Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Ion-exchange polymer or process of preparing
Reexamination Certificate
2001-08-21
2004-04-20
Lipman, Bernard (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Ion-exchange polymer or process of preparing
C521S031000, C521S032000, C521S033000, C525S359100, C525S416000
Reexamination Certificate
active
06723758
ABSTRACT:
FIELD OF THE INVENTION
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
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.
U.S. Pat. No. 4,012,303, reports the radiation grafting of &agr;,&bgr;,&bgr;-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.
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.
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.
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.
In the present membranes, one or more types of substituted TFS monomers and/or substituted &agr;,&bgr;,&bgr;-trifluorovinylnaphthylene (TFN) monomers are grafted to polymeric base films, the substituents being selected to offer particular advantages, for example:
(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.
(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.
(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.
(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.
(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.
(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
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):
where A
1
, A
2
, and B
1
, B
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
1
and A
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
1
, A
2
, B
1
, and B
2
can be the same or different, provided that in the selected monomer at least one of the substituents A
1
, A
2
, B
1
, B
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.
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-tetrafluoroethylene).
In embodiments in which a polymeric base film has been graft polymerized with a monomer of formula (I) in which A
1
is Ar and A
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.
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
1
and/or A
2
substituents, more than one monomer of formula (II) having different B
1
and/or B
2
substituents, more than one monomer of either formula (I) or formula (II) having the same substituents located at different positions, or monome
Choudhury Biswajit
Steck Alfred E.
Stone Charles
Ballard Power Systems Inc.
Seed IP Law Group PLLC
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