The Effects of Low Molar Mass Diluents
on the Microstructure of Ionomers
J. J. FITZGERALD* and R. A. WEISS, Polymer Science Program and
Department of Chemical Engineering, University of Connecticut,
97 North Eagkville Road, Storrs, Connecticut 06269-3136
Synopsis
The effect of polar and nonpolar low molar mass diluents on the microstructure of lightly
sulfonated polystyrene ( SPS) ionomers was studied using electron spin resonance spectroscopy,
small-angle x-ray scattering, and dynamic mechanical analysis. Nonpolar diluents primarily affected
the hydrocarbon-rich phase, while polar diluents partitioned into the ion-rich regions and disrupted
the supramolecular structure. The ionic clusters remained intact, even at elevated temperatures,
upon the addition of nonpolar solvents such as dodecane and dioctylphthalate. More polar solvents
such as methanol and glycerol swelled the ionic domains and promoted increased mixing of the
two phases.
INTRODUCTION
The relationship between the complex properties that ionomers exhibit and
their microstructure has been the subject of numerous studies, and several
recent review articles have extensively surveyed the ionomer literat~re.~-''
Relatively few studies have focused on the influence of solvents other than water
on the structure and properties of ionomers. Most of these have been based on
measurements of the viscosity of solutions. For example, Schade and Gartner l2
compared the solution behavior of copolymers of styrene with acrylic acid,
methacrylic acid, and maleic acid in tetrahydrofuran ( T H F ) and in dimethylformamide (DMF) . They observed polyelectrolyte behavior for the Na salts
at low polymer concentrations in a solvent capable of solvating the ion-pair,
such as DMF. With a solvent of lower dielectric constant, THF, for example,
the ion-pair was not dissociated and no polyelectrolyte effect was observed.
Several recent reports 13-18 have described the effect of solvent polarity on
the dilute solution viscosity of other ionomers. Lundberg and Phillips l4 studied
the solution properties of sulfonated polystyrene (SPS) ionomers in solvents
with dielectric constants t ranging from 2.2 (dioxane) to 46.7 (dimethylsulfoxide). They found that polar solvents tended to solvate the ions, whereas
nonpolar solvents promoted ion-pairs and interactions between the ionic dipoles.
Peiffer and Lundberg16correlated the solubility parameter of solvents with the
reduced viscosity of various ionomer-solvent systems and were able to qualitatively relate the molecular structure of a solvent to its ability to interact with
the ionic groups of the polymer.
* Present address: Research Laboratories, Eastman Kodak Co., Rochester, New York 14650.
Journal of Polymer Science: Part B: Polymer Physics, Vol. 28, 1719-1736 (1990)
0 1990 John Wiley & Sons, Inc.
CCC 0887-6266/90/01001719-018$04.00
1720
FITZGERALD AND WEISS
Even fewer studies have involved the effect of low concentrations of solvents
(i.e., plasticizers) on the bulk properties of i o n ~ m e r s . ' ~Lundberg
-~~
et al."
investigated the effect of several plasticizers on the melt rheology of SPS ionomers and proposed a dual plasticization mechanism for ionomers: polar diluents
(e.g., glycerol) preferentially partition into the ion-rich regions (clusters) and
disrupt the intermolecular ionic associations, while relatively nonpolar diluents
[ e.g., dioctylphthalate (DOP) ] plasticize only the continuous hydrocarbon phase.
The objective of the present study was to determine how the addition of
polar and nonpolar low molar mass diluents affects the microstructure and
properties of lightly sulfonated polystyrene ( SPS ) ionomers. The viscoelastic
response of the plasticized ionomers was studied by dynamic mechanical analysis
(DMA) and the microstructure of the ionomer was characterized by smallangle x-ray scattering (SAXS). The effects of diluents on the local interactions
between the anion and cation and between cations were studied p r e v i ~ u s l y ~ ~ * ~ *
using Fourier transform infrared spectroscopy (FTIR) and electron spin resonance (ESR) respectively. Additional ESR results that complement the SAXS
data are also discussed.
?
