Synthesis of high proton conducting nanoparticles by emulsion polymerization

Article abstract of DOI:10.1016/j.polymer.2010.11.055

High ion-exchange capacity (IEC) sulfonated polystyrene nanoparticles were synthesized by an emulsion copolymerization of styrene, divinyl benzene and sulfonated styrene (SS). The effects of varying the counterion of the sulfonated styrene monomer, the SS concentration, the surfactant and the addition of a crosslinking agent on the ability to stabilize the emulsion nanoparticles to high IEC were studied. Water-insoluble nanoparticles, 20-160 nm in diameter, with IEC as high as 5.2 meq/g were achieved using sulfonated styrene with a quaternary alkyl ammonium cation, a non-ionic surfactant and a crosslinking agent in the emulsion formulation. That IEC corresponds to fully sulfonated crosslinked polystyrene.

Full text of DOI:10.1016/j.polymer.2010.11.055

Polymer 52 (2011) 297e306
Contents lists available at ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Synthesis of high proton conducting nanoparticles by emulsion polymerization
,
b
,
,
Emmanuel Pitia a , M.T. Shaw 1, R.A. Weiss a
*
*
a University of Akron, Department of Polymer Engineering, Polymer Engineering Academic Center, 250 South Forge Street, Akron, OH 44325-0301, USA
b University of Connecticut, Polymer Program and Department of Chemical Engineering, Institute of Materials Science, 97 North Eagleville Road, Storrs, CT 06269-3136, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
High ion-exchange capacity (IEC) sulfonated polystyrene nanoparticles were synthesized by an emulsion
copolymerization of styrene, divinyl benzene and sulfonated styrene (SS). The effects of varying the
counterion of the sulfonated styrene monomer, the SS concentration, the surfactant and the addition of
a crosslinking agent on the ability to stabilize the emulsion nanoparticles to high IEC were studied.
Water-insoluble nanoparticles, 20e160 nm in diameter, with IEC as high as 5.2 meq/g were achieved
using sulfonated styrene with a quaternary alkyl ammonium cation, a non-ionic surfactant and a cross-
linking agent in the emulsion formulation. That IEC corresponds to fully sulfonated crosslinked
polystyrene.
Received 27 September 2010
Received in revised form
23 November 2010
Accepted 27 November 2010
Available online 4 December 2010
Keywords:
Conductivity
Nanoparticles
Styrene sulfonate
Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
of sulfonated styrene (SS) with (or without) styrene and a cross-
linker such as divinyl benzene (DVB) [15,25e27]. In the afore-
Ion-exchange resins such as sulfonated crosslinked polystyrene
(SXLPS) particles have applications in chromatography [1e3],
synthesis of magnetic particles [4e7], catalysis [8,9], polymeric
actuators [10,11] and water purification [12]. Recently, it has been
considered as a component of a composite proton-exchange
membranes (PEM) for fuel cells, because of its high ion-exchange
capacity (IEC [¼] meq/g) [13e18] and crosslinked structure. The
crosslinked structure stabilizes and prevents dissolution of the
particles in water. A practical range of the degree of crosslinking is
4e16% [19]. The IEC, degree of crosslinking, and particle size
depend on the application.
mentioned polymerization methods, the formulation typically
consists of monomers, water, surfactant, and initiator. The solu-
bility of the initiator in the water phase differentiates emulsion
from suspension polymerization.
Composite PEMs are of interest, because a two-phase system
provides the ability to decouple the transport and mechanical
properties, which is a problem with single ion membranes such as
NafionÒ and sulfonated homopolymers [28]. For a composite PEM
application, one wants SXLPS nanoparticles with high IEC, <100 nm
diameter and crosslinked to prevent excessive swelling or dissolu-
tion by water. Composite PEMs, containing sulfonated crosslinked
polystyrene (SXLPS) dispersed into various polymer matrices have
previously been described, though in those cases, usually micro-
High IEC, ∼4e5 meq/g, micrometer-size particles are commonly
obtained via suspension copolymerization of styrene with a cross-
linker such as divinyl benzene, followed by sulfonation of the
particles [20e23]. In suspension polymerization, the particle size
meter-size particles were used [13e18]. The conductivity (
those composite membranes was as high as 0.3 S/cm [16], which is
comparable to that of NafionÒ 117 (
∼ 0.1 S/cm). High IEC
(∼3.5e5.2 meq/g) SXLPS microparticles, ∼1 to 50 m, were obtained
s) of
ranges from 10 to 1000
mm. Another method of obtaining XLPS
s
particles is by emulsion polymerization which produces
m
50e500 nm size particles [14,24]. However, ionic nanoparticles
by either grinding conventional ion-exchange resins [15e18] or
post-sulfonation of crosslinked polystyrene particles [13,14]. A
disadvantage of micrometer-sized particles is that the particle size is
similar to the thickness of the membrane (e.g., Nafion 117 is
tend to coalesce into
mm-size particles during a post-poly-
merization sulfonation step. An alternative method for directly
producing SXLPS nanoparticles is by an emulsion copolymerization
∼180 mm thick), which produces a poor dispersion of the particles
and a relatively small ratio of surface area/volume (S/V) of the
conducting phase.
In summary, previous work [13e18], has demonstrated that
crosslinked sulfonated polystyrene micro- and nanoparticles can be
* Corresponding authors. Tel.: þ1 330 972 2581; fax: þ1 330 258 2339
E-mail addresses: esp9@zips.uakron.edu (E. Pitia), montgomery.shaw@uconn.
edu (M.T. Shaw), rweiss@uakron.edu (R.A. Weiss).
1
Tel.: þ1 860 486 3980.
0032-3861/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymer.2010.11.055

298
E. Pitia et al. / Polymer 52 (2011) 297e306
used to prepare composite PEMs and that alignment [18,29,30] of
the particles improved the proton conductivity. However, that work
was limited to either high IEC microparticles or low IEC nano-
particles. The hypothesis driving the research reported herein is that
one might expect better dispersion, higher proton conductivity, and
improved mechanical properties of a composite membrane if the
dispersed particles were nm-sized and had high IEC.
