Enhancing performance of perfluorinated ionomer membranes via dopant incorporation, method of making thereof and the membrane

The present invention describes a perfluorinated ionomer membrane having a improved transport characteristics. A surfactant species is added to a polymer mixture prior to film casting. The resulting membranes have a measurably altered membrane microstructure and improved transport characteristics over prior art membranes. The present invention describes the method of producing the improved membranes. The membranes of the present invention are useful in a number of separation processes, including the separation of NH.sub.3 from gaseous and liquid mixtures, in the production of NaOH and Cl.sub.2 gas from the electrolytic dissolution of NaCl, in the separation of toxic and radioactive metals from aqueous streams, and in solid polymer electrolyte H.sub.2 /O.sub.2 fuel cells.

FIELD OF THE INVENTION 
This invention relates to polyperfluorosulfonic acid (PFSA) separation 
membranes. Specifically, dopant molecules are used to modify the polymer 
structure of PFSA to improve the separation properties of the resulting 
membrane while retaining the characteristics of high mechanical strength, 
hydrophilicity, solvent and temperature resistance, and crystallinity. The 
membranes of the present invention are useful in a number of separation 
processes, including the separation of NH.sub.3, CO.sub.2, and H.sub.2 S 
from gaseous and liquid mixtures, in the production of NaOH and Cl.sub.2 
gas from the electrolytic dissolution of NaCl, and the separation of toxic 
and radioactive metals from aqueous streams. The membranes are also useful 
in solid polymer electrolyte H.sub.2 /O.sub.2 fuel cells. 
BACKGROUND OF THE INVENTION 
The use of an ion-exchange membrane as a support for facilitated transport 
offers several important advantages over other methods of separation: (1) 
in an ion-exchange membrane, the complexing agent is held in the membrane 
by electrostatic forces and cannot be leached out; (2) the concentration 
of complexing agent in the membrane is determined by ion-exchange site 
density and not physical solubility. The ion-exchange site density is 
normally much larger than the physical solubility; (3) the high charge 
density in the vicinity of the ion-exchange sites protects the complexing 
agent from redox reactions and extends its useful lifetime; and (4) 
solvent loss problems are reduced. The complexing agent is not removed if 
the solvent phase is removed. The membrane can be resolvated without a 
reduction in subsequent performance. 
Ionomers are polymeric materials containing ionic groups. Most of the 
research effort on ionomers has focused on only a small number of 
materials such as ethylenes, styrenes, rubbers, and fluorocarbon-based 
ionomers. Because of a high water permeability and cation selectivity, 
fluorocarbon-based ionomers have been used as ion-exchange membranes (Kyu 
(1985) in Materials Science of Synthetic Membranes, D. R. Lloyd, ed. 
(American Chemical Society, Washington, D.C.), pp. 365-405). 
Perfluorinated ion-exchange membranes are derived from copolymers of 
tetrafluoroethylene (TFE) and a perfluorovinyl ether terminated by a 
sulfonyl fluoride group. Examples of perfluorinated ion-exchange membranes 
include Nafion.RTM. (E. I. du Pont de Nemours), Femion.RTM. (Asahi Glass 
Co., Ltd.), and Neosepta-F.RTM. (Tokuyama Soda Co. Industry Company) (Kyu 
(1985) supra). 
Perfluorinated ionomer membranes are characterized by high chemical and 
thermal stability and strength, high water permeability, and cation 
permselectivity. These characteristics make them ideal membranes in many 
separation applications (Kyu (1985) supra). Perfluorinated ionomer 
membranes are widely used in chlor-alkali cells, water electrolyzers, 
batteries, and fuel cells (Kipling (1982) in Perfluorinated Ionomer 
Membranes, A. Eisenberg and H. L. Yeager, eds. (American Chemical Society, 
Washington, D.C.) pp. 475-487). They have been used in a number of 
nonelectrochemical applications including chemical separations, organic 
syntheses, and catalytic systems (Moore and Martin (1988) Macromolecules 
21:1334-1339). 
Perfluorinated ionomer membranes contain carbon backbones made up of 
fluorocarbon chains. Ionic groups are connected to the backbone through 
side chains. The side chains can terminate in a variety of ionic groups, 
including sulfonic acid, carboxylic acid, sulfonium, or quaternary 
ammonium (Fujimura et al. (1981) Macromolecules 14:1309-1315). In the case 
of an acidic side chain, the locations of the anions are fixed whereas the 
cation can transport through the polymer. 
Nafion.RTM., manufactured by E. I. du Pont de Nemours, is a cation exchange 
membrane that consists of a fluorocarbon backbone with fluorocarbon 
sidechains (Besso and Eisenberg (1981) in Proceedings of the Symposium on 
Ion Exchange Transport and Interfacial Properties, R. S. Yeo and R. P. 
