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Langmuir 2006, 22, 5952-5957
New Renewable Resource Amphiphilic Molecular Design for Size-Controlled and Highly Ordered Polyaniline Nanofibers P. Anilkumar and M. Jayakannan* Polymer Research Group, Chemical Sciences DiVision, Regional Research Laboratory, ThiruVananthapuram - 695019, India ReceiVed January 18, 2006. In Final Form: March 22, 2006 We demonstrate here, for the first time, a unique strategy for conducting polyaniline nanofibers based on renewable resources. Naturally available cardanol, which is an industrial waste and main pollutant from the cashew nut industry, is utilized for producing well-defined polyaniline nanofibers. A new amphiphilic molecule is designed and developed from cardanol, which forms a stable emulsion with aniline for a wide composition range in water (1:1 to 1:100 dopant/aniline mole ratio) to produce polyaniline nanofibers. The scanning electron microscopy and transmission electron microscopy analysis of the nanofibers reveals that the dopant/aniline ratio plays a major role in determining the shape and size of polyaniline nanofibers. The nanofiber length increases with the increase in the dopant/aniline ratio, and perfectly linear, well-defined nanofibers of lengths as long as 7-8 µM were produced. The amphiphilic dopant has a built-in head-to-tail geometry and effectively penetrates into the polyaniline chains to form highly organized nanofibers. Wide-angle X-ray diffraction (WXRD) spectra of the nanofibers showed a new peak at 2θ ) 6.3 (d spacing ) 13.9 Å) corresponding to the three-dimensional solid-state ordering of polyaniline-dopant chains, and this peak intensity increases with increase in the nanofiber length. The comparison of morphology and WXRD reveals that high ordering in polyaniline chains results in the formation of long, well-defined nanofibers, and this direct correlation for the polyaniline nanofibers with solid-state ordering has been established. The conductivity of the polyaniline nanofibers also increases with increase in the solid-state ordering rather than increasing with the extent of doping. The polyaniline nanofibers are freely soluble in water and possess high environmental and thermal stability up to 300 °C for various applications.
Introduction Polyaniline nanomaterials have gained significant interest in recent times because of their easy synthesis and potential application in sensors and electronic and optical devices.1-3 A large number of polymerization approaches such as “hard and soft” template,4,5 interfacial,6,7 rapid mixing,8 seeding,9 oligomerassisted,10 and dilute polymerization techniques11 have been reported for one- and three-dimensional (1D and 3D) polyaniline nanomaterials. Mineral acid-doped polyaniline nanofibers are very attractive, and their high solubility in water enhances applications in sensors; however, they have a few limitations, such as low yield, poor environmental stability, being highly amorphous due to poor solid-state ordering, and the need for excess mineral acids for their synthesis. On the other hand, polyaniline nanofibers based on aliphatic or aromatic sulfonic acids have high thermal and environmental stability compared to that of their inorganic counterparts.11-17 Most often, the * Corresponding author. Fax: 00914712491712. Tel: 00914712515316. E-mail:
[email protected]. (1) Janata, J.; Josowicz, M. Nat. Mater. 2002, 2, 19-24. (2) Haung, J.; Virji, S.; Weiller, B. H.; Kaner, R. B. Chem.sEur. J. 2004, 10, 1314. (3) 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, 604. (4) Wu, C. G.; Bein, T. Science 1994, 264, 1757. (5) Zhang, X.; Manohar, S. K. Chem. Commun. 2004, 2360. (6) Haung, J.; Virji, S.; Weiller, B. H.; Kaner, R. B. J. Am. Chem. Soc. 2003, 125, 314. (7) Haung, J.; Kaner, R. B. J. Am. Chem. Soc. 2004, 126, 851. (8) Haung, J.; Kaner, R. B. Angew. Chem. 2004, 116, 5941. (9) Zhang, X.; Goux, W. J.; Manohar, S. K. J. Am. Chem. Soc. 2004, 126, 4502. (10) Li, W.; Wang, H. L. J. Am. Chem. Soc. 2004, 126, 2278. (11) Chiou, N. R.; Epstein, A. J. AdV. Mater. 2005, 17, 1679. (12) Haung, K.; Wan, M. X. Chem. Mater. 2002, 14, 3486. (13) Zhang, L.; Long, Y.; Chen, Z.; Wan, M. X. AdV. Funct. Mater. 2004, 14, 693. (14) Zhang, L.; Wan, M. X. AdV. Funct. Mater. 2003, 13, 815.
