Langmuir 1993,9, 1220-1227
1220
Surface Chemistry of Emulsion Polymerization Marilyn E. Karaman, Laurence Meagher, and Richard M. Pashley' Department of Chemistry, The Faculties, Australian National University, Canberra, ACT, Australia Received July 20,1992. In Final Form: February 19,1993
The emulaion polymerization of styrene hae been studied using several surface chemical techniques. Evidence has been obtained from dye adsorption, latex morphology, and microelectrophoresis studies, which indicate that two quite different mechaniema can operate, dependingon the process conditions. Use of a water-insoluble initiator appears to favor micellar nucleation, whereas persulfate initiator tends to produce latex via homogeneous nucleation in the aqueous phase. An atomic force microscope adapted to measure surface forcea was used to study the interactions between polystyrene in water and surfactant solutions. The strong hydrophobic attraction observed presenta a likely explanation for the buildup of latex particles in the homogeneous nucleation process.
Introduction Emulsion polymerization processes have been extensively employed by the chemicals industry since the early production of rubber substitutes during the second world war. Today the process is used commercially for the polymerization of a wide range of monomers such as vinyl acetate, styrene, chloroprene,and several acrylates. Major products include automotive tires, adhesives, and latex paints.' Despite its obvious industrial importance, the mechanism of the polymerization process, which occurs within particles dispersed in an aqueous medium, is not fully understood. The particles are stabilized by surfactants, sometimes with added low molecular weight watersoluble polymers. The problem of a rapid increase in viscosity produced during bulk polymerization is then removed in the "emulsion" process, and thermal control of the exothermic reaction is easily obtained. The result is that in emulsion polymerization a low-viscosity (latex), high molecular weight polymer can be produced at rapid reaction rates. The polymer latex particles formed by this process are usually fairly spherical and about 0.1 pm in diameter. The latex may be used in this colloidal solution form (e.g., for paints or coatings) or can be further processed for the fabrication of plastics and rubbers. Because the process involves the stirred mixing of reactive monomer oils in water with the addition of surfactant, it has been misnamed "emulsion polymerization". It has been extensively demonstrated that in this general process polymerization does not, at any stage, occur within the relatively large (>lo pm) monomer oil droplets produced by vigorous stirring. That this is the case was first pointed out by Harkins2 using the observation that the latex particles produced were much smaller than the dispersed monomer droplets. Following the studies of Heller and Klevens3 on the influence of surfactant concentration and latex particle density, Harkins4 proposed that (monomer-swollen) surfactant micelles were the polymerization sites. Smith and Ewart5 used this model to quantitively describe rates of emulsion polym(1) Odian, G. Principles of Polymerization, 2nd ed.; John Wiley & Sons: New York, 1981. (2) Harkins, W. D. J. Chem. Phys. 1945, 13, 381. (3) Bassett, D. R., Hamielec, A. E., Eds. Emulsion Polymers and Emulsion Polymerization; ACS Symposium Series; 165; American Chemical Society: Washington, DC, 1981. (4) Harkins, W. D. J. Am. Chem. SOC.1947,69, 1428. (5) Smith, W. V.; Ewart, R. H. J. Chem. Phys. 1948,16,592; J. Am. Chem. SOC.1948, 70,3695; J. Am. Chem. SOC.1949, 71, 4077.
erization, and this model encapsulates the basis of the conventional understanding of the process. For processes involving surfactants above their critical micelle concentration and with monomers of low water solubility, this swollen aggregate polymerization model does seem appropriate. The number density of micellar species far outweighsthat of dispersed monomer droplets6 and statistically should dominate the initiation process. However, another factor that can play a role is the type of initiator used. For example, the commonly used persulfate thermal initiator generates sulfate ion radicals in the aqueous phase. We would not expect this ion to penetrate the swollen micelles, which are often aggregates of highly charged anionic Surfactants. However,the radical may, at a rate dependent on monomer solubility, react with several dissolved,monomer molecules and effectively produce a radical anionic surfactant. This active surfactant can readily exchange with micellar surfactant and hence initiate polymerization within the swollen micelle. Although this mechanism is appealing in its simplicity and does go some way toward explaining the relationship between latex density and surfactant concentration, an alternative method based on homogeneous nucleation in the aqueous phase has also been postulated. As early as the 194Os, Baxendale et aL7showed that the more watersoluble monomer methylmethacrylate can be polymerized in the absence of added surfactant. Although, it should be realized that the initial polymerization stage may itself produce surfactant-type molecules which may self-assemble to form micelles. However, recent work on molecular weight distributions8 and the rate of emulsion polymerizationghas led to further support for the homogeneous nucleation model, even for micellar systems. By contrast, unambiguous evidence for micellar polymerization has also been reported.1° In fact, there are difficulties which arise for both the micellar and homogeneous nucleation models. For example, in the micellar case it is hard to understand how (6) Kine, B. B.; Redlich, G. H. Surfactant Science Series; Marcel Dekker: New York, 1988, Vol. 28, Chapter 8. (7) Baxendale, J. H.; Evans, M. G.;Kilham, J. K. Trans. Faraday SOC. l946,42,668Baxendale, J. H.; Bywater, S.;Evans, M. G. Trans,Faraday SOC.1946, 42, 675. (8)Whang, B. C. Y.; Ballard, M. J.; Napper, D. H.; Gilbert, R. G. Auat. J. Chem. 1991,44, 1133. (9) Feeney, P. J.; Napper, D. H.; Gilbert, R. G . Macromolecules 1984, 17, 2520. (10) Lyons, C. J.; Elbing, E.; Coller, A. W.; McKinnon, I. R.; Wilson, 1. R. Chem. A u t . 1992, Feb, 66.
