Langmuir 1994,10, 2197-2201
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Polymerization of Styrene in a Winsor I-like System L. M. Gan,*N. Lian, C. H. Chew, and G. Z. Li? Department of Chemistry, National University of Singapore, Singapore, and Department of Chemistry, Shandong University, P. R. China Received January 16, 1994. In Final Form: May 2, 1994@ A new polymerization system, herein referred to as a Winsor I-like system, has been successfully used for polymerization of styrene at room temperature. The system consists of a microemulsion (lower)phase which is topped off with styrene. The polymerization takes place only in the microemulsion phase where a water-solubleredox initiator ammonium persulfatdN,Nfl,”-tetramethylethylenediamine (APSA’MIEDA) is present, while the styrene phase acts only as a single monomer reservoir. Unlike microemulsion polymerization which requires higher concentrations of surfactant, this new system needs only about 1.0 wt % dodecyltrimethylammoniumbromide (DTAB)for producingas high as 15wt % polystyrene microlatex. Rather monodispersed polystyrene particles (DJD,= 1.13)were obtained from the system that produced nanoparticles smaller than 100 nm in diameter. The factors that affect the polymerization are discussed.
Introduction Most of the studies on microemulsion polymerization reported in the last decade were carried out in fourcomponent (o/w) micro emulsion^'-^ consisting of water, monomer, surfactant, and cosurfactant. Recently, polymerizations of oil-soluble monomers have been successfully conducted in ternary (o/w) microemulsions without using a cosurfactant.8-11 Most of the ternary microemulsions used cationic surfactants, such as dodecyltrimethylammonium bromide (DTAB),12cetyltrimethylammonium bromide (CTAB),Sand cetyltrimethylammonium chloride (CTAC).l0 These microemulsions produced stable microlatexes with particle radii ranging from 20 to 50 nm. Larpent and Tadrosll have studied three methods for producing polystyrene and poly(methy1 methacrylate) microlatexes in ternary o/w microemulsions. The first method was thermally induced polymerization using K&Oa or AIBN initiators. The microlatexes produced by this method were in the range of 37 to 100 nm in diameter. The second polymerization procedure was chemically induced using a redox system of hydrogen peroxide and ascorbic acid. The system produced microlatexes with very small sizes (18-24-nm diameter). The third method of polymerization was based on UV irradiation in conjunction with &S208 or AIBN. The microlatexes obtained were also relatively small (30-60-nmdiameter) in size. The ternary systems usually require higher surfactant concentrations (as high as 15 wt %) for solubilizing
* To whom correspondence should be addressed. + Shandong University.
*
Abstract published in Advance ACSAbstracts, June 15,1994. (1) Stoffer, J. 0.;Bone, T. J . Dispersion Sci. Technol. 1980, I, 37. (2)Atik, S. S.; Thomas, J. K. J . Am. Chem. SOC.1981, 103, 4279. (3) Johnson, P. L.; Gulari, E. J . Polym. Sci., Polym. Chem.Ed. 1984, _22. _ 3967.(4) KUO,P. L.; Turro,N. J.; Tseng, C. M.; El-Aasser, M. S.;Vanderhoff, J. W. Macromoleculea 1987,20, 1216. (5) Guo, J. S.;El-Aasser, M. S.; Vanderhoff, J. W. J . Polym. Sci., Polym. Chem. Ed. 1989,27,691. (6)Holdcroft, S.; Guillet,J.E. J.Polym.Sci.,Polym.Chem.Ed. 1990, 28, 1823. (7) Feng, L.; Ng, K. Y. Macromolecules 1990,23, 1048. (8)Pe’rez-Luna, V. H.; Puig, J. E.; Castano, V. M.;Rodriguez, B. E.; Murthy, A. K.; Kaler, E. W. Langmuir 1990, 6, 1040. (9) Ferick, M. R.; Murtagh, J.; Thomas, J. K. Macromolecules 1989, 22, 1515. (10) Antonietti, M.; Bremser, W.; Miischenborn, D.; hsenauer, C.; Schupp, B.; Schmidt, M.Macromolecules 1991,24, 6636. (11) Larpent, C.; Tadros, T. F. Colloid Polym. Sci. 1991,269,1171. (12) Puig, J. E.; Pe’rez-Luna, V. H.; Pe’rez-Gonza’lez, M.; Macias, E. R.; Rodriguez, B. E.; Kaler, E. W. Colloid Polym. Sci. 1993,271, 114. I - - -
relatively low monomer contents of less than 10 wt %. This drawback needs to be overcomebefore microemulsion polymerization can be used as an attractive process for producing microlatexes of polymers. In our recent study, a new system has been successfully used for polymerizing styrene up to 15 wt % using only about 1wt % surfactant. This new system is rather similar to a Winsor I system, i.e., an organic phase containing small proportions of water and surfactant is in equilibrium with an o/w microemulsion. The slight difference between the two systems is that the new system consists ofa pure styrene phase (upper phase) which is placed on top of a ternary o/w microemulsion (lower phase). The new system is simply prepared by topping off a ternary microemulsion with a certain amount of styrene without disturbing both phases. The polymerization takes place only in the microemulsion using a water-soluble redox initiator.
