Langmuir 2006, 22, 3899-3905
3899
Facile Synthesis of Polyaniline-Sodium Alginate Nanofibers Yijun Yu, Si Zhihuai, Shuangjun Chen, Chaoqing Bian, Wei Chen, and Gi Xue* Nanjing National Laboratory of Solid State Microstructure, Department of Polymer Science and Engineering, School of Chemistry and Chemical Engineering, Nanjing UniVersity, Nanjing 210093, China ReceiVed July 14, 2005. In Final Form: October 14, 2005 This work demonstrates a facile route to the synthesis of large quantities of uniform polyaniline-sodium alginate (PANI-SA) nanofibers template-guided by SA. This approach is an easy, inexpensive, environmentally friendly, and scalable one-step method to produce uniform nanofibers with controllable average diameters in bulk quantities. We started with biopolymer-monomer complexes formed between the carboxylic groups of SA and the amino group of an organic monomer (aniline). When ammonium persulfate was added, such polymer-monomer complexes could be polymerized. Then, polyaniline-sodium alginate nanofibers with uniform diameters from 40 to 100 nm were successfully obtained in a high yield. The resultant PANI-SA nanofibers were characterized by means of different techniques, such as ultraviolet-visible spectroscopy, thermogravimetric analysis, wide-angle X-ray diffraction, Fourier transform infrared spectroscopy, and scanning and transmission electron microscopy methods. The mechanism governing the formation of the polyaniline-sodium alginate nanofibers is discussed.
1. Introduction
* To whom correspondence should be addressed. E-mail: xuegi@ nju.edu.cn.
final product. Besides template methods, an interfacial polymerization method has been employed to prepare polyaniline nanofibers.17 This synthesis procedure involves volatile organic solvents such as carbon tetrachloride, benzene, and carbon disulfide. A “nanofiber seeding” technique has been developed to prepare bulk quantities of PANI nanofibers,18 which decreases the use of organic solvents. This procedure requires seeds of polyaniline nanofibers or carbon nanotubes. When organic dopants with surfactant functionalities are used, emulsions or micelles can be formed, leading to microtubes, microfibers, or microrod-like structures.19 However, if polyaniline nanostructures with diameters e100 nm are desired, then very complex dopants with bulky side groups are needed, such as sulfonated naphthalene derivatives,19a,b fullerenes,19c or dendrimers.13 Electrochemical polymerization20 and some physical methods, such as electrospinning21 and mechanical stretching,22 can also produce conducting polymer nanofibers without templates, but these materials have only been made on a very limited scale, such as films on an electrode surface. Adding structure-directing molecules such as poly(acrylic acid) (PAA) or poly(styrenesulfonic acid) to the chemical polymerization bath is another way to obtain conducting polyaniline nanostructures.23 The method consists of dissolving the aniline monomer in a large amount of polyacid, followed by oxidative polymerization of the monomer. Among PANI-polyacid complexes, PANI-PAA has been studied quite extensively.24 The
(1) Huang, J.; Virji, S.; Weiller, B. H.; Kaner, R. B. J. Am. Chem. Soc. 2003, 125, 314. (2) Gao, J.; Sansinena, J. M.; Wang, H. L. Chem. Mater. 2003, 15, 2411. (3) Janata, J.; Josowicz, M. Nat. Mater. 2003, 2, 19. (4) Wu, C. G.; Bein, T. Science 1994, 264, 1757. (5) Anderson, M. R.; Mattes, B. R.; Reiss, H.; Kaner, R. B. Science 1991, 252, 1412. (6) Yang, J.; Burkinshaw, S. M.; Zhou, J.; Monkman, A. P.; Brown, P. J. AdV. Mater. 2003, 15, 1081. (7) Baba, A.; Knoll, W. AdV. Mater. 2003, 15, 1015. 2003, 15, 1382. (8) Huang, W. S.; Humphrey, B. D.; MacDiarmid, A. G. J. Chem. Soc., Faraday Trans. 1986, 82, 2385. (9) MacDiarmid, A. G. Synth. Met. 1997, 84, 27. (10) Martin, C. R. Science 1994, 266, 1961. (11) Huang, L.; Wang, Z.; Wang, H.; Cheng, X.; Mitra, A.; Yan, Y. J. Mater. Chem. 2002, 12, 388. (12) Jiang, J.; Yoon, H. Chem. Commun. 2003, 720. (13) Qiu, H.; Wan, M.; Matthews, B.; Dai, L. Macromolecules 2001, 34, 675. (14) Yang, J.; Wan, M. J. Mater. Chem. 2002, 12, 897. (15) Wei, Z.; Zhang, Z.; Wan, M. Langmuir 2002, 18, 917. (16) Yang, Y.; Liu, J.; Wan, M. Nanotechnology 2002, 13, 771.
