Microphase Separation in Sulfonated Polystyrene Ionomers

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Microphase Separation in Sulfonated Polystyrene Ionomers D. G. PEIFFER, * R. A. WEISS, t and R. D. LUNDBERG, Corporate Research Science Laboratories, Exxon Research and Engineering Company, P.O.

1 Microphase Separation in Sulfonated Polystyrene Ionomers D. G. PEIFFER, * R. A. WEISS, t and R. D. LUNDBERG, Corporate Research Science Laboratories, Exxon Research and Engineering Company, P.O. Box 45,1600 East Linden Avenue, Linden, New Jersey Synopsis Small- and wide-angle x-ray scattering results for a series of un-neutralized and neutralized sulfonated polystyrenes are presented for the range of sulfonation from 0 to 7.26 mol %. From the small-angle scattering it is shown that above the 3 mol % level for both the zinc and sodium salts, a Bragg spacing (37 A) and diameter ( A) of the scattering unit can be calculated. When the concentration of salt is increased, there is no appreciable change in the latter two measurements. The wide-angle data indicate that the cations do not influence to any large extent the basic intramolecular and intermolecular structure of polystyrene. All the data are consistent with the onset of clustering above a critical ion concentration. INTRODUCTION Considerable attention has been given recently to the study of the physical properties of ion-containing polymers.14 These materials are based primarily on ethylene, styrene, or butadiene backbones containing relatively low concentrations of pendant carboxylate or sulfonate functionalities dispersed randomly along the chain backbone. Because of the large difference in the dielectric properties of the ionic and hydrocarbon phases, aggregation of the ionic groups is favored, resulting in dramatic changes in the material properties. In spite of the wide range of efforts devoted to carboxylate ionomers, little work has been performed on sulfonate ionomers. The bulk of the published work has dealt with sulfonated fluorocarbons of the Nafion variety. Several studiess7 have been completed recently on sulfonated hydrocarbon-based polymers, but no systematic study of cluster formation in these materials was performed. It should be noted, however, that Earnest and co-workers6 measured the radius of gyration of sulfonated polystyrene ionomers over a range of sulfonation levels and found that it increases with increasing ion concentration. In this paper we report the results of a small-angle x-ray scattering (SAXS) study of a series of lightly sulfonated polystyrene ionomers. These polymers have been neutralized with salts of sodium and zinc. Specifically we have followed the formation of clusters within this material as the concentration of sulfonate groups is increased. These results are compared with cluster formation in carboxylated polystyrenes described in the literature. * To whom correspondence should be addressed. t Present address: Institute of Materials Science, University of Connecticut, Storrs, CT Journal of Polymer Science: Polymer Physics Edition, Vol. 20, (1982) John Wiley & Sons, Inc. CCC /82/ $01.70

1504 PEIFFER, WEISS, AND LUNDBERG EXPERIMENTAL Lightly sulfonated polystyrene was prepared from a commercial polystyrene, Styron 666 (Dow Chemical Company) that had number-average and weightaverage

2 1504 PEIFFER, WEISS, AND LUNDBERG EXPERIMENTAL Lightly sulfonated polystyrene was prepared from a commercial polystyrene, Styron 666 (Dow Chemical Company) that had number-average and weightaverage molecular weights, as determined by gel permeation chromatography, of 106,000 and 288,000, respectively. Sulfonation reactions were carried out in 1,2-dichloroethane at 50 C using acetyl sulfate as the sulfonating agent.8 The zinc and sodium salts were prepared by titrating the sulfonic acid derivatives with the appropriate metal acetate. The polymers were precipitated with methanol or boiling water for the acid derivative, washed with methanol, and vacuum dried. The sulfur content of the polymers was determined by Dietert sulfur analysis (ASTM D 1552) and was used to calculate the sulfonate concentrations (Table I). Films for use in the x-ray scattering experiments were compression molded at about 2OOOC under nitrogen and cooled under pressure between water-cooled platens. The molded films were dried to constant weight in a vacuum oven near their glass transition temperature and were stored until tested in a dessicator. In all instances the samples were prepared and treated at least one month before the SAXS experiment. Examination with a polarizing microscope revealed that the films were essentially devoid of birefringence. Wide-angle and small-angle scattering experiments were conducted using a Rigaku Rotaflex high-brilliance rotating-anode x-ray generator with samples mounted for symmetrical transmission. Cu K, radiation was used in conjunction with a nickel filter. Typical generator settings were 40 kv and 30 ma. A Norelco microcamera equipped with a 50-p pinhole collimator was used and specimento-film distances of 13.2 and 59.6 mm corresponded to the wide- and small-angle diffraction, respectively. The sample holder was purged with dried helium gas. Scattering was recorded on photographic film (Kodak No-Screen film), and the films were subsequently scanned with a Joyce-Lobe1 microdensitometer. Film exposure times of 48 h were employed, and each measurement included two scattering experiments: a background scan on the un-neutralized sulfonated polystyrene and a scattering scan on the neutralized material at equivalent sample thickness and sulfonation level. Conventional background corrections were made. TABLE I Sulfonation Level and Root-Mean-Square Radius of Gyration for Neutralized Sulfonated Polystyrene Wt % Sulfonation level Material sulfur Counterion (mol%) a (A) A 0.79 Zn B 1.04 Zn C 1.22 Zn D 1.41 Zn E 1.56 Zn F 1.86 Zn G 2.03 Zn K 0.54 L 1.28 M 1.66 Na Na Na