Cluster Morphology
Because the conclusions of this study are intimately tied to the existence of
microphase-separated ion-rich aggregates, clusters, a brief review of the evidence
for clusters and their morphology is useful. Small-angle x-ray scattering (SAXS)
and small-angle neutron scattering ( SANS ) have provided the most incontrovertible evidence for microphase separation in ionomers. The microphase separation occurs because of the large differences in polarity between the hydrocarbon backbone and the pendent acid groups. A maximum in the SAXS intensity, often referred to as the ionic peak, has been observed in most ionomers,
typically at a scattering vector k (k = 47r sin 8/ A, where X is the wavelength
of the x-ray radiation and 28 is the scattering angle) of 0.1-0.3 k'
. In addition
to the ionic peak, the SAXS curves of ionomers usually exhibit a large upturn
in intensity near zero scattering angle, which appears to be due to the presence
of the ions.25
MacKnight and Earnest and, more recently, Yarusso and Cooper, 26 have
reviewed in detail the various microstructure models that have been proposed
for ionomers to account for the SAXS peak. These may be divided into two
categories: those that attribute the scattering to intraparticle interference and
those that ascribe it to interparticle interference.
MacKnight et al.27proposed a shell-core model that attributes the scattering
to ion-rich spherical clusters, 16-20 A in diameter, with a very high electron
density. The clusters are shielded by the attached polymer chains, which form
a shell with the same electron density as the hydrocarbon matrix. At some
preferred distance, which is established by the electrostatic potential, there is
a shell of isolated ion-pairs with a lower electron density. The distance from
the core to the shell is responsible for the ionic peak. This model is qualitatively
able to predict the upturn in scattering intensity near zero angle and the scattering maximum, but it is not quantitative, and is not able to account for changes
in the position and the shape of the scattering maximum upon deformation.28
EFFECTS OF DILUENTS ON IONOMERS
1721
Yarusso and Cooper26proposed a cluster model morphology based on a hardsphere model first proposed by F ~ u r n e t . ~Their
'
modified hard-sphere model
attributes the ionic peak to interference between small ionic aggregates arranged
with a liquidlike degree of order. The distance of closest approach between
these aggregates is determined by steric limitations imposed by the attached
hydrocarbon chain. Typical calculations predict that the radius of the aggregates
in SPS is about 9 A and the distance of closest approach is about 18 A. Yarusso
and Cooper calculated that, on the average, only about 50% of the ionic groups
are incorporated in the microphase-separated clusters. The remaining ionic
groups are randomly dispersed in the hydrocarbon matrix as contact ion-pairs.
These conclusions are consistent with an earlier prediction by Ei~enberg,~'
who
envisioned two types of ionic environments: multiplets, which consist of up to
8 associated ion-pairs that exclude the hydrocarbon portion of the polymer,
and clusters that result from the aggregation of multiplets. Because of steric
limitations on the packing of a large number of bonded ionic groups, the clusters
were believed to contain a significant concentration of hydrocarbon chains.
The Yarusso-Cooper model does an excellent job of describing the experimentally determined SAXS ionic peak in SPS ionomers, but it is unable to fit the
zero-angle scattering region where an increase in the intensity is observed.
EXPERIMENTAL
The starting polystyrene was obtained from Dow Chemical Co. and had M,
of 110,000 and M, of 200,000 as determined by gel permeation chromatography.
The SPS was prepared according to the procedure of Makowski et al.3' The
reactions were carried out a t 50°C in dichloroethane using freshly prepared
acetyl sulfate as the sulfonating reagent. The sulfonated polymer was recovered
by steam distillation, washed with methanol, and dried. The polystyrene sulfonic
acid product was redissolved and neutralized with a stoichiometric amount,
based on elemental sulfur analysis, of the appropriate metal hydroxide or acetate,
and the polystyrene sulfonate was recovered by steam distillation, washed, and
dried under vacuum at 130°C for 72 h. The sample notation used in this report
is xx.x MSPS, where xx.x is the sulfonation level expressed as mole percent of
sulfonated styrene repeat units and M denotes the cation used.
Low-boiling diluents were added by either suspending compression-molded
ionomer films above the solvent or immersing them in the solvent at 25°C in
closed glass vials. Solvents with relatively low volatility, such as glycerol and
dioctylphthlate (DOP) were mixed with the polymer in a solution of 90% toluene
and 10% methanol. These samples were then air dried, placed in an air convection oven for 48 h a t 50"C, and finally dried in a vacuum oven at 80°C for
72 h to remove any residual toluene or methanol. The actual diluent concentration was determined gravimetrically. Sample films were prepared by
compression molding at temperatures of 170-180°C.
