The Effect of Aging on the Thermal Behavior of
Sulfonated Polystyrene
R. A. WEISS,* Corporate Research-Science Laboratories, Exxon Research
and Engineering Company, P.O. Box 45, Linden, New Jersey 07036
Synopsis
The thermal properties of an ionomer glass, lightly sulfonated polystyrene, was studied as a
function of aging at room temperature after b e i i cooled from the melt. An anomalous endothermic
event below T, was observed by differential scanning calorimetry;the intensity of this excess enthalpy
was a function of time and sulfonate concentration. It is suggested that the origin of this relaxation
may be due in part to morphologicalchanges that occur as a consequence of electrostatic interactions
of the sulfonate groups.
INTRODUCTION
A number of different experimental techniques, including small-angle x-ray
and neutron scattering; electron microscopy; dynamic mechanical, dielectric,
and rheological analyses; and infrared, b a n , and Mossbauer spectroscopyhave
been used to study the structure of ion-containing
Thermal
analyses such as differential scanning calorimetry (DSC) have been used primarily to determine glass transition temperatures Tg.Despite the complicated
morphologies of these materials, relatively little attention has been given to the
influence of the sample history on the nonequilibrium behavior of ionomers. For
example, Eisenberg and TrepmanlO recently demonstrated that for styrenesodium methacrylate ionomers containing greater than 6 mol % sodium methacrylate the thermal coefficient of expansion for the liquid varied substantially
depending upon the previous thermal history of the sample. These authors
attributed their results to changes in the state of ionic aggregation resulting from
different heat treatments.
An important implication of the findings of Eisenberg and Trepman is that
the conventional DSC procedures of preheating experimental specimens in order
to destroy any previous thermal history may in the case of ionomers actually
destroy the morphology one wants to characterize. Similarly, it is not clear
whether previous studies of the ionomer structure have been performed on
samples with equilibrium morphologies. This is unfortunate not only because
it raises serious questions as to the significance of many of these results, but it
also suggests the possibility of different results from different investigators
studying the same material if the thermal histories of their samples were substantially different.
The purpose of this investigation was to determine the thermal behavior of
a glassy ionomer, lightly sulfonated polystyrene (SPS),as a function of aging time
* Current Address: Institute of Materials Science,University of Connecticut, Storrs, CT 06268.
Journal of Polymer Science: Polymer Physics Edition, Vol. 20,6572 (1982)
0 1982 John Wiley & Sons, Inc.
CCC ooS8-1273/82/010065-08$01.00
WEISS
66
at room temperature after cooling the material from the melt. It is not the intent
of this communication either to define the equilibrium morphology of an ionomer
or to speculate as to the validity of previous studies describing the structure of
ionomers.
-dq
dt
-/J
AGING OF SULFONATED POLYSTYRENE
I
0.2
67
P-
mcallsec
(quenched)
40
60
80
100
120
140
1 D
TEMPERATURE (C)
Fig. 2. DSC thermograms (initial heating) as a function of annealing time at 25OC after quenching
from 200°C for PS.
Measurements of Tgand ACf,.weremade both by manual analysis of the strip
chart record and automatically with a Tektronics Model 31 programmable calculator which was interfaced with the DSC. Only relative changes in C, were
determined. AC, calculations were done manually. The precision of the
temperature measurements and the AC, (ACf,)measurements were 0.1OC and
0.001 cal/g "C, respectively. For a single sample the accuracies of the Tgand
AC, measurements were <0.3 and <5%. The accuracy of the ACf,measurements
for the higher sulfonated sample, 5.5 SPS, is estimated to be 10%; most of this
error results from difficulty in drawing a base line below Tg.
RESULTS
The initial heating thermograms of PS, 2.3 SPS, and 5.5 SPS are given as a
function of aging time in Figures 2,3, and 4, respectively. The curves have been
displaced along the ordinate in order to facilitate comparison. Tgand ACf, are
plotted against aging time in Figures 5 and 6. Figure 7 compares the initial and
second heating thermograms for a 5.5 SPS sample aged at room temperature for
235 days. The effect of cooling rate on the subsequent thermal behavior of PS
and 5.5 SPS are shown in Figures 8 and 9.
DISCUSSION
Aging PS at 25OC results in an increase of the magnitude of the endothermic
peak observed at the glass transition by DSC (Fig. 2). This is consistent with
the kinetic nature of the vitrification process, and the excess enthalpic peak
corresponds to the classical enthalpy relaxation process described by Petrie.12
As shown in Figures 5 and 6, Tgand ACL (AC,) remain constant over an aging
period of more than three months.
Similar results were observed for 2.3 SPS (Fig. 3); enthalpy relaxation occurred
at the glass transition and increased in magnitude with increasing aging time.
