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Silica nanoparticle decorated conducting polyaniline fibers and their electrorheology
Materials Letters 64 (2010) 154–156
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Silica nanoparticle decorated conducting polyaniline fibers and their electrorheology Ying Dan Liu, Fei Fei Fang, Hyoung Jin Choi ⁎ Department of Polymer Science and Engineering, Inha University, Incheon 402-751, Korea
a r t i c l e
i n f o
Article history: Received 21 August 2009 Accepted 14 October 2009 Available online 22 October 2009 Keywords: Nanomaterials Polymers Electrorheological fluid Polyaniline Silica Nanoparticle
a b s t r a c t Polyaniline (PANI) fibers as well as silica nanoparticle decorated PANI (silica-PANI) fibers were successfully synthesized as a dispersed phase of an electrorheological (ER) fluid. The fibers obtained through interfacial polymerization were about 300–400 nm in diameter and 2–5 μm long. Then the fibers were redispersed in ethanol containing tetraethyl orthosilicate (TEOS), and silica nanoparticles were formed on the surface of the fibers through a modified Stöber method. The ER characteristics of the ER fluids based on pure PANI fibers and silica-PANI fibers were examined under various electric field strengths using a rotational rheometer, demonstrating slight different flow curves for the silica-based ER fluid. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Electrorheological (ER) fluids [1,2] are known as a kind of smart material because their structural and rheological changes caused by electric fields are typically reversible. Commonly, ER fluids are composed of insulating fluids and polarizable particles. ER fluids exhibit a tremendous increase in their shear viscosity, from a liquidlike to a solid-like state, when they are exposed to a relatively high electric field. This phenomenon reportedly arises from the formation of chain-like structures of the dispersed particles aligned along the electric field [3], which occurs within a few milliseconds, along with their magnetically analogous magnetorheological suspensions [4,5]. The fluids perform like Newtonian fluids without an electric field when they are sheared. However, when an electric field is applied and increased, yield stresses are needed to make the fluids flow. ER fluids have been applied in various areas such as shock absorbers, engine mounts and clutches because of their quick response to an electrical field and their controllable mechanical properties [6]. Various substances, such as high dielectric inorganics [7], conducting polymers [8] as well as their composites [9,10], have been investigated for ER materials. Polyaniline (PANI) is a favorable conducting polymer which has been studied in this field because of its easy synthesis, thermal and chemical stability, high sensitivity to an electric field and controlled conductivity using a doping/dedoping process [11]. Many PANIs differing in inner structures or outer morphologies, including derivatives [12], nanocomposites, core-shell particles, nanospherical or fibrous PANI [13] and etc., have been developed in ER research. Recently, Yin et al. [14] reported PANI a
⁎ Corresponding author. Tel.: +82 32 860 8777; fax: +82 32 865 5178. E-mail address: [email protected] (H.J. Choi). 0167-577X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2009.10.031
nanofiber based ER fluid which possessed a higher yield stress and was more stable than normal PANI based ER fluids. PANI fibers can differ in shape (straight or dendritic) and diameter depending on method of polymerization. Additionally, modifications to the PANI fibers have been studied by researchers in order to give PANI new functionality in an attempt to improve its processability or other properties for special applications [15]. In this work, silica nanoparticle decorated PANI (silica-PANI) fibers were introduced as a dispersed phase of the ER fluid. The fibers were straight, and their diameters were larger than other reported results. Silica nanoparticles were formed by a modified Stöber method [16] and adhered to surface of the fibers by means of electrostatic attraction. These particles decreased the conductivity of the PANI fibers to a range that was suitable for the ER test. Therefore, the dedoping process was omitted before the ER fluid preparation. The products were characterized by scanning (SEM) and transmission (TEM) electron microscopies as well as FT-IR spectroscopy. The ER suspension performance was investigated in silicone oil. 2. Experimental PANI fibers were synthesized through an interfacial polymerization method. The monomer aniline (5.1 g) was first dissolved in 150 ml toluene, which was used as the oil phase; the initiator, ammonium persulphate (APS, 12.5 g), and dodecylbenzene sulfuric acid (DBSA) were dissolved in distilled water, which was used as the water phase. After magnetic stirring for 1 h, the oil phase was carefully transferred to the water phase without disturbing the interface. The reaction started the moment the two phases contacted each other, which was confirmed by the smoke-like PANI ascending from the interface. The reaction system was kept in a refrigerator for
Y.D. Liu et al. / Materials Letters 64 (2010) 154–156
155
Fig. 1. Schematic diagram of synthetic process of silica-PANI fiber.
