Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Ion-exchange polymer or process of preparing
Reexamination Certificate
2001-02-09
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
C521S028000, C525S203000, C525S204000, C525S376000, C525S376000, C525S376000, C525S327100, C525S340000, C525S344000
Reexamination Certificate
active
06723757
ABSTRACT:
The present invention relates to novel compatible binary and ternary cation-exchanger polymer and anion-exchanger polymer blend membranes.
The invention further relates to the use of such binary and ternary ionomer blend membranes in electromembrane processes, such as polymer electrolyte membrane fuel cells (PEFC), direct methanol fuel cells (DMFC), electrodialysis, and in other membrane processes, such as dialysis and inverse osmosis, diffusion dialysis, gas permeation, pervaporation and perstraction.
For ionomer membrane applications, such as polymer electrolyte membrane fuel cells (PEMFC), direct methanol fuel cells (DMFC), polymer electrolyte membrane electrolysis (PEM-E), a high chemical, mechanical and thermal stability of the membrane is necessary. The perfluorinated ionomer Nafion® (Grot, W. G.: Perfluorinated Ion-Exchange Polymers and Their Use in Research and Industry, Macromolecular Symposia, 82, 161-172 (1994)) is the only commercially available ionomer to date to meet the high requirements of chemical, mechanical and thermal stability (Ledjeff, K.; Heinzel, A.; Mahlendorf, F.; Peinecke, V.: Die reversible Membran-Brennstoffzelle, Dechema-Monographien Band 128, VCH Verlagsgesellschaft, 103-118 (1993)). However, it has various disadvantages which necessitate the search for alternative materials: It is very expensive (DM 1400.−/m
2
). The very complex production process comprises highly toxic intermediates (see Grot, W. G.). The environment-compatibility of Nafion® is to be evaluated critically: as a perfluorinated polymer, it is hardly degradable. The recyclability of Nafion® is questionable.
When applying Nafion® in direct methanol fuel cells (DMFC), it was discovered that it shows a very high methanol-permeability, especially when pure methanol is used (Surampudi, S., Narayanan, S. R.; Vamos, E.; Frank, H.; Halpert, G.; LaConti, A.; Kosek, J.; Surya Prakash, G. K.; Olah, G. A.: Advances in direct oxidation methanol fuel cells, J. Power Sources, 47, 377-385 (1994)), which greatly reduces the energy efficiency of the DMFC by mixed potential formation.
Possible alternative materials to the perfluorinated ionomers are arylene main chain ion-exchanger polymers, such as sulfonated polyethersulfone (Nolte, R.; Ledjeff, K.; Bauer, M.; Mülhaupt, R.: Partially Sulfonated poly(arylene ether sulfone)—A Versatile Proton Conducting Membrane Material for Modern Energy Conversion Technologies, Journal of Membrane Science 83, 211-220 (1993)) and sulfonated poly(etheretherketone) (Helmer-Metzmann, F.; Ledjeff, K.; Nolte, R., et al.: Polymerelektrolyt-Membran und Verfahren zu ihrer Herstellung, EP 0 574 791 A2), which have a disadvantage, however, in that they exhibit a high brittleness when drying out, which is unfavorable when used, for example, in membrane fuel cells.
Searching for polymers with high thermal and mechanical stability leads one to find polyimides, imidazole containing polymers and benzimidazoles which show excellent thermal stabilities, such as the polybenzimidazole (PBI) poly[(2,2′-m-phenylene)-5,5′-bibenzimidazole] of general formula
and the polyetherimide poly[2,2′-bis(3,4-dicarboxyphenoxy)phenyl-propane-2-phenylenebisimide] (Musto, P.; Karasz, F. E., MacKnight, W. J.: Fourier transform infra-red spectroscopy on the thermooxidative degradation of polybenzimidazole and of a polybenzimidazole/polyetherimide blend, Polymer, 34(12), 2934-2945 (1993)).
Polybenzimidazoles can be sulfonated by various methods. One possible way is the following sequence of reactions (Gieselman, M. B.; Reynolds, J. R.: Water-Soluble Polybenzimidazole-Based Polyelectrolytes):
1. Deprotonation of the imidazole N—H with LiH in DMAc;
2a. Reaction of the deprotonated polymer with propanesulfone to give the corresponding polybenzimidazole-N-propanesulfonate;
2b. Reaction of the deprotonated polymer with Na-4-(bromomethyl)-benzenesulfonate to give the corresponding polybenzimidazole-N-benzylsulfonate.
