Langmuir 1993,9, 2799-2803
2799
Polymerization of Methyl Methacrylate in Ternary Systems: Emulsion and Microemulsion L. M. Gan,* C. H. Chew, S. C. Ng,t and S. E. Loh Department of Chemistry, National University of Singapore, Republic of Singapore Received July 23, 1992. In Final Form: July 9,1993@ The ternary systems containing water, methyl methacrylate (MMA), and cetyltrimethylammonium bromide (CTAB) could be continuously changed from turbid emulsions to transparent microemulsions by merely increasing the surfactant concentration. Stable PMMA latexes (16-30nm in hydrodynamic radius) were obtainedfrom emulsionand microemulsionpolymerizationsinitiatedby potassiumpersulfate (KPS). Molecular weights of PMMA were in the range of (5 to 7) X 1Oe. The activation energies of the polymerizationsfor both systemswere about the same (81and 85 kJ mol-'). Each latex particle containing about one to two polymer chains was obtained from microemulsionPolymerization of 5 w t % MMA while it was about four from emulsion polymerization at the same MMA concentration. The number of polymer chainsper latex particle increased to about five for microemulsionsand nine for emulsions at 9 wt % MMA. The low dependency of polymerizationrate on [CTAB]0.31and ita strong dependency on [KPS]o~sz were observed for emulsion polymerizations in contrast with [CTAB]0.68and [KPS]o.33for microemulsion polymerizations. The possible polymerization mechanisms related to micellar nucleation, homogeneous nucleation, and monomer droplet initiation are discussed.
Introduction The kinetics of emulsion polymerizationcan be divided into two stages: particle nucleation and particle growth. The particle nucleation stage is more important and controversial. It is controversialbecause of experimental difficulties in determining the type of nucleation mechanism for different monomer systems and experimental c0nditions.l For example, micellar nucleationmechanism2 is valid only for those monomers solubilized in micelles, while the homogeneous nucleation3 and the coagulative nucleation4p5mechanisms are more appropriate for monomers of higher solubilities in the continuous phase. In addition, Ugelstad, El-Aasser, and Vanderhoff pioneered the research in emulsified monomer droplet initiatione in miniemulsion polymeri~ation.~-'~ This mechanism is possible only for small monomer droplets (1W4OOnm in diameter) of less water-soluble monomers in order to compete with micelles for capturing free radicals generated in the aqueous phase. As the colloidal dispersions are further reduced to less than 100 nm, transparent or translucent microemulsions will be formed. In spite of the intensive study of emulsion polymeri-
* To whom correspondence should be addressed. t Department of Physics. @Abstractpublished in Advance ACS Abstracts, September 1, 1993. (1) El-Asser, M. S. In Scientific Methods for the Study of Polymer
Colloids and Their Applications; Candau,F., Ottswill,R. H.,Eds.;Kluwer Academic Publishers: London, 1990; Chapter 1. (2) Harkins, W. D. J. Am. Chem. SOC.1947,69, 1428; J. Polym. Sei. 1947,5, 217. (3) Fitch, R. M.; Tsai,C. H. In Polymer Colloids; Fitch, R. M., Eds.; Plenum Prese: New York, 1971. Fitch, R. M.; Shih, L. B. Prog. Colloid Polym. Sci. 1975, 56, 1. (4) Lichti, G.; Gilbert, R. G.; Napper, D. H. J. Polym. Sci. 1983,21, 2GQ
(5) Feeney, P. J.; Napper, D. H.; Gilbert, R. G. Macromolecules 1984, 17,2520; Macromolecules 1987,20,2922. (6) Ugelstad, J.; El-Aasser,M. S.;Vanderhoff,J. W. Polym.Lett. 1973, 11,503.(7) Ugelstad, J.; Hansen, F. K.; Lange, S. Makromol. Chem. 1974,175, 507. (8)Hansen, F. K.; Ugelstad, J. J. Polym. Sci., Polym. Chem. Ed. 1979, 17, 3069. (9) Chamberlain,B. J.; Napper, D. H.; Gilbert, R. G. J. Chem. SOC., Faraday Trans. 1 1982, 78,591. (10) Choi, Y. T.; El-Aaeser, M. S.;Sudol, E. D.; Vanderhoff, J. W. J. Polym. Sci., Polym. Chem. Ed. 1986,23, 2973.
