Sulfonated Polystyrene Ionomers Prepared by
Emulsion Copolymerization of Styrene and Sodium
Styrene Sulfonate
R. A. WEISS,* S. R. TURNER,+and R. D. LUNDBERG, Corporate
Research Science Laboratories, Exxon Research and Engineering Company,
Route 22 East, Annandale, New Jersey 08801
Synopsis
Sulfonated polystyrene (S-PS),
which is of considerable scientific and technological interest,
has been traditionally prepared by the sulfonation of preformed polystyrene. This report
describes the preparation and properties of S-PS prepared by emulsion copolymerization of
styrene and sodium styrene sulfonate. S-PS prepared by copolymerization gave solubility,
solution behavior and thermal characteristics that are consistent with a n ionomeric structure.
The solubility characteristics indicated some chain-to-chain sulfonate heterogeneity. Thermal
analysis studies indicated that the glass transition does not increase with increasing sulfonate
content. This is contrary to what has been observed for S-PS prepared by sulfonation and
suggests that the S-PS prepared by copolymerization is fundamentally different in structure
than S-PS prepared by sulfonation of polystyrene.
INTRODUCTION
Metal sulfonate-containing ionomers have generated considerable scientific as well as technological interest because of the unique properties
that result from the associations of the ionic sulfonate groups.' The nature
of this sulfonate aggregation in a variety of polymers, e.g., EPDM, butyl
rubber, and polystyrene has received considerable a t t e n t i ~ n . Comparisons
~-~
of the degree of aggregation of carboxylate and sulfonate ionomers indicate
stronger aggregation of the more polarizable sulfonate species.6 Exploitation
of these stronger interactions has been duly reported, for example, thermoplastic elastomer^,^ stable foams: and solution proper tie^.^
Until recently most sulfonate ionomers have been prepared by sulfonation
of a preformed polymer backbone, e.g., EPDM'O and polystyrene." Some
recent reports have shown that sulfonate ionomers can be prepared by direct
emulsion copolymerization of a hydrocarbon monomer and a metal sulfonate-containing monomer. For example, the copolymerization of isoprene
and sodium styrene sulfonate12 and butadiene and sodium styrene
sulfonate13 have been shown to yield elastomeric sulfonate ion containing
copolymers that exhibit ionomeric characteristics.
Lightly sulfonated polystyrene (S-PSI has been one of the most thoroughly
studied of the metal sulfonate ionomers. Studies concerned with the solution
b e h a ~ i o r viscoelastic
,~
properties,14 EXAFS analyses,2 fluorescence mea-
+
Present address: Institute of Materials Science, University of Connecticut, Storrs, C
l
' 06268.
Present address: Eastman Kodak Company, Research Laboratories, Rochester, NY 14650.
Journal of Polymer Science: Polymer Chemistry Edition, Vol. 23, 525-533 (1985)
@ 1985 John Wiley & Sons, Inc.
CCC 0360-6376/85/02052549$04.00
526
WEISS, TURNER, AND LUNDBERG
~ u r e m e n t sthermal
,~
analysis,15 small-angle x-ray ~cattering‘.~
and electron
spin resonance5 have recently been reported. The S-PS used in the above
studies was prepared by the sulfonation of preformed polystyrene. We report
here that S-PS can be prepared by the emulsion copolymerization of styrene
and sodium styrene sulfonate. Our studies show that while S-PS prepared
by this copolymerization possesses ionomeric properties, these properties
significantly differ from S-PS prepared by polymer sulfonation. A concurrent publication16 describes the copolymerization reaction in more detail.
EXPERIMENTAL
Typical Copolymerizaton Procedure
All copolymerizations were carried out with freshly distilled styrene (St).
Sodium styrene sulfonate (NaSS) was used as received from Air Products,
Inc. (Allentown, PA). Distilled water was deareated before use by boiling
and then cooled in a nitrogen atmosphere. Sodium dodecyl sulfonate (SDS),
potassium persulfate (K2S208),and n-dodecylthiol (C12SH)were used as received.
