Particles Prepared by Seeded Dispersion Polymerization - American

Jun 12, 2008 - The size and number of dimples at the surfaces of the obtained golf ball-like ... layer of the seed particle, and, thus, dimples became...
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Ind. Eng. Chem. Res. 2008, 47, 6445–6449

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Effect of Polymer Polarity on the Shape of “Golf Ball-like” Particles Prepared by Seeded Dispersion Polymerization† Teruhisa Fujibayashi, Yoshifumi Komatsu, Natsumi Konishi, Hisamitsu Yamori, and Masayoshi Okubo* Graduate School of Engineering, Kobe UniVersity, Kobe 657-8501, Japan

Micrometer-sized, monodisperse, ”golf ball-like” particles that have numerous dimples at the surfaces were prepared by seeded dispersion polymerization of n-butyl methacrylate (n-BMA), with polystyrene/poly(styreneco-sodium styrene sulfonate) composite particles as seed, in the presence of dodecane droplets in a methanol/ water (80/20, w/w) medium, followed by the evaporation of dodecane. The dimples at the surface were formed by the volume reduction of poly(n-BMA)/dodecane (Pn-BMA/dodecane) domains, because of the evaporation of the dodecane. The size and number of dimples at the surfaces of the obtained golf ball-like particles decreased as the sodium styrene sulfonate content of the seed particles increased. The Pn-BMA/dodecane domains were engulfed deeper in the surface layer of the seed particle, and, thus, dimples became apparently smaller at the surface with an increase in the hydrophilicity of the surface of the seed particle. Introduction Nonspherical shape is one of the functional properties for the applications of polymer particles. Such particles can be utilized to synthesize materials with unique crystal structures,1,2 light-scattering properties, and responses to external fields such as shear field3 and electric field.4 Polymer particles prepared by heterogeneous polymerization systems (e.g., emulsion polymerization, suspension polymerization, dispersion polymerization, etc.) are usually spherical, because of the minimization of interfacial free energy between the particles and the medium. However, since we prepared nonspherical polymer particles in the mid-1970s,5 various nonspherical particles have been formed6–25 in a series of studies on the preparation of composite polymer particles by seeded polymerizations (e.g., seeded emulsion polymerization,5–16,20,22–24 and seeded dispersion polymerization (SDP).17,18,21,25 In most cases, they were formed by chance, and their formation mechanisms have been clarified in relation to the morphologies of the composite polymer particles, which were discussed in terms of a combination of thermodynamic and kinetic factors.26 The high viscosity within the particles during seeded polymerizations is one of the kinetic factors that lead to the formation of nonspherical particles with nonequilibrium morphology. In particular, because the viscosity within particles during SDP is high,27–30 compared with seeded emulsion polymerization, SDP is a powerful technique to prepare thermodynamically unstable nonspherical polymer particles. In our previous articles, we proposed a new possibility for the preparation of nonspherical particles by utilizing SDP. Micrometer-sized, monodisperse ”disk-like” particles were prepared by SDP of various methacrylate monomers with 1.57µm-sized31 and 2.67-µm-sized32 polystyrene (PS) seed particles in the presence of hydrocarbon droplets. Similarly, ”golf balllike” particles were also prepared via the SDP of styrene with poly(methyl methacrylate) (PMMA) seed particles in the presence of decalin (a mixture of trans- and cis-decalin) droplets.33 The formation mechanisms of golf ball-like particles were as follows. The decalin was predominantly absorbed by * To whom correspondence should be addressed. Tel./Fax: +8178-803-6161. E-mail address: [email protected]. † Part CCCX of the series ”Studies on Suspension and Emulsion”.

