Spherical and Vesicular Ionic Aggregates in Zn-Neutralized
Sulfonated Polystyrene Ionomers
BRIAN P. KIRKMEYER,1 ROBERT A. WEISS,2 KAREN I. WINEY1
1
Department of Materials Science and Engineering, University of Pennsylvania, 3231 Walnut Street, Philadelphia,
Pennsylvania, 19104-6272
2
Polymer Science Program and Department of Chemical Engineering, University of Connecticut, Storrs,
Connecticut 06269-3136
Received 25 May 2000; revised 6 December 2000; accepted 11 December 2000
Ionic aggregates in a series of Zn-neutralized poly(styrene-co-styrene sulfonate) (SPS) random ionomers have been imaged using scanning transmission electron
microscopy. The Zn-rich aggregates were found to have two shapes: solid spheres (Type
I) and shells or vesicles (Type II). Type I aggregates range in a maximum diameter from
4 to 10 nm, whereas Type II aggregates range in a maximum diameter from 9 to 55 nm
with a vesicle wall thickness of ⬃ 3 nm. Lightly neutralized ionomers exhibited only
Type I aggregates, whereas higher neutralization levels exhibited both Type I and II
aggregates. Lightly neutralized ionomers also showed evidence of macrophase separation at the micron size scale. These direct observations of ionic aggregates contradict
previous interpretations of small-angle X-ray scattering data with respect to size, size
dispersity, shape, and spatial distribution. In addition, the aggregates observed in SPS
differ markedly from the nearly monodisperse ⬃ 2-nm spherical aggregates observed in
Zn-neutralized poly(ethylene-co-methacrylic acid). The presence of vesicular aggregates encourages a re-examination of the morphologies and properties of styrenic
ionomers. © 2001 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 477– 483, 2001
Keywords: ionomers; sulfonated styrene; STEM; morphology
ABSTRACT:
INTRODUCTION
Typical ionomers are random copolymers with a
minority of ionizing monomeric units (typically
acids) and a majority of nonionizing monomeric
units. The ionizing groups can be partially or fully
neutralized with ions. The resulting ionic groups
microphase separate from the nonionic monomeric units to create ionic aggregates. Historically, academics have favored studies of amorphous polystyrene (PS) containing sulfonic- or
carboxylic-acid groups. The advantages of this
Correspondence to: K. I. Winey (E-mail: winey@lrsm.
upenn.edu)
Journal of Polymer Science: Part B: Polymer Physics, Vol. 39, 477– 483 (2001)
© 2001 John Wiley & Sons, Inc.
material include the availability of PS with narrow molecular weight distributions, the ease of
sulfonation, and the simplified interpretation of
small-angle X-ray scattering (SAXS) data because
of its amorphous structure. In the present article,
we apply our scanning transmission electron microscopy (STEM) methods to styrenic ionomers
for the first time.
Styrenic ionomers have been investigated by
methods including SAXS,1–10 dynamic mechanical analysis,10 –13 and extended X-ray absorption
fine structure14 –18 in efforts to discern the nature
of their ionic interactions as well as their morphology. SAXS of these materials detects a broad
peak in the range of q ⫽ 1.5–2.5 nm⫺1 and a
low-angle upturn at q ⬍ 0.5 nm⫺1.1–10 Weiss and
477
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KIRKMEYER, WEISS, AND WINEY
colleagues1 found that blending with Nylon-6,1
adding a diluent,2 and reducing the ionic content3
decrease the peak intensity and shift it to a lower
scattering vector. Weiss and Lefelar3 also found
that the thermal history of an ionomer impacts
the scattering results. Other factors that affect
the peak position and intensity include the identity of the neutralizing agent4 and acid groups.10
Regarding scattering at low angles, Ding et al.9
suggested that the upturn is due to a heterogeneous distribution of ionic groups in the matrix
and is associated with the neutralizing cation.
Models of ionomer morphology based on SAXS
data have been proposed by Marx et al.19 (paracrystalline lattice model), MacKnight et al.20 (intraparticle scattering model), and Yarusso and
Cooper4,5 (liquidlike interference model). All of
these models include an ion-rich, spherical aggregate. These three models have been previously
fitted to SAXS data collected from an 85% Znneutralized poly(styrene-co-styrene sulfonate)
(Zn-SPS) ionomer, where 6.91 mol % of the styrene groups are sulfonated, showing an ionomer
peak at q ⬇ 1.6 nm⫺1.5 The Marx model, using a
cubic lattice with equal disorder in all directions
and R1 ⫽ 0.98 nm, provides a reasonable fit to the
ionomer peak but an insufficient sulfonate group
concentration in the aggregates (see list of references for descriptions of these variables). The
MacKnight model, with parameters of R1 ⫽ 0.89
nm, R2 ⫽ 3.6 nm, and R3 ⫽ 6.8 nm, does not fit the
ionomer peak but does predict the low-angle upturn. The Yarusso–Cooper model, with parameters of R1 ⫽ 1.00 nm and RCA ⫽ 1.66 nm, fits the
ionomer peak but not the low-angle upturn. Thus,
it is apparent that the available scattering models
fail to fully describe the ionomer morphology.
