Phosphonate Ionomers Based on Phosphonylated Ethylene-Propylene
Copolymer
INTRODUCTION
The modification and control of polymer properties through the use of ionic functional groups
has received a considerable amount of attention in recent years. It is now well established that the
introduction of small amounts of salt groups into a hydrocarbon polymer can modify significantly
the physical and rheological behavior of the resultant polymer. A comprehensive review of the
ionomer literature can be found in several recent monographs1s2and review article^.^^^
For the specific case of an ionomer based on an ethylene-propylene copolymer (EP) or ethylene-propylene-diene terpolymer (EPDM) backbone, several references are worthy of mention.
Carboxylated E P and EPDM have been prepared by several routes including metallation of EPDM
in solution, followed by reaction with CO25; grafting polyacrylic acid to EP in an emulsion systems;
terminating a living styrene-maleic anhydride copolymer in the melt on a two-roll mill or in an extruder’; and graftiig maleic anhydride to E P in solution.s In carboxylated E P (EPDM), the metal
carboxylate groups act as ionic crosslinks which give rise to improved tensile strength, increased
viscosity during extension, and increased energy dis~ipation.~More recently, Lundberg et al.1”-16
have described the synthesis and properties of sulfonated EPDM. These ionomers display many
of the characteristics of crosslinked elastomers; however, the ionic crosslink is thermally reversible.
Phosphonylation of E P was reported in the early 1960s by researchers of the Union Carbide
Corporation.’6m They found that high-strength elastomers could be prepared by vulcanizing a
phosphonylated E P with lead oxide a t elevated temperatures. The resulting elastomers exhibited
outstanding resistance to heat, ozone, and oxygen, and the tensile properties increased and the
ultimate elongation decreased with increasing phosphonylation. It should be emphasized that these
authors describe vulcanized compositions, and it does not appear that they recognized the concept
of a reversible, ionic crosslink.
In this note, the synthesis and properties of an ionomer based on phosphonylated E P will be described. It is demonstrated that although these polymers behave in many respects as crosslinked
elastomers, the crosslink is a physical one and not covalent, and it is thermally reversible.
EXPERIMENTAL
Materials
The phosphonylated polymers were prepared by an oxidative chlorophosphonylation reaction
described by Leonard e t al.19 The starting EP, Vistalon 404, was obtained from Exxon Chemical
Co. (USA), contained 3% ethylene, and had a Mooney viscosity [ML 1 8 (21Z°F)] of 35.
The product of the chlorophosphonylation reaction, the polymer (phosphonyl dichloride) was
converted to the phosphonic acid derivative by pouring the reaction solution slowly over cracked
ice and allowing the reaction to proceed for 12-16 hr. The polymer precipitate was washed with
water until the pH of the wash was 5, ground to powder in a Wiley mill, washed with methanol in
a Soxhlet extractor for 12-14 hr, and dried under vacuum a t 35OC. The polymers were redissolved
in a toluene/alcohol mixture and reprecipitated in boiling water.
The sodium (Na) phosphonate derivative was prepared by titrating a 2.5% solution of the EPphosphonic acid with a 100% excesa of sodium hydroxide in ethanol. Where the phosphonic acid
derivatives were insoluble, the polymer was swollen in a toluene/methanol(90/10) mixture and then
neutralized. The polymer phosphonates were recovered by precipitation in methanol and were
subsequently washed with methanol in a Waring blender and dried under vacuum a t 35OC.
+
Journal of Polymer Science: Polymer Chemistry Edition, Vol. 18,2887-2899 (1980)
0 1980 John Wiley & Sons, Inc.
0360-6376/80/0018-2887$01.30
2888
J. POLYM. SCI.: POLYM. CHEM. ED. VOL. 18 (1980)
Ionemer Characterizations
Infrared (IR) spectra were obtained with a Digilab FTS model 14 IR spectrometer covering a
wavenumber range from 4000 to 400 cm-l. Intrinsic viscosities of the ummodified EPR and the
phosphonic acid derivatives were determined in toluene or a toluene/methanol mixture at 25OC using
a Ubbelohde capillary viscometer. Melt flow rates were determined a t 190OC with a Custom Scientific Extrusion Plastometer following the procedure outlined in ASTM-1238.
