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Conductive composites of polypyrrole and sulfonic-functionalized silica spheres
Materials Letters 61 (2007) 3142 – 3145 www.elsevier.com/locate/matlet
Conductive composites of polypyrrole and sulfonic-functionalized silica spheres Tingyang Dai, Xiaoming Yang, Yun Lu ⁎ Department of Polymer Science and Engineering, State Key Lab Coordination Chemistry, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, People's Republic of China Received 4 July 2006; accepted 2 November 2006 Available online 20 November 2006
Abstract Colloidal silica spheres were sulfonic-functionalized by the pretreatment of a mercapto-containing silane coupling agent and further oxidation. Starting off with these functionalized silica spheres, we successfully fabricated a silica–polypyrrole (PPy) core–shell nanostructure. FT-IR and TGA were used to characterize the resulting composite. Because of the complete coating of polypyrrole, the electric conductivity of the composite was as high as 12 S/cm, three orders of magnitude higher than that of conventional silica–polypyrrole composites. The colloidal stability can be improved greatly by introducing a common steric agent poly(N-vinylpyrrolidone) (PVP) into the system. © 2006 Elsevier B.V. All rights reserved. Keywords: Sulfonic-functionalized silica; Polypyrrole; Electric conductivity; Colloidal stability
1. Introduction Inherently conducting polymers are attractive materials, as they cover a wide range of functions from insulators to metals and retain the mechanical properties of conventional polymers [1,2]. Among conducting polymers, polypyrrole (PPy) is one of the most extensively studied materials due to its good electrical conductivity, redox properties, environmental stability and easy preparation by both electrochemical and chemical approaches in various organic solvents and in aqueous solution [3,4]. However, polypyrrole is infusible, insoluble and suffers from poor processability, mainly because of its rigid, highly conjugated backbone [5]. To overcome these limitations, many researches have focused on the preparation of hybrid materials such as organic sterically stabilized polypyrrole latexes [6] and composite materials using metal oxide [7] or silica particles [8,9] as inorganic substrate for the chemical polymerization of conducting polypyrrole. For instance, Armes et al. have reported the preparation and characterization of silica–polypyrrole nanocomposite colloids using small silica particles as a particulate dispersant [10,11]. In this approach, the ⁎ Corresponding author. E-mail address: [email protected] (Y. Lu). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.11.012
silica particles acted as a high surface area colloidal substrate for the precipitating polypyrrole, and the resulting nanocomposites were made up of microaggregates of the original silica particles ‘glued’ together by the polypyrrole component, which gave rise to a distinctive ‘raspberry’ particle morphology. The conductivity of the composites was as low as 10− 3–10− 4 S/cm because the surface was silica-rich. To increase the conductivity of the composites, silica should be coated completely with conductive polymer [12]. For this purpose, the hydrophilic surface of silica has been modified by various surfactants and macromolecules in order to create a surface environment in favor of the formation of polypyrrole [12– 15]. Layer-by-layer (LbL) self-assembly technique has also been applied to utilize charged silica colloidal particles as the adsorbing substrates to produce colloid-supported polyelectrolyte multilayer films [16,17]. Moreover, Lacaze et al. used a common silane coupling agent, aminopropyltriethoxysilane (APS), to modify the surface of the silica particles prior to pyrrole polymerization [18]. APS was effective in processing polypyrrole–silica hybrid materials which were of a much higher surface proportion of polypyrrole. But compressed pellet conductivity measurements still indicated a low conductivity of 10− 2 S/cm for the composite. It has been reported that charges on colloidal particles facilitate the formation of uniform conductive polymer thin film [19,20].
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Stober et al. [23]. Briefly, 44 mL of aqueous ammonia and 100 mL of absolute ethanol were introduced into a three neck round flask equipped with a refrigerating system and stirred to be homogenous. Then a solution of 10 mL of TEOS diluted in 40 mL of absolute ethanol was added into the above-prepared mixture continuously and slowly. The reaction mixture was kept stirring for 12 h to yield uniform silica spheres. Scheme 1. The route of sulfonic-functionalization of colloidal silica.
