The Emulsion Copolymerization of Styrene and Sodium Styrene Sulfonate

Transcription

1 The Emulsion Copolymerization of Styrene and Sodium Styrene Sulfonate S. RICHARD TURNER,* R. A. WEISS,' and ROBERT D. LUNDBERG, Corporate Research Science Laboratories, Exxon Research and Engineering Company, Route 22 East, Annandale, New Jersey Synopsis The emulsion copolymerization of styrene and sodium styrene sulfonate has been shown to be a feasible preparative route to ionomeric sulfonated polystyrene. The properties of these copolymers are reported elsewhere. The copolymerization rate was found to be dramatically enhanced when compared to that for the emulsion copolymerization of styrene under identical conditions. This copolymerization was studied in detail and two mechanisms were proposed to account for these rate differences. An increase in the number of polymerizing particles in the copolymerization with consequent rate enhancement was substantiated by electron microscopy. However, the data indicate that the rate differences cannot be fully accounted for by this effect. In addition, a gel effect is proposed as a second contributor to the enhanced rate. This gel effect is believed to result from the intermolecular association of the incorporated metal sulfonate units in the growing polymer particles. When a third monomer that plasticizes the ionic interactions is used the polymerization rate decreases. This supports the gel effect hypothesis. INTRODUCTION Lightly sulfonated polystyrene (S-PS) has received considerable attention recently because of the scientifically interesting and potential technologically important properties manifested by the ionic associations in this glassy hydrocarbon polymer. For example, plasticization,' solution behavior,2 viscoelastic proper tie^,^ EXAFS analyses of the aggregation aspect^,^ and fluorescence measurements on S-PS5 have all been recently reported. The technological interest in S-PS arises from the ionomer characteristics. Because of these properties S-PS has been shown to have outstanding potential as a polymer for forming rigid foams.6 Sulfonation of preformed polystyrene has been the principal synthetic route for preparation of S-PS.7 Sulfonation has several key attributes: (1) sulfonation of the preformed polymer probably yields a random distribution of sulfonate groups along the polymer chain; (2) a minimum of chain-tochain heterogeneity is expected from this process; (3) since the reaction has been shown to proceed without polymer degradation, the molecular weight of the functionalized polymer is known; and (4) preformed polystyrene can be obtained with narrow distributions, so that the interpretation of data from studies on the ionomers is simplified. * Present address: Eastman Kodak Company, Research Laboratories Rochester, NY Present address: Institute of Materials Science, University of Connecticut, Storrs, CT Journal of Polymer Science: Polymer Chemistry Edition, Vol. 23, John Wiley & Sons, Inc. CCC /85/ $04.00

2 536 TURNER, WEISS, AND LUNDBERG In concurrent publication^^.^ we communicate the preparation and properties of S-PS prepared by emulsion copolymerization, and describe the differences in properties manifested by S-PS from emulsion vis-a-vis S-PS from sulfonation. This article describes the unusual characteristics of the copolymerization reaction. EXPERIMENTAL Sodium Styrene Sulfonate (NaSS) NaSS was obtained from Air Products and used as received. HPLC analysis indicated only a very small amount of organic impurities, and a check of the salt content was consistent with the 44.5% NaBr and Na2S0, reported by Air Products. Styrene (St) All copolymerizations were carried out with freshly distilled St. Water Distilled water was deareated before use by boiling and then cooling in a nitrogen atmosphere. Sodium dodecyl sulfate (SDS), potassium persulfate (K2S20s), sodium thiosulfate (Na2S205), and n-dodecylthiol (C12H25SH) were used as received. Copolymerization Procedures Capped Bottles A small glass pressure bottle was charged with a,small magnetic stir bar to enhance agitation, 25 g of St (0.24 moll, 1.0 g of NaSS ( mol), 60 ml of H20, 0.1 g of C12SH, 1.6 g of SDS, and 0.1 g of K2S208. The flask was purged with nitrogen and capped with a two-hole crown cap containing a rubber septum. The bottle was placed into a safety screen in a thermostatted Erbach water shaker bath at 50 C and was agitated for a 6 h polymerization time. The bottle was removed and ca. 3 ml of a hydroquinone %hortstop solution was added via a syringe. The bottle was shaken for an additional 10 min, cooled, and opened. The polymer was coagulated by pouring the reaction solution into about 250 ml of methanol with some NaCl added. The very fine precipitate was difficult to filter, thus the slurry was centrifuged at ca rpm for about 45 min. The solid polymer was washed two to three times with distilled water and brought down by centrifugation each time. About 15 g (after drying under vacuum at 40 C) of white powder were obtained by this procedure. This represented about 58% conversion. Evaporation of the washes yielded the remainder of the expected product. These washes, of course, were contaminated with the surfactant and NaCl used to assist in coagulating the emulsion. Elemental analysis of the coagulated portion was 0.71% S. A control without NaSS present gave a S analysis of 0.21%. Therefore, the polymer actually contained 0.50% S, which is equiv-

