Surface chemistry of emulsion polymerization

1220 Langmuir 1993,9, 1220-1227

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, depending on 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 exten-

sively 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 water- soluble 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 outweighs that 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 aL7 showed that the more water- soluble monomer methyl methacrylate 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-as- semble to form micelles. However, recent work on molecular weight distributions8 and the rate of emulsion polymerizationg has 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


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 aqueous solution. These coagulate to produce swollen latex particles which further polymerize to form the final latex. The small (<50 A) precursor particles should be stabilized by surface charge and at a least partially covered with surfactant (if present). As wil l be shown later in this paper, current models used to explain the coagulation are probably incorrect and a more likely explanation is proposed.

At this stage it would seem prudent to postulate that the mechanism involved in any given situation will depend on a range of key factors such as monomer solubility, surfactant type and concentration, and initiator. In this study we present some results on the emulsion polymerization procees from the viewpoint of surface and colloid chemistry. We have used several techniques to study the formation of the latex particles. These include dye adsorption and the atomic force microscope adapted to measure latex particle interactions, Our results also have implications for some important properties of latex dispersions.

Langmuir, Vol. 9, No. 6, 1993 1221

Methods and Materials The emulsion polymerizations were carried out in a thee-

necked 500 cm3 round-bottomed flask, fitted with a mechanical stirrer, water condenser, and Nz bleed tube and immersed in a water bath at 70 "C. A typical procedure was to equilibrate the water/surfactant/monomer mixture under N2 at 70 "C with stirring at 350 rpm for '/2 h prior to the introduction of the initiator in a small volume of a suitable solvent (i.e., monomer or water); the reaction was allowed to proceed under these conditions over the next 2l/2 h. Two surfactants were used in these studies, sodium dodecyl sulfate (SDS) and the nonionic Triton X-100.

The resulting latex was examined using a Zeiss 109 transmission electron microscope (TEM), and a Cambridge 5360 or a Jeol 6400 F scanning electron microscope (SEM).

Microelectrophoresis measurements were carried out on latex samples diluted 10 OOO times in 0.1 M KC1 at 25 "C using a Rank Bros particle microelectrophoresis apparatus Mk 11.

All surface force measurements were obtained using a com- mercial atomic force microscope (AFM) produced by Nanoscope, SantaBarbara, CA. This device was adapted to enable the direct measurement of forces between the stylus (or stylus attachment) and a second (usually planar) surface. The displacement of one of the surfaces was varied using a piezoelectric crystal a t a rate in the range 0.2-2 pm s-I. The total force acting between the surface was measured by the deflection of the cantilever attached to the stylus using alaser reflectance system. The spring constant of this cantilever was 0.68 N m-1. Comparison of the deflection of the spring with the piezocrystal displacement gives a separation distance between the surfaces relative to the constant compliance position observed when the surfaces are forced into contact. It was possible to obtain a force sensitivity of less than 0.1 nN distance resolution of about 1-2 A.lz

For the experiments reported, the stylus was coated with either a melted polystyrene latex 5-pm sphere13 or a melted polystyrene droplet. The surface geometries of these samples were in each

(11) Hiemenz, P. C. Polymer Chemistry; Marcel Dekker: New York,

(12) Ducker, W. A.; Senden, T. J.; Pashley, R. M. Nature 1991,353,

(13) Tseng, C. M.; Lu, Y. Y.; El-Aasser, M. S.; Vanderhoff, J. W. J.

1984.

239.

Polym. Sci. A 1986,24, 2995.

Y C O O N a

Figure 1. Fluorescein dye, water-soluble disodium salt, with a maximum absorption at 491 nm.

case obtained after the force experiment using SEM. The mean radius of the stylus sample was required in order to scale the total force observed. The polystyrene probe was measured against a smooth, flat, premelted polystyrene block. The surface of thia block was imaged using the AFM and was found to have a standard deviation of 0.4 nm over a 1OOO-nm square region. Force experiments were carried out a t a temperature of about 25 "C, which is above the Krafft temperature of SDS.