Weiss and coworkers [25e27] previouslydescribed the synthesis
of the sodium salt of poly(styrene-co-sulfonated styrene) (PSSS-Na)
using an emulsion copolymerization, but they were concerned with
the development of ionomers with low IEC. Brijmohan et al. [31]
extended that work to prepare the sodium salt of crosslinked poly
(styrene-co-sulfonated styrene) (XLPSSS-Na) nanoparticles
(∼50 nm) using an emulsion copolymerization of styrene, sodium
styrene sulfonate (Na-SS) and divinyl benzene (DVB). The highest
IEC achieved in that work, however, was 2.2 meq/g, which is lower
than the theoretical limit for fully sulfonated polystyrene, 5.4 meq/g.
The IEC was limited bythe stabilityof the emulsion and the solubility
of the Na-SS in the polymerizing particles. In the emulsion copoly-
merization used, the styrene and DVB monomers were in the
dispersed oil phase and the Na-SS was in the continuous aqueous
phase. Weiss and coworkers [25e27] concluded that the locus of
the polymerizing species was the interface between the two
phases.
Kim et al. [32] studied the synthesis of emulsifier-free copoly-
merization of Na-SS and styrene where the targeted IEC was
∼0.50 meq/g. They proposed that homogenous nucleation was the
primary source of particles at low concentration of Na-SS. When
the Na-SS concentration increased, they proposed a dual particle
nucleation mechanism where particles were formed by homoge-
nous and micelle nucleation. They suggested that micelles formed
from aggregation of either the Na-SS or a water-soluble poly-
electrolyte rich in Na-SS. They concluded that synthesis of high IEC
particles was not possible due to formation of high IEC water-
soluble polymer. More recently, Arunbabu et al. [33] reported the
formation of water-soluble polymer due to homopolymerization of
Na-SS in the water phase. Hence, in order to eliminate the
production of water-soluble polymer and achieve high IEC nano-
particles, the solubility of sulfonated styrene needs to be altered
so that it is soluble in the oil phase. That, together with the
addition of a crosslinker, should improve its incorporation into the
nanoparticles.
of tetramethyl ammonium hydroxide (TMAH); tetrahexyl ammo-
nium bromide (99% THAB); tetraoctyl ammonium bromide (98%
TOAB); potassium persulfate (99.99% KPS); sodium dodecyl sulfate
(98% SDS); poly(oxyethylene-4-lauryl ether), (Brij 30; FW 362.6;
HLB 9.7); poly(oxyethylene-20-lauryl ether), (Brij 99; FW 1149.5;
HLB 15); lithium acetate dihydrate; hydroxide-exchange resin
(Amberlite IRN-78, 1.1 meq/mL); and a proton-exchange resin
(Amberlite IRN-77, 1.8 meq/mL) were obtained from Aldrich
Chemical Co. and used as received. A water-soluble non-ionic
initiator, 2,2020-azobis[2-(2-imidazolin-2-yl)propane] dihydro-
chloride (VA-044) was obtained from Wako Chemicals, and used as
received. Styrene (>99%) and divinyl benzene (∼80% mixture of
isomers, DVB) from Aldrich Chemical Co. were washed three times
with an aqueous solution consisting of 5 wt% NaOH and 25 wt%
NaCl, and then twice with de-ionized water to remove the inhibi-
tors. De-ionized water (18 M
polymerizations.
U
cm (@ 25 ꢀC) was used in all the
2.2. Synthesis of PSSS-Na nanoparticles
Although crosslinked nanoparticles are needed for a PEM
application to prevent dissolution in an aqueous medium, most of
the synthetic work reported in this paper was carried out on
uncrosslinked systems, i.e., DVB was not incorporated into the
formulation. However, some preliminary result using DVB in the
formulations are also reported.
The Na-SS was converted to the free acid form (H-SS) by passing
an aqueous solution through a bed of proton-exchange resin. The
extent of ion exchange was 95e98%, as determined by titration
with 0.01 N NaOH. The H-SS was neutralized to the QAA salt (QAA-
SS) by adding an aqueous solution of a QAA hydroxide to the
aqueous H-SS solution and agitating at 25 ꢀC for 0.5 h, see Fig. 1. In
a separate investigation, the neutralization reaction of H-SS with
QAA salt was determined to go to completion. This was done by
neutralizing a given amount of QAA hydroxide with excess H-SS.
The amounts of unreacted and reacted H-SS were determined by
titration using 0.01 N NaOH. Equal amounts of QAA hydroxide and
reacted H-SS indicated complete reaction.
Tetrahexyl and tetraoctyl ammonium hydroxides (THA-OH and
TOA-OH) were prepared by passing a solution of TOAB or THAB
(20 g/L) in a mixed solvent of water and methanol (30/70 g/g)
through a packed column of a hydroxide-exchange resin. Methanol
was used because TOAB and THAB are not soluble in water. The
addition of a small amount of water increased the ion-exchange.
The extent of ion exchange was 90e95%, as determined by titration
with 0.01 N HCl. Similarly, the Li-SS monomer was prepared by
passing an aqueous solution of Na-SS (20 g/L) through a packed
column of a Liþ-ion-exchange resin, which was prepared by
This paper reports on the synthesis and characterization of high
IEC (∼5.2 meq/g) crosslinked poly (styrene-co-sulfonated styrene)
(XLPSSS) nanoparticles prepared by emulsion copolymerization of
a
quaternary alkyl ammonium (QAA) neutralized-sulfonated
styrene, styrene, and divinyl benzene. The effects of varying the
counterion of the sulfonated styrene (SS) monomer (alkali metal
and QAA cations), SS concentration, and the addition of a cross-
linking agent (divinyl benzene) on the ability to stabilize the
nanoparticles to higher IECs were assessed. The QAA salts were
chosen specifically to shield the polar character of the sulfonate
group in SS and alter its partition between the water and the oil
phase in the emulsion polymerization. In addition, the QAA cations
were expected to be beneficial in membrane fabrication by sup-
pressing agglomeration of the nanoparticles and improving
dispersion in a hydrophobic matrix.
