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polyacrylamide microfibers seeding template method
EUROPEAN POLYMER JOURNAL
European Polymer Journal 42 (2006) 2108–2113
www.elsevier.com/locate/europolj
Synthesis and characterization of polyaniline microfibers by utilizing H4SiW12O40/polyacrylamide microfibers seeding template method Fengchun Wang a
a,b
, Rui Yang a,c, Jian Gong a,*, Chunhong Sui a, Yunqing Luo a, Lunyu Qu a
Faculty of Chemistry, Northeast Normal University, Changchun, Jilin 130024, PR China b Faculty of Chemistry, Tonghua Teachers College, Tonghua 134000, PR China c Department of Chemistry, Beihua University, Jilin 132013, PR China
Received 13 January 2006; received in revised form 28 March 2006; accepted 2 April 2006 Available online 5 June 2006
Abstract In this paper, polyaniline (PANI) microfibers with average diameter of 250 nm were synthesized by utilizing H4SiW12O40/polyacrylamide (HPA/PAM) microfibers seeding template method. The PANI microfibers were characterized by element analyses, FT–IR spectra, X-ray diffraction pattern (XRD), and Scanning electron micrograph (SEM). The microfibers seeding template significantly affected the fibrous morphology of the resulting PANI. However, the diameter of the PANI microfibers was almost not affected by the diameter of the microfibers seeding template in the experimental range. In addition, the conductivity of the PANI microfibers was also investigated. The result showed that the best conductivity of the PANI microfibers doped with H4SiW12O40 was 27.1 S/cm. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Polyaniline; Microfibers; Polyoxometalate; Polyacrylamide; Conductivity
1. Introduction Nanostructures, such as nanofibers, nanorods, nanowires, nanotubes, and nanoparticles etc. have attracted extensive attention as a result of their novel size-dependent properties [1–6]. Nanomaterials including polymer, metallic oxide etc., have been synthesized, and their properties have been investigated [7,8]. Recently, nanostructures of conductive *
Corresponding author. Tel.: +86 431 5099765. E-mail address: [email protected] (J. Gong).
polymer, such as nanoparticles, nanofibers, and nanorods, demonstrate improvement towards nanotechnology [9–12]. As we know, the discovery of conductive polymers has opened up many new possibilities for devices combining optical, electrochemical, and conductive properties [13,14]. Conductive polymers have been extensively studied during the past couple of decades, and they still remain the subject of intense investigations by many research groups worldwide [15]. The widely studied conductive polymers include polyaniline, polypyrrole, and polythiophene. Among the conductive polymers,
0014-3057/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2006.04.004
F. Wang et al. / European Polymer Journal 42 (2006) 2108–2113
PANI, one of the most promising intrinsically conductive polymers, has received considerable attention in recent years due to its straightforward polymerization, chemical stability, relatively high conductivity and potential applications in electrochromic displays, chargeable batteries, sensors, and electrorheological fluids [12–17]. In the past years, micro- and nanostructures (such as tubes, rods, and particles) of PANI have been synthesized by chemical and electrochemical oxidative polymerization of aniline [18–20]. Because nanomaterials with their inherent anisotropy are the smallest dimension structures that can be used for efficient transport of electrons and optical excitations. So, the controlled synthesis of nanoscale materials is critical for fundamental studies and potential devices. Among various approaches for the preparation of micro- and nanostructure of PANI, template method has been an effective and important method. Usually, template can be divided to ‘hard’ template, such as the channels of microporous membranes [21] and zeolite [22], anodized alumina [23], layered silicate [24], MCM-41 [16], and SBA15 [25], and ‘soft’ template, such as micelles [26], and organic acids [27]. However, the template removal is tedious for hard templates, and meanwhile soft templates limit the range of chemicals that can be used. Recently, a new ‘nanofiber seeding’ method was used for synthesizing conductive polymer nanostructure [28]. However, the selected seeding template, such as carbon nanofiber or nanotubes, cannot be moved from product, although the content of the seeding template can be ignored in the product. In this paper, we describe an extremely simple microfiber-seeding template method to synthesize microfibrous PANI doped polyoxometalate. In this system, the microfiber-seeding template can easy be removed from product because of the high hydrophilicity of polyacrylamide (PAM). The microfibrous morphology of the PANI is confirmed by SEM image. Element analysis, IR spectra, and XRD pattern characterize the PANI microfibers.