3 EMULSION COPOLYMERIZATION 537 alent to 1.57 mol % NaSS incorporation. The evaporated fraction was found to contain 1.21% S. 2-L Flasks The-reaction was scaled from the pressure bottles to a 2-L flask, and the components were charged in the same proportion as in the pressure bottles. This amounted to a lox scale-up, i.e., 16 g of SDS, 10 g of NaSS, 600 ml of H20, 250 g of St, 1.0 g of C12H25SH, and 1.0 g of K2S208. Agitation was provided by an overhead stirrer, and the system was carefully purged with N, and kept under a N, purge during the course of the reaction. The shorter induction periods were found when the reaction solution was sparged with N2 for ca. 15 min before bringing the reaction to 50 C. Heat was provided via a thermostatically controlled oil bath. For kinetic studies the initiator was added when a constant temperature of 50 C had been reached. Reactions were stopped after about 6 h and quenched with a hydroquinone solution. Yields and compositions were identical to those from the bottle reactions. It was discovered that these reactions develop a vigorous exotherm and a rapid temperature rise to C. 1-L Adiabatic Flask The copolymerization was done near adiabatic conditions in a l-l doublewalled, silvered, three-necked flask. The reaction contents, without initiator, were heated externally to about 55 C with nitrogen purge and then added to the flask which was also under a nitrogen purge. This procedure resulted in a temperature of 53 C for the reactants. The initiator was added at this time, and stirring was maintained along with a slow nitrogen purge. It was necessary to use the two-component, K2S208 Na2S205, initiator system in order to minimize the induction period caused by 0, picked up during the transfer of the heated reaction emulsion. In addition, in order to avoid the potential of boiling the reaction was intentionally diluted from that of the original charge in capped bottles and 2-L flasks above. In this configuration temperature was monitored or small samples were taken periodically. The reactant charges for these reactions were 500 ml of H20, 125 g of St, 5.0 g of NaSS, 8.0 g of SDS, 0.5 g of C12H25SH, 0.25 g of K2Sz08, and 0.25 g of Na2S205. RESULTS AND DISCUSSION The copolymerization of St and NaSS has been the subject of previous studies, but only for preparing lattices for various purposes.loj1 The NaSS was added to the styrene emulsion either to form a more stable latexlo or in the second case, which was an emulsifier free system, to control surface charge density and size of the polystyrene particles." In this study higher levels of NaSS were utilized, up to about 40 mol % NaSS, and the copolymerizations were done with the surfactant sodium dodecyl sulfate (SDS) as the emulsifier. As has been reported earlier the copolymerization could be effected with either water soluble initiators such as potassium persulfate or a potassium