the PS domains formed at the surfaces of the PMMA seed particles, and golf ball-like particles were obtained after evaporation of the decalin, as a result of the significant volume reduction of the phase comprising the PS and the decalin. The formation of dimples at the surface of the composite particles is dependent on whether the second polymer domains are formed at the surface of the seed particles or not. Thus, it is considered that the relative hydrophilicities of the seed polymer and the second polymer strongly affect the final particle shape. Such golf ball-like particles have interesting light-scattering properties, which may have various applications. In this article, to clarify the effect of relative hydrophilicities of seed and second polymers on the shape of the golf ball-like particles and reconfirm the formation mechanism of golf balllike particles, SDPs of n-butyl methacrylate (n-BMA) in the presence of decane were conducted with poly(styrene-co-sodium styrene sulfonate) (P(S-NaSS)) seed particles with various degrees of hydrophilicity, prepared by the copolymerization of styrene (S) and sodium styrene sulfonate (NaSS) at various S/NaSS ratios. Experimental Section Materials. S and n-BMA were purified by distillation under reduced pressure in a nitrogen atmosphere. Reagent-grade 2,2′azobisisobutyronitrile (AIBN) (Wako Pure Chemical Industries, Ltd., Osaka, Japan) was purified by recrystallization with methanol. Deionized water with a specific resistance of 5 × 106 Ω cm was used. NaSS (Wako Pure Chemical Industries, Ltd., Osaka, Japan), poly(N-vinylpyrrolidone) (PVP) (weightaverage molecular weight: 4 × 104), methanol, and dodecane (Nacalai Tesque, Inc., Kyoto, Japan) were used as received. Preparation of PS Particles. Monodisperse PS particles were prepared by dispersion polymerization in a four-necked 2-L reactor that was equipped with an inlet of N2, a reflux condenser, and a half-moon-type stirrer, according to the procedure described below. S (100 g), PVP (20 g), and methanol (550 g) were mixed in the reactor and heated at 60 °C with stirring at 120 rpm under nitrogen atmosphere until the mixture became homogeneous. AIBN (1 g) was dissolved in methanol (50 g) and poured into the reactor to start polymerization. After 24 h, monodisperse PS particles were obtained. The number-average

10.1021/ie800188f CCC: $40.75  2008 American Chemical Society Published on Web 06/12/2008

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diameter (Dn) and coefficient of variation (Cv) of the seed particles were measured using transmission electron microscopy (TEM) (JEM-1230, JEOL Ltd., Tokyo, Japan) using image analysis software (MacSCOPE, Mitani Co. Ltd., Fukui, Japan) for a Macintosh computer. PS particles were used after centrifugal washing three times with methanol, to ensure that soluble polymer generated during polymerization did not affect the kinetics of second-stage polymerization. Preparation of PS/P(S-NaSS) Composite Particles by SDP of S and NaSS with PS Particles. SDPs of S and NaSS (total weight: 5 g) in a solution of methanol (73 g) and water (12 g) containing dissolved PVP (0.5 g) were conducted with the washed PS particles (5 g) at 70 °C in a four-necked 200mL reactor that was equipped with the same systems as that previously described. The polymerization was started by the addition of AIBN (0.1 g) dissolved in methanol (5 g). PS/P(SNaSS) composite particles with various S/NaSS ratios (from 100/0 to 90/10, w/w) of P(S-NaSS) were obtained. The conversion of S was measured by gas chromatography; it was 88% in the case of PS/PS particles (S/NaSS ) 100/0). The conversions of the other copolymerizations were considered to be similar, although the conversion of NaSS was not measured, because of the nonvolatility of NaSS. These particles were used after centrifugal washing three times with methanol. Preparation of PS/P(S-NaSS)/PnBMA Compostite Particles by SDP of n-BMA in the Presence of Dodecane. SDPs of n-BMA were performed in the presence of decane in a methanol/water mixture in sealed glass tubes under a nitrogen atmosphere. PS/P(S-NaSS) composite particles (0.5 g), n-BMA (0.25 g), AIBN (3 mg), PVP (0.05 g), methanol (8 g), water (2 g), and dodecane (1 g) were mixed and poured into the glass tube. The glass tubes were sealed and then heated at 60 °C with shaking horizontally (3-cm strokes) at 60 cycles/min for 24 h in a water bath. Dodecane is partially soluble in the medium (0.65 wt %) and formed droplets during the polymerization. The obtained particles were observed using scanning electron microscopy (SEM) (S-2500, Hitachi Science Systems Ltd., Ibaraki, Japan), before and after the extraction of poly(n-BMA) (denoted as Pn-BMA hereafter) by acetic acid. The ultrathin cross sections of the obtained particles were observed via TEM. The dried particles, stained with ruthenium tetraoxide (RuO4) vapor at room temperature for 30 min in the presence of a 1% RuO4 aqueous solution, were dispersed in an epoxy matrix that was cured at room temperature for 24 h and subsequently microtomed. Interfacial Tensions. PS/P(S-NaSS) composite particles were dissolved in toluene. Interfacial tensions between the polymer/ toluene solution and the methanol/water (80/20 (w/w)) medium were measured by the pendant drop method, using a Drop Master 500 apparatus (Kyowa Interface Science Co., Ltd., Japan). Pendant drops of the polymer/toluene solution were formed at the tip of the stainless steel needle in a glass cell that was filled with a methanol/water (80/20 (w/w)) medium. All the measurements were performed at room temperature (ca. 20 °C). The accuracy of the measured interfacial tensions was on the order of (0.2 mN/m. Results and Discussion Dispersion copolymerization of S and NaSS was performed smoothly, resulting in copolymer particles with various S/NaSS ratios. However, the particle diameters increased as the NaSS content increased. The difference in the particle diameter of the seed particles may affect the particle shape in SDP.21 To avoid this problem, we tried to prepare PS-based particles that had

