449
Langmuir 1996,11, 449-454
Effects of Surfactant Concentration on Polymerizations of Methyl Methacrylate and Styrene in Emulsions and Microemulsions L. M. Gan, K. C. Lee, C. H. Chew, and S. C. Ng* Department of Chemistry, National University of Singapore, Republic of Singapore Received May 4, 1994. I n Final Form: August 15, 1994@ Two ternary systems which can be changed continuously from turbid emulsions to transparent microemulsions have been chosen for polymerization study of methyl methacrylate (MMA) and styrene. The effects of surfactant concentration, either tetradecyltrimethylammonium bromide (TTAB) or octadecyltrimethylammonium chloride (OTAC),on the rate of polymerization (R,), number of latex particles (Nd) and molecular weights ofthe polymers (M,) have been investigated. The rate of emulsion polymerization of MMA was found to be faster than that of the microemulsion polymerization. But it was reverse for the styrene systems. Weak dependency of R, on the surfactant concentration was observed for the emulsion polymerization of MMA (R,= [OTAClo.13).However, the dependency was stronger for its microemulsion polymerization (R,= [OTAC1°.56).Similarly, the dependency of Nd on the OTAC concentration was also different between emulsion and microemulsion polymerization of MMA. This is in contrast with the styrene polymerization, which showed only a single dependency for both emulsion and microemulsion systems (R,= ["TAB]0.52and Nd = [!M'AB]2,2). Possible mechanisms related to micellar as well as homogeneous nucleations are discussed to account for the similarities and the differences between the polymerization of MMA and styrene.
Introduction
This is why rather polydispersed particles of nanometer size are always observed from microemulsion polymeriThe general kinetics of emulsion polymerization involves zation. particle nucleation and particle growth. The particle For the past decade, the microemulsion polymerization nucleation has been generally described by the Smithof MMA or styrene has been carried out in a microemulsion Ewartl (micellar nucleation), the Roe2 (homogeneous system which requires a cosurfactant, such as pentanol. nucleation), and the HUFT (Hansen,Ugelstad, Fitch, and However, a ternary microemulsion system without a TsaiI3s4theories. The HUFT theory recognizes three cosurfactant has been found recently and the system has different loci for particle nucleation, i.e., micellar, homoalso been shown to be suitable for microemulsion polymgeneous and droplets nucleation. The main argument erization of styrenelOJ1and tetrahydrofurfuryl methacryfor the micellar theory is that micelles are so numerous late.12 With this new development, it is now possible to above the critical micelle concentration (cmc) that they study the polymerization of MMA or styrene in both have high possibility for nucleation. However, the hoemulsion and microemulsion systems using three identical mogeneous nucleation mechanisms may be important for components, i.e., water, monomer, and surfactant. The monomers of high water solubility in systems with low ternary system can be continuously changed from turbid surfactant concentrations. emulsions to transparent microemulsions by simply Although emulsion polymerization has been extensively increasing the surfactant concentration. studied for many decades, the first microemulsion poWe have reported the first continuous system13for the lymerization of methyl methacrylate (MMA) was only polymerization of MMA using a cationic surfactant reported in 1980 by Stoffer and Bone,5 followed by Atik hexadecyltrimethylammonium bromide (HTAB). It merand Thomas6 in 1981 on the polymerization of styrene. its further investigation on the effect of the surfactant For microemulsion polymerization of water-soluble acrylconcentration on the polymerization of different types of amide, Candau et al.7 suggested a continuous particle monomers. The aim of this paper is to study the nucleation process in the water-in-oil (w/o)microemulsion polymerization of polar MMA and nonpolar styrene in droplets. On the other hand, microemulsion polymeritwo ternary-component systems covering emulsions and zation of oil-soluble styrene is carried out in the oil-inmicroemulsions using cationic surfactant octadecyltriwater (o/w) microemulsion dropletss and the particle methylammonium chloride (OTAC) and tetradecyltrinucleation is also believed to be a continuous p r o c e ~ s . ~ methylammonium bromide (TTAB), respectively.