Garnier et al. and Yamanoto et al. have demonstrated that the presence of acidic functional groups (sulfonic or carboxylic acids) on the polystyrene (PS) particles were responsible for the formation of uniform polypyrrole coatings on the surface of the particles [21,22]. In this paper, we aim to report on the fabrication of the composite of sulfonic-functionalized silica spheres and polypyrrole. A core–shell nanostructure was gained and the electric conductivity was improved greatly due to the complete coating of polypyrrole. To improve the colloidal stability of the composite, poly(N-vinylpyrrolidone) (PVP) was introduced into the system acting as both steric agent and stabilizer. 2. Experimental section 2.1. Materials Pyrrole monomers were purchased from Aldrich and distilled under reduced pressure before use. Tetraethoxysilane (TEOS), (mercaptopropyl)trimethoxysilane, tert-butyl hydroperoxide (t-BuOOH), PVP and FeCl3 (Huakang Product Inc. Jiangsu Province, China) were purchased in their reagent grades and used without further purification. N,N-dimethylformamide (DMF), toluene, absolute ethanol and a 25% aqueous solution of ammonia were purchased from Shanghai Chemical Product Inc, China. 2.2. Synthesis of silica spheres Silica spheres in the size range of 500 nm were synthesized by the base-catalyzed hydrolysis of TEOS, as described by
2.3. DMF dispersion of colloidal silica Ethanol and water were slowly distilled out from 100 mL of the silica suspension already synthesized at the rate of about 1 mL/min, and meanwhile about 120 mL of DMF was added dropwise from an addition funnel into the system in order to keep a constant volume of dispersion. The distillation was continued until about 10 mL of distillate was collected at a constant boiling point of 152 °C. 2.4. Sulfonic-functionalization of colloidal silica In the atmosphere of nitrogen, (mercaptopropyl)trimethoxysilane (1.5 mL, 8 mmol) and 100 mL of colloidal silica dispersed in DMF (containing 1.5 g silica) were heated to 100 °C and kept at this temperature for 24 h. Then 30 mL of 3.0 M t-BuOOH (90 mmol) in toluene was added and the mixture was stirred at 25 °C for 24 h and at 60 °C for another 24 h to give a pale yellow dispersion. The solvent was evaporated until about 30 mL of solvent was left. Then 30 mL of water was added into the remains to precipitate the product. The functionalized silica was separated by centrifugation and washed with 2 M HCl and water sequently until neutral pH. Finally, the silica was dispersed in 30 mL of water and its concentration was determined by measuring the mass of a dried extract. 2.5. Synthesis of silica–polypyrrole composite 70 μL (1 mmol) of pyrrole monomer and 0.162 g (1 mmol) of FeCl3 were added into 20 mL of pretreated silica suspension (containing 0.2 g sulfonic-functionalized silica) sequently, and the mixture was stirred at room temperature for 24 h. The resulting composite was collected by centrifugation and dried in vacuum at 50 °C for 24 h. The effect of PVP on the system was
Fig. 1. TEM images of the composites of polypyrrole and (a) sulfonic-functionalized silica spheres; (b) un-modified silica spheres.
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Fig. 3. TEM image of the composite of sulfonic-functionalized silica spheres and polypyrrole synthesized in the presence of PVP. Fig. 2. FT-IR spectrum of the composite of sulfonic-functionalized silica spheres and polypyrrole.