ShenYang Chemical Co., Ltd. For FT–IR studies, an Alpha-Centauri 560 spectrophotometer (Nicolet, USA) was used with a number range of 400– 4000 cm 1. PE2400CHN element analyzer and an ICP emission spectrometer were used (Japan). XRD patterns were taken on a D/max-IIIc X-ray diffraction spectrometer with monochromatized Cu Ka radiation (Japan). Conductivity of the PANI was measured using a standard four-point probe method (China). Disk shape samples were prepared from powders using 20 MPa pressure at room temperature. For the scanning electron micrograph (SEM) investigation, an Amray 3000 SEM was used (Japan). 2.2. Preparation of HPA/PAM microfiber mats 1.6 g of PAM was dissolved in 25 ml of distilled water under the condition of constant stirring for 1 h. Then, 6.4 g of H4SiW12O40 was dissolved in the solution. The solution was stirred for 4 h at room temperature. The electrospinning apparatus was shown in Fig. 1. The HPA/PAM solution was contained in a plastic capillary tube. The capillary tube was then clamped to a ringstand that was above a grounded tubular layer. The tubular layer was covered by a piece of aluminum foil. A metal pin connected to a high-voltage generator was placed in the solution, and the solution was kept in the capillary by adjusting the angle between capillary and the aluminum foil. In order to prepare HPA/PAM microfibers with different diameters, the distance from tip to collector was changed from 8 to 20 cm. A voltage of 18 kV was applied to the solution and dense web of fibers was collected on the aluminum foil. The HPA/PAM fiber mats
2. Experimental 2.1. Instruments and reagents All chemical used in this study were analytical reagent grade and used as received. The commercial aniline was distilled twice under vacuum before used. The PAM was Mn 3,000,000, supplied by
2109
Fig. 1. Scheme of the electrospinning process.
2110
F. Wang et al. / European Polymer Journal 42 (2006) 2108–2113
with average diameter of ca. 250, 400 and 600 nm, respectively were obtained. The HPA/PAM microfiber mats were dried under vacuum for 24 h at 40 °C. 2.3. Preparation of PANI microfibers 0.2 g of (NH4)2S2O8 was dissolved in 8 ml deionized water with magnetic stirring at room temperature, 0.08 ml aniline was added to the solution in 0–5 °C. 0.03 g of HPA/PAM microfiber mats were added on the surface of the solution, and immobilized for 48 h in 0–5 °C. The entire color of the solution changed to a dark green. The precipitate was washed with deionized water, methanol, and ethyl ether several times, and then dried under vacuum for 24 h at 40 °C. According to the results of the elemental analysis, the following empirical formula was obtained. C6H4.5N(H4SiW12O40)0.021 (found: C, 47.51; N, 9.080; H, 3.551; Si, 0.201; W, 15.46%). 3. Results and discussion Polyoxometalates/PAM system has been widely investigated as inorganic and organic hybrids materials to date [29–31]. The function of hydrogen bond between polyoxometalates and PAM has been proven. As we know, the polymerization of aniline needs acidic condition. Polyoxometalates can provide acidic environment in the process for the polymerization of aniline. However, polyoxometalates as fibrous template is non-appropriate because it is difficult for polyoxometalates to form microfibers. PANI usually shows powders when the ordinary polyoxometalates are used for supplying acidic environment in polymerization of aniline [32,33]. Pure PAM microfibers are also non-appropriate, because PAM microfibers as a template cannot supply the acidic environment in the experiment. In this work, the reason why the PAM is selected for preparing the HPA/PANI microfibers seeding template is because of its high hydrophilicity, easy removing from the product, and easy forming microfibers in electrospinning condition. The polyoxometalates in the HPA/PAM microfibers can be released slowly from the HPA/PAM microfibers and form acidic environment around the microfibers template in the experiment. 3.1. SEM Morphology The HPA/PAM microfibers, as a template, with different average diameters from ca. 250, 400 to
Fig. 2. SEM images of HPA/PAM microfiber (a) and the corresponding PANI microfibers (b).
600 nm were used in the experiment, respectively. The resulting PANI microfibers almost showed the same average diameter with ca. 250 nm. As a example, Fig. 2 showed the SEM images of HPA/PAM microfiber (average diameter ca. 600 nm) and the corresponding PANI microfibers. As showed in the Fig. 2, the average diameter of the HPA/PAM microfibers was ca. 600 nm. However, the corresponding PANI showed microfibrous morphology with average diameter of ca. 250 nm (see Fig. 3). In fact, PANI microfibers with average diameter of 200–300 nm could be always obtained in this experiment no matter how to change the average diameter of the template from 250 nm to 400 nm or to 600 nm. Obviously, the diameter of the PANI microfibers was not affected by the diameter of the microfibers seeding template in the experimental ranges. We deduced that the polymerization of aniline occurred only on the surface of the HPA/ PAM microfibers template because of the bigger concentration of H+ around the HPA/PAM microfibers i.e., the polymerization of the aniline initiated on the surface of the HPA/PAM microfibers, and outspreaded along the HPA/PAM microfibers. The PANI microfibers would remove from the acidic environment when the average diameter of the PANI microfibers reached ca. 250 nm because of the weight of the PANI microfibers. As a template, the HPA/PAM microfibers was very important for forming the microfibrous morphology of PANI. In fact, the microfibrous morphology of PANI would be not found if there were no the HPA/PAM microfibers seeding template in the reaction system. Obviously, the microscale morphology of the ‘active’ seed was transcribed to the bulk precipitate of PANI. All above results indicated that the microfibers seeding template significantly affected the microfibrous morphology of the
F. Wang et al. / European Polymer Journal 42 (2006) 2108–2113 average diameter = 250nm
40 30 20 10 0 400
500
600
700
800
900
1000
Diameter (nm)
Frequency distribution (%)
Frequency distribution( %)
average diameter = 628nm 50
2111
60 50 40 30 20 10 0 180 200 220 240 260 280 300 320 340 360 380
Diameter (nm)
Fig. 3. Diameter distributing of HPA/PAM microfibers (left) and the PANI microfibers doped H4SiW12O40 (right).
