A composite membrane is provided in which a porous substrate is impregnated with a polymeric composition comprising various combinations of .alpha.,.beta.,.beta.-trifluorostyrene, substituted .alpha.,.beta.,.beta.-trifluorostyrene and ethylene-based monomeric units. Where the polymeric composition includes ion-exchange moieties, the resultant composite membranes are useful in electrochemical applications, particularly as membrane electrolytes in electrochemical fuel cells.

FIELD OF THE INVENTION 
This invention relates generally to composite membranes comprising a porous 
substrate and a polymeric composition comprising various combinations of 
.alpha.,.beta.,.beta.-trifluorostyrene, substituted 
.alpha.,.beta.,.beta.-trifluorostyrene and ethylene-based monomeric units. 
Where the polymeric composition includes ion-exchange moieties, the 
resultant composite membranes are useful in electrochemical applications, 
particularly as membrane electrolytes in electrochemical fuel cells. 
BACKGROUND OF THE INVENTION 
Dense films can be obtained from solutions of 
poly-.alpha.,.beta.,.beta.-trifluorostyrene. However, the brittleness of 
these films greatly limits their application. Films obtained from some 
sulfonated poly-.alpha.,.beta.,.beta.-trifluorostyrenes can be used as 
ion-exchange membranes. However, such films often have unfavorable 
mechanical properties when wet, and are known to be very brittle in the 
dry state (see, for example, Russian Chemical Reviews, Vol. 59, p. 583 
(1988)). Such films are of little practical use in fuel cells due to their 
poor physical properties. Some improvements in mechanical properties have 
been achieved by blending sulfonated 
poly-.alpha.,.beta.,.beta.-trifluorostyrene with polyvinylidene fluoride 
and triethyl phosphate plasticizer, but these films remained 
unsatisfactory for application in electrochemical cells (see Fuel Cell 
Handbook, A. J. Appleby, published by Van Nostrand Reinhold, p. 286 
(1989)). 
U.S. Pat. No. 5,422,411 and the related patent applications mentioned above 
describe various polymeric compositions incorporating substituted 
.alpha.,.beta.,.beta.-trifluorostyrenes and some cases further 
incorporating substituted ethylenes. Typically these compositions, as 
membranes, possess favorable mechanical properties compared to 
poly-.alpha.,.beta.,.beta.-trifluorostyrene and sulfonated 
poly-.alpha.,.beta.,.beta.-trifluorostyrene, although some of the 
membranes have a tendency to become brittle in the fully dehydrated state, 
depending, for example, on the equivalent weight. This effect is most 
apparent at equivalent weights below approximately 380 g/mol. Ion-exchange 
membranes derived from these polymeric compositions are suitable for many 
applications, including use in electrochemical applications, such as fuel 
cells. 
For ease of handling, for example, in the preparation of membrane electrode 
assemblies for use in electrochemical fuel cells, the mechanical strength 
of the membrane in the dry state is important. In electrochemical 
applications, such as electrolytic cells and fuel cells, the dimensional 
stability (changes in the dimensions of the membrane due to changes in the 
degree of hydration) of the membrane during operation is also important. 
However, to improve performance, it is generally desirable to reduce 
membrane thickness and to decrease the equivalent weight (thereby 
increasing the water content) of the membrane electrolyte, both of which 
tend to decrease both the mechanical strength in the dry state and the 
dimensional stability in the wet state. One way to improve mechanical 
strength and dimensional stability in ionomeric membranes is through use 
of a substrate or support material, to give a composite membrane. The 
substrate is selected so that it imparts mechanical strength and 
dimensional stability to the membrane. The substrate material can be 
combined with the membrane polymeric material to form a composite membrane 
in a variety of ways. For example, if possible, an unsupported membrane 
can be preformed and then laminated to the porous substrate. 
Alternatively, a solution of the polymer can be impregnated into the 
porous substrate material, and the composite membrane subsequently dried. 
The formation of composite membranes via impregnation provides a more 
intimate contact between the two components, thus giving advantages over 
standard lamination approaches. 
Composite ion-exchange membranes prepared by impregnating commercially 
available porous polytetrafluoroethylene film (Gore-tex.RTM.) with 
Nafion.RTM., a perfluorosulfonate ionomer, have been described in Journal 
of the Electrochemical Society, Vol. 132, pp. 514-515 (1985). The major 
goal in the study was to develop a composite membrane with the desirable 
chemical and mechanical features of Nafion.RTM., but which could be 
produced at low cost. Indeed, based on the polymer loadings necessary to 
produce these composite membranes, they are a low cost alternative to the 
costly perfluorosulfonic acid membranes. As indicated above, however, 
these perfluorosulfonate ionomers are known to form membranes suitable for 
use in electrochemical applications without the use of a substrate. 
It has been discovered that polymers which have a tendency to become 
brittle in the dehydrated state can be rendered mechanically stable, even 
in the fully dehydrated state, by impregnation into suitable substrates. 
Furthermore, it has been discovered that even polymers which are poor film 
formers, or polymers which form films with mechanical properties and is 
dimensional stability which would preclude their use in electrochemical 
and other applications, can be made into composite membranes through 
incorporation into a suitable substrate. The resulting composite membranes 
have the desired physical properties for use in a wide range of 
applications. 
SUMMARY OF THE INVENTION 
In one aspect of the present invention, a composite membrane comprises a 
porous substrate impregnated with a polymeric composition comprising 
.alpha.,.beta.,.beta.-trifluorostyrene monomeric units. 