investigated by adding 0.1 g of PVP to the suspension prior to the addition of the pyrrole monomer. 2.6. Instruments and measurements Transmission electron microscopy (TEM) experiments were performed with a JSM-6300 microscope (JEOL Company, Japan). Fourier-transform infrared spectra (FT-IR) were recorded on a Bruker VECTOR22 spectrometer. Thermogravimetric analysis (TGA) was carried out on a Shimadzu TGA-50 instrument at heating rate of 10 °C/min in air. The conductivities of compressed pellets of the silica–polypyrrole composites were determined using the standard four-point techniques at room temperature. 3. Results and discussion The approach employed for the fabrication of sulfonic-functionalized colloidal silica is shown in Scheme 1. It has been reported that the surface treatment of colloidal silica with (mercaptopropyl)trimethoxysilane in the original mixture of ethanol, water and ammonia is unsuccessful, and by using DMF to replace ethanol and water before the functionalization is necessary [24]. In our case, the surface sulfonic-functionalization of colloidal silica was achieved by the pretreatment of (mercaptopropyl) trimethoxysilane in DMF to introduce mercapto groups on the surface of silica and further oxidation with t-BuOOH in toluene to change mercapto groups to sulfonic ones. In a typical procedure, polymerization of pyrrole on the surface of the sulfonic-functionalized silica was performed in an aqueous solution, using FeCl3 as oxidant. TEM image of the resulting composite is shown in Fig. 1a. In contrast to the morphology of the composite of polypyrrole and un-modified silica (Fig. 1b), the functionalized silica core was coated with a uniform polypyrrole shell with the thickness of about 70 nm. Hence, we can conclude that sulfonic group is effective for the formation of polypyrrole coatings on the surface of silica, which is similar to the case of coating polypyrrole on sulfonic-functionalized polystyrene particles [21]. The FT-IR spectrum was used to characterize the functionalized silica–polypyrrole composite (Fig. 2). The peaks at 1545 and 1463 cm− 1 were due to the antisymmetric and symmetric ring-stretching modes, respectively. The peaks at 1302 and 1040 cm− 1 were attributed to C–N
stretching vibrations and C–H deformation vibrations. The strong peaks near 1178 and 924 cm− 1 implied the doping state of polypyrrole [25]. It should be pointed out that the strong peak at 1100 cm− 1, which is the characteristic peak of silica [26], cannot be observed here, indicating that the silica cores are completely coated with polypyrrole. Accordingly, the electric conductivity of the composite at room temperature is up to 12 S/ cm, three orders of magnitude higher than that of conventional silica– polypyrrole composites. It is worthwhile to notice here that the colloidal stability of the resulting silica–polypyrrole composite is poor, as the polypyrrole shell may act as a ‘binder’, which could promote the aggregation of the composite particles [10]. To improve the colloidal stability of the silica–polypyrrole composite, a common steric agent PVP was introduced into the system. In the presence of a very small quantity of PVP, polypyrrole formed a shell with the thickness of about 40 nm on the surface of sulfonicfunctionalized silica cores (Fig. 3). PVP has been used as a useful steric agent to promote a strong interaction between the silica sols and the pyrrole monomers [13]. Herein, it also acts as a stabilizer because of its steric hindrance effect just like many other macromolecules [12]. Correspondingly, the colloidal stability of the composite was improved greatly and cannot be collected by centrifugation (6000 rpm for 30 min) other than by adding acetone. The reason for the change of the thickness of the polypyrrole shell might be suggested as follows: The adsorbed PVP might provide active
Fig. 4. TGA curves of the composites of polypyrrole and (A) sulfonicfunctionalized silica spheres (with PVP); (B) un-modified silica spheres; (C) sulfonic-functionalized silica spheres (without PVP).
T. Dai et al. / Materials Letters 61 (2007) 3142–3145
sites on the silica for the polymerization of pyrrole monomers [27], and polypyrrole tends to form in the thin hydrophobic layer provided by PVP [28], resulting in the formation of a relatively thin shell. By adjusting the added quantity of PVP, the electric conductivity of the composites with corresponding colloidal stability can be controlled from 10− 2 to 101 S/cm. This method is a versatile technique for the construction of the core–shell nanostructural composites of silica and conducting polymers, which determine the properties of the materials in most cases [29]. So by appropriately varying the stoichiometry of the corresponding monomers, one can facilely change the properties in the desired directions. Results of the thermogravimetric analysis are shown in Fig. 4. The weight loss in the first step below 100 °C is attributed to the loss of residual moisture. The second step around 260 °C corresponds to the polypyrrole degradation. Residual sample mass after 700 °C belongs to silica. It can be seen that the decomposition temperature of the sulfonicfunctionalized silica–polypyrrole composites is about 10 °C higher than that of the un-modified silica–polypyrrole composite. This result could be interpreted by the stronger bonding interaction between polypyrrole and silica surface when silica was modified [12]. The weight loss corresponding to the polypyrrole degradation of the curve C (45%) is much bigger than that of curve A (18%). This is because the polypyrrole shell becomes much thinner in the presence of PVP. On the other hand, the composites show good time stability with no obvious decrease of electric conductivity when exposed to the air for two months.