The increase in Tg to l l l ° C for this polymer is due to the physical crosslinks
formed by intermolecular interactions of the metal sulfonate groups which re-
WEISS
68
d_q
dl
, / ,
40
uenched)
60
80
100
,
,
120
,
,
140
,
1
I
TEMPERATURE (C)
Fig. 3. Same as Figure 1, but for 2.3 SPS.
strict the segmental mobility of the styrene backbone. The apparent heat capacity change a t the glass transition, ACL, for 2.3 SPS remains fairly constant
with aging (Fig. 5 ) , but is about 15%lower than that for PS. Immediately after
being cooled from the melt, however, AC; for 2.3 SPS is very close to that of PS,
and the actual difference AC, between the specific heats of the liquid and solid
does not change with time. These results can be explained by a subtle but distinct increase in the slope of the thermogram below the glass transition. This
is difficult to see on the curves in Figure 3, but the experimental data for these
(21)
(12)
(4)
day)
,
, ( 2 brs),
40
60
,
,
80
,
,
100
,
,
120
,
,
140
TEMPERATURE (C)
Fig. 4. Same as Figure 1, but for 5.5 SPS.
,
1 0
AGING OF SULFONATED POLYSTYRENE
.
3
-
,
.
q
.
,
69
.
A POLYSTYRENE
T = 102.5
* I.OY
0
AGING TIME. (days)
Fig. 5. T, for PS and SPSs as a function of annealing time at 25OC after quenching from
200oc.
polymers definitely show an increase in apparent specific heat near 50°C. This
aging effect occurs rapidly, within several hours after cooling the polymer from
the melt, and it accounts for the discrepency between AC; and AC,.
Differences in AC; and AC, were also observed for 5.5 SPS. In this case there
was a much more dramatic decrease in AC; upon aging (Fig. 6), and a second
endothermicevent is clearly evident below Tgin the thermograms (Fig. 4). The
temperature of this endotherm increases and the endotherm broadens with increasing aging time. Tg remains constant with time at about 120°C. When 5.5
SPS is reheated for a second DSC scan, no pre-T, endotherm is observed and
AC; and AC, are identical (Fig. 7). Furthermore, AC, for 5.5 SPS does not
change with aging time, though the value measured, 0.056 callg "C f 5%, is
somewhat lower than that measured for PS, 0.071 cal/g "C f 5%. The decrease
.oq
1
1
P0LYSlY:ENE
,
,
,
,
,
,
0 2.3SPS
0 5.5SPS
0
0
20
40
60
AGING TIME days)
80
,
1
100
Fig. 6. Apparent heat capacity change at T, for PS and SPSs as a function of annealing time at
25OC after quenchingfrom 20OOC.
WEISS
70
I
I
I
I
I
60
I
#
I
#
80
100
TEMPERATURE ( C )
120
1
140
Fig. 7. Comparison of the DSC thermograms of 5.5 SPS after aging at 25OC for 235 days, initial
heating scan and reheating scan.
in AC, accompanying the increase in the Tg of this polymer is not, however,
without precedent; in fact the values obtained for AC,Tg of ca. 22-27 callg are
consistent with the near constancy of this product reported by Simha and Boyer13
for a variety of glassy liquids and amorphous polymers.
Sub-Tg endothermic events have been observed in polymers that have been
vitrified under pressure or mechanically stressed below Tg.14-16 In both cases
the endotherm below Tg represents an enthalpy relaxation resulting from aging,
distinct from the normal enthalpy recovery observed upon thermal annealing
of unstressed glasses. These aging processes are induced either by a change in
the conformational energy (pressure vitrified glasses) or by frozen-in stresses
(stressed glasses).16 Usually the observed endothermic event is followed by an
exothermic event that represents a relaxation of either conformational energy
dg
dt
(0.31Clmin)
(20)
(quenched)
40
60
80
100
120
TEMPERATURE ( C )
140
1 0
Fig. 8. DSC thermograms (initial heating) as a function of cooling rate from 200°C for PS.
AGING OF SULFONATED POLYSTYRENE
(quenched)
71
L/
!g
dt
____----.
40
60
00
100
1 0
TEMPERATURE (C)
140
'
1
Fig. 9. Same as Figure 6, but for 5.5 SPS.
or strain energy. During aging the glass densifies, and the sub-T, endotherm
represents the energy required for the volume to relax.16
The ionomer glasses studied here were neither vitrified under pressure nor
stressed and, therefore, the sub-T, endotherm observed in this investigation
would not at first appear to be related to those described above. It has been
demonstrated, however, by small-angle x-ray scattering17 that SPS ionomers
have a phase-separated microstructure of ion-rich domains dispersed in a styrene-rich continuous phase. It is believed that the domain morphology is destroyed a t elevated temperatures, and there is no reason to believe that an
equilibrium phase-separated morphology is achieved spontaneously upon cooling
the ionomer. This hypothesis suggests that perfection of the phase-separated
morphology occurs with time, and in order to accomplish this there must be a
finite internal mobility of the molecules.
