Effect of Stabilizer on Formation of Onionlike Multilayered Polystyrene

Faculty of Engineering, Kobe UniVersity, Kobe 657-8501, Japan. ReceiVed December 18, 2006. In Final Form: March 9, 2007. The effect of the kind of sta...
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Langmuir 2007, 23, 5978-5983

Effect of Stabilizer on Formation of “Onionlike” Multilayered Polystyrene-block-poly(methyl Methacrylate) Particles Naohiko Saito,† Ryu Takekoh,† Reiko Nakatsuru,† and Masayoshi Okubo*,†,‡ Graduate School of Science and Technology, and Department of Chemical Science and Engineering, Faculty of Engineering, Kobe UniVersity, Kobe 657-8501, Japan ReceiVed December 18, 2006. In Final Form: March 9, 2007 The effect of the kind of stabilizers on the formation of “onionlike” multilayered polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) particles was studied. The release of toluene from PS-b-PMMA/toluene droplets dispersed in aqueous medium resulted in the formation of onionlike multilayered structures in the particles for all stabilizers used. However, the surface composition of the particles was strongly affected by the kind of stabilizer. When sodium dodecyl sulfate (SDS) and poly(vinyl alcohol) (PVA) were used as stabilizers, the surface of the particles was occupied by PMMA phase. On the other hand, in the cases of Emulgen 911 (polyoxyethylene nonylphenyl ether) and Tween 80 (polyoxyethylene sorbitan monooleate) as stabilizers, the PS phase occupied the surfaces. These results for SDS, PVA, and Emulgen 911 are consistent with the surface layer of the PS-b-PMMA particles being occupied by the polymer phase, which gives a lower interfacial tension than that of another phase. However, in the case of Tween 80, interfacial tensions between water and toluene solutions of the polymer showed almost the same values making it impossible to predict the surface polymer phase.

Introduction The control of morphology of composite polymer particles is important for achieving desirable physical properties for coatings, for impact resistant plastics, and for other diverse applications. Consequently, extensive research has been aimed at controlling particle morphology over the past 20 years, resulting in the preparation of composite particles having a variety of morphologies.1-11 The topic of composite polymer particles has recently been reviewed.12 We have succeeded in the preparation of micrometer-sized, monodisperse, “onionlike” multilayered poly(methyl methacrylate) (PMMA)/polystyrene (PS) composite particles.13 This was achieved by reconstruction of the morphology of PMMA-core/ PS-shell composite particles, which were produced by seeded dispersion polymerization (SDP), using the solvent-absorbing/ releasing method (SARM). PS-g-PMMA or PS-b-PMMA, which was formed during the SDP, played an important role as a * To whom correspondence should be addressed. Tel/Fax: +81-78803-6161; e-mail: [email protected]. † Graduate School of Science and Technology, Kobe University. ‡ Faculty of Engineering, Kobe University. (1) Matsumoto, T.; Okubo, M.; Imai, T. Kobunshi Ronbunshu 1974, 31, 576586. (2) Okubo, M.; Katsuta, Y.; Yamada, A.; Matsumoto, T. Kobunshi Ronbunshu 1976, 36, 459-464. (3) Okubo, M.; Katsuta, Y.; Matsumoto, T. J. Polym. Sci., Polym. Lett. Ed. 1980, 18, 481-486. (4) Okubo, M.; Yamada, A.; Matsumoto, T. J. Polym. Sci., Polym. Chem. Ed. 1980, 16, 3219-3228. (5) Okubo, M.; Katsuta, Y.; Matsumoto, T. J. Polym. Sci., Polym. Lett. Ed. 1982, 20, 45-51. (6) Min, T. I.; Klein, A.; El-aasser, M. S.; Vanderhoff, J. W. J. Polym. Sci., Polym. Lett. Ed. 1983, 21, 2845-2861. (7) Cho, I.; Lee, K.-W. J. Appl. Polym. Sci. 1985, 30, 1903-1926. (8) Okubo, M. Makromol. Chem., Macromol. Symp. 1990, 35/36, 307-325. (9) Sheu, H. R.; El-aasser, M. S.; Vanderhoff, J. W. J. Polym. Sci., Part A: Polym. Chem. 1990, 28, 629-651. (10) Sundberg, D. C.; Casassa, A. P.; Pantazopoulos, J.; Muscato, M. R. J. Appl. Polym. Sci. 1990, 41, 1425-1442. (11) Durant, Y. G.; Sundberg, D. C.; Guillot, J. J. Appl. Polym. Sci. 1994, 52, 1823-1832. (12) Sundberg, D. C.; Durant, Y. G. Polym. React. Eng. 2003, 11, 379-432. (13) Okubo, M.; Izumi, J.; Takekoh, R. Colloid Polym. Sci. 1999, 277, 875880.