2. Experimental details
2.1. Materials
Sodium styrene sulfonate, NaSS; tetrabutyl ammonium
hydroxide (TBAH) 30-hydrate ; a 40 wt% aqueous solution of tet-
rabutyl ammonium hydroxide (TBAH); a 25 wt% aqueous solution
Fig. 1. Schematic of the preparation of QAA salt of styrene sulfonate. R ¼ CxH2xþ1
,
where x ¼ 1, 4, 6, 8.

E. Pitia et al. / Polymer 52 (2011) 297e306
299
regenerating the proton-exchange resin using an aqueous solution
of lithium acetate dihydrate. For some of the experiments, the
sodium dodecyl sulfate (SDS) surfactant, was converted to tetra-
hexyl ammonium-dodecyl sulfate (THA-DS) by first converting SDS
to the sulfonic acid derivative using a proton-exchange resin and
then neutralizing the sulfonic acid to THA-DS with THA-OH.
The nanoparticle synthesis was carried out in a 250-mL three-
necked jacketed reactor at 70 ꢀC under a continuous nitrogen feed.
The reactor was equipped with a condenser, natural rubber septa,
magnetic stirrer, and a nitrogen blanket. The reactor was first
purged with nitrogen for 5 min and then charged with de-ionized
water, surfactant (Brij 30, SDS, Brij 99, or a mixture of the two
surfactants) and M-SS, where M denotes the cation. After slightly
agitating the mixture, styrene and DVB (for the crosslinked nano-
particles) were added and the solution was agitated more vigor-
ously. The initiator (KPS or VA-044) was added 5 min later. A typical
reaction time was ∼6 h, at which time a 3-mL aliquot of 10%
hydroquinone solution was used to terminate the reaction.
and dried. For the other experiments, the emulsions were dialyzed,
but the particles were not precipitated. Unless otherwise stated, in
all formulations, the surfactant used was Brij 30e0.02e0.05 pph in
water and the initiator was KPS ∼1e3 pph based on the total
monomer feed.
2.3. Characterization of PSSS-M
2.3.1. Composition
A NicoletÔ 380 Fourier transform infrared (FTIR) spectrometer in
reflection mode was used to confirm the conversion of Na-SS to QAA-
SS or Li-SS, and also to confirm the incorporation of the sulfonated
monomer into the copolymer. The IR samples for the Na-SS, Li-SS,
QAA-SS monomers, and polymers were compression molded thin
films. Spectra covered a frequency range of 400e4000 cmꢁ1 using 32
scans, which provided a resolution of ∼2 cmꢁ1. Prior to analysis each
sample was dried in an oven at ∼95 ꢀC overnight and then placed in
a desiccator to cool to room temperature.
The product solution was dialyzed using cellulose dialysis
tubing (12,000 MWCO) placed in a de-ionized water bath. The solid
product was isolated by vacuum drying at 60 ꢀC overnight. In some
reactions, the product was isolated without dialysis to determine
the mass fraction of water-soluble/polymer and insoluble nano-
particles. This was achieved by first isolating the solid product by
drying the reaction solution. The resulting solid product was
washed and filtered several times using hot water (∼60 ꢀC) and
dried at 60 ꢀC. The water-soluble polymer was isolated by evapo-
rating the wash water. The nomenclature used for the poly(styrene-
co-sulfonated styrene) nanoparticles is xyPSSS-M where xy is the
concentration of M-SS in the product expressed as ion-exchange
capacity (meq/g) and M denotes the cation. Crosslinked nano-
particles are denoted as xyXLPSSS-M. The materials that were
produced in this research are summarized in Tables 1 and 2. Note
that all of these formulations produced nanoparticles, except the
samples indicated as bold faced. The conversion data are provided
only for the experiments where the nanoparticles were isolated
2.3.2. Ion-exchange capacity (IEC)
As described above, some of the samples in Table 1 were dia-
lyzed and others were not. All samples were vacuumed dried at
60 ꢀC and elemental analysis (C, H, N, S) was carried out by
combustion with an Elementar Vario Micro Cube. The S content was
used to calculate the IEC (based on the mass of the acid form of the
ionomer) using Eq. (1). The standard error of the mean determined
from analyses of multiple samples was ∼ꢂ0.01 meq/g.
%S
32ðg=eqÞ
IECðmeq=gÞ ¼
ꢃ 10
(1)
2.3.3. Morphology
Samples obtained from the emulsion solution were dialyzed and
diluted with water. A 3- L drop was then placed onto a carbon-
m
coated grid and the water was allowed to evaporate at room
temperature for at least 20 min. The particle sizes were measured
from images of unstained samples obtained with a Philips 420 or
a FEI Tecnai G2 BioTwin transmission electron microscope (TEM)
operating at 80 kV.
Table 1
Polymerization experiments: dialyzed samples.
Sample
Feed (mol% M-SS)a
Feed (mol%
styrene/DVB)
Product IEC
(meq/g)b
1.9 PSSS-Li
3.1 PSSS-Li
3.3 PSSS-Li
4.4 PSSS-Li
4.8 PSSS-Li
1.2 PSSS-Na
1.5 PSSS-Na
2.5 PSSS-Na
3.5 PSSS-Na
3.9 PSSS-Na
5.0 PSSS-Na
1.1 PSSS-TMA
1.5 PSSS-TMA
2.1 PSSS-TMA
3.5 PSSS-TMA
5.5 PSSS-TMA
1.1 PSSS-TBA
2.4 PSSS-TBA
3.7 PSSS-TBA
3.9 PSSS-TBA
4.0 PSSS-TBA
5.2 PSSS-TBA
10
20
50
32
80
15
20
31
50
53
75
15
20
30
50
80
11
28
20
50
50
75
90/0
80/0
50/0
68/0
20/0
85/0
80/0
69/0
50/0
47/0
25/0
85/0
80/0
70/0
50/0
20/0
89/0
72/0
80/0
50/0
50/0
25/0
1.9
3.1
3.3
4.4
4.8
1.2
1.5
2.5
3.5
3.9
5
1.1
1.5
2.1
3.5
5.5
1.1
2.4
3.7
3.9
4
3. Results and discussion
3.1. Conversion of Na-SS to M-SS
The exchange of the Naþ cation of Na-SS with QAAþ or Liþ was
confirmed by FTIR. Fig. 2 shows the FTIR spectral region from 850 to
1650 cmꢁ1 for the five M-SS monomers that were studied. This
spectral region includes the SO3ꢁ stretching vibrations. The sulfo-
nate absorptions for the different monomers are summarized in
Table 3. The interaction between the cation and the sulfonate anion
(SOꢁ3 ) affected the position of the symmetric and anti-symmetric
stretching vibrations of the sulfonate anion, which show up at
w1040 cmꢁ1 and ∼1200 cmꢁ1, respectively, for a solvated anion.
