Aluminum Hydroxide and Polystyrene

ARTICLE pubs.acs.org/Langmuir

Preparations of Polystyrene/Aluminum Hydroxide and Polystyrene/ Alumina Composite Particles in an Ionic Liquid Keigo Kinoshita,† Hideto Minami,†,* Yasunori Tarutani,† Kimitaka Tajima,† Masayoshi Okubo,† and Hiroshi Yanagimoto‡ † ‡

Graduate School of Engineering, Kobe University, Rokko, Nada, Kobe 657-8501, Japan Magnetic Material & Surface Modification Department, Metallic & Inorganic Material Engineering Division, Toyota Motor Corporation, Toyota-cho, Toyota, Aichi 471-8572, Japan ABSTRACT: Polystyrene (PS)/aluminum hydroxide (Al(OH)3) composite particles were successfully prepared by the sol
gel process of aluminum isopropoxide (Al(OPri)3) in a hydrophilic ionic liquid, 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim][BF4]) using ammonium hydroxide (NH4OH) as a catalyst in the presence of PS seed. Transmission electron microscopy observation of ultrathin cross-sections of the composite particles revealed that the composite particles had a core
shell morphology consisting of a PS core and a Al(OH)3 shell having high crystallinity. The amount of secondary nucleated Al(OH)3 could be reduced by dropwise addition of NH4OH. Moreover, PS/η-Al2O3 composite particles were successfully prepared by heat treatment of PS/Al(OH)3 at 300 °C in N2 atmosphere, which is below the decomposition temperature of PS.

’ INTRODUCTION Over the past decade, there has been increasing interest in the use of ionic liquids (ILs), which are a class of salts that consist of cations and anions but are liquid state at ambient temperature. ILs have been considered as environmentally friendly media because these liquids have unique properties such as nonvolatility, nonflammability, and high thermal stability.1
6 Many researchers were inspired by these unique properties and began to use them as a media for chemical syntheses, separations, electrochemical processes, and biochemical reactions.6
13 In the field of polymer chemistry, using ILs as polymerization media has attracted considerable attention in recent years.14
23 It was reported that free radical polymerization in ILs results in a polymerization rate and molecular weight higher than that in bulk or conventional organic solvents.21,24
26 Watanabe and coworkers discovered lower critical solution temperature-type phase separation of poly(benzyl methacrylate) (PBnMA) and reversible and discontinuous volume phase transition of PBnMA gel particles in ILs.27,28 Shimomura et al. reported the preparation of poly(p-phenylenevinylene) (PPV) particles from a PPV precursor solution by using a self-organized precipitation method with an IL as a poor solvent. PPV particles formed after annealing at 240 °C under reduced pressure in an IL, utilizing the advantage of high thermal stability and nonvolatility of ILs.29 Pringle et al. synthesized poly(terthiophene) particles containing metal (Au, Ag) nanoparticles by chemical oxidative polymerization.30 Landfester et al. prepared polyimide particles by heterophase polycondensation without any catalysts and stabilizers.31 Recently, we have reported the prepartion of polystyrene (PS) particles by dispersion polymerization in an IL, N,N-diethyl-Nr 2011 American Chemical Society

methyl-N-(2-methoxyethyl)ammonium bis(trifluoromethanesulfonyl)amide ([DEME][TFSA]).32 We also prepared PS particles by thermal polymerization in [DEME][TFSA] without a radical initiator, utilizing the advantage of nonvolatility and thermal stability of ILs. Furthermore, we prepared Nylon-6 particles33 and poly(acrylic acid) (PAA) particles34 in ILs. The preparation of composite polymer particles, which consist of a PS (hydrophobic) core and PAA (water-soluble) shell by seeded dispersion polymerization in [DEME][TFSA] using PS particles as seeds, was demonstrated.35 The syntheses of inorganic materials in ILs have also been reported.36
40 Some kinds of ILs will dissolve not only organic but also inorganic compounds by tuning anions and cations. Antonietti et al. reported the direct synthesis of highly crystalline titania nanoparticles and nanorods by the sol
gel process in IL at 80 °C without high-temperature (g1000 °C) calcination.41,42 This finding suggests the possibility of preparation of polymer/ highly crystalline metal oxide composite materials. Such materials could be expected to have a high thermal or electrical conductivity while being lightweight. Generally, highly crystalline metal oxide requires high-temperature calcination, and organic polymers such as PS particles are easily decomposed at high temperature. Endres and co-workers reported the synthesis of alumina (Al2O3) by the sol
gel process in hydrophobic ILs, in which IL was immiscible with water for hydrolysis.43,44 In their work, crystalline Al2O3 particles were obtained after calcination of the sol
gel Received: January 14, 2011 Revised: February 23, 2011 Published: March 18, 2011 4474