0743-746319312409-2799$04.00/0
zation for the past several decades, microemulsion polymerization was only reported by Stoffer and Bone" in 1980. They polymerized MMA and methyl acrylate (MA) in oil-in-water(o/w)microemulsionswithout much success due to phase separation during polymerization. Such a phase separation was also encountered by Jayakrishan and Shah.12 Although Atik and Thomas13 were able to polymerize styrene in an o/w microemulsion stabilized by CTAB and hesanol, the system contained only 1.9 wt % styrene. Gulari et al.14J6 and Vanderhoff et al.leJ7later successfully polymerized styrene of about 3-6 wt % in a microemulsionstabilizedby sodiumdodecylsulfate (SDS) and pentanol. The same system was further investigated by Holdcroft and Guillet18using a pulsed laser. Raman spectroscopy had also been employed by Feng and Ngl8 to monitor the polymerizationkinetics of styrene (ca. 6 wt %) and MMA (ca. 6 wt %). The above mentioned polymerizations of styrene and MMA were all carried out in the four-component olw microemulsions which consisted of an ionic surfactant, monomer, water, and a cosurfactant such as pentanol. It would be less complicated to study microemulsion polymerization in a ternary system without a cosurfactant. Perez-Luna et aL20 were able to polymerize styrene up to 8 wt % in a ternary o/w microemulsion which required about 14 wt % dodecyltrimethylammonium bromide (DTAB). Styrene had also been polymerized in a CTABmicellar system by Ferrick et al.21to produce polystyrene latexes (11-58 nm in diameter) which were stable upon (11)Stoffer, J. 0.;Bone, T. J . Dispersion Sci. Techno[. 1980, 1, 37. (12) Jayakrishnan, A.; Shah,D. 0. J. Polym. Sci., Polym. Lett. Ed. 1984, 22, 31. (13) Atik, S. S.; Thomas, J. K. J. Am. Chem. SOC.1981, 103,4279. (14) Tang,H. I.; Johnson, P. L.; Gulari, E. Polymer 1984,25,1357. (15) Johnson,P. L.; Gulari, E. J . Polym. Sci., Polym. Chem. Ed. 1984, 22, 3967. (16) Kuo,P.L.;Turro,N.J.;Tseng,C.M.;El-Arrsser,M.S.;Vanderhoff, J. W. Macromolecules 1987,20, 1216. (17) Guo, J. S.; E l - h e r , M. S.;Vanderhoff, J. W. J. Polym. Sci., Polym. Chem. Ed. 1989,27,691. (18) Holdcroft, S.; Guillet, J. E. J. Polym. Sci., Polym. Chem. Ed. 1990,28,1823. (19) Feng, L.; Ng, K. Y. Macromolecules 1990,23, 1048. (20) Perez-Luna, V. H.; Puig, J. E.; Castano, V. M.; Rodriguez, B. E.; Murthy, A. K.; Kaler, E. W. Langmuir 1990,6, 1040. (21) Ferrick, M. R.; Murtagh, J.;Thomas, J. K. Macromolecules 1989, 22, 1515.
0 1993 American Chemical Society
2800 Langmuir,
Gan et al.
Vol. 9,No. 11, 1993 100
6 0 4
\40
----__ ---I
1001
10
PO
30
40
50
100
CTAB
Figure 1. Partial phase diagram of CTAB-watel-MMA at 30 OC: E, emulsion region; M, fluid o/w microemulsionregion; shaded area, transparent/viscous region; L, liquid crystalline region.