* To whom correspondence should be addressed at the Depart-
ment of Physics, National University of Singapore. Abstract Dublished inAdvance ACSAbstracts, January 1,1995. @
(1)Smith, W. V.; Ewart, R. H. J . Chem. Phys. 1948, 16, -592. (2) Roe, C. P. Ind. Eng. Chem. 1968,60, 20. (3) Fitch, R. M.; Tsai, C. H. In Polymer Colloids; Fitch, R. M., Ed.; Plenum Press: New York, 1971; pp 73 and 103. (4) Hansen, F. K.; Ugelstad, J. In Emulsion Polymerization; Piirma, I., Ed.; Academic Press, New York, 1982; p 51. (5) Stoffer, J. 0.;Bone, T. T. Dispersion Sci. Technol. 1980, 1 , 37. (6)Atik, S. S.; Thomas, J. K. J . Am. Chem. SOC.1981, 103, 4279. (7) Candau, F.; Leong, Y. S.;Fitch, R. M. J . Polym. Sci., Polym. Chem. Ed. 1985, 23, 193. (8)Guo, J. S.; El-Aasser, M. S.; Vanderhoff, J. W. J . Polym. Sci., Polym. Chem. Ed. 1988,27, 691. (9) Guo, J. S.; Sudol, E. D.; Vanderhoff, J. W.; El-Aasser, M. S. J . Polym. Sci., Polym. Chem. Ed. 1992,30,691 and 703.
0743-746319512411-0449$09.00/0
Experimental Materials. OTAC and TTAB from Tokyo Chemical Industry were recrystallized from an ethanol-acetone mixture (1:3 by volume). MMA and styrene from Fluka were vacuum distilled at 2.5 Torr and 21 "C. Potassium persulfate ( U S )was
recrystallized from doubly distilled water. Standard polystyrene of known molecular weights was obtained from Polyscience Inc. (10) Perez-Luna, V. H.; Puig, J. E.; Castano, V. M.; Rodriguez, B. E.; Murthy, A. K.; Kaler, E. W. Langmuir 1990,6, 1040. (11)Antonietti, M.; Bremser, W.; Miischenborn, D.; Rosenauer, C.; Schupp, B.; Schmidt, M. Macromolecules 1991,24, 6636. (12) Full, A. P.; Puig, J. E.; Gron, L. U.; Kaler, E. W.; Minter, J. R.; Mourey, T. H.; Texter, J. Macromolecules 1992,25, 5157. (13) Gan, L. M.; Chew, C. H.; Ng, S. C.; Loh, S. E. Langmuir 1993, 9, 2799.
0 1995 American Chemical Society
Gan et al.