4. Conclusions In summary, for the first time we describe the synthesis of a silica core–polypyrrole shell nanostructural composite, using the pretreated sulfonic-functionalized silica spheres as cores. The surface of the composite is exclusive conducting polypyrrole, and the electric conductivity is up to 12 S/cm correspondingly. The composite exhibits a high colloidal stability in the presence of a very small quantity of PVP, which acts both as a steric agent and stabilizer. These conductive composites have potential applications in a wide variety of areas such as wave absorbent, conductive paint, chemical sensor and so on. Acknowledgements We are grateful for the support from the National Natural Science Foundation of China (No. 20574034) and Testing Foundation of Nanjing University.
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References [1] E.T. Kang, K.G. Neoh, K.L. Tan, Prog. Polym. Sci. 23 (1998) 277. [2] A.J. Heeger, J. Phys. Chem., B 105 (2001) 8475. [3] A.H. Chen, K. Kamata, M. Nakagama, J. Phys. Chem., B 109 (2005) 18283. [4] C.F. Zhou, S. Kumar, C.D. Doyle, Chem. Mater. 17 (2005) 1997. [5] H. Naarman, Synth. Met. 1 (1991) 41. [6] C. DeArmitt, S.P. Armes, Langmuir 9 (1993) 652. [7] R. Parth, S.G. Gangolly, E. Matijevic, W. Cai, S. Arajs, J. Colloid Interface Sci. 144 (1991) 27. [8] H. Chriswanto, H. Ge, G.G. Wallace, Chromatographia 37 (1993) 423. [9] S. Maeda, S.P. Armes, J. Mater. Chem. 4 (1994) 935. [10] M. Gill, S.P. Armes, D. Fairhurst, S.N. Emmett, G. Idzorek, T. Pigott, Langmuir 8 (1992) 2178. [11] S. Maeda, M. Gill, S.P. Armes, Langmuir 11 (1995) 1899. [12] X.M. Yang, T.Y. Dai, Y. Lu, Polymer 47 (2006) 441. [13] L.Y. Hao, C.L. Zhu, C.N. Chen, P. Kang, Synth. Met. 139 (2003) 391. [14] G.J. Cho, D.T. Glatzhofer, B.M. Fung, Langmuir 16 (2000) 4424. [15] G.P. Funkhouser, M. Arevalo, D.T. Glatzhofer, E.A. Orear, Langmuir 11 (1995) 1443. [16] X.Y. Shi, A.L. Briseno, R.J. Sanedrin, Macromolecules 36 (2003) 4093. [17] C. Perruchot, M.M. Chehimi, M. Delamar, S.F. Lascelles, S.P. Armes, Langmuir 12 (1996) 3245. [18] C. Perruchot, M.M. Chehimi, M. Delamar, P.C. Lacaze, Synth. Met. 102 (1999) 1194. [19] J. Stejskal, P. Kratochvil, S.P. Armes, S.F. Lascelles, A. Riede, M. Helmstedt, J. Prokes, I. Krivka, Macromolecules 29 (1996) 6814. [20] C. Barthet, S.P. Armes, M.M. Chehimi, C. Bilem, M. Omastova, Langmuir 14 (1998) 5032. [21] A. Yassar, J. Roncali, F. Garnier, Polym. Commun. 28 (1987) 103. [22] C.F. Liu, T. Maruyama, T. Yamamoto, Polym. J. 25 (1993) 363. [23] W. Stober, A. Fink, E. Bohn, J. Colloid Interface Sci. 26 (1968) 62. [24] R.D. Badley, W.T. Ford, J. Org. Chem. 54 (1989) 5437. [25] X.T. Zhang, J. Zhang, R.M. Wang, Chem. Phys. Chem. 5 (2004) 998. [26] D. Wang, R.A. Caruso, F. Caruso, Chem. Mater. 13 (2001) 364. [27] I. Seo, M. Pyo, G. Cho, Langmuir 18 (2002) 7253. [28] S. Reculusa, C.P. Legrand, S. Ravaine, C. Mingotaud, Chem. Mater. 14 (2002) 2354. [29] I. Fuks-Janczarek, I.V. Kityk, R. Miedzinski, E. Gondek, A. Danel, M. Zagorska, Spectrochim. Acta 64 (2006) 264.