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Langmuir products at 500 to 1000 °C, which is above the decomposition temperature of PS. Zheng et al. showed that the low-temperature and one-step synthesis of γ-Al2O3 in IL by an ionothermal synthetic method at 150 °C without high temperature (300
800 °C) calcination.45 Their method required high-pressure in an autoclave. We also successfully prepared highly crystalline aluminum hydroxide (Al(OH)3) particles by the sol
gel process of aluminum isopropoxide (Al(OPri)3) in hydrophilic IL, 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim][BF4]), under mild conditions (at 30 °C, without an autoclave), prior to preparation of PS/Al2O3 composite particles.46 [Bmim][BF4] worked as a template, in which Al(OH)3 was prepared by sol
gel along the local structure of [Bmim][BF4]. Moreover, crystalline η-Al2O3 particles were easily synthesized by heat treatment of the Al(OH)3 particles in N2 atmosphere at 300 °C, which is below the decomposition temperature of PS. In this work, we prepared PS/Al(OH)3 composite particles utilizing the sol
gel process of Al(OPri)3 in [Bmim][BF4] in the presence of PS seeds. We also investigated the influence of the surface charge of PS particles on the Al(OH)3 shell formation. Moreover, we tried to synthesize PS/η-Al2O3 particles by the heat treatment of PS/Al(OH)3 particles at 300 °C in N2 atmosphere.

’ EXPERIMENTAL SECTION Materials. Styrene was purified by distillation under reduced pressure in a N2 atmosphere. Reagent grade 2,20 -azobis(isobutyronitrile) (AIBN) and potassium persulfate (KPS) were recrystallized using methanol and water, respectively. 2,20 -Azobis(2-methylpropionamidine) dihydrochloride (V-50), poly(vinyl pyrrolidone) (PVP, K-30, weightaverage molecular weight 4  104 g/mol), aluminum isopropoxide (Al(OPri)3, purity of 99% or more), ammonium hydroxide (NH4OH, NH3: 28
30%), and hydrophilic IL 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim][BF4], purity of 97% or more) were used as received from Nacalai Tesque Inc., Kyoto, Japan. The water used in all experiments was obtained from an Elix UV (Millipore Co., Ltd.; Japan) purification system and had a resistivity of 18.2 MΩ cm.