dilution. Similarly,the size control of polystyrene latexes and the functionalization of microparticles had been discussed recently by Antionietti et al.22*23 for a ternary microemulsion polymerization of styrene. Fine polystyrene latxes with diameters ranging from 20 to 120 nm were obtained from the system stabilized either by cetyltrimethylammonium chloride (CTAC), DTAB, or mixtures of differently charged surfactants.23 Besides styrene and MMA, tetrahydrofurfury1 methacrylate had also been successfully polymerized in three-component mi~roemulsions.~~~~~ The use of slightly polar monomers suchas MMA results in forming smaller droplets. Due to ita polarity, MMA apparently tends to lie at the surface of the polymerized particles and it seems to act as a c ~ s u r f a c t a n t .A ~~ transparent o/wmicroemulsion region adjacent to a turbid o/w emulsion region can be identified from a phase diagram of a ternary system of MMA, CTAB, and water. The characteristics of MMA polymerization in both emulsions and microemulsions from this continuous region are discussed in this paper.
in a 60 A 0.1 "C water bath. The emulsion/microemuleion contained in a ground-glass tube was f i i t degassed at 10 Torr for one freezethaw cycle. It was then transferred directly into a dilatometer which was attached to a vacuum line. The change of liquid level in the capillary of the dilatometer was monitored by a cathetometer as a function of time. The fractional conversion of MMA was determined from the volume change which was calculated from the height of the capillary. After polymerization, PMMA was precipitated in a large quantity of distilled methanol. The polymer was washed repeatedly with distilled methanol and water in order to remove the residual CTAB. The conductivity of the washing was measured periodically in order to determine the cleanliness of PMMA obtained. Molecular Weight Determination. Molecular weights of PMMA 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 GMH6 in series and the eluent was the degassed tetrahydrofuran (THF) which contained 0.025 w t % 2,6-di-tert-butyl-p-cresol as a stabilizer. The flow rate was maintained at l.OmL/min. Polystyrene standards (Polyscience) (0.2 mg/mL in THF) were used for the calibration. Particle Size Determination. Particle sizes of the polymerized microemulsion and emulsion latexes were measured by quasielastic light scattering (QLS) using a Malvern 4700 light scattering spectrophotometer. Intensity correlation data were analyzed by the method of cummulanta to provide the average decay rate. Prior to measurements, the latexes were diluted with distilled water until the volume fractions of particles were in the range of 0.014.1. The hydrodynamic radius of latex particles (Rh)was calculated from the intrinsic diffusion coefficient (Do) using the well-known Stokes-Einstein equation.
Rssults
Phase Diagram. The partial phase diagram of Figure
(22) Antonietti, M.; Bremser, W.; Miischenbom, D.; Rosenauer, C.; Schupp, B.; Schmidt, M. Mucromolecules 1991,24, 6636. (23) Antonietti, M.; Lohmann, S.; Van Niel, C. Mucromolecules 1992, 25, 1139. (24) Texter, J.; Oppenhemier, L.; Minter, J. R. Polym. Bull. 1992,27,
1 shows the boundary between emulsions (turbid) and o/w microemulsions (clear and fluid region) together with the shaded area for the clear but viscous region. Lines A and B were drawn from fixed weight ratios of water to MMA at 18.8 and 9.0 to the apex of CTAB, respectively. The systems at 30 OC changed progressively from turbid emulsions in region E to transparent fluid microemulsions in region M, to transparent viscous mixtures in the shaded area, and finally to the liquid crystalline region L as the CTAB concentration increased gradually along l i e A or line B. The dotted curve represents the ill-defined phase boundary between a turbid emulsion and a transparent microemulsion. For series A, the CTAB concentration for the emulsion-microemulsion boundary was about 4 wt % while it was about 8wt % for series B. Those systems containing CTAB less than 4 wt % for series A or 8 wt % for series B were turbid emulsions which separated into two clear phases on long standing. The lower phase of MMA-saturatedmicellar solution was in equilibriumwith an excess MMA in the upper phase. Viscosities of the Systems. The effect of CTAB concentration on viscosities of the systems at 30 OC before polymerization is shown in Figure 2. The viscosities of both series A and B increased very slowly from about 1 to 3 CPs at 30 "C as the CTAB concentration was increased from 1to 8 wt 5%. But the viscosity increased quite rapidly on further increasing the CTAB concentration. The transition of spherical to rod-shaped micelles at higher CTAB concentrations may be responsible for the sharp change in the viscosity at room temperature. It is known26 that aqueous solutions containingmore than 9 wt 9% CTAB are deformed to elongated (rod-shaped)micelles owing to the increasing crowding of the strongly hydrated micelles
487. (25) Full,A. P.; Puig, J. E.; Gron, L. U.; Kaler, E.W.; Minter, J. R.; Mourey, T. H.; Texter, J. Mucromolecules 1992,25, 5157.