Figure 1. TEM photographs of PS/P(S-NaSS) composite particles prepared by SDPs of S and NaSS with PS seed particles at various S/NaSS ratios (w/w): (a) 100/0, (b) 98/2, (c) 96/4, (d) 94/6, (e) 98/2, and (f) 90/10.

Figure 2. SEM photographs of PS/P(S-NaSS)/Pn-BMA composite particles prepared by SDPs of n-BMA in the presence of dodecane droplets in a methanol/water (80/20, w/w) medium, with PS/P(S-NaSS) composite particles as seed, at various S/NaSS ratios (w/w) of P(S-NaSS): (a) 100/0, (b) 98/2, (c) 96/4, (d) 94/6, (e) 92/8, and (f) 90/10.

the same diameter and various NaSS contents by SDPs of S and NaSS at various NaSS contents with the same PS seed particles, of which the Dn and Cv values were 1.44 µm and 8%, respectively, based on previous work.27 Figure 1 shows TEM photographs of PS/P(S-NaSS) composite particles for various S/NaSS ratios in the copolymers (100/0, 98/2, 96/4, 94/6, 92/8, and 90/10 (w/w)). The composite particles were monodisperse at all S/NaSS ratios. The corresponding Dn and Cv values were 1.68, 1.65, 1.72, 1.67, 1.75, and 1.67 µm, and 9.8%, 9.7%, 8.6%, 11.7%, 4.9%, and 5.1%, respectively. Figure 2 shows SEM photographs of PS/P(S-NaSS)/P(nBMA) composite particles prepared via the SDP of n-BMA with PS/P(S-NaSS) composite particles that had various NaSS contents as seed, in the presence of dodecane, in a methanol/ water (80/20, w/w) medium. In the case of PS seed particles, the PS/P(n-BMA) composite particles obtained were almost spherical, with smooth surface. In our previous investigations on SDP, we have clarified that SDP has a great advantage for the preparation of core/shell particles.27–30 However, under the conditions that the rate of adsorption of polymer radicals formed in the media onto the seed particles is slow, the second-stage polymer has a tendency to form domains on the surface of the seed particle, and, thus, PS/P(n-BMA) egglike composite

