Method for forming fluorinated ionomers

Details Method for forming fluorinated ionomers Method for forming fluorinated ionomers

2002-07-01

2004-03-30

Lipman, Bernard (Department: 1713)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Mixing of two or more solid polymers; mixing of solid...

C525S326200, C526S236000, C526S243000, C526S248000

Reexamination Certificate

active

06713567

ABSTRACT:

FIELD OF THE INVENTION
This invention concerns ammonium ionomers, and a method for preparing them by contacting a polymer having a substantially fluorinated, but not perfluorinated, polyethylene backbone having pendant groups of fluoroalkoxy sulfonyl fluoride, with an excess of ammonium carbonate solution. This invention further concerns a method for forming ionomers by ion exchange with ammonium ionomers. The ionomers so formed are useful in electrochemical applications such as batteries, fuel cells, electrolysis cells, ion exchange membranes, sensors, electrochemical capacitors, and modified electrodes.
BACKGROUND OF THE INVENTION
Copolymers of vinylidene fluoride (VDF) with vinyl alkoxy sulfonyl halides are known in the art.
The disclosures in Ezzell et al. (U.S. Pat. No. 4,940,525) encompass copolymers of VDF with vinyl ethoxy sulfonyl fluorides containing one ether linkage. Disclosed is a process for emulsion polymerization of tetrafluoroethylene (TFE) with the vinyl ethoxy comonomer.
Banerjee et al. (U.S. Pat. No. 5,672,438) disclose copolymers of VDF with vinyl alkoxy sulfonyl fluorides containing more than one ether linkage.
Connolly et al. (U.S. Pat. No. 3,282,875) disclose the terpolymer of VDF with perfluorosulfonyl fluoride ethoxy propyl vinyl ether (PSEPVE) and hexafluoropropylene (HFP). They broadly teach an emulsion polymerization process said to be applicable to copolymerization of vinyl ethers with any ethylenically unsaturated comonomer, with greatest applicability to fluorinated monomers.
Barnes et al. (U.S. Pat. No. 5,595,676) disclose “substantially fluorinated” copolymers of a vinyl ether cation exchange group-containing monomer with a “substantially fluorinated” alkene. The copolymer is produced by controlled addition of the alkene in emulsion polymerization, followed by hydrolysis in NaOH. PSEPVE/TFE copolymers are exemplified.
Hietala et al., J. Mater. Chem. Volume 7 pages 721-726, 1997, disclose a porous poly(vinylidene fluoride) on to which styrene is grafted by exposing the PVDF to irradiation. The styrene functionality is subsequently functionalized to sulfonic acid by exposure of the polymer to chlorosulfonic acid. The resultant acid polymer, in combination with water, provides a proton-conducting membrane.
Formation of ionomers and acid copolymers by hydrolysis of the sulfonyl fluoride functionality in copolymers of TFE and fluoro alkoxy sulfonyl fluorides is well known in the art. The art teaches exposure of the copolymer to strongly basic conditions.
See for example, Ezzell et al. U.S. Pat. No. 4,940,525, wherein is used 25 wt % NaOH(aq) for 16 hours at 80-90° C.; Banerjee et al. U.S. Pat. No. 5,672,438, wherein is used 25 wt % NaOH for 16 hours at 90° C., or, in the alternative, an aqueous solution of 6-20% alkali metal hydroxide and 5-40% polar organic liquid (e.g., DMSO) for 5 minutes at 50-100° C.; Ezzell et al. U.S. Pat. No. 4,358,545 wherein is used 0.05N NaOH for 30 minutes for 50° C.; Ezzell et al. U.S. Pat. No. 4,330,654, wherein is used 95% boiling ethanol for 30 minutes followed by addition of equal volume of 30% NaOH (aq) with heating continued for 1 hour; Marshall et al. EP 0345964 A1, wherein is used 32 wt % NaOH (aq) and methanol for 16 hours at 70° C., or, in the alternative, an aqueous solution of 11 wt % KOH and 30 wt % DMSO for 1 hour at 90° C.; and, Barnes et al. U.S. Pat. No. 5,595,676, wherein is used 20 wt % NaOH (aq) for 17 hours at 90° C.
Because of its high dielectric constant, high electrochemical stability, and desirable swelling properties, poly(vinylidene fluoride) is known in the art of lithium batteries as a highly desirable material for use as a membrane separator. For example Gozdz et al. (U.S. Pat. No. 5,418,091) disclose porous PVDF homopolymer and copolymer containing solutions of lithium carbonates in aprotic solvents useful as separators in lithium batteries.