Buck, eds. (Electrochemical Society, Pennington, N.J.), pp. 197-209; 
Yeager and Eisenberg (1982) in Perfluorinated Ionomer Membranes, A. 
Eisenberg and H. L. Yeager, eds. (American Chemical Society, Washington, 
D.C.), pp. 1-6). Thin Nafion.RTM. films are particularly effective for the 
selective passage of water, cations, and water-soluble molecules, and as 
supports for facilitated transport separations. A primary application for 
Nafion.RTM. membranes is in the chlor-alkali industry (Yeager and Steck 
(1981) J. Electrochem. Soc. 128:1880-1884; Yeager and Eisenberg (1982) 
supra). Nafion.RTM. has also been used as a solid polymer electrolyte in 
an experimental photoelectrochemical cell (Sammells and Schmidt (1985) J. 
Electrochem. Soc. 132:520-522), and in a zinc bromide (ZnBr2) battery (Lim 
et al. (1977) J. Electrochem. Soc. 124:1154-1157; Will (1979) J. 
Electrochem. Soc. 126:36-42). A major limitation to more widespread use of 
Nafion.RTM. is its high resistance to mass transport through the polymer 
structure. 
Because of the technological importance of the perfluorinated ionomers, 
their microscopic structure and the relationship of structure to membrane 
transport properties have been extensively studied (Yeager and Steck 
(1981) supra; Yeager et al. (1982) J. Electrochem. Soc. 129:85-89; Yeo 
(1983) J. Electrochem. Soc. 130:533-538; Fales et al. (1986) in 
Proceedings of the Symposium on Engineering of Industrial Electrolytic 
Processes (Electrochemical Society, Pennington, N.J.), pp. 203-218; Sakai 
et al. (1986) J. Electrochemical Soc. 133:88-92; Fujimura et al. (1981) 
supra; Gierke et al. (1981) J. Polym. Sci. Polym. Phys. Ed. 19:1687-1704). 
Among the techniques used to probe different aspects of the structural 
features of perfluorinated ionomer membranes are small angle X-ray 
scattering, small angle neutron scattering, quasi-elastic neutron 
scattering, infrared nuclear magnetic resonance, and Mossbauer 
spectroscopy (Yeager and Eisenberg (1982) supra). 
Small angle X-ray scattering (SAXS) is a technique for studying material 
structural features that are on the order of a few nanometers in size 
(Kratky (1982) in Small Angle X-Ray Scattering, O. Glatter and O. Kratky, 
eds. (Academic Press, New York), Chapter 1). Any material with at least 
two phases having different electron densities will give a scattering 
pattern which is dependent on the shape and dimensions of the different 
regions (Porod (1982) in Small Angle X-ray Scattering, O. Glatter and O. 
Kratky, eds. (Academic Press, New York), Chapter 2). Electron density is 
defined as the moles of electrons per unit volume where each electron is a 
possible scattering sight for X-rays. 
For perfluorinated ionomers, a significant electron density difference 
between the ionic cluster and surrounding fluorocarbon region has been 
estimated (Roche et al. (1981) J. Polym. Sci. Polym. Phys. Ed. 19:1-11), 
supporting the idea that the PFSA microstructure contains at least two 
separate domains (Gierke et al. (1982) in Perfluorinated Ionomer Membranes 
(American Chemical Society, Washington, D.C.) pp. 195-216). One domain is 
composed primarily of the hydrophobic fluorocarbon backbone. A second 
domain contains ion-exchange sites that are part of the polymer backbone, 
called ionic clusters. A third domain is the interfacial region containing 
some side chain materials, small amounts of water, some sulfonate sites 
with cations and a relatively large fractional void volume. 
Ionic clusters are formed by the grouping of ionic sulfonate groups within 
the polymer (Roche et al. (1981) supra; Yeo and Cheng (1986) J. Appl. 
Polym. Sci. 32:5733-5741) and are small regions where ionic chemistry 
dominates. The average size of ionic clusters within Nafion.RTM. membranes 
has been estimated to be on the order of 40-50 .ANG. (Gierke et al. (1981) 
supra; Kyu (1985) supra). The ionic clusters may be linked by channels 
forming a network throughout the membrane. As with all ionomers, the 
sidechains are permanently attached to the polymer backbone at random 
intervals. The side chains are relatively immobile, but the counterion is 
free to move. Counterion motion makes these polymers ionic conductors. 
Specific molecules, which are polar or charged, can easily diffuse through 
a film of PFSA by way of the ionic cluster channels. 
Efforts have been made to modify the basic Nafion.RTM. homogeneous polymer 
film to produce materials with special characteristics, including 
lamination of fabric to the polymer film to increase its strength, 
composite membranes made up of layers of different equivalent weights of 
polymer film laminated together to increase anion rejection, and surface 
treatment to improve hydroxide ion rejection (Yeager and Eisenberg (1982) 
supra). 
