Far-Infrared Studies of Microphase Separation in
Sulfonated Ionomers
D. G. PEIFFER, B. L. HAGER, R. A. WEISS,’ P. K. AGARWAL, and R.
D. LUNDBERG, Exxon Research and Engineering Company, Clinton
Township, Route 22 East, Annandale, New Jersey 08801
Synopsis
Far-infrared spectra of a series of un-neutralized and neutralized lightly sulfonated polystyrenes with varying sulfonation levels have been investigated to seek spectroscopic evidence
for microphase separation known to control the physical properties of these polymers. Broad,
strong absorbance bands, not found in the spectrum of unmodified polystyrene, are o b s e ~ e d
in the spectra of the sulfonated analogs. The effects on the far-infrared spectra both of sulfonation level and of the mass and charge of the neutralizing cation are discussed in terms
of cation motion and the formation of ion-rich domains.
INTRODUCTION
The incorporation of low levels ( < 10 mol%) of ionic groups into the
structure of a n organic polymer profoundly influences the physical p r o p
erties of the resulting “ion+ontaining polymer” (i.e., ionomer) through both
intramolecular and intermolecular interactions. While these interactions
are Coulombic, their specific details remain a subject of considerable debate.
The mechanical and rheological properties of these systems exhibit complex
temperature dependences which appear to indicate that in many instances
the ionic species are present in a separate microphase.’s2 In recent years
have attempted to elucidate the structural factors
many
controlling the behavior of these materials in the solid state, solution, and
melt. For a specific ioncontaining polymer with low ion concentration, the
ionic groups are believed to aggregate as multiple ion pairs, multiplets,2
which give the polymer properties similar to those of a crosslinked system.
The crosslink junctions are due, however, to physical interactions and not
to chemical bonds. In fact, the versatility of these polymers lies in their
ability to break and reform these physical links between chains. Moreover,
as the concentration of ionic groups is further increased, a critical region
is reached where the behavior becomes dominated by aggregates of multiplets or ionic clusters.2
In most instances, the properties of ionomers can be related to specific
structural details of the polymer molecule. The influence of backbone structure is important not only in its effect on polymer conformation, but also
with respect to the dielectric properties of the material. Since ionomers are
largely hydrocarbon based, it may be argued2 that the extent of clustering
Present address: Institute of Materials Science, University of Connecticut, Storm, CT 06268.
Journal of Polymer Science: Polymer Physics Edition, Vol. 23, 1869-1881 (1985)
CCC 009&1273/85/091869-13$~.~
@ 1985 John Wiley & Sons, Inc.
1870
PEIFFER
ET AL.
depends primarily on the dielectric constant of the polymer matrix, rather
than on the nature of the ionic
Studies10-20have shown, however,
that the structure of both the anion and cation affects behavior when nonionic structural units remain invariant. These results can be correlated
with variations in Coulombic attraction between the polyion and its counterion. Low-frequency ( < 500 cm -9 far-infrared (FIR)21,23z and &mann
spectroscopies have ben used extensively to probe interactions between ionic
groups and their possible perturbation of the surrounding hydrocarbon
matrix. In general, one observes in this spectral region two bands which
are not present in the spectrum of the original (not ioncontaining) organic
polymer, with the frequency of at least one of these bands depending on
the mass of the cation. Based upon the FIR spectra of nonaqueous electrolyte
solutionsB*%and ionic glasses,32these bands have been attributed to motion
of cations in the potential field of their counterions. That two bands are
observed results from this cation motion occurring in environments with
differing states of aggregation believed to be multiplets and clusters, a
conclusion consistent with the dependences of the intensities of these bands
on temperature and ionic content of the ionomer.
The majority of low-frequency vibrational spectroscopic studies of ionomers have dealt with systems where the ion-containing functionality is a
neutralized or un-neutralized carboxylic acid g r o ~ p .Recently
~ ~ , ~ Mattera
and Risen= examined the FIR spectra of poly(styrene sulfonic acid) ionomers, originally prepared in these laboratories, which had been neutralized
with alkali and alkaline earth ions. While the authors observed in the
spectra of these materials only one absorption band due to cation motion,
they found for two of the neutralized forms (K+, Ba2+)that the position of
the band shifted to lower frequency with increasing ionic content (mol
percent sulfonate increasing from 3.4 to 16.7). The authors concluded that
this observation is consistent with a large number of cations being present
in large ionic clusters a t increased sulfonate concentrations.
The present investigation was undertaken to extend the study of the
influence of sulfonate concentration and counterion structure (ionic size
and charge) on microphase separation in sulfonated polystyrene ionomers.