ESR spectra were obtained with a Varian E-102E spectrometer a t X-band
frequency with 100 kHz field modulation. A Varian wavemeter with a crystal
detector was used to measure frequency. SAXS measurements were made at
the National Center for Small-Angle Scattering Research (NCSASR) at Oak
Ridge National Laboratory using the 10 m camera. This apparatus utilizes a
1722
FITZGERALD AND WEISS
rotating anode x-ray source ( A = 1.52 A ) , crystal monochromatization of the
incident beam, pinhole collimation, and an area detector, which was set 1.126
m from the sample for these experiments. Dynamic mechanical analysis (DMA)
was carried out with a Rheometrics System 4 mechanical spectrometer using
rectangular or parallel-plate geometries in torsion. A frequency of 1 Hz was
used and the temperature was varied from 25°C to 250°C.
RESULTS AND DISCUSSION
Dynamic Mechanical Analysis
The effect of glycerol and DOP on the viscoelastic behavior of a 1.65 MnSPS
is shown in Figure 1. The results are similar to those reported elsewhere3’ for
NaSPS. The unplasticized ionomer exhibited a distinct plateau region in G’
and a loss peak in G” above the glass transition temperature (T,). Both of
these features were absent in the unmodified polystyrene. The rubbery plateau
was due to the formation of an ionic network, and the loss peak has been
attributed to the glass transition of the ionic clusters in other i ~ n o m e r sPre.~
viously reported data33for SPS ionomers showed that a significant change in
the SAXS curve and the SAXS invariant occurred at about the same temperature as the high-temperature mechanical loss, which indicated that it was due
to local motions in the ionic clusters.
The addition of DOP lowered T, and shifted the entire modulus versus temperature curve to lower temperature, a result typically observed for plasticized
polymers. Although the plateau in G’ and the loss peak in G” moved to lower
temperature, these features, which are characteristic of ionic aggregation, persisted in the plasticized polymers. For the ionomer containing glycerol, however,
only a small decrease in Tgwas observed, the rubbery plateau was diminished,
and the high temperature loss peak moved to lower temperature. These results
clearly demonstrated the preferential plasticization of the two phases of an
ionomer by polar and nonpolar diluents as proposed by Lundberg et al., l9 who
measured the viscosities at 220°C of similar plasticized ionomer compositions.
The polar glycerol preferentially partitioned into the ionic domains and solvated
the ionic interactions, thus influencing primarily that part of the viscoelastic
behavior due to the ionic aggregation. The decrease in Tgcan be attributed to
the breakup of the ionic crosslinks by solvation by the glycerol. The relatively
nonpolar DOP affected only the polystyrene matrix, leaving the ionic associations intact.
An unanswered question arising from these data is why a matrix plasticizer
like DOP shifted the ionic transition in a manner identical to the Tgshift. This
may have been due in part to the presence of a significant hydrocarbon fraction
in the cluster, although the SAXS data presented later in this report do not
support this. It is more likely that the lowering of the cluster glass transition
occurred as a consequence of the enhanced mobility of the plasticized polymer
chains. As with block copolymers, in ionomers, the phases are connected owing
to the fact that individual chains pervade both the ionic domains and the hydrophobic continuous phase. Although DOP directly affects only the hydrocarbon-rich portion of the ionomer, there may be a synergistic effect, so that
relaxation in the clusters is influenced by the motion in the continuous phase.
EFFECTS OF DILUENTS ON IONOMERS
1723
II
10
9
8
7
6
\
5
I
100
I
200
Temperature ("C)
0
100
200
300
Temperature ("C)
Fig. 1. ( a ) Storage modulus and (b) loss modulus versus temperature for 1.65 MnSPS ionomers:
( A ) unplasticized, ( B ) plasticized with 25% DOP, ( C ) plasticized with 10%gycerol.
1724
FITZGERALD AND WEISS
Similar observations of dynamic interactions between phases have been reported
in styrene-butadiene block copolymer^.^^ In that case, it was proposed that the
highly mobile butadiene segments may induce premature molecular motions
in the polystyrene microphase.
Electron Spin Resonance
Electron spin resonance is useful for studying cation-cation interactions in
ionomers with paramagnetic cations such as Cu (11) or Mn ( 11) .35-38 The ESR
spectra for 0.5 wt % solutions of 2.25 MnSPS in THF and DMF are shown in
Figure 2. The sharp six-line spectrum for the DMF solution is typical of isolated
Mn (11) ions in solution, which shows that DMF disrupted the dipole-dipole
interactions of the metal sulfonate groups. Previous FTIR studies23also showed
that DMF solvated the ion-pair, that is, removed the cation from the anion
environment. These results were consistent with the observation of a polyelectrolyte effect for SPS in DMF.14,17For the THF solution, only a single
broadline ESR spectrum was observed. The broadening of the hyperfine structure was due to magnetic interactions such as occur in concentrated systems.