Furthermore, the models have not been independently verified.
Electron microscopy of the styrenic ionomers
may provide a means by which to confirm or refute the various scattering models because images can be interpreted directly to determine size,
shape, size dispersity, and spatial distribution of
ion-rich aggregates. A previous high-voltage
transmission electron microscopy (TEM) study of
solution-cast thin films of Zn-SPS21 showed promise in this regard, although the effect of solution
casting on ionomer morphology has since been
shown to depend on solution composition.22 Recently, Winey and colleagues23,24 imaged the ionic
aggregates of semicrystalline poly(ethylene-comethacrylic acid) random ionomers using STEM
equipped with a field emission electron gun
(FEG). The bulk morphologies of these ionomers
were maintained using cryo-ultramicrotomy for
sample preparation. This microscopy method reduces phase contrast and improves spatial resolution as well as atomic number contrast. In the
present study, FEG-STEM was used to image the
ionic aggregates of amorphous SPS random ionomers neutralized to varying levels with Zn.25
EXPERIMENTAL
Materials and Sample Preparation
The materials and preparations for the ionomers
used in the present study are comparable to those
used in previous studies. PS was synthesized by
bulk free-radical polymerization, yielding Mw
⫽ 64,600 g/mol (polydispersity ⫽ 1.09), and sulfonated to a level of 5.3 mol % using previously
described methods.1 Zn-SPS was prepared by
neutralizing SPS in toluene/methanol with zincacetate dihydrate in methanol. The neutralization levels were 25, 75, 100, and 125% neutralized
and were controlled by the relative amounts of
SPS and zinc-acetate solution. Sample designation for the ionomers is SPS-X, where X is the
percent Zn neutralization of the ionomer. Zn-neutralized SPS was precipitated in a large excess of
ethanol, filtered, washed several times with ethanol, and vacuum-dried at 70 °C for 1 week. Samples were compression-molded at ⬃6 Pa for 20
min at 150 °C. Visual inspection ensured that the
discs were clear, that is, free of voids.
SAXS
SAXS was conducted at the University of Connecticut using Cu K␣ radiation (␭ ⫽ 0.154 nm)
from a rotating-anode source operating at 40 kV
and 100 mA and a Bruker HiSTAR area detector.
The X rays were monochromated using a nickel
filter and collimated using a 150-␮m pinhole. A
background correction was made by subtracting
the signal at high q for a run with no sample.
STEM
The goal of preparing the specimens for electron
microscopy was to preserve the bulk structure of
the ionomers. To that end, thin sections (100-nm
nominal thickness) of the sample were microtomed at room temperature using a Reichert–
Jung Ultracut S ultramicrotome with a diamond
SPHERICAL AND VESICULAR IONIC AGGREGATES
479
knife at a cutting speed of 0.4 mm/s. The sections
were floated onto deionized water and collected on
copper TEM grids. The grids were touched to filter paper to remove the water droplets and then
allowed to dry.
Unlike previous studies of styrenic ionomers,1–18 the present study relies on imaging the
ionic aggregates with STEM. Electron microscopy
was performed on a JEOL 2010F FEG microscope
operated at 197 kV with a 10-cm STEM camera
length, a 1-nm probe size, and a 50-␮m condenser
aperture. Images were collected using a Gatan
bright field (BF) scintillating detector. The STEM
is equipped with a double-tilt specimen holder so
that it rotates about both the x and y axes, where
the z axis is the optical axis.
RESULTS
Figure 1. SAXS intensity as a function of scattering
vector for SPS-0 (E) and SPS-100 (F).
The room temperature SAXS data for SPS-0 and
SPS-100 are shown in Figure 1. SPS-0 does not
Figure 2. BF-STEM images show shape heterogeneity in the SPS-100 ionomer. The
double-tilt series provides multiple projections of the Zn-rich aggregates to assist in
determining shape characteristics. The horizontal and vertical sets of images were
collected after rotating the sample about the x and y axes, respectively, by the amounts
indicated in the figure relative to the untilted central image. The Type I aggregate is a
sphere of uniform composition, and the Type II aggregates are vesicles with wall
thicknesses of ⬃3 nm.
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KIRKMEYER, WEISS, AND WINEY
exhibit a scattering peak because the sample contains no Zn(II) ions and, thus, no ionic aggregates
form. When fully neutralizing with Zn (i.e., SPS100), an ionomer peak forms at q ⬇ 1.7 nm⫺1.