synthesis of polyaniline nanofibers based on sulfonic acids was found to be highly susceptible to the dopant/aniline amount in the feed, and good and homogeneous nanofibers were produced only for selected compositions.12,14,16 The reason for the inhomogeneity in the nanofiber formation is associated with the poor micelle formation in water. It was noticed that the structural design of the sulfonic acid is very crucial in the stabilization of micelles in water for the successful growth of polyaniline nanofibers.12 Therefore, the design and development of new amphiphilic dopants for stable emulsion polymerization in water for a wide range of aniline/dopant compositions is a very important issue to be addressed. It is very important to stabilize the polymerization in water for precise control of polyaniline nanofiber dimensions and also for reproducing nanomaterial synthesis. In general, the 3D solid-state ordering of the polyaniline chains was reported as being a very crucial factor in obtaining good properties such as high conductivity.18,19 The importance of solid-state ordering has been realized in polyaniline, and approaches such as hydrogen-bonding interactions or the utilization of 4-hexylresorcinol and m-cresol as solvents for camphorsulfonic acid (CSA) supramolecules have been reported.20-24 Nandi and co-workers reported sulfonic acid-doped thermo(15) Wei, Y.; Zhang, L.; Yu, M.; Yang, Y.; Wan, M. X, AdV. Mater. 2003, 15, 1382. (16) Wei, Y.; Wan, M. X. AdV. Mater. 2002, 14, 1314. (17) Wei, Y.; Wan, M. X.; Lin, T.; Dai, L. AdV. Mater. 2003, 15, 136. (18) Banka, E.; Luzny, W. Synth. Met. 1999, 101, 715. (19) Lunzy, W.; Banka, E. Macromolecules 2000, 33, 425. (20) Stockton, W. B.; Rubner, M. F. Macromolecules 1997, 30, 2717. (21) Pouget J. P.; Oblakowski, Z.; Nogami, Y.; Albouy, P. A.; Laridjani, M.; Oh, E. J.; Min, Y.; MacDiarmid, A. G.; Tsukamoto, J.; Ishiguro, T.; Epstein, A. J. Synth. Met. 1994, 65, 131. (22) Vikki, T.; Pietila, L. O.; Osterholm, H.; Ahjopalo, L.; Takala, A.; Toivo, A.; Levon, K.; Passiniemi, P.; Ikkala, O. Macromolecules, 1996, 29, 2945. (23) Kosonen, H.; Ruokolainen, J.; Knaapila, M.; Torkkeli, M.; Jokela, K.; Serimaa, R.; ten Brinke, G.; Bras, W.; Monkman, A. P.; Ikkala, O. Macromolecules 2000, 33, 8671.
10.1021/la060173n CCC: $33.50 © 2006 American Chemical Society Published on Web 05/19/2006
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reversible polyaniline gels from formic acid solutions. It was noticed that the formation of new unit cells in the lamella due to the microcrystallization of elongated surfactant molecules and the lamellar thickness is correlated to the cohesive force of the surfactant molecules within the lamella.25-27 The average fibrillar diameter and length of the thermoreversible gel based on the 2,7-dinonyl naphthalene 4,5-disulfonic acid systems were reported as ∼13 and 165 nm, respectively.28 Recently, we reported a systematic approach to producing ordered (crystalline), submicron-sized, and morphologically uniform polyaniline-doped materials using structurally different dopants in various polymerization methodologies such as interfacial and emulsion.29 We found that both the molecular design of the dopant and the polymerization methodologies play a significant role in ordering the polyaniline chains for higher conductivity. Although a substantial amount of information is known about the solid-state ordering of polyaniline chains via the simple doping approach, so far, much less effort has been devoted to the synthesis of highly ordered polyaniline nanofibers with good morphology and precise control over the structure-property relationships. Here, we report a new molecular design for addressing both the solid-state ordering of polyaniline nanomaterials and the tolerance for a larger window of dopant/aniline compositions for controlling the size and shape of the nanofibers. We have adopted a renewable resource strategy for the new molecular design because conducting polymers or nanomaterials from natural resources are very attractive due to their wide availability and lower cost compared to petroleum-based products. Additionally, the idea of interconnecting both renewable resources and conducting nanomaterials is very new and has tremendous opportunities for fundamental and applied research. Lignosulfonic acid, a waste from