We further seek to assess the state of multiplet/cluster formation in these
systems. Our results are compared with previous microphase separation
studies.%
EXPERIMENTAL
Lightly sulfonated polystyrene was prepared from a commercial polystyrene, Styron 666 (Dow Chemical Company), with number- and weight-average molecular weights as determined by gel permeation chromatography
of 106,000 and 288,000, respectively. Sulfonation reactions were carried out
in 1,2dichloroethaneat 50°C using acetyl sulfate as the sulfonating agent.n
The calcium, sodium, and zinc salts were prepared by titrating the sulfonic
acid derivatives with the appropriate metal acetate. Subsequently, the polymers were precipitated with methanol (boiling water for the acid derivative),
1871
FAR-IR STUDIES OF MICROPHASE SEPARATION
washed with methanol, and vacuum dried. The chemical composition of the
resulting “random copolymers” is given stoichiometrically by
tCHCH,
I
+, f CHCH,
-f,
I
where M is the metal cation (or proton in the case of the sulfonic acid) and
y/O, +XI
is the mole fraction of sulfonate groups. The sulfur content of each
polymer was determined by Dietert sulfur analysis (ASTM D1552)and used
to calculate sulfonate concentrations. Midinfrared spectra of the individual
polymers indicated that neutralization was complete. Table I lists the ionomers used in this study. In this table and subsequent discussion, we identify
each sulfonated polystyrene (SPS) by M S P S followed by a number, where
M refers to the cation (Na, Ca, Zn) or proton (H, for the sulfonic acid form)
and the number represents the sulfonation level y / ( y + x ) as a percentage.
Hence Na-SPS (4.2) refers to the sodium-neutralized form with sulfonation
level 4.2%.
The unmodified polystyrene, the sulfonic acid derivative, and its sodium
and zinc salts were compression molded at about 200°C under nitrogen and
TABLE I
Sulfur Content and Sulfonation Level for Un-neutralized and Neutralized
Sulfonated Polystyrene
Sulfur content
Material
Counterion
fwt 70)
A
B
C
D
E
F
G
H
I
J
K
L
M
N
0
P
H
H
H
H
H
H
H
H
Na
Na
Na
Zn
Zn
Zn
Zn
Zn
Zn
Zn
Zn
Ca
Ca
Ca
0.36
0.62
0.97
1.18
1.33
1.65
1.96
2.12
B
R
S
T
U
V
0.54
1.28
1.66
0.29
0.79
1.04
1.22
1.41
1.56
1.86
2.03
0.54
1.09
1.58
Sulfonation level
(mol%f
1.19
2.05
3.20
3.89
4.48
5.56
6.66
7.20
1.70
4.19
6.05
0.96
2.66
3.54
4.20
4.89
5.45
6.59
7.26
1.80
3.62
5.26
1872
PEIFFER ET AL.
cooled under pressure between water-cooled platens. In all instances the
molded films were dried to constant weight in a vacuum oven near their
glass transition temperatures and were stored in a desiccator until placed
in the sample holder of the spectrometer. The calcium salt could not be
compression molded to any satisfactory extent. Therefore films of this material were cast from solutions of xylene and methanol (normally 90/10%
by volume) onto Tefloncoated substrates. The solvent was evaporated at
room temperature for approximately 24 h. The films were then placed in
a vacuum oven near their "solventless" glass transition temperature, dried
to constant weight, and stored in a desiccator until tested. Examination
with a polarizing microscope revealed that all films were essentially devoid
of birefringence. The molded and cast films had thicknesses between 0.05
and 0.20 mm.
Far-infrared spectra (100-500 cm-'1 were obtained on a Digilab, Inc. FTS15 Fourier-transform infrared (FI'IR) spectrometer. Spectra were recorded
with 4-cm-1 resolution at ambient temperature. For each spectrum, 500
scans were averaged to achieve high signal-to-noise ratio.
RESULTS
To facilitate comparison among spectra, all are presented in absorbance.
The effect of sulfonation upon the FIR spectrum is readily observed by
comparing the spectrum of unmodified Styron 666 (Fig. 1)with the spectra
of a series of un-neutralized and neutralized sulfonated polystyrenes (Fig.
