Synthesis and Characterization of Poly
(styrene-co-vinyl phosphonate) Ionomers
QI WU, R.A. WEISS
Department of Chemical Engineering and Polymer Program, University of Connecticut, Storrs, Connecticut 06269-3136
Received 30 December 2003; revised 8 March 2004; accepted 10 March 2004
DOI: 10.1002/polb.20208
Published online in Wiley InterScience (www.interscience.wiley.com).
Poly(styrene-co-diethyl vinylphosphonate) copolymers were synthesized by
free radical copolymerization. The ester groups of the copolymers were hydrolyzed to
phosphonic acid groups, and the sodium and zinc salts ionomers were obtained by
neutralization. The structure and the thermal and viscoelastic properties of the copolymers and ionomers were characterized by nuclear magnetic resonance, Fourier transform infrared spectroscopy, differential scanning calorimetry, dynamic mechanical
analysis, and small-angle X-ray scattering. The phosphonate ester lowered the glass
transition temperature (Tg) of polystyrene. The free acid derivatives and metal phosphonates increased Tg and produced a rubbery plateau region in the viscoelastic
properties due to the formation of a physical network. The acid and salt ionomers
exhibited microphase-separated morphologies and were thermorheologically complex.
The phosphonic acid derivatives absorbed relatively little water, even for materials
with ion-exchange capacities greater than 1.0 mEq/g, and were not conductive, which
made them unsuitable for application as proton exchange membranes. © 2004 Wiley
ABSTRACT:
Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 3628 –3641, 2004
Keywords: synthesis; poly(styrene-co-vinyl phosphonate) ionomers; free radical
copolymerization
INTRODUCTION
The introduction of a small amount of ionic groups
into a hydrocarbon polymer can significantly modify
the physical and rheological properties of the resultant polymers, which are called ionomers.1,2 Although the exact details of the microstructure of
these materials have eluded researchers for over
three decades, it is generally agreed that strong
ion– dipole interactions between the ionic groups
produce a microphase-separated morphology of ionrich domains dispersed in a hydrocarbon-rich continuous phase.3–5 A range of factors affect the for-
Correspondence to: R.A. Weiss (E-mail: rweiss@mail.
ims.uconn.edu)
Journal of Polymer Science: Part B: Polymer Physics, Vol. 42, 3628 –3641 (2004)
© 2004 Wiley Periodicals, Inc.
3628
mation of the ionic aggregates and the properties of
the ionomers, including the dielectric constant of
the polymer backbone, the ion concentration, the
nature of the bound ion (e.g., carboxylate, sulfonate
or phosphonate), the choice of the counterion, the
location of the bound ion (e.g., attached to the backbone chain or to a side chain), the addition of low
molecular weight solvent or plasticizers, as well as
the solvent, thermal, and mechanical histories.
Ionomers based on an atactic polystyrene have
been popular as model systems in that the copolymerization of styrene with vinyl carboxylic acid
monomers and the sulfonation of preformed polystyrene are relatively easy reactions to perform
and control. The resulting polymers are amorphous, so that the effects of interactions of the
ionic species on the ionomer structure and properties are not complicated by the presence of crys-
SYNTHESIS OF POLY(STYRENE-CO-VINYL PHOSPHONATE) IONOMERS
talline phase. Lundberg and Makowski6 showed
that for a fixed level of functionality, the melt
viscosities of the sulfonated ionomers were two to
three orders of magnitude higher than those of
the carboxylate analogs. Other investigations7–9
also observed similar differences in the properties
of carboxylate and sulfonate ionomers. The differences are due to the fact that sulfonic acid (pKa
⬃1) is a much stronger acid than carboxylic acid
(pKa ⬃4 –5), which results in stronger electrostatic attractions between the sulfonate ion-pairs
than the carboxylate ion-pairs.
In comparison with the studies of styrenebased ionomers containing carboxylic or sulfonic
acids, relatively little work has considered phosphonic acid derivatives. Organophosphonic acids
(RPO3H2) possess two ionizable acid groups (pKa1
⬃2–3; pKa2 ⬃7). OneOPOOH has an ionization
potential intermediate between sulfonic and carboxylic acids. One might expect that with the
intermediate acid strength of the first acid group
and the higher ion exchange capacity of the phosphonic acid compared with sulfonic and carboxylic acids at the same acid concentration, phosphonate ionomers may have properties intermediate
between sulfonate and carboxylate ionomers or,
perhaps, even comparable to sulfonate ionomers.
Zoghbi10 prepared polystyrene ionomers by
postpolymerization reactions that attached either
phosphonate groups to the polystyrene backbone
or phosphinate groups to the styrl ring. The ionic
interactions in those ionomers were weaker than
comparable carboxylated and sulfonated polystyrene (SPS) ionomers, as judged by the rate of
increase of the glass transition temperature with
ionic group concentration, dTg/dcionic. An alternative way for preparing phosphonate–polystyrene
ionomers is to copolymerize styrene with a phosphonic acid or phosphonate-containing comonomer. Poly(styrene-co-diethyl vinylphosphonate)
copolymers, SDEVP, and poly(styrene-co-dimethyl vinylphosphonate) copolymers, SDMVP,
have been synthesized using free-radical chemistry by several research groups,11–20 and in some
cases the phosphonate ester was hydrolyzed to
produce a poly(styrene-co-vinyl phosphonic acid)
copolymer, but in none of the previous work were
the materials evaluated as ionomers.