The literature also contains numerous reports of structural and 
morphological modifications to PFSA polymers and film-casting strategies 
that attempt to improve the productivity of cast membranes (Moore and 
Martin (1988) supra and (1986) Anal. Chem. 58:2569; Liu and Martin (1990) 
J. Electrochem. Soc. 137:3114; Gebel et al. (1987) Macromolecules 
20:1425-1428; Heaney and Pellegrino (1989) J. Memb. Sci. 47:143-161). For 
example, Dow Chemical Corporation has a commercially available 
polyperfluorosulfonic acid (PFSA) material of substantially lower 
equivalent weight than Nafion.RTM. (U.S. Pat. No. 4,417,969 of Ezzell et 
al., issued Nov. 29, 1983). Equivalent weight is defined as the grams of 
polymer per one mole of ion exchange sites when the ionomer is in the acid 
form and dry (Yeager and Eisenberg (1982) supra). Low equivalent weight 
indicates a high density of ion-exchange sites per unit mass and often 
correlates well with lower mass transfer resistance (Gierke and Hsu (1982) 
supra; Yeo (1982) supra). Another report describes a procedure of heat 
treating Nafion.RTM. films that results in significantly increased 
permeability (Pellegrino et al. (1988) Gas Sep. and Purif. 2:126-130). 
Casting strategies such as forming very thin PFSA films on hollow fibers 
and other substrates to take advantage of more effective geometries also 
exist (U.S. Pat. No. 4,469,744 of Grot et al., issued Sep. 4, 1984). 
Attempts to increase the productivity of PFSA films often results in many 
of the advantageous chemical and physical properties being compromised. 
Two attempts to provide improved separator materials have included the use 
of a surfactant species in the polymer formation process. U.S. Pat. No. 
4,289,600, issued Sep. 15, 1981, to Lazarz et al. describes a microporous 
electrolytic cell separator produced from a mixture of 
polytetrafluoroethylene (PTFE), a particulate pore-forming material, and 
an organic fluorinated surfactant. The surfactant is added up to 50% by 
weight in an isopropanol-water solution to aid the blending of the PTFE 
and pore-forming material by lowering surface tension. However, the 
surfactant of the Lazarz et al. membrane is not incorporated into the 
polymer structure nor does it alter the membrane microstructure. Further, 
the surfactant does not improve membrane transport characteristics since 
the permeability of the Lazarz membrane results from addition of 
pore-forming particulate material. 
U.S. Pat. No. 4,741,744, issued May 3, 1988 to Wu et al. also describes a 
PFSA membrane with improved permeability characteristics produced from 
perfluorinated polymers containing pendant hydrated metal ionomer 
moieties. The Wu et al. membrane is produced by an emulsion polymerization 
of one or two types of monomers, a free radical initiator, a buffer and a 
fluorinated surfactant. The surfactant is used to aid in the 
polymerization process. The polymer formed is used to make membranes by a 
multitude of techniques, including casting films from solutions. The role 
of the surfactant in the Wu membrane specifically functions to form 
micelles in which polymerization takes place and to stabilize the polymer 
emulsion in the latex form throughout the reaction rather than to alter 
membrane microstructure so as to enhance membrane transport 
characteristics. 
The present invention describes the production of improved ionomer 
membranes from PFSA solutions that retain the desirable characteristics of 
high mechanical strength, high hydrophilicity, high solvent and 
temperature resistance, and high crystallinity, while having improved 
transport properties relative to membranes described in the prior art. 
This improvement is achieved by the addition of a surfactant species to 
the PFSA polymer solution prior to casting the polymer film, resulting in 
a membrane with a measurably altered membrane microstructure and improved 
transport characteristics. 
The improved membrane characteristics of the present invention may result 
from a number of possible routes. The presence of surfactant in the 
polymer solution may alter the internal microscopic structure of the 
membrane film, resulting in an improved polymer film morphology. The 
altered film morphology may be maintained with or without the persistant 
inclusion of the surfactant in the final membrane. If the surfactant 
persists in the final film, an improvement in the transport properties may 
result by interactions which include the surfactant. These include more 
ion-exchange sites for solvent, carriers, counterions, and transporting 
solutes. 
BRIEF SUMMARY OF THE INVENTION 
A variety of dopant species have been added to a PFSA polymer solution 
(Nafion.RTM.) prior to film casting. The resulting film membranes have 
improved microstructural crystallinity, decreased equivalent weights, and 
improved transport properties. 
