Microphase Separation in Sulfonated Polystyrene
Ionomers
D. G. PEIFFER, * R. A. WEISS, t and R. D. LUNDBERG, Corporate
Research Science Laboratories, Exxon Research and Engineering Company,
P.O. Box 45,1600 East Linden Avenue, Linden, New Jersey 07036
Synopsis
Small- and wide-angle x-ray scattering results for a series of un-neutralized and neutralized sulfonated polystyrenes are presented for the range of sulfonation from 0 to 7.26 mol %. From the
small-angle scattering it is shown that above the 3 mol % level for both the zinc and sodium salts,
a Bragg spacing (37 A) and diameter (6.9-8.4 A) of the scattering unit can be calculated. When the
concentration of salt is increased, there is no appreciable change in the latter two measurements.
The wide-angle data indicate that the cations do not influence to any large extent the basic intramolecular and intermolecular structure of polystyrene. All the data are consistent with the onset
of clustering above a critical ion concentration.
INTRODUCTION
Considerable attention has been given recently to the study of the physical
properties of ion-containing polymers.14 These materials are based primarily
on ethylene, styrene, or butadiene backbones containing relatively low concentrations of pendant carboxylate or sulfonate functionalities dispersed randomly
along the chain backbone. Because of the large difference in the dielectric
properties of the ionic and hydrocarbon phases, aggregation of the ionic groups
is favored, resulting in dramatic changes in the material properties.
In spite of the wide range of efforts devoted to carboxylate ionomers, little work
has been performed on sulfonate ionomers. The bulk of the published work has
dealt with sulfonated fluorocarbons of the Nafion variety. Several studiesS7
have been completed recently on sulfonated hydrocarbon-based polymers, but
no systematic study of cluster formation in these materials was performed. It
should be noted, however, that Earnest and co-workers6measured the radius
of gyration of sulfonated polystyrene ionomers over a range of sulfonation levels
and found that it increases with increasing ion concentration.
In this paper we report the results of a small-angle x-ray scattering (SAXS)
study of a series of lightly sulfonated polystyrene ionomers. These polymers
have been neutralized with salts of sodium and zinc. Specifically we have followed the formation of clusters within this material as the concentration of sulfonate groups is increased. These results are compared with cluster formation
in carboxylated polystyrenes described in the literature.
* To whom correspondence should be addressed.
t Present address: Institute of Materials Science, University of Connecticut, Storrs, CT 06268.
Journal of Polymer Science: Polymer Physics Edition, Vol. 20,1503-1509 (1982)
0 1982 John Wiley & Sons, Inc.
CCC 0098-1273/82/081503-07$01.70
PEIFFER, WEISS, AND LUNDBERG
1504
EXPERIMENTAL
Lightly sulfonated polystyrene was prepared from a commercial polystyrene,
Styron 666 (Dow Chemical Company) that had number-average and weightaverage molecular weights, as determined by gel permeation chromatography,
of 106,000and 288,000, respectively. Sulfonation reactions were carried out in
1,2-dichloroethane a t 50°C using acetyl sulfate as the sulfonating agent.8 The
zinc and sodium salts were prepared by titrating the sulfonic acid derivatives
with the appropriate metal acetate. The polymers were precipitated with
methanol or boiling water for the acid derivative, washed with methanol, and
vacuum dried. The sulfur content of the polymers was determined by Dietert
sulfur analysis (ASTM D 1552) and was used to calculate the sulfonate concentrations (Table I).
Films for use in the x-ray scattering experiments were compression molded
a t about 2OOOC under nitrogen and cooled under pressure between water-cooled
platens. The molded films were dried to constant weight in a vacuum oven near
their glass transition temperature and were stored until tested in a dessicator.
In all instances the samples were prepared and treated a t least one month before
the SAXS experiment. Examination with a polarizing microscope revealed that
the films were essentially devoid of birefringence.
Wide-angle and small-angle scattering experiments were conducted using a
Rigaku Rotaflex high-brilliance rotating-anode x-ray generator with samples
mounted for symmetrical transmission. Cu K, radiation was used in conjunction
with a nickel filter. Typical generator settings were 40 kV and 30 mA. A Norelco
microcamera equipped with a 50-p pinhole collimator was used and specimento-film distances of 13.2 and 59.6 mm corresponded to the wide- and small-angle
diffraction, respectively. The sample holder was purged with dried helium gas.
Scattering was recorded on photographic film (Kodak No-Screen film), and the
films were subsequently scanned with a Joyce-Lobe1 microdensitometer. Film
exposure times of 48 h were employed, and each measurement included two
scattering experiments: a background scan on the un-neutralized sulfonated
polystyrene and a scattering scan on the neutralized material a t equivalent
sample thickness and sulfonation level. Conventional background corrections
were made.