compatibilizer in the formation of the multilayered structure during the SARM treatment.14 Moreover, sub-micrometer-sized poly(i-butyl methacrylate)-b-PS particles prepared by two-step atom-transfer radical polymerization in aqueous medium had the same onionlike multilayered structure.15 It is well-known that a block copolymer operates as a compatibilizer in polymer blend. There are many reports on microphase separation in polymer blend film containing block copolymer.16-20 The unique properties of block copolymer materials significantly depend on their mesoscopic (10 nm scale) self-assembly (lamellar, hexagonalpacked cylinder, body-centered cubic, and bicontinuous cubic gyroid) in the molten and solid states. Three decades of theoretical development have culminated in remarkably predictive statistical theories that can account for the domain shapes, dimensions, connectivity, and ordered symmetry of many types.21 In the simplest case of a diblock copolymer (A-b-B), microphase separation depends on the following three parameters: overall degree of polymerization (N); the Flory-Huggins interaction parameter (χ), which characterizes the repulsive interaction between the two blocks A and B; and the volume fraction of A phase. Although the morphology of polymer film comprising homopolymers and a block copolymer has been investigated extensively, where there is an interplay between macrophase (because of presence of homopolymer) and microphase separations (of the block copolymer) depending on the relative lengths of the polymers and on the composition of the blend,22-26 there (14) Okubo, M.; Takekoh, R.; Saito, N. Colloid Polym. Sci. 2003, 281, 945950. (15) Kagawa, Y.; Minami, H.; Okubo, M.; Zhou, J. Polymer 2005, 46, 10451049. (16) Bates, F. S.; Fredrickson, G. H. Annu. ReV. Phys. Chem. 1990, 41, 525. (17) Hamley, I. W. The Physics of Block Copolymers; Oxford University Press: Oxford, U.K., 1998. (18) Bates, F. S.; Fredrickson, G. H. Phys. Today 1999, 52, 32-38. (19) Hamley, I. W. DeVelopments in Block Copolymer Science and Technology; John Wiley & Sons: New York 2004. (20) Ruzette, A.-v.; Leibler, L. Nat. Mater. 2005, 4, 19-31. (21) Masten, M. W. J. Phys.: Condens. Matter 2002, 14, R21-R47. (22) Inoue, T.; Soen, T.; Hashimoto, T.; Kawai, H. Macromolecules 1970, 3, 87-92. (23) Hong, K. M.; Noolandi, J. Macromolecules 1983, 16, 1083-1093. (24) Roe, R. J.; Zin, W. C. Macromolecules 1984, 17, 189-194.

10.1021/la063654f CCC: $37.00 © 2007 American Chemical Society Published on Web 04/20/2007