Single-distilled water was used in these experiments for the emulsion polymerizations and for the dye tests. AFM and electrophoretic mobility measurements were carried out in solutions using water purified by activated charcoal and reverse osmosis filtration, followed by distillation and storage in a filtered air laminar flow cabinet. All other chemicals were of analytical grade and used as purchased.

Results and Analysis

Dye Adsorption. Several polystyrene latex solutions were prepared under conditions which might be expected to proceed via either micellar or homogeneous nucleation. The same dye, fluorescein, was added to each reaction mixture prior to the polymerization step. This dye was chosen because of its chemical structure and high water solubility in the disodium salt form, used in these experiments. The structure of the disodium salt form is illustrated inFigure 1. Under the slightly acidic (pH -2- 3) aqueous solution conditions produced in the unbuffered emulsion polymerization with persulfate initiator, the alcohol group should be fully protonated, since the pK. values for this group is about 8 and the carboxylate group around 4. In order to remove any possibility of dye solubility variations (and hence adsorption), these experiments were also repeated at a controlled pH of about 7-8, and no differences were observed.

The presence of this particular dye molecule in the emulsion mixture for the production of polystyrene might be expected to give some interesting insight into the mechanism. This is due to the fact that the dye is not a surfactant and so will not be readily incorporated into micellar aggregates. However, we might expect some significant adsorption of the dye onto any oligomeric styrene species present in the aqueous phase. The behavior of the dye in the two extreme casea of micellar and homogeneous nucleation may then be quite markedly different.

In the first method, the styrene monomer was added to the surfactant/water/dye mixture and allowed to equilibrate for '/2 h before the introduction of the water-soluble potassium peradfate initiator. In the second method, the mixture was allowed to equilibrate for '/2 h prior the introduction of the oil-soluble 2,2-azobis[2-methylpropionitrile] and lauroyl peroxide initiators dissolved in a s d volume of the styrene monomer. In each case the reaction was then allowed to proceed for 2 V 2 h.

Both methods produced opaque yellow latex suspensions for SDS andTriton X-100 surfadant, but when these latex solutions were dialyzed in distilled water, quite different dye leaching properties were observed. Fluorescent green-

1222 Langmuir, Vol. 9, No. 5, 1993 Karaman et al.

yellow dye appeared to leach into the distilled water immediately for the oil-soluble nitrile/peroxide initiated latex, whereas no trace of dye leaching was observed for the water-soluble potassium persulfate initiated system, even after several weeks.

These results suggest that the reaction initiated via the water-soluble potassium persulfate proceeds via homogeneous nucleation and that these precursor particles of oligomeric styrene adsorb the dye. The dye then becomes permanently entrapped in the growing latex particle. By comparison, it is likely that in the oil-soluble nitrile/ peroxide initiated system the site of polymerization occurs inside the swollen micelle from which the water-soluble dye is totally excluded. The dye remaining in the aqueous phase easily passed through the dialysis membrane.

Although these results are by no means definitive, they do support the hypothesis that, under different conditions, two quite separate emusion polymerization processes are possible.

Microelectrophoresis and Electron Microscopy. Although the average size of typical latex particles is often below 1 pm, a significant but small proportion are always in the larger size range and can be observed in a dark-field illumination microscope. It is also reasonable to assume that the surface chemistry of these larger particles is identical to that of the majority and can be studied by microelectrophoresis. In this process, the average speed of the particles is determined for motion caused by an applied electric field. For micrometer-sized latex particles in a 0.1 M KC1 aqueous electrolyte solution, we can apply the Smoluchowski equation:

r = pr l /ep (1) where the particle's surface electrostatic potential (0 is simply related to the electromobility (p ) of the particle, solution viscosity (a), and permittivity (ea). This surface potential can in this case be related to the particle surface charge density (a) via the relation

Figure 2. SEM micrograph of one of the larger sized latex particles present in the sample produced by the emulsion polymerization of styrene using Triton X-100 as emulsifier and persulfate as initiator. The average particle size was about 0.1 m.

a = sinh(qJ2kT) (2)

where PB is the bulk concentration of (monovalent) electrolyte and qe is the positive value of the electronic charge.