This result indicated association of the ionic groups, and no polyelectrolyte
effect was reported for this ~ y s t e m . ' ~ , ~ ~
Lundberg and Phillips14 reported that the addition of water ( t = 78.5) to an
SPS/THF solution resulted in solvation of the ion-pair and polyelectrolyte
behavior. Spectroscopic evidence for this is shown in Figure 3 by the ESR
spectra of 2.25 MnSPS in THFlwater mixed solvents. Although the six hyperfine lines were resolved when water was added to the THF solutions, the
spectra were built on a broad background. This broad background was probably
due to incomplete solvation of the cation, indicating that both isolated Mn (11)
and associated species were present in the solution.
3hOG
Fig. 2. ESR spectra of 0.5% solutions of 2.25 MnSPS in ( A ) THF and ( B ) DMF.
EFFECTS OF DILUENTS ON IONOMERS
1725
I
31 OOG
Fig. 3. ESR spectra of 0.5% solutions of 2.25 MnSPS in THF/water: ( A ) 0%water, ( B ) 3%
water, ( C ) 6% water.
Similar results were observed38in mixed solvent systems containing toluene
= 2.4) and methanol ( t = 32.6). A toluene-swollen 2.25 MnSPS ionomer
exhibited a single broadline ESR spectrum characteristic of associated cations
(Fig. 4).A polar cosolvent was required in order to dissolve the MnSPS. Upon
the addition of 3% methanol, the hyperfine structure began to be resolved.
Increasing the methanol concentration up to 6% had no further influence on
the ESR spectrum.
The effect of the polarity of the cosolvent was also studied using ESR. Figure
5 shows the effect of the addition of equal molar quantities of primary aliphatic
alcohols ranging from methanol ( C , ) to heptanol (C7) to a 1.65 MnSPS in
toluene. Addition of any of the alcohols resulted in resolution of the ESR hyperfine structure, but the six-line spectra were built on broad absorption backgrounds, indicating that both isolated and associated Mn (11)ions were present
in these mixed solvent solutions. As the molar mass of the alcohol cosolvent
was increased, the ESR spectra became broader and the hyperfine structure
was less resolved, indicating that fewer isolated Mn ( I1 ) ions were present in
solution. That is, as the nonpolar character of the solvent increased, its ability
to solvate the ionic interactions decreased. This observation was consistent
with earlier viscosity results14 which showed that when a higher molar mass
alcohol was used for the cosolvent, higher solution viscosities resulted due to
increased association of the ionic species.
Although the ESR spectra for the two mixed solvent systems discussed above
were similar, THF/water solutions of SPS exhibited polyelectrolyte behavior
while toluene /methanol solutions did not.14717FTIR analysis indicated that
both water and methanol could ionize the sulfonate ion-pair. The reason for
the similarity of the ESR spectra, but differences in the dilute solution behavior
(t
1726
FITZGERALD AND WEISS
P,
/"
31 OOG
Fig. 4. ESR spectra of 0.5%solutions of 2.25 MnSPS in toluene/methanol: ( A ) 0%methanol,
( B ) 3% methanol, ( C ) 6% methanol.
for the THF/water and toluene/methanol systems is not clear. One explanation
may be differences in the relative populations of isolated and associated species
in the two solutions. For example, Lundberg3' has observed from Na nuclear
L
3500G
I
3500G
Fig. 5. ESR spectra of solutions of 1.65 MnSPS in ( A ) toluene and toluene/alcohol mixtures
containing equimolar amounts of ( B ) methanol, ( C ) ethanol, ( D ) n-butanol, ( E ) n-amyl alcohol,
(F)heptanol.
EFFECTS OF DILUENTS ON IONOMERS
1727
magnetic resonance ( NMR) experiments that THF/water solutions of NaSPS
contained a higher concentration of isolated Na ions than did toluene /methanol
solutions.