However, the interaction had no effect on the SeO bending band at
∼905 cmꢁ1. The bands at ∼950 cmꢁ1 and ∼865e905 cmꢁ1 for TMA-
SS and TBA-SS respectively arise from vinyl CeH out of plane
bending.
c
The Coulombic field strength of the ion pair, i.e., the cation and
the sulfonate anion, is F ¼ kqcqa/a2, where qc and qa are the charges
on the cation and anion respectively, k is Coulomb’s constant
5.2
Bold face: Samples where the product was only water-soluble polymer, i.e., no
particles.
(8.99 ꢃ 109 N m2
C
ꢁ2), and a is the ionic radius of the cation. In
general, as the Coulombic field strength between ion pair increases,
a
mol% M-SS was based on the total monomer feed (styrene, DVB, and M-SS).
The product IEC was measured by elemental analysis.
SDS was used as the surfactant in this batch.
b
c
the symmetric stretching vibration moved to higher frequency and

300
E. Pitia et al. / Polymer 52 (2011) 297e306
Table 2
Polymerization experiments: fractionated product.
Sample
Feed
Feed
Conversion (%)
Product
Water-soluble
fraction (wt%)
IEC of water-soluble
fraction
(meq/g)
IEC of
nanoparticles
(meq/g)
(mol% M-SS)a
(mol% styrene/DVB)
IEC (meq/g)b
2.4 PSSS-Li
4.6 PSSS-Li
1.7 PSSS-Na
2.0 PSSS-Na
4.7 PSSS-Na
5.1 PSSS-Na
31
65
17
30
55
95
12
27
95
14
27
30
49
95
62
68
74
11
21
29
51
76
80
69/0
35/0
83/0
70/0
45/0
5/0
88/0
73/0
5/0
86/0
73/0
70/0
51/0
5/0
0/38
0/32
0/26
76/12
67/12
59/12
38/11
13/11
9/11
75
79
45
99
58
80
97
98
77
81
87
87
78
90
e
2.2
4.6
1.7
2
4.7
5.1
1.1
2.1
4.9
1.5
2.4
2.6
4
4.6
3.8c
4.1c
4.4d
1.3
1.7
2.4
3.4
4.8
4.5
42
100
35
31
100
100
1
22
100
28
55
63
84
100
0
0
0
8
15
29
43
100
100
4.5
4.6
4.4
4.7
4.7
5.1
5.8
5.4
4.9
3.8
3.4
3.4
4.4
4.6
e
0.9
e
0.3
0.8
e
e
1.1 PSSS-TMA
2.1 PSSS-TMA
4.9 PSSS-TMA
1.5 PSSS-TBA
2.4 PSSS-TBA
2.6 PSSS-TBA
4.0 PSSS-TBA
4.6 PSSS-TBA
3.8 XLPSSS-THA
4.1 XLPSSS-THA
4.4 XLPSSS-THA
1.3 XLPSSS-Na
1.7 XLPSSS-Na
2.4 XLPSSS-Na
3.4 XLPSSS-Na
4.8 XLPSSS-Na
4.5 XLPSSS-Na
1.1
1.2
e
0.6
1.1
1.2
2.4
e
4.5
4.9
5.4
1
1.3
1.8
2.2
e
91
90
96
99
95
93
99
96
e
e
5.5
4.3
3.8
5
4.8
4.5
e
Bold face: Samples where the product was only water-soluble polymer, i.e., no particles.
a
mol% M-SS was based on the total monomer feed (styrene, DVB, and M-SS).
The product IEC was calculated by IEC ¼ x$IEC(soluble) þ (1 ꢁ x)$IEC (nanoparticles), where x is the mass fraction of the water-soluble polymer. The IEC of water-soluble
b
polymer and nanoparticles were measured by elemental analysis.
c
VA-044 was used as initiator. A mixture of surfactants was used, Brij 30 and Brij 99 with mass proportions of 1:1.4. The product was ∼70 wt% stable emulsion and ∼30%
coagulated polymer. The product IEC was calculated assuming that the concentration of M-SS in the product was the same as in the feed.
d
VA-044 was used as initiator. Brij 99 was used as a surfactant. The product was 84 wt% stable emulsion and 16 wt% coagulated polymer. The product IEC was calculated
assuming that the concentration of M-SS in the product was the same as in the feed.
the splitting of the two asymmetric stretching bands, which
constitute a doublet centered around 1200 cmꢁ1, also increases as
was expected for sulfonated polystyrenes [34]. The in plane skel-
eton vibration of M-SS at ∼1140 cmꢁ1 also followed a similar trend
as the symmetric stretching. The symmetric stretching frequency
increased, but the splitting of the asymmetric stretching doublet
decreased as the cation was changed from Naþ to Liþ. The increase
in frequency is expected, because for alkali metal cations the
Coulombic force decreases with increasing size of the cation [34].
However, the splitting of the asymmetric stretching doublet is also
expected to increase. The deviation in that behavior observed here
can probably be attributed to difficulty in removing water from the
Li-SS. The stronger Coulombic field strength allows Liþ to hold onto
water more strongly, and the hydrogen bonding of water with Liþ
weakens the interaction between Liþ and sulfonate anion (SO3ꢁ)
[34]. For the TMA-SS, TBA-SS, THA-SS, and TOA-SS monomers, the
symmetric stretching frequency decreased and the splitting of the
asymmetric stretch doublet also decreased, see Table 3, which is
due to the weaker Coulombic force of the ion-pair. The splittings of
the asymmetric stretching doublet of the anion for THA-SS and
TOA-SS were slightly higher than for TMA-SS and TBA-SS, which
may be due to residual hydrogen-bonded water in the latter two
monomers, which are more hydrophilic than the former.