2) where the sulfonation level is ca. 5 mol %. We observe that the spectrum
of polystyrene is essentially the same as previously reportedBSmand contains
no special features of interest to this study below 350 cm-'. Sulfonation
produces a broad, relatively intense absorption centered at about 250 cm- '
for the un-neutralized ionomer. Neutralization produces a broad, very intense absorption below 300 cm-' which shifts to lower frequency as the
mass of the counterion increases, a n observation consistent with the results
of Mattera and Risena and assigned to cation motion. However, the central
frequency of absorption of the CaSPS, the counterion of lowest mass-charge
ratio studied here, actually lies about 10-15 cm-' above that of the unneutralized HSPS. The bands at 446 and 326 cm-I are also influenced by
sulfonation. The 446-cm- absorption becomes generally more diffuse with
little change in central peak position, while the 326-cm- absorption, which
is very weak in the unmodified polystyrene, becomes more intense with
sulfonation. It is of interest to note that sulfonation has no apparent effect
on the position of the 446-cm-' absorption, the V16A out-of-plane ring vibration of polystyrene, in the SPS spectra in Figure 2. It was found that within
experimental error, the intensity of the 406-cm-I absorption varied linearly
with measured film thickness, thus indicating that this band is a practical
measure of film thickness.
The asymmetric band shapes and apparent shoulders on the broad SF'S
absorptions in Figure 2 raise the question of whether we are observing two
cation motion bands as seen by Rouse et al. for poly(styrene methacrylic
acid) ionomers,21 or only one cation motion band as seen by Mattera and
Risen for SPS ionomema To address this question, we examined the effect
FAR-IR STUDIES OF MICROPHASE SEPARATION
500
I
I
I
400
300
200
1873
100
Lh i l l
f IGURE 1
Fig. 1. Far-infrared spectrum (100-500cm-') of unsulfonated polystyrene.
of sulfonation level on the FIR spectra of several series of SPS ionomers.
Figure 3 shows the spectra of a series of H-SPS ionomers with sulfonation
levels varying from 1.2 to 7.2 mol %. In this and the following two figures,
the spectra have been normalized so that the 406cm-' absorption is of
constant intensity. While there exists a shoulder at 185 cm-', it appears
that the band intensity at 185 cm-' increases with sulfonation level at
about the same rate as the band maximum at 250 cm-I. This topic is further
discussed below. From Figure 3, we again observe that the 446cm-I absorption becomes quite diffuse with increasing sulfonation level, while the
326cm-l absorption becomes more intense at low (ca. 3%) levels of sulfonation but remains unchanged with further increase of sulfonation level.
Upon increasing the sulfonation level for a series of NaSPS ionomers
(Fig. 4) the intensity of the 21&m-' cation motion absorption increases,
while the shoulders at 250 and 190 cm-' observed at the lowest sulfonation
level (1.7%) are lost. The 446- and 326-cm-l absorptions in the Na-SPS
system behave as in the H-SPS system, while a new absorption is observed
at 385 cm-' and increases with sulfonation level.
The behavior of the ZnSPS system differs from that of the H-SPS and
Na-SPS systems described above. Most importantly, the broad cation motion
1874
PEIFFER
ET AL.
8
Q
9
n
P
4
500
I
I
I
400
300
200
1 0
0 (cm-1)
Fig. 2. Far-infrared spectra (100-500 cm-') of M-SPS (approximately5 mol %) in the acid
and neutralized forms (Ca,Na, and Zn).
band appears to be composed of two separate absorptions at 210 and 165
cm-l which increase in intensity with sulfonation level at different rates,
the 165-cm-l absorption increasing faster. The 446cm-I band becomes diffuse initially, but increases in intensity when the sulfonation level exceeds
ca. 4.9 mol %. The 326cm-I absorption also appears to increase when sulfonation exceeds ca. 4.9 mol %.
DISCUSSION
Our results show that the FIR spectra of sulfonated polystyrenes contain
a broad strong absorption, the peak frequency and intensity of which depend
upon cation mass and sulfonation level, respectively. For at least one system,
the zinc-neutralized ionomer, there is evidence that this absorption band
contains two spectroscopic components.
Mattera and Risen= have assigned the FIR absorption of sulfonated polystyrene ionomers as a cation motion vibrational band due to motion of the
cation in the electrostatic field of the anionic sulfonate group. A key feature
in this assignment is the use of a pseudo-diatomic-oscillator model which
allows the reduced mass of the vibrating species to vary as that of the cation
alone, so that the vibrational frequency varies with cation mass as M&
for an isovalent series of cations. Figure 6 depicts this behavior and provides
a convenient format for comparing our results with those of Mattera and
Risen. We observe that for Na+ and Ca2+, our results are in excellent
agreement with those previously reported. This is of particular note since
FAR-IR STUDIES OF MICROPHASE SEPARATION
1875
7.20
6.66
5.56
4.48
3.89
3.20
2.05
1.19
500
I
I
1
400
300
200
100
ij (cm-1)
Fig. 3. Far-infrared spectra of a series of un-neutralized sulfonated polystyrenes covering
a range of sulfonation levels (mol %) as indicated.
the ionomer films from which these spectra were obtained were prepared
in a manner entirely different from that of Mattera and Risen. Of greater
interest are our results for ZnSPS. As shown in Figure 5, the absorption
band for this material appears to indicate the presence of two
spectroscopic components with peak frequencies of 210 and 165 cm-'.