The membranes of the present invention are useful in a number of ways, 
including: in the production of NaOH and Cl.sub.2 gas from the 
electrolytic dissolution of NaCl, the dehydration of gases used in medical 
and industrial processes, the separation of NH.sub.3, CO.sub.2, and 
H.sub.2 S from gaseous and liquid mixtures, the separation of acid gases 
from natural gas, the separation of water from alcohols and other organic 
solvents, including the separation of azeotropic compositions, the 
separation of olefin/saturate, olefin/olefin, and olefin isomer 
separations, electrodialytic separation of toxic and radioactive metals 
from aqueous streams, separation of organic liquids from each other for 
waste minimization and recycle, coatings on protective garments to guard 
against chemical and biological agents, and ion-specific coatings on 
electrochemical sensors. Further, the membranes of the present invention 
are useful in solid polymer electrolyte H.sub.2 /O.sub.2 fuel cells.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present invention describes an ion-exchange membrane with improved 
transport and permselectivity characteristics. The membrane of the present 
invention is achieved by the addition of surfactant species to the polymer 
solution prior to film casting. The resulting film membranes have a 
membrane microstructure with measurably altered membrane microstructure 
and exhibit improved transport characteristics. The membranes of the 
present invention are useful in a number of separation processes, 
including the separation of NH.sub.3, CO.sub.2, and H.sub.2 S from gaseous 
and liquid mixtures, in the production of NaOH and Cl.sub.2 gas from the 
electrolytic dissolution of NaCl, and the separation of toxic and 
radioactive metals from aqueous streams. 
The term "dopant" as used in the present disclosure refers to the 
surfactants mixed with the polymer solution, and the term "doped 
membranes" refer to the membranes formed from the polymer solution to 
which a surfactant species has been added. The doped membranes exhibit an 
altered microstructure resulting in improved transport properties. In some 
but not all cases, the doped membrane retains the dopant material. 
The present invention discloses the use of ammonium perfluoro-octane 
sulfonate (FC93), potassium perfluorooctane sulfonate (FC95), potassium 
perfluoroalkylcyclohexyl sulfonate (FC98), sodium butane sulfonate 
(ButSO.sub.3 Na), and sodium octane sulfonate (OctSO.sub.3 Na) as 
surfactant species added to the polymer solution prior to formation of the 
film membrane. In the preferred embodiment the dopant is a surfactant, and 
in the most preferred embodiment the dopant is a sulfonate. The present 
invention encompasses use of other surfactant molecules, the selection of 
which will be obvious to one skilled in the art as a result of this 
disclosure. The present invention discloses use of surfactant molecules 
resulting in a concentration of up to 60 mol % (as a percentage of total 
solids) in the cast membrane. 
The term "polymer solution" as used in the present disclosure refers to the 
mixing of pre-formed, soluble polymer (not monomers) materials, solvents 
and additives, not restricted to but including surfactants and organic 
salts. In a preferred embodiment the polymer solution is comprised of a 5% 
PFSA (Nafion.RTM., 1100 equivalent weight, obtained commercially as a 5% 
weight/weight (w/w) solution in light alcohols and ethers) diluted to 
between 0.5-5% (w/w) in a solvent. Upon evaporation of the solvent from 
the polymer solution, a membrane film is formed. The term "evaporation" as 
used in the present disclosure refers to the formation of a solid film 
from the polymer solution by a number of means including lowering the 
pressure and heating. Upon curing, which may be simultaneous with 
evaporation or subsequent to it, the polymer film is rendered mostly 
insoluble in common solvents (including those used in the original polymer 
solution). This insolubility persists at the temperatures and pressures 
that are likely to be encountered in current uses of the membrane film. 
"Curing" entails heating of the polymer film for 30 minutes or more at 
temperatures of 90.degree. C. or more. Curing is thought to involve the 
formation of crystalline regions within the region. This is commonly 
referred to as physical crosslinking, as differentiated from chemical 
crosslinking. The present invention encompasses use of other methods of 
curing a membrane film known to those skilled in the art, and will depend 
on specific experimental objectives. 
In the present invention, the polymer material and the surfactant are 
dissolved in a number of solvents, including dimethyl sulfoxide (DMSO), 
dimethylformamide (DMF), and methanol (MeOH). The invention encompasses 
the use of other appropriate solvents, the selection of which will become 
apparent to those skilled in the art as a result of the present 
disclosure. For example, Nafion.RTM. comes as a 5% solution in light 
alcohols, water, and ethers. 
Perfluorinated ion-exchange membranes are thought to contain at least two 
domains. One domain is a cluster which contains ion-exchange sites that 
are connected to the polymer backbone, called ionic clusters. A second 
domain is primarily fluorocarbon, throughout which are distributed the 
ionic clusters. Polar or charged molecules diffuse through the membrane by 
way of the ionic clusters. The speed of their diffusion depends on their 
ability to move from one cluster to another through the fluorocarbon 
region. The movement from one cluster to another can be made faster by 
several means, including decreasing the distance between clusters, and 
providing some ion exchange sites in the intervening fluorocarbon region. 
The presence of dopants in the formation of the membranes of the present 
invention result in faster permeation of diffusant molecules. Although not 
limited by theory, this may result from a dopant-induced increase in the 
size of the ionic clusters, thereby decreasing the distance between them. 
