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Langmuir 2006, 22, 11384-11387
Preparation of Submicron Polypyrrole/Poly(methyl methacrylate) Coaxial Fibers and Conversion to Polypyrrole Tubes and Carbon Tubes Hong Dong and Wayne E. Jones, Jr.* Department of Chemistry and Institute for Materials Research, State UniVersity of New York at Binghamton, Binghamton, New York 13902 ReceiVed May 17, 2006. In Final Form: September 12, 2006 In an effort to prepare electrically conductive nanofiber and nanotube materials, polypyrrole/poly(methyl methacrylate) coaxial fibers have been prepared using polymer fibers produced from an electrospinning process. Poly(methyl methacrylate) (PMMA) fibers with an average diameter of 230 nm were initially fabricated by electrospinning as core materials. The PMMA fibers were subsequently coated as templates with a thin layer of polypyrrole (PPy) by in-situ deposition of the conducting polymer from aqueous solution. Hollow PPy tubes were produced by dissolution of the PMMA core from PPy/PMMA coaxial fibers. High-temperature (1000 °C) treatment under inert atmosphere converted PPy/PMMA coaxial fibers into carbon tubes by complete decomposition of PMMA fiber core and carbonization of the PPy wall. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), and FT-IR spectroscopy confirmed the formation of the PPy/PMMA coaxial fibers, PPy tubes, and carbon tubes.
1. Introduction Nanofiber and nanotube materials are predicted to possess unique physical, chemical, and electronic properties. Conducting polymer nanofibers and nanotubes have attracted considerable interest for future applications in nanoelectronic junctions and devices because of their improved electronic properties.1 Polypyrrole (PPy) has been studied in particular among conducting polymers because of its high electrical conductivity and good stability under ambient conditions. Various methods for the preparation of polypyrrole nanotubes have been reported. Porous membranes such as anodic alumina membranes and track-etched polycarbonate membranes have been often used as templates to fabricate fibrils and tube materials of conducting polymers including polyaniline, PPy, and polythiophene.2-4 Fibrils and tubes were obtained by filling the nanopores of the membrane through chemical or electrical polymerization and subsequently removing the membrane. Template-free methods, such as selfassembly using large organic dopant anions such as β-naphthalenesulfonic acid during the synthesis5 and reverse microemulsion polymerization,6 have also been utilized to prepare PPy microtubes and nanotubes. A recent interest in our group has focused on the use of a fiber-templating approach for the preparation of electrically conductive coaxial nanofiber and nanotube materials. The tubes by fiber template (TUFT) was first proposed by Bognitzki et al. and Hou et al.7,8 This method involves the fabrication of polymer fibers using a simple, electrostatic method known as electro* Author to whom correspondence should be addressed. Tel.: (607) 7772421. Fax: (607) 777-4478. E-mail:
[email protected]. (1) MacDiarmid, A. G. Angew. Chem., Int. Ed. 2001, 40, 2581. (2) Martin, C. R. Acc. Chem. Res. 1995, 28, 61. (3) Menon, V. P.; Lei, J.; Martin, C. R. Chem. Mater. 1996, 8, 2382. (4) Joo, J.; Park, K. T.; Kim, B. H.; Kim, M. S.; Lee, S. Y.; Jeong, C. K.; Lee, J. K.; Park, D. H.; Yi, W. K.; Lee, S. H.; Ryu, K. S. Synth. Met. 2003, 135, 7. (5) Liu, J.; Wan, M. J. Mater. Chem. 2001, 11, 404. (6) Jang, J.; Yoon, H. Chem. Commun. 2003, 6, 720. (7) Bognitzki, M.; Hou, H.; Ishaque, M.; Frese, T.; Hellwig, M.; Schwarte, C.; Schaper, A.; Wendorff, J. H.; Greiner, A. AdV. Mater. 2000, 12, 637. (8) Hou, H.; Jun, Z.; Reuning, A.; Schaper, A.; Wendorff, J. H.; Greiner, A. Macromolecules 2002, 35, 2429.