Stabilizer and Formation of Onionlike Particles

are only few reports dealing with morphology control of composite polymer particles using block copolymer as compatibilizer.27,28 Similarly, not much work has been reported on the preparation of polymer particles consisting of block copolymer only.15,29 In our previous work, investigations employing well-defined PSb-PMMA for preparation of composite polymer particles by release of toluene from toluene droplets of PS/PS-b-PMMA/ PMMA (equal weight fractions of PS and PMMA segments) led to formation of onionlike multilayered particles.30 In the absence of PS-b-PMMA, such microphase-separated morphology was not formed, thus revealing the crucial role of PS-b-PMMA or PS-g-PMMA as compatibilizer.31,32 In this article, the formation mechanism of the onionlike multilayered particles will be further investigated by clarifying the effect of the kind of colloidal stabilizer (ionic surfactant, nonionic surfactant, and polymeric stabilizer) from the viewpoint of interfacial tensions. Experimental Section Materials. Styrene and methyl methacrylate (MMA) were distilled under reduced pressure in a nitrogen atmosphere. Reagent grade 2,2′-azobis(isobutyronitrile) (AIBN) was purified by recrystallization. PS-b-PMMA (Mn: 2.63 × 105, φPS: 0.49, Mw/Mn ) 1.15) purchased from Polymer Source Inc. (Canada) was used as received. Commercial grade polyoxyethylene nonylphenyl ether (Emulgen 911, Kao Co., Japan) with an average ethylene oxide (EO) chain length of 10.9 units and polyoxyethylene sorbitan monooleate (Tween 80, Nacalai Tesque Inc., Japan) with an average EO chain length of 20 units, nonionic surfactants, were used as received. Poly(vinyl alcohol) (PVA) (Gohsenol GH-17: degree of polymerization, 1700; degree of saponification, 88%) was supplied from Nippon Synthetic Chemical, Osaka, Japan. Deionized water with a specific resistance of 5 × 106 Ω cm was used after distillation. Sodium dodecyl sulfate (SDS) and toluene were used as received from Nacalai Tesque, Inc. Preparation of Onionlike Multilayered PS-b-PMMA Particles. A homogeneous toluene solution (1.1 g) of PS-b-PMMA (PS-bPMMA/toluene ) 1/10, w/w) was mixed with a 0.067 wt % stabilizer aqueous medium (30 g), and the mixture was stirred vigorously using a NISSEI ABM-2 homogenizer at 2000 rpm for 2 min in a 50 mL glass cylindrical vial. The toluene was evaporated from the dispersion stirred with a magnetic stirrer at room temperature for 24 h in the uncovered glass vial (surface area between dispersion and air was 8 cm2). Four kinds of stabilizers were used as shown in Figure 1: SDS, PVA, Emulgen 911, Tween 80. Observation of Ultrathin Cross Sections of Particles. The dried particles were stained with ruthenium tetraoxide (RuO4) vapor at room temperature for 30 min in the presence of 1% RuO4 aqueous solution, were dispersed in an epoxy matrix, were cured at room temperature for 24 h, and were subsequently microtomed. The ultrathin cross sections were observed with a Hitachi H-7500 transmission electron microscope (TEM). X-ray Photoelectron Spectroscopy. X-ray photoelectron spectroscopy (XPS) data of PS-b-PMMA particles were obtained with a Shimadzu ESCA-3400 apparatus using magnesium KR radiation (1253.6 eV) at a potential of 10 kV and an X-ray current of 20 mA. The pressure in the measurement chamber was ca. 8.0 × 10-7 Pa. (25) Hashimoto, T.; Tanaka, H.; Hasegawa, H. Macromolecules 1990, 23, 4378-4386. (26) Tanaka, H.; Hasegawa, H.; Hashimoto, T. Macromolecules 1991, 24, 240-251. (27) Rajatapiti, P.; Dimonie, V. L.; El-aasser, M. S. J. Appl. Polym. Sci. 1996, 61, 891-900. (28) Herrera, V.; Pirri, R.; Leiza, J. R.; Asua, J. M. Macromolecules 2006, 39, 6969-6974. (29) Ding, J.; Liu, G. Macromolecules 1999, 32, 8413-8420. (30) Okubo, M.; Saito, N.; Takekoh, R.; Kobayashi, H. Polymer 2005, 46, 1151-1156. (31) Okubo, M.; Saito, N.; Fujibayashi, T. Colloid Polym. Sci. 2005, 283, 691-698. (32) Okubo, M.; Saito, N.; Kagari, Y. Langmuir 2006, 22, 9397-9402.

