Journal of New Materials for Electrochemical Systems 6, 9-15 (2003)
© J. New. Mat. Electrochem. Systems
Effect of Equivalent Weight on Water Sorption, PTFE-Like Crystallinity,
and Ionic Conductivity in Bis[(Perfluoroalkyl)Sulfonyl]
Imide Perfluorinated Ionomers
J. R. Atkins, C. R. Sides, S. E. Creager*, J. L. Harris, W. T. Pennington, B. H. Thomas
and D. D. DesMarteau
Department of Chemistry, Clemson University, Clemson, South Carolina 29634-0973 USA
(Received February 22, 2002; received revised form August 20, 2002)
Abstract: Measurements of water absorption and ionic conductivity as a function of relative humidity (RH) were carried out on membranes
comprised of bis[(perfluoroalkyl)sulfonyl] imide ionomers of equivalent weights 1470, 1200, and 1075 g equiv-1, and on a sample of the
perfluorosulfonic ionomer Nafion™ ionomer of equivalent weight 1100 g equiv-1 for comparison. All of the ionomers exhibited decreased water
absorption and ionic conductivity with decreasing RH. Within the sulfonyl imide series, the extents of the decreases correlated with ionomer
equivalent weight, such that the highest equivalent weight ionomer (1470 g equiv-1) exhibited the lowest conductivity and the sharpest drop in
conductivity with decreasing RH compared with the lower equivalent weight ionomers (1200 g equiv-1 and 1075 g equiv-1). This observation also
correlates with WAXD and DSC data which indicate an increase in PTFE-like crystallinity with increasing ionomer equivalent weight. The
observed dependencies of water absorption and ionic conductivity on RH and ionomer equivalent weight for the bis[(perfluoroalkyl)sulfonyl] imide
ionomers are similar to that which has been reported for Nafion™, which suggests that the phase-separated ionomer structures and the effect of
water sorption on the phase-separated structure are qualitatively similar for the two ionomer classes.
Keywords: PEM Fuel Cell, Membrane, Fluorinated Ionomer.
[1, 2] indicate the presence of hydrophilic ionic clusters
within the ionomer. The size and shape of the clusters vary
only slightly with ionomer equivalent weight (EW), however
the connections between the clusters are strongly dependent
upon the swelling of the ionomer with the absorption of water,
which in turn is strongly dependent upon ionomer EW.
Volume expansion of the ionomer is inhibited by the PTFElike crystallinity in the ionomers, which tends to be greater for
higher-EW ionomers. This explains the observation that ionic
conduction are also very strongly dependent on ionomer
equivalent weight, being greatest at low ionomer EW[3] when
water absorption is greatest and volume expansion on water
absorption is permitted[7]. Unfortunately, most ionomers also
become softer and more water soluble with decreasing EW,
and the resulting poor mechanical properties can limit the
useful EW range of the ionomers for fuel cell applications.
Thus, the ideal ionomer for fuel cell applications is one which
achieves high conductivity by enabling volume expansion on
1. INTRODUCTION
The membrane electrolyte in a PEM fuel cell serves to
conduct protons from the anode to cathode, separate the
reactant gases, and provide a physical structure onto which
the electrodes are attached. The material used to fabricate the
membrane should therefore possess high ionic conductivity,
good chemical stability, and good mechanical stability.
Perfluorinated sulfonic acid ionomers such as Nafion™
exhibit all of these qualities and are therefore leading
candidates for use in PEM fuel cells.
Perfluorinated sulfonic acid ionomers such as Nafion™
consist of hydrophilic acid groups bound to a hydrophobic
PTFE backbone in a phase-separated morphology. Previous
studies of Nafion™ by small-angle x-ray diffraction (SAXD)
*To whom correspondence should be addressed: E-mail:
[email protected]; Fax: +1(864) 656-6613
9
10
J.R. Atkins et al./ J. New. Mat. Electrochem. Systems 6, 9-15 (2003)
water absorption, while not allowing so much volume
expansion as to cause the membrane to become so soft that it
loses dimensional stability.
