Polymer International
Polym Int 54:1220–1223 (2005)
DOI: 10.1002/pi.1836
Sulfonated poly(ethylene-ran-styrene)
ionomers
Sang-Yeon Shim1 and RA Weiss1,2∗
1 Department
2 Polymer
of Chemical Engineering, University of Connecticut, Storrs, CT 06269-3136, USA
Program, University of Connecticut, Storrs, CT 06269-3136, USA
Abstract: Ionomers were prepared by partially sulfonating the styrene units of a poly(ethylene-ranstyrene) (ES) copolymer. The metal salt derivatives of sulfonated ES had higher Tg , melt viscosity and
rubbery modulus than did the parent ES because of the formation of labile, physical crosslinks from
intermolecular dipole–dipole associations between metal sulfonate groups. Microphase separation of ionrich aggregates with a characteristic size of 3–9 nm also occurred as a result of the strong intermolecular
associations.
 2005 Society of Chemical Industry
Keywords: ionomer; sulfonation; ethylene–styrene interpolymer
INTRODUCTION
An important application of thermally reversible networks is thermoplastic elastomers. At use temperature, these materials possess properties similar to a
crosslinked rubber, yet at higher temperatures the
network relaxes and the polymer can be processed
like a thermoplastic. For example, polystyrene-blockpolydiene elastomeric copolymers can be meltprocessed by conventional processing technology, but
at room temperature exhibit many of the physical
properties of vulcanized rubber.1 These features are a
result of the formation of microphase-separated glassy
polystyrene domains that act as physical crosslinks
dispersed within the rubbery matrix. However, above
the glass transition of the polystyrene phase, the physical crosslinks are easily deformed. Ionomers similarly
possess a thermally reversible network as a consequence of the thermally labile physical crosslinks
formed from intermolecular association of the ionic
species.2 An example of a ionomeric thermoplastic
elastomer is sulfonated poly(ethylene-co-propylene-codiene monomer) (sulfonated-EPDM).3
Recent developments in metallocene catalyst technology facilitate the copolymerization of ethylene
with relatively large amounts of styrene.4,5 These
poly(ethylene-ran-styrene) copolymers represent a
new family of materials that are being used in a variety of applications such as calendaring, filled systems,
injection molding and blends.6,7 In this communication, we report the synthesis and properties of thermoplastic elastomers derived from the ES copolymers,
namely lightly sulfonated poly(ethylene-ran-styrene)
ionomers.
EXPERIMENTAL
Polymers
Poly(ethylene-ran-styrene) (ES) with a composition of
42 wt% styrene (16 mol% styrene) was provided by
the Dow Chemical Co. The molecular weight averages
determined by gel permeation chromatography using
THF as solvent and polystyrene standards were
Mn = 96 kDa and Mw = 170 kDa. The styrene
units were partially sulfonated with acetyl sulfate
following the procedure of Makowski et al.8 A 5 wt%
solution of the ES was prepared in a 3/1 mixture of
1,2-dichloroethane (DCE)/hexane and purged with
nitrogen for 30 min. The mixed solvent was used
because of the poor solubility of the copolymer in
DCE. Acetyl sulfate was prepared by reaction of excess
acetic anhydride and sulfuric acid and this was added
to the polymer solution at 50–55 ◦ C. The reaction
was allowed to proceed at 50–55 ◦ C for 3 h, and
then was terminated with 2-propanol. The sulfonated
polymer was precipitated into stirred boiling water.
To remove residual sulfuric acid, the polymer crumb
was stirred in de-ionized water and then heated at
40–50 ◦ C for 20 h, changing the water every 4 h. The
crumb was then isolated from the water and dried in
vacuum at 60 ◦ C for 3 days. The extent of sulfonation
was determined by titration of the free acid derivative
to a phenolphthalein end-point in a mixed solvent
of toluene/methanol (9/1 v/v) using a standardized
solution of methanolic sodium hydroxide. Elemental
sulfur analysis (Galbraith Laboratories, Knoxville,
TN) was used to determine the degree of sulfonation.