resulting PANI. However, the different diameters of the microfibers seeding template in the experimental ranges could not affect the average diameter of the PANI microfibers. The formation mechanism of the PANI microfibers was not clear and needed to investigate further. 3.2. Structural characterization Figs. 4 and 5 showed the IR spectrum and XRD pattern of the PANI microfibers, respectively. The peaks in the IR spectral range 700–1100 cm 1 corresponding to H4SiW12O40 structural vibrations could be distinguished easily [32] i.e., at 926.85, 980.86, 881.92 and 782.70 cm 1, which were attributed to the mas(Si = Oa), mas(W = Ot), mas(W–Ob–W), and mas(W–Oc–W), respectively [32,33]. The characteristic peaks of PANI could be also noted. The peaks at 1575.87 and 1498.04 cm 1 were the characteristic peaks of benzene and quinoid ring stretching, respectively. The peaks in the range of 1200– 1400 cm 1 were C–N stretching peak of an aromatic
Fig. 4. IR spectra of polyaniline microfibers doped with H4SiW12O40.
Fig. 5. XRD pattern of polyaniline microfibers doped with H4SiW12O40.
amine. The peak at 1146.00 cm 1, which was the characteristic peak of protonated state, appeared. The peaks at 3441.11 and 3370.10 cm 1 were the N–H stretching vibration absorption peaks [32,33]. The IR spectrum suggested that the resulting product was the PANI doped with H4SiW12O40. XRD pattern confirmed the primarily amorphous character of PANI (see Fig. 5). Compared with previously reported XRD pattern of non-fibrous PANI powders or films, the XRD pattern of the PANI microfibers showed similarly structural order [34]. The XRD pattern of the PANI microfibers showed ˚ ), which close to a sharp peak at 7.16° (d = 12.34 A the distance of polymer repeat unit with relatively distinct Bragg reflections. This indicated that the PANI doped H4SiW12O40 was ordered in short distance [32]. At the same time, two broad bands cen˚ ) and 2h = 25.64° tered at 2h = 20.06° (d = 4.42 A ˚ ), which were ascribed to the periodicity (d = 3.47 A parallel and perpendicular to the PANI chains, respectively, also appeared [35]. This also indicated that the PANI microfibers were ordered. No peaks
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F. Wang et al. / European Polymer Journal 42 (2006) 2108–2113
Fig. 6. Conductivity of the PANI microfibers prepared with 30 mg of 80 wt.% HPA/PAM microfiber mats as seeding template and 0.2 g (NH4)2S2O8.
from any crystalline form of polyoxometalates could be observed indicating that Keggin units were inserted into the polymer matrix [32]. 3.3. Conductivity of the PANI microfibers PANI base was insulating with the conductivity of the order of 10 12 S cm 1, and its protonation made the rise to increase of electronic conductivity by several orders of magnitude [32]. Polyoxometalates as a stronger protonic acid could be doped into the PANI base. The protonation occurred at imine nitrogen sites to yield polysemiquinone, in which the polarons delocalized along the PANI chain. Usually, the conductivity of PANI doped with polyoxometalates was 10 2 10 4 S cm 1 [32,33]. As we know that improvement of synthesis method is very important since different synthesis method will bring some changes of property for the synthesized materials [33]. Here, the effect of different ratio of (NH4)2S2O8 to aniline on the conductivity of the PANI microfibers were investigated. The result was shown in Fig. 6. The conductivity of the PANI microfibers changed with changing the ratio of (NH4)2S2O8 to aniline. The best conductivity of the PANI microfibers was 27.1 S cm 1 with (NH4)2S2O8/aniline = 1:1 (mol/mol). 4. Conclusion In this paper, PANI microfibers were prepared by using microfiber seeding template method. (i) The microfibrous morphology of PANI was found,