The direct copolymerization of styrene and vinylphosphonic acid is difficult, because of the
large difference in the polarity of the two monomers. The monomer reactivity ratios for the free
radical copolymerization of styrene (M1) and diethyl vinylphosphonate (M2) are r1 ⫽ 3.25 and r2
3629
⫽ 0,11 which indicates that the copolymerization
should produce long blocks of styrene and randomly distributed short blocks of DEVP.13 For
relatively low DEVP mole fractions, a random
copolymer is approximated.
The subject of the present article is the synthesis of polystyrene-based phosphonate ionomers by
free radical copolymerization of styrene and
DEVP. Ionomers containing acid and metal–salt
groups were obtained by hydrolysis of the ester
group and subsequent neutralization. The structure and properties of the phosphonate–polystyrene ionomers are reported and compared with
those of sulfonated and carboxylated polystyrenebased ionomers.
EXPERIMENTAL
Synthesis
Styrene (Acros Organics, 99%) was passed
through a short column packed with Al2O3 to
remove the 4-tert-butylcatechol inhibitor. DEVP
(Acros Organics, 97%) and tetrabutyl hydroperoxide (TBHP) (Acros Organics, 5– 6-M solution in
decane) were used as received. Copolymers of styrene and DEVP were synthesized by free radial
copolymerization in toluene (or in bulk for the
higher DEVP feed concentrations) under argon
protection at 100 °C for 24 h with TBHP as the
initiator. The copolymers were precipitated in
methanol, washed with refluxing methanol for
48 h in a Soxhlet extractor, and dried to constant
mass in a vacuum oven at 70 °C. The reactions
and poly(styrene-co-DEVP) products are summarized in Table 1.
The phosphonate ester groups were hydrolyzed to phosphonic acid groups by dissolving
the copolymer in a 1 : 5 mixture of concentrated
hydrochloric acid (36.5 to 38%) and dioxane that
was refluxed at ca. 110 °C for 6 –12 days. The
hydrolysis reaction was followed by monitoring
the disappearance of the ester protons with
1
H NMR spectroscopy. The styrene–vinyl phosphonic acid copolymer was precipitated in
deionized water, filtered, washed several times
with deionized water, and dried in a vacuum
oven at 70 °C. The sodium phosphonate ionomers were obtained by adding a 100% excess of
a 2-M methanolic sodium hydroxide (NaOH) solution, based on neutralization of both acid
groups of vinylphosphonic acid, to an agitated
solution of the styrene–vinyl phosphonic acid
3630
WU AND WEISS
Table 1. Properties of Styrene-DEVP Copolymers
2.4E-SVP
4.3E-SVP
6.4E-SVP
12.8E-SVP
15.4E-SVP
a
b
DEVP mol% in
Copolymer
S: DEVP
in Feed
Reaction
Conversion
(%)
Theoreticala
9:1
5:1
3:1
3:2
1:1
60
44
33
17
19
3.2
5.5
8.5
14.5
19
H NMR
Mw (kDa)
from GPC
Mw/Mn
Tg (°C)
from DSC
IECb
(mEq/g)
2.4
4.3
6.4
12.8
15.4
70
70
82
61
57
1.4
1.5
1.3
1.5
1.5
104.5
103.3
99.6
88.0
86
0.23
0.41
0.64
1.22
1.47
1
Calculated based on reactivity ratio: r1 (styrene) ⫽ 3.25, r2 (DEVP) ⫽ 0 (ref. 11).
IEC ⫽ ion exchange capacity.
copolymer in tetrahydrofuran (THF) or 1methyl-2-pyrrolidinone (NMP). The solution
was stirred for 6 h, and the neutralized polymer
was then recovered by precipitation in deionized water, washed with deionized water a few
times to remove the excess NaOH, and dried
under vacuum at 70 °C. The zinc salt was prepared in a similar manner, except that 1 M
methanolic zinc acetate dihydrate (ZnAc) was
used as the neutralizing agent.
The sample notation used for the ionomers in
this article is x.yM-SVP, where x.y is the degree
of phosphonation expressed as mol % of DEVP
in the copolymer and M denotes the cation (i.e.,
M ⫽ H⫹, Na⫹, or Zn2⫹ for the free acid, sodium,
and zinc salts, respectively). The styrene–
DEVP copolymers are denoted as x.yE-SVP. It
was assumed that the composition and backbone molecular weight of the E-SVP copolymer
did not change during the hydrolysis and neutralization reactions.
Sulfonated polystyrene ionomer was prepared according to the procedure of Makowski
et al.21 A commercial polystyrene, Styron™ 666
(Dow Chemical Company, Mn ⫽ 103 kDa, Mw
⫽ 288 kDa) was sulfonated with acetyl sulfate
in 1,2-dichloroethane at 50 °C. The sulfonation
level was determined by titration of the acid
derivative with methanolic NaOH to a phenolphthalein end point. Na- and Zn-SPS were prepared by neutralizing the H-SPS with ca. 20%
excess of NaOH and ZnAc, respectively. The
sample notation used for the sulfonate ionomers
is x.yM-SPS ionomers, where x.y represents the
sulfonation level (mol %) of the ionomer.
Polymer Characterization
Copolymer and ionomer films were compression
molded in a vacuum oven using two 10 ⫻ 10-cm
spring loaded steel plates. A vacuum oven was
used to heat the sample and to degas the material
during the molding process to produce bubble-free
films. The mold had steel springs at each corner
that provided a compressive stress that pushed
the plates together as the molten sample flowed
into a film.