24 h without being moved. After that, the upper oil layer was removed using a syringe. The solid PANI mass in the aqueous phase was centrifuged and washed with distilled water/ethanol several times. The solid product was dried in a vacuum oven at 60 °C for 3 days. Silica-PANI fibers were synthesized through the following method. First, 0.5 g dried fibers and 0.5 g tetraethyl orthosilicate (TEOS) were dispersed in 24 ml ethanol in a glass vial and sonicated for half an hour to ensured that they were well dispersed. Then the vial was placed into water bath at 40 °C for several minutes until a constant temperature was reached. Then 0.76 g of 35% hydrous ammonia (NH3·H2O, catalyst) and 0.17 g of distilled water were quickly stirred into the mixture. After 15 h, the products were centrifuged washed with distilled water/ethanol several times, and then dried in a vacuum oven at 60 °C. SEM studies were carried out using a Hitachi Electron Microscope (S-4300, Japan). The silica surface decoration of PANI was identified by FT-IR (Perkin-Elmer, Spectrum 2000 Explorer). Pure PANI fibers were dedoped with 1 M NaOH before ER fluid preparation. Then 13 wt.% of both the PANI and silica-PANI fibers were dispersed in the silicone oil (50cS) by shaking and sonication. The ER properties of the product based ER fluids were measured using a rotational rheometer (MC120, Paar-Physica, Germany) which was equipped with a high
Fig. 2. SEM image of silica-PANI fiber (inset: PANI fiber).
direct current voltage generator. The flow curves were obtained for various electric fields by measuring the shear stress as a function of the shear rate in the range of 0.01 to 1000 s− 1. 3. Results and discussion Fig. 1 is a schematic illustration of the synthesis process for the silica nanoparticle decorated PANI fiber. The fiber was positively charged during polymerization due to its emeraldine salt form with DBSA. The fibers are surrounded by TEOS, the precursor to silica, in an ethanol system. Hydrolysis and condensation were started after the addition of NH3·H2O. The formed silica nanoparticles were negatively charged, and therefore, easy to locate on the surface of the PANI fibers because of an electrostatic attraction. The SEM images of both the PANI fibers (inset) and silica-PANI fibers are shown in Fig. 2. The pure fibers had diameters of about 300–400 nm and length of 2–5 μm with smooth surfaces. On the other hand, the silica decorated fibers were much rougher than the pure fibers. Silica nanoparticles were easy recognized on the surface of fibers. These nanoparticles were not ideal spheres that were distributed with various particle sizes on the order of a few to dozens of nanometers. Fig. 3 represents the FT-IR spectrum of two samples. The main characteristic bands for the pure fiber in Fig. 3(a) were assigned as follows: the band in the region between 3420 and 3450 cm− 1 was related to the N–H stretching [17]; the bands at 1570 and 1497 cm− 1 corresponded to the C C stretching vibration of the quinoid ring and
Fig. 3. FT-IR spectra of PANI fiber (a) and silica-PANI fiber (b).
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Y.D. Liu et al. / Materials Letters 64 (2010) 154–156
structures. In the shear field, these structures were destroyed and reformed under the cooperation of the electrostatic and hydrodynamic forces. The silica particles obstructed the repacking of the fibers. Moreover the time needed for the relaxation process lagged behind the action of the shear rate. Therefore, the shear stress decreased in the electrostatic dominated region (low shear rate region) and increased in the hydrodynamic force dominated region (high shear rate region) thereafter. This observation was probably caused by the obstruction effect of the silica nanoparticles on the packing of the fibers in chain-like structure formation. 4. Conclusions
Fig. 4. Flow curves of PANI fiber and silica-PANI fiber based ER fluids under electric field strengths (PANI: closed; silica-PANI: open).