and the average diameter of the PANI microfibers was ca. 250 nm. (ii) The best conductivity of the PANI microfibers doped with H4SiW12O40 was 27.1 S cm 1 with the condition of 30 mg of 80 wt.% HPA/PAM microfiber mats as a seeding template and 1:1 (mol/mol) ratio of (NH4)2S2O8 to aniline. (iii) The microfibers seeding template significantly affected the microfibrous morphology of the resulting PANI. However, the diameter of the PANI microfibers was not affected by the diameter change of the microfibers seeding template in the experimental ranges. (iiii) A possible formation process of the PANI microfibers was suggested i.e., the polymerization of the aniline initiated on the surface of the microfibers, and outspreaded along the HPA/ PAM microfibers because of the higher concentration of H+ around the HPA/PAM microfibers template. The PANI microfibers would remove from the HPA/PAM microfibers template when the average diameter of the PANI microfibers reached ca. 250 nm because of the weight of PANI microfibers. Acknowledgement We greatly appreciated the financial supports from the Science Foundation of Jilin province and the project-sponsored by SRF for ROCS, SEM. References [1] Favier F, Walter EC, Zach MP, Benter T, Penner RM. Hydrogen sensors and switches from electrodeposited palladium mesowire arrays. Sci 2001;293:2227–31. [2] Cao YW, Jin R, Mirkin CA. DNA-modified core-shell Ag/ Au nanoparticles. J Am Chem Soc 2001;123:7961–2. [3] Xia Y, Yang P, Sun Y, Wu Y, Mayers B, Yin Y, et al. Onedimensional nanostructures: synthesis, characterization, and applications. Adv Mater 2003;15:353–89. [4] Rao CNR, Nath M. Inorganic nanotubes. J Chem Soc Dalton Trans 2003;1:1–24. [5] Wu Y, Yan H, Huang M, Messer B, Song JH, Yang P. Inorganic semiconductor nanowires: rational growth, assembly, and novel properties. Chem Eur J 2002;8:1260–8. [6] Cho MS, Park SY, Hwang JY, Choi HJ. Synthesis and electrical properties of polymer composites with polyaniline nanoparticles. Mater Sci Eng 2004;24:15–8. [7] Katta P, Alessandro M, Ramsier RD, Chase GG. Continuous electrospinning of aligned polymer nanofibers onto a wire drum collector. Nano Lett 2004;4:2215–8. [8] Cao MH, Hu CW, Wang EB. The first fluoride onedimensional nanostructures: microemulsion-mediated hydrothermal synthesis of BaF2 whiskers. J Am Chem Soc 2003;125:11196–7. [9] Jang J, Bae J. Formation of polyaniline nanorod/liquid crystalline epoxy composite nanowires using a temperaturegradient method. Adv Fun Mater 2005;15:1877–82.
F. Wang et al. / European Polymer Journal 42 (2006) 2108–2113 [10] Lee IS, Cho MS, Choi HJ. Preparation of polyaniline coated poly(methyl methacrylate) microsphere by graft polymerization and its electrorheology. Polym 2005;46:1317–21. [11] Zhang XY, Manohar SK. Bulk synthesis of polypyrrole nanofibers by a seeding approach. J Am Chem Soc 2004;126:12714–5. [12] Cho MS, Cho YH, Choi HJ, Jhon MS. Synthesis and electrorheological characteristics of polyaniline-coated poly (methyl methacrylate) microsphere: Size effect. Langmuir 2003;19:5875–81. [13] MacDiarmid AG. Synthetic metals: a novel role for organic polymers. Synth Met 2002;125:11–22. [14] Heeger AJ. Semiconducting and metallic polymers: the fourth generation of polymeric materials. Synth Met 2002;125:23–42. [15] Malinauskas A. Chemical deposition of conducting polymers. Polym 2001;42:3957–72. [16] Cho MS, Choi HJ, Ahn WS. Enhanced electrorheology of conducting polyaniline confined in MCM-41 channels. Langmuir 2004;20:202–7. [17] Choi HJ, Kim TW, Cho MS, Kim SQ, Jhon MS. Electrorheological characterization of polyaniline dispersions. Eur Polym J 1997;33:699–703. [18] Jaroslav S, Milena S, Andrea R, Martin H, Pavlina M, Jan P. Polyaniline dispersions 8. The control of particle morphology. Polym 1999;40:2487–97. [19] Gao QX, Yi CQ, Zhu L, Yang Q, Xie Y. Chemical synthesis of cross-linked polyaniline by a novel solvothermal metathesis reaction of p-dichlorobenzene with sodium amide. Polym 2005;46:3185–9. [20] Gao HX, Jiang T, Han BX, Wang Y, Du JM, Liu ZM, et al. Aqueous/ionic liquid interfacial polymerization for preparing polyaniline nanoparticles. Polymers 2004;45:3017–9. [21] Martin CR. Membrane-based synthesis of nanomaterials. Chem Mater 1996;8:1739–46. [22] Wu CG, Bein T. Conducting Polyaniline Filaments in a mesoporous. Chan Host Sci 1994;264:1757–9. [23] Wang CW, Wang Z, Li HL. Well-aligned polyaniline nanofibril array membrane and its field emission property. Chem Phys Lett 2001;341:431–4.