The softening behavior of the ionomers was measured with a Perkin-Elmer model TMS-2 thermomechanical analyzer (TMA) using a quartz compressionprobe with a hemispherical tip (0.46-mm
radius), a load of 40 g, and a heating rate of 10°C/min. Tensile properties were measured with an
Instron Universal Testing Machine following the procedure outlined in ASTM Standard D-412.
Microdumbbell specimens were cut from compression-molded films approximately 1mm thick.
RESULTS AND DISCUSSION
IR Spectroscopy
The IR spectra of the starting E P and two phosphonic acid derivatives of phosphonylated EP,
one for a relatively low degree of phosphonylation (13.2 mmole of phosphonic acid per 100 g total
of rubber, phr) and one for a relatively high degree of phosphonylation (122.3 mmole phr), are given
in Figures 1-3. The IR absorption bands characteristic of organophosphorus compounds21appear
in the spectra of the phosphonylated EPs even at relatively low degrees of substitution. At low levels
of phosphonylation, however, the phosphonyl absorption and the acid absorptions are a t higher
frequencies than normally observed in compounds containing phosphonic acids. For example, for
a phosphonic acid concentration of 1mmole of acid phr the phosphonyl absorption occurs a t 1262
cm-l and two P-0-H absorptions occur a t 1100 and 1020 cm-'. The higher-frequency P = O band
is consistent with a phosphonyl vibration in the absence of hydrogen bonding, as found, for example,
in the phosphonate esters.2l That strong hydrogen-bonding effects are not prevalent a t low degrees
of phosphonic acid substitution is not altogether surprising considering the average separation of
the acid groups. For example, an acid concentration of 1mmole phr corresponds to approximately
one phosphonic acid group for every 5700 chain carbon atoms.
The phosphonyl and phosphonic acid absorptions shift to lower frequencies as the concentration
of phosphonic acid in the polymer increases. For example, at an acid concentration of 62.6 mmole
phr, the phosphonyl doublet occurs a t 1185and 1115cm-l which is consistent with the doublet observed in lower-molecular-weightorganophosphonicacids. Similarly, acid absorptions are observed
a t 2320,1650,1005,935, and 530 cm-' which agree with those found in lower-molecular-weightorganophosphonic acids21and indicate that strong hydrogen bonding occurs in this material even
though the phosphonic acid groups are on the average separated by 83 chain carbon atoms. The
IR results do not, however, indicate whether the hydrogen bonding is intramolecular or intermolecular. In the discussionthat follows it w
illbe demonstrated that hydrogen bonding occurs, a t least
in part, between phosphonic acid groups on different polymer chains, and in doing so forms a temporary, physical crosslink.
Molecular Weight
The chlorophosphonylation reaction proceeds by a free radical reaction which can lead to crosslinking or chain scission in addition to chlorophosphonylation of the polymer. Because E P has a
high concentration of tertiary carbon atoms, the methine carbon of the propylene monomer, chain
degradation is a likely side reaction. This is demonstrated by the intrinsic viscosity data in Table
I for several phosphonic acid derivatives,which indicate that the polymer molecular weight decreases
as the extent of the grafting reaction increases. Although it was necessary to add some alcohol cosolvent in order to dissolve the more highly phosphonylated polymers, it is unlikely that the large
decreases in the intrinsic viscosities of these samples are due solely to the addition of alcohol.
A reduction in the polymer molecular weight should also be reflected by a decrease in the melt
viscosity of the polymer or an increase in its melt index, and this is demonstrated in Table 11. For
most samples the melt index increased compared with that of the starting EP, though in some cases
a marked decline in the melt index occurred. These results indicate that both chain scission and
crosslinking side reactions &curred during the preparation of these polymer samples. Although
the reaction conditions which favor one side reaction over the other were not determined, chain
degradation is most likely favored by a relatively high PCl3:polymer ratio and crosslinking by a lower
PCl3:polymer ratio in the reaction mixture.
dI-
5z
2k
0
2
W
a
0
z
w
i=
-
-
-
I
3500
1
4doo
0-
20
40
60-
80
2.5
100
3000
2500
I
I
I
I
J
4
3.5
3
2000
WAVENUMBER (CM-l)
1800
1
I
5.5
I
WAVELENGTH ( p m )
5
I
1400
7
Fig. 1. Infrared spectrum of Vistalon 404.