3 MICROPHASE SEPARATION IN IONOMERS 1505 RESULTS AND DISCUSSION Small-angle x-ray scattering (SAXS) permits the evaluation of the mean separation of ionic clusters and the determination of size within a bulk ionomer ~ample.3?~- ~ In addition the variation of these parameters with the degree of sulfonation and cation size can be studied. Shown in Figure 1 are the SAXS results for zinc sulfonates. It is immediately apparent that a Bragg peak appears in the scattering curves above approximately 3 mol % sulfonation. This is undoubtedly due to an interference phenomenon caused by the mean separation of ionic domains or by the short-range ordering of ionic groups around individual ionic cluster^.^ The Bragg peak was completely absent in the un-neutralized sulfonic acid derivatives. In any case, the angular position of this peak does not change with increasing sulfonation, and the calculated Bragg distance was found to be 37 A. It is also observed that the sharpness of the Bragg peak changes with increasing sulfonate concentration. At low ion levels, the peak is broader in the scattering angle (2 0) and lower in peak height as compared to higher sulfonation levels. This may be due to a lower number of scattering sites at low ion concentrations and to a somewhat broader size distribution of the scattering units. Similar trends can be observed when the zinc counterion is replaced with sodium (Fig. 2). Apparently, the nature of the counterion does not appreciably affect the size of the scattering entity in sulfonated polystyrene, as has been observed in other ionomeric materials.1 J3 Guinier15 showed that the SAXS from noninteracting particles is related to the specific size of the scattering entity. It is assumed, however, in the Guinier Q$ a (RADIANS) Fig. 1. SAXS curves for polystyrene and for zinc-neutralized ( mol %) sulfonated polystyrene.

&; SULFONATION 1506 PEIFFER, WEISS, AND LUNDBERG I I I I t 1 I l l.022.032.042.052.062.072.082.092 28 (RADIANS) Fig. 2. SAXS curves for sodium-neutralized (2-7 mol %) sulfonated polystyrene.

4 &; SULFONATION 1506 PEIFFER, WEISS, AND LUNDBERG I I I I t 1 I l l (RADIANS) Fig. 2. SAXS curves for sodium-neutralized (2-7 mol %) sulfonated polystyrene. relation that interparticle scattering is negligible and that the particles are of uniform size. According to this relation, the plot of the logarithm of the corrected scattering intensity versus t12 should yield a straight line with a slope proportional to the size R of the scattering unit. In order to study the size of the scattering entity, the assumption is made that the Bragg peak has a negligible effect on the scattering curve. A Guinier analysis can be performed on the scattering data. However, it is realized that the applicability of the method to ionomeric systems is undoubtedly only an approximation since deviations could result from nonuniformity in the size of the scattering entity and interference effects. The nature of the interference effect at- tributable to the maximum is presently a matter of contro~ersy.~ The results obtained from the treatment of the data according to this method is presented for zinc and sodium ionomers in Figures 3 and 4, respectively. The logarithm of intensity is plotted as a function of t12 in Figures 3 and 4 for several of the metal-sulfonated polystyrenes. A linear relation was found for all sulfonation levels studied, and the results of this analysis are summarized in Table I. The mean scattering size is approximately 7.4 A and is essentially independent of the counterion. These SAXS results for the sulfonated polystyrenes are consistent with those of Moudden and co-workers13 in their studies of ionic clustering in telechelic butadiene-methacrylic acid copolymers containing 2 mol % acid and neutralized (5-100%) with various salts. In that study the salt groups were found to form clusters with an average size of approximately 8 A, and the cluster size appeared to be independent of the nature of the cation and the degree of neutraliz&m. The existence of a low-angle maximum in the scattering intensity indicated a mean separation distance of 80 A between clusters in the fully neutralized sample, a value appreciably larger than the 37 A separation found in the present SAXS investigation. The difference may be due to differences in the distribution of ionic groups along the main chain. In the telechelic materials the ionic groups are present only at the ends of each chain, while in sulfonated polystyrene the sulfonate groups are randomly distributed along the polymer backbone. A Bragg spacing corresponding to A has been observed in many carboxylate monomers.l