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[24] Kim JW, Kim SG, Choi HJ, Jhon MS. Synthesis and electrorheological properties of polyaniline-Na+-montmorillonite suspensions. Macromol Rapid Commun 1999;20: 450–2. [25] Cho MS, Choi HJ, Kim KY, Ahn WS. Synthesis and characterization of polyaniline/mesoporous SB A-15 nanocomposite. Macromol Rapid Commun 2002;23:713–6. [26] Yang YS, Wan MX. Microtubules of polypyrrole synthesized by an electrochemical template-free method. J Mater Chem 2001;8:2022–7. [27] Sunil KP, Frank DB, Akira TT, Story JG, Massimo FB. Radiolytic synthesis of polyaniline nanofibers: A new templateless pathway. Chem Mater 2005;17:227–9. [28] Zhang XY, Goux WJ, Manohar SK. Synthesis of polyaniline nanofibers by ‘nanofiber seeding’. J Am Chem Soc 2004;126:4502–3. [29] Feng W, Zhang TR, Liu Y, Lu R, Zhao YY, Li TJ, et al. Sonochemical preparation of photochromic nanocomposite thin film based on polyoxometalates well dispersed in polyacrylamide. J Solid State Chem 2002;169:1–5. [30] Feng W, Zhang TR, Liu Y, Lu R, Guan C, Zhao YY, et al. Evaluation of photochromic properties in heteropolyoxometallate-based inorganic polymeric thin films. Mater Chem Phys 2002;77:294–8. [31] Wang J, Wang Z, Wang HS, Liu FY, Fu LS, Zhang HJ. Selfassembled multilayer films of europium-substituted polyoxometalate and their luminescence properties. J Alloys Comp 2004;376:68–72. [32] Gong J, Hua R, Xie Z, Wang S, Qu LY. Chemical syntheses of the conducting material formed by heteropolyacids and poly aniline. Polym J 2001;33:377–82. [33] Gong J, Cui XJ, Xie ZW, Wang SG, Qu LY. The solid-state synthesis of polyaniline/H4SiW12O40 materials. Synth Met 2002;129:187–92. [34] Chiou NR, Epstein AJ. Polyaniline nanofibers prepared by dilute polymerization. Adv Mater 2005;17:1679–83. [35] Pouget JP, Jozefowicz ME, Epstein AJ, Tang X, MacDiarmid AG. X-ray structure of polyaniline. Macromolecules 1991;24:779–89.
European Polymer Journal 42 (2006) 2108–2113
www.elsevier.com/locate/europolj
Synthesis and characterization of polyaniline microfibers by utilizing H4SiW12O40/polyacrylamide microfibers seeding template method Fengchun Wang a
a,b
, Rui Yang a,c, Jian Gong a,*, Chunhong Sui a, Yunqing Luo a, Lunyu Qu a
Faculty of Chemistry, Northeast Normal University, Changchun, Jilin 130024, PR China b Faculty of Chemistry, Tonghua Teachers College, Tonghua 134000, PR China c Department of Chemistry, Beihua University, Jilin 132013, PR China
Received 13 January 2006; received in revised form 28 March 2006; accepted 2 April 2006 Available online 5 June 2006
Abstract In this paper, polyaniline (PANI) microfibers with average diameter of 250 nm were synthesized by utilizing H4SiW12O40/polyacrylamide (HPA/PAM) microfibers seeding template method. The PANI microfibers were characterized by element analyses, FT–IR spectra, X-ray diffraction pattern (XRD), and Scanning electron micrograph (SEM). The microfibers seeding template significantly affected the fibrous morphology of the resulting PANI. However, the diameter of the PANI microfibers was almost not affected by the diameter of the microfibers seeding template in the experimental range. In addition, the conductivity of the PANI microfibers was also investigated. The result showed that the best conductivity of the PANI microfibers doped with H4SiW12O40 was 27.1 S/cm. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Polyaniline; Microfibers; Polyoxometalate; Polyacrylamide; Conductivity
1. Introduction Nanostructures, such as nanofibers, nanorods, nanowires, nanotubes, and nanoparticles etc. have attracted extensive attention as a result of their novel size-dependent properties [1–6]. Nanomaterials including polymer, metallic oxide etc., have been synthesized, and their properties have been investigated [7,8]. Recently, nanostructures of conductive *
Corresponding author. Tel.: +86 431 5099765. E-mail address: [email protected] (J. Gong).