A new class of ionomer materials being developed for PEM
fuel
cell
applications
is
based
upon
the
bis[(perfluoroalkyl)sulfonyl imide acid group whose structure
is illustrated in Figure 1.
When copolymerized with
tetrafluoroethylene, these materials produce ionomers with a
primary structure that is very similar to Nafion™ except that
the perfluoroalkylsulfonic acid group in Nafion™ is replaced
with a bis[(perfluoroalkyl)sulfonyl] imide group in the new
materials. The bis[(perfluoroalkyl)sulfonyl]imide group has
been shown to exhibit greater thermal stability [8] and
stronger gas phase acidity [9] than the perfluorosulfonic acid
group which suggests that ionomers based on that acid group
may be well suited for use in PEM fuel cells.
a)
(CF2CF2)n
(CF2CF)
x
OCF2CFOCF2CF2
CF3
b)
(CF2CF2)n
(CF2CF)
SO3H
H
x
OCF2CFOCF2CF2
SO2NSO2CF3
CF3
Figure 1. Chemical structures of (a) Nafion™ and
(b) bis[(perfluoroalkyl)sulfonyl]imide.
Previous work has showed that early-generation sulfonyl
imide ionomers have ionic conductivities similar to Nafion™
[6, 10]. In the present work we explore more thoroughly the
effect of ionomer EW on water absorption and ionic
conductivity
for
the
bis[(perfluoroalkyl)sulfonyl]imide
ionomers. We find strong correlations among ionomer EW,
water absorption, and ionic conductivity, with both water
absorption and ionic conductivity being greater for low
equivalent weight materials. The strong similarities in water
absorption and ionic conductivity between Nafion™ and the
sulfonyl imide ionomers suggest that the two materials
possess similar phase-separated ionomer structures and water
absorption mechanisms.
emulsion polymerization process in a pressurized semi-batch
reactor system. Reaction temperatures were between 6° - 12°C
and pressures were between 145 - 150 psig. The
polymerization reactions were initiated with bisulfite/
persulfate using Dow FC-143 (C7F15CO2NH4) as emulsifier and
a phosphate buffer to control pH. The monomer in aqueous
solution was pumped continually into the system as TFE was
added batchwise from a pressurized tank to maintain the feed
ratio approximately constant.
The TFE consumption was
determined through monitoring pressure drops during the
reaction.
Polymerization reactions were terminated with
hydrochloric acid upon the consumption of the desired
amount of TFE to achieve a desired target equivalent weight.
Ionomers were recovered from polymerization reactions by
coagulation with 70% hydrochloric acid and collected as
white solids. Equivalent weights (EW) were determined by
titration of ionomers in acid form with standard base (NaOH).
Ionomer membranes were formed by solvent casting of
solubilized ionomers in N,N’-dimethylformamide (DMF)
solvent onto stainless steel plates.
Solvent was slowly
removed in a vacuum oven under a flow of nitrogen to give
plastic membrane films. The membranes were then annealed
at 220 – 250oC for 2-4 hours. The resultant membranes were
then boiled in 70% nitric acid to clean and convert to the H+
form, then boiled in deionized (DI) water with regular changes
of the water until the wash water was neutral. Prior to use, the
washed membranes were treated sequentially with DI water,
3% hydrogen peroxide, 5% hydrochloric acid, and again DI
water[12]. Nafion™ 117 membranes were obtained from
commercial sources (CG Processing, Rockdale, DE, USA), and
were cleaned and converted to the H+ form by boiling for one
hour each in 70% nitric acid, 0.5M sulfuric acid, and DI water
prior to use.