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particles were obtained via the SDP of n-BMA with PS seed particles.17 Furthermore, the slow adsorption rate of Pn-BMA gave snowman- and confetti-like particles, using 1.28- and 2.67µm-sized PS seed particles, respectively.21 In this study, although the addition of dodecane is different from the previous case and this might influence the polymerization kinetics, the rate of adsorption of Pn-BMA radicals was fast enough to cover the 1.68-µm-sized PS seed particles, resulting in the formation of PS/Pn-BMA core/shell composite particles with a smooth surface. This result was considered to be reasonable from the comparison of the polymerization conditions with the previous cases.21 The reason why the particles did not appear to be spherical in Figure 2a is that the PS surface was partially exposed and the volume reduction of the Pn-BMA incomplete shell after evaporation of dodecane caused the formation of polyhedral particles with a smooth surface. On the other hand, when PS/P(S-NaSS) composite particles were used as seed, golf ball-like particles were formed, regardless of the NaSS content. This indicates that the formation of Pn-BMA domains was facilitated in the hydrophilic surface layer, in comparison with Pn-BMA easily covering the hydrophobic surface of PS/PS seed particles. In addition, as the NaSS content in the P(S-NaSS) increased, the dimples at the surface of the composite particles became smaller and fewer. In previous work,33 the size and number of the dimples at the surfaces of golf ball-like PMMA/PS composite particles prepared via the SDP of S with PMMA seed particles in the presence of decalin droplets were affected by the viscosity of the particles, because the rate of growth of second polymer domains is influenced by the viscosity of the seed particles. In this study, it is considered that the amount of dodecane absorbed by the seed particles might be influenced by the NaSS content. Accordingly, the viscosities of the PS phase were evaluated by measuring the glass-transition temperatures (Tg) of PS/P(S-NaSS) seed particles swollen by dodecane. The Tg values of the PS/P(S-NaSS) (S/NaSS ) 98/ 2, 90/10 (w/w)) seed particles swollen with dodecane were almost the same as that of the PS particles measured in previous work.32 This indicates that the NaSS content did not affect the viscosity of the seed particles. Thus, the polarity of the second polymer, relative to the seed polymer, affected not only whether the golf ball-like particles were formed or not, but also the size and number of the dimples at the surface of the PS/P(S-NaSS)/ Pn-BMA composite particles by the different mechanism from varying viscosities of seed particles. For further investigation, PS/P(S-NaSS)/Pn-BMA composite particles were immersed in acetic acid for 3 days to extract Pn-BMA that was present on the particle surfaces. The extraction of Pn-BMA was required to observe the particle morphology accurately, because Pn-BMA that contained dodecane was able to move from the original position during the evaporation of dodecane and medium. Figure 3shows SEM photographs of particles after the extraction of Pn-BMA from PS/P(S-NaSS)/Pn-BMA composite particles, as well as TEM photographs of their ultrathin cross sections. Although Pn-BMA at the surface of PS/P(SNaSS)/Pn-BMA composite particles was only partially removed, the difference in the shape of the particles between the S/NaSS ratios (w/w) of 98/2 (Figure 3a) and 90/10 (Figure 3b) was more clearly observed by SEM, compared with Figures 2b and 2f, respectively. The size and number of the dimples were smaller at the S/NaSS ratio of 90/10 than at the S/NaSS ratio of 98/2. The reason for this difference could be explained by the TEM photographs of ultrathin cross sections of the two particles. Because RuO4 selectively stains

Figure 3. (a, b) SEM photographs of particles after the extraction of PnBMA from PS/P(S-NaSS)/Pn-BMA composite particles prepared by SDPs of n-BMA in the presence of dodecane droplets in a methanol/water (80/ 20, w/w) medium with PS/P(S-NaSS) composite particles as seed; (c, d) TEM photographs of their ultrathin cross sections after RuO4 staining; and (e, f) magnified portions of the photographs in panels c and d, as indicated by the white square. S/NaSS ratios (w/w) of P(S-NaSS): (a, c, e) 98/2; (b, d, f) 90/10.