Polymer molecular weight averages were determined by gel permeation chromatography
(GPC) of E-SVP copolymer solutions in THF using
a Waters 150-C ALC/GPC equipped with an evaporative light scattering detector. Polystyrene
standards with narrow molecular distribution
were used for calibration. Fourier transform infrared (FTIR) spectra were obtained with a Nicolet Magna-IR 560 FTIR spectrometer by signal
averaging a total of 32 scans. Thin films were
prepared by casting polymer/THF solutions onto
KBr discs. 1H NMR spectra were collected for
E-SVP/CDCl3 and H-SVP/CDCl3 (or DMSO-d6)
solutions using a Bruker 500 MHz NMR. The
compositions of the copolymers were determined
by calculating the relative areas of the styrene
phenyl peaks at 6.5–7.3 ppm and the DEVP ester
peaks at 3.2–3.9 ppm.
Solutions of E-SVP and Na-SVP polymers in
toluene/methanol (95/5, v/v) and in NMP were
prepared in volumetric flasks. The mixtures were
sonicated to facilitate dissolution of the polymers.
The E-SVP copolymers were readily soluble,
while some ionomers required 1 or 2 days to dissolve. Dilute solution viscosity was measured
with an Ubbelohde capillary viscometer in a temperature-controlled bath at 25 °C. The polymer
solution was filtered through 0.45 or 1-␮m microfilter before introduction to the viscometer. Before
measurement, the temperature of the solution
was allowed to equilibrate for 20 min. Measurements at each concentration were repeated until
SYNTHESIS OF POLY(STYRENE-CO-VINYL PHOSPHONATE) IONOMERS
the relative error of three consecutive measurements was less than 0.5%.
Differential scanning calorimetry (DSC) of the
polymers was performed with a TA Instruments
DSC 2929 using a nitrogen atmosphere and a
heating rate of 20 °C/min. Dynamic mechanical
analysis (DMA) was carried out with a TA instrument DMA 2980 using a tensile fixture, a dry
nitrogen atmosphere, a heating rate of 2 °C/min,
and a fixed frequency of 1 Hz. Isothermal frequency sweeps from 0.04 to 30 Hz were also obtained at temperatures between 135 and 295 °C
using a shear sandwich fixture, which is particularly suited for measurements on extremely viscous melts such as ionomers.22 All measurements
were made within the linear viscoelastic region,
which was determined from strain sweeps. Time–
temperature superposition was attempted by
manually shifting the isothermal modulus–frequency curves horizontally with Tref ⫽ Tg
⫹ 50 °C. Vertical corrections of Tref/T were made
to compensate for the temperature dependence of
the modulus.
Small-angle X-ray scattering (SAXS) was measured with a Bruker Anton-Paar small-angle
scattering instrument using a Rigaku RU-300 rotating anode operating at 40 kV and 100 mA with
a CuK␣ (␭ ⫽ 0.1542 nm) X-ray source. The SAXS
data were radially averaged and corrected for
background scattering.
The H-SVP ionomers were evaluated for use as
proton exchange membranes (PEM). Sorption isotherms were measured gravimetrically. Predried
(1 day in a vacuum at 70 °C) H-SVP membranes
(70 –140 ␮m thick) were immersed in deionized
water at room temperature and removed from the
water and weighed on an analytical balance at
various times over a period of 176 h. The water
sorption was calculated from eq 1.
3631
Figure 1. 1H NMR spectra of (a) E-SVP copolymer
and (b) H-SVP ionomer in CDCl3. The resonances at 1.3
and 7.25 ppm are due to impurities and the CDCl3
solvent, respectively.
point probe cell consisting of two outer currentcarrying and inner potential-sensing platinum
electrodes was used.
RESULTS AND DISCUSSION
Wt% water sorption ⫽
Wwet ⫺ Wdry
⫻ 100
Wdry
(1)
where Wdry was the dry membrane mass and Wwet
was the mass of the hydrated membrane.
Proton conductivity of hydrated H-SVP membranes (ca. 70 ␮m thick) was measured at room
temperature by impedance spectroscopy over a
frequency range of 10⫺2 to 106 Hz and an AC
voltage amplitude of 50 mV using a frequency
response analyzer (Solartron SI 1260, impedance/
gain-phase analyzer) and a potentiostat (Solartron SI 1287, electrochemical interface). A two-
Table 1 lists the compositions, molecular weights
and Tgs of the E-SVP copolymers. The compositions of the copolymers were determined by
1
H NMR spectra of the E-SVP copolymers,
[Fig. 1(a)] using the ratio of the integrated area
for the styrl peaks at 6.5–7.3 ppm and the DEVP
ester peaks at 3.2–3.9 ppm. The DEVP content
determined by 1H NMR was consistently about
25% lower than the theoretical value calculated
from the reactivity ratios reported in ref. 11. The
molecular weights for all five samples were
higher than the entanglement molecular weight
of PS (Me ⬃18 kDa23,24), and the polydispersities
3632
WU AND WEISS
Figure 2. FTIR spectra of PS and E-SVP copolymers.
were relatively narrow, 1.3–1.5. The Tg of the
E-SVP copolymers decreased with increasing
DEVP concentration, which is a consequence of
the presence of the bulky phosphonate ester
group.