ARTICLE

On the other hand, in the case of anionic and cationic PS seeds, the medium of PS emulsion (8.3 g, solid content: 3 wt %) was changed from water to methanol by centrifugal washing. The emulsion was mixed with [Bmim][BF4] (2.5 g), dissolving Al(OPri)3 (0.125 g), and then the medium was replaced by evaporation of methanol at 60 °C. Conditions of the sol
gel process were the same as those in the case of the nonionic PS seed. Preparation of PS/Al2O3 Composite Particles. PS/Al(OH)3 particles were centrifugally washed with methanol three times (10 000 rpm, 10 min, rotational radius was 10 cm) and then dried at room temperature under vacuum. PS/η-Al2O3 composite particles were prepared by heat treatment of dried PS/Al(OH)3 at 300 °C for 24 h in a muffle furnace under N2 flow. Characterization. Scanning electron microscopy (SEM, JSM6510, JEOL Ltd., Tokyo, Japan) observation of the particles coated with osmium was performed at 20 kV. Transmission electron microscopy (TEM, JEM-1230, JEOL Ltd., Tokyo, Japan) observation of the particles was performed at 100 kV. To observe the interior morphology of the particles, the dried particles were stained with ruthenium tetroxide (RuO4) vapor at room temperature for 30 min in the presence of 0.5 wt % RuO4 aqueous solution, embedded in an epoxy matrix, cured at room temperature overnight, and subsequently microtomed. The ultrathin cross-sections of approximately 100 nm thickness were observed with TEM. The zeta potential of PS particles dispersed in water was measured with a zeta potential and particle size analyzer (ELSZ-2, Otsuka Electronics Co., Ltd., Osaka, Japan). Qualitative analyses of the composite particles were conducted with a Fourier transform infrared spectrometer (FT-IR, FT/IR-6200, JASCO Corporation, Tokyo, Japan) using a pressed KBr pellet technique and X-ray diffractometer (XRD, RINTTTR, Rigaku Co. Ltd., Tokyo, Japan) using 2θ/θ method with Cu KR radiation generated at 50 kV and 300 mA at a scanning rate of 4°/min. The Al(OH)3 and Al2O3 content of the composite particles were calculated from the weight loss of the composite particles which was measured by thermogravimetry (TGA, EXSTAR TG/DTA6200, SII NanoTechnology Inc., Chiba, Japan) at a heating rate 10 °C/min from 30 °C to 500 °C under N2 atmosphere.

’ RESULTS AND DISCUSSION

Preparation of PS Seed Particles Having Different Charges. PS particles having a nonionic surface were prepared by

Sol
Gel Process in [Bmim][BF4] in the Presence of the PS Particles. Figure 1 shows the SEM photographs of PS seed

dispersion polymerization in [Bmim][BF4] as reported in our previous paper.35 PVP (12.5 mg) was dissolved in [Bmim][BF4] (2.13 g) at 120 °C in an oven for 3 h. The mixture was cooled to room temperature, and a mixuture of 0.25 g of styrene and 2.5 mg of AIBN was added. The polymerization was conducted at 70 °C for 24 h with magnetic stirring at 400 rpm. PS particles having a negative charge (PSKPS) were prepared by emulsifier-free emulsion polymerization in aqueous medium in a fournecked 1-L round-bottom flask equipped with inlet of N2, reflux condenser, and a half-moon type stirrer under the following conditions: 600 g of water added to the reactor was heated to 70 °C, and 18 g of styrene was poured into the reactor. The mixture was deoxygenated with a stream of N2 for 30 min, and then 0.36 g of KPS aqueous solution was added to the reactor. The polymerization was conducted for 24 h with stirring at 120 rpm. PS particles having a positive charge (PSV-50) were prepared by emulsifier-free emulsion polymerization under the same conditions except that the KPS was replaced with V-50 (0.36 g), and the polymerization temperature was changed to 60 °C.

particles prepared by dispersion polymerization in [Bmim][BF4] and obtained particles prepared by the seeded sol
gel process of Al(OPri)3 in [Bmim][BF4] in the presence of the PS particles, and the TEM photograph of the obtained particles before centrifugal washing. From the SEM photographs, the seed particles had comparatively high monodispersity and a slightly rough surface, and their diameter was approximately 400 nm (Figure 1a). The obtained particles after the sol
gel process were polydisperse in diameter from a few nanometers to 500 nm (Figure 1b). From the FT-IR analysis, the broad peak of OH vibration of Al(OH)3 at 3300 cm
1 was observed, indicating that the obtained particles contained Al(OH)3. As shown in Figure 1c, nanosized particles were clearly observed, which should be secondary nucleated Al(OH)3 particles. During the seeded dispersion sol
gel process, Al(OPri)3 dissolved in IL was hydrolyzed by OH
, resulting in precipitation of Al(OH)3. Generated Al(OH)3 in IL tended to deposit on the surface of the PS seed particles. The crucial point of efficient shell formation should be a competition between “formation rate of Al(OH)3” and “deposition (adsorbing) rate of Al(OH)3”. In the case of above conditions, the rate of sol
gel process was faster than that of deposition of Al(OH)3 on the seed surface. The generated Al(OH)3 should nucleate and grow in IL medium (secondary nucleation).