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Figure 1. Structural formulas of stabilizers. Dried particles were spread on an indium plate with a spatula and were stored under reduced pressure by continuous operation of an oil rotary pump just before the measurement. Interfacial Tension. PS (Mw ) 10.2 × 104, Mw/Mn ) 1.63) and PMMA (Mw ) 9.7 × 104, Mw/Mn ) 1.88) were prepared by solution polymerizations in toluene for 24 h, respectively, at 70 and 60 °C and at monomer concentration of 60 and 40 wt % and at AIBN concentration of 0.2 and 0.1 wt %. Interfacial tensions of PS and PMMA solutions between polymer/toluene solution (polymer/toluene ) 1/5, w/w) and water were measured by the pendant drop method with a Drop Master 500 (Kyowa Interface Science Co., Ltd.) at room temperature (ca. 20 °C). Pendant drops of the polymer solutions were formed at the tip of the stainless steel needle in a glass cell filled with water. Before the measurements, the glass cell, syringe, and needle were cleaned with tetrahydrofuran and were rinsed several times with distilled water to remove residual polymer and surfactant. The accuracy of the measured interfacial tensions is about (0.2 mN/m. Partition of Nonionic Surfactant between Toluene and Water. Partition isotherms of Emulgen 911 and Tween 80 between water and toluene were obtained as follows. Aqueous solution (300 g) and toluene (0.83-3.3 wt % vs water) mixture, in which nonionic surfactant (0.067 wt % vs water) was dissolved, were poured into a glass jar, were shaken for 1 min after tightly closing the jar, and were left standing for 1 month at room temperature. After 1 month, the concentration of the nonionic surfactant in the aqueous phase was measured by gravimetry. The concentration in the toluene phase was thus calculated by subtracting it from the total amount. Partitioning experiments were performed by preparing a surfactant solution in either the water or toluene keeping the total amount of a nonionic surfactant constant. Partition of Nonionic Surfactant between PS and PMMA Phases. Partition isotherms of Emulgen 911 and Tween 80 between toluene phases of PS and PMMA were obtained as follows. PS/ PMMA/toluene (1/1/10, w/w/w) solution (1.2 g) containing the nonionic surfactant (0.69∼3.3 wt % vs polymer solution) was poured into graduated cylinders. The solutions were left standing in the closed cylinders at room temperature for 3 days. When each solution separated into transparent PS and PMMA phases, a part of the solutions in both phases were dried under reduced pressure, were dissolved into THF (5 g), and were measured by gel permeation chromatography (GPC) with two styrene/divinyl benzene gel columns (Tosoh, TSKgel GMHHR-H, 7.8 mm i.d. × 30 cm) using THF as an eluent at 40 °C at a flow rate of 1.0 mL/min employing a refractive index detector (Tosoh RI-8020/21). Just before the GPC measurement, 5 mg of standard PS (Mw ) 5.48 × 106) was added to the sample to compare with the adsorption area of the surfactant. The partition of the surfactant between toluene phases of PS and PMMA was determined by calculating the area ratio of the nonionic surfactant to the PS standard.

Results and Discussion Figure 2 shows TEM photographs of ultrathin cross sections of the RuO4-stained PS-b-PMMA particles prepared by release of toluene from PS-b-PMMA/toluene (1/10, w/w) droplets dispersed in 0.067 wt % various stabilizer aqueous medium. The

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Figure 2. TEM photographs of ultrathin cross sections of RuO4stained PS-b-PMMA (Mn: 26.3 × 104, φPS: 0.49) particles prepared by release of toluene from polymer/toluene (1/10, w/w) droplets dispersed in 0.067 wt % various stabilizer aqueous solutions: (a) SDS, (b) PVA, (c) Emulgen 911, and (d) Tween 80.

amounts of residual toluene after 24 h in the dispersions measured by gas chromatography were less than 1 wt % of its initial amount. Because RuO4 stains PS but not PMMA, the dark and white parts in the TEM photographs show the PS and PMMA phases, respectively.33 Spherical particles with onionlike multilayered structure were obtained after toluene evaporation for all stabilizers. This result is consistent with PS/PS-b-PMMA/PMMA (equal weight fractions of PS and PMMA segments) composite particles leading to formation of onionlike multilayered morphology.30 The formation mechanism of the multilayered morphology was discussed in our previous study.34 No effect of the kind of surfactant on the inner morphology was detected. Moreover, the rate of toluene evaporation did not influence the morphology for evaporation rates as low as those employed in the present work.30 The small difference of the layer thicknesses among the particles is probably due to the differences of cutting position of the ultrathin cross sections or the degree of the swelling by epoxy resin in the process of the embedding. The surface compositions of the particles were investigated with XPS. XPS can measure the surface composition up to ca. 10 nm in depth, which is less than the layer thickness of the PS-b-PMMA particles (ca. 35 nm estimated from the TEM analysis). Thus, XPS measurement is suitable for analysis of the surface composition of the particles. Figure 3 shows C1s XPS spectra of PS (a) and PMMA (b) particles as references for the calculation of surface composition. Before measurement, the particles were centrifugally washed with methanol (SDS, Emulgen 911, and Tween 80) or water (PVA) three times to remove the adsorbed stabilizer.35 The spectra of the washed particles were different from those corresponding stabilizers in all cases leading us to the conclusion that the spectra shown in Figure 3 indicate the surface polymer of the particles but not the adsorbed stabilizer. In the C1s XPS spectrum of PS particles, a strong peak appears at 285 eV with a small satellite peak at approximately 292 eV because of the phenyl substituent (Figure 3a). The PMMA spectrum exhibits a strong peak at 285 eV with a small peak due to the carbonyl group at approximately (33) Trent, J. S.; Scheinbeim, J. I.; Couchman, P. R. Macromolecules 1983, 16, 589-598. (34) Okubo, M.; Takekoh, R.; Saito, N. Colloid Polym. Sci. 2004, 282, 11921197. (35) Chaiyasat, A.; Kobayashi, H.; Okubo, M. Colloid Polym. Sci. 2007, 285, 557-562.