(17) Huang, J.; Virji, S.; Weiller, B. H.; Kaner, R. B. J. Am. Chem. Soc. 2004, 126, 851. (18) Zhang, X.; Goux, W. J.; Manohar, S. K. J. Am. Chem. Soc. 2004, 126, 4502. (19) (a) Kinlen, P. J.; Liu, J.; Ding, Y.; Graham, C. R.; Remsen, E. E. Macromolecules 1998, 31, 1735. (b) Wei, Z. X.; Wan, M. X. J. Appl. Polym. Sci. 2003, 87, 1297. (c) Langer, J. J.; Framski, G.; Joachimiak, R. Synth. Met. 2001, 121, 1281. (20) (a) Liu, J.; Lin, Y.; Liang, L.; Voigt, J. A.; Huber, D. L.; Tian, Z. R.; Coker, E.; Mckenzie, B.; Mcdermott, M. J. Chem.sEur. J. 2003, 9, 605. (b) Liang, L.; Liu, J.; Windisch, C. F.; Exarhos, G. J.; Lin, Y. H. Angew. Chem., Int. Ed. 2002, 41, 3665. (21) MacDiarmid, A. G.; Jones, W. E.; Norris, I. D.; Gao, J.; Johnson, A. T.; Pinto, N. J.; Hone, J.; Han, B.; Ko, F. K.; Okuzaki, H.; Llaguno, M. Synth. Met. 2001, 119, 27. (22) He, H. X.; Li, C. Z.; Tao, N. J. Appl. Phys. Lett. 2001, 78, 811. (23) (a) Liu, J. M.; Yang, S. C. Chem. Commun. 1991, 21, 1529. (b) Hwang, J. H.; Yang, S. C. Synth. Met. 1989, 29, E271. (c) Li, S.; Dong, H.; Cao, Y. Synth. Met. 1989, 29, E329. (d) Lu, X. F.; Yu, Y. H.; Chen, L.; Mao, H. P.; Wang, L. F.; Zhang, W. J.; Wei, Y. Polymer 2005, 46, 5329.
Among the different morphologies of nanomaterials, onedimensional nanostructures of conducting polymers, for instance, have generated a great deal of interest in nanotechnology, since they have offered a variety of different functions such as chemical sensors or actuators,1-3 polymeric conducting molecular wires,4 gas-separation membranes,5,6 and neuron devices.7 Polyaniline (PANI) is unique among the family of conjugated polymers since it can be relatively tailored for specific applications through a nonredox acid/base doping process.8 The wide range of associated electrical, electrochemical, and optical properties, coupled with excellent environmental stability, makes PANI potentially attractive for use as an electronic material in a variety of applications.1,9 Recently, various approaches for the preparation of nanostructures of PANI including nanofibers and nanotubes have been reported with or without the aid of templates. “Hard templates” such as aluminosilicate MCM-41,4 track-etched polymeric membranes, and porous alumina10 and “soft templates” such as liquid crystalline phases,11 reverse microemulsions,12 and micelles13-16 have been used to prepare polyaniline or polypyrrole nanofibers or nanotubes. Although these templates, whether hard or soft, allow the desired high aspect ratios to be achieved, they complicate the synthesis and the extraction of the
10.1021/la051911v CCC: $33.50 © 2006 American Chemical Society Published on Web 03/11/2006
3900 Langmuir, Vol. 22, No. 8, 2006 Scheme 1. Chemical Structure of SA
morphology of the PANI-PAA film shows the presence of small clusters resembling those of a conventional PANI film and large globule-like conglomerates.24a A fibrillar network morphology of PANI-PAA can be obtained by using N-methyl-2-pyrrolidinone (NMP) as the solvent.24b Colloidal polyaniline fibrils were made by template-guided chemical polymerization.23a Polyaniline hollow microspheres have also been synthesized with the alkali-metal-guided method.25 Here, we demonstrate a facile route to the synthesis of large quantities of uniform polyaniline-sodium alginate nanofibers by a template-guided process in a dilute solution of sodium alginate. It is an easy, inexpensive, environmentally friendly, and scalable one-step method to produce uniform nanofibers with controllable average diameters in bulk quantities. We started with biopolymer-monomer complexes