In another aspect, a composite membrane comprises a porous substrate 
impregnated with a polymeric composition comprising substituted 
.alpha.,.beta.,.beta.-trifluorostyrene monomeric units. Substituted 
.alpha.,.beta.,.beta.-trifluorostyrene monomeric units have at least one 
non-hydrogen substituent on the aromatic ring. In a preferred embodiment, 
the polymeric composition comprises at least two different substituted 
.alpha.,.beta.,.beta.-trifluorostyrene monomeric units. 
In a first embodiment of a composite membrane comprising a porous substrate 
impregnated with a polymeric composition comprising 
.alpha.,.beta.,.beta.-trifluorostyrene monomeric units, the polymeric 
composition further comprises ethylene monomeric units, the polymeric 
composition derived from a copolymerization reaction involving at least 
ethylene and .alpha.,.beta.,.beta.-trifluorostyrene. 
In a second embodiment of a composite membrane comprising a porous 
substrate impregnated with a polymeric composition comprising 
.alpha.,.beta.,.beta.-trifluorostyrene monomeric units, the polymeric 
composition further comprises partially fluorinated ethylene monomeric 
units, the polymeric composition derived from a copolymerization reaction 
involving at least .alpha.,.beta.,.beta.-trifluorostyrene and, for 
example, CH.sub.2 .dbd.CHF, CHF.dbd.CHF, CF.sub.2 .dbd.CH.sub.2, or 
CF.sub.2 .dbd.CHF. 
In a third embodiment of a composite membrane comprising a porous substrate 
impregnated with a polymeric composition comprising 
.alpha.,.beta.,.beta.-trifluorostyrene monomeric units, the polymeric 
composition further comprises tetrafluoroethylene monomeric units, the 
polymeric composition derived from a copolymerization reaction involving 
at least tetrafluoroethylene and .alpha.,.beta.,.beta.-trifluorostyrene. 
In a fourth embodiment of a composite membrane comprising a porous 
substrate impregnated with a polymeric composition comprising 
.alpha.,.beta.,.beta.-trifluorostyrene monomeric units, the polymeric 
composition further comprises: 
##STR1## 
where m is an integer greater than zero; Y is selected from the group 
consisting of chlorine, bromine, iodine, C.sub.x H.sub.y F.sub.z (where x 
is an integer greater than zero and y+z=2x+1), O--R (where R is selected 
from the group consisting of C.sub.x H.sub.y F.sub.z (where x is an 
integer greater than zero and y+z=2x+1) and aryls), CF.dbd.CF.sub.2, CN, 
COOH and CO.sub.2 R.sup.1 (where R.sup.1 is selected from the group 
consisting of perfluoroalkyls, aryls, and NR.sup.2 R.sup.3 where R.sup.2 
and R.sup.3 are selected from the group consisting of hydrogen, alkyls and 
aryls). 
In a fifth embodiment of a composite membrane comprising a porous substrate 
impregnated with a polymeric composition comprising 
.alpha.,.beta.,.beta.-trifluorostyrene monomeric units, the polymeric 
composition further comprises styrene monomeric units, the polymeric 
composition derived from a copolymerization reaction involving at least 
styrene and .alpha.,.beta.,.beta.-trifluorostyrene. 
In a sixth embodiment of a composite membrane comprising a porous substrate 
impregnated with a polymeric composition comprising 
.alpha.,.beta.,.beta.-trifluorostyrene monomeric units, the polymeric 
composition further comprises substituted styrene monomeric units, the 
polymeric composition derived from a copolymerization reaction involving 
at least a substituted styrene and .alpha.,.beta.,.beta.-trifluorostyrene. 
Substituted styrene monomeric units have at least one non-hydrogen 
substituent on the aromatic ring. 
In a first embodiment of a composite membrane comprising a porous substrate 
impregnated with a polymeric composition comprising substituted 
.alpha.,.beta.,.beta.-trifluorostyrene monomeric units, the polymeric 
composition comprises: 
##STR2## 
where m is an integer greater than zero. In a further embodiment the 
polymeric composition comprises: 
##STR3## 
where m is an integer greater than zero, and at least one of n, p and q is 
an integer greater than zero; A.sub.1, A.sub.2 and A.sub.3 are selected 
from the group consisting of hydrogen, halogens, C.sub.x H.sub.y F.sub.z 
(where x is an integer greater than zero and y+z=2x+1), CF.dbd.CF.sub.2, 
CN, NO.sub.2 and OH, O--R (where R is selected from the group consisting 
of alkyls and perfluoroalkyls and aryls). In a still further embodiment, 
the group from which A.sub.1, A.sub.2 and A.sub.3 are selected further 
consists of SO.sub.3 H, PO.sub.2 H.sub.2, PO.sub.3 H.sub.2, CH.sub.2 
PO.sub.3 H.sub.2, COOH, OSO.sub.3 H, OPO.sub.2 H.sub.2, OPO.sub.3 H.sub.2, 
NR.sub.3.sup.+ (where R is selected from the group consisting of alkyls, 
perfluoroalkyls and aryls) and CH.sub.2 NR.sub.3.sup.+ (where R is 
selected from the group consisting of alkyls, perfluoroalkyls and aryls), 
and at least one of A.sub.1, A.sub.2 and A.sub.3 is selected from the 
group consisting of SO.sub.3 H, PO.sub.2 H.sub.2, PO.sub.3 H.sub.2, 
CH.sub.2 PO.sub.3 H.sub.2, COOH, OSO.sub.3 H, OPO.sub.2 H2, OPO.sub.3 
H.sub.2, NR.sub.3.sup.+ (where R is selected from the group consisting of 
alkyls, perfluoroalkyls and aryls) and CH.sub.2 NR.sub.3.sup.+ (where R 
is selected from the group consisting of alkyls, perfluoroalkyls and 
aryls). 