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Figure 3. C1s X-ray photoelectron spectra of PS (a), PMMA (b), and PS-b-PMMA (Mn: 2.63 × 105, φPS: 0.49) (c-f) particles prepared by release of toluene from polymer/toluene (1/10, w/w) droplets dispersed in 0.067 wt % various stabilizer aqueous solutions: (a, b, c) SDS, (d) PVA, (e) Emulgen 911, and (f) Tween 80.

Figure 4. Interfacial tensions (pendant drop method) between polymer (O, PS; 0, PMMA)/toluene (1/5, w/w) solutions and 0.067 wt % stabilizer [(a) SDS; (b) PVA] aqueous solutions as a function of the time.

289 eV (Figure 3b). XPS spectra of the PS-b-PMMA particles (Figure 3c-f) were fitted with the reference spectra of PS (Figure 3a) and PMMA (Figure 3b) to calculate the surface compositions. When SDS and PVA were used as stabilizer, the surfaces of both PS-b-PMMA particles were completely occupied by the PMMA phase (Figure 3c, d). On the other hand, in the cases of Emulgen 911 and Tween 80, the PS phase occupied the surfaces (Figure 3e, f). Figure 4 shows the interfacial tensions between polymer/ toluene (1/5, w/w) solutions and SDS (a) and PVA (b) aqueous solutions (γP-T/W), as a function of time, measured by the pendant drop method. In the absence of stabilizer, γPS-T/W and γPMMA-T/W were 34.4 and 19.2 mN/m, respectively, and no time dependence was observed. The interfacial tension between toluene and water without stabilizer was 35.2 mN/m. These results indicate that PMMA would adsorb on the toluene/water interface to decrease the interfacial tension, whereas PS would not behave as a surfaceactive molecule. When stabilizer was added to the systems, γP-T/W decreased drastically. In the cases of SDS and PVA, γPMMA-T/W was lower than γPS-T/W as shown in Figure 4. Thus, the surface of the PS-b-PMMA/toluene droplets is occupied by the PMMA phase after microphase separation between PS and PMMA segments in the PS-b-PMMA. The influence of the stabilizer on the morphology is different for “macrophase” and “microphase” separated particles as discussed as follows. Winzor and Sundberg10,36 and Chen et al.37 clarified that the morphology of macrophase-separated composite particles changed from core(36) Winzor, C. L.; Sundberg, D. C. Polymer 1992, 33, 3797-3810. (37) Chen, Y. C.; Dimonie, V. L.; El-aasser, M. S. J. Appl. Polym. Sci. 1992, 45, 487-499.