formed between a biopolymer (SA) with carboxylic groups and an organic monomer (aniline) with an amino group. When ammonium persulfate (APS) was added, such polymer-monomer complexes could be polymerized. Then polyaniline-sodium alginate nanofibers with uniform diameters from 40 to 100 nm were successfully obtained in a high yield. The resultant PANI-SA nanofibers were characterized by means of different techniques, such as ultraviolet-visible (UV-vis) spectroscopy, thermogravimetric analysis (TGA), wide-angle X-ray diffraction (WXRD), Fourier transform infrared (FTIR) spectroscopy, and scanning and transmission electron microscopy methods. The mechanism governing the formation of the polyaniline-sodium alginate nanofibers is discussed. 2. Experimental Section 2.1. Reagents. The monomer of aniline (99% purity, Aldrich) was distilled under reduced pressure to remove the inhibitors. APS ((NH4)2S2O8), NMP, and methanol were analytical grade and used without further purification. SA (Shanghai Chemical Reagent Co.) was analytical grade, and its molecular weight (Mw) was about 2.1 × 105. Its chemical structure is shown in Scheme 1. 2.2. Synthesis. The purified SA was dissolved in 50 mL of deionized water to make a 0.2 wt % aqueous solution. Then 2 mmol of aniline was added to the homogeneous solution and the resulting solution cooled to 0 °C. On stirring, a precooled solution of deionized water (5 mL) containing 2 mmol of APS was added to the homogeneous solution and the resulting solution stirred for 20 min. Then the reaction mixture was allowed to stand at 0 °C under nitrogen without stirring for 48 h. The resulting product was collected by filtration and washed with deionized water and methanol several times until the washing solution became clear. Finally, the product was dried in a dynamic vacuum for 48 h at room temperature before characterization. For comparison, pure PANI was synthesized without SA by means of this process. 2.3. Instrumentation. The scanning electron microscopy (SEM) images of polyaniline-sodium alginate nanofibers were obtained using an LEO 1530VP SEM instrument. Samples for the SEM experiment were made on a conducting stage and observed with gold coating. Samples for transmission electron microscopy (TEM; JEM-200 CX) were extracted from the reaction and then washed with deionized (24) (a) Hu, H. L.; Saniger, J. M.; Banuelos, J. G. Thin Solid Films 1999, 347, 241. (b) Chen, S. A.; Lee, H. T. Marcromolecules 1995, 28, 2858. (25) Wang, X.; Liu, N.; Yan, X.; Zhang, W. J.; Wei, Y. Chem. Lett. 2005, 34, 42.
Yu et al. water and methanol. All samples were sonicated in a water/ethanol mixture for 10 min before the TEM observation. Ultraviolet-visible absorption spectra were recorded on a PerkinElmer Instruments Lambda 35 in the range of 1100-190 nm at a scanning rate of 480 nm/min. FTIR spectra were recorded on a Bruker IFS66V spectrometer. The spectra were collected from 4000 to 400 cm-1 with 4 cm-1 resolution over 40 scans. TGA analysis was carried out on a TA Instruments 2100 thermogravimetric analyzer with a heating rate of 20 °C/min under a dynamic dry O2 flow of 100 cm3/min. Electrical conductivity was measured by a conventional fourprobe method under laboratory conditions. The composite powder was compacted into a disk pellet 12.7 mm in diameter under a pressure of 10 (kg N)/cm2. All the pellets were about 0.1 mm in thickness and had almost the same density of 0.48 g/cm3. WXRD patterns were taken on a Shimadzu XD-3A instrument with a Cu KR X-ray source. The pH values were obtained with a PHS-2C precision pH/mV meter.