Preparation of PS/Al(OH)3 Composite Particles by the Sol
Gel Process in [Bmim][BF4]. In the case of the nonionic PS seed, residual monomer was removed from the as-prepared PS emulsion under vacuum. [Bmim][BF4] (0.37 g) solution of Al(OPri)3 (0.125 g) was added to the emulsion and magnetically stirred at 400 rpm at 30 °C. The sol
gel reactions were started by adding 0.1 M NH4OH (1.0 g) as catalyst at 30 °C and were carried out for 24 h.

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Figure 1. SEM photographs of PS seed particles (a) prepared by dispersion polymerization in [Bmim][BF4] and particles (b) prepared by sol
gel process of Al(OPri)3 in [Bmim][BF4] at 30 °C using 0.1 M NH4OH (batch addition) as catalyst in the presence of the PS seed particles, and TEM photograph (c) of the particles after the sol
gel process.

Figure 3. TGA curves of PS seed particles (line 1), Al(OH)3 (line 2), and PS/Al(OH)3 composite particles (lines 3, 4, and 5) prepared by the sol
gel process of Al(OPri)3 in [Bmim][BF4] at 30 °C using 0.1 M NH4OH as catalyst in the presence of PS seed particles, measured at the heating rate of 10 °C/min. The lines 3, 4, and 5 correspond to the composite particles of Figures 2, 7a and 7b, respectively. Figure 2. TEM (a) and SEM (b) photographs of particles prepared by the sol
gel process of Al(OPri)3 in [Bmim][BF4] at 30 °C using 0.1 M NH4OH (0.20 g  5, every 30 min) as catalyst in the presence of PS seed particles, and TEM photographs of ultrathin cross-sections of the RuO4stained particles before (c) and after (d) extraction of PS with toluene.

In order to reduce the formation rate of Al(OH)3 and to avoid secondary nucleation of Al(OH)3, 1.0 mL of NH4OH was added dropwise (0.2 mL  5, every 30 min). Figure 2 shows TEM and SEM photographs of the obtained particles prepared in the catalyst dropwise-system and TEM photographs of the ultrathin cross-sections of the RuO4-stained particles. From the TEM observation, the amount of the small particles decreased, although tens of nanometer-sized particles were still obtained. These results suggest that the rate of the sol
gel process should be suppressed by dropwise addition of NH4OH; thus, nucleation of Al(OH)3 in the medium was depressed. The small particles were easily removed by centrifugation (10 000 rpm, 10 min, rotational radius was 10 cm) with methanol. As shown in the SEM photograph, the obtained particles after centrifugation were monodisperse and had no secondary particles. Moreover, the surface of the obtained particles was smoother than that of the seed particles (Figure 1a), which indicates that Al(OH)3 effectively deposited on the surface of the PS seed particles. The TEM observation of the ultrathin cross-sections of the particles clearly indicates that the obtained particles had core
shell morphologies (Figure 2c). RuO4 stains PS as well as Al(OH)3, and Al(OH)3 is