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shell to hemispherical depending on the kind of the stabilizer, in good agreement with thermodynamic simulations using interfacial tensions. In the case of macrophase-separated composite particles, hemispherical morphology, in which PMMA phase partially (approximately 50%) occupies the surface, often becomes thermodynamically more stable than PS-core/PMMAshell even if the interfacial tension between PMMA/toluene solution and water (γPMMA-T/W) is lower than that between PS/ toluene solution and water (γPS-T/W). This is a result of the system minimizing its total interfacial free energy by decreasing the interfacial area between the PS and PMMA phases.32 In contrast, in the case of microphase-separated A/B composite particles containing A-b-B, the inner ordered morphology (which defines the total interfacial area between A and B phases) would be determined by polymer composition, A-b-B composition with respect to total polymer, molecular weight of A-b-B, and chain length of A segment in A-b-B.30 Thus, the interfacial area between the A and B phases would not affect the surface composition of microphase-separated A/B particles as opposed to the case of macrophase-separated polymer particles. In other words, in the case of microphase-separated polymer particles, the effect of the stabilizer is to determine which polymer phase is more thermodynamically stable to occupy the surface. Onionlike multilayered structure consisting of a mixture of A-B block copolymer and homopolymer has often been observed in the solution-cast films.38-40 Koizumi et al. prepared solutioncast films of binary mixtures of PS-block-polyisoprene (PI) (Mn: 1.0 × 105, wPS: 0.5, Mw/Mn ) 1.10) and polystyrene (Mn: 5.8 × 105, Mw/Mn ) 1.34), in which the macrophase transition between the PS-b-PI and PS dominates the microphase separation, since Mn of the PS is considerably higher than that of the PSb-PI.40 The shape of the onionlike PS-b-PI domain in the PS matrix changed from spherical to oblate spheroidal with increasing domain size. They explain that the onionlike PS-b-PI domains are expected to be spherical to minimize the interfacial free energy in the PS matrix, but the interfaces tend not to be bent as much as possible to minimize the elastic free energy. Bending of the lamellar interfaces costs the elastic free energy for the stretching of chains to maintain a uniform density in the domain. The oblate spheroidal shape is thus a result of the system trying to minimize its overall energy. In contrast, spherical onionlike PS-b-PMMA particles were obtained regardless of the particle size under the current conditions. This might be because the interfacial tension between PS-b-PMMA and water is high enough for the cost in elastic free energy to be neglected. If PA-b-PB particles were prepared by solvent evaporation in a system where interfacial tensions between both polymer phases and medium are the same (γPA/medium ) γPB/medium), spherical particles with flat lamellar interfaces might be obtained (both polymers exist at the surface). In fact, spherical block polymer particles with flat lamellar morphology, prepared by evaporation of a good solvent from a polymer solution consisting of a nonvolatile poor solvent and a volatile good solvent, has been observed recently.41 Although SDS and PVA are only soluble in water, Emulgen 911 and Tween 80 are soluble in both phases (water and toluene). To estimate the interfacial tension between the droplets and the aqueous medium, the partition isotherms of Emulgen 911 and Tween 80 in the toluene-water systems were investigated by gravimetry. Because the influence of the polymer in this partition

system (toluene-water) was negligible (data not shown), the partition isotherms were examined in the absence of polymer. Figure 5 shows variations of Emulgen 911 and Tween 80 concentrations in toluene (a, c) and water (b, d) as a function of the weight fraction of toluene relative to total amount of toluene and water. The Emulgen 911 partitioning between water (a) and toluene (b) was independent of whether the surfactant had been initially dissolved in water (O) or toluene (0) before mixing, indicating equilibrium conditions. Similar partition experiments carried out for a short time (a few hours) indicate that the partitionings of Emulgen 911 presented in the current paper remained the same; a more detailed discussion with regard to the time of the equilibration will be presented soon.42 Lowering toluene concentration resulted in increases of Emulgen 911 concentrations in both phases, with the concentration in toluene being much higher than that in water. Thus, interfacial tension between the PS-b-PMMA/toluene droplets and aqueous medium would decrease during toluene evaporation because of the increases of Emulgen 911 concentrations in both phases. On the other hand, the partitionings of Tween 80 between toluene and water were significantly different depending on the initial locations of the surfactant even after 1 month as shown in Figure 5c and d, though total amount of Tween 80 in the system was the same. This result suggests that the exchange of Tween 80 between toluene and water is a very slow process to reach equilibrium. This implies that the Tween 80 concentrations changed under nonequilibrium state during toluene evaporation in the PS-bPMMA/toluene droplets. This might cause the difference of interfacial tension between the droplets and the aqueous medium in each particle. These results (Figure 5) are also important from the viewpoint of emulsion polymerization. Emulsion polymerization using nonionic emulsifier sometimes results in a very broad particle size distribution, inconsistent with Smith-Ewart theory.43-48

(38) Riess, G.; Schlienger, M.; Marti, S. J. Macromol. Sci., Phys. 1980, B17, 355-374. (39) Gebizlioglu, O. S.; Argon, A. S.; Cohen, R. E. Polymer 1985, 26, 519528. (40) Koizumi, S.; Hasegawa, H.; Hashimoto, T. Macromolecules 1994, 27, 6532-6540. (41) Yabu, H.; Higuchi, T.; Shimomura, M. AdV. Mater 2005, 17, 2062-2065.