In a second embodiment of a composite membrane comprising a porous 
substrate impregnated with a polymeric composition comprising substituted 
.alpha.,.beta.,.beta.-trifluorostyrene monomeric units, the polymeric 
composition comprises: 
##STR4## 
where at least one of n, p and q is an integer greater than zero; A.sub.1, 
A.sub.2 and A.sub.3 are selected from the group consisting of 
CF.dbd.CF.sub.2, CN, NO.sub.2 and OH, O--R (where R is selected from the 
group consisting of C.sub.x H.sub.y F.sub.z (where x is an integer greater 
than three and y+z=2x+1), and aryls). 
In a third embodiment of a composite membrane comprising a porous substrate 
impregnated with a polymeric composition comprising substituted 
.alpha.,.beta.,.beta.-trifluorostyrene monomeric units, the polymeric 
composition comprises: 
##STR5## 
where m is an integer greater than zero. 
In a fourth embodiment of a composite membrane comprising a porous 
substrate impregnated with a polymeric composition comprising substituted 
.alpha.,.beta.,.beta.-trifluorostyrene monomeric units, the polymeric 
composition comprises: 
##STR6## 
where m is an integer greater than zero; X is selected from the group 
consisting of PO.sub.2 H.sub.2, PO.sub.3 H.sub.2, CH.sub.2 PO.sub.3 
H.sub.2, COOH, OSO.sub.3 H, OPO.sub.2 H.sub.2, OPO.sub.3 H.sub.2, 
OArSO.sub.3 H where Ar is an aryl, NR.sub.3.sup.+ (where R is selected 
from the group consisting of alkyls, perfluoroalkyls and aryls) and 
CH.sub.2 NR.sub.3.sup.+ (where R is selected from the group consisting of 
alkyls, perfluoroalkyls and aryls). 
In a fifth embodiment of a composite membrane comprising a porous substrate 
impregnated with a polymeric composition comprising substituted 
.alpha.,.beta.,.beta.-trifluorostyrene monomeric units, the polymeric 
composition comprises: 
##STR7## 
where m is an integer greater than zero, and at least one of n, p and q is 
an integer greater than zero; X is selected from the group consisting of 
SO.sub.3 H, PO.sub.2 H.sub.2, PO.sub.3 H.sub.2, CH.sub.2 PO.sub.3 H.sub.2, 
COOH, OSO.sub.3 H, OPO.sub.2 H.sub.2, OPO.sub.3 H.sub.2, OArSO.sub.3 H 
where Ar is an aryl, NR.sub.3.sup.+ (where R is selected from the group 
consisting of alkyls, perfluoroalkyls and aryls) and CH.sub.2 
NR.sub.3.sup.+ (where R is selected from the group consisting of alkyls, 
perfluoroalkyls and aryls) ; A.sub.1, A.sub.2 and A.sub.3 are selected 
from the group consisting of halogens, C.sub.x H.sub.y F.sub.z (where x is 
an integer greater than zero and y+z=2.times.1), CF.dbd.CF.sub.2, CN, 
NO.sub.2 and OH, O--R (where R is selected from the group consisting of 
alkyls and perfluoroalkyls and aryls). In a further embodiment, the group 
from which A.sub.1, A.sub.2 and A.sub.3 are selected further consists of 
hydrogen. In a still further embodiment, the group from which A.sub.1, 
A.sub.2 and A.sub.3 are selected further consists of SO.sub.3 H, PO.sub.2 
H.sub.2, PO.sub.3 H.sub.2, CH.sub.2 PO.sub.3 H.sub.2, COOH, OSO.sub.3 H, 
OPO.sub.2 H.sub.2, OPO.sub.3 H.sub.2, NR.sub.3.sup.+ (where R is selected 
from the group consisting of alkyls, perfluoroalkyls and aryls) and 
CH.sub.2 NR.sub.3.sup.+ (where R is selected from the group consisting of 
alkyls, perfluoroalkyls and aryls), and at least one of A.sub.1, A.sub.2 
and A.sub.3 is selected from the group consisting of SO.sub.3 H, PO.sub.2 
H.sub.2, PO.sub.3 H.sub.2, CH.sub.2 PO.sub.3 H.sub.2, COOH, OSO.sub.3 H, 
OPO.sub.2 H.sub.2, OPO.sub.3 H.sub.2, NR.sub.3.sup.+ (where R is selected 
from the group consisting of alkyls, perfluoroalkyls and aryls) and 
CH.sub.2 NR.sub.3.sup.+ (where R is selected from the group consisting of 
alkyls, perfluoroalkyls and aryls). 