A patent describes another method for obtaining sulfonated polybenzimidazoles (Sansone, M. J.; Gupta, B.; Forbes, C. E.; Kwiatek, M. S.:
Sulfalkylation of Hydroxyethylated Polybenzimidazole Polymers, U.S. Pat. No. 4,997,892) which involves the following sequence of reactions:
1. Reaction of polybenzimidazole at the N—H group of the imidazole ring with ethylene carbonate in a dipolar-aprotic solvent, such as N-methylpyrrolidinone, to give the hydroxyethylated polybenzimidazole N—(CH
2
)
2
OH;
2. Deprotonation of the OH group of the hydroxyethylated polybenzimidazole with a suitable base to give the hydroxyethylated polybenzimidazole anion N—(CH
2
)
2
O
−
;
3. Reaction of the hydroxyethylated polybenzimidazole anion N—(CH
2
)
2
O
−
with a sulfone, e.g., propanesulfone, to give the sulfoalkylated polymer N—(CH
2
)
2
O(CH
2
)
3
—SO
3
−
.
It has been found that the excellent thermal stability of polybenzimidazoles is partially retained with these sulfonating methods (see Gieselman et al.). For some applications of the sulfonated polybenzimidazoles mentioned, such as their use in membrane fuel cells, it may be a disadvantage, however, that they contain —CH
2
— groups which result in a lower oxidation stability than that of purely aromatic sulfonated polymers. In addition, the sulfonated polybenzimidazoles can form inner salts in their protonated form which reduce the proton conduction according to the following reaction scheme:
Further, the sulfonated polybenzimidazoles may lose part of their mechanical stability by interference of the substituent with the chain conformation.
Polybenzimidazole can be alkylated at both imidazole nitrogens by the following method to obtain an anion-exchanger polymer which may also be water-soluble (Hu, Ming; Pearce, Eli.M.; Kwei, T. K.: Modification of Polybenzimidazole: Synthesis and thermal stability of Poly(N
1
-methylbenzimidazole and Poly(N
1
,N
3
-dimethylbenzimidazole), Salt Journal of Polymer Science: Part A: Polymer Chemistry, 31, 553-561, 1993):
1. Deprotonation of the imidazole N—H with LiH in DMAC or NMP to give the N—Li salt;
2. Alkylation of the Li salt≈N—Li with methyl iodide to give ≈N—CH
3
;
3. Reaction of the methylated polybenzimidazole with an excess of methyl iodide at 80° C. to obtain poly(N
1
,N
3
-dimethylbenzimidazplium)diiodide.
A disadvantage of this poly(N
1
,N
3
-dimethylbenzimidazolium)iodide is its poor thermal stability (thermogravimetry: onset loss of weight at 180° C. (heating rate 10°/min)). This loss of weight can be explained by the cleavage of methyl iodide to form the monomethylated polybenzimidazole, which results in a loss of the anion-exchanger properties of the polymer.
In the patent literature a work claiming blends/mixtures of low-molecular non-aqueous amphoters and high-molecular acids (proton donors) or high-molecular amphoters and low-molecular acids is found (Kreuer, K. D.; Fuchs, A.; Maier, J.; Frank, G.; Soczka-Guth, Th.; Clau&bgr;, J; Protonenleiter mit einer Temperaturbeständigkeit in einem weiten Bereich und guten Protonenleitfähigkeiten, DE 196 32 285 A1). Said amphoters are heterocyclic and heteroaromatic N-containing compounds including, among others, also imidazole or benzimidazole and imidazole or benzimidazole containing organic low-molecular, oligomer, or high-molecular compounds functioning as proton solvents, the acids being present in the system being the proton donors for the amphoters. “Proton solvent” indicates that the protons are directed by the molecules or groups of the amphoters.
In the application examples of DE 196 32 285 A1 only the preparation and characterization of blends of sulfonated polyetherketones and imidazole and/or pyrazole are quoted, the blends containing 10 imidazole and/or pyrazole molecules showing the best proton conductivities. The good proton conductivity of said blends is presumably due to the high mobility of the imidazole and/or pyrazole molecules within the polymer matrix. This high mobility of the low-molecular heterocycles involves the danger that said molecules possibly can be re-discharged from the ac
Häring Thomas
Kerres Jochen
Ullrich Andreas
Hunton & Williams LLP
Lipman Bernard
Universitat Stuttgart Lehrstuhl
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