(42) Saito, N.; Nakatsuru, R.; Kagari, Y.; Okubo, M. Langmuir submitted. (43) Ozdeger, E.; Sudol, E. D.; El-Aasser, M. S.; Klein, A. J. Polym. Sci. Part A: Polym. Chem. 1997, 35, 3813-3825. (44) Ozdeger, E.; Sudol, E. D.; El-Aasser, M. S.; Klein, A. J. Polym. Sci. Part A: Polym. Chem. 1997, 35, 3827-3835. (45) Ozdeger, E.; Sudol, E. D.; El-Aasser, M. S.; Klein, A. J. Polym. Sci. Part A: Polym. Chem. 1997, 35, 3837-3846.

Figure 5. Emulgen 911 (a, b) and Tween 80 (c, d) concentrations in toluene (a, c) and aqueous (b, d) layers as a function of the weight fraction of toluene (relative to total amount of toluene and water) after the dispersions were left standing for 1 month in a covered glass jar. The dispersions were prepared by mixing with toluene and aqueous solution, in which nonionic surfactant had been previously dissolved in water (O) or toluene (0).

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Figure 6. Partitionings of Emulgen 911 (O) and Tween 80 (0) between phase-separated PS and PMMA layers as a function of weight fraction of the nonionic surfactant (relative to total amount of PS, PMMA, toluene, and surfactant) after the PS/PMMA/toluene solutions were left standing (1/1/10, w/w/w) for 3 days.

Pirma and Chang49 proposed that secondary nucleation occurs during the polymerization because of release of emulsifier from the monomer droplets to the aqueous solution. Recently, we reported that a significant fraction of nonionic emulsifier was incorporated into the final particles depending on the distribution of emulsifier between the monomer and aqueous phases, which is affected by the hydrophilic-lipophilic balance (HLB) of the emulsifier.35,50 To estimate the γPMMA-T/W and γPS-T/W containing nonionic surfactant, not only the partitioning of the surfactant between polymer/toluene and aqueous solutions but also the partitioning between PS and PMMA phases should be considered. Figure 6 shows the partition isotherms of Emulgen 911 and Tween 80 between PS and PMMA phases. The partitionings of the nonionic surfactants to both phases were similarly independent of the surfactant concentration relative to the total amount of the polymers, the surfactant, and the toluene in each case. Thus, the concentrations of the nonionic surfactants in the PS and PMMA phases were the same at any toluene amounts for the interfacial tension measurement in this study. Strictly speaking, PMMA phase contained the surfactant a little more than the PS phase, while toluene partitioned more to the PS phase than the PMMA phase as we have discussed elsewhere.26 The reason why the surfactant partitions between PS and PMMA phases could not be explained simply by toluene fraction of the polymer phase is still unclear. As far as we know, no one has investigated partition of nonionic surfactant between polymer solutions. No attempt will be made here to investigate the partition of the nonionic surfactant among polymer solutions. The adsorption process of the nonionic surfactant at the interface is affected by its solubilities in both phases (diffusioncontrolled adsorption),51-55 which is in practice never negligible, (46) Lin, S. Y.; Capek, I.; Hsu, T. J.; Chern, C. S. J. Polym. Sci., Part A: Polym. Chem. 1999, 37, 4422-4431. (47) Capek, I.; Chudej, J.; Janickova, S. J. Polym. Sci., Part A: Polym. Chem. 2003, 41, 804-820. (48) Capek, I. Polym. J. 2004, 36, 96-107. (49) Piirma, I.; Chang, M. J. Polym. Sci., Polym. Chem. Ed. 1982, 20, 489498. (50) Okubo, M.; Kobayashi, H.; Matoba, T.; Oshima, Y. Langmuir 2006, 22, 8727-8731. (51) Ravera, F.; Liggieri, L.; Passerone, A.; Steinchen, A. J. Colloid Interface Sci. 1994, 163, 309-314. (52) Ferrari, M.; Liggieri, L.; Ravera, F.; Amodio, C.; Miller, R. J. Colloid Interface Sci. 1997, 186, 40-45. (53) Liggieri, L.; Ravera, F.; Ferrari, M.; Passerone, A.; Miller, R. J. Colloid Interface Sci. 1997, 186, 46-52. (54) Hansen, F. K.; Hveem, J. J. Colloid Interface Sci. 1999, 210, 144-151. (55) Ravera, F.; Ferrari, M.; Liggieri, L. AdV. Colloid Interface Sci. 2000, 88, 129-177.