450 Langmuir, Vol. 11, No. 2, 1995 Table 1. Compositions for Polymerization of MMA MMA OTAC watel.b waterl type of system (wt%) ( w t % ) (wt%) MMA polymerizationQ 9 E 4.0 86.4 M01 9.6 9 E 6.0 84.6 M02 9.4 M03 8.0 82.8 9 E 9.2 81.0 9 E/ME 10.0 M04 9.0 9 ME 12.0 79.2 M05 8.8 9 ME 13.0 78.3 M06 8.7 9 ME 14.0 77.4 M07 8.6 E, emulsion polymerization (turbid) at 60 "C; ME, microemulsion polymerization (transparentkranslucent)at 60 "C. KPS,0.15 mM based on the water content. a
Polymerization. The dilatometric method was used to study the kinetics of microemulsion polymerization. The dilatometer consisted of a 10-mL Erlenmeyer flask with an attached 40 cm long capillary, 2 mm in diameter. The microemulsion in a groundglass tube was first frozen by liquid nitrogen and degassed at about 10 Torr for one freeze-thaw cycle. It was then introduced directly into the dilatometer and placed in a thermostatic bath at 60 0.1 "C. The change of liquid level in the capillary of the dilatometer was monitored by a cathetometer as a function of time. The polymer conversion was calculated from the volume change of liquid in the capillary. Particle Size Determination. Particle sizes of microemulsion latexes were determined using a Malvern 4700 light scattering spectrophotometer. Prior to t h e measurements, the microemulsion latexes were diluted with Millipore filtered water until the volume fractions of particles were in the range of 0.01 to 0.1. An average hydrodynamic radius of latex particles (Rh) was calculated from the intrinsic diffusion coefficient (Do), i.e., Rh = kT/6nqDo, where q is the Viscosityofthe continuous medium. The polydispersity ( P d ) which corresponds t o t h e variance of t h e size distribution was obtained using the computer software provided with the Malvern 4700. Molecular Weight Determination. The polymerized microemulsion latexes were precipitated in a large quantity of methanol. The polymer was washed until free of the surfactant. Molecular weights of polystyrene (PSI and poly(methy1 methacrylate) (PMMA) were determined by gel permeation chromatography (GPC) using a Varian 5500 liquid chromatography system equipped with 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 wt % 2,6-di-tert-butyl-p-cresol as a stabilizer. The flow rate was maintained at 1.0 m u m i n . Polystyrene Standards (0.2 mg/mL in THF) were used for the calibration. Since some fractions of the polymer have very high molecular weights, the GPC results are not absolute due to the exclusion limits of the columns (-4 x lo8) and they are only used for qualitative study.
*
Results The phase diagrams of two ternary-component systems consisting of MMA/OTAC/H20 and styrene/"AB/HzO which have been investigated exhibit relatively large olw microem~lsions.~~J5 The change from a turbid emulsion to a transparent microemulison for each of these systems can readily be attained by merely increasing the surfactant concentration. This unique phase behavior renders the systems ideal for studying and comparing the characteristics of polymerization of MMA and styrene due solely to the effect of the surfactant concentration. Tables 1 and 2 list the compositionsused in this study for polymerization of MMA and styrene, respectively. For the MMA system, the weight ratio of water to MMA was kept constant a t 9, while the surfactant (OTAC) concentration was increased from 4 to 14 wt % which covered both emulsion and microemulsion regions. The (14)Gan, L. M.; Chew, C. H.; Lee, K. C.; Ng, S. C. Polymer 1993,34, 3064. (15) Gan, L. M.; Chew, C. H.; Lee, K. C.; Ng, S. C. Polymer 1994,12, 2659.
Table 2. ComDositions for Polmerization of Stwene styrene TTAB watel.b waterl type of system (wt %) (wt %) (wt %) styrene polymerizationa 89.5 12 E 7.5 3.0 ST1 87.7 12 E 7.3 5.0 ST2 85.8 12 E 7.2 7.0 ST3 84.0 12 ME 7.0 9.0 ST4 82.6 12 ME 6.9 10.5 ST5 81.2 12 ME 6.8 12.0 ST6 ~
~~
a E, emulsion polymerization (turbid) at 60 "C; ME, microemulsion polymerization(transparentltrans1ucent)at 60 "C. KPS, 0.15 mM based on the water content.