4 538 TURNER, WEISS, AND LUNDBERG persulfate and sodium thiosulfate mixture or oil soluble/water soluble redox couples such as diisopropyl benzene hydroperoxide/ trimethylenetetramine. Similar copolymer compositions were obtained in both cases.8 All copolymerizations in this study were done with the water-soluble systems. Composition Versus Comonomer Charge The NaSS incorporation into the S-PS copolymer increases as the amount of NaSS in the comonomer charge is increased (Figs. 1 and 2.) This increase in NaSS content is reflected in the polymer properties. For example, as the NaSS charge is increased, the copolymer becomes more dispersable in water and more difficult to work up. The dispersed phase (Fig. 2) can be separated from a precipitated phase (Fig. 1). As expected, the dispersed phase contains a higher level of NaSS than the precipitated phase. This heterogeneity may proceed to such a degree that significant amounts of water-soluble polymer product are formed. This is suggested by the asymptotic behavior shown in Figure 2. The properties of these copolymers are discussed elsewhere.8 Copolymerization Chemistry It was expected that with a high St charge ratio (96 wt % St) in the emulsion copolymerization of NaSS and St that the water soluble NaSS comonomer would not be located in sufficient concentration at the locus of polymerization of the hydrophobic particle to influence significantly the reaction kinetics, even though, as shown in this study, NaSS can readily 10.0 U Y I t -1 0, I I I I I MOL % NaSS IN FEED Fig. 1. Plot of mol % NaSS in copolymer from precipitated phase versus mol % NaSS in copolymerization feed for 21 reaction, standard condition from Experimental section.

5 EMULSION COPOLYMERIZATION I I I I MOL % N.SS IN FEED Fig. 2. Plot of mol % NaSS in copolymer from dispersed phase versus mol % NaSS in copolymerization feed for 21 reaction, standard conditions from Experimental section. be incorporated into the backbone. Our observations indicate otherwise, namely that inclusion of these small amounts of NaSS into the copolymer result in a dramatic alteration of the polymerization kinetics. To understand the origin of this effect, we have studied this reaction in some detail, and the results are reported in the following discussion. When NaSS and St are copolymerized under emulsion conditions, as outlined in the Experimental section, a vigorous exotherm is noted that is significantly faster and reaches a higher temperature than the exotherm observed for the corresponding styrene polymerization control. These observations are plotted in Figure 3. The temperature peaks and then drops because of the fast heat dissipation from the 2-L glass flask. This exothermic behavior suggests a very fast copolymerization reaction, and this has been verified by following the rate of disappearance of NaSS by HPLC and by gravimetric determination of reaction conversion. These data are plotted in Figures 4 and 5. Figure 5 also shows a comparison of the copolymerization versus homopolymerization. From these data overall rates of polymerization can be calculated. For the copolymerization a value of 16.7 x mol/l s was obtained compared with 2.0 x mol/l s

6 540 TURNER, WEISS, AND LUNDBERG Time (hn.) Fig. 3. Plot of reaction temperature versus time for (- -) St/NaSS copolymerization (a) and St (-Cl-) homopolymerization (b) in 21 reaction standard conditions from Experimental section. for the homopolymerization. We have considered two possible mechanisms that could contribute to this enhanced rate of the copolymerization of NaSS and St over the homopolymerization of styrene: gel effect and particle size. Gel Effect One possible mechanism for this unique kinetic effect would involve an early onset of a gel effect in the growing polymer particles. The large Time (minutes) Fig. 4. Plot of NaSS as determined by HPLC in reaction mixture versus reaction time for 21 reaction, standard conditions, from Experimental section.

7 EMULSION COPOLYMERIZATION 541 Time (minuter) Fig. 5. Plot of conversion (gravimetric determination) versus time for St/NaSS copolymerization (+) run 1, (-C-) run 2, and for St homopolymerization ( ). rate difference in the copolymerization vis-a-vis the homopolymerization would originate from the associations of the metal sulfonate groups in the growing polymer particles. As the apparent molecular weight of the copolymer increased due to these associations, the termination rate would be lowered, and thus there would be an increase in the rate of polymerization as compared with the homopolymerization of styrene where these associations are not present. The rate of polymerization is given by where k, is the rate constant for propagation, k, is the rate constant for termination, f is initiator efficiency, kd is the rate constant for initiator decomposition, and [I] and [MI are concentrations of initiator and monomer, respectively. Inspection of this equation clearly shows that a decrease in k, increases Rp The occurrence of gel effects in emulsion polymerizations is well documented. Early reports include the work of Gerrens12 on styrene, van der H0ffl3 on styrene, and ZirnmtI4 on methyl methacrylate. Styrene was shown to exhibit this effect at about 60% conversion. All styrene emulsion polymerizations have been said to exhibit gel effects noted by a change in slope of the conversion versus time curve at 5540% conversion. Recent work has been extended in developing models for understanding these effects for glassy polymers in terms of a critical molecular weight for chain entanglements that lead to reduction in diffusion controlled terminati~n. ~. ~ The idea of ionic association enhancing the gel effect in the copolymerization system appears to be consistent with these models. A low critical molecular weight, because of the associations, would lead to an earlier onset