the paper pulp industry, is used for the preparation of conducting polyaniline, polyaniline-ferromagnetic composites, and self-assembled films of poly(o-ethoxyaniline), and the exploration of polyaniline for biomedical applications.30-33 However, so far, there have been no attempts to synthesize and control the properties of polyaniline nanofibers based on lignosulfonic acid. The present approach is the first example of high-performance nanostructured conducting materials based on renewable resourcessmore specifically, polyaniline nanofibers. The renewable resource raw material cardanol, which is an industrial waste and pollutant from the cashew nut industry, is utilized for new dopant synthesis. The new renewable resource dopant has an built-in head-to-tail molecular design for an amphiphilic nature and forms a stable emulsion with aniline in water in wider compositions. Well-defined polyaniline nanofibers, with precise control over the size, shape, water solubility, and high solid-state ordering, were prepared by controlling the composition of the amphiphilic dopant/aniline ratio from 1 to 100 mol %. The doped materials were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared (FT-IR), ultraviolet-visible (UV-vis) spectroscopy, thermogravimetric analysis (TGA), (24) Moon, H. S.; Park, J. K. J. Polym. Sci., Part A: Polym. Chem. 1998, 36, 1431. (25) Jana, T.; Nandi, A. K. Langmuir 2000, 16, 3141. (26) Jana, T.; Nandi, A. K. Langmuir 2001, 17, 5768. (27) Jana, T.; Chatterjee, J.; Nandi, A. K. Langmuir 2002, 18, 5720. (28) Jana, T.; Roy, S.; Nandi, A. K. Synth. Met. 2003, 132, 257. (29) Jayakannan, M.; Annu, S.; Ramalekshmi, S. J. Polym. Sci., Part B: Polym. Phys. 2005, 43, 1321. (30) Viswanathan, T.; Berry, B. US Patent 6764617, 2004. (b) Bourdo, S. E.; Berry, B. C.; Viswanathan, T. J. Appl. Polym. Sci. 2005, 98, 29. (31) Paterno, L. G.; Mattoso, L. H. C. Polymer 2001, 42, 5239. (32) Nikolaidis, G. M.; Sejdic, T. J.; Bowmaker, G. A.; Cooney, R. P.; Kilmartin, P. A. Synth. Met. 2004, 140, 225. (33) Jayakannan, M.; Amrutha, S. R.; Sindhu, K. V. J. Appl. Polym. Sci. 2006, in press.
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wide-angle X-ray diffraction (WXRD), and four-probe conductivity measurements. The mechanism of nanofiber formation was studied, and the influence of the amphiphilic dopant on the morphology, solid-state ordering, and conductivity were also investigated in detail. The relationship between the solid-state ordering with the morphology and conductivity of the polyaniline nanofibers was also established. Experimental Section Materials. Aniline, ammonium persulfate (APS), sulfanilic acid, hydrochloric acid, and sodium hydroxide were purchased locally and purified. Cardanol was purified by double-vacuum distillation at 3-4 mm of Hg, and the fraction distilled at 220-235 °C was collected.34 Measurements. NMR spectra of the compounds were recorded using a 300-MHz Brucker NMR spectrophotometer in d6-dimethyl sulfoxide (DMSO) containing small amount of tetramethylsilane (TMS) as the internal standard. Infrared spectra of the polymers were recorded using an Impact 400 D Nicolet FT-IR spectrophotometer in the range of 4000-400 cm-1. The purity of the compounds was determined by fast atom bombardment high-resolution mass spectrometry (FAB-HRMS; JEOL JSM600). For conductivity measurements, the polymer samples were pressed into a 10 mm diameter disk and analyzed using a four-probe conductivity instrument by applying a constant current. The resistivity of the samples was measured at five different positions, and at least two pellets were measured for each sample; the average of 10 readings was used for the conductivity calculations. The thermal stability of the polymers was determined using a TGA-50 Shimadzu thermogravimetric analyzer at a heating rate of 10 °C/min in nitrogen. For the SEM measurements, polymer samples were subjected to a thin gold coating using a JEOL JFC-1200 fine coater. The probing side was inserted into a JEOL JSM- 5600 LV scanning electron microscope for taking photographs. WXRDs of the finely powdered polymer samples were recoded by a Philips analytical diffractometer using Cu KR emission. The spectra were recorded in the range of 2θ ) 0-50 and analyzed using X’Pert software. UV-vis spectra of the polyaniline nanofiber (PANI) in water and drop-cast