I
4.5
1200
I
1000
I
800
1
600
1
460
CD
h3
03
03
2890
J. POLYM. SCI.: POLYM. CHEM. ED. VOL. 18 (1980)
NOTES
2891
8
f
I
,
1
0
m
I
1
8
8
1
w
(lN33t13d) 33NVlllWSNVtil
C
2892
J. POLYM. SCI.: POLYM. CHEM. ED. VOL. 18 (1980)
TABLE I
Effect of Phosphonylation in the Polymer Intrinsic Viscosity
Polymer
mmole Acid
phr
Solvent
(dl/d
V-404
6180-35
6180-59
6180-69
6180-45
0
0.97
3.87
62.6
73.2
Toluene
Toluene
ToluenehleOH (99.50.5)
ToluenehleOH (946)
Toluene/MeOH (946)
1.70
1.81
1.13
0.38
0.21
[Sl
Because the effects of ionic interactions on the physical behavior of an ionomer can be ambiguous
if the material is covalently crosslinked, the latter six samples listed in Table I1 are not considered
in the remainder of this note. It is important to recognize that the rest of the samples were not covalently crosslinked as was demonstrated by their solubility in an appropriate solvent, a fact which
distinguishes these materials from the cured phosphonylated EPRs described in refs. 1G20.
One striking difference between the solubility characteristics of these polymers and sulfonated
EPDM is noteworthy. The sulfonic acid derivatives of sulfonated EPDM containing up to 30 mmole
phr are soluble in nonpolar hydrocarbon solvents such as toluene or xylene.'O The phosphonic acid
derivatives of phosphonylated EPR, however, were insoluble in toluene even a t very low degrees
of modification, and a small amount of a polar cosolvent such as methanol was required in order to
dissolve these materials. This suggests that the intermolecular interactions between phosphonic
acid groups are stronger than those between sulfonic acid groups-that is, a more polar solvent is
necessary in order to dissociate adequately the interactions arising from hydrogen bonding for the
phosphonic acids than for the sulfonic acids. This result may suggest that the strength of the intermolecular associations possible in a phosphonated elastomer might be comparable to or greater
than those occurring in the sulfonated elastomer previously described.
The melt indices (MI) of the sodium phosphonate derivatives of phosphonylated EPR are given
TABLE I1
Melt Indices of PhosDhonated EPR
Sample
Functional
group conc.
(mmole phr)
Vistalon 404
0
acid
Melt index a t 1 9 0 T ("C/min)
43 psi
250 psi
Na salt
acid
Na salt
7.8
0.23
6180-35
5916-17
5916-12
5916-116
5916-138
5916-53
5916-25
6180-1
5916-66
5916-99
6180-45
5916-88
6180-15
5916-79
1.0
3.5
4.2
6.1
8.4
9.0
10.0
10.3
13.2
13.2
73.2
102.3
106.8
122.3
0.27
0.30
0.34
0.21
0.22
0.38
0.30
0.25
0.32
0.25
20.7
3.9
2.5
11.1
6180-59
5916-150
5916-127
5916-37
6180-69
6180-25
3.9
5.2
21.9
23.2
62.6
82.6
0.0
0.04
0.01
0.11
0.03
0.32
0.18
0.14
0.13
0.20
0.20
0.10
0.17
0.22
0.10
0.13
0
0
0
0
8.0
10.3
11.4
7.8
7.6
14.1
10.6
7.9
11.2
7.8
>60
65.6
28.1
>60
0.82
2.1
0.56
3.2
0.61
3.3
8.9
9.1
4.1
6.6
6.4
3.6
5.5
6.7
4.1
5.5
0
0.06
0
0.07
NOTES
2893
in Table 111. Neutralization of the phosphonic acid results in a decrease in the MI, indicating an
increase in the apparent molecular weight of the ionomer. At high levels of phosphonation the decline
of the MI is truly remarkable. For example, the phosphonic acid derivative of sample 6180-45 (73.2
mmole of acid phr) had such a low viscosity a t 190°C and 250 psi that no accurate determination
of the MI could be made. The sodium phosphonate derivative, however,would not flow under these
same conditions. The effect of neutralization on the MI of these ionomers is consistent with that
observed in the sulfonated-EPDM systems,’O and both results clearly indicate the occurrence of
strong interchain ionic associations.