*
turbid emulsions, designated by E, were obtained for the samples M 0 1 to M 0 3 containing less than 9 wt % OTAC. With the OTAC concentration greater than 9 w t %, transparent microemulsions designated by ME were formed as represented by samples M05 to M07. Similarly, the weight ratio of water to styrene was fmed a t 12 for the styrene system using "TAB which was increased from 3 to 12 wt %. Samples ST1 to ST3 were emulsions while samples ST4 to ST6 were microemulsions. Polymerization of MMA. The MMA polymerization as a function of OTAC concentration in both emulsions ( M o l and M03) and microemulsions (M04 and M07) is shown in Figure 1. It is to be noted that the rates of emulsion polymerization were faster than those of microemulsion polymerization. In addition, only two polymerization rate intervals (Figure 1B)were observed for both emulsion and microemulsion polymerizations. The polymerization was characterized by a n increase in the rate of polymerization (R,) to a maximum, interval I, and then a decrease in interval 11. The maximum R , for the emulsion polymerization occurred at about 40% conversion, while it was a t a lower conversion (20-30%) for microemulsion polymerization. The effect of OTAC concentration on the initial rates of MMA polymerization obtained a t 5% conversion (RPhis shown in Figure 2 for both emulsion and microemulsion systems. In order to normalize the slight variation of MMA concentrations in both systems, Figure 2 is plotted as log (RJiversus log([OTACY[MMAl) instead of log[OTAC]. (Rp)lvaried with [OTAClo,13for emulsions but with [OTAC]o.56for microemulsions. R, was higher for the emulsion polymerization but its dependence on the surfactant concentration was less than that of the microemulsion polymerization. It decreased substantially across the emulsion to the microemulsion boundary region before it increased slightly again. This is a n unexpected observation. Other properties of MMA latexes are listed in Table 3. The average Rh of the final PMMA latex particles was generally very small. Rh decreased rather linearly from about 30 to 23 nm with the increase of OTAC from 4 to 8 w t % in the emulsion region. But it seems to increase very slightly in the microemulsion region with the OTAC concentration increasing from 12 to 14 wt %. Such a trend of variation in the size of PMMA latexes has also been observed in another ternary system using HTAB.13 The number of latex particles per milliliter (Nd) was calculated from the volume fraction of the latex. From M , andNd, the number ofpolymer chains per latex particle (n,) can be roughly estimated. High molecular weights (M,) of PMMA (7-9 x 109 were obtained as shown in Table 3. Figure 3 illustrates the effect of the OTAC concentration on Nd which increased in emulsions up to 8 wt % OTAC and then decreased in microemulsions with OTAC greater than 10 w t %. The dependency obtained is Nd = [OTAC]1,6for the emulsion region. But inverse dependence of Nd = [OTAC]-0.39is found for the microemulsion region. It should be noted that Nd obtained
Surfactant Concentration on Emulsions
Langmuir, Vol. 11, No. 2, 1995 451 -3.35 T -3.4
h
2In E
-3.46 I
Q
'4
I
v
I
E E.
-3.5
I
I I
0
9
-3.55
1
5
0
1 0
-3.8 -1
15 20 25 30 35 40 45 50 55 60
TIME
(min)
I1
,
I
-0.9 -0.8 -0.7
,
-0.6 -0.5
~l
-0.4
I
-0.3 -0.2
LOG ([OTAC]/[MMA])
Figure 2. Effect of OTAC concentrationon the initial rates of MMA polymerization in both (A) emulsions (E) and ( 0 )
-3
.
.
-3.2
microemulsions (ME).
Table 3. Effect of OTAC Concentration on Some Properties of PMMA Latexesa
. i In
M01 (E) M02(E) M03 (E) M04(ME) M05(ME) M06(ME) M07(ME)
al -
-3.4 Y
Q
K
P
0
0
9
0 0
-3.6
. -.-
0
I I 20
10.0
12.0 13.0 14.0
30.0 25.3 23.3 23.1 22.0 22.6 23.2
0.199 0.151 0.120
1.09 2.09 3.05 3.55 4.47 4.30 4.13
0.108
0.115 0.114 0.111
7.6 3.7 2.3 1.8 1.5 1.6 1.7
7.03 12.94 7.47 9.72 8.09 7.89 8.78 7.08 8.13 9.46 7.66 10.21 7.41 11.87
a P d , polydispersity of particle size; Nd, number of particles per mL of latex;np, number of PMMA chains per particle of latex; M w / M,,molecular weight distribution.
