Ionomers and ionically conductive compositions

Details Ionomers and ionically conductive compositions Ionomers and ionically conductive compositions

2000-06-07

2001-07-31

Yoon, Tae H. (Department: 1714)

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

Synthetic resins

At least one aryl ring which is part of a fused or bridged...

C524S108000, C524S504000, C525S069000, C525S250000, C528S491000, C204S280000, C204S291000, C204S660000

Reexamination Certificate

active

06268430

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to ionomers comprising functionalized polyolefins having fluoroalkyl sulfonate pendant groups and to ionically conductive compositions formed therefrom by the addition of solvents thereto. The ionically conductive compositions of the invention are useful in batteries, fuel cells, electrolysis cells, ion exchange membranes, sensors, electrochemical capacitors, and modified electrodes.
TECHNICAL BACKGROUND OF THE INVENTION
It has long been known in the art to form ionically conducting membranes and gels from organic polymers containing ionic pendant groups. Such polymers are known as ionomers. Particularly well-known ionomer membranes in widespread commercial use are Nafion™ Membranes available from E. I. du Pont de Nemours and Company. Nafion™ is formed by copolymerizing tetra-fluoro ethylene (TFE) with perfluoro(3,6-dioxa-4-methyl-7-octenesulfonyl fluoride), as disclosed in U.S. Pat. No. 3,282,875. Also known are copolymers of TFE with perfluoro (3-oxa-4-pentene sulfonyl fluoride), as disclosed in U.S. Pat. No. 4,358,545. The copolymers so formed are converted to the ionomeric form by hydrolysis, typically by exposure to an appropriate aqueous base, as disclosed in U.S. Pat. No. 3,282,875. Lithium, sodium and potassium are all well known in the art as suitable cations for the above cited ionomers.
In the polymers above-cited, the fluorine atoms provide more than one benefit. The fluorine groups on the carbons proximate to the sulfonyl group in the pendant side chain provide the electronegativity to render the cation sufficiently labile so as to provide high ionic conductivity. Replacement of those fluorine atoms with hydrogen results in a considerable reduction in ionic mobility and consequent loss of conductivity.
The remainder of the fluorine atoms afford the chemical and thermal stability to the polymer normally associated with fluorinated polymers. This has proven to be of considerable value in such applications as the well-known “chlor-alkali” process. However, highly fluorinated polymers also have disadvantages where there is less need for high chemical and thermal stability. The fluorinated monomers are more expensive than their olefin counterparts, require higher processing temperatures, and often require expensive corrosion resistant processing equipment. Furthermore, it is difficult to form solutions and dispersions of fluoropolymers. Additionally, it is difficult to form strong adhesive bonds with fluoropolymers. In materials employed in electrochemical cells, for example, it may be advantageous to have better processibility at some cost to chemical and thermal stability. Thus, there is an incentive to develop ionomers with highly labile cations having non-fluorinated polymer backbones.
Numerous publications disclose polyethers with either proximal ionic species in the polymer or in combination with ionic salts. Conductivities are in the range of 10
5
S/cm and less. Le Nest et al., Polymer Communications 28, 303 (1987) disclose a composition of polyether glycol oligomers joined by phosphate or thiophosphate moieties hydrolyzed to the related lithium ionomer. In combination with propylene carbonate, conductivity in the range of 1×10×10
−4
S/cm was realized. A review of the related art is found in Fauteux et al., Electrochimica Acta 40, 2185 (1995).
Benrabah et al, Electrochimica Acta, 40 2259 (1995) disclose polyethers crosslinked by lithium oxytetrafluorosulfonates and derivatives. No aprotic solvents are incorporated. With the addition of lithium salts conductivity of <10
−4
S/cm was achieved.