CF 3SO 3H
NaOH
P 4O 10
O(SO2CF 3)2
CF 3SO2NHNa
NH 3
(MeSi)2NH
CF 3SO2NH 2
CF 3SO2NHNaSiMe 3
CF 2BrCF BrOCF 2CF(CF 3)OCF 2CF 2SO 2F
CF 2BrCF BrOCF 2CF(CF 3)OCF 2CF 2SO 2N(Na)SO2CF 3
1) Zn/CH3CN
+
2) H
3) distillation
2. EXPERIMENTAL
Perfluorinated sulfonyl imide ionomers were synthesized from
the relevant perfluorinated vinyl ether monomer and
tetrafluoroethylene (TFE) using adaptations of previously
described methods [11] (Fig. 2). The sulfonyl imide vinyl
ether monomer was synthesized in the sodium salt form as
described previously [11]. Monomer in the sodium salt form
was co-polymerized with TFE via an aqueous free-radical
CF 2
CFOCF 2CF(CF 3)OCF 2CF 2SO2N(H)SO 2CF 3
CF 2
CFOCF 2CF(CF 3)OCF 2CF 2SO2N(Na)SO2CF 3
NaOH
Figure 2. Reaction scheme for the synthesis of the perfluorinated
vinyl ether monomers.
11
Effect of Equivalent Weight on Water Sorption, PTFE-Like Crystallinity / J. New. Mat. Electrochem. Systems 6, 9-15 (2003)
Ionic conductivities were measured in ionomers as a function
of RH at ambient temperature using AC impedance
spectroscopy. A two-point probe conductivity cell with
platinized platinum electrodes similar to that used by
Zawodzinski [15] was used to measure conductivity in lateral
configuration, parallel to the membrane plane. The cell was
fabricated from PTFE and had a 1 cm pathlength. Membranes
were cut into strips approximately 3 cm long and 1 cm wide
(more precise measurements of the length and width were
made with each sample) then affixed in the conductivity cell,
which in turn was placed in the controlled RH chamber.
Membrane impedances were measured in potentiostatic mode
over a frequency range of 20,000 to 100 Hz with 0 V DC bias
and AC voltage amplitudes between 1 and 100 mV using a
frequency response analyzer (1280B, Solartron, Inc., Houston,
Texas, USA) controlled by Zplot for Windows software 2.1a
(Scribner Associates, Inc., Southern Pines, North Carolina,
USA). Phase angles were always found to be within five
degrees of zero over the entire frequency range, signifying
that the measured impedance was primarily resistive and
contains little or no contribution from contact impedance.
Wide-angle X-ray diffraction measurements were made under
ambient RH and temperature conditions using a Scintag XDS/
2000 q - q diffractometer with Cu-Ka1 radiation (l = 1.54060
Å) and an intrinsic germanium solid-state detection system.
Step scans (at one second step-1 with a step size of 0.01° step1) were taken between 10° and 24° (2q) on 5 mm x 5 mm
membrane samples on a zero-background quartz sample pan.
Differential scanning calorimetry measurements were made
using a Perkin Elmer series 7 thermal analysis system. Prior to
analysis samples were dried under vacuum overnight at 120°
C. Samples were run from 30° to 340° C at a scan rate of 10°
minute-1 in stainless steel pans.
3. RESULTS AND DISCUSSION
Perfluorinated ionomers such as Nafion™ and the present
sulfonyl imide ionomers exhibit a strong interdependence
between water content and ionic conductivity[6, 10]. In the
case of Nafion™, this interdependence comes about because
the connectivity among water-filled clusters is strongly
dependent on the ionomer water content. As water is lost from
the ionomer, the channels linking the clusters together
become narrower and eventually close off, and the ionic
conductivity is strongly diminished.
The dependence of
ionomer conductivity on water content is also strongly
dependent on ionomer equivalent weight (EW); in the case of
Nafion™, low EW ionomers have larger water-filled clusters
and exhibit higher conductivity than higher EW ionomers.