In this paper, the sulfonation is reported as meq
sulfonate (g polymer)−1 , meq g−1 . An alternative
∗
Correspondence to: RA Weiss, Department of Chemical Engineering and Polymer Program, University of Connecticut, Storrs, CT 062693136, USA
E-mail: [email protected]
(Received 3 January 2005; revised version received 7 February 2005; accepted 11 March 2005)
Published online 3 May 2005
 2005 Society of Chemical Industry. Polym Int 0959–8103/2005/$30.00
1220
Sulfonated poly(ethylene-ran-styrene) ionomers
measure is the mol% styrene groups sulfonated; the
conversion is
where y = meq sulfonate (g polymer)−1
(1)
Sodium (Na-sES), zinc (Zn-sES) and cesium
(Cs-sES) salts were prepared by neutralizing the
free acid derivative (H-sES) in a toluene/methanol
solution (9/1 v/v) with a methanolic solution of
sodium hydroxide, zinc acetate dihydrate and cesium
hydroxide hydrate, respectively.
6.5
1.3
6.0
1.2
5.5
1.1
5.0
1.0
4.5
0.9
4.0
4
Polymer characterization
Thin films for infrared spectroscopy were prepared by solvent casting a polymer solution (9/1
toluene/methanol) onto a KBr disk. The IR absorption was measured using a Nicolet 60 SX FTIR
spectrometer by averaging over 30 scans at a resolution of 4 cm−1 . Thermal analysis was carried out
with a TA Instruments DSC model 2920. Samples
were heated in a nitrogen atmosphere from—50 ◦ C
to 150 ◦ C using a heating rate of 20 ◦ C min−1 .
Dynamic mechanical analysis (DMA) was performed
on compression-molded films using a TA Instruments
dynamic mechanical analyzer, model 2980, with a
tensile fixture and a heating rate of 2 ◦ C min−1 . The
frequency was fixed at 1 Hz and the temperature
was varied from −50 ◦ C to 50 ◦ C. Small-angle X-ray
scattering (SAXS) data were obtained at room temperature with a Bruker SAXS instrument equipped
with a 2-D HiStar detector. A rotating anode source
was used with an accelerating voltage and electric current of 40 kV and 100 mA, respectively. Radial scans of
intensity versus wavevector, q, from 0.5 to 5 nm−1 were
obtained, where q = 4π sin θ/λ, 2θ is the scattering
angle and λ = 0.154 nm is the X-ray wavelength.
RESULTS AND DISCUSSION
Copolymers were prepared with sulfonation varying
from about 0.9 to 1.3 meq g−1 (4.2–6.3 mol% styrene
units) by varying the amount of acetyl sulfate used
(Fig 1). Those sulfonate concentrations correspond to
about 0.7–1.0 sulfonate groups per 100 repeat units of
the chain, which is comparable with that in sulfonated
EPDM. Ionomers with higher sulfonation levels were
difficult to work with because of the very high melt
viscosities. The stoichiometry of the acetyl sulfate used
was based on the styrene content of the copolymer,
since only the styrene is sulfonated. The sulfonation
efficiency for the copolymers was about 56 % on the
basis of the acetyl sulfate feed, which was considerably lower than the 87 % conversion achieved for the
sulfonation of polystyrene homopolymer.9 The lower
conversion of the sulfonation reaction was similar to
the results for the sulfonation of polystyrene-blockpoly(ethylene-co-butylene)-block-polystyrene terpolymers (SEBS), where it was concluded that the differences in the solubility and chain conformation of
Polym Int 54:1220–1223 (2005)
Sulfonate conc (meq g-1 polymer)
4300y
1000 − 80y
1.4
Sulfonate conc (mol% styrene)
mol%styrene sulfonated =
7.0
6
8
10
12
14
Acetyl sulfate (mol%)
Figure 1. Sulfonation conversion versus acetyl sulfate in the feed.
The acetyl sulfate concentration is based on the styrene content of
the parent ES copolymer.
Table 1. sES copolymer ionomers
Sample
Sulfonation
(meq g−1
polymer)
Sulfonation
(mol%
styrene)
0
0.9
1.1
1.2
1.3
0
4.2
5.3
5.8
6.3
ES
0.9M-sES
1.1M-sES
1.2M-sES
1.3M-sES
Tg
(◦ C)
Tm
(◦ C)
−19
−20
−20
36
34
34
M = H, Na, Cs, Zn.
the two polymers may have shielded the polystyrene
blocks from the reaction media.9 This may be due
to differences in the solvent quality for ethylene and
styrene and the screening effect of the ethylene units in
the random copolymer. The sulfonated-ES ionomers
(sES) prepared are summarized in Table 1. The sample designation used is x.yM-sES, where x.y is the
degree of substitution of the styrene in meq g−1 polymer and M denotes the counterion, H, Na, Zn or Cs.