benzenoid ring, respectively [18]; a higher peak at 2923 cm− 1 resulted from the crosslinking moieties [19]; and the peak at 1117 cm− 1 was caused by the C–C stretching. The characteristic peaks of silica appeared when the fibers were modified with silica. The band at 1100 cm− 1 was attributed to the Si–O–Si asymmetric stretching. The stretching vibration of Si–OH was assigned to the peak at 829 cm− 1. Additionally, the Si–O–Si bending vibration peak appeared at 470 cm− 1 [20]. Expressed by the flow curves in Fig. 4, the ER performances of the ER fluids (13 wt.%) based on the pure PANI fibers and silica-PANI fibers were tested in a controlled shear rate mode. Without an electric field, both of the ER fluids act as ideal viscous fluid where the shear stress grew linearly with the shear rate. When an electric field was applied, yield stresses appeared and increased step-wise with the electric field strengths. The yield stresses of the two ER fluids were similar. For instance, they were ca. 12 Pa, 70 Pa, and 180 Pa for field strengths of 0.5 kV/mm, 1 kV/mm and 2.0 kV/mm respectively, which also showed better dependence of shear stress on the electric field strength than our previously reported silica-based mesoporous-PANI system [21]. However, the pure PANI based ER fluid showed more ER behavior stability with respect to shear rate for a long range. The same property was also reported for the PANI fiber based ER fluid by Yin et al. [7]. The flow curves for the silica-PANI based ER fluid were much different. After the appearance of the yield stress, the shear stress decreased as a function of the shear rate to a minimum, called the critical shear rate, before increasing again. This phenomenon was considered the result of the changing fiber microstructures and was observed by various researchers [22]. In the presence of an electric field, the silica-PANI fibers were polarized and connected to each other to form chain-like
PANI fibers and silica nanoparticle decorated PANI fibers were successfully synthesized. The morphologies of these were characterized by SEM images. The diameters of the fibers were about 300–400 nm, and the silica nanoparticles located on the surfaces of fibers were multi distributed in size ranging a few decades of nanometers. The formation of silica was confirmed using the FT-IR spectra. The flow curves of the ER fluid based on both pure PANI fibers and silica-PANI fibers were tested in the presence of various electric field strengths, while the silica-PANI based ER fluid showed different behaviors from that of pure fibers. Acknowledgements This work was supported by research fund from ETRI (2009). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]
Han SJ, Kim B, Suh KD. Mater Lett 2007;61:3995–9. Kim YD, Park DH. Synth Met 2004;142:147–51. Choi HJ, Jhon MS. Soft Matter 2009;5:1562–7. Kim JH, Fang FF, Choi HJ, Seo Y. Mater Lett 2008;62:2897–9. Ko SW, Yang MS, Choi HJ. Mater Lett 2009;63:861–3. Han YM, Lim SC, Lee HG, Choi SB, Choi HJ. Mater Design 2003;24:53–61. Yin JB, Zhao XP. Nanotechnology 2006;17:192–6. Cho CH, Choi HJ, Kim JW, Jhon MS. J Mater Sci 2004;39:1883–5. Ilieva M, Ivanov S, Tsakova V. J Appl Electrochem 2008;38:63–9. Cheng Q, Pavlinek V, He Y, Li C, Saha P. Colloid Polym Sci 2009;287:435–41. Huang J. Pure Appl Chem 2006;78:15–27. Kim JW, Jang WH, Choi HJ, Joo J. Synth Met 2001;119:173–4. Oz K, Yavuz M, Yilmaz H, Unal HI, Sari B. J Mater Sci 2008;43:1451–9. Yin JB, Zhao XP, Xia X, Xiang LQ, Qiao YP. Polymer 2008;49:4413–9. Wang X, Feng X, Zhao Y, Zhang R, Sun D. Eur Polym J 2007;43:3679–82. Stöber W, Fink A, Bohn E. J Colloid Interf Sci 1968;26:62–9. Jang WH, Kim JW, Choi HJ, Jhon MS. Colloid Polym Sci 2001;279:823–7. Trchova M, Sedenkova I, Tobolkova E, Stejskal J. Polym Degrad Stabil 2004;86: 179–85. Karim MR, Lim KT, Lee CJ, Bhuiyan TI, Kim HJ, Park LS, et al. J Polym Sci Pol Chem 2007;45:5741–7. Zhang X, Wu Y, He S, Yang D. Surf Coat Technol 2007;201:6051–8. Fang FF, Choi HJ, Ahn WS. Compos Sci Technol 2009;69:2088–92. Wang B, Zhou M, Rozynek Z, Fossum JO. J Mater Chem 2009;19:1816–28.