I
10
4.0 6.0 8.0
30
40
50
BO
70
POLYMER CONVERSION (%)
Figure 1. (A, top) Effect of OTAC concentration on MMA polymerizationusing 0.15 mM KPS at 60 "C: B, M O l ( 4 wt %); V, M 0 3 (8 wt %); A, M 0 4 (10wt %); 0, M 0 7 (14 wt %). (B, bottom) Effect of OTAC concentration on the rate of polymerization as a function ofpolymerconversion: B, M O l ( 4 wt %); V, M 0 3 (8 wt %); A, M 0 4 (10wt %); 0, M 0 7 (14 W t %).
I
.I
15.5-
. y 3 E
15.4-
0
from the boundary region at 10 wt % OTAC did not fit into these linear relationships. Polymerizationof Styrene. From the investigation of the ternary phase diagram, cationic surfactant OTAC is found to be suitable for forming a relatively wide range of MMA-containing microemulsions, but it only produced a narrow range of the styrene-containing microemulsions. However, a rather broad region of styrene-containing microemulsions can be prepared using a relatively more water-soluble TTAB. This is the reason why OTAC was used in the MMA system and TTAB in the styrene system. The effect of TTAB concentration on the rate of styrene polymerization as a function of polymer conversion is shown in Figure 4 for both emulsions (ST1 and ST2) and microemulsions (ST4 and ST6). The rates of styrene polymerization (R,) in microemulsion were found to be higher than those in emulsions. This rate behavior, which isjust reverse to that ofthe MMA system, is quite expected. In addition, three distinct regions of R, were observed for the styrene-emulsion system as compared to only two rate intervals for the MMA-emulsion system. But the rate-plateau region (interval I1in Figure 4) became narrow
l-
-
a
8
15.3-
0
2 ~
15.2-
9 l5.I
t /
15 -1
I
-0.9
-0.8
I
1
1
I
1
-0.7
-0.6
-0.5
-0.4
-0.3
-1 .2
(1OTAC]/[MMA]) Figure 3. Effect of OTAC concentration on the number of PMMA particles per mL of latex (Nd) in both (B) emulsions (E) LOG
and ( 0 )microemulsions (ME).
as the concentration of TTAB was increased and it finally disappeared in the microemulsion region. In contrast to the MMA polymerization, only a single linear line was found for the plot of log (R,)i vs log([TTABl/ [styrene]) for the styrene polymerization throughout the emulsion and microemulsion regions as shown in Figure 5. The dependency is (R,)i = [TTAB]0.52.Table 4 shows some properties of polystyrene latexes. It can be seen
Gun et al.
452 Langmuir, Vol. 11, No. 2, 1995 -3.4
i5 . 5
1 2 E 5 W
15-
A
0
k
4:
&
14.5-
-0
z
0
9
14-
0
I -4.2 4-a
c
I
I I._
0
1 0
20
40
30
50
60
-1
70
-0.9 -0.8
-0.7
-0.6
-0.5
-0.4
-0.3
-0.2
LOG ([TTAB]/[ STY RENE])
POLYMER CONVERSION (%)
Figure 4. Effect of TTAB concentration on the rate of polymerization as a function of polymer conversion: V,ST1 (3 wt %); 0 , ST2 (5 w t %); U, ST4 (9 wt %); A, ST6 (12 wt %).
Figure 6. Dependency of number of particles (Nd)on the TTAB concentration in both ( 0 )emulsions(E)and (0) microemulsions (ME). Table 4. Effect of TTAB Concentrationon Some Properties of PS Latexesa
-3.5
-3.8
ST1 (E) ST2(E) ST3 (E) ST4(ME) ST5 (ME) ST6 (ME)
-
3.0 5.0 7.0 9.0 10.5 12.0
66.0 52.4 39.5 33.7 29.9 28.2
0.256 0.244 0.139 0.126 0.126 0.122
0.78 1.83 4.91 8.94 13.5 17.9
45.6 18.2 7.2 3.8 2.3 1.7
1.29 14.5 1.34 8.08 1.23 5.44 1.27 6.36 1.33 8.12 1.37 5.47
2-
-3.7
4
-3.8
a P d , polydispersity of particle size; Nd, number of particles per mL of latex; np, number of PS chains per particle of latex; MwiMn, molecular weight distribution.