A small glass pressure bottle (250 mL) was charged with a small magnetic
stir bar to enhance agitation, 25 g of St (0.24 moll, 1.0 g of NaSS (0.0048
moll, 60 mL of HzO, 0.1 g of Cl2SH, 1.6 g of SDS, and 0.1 g of KzS208.The
flask was purged with nitrogen and capped with a two-hole crown cap
containing a rubber septum. The bottle was placed into a safety screen in
a thermostatted Erback water shaker bath at 50°C and was agitated for a
6-h polymerization time. The bottle was removed and ca. 3 mL of a hydroquinone “shortstop” solution was added via a syringe. The bottle was shaken
for a n additional 10 min, cooled, and opened. This polymer was coagulated
by addition of ca. 250 mL of methanol with some NaCl added. The very
fine precipitate was difficult to filter, thus the slurry was centrifuged at
ca. 2000 rpm for about 45 min. The solid polymer was washed two to three
times with distilled water and brought down by centrifugation each time.
About 15 g (after drying under vacuum at 40°C) of white powder were
obtained by this procedure. This represented about 58% conversion. Evaporation of the washes yielded the remainder of the expected product. These
washes, of course, were contaminated with surfactant and the NaCl used
to assist in coagulating the emulsion. Elemental analysis of the coagulated
portion was 0.71% S. A control without NaSS present gave a S analysis of
0.21%. Therefore, the polymer actually contained 0.50% S, which is equivalent to 1.57 mol % NaSS incorporation. The evaporated fraction was found
to contain 1.21% S.
Similar results were obtained when the redox couple initiator triethylenetetramine and diisopropylbenzene hydroperoxide was used at room temperature.
Alternative Workup
In some cases, especially at high sulfonate charges, the copolymer was
isolated by evaporation of the water, dissolution in a n appropriate solvent
and reprecipitation into methanol.
SULFONATED POLYSTYRENE IONOMERS
527
Polymer Characterization
Sulfur content was determined by Dietert sulfur analysis and was used
to calculate the copolymer composition. Thiol and persulfate contributions
to total sulfur were determined on for polystyrene samples polymerized by
the same procedure and subtracted in order to obtain accurate sulfonate
contents.
Glass transition temperatures Tgwere measured with a Perkin-Elmer
differential scanning calorimeter (DSC) model 2, using a scanning rate of
20"/min and a nitrogen atmosphere. Tgwas taken as the temperature at
which the change in heat capacity associated with the transition reached.
one-half of its ultimate value. Softening behavior was measured under a
helium atmosphere with a Perkin-Elmer TMS-2 thermomechanical analyzer (TMA) using a quartz compression probe with a hemispherical tip
(1.52 mm radius) and a heating rate of 10"/min.
Dilute solution viscosities were determined in a Ubbelohde capillary viscometer for which the kinetic energy correction was negligible.
RESULTS AND DISCUSSION
Copolymer Preparation
Previous studies of the emulsion copolymerization of styrene and sodium
styrene sulfonate were concerned with the preparation of stable lattices at
10.0
0
0
I
/
/
10
0
20
30
40
MOL X N S IN FEED
Fig. 1. NaSS content in the copolymer versus NaSS content in the copolymerization feed
diisopropyl benzene hydroperoxide/ triethylenetetramine;
for the precipitated fractions: (-)
(-)
potassium persulfate-initiated copolymerization.