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Silica nanoparticle decorated conducting polyaniline fibers and their electrorheology Ying Dan Liu, Fei Fei Fang, Hyoung Jin Choi ⁎ Department of Polymer Science and Engineering, Inha University, Incheon 402-751, Korea
a r t i c l e
i n f o
Article history: Received 21 August 2009 Accepted 14 October 2009 Available online 22 October 2009 Keywords: Nanomaterials Polymers Electrorheological fluid Polyaniline Silica Nanoparticle
a b s t r a c t Polyaniline (PANI) fibers as well as silica nanoparticle decorated PANI (silica-PANI) fibers were successfully synthesized as a dispersed phase of an electrorheological (ER) fluid. The fibers obtained through interfacial polymerization were about 300–400 nm in diameter and 2–5 μm long. Then the fibers were redispersed in ethanol containing tetraethyl orthosilicate (TEOS), and silica nanoparticles were formed on the surface of the fibers through a modified Stöber method. The ER characteristics of the ER fluids based on pure PANI fibers and silica-PANI fibers were examined under various electric field strengths using a rotational rheometer, demonstrating slight different flow curves for the silica-based ER fluid. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Electrorheological (ER) fluids [1,2] are known as a kind of smart material because their structural and rheological changes caused by electric fields are typically reversible. Commonly, ER fluids are composed of insulating fluids and polarizable particles. ER fluids exhibit a tremendous increase in their shear viscosity, from a liquidlike to a solid-like state, when they are exposed to a relatively high electric field. This phenomenon reportedly arises from the formation of chain-like structures of the dispersed particles aligned along the electric field [3], which occurs within a few milliseconds, along with their magnetically analogous magnetorheological suspensions [4,5]. The fluids perform like Newtonian fluids without an electric field when they are sheared. However, when an electric field is applied and increased, yield stresses are needed to make the fluids flow. ER fluids have been applied in various areas such as shock absorbers, engine mounts and clutches because of their quick response to an electrical field and their controllable mechanical properties [6]. Various substances, such as high dielectric inorganics [7], conducting polymers [8] as well as their composites [9,10], have been investigated for ER materials. Polyaniline (PANI) is a favorable conducting polymer which has been studied in this field because of its easy synthesis, thermal and chemical stability, high sensitivity to an electric field and controlled conductivity using a doping/dedoping process [11]. Many PANIs differing in inner structures or outer morphologies, including derivatives [12], nanocomposites, core-shell particles, nanospherical or fibrous PANI [13] and etc., have been developed in ER research. Recently, Yin et al. [14] reported PANI a
⁎ Corresponding author. Tel.: +82 32 860 8777; fax: +82 32 865 5178. E-mail address: [email protected] (H.J. Choi). 0167-577X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2009.10.031
nanofiber based ER fluid which possessed a higher yield stress and was more stable than normal PANI based ER fluids. PANI fibers can differ in shape (straight or dendritic) and diameter depending on method of polymerization. Additionally, modifications to the PANI fibers have been studied by researchers in order to give PANI new functionality in an attempt to improve its processability or other properties for special applications [15]. In this work, silica nanoparticle decorated PANI (silica-PANI) fibers were introduced as a dispersed phase of the ER fluid. The fibers were straight, and their diameters were larger than other reported results. Silica nanoparticles were formed by a modified Stöber method [16] and adhered to surface of the fibers by means of electrostatic attraction. These particles decreased the conductivity of the PANI fibers to a range that was suitable for the ER test. Therefore, the dedoping process was omitted before the ER fluid preparation. The products were characterized by scanning (SEM) and transmission (TEM) electron microscopies as well as FT-IR spectroscopy. The ER suspension performance was investigated in silicone oil. 2. Experimental PANI fibers were synthesized through an interfacial polymerization method. The monomer aniline (5.1 g) was first dissolved in 150 ml toluene, which was used as the oil phase; the initiator, ammonium persulphate (APS, 12.5 g), and dodecylbenzene sulfuric acid (DBSA) were dissolved in distilled water, which was used as the water phase. After magnetic stirring for 1 h, the oil phase was carefully transferred to the water phase without disturbing the interface. The reaction started the moment the two phases contacted each other, which was confirmed by the smoke-like PANI ascending from the interface. The reaction system was kept in a refrigerator for
Y.D. Liu et al. / Materials Letters 64 (2010) 154–156
155
Fig. 1. Schematic diagram of synthetic process of silica-PANI fiber.