Experimental Section Materials. N-Dodecyltrimethylammonium bromide (DTAB) from Tokyo Chemical Industry (TCI) was recrystallized from a mixture of ethanol-acetone (1:3by volume). 2,2’-Azobis(isobutyronitrile) (AIBN) from TCI was recrystallized from methanol. Styrene (ST) from Fluka was vacuum distilled to remove the inhibitor. Ammonium persulfate (APS)from Fluka and NJV,W,W-tetramethylethylenediamine (TMEDA) from Aldrich were used as received. Microemulsion Polymerization in Winsor I-like Systems. The polymerization of styrene in a transparent ternary microemulsion (lower phase) with a certain amount styrene (upper phase) was carried out in a glass tube with mild magnetic stirring. The composition of DTAB/ST/H20 in a microemulsion was varied. The redox initiator was dissolved in the microemulsion. After the microemulsion was bubbled with N2 for about 15min and topped off with a certain amount of styrene, the polymerization was carried out at 30 “C for about 20 h. Polystyrene (PS)was recovered from precipitating the latex in a large quantity of distilled methanol. The polymer was washed repeatedly with distilled methanol in order to remove the residual DTAB. Particle-Size Determination. Particle sizes of PS latexes were determined by photon correlation spectroscopy (PCS) using Malvern 4700 light scattering spectrophotometer, Prior to measurements, the microemulsion latexes were diluted with distilled water until the volume fractions of particles were in the range of 0.01 to 0.1. The average hydrodynamic radius of latex particle (Rh)was
0743-746319412410-2197$04.50/00 1994 American Chemical Society
Gan et al.
2198 Langmuir, Vol. 10,No. 7, 1994
Styrene
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DTAB Figure 1. Partial phase diagram of DTAB/ST/H20 at 30 "C.
calculated from the well-known Stokes-Einstein equation, Rh = RT/6n7D0, where Do is the intrinsic diffusion coefficient and 7 is the viscosity of the dispersion medium. PS latexes were also examined by a JEOL JEM-100CX electron microscope. One drop of each latex was thoroughly mixed with 2 mL of 0.2% phosphotungstic acid (PTA). A drop of the mixture was then put on a copper grid coated with a thin layer of Formvar. The diameters of particles were measured directly from each transmission electron micrograph (TEM). The number average diameter (D,) and weight average diameter (D,)were calculated from the following equations: l4
D, =
z~p~/z~~
At least 300 particles (N)were counted for each calculation. The polydispersity of particle size is expressed as DJD,. Molecular Weight Determination. Molecular weights of PS were determined by gel permeation chromatography (GPC), using a Varian 5500 liquid chromatography system equipped with a RI-3 detector. The columns used were Varian Micropak TSK 7000H and GMH 66 in series and the eluent was the HPLC tetrahydrofuran (THF) which contained 0.25 wt % 2,6-di-tertbutyl-p-cresol as a stabilizer. The flow rate was maintained a t 0.8 mumin. PS standards (Polyscience, 0.15 mg/mL in THF) were used for the calibration.
