employed in a
membrane electrode assembly in the electrochemical fuel cell
operating on methanol-air.
[0084] In addition to the utility of the grafted membranes
described herein in ion exchange membranes for electrochemical fuel
cells, it is contemplated that such membranes will also have
utility in the following applications:
[0085] (1) as membranes in filtration and ultrafiltration
applications;
[0086] (2) as proton exchange membranes in water electrolysis,
which involves a reverse chemical reaction to that employed in
hydrogen/oxygen electrochemical fuel cells;
[0087] (3) as membranes in chloralkali electrolysis, which
typically involves the electrolysis of a brine solution to produce
chlorine and sodium hydroxide, with hydrogen as a by-product;
[0088] (4) as electrode separators in conventional batteries,
provided the membrane has the requisite chemical inertness and high
electrical conductivity;
[0089] (5) as ion-selective electrodes, particularly those employd
for the potentiometric determination of a specific ion such as
Ca.sup.2+, Na.sup.+, K.sup.+ and like ions;
[0090] (6) as sensor materials for humidity sensors based on ion
exchange membranes, as the electrical conductivity of an ion
exchange membrane varies with humidity;
[0091] (7) as ion exchange membranes for separations by ion
exchange chromatography--typical such applications are deionization
and desalination of water, ion separations, removal of interfering
ionic species, and separation and purification of biomolecules;
[0092] (8) as ion exchange membranes employed in analytical
pre-concentration techniques (for example, Donnan Dialysis);
[0093] (9) as ion exchange membranes in electrodialysis, in which
membranes are employed to separate components of an ionic solution
under the driving force of an electrical current--industrial
applications include desalination of brackish water, preparation of
boiler feed make-up and chemical process water, de-ashing of sugar
solutions, deacidification of citrus juices, separation of amino
acids, and the like;
[0094] (10) as membranes in dialysis applications, in which solutes
diffuse from one side of the membrane (the feed side) to the other
side according to their concentration gradient--applications
include hemodialysis and the removal of alcohol from beer;
[0095] (11) as membranes in gas separation (gas permeation) and
pervaporation (liquid permeation) techniques; and
[0096] (12) as bipolar membranes employed in water splitting and
subsequently in the recovery of acids and bases from waste water
solutions.
[0097] While particular elements, embodiments and applications of
the present invention have been shown and described, it will be
understood, of course, that the invention is not limited thereto
since modifications may be made by those skilled in the art,
particularly in light of the the foregoing teachings. It is
therefore contemplated by the appended claims to cover such
modifications that incorporate those features coming within the
scope of the invention.
* * * * *