24 h without being moved. After that, the upper oil layer was removed using a syringe. The solid PANI mass in the aqueous phase was centrifuged and washed with distilled water/ethanol several times. The solid product was dried in a vacuum oven at 60 °C for 3 days. Silica-PANI fibers were synthesized through the following method. First, 0.5 g dried fibers and 0.5 g tetraethyl orthosilicate (TEOS) were dispersed in 24 ml ethanol in a glass vial and sonicated for half an hour to ensured that they were well dispersed. Then the vial was placed into water bath at 40 °C for several minutes until a constant temperature was reached. Then 0.76 g of 35% hydrous ammonia (NH3·H2O, catalyst) and 0.17 g of distilled water were quickly stirred into the mixture. After 15 h, the products were centrifuged washed with distilled water/ethanol several times, and then dried in a vacuum oven at 60 °C. SEM studies were carried out using a Hitachi Electron Microscope (S-4300, Japan). The silica surface decoration of PANI was identified by FT-IR (Perkin-Elmer, Spectrum 2000 Explorer). Pure PANI fibers were dedoped with 1 M NaOH before ER fluid preparation. Then 13 wt.% of both the PANI and silica-PANI fibers were dispersed in the silicone oil (50cS) by shaking and sonication. The ER properties of the product based ER fluids were measured using a rotational rheometer (MC120, Paar-Physica, Germany) which was equipped with a high
Fig. 2. SEM image of silica-PANI fiber (inset: PANI fiber).
direct current voltage generator. The flow curves were obtained for various electric fields by measuring the shear stress as a function of the shear rate in the range of 0.01 to 1000 s− 1. 3. Results and discussion Fig. 1 is a schematic illustration of the synthesis process for the silica nanoparticle decorated PANI fiber. The fiber was positively charged during polymerization due to its emeraldine salt form with DBSA. The fibers are surrounded by TEOS, the precursor to silica, in an ethanol system. Hydrolysis and condensation were started after the addition of NH3·H2O. The formed silica nanoparticles were negatively charged, and therefore, easy to locate on the surface of the PANI fibers because of an electrostatic attraction. The SEM images of both the PANI fibers (inset) and silica-PANI fibers are shown in Fig. 2. The pure fibers had diameters of about 300–400 nm and length of 2–5 μm with smooth surfaces. On the other hand, the silica decorated fibers were much rougher than the pure fibers. Silica nanoparticles were easy recognized on the surface of fibers. These nanoparticles were not ideal spheres that were distributed with various particle sizes on the order of a few to dozens of nanometers. Fig. 3 represents the FT-IR spectrum of two samples. The main characteristic bands for the pure fiber in Fig. 3(a) were assigned as follows: the band in the region between 3420 and 3450 cm− 1 was related to the N–H stretching [17]; the bands at 1570 and 1497 cm− 1 corresponded to the C C stretching vibration of the quinoid ring and
Fig. 3. FT-IR spectra of PANI fiber (a) and silica-PANI fiber (b).
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Y.D. Liu et al. / Materials Letters 64 (2010) 154–156
structures. In the shear field, these structures were destroyed and reformed under the cooperation of the electrostatic and hydrodynamic forces. The silica particles obstructed the repacking of the fibers. Moreover the time needed for the relaxation process lagged behind the action of the shear rate. Therefore, the shear stress decreased in the electrostatic dominated region (low shear rate region) and increased in the hydrodynamic force dominated region (high shear rate region) thereafter. This observation was probably caused by the obstruction effect of the silica nanoparticles on the packing of the fibers in chain-like structure formation. 4. Conclusions
Fig. 4. Flow curves of PANI fiber and silica-PANI fiber based ER fluids under electric field strengths (PANI: closed; silica-PANI: open).