528
WEISS, TURNER, AND LUNDBERG
15
0
10
a
I
5
8
E
4
4
2
z
dI
Q
5
i
I
I
I
I
P
~
10
20
r)
40
60
MOL X NISS IN FEED
Fig. 2. NaSS content in the copolymer versus NaSS content in the copolymerization feed
for the dispersed fractions: (-)
diisopropyl benzene hydroperoxide/triethylenetetramine;
(-)potassium persulfate-initiatedcopolymerization.
low NaSS levels in the absence of s u r f a ~ t a n t s . 'The
~ resulting copolymers
were not isolated and consequently the properties of these copolymers were
not reported. In this study higher levels of NaSS were utilized, up to about
40 mol % NaSS, and the copolymerizations were conducted in the presence
of sodium dodecyl sulfate (SDS). Polymers were isolated by coagulation into
methanol with the addition of sodium chloride. The coagulated polymers
were extremely difficult to filter, thus centrifugation was used to effect the
isolation with some solid remaining dispersed. The precipitate was washed
three times with water. The washes were combined with the dispersed solids
from the coagulation and evaporated. Thus two fractions of copolymer and
copolymer contaminated with surfactant and salt were obtained. The sulfonate content of these fractions was dependent on the NaSS charge and
was always lower than the initial charge of NaSS. The nondispersed or
precipitated fraction, which was normally 60-70% of the product, contained
lower sulfonate levels than the dispersed fraction (Figs. 1 and 2).
SULFONATED POLYSTYRENE IONOMERS
529
This behavior appeared to be almost independent of the initiator, whether
it was a water soluble persulfate or a two-component redox system with
one component present in the hydrocarbon phase and the other in the
aqueous phase. These results contrast sharply with those that have been
previously reported for the isoprene/NaSS copolymerization where the redox system was claimed to be necessary for the copolymerization to occur.12
We will discuss the copolymerization reaction of styrene and sodium styrene
sulfonate in more detail in a subsequent publication.
Copolymer Properties
Solubility Behavior
Ionomers, in general, and particularly sufonate-containing ionomers, exhibit such strong ionic associations that dissolution in solvents of low polarity is difficult. Depending upon the concentration of ionic groups, and
to some degree upon the polymer concentration, attempts to dissolve a
sulfonate-ionomer in a hydrocarbon solvent, such as toluene, results in
either a n extremely viscous solution or insolubility. In some cases, the
system may form a highly swollen gel.
The addition of a small amount of a polar cosolvent such as a n alcohol,
which selectively solvates the ionic species, weakens the ionic associations
and permits dissolution of the ionomer. Lundberg and Markowski18 have
explained this behavior as a equilibrium between a solvated species and a n
aggregated species of multiple ion pairs.
The solubility behavior of the copolymers prepared in this study indicate
strong association of the metal sulfonate groups. Under 3 mol % sulfonate
level the copolymers were soluble in toluene alone. Above this concentration
they were insoluble in toluene, but the addition of 10% methanol as cosolvent improves the solubility. The addition of methanol also results in a
visible reduction in viscosity of the soluble copolymers. The solubility could
be further improved by using a solvent with a relatively high dielectric
constant such as dimethylformamide (DMF). These solubility data versus
sulfonate level content are summarized in Table I.
TABLE I
Solubility Behavior of S-PS from Copolymerization'
% Soluble copolymer
Mol 70
SO,-Na+
1.6
2.9
4.1
8.1
9.3
11.9
17.7
31.2
a
To1u en e
97
97
32
9
8
14
<1
<1
Prepared with redox initiator
90/10
toluene/methanol
96
96
97
56
50
44
<1
<1
DMF
100
100
100
-
74
78
55
41
WEISS, TURNER, AND LUNDBERG
530
1 .o
0.8
Tolume/Mahanol
\
>
s
/
O
H
0.6
5
::
V
2
0.4
u
a
D ~ F
0.2
0
I
1
I
I
0.5
1.0
1.5
2.0
CONCENTRATION (pm/lOOml)
Fig. 3. Reduced viscosity versus concentration of 4.3 mol % S-PS from copolymerization
in 90110 (v/v) toluene/methanol and dimethylformamide.
1
I
I
I
I
5
10
15
20
25
I
30
MOL K METAL SULFONATE
Fig. 4. Glass transition temperature versus sulfonate level for S-PS from sulfonation of
polystyrene and S-PS from copolymerization.