In a sixth embodiment of a composite membrane comprising a porous substrate 
impregnated with a polymeric composition comprising substituted 
.alpha.,.beta.,.beta.-trifluorostyrene monomeric units, the polymeric 
composition comprises: 
##STR8## 
where m is an integer greater than zero; B and D are selected from the 
group consisting of hydrogen, SO.sub.2 F, SO.sub.3 H, PO.sub.2 H.sub.2, 
PO.sub.3 H.sub.2, CH.sub.2 PO.sub.3 H.sub.2, COOH, OSO.sub.3 H, OPO.sub.2 
H.sub.2, OPO.sub.3 H.sub.2, NR.sub.3.sup.+ (where R is selected from the 
group consisting of alkyls, perfluoroalkyls and aryls) and CH.sub.2 
NR.sub.3.sup.+ (where R is selected from the group consisting of alkyls, 
perfluoroalkyls and aryls). In a further embodiment, the polymeric 
composition comprises: 
##STR9## 
where m is an integer greater than zero, and at least one of n, p and q is 
an integer greater than zero; B and D are selected from the group 
consisting of hydrogen, SO.sub.2 F, SO.sub.3 H, PO.sub.2 H.sub.2, PO.sub.3 
H.sub.2, CH.sub.2 PO.sub.3 H.sub.2, COOH, OSO.sub.3 H, OPO.sub.2 H.sub.2, 
OPO.sub.3 H.sub.2, NR.sub.3.sup.+ (where R is selected from the group 
consisting of alkyls, perfluoroalkyls and aryls) and CH.sub.2 
NR.sub.3.sup.+ (where R is selected from the group consisting of alkyls, 
perfluoroalkyls and aryls); A.sub.1, A.sub.2 and A.sub.3 are selected from 
the group consisting of hydrogen, SO.sub.2 F, halogens, C.sub.x H.sub.y 
F.sub.z (where x is an integer greater than zero and y+z=2x+1), 
CF.dbd.CF.sub.2, CN, NO.sub.2 and OH, O--R (where R is selected from the 
group consisting of alkyls and perfluoroalkyls and aryls). In a still 
further embodiment, the group from which A.sub.1, A.sub.2 and A.sub.3 are 
selected further consists of SO.sub.3 H, PO.sub.2 H.sub.2, PO.sub.3 
H.sub.2, CH.sub.2 PO.sub.3 H.sub.2, COOH, OSO.sub.3 H, OPO.sub.2 H.sub.2, 
OPO.sub.3 H.sub.2, NR.sub.3.sup.+ (where R is selected from the group 
consisting of alkyls, perfluoroalkyls and aryls) and CH.sub.2 
NR.sub.3.sup.+ (where R is selected from the group consisting of alkyls, 
perfluoroalkyls and aryls), and at least one of A.sub.1, A.sub.2 and 
A.sub.3 is selected from the group consisting of SO.sub.3 H, PO.sub.2 
H.sub.2, PO.sub.3 H.sub.2, CH.sub.2 PO.sub.3 H.sub.2, COOH, OSO.sub.3 H, 
OPO.sub.2, OPO.sub.3 H.sub.2, NR.sub.3.sup.+ (where R is selected from 
the group consisting of alkyls, perfluoroalkyls and aryls) and CH.sub.2 
NR.sub.3.sup.+ (where R is selected from the group consisting of alkyls, 
perfluoroalkyls and aryls). In preferred embodiments B is hydrogen. 
In a seventh embodiment of a composite membrane comprising a porous 
substrate impregnated with a polymeric composition comprising substituted 
.alpha.,.beta.,.beta.-trifluorostyrene monomeric units, the polymeric 
composition further comprises ethylene monomeric units. 
In an eighth embodiment of a composite membrane comprising a porous 
substrate impregnated with a polymeric composition comprising substituted 
.alpha.,.beta.,.beta.-trifluorostyrene monomeric units, the polymeric 
composition further comprises partially fluorinated ethylene monomeric 
units, the polymeric composition derived from a copolymerization reaction 
involving, for example, CH.sub.2 .dbd.CHF, CHF.dbd.CHF, 
In a ninth embodiment of a composite membrane comprising a porous substrate 
impregnated with a polymeric composition comprising substituted 
.alpha.,.beta.,.beta.-trifluorostyrene monomeric units, the polymeric 
composition further comprises tetrafluoroethylene monomeric units. 
In a tenth embodiment of a composite membrane comprising a porous substrate 
impregnated with a polymeric composition comprising substituted 
.alpha.,.beta.,.beta.-trifluorostyrene monomeric units, the polymeric 
composition further comprises: 
##STR10## 
where m is an integer greater than zero; Y is selected from the group 
consisting of chlorine, bromine, iodine, C.sub.x H.sub.y F.sub.z (where x 
is an integer greater than zero and y+z=2x+1), O--R (where R is selected 
from the group consisting of C.sub.x H.sub.y F.sub.z (where x is an 
integer greater than zero and y+z=2x+1) and aryls), CF.dbd.CF.sub.2, CN, 
COOH and CO.sub.2 R.sup.1 (where R.sup.1 is selected from the group 
consisting of alkyls, perfluoroalkyls, aryls, and NR.sup.2 R.sup.3 where 
R.sup.2 and R.sup.3 are selected from the group consisting of hydrogen, 
alkyls and aryls). 
In an eleventh embodiment of a composite membrane comprising a porous 
substrate impregnated with a polymeric composition comprising substituted 
.alpha.,.beta.,.beta.-trifluorostyrene monomeric units, the polymeric 
composition further comprises styrene monomeric units. 
In a twelfth embodiment of a composite membrane comprising a porous 
substrate impregnated with a polymeric composition comprising substituted 
.alpha.,.beta.,.beta.-trifluorostyrene monomeric units, the polymeric 
composition further comprises substituted styrene monomeric units. 