Results PolymerizationSystem. The partial phase diagram of the ternary system of DTAB/ST/H20 is shown in Figure 1. The transparent o/w microemulsion region is denoted by ,DE,and the turbid emulsionregion is roughly presented by the shaded area. The microemulsion region is extremely small and narrow near the corner of water apex, where low DTAB is used. As shown in the Figure 1, a line is also drawn from the styrene apex to intercept at 3 wt % DTAB. This is to show that microemulsions will change to emulsions as the styrene concentration is continuously increased along this line. For instance, the composition a t point A is a transparent microemulsion, whereas it is a turbid emulsion at point B. However, if the turbid emulsion with the composition at point B is left overnight for equilibrium, it will separate into two clear phases. The lower phase is a microemulsion of which the composition is identical to that a t point A. The upper phase (13)Gan, L. M.;Chew, C. H.; Lee, K. C.; Ng, S. C. Polymer 1993,34, 3064. (14)Barth, H.G.Modern Methods of Particle Size Analysis; Wiley: New York, 1984;p 111.
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Styrene (wt%) in the Upper Phase Figure 2. Effect of the amount of styrene in the oil phase on particle sizes and polydispersities of polystyrene latexes. System: The system contained 1wt % DTAB,O.5wt % styrene, 98.5 wt % HzO as the microemulsion (lower phase) and varying amounts of styrene in the upper phase. Initiator: 1 mmoVL of equimolar APSRMEDA. Polymerization: 30 "C for 20 h.
consists mainly of styrene and a small amounts of water as well as DTAB. Both phases are in equilibrium, and such a system is known as Winsor I system. In this study, an o/w microemulsion (for example, composition at point A) was first prepared and it was then topped off with styrene up to a certain level, say up to point B. Strictly speaking, this system is not identical with the above mentioned Winsor I system. Hence, we refer to this new polymerization system as "Polymerization in a Winsor I-like System". The actual polymerization occurs in the microemulsion phase which is interfaced with the styrene phase. During the polymerization, a mild magnetic stirring was applied to the lower phase of microemulsion without disturbing the upper phase of styrene. This styrene phase acts only as a monomer reservoir for monomer to diffuse to the microemulsion for the continuous polymerization. A very reactive redox initiator15 of APSPTMEDA was selected for this styrene polymerization which was mainly carried out at room temperature. The stability of polystyrene latexes prepared by this new method was affected by the stirring in the microemulsion phase, type of initiator used, and the polymerization temperature. This can be illustrated by a system consisting of a microemulsion (1wt % DTAB, 0.5 wt% ST, and 98.5 wt% H20)which was topped off with an additional 10 wt % styrene (based on the total weight of the microemulsion). The concentration of the redox initiator (APSPTMEDA at equiriolar ratio) used was 1 mmoVL based on the total weight of the microemulsion. When the polymerization was carried out at 30 "C with or without stirring in the microemulsion phase, both polystyrene latexes produced were stable for more than 6 months. However, if the polymerization was conducted a t 60 "C, a stable polystyrene latex was only obtained from the nonstirring system, but a crumb latex for the stirring system. On the other hand, when the redox initiator was replaced by APS or AIBN alone for the polymerization at (15)Guo, X.Q.; Qiu, K. Y.; Feng, X. D. Mukromol. Chem. 1990,191, 577.
Polymerization in a Winsor I-like System Lamr-
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Figure 3. "EM of polystyrenelatexes with different amount of styrene in the upper oil phase. Microemulsion compositions and other conditions are identical to those of Figure 2: (a)D, = 45.5 nm, DJD, = 1.35 for 1.8 w t % styrene in the upper phase; (b) D, = 97.3 nm, DJD, = 1.15 for 15 wt % styrene in the upper phase.
