Langmuir 1990,6, 1040-1044
1040
Conclusions Examination of six dyes of increasing amphiphilicity in water-rich microemulsions incorporating oils of different polarity and surfactants with different charge reveals important factors for optimizing dye solubility. In almost all cases, the solubility of dye is much enhanced in microemulsions over that found in pure solvents. It appears that the primary variable is dye amphiphilicity: the more surfactant-like the dye becomes, the greater the solubility in water-rich microemulsions. From the data, it is inferred that the primary site for solubilization of polar
dyes is the surfactant-rich interfacial region separating Oil and water domains*
Acknowledgment. This work was supported by IBM, and we are grateful for the comments Of Dr*A. Registry No. SDS,151-21-3;C12E6,3055-96-7;DTAB, 1119629-14-1; pentanoi, 71-41-0; butanol, 71-36-3; 94-4; c20c20c2, toluene, 108438-3; decane, 124-18-5; Sudan 111, 85-86-9; Ethyl Orange, 13545-67-0; Acid Violet 17, 4129-84-4; Naphthyl Red, 63494-91-7; Methyl Orange, 547-58-0; Orange IV, 554-73-4.
Styrene Polymerization in Three-Component Cationic Microemulsions V. H. PBrez-Luna,?J. E. Puig,? V. M. Castaho,$ B. E. Rodriguez,§ A. K. Murthy,§ and E. W. Kaler*y§ Facultad de Ciencias Quimicas, Universidad de Guadalajara, Guadalajara, Jal44430 Mexico, Instituto de Fisica, U N A M , Mexico, D. F. Mexico, and Department of Chemical Engineering, University of Delaware, Newark, Delaware 19716 Received July 19, 1989 The polymerization of styrene in three-component dodecyltrimethylammonium bromide (DTAB) microemulsions is reported. The structure of the unpolymerized microemulsions, determined by conductimetry and quasielastic light scattering (QLS), is consistent with styrene-swollen micelles in equilibrium with regular micelles, both dispersed in an aqueous phase. Polymerization of these transparent microemulsions, monitored by QLS and dilatometry, produced stable, bluish monodisperse microlatices with particle radii ranging from 20 to 30 nm, depending on styrene content. Polymerization initiation appears to occur in the styrene-swollen micelles, and the polymer particles grow by recruiting monomer and surfactant from uninitiated droplets and small micelles.
Introduction Microemulsions are transparent or translucent microstructured phases which contain two ordinarily immiscible liquids (i.e., water and oil) and surfactank3.l Because microemulsions demonstrate a large interfacial area between oil and aqueous domains, they are being used in many applications such as phase-transfer catalysis, liquid membrane separations, enhanced oil recovery, and especially as substrates for polymerization reaction^.^^^ Microemulsion polymerization was first reported by Stauffer and B ~ n e ;they ~ , ~incorporated either methyl acrylate or methyl methacrylate in the continuous phase of water-in-oil microemulsions and found the kinetics of poly+
Facultad de Ciencias Qufmicas.
8
University of Delaware.
* Instituto de Fisica.
(1) Langevin, D. Acc. Chem. Res. 1988, 21, 255.
(2) Leung, R.; Hou, M. J.; Shah, D. 0. In Surfactants in Chemical/ Processing Engineering; Wasan, D. T., Ginn, M. E., Shah, D. O., Eds.; Marcel Dekker: New York, 1988. (3) Dun, A. S. In ComprehensioePolymer Science. Vol. 4. Chain Polymerization ZI;Aggarwal, S. L., Ed.;Pergamon Press: London, 1989; Chapter 12. (4) Stoffer, J. 0.; Bone, T. J. Dispersion Sci. Technol. 1980, 1, 37. Bone, T. J . Polym. Sci., Polym. Chem. Ed. 1980, (5) Stoffer, J. 0.; 18, 2641.