Armand et al., U.S. Pat. No. 5,627,292 disclose copolymers formed from vinyl fluoroethoxy sulfonyl fluorides or cyclic ethers having fluoroethoxy sulfonyl fluoride groups with polyethylene oxide, acrylonitrile, pyridine and other monomers. Lithium sulfonate ionomers are formed. No aprotic solvents are incorporated. Conductivity was <104 S/cm.
Narang et al., U.S. Pat. No. 5,633,098 disclose polyacrylate copolymers having a functionalized polyolefin backbone and pendant groups containing tetrafluoroethoxy lithium sulfonate groups. The comonomers containing the sulfonate groups are present in molar ratios of 50-100%. Compositions are disclosed comprising the polymer and a solvent mixture consisting of propylene carbonate, ethylene carbonate, and dimethoxyethane ethyl ether. Ionic conductivity of those compositions was in the range of 10
−4
-10
−3
S/cm.
Brookhart et al., WO 9623010A2, discloses a copolymer formed from ethene and 1,1,2,2-tetrafluoro-2-[(1,1,2,2,3,3,4,4-octafluoro-9-decenyl)oxy] ethanesulfonyl fluoride via a catalyzed reaction employing diimine-transition metal complexes. The polymer so-formed comprises a polyethylene backbone having randomly distributed pendant groups of 1,1,2,2-tetrafluoro-2-[(1,1,2,2,3,3,4,4-octafluoro-(mostly)octoxy] ethanesulfonyl fluoride, as well as alkyl branches.
SUMMARY OF THE INVENTION
This invention provides for an ionomer comprising a backbone and pendant groups, the backbone consisting essentially of methylene units and the pendant groups comprising ionic radicals of the formula
—R
n
—R
f
CF
2
CF
2
—SO
2
—X—(SO
2
R
f
)
a
−
M
+
where M
+
is a univalent metal cation; the R
f
groups are independently selected from the group consisting of linear or branched perfluoroalkylene radicals, perfluoroalkylene radicals containing O or Cl, and perfluoroaryl radicals; R is hydrocarbyl where n=0 or 1; a=0-2; and X=O, N or C; said ionic radicals being further limited in that a=0 when X=O, a=1 when X=N, and a=2 when X=C.
This invention farther provides for an ionically conductive composition comprising said ionomer described above and a liquid imbibed therewithin.
This invention also discloses a process for forming an ionomer, the process comprising contacting a polyolefin comprising a backbone and pendant groups, the backbone consisting essentially of methylene and methine units and the pendant groups comprising ionic radicals of the formula
XSO
2
—CF
2
CF
2
—R
f
—R
n
—
where X is F or Cl, R
f
is a linear or branched perfluoroalkylene, perfluoroalkylene containing O or Cl, or perfluoroaryl radical, and R is hydrocarbyl where n=0 or 1, with a solution of an alkali metal base.
Further disclosed is a process for forming a conductive composition the process comprising contacting the above ionomer with a liquid.
Also included herein is an electrode comprising at least one electrode active material, the ionomer disclosed herein mixed therewith, and a liquid imbibed therewithin.
Further disclosed is an electrochemical cell comprising a positive electrode, a negative electrode, a separator disposed between the positive and negative electrodes, and a means for connecting the cell to an outside load or source wherein at least one of the group consisting of the separator, the cathode, and the anode, comprises the above ionomer.
DETAILED DESCRIPTION
In a preferred embodiment of the polyolefin ionomer of the invention, the backbone consists essentially of olefinic radicals whereof 1-20 mol-% have pendant groups in the form of a radical of the formula
M
+ −
SO
3
—CF
2
CF
2
—O—[(CFR
1
CF
2
)
x
—O
y
]
n
—(CH
2
)
z
—  (I)
where M
+
is an alkali metal cation, R
1
is perfluoroalkyl or fluorine, x=0,1,2, or 3, y=0 or 1, n=0,1,2, or 3, and z is an integer in the range of 2 to 6. Most preferably M
+
is a lithium cation, R
1
is fluorine, x=1, y=0, n=1 or 2, z=4.
The olefinic radicals making up the backbone of the polyolefin ionomer of the invention are substantially unsubstituted except that 1-20 mol-% of the olefinic radicals of the backbone in a preferred embodiment of the invention have a pendant group in the form of the radical (I). In a most preferred embodiment, 2-10 mol-% of the olefinic radicals of the backbone have a pendant group in the form of the radical (I).
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