60 "C, only the AIBN system under the nonstimng condition produced polystyrene latexes which were stable for about 2 weeks. If it was stirred, the latexes were unstable and precipitated. As for APS-initiated systems, only unstable polystyrenelatexes were obtained regardless of whether they were stirred or not. Thus, the subsequent investigations were based on the systems using APSI TMEDA redox initiator with stirring in the microemulsion phase a t 30 "C. Particle Size of Microlatexes. Although polymerization of monomers in common o/w microemulsions produce nanoparticles, they are usually polydispersied, i.e., DJDn = 1.3-1.5.16 However, rather monodispersed nanoparticles of polystyrene can be obtained by this new method. At least three factors would affect the polydispersity of polystyrene nanoparticles prepared by this method. They are concentrations of the redox initiator and DTAB in the microemulsion (lower phase) as well as the amount of styrene in the upper phase. Figure 2 shows that average particle sizes (Dh,D,,and Dn)of polystyrene increased, but their polydispersities (DJDn) decreased slightlywith the increasing amount of styrene in the upper oil phase from 1.8 to 15 w t % (based on the initial total weight of the microemulsion). The hydrodynamic radius of the particles (Rh) is the distance from the center of the particle to the shear plane at the surface. The shear plane is assumed to be that of nondraining particles. It is known that Rh is always greater than R, and Rn which measure the core of the particles. The dependence of D, (nm) on styrene content (wt %) was found to be D, = [ST1°.38.On the other hand, DJD, decreased linearly from 1.35 (1.8 wt % ST) to about 1.15 (15w t % ST). Figure 3 shows two TEMs of the latex particles with different sizes obtained from using 1.8 and 15 w t % styrene respectively. The system was unstable (precipitated) when 20 wt % styrene was used. The effect of the concentration of the redox initiator (equimolar APSA'MEDA) on particle sizes and polydispersities of polystyrene latexes is shown in Figure 4. D, decreased substantially from about 90 to 60 nm as the initiator concentration in the microemulsionwas increased from 0.5 to 3 mmoyL. It approached about 50 nm at 6 mmol/L of the initiator. However, DJDn increased only marginally from 1.13to 1.22. Figure 5 shows two TEMs (16)Gan, L. M.; Chew, C. H.; Lye, I.; Ma, L.; Li, G. P o Z ~ 1993, ~ F 34, 3861.
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Figure 4. Effect of the redox initiator concentrationon particle sizes and polydispersitiesof polystyrenelatexes. System: The system contained 1wt % DTAB, 0.5 wt % ST, 98.5 wt % HzO as the microemulsion (lower phase) and 10 wt % styrene in the upper phase. Initiator: varying concentration (mmoVL) of equimolar APS/TMEDA. Polymerization: 30 "C for 20 h. of such latex particles obtained from the systems containing 1.0 and 6 mmom of the initiator, respectively. It is known that surfactant concentration of a microemulsion will affect particle sizes of latexes. In this study, the surfactant concentrationof the microemulsion (bottom phase) was varied from 0.5 to 3 w t %, while its styrene concentrations was fmed at 0.5 wt % and the remainder was made up with water to 100 w t % for each microemulsion. These microemulsions were then topped off with 10 wt % styrene as the upper oil phase. As can be seen from Figure 6, D, substantially decreased linearly from 89.3 to 67.2nm, WhileDJD, increased quite linearly from 1.13 to 1.26 as the surfactant concentration was increased from 0.5 to 3 wt %. The dependence ofD, (nm) on surfactant content (wt %) was found to be D, = [DTAB]-0.16 for the DTAB concentrations ranging from 0.5 to 3.0 wt %. Molecular Weights of Polystyrene. Molecular weights of polystyrene obtained from the systems which
2200 Langmuir, Vol. 10, No. 7, 1994
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Figure 5. TEM of polystyrene particles obtained from the systems which are identical to those of Figure 4: (a)D, = 83.6 nm and DJDn = 1.13 for 1.0 mmoVL of equimolar APS/TMEDA, (b)Dw = 46.5 nm and DJDn = 1.22 for 6.0 mmoVL of equimolar APS/ TMEDA. Table 1. Summary of Particle Sizes and Polydispersities of Polystyrene Latexes series fixed compositionsa variables D w DJDn MW x 106 MwIMn A lwt%DTAB 0.5 95.5 1.18 5.4 3.9 1.0 83.6 1.13 6.5 6.5 0.5 wt 5% ST, 98.5 wt % H20 3.0 62.0 1.22 6.9 15.4 10 wt 9% ST in oil phase 6.0 46.5 1.22 5.0 12.9 variable, I (mmoVL) B lwt%DTAB 3.0 59.2 1.35 4.3 6.4 5.0 66.6 1.23 4.6 6.0 0.5 wt % ST, 98.5 wt 9% H2O 1mmoVL of I 10.0 83.6 1.13 6.5 6.5 15.0 97.2 1.15 7.3 6.1 variable, ST wt % in oil phase C 0.5 wt % ST in microemulsion 0.5 89.3 1.16 6.8 10.4 1mmol/L of I 1.o 83.6 1.13 6.5 6.5 2.0 74.5 1.27 6.1 7.1 10 wt % ST in oil phase 3.0 67.2 1.26 6.4 8.7 varible, DTAB ( w t %) I = equimolar of APS/TMEDA, the concentration used was based on the water content in the microemulsion. ST = styrene. 0
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Dw: Weight Avenge Diameter by TEM
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increased from 0.5 to 3 mmoVL. When the DTAB of the microemulsion (bottom phase) for series B was increased from 1to 3 wt%, gw remained about the same at 6.6 x lo6, but MJMn incrsased significantly from 6.5 to 8.7. On the other hand, &fwfor series C increased from 4.3 x lo6to 7.3 x lo6,while MJMn remained about the same (6.1-6.5) as the amount of the styrene was increased from 3 to 15 w t %. It is thus established that a Winsor I-like system can be used to prepare higher contents of polystyrene (ca. 15 w t 96) with rather monodispersed nanoparticles at room temperature using only 1wt 9%DTAB and 1mmoVL of redox initiator (equimolar APSlTMEDA). The similar investigation has now been extended to other polymerization systems.