Substituted styrene monomeric units have at least one non-hydrogen 
substituent on the aromatic ring. 
In the aspects and embodiments described above, the substrate is preferably 
a porous film or sheet material. For electrochemical applications, for 
example, preferred porous substrates comprise, or consist essentially of, 
porous polyolefins. Preferred polyolefins are polyethylene and 
polypropylene. Particularly preferred substrates comprise, or consist 
essentially of, porous polytetrafluoroethylene, also known as expanded 
polytetrafluoroethylene. 
In a preferred aspect, a composite membrane comprises a porous substrate 
impregnated with a polymeric composition comprising: 
##STR11## 
where m and n are integers greater than zero and A.sub.1 is selected from 
the group consisting of fluorine, CF.sub.3 and para-phenoxy. In a further 
embodiment of this preferred aspect, the group from which A.sub.1 is 
selected further consists of hydrogen. 
In another preferred aspect, a composite membrane comprises a porous 
substrate impregnated with a polymeric composition comprising: 
##STR12## 
where m, n, and p are integers greater than zero and A.sub.1 and A.sub.2 
are selected from the group consisting of hydrogen, fluorine, CF.sub.3, 
and para-phenoxy. 
In another preferred aspect, a composite membrane comprises a porous 
substrate impregnated with a polymeric composition comprising: 
##STR13## 
where m and n are integers greater than zero and X is selected from the 
group consisting of para-SO.sub.2 F, meta-SO.sub.3 H and para-SO.sub.3 H. 
In yet another preferred aspect, a composite membrane comprises a porous 
substrate impregnated with a polymeric composition comprising: 
##STR14## 
where m and q are integers greater than zero, n and p are zero or an 
integer greater than zero; X is selected from the group consisting of 
para-SO.sub.2 F, meta-SO.sub.3 H and para-SO.sub.3 H; and A.sub.1 and 
A.sub.2 are selected from the group consisting of hydrogen, fluorine, 
CF.sub.3, and para-phenoxy. In a further embodiment of this preferred 
aspect, n is an integer greater than zero. 
In still another preferred aspect, a composite membrane comprises a porous 
substrate impregnated with a polymeric composition comprising: 
##STR15## 
where m and q are integers greater than zero, n and p are zero or an 
integer greater than zero; X is selected from the group consisting of 
para-SO.sub.2 F, meta-SO.sub.3 H and para-SO.sub.3 H; and A.sub.1 and 
A.sub.2 are selected from the group consisting of hydrogen, fluorine, 
CF.sub.3, and para-phenoxy. In a further embodiment of this preferred 
aspect, n is an integer greater than zero. 
In the aspects and embodiments described above, the polymeric compositions 
can consist essentially of the described monomeric units. 
In all of the above preferred aspects, preferably the porous substrate 
comprises polytetrafluoroethylene. A preferred porous substrate consists 
essentially of polytetrafluoroethylene. 
In the aspects and embodiments described above, the A.sub.1, A.sub.2, 
A.sub.3 substituents may be further elaborated by known means such as, for 
example, by hydrolysis of the CN group to form COOH or by reduction with 
common reducing agents (such as, for example, Raney nickel) to form a 
primary amine, thereby transforming the A.sub.1, A.sub.2 and A.sub.3 
substituents into ion-exchange moieties. The resulting polymeric 
composition may thus comprise one or more type of ion-exchange moiety, and 
may also comprise both cation and anion exchange moieties. 
The term "monomeric unit" as used herein indicates that the polymeric 
composition contains the described fragment or unit, and is obtained by a 
polymerization reaction involving the corresponding unsaturated monomer. 
The substituents on the aromatic rings (including, for example, A.sub.1, 
A.sub.2, A.sub.3, X, B and D) may each be located in the ortho, meta or 
para positions, as indicated in the formulas wherein the chemical bond 
drawn for the substituents intersects the aromatic ring. In preferred 
aspects of the described embodiments, the substituents are in the meta or 
para positions. 
As used herein, the term "aryl" refers to a substituted or unsubstituted 
phenyl group. The formula C.sub.x H.sub.y F.sub.z (where x is an integer 
greater than zero and y+z=2x+1) is used to indicate alkyl, perfluoroalkyl 
or partially fluorinated alkyl groups. 
In accordance with convention in the art, the above chemical formulas for 
polymeric compositions containing more than two monomeric units (where at 
least three of m, n, p and q are greater than zero) are intended to 
indicate that the monomeric units are present in the polymeric 
composition, but are not limited to the particular order in which the 
monomeric units are set forth in each general formula. For example, random 
linear copolymers, alternating copolymers and linear block copolymers, 
formed from the indicated monomeric units, are contemplated.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Methods for preparing the polymeric compositions described herein are 
described in the related applications or will be apparent to those skilled 
in the art. 