(26) Ekwall,P.; Mandell, L.; Solyom, P. J. ColloidZnterfuceSci. 1971, 35, 519.
Experimental Section Materials. Methyl methacrylate (Fluka)was vacuum-distilled at 2.5 Torr (21 OC). Cetyltrimethylammonium bromide (CTAB) and potassium persulfate (KPS) from Fluka were recrystallized fromethanol-acetonemixture (1:3by volume) anddietilled water, respectively. Phase Diagram of Three-Component System. The clear region (o/w microemulsion) was determined visually by titration of distilled water into a certain amount of MMA and CTAB mixture in a screw-capped tube at 30 OC. It was thoroughly mixed with a vortex mixer. The clear-turbid boundaries were established from systematic titrations. The partial phase diagram is shown in Figure 1. The shaded area represents the clear but rather viscous region. Polymerization. Polymerization of MMA from both clear and turbid regions was carried out in a glaes dilatometer immersed ~
Polymerization of MMA in Ternary Systems
Langmuir, Vol.9,No. 11, 1993 2801
A
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Table I. Compositions for MMA Polymerization in Ternary Systems appearance of system CTAB M m water at30and6O0Ca system ( w t % ) ( w t % ) (*%) BP APb 1.00 2.00 3.00 5.00 7.00 8.00 9.00 10.00
5.00 4.95 4.90 4.80 4.70 4.65 4.60 4.55
94.00 93.05 92.10 90.20 88.30 87.35 86.40 85.45
B1
3.00 4.00 5.00 7.00 10.00 11.00 12.00
9.70 9.60 9.50 9.30 9.00 8.90 8.80
87.30 86.40 85.50 83.70 81.00 80.10 79.20
B2
B3 B4
B5 B6 B7
T
T T CF CF CF CF CV (30 "C) CF (60"C) T T
T T CF CF CF
1
20
,
30
I
40
Polymer conversion (%)
Figure 2. Effect of CTAB concentration on viscosities of series A and B at 30 "C before polymerization. The dashed lines represent the viscosities (30 "C) of series A and B after polymerization.
A1 A2 A3 A4 A5 A6 A7 A8
1
10
TF TF TF TF TF TF TF TF TF TF TF TF TF TF TF
a BP,before polymerization;AP,after polymerization; T, turbid; CF, clear fluid;CV, clear viscous;TF, translucentfluid. * All systems were polymerized at 60 "C using 0.6 mM KPS (baaed on water).
in the solution. But these solutions contain only spherical micellesz632' at temperature above 45 "C. The change in viscosity for both series A and B after polymerization is represented by dashed lines as also shown in Figure 2. The increase in viscosity for the polymerized systems containing higher CTAB concentrations was also probablydue to a large amount of surfactant, which would form elongated micelles (as in unpolymerized systems), present at the end of polymerization. However, we are uncertain about viscosities of the polymerized systems which were generally lower than their corresponding unpolymerized systems. Kinetics of Polymerization. Two series of compositions with increasing CTAB concentrations as shown in Table I were used for the polymerization study. Series A contained about 5 wt 5% MMA, while it was about 9 wt 5% MMA for series B. Samples A1 to A3 and B1 to B4 were emulsions which appeared rather turbid before polymerization and they became translucent after polymerization. SamplesA4to A7 and B5 to B7 were microemulsions which were less transparent after polymerization. The polymerization rate-conversioncurves (Figure 3) show only two distinct regions (interval I and interval 111) for (27)Sepulveda, L.; Gamboa, C. J. ColloidZnterface Sci. 1987,118,87.