polyaniline samples on a glass plate were recorded using a Perkin-Elmer Lambda-35 UV-vis spectrophotometer. TEM images were recorded using a Hitachi H-600 instrument at 75 kV. For TEM measurements, a water suspension of nanofibers was prepared under ultrasonic conditions and deposited on a Formvar-coated copper grid. Synthesis of 4-[4-Hydroxy-2((Z)-pentadec-8-enyl)phenylazo]benzenesulfonic Acid (Dopant 1). Sulfanilic acid (31.5 g, 0.18 mol) and sodium carbonate (7.95 g, 0.08 mol) were added into 300 mL of water and heated to 60-70 °C to dissolve the entire solid. It was then cooled to 5 °C, and a cold solution of sodium nitrite (11.1 g, 0.16 mol) in water (32 mL) was added. The resultant yellow solution was poured into ice (200 g) containing concentrated HCl (31.5 mL) and stirred using a mechanical stirrer for 30 min at 5 °C. It was then added into a flask containing sodium hydroxide (18 g, 0.45 mol) and distilled cardanol (45 mL, 0.15 mol) in water (150 mL). The coupling reaction was continued with stirring for 3 h in the ice-cold conditions using a mechanical stirrer. The reaction mixture was neutralized by the addition of concentrated HCl (150 mL) in crushed ice (300 g). The red precipitate was filtered using a Buchner funnel and washed with water. It was further purified by recrystallization from a chloroform/methanol mixture. The dried product weighed 64.18 g (88% yield). Melting point: 205-207 °C. 1H NMR (d -N,N-DMSO, δ): 7.75 ppm (b, 4H, Ar-H), 7.57 ppm 6 (d, 1H, Ar-H), 7.75 ppm (s, 1H, Ar-H), 7.70 ppm (d, 1H, Ar-H), 5.24 ppm (b, 2H, CHdCH), 3.1-0.6 ppm (m, 27H, aliphatic-H). 13C NMR (d -N,N-DMSO, δ): 161.25, 152.59, 149.14, 145.82, 6 142.98, 129.69, 126.75, 121.76, 116.88, 116.36, 114.23, 31.75, 31.24, 30.77, 29.14, 28.85, 28.59, 28.38, 26.70, 26.62, and 25.33. FT-IR (KBr): 3006.7, 2923.6, 2852.9, 1600.1, 1533.7, 1498.7, 1369.3, 13338.7, 1236.9, 1174.7, 1116.5, 1033.2, 1007.6, 820.1, 706.4, and (34) Santos, M. L.; Magabhaes, G. C. J. Braz. Chem. Soc. 1999, 10, 13.
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Table 1. Conductivity, Dimensions, and WXRD Values of Polyaniline Nanofibers Prepared from Renewable Resource Dopant 1 size of nanofibersa sample PANI-1 PANI-2 PANI-3 PANI-4
dopant/aniline (moles) 1:1 1:10 1:50 1:100
yield (%)
c
d
e
peaks in WXRDb
S/N ratio (%)
σ (S/cm)
diameter (nm)
length (µm)
peak at 2θ
d-spacing (Å)
39.4 34.8 30.1 26.7
6× 0.9 × 10-2 1.0 × 10-2 1.1 × 10-2
spheres 130-160 160-200 180-260
dendritic 0.90-1.85 0.90-2.60 2.10-8.00
20.3, 25.1 6.3, 20.1, 25.1 6.5, 20.1, 25.6 6.4, 20.1, 25.6
4.4, 3.6 13.9, 4.4, 3.6 13.6, 4.4, 3.5 13.7, 4.4, 3.5
16 78 83 76
10-5
a Calculated from the SEM images. b Determined by WXRD technique at 30 °C. c Calculated for isolated product. d Determined from the elemental analysis by following standard procedure (ref 17). e Determined using four-probe conductivity measurement units at 30 °C.
Scheme 1. Synthesis of Dopant 1 and Preparation of Polyaniline Nanofibers
559.3 cm-1. UV-vis (CH3OH): λmax ) 336 nm. FAB-HRMS (MW ) 486.0) m/z: 486.3 (M+). Preparation of Polyaniline Nanofibers (PANI-1 to PANI-4). The synthesis of polyaniline nanofibers is described in detail for PANI-1, and the other nanofibers were prepared following the same procedure. Dopant 1 (5.30 g, 11 mmol) was dissolved in doubly distilled water (20 mL) and stirred under ultrasonic conditions for 1 h at 30 °C. Distilled aniline (1 mL, 1.02 g, 11 mmol dopant/aniline ) 1:1) was added to the dopant solution and stirred under ultrasonic conditions for an additional 1 h at 30 °C. At the end of the stirring, the formation of emulsion was noticed. APS (1.1 M solution) was added at 5 °C and stirred under ultrasonic conditions for 1 h at 5 °C. The resultant green content was allowed to stand at 5 °C for 15 h without disturbing. The solid mass was filtered and washed with distilled water, methanol, and diethyl ether several times until the filtrate became colorless. The solid product was dried in a vacuum oven at 60 °C for 48 h (0.01 m Hg). Yield ) 1.00 g (16%). FT-IR (cm-1): 3010.8, 1579.7, 1500.6, 1305.8, 1224.8, 1159.2, 1031.9, 825, 705.9, and 628.7. PANI-2, PANI-3, and PANI-4 were synthesized by following the same procedure but varying the dopant/aniline ratio in the feed to 1:10, 1:50, and 1:100, respectively. The yields of the nanofibers are given in the Table 1.