3. Results and Discussion The morphology of the products can be imaged using electron microscopy. Highly uniform nanofibers are observed in samples prepared from an aqueous system under both TEM and SEM. For the powders obtained after filtration, scanning electron microscopy images reveal that they are actually agglomerations of nanofibers (Figure 1). The morphology of the polyanilinesodium alginate nanofibers is unaffected when the nanofibers are dedoped with an aqueous solution of ammonium hydroxide. Note that when polyaniline was prepared without SA, only spherical PANI nanoparticles were found (Figure 2). Traditional chemical polymerization using common mineral acids yields granular polyaniline.26 Uniform polyaniline-sodium alginate nanofibers synthesized with different concentrations of aniline at 0 °C are observed in scanning electron microscopy images (Figure 3). It can be seen that these wires are curved and entangled, demonstrating that the PANI-SA nanofibers are flexible. The diameters of the resulting nanofibers are affected by the concentration of aniline in the polymerization. When the concentration of aniline varies from 0.02 to 0.10 M, the average diameter of all the PANI-SA nanofibers increases from 40 to 100 nm as estimated from these SEM images. The average diameter of the polyaniline-sodium alginate nanofibers is found to increase with increasing concentration of aniline. The effects of the concentrations of SA and polymerization reaction temperature on the morphologies of PANI nanostructures are investigated. The average diameter of PANI-SA nanofibers appears to be sensitive to the SA concentration. When the SA concentration decreases from 0.2 to 0.025 wt %, the average diameter of the PANI-SA nanofibers increases from 35 to 100 nm. These images reveal that the average diameter of the PANISA nanofibers can be controlled by changing the concentration of SA; in addition, these kinds of nanofibers can be repeatedly produced, and the yield is very high. From the TEM images with high magnification shown in Figure 4a, it can also be seen that most of the nanofibers are bent in length, indicating their high flexibility. As shown in Figure 4b,c, the bending PANI-SA nanofibers are common in the product when the concentration of SA decreases to 0.05 wt %. The bending PANI-SA nanofibers may result from the bending of SA-aniline complexes during the polymerization reaction. When the concentration of SA (26) (a) Avlyanov, J. K.; Josefowicz, J. Y.; MacDiarmid, A. G. Synth. Met. 1995, 73, 205. (b) Chandrasekhar, P. Conducting Polymers, Fundamentals and Applications: A Practical Approach; Kluwer Academic Publishers: Boston, 1999.
Facile Synthesis of PANI-SA Nanofibers
Langmuir, Vol. 22, No. 8, 2006 3901
Figure 1. Polyaniline-sodium alginate powders obtained after filtration. Scanning electron microscopy images show that the powders (left; low magnification, 912×) are agglomerations of nanofibers (right; high magnification, 60000×).
Figure 2. TEM (a) and SEM (b) images of polyaniline nanoparticles synthesized without SA.
decreases to 0.025 wt %, PANI-SA nanofibers with a diameter of about 100 nm are formed (Figure 4d). Nevertheless, the flexibility of PANI-SA nanofibers is decreased, since polyaniline-sodium alginate nanofibers could not be folded in length, and a fracture could be seen in the length caused by sonication. Figure 4e is very interesting, showing a three-dimensional network with spherical nodes (about 60-70 nm) as the ‘‘crosslinking points”, when the polymerization is carried out at room temperature. These data indicate that the SA-aniline complexes