The physics of phase separation in ionomers has been treated semiquantitatively by Eisenberg.18 According to this theory, electrostatic interactions provide
the driving force for phase separation. As a consequence of the aggregation of
the ionic species, densification of the polymer should occur and the chain segments between neighboring ionic groups stretched. It is conceivable that below
Tg this reorganization of the chain segments could give rise to significant internal
stresses, resulting in thermal behavior similar to that observed in the externally
stressed glasses. The aging effect is more clearly exhibited in the more highly
sulfonated polymer, though the difference observed between ACL and AC, in
the less sulfonated material suggests that a similar aging effect occurs in that
ionomer.
It should be noted that these aging results were obtained with samples stored
in an atmosphere of 50%RH. Although the polymers were sealed inside aluminum sample pans that were stored in closed, unsealed paper envelopes, there
was concern, because of the water sensitivity of these materials, that moisture
might be responsible for the aging effects described here. A limited number of
replicate experiments were subsequently run on samples aged at 25°C in dry
nitrogen. The thermal behavior versus time effects were identical with those
reported here; thus the environment was not responsible for these aging phenomena.
72
WEISS
The results of cooling PS and 5.5 SPS at different rates from the melt and then
heating at 20”C/min are shown in Figures 8 and 9. The “quenched” rate designates samples cooled in the DSC from the melt at the maximum rate possible,
in this case between 160 and 320°C/min. For PS, the effect of decreasing the
cooling rate is the same as increasing the annealingtime: the amount of enthalpy
relaxation occurring at T, increases. Decreasing the cooling rate for 5.5 SPS
also enhances the enthalpy relaxation at T,,but it also results in a decrease in
AC; due to an increase in the apparent specific heat near 6OOC.
CONCLUSIONS
The thermal behavior of lightly sulfonated polystyrene samples is dependent
upon thermal history. Upon aging at room temperature after cooling from the
melt, an excess enthalpic process becomes evident well below Te It has been
suggested that this aging phenomenon is similar to that previously observed in
pressure-vitrified and mechanically stressed glasses. In this case, the pre-T,
relaxation observed in the ionomeric glasses may be related to densification of
the material as it preceeds towards an equilibrium phase-separated morphology.
At this stage, this conclusion is highly speculative, and future experiments such
as time-dependent small-angle x-ray scattering measurements will be made in
order to evaluate this hypothesis.
The author wishes to thank Mr. William Petrik for his contribution to the DSC results and Drs.
William Prest and William MacKnight for helpful conversations.
References
1. A. Eisenberg, Pure Appl. Chem., 46,171 (1976).
2. A. Eisenberg, in Contemporary Topics in Polymer Science, M. Shen, Ed., Plenum, New York,
Vol. 3,p. 231.
3. A. Eisenberg and M. King, Ion-Containing Polymers, Academic, New York, 1977.
4. Ionic Polymers, L. Holliday, Ed., Applied Science, London, 1975.
5. Ions in Polymers, A. Eisenberg, Ed., Adv. Chem. Ser., Vol. 187,American Chemical Society,
Washington, DC, 1980.
6. G. B. Rouse, W. M. Rosen, Jr., A. T. Tsatsas, and A. Eisenberg, J. Polym. Sci. Polym. Phys.
Ed., 17,81 (1979).
7. A. Neppel, I. S. Butler, and Eisenberg, J. Polym. Sci. Polym. Phys. Ed., 17,2145 (1979).
8. A. Neppel, I. S. Butler, and A. Eisenberg, Macromolecules, 12,949 (1979).
9. E. J. Roche, M. Pireri, R. Duplessix, and A. M. Levelut, J.Polym. Sci. Polym. Phys. Ed., 19,
l(1981).
10. A. Eisenberg and E. Trepman, J. Polym. Sci. Polym. Phys. Ed., 16,1381 (1978).
11. H.S.Makowski, R. D. Lundberg, and G. H. Singhal, U.S.Patent 3,870,841(1975).
12. S.E. B. Petrie, J. Polym. Sci. A-2,10,1255(1972).
13. R. Simha and R. F. Boyer, J. Chem. Phys., 35,1003 (1962).
14. M. J. Richardson, in Developments in Polymer Characterization, J. V. Dawkins, Ed., Applied
Science, London, 1978,p. 205.
15. W. M. Prest, Jr., J. M. O’Reiy, F. J. Roberts, Jr., and R. A. Mosher, Polym. Prepr., Am. Chem.
SOC.Diu. Polym. Chem., 21,12 (1980).
16. W. M Prest, Jr., and F. J. Roberta, Jr., in Contemporary Topics in Polymer Science, Vol. 4,
W. J. Bailey, Ed., Plenum, New York, 1981.
17. D. G.Peiffer, R. A. Weiss, R. D. Lundberg, N. C. Payne, and J. Richardson, unpublished.
18. A. Eisenberg, Macromolecules, 3,147 (1970).
Received August 25,1980
Accepted July 20,1981