Both SPS-0 and SPS-100 also exhibit an increase
in intensity at q ⬍ 0.5 nm⫺1. The peak and low-q
upturn were also observed in samples with other
levels of neutralization: 25, 50, 75, and 125%.
These data are provided to demonstrate that the
materials used in the present study are typical of
Zn-SPS ionomers studied previously.1–10
In BF-STEM, feature contrast arises from differences in specimen density and/or thickness.
Given that these sections are microtomed with
approximately uniform thickness, the dark domains in the images correspond to regions with
higher atomic-number species. In Zn-SPS, the
dark domains are Zn-rich aggregates relative to
the PS-rich matrix. Annular dark-field STEM images in which contrast arises from atomic-number
differences confirm that the features discussed
are Zn-rich.
The Zn-rich aggregates in the fully neutralized
ionomer, SPS-100, exhibit two shapes and a range
of sizes (Fig. 2). One type of aggregate, Type I,
appears spherical with a uniform intensity across
the feature. When tilted about the x or y axis, the
shape and intensity of Type I aggregates remain
constant. This indicates that Type I aggregates
are homogeneous solid spheres. Type I aggregates
in SPS-100 range in size from ⬃4 to 8 nm in
diameter.
Sample SPS-100 also contains aggregates that
vary in intensity between the edges and the centers (Type II). When a Type II aggregate is tilted
about the x or y axis, the projected shapes and
intensity profiles of the aggregate change. For
example, the Type II aggregate indicated in Figure 2 has a perimeter that is darker than the
interior when viewed without tilting. With tilting
about either the x or y axis, the edges of the
projected aggregate become darker, whereas a
sliver of a lighter interior remains. These images
suggest that Type II aggregates are shells or vesicles. The gray level inside the vesicles is comparable to that of the matrix, indicating that the
compositions inside and outside the vesicles are
comparable. The size of Type II aggregates in
SPS-100 range in diameter from ⬃12 to 34 nm,
and the average thickness of the vesicle wall is
approximately uniform at ⬃2– 4 nm.
Two other neutralization levels exhibit a similarly complex microstructure. Partially neutralized
ionomer SPS-75 and overneutralized ionomer SPS-
Figure 3. BF-STEM images of SPS-75 ionomer: (a) at
this magnification, multiple ionic aggregates are observed with random distribution; (b) the Zn-rich aggregate indicated by an arrow in (a), shown at higher
magnification, has an intensity gradient across the aggregate, Type II.
125 contain both Types I and II aggregates (Figs. 3
and 4). The Type I aggregates range in diameter
from ⬃4 to 10 nm, and the Type II aggregates range
in diameter from ⬃9 to 55 nm for both SPS-75 and
SPS-125. The vesicle wall thickness for the Type II
aggregates remains ⬃3 nm.
A somewhat different microstructure is observed in the SPS-25 ionomer (Fig. 5). The principal observation is the macrophase separation of
the aggregates on the micron size scale [Fig. 5(a)].
Large regions of the sample are void of aggregates. The second observation is the presence of
only Type I ionic aggregates in SPS-25; no Type II
SPHERICAL AND VESICULAR IONIC AGGREGATES
481
compare the expected morphology with the macrophase-separated ionomer, SPS-25, and then the
homogeneous ionomers, SPS-75, -100, and -125.
The STEM images of SPS-25 are only consistent with the various SAXS models with respect
to aggregate-size dispersity (monodisperse) and
shape (spheres). The observed aggregate size is at
least twice the predicted size, and more importantly the spatial distribution is inhomogeneous
due to macrophase separation. This macrophase
separation could be caused by inhomogeneous,
Figure 4. BF-STEM images show shape heterogeneity in SPS-125 ionomer. Type I aggregates are uniform
spheres (left arrow), and Type II aggregates are vesicles (right arrow).
aggregates were observed. The Type I aggregates
range in diameter from ⬃4 to 10 nm for SPS-25.
Finally, a higher magnification STEM image of
SPS-100 is presented in Figure 6. Under these
imaging conditions JEOL-2010 can resolve features ⱖ 1 nm in diameter. Thus, we can conclude
that this PS-rich matrix is devoid of ion-rich aggregates with diameters ⱖ 1 nm. High-magnification images were collected at the other neutralization levels with the same result. For these
reasons, the ionic aggregates described previously
are the only Zn-rich aggregates larger than 1 nm
within the SPS materials.