The solubility of M-SS in water and styrene were determined by
adding excess M-SS to the solvent, extracting a known volume of
sample from the clear phase and measuring the monomer
concentration gravimetrically. The solubility behavior is described
in Table 4. The TMA-SS, TBA-SS and Li-SS monomers formed clear,
viscous solutions in water with relatively high concentration
(>37 g/100 g water), which may have been a consequence of the
formation of micelles. Similar to the metal salts of the SS monomer,
TMA-SS was insoluble in styrene; however, TBA-SS was slightly
soluble in styrene. As the alkyl chain length of the QAA-SS
increased, the monomer became less soluble in water and more
soluble in the styrene phase. It was thought that this would be
beneficial with regard to incorporation of the SS monomer into the
emulsified styrene phase and, consequently into the nanoparticles
during the emulsion polymerization. Neat TOA-SS and THA-SS were
viscous oils that were insoluble in water. Thus, they can be used
Fig. 2. FTIR absorption spectra of M-SS monomers. The dashed lines represent anti-
symmetric, symmetric SO3ꢁ stretching, and the bending of SOꢁ
.

E. Pitia et al. / Polymer 52 (2011) 297e306
301
Table 3
IR assignments for M-SS monomers.
Group
Wavenumber (cmꢁ1
)
Li-SS
Na-SS
TMA-SS
TBA-SS
THA-SS
TOA-SS
SOꢁ bending
905
1051
1130
1232
1188
44
905
1046
1130
1235
1182
53
e
e
904
904
SOꢁ3 symmetric stretching
1030
1120
1206
1191
15
1032
1120
1206
1193
13
1032
1120
1217
1198
19
1032
1120
1216
1199
17
In-plane skeleton vibration of substituted styrene
Asymmetric stretching doublet of sulfonate
Splitting of doublet
Methylene/methyl CeH sym./asym. bending
e
e
1465e1510
1430e1510
1410e1510
1410e1510
directly as the oil phase in an emulsion polymerization without
including styrene.
emulsion polymerizations as a function of the sulfonate monomer
concentration in the feed. The feed compositions were corrected for
loss of styrene due to absorption by the rubber septa. The solubility
of styrene in the rubber septa was determined separately by
exposing the septa to styrene vapor. The water-soluble fraction of
the PSSS-TBA samples included some water-insoluble nano-
particles due to insufficient separation. As a result, the IEC of the
water-soluble fractions were slightly lower compared to the other
cations for the feed composition range of ∼1.5e2.5 meq/g; see
Fig. 6.
Fig. 4 shows that as the sulfonate monomer concentration in the
feed increased, the fraction of nanoparticles produced decreased
and the amount of water-soluble polymer increased. Above a feed
composition of about 50 mol% M-SS, essentially only water-soluble
polymer was produced. The relative amounts of water-soluble
polymer and nanoparticles produced by the emulsion polymeri-
zation appeared to be insensitive to the sulfonate salt used and to
the use of divinyl benzene as a crosslinking agent. However, the
incorporation of divinyl benzene significantly lowered the relative
amount of water-soluble polymer produced, due to stabilization of
the high IEC nanoparticles by crosslinking.
3.2. Polymerization of PSSS-M nanoparticles
The emulsion polymerization produced two different products:
1) dispersed nanoparticles and 2) water-soluble polymer. The IECs
of the total PSSS-M products from the emulsion polymerizations
are summarized in Table 1 and plotted as a function of the feed
composition in Fig. 3. The IEC calculated from the elemental sulfur
analysis (see Eq. (1)) was based on the free acid derivative. The solid
line in Fig. 3 represents stoichiometric conversion of M-SS in the
product. Data from Refs. [31] and [27] are included in Fig. 3. Only
the experimental data from the present study for the polymers that
were dialyzed, see Table 1, were used in this graph. The polymers
from Ref. [31] were also dialyzed, but the products from Ref. [27]
were not.
In general, the polymerization reaction produced a near stoi-
chiometric incorporation of the sulfonated monomer into the
product, regardless of what monomer was used. Where the data
deviated significantly from stoichiometry (data above the line in
Fig. 3), a higher concentration of the sulfonated monomer was
incorporated into the product as a result of a loss of styrene
monomer during the reaction, primarily because of absorption by
the rubber septa. That problem was resolved by using glass stop-
pers. Nonetheless, some styrene was still carried away by nitrogen
gas stream. The results from Weiss et al. [27] were generally lower
than the stoichiometric line, which may be due to the presence of
buffer, reducing agent and/or chain transfer agent residues, as
a consequence of not dialyzing the product.
As might be expected, the water-soluble polymer was rich in the
sulfonated monomer. The IECs for the water-soluble fraction
ranged between 3.0 and 5.4 meq/g. The water-insoluble nano-
particles had much lower IEC than the water-soluble polymer, see
Fig. 5. For most of the M-SS monomers used, the nanoparticles had
an IEC ∼ 1 meq/g. The one notable exception was a single experi-
ment with TBA-SS, where an IEC >2 meq/g was achieved. Also
noteworthy is that the IECs for the nanoparticles from the QAA-SS
The water-soluble polymer was separated from the nano-
particles by first forming a film from the entire polymer product on
a glass substrate, drying it and then washing the film with 60 ꢀC
water several times to remove the water-soluble fraction. The
water-soluble polymer was isolated by filtering the wash solutions
with a cotton-filled syringe that served to remove any water-
insoluble material transferred from the glass surface during the
washing steps. Fig. 5 plots the mass fraction of the water-soluble
polymer and water-insoluble nanoparticles produced by the
Table 4
Solubility of M-SS in water and styrene at room temperature.