Figure 7. Interfacial tensions between the polymer/toluene (1/5, w/w) and aqueous solutions as functions of the initial concentrations of Emulgen 911 (a, b) and Tween 80 (c, d), which had been previously dissolved in either polymer/toluene solution (a, c) or water (b, d). Polymers: O, PS; 0, PMMA. Interfacial tensions were measured by the pendant drop method after 30 min.

and in some cases the diffusion-controlled adsorption can be the main process controlling the adsorption. Actually, an unusual relationship between the interfacial tension (pendant drop technique) and time has been observed under particular conditions, where adsorption is expected to pass through a maximum when adsorbing surfactant transferred from one phase is no longer sufficient to balance the transfer into the second phase.52,53 It takes a long time to reach interfacial tension equilibrium because of the transfer of nonionic surfactant measured by the pendant drop method. Furthermore, because pendant drops often fall off when the interfacial tension becomes low, the interfacial tension for only 30 min from the beginning of the formation of pendant drop was measured. Therefore, we only will qualitatively discuss the difference between γPMMA-T/W and γPS-T/W. γP-T/W (pendant drop method) was measured with the surfactant initially located entirely in the water or polymer/toluene solution (the initial surfactant concentration would be changed because of the transfer into the other phase). Figure 7 shows γP-T/W as functions of the initial nonionic surfactant concentrations in polymer/toluene solution (a, c) or in water (b, d). The partition isotherms of Emulgen 911 shown in Figure 5 indicate that the surfactant concentrations in both phases would increase with decreasing the toluene concentration relative to the total amount of toluene and water (Figure 5a, b). Thus, γP-T/W would decrease as toluene evaporates from the droplets. When Emulgen 911 was used as a stabilizer, γPS-T/W was lower than γPMMA-T/W above 0.1 wt %, as shown in Figure 7a and b, in both cases (stabilizer previously dissolved into either toluene or water). Because Emulgen 911 concentration in the toluene phase was much higher than that in water (approximately 10 times) at equilibrium state (Figure 5a, b), the γP-T/W would be determined by the contribution of Emulgen 911 adsorption from the polymer/toluene solution. The γP-T/W of the droplets was 0-2 mN/m by fitting Figure 7a with the concentration in toluene phase in Figure 5a. In Figure 7a, γPS-T/W was lower than γPMMA-T/W over the range 0.166-2.66 wt % Emulgen 911 versus toluene. Over 2.66 wt %, although the difference between γPS-T/W and γPMMA-T/W converged more to the error range, this relationship, that γPS-T/W was lower than γPMMA-T/W, would be kept. These results are consistent with the surface of the PS-b-PMMA

Stabilizer and Formation of Onionlike Particles

particles being occupied by the polymer phase which has the lower γP-T/W (PS phase occupied the surface in the case of Emulgen 911). On the other hand, in the case of Tween 80, γPS-T/W and γPMMA-T/W show almost the same values at all surfactant concentrations (Figure 7c and d), and it is thus difficult to rationalize why PS phase occupied the particle surface. Further work is needed for clarification of these points in the future.

Conclusions This paper is the first report dealing with the effect of surfactant on surface morphology of microphase-separated polymer particles. Spherical onionlike multilayered particles were prepared by release of toluene from PS-b-PMMA/toluene droplets dispersed in aqueous media containing various stabilizers. No effect of the surfactant on the inner morphology of the particles was observed. However, the first (surface) layer of the particles was strongly affected by the kind of stabilizer. When SDS or

Langmuir, Vol. 23, No. 11, 2007 5983

PVA was used as a stabilizer, the surface of the particles was occupied by PMMA phase. On the other hand, in the cases of Emulgen 911 and Tween 80, the PS phase occupied the surfaces. This difference of the surface layer can be interpreted from the viewpoint of interfacial tension. The surface layer consisted of one polymer phase which gives a lower interfacial tension with water than that of another phase. Interfacial area between polymer phases would not affect the surface composition of microphaseseparated composite particles as opposed to the case of the macrophase-separated polymer particles. Acknowledgment. This work was supported by Creation and Support program for Start-ups from Universities (No. 1509) from the Japan Science and Technology Agency (JST) and by Research Fellowships of the Japan Society for the Promotion of Science (JSPS) for Young Scientists (given to N. S.). LA063654F