-3.9
Table 5. Some Linear Relationships for Emulsion and Microemulsion Polvmerizations of MMA and Stvrenea
-E
styrene
MMA --
I
I
1
-0.9 -0.8 -0.7
-0.6
parameters
I
E
ME
E
ME
~
-1
-0.5
-0.4
-0.3 -0.2
LOG ([TTAB] / [STY R E NE])
Figure 5. Effect of TTAB concentrationon the initial rates of styrene polymerization in both ( 0 ) emulsions (E) and (0) microemulsions (ME).
-
Nd a
that with increasing TTAB concentration, Rh decreased substantially from 66 to 40 nm in the emulsion region, but it decreased only slightly from 34 to 28 nm in the microemulsion region. Figure 6 is a linear plot of log Nd vs log([TTABMstyrenel) and the dependency is IL'd 0~
[TTAB12.2.
Molecular weights obtained for PS were unexpectedly as shown in Table 4. No significant high (ca. 1.3 x effect of the TTAB concentration on M , of PS can be observed. But a very significant effect was observed for n, which decreased continuously from about 45 to 2 a s the TTAB concentration was increased from 3 to 12 wt %. The decrease ofn, was rather sharp from 45 to 7 in the emulsion region, while i t only decreased slightly from 4 to 2 in the microemulsion region.
Discussion Some characteristics of emulsion and microemulsion polymerization of MMA and styrene are summarized in Table 5 for easy comparison. For the styrene polymerization, it shows only a single dependency for R, and N d on the TTAB concentration throughout the emulsion and
-
(Rp)i
I
[OTAC]0.13 [OTAC1°.56 [TTAB1°.52 [TTABlo.52 [OTAC]',6 [OTACl-o.39 [TTABI'.' [TTABJ'.'
E, emulsion; ME, microemulsion; (R,,),,,
maximum rate of
polymerization;(I?,)!, initial rate ofpolymerization at 5% conversion; Nd, number of particles per mL of latex. microemulsion regions. The visual phase boundary between emulsion and microemulsion does not seem to alter the course of styrene polymerization for this ternary system. But a marked effect of the OTAC concentration on these parameters appears across the phase boundary between for emulsion and microemulsion polymerizations of MMA. The striking difference between these two ternary systems arises from the use of different monomers and surfactants. The solubilities of MMA and styrene and their oligomers in the aqueous phase are different. Surfactants with high hydrophile-lipophile balance (HLB) are generally known to be good stabilizers for oil-in-water emulsions and microemulsions. It has been reported16 that optimum stability of PS latexes was obtained from using surfactantsofhigher HLB, 13-16, while lower HLB surfactants, 12.1-13.7,gave optimumstabi1ityforPMM.A (16)El-Aasser, M. S. In Scientific Methods for the Study of Polymer Colloids and TheirApplications; Candau, F., Ottewill, R. H., Eds.; NATO AS1 Series C No. 303; Kluwer Academic: Dordrecht, 1990; p 1.