formed at low temperature may play a template-like role in the synthesis of PANI-SA nanofibers. The UV-vis spectra of SA, the as-synthesized PANI-SA nanofibers, and the ammonium hydroxide treated PANI-SA nanofibers were measured, as shown in Figure 5. An absorption band at about 280 nm was observed in the UV-vis absorption spectrum of SA (see Figure 5a). It can be assigned to double bonds of alginate formed after main chain scission.27 For PANISA nanofibers in water, three bands are seen with maxima at 290, 443, and 800 nm (Figure 5b). The band around 290 nm can be attributed to the absorption of SA. A shoulder peak at about 443 nm and a broad band at about 800 nm with a long tail are assigned to the polaron transition, which is a typical protonation characterization, identical to that of the emeraldine salt form of PANI. When the PANI-SA nanofibers are dedoped with ammonium hydroxide, only two peaks at 333 and about 643 nm (Figure 5c) can be observed in the solution UV-vis spectra of dedoped PANI-SA nanofibers in NMP, and this agrees with the undoped PANI.28 These results suggest that the PANI can be doped in situ by sodium alginate and can be dedoped by treatment with an aqueous solution of ammonium hydroxide.24b At the beginning of polymerization, the solution pH of both the control and SA-mediated systems was about 3.8 and 5.8, respectively. Then the solution pH fell as the polymerization proceeded, similar to what had been observed by Zhang et al.25 After polymerization, both of the mother liquids were strongly acidic with a pH of about 1.6. It is well-known that APS was reduced to sulfate or hydrosulfate radical; thus, the solution pH fell during the polymerization process. The four-probe pressedpellet conductivity of the PANI-SA nanofibers (the weight ratio of sodium alginate and aniline is about 1:1) is about 0.2 S‚cm-1. The conductivity of PANI synthesized without SA is about 0.6 S‚cm-1. To further corroborate the existence of SA and possible interaction between SA and PANI, bulk measurements on the PANI-SA nanofibers were carried out. These polyanilinesodium alginate nanofibers were synthesized with concentrations of aniline and SA of 0.04 M and 0.2 wt %, respectively, [APS]:[An] ) 1:1, at 0 °C for 48 h. Figure 6 shows the TGA (27) (a) Nagasawa, N.; Mitomo, H.; Yoshii, F.; Kume, T. Polym. Degrad. Stab. 2000, 69, 279. (b) Wasikiewicz, J. M.; Yoshii, F.; Nagasawa, N.; Wach, R. A.; Mitomo, H. Radiat. Phys. Chem. 2005, 73, 287. (28) (a) Athawale, A. A.; Kulkarni, M. V.; Chabukswar, V. V. Mater. Chem. Phys. 2002, 73, 106. (b) Chiou, N. R.; Epstein, A. J. AdV. Mater. 2005, 17, 1679. (c) Zhong, W. B.; Deng, J. Y.; Yang, Y. S.; Yang, W. T. Macromol. Rapid Commun. 2005, 26, 395.
3902 Langmuir, Vol. 22, No. 8, 2006
Yu et al.
Figure 3. Scanning electron microscopy images of polyaniline-sodium alginate nanofibers synthesized with different concentrations of aniline at 0 °C for 48 h: (a) [An] ) 0.04 M, (b) [An] ) 0.06 M, (c) [An] ) 0.08 M, (d) [An] ) 0.10 M. The content of SA is 0.2 wt %, and the molar ratio of APS to aniline is 1:1.
of the samples of SA, PANI-SA nanofibers, and pure PANI prepared without SA. Here, the first small fraction of weight loss from room temperature to ca. 100 °C arises mainly from the expulsion of absorbed water trapped in the samples. Pure PANI powder shows one principal weight loss between 330 and 700 °C (Figure 6a), corresponding to the complete decomposition of its backbone structure.29-31 In PANI-SA nanofibers (Figure 6b), an additional important weight loss of about 10-15% between 200 and 300 °C can be observed. Then a larger mass loss of PANI-SA nanofibers begins at 335 °C. By looking at the thermal behavior of SA (Figure 6c), sodium alginate showed only one weight loss step at 200-280 °C in the course of thermal degradation.32 It becomes clear that the additional weight loss in PANI-SA nanofibers is linked to the presence of SA in the PANI. The additional weight loss at about 240 °C for PANI-SA nanofibers can be attributed to the decomposition of alginate. The decomposition temperature of alginate in the PANI-SA (29) Kulkarni, V. G.; Campbell, L. D.; Mathew, W. R. Synth. Met. 1989, 30, 321. (30) Palaniappan, S.; Narayana, B. H. J. Polym. Sci., Part A: Polym. Chem. 1994, 32, 2431. (31) Wei, Y.; Hsueh, K. F. J. Polym. Sci., Part A: Polym. Chem. 1989, 27, 4351. (32) Liang, C. X.; Hirabayashi, K. J. Appl. Polym. Sci. 1992, 45, 1937.