ESR results on plasticized bulk ionomers did not indicate solvation of the
ionic associations. For example, Figure 6 shows ESR spectra of a 1.82 MnSPS
plasticized with 10% glycerol. Single broadline spectra were observed below
and above Tg,
and no hyperfine structure was resolved. This result was surprising
in light of the viscosity studies of Lundberg et al., l9 which demonstrated that
glycerol was very effective at diminishing the intermolecular interactions in
SPS melts. In fact, in the DMA discussion above it was shown that the microphase-separated clusters were substantially eliminated by the addition of glycerol. Thus, it appears that although a polar plasticizer such as glycerol broke
up the supramolecular structure in these ionomers, some level of ionic association, presumably multiplets, still existed. Additional evidence for the interaction of glycerol with the ionic clusters is presented in the discussion that
follows.
Small-Angle X-ray Scattering
Bulk Ionomers
A maximum in S A X S intensity was not observed for compression-molded
films of either the unmodified polystyrene or the S P s - a ~ i dA. ~SAXS
~
maximum
was, however, observed for all the SPS salts studied, even a t sulfonate concentrations as low as 0.90 mol %. S A X S results for CsSPS are shown in Figure 7.
Since the scattering intensity was normalized for both thickness and the transmission coefficient of the sample, the differences in the intensities of those
scattering curves were due to differences in the number of ionic groups participating in the scattering. As the sulfonate concentration increased, the angular
400G
3000G
Fig. 6.
ESR spectra of 1.82 MnSPS plasticized with 10% glycerol a t ( A ) 40°C and (B)13OoC.
1728
FITZGERALD AND WEISS
d%
0
0
0
0
0
0
0
0
0
sp
0
0
0
0
A
0.0
Fig. 7.
CsSPS.
0.1
0.2
0.3
0.4
SAXS intensity vs. wavevector for ( A ) 0.90 CsSPS, (0)
2.65 CsSPS, and ( 0 )4.85
position of the scattering maximum moved to a higher scattering vector, which
corresponded to a smaller characteristic dimension.
Detection of clustering of the ionic groups by SAXS has not been previously
reported for sulfonation levels below 1mol %. However, electron spin resonance
studies have shown that Mn (11) ions are associated in the solid state at this
The fact that clustering occurs at very low ionic functionality in SPS
ionomers was further supported by dynamic mechanical analysis that showed
a high temperature loss peak in G" at a sulfonate concentration of 1.65
mol %.40
Effect of Polar Diluents
The effect of methanol on the SAXS curve of 3.85 ZnSPS is shown in Figure
8. The position of the scattering maximum for the neat ionomer was at k = 0.162
A?', which corresponds to a characteristic size of 40.9 A.When 2.3% methanol
(based on dry weight) was absorbed by the sample, the position of the scattering
maximum moved to slightly lower scattering vector and the intensity of the
scattering maximum decreased. At 6.3% methanol, the intensity of the scattering
curve was substantially reduced and the position of the peak moved to lower
k. The scattering invariant, which is related to the electron density contrast
between phases, decreased by a factor of three upon the addition of 6.3% meth-
EFFECTS OF DILUENTS ON IONOMERS
1729
4
3.2 h
IJY
c
.-
*
2.4-
9
.-c
-P
9
1.6-
C
2
x
c
.-
IJY
C
c
a,
-
.8
C
-
0
1
1
I
I
anol. Similar results were obtained for a 1.65 MnSPS (Fig. 9 ) . The addition
of 5% methanol lowered the scattering intensity and shifted the SAXS maximum to lower k, and a t 10% methanol the peak was no longer observed. The
increase in the lower angle scattering in the latter sample suggested that the
80
K
-
30 20
0
-
10 3
0.0
I
I
0.I
0.2
I
0.3
0.4
k(i-’)
Fig. 9. SAXS intensity vs. wavevector for 1.65 MnSPS containing (0)
0% methanol, ( 0 )5%
methanol, and (0)
10% methanol.
1730
FITZGERALD AND WEISS
peak had shifted to a scattering vector below which resolution was possible on
this experimental geometry.
The ESR and FTIR results discussed earlier indicated that low molar mass
polar compounds, such as methanol and water, partitioned into the ionic-rich
phase and swelled the ionic clusters. This process would involve reorganization
of the microstructure and possibly allow more of the hydrocarbon polymer to
be incorporated into the ionic phase. This may also allow the incorporation
into the cluster of ion-pairs that were previously excluded. The SAXS results
suggest that this process increases the size of the cluster, but reduces the electron
density difference between the clusters and the polystyrene-rich phase.