Figure 3. MMA polymerization rate curves at 60 O C for emulsions (BIand B3) and microemulsions (Bsand BY). -3.25
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microemulsion polymerizations of samplesB5 and B7. But an additional rate plateau region (interval 11)appears in emulsion polymerizations of samples B1 and B3. Such a rate-plateau region is commonly observed in general emulsion polymerizations.28 The polymerization rate curves for series A are quite similar to those of series B. The rate of polymerization (R,) increased with polymer conversion up to a maximum of about 15% conversion for the emulsions and to about 20% for the o/w microemulsions. This is perhaps the first paper to report on the disappearanceof interval I1of an emulsion polymerization when the CTAB concentration in the emulsion is further increased to become a microemulsion. Further work on this aspect of polymerization will be published later. The effect of CTAB concentration on the initial rate of polymerization at about 5 % conversion (Rp)ifor both series A and B is shown in Figure 4. In order to normalize the slight variation of MMA concentrations in both series,
Figure4isplottedaslog(R,)iagainstlog([CTABI/EMMAl) instead of log[CTABl. The R, for both series A and B varied with [CTAB]0.31for emulsions but [CTAB1°.58for microemulsions. Although R, for series A decreased substantially across the emulsion boundary to the microemulsion region, no significant change in R, across the boundary was observed for series B. The effect of MMA concentration on R, in this boundary region is not yet clear and further study in this aspect is underway. The (28) Gardon, J. L. In Polymerization Processes; Schildknecht, C. E., Skeist, I., Eds.; Wiley-Interscience: New York, 1977; Chapter 6.
Can et al.
2802 Langmuir, Vol. 9, No.11, 1993 -3.2 I
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Table 11. Some Information about PMMA Latexes Prepared at 60 OC Using 0.6 mM KPS system A1 A2 A3 A4 A5 A7 B1 B3 B5 B6
-3.55
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1
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I
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I
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Log [KPS] (m%is/L/sec) Figure 5. Effect of KPS concentration on the initial rate of MMA polymerization at 60 OC.
14 0
2
4
6
8
10
12
CTAB (wt%)
Figure 6. Effect of CTAB concentrationon hydrodynamicradius (&) of PMMA latexes. dependency of R, on KPS concentration (Figure 5) was found to be 0.82 and 0.33 for emulsion A3 and microemulsion A5, respectively. The overall activation energies of MMA polymerization (E,) were about the same for emulsion A3 (81kJ m o l 3 and microemulsion A5 (85 kJ mol-') as obtained from the Arrhenius plots. Particle Sizes and Molecular Weights. The hydrodynamic radii (&) of latexes for both series A and B decreased in parallel when the CTAB concentration was increased from 1to about 8 wt % as shown in Figure 6. R h approached the minimum at about 9 wt % CTAB, i.e., 16 and 25 nm for series A and B, respectively. But Rh for both series increased on further increasing CTAB concentrations. High molecular weights of PMMA (Mw= (5-7) X lo6) were obtained for both series A and B. They did not seem to be significantly affected by the CTAB concentration. Some information about polymer latexes is summarized in Table 11. The number of polymer particles per milliliter of latex is denoted by Nd, while the number of polymer chains contained in each polymer particle is denoted by Np' Nd was calculated from the volume fraction of latex (9) and Rh, while N p was calculated from Mw and Nd. In series A, Nd increased from about 9 X 1014mL-' of an emulsion containing 1w t % CTAB to about 7 X 1015mL-l for a 9 wt % CTAB microemulsion. At a given CTAB concentration, Nd was much smaller for series B than that of series A. The values of N p for series A were about 1to