Thermal Analysis
A qualitative determination of the modulus-temperature behavior of a polymer can be obtained
conveniently by thermomechanical analysis (TMA), and the TMA curves for some of the phosphonylated polymers are given in Figures 4-6. In Figure 4 the TMA curves for Vistalon 404 and
three phosphonic acid derivatives of phosphonylated EPR with varying degrees of substitution are
shown. At low phosphonic acid concentrations (samples 6180-35 and 5916-66) the curves are not
much different than that of the starting EPR. The apparent glass transitions Tgof all three materials
correspond to approximately -5OoC, and although small differences can be seen in the rubbery
plateau and flow transition regions of these curves, these may be due simply to differences in the
backbone molecular weight and experimental error.
A t a higher level of phosphonic acid concentration (cf. sample 5916-88), Tgis shifted to a higher
temperature, and the modulus decrease at Tgappears to be less pronounced. For these materials
the rubbery plateau is maintained 2OoChigher than for the Vistalon 404 and the samples with lower
phosphonic acid concentrations. The extended rubbery plateau suggests a certain degree of chain
interaction which, in effect, increases the apparent polymer molecular weight. Similarly,the increase
in the Tgcan be explained by the interaction of phosphonic acid groups, presumably through hydrogen bonding which retards the relaxation of the polymer backbone.
The effect of neutralization of the phosphonic acid groups on the TMA behavior is shown in Figures
5 and 6 for a fairly low degree of substitution (13.2 mmole phr) and a higher phosphonate concentration (102 mmole phr), respectively. At a degree of modification of 13.2 mmole phr the effect of
neutralization is to shift the modulus-temperature curve along the temperature axis (cf. Fig. 5); Tg
increases by some 5OC and the rubbery plateau for the salt persists 3OoC higher than for the acid.
These results demonstrate that the interactions of the ionic species are stronger than for the acid
and the chain relaxations are retarded to a greater degree.
A t a sodium phosphonate concentration of 102 mmole phr the TMA behavior curve is similar to
what one might expect from a crosslinked elastomer (cf. Fig. 6). In this case, Tgdecreases 8°C as
opposed to Tgof the acid and the rubbery plateau which continues to nearly 200OC. No clear
transition is observed in the temperature range covered. This behavior is analogous to that of sulfonated EPDM, and in both cases the crosslinks arise from strong intermolecular associations of
the ionic species.
Tensile Properties
As demonstrated by the previously discussed results, an important characteristic of these ionomen
is that intermolecular associations of the ionic species behave as thermally reversible crosslinks. In
sulfonated EPDM these associations, or crosslinks, are stable a t room temperature and have a profound effect on the mechanical properties of the polymer.1° Thus, the incorporation of metal sulfonate functionality into EPDM will improve the tensile modulus and tensile strength and lower
the ultimate elongation of the resultant polymer. For example, a modified EPDM containing 31
mmole of sodium sulfonate phr can have a room temperature tensile strength of 960 psi a t an elongation of 350%,1° whereas the unmodified EPDM has a tensile strength of less than 100 psi and an
ultimate elongation of greater than 800%.
The tensile properties of the phosphonylated EPRs are given in Table 111. In many cases considerable chain scission occurred during the phosphonylation reactions and, therefore, comparisons
between the phosphonylation products and the starting EPR are not straightforward. In the case
of EPR, however, lowering the polymer molecular weight should, if anything, result in a decrease
in the tensile modulus and strength. Therefore, any improvements in the mechanical properties
of the phosphonylation products versus the starting Vistalon 4 should be even more distinctive if
compared with a lower-molecular-weightEPR. As was the case with the MI data, direct comparisons
between the properties of the phosphonic acid and the sodium phosphonate derivatives are valid.
a Elm
1.o
3.5
4.2
6.1
8.4
9.0
10.0
10.3
13.2
13.2
73.2
102.3
106.8
122.3
0
28
32
28
26
29
27
21
29
25
25
21
53
47
91
53
EIW(psi)
= secant modulus at an elongation of 100%
Vistalon 404
6180-35
5916-17
5916-12
5916-116
5916-138
5916-53
5916-25
6180-1
5916-66
5916-99
6180-45
5916-88
6180-15
5916-79
Sample
Functional
group conc.