Monomer
Li-SS
Water
(g/100 g)
Styrene
(g/100 g)
Clear, viscous solution (gelled at
0
concentration >68% > 44)
Na-SS
TMA-SS
18
0
0
Clear, viscous solution (gelled at
Fig. 3. IEC (meq/g) of emulsion products (based on the acid form of the ionomer)
versus the IEC of the M-SS in the feed. The IEC of the product was calculated by
elemental analysis, except for the data from Ref. [8], where the IEC was calculated by
titration. The solid line represents stoichiometric conversion of the M-SS in the
concentration >68%)
Clear, viscous solution (gelled at
concentration >37%)
0
0
TBA-SS
4.0
THA-SS
TOA-SS
Miscible (viscous solution)
Miscible (viscous solution)
q
product. The products are: (Δ) PSSS-Li, ( ) PSSS-Na, (,) PSSS-TMA, (B) PSSS-TBA,
(
) XLPSSS-Na from Ref. [31], and (C) PSSS-Na from Ref. [27].
;

302
E. Pitia et al. / Polymer 52 (2011) 297e306
Fig. 4. Mass fraction of water-soluble polymer and water-insoluble nanoparticles in
the emulsion polymerization product versus concentration of M-SS in the feed. The
filled symbols represent the water-soluble fraction and the open symbols are the
q
nanoparticle fraction: (Δ,:) Li,-SS ( ,+) Na-SS, (,,-) TMA-SS, (B,C) TBA-SS, and
(>,A) XLPSSS-Na.
Fig. 6. FTIR absorption spectra of PSSS-M nanoparticles. The dashed lines the anti-
symmetric stretching doublet of the SOꢁ3 , the symmetric stretching of the SOꢁ3 and SeO
polymerizations had a mean value of 1.3 ꢂ 0.2 meq/g, while for the
metal salts, the mean value was 0.54 ꢂ 0.03 meq/g. The values after
the ꢂ sign are the 95% confidence interval from the mean. These
two observations are consistent with the earlier hypothesis that the
QAA-SS monomer is more soluble in the oil-phase micelles and
thus should be more efficient at incorporation into the nano-
particles than the more water-soluble monomers, see Table 4.
Brijmohan et al. [31] reported the synthesis of an XLPSSS-Na
emulsion where the isolated polymer had an IEC ∼ 2.2 meq/g for
16mol%NaSSinthefeed. Inthatwork, however, noattemptwasmade
to isolate a water-soluble fraction, so the IEC of the nanoparticles per
se was not known. Fig. 5 shows that by crosslinking the nanoparticles
as they are produced, higher IEC nanoparticles were stabilized, since
the particles were no longer water soluble. Crosslinked nanoparticles
had IEC ∼ 1e2 meq/g, which was similar to that obtained by using
TBA-SS as the sulfonate monomer, though a higher concentration of
nanoparticles was obtained using the crosslinker.
bendingꢁ
.
require using the crosslinker in combination with a completely oil-
soluble QAA-SS monomer to maximize the concentration of
sulfonated monomer in the oil domains. THA-SS was chosen to test
that hypothesis, because it was observed to be soluble in styrene
and it produced a lower-viscosity mixture with styrene than did
the higher molecular weight TOA-SS. A higher viscosity within
the emulsified nanoparticle domains presented two problems:
1) poorer dispersion of the two monomers (styrene and the QAA-
SS) and 2) increased difficulty of exchanging the QAA cation with an
acid once the polymer was produced.
With a combination of the THA-SS monomer and the addition of
divinyl benzene, very high IEC nanoparticles, ranging from 4.5 to
5.4 meq/g, were obtained, see Fig. 5. The data deviated from stoi-
chiometry (i.e., the point is above the line in Fig. 5), which suggests
some monomer was lost either due to evaporation into the nitrogen
purge stream or by less than complete conversion. The amount of
monomer (DVB) lost by evaporation was estimated to be >50 wt%
of its initial amount, assuming the amount lost is the only cause for
IEC to deviate from stoichiometry. Note that the crosslink density of
the crosslinked nanoparticles particles produced is not known. This
is not a trivial characterization for nanoparticles and was consid-
ered beyond the scope of this project.
Figs. 4 and 5 show, however, that crosslinking alone was not
sufficient for producing nanoparticles with IEC >2 meq/g. That may
In the synthesis of XLPSSS-THA, a non-ionic initiator (VA-044)
was used, because of concern that the THA-SS would be converted
to M-SS and become water soluble in the presence of ionic initiator
or surfactants containing metal cations. Attempts to produce stable
nanoparticles were unsuccessful, except when the non-ionic
surfactant, Brij 99, was used. Nanoparticles are stabilized electro-
statically when similar charges are located at the surface of adja-
cent particles, i.e., in the electrical double layer. This electrostatic
repulsion is greatly reduced when a non-ionic surfactant and the
bulky tetraalkylammonium cation are present, as in the case of the
XLPSSS-THA nanoparticles. The QAA may actually behave as an
organic electrolyte, which is known as a latex coagulant [35,36].
The strong attraction between water molecules, due to hydrogen
bonding, forces the QAA out of the aqueous phase, resulting in
neutral particles which can coagulate the nanoparticles because of
their weak repulsive interactions.
Fig. 5. IEC of the water-soluble and nanoparticle fractions of the emulsion polymeri-
zation product versus the concentration of M-SS in the feed. The filled symbols are for
the water-soluble polymer and the open symbols are for the insoluble nanoparticles:
In sterically stabilized emulsions, a non-ionic surfactant is used
to increase the separation of adjacent particles, which prevents
q
(Δ,:) Li,-SS ( ,+) Na-SS, (,,-) TMA-SS, (B,C) TBA-SS, (ꢃ) from Ref. [25], (>,A)
XLPSSS-Na, and (4) XLPSSS-THA.

E. Pitia et al. / Polymer 52 (2011) 297e306
303
coagulation. Sterically stabilized emulsions depend on many
factors. For example, the molecule must adsorb onto the particle
surface, it must be large enough to separate adjacent particles and it
should be soluble in the dispersion medium. Although Brij 99
successfully stabilized the XLPSSS-THA emulsion, THA-dodecyl
sulfate and Brij 30 were unable to stabilize the nanoparticles. Brij 99
has a higher molecular weight and is more soluble in the water
phase. The difference between Brij 30 and Brij 99 is primarily that
the latter has 20 oxyethylene groups, while Brij 30 has only 4. In
addition to its better solubility in water, the larger size of the Brij 99
molecule probably improved its ability to separate adjacent nano-
particles. The poor efficacy of the THA-dodecyl sulfate is probably
due to poor solubility in water.