CTAB
Rh
(wt %)
(nm) 25.4 24.1 22.9 19.6 17.0 15.8 31.8 26.7 25.4 30.4
1 2 3 5 7 9 3 5 10 11
Nd iw, (1016mL-1) (lo6) hTw/Mn N p 0.89 1.08 1.41 5.0 4.9 3.8 5.8 4.0 1.9 2.76 5.10 6.1 3.6 0.9 7.39 6.2 3.6 0.8 0.91 6.0 3.6 9.4 1.62 6.6 3.6 4.6 2.36 6.7 3.2 3.2 1.48 6.3 4.0 4.0
2 from microemulsion polymerizations and about 4 from emulsion polymerizations. They increased to 3-5 for microemulsion systems (10-11 w t % CTAB) and 5-9 for emulsion systems (3-5 w t % CTAB) of series B. Discussion The polymerization loci (monomer-swollen micelles or microemulsion droplets) are known to increase with the emulsifier concentration. The rate of polymerization is thus strongly dependent on the emulsifier concentration if the micellar nucleation mechanism is prevailed. Though we could not confidently determine the number of monomer-swollenmicelles and microemulsion droplets by quasielastic light scattering measurements, they are estimated to be in the order of 10'' per mL of emulsion or 10l8 per mL of microemulsion. On the other hand, the number of polymer particles per mL of latex (Nd)for the systems listed in Table I1 were confidently determined. As can be clearly seen, Nd increased from about 10l6to 7 X 1015for series A and 1015to 2.5 X 1015for series B as the CTAB concentration was increased from 1to 9 w t % and 3 to 10 w t %, respectively. Due to the relatively high solubility of MMA in the aqueous phase, micellar as well as homogeneous nucleation mechanisms will be discussed for the microemulsion polymerization. As for the emulsion polymerization, an additional nucleation mechanism of polymer particles in the fiie emulsified oil droplets is also considered in the subsequent discussion. As mentioned in the previous section, the emulsions before polymerization could be separated into two clear phases on long standing, i.e., an upper phase of excess MMA which was in equilibrium with the lower phase of MMA-saturated micellar solution. The percentages of excess MMA in samples A l , A2, A3, B1, and B3 were determined at 60 OC and they constituted 61,57,27,81, and 55 wt % of the total amount of MMA, respectively. The observed interval I1 regions for both series A and B were due to the presence of monomer droplets which acted as monomer reservoirs in emulsion polymerizations. Baxendale et alZ9in 1946 suggested that the initiation of MMA emulsion polymerization occurred in the aqueous phase (homogeneous nucleation) and the rate of polymerization was constant until high conversion. But Zimmt30 found that the rate of MMA polymerization was only constant for small particles (a true steady-state rate only for swollen diameter less than 100 nm), while the rate for larger particles showed an increase in time due to the gel effect. In an extensive study by Germes31in latexes with small particle sizes (50-100 nm swollen diameter), it was (29) Baxendale,J. H.; Evans,M.G.;Kilham, S . K. J . Polym. Sci. 1946, I , 466. Baxendale,J. H.; Bywater, S.;Evans, M. G. Tram.Faraday SOC. 1946, 42, 675. (30)Zimmt, W.S . J. Appl. Polym. Sci. 1959, 1 , 323. (31)Gerrens, H. 2.Elektrochen. 1963, 67, 741.