Surfactant Concentration on Emulsions
latexes. Though we are unable to quote the HLB values for TTAB and OTAC, it is predicted that TTAB has higher HLB than that of OTAC because of its shorter hydrophobic chain length. This is consistent with the combinations of MMA with OTAC and styrene with TTAB for the present study. Similar strong dependency of R, on the surfactant concentration is observed not only for styrene system (0.52) but also for the MMA polymerization in microemulsions (0.56). The dependency of 0.52 or 0.56 is close to the predicted value of 0.60 based on the Smith-Ewart micellar nucleation mechanism. However, it is in sharp contrast with the weak dependency of 0.13 obtained for the MMA polymerization in emulsions. This is attributed to the significant contribution by homogeneous nucleation as described by Baxendale et al.,17 in addition to micellar nucleation, in the emulsion polymerization of MMA. In view of the relatively high solubility of MMA in the aqueous phase (150 mM), free radicals generated by the water-soluble KPS would react with MMA solubilized in the aqueous phase to form soluble oligomeric radicals. The oligomeric radicals grow by further addition of MMA units. It is estimatedls that the oligomeric radicals with four or more MMA units correspond to species with sufficient surface activity for the usual micellar entry. In addition, when the oligomeric radicals grow to more than 10 units of MMA, the solubility limit causes them to precipitate. The precipitated oligomericradicals can then adsorb surfactant molecules to form particles (homogeneous nucleation) which swell with monomers and grow by propagation a s micellar particles. The number of the growing radicals is thus determined by the competition between the entry of oligomeric radicals into micelles and other already formed particles (micellar nucleation) and the oligomer precipitation (homogeneousnucleation). Both mechanisms may be operating in the emulsion polymerization of MMA as reflected by the low exponent dependency (0.13) of (R,)i on the OTAC concentration. In addition, nucleation in fine monomer droplets is also possible. On the other hand, the micellar nucleation is predominant in the emulsion polymerization of styrene due to its low solubility in aqueous phase (3.5 mM). When the systems contain much higher surfactant concentrations as in microemulsions, the micellar nucleation mechanism prevails irrespective of polymerization of MMA or styrene. This is because an enormous number of micelles are readily available for capturing oligomeric radicals before they grow to a critical size for precipitation,Is Le., 11 units of MMA or 6 units of styrene. The “shell”structure formed by absorption of surfactant on the microemulsion droplets may slightly retard the entry of oligomeric radicals and lead to a lower radical capture efficiency.8 However, the formation of polymer particles via homogeneous nucleation allows oligomeric radicals to react readily with monomers in the aqueous phase and then adsorb surfactant molecules for stabilization. This may be the main reason for the rate of emulsion polymerization of MMA to be greater than that of the microemulsion polymerization. But the decisive factor for the rate of styrene polymerization is the number of monomer-swollen micelles and microemulsion droplets, both of which are strongly dependent on the surfactant concentration. This is because the micellar nucleation mechanism is dominant in the styrene system. Hence, the rate of microemulsion polymerization of styrene is (17) Baxendale, J. H.; Evans, M. G.; Kilham, S. K. J . Polym. Sci.
1946, 1, 466.
(18)Maxwell, I. A.; Morrison, B. R.; Napper, D. H.; Gilbert, R. G. Macromolecules 1991,24, 1629.