Results and Discussion Cardanol is a phenolic compound containing an unsaturated C15-alkyl chain at the meta position and is available largely as waste from the cashew nut processing industry. The synthesis of sulfonic acid derivatives through direct sulfonation at the phenyl ring was not possible because of the presence of unsaturated double bonds in the pendent chains. Therefore, we adopted a diazotization coupling reaction in water using the diazonium salt of sulfanilic acid to prepare a novel sulfonic acid dopant, dopant 1.35 The synthesis of dopant 1 is represented in Scheme 1. The structure of dopant 1 was confirmed by NMR,
FT-IR, and mass techniques. The 1H NMR spectrum of dopant 1 is given in Figure 1, and all the different types of protons in the structure are designated with letters. The four protons in the aromatic ring containing the sulfonic acid group appear together at 7.8 ppm, whereas the aromatic protons from the other ring appear with a characteristic splitting pattern at 7.6, 6.85, and 6.80 ppm. The double bond in the pendent group appears at 5.4 ppm, and all other aliphatic protons appear below 3.2 ppm. The 13C NMR spectrum also has the required amount of peaks for dopant 1. The peak intensities are in accordance with the expected structure, which confirms the formation of the expected amphiphilic dopant. Dopant 1 has a unique built-in amphiphilic design in which the hydrophilic sulfonic acid part behaves as a polar head, and the long alkyl chain as behaves as a hydrophobic tail. The free phenolic group (-OH) in the polar head also provides a handle for hydrogen-bonding interactions for the dopant molecule with the polyaniline backbone. The new dopant 1 is freely soluble in water and various water-loving organic solvents, which is an added advantage for preparing polyaniline nanostructures under various conditions. The polyaniline nanofibers PANI-1 to PANI-4 were prepared by varying the dopant/aniline ratio in the feed to 1:1, 1:10, 1:50, and 1:100 (in moles), respectively. The amphiphilic dopant 1 forms an emulsion with aniline in water, which behaves as a self-assisted template for the growth of nanofibers. The polymerization was carried out by oxidizing the micelles using APS as the oxidizing agent in water. PANI-1 was obtained as a brownish-green solid in low yield, whereas the others were obtained in good yield as green solids (see Table 1). The dopant/ aniline ratio in the feed significantly influences the solubility of anilinium salt as well as the stability of the micelles in water. (35) Jayakannan, M.; Pillai, C. K. S.; Dhawan, S. K.; Radhakrishnan, S.; Trivedi, D. C. Indian Patent Application 3488 DEL 2005, December 27, 2005.
Renewable Resource-Based Polyaniline Nanofibers
Figure 1. 1H and 13C NMR spectra of dopant 1 in d6-DMSO.
Figure 2. FT-IR spectra of dopant 1, PANI-EB, and the polyaniline nanofibers.
At higher dopant concentrations (for 1:1 and >1:1 of dopant/ aniline) in the feed, the anilinium salt was found to be partially soluble in water, and the entire polymerization mixture appeared as a mixture of emulsion plus precipitate. At lower dopant concentrations, the anilinium salt in water was observed as a stable emulsion, and the stabilization increased with increase in the dopant/aniline ratio (1:10 to 1:100). The stability of the anilinium salt significantly affects the yield of the polyaniline nanofibers, and relatively low yield was obtained for the 1:1 composition compared to that of other compositions. The polyaniline nanofibers were purified very well by continuous washing with water and methanol. The unreacted aniline, dopant 1, anilinium salts, and oligomers were freely soluble in these solvents; therefore washing with these solvents improved the purity of the polyaniline nanofibers. Furthermore, the polyaniline nanofibers were also subjected to FT-IR analysis (Figure 2) to understand the doping behaviors. Four new peaks are present in the doped samples at 1307, 1026, 830, and 630 cm-1 for the OdSdO (sym) and NH+‚‚‚SO3- interactions between the polymers chain and dopant 1, and the S-O (unsym) and C-S stretching vibrations, respectively.29 Interestingly, the peak intensities at 1026 and 630 cm-1 increase with decrease in the dopant/aniline ratio in the feed, which confirms that all the samples were effectively doped by dopant 1. The degree of doping was calculated from the elemental analysis of the doped materials, and the sulfur/nitrogen ratios were obtained as 39.4, 34.8, 30.1, and 26.7% for PANI-1 to PANI-4, respectively (see Table 1). The values are in the range of those reported for the sulfonic acid-doped polyaniline, and, as expected, the degree of doping decreases with a decrease in the amount of dopant in the feed.