This interdependence among ionomer EW, water content, and
ionic conductivity is a central feature of the properties of
these ionomers, and has important consequences for their
usage in fuel cell technology.
In the case of bis[(perfluoroalkyl)sulfonyl]imide ionomers,
few data are available on the interdependence among ionomer
EW, water content, and conductivity. Thus, we have made
measurements of water content and ionic conductivity for a
series of sulfonyl imide ionomers over an EW range from
1470 to 1075 g equiv-1. Figure 3 and Table 1 present water
Water Absorption (# H2Os per acid)
Water absorption was measured gravimetrically at ambient
temperature and in different RH environments by first soaking
the membranes in DI water for four hours and then suspending
them in a specific RH environment for ninety minutes to allow
for equilibration with the environment. Tests using
equilibration times up to six hours showed no mass change
after ninety minutes, which we take to mean that the water
uptake/loss reaction in these membranes has reached
equilibrium after ninety minutes. Relative humidities were
controlled using saturated salt solutions in an enclosed
chamber. DI water and saturated ammonium sulfate, sodium
bromide, calcium chloride, and potassium hydroxide
solutions were used to produce 100%, 81%, 58%, 31%, and
9% RH environments respectively [13, 14]. Water absorption
was measured in descending order, beginning with the highest
RH and working down to the lowest. Following the last
measurement, membranes were dried under vacuum at 110° C
for four hours to obtain the dry mass, and water absorption
was calculated from the difference between the mass of the
membrane at a specific RH and the dry mass.
50
Sulfonyl Imide EW 1470
Sulfonyl Imide EW 1200
Sulfonyl Imide EW 1075
Nafion EW 1100
40
30
20
10
0
0
20
40
60
80
100
% Relative Humidity
Figure 3. Water absorption of the sulfonyl imide ionomers and Nafion
1100 at various relative humidities and ambient temperature and
pressure.
Table 1. Water Absorption of Sulfonyl Imide Ionomers and Nafion™
1100.
Water absorption ( # H2Os per acid site)
% Relative
Humidity
100
81
58
31
9
Imide 1470
Imide 1200
Imide 1075
NafionTM 1100
21
8
4
3
2
28
8
5
4
1
48
17
3
2
1
19
10
5
3
1
12
J.R. Atkins et al./ J. New. Mat. Electrochem. Systems 6, 9-15 (2003)
Figure 4 and Table 2 present ionic conductivities for the three
different EW sulfonyl imide ionomers at ambient temperature
The coupled nature of water content (which is set by the RH)
and ionic conduction may be further analyzed via plots of
conductivity vs. water content, as presented in Figure 5 for the
three different EW sulfonyl imide ionomers and the 1100 EW
Nafion™ sample. Two regions are observed in each of these
plots. At lower water contents, λ ≤ 5 (λ = # H2Os per acid
-1
Conductivity (S cm)
absorption data (in units of water molecules retained per acid
group) for these ionomers at ambient temperature under
various RH conditions.
All of the ionomers exhibit an
increase in water content with increasing RH. The greatest
differences are seen at the higher RH values, with the lowest
EW ionomer (1075 EW) showing the highest water content.
At lower RH values all of the ionomers studied (including the
1100 EW Nafion™ sample) behave quite similarly, exhibiting
water contents of 5 waters / acid or less for 58% RH and
below.
2.0e-2
1.5e-2
1.0e-2
5.0e-3
0.0
a
0
10
20
30
# H 2 Os per acid
-1
Conductivity (S cm)
1e+0
1e-2
1e-3
1e-4
6.0e-2
4.0e-2
2.0e-2
0.0
10
20
30
1e-9
0
20
40
60
80
100
% Relative Humidity
Figure 4. The ionic conductivity of the sulfonyl imide ionomers and
Nafion 1100 at various relative humidities. Measurements were made
at using AC impedance spectroscopy at ambient temperature and
pressure.