Conductive composites of polypyrrole and sulfonic-functionalized silica spheres Tingyang Dai, Xiaoming Yang, Yun Lu ⁎ Department of Polymer Science and Engineering, State Key Lab Coordination Chemistry, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, People's Republic of China Received 4 July 2006; accepted 2 November 2006 Available online 20 November 2006
Abstract Colloidal silica spheres were sulfonic-functionalized by the pretreatment of a mercapto-containing silane coupling agent and further oxidation. Starting off with these functionalized silica spheres, we successfully fabricated a silica–polypyrrole (PPy) core–shell nanostructure. FT-IR and TGA were used to characterize the resulting composite. Because of the complete coating of polypyrrole, the electric conductivity of the composite was as high as 12 S/cm, three orders of magnitude higher than that of conventional silica–polypyrrole composites. The colloidal stability can be improved greatly by introducing a common steric agent poly(N-vinylpyrrolidone) (PVP) into the system. © 2006 Elsevier B.V. All rights reserved. Keywords: Sulfonic-functionalized silica; Polypyrrole; Electric conductivity; Colloidal stability
1. Introduction Inherently conducting polymers are attractive materials, as they cover a wide range of functions from insulators to metals and retain the mechanical properties of conventional polymers [1,2]. Among conducting polymers, polypyrrole (PPy) is one of the most extensively studied materials due to its good electrical conductivity, redox properties, environmental stability and easy preparation by both electrochemical and chemical approaches in various organic solvents and in aqueous solution [3,4]. However, polypyrrole is infusible, insoluble and suffers from poor processability, mainly because of its rigid, highly conjugated backbone [5]. To overcome these limitations, many researches have focused on the preparation of hybrid materials such as organic sterically stabilized polypyrrole latexes [6] and composite materials using metal oxide [7] or silica particles [8,9] as inorganic substrate for the chemical polymerization of conducting polypyrrole. For instance, Armes et al. have reported the preparation and characterization of silica–polypyrrole nanocomposite colloids using small silica particles as a particulate dispersant [10,11]. In this approach, the ⁎ Corresponding author. E-mail address: [email protected] (Y. Lu). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.11.012
silica particles acted as a high surface area colloidal substrate for the precipitating polypyrrole, and the resulting nanocomposites were made up of microaggregates of the original silica particles ‘glued’ together by the polypyrrole component, which gave rise to a distinctive ‘raspberry’ particle morphology. The conductivity of the composites was as low as 10− 3–10− 4 S/cm because the surface was silica-rich. To increase the conductivity of the composites, silica should be coated completely with conductive polymer [12]. For this purpose, the hydrophilic surface of silica has been modified by various surfactants and macromolecules in order to create a surface environment in favor of the formation of polypyrrole [12– 15]. Layer-by-layer (LbL) self-assembly technique has also been applied to utilize charged silica colloidal particles as the adsorbing substrates to produce colloid-supported polyelectrolyte multilayer films [16,17]. Moreover, Lacaze et al. used a common silane coupling agent, aminopropyltriethoxysilane (APS), to modify the surface of the silica particles prior to pyrrole polymerization [18]. APS was effective in processing polypyrrole–silica hybrid materials which were of a much higher surface proportion of polypyrrole. But compressed pellet conductivity measurements still indicated a low conductivity of 10− 2 S/cm for the composite. It has been reported that charges on colloidal particles facilitate the formation of uniform conductive polymer thin film [19,20].
T. Dai et al. / Materials Letters 61 (2007) 3142–3145
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Stober et al. [23]. Briefly, 44 mL of aqueous ammonia and 100 mL of absolute ethanol were introduced into a three neck round flask equipped with a refrigerating system and stirred to be homogenous. Then a solution of 10 mL of TEOS diluted in 40 mL of absolute ethanol was added into the above-prepared mixture continuously and slowly. The reaction mixture was kept stirring for 12 h to yield uniform silica spheres. Scheme 1. The route of sulfonic-functionalization of colloidal silica.