Effectof Nonpolar Diluents
The effect of a relatively nonpolar solvent, dodecane, on the SAXS curve is
shown in Figure 10. In this case, the absorption of 3.4 wt % dodecane had no
effect on the SAXS curve. If the dodecane were present in the ionic clusters,
the reduction of the electron density difference between the ionic aggregates
and the hydrocarbon matrix would have reduced the intensity of the scattering
peak. The fact that no intensity change of the SAXS peak was observed indicated that the dodecane must have preferentially partitioned into the hydrocarbon phase.
2.25
-
B
2.00 1.75 1.50
-
1.25
-
1.00 -
0.75 -
0.0
0.1
0.2
0.3
0.4
k(W-’)
Fig. 10. SAXS intensity vs. wavevector for 3.85 ZnSPS: ( 0 )neat ionomer, ( 0 )containing
3.4% dodecane.
EFFECTS OF DILUENTS ON IONOMERS
1731
Effect of Higher Boiling Diluents (Plasticizers)
DMA data on SPS ionomers plasticized with DOP and glycerol were discussed
earlier. The microstructure of these materials was also studied by SAXS, and
the results are discussed here and compared with both the DMA results and
the SAXS data for the lower-boiling diluents.
The effect of DOP on the microstructure of 2.65 MnSPS is shown in Figure
11. In this case, the intensity of the scattering maximum decreased as the
concentration of the DOP increased, yet the position of the scattering maximum
and the shape of the peak did not change. This would be expected if the DOP,
like the dodecane, preferentially partitioned into the hydrocarbon phase. DOP
would have only a small effect on the electron density of the hydrocarbon-rich
phase. If anything, DOP in the hydrocarbon phase would tend to lower the
electron density contrast between the matrix and the clusters, which may account for the observed decrease in the intensity of the SAXS curves. A change
in the position of the scattering maximum should have occurred if DOP swelled
the clusters. These SAXS results were consistent with the dynamic mechanical
results discussed earlier, where the major effect of adding DOP was a reduction
in the glass transition temperature of the hydrocarbon phase. The loss peak in
G" that represented the Tgof the ionic domains was still present, indicating
that the clusters were intact [ cf., Fig. 1( b ) 1 .
The addition of glycerol to 2.65 MnSPS increased the ionic peak intensity
and shifted it to lower scattering vector (larger characteristic size) (Fig. 12).
The shape of the peak sharpened, however, which indicated that the characteristic size of the scattering entity became more homogeneous. This suggests
that like methanol, glycerol preferentially swelled the ionic clusters. The increase
0
aA
ioh
0.0
I
0.I
0.2
0.3
0.4
k(A-')
Fig. 11. SAXS intensity vs. wavevector for 2.65 MnSPS containing ( 0 )0% DOP, (0)
1%
DOP, ( 0 )5% DOP, and ( A ) 10%DOP.
FITZGERALD AND WEISS
1732
"9 +
80
s
70
Ob
Ox)
I
I
0.1
0.2
I
0.3
0
k(;-')
Fig. 12. SAXS intensity vs. wavevector for 2.65 MnSPS containing (0)
0%glycerol, ( 0 )1%
glycerol, ( A ) 5% glycerol, and ( + ) 10% glycerol.
in the intensity may be the result of an increase in the volume fraction occupied
by the clusters. In swelling the cluster, glycerol must also partially solvate the
cation-cation interactions, which would reduce the strength on the ionic interactions a t elevated temperatures. This is consistent with the DMA results
presented earlier that showed that the high-temperature loss peak in G" was
substantially diminished upon the addition of glycerol [ cf., Fig. 1 ( b ) ] . Although
the glycerol-swollen samples appeared to be optically clear, recent low-temperature dynamic mechanical studies suggest that the glycerol was partially
phase separated when the bulk concentration was
Effectof Temperature
The microstructure of the neat SPS ionomers is especially resistant to temperature. Weiss and Lefelar33showed that the for NaSPS, the cluster morphology persisted to temperatures as high as 260°C; and although phase mixing
between the clusters and the matrix did occur at elevated temperatures in
ZnSPS, the SAXS peak did not completely disappear at 260°C. SAXS curves
for 2.65 MnSPS are given in Figure 13 for four different temperatures from
25°C to 265OC. The position and shape of the scattering maximum was essentially unaffected by temperature and it is clear that the ionic clusters were
intact a t 265°C.