The dopants may also increase the diffusion of specific molecules through 
the membrane by an altered distribution of ionic pockets throughout the 
fluorocarbon region. Also, the dopants may alter the free volume of 
intervening interfacial regions between ionic clusters. 
The preferred method of making the improved membranes of the present 
invention is as follows: 
1) PFSA [Nafion.RTM., 1100 equivalent weight (EW)] is obtained commercially 
as a 5% (w/w) solution in light alcohols and ethers. The solution is 
diluted to between 0.5-5% (w/w) polymer in a solvent, such as dimethyl 
sulfoxide (DMSO), neutralized with an equivalent amount of sodium 
hydroxide, and heated to eliminate all solvents except DMSO; 
2) A dopant species, such as ammonium perfluorooctane sulfonate (FC93), 
potassium perfluoro-octane sulfonate (FC95), potassium 
perfluoro-alkylcyclohexyl sulfonate (FC98), sodium butane sulfonate 
(ButSO.sub.3 Na), and sodium octane sulfonate (OctSO.sub.3 Na), is 
prepared as a dilute solution (0.1%-3%) in the solvent; 
3) The above two solutions are mixed in proportions to give a specific 
ratio of dopant to polymer. The concentration of dopant plus polymer is 
adjusted to 1% and the solution delivered to a glass casting dish; 
4) The polymer-surfactant solution may be allowed to evaporate to dryness 
or the casting dish may be placed in an oven and heated to 175.degree. C. 
for 2 hours resulting in a thin, dry membrane film. 
The resulting membrane film contains 40-100 mol % polymer and 0-60 mol % 
surfactant (both as a percentage of total solids). In the preferred 
embodiment of the invention, the membrane film is comprised of between 
1-60 mole % surfactant. In the most preferred embodiment, the membrane 
film is comprised of more than 10 mole % surfactant. The membrane film may 
be used directly or detached from the glass surface by rinsing with warm 
water. 
The described general procedure results in high quality membranes with 
improved transport properties. However, the described procedure is by no 
means the only method for producing PFSA membranes of increased 
productivity by addition of surfactant molecules. It is envisioned that 
other methods of producing the membranes of the present invention will be 
obvious to those skilled in the art as a result of the present disclosure. 
It is envisioned that, as a result of the present invention, other casting 
solvents and other surfactant-type dopant molecules would be effective in 
improving PFSA film productivity. 
It is to be understood that both the foregoing general description and the 
following detailed description are exemplary and explanatory only, and are 
not restrictive of the invention, as claimed. 
Example 1 describes general methods known to the art for casting membranes. 
The present invention encompasses other methods known to the art for 
membrane formation, the selection of which will depend on the specific 
type of membrane required in each specific case. 
Example 2 describes the conditions under which SAXS profiles were obtained 
for several doped membranes. The results indicate that the presence of 
dopant added prior to membrane formation causes the semi-crystalline ionic 
cluster regions to achieve a more ordered arrangement within the membrane 
microstructure. 
Example 3 describes the equivalent weight determinations conducted with 
doped and undoped membranes. The results show that the addition of dopant 
to the polymer solution prior to membrane formation results in membranes 
with a lower equivalent weight. 
Example 4 describes vapor phase permeation measurements conducted with 
doped and undoped membranes. The results show that doped membranes are 
significantly more permeable to water. 
Example 5 describes experiments correlating the observed SAXS data with 
water or isopropanol permeation for membranes made up with DMSO or MEOH as 
the added solvent and varying percentages of FC-95 dopant. The results 
show that the best permselectivity was achieved with membranes composed of 
10% and 20% dopant. 
EXAMPLE 1 
General Methods for Casting Membranes 
General methods for casting membranes are well known to those of ordinary 
skill in the art. Kesting (1985) in Materials Science of Synthetic 
Membranes, D. R. Lloyd, ed. (American Chemical Society, Washington, D.C.) 
pp. 131-164, specifically incorported herein by reference, describes 
several methods of casting membranes, including the evaporative technique 
herein described. The invention encompasses other methods of forming 
membranes from solutions known to the art (U.S. Pat. No. 4,908,235). 
Nafion.RTM. polymer is commercially available as a 5% solution in mixed 
light alcohols, primarily isopropanol. This solution is diluted in a 
secodary casting solvent containing the surfactant, and the resulting 
mixture is evaporated to dryness. Heating of the resulting solid film 
creates an insoluble membrane. 
Specific Casting Conditions 
In one embodiment of the invention, a 4.32% solution of FC-95 in MeOH was 
added to the 5% Nafion.RTM. solution in a quantity such that the mol % of 
solids contributed by the surfactant was up to 40 mol %. Sufficient 
additional MeOH was then added to bring the total percent solids of the 
casting solution to about 1.7%. The membranes were allowed to evaporate 
overnight at ambient temperature and pressure in a glass dish. 