. - - - ' - -Db:-tfydrudynamic . ' - Diameter - . by- FCS ' - - - concentration -
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DTAB ( w t % ) Figure 6. Effect of surfactant concentration on particle sizes and polydispersities of polystyrene latexes. System: The microemulsioncontained 0.5w t 96 styrene and varying amounts of DTAB with 10 w t 96 styrene in the upper phase. Initiator: 1 mmoVL of equimolar APS/TMEDA. Polymerization: 30 "C for 20 h. were affected by three factors as discussed earlier are summarized in Table 1. In general, M, ranged from 4 x lo6to 7 x IO6were obtained. For series!, Mwincreased slightly from 5.4 x lo6to 6.9 x lo6but MJM,, increased markedly from 4.0 to 15.4 as the redox initiator was
The Winsor I-like polymerization system consists of two phases, i.e., a styrene (upper) phase and an o/w microemulsion (lower) phase. Since the redox initiator (APS/ TMEDA)is solubilized in the aqueous medium of the o/w microemulsion, it is believed that the loci of styrene polymerization are in microemulsion droplets. The free radicals generated in the aqueous medium might first react with the dissolved styrene (ca. 0.12 M) in the aqueous medium to form oligomeric radicals of higher hydrophobicities. *'*I8 These oligomeric radicals could then diffuse more favorably into microemulsion droplets to continue (17)Maxwell, I. A.; Morrison, B. R.; Napper, D. H.;Gilbert, R. G. Macromolecules 1991,24, 1629. (18)Normula, M.;Ikoma, J.; Fujita, I(.J . Polym. Sci., Polym. Chem. 1993,31, 2103.
Polymerization in a Winsor I-like System the polymerization. For styrene-emulsionpolymerization initiated by potassium persulfate, the critical size of oligomeric radicals for entering latex particles is believed to be dimeric.17 The stirring in the microemulsion phase is to facilitate the homogeneous distribution of the styrene which diffises continuously from the upper phase of styrene reservoir into the lower phase of polymerizing microemulsion. Once polymer particles are formed in the microemulsion, they continue to grow by recruiting styrene from the styrene phase which only acts as a single monomer reservoir. The capacity of this single monomer reservoir can easily be adjusted by adding any amount of monomer in the upper phase. The AF'SPTMEDA is an excellent water-soluble redox initiator for the polymerization of vinyl monomers at room temperature. It has been shown that the (dialky1amino)methyl radical KCH&NCH2CH2N(CH)&H2*1 and the sulfate free radical (H03SO') are responsible for the initiation of vinyl polymerization using this redox initiaUnlike APSPTMEDA, APS alone produces only an anionic radical (-OSOZO'). Such anionic radicals could interact with cationic DTAB resulting in forming unstable latex particles at 60 "C for the system containing only 1 wt % DTAE? in the microemulsion phase and 10 wt % styrene in the upper phase. However, the use of APS/ TMEDA or uncharged AIBN could produce stable latexes at 60 "Cprovided the same microemulsion phase was not stirred. If it was stirred, the latexes produced became unstable and eventually precipitated. This could be due t o the coalescence of latex particles which were barely protected by only 1wt % DTAB. The coalescence between particles might be accelerated by stirring at higher temperature. But the similar system using APSiTMEDA could still produce stable microlatexes under the stirring condition if the polymerization was carried out at about 30 "C. This indicates that 1 wt % DTAB was sufficient to protect latex particles from coalescence at room temperature, but not greater than 40 "C. The polymerization mechanism for this new method deals with loci of initiation and polymerization. It may be better illustrated by the following two simple polymerization systems. The first system consisted of the aqueous solution of the redox initiator and it was topped up with styrene. The other system was similar to the first one, except its aqueous solution also contained 1wt % DTAB. The former system could also produce a milky latex but it slowly precipitated. On the other hand, the latter system