Even with the use of Brij 99, a slight amount of coagulated
polymer formed, which was probably due to the high shear rate
used during the polymerization and may be insufficient surfactant
[24]. A mixture of Brij 99 and Brij 30 also produced a relatively
stable emulsion, but the dilution of the Brij 99 increased the
amount of coagulated polymer. The mixture of Brij 30 and Brij 99,
however, produced smaller particles, ∼80 nm, than Brij 99 alone.
The formation of smaller particles is due to the lower critical
micelle concentration of Brij 30 (0.004 mM versus 0.265 mM for
Brij 99), which allows the Brij 30 to produce more micelles at the
feed concentrations used in these experiments.
Fig. 7. Normalized intensity of the IR sulfonate symmetric stretching vibration as
a function of IEC for the nanoparticles. The dashed curve is a least-squares fit of
a quadratic equation to the data and the dotted curves represent the 95% confidence
interval.
relationship. However, the quadratic fit was good and provides
a reasonable estimate of the sulfonation level in these copolymers.
3.3. FTIR analysis of the copolymers
3.4. Nanoparticle size distribution
Incorporation of M-SS into the copolymer was confirmed by the
presence of the IR bands associated with the sulfonate groups; see
Fig. 6. The frequencies and assignments for the sulfonate vibrations
are listed in Table 5. The IR bands associated with SOꢁ3 appeared at
lower frequency for the polymers than in the M-SS monomer,
which was due to the greater difficulty in removing water from the
polymer. That was confirmed by the presence of the OeH stretch-
ing vibration of water between 3000 and 3700 cmꢁ1 in the IR
spectra of the polymers. The alkali metal cations had the most
prominent OeH stretching vibration in the IR spectrum, which is
expected, since they were the more hygroscopic copolymers.
The absorbance intensity of the symmetric stretching vibration
of SOꢁ3 at ∼1040 cmꢁ1 for PSSS-M nanoparticles normalized by the
absorption for CeC in-plane stretching of benzene ring at
1600 cmꢁ1 is plotted against the IEC measured by elemental anal-
ysis in Fig. 7. The dashed line in Fig. 7 is a quadratic (y ¼ ax þ bx2),
least-squares fit of the data and the dotted lines represent the 95%
confidence band of the fit. The deviation of the confidence band at
low IEC is a consequence of the quadratic fit, which is unavoidable
since we had no measure of the pure error for a single condition.
Although one expects a linear fit from Beer’s law, errors in the
elemental analyses and/or in the measurement of the symmetric
stretching peak intensity due to broadening of the peak (see Fig. 6)
may be responsible for the non-linearity of the IR-concentration
Representative micrographs of the PSSS-M and XLPSSS-M
nanoparticles produced by the emulsion copolymerizations are
shown in Fig. 8. The nanoparticles were spherical, and the average
volume-equivalent particle diameter, D, was calculated from Eq.
(2),
0
1
Nbins
P
1=3
nid3
B
B
B
B
@
C
C
C
C
A
i
i ¼ 1
D ¼
(2)
Nbins
P
ni
i ¼ 1
where ni is the number of particles in the bin with diameters
di ꢂ Δd/2 and Δd ¼ di ꢁ diꢁ1. The values of D are summarized in
Table 6. The distortion seen in some of the micrographs is probably
due to the drying and handling. The slightly dark background may
be due to a film of the water-soluble polymer that was rich in the
M-SS.
In a classical emulsion polymerization of styrene, the primary
site for particle formation is the monomer-swollen micelles
[37e40]. Styrene is initiated in the aqueous phase, forming oligo-
meric radicals that propagate until captured by the micelles. As
the reaction progresses, micelles are consumed by growth and
formation of particles. When the micelles are depleted, secondary
Table 5
IR assignments for PSSS-M and XLPSSS-M copolymers.
Group
Wave number (cmꢁ1
)
1.0PSSS-Li
0.8PSSS-Na
1.1PSSS-TMA
1.1PSSS-TBA
4.4 XLPSSS-THA
1.2PSSS-TOA
SOꢁ bending
905
905
905
1033
1120
1213
1184
29.0
1430e1500
1600
905
1033
1120
1216
1196
20.0
1430e1500
1600
e
e
SOꢁ3 symmetric stretching
1046
1130
1232
1179
53.0
e
1040
1127
1221
1177
44.0
e
1033
1120
1214
1193
21
1033
1120
1217
1199
18
In-plane skeleton vibration of substituted styrene
Asymmetric stretching doublet of sulfonate
Splitting of doublet
Methylene/methyl CeH sym/asym. bending
CeC in-plane stretching of styrene ring
1430e1500
1600
1430e1500
1600
1600
1600

304
E. Pitia et al. / Polymer 52 (2011) 297e306
Fig. 8. TEM images of xyPSSS-M nanoparticles.

E. Pitia et al. / Polymer 52 (2011) 297e306
305
Table 6
Table 7
Average nanoparticle size for PSSS-M and XLPSSS-M copolymers.
Kink function parameters.
Samples
Number of
particles
counted
Diameter
(std dev)
(nm)
a (nm)
q/a
Parametera
Value
95% confidence interval
p valuesb
(nmꢁ1
)
b
d
x0
y0
s
1.3
ꢂ1.2
ꢂ1.4
ꢂ0.4
ꢂ0.3
e
0.017
0.028
<0.001
<0.001
e
ꢁ1.3
1.5 PSSS-Na
3.5 PSSS-Na
5.2 PSSS-Na
1.4 PSSS-Li
104
67
None
58
45
53
None
70
87
91
74
42 (17)
34 (7)
e
0.097 [41]
0.097
0.097
0.068 [42]
0.068
0.347 [42,43]
0.347
0.494 [44]
0.494
0.560 [45]
0.560
10.3
10.3
10.3
14.7
14.7
2.88
2.88
2.02
2.02
1.78
1.78
1.0
2.2
0.1 (fixed)
73 (19)
18 (3)
42 (4)
e
a
See Eq. (3) for definition of parameter symbols
3.1 PSSS-Li
b
Probability of Type I error by rejecting zero value hypothesis using one-tail t
test; see text for explanation.