5 MICROPHASE SEPARATION IN IONOMERS e2 lo4, RADIANS~ Fig. 3. Guinier plots of scattering intensities (in relative units) for several zinc-neutralized sulfonated polystyrenes. As previously shown in Table I, the ion concentration at which clusters become evident in the SAXS data is between 2 and 3 mol % (corresponding to 2-3 sulfonate groups per 100 styrene monomer units). Eisenberg et al. have studied t 2 e2 x lo4, RADIANS Fig. 4. Guinier plots of the scattering intensities in relative units for several sodium-neutralized sulfonated polystyrenes.

1508 PEIFFER, WEISS, AND LUNDBERG the effect of metal carboxylate1j6 and sulfonatel7 groups on the viscoelastic behavior of styrene-based ionomers.

6 1508 PEIFFER, WEISS, AND LUNDBERG the effect of metal carboxylate1j6 and sulfonatel7 groups on the viscoelastic behavior of styrene-based ionomers. In both systems a secondary relaxation process believed to be of a viscous origin was observed in ionomers containing greater than ca. 5-6 mol 5% ionic groups. This critical concentration, which marked the ionic concentration at which time-temperature superposition was no longer valid for these materials, was initially interpreted as marking the onset of clustering.' Recent dielectric and Raman evidence, however, indicates that ionic clusters exist in styrene-based ionomers at concentrations as low as 2 mol '%.I6 The SAXS results presented here are consistent with these findings. Wide-angle x-ray scattering (WAXS) measurements were also made on all samples. Previous WAXS studied8 of semicrystalline ethylene-methacrylate ionomers established that the introduction of ionic groups reduced the total crystallinity of the copolymer, but did not affect the crystal lattice spacings of the polyethylene component. Presumably, this is due to the inability of the bulky ionic groups to incorporate into the crystal lattice. It is of interest to investigate the affect of sulfonate groups on the diffraction pattern of an amorphous polymer such as atactic polystyrene. Analyses of the x-ray scattering from polystyrene have been reported by several gro~ps.l~,~~ The results indicate that both intramolecular and intermolecular scattering contribute to the radial distribution function at distances beyond 3.7 A, and the molecular structure is dominated by the steric interaction of the phenyl groups. For example, Wecker et a1.20 have assigned the peak at 20 = 19" (d = 4.67 A) to phenyl-phenyl interchain and phenyl-chain interchain interactions and 20 = 11' (d = 8.04 A) mainly to phenyl-chain interchain interactions. Shown in Figure 5 are typical WAXS results from the unmodified polystyrene and three sulfonated materials for low 20 values (d > 3.7 A). The position of the diffraction peaks in all cases is unaffected by sulfonation; however, with increasing levels of sulfonation, the peaks become broader. It can be concluded that the large counterion on the sulfonate group does not affect the average distance of the phenyl groups with respect to the main chain or with other phenyl groups. The broadening of the peak width indicates, however, that the neutralized sulfonate group increases the distribution of the above-mentioned distances. 1 In - I- z a w - > I- 4 - W E - a (DEGREES) SULFONATION LEVEL (MOLE yo) 7.26 Mole % Zinc 6.05 Mole 70 Sodium 7.20 Mole Acid Polvstvrene Fig. 5. WAXS patterns from polystyrene, unneutralized and neutralized (7 mol % sodium, 7.26 mol % zinc) sulfonated polystyrene.