Garnier et al. and Yamanoto et al. have demonstrated that the presence of acidic functional groups (sulfonic or carboxylic acids) on the polystyrene (PS) particles were responsible for the formation of uniform polypyrrole coatings on the surface of the particles [21,22]. In this paper, we aim to report on the fabrication of the composite of sulfonic-functionalized silica spheres and polypyrrole. A core–shell nanostructure was gained and the electric conductivity was improved greatly due to the complete coating of polypyrrole. To improve the colloidal stability of the composite, poly(N-vinylpyrrolidone) (PVP) was introduced into the system acting as both steric agent and stabilizer. 2. Experimental section 2.1. Materials Pyrrole monomers were purchased from Aldrich and distilled under reduced pressure before use. Tetraethoxysilane (TEOS), (mercaptopropyl)trimethoxysilane, tert-butyl hydroperoxide (t-BuOOH), PVP and FeCl3 (Huakang Product Inc. Jiangsu Province, China) were purchased in their reagent grades and used without further purification. N,N-dimethylformamide (DMF), toluene, absolute ethanol and a 25% aqueous solution of ammonia were purchased from Shanghai Chemical Product Inc, China. 2.2. Synthesis of silica spheres Silica spheres in the size range of 500 nm were synthesized by the base-catalyzed hydrolysis of TEOS, as described by
2.3. DMF dispersion of colloidal silica Ethanol and water were slowly distilled out from 100 mL of the silica suspension already synthesized at the rate of about 1 mL/min, and meanwhile about 120 mL of DMF was added dropwise from an addition funnel into the system in order to keep a constant volume of dispersion. The distillation was continued until about 10 mL of distillate was collected at a constant boiling point of 152 °C. 2.4. Sulfonic-functionalization of colloidal silica In the atmosphere of nitrogen, (mercaptopropyl)trimethoxysilane (1.5 mL, 8 mmol) and 100 mL of colloidal silica dispersed in DMF (containing 1.5 g silica) were heated to 100 °C and kept at this temperature for 24 h. Then 30 mL of 3.0 M t-BuOOH (90 mmol) in toluene was added and the mixture was stirred at 25 °C for 24 h and at 60 °C for another 24 h to give a pale yellow dispersion. The solvent was evaporated until about 30 mL of solvent was left. Then 30 mL of water was added into the remains to precipitate the product. The functionalized silica was separated by centrifugation and washed with 2 M HCl and water sequently until neutral pH. Finally, the silica was dispersed in 30 mL of water and its concentration was determined by measuring the mass of a dried extract. 2.5. Synthesis of silica–polypyrrole composite 70 μL (1 mmol) of pyrrole monomer and 0.162 g (1 mmol) of FeCl3 were added into 20 mL of pretreated silica suspension (containing 0.2 g sulfonic-functionalized silica) sequently, and the mixture was stirred at room temperature for 24 h. The resulting composite was collected by centrifugation and dried in vacuum at 50 °C for 24 h. The effect of PVP on the system was
Fig. 1. TEM images of the composites of polypyrrole and (a) sulfonic-functionalized silica spheres; (b) un-modified silica spheres.
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Fig. 3. TEM image of the composite of sulfonic-functionalized silica spheres and polypyrrole synthesized in the presence of PVP. Fig. 2. FT-IR spectrum of the composite of sulfonic-functionalized silica spheres and polypyrrole.
investigated by adding 0.1 g of PVP to the suspension prior to the addition of the pyrrole monomer. 2.6. Instruments and measurements Transmission electron microscopy (TEM) experiments were performed with a JSM-6300 microscope (JEOL Company, Japan). Fourier-transform infrared spectra (FT-IR) were recorded on a Bruker VECTOR22 spectrometer. Thermogravimetric analysis (TGA) was carried out on a Shimadzu TGA-50 instrument at heating rate of 10 °C/min in air. The conductivities of compressed pellets of the silica–polypyrrole composites were determined using the standard four-point techniques at room temperature. 3. Results and discussion The approach employed for the fabrication of sulfonic-functionalized colloidal silica is shown in Scheme 1. It has been reported that the surface treatment of colloidal silica with (mercaptopropyl)trimethoxysilane in the original mixture of ethanol, water and ammonia is unsuccessful, and by using DMF to replace ethanol and water before the functionalization is necessary [24]. In our case, the surface sulfonic-functionalization of colloidal silica was achieved by the pretreatment of (mercaptopropyl) trimethoxysilane in DMF to introduce mercapto groups on the surface of silica and further oxidation with t-BuOOH in toluene to change mercapto groups to sulfonic ones. In a typical procedure, polymerization of pyrrole on the surface of the sulfonic-functionalized silica was performed in an aqueous solution, using FeCl3 as oxidant. TEM image of the resulting composite is shown in Fig. 1a. In contrast to the morphology of the composite of polypyrrole and un-modified silica (Fig. 1b), the functionalized silica core was coated with a uniform polypyrrole shell with the thickness of about 70 nm. Hence, we can conclude that sulfonic group is effective for the formation of polypyrrole coatings on the surface of silica, which is similar to the case of coating polypyrrole on sulfonic-functionalized polystyrene particles [21]. The FT-IR spectrum was used to characterize the functionalized silica–polypyrrole composite (Fig. 2). The peaks at 1545 and 1463 cm− 1 were due to the antisymmetric and symmetric ring-stretching modes, respectively. The peaks at 1302 and 1040 cm− 1 were attributed to C–N