8 542 TURNER, WEISS, AND LUNDBERG of the gel effect and thus a higher exothermicity and a faster reaction rate. An inspection of Figure 6 shows the effect of the charge of NaSS on the copolymerization kinetics as measured by exothermic temperature. The copolymerizations were done with the standard surfactant concentration. As the NaSS concentration is increased, small but significant rate enhancements are noted. It is interesting to note that even at a NaSS concentration of 1.0 g or 99.5% molar ratio of styrene, a considerably faster reaction is observed than that for styrene. These data are consistent with the gel effect explanation in that higher ionic content should correspondingly lead to an increase in intermolecular association and an earlier entry of the polymer particle into a gel state. Also consistent with the gel effect explanation is the observation of. an increase in molecular weight, as measured by viscosity, as the NaSS level was increa~ed.~ Attempts have been made to plasticize the growing polymer chain in a styrene emulsion with known plasticizers for styrene, e.g., octane, Nujol, and dioctylphthalate, and with various low-molecular-weight compo~nds. ~J~ In both cases the authors observed rate decreases and attributed these to phase separation in one and to moderation of the gel effect in the other study.18 However, it is noted that these conclusions have recently been questioned by Azod, Fitch, and Haynes.lg In recent years it has been shown that it is possible to plasticize the ionic associations that are present in lightly sulfonated S-PS2 For example, these polymers can be swollen in an aromatic solvent and then addition of an alcohol can break the association and bring about solution. Alcohols, acids, amides, etc., have been shown to be effective in diminishing the rubbery I I Time (minutes) Fig. 6. Plot of reaction temperature versus reaction time for St/NaSS copolymerization at various NaSS feeds as measured in adiabatic reactor as described in the Experimental section: ( g of NaSS, ( g of NaSS, ( NaSS. - - ) 2.5 g of NaSS, ( ) 1.0 g of

9 EMULSION COPOLYMERIZATION 543 plateau exhibited by S-PS. Recent work by LundbergZ0 has established the concept of internal plasticization by incorporating a termonomer such as acrylamide. We have thus investigated the copolymerization in the presence of known plasticizers to see if the reaction rate is moderated. In addition, we have attempted terpolymerizations to see if internal plasticizations occurs in the growing polymer particles. We have observed that addition of hexanol or diluted phthalate, both known ionic plasticizers, show no effect on the copolymerization. We believe that these compounds never really are dispersed in the polymer particles and are either in the monomer droplets or emulsified separately, and hence they have no effect. On the other hand, incorporation of acrylamide or acrylic acid as termonomers into the polymerization leads to a significant decrease in the reaction exotherm in the nonadiabatic reaction. Under adiabatic conditions this is shown as a decrease in the copolymerization rate as measured by rate of temperature increase in Figure 7. This result is consistent with a picture of ionic associations governing the reaction kinetics. The plasticization of the ionic associations thus could raise the critical molecular weight for onset of the gel effect, and, therefore, slower overall rate is observed. Particle Size However, before we can say with certainty that the gel effect contributes to all or to even part of the rate enhancement, we must consider the effect of particle size and number of the copolymerization kinetics. In the classical Smith-Ewart scheme of emulsion polymerization, the rate of polymerization is proportional to the number of polymer particles formed, 8E 75 i I I I I -0,- StyINaSSIAA.&..O.O...o..O...o...o StylNaSSlAm o 70. f 1 65 I I I I I Time (minutes) Fig. 7. Plot of reaction temperature versus reaction time for St/NaSS copolymerization, St/NaSS/Am, and St/NaSS/AA terpolymerization and St homopolymerizations as measured in adiabatic reactor as described in the Experimental section.