polymer, such as nanoparticles, nanofibers, and nanorods, demonstrate improvement towards nanotechnology [9–12]. As we know, the discovery of conductive polymers has opened up many new possibilities for devices combining optical, electrochemical, and conductive properties [13,14]. Conductive polymers have been extensively studied during the past couple of decades, and they still remain the subject of intense investigations by many research groups worldwide [15]. The widely studied conductive polymers include polyaniline, polypyrrole, and polythiophene. Among the conductive polymers,
0014-3057/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2006.04.004
F. Wang et al. / European Polymer Journal 42 (2006) 2108–2113
PANI, one of the most promising intrinsically conductive polymers, has received considerable attention in recent years due to its straightforward polymerization, chemical stability, relatively high conductivity and potential applications in electrochromic displays, chargeable batteries, sensors, and electrorheological fluids [12–17]. In the past years, micro- and nanostructures (such as tubes, rods, and particles) of PANI have been synthesized by chemical and electrochemical oxidative polymerization of aniline [18–20]. Because nanomaterials with their inherent anisotropy are the smallest dimension structures that can be used for efficient transport of electrons and optical excitations. So, the controlled synthesis of nanoscale materials is critical for fundamental studies and potential devices. Among various approaches for the preparation of micro- and nanostructure of PANI, template method has been an effective and important method. Usually, template can be divided to ‘hard’ template, such as the channels of microporous membranes [21] and zeolite [22], anodized alumina [23], layered silicate [24], MCM-41 [16], and SBA15 [25], and ‘soft’ template, such as micelles [26], and organic acids [27]. However, the template removal is tedious for hard templates, and meanwhile soft templates limit the range of chemicals that can be used. Recently, a new ‘nanofiber seeding’ method was used for synthesizing conductive polymer nanostructure [28]. However, the selected seeding template, such as carbon nanofiber or nanotubes, cannot be moved from product, although the content of the seeding template can be ignored in the product. In this paper, we describe an extremely simple microfiber-seeding template method to synthesize microfibrous PANI doped polyoxometalate. In this system, the microfiber-seeding template can easy be removed from product because of the high hydrophilicity of polyacrylamide (PAM). The microfibrous morphology of the PANI is confirmed by SEM image. Element analysis, IR spectra, and XRD pattern characterize the PANI microfibers.
ShenYang Chemical Co., Ltd. For FT–IR studies, an Alpha-Centauri 560 spectrophotometer (Nicolet, USA) was used with a number range of 400– 4000 cm 1. PE2400CHN element analyzer and an ICP emission spectrometer were used (Japan). XRD patterns were taken on a D/max-IIIc X-ray diffraction spectrometer with monochromatized Cu Ka radiation (Japan). Conductivity of the PANI was measured using a standard four-point probe method (China). Disk shape samples were prepared from powders using 20 MPa pressure at room temperature. For the scanning electron micrograph (SEM) investigation, an Amray 3000 SEM was used (Japan). 2.2. Preparation of HPA/PAM microfiber mats 1.6 g of PAM was dissolved in 25 ml of distilled water under the condition of constant stirring for 1 h. Then, 6.4 g of H4SiW12O40 was dissolved in the solution. The solution was stirred for 4 h at room temperature. The electrospinning apparatus was shown in Fig. 1. The HPA/PAM solution was contained in a plastic capillary tube. The capillary tube was then clamped to a ringstand that was above a grounded tubular layer. The tubular layer was covered by a piece of aluminum foil. A metal pin connected to a high-voltage generator was placed in the solution, and the solution was kept in the capillary by adjusting the angle between capillary and the aluminum foil. In order to prepare HPA/PAM microfibers with different diameters, the distance from tip to collector was changed from 8 to 20 cm. A voltage of 18 kV was applied to the solution and dense web of fibers was collected on the aluminum foil. The HPA/PAM fiber mats
2. Experimental 2.1. Instruments and reagents All chemical used in this study were analytical reagent grade and used as received. The commercial aniline was distilled twice under vacuum before used. The PAM was Mn 3,000,000, supplied by
2109
Fig. 1. Scheme of the electrospinning process.