The FTIR spectra of PS and E-SVP copolymers
showing the phosphonate spectral region, 1550 –
900 cm⫺1, are given in Figure 2. The vinylphosphonate group in the E-SVP copolymer was confirmed by the phosphoryl (PAO) stretching vibration at 1238 cm⫺1, the (P)OOC stretching
vibration at 1098 cm⫺1 and the POO stretching
at 1055 cm⫺1.25 The absorbance ratio of the PAO
stretching 1238 cm⫺1 and the CAC stretching of
the styrl ring at 1452 cm⫺1 was linear with DEVP
concentration in the copolymer (determined by
1
H NMR), which provided the following correlation for the copolymer composition (see Fig. 3).
A1238/A1452 ⫽ 0.0545cDEVP ⫺ 0.0545
Figure 3. Ratio of FTIR absorbances for PAO
stretching at 1238 cm⫺1 and CAC stretching at 1452
cm⫺1 versus DEVP concentration in the E-SVP copolymers.
3.5 ppm and the new broad resonance between
4 and 5 ppm were due to the formation of phosphonic acid. The PAO stretching vibration for the
Na-salt ionomer was a shoulder at ca. 1200 cm⫺1
in the FTIR spectrum shown in Figure 4, and the
disappearance of the (P)OOH absorption at
990 cm⫺1 and new vibrations at ca. 1135 and
1050 cm⫺1 indicated the presence of the PO32⫺
anion.
The glass transition temperatures (Tg) of the
E-SVP copolymers and the M-SVP ionomers determined by DSC are plotted against phosphonate composition in Figure 5. The Tg of E-SVP
decreased and the Tg of the H, Na, and Zn-ionomers increased with increasing phosphonate concentration. The decrease in Tg for the phospho-
(2)
Figure 4 compares the FTIR spectra of 6.4E-,
H-, and Na-SVP. After hydrolysis, the PAO
stretching vibration shifted from 1238 cm⫺1 for
the ester to 1194 cm⫺1 for the phosphonic acid,
which is due to hydrogen bonding with the (P)OH
groups in the acid derivative. The (P)OOC absorptions for the ester group at 1098 and
1055 cm⫺1 disappeared after hydrolysis, and the
(P)OOH bending vibration at 990 cm⫺1 and a
broad (P)OOH stretching band at 2300 cm⫺1 appeared for the H-SVP copolymers. 1H NMR was
also used to confirm the hydrolysis of the phosphonate ester groups. As shown in Figure 1(b),
the disappearance of the DEVP ester group at ca.
Figure 4. FTIR spectra of 6.4E-, 6.4H-, and 6.4NaSVP copolymers.
SYNTHESIS OF POLY(STYRENE-CO-VINYL PHOSPHONATE) IONOMERS
Figure 5. Tg measured from DSC as a function of
composition for E-SVP and M-SVP.
nate esters is a consequence of the increase in free
volume of the copolymer due to the bulky phosphonate substituent and the lack of any intermolecular associative interactions in the ester derivative. In contrast, the H-SVP copolymers exhibit
intermolecular hydrogen bonding and strong dipole– dipole associations occur in the phosphonate
ionomers that act as physical crosslinks and restrict the mobility of the backbone.
The change in Tg as a function of ion concentration (dTg/dc) for the acid and metal salt SVP
ionomers was ⬃3.4 °C/mol %. That value is similar to the dTg/dc values that have been reported
for other styrene ionomers, such as SPS (2.8 –
3.8),26 –29 p-carboxylated polystyrene (CPS) (3.3–
3.9),22,28 styrene–methacrylate copolymers (3.3–
3.8)28 –30 and styrene–acrylate copolymers (3.2–
3.6).31,32 That result is consistent with the reports
by Risen et al.27 and Eisenberg et al.28,33 that the
Tg of styrene-based ionomers is relatively independent of the nature of the bound anion or the
mobile cation.
The dynamic mechanical properties of 6.4E-,
H-, Na-, and Zn-SVP and 6.4Na- and Zn-SPS are
compared in Figure 6. For the phosphonate copolymers with equivalent compositions, the storage
modulus, E⬘, above Tg increased in the order E
⬍ H ⬍ Na ⫽ Zn. The phosphonate ester copolymer showed no rubbery plateau region, which is
consistent with the absence of intermolecular interactions for the ester derivative. The phosphonic acid derivative exhibited a small, but weak
plateau-like region above Tg, which is due to the
physical crosslinks formed by hydrogen bonding
3633
between phosphonic acid groups. That “plateau”
region persisted for only 10 –20 °C, due to weakening of the hydrogen bonds at elevated temperatures. The rubbery plateau above Tg was much
better developed in the salts than in the phosphonic acid derivative, which indicates that the
physical crosslinks due to the dipole– dipole interactions were stronger and much more temperature resistant than those due to hydrogen bonding. For the 6.4 mol % zinc salt, melt flow was not
apparent from the DMA data for temperatures as
high as 320 °C. However, melt flow did occur at
the higher applied stresses used to compression
mold the ionomers, which illustrates the nonlinearity of the viscoelastic behavior of these materials. Somewhat surprisingly, even though the
Figure 6. Dynamic mechanical behavior of 6.4E-, H-,
Na-, Zn-SVP, and 6.4Na- and Zn-SPS (f ⫽ 1 Hz): (a) E⬘;
(b) tan ␦.