10 544 TURNER, WEISS, AND LUNDBERG i.e., the greater the number of polymerizing sites the faster the polymerization. This is represented by R, = Nkp[M]/2 (2) where R, is the rate of polymerization, Nis the number of polymer particles, k, is the rate constant of polymerization, [MI is the monomer concentration, and the one-half comes from the theory that one-half the particles are polymerizing at any one time. Since we have shown that the copolymerization rate is about eight times faster than the rate of homopolymerization, it should require eight times the number of particles present in the copolymerization reaction to account for the rate enhancement. Electron micrographs of emulsions from the homopolymerization and a corresponding copolymerization, done under identical conditions with the addition of NaSS, are shown in Figures 8 and 9. Analysis of these pictures clearly indicates the larger size of the homopolymer emulsion, however, actual analyses show the copolymer particles to have a mean size of 39 nm (+ 22 nm) and the homopolymer to have a mean size of 54 nm (+ 53 nm). Although we do not have a direct measure of the number of particles, the size differences do not appear to be sufficient to account for the entire rate enhancement. These data also indicate that the copolymerization yields a much narrower particle size distribution than the homopolymerization. These distributions are plotted in Figure 10. Since the number of polymer particles in an emulsion polymerization is dependent on the surfactant concentration, we have increased the surfactant level in the styrene homopolymerization so that the total molar concentration of SDS is equivalent to the molar concentration of SDS and NaSS in the copolymerization. As expected, the rate of homopolymerization in- Fig. 8. Electron micrograph of St/NaSS copolymer emulsion after dialysis.

11 EMULSION COPOLYMERIZATION 545 Fig. 9. Electron micrograph of St homopolymer emulsion after dialysis. creased (Fig. 11). However, it remained significantly below that observed for the copolymerization. Substitution of sodium-p-toluene sulfonate (NaTos) for NaSS also led to some rate enhancement for styrene, but this rate was also significantly below the copolymerization rate (Fig. 11). The experiments with NaTos show that two other potential rate-controlling events are not operative in the copolymerization. The first of these is a rate increase due to an enhanced initiator decomposition due to the presence of an aromatic sulfonate group. It would be expected that this effect would be the same for NaSS or NaTos, and this clearly is not the case. A second consideration is that the comonomer-nass-somehow changes the activity of styrene in the monomer droplets in a manner described by Azod, Fitch, and Haynes et al.19 Here again the same effect would be expected for NaTos, and this was not observed. The level of SDS also has an effect on the rate of the copolymerization. Particle Size Distribution 70 Diameter (nm) Fig. 10. Particle size distributions for (0) St/NaSS copolymer (Latex 94) and (0) St homopolymer (Latex 97).

12 546 TURNER, WEISS, AND LUNDBERG I I J Time (minuter) Fig. 11. Plot of reaction temperature versus reaction time for St homopolymerization with various surfactant levels and with sodium tosylate as measured in adiabatic reactor as described in the Experimental section: (-O-) with extra NaLS, (+) with NaTos + NaLS, (+I standard NaLS. Lowering the surfactant level at constant monomer ratio and concentration led to a depressed rate and also a lower exothermic maximum temperature. This was reflected in a lower copolymer conversion (Fig. 12). In our studies copolymerization occured in the absence of surfactant, however, at a much slower rate. Flocculation also was observed during the reaction. Initiation Mechanism In obtaining conversion data on these fast copolymerization, we have obtained samples from the early reaction stages. The sulfur content was observed to be high and to decrease on conversion. The observation of smaller polymer particles vis-6-uis the homopolymerization (and these composition data) suggest a mechanism for initiation that we envision as follows. The water-soluble persulfate initiates some NaSS in the aqueous phase which yields NaSS oligomers. These begin to capture styrene in the early stages of the polymerization and very quickly aggregate to stabilized particles, similar to the mechanism proposed by Krieger in an emulsifier free copolymerization. This would be a type of homogeneous nucleation as proposed by Fitch.21 It is possible that such particles could be much smaller than particles without the NaSS present due to the hydrophilic nature of the S03-Na+ groups which, if located at the H,O/polymer interface, could lead to a stable particle with the aggregation of a relatively small number of chains. This argument has been proposed in a recent paper on seeded emulsion copolymerization of water soluble/nonsoluble vinyl monomers.22