The FTIR spectra for ES, 5.3H-sES and 5.3ZnsES are shown in Fig 2 for the spectral range from
800–1400 cm−1 . The results of FTIR analyses indicated the presence of sulfonate groups in the polymer
after reaction, as evident from the characteristic bands
corresponding to the S=O and the S–O bond of the
sulfonate groups: 907 cm−1 for S–O stretching and
1176 and 1350 cm−1 for the O=S=O symmetric and
antisymmetric stretching vibrations, respectively. The
absorbance at 1040 cm−1 is due to the symmetric
stretch of the SO3 − anion and a band at 1200 cm−1 is
assigned to the antisymmetric stretching vibration of
SO3 − anion.
DSC thermograms (not shown) did not resolve
any appreciable differences in Tg when the ES
was sulfonated, but Tm of the ES decreased upon
sulfonation. Both transitions were fairly broad. DMTA
did resolve an increase in Tg as evident from the shift in
the peak temperature of tan δ, see Fig 3. Sulfonation
1221
S-Y Shim, RA Weiss
Sulfonate Conc. (mol% styrene)
SO31200
1040
0
4
6
40
1.1Zn-sES
S=O 1176
1350
2
S-O
Tg or Tm (°C)
907
1.1H-sES
20
0
ES
1500
1000
500
Wavenumber (cm-1)
-20
0.0
Figure 2. Comparison of FTIR spectra of ES, 5.3H-sES and
5.3Zn-sES in the range 800–1400 cm−1 .
1.0
0.4
0.6
0.8
Sulfonate conc (meq
g-1
1.0
1.2
1.4
polymer)
Figure 4. Tg (open symbols) and Tm (filled symbols) as a function of
the sulfonate concentration and cation for M-sES ionomers: ( ) ES,
() H-sES, () Na-sES, ( ) Zn-sES.
°
1.2
ES
1.1H-sES
0.2
1.1Zn-sES
250
1.1Cs-sES
0.6
200
1.1Na-sES
0.4
0.2
0.0
-40
-20
0
20
40
Temperature (°C)
Figure 3. Tan δ versus temperature for 1.1 M-sES ionomers
(f = 1 Hz).
of the ES slightly lowered the Tg for the free acid
form of the ionomer, but the Tg increased ∼5 ◦ C upon
neutralization. The lowering of the Tg by sulfonation
to the free acid derivative may be a consequence of
the lower crystallinity, ie a higher concentration of
the more flexible ethylene units in the amorphous
phase offsets any effect of intermolecular hydrogen
bonding of the sulfonic acid groups. However, the
stronger dipole–dipole interactions of the metal salts
did raise Tg . The choice of the counter-ion had little
effect on either Tg or Tm . An increase of the Tg with
sulfonation is commonly observed for ionomers and
is due to the restrictions on segmental motion that
arise from intermolecular dipole–dipole interactions
between sulfonate groups. The decrease of the Tm
may be kinetic in origin. Sulfonation does not affect
the concentration of crystallizable units, which would
be expected to control the equilibrium Tm , since
the styrene units were already non-crystallizable, but
the intermolecular interactions that occur in these
1222
Scattering Intensity (au)
tan δ
0.8
ES
1.2Cs-sES
1.3Cs-sES
150
100
50
0
0
1
2
q
3
(nm-1)
Figure 5. SAXS for ES, 1.2Cs-sES and 1.3Cs-sES.
materials increase the melt viscosity and suppress
diffusion of the ethylene units during crystallization.
As a result, smaller, less perfect crystallites may result,
which produces a lower melting temperature. The
thermal transitions for the sES ionomers determined
by DSC and DMTA are summarized in Fig 4.
Microphase separation of an ion-rich phase is
usually observed in ionomers as a consequence of the
strong intermolecular associations. The most common
evidence for this is a peak in the SAXS curves. The
SAXS curves for ES, 1.2Cs-sES and 1.3Cs-sES are
shown in Fig 5. A broad shoulder occurs between ca
q = 0.7–1.7 nm−1 , which corresponds to characteristic
Polym Int 54:1220–1223 (2005)
Sulfonated poly(ethylene-ran-styrene) ionomers
flow was observed. That, too, is a result of the strong
dipole–dipole interactions of the ionic groups that
persist at high temperatures.