3634
WU AND WEISS
Figure 7. Dynamic mechanical tensile properties of
6.4E-SVP and H-SVP ionomers (f ⫽ 1 Hz).
acid strength of the phosphonic acid is less than
that of the sulfonic acid (i.e., lower pKa), the MSVP ionomers exhibited a higher rubbery plateau
modulus and much more extended rubbery region
than did the Na- and Zn-SPS salts (see Fig. 6).
That may be a consequence of the placement of
the phosphonate groups closer to the backbone in
the SVP ionomers, as opposed to attachment to
the styrl ring in the SPS ionomers.
Ionomers often exhibit two peaks in the tan ␦
versus temperature response: a lower temperature peak due to the glass transition of the hydrocarbon matrix and a higher temperature peak due
to a transition of an ion-rich microphase. The
temperature dependence of tan ␦ is shown for the
6.4M-SVP copolymers in Figure 6(b). Only a single tan ␦ peak was observed for 6.4E-SVP, which
is consistent with the absence of strong intermolecular interactions that can produce microphase
separation. The 6.4H-SVP, however, showed two
loss peaks, indicating that microphase separation
did occur in that material.
Figure 7 shows the dynamic mechanical behavior of phosphonic acid derivatives as a function of
the phosphonic acid concentration from 2.4 to
15.4 mol %. Two loss processes were observed for
all the samples, which indicates that the hydrogen-bonded species was microphase separated. As
the acid content increased from 2.4 to 6.4 mol %,
the lower temperature peak decreased in intensity and broadened a bit, and the intensity of the
higher temperature peak increased. Those result
are consistent with an increase in the volume of
the ionic microphase (in this case the acid-rich
microphase) or a region of restricted mobility of
the polymer due to intermolecular hydrogen
bonding similar to what Eisenberg et al.5 postulated for metal salt ionomers. At higher acid content, 12.8 and 15.4 mol %, the lower temperature
loss process occurred around 210 –220 °C and was
considerably broadened compared to that for the
copolymers with lower phosphonate concentrations. For those two samples, a very broad higher
temperature relaxation occurred between 250 –
300 °C, which indicates a rather broad distribution of relaxation times for the higher temperature relaxation process.
In contrast to the results shown in Figure 7 for
the styrene–vinyl phosphonic acid copolymers,
microphase separation does not occur in the acid
derivatives of carboxylate ionomers.34 For the sulfonic acid derivative of SPS, microphase separation has been reported by some,7,35 but not all,
researchers. The difference in the phase behavior
of the carboxylic and the sulfonic and phosphonic
acid containing polystyrene ionomers is due to the
stronger acid strengths of the two latter acids. In
those cases, the hydrogen bonding interactions
should be stronger and provide a greater driving
force for phase separation. It would also appear
that the tendency for microphase separation is
stronger in the styrene–vinyl phosphonic acid
ionomers than in SPS copolymers, in that the
phosphonic acid ionomers exhibited microphase
separation at acid concentrations as low as
2.4 mol %, while microphase separation has only
been reported for SPS ionomers above a sulfonic
acid concentration of 5.8 mol %. The difference for
the two types of ionomers may be a consequence
of the phosphonic acid being attached to the more
flexible vinyl group, whereas the sulfonic acid in
SPS is attached to the styryl ring, but a more
complete study of the differences in the two materials is needed to confirm and gauge the importance of this observation.
The tan ␦ curves for the 6.4 mol % Na- and
Zn-salts of phosphonate ionomers shown in
Figure 6(b) only exhibit one clear loss process, for
the glass transition. A very weak and broad peak
may be visible in the data for the Na-salt between
240 –300 °C [see insert in Fig. 6(b)], but no higher
temperature relaxation is observed in the data for
the Zn-salt. That result was surprising, because,
as will be shown by the small-angle X-ray scattering data discussed later in this article, both
those ionomers exhibited microphase separation
of ion-rich aggregates. It is possible that the ionic
phase relaxation in these ionomers occurs above
300 °C, which would be consistent with the elastic
modulus data that show some flow for the Na-
SYNTHESIS OF POLY(STYRENE-CO-VINYL PHOSPHONATE) IONOMERS
Figure 8. Tensile storage modulus for Na-SVP ionomers (f ⫽ 1 Hz).
salt, but relatively little for the Zn-salt up to 320
°C. Unfortunately, because of degradation of the
polystyrene, dynamic mechanical data above 300
°C are not very reliable. In contrast, the tan ␦
data for the Na- and Zn-SPS ionomers shown in
Figure 6(b) clearly show two loss processes.
Tan ␦ curves for Na- and Zn-salts of the SVP
ionomers with other phosphonate concentrations
likewise did not exhibit a distinctive peak or relaxation process above Tg. That observation is in
contrast to the many studies of metal salts of SPS
and carboxylate–styrene ionomers where a second, high temperature, loss process was nearly
always observed. The position of the relaxation
associated with the ion-rich microphase, relative
to Tg, is dependent on the strength of the dipole–
dipole interactions, which in turn, should be a
function of the ionization potential of the ion-pair.
For that reason, the ionic relaxation for a Na-SPS
ionomer containing 6 mol % sulfonate groups,
occurs at ca. 245 °C, while the transition occurs
some 60 – 80 °C lower for Na-carboxylate styrene
ionomers with comparable ionic group concentration.28 The higher transition temperature (or failure to detect a distinct high temperature process)
for the Na-SVP ionomers may indicate that those
materials possess stronger, or more efficient, dipole– dipole interactions, even though that would
not be expected on the basis of the pKa values.