Table 2. Ionic Conductivity of Sulfonyl Imide Ionomers and Nafion™
1100.
Conductivity (S cm-1)
Imide 1470
Imide 1200
Imide 1075
1.1e-2
2.1e-3
2.3e-4
2.9e-6
1.0e-8
5.3e-2
8.5e-3
2.7e-3
3.8e-4
5.9e-5
5.2e-2
2.1e-2
5.1e-3
4.6e-4
6.2e-5
TM
Nafion
1100
6.2e-2
2.1e-2
8.0e-3
1.4e-3
1.1e-4
under various RH conditions.
Conductivities are also
strongly dependent on RH, and are always highest at the
highest RH. Conductivity is also dependent upon ionomer
EW but in a complex way. Specifically, the two lower EW
imide ionomers (1075 and 1200 EW) exbibit behavior similar
to that of the 1100 EW Nafion™ ionomer, whereas the highest
EW imide ionomer (1470 EW) exhibits a much smaller
conductivity than the two lower EW ionomers over the full
RH range studied. Additionally, the higher EW ionomer
exhibits a stronger dependence of conductivity on RH, i.e. the
conductivity drops much more steeply as RH is diminished
for this ionomer compared with the lower EW ionomers.
6.0e-2
4.0e-2
2.0e-2
0.0
c
0
10
20
30
40
50
60
# H 2 Os per acid
-1
1e-8
-1
Sulfonyl Imide EW 1470
Sulfonyl Imide EW 1200
Sulfonyl Imide EW 1075
Nafion EW 1100
1e-7
Conductivity (S cm)
# H 2 Os per acid
1e-6
% Relative
Humidity
100
81
58
31
9
b
0
1e-5
Conductivity (S cm)
-1
Conductivity (S cm )
1e-1
8.0e-2
6.0e-2
4.0e-2
2.0e-2
d
0.0
0
10
20
30
# H 2 Os per acid
Figure 5.
Ionic conductivity vs. water absorption for the
bis[(perfluoroalkyl)sulfonyl]imide ionomers EW 1470 (a), EW 1200
(b), EW 1075 (c), and NafionTM 1100 (d).
group), conductivity increases approximately exponentially
with water content, whereas at high water content, λ ≥ 5,
conductivity depends approximately linearly on water
content.
The presence of two regions in these plots is
consistent with a two-step hydration process. In the case of
Nafion™, it is thought that the first step involves water
localized near acid groups by hydrogen bonding[16], and the
second step involves additional absorbed water that fills the
ionic clusters of the ionomer, thereby swelling it.
PTFE-like crystallinity is thought to be crucial in
perfluorinated ionomers since it provides the structural
rigidity to hold the ionomer together as it absorbs water.
Wide-angle X-ray diffraction (WAXD) was therefore used to
study
PTFE-like
crystallinity
in
the
present
bis[(perfluoroalkyl)sulfonyl]imide ionomers as a function of
13
S c a n n in g In t e n s it y
Effect of Equivalent Weight on Water Sorption, PTFE-Like Crystallinity / J. New. Mat. Electrochem. Systems 6, 9-15 (2003)
80000
a
60000
40000
20000
S c a n n in g In t e n s it y
0
12
16
20
24
2 -th e ta (d e g re e s )
80000
Table 3. PTFE-like Crystallinity of Sulfonyl Imide Ionomers and
Nafion™ 1100.
b
60000
40000
20000
0
S c a n n in g In t e n s it y
12
16
20
24
2 -th e ta (d e g re e s )
80000
C
Technique
XRD
(percent
crystallinity)
DSC
(Heat of
fusion, J g-1)
Imide 1470
Imide 1200
Imide 1075
NafionTM 1100
66.3
57.9
33.6
6.7
3.92
3.27
2.10
--
60000
40000
20000
0
12
S c a t t e r in g In t e n s it y
EW ionomers always exhibit the highest PTFE-like
crystallinity, as expected given that they also contain the
greatest TFE content. All three of the sulfonyl imide ionomers
exhibit much higher PTFE-like crystallinities than the
Nafion™ sample, despite the fact that two of the three imide
ionomers have equivalent weights that are not greatly
different from that of Nafion™. Possible reasons for this
discrepancy are discussed below.