TABLE I
Sulfonation Level and Root-Mean-Square Radius of Gyration for Neutralized Sulfonated
Polystyrene
Wt %
sulfur
Counterion
Sulfonation
level
(mol%)
C
D
E
F
G
0.79
1.04
1.22
1.41
1.56
1.86
2.03
Zn
Zn
Zn
Zn
Zn
Zn
Zn
2.66
3.54
4.20
4.89
5.45
6.59
7.26
7.4
7.5
7.1
7.2
6.9
8.4
K
L
M
0.54
1.28
1.66
Na
Na
Na
1.70
4.19
6.05
7.0
7.4
7.4
Material
A
B
a (A)
...
MICROPHASE SEPARATION IN IONOMERS
1505
RESULTS AND DISCUSSION
Small-angle x-ray scattering (SAXS) permits the evaluation of the mean
separation of ionic clusters and the determination of size within a bulk ionomer
In addition the variation of these parameters with the degree of
~ample.3?~-’~
sulfonation and cation size can be studied. Shown in Figure 1 are the SAXS
results for zinc sulfonates. It is immediately apparent that a Bragg peak appears
in the scattering curves above approximately 3 mol % sulfonation. This is undoubtedly due to an interference phenomenon caused by the mean separation
of ionic domains or by the short-range ordering of ionic groups around individual
ionic cluster^.^ The Bragg peak was completely absent in the un-neutralized
sulfonic acid derivatives. In any case, the angular position of this peak does not
change with increasing sulfonation, and the calculated Bragg distance was found
to be 37 A.
It is also observed that the “sharpness” of the Bragg peak changes with increasing sulfonate concentration. A t low ion levels, the peak is broader in the
scattering angle (2 0) and lower in peak height as compared to higher sulfonation
levels. This may be due to a lower number of scattering sites at low ion concentrations and to a somewhat broader size distribution of the scattering units.
Similar trends can be observed when the zinc counterion is replaced with sodium
(Fig. 2). Apparently, the nature of the counterion does not appreciably affect
the size of the scattering entity in sulfonated polystyrene, as has been observed
in other ionomeric materials.1°J3
Guinier15showed that the SAXS from noninteracting particles is related to
the specific size of the scattering entity. It is assumed, however, in the Guinier
Q$
5.45
4.89
4.20
2.6603.54
.4
.022 .a32 .042 .052 .062
.072 .082 .092
28 (RADIANS)
Fig. 1. SAXS curves for polystyrene and for zinc-neutralized (2.66-7.26 mol %) sulfonated polystyrene.
&;
PEIFFER, WEISS, AND LUNDBERG
1506
SULFONATION
I
I
.022
I
I
t
1
I
l
l
.032 .042 .052 .062 .072 .082 .092
28 (RADIANS)
Fig. 2. SAXS curves for sodium-neutralized(2-7 mol %) sulfonated polystyrene.
relation that interparticle scattering is negligible and that the particles are of
uniform size. According to this relation, the plot of the logarithm of the corrected
scattering intensity versus t12should yield a straight line with a slope proportional
to the size R of the scattering unit.
In order to study the size of the scattering entity, the assumption is made that
the Bragg peak has a negligible effect on the scattering curve. A Guinier analysis
can be performed on the scattering data. However, it is realized that the applicability of the method to ionomeric systems is undoubtedly only an approximation since deviations could result from nonuniformity in the size of the scattering entity and interference effects. The nature of the interference effect atThe results
tributable to the maximum is presently a matter of contro~ersy.~
obtained from the treatment of the data according to this method is presented
for zinc and sodium ionomers in Figures 3 and 4, respectively.
The logarithm of intensity is plotted as a function of t12in Figures 3 and 4 for
several of the metal-sulfonated polystyrenes. A linear relation was found for
all sulfonation levels studied, and the results of this analysis are summarized in
Table I. The mean scattering size is approximately 7.4 A and is essentially independent of the counterion. These SAXS results for the sulfonated polystyrenes are consistent with those of Moudden and co-workers13in their studies
of ionic clustering in telechelic butadiene-methacrylic acid copolymers containing
2 mol % acid and neutralized (5-100%) with various salts. In that study the salt
groups were found to form clusters with an average size of approximately 8 A,
and the cluster size appeared to be independent of the nature of the cation and
the degree of neutraliz&m. The existence of a low-angle maximum in the
scattering intensity indicated a mean separation distance of 80 A between clusters
in the fully neutralized sample, a value appreciably larger than the 37 A separation found in the present SAXS investigation. The difference may be due to
differences in the distribution of ionic groups along the main chain. In the
telechelic materials the ionic groups are present only at the ends of each chain,
while in sulfonated polystyrene the sulfonate groups are randomly distributed
along the polymer backbone. A Bragg spacing corresponding to 20-40 A has
been observed in many carboxylate monomers.l
MICROPHASE SEPARATION IN IONOMERS
1507
10
9
8
7
6
1
e2 lo4,
RADIANS~
Fig. 3. Guinier plots of scattering intensities (in relative units) for several zinc-neutralizedsulfonated polystyrenes.