MICROPHASE SEPARATION IN IONOMERS 1509 CONCLUSIONS SAXS results from lightly sulfonated polystyrene ionomers indicate the presence of a scattering peak below 28 = 2O in materials containing ca.

7 MICROPHASE SEPARATION IN IONOMERS 1509 CONCLUSIONS SAXS results from lightly sulfonated polystyrene ionomers indicate the presence of a scattering peak below 28 = 2O in materials containing ca. 2-3 mol % metal sulfonate groups. This peak corresponds to a Bragg spacing of about 37 8 and a characteristic size of the scattering unit of 7 8. The small-angle Bragg peak is insensitive to cation type (zinc versus sodium) and concentration, but it is completely absent in the unsulfonated polymer and in the un-neutralized sulfonic acid derivatives. These results are consistent with previous SAXS studies of carboxylate ionomers both in the characteristic Bragg spacings observed and in that the scattering unit is found only in neutralized ionomers containing greater than a critical ion concentration. It is believed that the origin of the SAXS scattering is identical to that reported for carboxylate ionomersthat is, ion-rich aggregates or clusters, though no attempt has been made in this study to determine the geometry or the composition of the cluster. Complementary wide-angle x-ray diffraction data indicate that the sulfonate groups have little influence on the basic intramolecular and intermolecular structure of polystyrene. It is a pleasure to thank Professor N. C. Payne and Dr. John Richardson of the Department of Chemistry of the University of Western Ontario for obtaining the x-ray photographs. References 1. A. Eisenberg and M. King, Ion Containing Polymers: Physical Properties and Structure, Academic, New York, Ionic Polymers, L. Holliday, Ed., Applied Science, London, E. P. Otocka, J. Macromol. Sci. Reu. Macromol. Chem., 5,275 (1971). 4. W. J. MacKnight and T. R. Earnest, Jr., J. Polym. Sci. Macromol. Reo., 16,41 (1981). 5. H. S. Makowski, R. D. Lundberg, L. Westerman, and J. Bock, Ado. Chem. Ser., 187, 3 (1980). 6. T. R. Earnest, J. S. Higgins, D. L. Handlin, and W. J. MacKnight, Macromolecules, 14,192 (1981). 7. D. Rahrig and W. J. MacKnight, Macromolecules, 187,77 (1980). 8. H. S. Makowski, R. D. Lundberg, and G. S. Singhal, U.S. Pat. 3,870,841 to Exxon Research and Engineering Company, W. J. MacKnight, W. P. Taggart, and R. S. Stein, J. Polym. Sci. Polym. Symp., 45, 113 (1974). 10. (a) A. Eisenberg and M. Navratil, Macromolecules, 7,84 (1974);(b) Ibid., 1,90 (1974). 11. C. T. Meyer and M. Pineri, J. Polym. Sci. A-2,16,569 (1978). 12. M. Pineri, C. Meyer, A. M. Levelut, and M. Lambert, J. Polym. Sci. A-2,12,115 (1974). 13. A. Moudden, A. M. Levelut, and M. Pineri, J. Polym. Sci. A-2,15,1707 (1977). 14. E. J. Roche, R. S. Stein, and W. J. MacKnight, J. Polym. Sci., 18,1035 (1980). 15. A. Guinier and G. Fournet, Small Angle Scattering of X-Rays, Wiley, New York, A. Eisenberg, Contemp. Top. Polym. Sci., 3,231 (1979). 17. M. Rigdahl and A. Eisenberg, J. Polym. Sci. A-2,19,1641 (1981). 18. R. Longworth and D. J. Vaughin, Nature, 218,85 (1968). 19. S. Krimm, J. Phys. Chem., 57,22 (1953). 20. S. M. Wecker, T. Davidson, and J. B. Cohen, J. Mater. Sci., 7,1249 (1972). Received August 11,1981 Accepted February 9,1982

Control of Ionic Interactions in Sulfonated Polystyrene Ionomers by the Use of Alkyl- Substituted Ammonium Counterions

Control of Ionic Interactions in Sulfonated Polystyrene Ionomers by the Use of Alkyl- Substituted Ammonium Counterions R. A. WEISS,* P. K. AGARWAL, and R. D. LUNDBERG, Corporate Research Science Laboratories,