The {potential and corresponding surface charge density were measured as a function of pH for a polystryrene latex formed using only nonionic surfactant (Triton X-100) as stabilizer. This surfactant was used so that the fiial charge density on the particle surface must be due solely to the sulfate groups produced by the potassium persulfate thermal initiator. The latex particles produced by this method were studied using both SEM and TEM. An SEM micrograph of one of the larger latex particles is shown in Figure 2. These particles are clearly structured and so seem to have been formed via coagulated precursor particles, consistent with the dye results. Higher magnification TEM also clearly demonstrated the internal structure of these latex samples. The formation structure is probably retained because of the relatively high glass transition temperature (- 100 "C) of this polymer. In the following section we investigate the process by which the precursor polymer particles aggregate to form the final latex.

Latex produced by the oil-soluble initiator was also studied, and a typical SEM micrograph of the larger particles is shown in Figure 3. The latex particles produced appear smooth, spherical, and dimpled, and were quite different from the latex produced using the water-soluble initiated system. High magnification TEM micrographs

Figure 3. SEM micrograph of one of the larger sized latex particles in the sample produced by the emulsion polymerization of styrene using Triton X-100 with lauroyl peroxide and 2 3 mobis[ 2-methylpropionitrile] as initiator. The average particle size was about 0.1 pm.

showed no intemal structure. We believe that the latex produced using the oil-soluble initiated system goes via the micellar model, where the loci of polymerization are located inside the swollen micelles.

The potentials obtained by microelectrophoresis measurements on the Triton X-lOO/persulfate latex, as a function of pH in 0.1 M KCl, are shown in Figure 4. To carry out these measurements at a reasonable particle density, the original latex was diluted 10 0o0 times in 0.1 M KC1. At this level of electrolyte the Debye length of the solution would be 9.6 A, which is small compared with the particle radius (of about 1 pm) and hence validates the use of the Smoluchowski equation. The results shown in Figure 4 indicate that a plateau in potential in the pH range 4-6 should correspond to fully ionized sulfate groups on the latex surface. At higher pH values the ethylene oxide-alcohol groups on Triton X-100, adsorbed at high density on the surface of the latex, will begin to ionize with a pKa above 10. Under strongly acidic conditions the sulfate groups will begin to protonate, as observed at low pH values (below 3). Similar < potentials were also obtained in 0.1 M NaC1.

The observed plateau in potential corresponds to a surface charge density of about -0.015 C m-2. If we accept this value as a resonable estimate of the density of charged

Surface Chemistry of Emulsion Polymerization Langmuir, VoZ. 9, No. 5, 1993 1223

0

-5

-1 0

g - 1 5 v - I -

-20 - 0 G m - -25 ;

-30

-35

- 4 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 , , , 1 , ,

0 1 2 3 4 5 6 7 8 9 1 0 1 1 1 2

PH Figure 4. f potentials of the Triton X-l00/persulfate latex as a function of pH in aqueous 0.1 M KCl solution.

sulfate groups on the latex surface, then a comparison can be made with approximate values expected from the two emulsion polymerization models, namely, homogenous nucleation and micellar aggregation. An estimate of the average molecular weight of the Triton X-100 stabilized polystyrene latex was obtained by viscosity measurements in toluene solution. Using the Mark-Houwink equation, the average molecular weight was found to be 58 000. Using the average polystyrene molecular weight and the value of its density, the average packed globular size of each p o 1 . r chain is estimated to have a diameter of about 57 and a cross-sectional area of 2500 Hi2. Each polymer chain must have two sulfate groups which will remain at the surface of the growing swollen micelle in the micellar model, and hence all of the initiating and terminating sulfate groups would be expected to remain at the surface of the final latex particle. Since the average latex diameter is known (about 0.1 pm), the number of sulfate groups present in the latex can be estimated. From this calcu- lation the expected latex charge density would be about -0.055 C m-2, which is substantially greater than that observed. For comparison, we have also carried out { potential measurements on polystyrene latex fully coated with adsorbed SDS (at the critical micelle concentration (cmc)). Under these conditions in 0.1 M NaCl the maximum charge density in the pH range 4-6 was found to be about -0.031 C m-2, which must represent the upper limit in latex charge. Although it is difficult to estimate the expected final latex charge for the homogeneous nucleation case, we would expect it to be significantly lower than for the micellar case. The results strongly suggest that latex produced using water-soluble initiator, even in micellar surfactant solution, can be formed via the homogeneous nucleation mechanism.