produced a very stable latex. This clearly demonstrates that styrene from the upper phase could readily diffise into the aqueous phase where the initiation of styrene polymerization took place upon capturing the newly generated radicals to form oligomeric radicals. It has been reported17that the surface activity for anionic free radical (SO,-)could be expected if the oligomeric radical of styrene is a dimer. When the oligomericradicals of styrene grow to 4 or 5 units, they are no longer soluble in the aqueous phase resulting in precipitation. When a small amount of DTAE?is present, the oligomeric radicals would be captured by monomer-swollenmicelles and stable latex particles grow almost in~tantaneous1y.l~It is thus surmized that the locus of styrene initiation is in the
Langmuir, Vol. 10, No. 7, 1994 2201 aqueous phase and the polymerization is in microemulsion droplets, while the styrene phase simply serves as a single monomer reservoir for the Winsor I-like system. The formation of polystyrene in the microemulsion phase may be viewed as seeds for further growth of polymer particles by continuously recruiting monomer from the styrene phase. Once the polymer is formed in microemulsion droplets, the polymer-containing droplets will rapidly take up more monomer and swell more effectively than those of the unpolymerized dr0p1ets.l~For an o/w microemulsion, a thermodynamic modells predicts that the depletion of oil cores of microemulsion droplets would be at about 4% monomer conversion. This indicates that the monomer is highly preferred to diffise into the polymer-containing droplets. The unpolymerized microemulsion droplets will then be reduced in number via adsorption of surfactant onto the rapidly growing polymer particles. This will be an ideal situation for preparing particles with uniform size. Our results do show that the particle size distribution became narrower (DJD, = 1.351.15)as the amount of styrene phase was increased from 3 to 15 wt %. However, these polymer particles are still far from monodispersed. This implies that some continuous nucleations of polymer particles still occurred in the system where even as little as 0.5 wt % DTAB was used. The continuous nucleation of polymer particles is expected to increase with the increase of surfactant concentration. It is thus not surprising to find that broader particle size distributions (DJD,) increased from 1.16to 1.26 as DTAB was increased from 0.5 to 3.0 wt %. The general condition for obtaining monodispersed polymer particles is to control the nucleation and the growth stages. The most ideal situation is the combination of a high nucleation rate and a slow growth rate. In this new polymerization system, a slow growth rate of polymerization is controlled by the monomer diffision from the oil phase to the microemulsion phase. The nucleation rate was increased by increasingthe concentration of redox initiator (APSiTMEDA) from 0.5 to 6 mmoVL. But the result of DJD, did not seem to change significantly as shown in Table 1. In fact, the lowest DJDn (1.13) was obtained from the system using 1 mmol/L of the redox initiator rather than that of 6 mmol/L. This unexpected observation is still under investigation.
Conclusion The Winsor I-like system is successfully used for the first time to polymerize styrene at room temperature using redox initiator APSiTMEDA. The locus of polymerization is in the microemulsion (lower) phase while the styrene (upper) phase serves only as a single monomer reservoir. The stability of polystyrene microlatex is affected by the stirring in the microemulsion phase, type of initiator, polymerizationtemperature, the amount of styrene phase, and the concentration of DTAB. The new system can produce as high as 15w t % polystyrene stable latex using only about 1.0 wt % DTAB. This is in contrast with the normal microemulsion polymerization of styrene, which usually requires about 10 wt % DTAB in order to produce about 8 wt % polystyrene of a stable latex. (19)Guo, L. M.; Sudol,E. D.; Vanderhoff, J. W.; El-Aasser, M. S. J . Polym. Sei., Polym. Chem. 1992, 30, 691;703.