The preferred substrate material is dependent on the application in which 
the composite membrane is to be used. The substrate material preferably 
has good mechanical properties, is chemically and thermally stable in the 
environment in which the composite membrane is to be used, is tolerant of 
the solvent used for impregnation, and in most applications is preferably 
flexible. For example, the porous substrate can be a woven or nonwoven 
fabric or cloth, or can be made of paper, fiber glass, cellulosics or a 
ceramic material. Preferred substrates for electrochemical applications 
are porous polymeric materials. Preferred polymeric materials are, for 
example, hydrocarbons such as porous polyolefins, especially polyethylene 
and polypropylene. In some applications, a perfluorinated polymeric 
substrate may be preferred, for example, a preferred substrate material, 
when the composite membrane is to be used in an electrochemical fuel cell, 
is porous polytetrafluoroethylene, also known as expanded 
polytetrafluoroethylene. Porous polyolefins and polytetrafluoroethylenes 
typically have excellent mechanical strength, flexibility and do not swell 
in water. Polytetrafluoroethylene offers additional advantages in that it 
is also chemically inert, and porous polytetrafluoroethylene films with 
different characteristics are commercially available from various sources. 
It may be possible to obtain or prepare other suitable porous polymeric 
substrates from, such as, for example, polyvinylidene fluoride or 
polysulfones. Copolymeric substrates such as, for example, 
poly(ethylene-co-tetrafluoroethylene) and 
poly(tetrafluoroethylene-co-hexafluoropropylene), may also be used. 
The degree of porosity, pore size and thickness of the substrate used in 
the composite membrane can be selected to suit the application. For use of 
the composite membrane as an electrolyte in an electrochemical fuel cell, 
the substrate thickness is preferably 10-200 .mu.m, and more preferably 
25-50 .mu.m, the preferable average pore diameter is 0.1-1.0 .mu.m, and 
the preferable porosity is 50-98%, more preferably 75-90%. 
Depending on the application the resultant composite membrane may be gas 
permeable or gas impermeable. The loading of the polymeric composition on 
the substrate can be varied in order to control the porosity of the 
resultant composite membrane. For fuel cell applications, the composite 
membrane is preferably substantially gas impermeable, thus the degree of 
impregnation and loading is such that the porosity of the composite 
membrane is reduced essentially to zero. 
In a method for preparing composite membranes, the polymeric composition is 
dissolved in a solvent, typically an organic solvent, to form a solution. 
The solvent used will depend, for example, on both the nature of the 
polymeric composition and the substrate. For impregnation of porous 
polyolefins with the type of polymeric compositions described herein, 
suitable solvents include N,N-dimethylformamide, N-methylpyrrolidone, 
dimethylsulfoxide and N,N-dimethylacetamide. When polytetrafluoroethylene 
is the substrate, an alcohol or mixture of alcohols (chosen, for example, 
from methanol, ethanol and propan-2-ol) is often the preferred solvent. 
The concentration of the solution will depend on the loading desired, and 
whether the composite membrane is to be porous or not. For example, if the 
composite membrane is to be gas permeable a lower concentration is 
generally preferred. 
The porous substrate is then impregnated, for example, by constraining the 
substrate in a frame and dipping or soaking it in the solution. The 
contact time is dependent on the viscosity and percentage solids of the 
solution. Other techniques known in the art, such as ultrasonication, may 
be used to facilitate impregnation. Also, multiple impregnations, possibly 
with different polymeric compositions, may be desirable for some 
applications. The substrate is then removed from the solution and the 
composite membrane dried preferably in a humidity controlled atmosphere 
(generally at less than or equal to 2% relative humidity) at above ambient 
temperatures. 
If the composite membrane includes proton-exchange moieties and is to be 
used in, for example, a proton-exchange membrane fuel cell, it is removed 
from the frame, treated with 1M hydrochloric acid and washed with 
deionized water prior to use. 
The means by which the process described above could be modified for 
impregnation of non-membrane substrates, and also for a continuous 
composite membrane manufacturing process will be apparent to those skilled 
in the art. 
In the preparation of composite ion-exchange membranes, the ion-exchange 
moieties can be: 
(i) present in the polymeric composition prior to its impregnation into the 
substrate; or 
(ii) introduced post-impregnation through further reaction of the polymeric 
composition on the substrate; or 
(iii) introduced via conversion of precursor groups, present in the 
polymeric composition, after impregnation. 
If the ion-exchange moieties are to be introduced via a post-impregnation 
conversion or reaction, the substrate needs to selected such that it can 
withstand the post-impregnation treatment step. For example, in 
post-impregnation introduction of ion-exchange moieties, the ion-exchange 
moieties may be introduced into polymeric compositions containing 
unsubstituted .alpha.,.beta.,.beta.-trifluorostyrene units (so called 
"base polymers") via aromatic substitution of at least a portion of those 
units, after preparation of a composite membrane. For example, pendant 
unsubstituted phenyl rings in the composite membrane can be conveniently 
sulfonated (see U.S. Pat. No. 5,422,411) to produce a composite 
cation-exchange membrane. Similarly, such pendant unsubstituted phenyl 
rings may be phosphorylated, carboxylated, quaternary-aminoalkylated or 
chloromethylated, and further modified to include --CH.sub.2 PO.sub.3 
H.sub.2, --CH.sub.2 NR.sub.3.sup.+ where R is an alkyl, or --CH.sub.2 
NAr.sub.3.sup.+ (where Ar is a substituted or unsubstituted phenyl group) 
and other substituents, to provide cation-exchange or anion-exchange 
composite membranes. Further still, the pendent phenyl moiety may contain 
a hydroxyl group which can be elaborated by known methods to generate 
--OSO.sub.3 H, --OPO.sub.2 H.sub.2 and --OPO.sub.3 H.sub.2 cation-exchange 
sites on the composite membrane. 