13 EMULSION COPOLYMERIZATION 547 q..o-o limo (minutor) Fig. 12. Plot of reaction temperature versus reaction time for St/NaSS copolymerization with various surfactant levels: ( ) 8.0 g of NaLS, (+) 4.0 g of NaLS, (- -A-) 2.0 g of NaLS. CONCLUSIONS The copolymerization rate of St and NaSS in emulsion is dramatically enhanced over that observed for the homopolymerization of styrene. Two mechanisms, completely different in nature, are proposed to account for these rate differences. An increase in the number of polymerizing particles in the copolymerization with thus subsequent rate enhancement, was substantiated by electron microscopy. However, available data indicate that the rate differences are too large to be fully accounted for by this effect. Therefore, it seems reasonable to propose a gel effect over that for the styrene homopolymerization as a significant contributor to the rate enhancements observed. This gel effect would be manifested by intermolecular association of incorporated metal sulfonate units in the growing polymer particles. Terpolymerization data with acrylamide and acrylic acid, where internal plasticization of the ionic association depresses the reaction rate, are consistent with the gel effect occurring in the styreneinass copolymerization. The authors acknowledge the contribution of Dr. E. B. Prestridge for electron microscopy, Dr. W. Schulz for HPLC, and by N. Brown for technical assistance. References 1. R. D. Lundberg, H. S. Makowski and L. Westerman in Ions in Polymers, A. Eisenberg, Ed., Adv. Chem. Ser. No. 187, Am. Chem. Soc., Washington, DC, 1980, p R. D. Lundberg and H. S. Makowski, J. Polym. Sci. Polym. Phys. Ed., 18, 1821 (1980).

14 548 TURNER, WEISS, AND LUNDBERG 3. M. Rigdahl and A. Eisenberg, J. Polym. Sci. Polym. Phys. Ed., 19, 1041 (1981). 4. D. J. Yarusso, S. L. Cooper, G. S. Knopp, and P. Georgopoulos, J. Polym. Sci. Polym. Lett. Ed., 18, 557 (1980). 5. Y. Okamoto, Y. Ueba, N. F. Dzhanibekov, and E. Banks, Macromolecules, 14,17 (1981). 6. D. Brenner and R. D. Lundberg, U. S. Pat. 4,186,163 (January 29,1980) to Exxon Research and Engineering Company. 7. H. S. Makowski, R. D. Lundberg, and G. H. Singhal, US. Pat. 3,870,841 (March 11, 1975) to Exxon Research and Engineering Company. 8. R. A. We&, S. R. Turner, and R. D. Lundberg, J. Polym. Sci. Polym. Chem. Ed., to appear. 9. R. A. Weiss, S. R. Turner, and R. D. Lundberg, J. Polym. Sci. Polym. Chem. Ed., to appear. 10. V. D. Floria, U. S. Pat. 2,971,935 (February 14, 1961) to Dow Chemical Co. 11. M. S. Juang and I. M. Krieger, J. Polym. Sci. Polym. Chem. Ed., 14, 2089 (1976). 12. H. Gerrens, Z. Electrochem., 60, 400 (1956). 13. B. M. E. van der Hoff, J. Polym. Sci., 33, 487 (1958). 14. H. S. Zimmt, J. Appl. Polym. Sci., 1, 323 (1959). 15. D. C. Sundberg, J. Y. Hsieh, S. K. Soh, and R. F. Baldus, Am. Chem Soc. Symp. Ser., 165, 327 (1981). 16. B. Harris, A. E. Hamielec, and L. Marten, Am. Chem. SOC. Symp. Ser., 165, 313 (1981). 17. D. R. Owen, D. McLemore, W. Liu, R. B. Seymour, and W. N. Tinnerman, Am. Chem. SOC. Symp. Ser., 24, 299 (1976). 18. D. C. Blackley and A. C. Haynes, Br. Polym., 9, 312 (1977). 19. A. R. M. Azod, R. M. Fitch, and A. C. Haynes, Am. Chem. Soc. Symp. Ser., 165,357 (1981). 20. R. D. Lundberg, unpublished results. 21. R. M. Fitch, Am. Chern. SOC. Symp. Ser., 165, 1 (1981). 22. C. Hagiopol, V. Dimonie, M. Georgescu, L. Deaconescu, T. Deleanu, and M. Marianescu, Acta Polym., 32,390 (1981). Received June 13, 1984 Accepted July 26, 1984