nanofibers is higher than that of SA, indicating relatively high thermal stability because of the enhancement of intermolecular interaction between two polymers. Figure 7 shows the FTIR spectra for SA, pure PANI, and PANI-SA nanofibers in the region from 4000 to 400 cm-1. In the FTIR spectrum of SA (Figure 7a), the peaks around 3447, 1615, 1421, and 1035 cm-1 are attributed to the stretching of O-H, -COO- (asymmetric), -COO- (symmetric), and C-OC, respectively.33 FTIR spectra of both PANI-SA nanofibers (Figure 7b) and pure PANI (Figure 7c) exhibit absorption peaks corresponding to the stretching of quinonoid (1573 cm-1) and benzenoid (1494 cm-1) rings and to C-N and CdN stretching vibrations at about 1300 and 1120 cm-1, respectively, in good agreement with previous spectroscopic characterizations of the emeraldine salt of polyaniline.34-36 However, there are several differences between the FTIR spectrum of the PANI-SA nanofibers in the ES form and the (33) Sartori, C.; Finch, D. S.; Ralph, B.; Gilding, K. Polymer 1997, 38, 43. (34) Sun, Y.; MacDiarmid, A. G.; Epstein, A. J. J. Chem. Soc., Chem. Commun. 1990, 7, 529. (35) Wang, D.; Caruso, F. AdV. Mater. 2001, 13, 350. (36) Lei, Z.; Zhang, H.; Ma, S.; Ke, Y.; Li, J.; Li, F. Chem. Commun. 2002, 676.
Facile Synthesis of PANI-SA Nanofibers
Langmuir, Vol. 22, No. 8, 2006 3903
Figure 4. TEM image of polyaniline-sodium alginate nanofibers synthesized with different concentrations of SA at 0 °C for 48 h: (a) 0.2 wt %, (b) 0.1 wt %, (c) 0.05 wt %, (d) 0.025 wt %. The concentration of aniline is 0.04 M, and the molar ratio of APS to aniline is 1:1. (e) TEM image of polyaniline-sodium alginate nanoparticles synthesized with 0.2 wt % SA at room temperature for 24 h. The concentration of aniline is 0.04 M, and [APS]:[An] ) 1:1.
spectrum of the neat ES PANI. In the spectrum of PANI-SA nanofibers (Figure 7b), a new band at 1720 cm-1 is observed due to the asymmetric stretching of -COO- groups of SA. This -COO- stretch peak exhibits a large shift to higher wavenumbers, as well as a decrease in intensity. This peak is specific to ionic binding. As protonated amino groups (-NH2+-C6H4-) from ES of PANI replace sodium ions in the alginate blocks, the charge density, the radius, and the atomic weight of the cation are changed, creating a new environment around the carbonyl group. Hence, a peak shift would be expected.37 A striking difference between parts b and c of Figure 7 is found near 3400 cm-1. This signal is broad and strong in the PANI-SA nanofibers yet very weak in the pure ES polyaniline spectrum. To rule out moisture effects, spectra from identical samples saturated with water were compared as a function of time and drying. Water was completely removed under the drying conditions used for the samples. The evidence suggests that SA exists in the PANI-SA nanofibers. The spectra confirmed that the carboxylate groups of sodium alginate were dissociated to COO- groups which complexed with protonated amino groups from PANI-SA nanofibers through (37) Sartori, C.; Finch, D. S.; Ralph, B.; Gilding, K. Polymer 1997, 38, 43.
electrostatic interaction. The interaction may result in SA functioning as a chemical dopant for PANI. Moreover, as the PANI-SA nanofiber formation proceeded, O-H bonding would also be expected because of an increase in intermolecular interaction such as hydrogen bonding between sodium alginate and PANI.38 The ordered structures of pure PANI, SA, and PANI-SA nanofibers were studied by WXAD. Shown in Figure 8 are the typical powder diffraction patterns for pure PANI, SA, and PANI-SA nanofibers. One typical peak at 2θ ) 25° is observed for pure PANI. This peak may be ascribed to the periodicity parallel to the polymer chain.39 The diffraction of SA shows typical peaks around 14° and 23°.40 The spectrum for the PANISA nanofibers clearly reveals two new diffractions (namely, at 2θ angles of 14° and 23°). The new additional ordered structure is introduced by SA. Furthermore, the diffraction intensities of new peaks at around 14° and 23° increase drastically in the PANI-SA nanofibers and become the dominant feature in the composites. It is obvious that the crystallinities of PANI-SA (38) Kanti, P.; Srigowri, K.; Madhuri, J.; Smitha, B.; Sridhar, S. Sep. Purif. Technol. 2004, 40, 259. (39) Moon, Y. B.; Cao, Y.; Smith, P.; Heeger, A. J. Polym. Commun. 1989, 30, 196. (40) Zhou, J.; Zhang, L. J. Polym. Sci., Part B: Polym. Phys. 2001, 39, 451.