Figure 8 shows the storage modulus versus
temperature data for a series of Na-SVP ionomers
with phosphonate concentrations ranging from
2.4 to 15.4 mol %. The Tg and the modulus of the
rubbery plateau increase with increasing phosphonate concentration. For comparable ionic
3635
group concentrations, the modulus of the rubbery
plateau was similar for the Na- and Zn-salts, but
in all cases, the Zn-salt was much more resistant
to flow at high temperature. Hara et al.36 reported similar results for the Na- and Ca-salts of
SPS ionomers, in which the SPS neutralized with
the divalent Ca cation showed a much more extended rubbery plateau region than the Na-salt
derivative. Similarly, Lefelar and Weiss37 observed a broader rubbery plateau for Zn-SPS compared with Na-SPS and a higher storage modulus
for the Na-salt. In contrast, the DMA results for
6.4M-SPS measured under the same conditions
as the SVP ionomers and shown in Figure 6(a)
show a broader rubbery plateau and higher storage modulus for the Na-salt. And Weiss et al.38
reported that the peak in the relaxation time
spectra due to the ionic phase occurred at shorter
times for the Zn-salt than for the Na-salt. Tomita
and Register22 also observed that the ionic associations in Zn-CPS were weaker than in Na-CPS.
The increase in the rubbery modulus with
phosphonate concentration is consistent with rubber elasticity theory if the intermolecular dipole–
dipole associations act as physical crosslinks. The
rubbery modulus is expected to be proportional to
the crosslink density. Because the rubbery modulus for the two different salts of SVP was essentially the same, it would appear that the effective
crosslink density of the ionomers was dependent
only on the ionic group concentration and was
independent of the nature of the counterion (one
was a monovalent, alkali metal, and the other a
divalent, transition metal).
If one assumes that all the ionic groups participate in intermolecular interactions, that is, all
are part of a network chain, the average molecular weight of an effective network chain is the
average molecular weight between ionic groups.
Accordingly, to a first approximation, the rubbery
modulus is given by rubber elasticity theory39:
E⬘ ⫽ 3 ␳ RT/M c
(3)
where ␳ is the mass density of the polymer, R is
the gas constant, T is the absolute temperature,
and Mc is the average molecular weight between
crosslinks. The crosslink density, ␮, can be calculated from eq 440:
␮⫽
2␳
␸Mc
(4)
3636
WU AND WEISS
Table 2. Comparison of Theoretical and
Experimentally Measured Crosslink Densities of
M-SVP Ionomers
Crosslink Density (10⫺4 mol/cm3)
Phosphonation
Level (mol %)
2.4
4.3
6.4
12.8
15.4
Calc.
Na-SVPb
(⫻10⫺4)
Zn-SVPb
(⫻10⫺4)
1.6
3.0
4.6
9.8
12
2.4
5.2
14
125
185
2.6
6.7
14
99
182
a
a
Calculated by assuming each phosphonate group participates in one-half of a crosslink.
b
Calculated from eqs 3 and 4 assuming a crosslink functionality of 3.
where ␸ is the average functionality of the
crosslinks.
Table 2 compares the crosslink densities calculated by assuming each phosphonate group participates in one-half of a crosslink and the experimentally determined crosslink densities calculated from eqs 3 and 4, assuming ␸ ⫽ 3, that is,
each phosphonate group is attached to two chain
segments and coordinated with another phosphonate group. For all of the ionomers, the crosslink
density calculated from the plateau modulus exceeded the theoretical value, which indicates that
the physical crosslinks arising from ionic interactions either create a large number of trapped
chain entanglements, similar to the formation of
a semi-interpenetrating network, and/or the functionality of the physical ionic crosslinks was
greater than three. The latter explanation is
probable, especially given the evidence for microphase separation of the ionic species, which
indicates that the phosphonate groups participate
in multifunctional crosslinks. Still, the incidence
of trapped chain entanglements is also probable
in these, as well as other ionomers, but the DMA
data cannot resolve the two phenomena.
Thermorheologically simple materials obey the
principle of time–temperature equivalence and
viscoelastic master curves for such materials can
be constructed by superposing data obtained over
a range of time (or frequency) and different temperatures. The basis for time–temperature superposition is that all the relaxation times for the
material have the same temperature dependence.
Usually, the viscoelastic properties of polymers
with complex microstructures, such as block co-
polymers and ionomers, do not obey time–temperature superposition (TTS). Viscoelastic master
curves for the dynamic shear (G⬘) and loss (G⬙)
moduli for the ester, acid, and Na-, Zn-salt of
2.4-SVP copolymers are shown in Figure 9. The
shear moduli–frequency curves were shifted horizontally using a reference temperature of Tref
⫽ Tg ⫹ 50 °C, and vertical shifts of Tref/T were
made to compensate for the temperature dependence of modulus.
The lowest phosphonate concentration was
chosen for discussion here, because if TTS was
going to be applicable, it should work at the lower
ionic concentrations. For the 2.4M-SVP copoly-
Figure 9. Dynamic mechanical shear moduli master
curves for 2.4E-SVP and pseudomaster curves for
2.4H-, Na-, and Zn-SVP ionomers; Tref ⫽ Tg ⫹ 50 °C: (a)
G⬘ and (b) G⬙ temperature range: 2.4E- (135–165 °C);
2.4H- (145–245 °C); 2.4Na- and Zn- (155–255 °C).