In another embodiment of the present invention, a 4.23% solution of FC-95 
in DMSO was added to the 5% Nafion.RTM. solution in a quantity such that 
the mol % of solids contributed by the surfactant was up to 60 mol %. 
Sufficient MeOH was then added to bring the total percent solids of the 
casting solution to about 1.7%. The membranes were allowed to evaporate at 
308K (35.degree. C.) and 3 torr in a glass dish. 
The dry films were either used directly or were hydrated with water and 
easily lifted off the casting dish. The membranes were heated for 1 hr at 
373 K (100.degree. C.). The resulting films were insoluble in aqueous 
solvent. The solids mass in each film was nominally 0.218 g and the 
thickness about 25 .mu.m. 
Table 1 lists samples of membranes formed by the methods herein described. 
The PFSA was diluted in one of three solvents: dimethylsulfoxide (DMSO), 
dimethylformamide (DMF), or methanol (MeOH). The pressure and temperature 
at which the solvent was evaporated to produce the film was either ambient 
pressure and temperature (84 kPa and 22.degree. C.)("amb"), or evacuation 
in a vacuum oven to 8 kPa ("vac"). The surfactants added to the casting 
solution where one of the following: CF.sub.3 --(CF.sub.2).sub.7 
--SO.sub.3 K ("FC95"), CF.sub.3 --(CF.sub.2).sub.7 --SO.sub.3 NH.sub.4 
("FC93"), CH.sub.3 --(CH.sub.2).sub.3 --SO.sub.3 Na ("ButSO.sub.3 "), or 
CH.sub.3 --(CH.sub.2).sub.7 --SO.sub.3 Na ("OctSO.sub.3 "). The percent 
solids at casting is the percent weight of PFSA plus the surfactant in the 
casting solution. The dopant mole fraction is the mole fraction of dopant 
based on total solids. 
TABLE 1 
______________________________________ 
Samples of Doped PFSA Membranes 
Dopant 
Mole 
Sample 
Solvent Pres/Temp. 
Dopant % Solids 
Fraction 
______________________________________ 
10301 DMSO vac/35 FC95 0.1 
10501 DMSO vac/35 FC95 0.6 
10703 DMSO vac/35 FC95 1.66 0.2 
11201 DMSO vac/35 FC95 1.65 0.4001 
11901 MEOH amb none 1.67 0.00 
11902 MEOH amb FC95 1.87 0.198 
12001 DMSO vac/35 none 1.67 0.00 
12002 MEOH amb FC95 2.78 0.398 
12101 DMSO vac/35 FC95 2.87 0.597 
22501 DMSO amb/165 none 0.925 0.00 
22502 DMSO amb/165 none 0.931 0.00 
22503 DMSO amb/165 FC95 0.89 0.399 
22504 DMSO amb/165 FC95 0.892 0.398 
42301 DMF vac/23 none 1.00 0.00 
42501 DMF vac/23 FC95 1.7 0.414 
42601 DMF vac/23 FC95 1.5 0.206 
51301 DMF amb/135 none 1.0 0.00 
51302 DMF amb/135 FC95 1.0 0.199 
51303 DMF amb/135 FC95 1.0 0.308 
51304 DMF amb/135 FC98 1.0 0.2 
51305 DMF amb/135 FC98 1.0 0.301 
51306 DMF amb/135 FC93 1.0 0.205 
51307 DMF amb/135 FC93 1.0 0.3 
51308 DMF amb/135 BuSO.sub.3 Na 
1.0 0.198 
51309 DMF amb/135 BuSO.sub.3 Na 
1.0 0.298 
51310 DMF amb/135 OctSO.sub. 3 Na 
1.0 0.209 
51311 DMF amb/135 OctSO.sub.3 Na 
1.0 0.3 
51401 DMF vac/23 none 3.0 0.00 
51402 DMF vac/23 FC93 3.0 0.207 
51403 DMF vac/23 FC93 3.0 0.3 
51404 DMF vac/23 FC95 3.0 0.199 
51405 DMF vac/23 FC95 3.0 0.299 
51406 DMF vac/23 FC98 2.9 0.202 
51407 DMF vac/23 FC98 2.8 0.3 
51408 DMF vac/23 BuSO.sub.3 Na 
2.7 0.198 
51409 DMF vac/23 BuSO.sub.3 Na 
2.4 0.3 
51410 DMF vac/23 OctSO.sub.3 Na 
2.9 0.202 
51411 DMF vac/23 OctSO.sub.3 Na 
2.7 0.3 
52301 DMF vac/23 none 1.0 0.00 
52302 DMF vac/23 none 1.0 0.00 