While it appears that 210 cm-* is the frequency in better agreement with
existing data, the nature of the concentrationdependent ZnSPS spectra
warrants further discussion. If we assert that the 165cm-l absorption arises
from cation motion in a microphase region, there are three possible explanations for the presence of two spectroscopic components. First, the 210cm-' component might not be due to cation motion. It could be a spectral
1876
PEIFF'ER
ET AL.
6.05
4.19
1.70
I
I
I
400
300
200
1 0
Vknil)
Fig. 4. Far-infrared spectra of a series of NaSF'S covering a range of sulfonation levels
(mol %) as indicated.
artifact or impurity, although we have no idea as to the nature of such a n
impurity. Since zinc is a Group IIB cation, it might interact with the sulfonate anionic field in a manner different from the divalent cations of Group
IIA, thus resulting in a lower cation motion frequency. Second, the 210cm- absorption could arise from hydration. In this instance water solvates
the ionic domain so that the metal-sulfonate site interaction is not screened
by the presence of other metal-sulfonate pairs. The result is a greater
effective force constant and, therefore, a higher cation motion frequency.%
Third, the 210- and 165cm-l absorptions could represent cation motion of
zinc sulfonate sites in multiplets and clusters, respectively.
The cluster vibrational mode has previously been observed in the FIR
spectra of poly(styrene methacrylic acid) ionomers.21More recently, Mattera
FAR-IR STUDIES OF MICROPHASE SEPARATION
1877
7.26
6.59
5.45
4.89
4.20
3.54
2.66
0.96
500
400
300
200
100
B (cm-’)
Fig. 5. Far-infrared spectra of a series of ZnSPS spanning a range of sulfonation levels
(mol %) as indicated.
and Risen suggested that cluster formation may be responsible for the
apparent shift to lower frequencies of the FIR cation motion absorption of
the K+- and Ba2+-neutralized forms of SPS. Since these authors do not
observe a similar peak shift in the other neutralized SPS systems studied,
they suggest that clustering interactions may be favored in the KSF’S and
Ba-SPS systems because of configurational arguments as K+ and Ba2+ have
essentially the same ionic radius, ca. 1.34 A. The Zn-SPS system does not
conform to this argument since the ionic radius of Zn2+ is 0.74 A.
Raman studies of carboxylatecontaining ionomers have examined the
concentrationdependent intensities of peaks assigned to cation motion in
multiplets and clusters as evidence of cluster formation.22-24Although FIR
1878
PEIFFER ET AL.
0.1
Me-n -‘/z(amu)-fi
0.2
Fig. 6. Graph of the cation motion frequency peakii Y (cm-I) vs. cation mass M,+ for the
ionomers described in this work (filled points) and those of Risen et al.” (open points); (A)
1 6 5 - m - I absorption of ZnSPS discused in the text.
cation motion absorptions are diffuse relative to their Raman counterparts,
such a n analysis of our data is possible since the spectra have been “normalized” to constant film thickness. Figure 7 shows the results for zinc
cation motion absorbances taken as peak heights above a tangential baseline.* If the absorptions at 210 and 165 cm-’ are assigned to cation motion
vibrations in multiplets and clusters, respectively, then the resemblance of
Figure 7 to analogous Raman results is striking. We note that this analysis
suggests that the onset of cluster formation occurs around 1-3 mol % sulfonation, a result consistent with SAXS,= EXAFS,= and rheologica131measurements of sulfonated polystyrenes.