0743-7463/90/2406-1040$02.50/0
merization to be similar to that of solution polymerization. Monodisperse microlatices result from polymerization of styrene and divinylbenzene-styrene in dilute oilin-water microemulsions initiated with AIBN,G while the polymerization of styrene in oil-in-water microemulsions using oil-soluble and water-soluble initiators yields latices with bimodal size distributions.' Polymer affects the stability of monomer-based microemulsions,also.s Jayakrishnan and Shah9 compared the kinetics of polymerization of styrene and methyl methacrylate in emulsion and microemulsion systems made with the same components and found notable differences, although the studied microemulsions were neither transparent nor stable during polymerization. Large polydisperse particles can be produced by photoredox initiation of oil-in-water microemulsions containing a persulfate surfactant,1° and Murtagh et al.I1 have shown that microlatices produced (6)Atik, S. S.; Thomas, J. K. J. Am. Chem. SOC.1981, 103, 4279; 1983, 105,4515. (7) Johnson, P. L.; Gulari, E. J . Polym. Sci., Polym. Chem. Ed. 1984, 22, 3967. ( 8 ) Gan, L. M.; Chew, C. H.; Friberg, S. E. J . Macromol. Sci. Chem. 1983, A19,739. (9) Jayakrishnan, A.; Shah, D. 0. J . Polym. Sci., Polym. Lett. Ed. 1984, 22, 31. (10) Gratzel, C. K.; Jirousek, M.; Gratzel, M. Langmuir 1986,2, 293.
0 1990 American Chemical Society
Langmuir, Vol. 6, No. 6, 1990 1041
Styrene Polymerization in Microemulsions from rod-like micellar solutions are similar to those produced from microemulsions. Kuo et a1.12 reported monodisperse microlatices from photoinitiated oil-in-water microemulsions that remain transparent during the whole polymerization process, the kinetics of which was monitored by dilatometry. Solid porous materials can be made by polymerizing anionic and nonionic water-in-oil, oil-inwater, and bicontinuous mi~roemulsions.~~ Finally, Candau and c ~ - w o r k e r s have ~ ~ - ~reported ~ the polymerization of acrylamide and of acrylamide and sodium acrylate together in inverse microemulsions. All reported polymerizations have been made in microemulsions containing four or more components in addition to the initiator. The presence of a fourth component such as an alcohol cosurfactant places substantial limits on the utility of microemulsion polymerization for two reasons. First, the cosurfactant needlessly complicates the phase behavior of the microemulsion system. Ternary microemulsion formulations are substantially easier to interpret and modify.20 Second, alcohol present as a cosurfactant can act as a chain-transfer agent and so interfere with or prevent the desired polymerization. Here we report for the first time the polymerization of styrene in three-component cationic microemulsions. The polymerization kinetics was followed by quasielastic light scattering (QLS), gravimetry, and dilatometry. In all cases, monodisperse latices were obtained with radii in the range 20-30 nm. The latices are bluish and have remained stable for months. The use of QLS provides insight into the nature of microemulsion polymerization, and we propose a new mechanism to account for the evolution in microstructural sizes observed.