52303 DMF vac/23 FC93 1.0 0.2 
52307 DMF vac/23 FC95 1.0 0.2 
52308 DMF vac/23 FC95 1.0 0.2 
52309 DMF vac/23 FC95 1.0 0.31 
52310 DMF vac/23 FC95 1.0 0.3 
52311 DMF vac/23 FC98 1.0 0.21 
52312 DMF vac/23 FC98 1.0 0.21 
52313 DMF vac/23 FC98 1.0 0.3 
52314 DMF vac/23 FC98 1.0 0.3 
52315 DMF vac/23 BuSO.sub.3 Na 
1.0 0.2 
52316 DMF vac/23 BuSO.sub.3 Na 
1.0 0.2 
52317 DMF vac/23 BuSO.sub.3 Na 
1.0 0.3 
52318 DMF vac/23 BuSO.sub.3 Na 
1.0 0.3 
60401 DMSO vac/23 none 3.0 0.00 
60402 DMSO vac/23 FC93 3.0 0.21 
60403 DMSO vac/23 FC93 3.0 0.3 
60404 DMSO vac/23 FC95 3.0 0.2 
60405 DMSO vac/23 FC95 3.0 0.31 
60406 DMSO vac/23 FC98 3.0 0.2 
60407 DMSO vac/23 FC98 3.0 0.32 
60408 DMSO vac/23 BuSO.sub.3 Na 
3.0 0.21 
60409 DMSO vac/23 BuSO.sub.3 Na 
3.0 0.3 
60410 DMSO vac/23 OctSO.sub.3 Na 
3.0 0.2 
60411 DMSO vac/23 OctSO.sub.3 Na 
3.0 0.3 
61101 DMSO vac/75 none 3.0 0.00 
61102 DMSO vac/75 none 3.0 0.00 
61103 DMSO vac/75 FC93 3.0 0.21 
61104 DMSO vac/75 FC93 3.0 0.21 
61105 DMSO vac/75 FC93 3.0 0.3 
61106 DMSO vac/75 FC93 3.0 0.3 
61107 DMSO vac/75 FC95 3.0 0.21 
61108 DMSO vac/75 FC95 3.0 0.2 
61109 DMSO vac/75 FC95 3.0 0.29 
61110 DMSO vac/75 FC95 3.0 0.3 
61111 DMSO vac/75 FC98 3.0 0.2 
61112 DMSO vac/75 FC98 3.0 0.2 
61113 DMSO vac/75 FC98 3.0 0.31 
61114 DMSO vac/75 FC98 3.0 0.32 
61115 DMSO .vac/75 BuSO.sub.3 Na 
3.0 0.19 
61116 DMSO vac/75 BuSO.sub.3 Na 
3.0 0.2 
61117 DMSO vac/75 BuSO.sub.3 Na 
3.0 0.33 
61118 DMSO vac/75 BuSO.sub.3 Na 
3.0 0.31 
61119 DMSO vac/75 OctSO.sub.3 Na 
3.0 0.19 
61120 DMSO vac/75 OctSO.sub.3 Na 
3.0 0.2 
61121 DMSO vac/75 OctSO.sub.3 Na 
3.0 0.3 
61122 DMSO vac/75 OctSO.sub.3 Na 
3.0 0.3 
62304 DMSO vac/175 FC95 1.0 0.21 
62305 DMSO vac/175 FC95 1.0 0.31 
62306 DMSO vac/175 FC98 1.0 0.21 
62307 DMSO vac/175 FC98 1.0 0.31 
62308 DMSO vac/175 BuSO.sub.3 Na 
1.0 0.2 
62309 DMSO vac/175 BuSO.sub.3 Na 
1.0 0.32 
62310 DMSO vac/175 OctSO.sub.3 Na 
1.0 0.2 
62311 DMSO vac/175 OctSO.sub.3 Na 
1.0 0.29 
122010 
DMSO vac/35 FC95 0.3 
______________________________________ 
EXAMPLE 2 
Small-Angle X-Ray Scattering (SAXS) 
This technique has been used extensively to investigate the microstructure 
of ionomer membranes. SAXS peaks indicate dimensions and ordering of 
semicrystalline ionic cluster regions within the membrane. Since material 
transport occurs primarily through the ionic cluster regions, SAXS is a 
particularly useful technique to characterize the doped membranes of the 
present invention. 
A 10-m, digital SAXS camera at the National Institute of Standards of 
Technology (Gaithersburg, Md.) was used to obtain the scattering data. The 
camera uses pinhole optics for the collimation of the incident beam. The 
source was a 12-kW rotating copper anode operated at 45 kV and 180 mA with 
the Cu K .alpha. line selected. The sample to detector distance was set at 
2808 mm, and the two dimensional position sensitive detector was 
interfaced to a PDP-11 and VAX for graphics and data processing. 
Small lead masks with 2-mm diameter holes were used to select homogeneous 
regions of the samples. The sample exposures were approximately 1 hr with 
additional exposures for empty beam, dark current, and reference standard. 
No drift was observed in the empty beam exposures. 