0743-7463/93/2409-1220$04.00/0 0 1993 American Chemical Society
Surface Chemistry of Emulsion Polymerization
the latex particle grows from one micelle, when the fiial particle may typically contain over 100 polymer chains. An "on-ofr mechanism has been suggested,ll where a second radical entry into the swollen micelle terminates the firat and so on. But since the initial micellar density is much higher than the f i i latex particle density, why should only relatively few micelles grow? On the other hand, the homogeneous nucleation models have to address the problem of precursor polymer particles produced in aqueoussolution. These coagulateto produce swollen latex particles which further polymerize to form the final latex. The small (90°) of polystyrene. Since the polystyrene latex particles were prepared in a different proceea, a comparison of forces was made under the eame conditions (i.e., in water), and the results are shown in Figure 8. A very similar jump distance was observed. For latex particle probes the larger radius can be more accurately measured, and so the forces in Figure 8 are scaled with the mean radius of the probe particle. In this form it is possible to compare the force results with theory using the Derjaguin approximation, which should be valid for these size particles. In this figure we have also plottad the expected van der Waals attraction potential
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Figure 10. Forces measured between a polystyrenelatex particle and a flat sheet immersed in 8 X lo-' M SDS solution (i.e., 10% of the cmc value). Although the repulsive forces are slightly stronger, the surfaces were again pulled into contact with a force stronger than the van der Waals force (the solid line is the DLVO best fit).
for the polystyrene/water/polystyrene system. The surfaces were clearly pulled together by a much longer range force. Addition of the anionic surfactant sodium dodecyl sulfate (SDS)to the aqueoussolution substantiallychanges the forces between polystyrene surfaces. In Figure 9 the force measurementswere obtained at an SDS concentration of 2.5% of the cmc value (8 X 10-3M).Under these conditions the surfactant clearly adsorbs to the polystyrene, producing a surface charge density of about -0.0052 C m-2 and a surface potential of about -95 mV. The repulsive forces can be explained at separations greater than about 10-20 nm by the nonlinear Poisson-Boltzmann equation for this surface potential and the expected Debye length for this solution (-20 nm). However, the van der Waals attraction should only be capable of pulling the surfaces together at very short range (-2 nm), whereas a much stronger attraction was observed pulling the surfaces together from a separation of about 12nm. These results clearly indicate adsorption of a submonolayer of SDS at this concentration, which gave rise to an increase in charge but still with a significant hydrophobic nature. Increasing the SDS to 10%of the cmc gave interaction forces, shown in Figure 10,of a similar nature, although with an increased magnitude of surface charge (-0.021 C m-2) and potential (-130 mV). Again, the charge was not sufficient to overcome the hydrophobic attraction at separations less than about 15 nm. The Debye length in this case was about 10.7 nm. It is of some interest to note that, in these two cases of partial SDS coverage, addition of electrolyte would completely remove the long-range
Langmuir, Vol. 9, No. 5, 1993 1225
Surface Chemietry of Emulsion Polymerization I
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Figure 11. Forces measured betweena polystyrene latex particle and a flat sheet immersed in 8 X le3M SDS (i-e.,at the cmc). Under these conditions the surfaces should be fully coated with a layer of adsorbed SDS molecules. The forces observed are now quite similar to those expected from the DLVO theory (solid line). A small inward jump from the DLVO maximum may also be present but was difficult to measure precisely.