Polymerization of MMA in Ternary Systems found that there was a constant rate of polymerization when monomer droplets were present. Ballard et al.32 concluded that MMA emulsion polymerization in seeded systems of particle diameter ranging from 74 to 136 nm also showed the rate of interval I1 (rate plateau region). The interval I1 regions observed in these MMA emulsion polymerizationsare thus expected because of the presence of monomer droplets and small latex particles (46-64 nm in diameter) in these systems. In the absence of oil droplets, the MMA-microemulsion polymerization showed only two rate intervals; i.e., the rate increased to a maximum in interval I due to continuous particle nucleations and then it decreased in interval I11 due to the depletion of monomer concentration in the growing polymer particles. The similar two rate intervals have also been observed for microemulsion polymerization of and MMA.35 It is envisaged that free radicals generated by KPS in the aqueous phase would not only react with MMA via the usual micellar nucleation but also likely react with a relatively large amount of MMA (0.15mol dm-3)dissolved in the aqueous phase to form oligoradicals (homogeneous nucleations). The homogeneous nucleation mechanism may be important for the system containing low surfactant concentration as in an emulsion. As chain lengths of the oligoradicals increased, they could diffuse into the monomer-swollen micelles and the monomer droplets to continue the polymerization. In other words, anion radicals produced by KPS might react with some MMA molecules in the aqueous phase to form surface-active compounds of anion radicals.36 These surface-active radicals can readily exchange with micellar surfactants and those surfactants stabilizing oil droplets to continue the polymerization. The low CTAB concentration dependency (0.31) on R, for emulsion polymerizations of MMA may be due to this significant homogeneous nucleation. On the other hand, the micellar nucleation mechanism is possibly predominant in microemulsionpolymerizations due to larger interfacial areas available for capturing the free radicals generated in the aqueous phase. The homogeneous nucleation of MMA in microemulsions is thus relatively less significant. This is reflected in a relatively higher CTAB concentration dependency (0.58) on R, for microemulsionpolymerizations as compared to that (0.31) of emulsion polymerizations. The strong dependency of R, on KPS concentration (0.82) for the emulsion polymerization of sample A3 (3 w t % CTAB) implies that the homogeneous nucleation of MMA in the aqueous phase as well as the initiation in emulsified monomer droplets might be very important on top of the micellar nucleation. This is in contrast with the low power dependency (0.33) of R, on KPS concentration for the microemulsion polymerization of sample A5 (7 wt % CTAB) which contained no excess oil droplets. This low dependency (0.33) is quite comparable to that (0.47) for the microemulsion polymerization of styrene.34 (32) Ballard,M. J.; Napper, D. H.; Gilbert, R. G.J. Polym. Sci., Polym. Chem. Ed. 1984,22,3225. (33) Gan, L. M.; Chew, C. H.; Lye, I.; Imae, T. Polym. Bull. 1991,25, 193. (34) Gan,L. M.;Chew,C. H.;Lye, 1.Makromol. Chem. 1992,193,1249. (35) Gan, L. M.; Chew, C. H.; Ng, S. C.; Lee, K.C. Polymer, in press. (36) Karaman, M. E.; Meagher, L.; Pashley, R. M. Langmuir 1993,9, 1220.
Langmuir, Vol. 9, No. 11, 1993 2803
Figure 7. Transmission electron micrograph of latex particles for sample & (emulsion).
The particle sizes for series B (Rh = 25-32 nm) were generally larger than those of series A ( R h = 16-25 nm) due to higher MMA concentrations in the former systems. Emulsion systems also produced larger polymer particles than those from microemulsion systems. This is mainly due to a relatively smaller number of monomer-swollen micelles in emulsions as compared to a larger number of microemulsiondroplets. The increasein& for the systems containing more than 9 wt % CTAB may be simply due to interactions between CTAB micelles and polymer particles as measured by QLS. Figure 7 shows a transmission electron micrograph of latex particles (emulsion A3) which appear to be heterogeneous in sizes (ca. 20-60 nm in diameter). Conclusion Turbid emulsions can be changed progressively to transparent microemulsions by merely increasing the CTAB concentration in three-component systems which consisted of water, CTAB, and low amounts of MMA (ca. 5-9 wt %). Polymerizations of MMA in both emulsions and microemulsions produce stable latexes (translucent) of microparticles ranging from 16 to 30 nm in R h . High molecular weights of PMMA ((5-7) X lo6) can also be obtained from both systems. Only two rate intervals are observed for the microemulsion polymerizations using higher CTAB concentrations (ca. 7-11 wt %). However an additional constant rate interval is observed for emulsionpolymerizationsusing lower CTAB concentrations (ca. 1-4 wt %). The concentration dependencies of CTAB and KPS on the rate of MMA polymerization are found to be the power of 0.58 and 0.33, respectively, for the microemulsions, but they are 0.31 and 0.82 for the emulsions. This is attributed to a significant particle nucleation in fine emulsified oil droplets in emulsions, in addition to micellar and homogeneous nucleations. Acknowledgment. This work is supported by the National University of Singaporeunder Grants RP 840038 and RP 890638. The authors thank Mdm. Loy (Zoology Department) for the help in taking transmission electron micrographs.