Porous membranes of the type described by Gozdz, however, conduct both the cation and the anion back and forth across the separator, and are thus subject to concentration polarization during use, which degrades the performance of the battery in which it is used. So-called single ion conducting polymeric membranes, wherein the ionic carbonate is attached to the polymer backbone, thereby immobilizing either the cation or the anion, offer a solution to the concentration polarization problem, and are known in the art. One particularly well-known such single ion conducting polymer is Nafion® Perfluoroionomer Resin and Membranes available from DuPont, Wilmington, Del. Nafion is a copolymer of TFE and perfluoro(3,6-dioxa-4-methyl-7-octenesulfonyl fluoride) which has been hydrolyzed by treatment with an alkali metal hydroxide according to the teachings of the art as hereinabove described.
It is further known in the art, and hereinbelow shown, that PVDF homopolymers and copolymers are subject to attack by strong bases such as the alkali metal hydroxides taught in the art hereinabove cited. Of particular importance is that the attack of basic nucleophiles on a copolymer of VDF and perfluorovinyl ethers results in the removal of the vinyl ether moiety from the polymer, see W. W. Schmiegel in Die Angewandte Makromolekulare Chemie, 76/77 pp 39ff, 1979. Since the highly preferred monomer species taught in the art, and exemplified by DuPont's Nafion and similar products, for imparting ionomeric character to various polymers is a vinyl ether terminated by a sulfonyl halide functionality, the sensitivity to base attack of the VDF copolymer formed therewith has prevented the development of a single-ion conducting ionomer based upon VDF. There simply is no means taught in the art for making the ionomer.
Doyle et al, U.S. Pat. No. 6,025,092, discloses ionomers formed with vinylidene fluoride copolymers by subjecting sulfonyl fluoride containing precursors to hydrolysis with alkali and alkaline earth metal carbonates, such as lithium carbonate, under mildly basic conditions. The method of Doyle et al, however is limited in that any excess over stoichiometric amounts of the hydrolyzing agent results in attack on the VDF backbone, causing polymer degradation. Thus the method of Doyle et al is limited in industrial applicability.
Barton et al, WO 0052085A1, discloses melt processible compositions comprising alkali metal ionomers having vinylidene fluoride monomer units, and liquids imbibed therewithin. The ionomers of Barton et al are not melt processible without incorporation of the liquids.
SUMMARY OF THE INVENTION
The present invention provides for an ionomer comprising monomer units of vinylidene fluoride and 0.5-50 mole % of a perfluoroalkenyl monomer having a pendant group of the formula
—(O—CF
2
CFR)
a
O—CF
2
(CFR′)
b
SO
3
−
NH
4
+
wherein R and R′ are independently selected from F, Cl or a perfluorinated alkyl group having 1 to 10 carbon atoms, a=0, 1 or 2, b=0 to 6.
The present invention also provides for a process for forming an ionomer comprising
contacting a polymer comprising
monomer units of vinylidene fluoride and 0.5-50 mole % of a perfluoroalkenyl monomer having a pendant group of the formula
—(O—CF
2
CFR)
a
O—CF
2
(CFR′)
b
SO
2
F
 wherein R and R′ are independently selected from F, Cl or a perfluorinated alkyl group having 1 to 10 carbon atoms, a=0, 1 or 2, b=0 to 6,
with an excess of a solution of ammonium carbonate for a period of time sufficient to obtain the degree of conversion desired to the ammonium sulfonate form of the polymer.
DETAILED DESCRIPTION OF THE INVENTION
For the purposes of description in the present invention, the generic term “ionomer” will be taken to encompass the ammonium sulfonate and the sulfonic acid forms of the polymer of the invention, as well as the alkali and alkaline earth salts thereof. For the purpose of the present invention, the term “excess” when applied to the ammonium carbonate solution of the present invention means that the solution contains more, preferably many fold, more than the amount of ammonium carbonate necessary to achieve complete h

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