The effect of temperature on a 2.65 MnSPS plasticized with 10% DOP is
shown in Figure 14. As for the neat ionomer, temperature had little effect on
the position and shape of the scattering curve for the DOP-plasticized material.
This result is consistent with the conclusion that DOP does not disrupt the
ionic domains, even at elevated temperatures. The data in Figures 13 and 14
were not corrected for thermal density fluctuations, which accounts for the
increases in scattering intensity at higher temperatures.
EFFECTS OF DILUENTS ON IONOMERS
40
-
30
-
20
-
10
-
0
1.
0.0
I
I
I
0.I
0.2
0.3
1733
0.4
k(L-')
25OC, (0)
70°C, ( + ) 150"C,
Fig. 13. SAXS intensity vs. wavevector for 2.65 MnSPS a t (0)
and ( 0 )265'C.
The effect of temperature on the SAXS curves of the 2.65 MnSPS containing
10% glycerol is shown in Figure 15. In this case, temperature had a strong
effect on both the scattering intensity and the peak position. As the temperature
increased, the scattering intensity decreased and the peak broadened and moved
to higher scattering vector.
v)
40-
c
._
C
3
x
L
9
c
m
30-
v
z1
._
v)
C
a,
c
5 20-
lob
0.0
I
I
I
0.I
0.2
0.3
c
4
k(L-')
Fig. 14. SAXS intensity vs. wavevector for 2.65 MnSPS plasticized with 10% DOP a t (0)
25"C, ( 0 )70°C.( + ) 15OoC, and (0)
265°C.
1734
FITZGERALD AND WEISS
'-I
. .-
120
A
v)
c
.E
=
z
O $0
oo 0
110-
loo90-
80._
70c
m
v
A
c
.-
60-
-
40-
za,
!XI30-
20 10
-
0
0.0
0. I
0.2
0.3
0
07
k(i-')
Fig. 15. SAXS intensity vs. wavevector for 2.65 MnSPS plasticized with 10%glycerol at (0)
25"C, ( 0 )70"C, ( 0 )140"C,and (+) 240OC.
The largest change occurred between 70°C and 140"C, which bracketed the
glass transition of the matrix. These results further support the premise that
glycerol associated with the ionic regions of the ionomer, and unlike DOP,
glycerol promoted rearrangement of the microstructure of the ionomer at relatively low melt temperatures. The decreased intensity of the SAXS peak a t
higher temperatures suggests that considerable phase mixing occurred in this
system. The effect of temperature on the SAXS of the glycerol-plasticized ionomer agreed well with the DMA results ( c f . , Figs. 1 and 1 5 ) . The DMA showed
a cluster T, at about 140-150°C, which was consistent with the temperature
interval in which the SAXS curves changed the most.
CONCLUSIONS
Diluents of low molar mass were chosen as model compounds to determine
the effect of polar and nonpolar additives on the microstructure of SPS ionomers. The addition of polar diluents changed the intensity of the SAXS peak
characteristic of ionic clusters and shifted the peak to a lower scattering vector
(i.e., larger characteristic size). The addition of methanol decreased peak intensity, while glycerol increased intensity, although the peak became narrower
in the latter case. The intensity changes and the peak shift indicated that polar
diluents disrupted the structural heterogeneities responsible for the peak. ESR
data confirmed that polar molecules such as methanol and water solvated the
interactions between the ionic groups. Thus, polar diluents weaken the ionic
interactions and promote phase mixing (i.e., break-up or swelling of the clusters). No changes in the position or shape of the SAXS peak occurred when a
nonpolar diluent such as dodecane or DOP was added. These molecules ap-
EFFECTS OF DILUENTS ON IONOMERS
1735
parently partitioned preferentially into the ion-poor, hydrocarbon phase, leaving
the ionic clusters essentially unchanged.
These conclusions were supported by SAXS data for the plasticized ionomers
at elevated temperatures. For the DOP plasticized sample, the position and
shape of the scattering maximum were unaltered at temperatures as high as
240°C. The SAXS peak of the glycerol-plasticized sample, however, decreased
significantly a t elevated temperatures, indicating that a substantial amount of
phase mixing occurred. These results agreed well with the results obtained from
dynamic mechanical studies, which also indicated that, whereas DOP plasticized
the hydrocarbon matrix of an SPS ionomer, glycerol plasticized the ionic
clusters.
This research was supported by a grant (DMR 8805981) from the Polymers Section of the
Division of Materials Research of National Science Foundation.
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Accepted January 5, 1990