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The thermal analysis indicated that the polyaniline fibers were very stable up to 300 °C and therefore useful for high-temperature applications. The morphologies of the nanomaterials were recorded using a JEOL JSM-5600 LV scanning electron microscope, and the SEM images of PANI-1 to PANI-4 are given in Figure 3. The average length and diameters of the nanofibers were determined from the SEM images and summarized in Table 1. It is very clear from the images that the dopant/aniline ratio plays a significant role in the formation of polyaniline nanofibers. PANI-1 has both microspheres of 1-2 µm diameter and short dendritic nanofibers. It was observed that, at higher dopant concentrations (for 1:1), the dopant-anilinium salts form inhomogeneous emulsions, and the subsequent oxidation leads to mixtures of microspheres and nanofibers. The samples PANI-2 to PANI-4 did not show any traces of microspheres, which indicates that, at lower dopant concentrations, the anilinium salts form homogeneous micelles in water, which in turn produce only nanofibers. The nanofiber length and diameter are almost the same in PANI-2 and PANI3, but the images reveal that the former has a more dendritic nature compared to the latter. The dendritic nanofibers are formed by the aggregation of micelles in PANI-2 because of the higher anilinium concentration in water. Interestingly, further dilution of the dopant in the feed (from 1:10 to 1:100) results in the isolation of micelles in water, leading to linear nanofibers in PANI-3 and PANI-4. In PANI-4, perfectly linear nanofibers were observed, and their lengths were as long as 7-8 µm, the longest polyaniline nanofibers reported so far in the literature for sulfonic acid dopants. With the decrease in the dopant concentration in the feed, the diameter of the nanofibers increased from 130 to 260 nm, and their length also increased from 1.0 to as long as 8.0 µm (see Table 1). Polyaniline nanofibers using HCl instead of the new dopant 1 were also synthesized, and it was found that the morphological features of the HCl-doped nanofibers are comparable to those reported by others under identical experimental conditions (see Supporting Information).9,36-38 This indicates that the experimental conditions adopted in the present approach are very good for polyaniline nanomaterials, and the differences in the morphologies of PANI-1 to PANI-4 are less significant to the experimental conditions. The nanofiber thickness is much higher than that reported for the interfacial route,10 but the linear shape and length is much more superior compared to many sulfonic acid-based examples.16 It was reported earlier that the sulfonic acid-doped polyaniline nanomaterials prepared through emulsion polymerization were highly sensitive to micelle formation, and either nanotubes, nanofibers, or mixtures of both were produced, depending upon the nature and stability of the micelles in water.39,40 Since the SEM technique is not adequate to differentiate between nanotubes and nanofibers, we subjected the samples to TEM analysis. PANI-4 was dispersed in water under ultrasonic conditions, and a drop of the solution was evaporated on the Cu grid for TEM analysis. The TEM images are shown in Figure 4. It is very clear from the TEM-images that all of the nanomaterials have a uniform fibrous morphology, and their diameters and lengths are in the ranges of 120-200 nm and 800 nm to 1.2 µm, respectively. A comparison of the TEM and SEM images reveals that the nanofiber diameters are almost matching, but their lengths were somewhat shorter in the TEM images. We assume that the nanofiber length was disturbed by (36) Chiou, N. R.; Epstein, A. J. Synth. Met. 2005, 153, 69. (37) Jing, X.; Wang, Y.; Wu, D.; She, L.; Guo, Y. J. Polym. Sci., Part A: Polym. Chem. 2006, 44, 1014. (38) Li, D.; Kaner, R. B. J. Am. Chem. Soc. 2006, 128, 968. (39) Wei, Z.; Zhang, Z.; Wan, M. Langmuir, 2002, 18, 917-921. (40) Zhang, Z.; Wei, Z.; Wan, M. Macromolecules 2002, 35, 5937-5942.
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Figure 3. SEM images of PANI-1 (a), PANI-2 (b), PANI-3 (c), and PANI-4 (d).
Figure 4. TEM images of PANI-4. The samples were prepared by being dissolved in water and evaporated on a grid.