stained stronger than PS, thus, the PS phase appeared to have less contrast than that of Al(OH)3. In order to confirm the core
shell structure in more detail, PS was extracted with toluene from the ultrathin cross-sections of composite particles. After extraction, the cores of the particles were partially removed (Figure 2d), which indicated that PS/Al(OH)3 composite particles consisting of the PS core and the Al(OH)3 shell were successfully prepared. The PS/Al(OH)3 ratio of the composite particles was found to be 93/7 (w/w), which was calculated from the weight loss of the PS, Al(OH)3, and composite particles measured by TGA (Figure 3, line 3). The shell thickness calculated from TGA was 7 nm; on the other hand, theoretical shell thickness, which is calculated from the initial amount of Al(OPri)3, was 9 nm at completion of the sol
gel process for Al(OPri)3. From these results, the amount of secondary nucleated Al(OH)3 was calculated to be 22%. Moreover, from the SEM and TEM observation shown in Figure 2, the shell thicknesses were directly measured to be 14 and 17 nm, respectively, which were calculated from data based on an excess of 100 particles. There seemed to be better agreement between the theoretically calculated shell thickness and that of the experimentally measured thickness. The slight difference might be based on the sparse shell of the Al(OH)3, as shown in Figure 2c; thus, the apparatus shell thickness was larger than the theoretical shell thickness. Figure 4 shows the SEM photograph of composite particles prepared in 2-propanol in the presence of the PS seed for comparison. The composite particles prepared in 2-propanol 4476

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Figure 5. SEM photographs of PS seed particles prepared by emulsifierfree emulsion polymerizations in aqueous medium with different initiators: (a) KPS; (b) V-50. Figure 4. The SEM photograph of composite particles prepared by the sol
gel process of Al(OPri)3 in 2-propanol at 30 °C using 0.1 M NH4OH as catalyst.

had a rough surface and were coagulated. This was because Al(OPri)3 did not dissolve in 2-propanol but rather in [Bmim][BF4]. In our previous study, it was revealed that samples obtained by the sol
gel process of Al(OPri)3 in 2-propanol were low crystalline AlO(OH), which do not transform to alumina at a calcination temperature of 300 °C. On the other hand, the sample obtained in [Bmim][BF4] was highly crystalline Al(OH)3 which transform to crystalline alumina at mild conditions.46 We concluded in that report that the “local structure” of [Bmim][BF4] based on the interactions of alkylimidazolium cation worked as a template for preparation of highly crystalline materials. Thus, using IL as a medium for the sol
gel process is important for the efficient preparation of organic/ inorganic composite particles having a highly crystalline metal hydroxide and metal oxide shell. Influence of the Particle Surface Charge on the Shell Formation. It is well-known that colloidal Al(OH)3 particles are positively charged in aqueous medium.47 In order to clarify the influence of electrostatic interaction between PS seed particles and Al(OH)3 upon formation of the Al(OH)3 shell, PS particles having anionic (PSKPS) and cationic (PSV-50) surface charge were used as seeds, which were prepared by emulsifierfree emulsion polymerization using KPS and V-50 as initiators in aqueous media. Figure 5 shows the SEM photographs of the PSKPS and PSV-50 particles prepared by emulsifier-free emulsion polymerization using KPS and V-50 as initiators, respectively. In both cases, 400 nm-sized, monodispersed particles having a smooth surface were obtained. The zeta potential value in water of PSKPS and PSV-50 exhibited -46.7 and þ66.7 mV, respectively. Each PS emulsion was centrifuged with methanol three times and then added to the [Bmim][BF4] solution of Al(OPri)3 with magnetic stirring. Methanol was evaporated in an oven at 60 °C until that no residual methanol was detected by gas chromatography. In aqueous medium, PSKPS and PSV-50 particles are colloidally stable by electrostatic repulsive force derived from the end group (OSO3
of KPS, and [C(NH2)NH2]þ of V-50, respectively). However, in ILs, it was expected that the particles aggregate because electrostatic repulsive force was depressed because of the high ionic strength of ILs.48 Surprisingly, PS particles after evaporation of methanol were well dispersed and highly stable in [Bmim][BF4] without any stabilizers. This type of phenomenon was reported by Watanabe and co-workers, in which hydrophilic silica nanoparticles having surface silanol (Si
OH)