(mmole phr)
Acid
(psi)
3
6
8
8
9
6
3
6
2
3
3
160
8
190
12
Ub
Na salt
-
1910
-
-
-
23
23
35
27
30
25
26
22
22
24
a m (psi)
1010
1650
760
30
36
49
37
39
31
37
34
31
35
Elm (psi)
= stress at break; € b = ultimate elongation.
920
960
780
820
540
660
650
750
640
800
560
760
>4000
990
>4000
c b (%)
u 3 =~stress at an elongation of 300%; IJb
19
23
19
18
14
17
13
18
16
17
14
67
40
99
48
am (psi)
TABLE I11
Tensile Properties* of Phosphonated EPR
(Psi)
17
14
29
4
11
14
6
1
13
2
1010
1240
2150
2280
ab
1350
330
310
1400
1090
2000
1400
820
1460
930
40
160
190
360
cb (%)
W
00
0
v
r
00
+
p
c
P
M
NOTES
2895
-
L
.c
n
0
i
’
E
//
/ 4
/
I
0
I
0
I
I
I
I
I
I
I
I
I
N
m
U
In
u3
0
0
0
0
0
(WW)
UOL’+JJUau JqOJd
0.6
0.5
0.4
0.3
0.2
- 80
- 60
-40
- 20
T
0
("C)
20
40
60
80
100
Fig. 5. Softening behavior of acid and salt derivatives of sample 5916-66 (13.2 mmole phr). Penetration probe: 40-g weight, dT/dt = 10°C/min.
p.
0
n
al
0
al
,
u-"
c,
u
.r
C
v
E
-
0.1
0
h
2
cd
P
R..
cn
NOTES
2897
1
,>
>
>t
IJ
2898
J. POLYM. SCI.: POLYM. CHEM. ED. VOL. 18 (1980)
Below a phosphonic acid concentration of 13 mmole phr the tensile properties of the modified
polymers were, in general, not much different than the starting EPR. Similarly, no significant
differences are observed in the tensile properties of the sodium salts below 13mmole phr. It appears
that there may have been a slight improvements in the strength and the elongation upon converting
the acid to the salt, but since each datum point in Table I11 represents the average of only three determinations, it is difficult to say whether these differences are significant.
Dramatic differences between the different phosphonylation products and the starting EPR are
apparent, however, a t levels of substitution greater than 70 mmole phr. The acid derivatives exhibit
a t least a twofold increase in modulus. Samples 6180-45 and 6180-15 have relatively high tensile
strengths (150-200 psi), while samples 5916-88 and 5916-79exhibit a yield point a t a low elongation
and a relatively low stress gnd high elongation a t failure. The stress at yield for these materials is,
however, much greater than the tensile strengths of the polymers with lower phosphonic acid concentrations. The explanation for the differences in the stress-strain behavior of these four samples
is not clear but may be a result of differences in the backbone molecular weight. The important
point concerning these data is that a t sufficiently high phosphonic acid concentrations, interactions
between the acid groups are strong enough to improve the room temperature tensile modulus and
the tensile strength of these polymers. These results are consistent with the IR spectra which indicate
strong hydrogen bonding a t the higher phosphonic acid concentrations.
The effect of neutralization of the phosphonic acid groups on the tensile properties of the higher
substituted materials is even more remarkable. The moduli of these materials are of the order of
750-1500 psi and the tensile strengths are from loo0 to 2300 psi, even though the ultimate elongations
were extremely low, 40-360%. The low elongations can be explained in part by imperfections in
the tensile samples due to the difficulties in compression molding these polymers. It is particularly
significant that the solubility behavior, the tensile data, and the melt flow data for the free acid derivatives of these samples indicate that these materials are not crosslinked covalently. Therefore,
the phosphonate results represent positive evidence for strong ionic interactions in these ionomers.
