Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Ion-exchange polymer or process of preparing
Reexamination Certificate
1997-11-12
2002-03-19
Zitomer, Fred (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Ion-exchange polymer or process of preparing
C429S010000, C521S025000, C521S030000, C521S032000, C521S033000, C525S276000, C526S251000
Reexamination Certificate
active
06359019
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 preformed polymeric base film. Where 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 used 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 co-irradiation, followed by the introduction of various ion-exchange substituents to the pendant aromatic rinas 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 phosphoration are among the reactions subsequently used to introduce ion-exchange groups into the grafted polymeric chains. Post-sulfonation rates of 40% to 100% are reported.
In either case the prior art TFS-based grafted membranes incorporate 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 invention, one or more types of substituted TFS monomers are grafted to polymeric base films, the substituents being selected to offer particular advantages, for example:
(a) substituted TFS monomers which are activated by virtue of their aromatic substituents have increased reactivity in the grafting reaction facilitating grafting and/or are activated in subsequent reactions to introduce ion-exchange functionality into the grafted chains;
(b) 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.
(c) substituted TFS 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.
Other trifluorovinyl aromatic or heteroaromatic monomers offering one or more of the above described advantages similarly may be grafted to polymeric base films to give novel graft polymeric membranes.
SUMMARY OF THE INVENTION
Graft polymeric membranes comprise one or more trifluorovinyl aromatic monomers radiation graft polymerized to a preformed polymeric base film. In a first embodiment, a graft polymeric membrane comprises a preformed polymeric base film to which has been graft polymerized a substituted &agr;, &bgr;, &bgr;-trifluorostyrene monomer selected from the group consisting of monomers of formula (I):
The group A may be OR, SR, NRR′ (where R and R′ are independently selected from the group consisting of alkyl, fluoroalkyl and aryl), which are substituents that activate the monomer.
The monomer of formula (I) may have more than one aromatic ring, for example when A is Ph, OPh, SPh, N(R)Ph (where R is selected from the group consisting of hydrogen, Ph, alkyl and fluoroalkyl), (CH
2
)
n
Ph or (CF
2
)
n
Ph (where n is an integer greater than zero). Of these, the groups Ph, OPh, SPh, N(R)Ph are particularly preferred.
Other substituents, A, which are useful precursors to ion-exchange groups can be advantageously selected, for example, SO
2
X (where X is selected from the group consisting of F, Cl, Br, I), OH, NH
2
, CN, and NO
2
.
In a second embodiment, a graft polymeric membrane comprises a preformed polymeric base film to which has been graft polymerized a trifluorovinyl naphthalene monomer selected from the group consisting of monomers of formula (II):
B may be hydrogen, or may be selected from the preferred substituents described above for A in formula (I).
These and other trifluorovinyl polyaromatic and heteroaromatic monomers may be advantageously graft polymerized to polymeric base films, as such monomer units provide more sites for subsequent introduction of functional groups, ard in many cases, the aromatic rings are electron rich and activated compared to that of TFS.
In any of the embodiments described above, the preformed polymeric base film may be grafted with a single monomer whereby the grafted chains are homopolymeric. Alternatively, the preformed polymeric base film may be grafted with a mixture of monomers to give grafted chains which are copolymeric. The monomer mixture may comprise, or may consist predominantly of, one or more monomers described by the formulae. In some embodiments the mixture may consist of monomers described by the formulae.
Preferred vinyl monomers for co-grafting with those described by the formulae include styrene and ethylene-based monomers, fluorinated ethylene-based monomers, and other &agr;, &bgr;, &bgr;-trifluorostyrene monomers. For example, one or more monomers of formula (III) may be included in the mixture:
where D is selected from the group consisting of hydrogen, halomethyl, perfluoroalkyl, perfluoroalkenyl and fluorine and SO
3
−
M
+
, the latter representing a sulfonic acid salt where M
+
is a counterion.
Depending on the nature of the monomers incorporated into the grafted chains of the membranes described above, ion-exchange groups may be introduced by the transformation of precursor groups already present as aromatic substituents in the monomers, and/or via post-graft reaction processes. For example, the membranes may be subjected to a reaction process selected from the group consisting of sulfonation, phosphonation, phosphorylation, amination, carboxylation, hydroxylation and nitration whereby ion-exchange groups are introduced into pendant aromatic rings of the grafted chains, directly by these reaction processes or via these reaction processes in combination with subsequent steps.
In one embodiment, an ion-exchange membrane comprises a preformed polymeric base film with grafted chains comprising monomer units of formula (IV):
where A is OH, OR, SR or NRR′ (where R and R′ are independently selected from the group consisting of alkyl, fluoroalkyl and aryl) and at least a portion of the monomer units include at least one ion-exchange substituent on the aromatic ring thereof. In preferred embodiments at least a portion of the monomer units include at least two ion-exchange substituents, which ma
Steck Alfred E.
Stone Charles
Ballard Power Systems Inc.
McAndrews Held & Malloy Ltd.
Zitomer Fred
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