PS but not Pn-BMA, the black part in the particle is PS phase and the white part is Pn-BMA. Several small Pn-BMA domains were observed in the core part of the particles. This suggests that n-BMA was absorbed in the core, where the polymerization proceeded, but the concentration was much lower in the core than in the surface layer. The volumes of the dimples in the surface layer were almost the same at ratios of 98/2 and 90/10. However, the dimples existed more inside of the surface layer at a ratio of 90/10 than at a ratio of 98/ 2. This is the reason why apparently smaller dimples were observed at the particle surface in the case of 90/10. The dimples indicate the existence of Pn-BMA domains, and, thus, the phase structure of PS/Pn-BMA/dodecane is speculated from the shape of dimples at the surfaces of the particles. Pn-BMA/dodecane domains were engulfed deeper in the seed particle at the S/NaSS ratio of 90/10 than at the S/NaSS ratio of 98/2 (see Figures 3e and 3f). This indicates that relatively hydrophobic Pn-BMA/dodecane domains were more engulfed in the more-hydrophilic surface layer of the seed particles in the aqueous dispersed system to achieve a thermodynamically favored morphology. It is concluded that the hydrophilicity of the surface layer of the seed particle influenced the formation of dimples at the surface of the golf ball-like particle. To compare of the hydrophilicities of the surface layer of the PS/P(S-NaSS) composite seed particles, we tried to quantify the amount of NaSS units actually introduced in the seed particles. The amount of NaSS units was too small to detect by X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR) spectroscopy. Figure 4 shows the interfacial tensions between water and toluene solutions, in which PS/PS and PS/P(S-NaSS) seed particles were dissolved. Interfacial tensions with PS/P(S-NaSS) were always lower than that with PS/PS particles (S/NaSS ) 100/0

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Figure 4. Interfacional tension between a PS/P(S-NaSS)/toluene solution and water, as a function of time, measured by the pendant drop method at various S/NaSS ratios (w/w) of P(S-NaSS): (O) 100/0, (0) 98/2, (4) 96/4, (b) 94/6, (9) 92/8, and (2) 90/10.

(w/w)). However, there was no obvious tendency in the relationship between the interfacial tension and the NaSS content. A possible explanation of this unexpected phenomenon is the inclusion of water in the droplets of the polymer solution. Some water domains were observed with the naked eye in the droplet of polymer solutions during the measurement of the interfacial tension with high NaSS content copolymer. Recently, Tauer et al.34 reported spontaneous emulsification at the interface between pure oil and pure water. In the case of this study, water domains were not observed during measurement of the interfacial tension between water and toluene in which PS/PS particles were dissolved, indicating that the formation of the water domains was caused by the P(S-NaSS) copolymer. The consumption of P(S-NaSS) copolymer to stabilize the water domains in the droplet prevented reduction of the interfacial tension at the aqueous medium. The amount of consumed P(S-NaSS) copolymer increased as the NaSS content increased. Conclusion In preparing golf ball-like particles via the seeded dispersion polymerization (SDP) of n-BMA with PS/P(S-NaSS) composite seed particles in the presence of dodecane droplets in the aqueous medium, the relative hydrophilicities of the seed polymers and the second polymer are key factors. It is possible to control the size and number of dimples on the surfaces by changing the relative hydrophilicities. Acknowledgment This work was partially supported by Creation and Support Program for Start-ups from Universities (No. 1509) from the Japan Science and Technology Agency (JST). Literature Cited (1) Yin, Y.; Xia, Y. Self-Assembly of Monodispersed Spherical Colloids into Complex Aggregates with Well-Defined Sizes, Shapes, and Structures. AdV. Mater. 2001, 13, 267–271. (2) Lu, Y.; Yin, Y.; Li, Z.-Y.; Xia, Y. Colloidal Crystals Made of Polystyrene Spheroids: Fabrication and Structural/Optical Characterization. Langmuir 2002, 18, 7722–7727. (3) Jogun, S. M.; Zukoski, C. F. Rheology and Microstructure of Dense Suspensions of Plate-Shaped Colloidal Particles. J. Rheol. 1999, 43, 847– 871. (4) Ho, C. C.; Ottewill, R. H.; Yu, L. Examination of Ellipsoidal Polystyrene Particles by Electrophoresis. Langmuir 1997, 13, 1925–1930.

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ReceiVed for reView February 1, 2008 ReVised manuscript receiVed March 31, 2008 Accepted April 1, 2008 IE800188F