531
SULFONATED POLYSTYRENE IONOMERS
Solution Behavior
Lundberg and Phillipslg have demonstrated that the solution properties
of sulfonated ionomers can vary from classical polyelectrolyte behavior to
behavior characteristic of strong ion pair interactions. In nonpolar media
the ion pair interactions predominate, while in more polar solvents there
is sufficient ionization to exhibit a significant polyelectrolyte effect. This
behavior is exhibited in Figure 3 for a copolymer containing 4.3 mol %
NaSS in 901 10 toluene/methanol (nonpolar solvent) and in dimethylformamide (DMF).
Thermal Analysis
Eisenberg20 has reviewed the effect of pendant ionic groups on the glass
transition temperature of carboxylate ionomers. In general, Tg increases
with increasing ionic group concentration, and the effects of different counterions can be superimposed if the cation charge and the distance between
centers of charge are considered. Studies of the effect of sulfonation on the
Tgof polysulfone22p ~ l y p e n t e n a r n e r ,Nafion,14
~ ~ . ~ ~ p o l y ~ t y r e n eand
, ~ ~EPDM2'j
yielded similar results.
The increase in the T, with increasing ionic group concentration may be
due to either a copolymer effect-i.e., the T, of the copolymer is between
the Tg's of the homopolymers prepared from the two comonomer u n i t s - o r
it may be a result of the decrease in free volume produced by the formation
of ionic crosslinks. Arguments have been made for both explanations; however, there is no universal agreement as to which mechanism prevails.
The Tg's of the styrene-NaSS copolymers and sulfonated polystyrene
determined by DSC are plotted versus sulfonate concentration in Figure 4.
The T, data for the S-PS by copolymerization are inconsistent with previous
(17.7)
\
(13.1)
(Polystyrene)
dTldt = 10°lminute
I . . 50,
25
l
l
l
.
l
100
l
.
.
.
l
150
.
.
.
l
l
200
.
,
.
.
Temperature ('C)
Fig. 5. TMA penetration curves for S-PS from copolymerization.
l
.
2 0
532
WEISS, TURNER, AND LUNDBERG
observations of an increase in T,with increased ionic group content. These
results suggest phase separation and may be indicative of a nonrandom
distribution of sulfonate groups along the polymer chain.
Thermomechanical Analysis
Intermolecular physical interactions such as occur between ionic species
can have profound effects on the mechanical and rheological behavior of a
polymer, especially at temperatures above the Tgwhere the normal relaxation rates for linear polymers are relatively rapid. The ionic groups decrease the rate of stress relaxation processes. This results in increased
softening points and increased melt viscosities. Such results have been observed by Rigdahl and Eisenberg14 for sulfonated polystyrene and by Lundberg and Makowski2'j for the sodium salts of sulfonated and carboxylated
polystyrene. Thermomechanical analyses (TMA) of the NaSS styrene copolymers prepared in this study show a significant increase in the rubbery
plateau region with increasing ionic content as has been observed for sulfonated and carboxylated prepolymers as described above. These results are
shown in Figure 5.
CONCLUSIONS
The results presented in this study clearly show that ionomeric sulfonated
polystyrene can be synthesized by emulsion copolymerization of styrene
and sodium styrene sulfonate. Polymer properties such as solubility, solution behavior, Tp and viscoelastic behavior exhibited ionomeric characteristics. The results also suggest, especially T,data, that the S-PS prepared
by copolymerization is fundamentally different from S-PS prepared by sulfonation of preformed polystyrene. This topic will be covered in detail in a
subsequent paper.27In another paper,l6 we report on the unusual copolymerization chemistry of styrene and NaSS.
The authors acknowledge the technical assistance of Mr. William Petrik and Mr. Nate
Brown.
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Received June 13, 1984
Accepted July 20, 1984