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Figure 12. Forces measured betweenan ill-definedpolystyrenecoated (low-radius)probe anda polystyrene flat surface immereed
in an aqueous solution at the cmc value of SDS. As the lowradius probe approached the surface, an electrostatic repulsive force was detected, but at smaller separations the probe could be forced through the adsorbed SDS layers and an estimate of their thickness obtained.
repulsive component of the forces and the attractive hydrophobic force would then cause latex coagulation. At the cmc the forcesobserved were completely repulsive and are shown in Figure 11. The forces can be quite well described by Poisson-Boltzmann theory using the expected Debye length and a surface charge and potential of -0.017 C m-2 and -65 mV, respectively. No adhesion minimum was observed in this case, although we might have expected a short-range van der Waals attraction as indicated by the theoretical line in the figure. It is possible that hydration effects associated with the sulfate headgroup may produce a short-range repulsive force which would remove this minimum. The "zero" distance on this graph corresponds to the distance obtained on application of relatively large forces and should correepond to a closely packed monolayer of SDS on each polystyrene surface. This expectation was confirmed in a separate experiment using a sharp, but ill-defined (coated),probe pressed against polystyrene and immersed in SDSsolution at the cmc. The results obtained are shown in Figure 12 and indicate that two adsorbed SDS layers had a thickness of about 2.5 nm, which is in reasonable agreement with that expected from the size of the SDS molecule. It is possible to use these techniques to obtain the thickness of adsorption layers because of the high local pressures generated using a probe with a radius of only about 5-10 nm. The charge density of 4.017 C m-2 obtained from the theoretical beat fit to the measured force curves at the cmc was substantially lower than the value obtained
(-0.031 C m-2) under similar solution conditions using microelectrophoresis (see earlier). These differences cannot be explained by experimentalerror but must be related to the methods used in the two experiments. The results are, however, hard to explain in this way because force measurements are normally expected to give the higher values, due to the shear-plane effect on microelectrophoresis or to surface flattening in the force studies. Also, any correction to the Poisson-Boltzmann theory for surface charge density14would be expected to increase the value only by about 25%, which is not sufficient to explain the differences. One poseible explanation might be considered, that a residual hydrophobic interaction reduces the electrostatic repulsion observed. Discussion We have used several interfacial chemistry techniques to investigate the emulsion polymerization process from a viewpoint different fromthat usually taken. The results serve to highlight some problems with current models. Overall, the results support the suggestion that at least two main types of mechanism can operate during emulsion polymerization. This is dramatically illustrated with dye adsorptiontests. Depending on initial reaction conditions, the process may proceed via polymerizationwithin swollen micelles or via the coagulation of precursor, low molecular weight polymers produced by homogeneous nucleation. Both of these models have difficulties in providing a complete description of the process. It has been proposed that the coagulation of small precursor polymer particles, produced by homogeneous nucleation, occurred because of an electrostatic attraction between particles with dissimilar size and surface potentials, but of the same signal5 The variation in potential was assumed to arise because of unequal amounts of adsorbed (charged) surfactant or incorporated sulfate groups (from persulfate initiator) on the numerous small particles. If these particles were fully coated with a layer of adsorbed surfactant, no coagulation would be possible, as is illustrated by the results in Figure 11. It is debatable as to how large a variation in potential might be possible for these precursor particles. But even more serious difficulties arise on closer examination of the origin of this electrostatic attraction. For the case of the electrostaticinteraction between two charged surfaces of unequal (same sign) potentials, it is indeed the case that the Poisson-Boltzmann equation used to describe the diffuse double layer does predict an attractive electrostatic force at small separations (typically, at separations less than a Debye length). But this can only arise if the potentials of the interacting surfaces remain substantially constant as the surfaces come together. Under these conditions, the higher potential surface causes charge reversal on the other interacting surface as it comes into close proximity. This is a valid theoretical prediction, but it is flawed for most real surfaces by the chemical nature of the surface charge. That is, the surface potential is not determined only by the amount of adsorbed (ionic) surfactant, but also by the degree of surface dissociation of the counterion (Na+ in this case). During the interaction process the charged surfaceswill be expected to regulate their potential in response to the approachingfield via the adsorption or desorption of ions. Recent studies have shown that usually surfaces interact under conditions somewherebetween the (14) Attard, P.; Mitchell, D. J.; Ninham, B. W. J. Chem. Phys. 1988, 89 (7), 4358.
(15) Feeney, P. J.; Napper, D. H.; Gilbert, R. G .Macromolecules 1987,
20, 2922.
Karaman et al.
1226 Langmuir, Vol. 9,No. 5,1993 constant surface charge and potential limits.16 Only repulsiveelectrostaticforcesare possiblebetween similarly charged surfaces, even with different magnitudes, a situation that almost certainly obtains for these precursor particles. The alternative is that one particle adsorbs excess Na+ ions and becomes positively charged, which appears likely or even only possible for amphoteric surfaces. Further, rapid collisionsbetween small particles will occur under essentially constant charge conditions because of the relatively slow ion diffusion rates. Hence, only repulsive forces are generated. Also, at the relatively high electrolyte levels used, the partially coated particles will have such low electrostatic potentials that they are insufficient to produce a strong attraction. In this evidentlyartificialelectrostaticheterocoagulation model is unlikely, what then is the mechanism by which nucleated particles grow to form latex? The partial surfactant coverage of the large number of precursor particles would indicate that they will be essentially hydrophobic. In recent years it has been shown by direct measurement that hydrophobic surfacesin water strongly attract over distances up to 100 nm.17 The magnitude of the attraction at short range (