Figure 5. UV-vis spectra of dopant 1 and polyaniline nanofibers in water at 30 °C.
the mode of sample preparation for TEM analysis, which is commonly observed by others.39 Interestingly, the amphiphilic dopant is very efficient in forming the homogeneous micelles and producing only nanofibers, and there are no particles or tubes observed in the entire matrix. The nanofibers were freely soluble in water by simple mixing under ultrasonic conditions at ambient temperature, and the resultant solutions were green. The UV-vis spectra of the samples were recorded in water and are shown in Figure 5. In dopant 1, the peaks at 240 and 350 and the shoulder at 450 nm correspond to the π-π* (cis), π-π* (trans), and n-π* (cis) of the azobenzene linkages, respectively. The absorption spectra of PANI-1 to PANI-4 are free from quinoid rings (at 650 nm), which confirms the efficient doping of 1. PANI-2 to PANI-4 showed two distinct peaks at 420-450 nm and at the near-IR region above 850 nm, corresponding to the formation of polaron species in the nanofibers.6 The solid-state UV-vis spectra (not shown) also showed a trend similar to that observed in solution, which indicates that the difference among the samples directly arose from the difference in their doping behavior. To understand the reversibility of the doping behavior of the polyaniline nanofibers, control experiments were carried out in water, and their UV-vis spectra were recorded (available in the Supporting Information). The polyaniline nanofibers were dedoped by adding aq NH3 into the PANI-3 and PANI-4 solutions. The color of the solution
immediately changed to blue, corresponding to the formation of a polyaniline emeraldine base, and the UV-vis spectra showed a new peak at 650 nm corresponding to the quinoid ring in the emeraldine base. The blue solution was redoped followed by the addition of HCl. The color of the solution immediately turns green, and the peak for the polaron species reappeared at 850 nm, corresponding to the formation of emeraldine salt. This experiment confirms that the optical properties of the polyaniline nanofibers are completely reversible in nature. The conductivity of the polyaniline nanofibers was determined by four-probe conductivity measurements for compressed pellets at room temperature. PANI-1 has very low conductivity in the range of 6.5 × 10-5 S/cm, whereas nanofibers PANI-2 to PANI-4 showed an increase in the conductivity, and the values were obtained as (0.9-1.1) × 10-2 S/cm. The conductivity of PANI-2 to PANI-4 is a little lower than that of conventional doped systems (100 S/cm), which may be due to the lower concentration of the dopant in the matrix.35 A similar trend of low conductivities for polyaniline nanofibers based on sulfonic acid dopants was also noticed by other researchers.15,25 Interestingly, in the present investigation, the morphology of the nanofibers played a major role in their conductivity values. The low conductivity of the PANI-1 is also supported by the absence of polaron peaks in the UV-vis spectra. The conductivity of PANI-1 is very low, despite the fact that the dopant concentration is relatively high compared
Renewable Resource-Based Polyaniline Nanofibers
Figure 6. WXRD patterns of dopant 1 and polyaniline nanofibers.
to the other samples (S/N ratio, Table 1), which indicates that the extent of doping is not directly increasing the conductivity of the polyaniline nanofibers. It is also very evident from the SEM images that PANI-1 has poor morphology compared to those of PANI-2 to PANI-4, and therefore the morphology plays the major role in the solid-state properties such as the conductivity of the polyaniline nanofibers rather than the extent of doping. To check the reversibility of the electronic properties of the polyaniline nanofibers, control experiments were also performed on compressed pellets of PANI-2 to PANI-4 (see table in the Supporting Information). The pellets were dedoped by stirring in 10 mL of a 25% solution of aq NH3 for 1 h, and dried, and the conductivities were measured. As expected, the polyaniline nanofibers were dedoped completely, and the color of the pellets was changed to blue, indicating the formation of emeraldine base. The conductivity of the dedoped pellets was dropped by 1000 times (in the range of 10-5-10-6 S/cm) compared to that of the original (in the range of 10-2 S/cm) The blue pellets were redoped by treating with 10 mL of 50% v/v HCl solution for 1 h. The colors of the pellets were turned into the original green, and the conductivity values showed a 100-fold (in the range of 10-2-10-3 S/cm) increase compared to the dedoped pellets. The decrease in the conductivity upon dedoping (by aq NH3) and increase followed by redoping (by HCl) confirm the complete reversibility of the polyaniline nanofibers. The combination of UV-vis and conductivity measurements reveals that the electronic and optical properties of the polyaniline nanofibers are reversible and intrinsic to the polymer backbone. To further investigate the solid-state properties of polyaniline nanofibers, they were finely