2110
F. Wang et al. / European Polymer Journal 42 (2006) 2108–2113
with average diameter of ca. 250, 400 and 600 nm, respectively were obtained. The HPA/PAM microfiber mats were dried under vacuum for 24 h at 40 °C. 2.3. Preparation of PANI microfibers 0.2 g of (NH4)2S2O8 was dissolved in 8 ml deionized water with magnetic stirring at room temperature, 0.08 ml aniline was added to the solution in 0–5 °C. 0.03 g of HPA/PAM microfiber mats were added on the surface of the solution, and immobilized for 48 h in 0–5 °C. The entire color of the solution changed to a dark green. The precipitate was washed with deionized water, methanol, and ethyl ether several times, and then dried under vacuum for 24 h at 40 °C. According to the results of the elemental analysis, the following empirical formula was obtained. C6H4.5N(H4SiW12O40)0.021 (found: C, 47.51; N, 9.080; H, 3.551; Si, 0.201; W, 15.46%). 3. Results and discussion Polyoxometalates/PAM system has been widely investigated as inorganic and organic hybrids materials to date [29–31]. The function of hydrogen bond between polyoxometalates and PAM has been proven. As we know, the polymerization of aniline needs acidic condition. Polyoxometalates can provide acidic environment in the process for the polymerization of aniline. However, polyoxometalates as fibrous template is non-appropriate because it is difficult for polyoxometalates to form microfibers. PANI usually shows powders when the ordinary polyoxometalates are used for supplying acidic environment in polymerization of aniline [32,33]. Pure PAM microfibers are also non-appropriate, because PAM microfibers as a template cannot supply the acidic environment in the experiment. In this work, the reason why the PAM is selected for preparing the HPA/PANI microfibers seeding template is because of its high hydrophilicity, easy removing from the product, and easy forming microfibers in electrospinning condition. The polyoxometalates in the HPA/PAM microfibers can be released slowly from the HPA/PAM microfibers and form acidic environment around the microfibers template in the experiment. 3.1. SEM Morphology The HPA/PAM microfibers, as a template, with different average diameters from ca. 250, 400 to
Fig. 2. SEM images of HPA/PAM microfiber (a) and the corresponding PANI microfibers (b).
600 nm were used in the experiment, respectively. The resulting PANI microfibers almost showed the same average diameter with ca. 250 nm. As a example, Fig. 2 showed the SEM images of HPA/PAM microfiber (average diameter ca. 600 nm) and the corresponding PANI microfibers. As showed in the Fig. 2, the average diameter of the HPA/PAM microfibers was ca. 600 nm. However, the corresponding PANI showed microfibrous morphology with average diameter of ca. 250 nm (see Fig. 3). In fact, PANI microfibers with average diameter of 200–300 nm could be always obtained in this experiment no matter how to change the average diameter of the template from 250 nm to 400 nm or to 600 nm. Obviously, the diameter of the PANI microfibers was not affected by the diameter of the microfibers seeding template in the experimental ranges. We deduced that the polymerization of aniline occurred only on the surface of the HPA/ PAM microfibers template because of the bigger concentration of H+ around the HPA/PAM microfibers i.e., the polymerization of the aniline initiated on the surface of the HPA/PAM microfibers, and outspreaded along the HPA/PAM microfibers. The PANI microfibers would remove from the acidic environment when the average diameter of the PANI microfibers reached ca. 250 nm because of the weight of the PANI microfibers. As a template, the HPA/PAM microfibers was very important for forming the microfibrous morphology of PANI. In fact, the microfibrous morphology of PANI would be not found if there were no the HPA/PAM microfibers seeding template in the reaction system. Obviously, the microscale morphology of the ‘active’ seed was transcribed to the bulk precipitate of PANI. All above results indicated that the microfibers seeding template significantly affected the microfibrous morphology of the
F. Wang et al. / European Polymer Journal 42 (2006) 2108–2113 average diameter = 250nm
40 30 20 10 0 400
500
600
700
800
900
1000
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Frequency distribution (%)
Frequency distribution( %)
average diameter = 628nm 50
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Fig. 3. Diameter distributing of HPA/PAM microfibers (left) and the PANI microfibers doped H4SiW12O40 (right).
resulting PANI. However, the different diameters of the microfibers seeding template in the experimental ranges could not affect the average diameter of the PANI microfibers. The formation mechanism of the PANI microfibers was not clear and needed to investigate further. 3.2. Structural characterization Figs. 4 and 5 showed the IR spectrum and XRD pattern of the PANI microfibers, respectively. The peaks in the IR spectral range 700–1100 cm 1 corresponding to H4SiW12O40 structural vibrations could be distinguished easily [32] i.e., at 926.85, 980.86, 881.92 and 782.70 cm 1, which were attributed to the mas(Si = Oa), mas(W = Ot), mas(W–Ob–W), and mas(W–Oc–W), respectively [32,33]. The characteristic peaks of PANI could be also noted. The peaks at 1575.87 and 1498.04 cm 1 were the characteristic peaks of benzene and quinoid ring stretching, respectively. The peaks in the range of 1200– 1400 cm 1 were C–N stretching peak of an aromatic
Fig. 4. IR spectra of polyaniline microfibers doped with H4SiW12O40.