3904 Langmuir, Vol. 22, No. 8, 2006
Yu et al.
Figure 6. TGA of (a) pure PANI, (b) PANI-SA nanofibers, and (c) SA.
Figure 7. FTIR spectra of the (a) sodium alginate, (b) PANI-SA nanofibers, and (c) pure PANI.
Figure 5. UV-vis spectra of (a) SA dissolved in water, (b) the as-synthesized PANI-SA nanofibers dispersed in water, and (c) the ammonium hydroxide treated PANI-SA nanofibers dissolved in NMP.
nanofibers increased with the introduction of sodium alginate, which can be explained by the enhancement of intermolecular interaction. To further understand the formation mechanism of the PANISA nanofibers, the influence of the concentration of SA, aniline, and APS on the morphologies of the PANI-SA nanofibers was systematically investigated (reagents are added in the order of SA, aniline, and APS, with stirring for 20 min; the concentration of SA was 0.2 wt %). The results are briefly summarized as
follows: (1) if no SA is used in the reaction solution, only spherical PANI nanoparticles are found (shown in Figure 2); (2) when the concentration of aniline is between 0.04 and 0.10 M, PANI-SA nanofibers are obtained; (3) if the aniline is oxidized by FeCl3, only nanoparticles appear; (4) when the strong mineral acid dopant is added to the system, the morphology of PANI also transforms into particles. These changes in morphology of the synthesized PANI-SA nanofibers with the synthesis conditions suggest that the sodium alginate-aniline complexes play an important role in the formation of the polyaniline-sodium alginate nanofibers. On the basis of the above results, a model describing the mechanism of polyaniline-sodium alginate nanofiber formation using biopolymer SA is as follows: When sodium alginate and aniline are added in water, the sodium alginate acts as a template and then aniline with an amino group reacts/interacts with the
Facile Synthesis of PANI-SA Nanofibers
Langmuir, Vol. 22, No. 8, 2006 3905
biopolymer-monomer complexes might be connected by junctions. Eventually, the SA-aniline complexes could play a template-like role in the synthesis of PANI-SA nanofibers. When APS is added to the reaction solution, conjugation of the polyaniline chains is promoted with limited parasitic branching, thus forming the polyaniline-sodium alginate nanofibers.
4. Conclusions
Figure 8. X-ray diffraction patterns of (a) SA, (b) pure PANI, and (c) PANI-SA nanofibers.
SA with carboxylic groups. Upon the sodium alginate, the aniline monomers preferentially align to form biopolymer-monomer complexes. In addition, sodium alginate has many carboxyl groups and a -OH group. It is reasonable to expect that hydrogen bonds could be formed through the hydrogen and oxygen of the adjacent SA or via the -OH group of SA with the amine of aniline. Thus, the structure of a junction between two biopolymer-monomer complexes could be constructed, and then more and more
In summary, large quantities of uniform polyaniline-sodium alginate nanofibers were synthesized by a template-guided process in a dilute solution of sodium alginate. This approach is an easy, inexpensive, environmentally friendly, and scalable one-step method to produce uniform nanofibers with controllable average diameters in bulk quantities. It was found that the diameter of the PANI-SA nanofibers could be easily controlled by the concentration of aniline and SA. Experimental results showed biopolymer-monomer complexes composed of sodium alginate and aniline played an important role in the formation of PANISA nanofibers. The proposed method provides a wider, simpler, and repeatable route for the preparation of nanofibers of conducting PANI. Acknowledgment. We gratefully acknowledge financial support by the National Science Foundation of China (NNSFC; Grant Nos. 20374027, 50533020, and 90403013). LA051911V