16
20
24
20
24
2 -th e ta (d e g re e s )
80000
d
60000
40000
20000
0
12
16
2 -th e ta (d e g re e s )
Figure 6.
Wide angle X-ray diffraction data for the
bis[(perfluoroalkyl)sulfonyl]imide ionomers EW 1470 (a), EW 1200
(b), EW 1075 (c), and Nafion™ 1135 (d).
Another way of studying crystallinity in polymers is via
scanning calorimetry. Crystalline regions exhibit a melting
endotherm on heating, and in favorable cases the heat of
fusion for this endotherm may be used to estimate the degree
of crystallinity. [19, 20] Thus, as a complement to the WAXD
studies, we sought to study the PTFE-like crystallinity in the
present ionomers by differential scanning calorimetry (DSC),
which measures the heat of fusion associated with the melting
of crystalline PTFE-like regions. Figure 6 and Table 3 present
the results of these studies. Endothermic peaks near 325° C in
the thermograms in Figure 7 correlate with melting of PTFElike regions in the ionomers. Table 3 presents estimates of the
heat of fusion, normalized to ionomer mass, for each of the
three sulfonyl imide ionomers. It is difficult to directly
convert these values into degrees of PTFE-like crystallinity
70
EW 1470
Heat Flow (mW)
ionomer EW. Figure 6 presents plots of scattering intensity
vs. scattering angle (2θ) for the three sulfonyl imide ionomers
in question and an 1100 EW Nafion™ sample. All four
ionomers feature a broad diffraction peak at approximately 2θ
= 16° which is thought to be related to the short-range order of
perfluoroalkyl chains near the ionic clusters.
The three
sulfonyl imide ionomers also exhibit a sharper diffraction
peak at approximately 2θ = 18° which is thought to be related
to the PTFE-like crystalline regions [7, 17, 18]. The Nafion™
sample also exhibits this peak but it is much less prominent
and exists only as a shoulder on the larger peak. By assuming
that scattering intensity is directly proportional to the amount
of material giving rise to the scattering, the overall degree of
PTFE-like crystallinity may be determined as the ratio of the
area of the crystalline peak to the overall area of both peaks.
Table 3 presents values for the PTFE-like crystallinities of all
the ionomers in question determined in this way. The highest
65
60
EW 1200
55
EW 1075
50
45
0
100
200
300
400
Temperature (° C)
Figure 7.
Differential scanning calorimetry data for the
bis[(perfluoroalkyl)sulfonyl]imide ionomers EW 1470, EW 1200, and
EW 1075. The heat of fusion was calculated from the peaks seen
between 320° and 330° C, corresponding to melting point of pure
PTFE.
14
J.R. Atkins et al./ J. New. Mat. Electrochem. Systems 6, 9-15 (2003)
of fusion probably includes contributions from other
processes besides just the melting enthalpy of crystalline
PTFE regions. Even so, we note that the measured heats of
fusion increase monotonically with ionomer EW, which is
consistent with the conclusion from the WAXD studies that
the higher EW ionomers have a greater degree of PTFE-like
crystallinity.
Both Nafion™ and the bis[(perfluoroalkyl)sulfonyl]imide
ionomers show a strong dependence of water absorption and
ionic conductivity on ionomer EW and the relative humidity
of the ionomer environment.
While changes in acid
concentration
(equivalent weight) in the ionomer are
expected to affect conductivity, the observed dependencies in
Nafion™ are much greater than anticipated based on
predictions from simple theories of electrolyte solutions, e.g.
even under highly humidifying conditions conductivity
diminishes by nearly an order of magnitude for modest
increases in ionomer EW.