stretching vibrations and C–H deformation vibrations. The strong peaks near 1178 and 924 cm− 1 implied the doping state of polypyrrole [25]. It should be pointed out that the strong peak at 1100 cm− 1, which is the characteristic peak of silica [26], cannot be observed here, indicating that the silica cores are completely coated with polypyrrole. Accordingly, the electric conductivity of the composite at room temperature is up to 12 S/ cm, three orders of magnitude higher than that of conventional silica– polypyrrole composites. It is worthwhile to notice here that the colloidal stability of the resulting silica–polypyrrole composite is poor, as the polypyrrole shell may act as a ‘binder’, which could promote the aggregation of the composite particles [10]. To improve the colloidal stability of the silica–polypyrrole composite, a common steric agent PVP was introduced into the system. In the presence of a very small quantity of PVP, polypyrrole formed a shell with the thickness of about 40 nm on the surface of sulfonicfunctionalized silica cores (Fig. 3). PVP has been used as a useful steric agent to promote a strong interaction between the silica sols and the pyrrole monomers [13]. Herein, it also acts as a stabilizer because of its steric hindrance effect just like many other macromolecules [12]. Correspondingly, the colloidal stability of the composite was improved greatly and cannot be collected by centrifugation (6000 rpm for 30 min) other than by adding acetone. The reason for the change of the thickness of the polypyrrole shell might be suggested as follows: The adsorbed PVP might provide active
Fig. 4. TGA curves of the composites of polypyrrole and (A) sulfonicfunctionalized silica spheres (with PVP); (B) un-modified silica spheres; (C) sulfonic-functionalized silica spheres (without PVP).
T. Dai et al. / Materials Letters 61 (2007) 3142–3145
sites on the silica for the polymerization of pyrrole monomers [27], and polypyrrole tends to form in the thin hydrophobic layer provided by PVP [28], resulting in the formation of a relatively thin shell. By adjusting the added quantity of PVP, the electric conductivity of the composites with corresponding colloidal stability can be controlled from 10− 2 to 101 S/cm. This method is a versatile technique for the construction of the core–shell nanostructural composites of silica and conducting polymers, which determine the properties of the materials in most cases [29]. So by appropriately varying the stoichiometry of the corresponding monomers, one can facilely change the properties in the desired directions. Results of the thermogravimetric analysis are shown in Fig. 4. The weight loss in the first step below 100 °C is attributed to the loss of residual moisture. The second step around 260 °C corresponds to the polypyrrole degradation. Residual sample mass after 700 °C belongs to silica. It can be seen that the decomposition temperature of the sulfonicfunctionalized silica–polypyrrole composites is about 10 °C higher than that of the un-modified silica–polypyrrole composite. This result could be interpreted by the stronger bonding interaction between polypyrrole and silica surface when silica was modified [12]. The weight loss corresponding to the polypyrrole degradation of the curve C (45%) is much bigger than that of curve A (18%). This is because the polypyrrole shell becomes much thinner in the presence of PVP. On the other hand, the composites show good time stability with no obvious decrease of electric conductivity when exposed to the air for two months.
4. Conclusions In summary, for the first time we describe the synthesis of a silica core–polypyrrole shell nanostructural composite, using the pretreated sulfonic-functionalized silica spheres as cores. The surface of the composite is exclusive conducting polypyrrole, and the electric conductivity is up to 12 S/cm correspondingly. The composite exhibits a high colloidal stability in the presence of a very small quantity of PVP, which acts both as a steric agent and stabilizer. These conductive composites have potential applications in a wide variety of areas such as wave absorbent, conductive paint, chemical sensor and so on. Acknowledgements We are grateful for the support from the National Natural Science Foundation of China (No. 20574034) and Testing Foundation of Nanjing University.
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