Interaction Forces between Polystyrene Latex Particles. Any homogeneous nucleation model of emulsion polymerization must be critically dependent on the mechanism by which the precursor particles fuse together prior to further polymerization. Forces acting between

- L 2 4 '4 .f &- ..: 4

Figure 5. SEM micrograph of an AFM silicon nitride stylus covered with previously molten polystyrene.

Figure 6. SEM micrograph of several polystyrene latex particles attached by partial melting to the AFM stylus. The mean radius of the interacting particle can be obtained from these photographs.

fully grown latex particles are also of major importance in the film formation/drying process. In this section we present the first reported results on the direct measurement of latex particle interaction forces in aqueous solution.

In these experiments two types of AFM probes were used. In the fiit, the standard AFM silicon nitride, square- pyramid stylus was coated with a molten layer of polystyrene. On cooling, this remains firmly attached to the probe, as can be seen in Figure 5. A second type of probe was made by attaching one or several 5-pm-diameter polystyrene latex spheres to the stylus. The stylus was then annealed at approximatey 200 "C to firmly attach the latex and to smoothen the surface (see Figure 6). Interaction forces measured by either probe, when scaled with the corresponding radius (measured by SEM), are the same within experimental error. However, the latex probes had larger radii, were easier to produce, and were therefore used for most of the following measurements. In all cases the polystyrene probe was used for the measurement of interaction forces on approach to a polystyrene flat surface, premelted for smoothness.

The first experiments were between a polystyrene- coated probe of the type shown in Figure 5. The forces measured as a function of the closest distance of separation between the curved probe and the flat surface when immersed in distilled water are given in Figure 7. The probe approached the surface at a rate of about 0.6 pm s-l, and no interaction force was detected until the surfaces

1224 Langmuir, Vol. 9, No. 5, 1993 Karaman et al.

0 20 40 Distance (nm)

Figure 7. Forces measured wing the polystyrene-coated AFM tip against a flat premelted sheet of polystyrene immersed in distilled water. The arrow indicates the surface separation at which the tip was pulled inward by attractive surface forces.

i


Figure 8. Forces measured using the polystyrene latex particle attached to the AFM tip against a polystyrene flat surface i m m e d i n distilled water. The surfaces were pulled together by a fairly long-range attractive force as observed for the polystyrene-coated tip. However, in this case the radius of the latex particle can be accurately measured (see Figure 61, and hence the total force can be scaled with the mean radius (FIR). The scaled values can be compared with the forces expected from DLVO theory, i.e., van der Waals and double-layer forces. In this case no surface charge was observed (as expected), and hence only van der Waals attractive forces are expected. The solid line corresponds to the nonretarded van der Waals force using a Hamaker constant of 0.95 X

were about 23 nm apart. At this distance the inward motion increased rapidly and the surfaces jumped into an adhesive contact. On further forcing the surfaces together, no displacement in distance was detected.

These forces are consistent with hydrophobic surfaces with no significant electrostatic charge. In water, the Debye length is typically of the order of about 150 nm, and hence any significant surface charge would be detected as a relatively long-range repulsive force. If only van der Waals attractive forces were present, we would expect the surfaces to jump together from a much closer separation of leee than 5 nm (see later). For this reason we can identify this longer range attraction as due to the hydrophobic interaction. This result is, of course, not unexpected because of the hydrophobic nature and correspondingly high water contact angle (>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

J.

0 50 100 150 Distance (nm)

Figure 9. Forces measured between a polystyrene latex particle and a flat sheet immersed in 2 X 1o-L M SDS solution (Le., 2.5% of the cmc value). Significant surfactant adsorption changed the surfaces, and the repulsive force generated was fitted using a numerical solution to the Poisson-Boltzmann equation. At close separations the surfaces were pulled into contact with an attractive force much stronger than the van der Waals force.