10000
1000
1.1Na-sES
E ′ (MPa)
100
1.1Cs-sES
1.1Zn-sES
10
1.2H-sES
1
ES
0.1
0.9H-sES
1.1H-sES
0.01
-100
-50
0
50
100
150
200
Temperature (°C)
Figure 6. E versus temperature for ES and M-sES ionomers.
sizes (d) in real space (d = 2π/q) of 3.7–9.0 nm. No
such peak was observed in the parent ES copolymer.
In addition, a strong intensity upturn occurs at low
q, which is also typical of ionomers and is believed to
arise from an inhomogeneous distribution of isolated
ionic groups or multiplets.10,11
The effect of sulfonation on the tensile storage
modulus, E , for the ionomers is shown in Fig 6.
There was little difference between the ES and the
sulfonic acid derivatives in the transition region. If
anything, there was a small decrease in E as the
sulfonic acid concentration increased, which like the
decrease discussed earlier for Tg may be a consequence
of the lower crystallinity of the ionomers compared
with the parent ES. Above Tm , however, the influence
of the intermolecular hydrogen bonding of the sulfonic
acid groups became more apparent, and the modulus
generally increased with increasing sulfonation level.
The moduli of the metal salt derivatives were
substantially higher above Tg than those of the
free acid derivatives and the parent ES copolymer.
That, again, is due to the stronger intermolecular
dipole–dipole associations and microphase separation
of the ionic species, which act as physical crosslinks.
The modulus differences for the three salts were
not significant. Whereas the hydrogen bonding of
the H-sES derivatives weakened enough at elevated
temperatures, >80 ◦ C so as to allow viscous flow of
those polymers, the data for the Cs-salt in Fig 6 shows
a distinct rubbery plateau region between 80–180 ◦ C
where the modulus was relatively flat and no viscous
Polym Int 54:1220–1223 (2005)
CONCLUSIONS
Ionomers were prepared by partially sulfonating the
styrene units of a poly(ethylene-ran-styrene) (ES)
copolymer. The sulfonic acid derivatives exhibited
lower glass transition temperatures than the ES,
presumably as a result of lower crystallinity. The metal
salt derivatives of sulfonated ES, however, had higher
Tg , melt viscosity and rubbery modulus than did the
parent ES, which was a result of the development of
physical crosslinks from intermolecular dipole–dipole
associations between metal sulfonate groups. The
strong intermolecular associations also produced
microphase separation of ion-rich aggregates, similar
to those observed in other ionomers. The distinct
rubbery plateau observed for the metal salts above Tg
and the labile nature of the physical crosslinks in these
ionomers make them candidates for thermoplastic
elastomers.
ACKNOWLEDGEMENTS
We thank Dr Stephen Chum and Dow Chemical Co
for the ES copolymer. Dr S-Y Shim thanks Kangnung
National University in Korea for providing him the
opportunity to work on this project.
REFERENCES
1 Noshay A and McGrath JE, Block Copolymers, Academic Press,
NY (1977).
2 Lundberg RD, in Ionomers, ed by Tant MR, Mauritz KA and
Wilkes GL, Blackie Academic, London, pp 477–503 (1997).
3 Makowski HS, Lundberg RD, Westerman L and Bock J, Ions
in Polymers, ed by Eisenberg A, Adv Chem. Ser, American
Chemical Society, Washington, DC, Vol 187, p 37 (1980).
4 Cheung YW and Guest MJ, Proc Ann Tech Conf Soc Plast Eng
62:1634 (1996).
5 Mudrich SF, Cheung YW and Guest MJ, Proc Ann Tech Conf
Soc Plast Eng 63:1783 (1997).
6 Ronesi VM and Cheung YW, Hiltner A and Baer E, J Appl
Polym Sci 89:153–162 (2003).
7 Cheung YW and Guest MJ, J Polym Sci, Polym Phys Ed
38:2976–2987 (2000).
8 Makowski HS, Lundberg RD and Singhal GH, US Patent
3870841 (1975).
9 Weiss RA, Sen A, Willis C and Pottick LA, Polymer
32:1867–1874 (1991).
10 Ding YS, Hubbard SR, Hodgson KO, Register RA and Cooper
SL, Macromolecules 21:1698–1703 (1988).
11 Moore RB, Gauthier M, Williams CE and Eisenberg A, Macromolecules 25:5769–5773 (1992).
1223