SYNTHESIS OF POLY(STYRENE-CO-VINYL PHOSPHONATE) IONOMERS
Figure 10. Small-angle X-ray scattering data for (a)
Na- and (b) Zn-SVP ionomers.
mers, TTS was obeyed for the phosphonate ester,
but not for the phosphonic acid or salt derivatives.
Those results demonstrate the complex viscoelastic behavior of the ionomers, which is consistent
with a microphase separated morphology that
will be demonstrated by the SAXS data discussed
below. The viscoelastic data for the higher phosphonate concentrations also did not obey TTS; a
more complete discussion of the rheological behavior of these ionomers will be presented in a
subsequent publication.
Although the DMA data showed that the phosphonic acid derivatives were microphase separated, it did not show clear evidence for this for
the metal phosphonate derivatives. Microphase
separation in those ionomers, however, was clear
from the observation of a peak in the structure
factor measured by SAXS (Fig. 10). The single,
3637
broad scattering peak seen in Figure 10 for each
of the samples, except for maybe the 2.4-SVP
ionomers, is similar to the “ionic peak” seen in
most ionomers that is due to an ion-rich microphase.41 In addition, the scattering intensity
upturn at low q (q ⫽ 4␲sin␪/␭ where ␭ is the X-ray
wavelength and ␪ is one-half the scattering angle)
is also characteristic of ionomers and is believed
to arise from an inhomogeneous distribution of
isolated ionic groups or multiplets.42,43 A clear
peak was not observed in the structure factor for
the 2.4Na-SVP, but a distinct shoulder can be
seen in the SAXS curve for the 2.4Zn-SVP. This is
understandable, because the electron density of
Zn is about three times that of Na, which greatly
enhances the scattering contrast for the Zn-salts.
The failure of TTS of the viscoelastic data for the
2.4 M-SVP ionomers also supports the conclusion
that those ionomers were microphase separated
as well, and the failure to detect that with SAXS
for the 2.4Na-SVP is probably due to the low
concentration of the ionic phase.
SAXS of the H-SVP copolymers did not resolve
a peak indicating a second phase; the data for the
sample with the highest phosphonic acid concentration, 15.4H-SVP, are shown in Figure 11, although the upturn of intensity at low q may be
indicative of concentration inhomogenities arising from aggregation of the phosphonic acid
groups. The failure of TTS of the viscoelastic data
(Fig. 9), also suggests two phases in these materials. The failure to see a peak with SAXS is
probably due to the very low electron density contrast between the phosphonic acid groups and the
polystyrene matrix.
Figure 10 shows that for the metal phosphonate ionomers the intensity of the ionic peak in-
Figure 11. Small-angle X-ray scattering data for
15.4H-SVP.
3638
WU AND WEISS
Table 3. Characteristic Size Determined by SAXS for Na- and Zn-SVP Ionomers
Na-SVP
Zn-SVP
Phosphonation
Level (mol %)
Scattering
Vector, q (nm⫺1)
Bragg
Distance,
d (nm)
Scattering
Vector, q (nm⫺1)
Bragg
Distance,
d (nm)
2.4
4.3
6.4
12.8
15.4
—
2.10
2.52
2.81
2.82
—
2.99
2.49
2.24
2.23
—
2.47
2.65
2.98
3.10
—
2.54
2.37
2.11
2.03
creased as the ion content increased and the position of the peak shifted to higher q (see Table 3).
The increase in the position of the scattering
peak, qmax, corresponds to a decrease in the characteristic size, d, in real space (d ⫽ 2␲/q), which
most likely represents the average spacing between ionic aggregates. The range of interdomain
spacings for these ionomers was about 2–3 nm,
which is comparable to the spacings observed for
styrene–methacrylate and styrene–acrylate ionomers (2–2.2 nm)32,44 and CPS ionomers
(⬃3.0 nm),32 but smaller than that for SPS ionomers (3.6 – 4.7 nm).35 In general, the interdomain
spacings measured for the Zn-salts were smaller
than for the Na-salts.
The introduction of small amount of ionic
groups into hydrocarbon polymers not only modifies the bulk properties of the polymer, but also
affects the solution behavior of the polymer. Systematic studies on SPS ionomer solutions by Lundberg et al.45,46 showed that two types of behavior
were observed depending on the polarity of the
solvent: (1) ionic aggregation in relatively lowpolarity solvents, and (2) polyelectrolyte behavior
in more polar solvents. These two characteristic
behaviors were observed in the dilute solution
properties of the SVP ionomers as shown in
Figure 12 for the 4.3E-SVP and 4.3Na-SVP. No
inter- or intramolecular interactions are expected
in the E-SVP copolymer, so the solution behavior
of that material should be similar to that of polystyrene.