SUMMARY
The preparation and characterization of the phosphonic acid and sodium phosphonate derivatives
of phosphonylated ethylene-propylene copolymer were described. The phosphorus-containing
substituent group is attached directly to the polymer backbone by a C-P bond using an oxidative
chlorophosphonylation reaction. The presence of the phosphonic acid derivative was confirmed
by IR spectroscopy which also indicated the presence of strong hydrogen bonding arising from interactions of the phosphonic acid groups. Conversion of the phosphonic acid groups to metal
phosphonate groups increases the melt viscosity, the tensile modulus, the tensile strength, and decreases the ultimate polymer elongation. These results are consistent with the concepts of ionic
crosslinking resulting from intermolecular interactions of the salt groups. These interactions are
reversible, however, upon the application of heat, and it is demonstrated that these ionomers are
not crosslinked covalently.
C0NCLU SI 0NS
The data described in this note clearly demonstrate that strong intermolecular interactions occur
in elastomers containing modest amounts of phosphonic acid or metal phosphonate pendant groups.
In general, these interactions result in an increase in the melt viscosity, tensile modulus, and tensile
strength, and a decrease in the ultimate elongation. The effect is much more pronounced in the
metal phosphonate derivatives, and for these materials the mechanical behavior is analogous to a
crosslinked rubber. In this case, however, the crosslink is of a physical rather than chemical nature.
The author acknowledgesthe contributions of Mr. Madhukar Rao who conducted the polymer
modifications, Mr. Salvatore Pace who assisted with much of the polymer characterizations, and
Ms.Stephanie Dexter who helped prepare the manuscript.
NOTES
2899
References
1. L. Holliday, Ed., Ionic Polymers, Halstead, New York, 1975.
2. A. Eisenberg and M. King,Ion-Containing Polymers, Academic, New York, 1977.
3. E. P. Otocka, J. Macromol. Sci. Rev. Macromol. Chem., 5,275 (1971).
4. W. J. MacKnight and T. R. Earnest, Jr., unpublished.
5. A. J. Amass, E. W. Duck, J. R. Hawkins, and J. M. Locke, Eur. Polym. J., 8,781 (1972).
6. A Crugnola, M.Pegoraro, and F. Severine, J. Polym. Sci. C, 16,4547 (1969).
7. N. G. Gaylord, A. Takahashi, S. Kikuchi, and R. A. Guzzi, Polym. Lett., 10,95 (1972).
8. Br. Pat. 1,148,855 (1968).
9. E. N. Kresge, “Ionic Bonding in Elastomeric Networks,” paper presented at the 18th Canadian
High Polymer Forum, Hamilton, Ontario, 1975.
10. H. S. Makowski, R. D. Lundberg, L. Westerman, and J. Bock, Polym. Prepr., 19(2), 292
(1978).
11. H. S.Makowski and R. D. Lundberg, Polym. Prepr., 19(2), 304 (1978).
12. R. D. Lundberg, W. J. MacKnight, and R. M. Neumann, Polym. Prepr., 19(2),298 (1978).
13. R. D. Lundberg, Polym. Prepr., 19(1), 455 (1978).
14. R. D. Lundberg and D. Brenner, Chemtech, 748 (December 1977).
15. H. S.Makowski, P. K. Agarwal, R. A. Weiss, and R. D. Lundberg, Polym. Prepr., 20(2), 281
(1979).
16. J. Schroeder and E. Leonard, Jr., Br. Pat., 849,058(1960).
17. J. Schroeder and E. Leonard, Jr., Br. Pat., 849,059(1960).
18. E. Leonard, Jr., and W. Wheelwright, US.Pat. 3,097,194(1963).
19. E. Leonard, Jr., W. I m b , J. Mason, and W. Wheelwright, J. Appl. Polym. Sci., 5, 157
(1961).
20. E. Leonard, W. Loeb, J. Mason, and J. Stenstrom, J.Polym. Sci., 55,799 (1961).
21. L. C. Thomas, Interpretations of the Infrared Spectra of Organophosphorus Compounds,
Heyden, London, 1974.
R. A. WEISS
Corporate Research-Science Laboratories
Exxon Research and Engineering Company
P.O. Box 45
Linden, New Jersey 07036
Received July 27,1979
Accepted June 12,1980