Experimental Section Dodecyltrimethylammonium bromide (DTAB), with purity greater than 99%, was purchased from Tokyo Kasei or Ald-
rich. Reagent grade styrene, from either Scientific Polymer Products or Resinas Guadalajara, was passed through a column to remove inhibitor (DHR-4 from Scientific Polymer Products) or distilled at 25 "C/6 mmHg to remove the inhibitor and any polymeric residue. Doubly distilled water was used. HPLC grade tetrahydrofuran (Merck) was used as the mobile phase for molecular weight determinations. Hydroquinone and potassium persulfate were reagent grade (Aldrich). The one-phase microemulsion region at 25 "C was determined visually by styrene titration of aqueous micellar solutions of DTAB. Phase boundaries were checked by preparing samples by weight with compositions below and above the titration-determined phase boundaries in sealed glass ampules. Phase diagrams at 60 "C were made with styrene containing a few parts per million of hydroquinone to inhibit thermal polymerization. Conductivity was measured at 1000 Hz with an Orion 101 digital conductimeter and a Yellow Spring immersion cell (cell constant of 1 cm-l). Viscosities were measured at 25 "C with a (11) Murtagh, J.; Ferrick, M. R.; Thomas, J. K. ACS Polym. P r e p . 1987, 28,441. (12) Kuo, P.-L.; Turro, N. J.; Tsang, C.-M.; El-Aasser, M. S.; Vanderhoff, J. W. Macromolecules 1987,20, 1216. (13) Haque, E.; Qutubuddin, S. J. Polym. Sci., Polym. Lett. Ed. 1988, 26. 429. (14) Leong, Y. S.; Caudau, F. J. Phys. Chem. 1982,86, 2269. (15) Candau, F.; Leong, Y. S.; Pouget, G.; Candau, S. J. Colloid Interface Sci. 1984,101,167. (16) Candau, F.; Leong, Y. S.;Fitch, R. J.Polym. Sci., Polym. Chem. Ed. 1985,23, 193. (17) Candau, F.; Zekhnini, 2.;Durand, J.-P. h o g . Colloid Polym. Sci. 1987, 73, 33. (18) Carver, M. T.; Candau, F.; Fitch, R. M. J. Polym. Sci., Polym. Chem. Ed. 1989,27,2179. (19) Carver, M. T.; Dreyer, U.; Knoesel, R.; Candau, F.; Fitch, R. M. J . Polym. Sci., Polym. Chem. Ed.; 1989,27, 2161. (20)Kahlweit, M.; Strey, R. Angew. Chem., Int. Ed. Engl. 1985,24, 654.
DTAB
90
80
70
60
50
STYRENE
H2O
Figure 1. Extent of the single-phase region in a partial phase diagram of DTAB/styrene/water at 25 and 60 "C. Samples at high surfactant concentration are viscous, and the boundaries were not determined exactly (dashed lines). Microemulsions examined in detail are shown along the solid line (DTAB/ water = 15/85) with styrene contents of 4 wt % (A), 6 wt % (B), 8 wt 76 (C);the sample at 15 wt % (D)is an emulsion.
calibrated Cannon-Fenske viscometer 25. Gel permeation chromatography (GPC) measurements were made with a Hewlett Packard 1090M liquid chromatograph equipped with a HP diode array UV detector and Shodex columns in a molecular weight range 105-107. Quasielastic light scattering (QLS) measurements were made by using equipment previously described.21 The sample temperature was controlled within 0.1 "C during measurements. The sample cells used were 20-mL glass vials,with plastic screw caps. The magnitude of the scattering vector, q = (4?m/b) sin (8/2), was varied by changing the scattering angle 0 from 30" to 120". Here n is the index of refraction and XO, 488 nm, is the wavelength of the light in a vacuum. The measured diffusion coefficients are represented in terms of apparent radii by using Stokes law and assuming the solvent has the viscosity of water. Polymerization of styrene was carried out at 60 "C in a 100-mL glass reaction vessel or in a 10-mL calibrated glass dilatometer. The reaction vessel was loaded with microemulsion and heated to 60 "C before addition of K2SzOs(1wt % with respect to the monomer). The mixture was continuously stirred and sparged with nitrogen, and the polystyrene product was isolated by filtration after precipitation with methanol. Conversion was proportional to changes in liquid height of the reacting sample in the capillary tube of a dilatometer. The dilatometer was calibrated by comparing changes in the capillary height with conversion curves obtained by a gravimetric method. The time course of structural changes in the microemulsion during polymerization was followed by QLS. Samples (2 mL) were taken regularly from the reactor, quenched with hydroquinone and cooled to room temperature to stop the reaction, and examined within 18 h by QLS. Latex size was determined with a JEOL-100 CX transmission electron microscope (TEM) operating at 100 kV. The latices were examined both with and without phosphotungstic acid (PTA) staining. The polymerized latex was diluted lox with deionized distilled water, and one drop of the diluted dispersion was allowed to dry on a 200-mesh carbon-coated gold grid. Alternatively, adequate staining was obtained by pressing the latex-containing grid against a Teflon surface holding a drop of 10% PTA aqueous solution. Results One-phase microemulsion regions of styrene/water/ DTAB mixtures exist at 25 and 60 "C (Figure 1). Samples within the one-phase region were transparent and fluid except at high surfactant concentrations, where samples were transparent but highly viscous (gel-like). Thus the upper boundaries were not determined exactly (dot(21) Chang, N. J. Ph.D. Thesis, University of Washington, 1986.