DISCUSSION
As presented in our Introduction, previous analyses of SAXS data have concluded that the Znrich aggregates in highly (85%) neutralized SPS
(6.91 mol % acid) are homogeneously distributed
and spherical with a diameter of 1.8 –2.0 nm.5
Furthermore, according to the interpretations of
SAXS data, the diameter remains approximately
constant as both the neutralization level12 and
the acid content increase.5 These prior results
suggest that our Zn-SPS (5.3 mol % acid) with
25–125% neutralization levels would possess homogeneously distributed spherical aggregates ⬃2
nm in diameter.5 No such morphology was observed in these materials using STEM. First, we
Figure 5. BF-STEM images show shape homogeneity
and spatial heterogeneity in SPS-25 ionomer: (a) at this
magnification, macrophase separation of ionic aggregates is observed in that the ionic aggregates are confined to a ⬃200-nm band across the field of view; (b) the
Zn-rich aggregates indicated by the box and arrow in
(a) are shown at higher magnification [only Type I
aggregates (uniform spheres) exist in this sample].
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KIRKMEYER, WEISS, AND WINEY
Figure 6. High-magnification BF-STEM image shows
the absence of small features (ⱕ1 nm in diameter) in
the region within or surrounding the Type II ionic
aggregate in SPS-100 ionomer.
yet random, sulfonation of PS within and/or between chains. A statistical distribution of sulfonated groups per molecule displayed in Figure 12
of Ref. 26 indicates that for the sulfonation level
studied in the present paper, it is likely that PS
molecules have a broad range of sulfonation levels. There is also evidence that PS and SPS (1.67mol % sulfonated) are immiscible over a wide
composition range.27 These two observations suggest that the macrophase separation observed in
SPS-25 could result from copolymer-copolymer incompatibility in which the phase with more sulfonic-acid groups is more neutralized, and thus
exhibits ionic aggregates, than the aggregate-free
phase. Such macrophase separation would result
in scattering at low q.
At neutralization levels ⱖ 75%, the comparison
between the various interpretations of the SAXS
data and the STEM images change. For SPS-75,
-100, and -125, the spatial distribution observed
in the images is more homogeneous, as proposed
by the models, although the spacing between the
aggregates is larger than the radius of closest
approach proposed by any model. In fact, the
spacing between aggregates is larger than the
radius of gyration of the polymers, which raises
questions about the meaning of “fully” neutralized. Analytical TEM methods are under development to determine the Zn concentration in the
matrix away from the Types I and II aggregates.
Also contrary to the scattering models, two
types of ionic aggregates are present in styrenic
ionomers at the higher neutralization levels. Type
I aggregates are spherical domains with uniform
composition across the aggregate. These aggregates are similar to spherical micelles in which all
the polymer chains participating in the micelle
cross the same spherical interface into the matrix.
Type II aggregates are vesicular domains in
which the Zn ions are constrained to a ⬃2– 4 nm
shell. Here, the polymer chains participating in
the vesicular aggregate have the freedom to exit
the vesicular wall either by entering the interior
of the vesicle or by entering the matrix. Recall
that the vesicular aggregates were not observed
at 25% neutralization. Apparently, the neutralization level is critical in determining whether
one or both types of ionic aggregates are present.
The Type II vesicular aggregates have not been
previously proposed or observed in ionomers. Socalled “inverted multiplets” with similar architectures to Type II aggregates have been suggested
for oligomer/ionomer blends.28 Recent STEM images of a poly(styrene-co-methacrylic acid) random ionomer neutralized with Cs also show Type
II aggregates.29 Therefore, at higher neutralization levels, the need for new SAXS models is even
more pronounced than in SPS-25. The general
conclusion for all neutralization levels is that the
microstructures observed in the STEM images
encourage researchers to re-examine their current concepts of styrenic ionomer morphology.
CONCLUSION
FEG-STEM is a viable tool for directly imaging
ionic aggregates in typical amorphous SPS random
ionomers neutralized with Zn. The lightly neutralized (25%) ionomer exhibits only Type I solid spherical aggregates, whereas more neutralized
(75–125%) ionomers exhibit Types I and II vesicular
aggregates. Macrophase separation is also observed
in the lightly neutralized ionomer. These results are
inconsistent with previous interpretations of SAXS
data with respect to size, size dispersity, shape, and
spatial distribution of the ionic aggregates. Obviously, new interpretations of SAXS data are required to reconcile the morphological models as well
as these STEM results. More importantly, these
images suggest the presence of vesicular aggregates
that are a new proposition in ionomers.
We thank Dr. J. H. Laurer (Lexmark International)
and Dr. A. H. Taubert (Univ. of Pennsylvania) for help-
SPHERICAL AND VESICULAR IONIC AGGREGATES
ful discussions. We acknowledge Dr. R. E. Lakis and
Dr. D. M. Yates for technical assistance with the JEOL
2010F. This work was supported by NSF-DMR 9906829. The analytical electron microscopy was funded
by NSF-DMR 94-13550 and the Laboratory for the Research on the Structure of Matter at the University of
Pennsylvania. Dr. R. A. Weiss acknowledges support
from NSF-DMR 97-12194.
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