As previously shown in Table I, the ion concentration at which clusters become
evident in the SAXS data is between 2 and 3 mol % (corresponding to 2-3 sulfonate groups per 100 styrene monomer units). Eisenberg et al. have studied
10
9
8
7
6
t
e2
x
lo4, RADIANS 2
Fig. 4. Guinier plots of the scattering intensities in relative units for several sodium-neutralized
sulfonated polystyrenes.
PEIFFER, WEISS, AND LUNDBERG
1508
the effect of metal carboxylate1J6 and sulfonatel7 groups on the viscoelastic
behavior of styrene-based ionomers. In both systems a secondary relaxation
process believed to be of a viscous origin was observed in ionomers containing
greater than ca. 5-6 mol 5% ionic groups. This critical concentration, which
marked the ionic concentration at which time-temperature superposition was
no longer valid for these materials, was initially interpreted as marking the onset
of clustering.' Recent dielectric and Raman evidence, however, indicates that
ionic clusters exist in styrene-based ionomers at concentrations as low as 2 mol
'%.I6 The SAXS results presented here are consistent with these findings.
Wide-angle x-ray scattering (WAXS) measurements were also made on all
samples. Previous WAXS studied8 of semicrystalline ethylene-methacrylate
ionomers established that the introduction of ionic groups reduced the total
crystallinity of the copolymer, but did not affect the crystal lattice spacings of
the polyethylene component. Presumably, this is due to the inability of the
bulky ionic groups to incorporate into the crystal lattice. It is of interest to investigate the affect of sulfonate groups on the diffraction pattern of an amorphous
polymer such as atactic polystyrene. Analyses of the x-ray scattering from
polystyrene have been reported by several g r o ~ p s . l ~
The
, ~ results
~
indicate that
both intramolecular and intermolecular scattering contribute to the radial distribution function at distances beyond 3.7 A, and the molecular structure is
dominated by the steric interaction of the phenyl groups. For example, Wecker
et a1.20have assigned the peak at 20 = 19" (d = 4.67 A) to phenyl-phenyl interchain and phenyl-chain interchain interactions and 20 = 11' (d = 8.04 A) mainly
to phenyl-chain interchain interactions.
Shown in Figure 5 are typical WAXS results from the unmodified polystyrene
and three sulfonated materials for low 20 values (d > 3.7 A). The position of
the diffraction peaks in all cases is unaffected by sulfonation; however, with increasing levels of sulfonation, the peaks become broader. It can be concluded
that the large counterion on the sulfonate group does not affect the average
distance of the phenyl groups with respect to the main chain or with other phenyl
groups. The broadening of the peak width indicates, however, that the neutralized sulfonate group increases the distribution of the above-mentioned distances.
1
In
Iz
a
SULFONATION LEVEL
(MOLE yo)
w
7.26 Mole % Zinc
>
I-
4
W
--E
6.05 Mole 70Sodium
a
7 . 2 0 Mole
Polvstvrene
Acid
20
10
28 (DEGREES)
Fig. 5. WAXS patterns from polystyrene, unneutralized and neutralized (7 mol % sodium, 7.26
mol % zinc) sulfonated polystyrene.
MICROPHASE SEPARATION I N IONOMERS
1509
CONCLUSIONS
SAXS results from lightly sulfonated polystyrene ionomers indicate the
presence of a scattering peak below 28 = 2O in materials containing ca. 2-3 mol
% metal sulfonate groups. This peak corresponds to a Bragg spacing of about
37 8 and a characteristic size of the scattering unit of 7 8. The small-angle Bragg
peak is insensitive to cation type (zinc versus sodium) and concentration, but
it is completely absent in the unsulfonated polymer and in the un-neutralized
sulfonic acid derivatives. These results are consistent with previous SAXS
studies of carboxylate ionomers both in the characteristic Bragg spacings observed and in that the scattering unit is found only in neutralized ionomers
containing greater than a critical ion concentration. It is believed that the origin
of the SAXS scattering is identical to that reported for carboxylate ionomersthat is, ion-rich aggregates or clusters, though no attempt has been made in this
study to determine the geometry or the composition of the cluster. Complementary wide-angle x-ray diffraction data indicate that the sulfonate groups have
little influence on the basic intramolecular and intermolecular structure of
polystyrene.
I t is a pleasure to thank Professor N. C. Payne and Dr. John Richardson of the Department of
Chemistry of the University of Western Ontario for obtaining the x-ray photographs.
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Received August 11,1981
Accepted February 9,1982