1042 Langmuir, Vol. 6, No. 6, 1990
P6rez-Luna et al.
"
0
2
4
6
8
10
1 2 1 4
% STYRENE Figure 2. Electrical conductivities of DTAB/styrene/water microemulsions at 25 and 60 O C at various DTAB/water ratios.
The electrical conductivity decreases monotonically with increasing styrene content and suggests the growth of microemulsion droplets. Table I. Apparent Sizes in DTAB/Styrene/Water Unpolymerized Microemulsions As Estimated by QLS. water radii, nm (% contribution to QLS signal) styrene 0
2 4 6 8 2 2 2 a
15.0 14.7 14.4 14.1 13.8 5.0 15.0 20.0
85.0 83.3 81.6 79.9 78.2 93.0 83.0 78.0
0.60 0.62 0.65 0.59 0.57 0.55 0.63 0.56
(100) (15) (18) (18) (10) (19) (16) (20)
5.8 8.0 9.3 14.8 8.8 5.3 2.4
(85) (82) (82) (90) (81) (84) (80)
Compositions are in weight percent.
ted line). The letters A to D in the phase diagram (Figure 1; expressed in wt % ) indicate the composition of samples studied. Sample D lies outside the one-phase region; it was a milky white mini- or macroemulsion. The sizes of the one-phase regions are similar at 25 and 60 "C. Microemulsion conductivities at 25 and 60 O C were measured along lines of constant surfactant-to-water ratio as a function of styrene content (Figure 2). Conductivities are high and increase with surfactant concentration and with temperature and decrease fairly linearly with increasing styrene content. The structures in the unpolymerized and polymerized microemulsions were characterized by QLS. The autocorrelation functions measured from unpolymerized microemulsions deviated substantially from a single exponential. Analysis of scattering from such strongly interacti n g dispersions is problematic, b u t we used a multiexponential fitting routine22 to characterize the observed relaxations. In DTAB/water without styrene, an apparent size of 0.6 nm is observed. Two apparent particle sizes are always observed in the presence of styrene: a small size of ca. 0.6-0.8 nm in hydrodynamic radius and a larger composition-dependent size (Table I). As discussed below, we believe these sizes correspond to DTAB micelles and styrene-swollen droplets, respectively. The correlation functions measured for the polymerized samples were single exponentials. The sizes of the polymerized microemulsion droplets (microlatices) are consis(22) Morrison, I. D.; Grabowski, E. F. Langmuir 1985, 1, 496
tently larger than the parent unpolymerized droplets (Table 11). Final conversions and particle sizes of the resulting latices after polymerization a t 60 "C under Nz are reported in Table 11. Unpolymerized samples were transparent, but as polymerization proceeded samples became turbid, and the final solutions had a bluish tinge. No phase separation was detected visually. Initial reaction rates were fast (Figure 3) but slowed after about 5 min, concomitant with an increase in turbidity. Molecular weights and particle sizes increased with increasing styrene content (Table 11). The polydispersity (M,.,/M,) of the polymers was always approximately 3. However, the microlatices were monodisperse with particle radii of 9.8 nm a t the lowest styrene content, and the radii increased with styrene concentration (Table 11). The microlatices have remained stable with respect to coagulation for months. The particles are spherical and apparently monodisperse when observed with TEM (Figure 4). The changes in the microemulsion during polymerization at 40 O C were followed by QLS as described above. The correlation functions displayed two decays a t early times and one a t later times (Figure 5 ) , and the fraction of small size decreases steadily as polymerization proceeds. Similar results were obtained at 50 and 60 "C except that the disappearance of the small size occurred within the first 1-3 min.
Discussion This is the first report of polymerization in a true ternary microemulsion system made without the addition of a cosurfactant. All microemulsionswere optically transparent and of low (ca.