1.5 PSSS-TMA
3.5 PSSS-TMA
1.1 PSSS-TBA
3.7 PSSS-TBA
3.8 XLPSSS-THA
4.4 XLPSSS-THA
19 (3)
61 (17)
83 (22)
156 (27)
ꢀ
ꢁ
ꢂꢃ
x ꢁ x0
y ꢁ y0 ¼ bðx ꢁ x0Þ þ sðd ꢁ bÞln 1 þ exp
(3)
s
where b and d are the slopes of the straight lines, x0 and y0 are the
intercepts and s is known as the “sharpness” parameter. The statis-
tics obtained from fitting the datawith the kink function are listed in
Table 7. We hypothesize the kink in the curve is due to distinct
differences in the mechanism of particle formation and stabilization
of QAA and metal neutralized particles. The positive and negative
slope regions of the curve illustrate the behavior of QAA and metal
neutralized particles, respectively. For QAA neutralized particles, the
particle size increased with increasing cq/a, or since q/a was rela-
tively constant for those data, with increasing sulfonate ion
concentration, c. On the other hand, for the metal neutralized
particles, the particle size decreased with increasing cq/a.
The difference in the number of particles formed during the
reaction may be responsible for these two opposite effects of cq/a on
particle size, at low sulfonate concentration. Na-SS is reported to
aggregate in aqueous solution at concentrations >0.1 M, due to is
amphiphilic nature, i.e., a hydrophobic phenyl ring and a hydrophilic
sulfonate group [32]. The lower hydrophilic-lipophilic balance (HLB)
of the QAA-SS salt is expected to lower the critical micelle concen-
tration (CMC) of M-SS and also increase the number of micelles
formed in the order of THAþ ꢄ TBAþ ꢄ TMAþ ꢄ Naþ ꢄ Liþ. Higher
micelle concentration produces more primary particles. More
primary particles reduces the average particle size. At high
concentration of sulfonated monomer, the effect of the cation on the
particle size reverses as a consequence due to poor electrostatic
stabilization of the emulsion by the QAA cations, reducing the effect
of the electrical double layer.
particles form by homogenous nucleation. Nonetheless, the particle
size distribution usually has a relatively low polydispersity.
For the copolymerization of styrene and Na-SS, monomer-
swollen micelles were not the only source of particle formation.
Particles also form simultaneously by homogenous nucleation in
the aqueous phase [32]. Water-soluble Na-SS oligomeric radicals
grow to a critical length by capturing styrene prior to precipitating
and forming aggregates that absorb styrene and behave as styrene-
swollen micelles. Homogenous nucleation of particles occurs
throughout the reaction; and, as a result, particles grow at different
rates which leads to a broader particle size distribution compared
to polymerization of styrene, see Table 6.
The diameter data are plotted against the quantity cq/a in Fig. 9
as a logelog plot, where c is the sulfonate ion concentration of
the emulsion product, a is the ionic radius of the cation and q is the
charge of the cation. Values for a are listed in Table 6. The
Coulombic energy of the ion-pair is Ec ¼ kqcqa/a, where qc and qa are
the charges on the cation and anion, respectively, and k is
Coulomb’s constant (8.99 ꢃ 109 N m2 Cꢁ2). For a fixed anion, in this
case the sulfonate anion, qa is a constant and the Coulombic energy
is proportional to q/a, where q ¼ qc. For ionomers, many properties
scale with the product of the Coulombic energy and the ion
concentration (cq/a) [47e51]. The data in Fig. 9 were fitted using
a continuous kink function [46]. The kink function consists of two
straight lines connected with a small curve of certain width. The
equation used for the continuous kink function is:
4. Conclusions
Water-insoluble sulfonated polystyrene nanoparticles with IEC
up to ∼5.4 meq/g, can be synthesized by an emulsion copolymeri-
zation of styrene, divinyl benzene and a salt of p-styrene sulfonic
acid (M-SS). The choice of the sulfonate cation affects the poly-
merization reaction; the most significant effect occurs for bulky
quaternary alkyl ammonium (QAA) salts, such as tetrahexyl
ammonium. Changing the cation from an alkali metal to a quater-
nary alkyl ammonium makes the M-SS monomer less water soluble
and more soluble in the oil phase of the emulsion. The IEC of the
nanoparticles increased with increasing solubility of the M-SS
monomer in the oil phase, but increasing the hydrophobic nature of
the sulfonated polymer is not by itself sufficient to achieve nano-
particles with IEC >∼2.4 meq/g.
The addition of crosslinking agent, such as divinyl benzene,
stabilizes the nanoparticles to higher IEC by preventing their
dissolution into the water phase. But, if the sulfonate monomer is
water soluble, adding a crosslinker has limited effectiveness for
achieving nanoparticles with IEC >2.4 meq/g. An emulsion poly-
merization using a combination of a water-insoluble sulfonated
styrene monomer (e.g., using the tetrahexyl ammonium salt) and
Fig. 9. Nanoparticles size versus the product of Coulombic energy and sulfonate
concentration for PSSS-M and XLPSSS-M; a is the ionic radius of the cation, q is the
charge, and c is the concentration of sulfonate ion of the emulsion product expressed
as ion-exchange capacity: (,) M ¼ Li; (B) Na; (Δ) TMA; (>) TBA; (4) THA. Filled
symbols are polymers with IEC between 1.1 and 1.5 meq/g and open symbols represent
polymers with IEC between 3.1 and 5.4 meq/g. The curve is a kink function [46] with
s ∼ 0.1, Eq. (3).

306
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Acknowledgment
This work was supported by the Civil, Mechanical and Manufacturing
Innovation Program (Directorate of Engineering) of the National
Science Foundation, Grant CMMI 0727545.
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