For a relatively nonpolar solvent such as a
toluene/methanol (95/5) mixture, both the E-SVP
copolymer and the ionomer show classical dilute
solution behavior, that is, a linear decrease of the
reduced viscosity with decreasing polymer concentration. However, the intrinsic viscosity (␩red
as c 3 0) of the Na-salt was much lower than for
the ester copolymer, which is a consequence of
intramolecular ionic associations that tend to decrease the size of the ionomer molecule in dilute
solution. The increase of ␮red with increasing the
polymer concentration was greater for the ionomer, which indicates that at some critical polymer
concentration, the ionomer viscosity becomes
greater than that of the ester copolymer. Again,
that behavior is similar to that of the SPS ionomers in toluene/methanol45,46 and is due to intermolecular ionic associations that become dominant above a critical overlap concentration. In the
case of the more polar NMP solvent, the ester
copolymer still shows classical viscosity versus
concentration behavior, although the intrinsic
viscosity was higher than for the toluene/methanol solvent. In contrast, the ionomer solution exhibits an upturn in the viscosity at low polymer
concentration, which is typical of polyelectrolyte
behavior that arises from solvation of the ion-pair
Figure 12. Reduced viscosity versus concentration
for 4.3E-SVP (circles) and 4.3Na-SVP(squares) in 95/5
(v/v) toluene/methanol mixture (open symbols) and
1-methyl-2-pyrrolidone (closed symbols) at 25 °C.
SYNTHESIS OF POLY(STYRENE-CO-VINYL PHOSPHONATE) IONOMERS
Figure 13. Sorption isotherms for H-SVP membranes in water at 25 °C. The numbers in parentheses
denote the phosphonate concentration (IEC) in mEq/g.
and repulsion of unshielded anions along the
chain. This is similar to what was seen for SPS
ionomers in solvents such as dimethyl formamide.
One application of ionomers is the proton exchange membrane used in hydrogen fuel cells.
The most common PEMs are perfluorosulfonate
ionomers,47 but considerable contemporary ionomer research has focused on the development of
sulfonated hydrocarbon polymers as substitutes
for the relatively expensive perfluorosulfonates.48 –51 Phosphonate ionomers have also been
studied for PEM applications,52–59 but that represents only a relatively small part of the materials development for PEMs conducted over the
past decade. In the present study, the styrene–
phosphonate ionomers were evaluated for their
applicability as PEMs.
The ion-exchange capacities (IEC) of the HSVP ionomers are listed in Table 1. For comparison, the IEC of Nafion™ 117, a common commercial PEM, is 0.9 mEq/g. Sorption isotherms at
25 °C for the H-SVP membranes with an IEC
ranging from 0.2 to 1.5 are given in Figure 13.
Equilibrium sorption was achieved after about
40 h. Although the water sorption increased, as
expected, with increasing IEC, the equilibrium
values were unexpected low. The ionomer with
highest phosphonate concentration, 15.4H-SVP,
only absorbed 3.6 wt % water. In contrast, an
H-SPS with IEC of 0.9 absorbed nearly 14% water.60 The reason for the low water sorption of the
SVP ionomers is not clear from this study, but it
may be that the self-hydrogen bonding in these
3639
materials is so effective that few phosphonic acid
sites are available for hydrogen bonding with
water.
In ionomer PEMs, high levels of hydration are
necessary to sufficiently swell the ionic microdomains to form a percolation pathway for ion
transport and to facilitate the transport by solvating the ion-pair. Therefore, as might be expected
for materials with such low water sorption characteristics as the H-SVP ionomers, AC impedance
measurements revealed no measurable conductivity. Even for IECs of 1.22 and 1.47 mEq/g, the
H-SVP membranes were resistive. In contrast,
the conductivity of a H-SPS membrane with IEC
of 1.35 mEq/g, measured with the same experimental set-up, was 0.003 S/cm. Thus, the H-SVP
ionomers have no value as PEMs, in marked contrast to other phosphonic acid ionomers based on
a perfluorocarbon backbone53,54 that have shown
demonstrable performance as PEM in fuel cells.
CONCLUSIONS
The copolymerization of styrene and DEVP, followed by hydrolysis of the phosphonate ester to
the phosphonic acid, provided a facile route for
preparing styrene-based phosphonate ionomers.
The incorporation of DEVP reduced the Tg of polystyrene due to an increase in free volume by the
bulky phosphonate ester group and/or the introduction of more flexible links in the chain. Conversion to the phosphonic acid and metal phosphonates increased Tg due to intermolecular
hydrogen bonding and ionic interactions, respectively.
Hydrogen bonding also produced a small rubbery plateau in the temperature dependence of
the modulus, while the ionic interactions produced a rubbery region that persisted for as much
as 250 °C, depending on the ionic concentration.
The values of the plateau moduli for the ionomers
were higher than could be accounted by simple
crosslinks derived from pairwise interactions of
the phosphonate groups, which indicates that the
crosslinks were multifunctional and/or ionic
crosslinking produces significant trapped chain
entanglements. In general, the Na- and Zn-salts
exhibited similar plateau moduli, but the Zn-salts
were more resistant to flow in a low strain, dynamic mechanical experiment. The styrene–
DEVP copolymers were thermo-rheologically simple, but the viscoelastic behavior of the acid and
salt derivatives does not obey time–temperature
3640
WU AND WEISS
superposition as a result of microphase separation of the polar species.
The styrene–phosphonate ionomers absorbed
relatively little water, even for ionomers with ionexchange capacities greater than 1.0 mEq/g. The
reason for this was not determined, but may be
due to the nature and effectiveness of the hydrogen bonding between phosphonic acid groups that
makes them unavailable for hydrogen bonding
with water. Additional study of this phenomenon
is needed. However, the low water absorption
makes these materials unsuitable for application
as proton exchange membranes, as they are not
conductive.
This research was supported in part by grants from the
Polymer Program of the National Science Foundation
(DMR 97-12194) and from the Global Fuel Cell Center
at the University of Connecticut.
16.
17.
18.
19.
20.
21.
22.
23.
24.
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