- E

l I


Figure 10. Forces measured between a polystyrene latex 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 substantially changes the forces between polystyrene surfaces. In Figure 9 the force measurements were obtained at an SDS concentration of 2.5% of the cmc value (8 X 10-3 M). 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 12 nm. 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

Surface Chemietry of Emulsion Polymerization Langmuir, Vol. 9, No. 5, 1993 1225

(-0.031 C m-2) under similar solution conditions using microelectrophoresis (see earlier). These differences cannot be explained by experimental error 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 density14 would 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.

I I I

0 10 20 30 Distance (nm)

Figure 11. Forces measured betweena polystyrene latex particle and a flat sheet immersed in 8 X le3 M 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.

0 5 10 15 Distance (nm)

Figure 12. Forces measured between an ill-defined polystyrene- coated (low-radius) probe anda polystyrene flat surface immereed in an aqueous solution at the cmc value of SDS. As the low- radius 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 forces observed 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 head- group 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 SDS solution 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

Discussion We have used several interfacial chemistry techniques

to investigate the emulsion polymerization process from a viewpoint different from that 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 adsorption tests. Depending on initial reaction conditions, the process may proceed via polymerization within 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 electrostatic interaction 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 surfaces will be expected to regulate their potential in response to the approaching field via the adsorption or desorption of ions. Recent studies have shown that usually surfaces interact under conditions somewhere between the

(14) Attard, P.; Mitchell, D. J.; Ninham, B. W. J. Chem. Phys. 1988,

(15) Feeney, P. J.; Napper, D. H.; Gilbert, R. G . Macromolecules 1987, 89 (7), 4358.

20, 2922.

1226 Langmuir, Vol. 9, No. 5,1993

constant surface charge and potential limits.16 Only repulsive electrostatic forces are possible between 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 collisions between small particles wil l 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 evidently artificial electrostatic heterocoagulation 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 surfaces in water strongly attract over distances up to 100 nm.17 The magnitude of the attraction at short range (<5 nm) is substantially larger than for van der Waals forces and the relatively weak electrostatic repulsion expected for these particles. That these forces exist between polystyrene latex particles both uncoated and partially covered with adsorbed surfactant has been unambiguously demonstrated by our work.

If the hydrophobic interaction is the dominant force causing coagulation and buildup of the final latex particle, we might expect that each added precursor particle brings with it further surfactant. As the process continues, we can expect that surfactant will be forced to the outer aqueous layer and hence eventually substantially coat the aggregated particle, preventing any further addition. We would expect then that the surfactant levels in emulsion polymerization affect the final particle size and density, as in indeed the case.6 This model predicts that higher surfactant levels produce larger numbers of smaller-sized latices.

It is interesting to speculate further that this attractive force may also be important during the latex film formation processes. Here evaporation must increase the electrolyte level and suppress electrostatic repulsion between latex particles, so that the partially hydrophobic nature of even the coated particles may draw them together in the initial stage of film formation.

Although we have obtained only limited data on interaction forces between latex particles, it is worth noting that the range of the uncoated polystyrene attractive forces is substantially lower than that reported for macroscopic surfaces of similar hydrophobicity.17 For these, the maximum reported range of about 100 nm cannot be explained by the influence of the surface on the structure of adjacent water layers, which would be expected to decay within no more than about 10 molecular layers (i.e., about 3 nm per surface). When hydrophobic surfaces are brought together to close distances (within about 2 nm), cavitation of intervening water has been observed.'B It is, perhaps, possible that the long-range attraction might have more to do with the production of transient vapor cavitation than the propagation of water structure. Bridging of such cavities would give rise to attraction between surfaces. We might expect a reduction in size of the interacting surfaces to reduce this effect, leading, perhaps, to forces

(16) Pashley, R. M. J . Colloid Interface Sci. 1981, 83, 531. (17) Christenson, H. K.; Claeseon, P. M. J. Phys. Chem. 1988,92,1650. (18) Christenson, H. K.; Claesson, P. M. Science 1988,239, 390.