groups did not flocculate in BF4 anion-based ILs without any surfactant.49 A mechanism of colloidal stability in ILs was proposed, in which solvation layers of IL molecules were formed and the repulsive solvation force derived from IL molecules can enhance the colloidal stability of silica particles in BF4 anionbased ILs. In our case, the colloidal stability in BF4 anion-based IL was observed for the particles having both surfaces, OSO3
and [C(NH2)NH2]þ groups. This should suggest that BF4 anion interacted with not only Si
OH but also with OSO3
and [C(NH2)NH2]þ, and solvation layers could be formed in those systems. The detailed mechanism may be described after further research. Figure 6 shows the SEM photographs of the composite particles prepared by the sol
gel process in [Bmim][BF4] in the presence of the PSKPS and PSV-50 and TEM photographs of the ultrathin cross-sections of the RuO4-stained composite particles. As shown in the SEM photographs, secondary particles were mostly removed by centrifugal washing. Both of the particles were monodisperse and had a smooth surface. From the TEM observation of ultrathin cross-sections, each particle had a core
shell structure having a PS core and a Al(OH)3 shell, which was observed to be the same as the particles in the case of the nonionic PS seed (Figure 2c). Although the seed particles had different charges, there seemed to be no difference in the shell thickness and homogeneity in both particles. These results suggest that the electrical double layer of the seed particles and Al(OH)3 were strongly suppressed in IL; thus, electrostatic interactions between the seed particles and Al(OH)3 were negligible in the shell formation. Influence of the Sol
Gel Conditions on the Shell Formation. In order to increase shell thickness, the amounts of Al(OPri)3 and NH4OH were increased to twice as much as the initial conditions. We expected that the shell thickness increased with increasing amounts of Al(OPri)3 and NH4OH; however, there was little difference in the shell thickness (Figure 7a). TGA analysis revealed that the amount of PS/Al(OH)3 ratio was found to be 95/5 (w/w) (Figure 3, line 4). These results showed that the increase in the amount of Al(OPri)3 and NH4OH did not lead to an increase in shell thickness, but that the amount of secondary nucleated Al(OH)3 particles would increase in this system. Another approach to increase shell thickness was a multistep sol
gel process, which was conducted in the presence of the PS/ Al(OH)3 composite particles after washing secondary Al(OH)3 particles prepared by the first sol
gel process. The shell thickness clearly increased by conducting this sol
gel process (Figure 7b). 4477

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Figure 8. SEM photograph of composite particles after heat treatment of PS/Al(OH)3 composite particles at 300 °C in N2 atmosphere prepared by the sol
gel process in [Bmim][BF4] at 30 °C using 0.1 M NH4OH as catalyst in the presence of PS seed particles.

Figure 6. SEM photographs (a, b) of PS/Al(OH)3 composite particles prepared by the sol
gel process of Al(OPri)3 in [Bmim][BF4] at 30 °C using 0.1 M NH4OH as catalyst in the presence of PS seed particles, which were prepared with different initiators, and TEM photographs (a0 , b0 ) of ultrathin cross-sections of the RuO4-stained composite particles. Initiators: (a, a0 ) KPS; (b, b0 ) V-50.

Figure 9. FT-IR spectra of composite particles before (a) and after (b) heat treatment at 300 °C in N2 atmosphere prepared by the sol
gel process of Al(OPri)3 in [Bmim][BF4] at 30 °C using NH4OH as catalyst in the presence of PS seed particles. Figure 7. TEM photographs of ultrathin cross-sections of RuO4stained PS/Al(OH)3 composite particles prepared by the sol
gel process in [Bmim][BF4] at 30 °C under different conditions: (a) amount of Al(OPri)3 and NH4OH were twice as much as the initial condition; (b) the sol
gel process was conducted in the presence of the PS/Al(OH)3 composite particles prepared by the first sol
gel process.

Moreover, from the TGA analysis (Figure 3, line 5), the PS/ Al(OH)3 ratio was found to be 89/11 (w/w). These results suggest that shell thickness can be controlled by the multistep sol
gel process. From TEM (Figure 7), the shell layers were observed to consist of small Al(OH)3 particles. The generated Al(OH)3 particles in the medium diffused and deposited on the PS seed particles to decrease the interfacial free energy. The interfacial tensions between PS and IL should be higher than that of Al(OH)3 and IL; thus, generated Al(OH)3 deposited on the surface covering the PS seed particles. The interactions between PS and IL would be van der Waals forces, but not electrostatic interaction, as described above.