The approach in which the ion-exchange functionality is introduced 
post-impregnation via conversion of a precursor using simple 
post-impregnation procedure, such as hydrolysis, can be advantageous. For 
example, composite membranes comprising polymers containing sulfonyl 
fluoride moieties (--SO.sub.2 F) can be hydrolyzed to generate --SO.sub.3 
H cation-exchange sites. In a typical hydrolysis reaction, the sulfonyl 
fluoride is converted to the free sulfonic acid functionality by treatment 
of the composite membrane with concentrated aqueous alkali metal hydroxide 
at elevated temperatures. This and other procedures for the hydrolysis of 
--SO.sub.2 F to --SO.sub.3 H are well-known to those skilled in the art. 
The latter approach to the introduction of --SO.sub.3 H moieties offers 
advantages over sulfonation of a base polymer in the composite membrane. 
For example, it permits greater control over the ion-exchange capacity of 
the resultant composite membrane. 
Membranes including sulfonyl fluoride substituted 
.alpha.,.beta.,.beta.-trifluorostyrene monomeric units are described in a 
related application. Unsupported membranes containing a significant 
proportion of sulfonyl fluoride substituted 
.alpha.,.beta.,.beta.-trifluorostyrene monomeric units can be very 
fragile. The mechanical properties of these precursor ion-exchange 
membranes can be significantly enhanced through incorporation into a 
porous substrate. 
It may be advantageous to introduce ion-exchange moieties after preparation 
of the composite membranes, as described in (ii) and (iii) above. For 
example, in electrochemical applications where the preferred substrates 
are typically hydrophobic, the preparation of a composite membrane by 
first impregnating the substrate with a solution of a non-ionic polymer 
which is also essentially hydrophobic may lead to more facile and improved 
impregnation. 
The following examples are for purposes of illustration and are not 
intended to limit the invention. Examples 1-3 describe the preparation of 
composite ion-exchange membranes in which porous, high density 
polyethylene is used as the substrate. Examples 4 and 5 describe the 
preparation of composite ion-exchange membranes in which expanded 
polytetrafluoroethylene is used as the porous substrate. In Examples 1, 2, 
4 and 5 the ion-exchange moieties were present in the polymeric 
composition prior to its impregnation into the substrate. In Example 3 the 
ion-exchange moiety was generated by hydrolysis of sulfonyl fluoride 
moieties after preparation of the composite membrane. Example 6 sets forth 
the procedure used to test the composite ion-exchange membranes, prepared 
as described in Examples 1-5, as membrane electrolytes in an 
electrochemical fuel cell. 
EXAMPLE 1 
Porous polyethylene impregnated-with a sulfonated copolymer of 
.alpha.,.beta.,.beta.-trifluorostyrene and 
m-trifluoromethyl-.alpha.,.beta.,.beta.-trifluorostyrene (Composite 
Membrane A) 
The substrate, a 9 inch.times.9 inch piece of high density polyethylene 
(obtained from 3M, product ID #43-9100-6770-1, 81% porosity, approximately 
50 micron) was clamped in a frame and immersed in a N,N-dimethylformamide 
solution (7% w/w) of a sulfonated copolymer of 
.alpha.,.beta.,.beta.-trifluorostyrene and 
m-trifluoromethyl-.alpha.,.beta.,.beta.-trifluorostyrene (equivalent 
weight 384 g/mol) in a glass container. The container was covered to 
exclude moisture and particulate contaminants. After 1 hour excess polymer 
solution was removed and the transparent, wetted substrate was placed to 
dry in a chamber at approximately 2% relative humidity, at 50.degree. C. 
After approximately 3 hours the dry composite membrane, now opaque, was a 
mechanically strong flexible film. On immersion in 1M hydrochloric acid 
(to ensure protonation of all the sulfonic acid moieties), and subsequent 
washing with deionized water, the composite membrane once again became 
transparent. The wet composite membrane (50-60 micron thick) was also 
strong and flexible. 
EXAMPLE 2 
Porous polyethylene impregnated with sulfonated 
poly-.alpha.,.beta.,.beta.-trifluorostyrene (Composite Membrane B) 
The substrate, a 10 inch.times.10 inch piece of high density polyethylene 
(from 3M, product ID #43-9100-6770-1, 81% porosity, 50 micron) was clamped 
in a frame and immersed in a N,N-dimethylformamide solution (7% w/w) of a 
sulfonated polymer of .alpha.,.beta.,.beta.-trifluorostyrene (equivalent 
weight 430 g/mol) in a glass container. The container was covered to 
exclude moisture and particulate contaminants. After 2 hours excess 
polymer solution was removed and the transparent, wetted substrate was 
placed to dry in a chamber at approximately 2% relative humidity, at 
50.degree. C. After approximately 3 hours the dry composite membrane, now 
opaque, was a mechanically strong flexible film, in contrast to the 
analogous unsupported membrane which would be extremely fragile in the dry 
state. On immersion in 1M hydrochloric acid (to ensure protonation of all 
the sulfonic acid moieties), and subsequent washing with deionized water, 
the composite membrane once again became transparent. The wet composite 
membrane (approximately 100 micron thick) was also strong and flexible. 