Karaman et al.

extending only a few molecular diameters for small organic molecules interacting in water. Whatever the origin of these fascinating forces, it seems clear that they must operate in emulsion polymerization processes involving the buildup of the final aggregated latex particle from hydrophobic precursor particles. Most monomers used in this process are hydrophobic.

The hydrophobic interaction between molecules is, of course, the driving force for the self-assembly of surfactant molecules in water into micelles, and these provide the sites for polymerization in the second mechanism. That polymerization may occur within the monomer-swollen micelle has been shown convicingly recently via a study of the rates of the reactionlo as well as by the radiation polymerization of clear microemulsion fluids.lg The dye studies reported in the present work also support this mechanism, under conditions where polymerization is favored in the aggregate by the use of aqueous-insoluble monomer and initiator. Electron micrographs of these latices show much smoother particles than by homogeneous nucleation. The micelle-produced latex also showed some uniformly spaced surface dimpling, indicative of phase separation in the growing polymer-monomer fluid core.

In the micellar model two mechanisms are possible at the initial polymerization stage. Systems containing oil- soluble initiator may be expected to proceed via thermal initiation within the micellar core, whereas for a water- soluble initiator, such as persulfate, an oligomeric surfactant may be formed in solution. This then freely exchanges with the original micellar surfactant. This mechanism has been proposed to overcome the barrier presented to a negatively charged ionic (sulfate) radical entering a negatively charged hydrophobic sphere. It would be of some interest to study the aggregation properties of stable, nonradical oligomeric surfactants synthesized from typical monomers. It may also be useful to study an initiator such as persulfate with a cationic, micelle-forming surfactant, where micellar exchange would be electrostatically favored. However, our initial attempts proved difficult because of the coagulating power of divalent persulfate ions on common cationic surfactants.

In the micelle model, it is assumed that the presence of two growing radicals in the restricted space of the micelle core rapidly produces termination. Hence, polymerization ceases until another radical enters the micelle and the reaction proceeds in an on-off manner. The question then arises as to why the same partially polymerized micelle should continue to absorb oligomer radicals (up to lo00 times) rather than all of the original micelles taking up at least one radical. The high initial micelle density would strongly favor an equal distribution of radicals, but the final latex density is much lower than the original micelle density.6 Presumably, the micelles containing some polymer will take up monomer and swell more effectively than the original micelles. The latter will then be reduced in number via adsorption onto the rapidly growing polymer micelles.

In this work we appear to have demonstrated two extreme cases of micellar and homogeneous nucleation. However, intermediate mechanisms may also occur under appropriate conditions. For example, it could be envisaged that oligomeric precursor particles in the aqueous phase grow by adsorption of monomer and surfactant to produce the final polymer particle. These particles then coagulate because of their incomplete surfactant coverage to produce fully coated, stable latex particles.

(19) Napper, D. H. Private communication.


Conclusions

Surface chemical techniques can be applied to the emulsion polymerization process to elucidate the mechanisms involved. Morphological and dye adsorption studies indicate two extreme types of latex formation and product. These have been identified with the two currently postulated models of homogeneous and micellar nucleation. It seems most probable that either process can occur depending on polymerization conditions. The nature of the radical initiator appears to be a key factor, although monomer solubility and surfactant type are also important. Water-soluble persulfate initiators can induce latex formation via either micellar or homogeneous nucleation,

Langmuir, Vol. 9, No. 5,1993 1227

whereas oil-soluble initiators tend to induce micellar nucleation.

The current explanation for the powth of latex particles via the electrostatic coagulation of precursor particles has been shown to be unreasonable. Direct measurements reported here demonstrate that a strong hydrophobic attraction is the more likely mechanism.

Acknowledgment. The authors would like to thank Professor M. S. El-Aasser for supplying a sample of 5-pm spherical polystyrene latex. We would also like to thank Professor B. W. Ninham and Carinna Tong for useful comments and discussions.

Surface chemistry of emulsion polymerization

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