Langmuir, Vol. 11, No. 2, 1995 453
expected to be faster as observed due to a large number of polymerization loci than that of the emulsion polymerization. The number of PMMA latex particles mL-l) was found to be much higher than that of PS latex particles mL-l). This may be taken as a n indication of the important role of homogeneous nucleation mechanism for particle formation in the MMA system. According to Smith and Ewart’s theory, which was based on the micellar nucleation mechanism, the number of latex particles formed is proportional to the surfactant concentration to the power 0.60. It should be emphasized that the profile of the particle number during the entire polymerization process depends not only on the number of particles generated but also on their stabilities against flocculation. In this study, the dependencies of Nd on the surfactant concentration for both MMA and styrene systems markedly deviated from the power 0.60. It was 1.6 for emulsion polymerization of MMA but became -0.39 in the microemulsion polymerization. On the other hand, rather high dependency (2.2) was obtained for both emulsion and microemulsion polymerization of styrene. These unexpected high dependencies are also observed for other systems. The particle number found by Fitch et aL3 was proportional to the surfactant concentration to the power 1.1 when persulfate-bisulfite-iron initiator was used for emulsion polymerization of MMA. It increased to 3.9 when hydrogen peroxide-iron initiator was used indicating greater dependency on the surfactant concentration when nonionic initiator system was employed. Nomuralg has also discussed the desorption and readsorption of free radicals in emulsion polymerization. It is believed that the radical desorption leads to a n increase in the number of latex particles. This is because the desorbed radicals can re-enter the monomer-swollen micelles and take part in nucleation. Hence, the dependency can increase beyond the power 0.6 with increasing radical desorption. On the basis of the thermodynamic considerations for microemulsion polymerization as discussed by Guo et al.,8 the present results may be explained by radical desorption and continuous nucleation mechanisms. The termination of chain growth inside the polymer particles is due very likely to the chain transfer reaction resulting in monomeric radicals. The rate of desorption of monomeric radicals from the small particles is deemed to be fast resulting in low number of radicals per particle. The monomeric radicals of MMAzOand styrenez1 can then re-enter the particles during polymerization. Moreover, Guo et al.s and Gan et al.15 also found that the number of particles increased continuously throughout the microemulsion polymerization of styrene because of the continuous nucleation. I t is the increasing number of latex particles that leads to a higher dependency greater than the power 0.6 as observed in this study. Contrary to the styrene system, Gan et alazzfound that the number of PMMA particles decreased continuously after completed polymerization, especially a t higher surfactant concentrations. It is attributed to flocculation of particles as evidenced by the increase in Rh. This explains the negative dependency of the power -0.39 which is only observed for the microemulsion polymeri(19) Nomura, M. In Emulsion Polymerization; Piirma, I., Ed.; Academic Press: New York, 1982; p 191. (20) Ballard, M. J.;Napper, D. H.; Gilbert, R. G.J . Polym. Sci., Polym. Chem. Ed. 1984,22, 3225. (21) Lichti, G.;Sangster,D. F.;Whang, B. C.Y.; Napper, D. H.; Gilbert, R. G. J . Chem. SOC.,Faraday Trans. 1 1984, 80, 2911. (22)Gan. L. M.: Lee. K. C.: Chew, C. H.; Tok, E. S.; Ng, S. C. Submitted to J . Polym. Sci., Polym. Chem.
454 Langmuir, Vol. 11, No. 2, 1995
zation of MMA. It is known that the electrolyte concentration (counterion) is appreciably increased at much higher surfactant concentration^.^^ Consequently, the compression of the electrical double layers of particles occurs leading to the instability of latex particles. But this is not the case for microemulsion polymerization of styrene. MMA, but not styrene, is known to function also as a cosurfactant in a microemulsion. The self-surfacting affect of MMA a t the interface of particles may reduce the bending rigidity of the interface and thus facilitate the flocculation of particles.
Conclusion Similar dependency (ca. 0.52) of the polymerization rate on the surfactant concentration is observed not only for (23) Ottewill, R. H. In Emulsion Polymers and Emulsion Polymerization; Bassett, D. R., Hamielec, A. E., Eds.; ACS Symp. Ser. 165; American Chemical Society: Washington, DC, 1981; p 31.
Gan et al.
emulsion and microemulsion polymerizations of styrene but also for microemulsion polymerization of MMA. This dependency is only slightly smaller than the theoretical value of 0.60 based on the Smith-Ewart theory, indicating that micellar nucleation mechanism prevails in these systems. On the other hand, the weak dependency (0.13) obtained for the emulsion polymerization of MMA is attributed to the significant contribution by homogeneous nucleation in addition to micellar nucleation. But the dependency of the number of latex particles (Nd)on the surfactant concentration for both MMA and styrene polymerizationsystems deviates markedly from the power 0.60. This is the consequence of the chain transfer reaction of growing polymer chains, which leads to the desorption and readsorption of monomeric radicals for the continuous nucleation and polymerization. LA940353+