powdered and subjected to WXRD analysis. In general, a polymer chain has both amorphous and crystalline domains in the matrix, and the percentage of the respective domains varies depending upon their backbone. A polyaniline backbone is highly rigid because of its linear structure and less flexible to chain folding to induce a crystalline domain. Therefore, undoped polyaniline chains (emeraldine base) are normally observed as highly amorphous polymers.29 In the presence of sulfonic acids, the dopant-polymer undergoes various interactions, which tend to organize the polymer chains in 3D highly ordered fashions. The highly organized and ordered structures reflect on the low angle region of the WXRD plot (low 2θ values, higher d spacing).29 The WXRD patterns of polyaniline nanofibers and the dopant are shown in Figure 6. It is very clear from the plots that all the doped samples were free from the peaks corresponding to the free dopant, which confirm their high purity. The WXRD patterns of nanofibers (Figure 6) showed three distinct peaks at 2θ ) 6.4, 20.1, and 25.5 (d spacing )13.6, 4.4, and 3.5 Å). The two peaks at 20.1 and 25.5 are generally observed in doped polyaniline, but the new peak at 2θ ) 6.4 is only observed for highly ordered samples in which the polyaniline interplanar distance increased by the effective penetrating of
Langmuir, Vol. 22, No. 13, 2006 5957
dopant molecules. The new peak at 2θ ) 6.4 arises from the formation of a lamella between the polyaniline chains. The lamella is formed because of the interdigitations and crystallization of the side chains of the comb-shaped doped polyaniline.25-27 In PANI-1, the new peak is absent, indicating less ordering of nanofibers because its interlamellar peak is not distinct for the same kind of mixed structures (spheroids), which is further evident from its SEM photographs. The increase in the peak intensity at 2θ ) 6.4 (d ) 13.6 Å) from PANI-2 to PANI-4 indicates that the solid-state ordering in the polyaniline chains increases with increase in the nanofiber length. The comparison of SEM and WXRD reveal that the highly ordered polyaniline chains produce longer nanofibers. This direct correlation for the polyaniline nanofibers with solid-state ordering has been seen for the first time for polyaniline nanofibers. Therefore, the high solid-state ordering of polyaniline nanofibers in PANI-2 to PANI-4 contributing to their higher conductivity compared to PANI-1, despite the degree of doping, follows the opposite trend. It suggests that the morphology as well as the solid-state ordering of polyaniline play a major role in increasing the conductivity rather than the extent of doping. A longer polyaniline nanofiber length results in more electrical conductivity.
Conclusion In summary, we have developed a renewable resource strategy for conducting highly ordered and water soluble polyaniline nanofibers directly from an industrial waste, the renewable resource cardanol. The present approach is demonstrated for a larger window of dopant/aniline ratios in the feed under ecofriendly conditions, and the dimensions and solid-state ordering of the polyaniline nanofibers are systematically controlled synthetically by adjusting the reactants in the feed. The present approach has many advantages: (i) it is the first demonstration, in general, of renewable resource-based conducting nanomaterials; (ii) the dopant has a built-in head-to-tail amphiphilic nature for producing micelles, and no additional emulsifiers or templates are required; (iii) cardanol is available in abundance as industrial waste, it is cheap, and its utilization reduces environmental pollution; and (iv) the resulting polyaniline nanofibers are linear, longer, thin, highly ordered, conducting, thermally stable, and water soluble. The polyaniline nanofibers are freely soluble in water and bear optically switchable azo groups, and therefore they are suitable for various applications in biosensors and electronic, electrical, and optical devices. The dopant also has free phenolic functionality, which may be very attractive in substituting hydrophilic or hydrophobic units to fine-tune polyaniline nanostructured materials. In a nut shell, the present communication demonstrates the feasibility for highly ordered and size-controlled polyaniline nanofibers by completely adopting a renewable resource strategy. Acknowledgment. We thank KSCTSE, Thiruvananthapuram, Kerala, India (082/SRSPS/2004/CSTE), and the CSIR Task Force (COR 0004), New Delhi, India, for financial support. The authors thank Dr. Peter Koshy, Mr. M. R. Chandran, Dr. U. Syamaprasad, and Mr. P. Gurusamy at RRL-Trivandrum for SEM and WXRD analysis. We also thank Dr. Annie John, SCTIMST, Trivandrum, for TEM analysis. P.A. thanks UGC-New Delhi, India, for junior research fellowship. Supporting Information Available: SEC plot, TGA plot, solidstate UV-vis spectra, dedoping and redoping UV-vis spectra, and a table containing the conductivity of dedoped and redoped samples. This material is available free of charge via the Internet at http://pubs.acs.org. LA060173N