EXAMPLE 3 
Porous polyethylene impregnated with a copolymer of 
.alpha.,.beta.,.beta.-trifluorostyrene, 
m-trifluoromethyl-.alpha.,.beta.,.beta.-trifluorostyrene and p-sulfonyl 
fluoride-.alpha.,.beta.,.beta.-trifluorostyrene, and subsequent hydrolysis 
(Composite Membrane C) 
The substrate, a 10 inch.times.10 inch piece of high density polyethylene 
(from 3M, product ID #43-9100-6770-1, 81% porosity, approximately 50 
micron) was clamped in a frame and immersed in a N,N-dimethylformamide 
solution (5% w/w) of a copolymer of 
.alpha.,.beta.,.beta.-trifluorostyrene, m-trifluoromethyl-.alpha.,.beta.,. 
beta.-trifluorostyrene and p-sulfonyl 
fluoride-.alpha.,.beta.,.beta.-trifluorostyrene (equivalent weight 480 
g/mol after hydrolysis) in a glass container. The container was covered to 
exclude moisture and particulate contaminants. After 2 hours excess 
polymer solution was removed and the transparent, wetted substrate was 
placed to dry in a chamber at approximately 2% relative humidity, at 
50.degree. C. After approximately 3 hours the dry composite membrane was a 
mechanically strong flexible film. The sulfonyl fluoride moieties were 
hydrolyzed by treatment of the composite membrane with potassium hydroxide 
solution (approximately 6% w/w, in 5:1 w/w water:1-methoxy-2-propanol) at 
60.degree. C. (see U.S. Pat. No. 5,310,765). The composite membrane was 
then immersed in 1M hydrochloric acid to ensure protonation of all the 
sulfonic acid moieties in the composite membrane, and subsequently washed 
with deionized water. The wet, hydrolyzed composite membrane (50-70 micron 
thick) was also strong and flexible. 
EXAMPLE 4 
Expanded polytetrafluoroethylene impregnated with a sulfonated copolymer of 
.alpha.,.beta.,.beta.-trifluorostyrene and 
m-trifluoromethyl-.alpha.,.beta.,.beta.-trifluorostyrene (Composite 
Membrane D) 
The substrate, an 8 inch.times.8 inch piece of expanded 
polytetrafluoroethylene (Tetratex.RTM. obtained from Tetratec Corporation, 
80-90% porosity, approximately 38 micron, 0.45 micron pore size) was 
clamped in a frame and immersed in a methanol/propan-2-ol (3:1) solution 
(approximately 5% w/v) of a sulfonated copolymer of 
.alpha.,.beta.,.beta.-trifluorostyrene and 
m-trifluoromethyl-.alpha.,.beta.,.beta.-trifluorostyrene (equivalent 
weight 412 g/mol) in a glass container. The container was covered to 
exclude moisture and particulate contaminants. After 18 hours excess 
polymer solution was removed and the transparent, wetted substrate was 
placed to dry in a chamber at approximately 2% relative humidity, at 
50.degree. C. After approximately 1.5 hours the dry composite membrane, 
now opaque, was a mechanically strong flexible film. On immersion in 1M 
hydrochloric acid (to ensure protonation of all the sulfonic acid 
moieties), and subsequent washing with deionized water, the composite 
membrane once again became transparent. The wet composite membrane (50-60 
micron thick) was also strong and flexible. 
EXAMPLE 5 
Expanded polytetrafluoroethylene impregnated with a sulfonated copolymer of 
.alpha.,.beta.,.beta.-trifluorostyrene and 
m-trifluoromethyl-.alpha.,.beta.,.beta.-trifluorostyrene (Composite 
Membrane E) 
The composite membrane was prepared as described in Example 4, using a 
sulfonated copolymer of .alpha.,.beta.,.beta.-trifluorostyrene and 
m-trifluoromethyl-.alpha.,.beta.,.beta.-trifluorostyrene with a lower 
equivalent weight (362 g/mol) and impregnating the substrate for 30 
minutes. The resulting dry composite membrane was a mechanically strong 
flexible film, in contrast to the analogous unsupported membrane which, at 
this low equivalent weight, is extremely fragile and readily reduced to a 
powder on handling. The wet composite membrane (25-40 micron thick) was 
also strong and flexible, again in contrast to the unsupported membrane 
which is fragile and dimensionally unstable, and is therefore of limited 
use in electrochemical fuel cells. 
EXAMPLE 6 
Each of the composite membranes prepared as described above was bonded to 
two catalyzed carbon fiber paper electrodes at room temperature under 
7,500 pounds of pressure. Each membrane electrode assembly was tested in a 
Ballard Mark IV single cell fuel cell (see U.S. Pat. Nos. 4,988,583; 
5,108,849; 5,170,124; 5,176,966 and 5,200,278; all incorporated herein by 
reference in their entirety). The following operating conditions applied 
to the fuel cell in which the membranes were tested: 
Temperature: 70.degree. C. 
Reactant inlet pressure: 
24 psi for both air and hydrogen Reactant stoichiometries: 
2.0 air and 1.15 hydrogen. 
The membrane electrode assemblies incorporating the composite membranes 
were tested for 200-1400 hours, depending on availability of testing 
equipment. 
FIGS. 1-5 are polarization plots of voltage as a function of current 
density for composite membranes A-E, respectively, employed in membrane 
electrode assemblies in the electrochemical fuel cell. The data is 
comparable to data reported for unsupported membranes in related U.S. Pat. 
No. 5,422,411. 
While particular elements, embodiments and applications of the present 
invention have been shown and described, it will be understood, of course, 
that the invention is not limited thereto since modifications may be made 
by those skilled in the art, particularly in light of the foregoing 
teachings. It is therefore contemplated by the appended claims to cover 
such modifications as incorporate those features which come within the 
spirit and scope of the invention.