benzenoid ring, respectively [18]; a higher peak at 2923 cm− 1 resulted from the crosslinking moieties [19]; and the peak at 1117 cm− 1 was caused by the C–C stretching. The characteristic peaks of silica appeared when the fibers were modified with silica. The band at 1100 cm− 1 was attributed to the Si–O–Si asymmetric stretching. The stretching vibration of Si–OH was assigned to the peak at 829 cm− 1. Additionally, the Si–O–Si bending vibration peak appeared at 470 cm− 1 [20]. Expressed by the flow curves in Fig. 4, the ER performances of the ER fluids (13 wt.%) based on the pure PANI fibers and silica-PANI fibers were tested in a controlled shear rate mode. Without an electric field, both of the ER fluids act as ideal viscous fluid where the shear stress grew linearly with the shear rate. When an electric field was applied, yield stresses appeared and increased step-wise with the electric field strengths. The yield stresses of the two ER fluids were similar. For instance, they were ca. 12 Pa, 70 Pa, and 180 Pa for field strengths of 0.5 kV/mm, 1 kV/mm and 2.0 kV/mm respectively, which also showed better dependence of shear stress on the electric field strength than our previously reported silica-based mesoporous-PANI system [21]. However, the pure PANI based ER fluid showed more ER behavior stability with respect to shear rate for a long range. The same property was also reported for the PANI fiber based ER fluid by Yin et al. [7]. The flow curves for the silica-PANI based ER fluid were much different. After the appearance of the yield stress, the shear stress decreased as a function of the shear rate to a minimum, called the critical shear rate, before increasing again. This phenomenon was considered the result of the changing fiber microstructures and was observed by various researchers [22]. In the presence of an electric field, the silica-PANI fibers were polarized and connected to each other to form chain-like
PANI fibers and silica nanoparticle decorated PANI fibers were successfully synthesized. The morphologies of these were characterized by SEM images. The diameters of the fibers were about 300–400 nm, and the silica nanoparticles located on the surfaces of fibers were multi distributed in size ranging a few decades of nanometers. The formation of silica was confirmed using the FT-IR spectra. The flow curves of the ER fluid based on both pure PANI fibers and silica-PANI fibers were tested in the presence of various electric field strengths, while the silica-PANI based ER fluid showed different behaviors from that of pure fibers. Acknowledgements This work was supported by research fund from ETRI (2009). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]
Han SJ, Kim B, Suh KD. Mater Lett 2007;61:3995–9. Kim YD, Park DH. Synth Met 2004;142:147–51. Choi HJ, Jhon MS. Soft Matter 2009;5:1562–7. Kim JH, Fang FF, Choi HJ, Seo Y. Mater Lett 2008;62:2897–9. Ko SW, Yang MS, Choi HJ. Mater Lett 2009;63:861–3. Han YM, Lim SC, Lee HG, Choi SB, Choi HJ. Mater Design 2003;24:53–61. Yin JB, Zhao XP. Nanotechnology 2006;17:192–6. Cho CH, Choi HJ, Kim JW, Jhon MS. J Mater Sci 2004;39:1883–5. Ilieva M, Ivanov S, Tsakova V. J Appl Electrochem 2008;38:63–9. Cheng Q, Pavlinek V, He Y, Li C, Saha P. Colloid Polym Sci 2009;287:435–41. Huang J. Pure Appl Chem 2006;78:15–27. Kim JW, Jang WH, Choi HJ, Joo J. Synth Met 2001;119:173–4. Oz K, Yavuz M, Yilmaz H, Unal HI, Sari B. J Mater Sci 2008;43:1451–9. Yin JB, Zhao XP, Xia X, Xiang LQ, Qiao YP. Polymer 2008;49:4413–9. Wang X, Feng X, Zhao Y, Zhang R, Sun D. Eur Polym J 2007;43:3679–82. Stöber W, Fink A, Bohn E. J Colloid Interf Sci 1968;26:62–9. Jang WH, Kim JW, Choi HJ, Jhon MS. Colloid Polym Sci 2001;279:823–7. Trchova M, Sedenkova I, Tobolkova E, Stejskal J. Polym Degrad Stabil 2004;86: 179–85. Karim MR, Lim KT, Lee CJ, Bhuiyan TI, Kim HJ, Park LS, et al. J Polym Sci Pol Chem 2007;45:5741–7. Zhang X, Wu Y, He S, Yang D. Surf Coat Technol 2007;201:6051–8. Fang FF, Choi HJ, Ahn WS. Compos Sci Technol 2009;69:2088–92. Wang B, Zhou M, Rozynek Z, Fossum JO. J Mater Chem 2009;19:1816–28.