Preparation of PS/Al2O3 Composite Particles. Finally, in order to prepare PS/Al2O3 composite particles, heat treatment of PS/Al(OH)3 composite particles, which were washed with methanol three times and dried at room temperature under vacuum, was conducted at 300 °C for 24 h in N2 atmosphere. Figure 8 shows the SEM photographs of the composite particles before and after heat treatment. As a result of the heat treatment, sintering and aggregation of the particles were observed; however, the particles maintained their particulate shape and size without collapse although their surface became rough. Figure 9 shows the FT-IR spectra of the composite particles before and after heat treatment at 300 °C for 24 h in N2 atmosphere. The FT-IR spectra showed that the composite particles before heat treatment (Figure 9a) had characteristic peaks of PS at 697, 755, 1452, and 1493 cm
1 (encircled in Figure 9), and the broad peaks of Al2O3 from 450 to 900 cm
1 and 1650 cm
1 were not observed. On the other hand, after the heat treatment (Figure 9b), the particles have characteristic broad peaks of Al2O3 as illustrated by the gray domain, as well as the narrow peaks of PS. Figure 10 shows the XRD spectrum of 4478

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described in this study will expand the utility of ILs as media for synthesis of polymer/crystalline metal oxide composite particles.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Tel and fax: (þ81) 78 803 6197. Figure 10. The XRD spectrum of the PS/Al(OH)3 composite particles after heat treatment at 300 °C in N2 atmosphere, prepared by the sol
gel process of Al(OPri)3 in [Bmim][BF4] at 30 °C using NH4OH as catalyst in the presence of PS seed particles. Boehmite [AlO(OH)] (1); η-Al2O3 (b)

’ ACKNOWLEDGMENT This work was partially supported by Grant-in-Aid for Scientific Research (Grant No. 21655082) from the Japan Society for the Promotion of Science (JSPS). The authors thank Professor Minoru Mizuhata (Kobe University) for ζ-potential and XRD measurements. ’ REFERENCES

Figure 11. TGA curves of PS seed particles (line 1), Al(OH)3 (line 2), and composite particles before (line 3) and after (line 4) heat treatment at 300 °C in N2 atmosphere prepared by the sol
gel process of Al(OPri)3 in [Bmim][BF4] at 30 °C using 0.1 M NH4OH as catalyst in the presence of PS seed particles.

the PS/Al(OH)3 composite particles after heat treatment at 300 °C for 24 h in N2 atmosphere, prepared by the sol
gel process of Al(OPri)3 in [Bmim][BF4] in the presence of PS seed particles. Although peaks were rather small, the XRD pattern of the composite particles showed two types of peaks which can be assigned to boehmite [AlO(OH)] and η-Al2O3. In our previous study (PS absence system),46 similar results were observed indicating that the presence of PS particles did not affect the transformation from Al(OH)3 to η-Al2O3. Quantitative analysis of decomposed PS from the TGA curve of PS/η-Al2O3 (Figure 11, line 4) was found to be 4.2%, which was insignificant. Moreover, the η-Al2O3 ratio was estimated to be 8%, bearing in mind the decomposed PS and complete transformation of Al(OH)3. These results indicated that the PS/Al2O3 composite particles were successfully prepared by the heat treatment of PS/ Al(OH)3 composite particles.

’ CONCLUSIONS We have demonstrated the preparation of PS/Al(OH)3 composite particles by the sol
gel process in [Bmim][BF4] in the presence of PS seed. The composite particles had a core
shell morphology having PS core and highly crystalline Al(OH)3 shell. The effect of electrostatic interaction on the shell formation was negligible because of the high ionic strength of ILs. Moreover, PS/η-Al2O3 particles were easily synthesized by heat treatment of PS/Al(OH)3 composite particles. The results

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