Fig. 5. XRD pattern of polyaniline microfibers doped with H4SiW12O40.
amine. The peak at 1146.00 cm 1, which was the characteristic peak of protonated state, appeared. The peaks at 3441.11 and 3370.10 cm 1 were the N–H stretching vibration absorption peaks [32,33]. The IR spectrum suggested that the resulting product was the PANI doped with H4SiW12O40. XRD pattern confirmed the primarily amorphous character of PANI (see Fig. 5). Compared with previously reported XRD pattern of non-fibrous PANI powders or films, the XRD pattern of the PANI microfibers showed similarly structural order [34]. The XRD pattern of the PANI microfibers showed ˚ ), which close to a sharp peak at 7.16° (d = 12.34 A the distance of polymer repeat unit with relatively distinct Bragg reflections. This indicated that the PANI doped H4SiW12O40 was ordered in short distance [32]. At the same time, two broad bands cen˚ ) and 2h = 25.64° tered at 2h = 20.06° (d = 4.42 A ˚ ), which were ascribed to the periodicity (d = 3.47 A parallel and perpendicular to the PANI chains, respectively, also appeared [35]. This also indicated that the PANI microfibers were ordered. No peaks
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F. Wang et al. / European Polymer Journal 42 (2006) 2108–2113
Fig. 6. Conductivity of the PANI microfibers prepared with 30 mg of 80 wt.% HPA/PAM microfiber mats as seeding template and 0.2 g (NH4)2S2O8.
from any crystalline form of polyoxometalates could be observed indicating that Keggin units were inserted into the polymer matrix [32]. 3.3. Conductivity of the PANI microfibers PANI base was insulating with the conductivity of the order of 10 12 S cm 1, and its protonation made the rise to increase of electronic conductivity by several orders of magnitude [32]. Polyoxometalates as a stronger protonic acid could be doped into the PANI base. The protonation occurred at imine nitrogen sites to yield polysemiquinone, in which the polarons delocalized along the PANI chain. Usually, the conductivity of PANI doped with polyoxometalates was 10 2 10 4 S cm 1 [32,33]. As we know that improvement of synthesis method is very important since different synthesis method will bring some changes of property for the synthesized materials [33]. Here, the effect of different ratio of (NH4)2S2O8 to aniline on the conductivity of the PANI microfibers were investigated. The result was shown in Fig. 6. The conductivity of the PANI microfibers changed with changing the ratio of (NH4)2S2O8 to aniline. The best conductivity of the PANI microfibers was 27.1 S cm 1 with (NH4)2S2O8/aniline = 1:1 (mol/mol). 4. Conclusion In this paper, PANI microfibers were prepared by using microfiber seeding template method. (i) The microfibrous morphology of PANI was found,
and the average diameter of the PANI microfibers was ca. 250 nm. (ii) The best conductivity of the PANI microfibers doped with H4SiW12O40 was 27.1 S cm 1 with the condition of 30 mg of 80 wt.% HPA/PAM microfiber mats as a seeding template and 1:1 (mol/mol) ratio of (NH4)2S2O8 to aniline. (iii) The microfibers seeding template significantly affected the microfibrous morphology of the resulting PANI. However, the diameter of the PANI microfibers was not affected by the diameter change of the microfibers seeding template in the experimental ranges. (iiii) A possible formation process of the PANI microfibers was suggested i.e., the polymerization of the aniline initiated on the surface of the microfibers, and outspreaded along the HPA/ PAM microfibers because of the higher concentration of H+ around the HPA/PAM microfibers template. The PANI microfibers would remove from the HPA/PAM microfibers template when the average diameter of the PANI microfibers reached ca. 250 nm because of the weight of PANI microfibers. Acknowledgement We greatly appreciated the financial supports from the Science Foundation of Jilin province and the project-sponsored by SRF for ROCS, SEM. References [1] Favier F, Walter EC, Zach MP, Benter T, Penner RM. Hydrogen sensors and switches from electrodeposited palladium mesowire arrays. Sci 2001;293:2227–31. [2] Cao YW, Jin R, Mirkin CA. DNA-modified core-shell Ag/ Au nanoparticles. J Am Chem Soc 2001;123:7961–2. [3] Xia Y, Yang P, Sun Y, Wu Y, Mayers B, Yin Y, et al. Onedimensional nanostructures: synthesis, characterization, and applications. Adv Mater 2003;15:353–89. [4] Rao CNR, Nath M. Inorganic nanotubes. J Chem Soc Dalton Trans 2003;1:1–24. [5] Wu Y, Yan H, Huang M, Messer B, Song JH, Yang P. Inorganic semiconductor nanowires: rational growth, assembly, and novel properties. Chem Eur J 2002;8:1260–8. [6] Cho MS, Park SY, Hwang JY, Choi HJ. Synthesis and electrical properties of polymer composites with polyaniline nanoparticles. Mater Sci Eng 2004;24:15–8. [7] Katta P, Alessandro M, Ramsier RD, Chase GG. Continuous electrospinning of aligned polymer nanofibers onto a wire drum collector. Nano Lett 2004;4:2215–8. [8] Cao MH, Hu CW, Wang EB. The first fluoride onedimensional nanostructures: microemulsion-mediated hydrothermal synthesis of BaF2 whiskers. J Am Chem Soc 2003;125:11196–7. [9] Jang J, Bae J. Formation of polyaniline nanorod/liquid crystalline epoxy composite nanowires using a temperaturegradient method. Adv Fun Mater 2005;15:1877–82.
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