The membrane samples were enclosed in small polypropylene (PP) bags after 
having soaked in deionized (DI) H.sub.2 O (18 mega-ohm resistance and less 
than 10 ppb total organic carbon) and folded. The samples were folded so 
that the beam passed through 4 layers of PFSA. In addition to the empty 
beam and polyethylene reference standard, a moist PP bag was also measured 
as a blank. The signal from the blank was subtracted from the radially 
averaged membrane sample signals. 
FIGS. 1 and 2 present a way of viewing the X-ray scattering data, called 
the Lorentz-corrected (or normalized) intensity versus Bragg spacing. FIG. 
1 is for samples made with DMSO as the added solvent, and FIG. 2 is for 
samples using MeOH. The percent solids in all samples was between 1.6 and 
2.8. The graph labels indicate the mol % of solids contributed by the 
FC95. Two samples are included that do not have dopant: NE111 is a 
developmental sample of Nafion.RTM. (supplied by Dupont Co.), and the 
other is a sample cast with 0% dopant but the same solvent. 
As seen from FIGS. 1 and 2, the doped samples show: a) a sharper peak at 40 
.ANG., and b) substantially greater peak intensity for similar sample 
sizes. Both of these characteristics indicate that the dopant is causing 
the semi-crystalline ionic cluster regions to achieve a more ordered 
arrangement in the membrane microstructure. 
EXAMPLE 3 
Equivalent Weight Determinations 
Ion-exchange experiments to determine whether the dopants were being 
incorporated into the membrane structure were done on three membranes: one 
undoped, one doped with potassium perfluoro-octane sulfonate, and one 
doped with sodium butane sulfonate. Equivalent weight is determined by 
measurement of an ion-exchange membrane's dry mass in each of two 
different cationic forms. The results are shown in Table 2: 
TABLE 2 
______________________________________ 
Membrane Equivalent Weights 
Membrane Equivalent Weight 
Dopant (g polymer/mole ion exchange sites) 
______________________________________ 
None 1110 
Potassium perfluoro- 
1010 
octane sulfonate 
Sodium butane sulfonate 
940 
______________________________________ 
EXAMPLE 4 
Vapor Permeations Measurements 
Vapor phase permeation experiments were done to determine the permeability 
of various molecules through doped and undoped membranes. The experiment 
involved placing a liquid in an open-cap vial fitted with a piece of 
membrane sealed into the cap. The permeation rate was measured by 
following the decrease in vial mass over time. The steady-state 
permeabilities for water transport through the membrane obtained are shown 
in Table 3: 
TABLE 3 
______________________________________ 
Membrane Water Permeability 
H.sub.2 O Permeability .times. 10.sup.-9 
Dopant (mol-cm)/(cm.sup.2 -sec-cmHg) 
______________________________________ 
None 7.6 
FC93 15.0 
FC95 8.1 
FC98 12.0 
ButSO.sub.3 
-- 
OctSO.sub.3 
15.0 
______________________________________ 
As seen in Table 3, the permeability of water in doped samples is 
significantly higher than the undoped sample, indicating less resistive 
transport paths in the doped membranes. 
Transport of a volatile non-permeating species (benzene) was also measured 
to verify that the membranes had no significant pinholes. The average 
benzene permeability for all samples was 5.times.10.sup.-13 
mol-cm/cm.sup.2 -sec-cm Hg. 
EXAMPLE 5 
Structure-Function Relationships 
Water and isopropanol vapor permeation experiments were conducted with the 
membranes of Example 2. The results of those tests are shown in Table 4: 
TABLE 4 
______________________________________ 
Water and Isopropanol Vapor Permeation 
Isopropanol 
H.sub.2 O 
H.sub.2 O/Isopropanol 
Solvent 
% FC-95 (.times. 10.sup.-7 mol-cm/cm.sup.2 min) 
______________________________________ 
DMSO 0 0.11 0.71 6.35 
DMSO 10 0.04 0.85 19.47 
DMSO 20 0.08 0.62 8.06 
DMSO 30 0.25 0.82 3.27 
DMSO 40 0.37 0.83 2.26 
DMSO 60 0.10 0.43 4.25 
MeOH 0 0.11 0.34 3.15 
MeOH 10 0.05 0.54 10.19 
MeOH 20 0.05 0.82 16.44 
MeOH 30 0.11 0.73 6.69 
MeOH 40 0.27 1.01 3.79 
NE111 0 0.12 0.69 5.79 
______________________________________ 
These data show significant variations in both the amount and ideal ratio 
of water or isopropanol that permeate the membranes. Ideal ratio (H.sub.2 
O/isopropanol) is a measure of the selectivity of membrane permeability. 
Table 4 shows that the addition of dopant significantly alters 
permeability, with the biggest improvement in ideal water selectivity 
versus isopropanol is seen in samples with 10% and 20% dopant.