As explained above, the
explanation that has been put forth for this behavior in
Nafion™ is that the major determinant of conductivity is not
simply acid concentration, but the action of a network of ionconducting channels that link ion-rich clusters together. Our
observation of similar behavior in the imide ionomers leads us
to propose that the imide ionomers possess a similar tertiary
structure whereby conductivity is determined in large part by
a network of ion-conducting channels that link acid-rich
clusters togethers. This proposal is quite reasonable given the
obvious similarity in the primary structures of Nafion and the
imide ionomers (see Figure 1).
Independent evidence for a clustering morphology in
bis[(perfluoroalkyl)sulfonyl]imide
ionomers
has
been
reported from fluorescence spectroscopy [21-23] data in
membranes, and in solution from small angle neutron
scattering (SANS) studies [24].
SANS data have been
interpreted in terms of cylindrical micelles that maintain the
same radii but increase in length with increasing ionomer EW.
These findings, along with the similarities in water absorption
and conductivity between Nafion™ and the experimental
bis[(perfluoroalkyl)sulfonyl]imide ionomers reported here and
previously [6, 10], support the idea that the two materials
probably have similar internal structures.
Membrane mechanical properties and the detailed structure of
the network of ion-conducting channels are strongly
dependent upon the PTFE-like crystallinity of the ionomers.
The present WAXD studies show that the PTFE-like
crystallinity
of
the
bis[(perfluoroalkyl)sulfonyl]imide
ionomers increases with increasing ionomer EW.
It is
believed that the greater PTFE-like crystallinity for the higher
EW ionomers stiffens the membranes and inhibits the volume
expansion required for extensive water absorption. Therefore,
the formation of a percolation network for transport of ions
between and among the ion-rich clusters is hindered.
Finally, despite the similarities between Nafion™ and the
bis[(perfluoroalkyl)sulfonyl]imide ionomers in the effects of
EW and RH on water absorption and ionic conductivity, we
note that differences also exist in the absolute magnitudes of
the conductivities and the effects of water and RH on
conductivity, as well as in the overall PTFE-like crystallinity
of ionomers of similar EW. While the different structures of
the acid groups in the two ionomers may explain some of
these differences, differences in the details of membrane
synthesis and processing may also be important. For example,
Nafion™ is typically processed in the sulfonyl fluoride form
which is a thermoplastic material that may be melt processed,
whereas the bis[(perfluoroalkyl)sulfonyl]imide ionomers must
be fabricated as membranes by solution casting. Also, while
Nafion™ is a commercial product for which the synthesis is
reasonably well-developed and is conducted on a
comparatively large scale, the synthetic methods used to
prepare the bis[(perfluoroalkyl)sulfonyl]imides are still being
developed and are conducted on a much smaller scale which
may lead to greater lot-to-lot variation and greater variation in
the degree of randomness of the TFE / vinyl ether co-polymer
even within a given batch. This factor in particular may be
the cause of the higher overall PTFE-like crystallinities
observed within the imide ionomer series.
4. CONCLUSION
Ionomer equivalent weight has shown to affect both water
absorption and ionic conductivity for a series of
bis[(perfluoroalkyl)sulfonyl]imide ionomers.
Lower equivalent weight ionomers are less crystalline, as shown by
WAXD and DSC, allowing for the absorption of greater
amounts of water. This greater water absorption, in turn, leads
to greater connectivity among ionic clusters and channels,
which in turn leads to higher ionic conductivities.
The
similarities in water absorption and ionic conductivity
between Nafion™ and the bis[(perfluoroalkyl)sulfonyl]imide
membranes suggest similar water absorption mechanisms and
internal structures for the two materials.
5. ACKNOWLEDGEMENTS
Financial support of this work is by the U.S. Department of
Energy and the State of South Carolina under the DOEEPSCoR program is gratefully acknowledged.
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