A question left open by the above analysis concerns the nature of the
cation motion vibration in the other neutralized ionomers. Mattera and
Risen conclude from the weak dependence of cation -anion site forcefield
elements on sulfonation level that a clustered or ordered morphology may
be present in even the un-neutralized HSF’S form. Presumably this would
be true of the neutralized forms as well. Rheological measurements on the
Zn-, Ca-, and Na-neutralized materials suggest that ionic interactions are
weakest in the Zn-SPS system.37 This is consistent with the assignment of
the 210- cm- 1 FIR absorption for Zn-SPS as due to multiplets or low-order
Normalized absorbances appearing in Fig. 7 were obtained as follows. After computer
“normalization” of the ZnSPS spectra to constant film thickness, a baseline (essentially tangential) was drawn from ca. 300 to 100 cm-’, the terminal point in the spectrum. This baseline
has a natural negative slope; conversion to zeroslope baseline (so-called “baseline correction”)
was not attempted. Absorbancea were measured as peak heights at 210 and 165 cm-L (fie
quencies chosen as local maxima) above the baseline. In similar fashion, the “background”
absorbances due to polystyrene at 210 and 165 cm-I were determined from the normalized
absorbance spectrum of Styron 666 (Fig. 1) as the peak heights above a tangential baseline
drawn from ca. 280 to 100 cm-’. The differences in absorbance between the ZnSPS spectra
and the Styron 666 spectrum at 210 and 165 cm-1 were then plotted on Fig. 7.
Qualitatively similar absorbance vs. concentration profiles are obtained when integrated
absorbance are used, although absolute integrated values depend strongly upon choice of
baseline and nature of band deconvolution. The integrated absorbance mentioned here were
obtained graphically above a variety of baselines. Studies currently under way in our l a b
ratories employ corrected baselines and peak deconvolution by computer to seek further evidence of microphase separation in both the FIR and Raman spectra of these systems.
FAR-IR STUDIES OF MICROPHASE SEPARATION
1879
Sulfonation Level (Mole Percent)
Fig. 7. Normalized absorbance of the bands at 210 cm-I (assignedto multiplet absorbance)
and 165 cm-' (assigned to cluster absorbance) vs. mole percent sulfonation for the complete
series of ZnSPS ionomers investigated; (0)165, (0)
210 cm-' absorbance.
clusters. Current variable-temperature Raman experiments seek to understand further the multiplet/cluster equilibrium in these systems.
In review of the original version of this manuscript, one referee suggested
two alternative explanations based on Zn2+ chemistry for the origin of two
FIR absorptions in the Zn-SPS spectra. To be sure, it is impossible to choose
unambiguously a single explanation for our observations based solely on
this study. One alternative explanation is that there is often a tetrahedral/
octahedral Zn2+ coordination equilibrium in Zn2+ systems. Since both
0,,-Zn06 and Td-ZnO,- species have one allowed stretching mode in the
FIR, such species might cause the two absorptions observed.
A second alternative explanation involves the formation of specific compounds and is complementary to the possibility stated above that one of
the FIR absorptions might be due to a n impurity. One possible candidate
is zinc oxoacetate, ZnrO(OCXH3),, formed from the zinc acetate used to
neutralize HSPS. Another possibility, by analogy to the formation of a n
oxoacetate, is the formation of zinc oxosulfonate via reaction of Zn2+ with
H,O (present due to the hydroscopic nature of ZnSPS) to produce ZnOH+,
followed by neutralization with RS03H to yield zinc oxosulfonate
Zn,CXOSO,R),. Of course the chemical environments seen by Zn2+ in
Zn,O(OSO,R), and in a cluster are probably quite similar.
A final comment in this discussion concerns the FIR spectra of un-neutralized H S P S shown in Figure 3. To our knowledge these are the first
spectra to show a concentrationdependent vibrational mode in the lowfrequency (FIR) spectral region of an ionomer. Because we expect the
proton-sulfonate interaction to possess little ionic character, we cannot
attribute this absorption solely to cation motion. The spectroscopic behavior
of the H S P S system warrants further study. Interestingly, Rigdahl and
Eisenberg38 examined the viscoelastic properties of un-neutralized and neutralized SPS, observing that HSPS has a small amount of acid groups located
in clusters. These aggregates, as noted by Eisenberg, are capable of being
PEIFFER ET AL.
formed in sulfonated materials as opposed to carboxylate-containing polymers, where cluster formation is markedly absent. This difference is most
likely due to the fact that the sulfonic group is a much stronger acid.
CONCLUSION
From the far-infrared measurements, it is apparent that the ionic groups
within SPS ionomers are in different environments defined to a large extent
by the magnitude of the force field surrounding the counterions. Clusters,
at least low-order ones, appear to exist at sulfonation levels of 2-3 mol %
in the Zn-SPS system, and may exist at even lower sulfonation in other
neutralized SPS systems. This is consistent with the strong ionic interactions present in sulfonated ionomers, as manifested particularly by their
enhanced rheological properties2 Further vibrational spectroscopic studies
are necessary to explore the nature of the ionic interactions leading to
cluster formation in these systems.
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Received July 17, 1984
Accepted March 27, 1985