spinning that has been described elsewhere.9-11 The long fibers produced have diameters ranging from several microns down to tens of nanometers, and the diameter is controllable by adjusting the physical parameters of the initial polymer solution.12,13 The deposition of different wall materials on the surface of the ultrafine fibers was achieved by using various techniques including chemical vapor deposition, sol-gel, in-situ polymerization, and electroless plating. Selective removal of the fiber core leads to the formation of tube materials.7 We have successfully synthesized conductive coaxial fibers and tubes of polyaniline13,14 by in-situ deposition and metal (e.g., Au, Cu, Ni) tubes15 by electroless plating using this fiber-templating approach. Conductive poly(3,4-ethylenedioxythiophene) (PEDOT) nanotubes have also been prepared by electrochemical deposition of PEDOT around electrospun polymer fibers.16 Compared to other methods, this approach provides a simple method for relatively large-scale fabrication of conductive coaxial fiber and tube materials. In this report, we describe the preparation of polypyrrole/ poly(methyl methacrylate) (PPy/PMMA) coaxial fibers using electrospun polymer fibers as templates and subsequent formation of PPy tubes and carbon tubes. Poly(methyl methacrylate) (PMMA) was fabricated as the fiber core by electrospinning. The preparation of electrically conductive PPy/PMMA coaxial fibers was achieved by depositing the conducting polymer PPy on the surface of the PMMA fibers via in-situ polymerization in aqueous solution. In-depth characterization of the products of the solvent extraction and thermal treatment of these composite fibers provides insight into the formation of both conducting polymer and carbon-based tubes. (9) Reneker, D. H.; Chun, I. Nanotechnology 1996, 7, 216. (10) Huang, Z. M.; Zhang, Y. Z.; Kotaki, M.; Ramakrishna, S. Compos. Sci. Technol. 2003, 63, 2223. (11) MacDiarmid, A. G.; Jones, W. E., Jr.; Norris, I. D.; Gao, J.; Johnson, A. T., Jr.; Pinto, N. J.; Hone, J.; Han, B.; Ko, F. K.; Okuzaki, H.; Llaguno, M. Synth. Met. 2001, 119, 27. (12) Fong, H.; Chun, I.; Reneker, D. H. Polymer 1999, 40, 4585. (13) Dong, H.; Nyame, V.; Macdiarmid, A. G.; Jones, W. E., Jr. J. Polym. Sci., Part B: Polym. Phys. 2004, 42, 3934. (14) Dong, H.; Prasad, S.; Nyame, V.; Jones, W. E., Jr. Chem. Mater. 2004, 16, 371. (15) Ochanda, F.; Jones, W. E., Jr. Langmuir 2005, 21, 10791. (16) Abidian, M. R.; Kim, D.-H.; Martin, D. C. AdV. Mater. 2006, 18, 405.
10.1021/la061399t CCC: $33.50 © 2006 American Chemical Society Published on Web 11/07/2006
Coaxial Fibers and ConVersion to Polypyrrole Tubes
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2. Experimental Section 2.1. Materials. Pyrrole (Aldrich) was distilled before use. PMMA (Aldrich, Mw 350 000), ammonium persulfate (Aldrich), p-toluene sulfonic acid (Aldrich), tetrabutylammonium chloride (Fluka), and N,N-dimethylformamide (DMF) (Fisher) were used as received. 2.2. Fabrication of PMMA Fibers by Electrospinning. The PMMA fiber core was fabricated by the electrospinning process.13 The electrical field was generated by a variable high-voltage power supply (SPELLMAN), which can provide voltage up to 30 kV. The PMMA solution was prepared by dissolving 60 mg/mL PMMA in DMF, followed by addition of 5 wt % (relative to PMMA) tetrabutylammonium chloride (TBAC). The PMMA solution was placed in a glass pipet. A copper wire was inserted into the polymer solution and was connected to one end of the power supply. Aluminum foil connected to the other end of the power supply was used as a collection screen. The pipet was tilted at approximately 5-10° from horizontal so that a small viscous drop was maintained at the capillary tip. An electrical voltage of 20 kV and a distance of 25 cm were applied between the polymer solution and the collection screen. In the electric field, a jet was ejected and traveled to the opposite electrode. With the evaporation of the solvent, long and dry fibers accumulated on the collection screen. 2.3. Deposition of PPy on PMMA Nanofibers. The PPy was coated on electrospun PMMA fibers by in-situ polymerization using ammonium persulfate as an oxidizing agent. A thin PMMA fiber mat was detached from the aluminum foil and was immersed in a 0.008 M aqueous solution of pyrrole (10 mL) containing 2 equiv amounts of p-toluene sulfonic acid as a dopant. The polymerization of pyrrole and the deposition of PPy on PMMA fibers at room temperature were initiated by the addition of 0.008 M aqueous ammonium persulfate solution (10 mL). The mixture was shaken gently for 8 min. The surface of the PMMA fibers gradually changed from white to dark with a black coating of polypyrrole. These coated fibers were rinsed with distilled water to remove loosely bonded polypyrrole particles and then were dried in air prior to treatment. 2.4. Hollow PPy Tube and Carbon Tube Production. Hollow PPy tubes were prepared by solvent extraction. Excess chloroform was added slowly along the vessel wall to a dried PPy/PMMA coaxial fiber mat at room temperature, and this solution was left to stand overnight. PPy residues were removed from solution, were washed several times with chloroform to ensure complete core removal, and were air-dried. For the production of carbon tubes from PPy/PMMA coaxial fibers, the dried PPy/PMMA coaxial fiber mat was put in a tube furnace for thermolysis. The tube furnace was purged with N2 for 0.5 h before heating. Under a N2 atmosphere, the furnace temperature was raised to 1000 °C at a constant rate of 10 °C/min, and then it was held at 1000 °C for 1 h before cooling down to room temperature. 2.5. Characterization. Morphological studies were carried out using a Hitachi S-570 scanning electron microscope (SEM). Samples were sputter-coated with a thin layer of Au/Pd prior to the SEM observations to prevent sample-charging problems. Transmission electron microscopic (TEM) images were obtained using a Hitachi H-7000 with 100 kV accelerating voltage of the electron beam. Tube samples for TEM were prepared by ultrasonic dispersion of tube materials in anhydrous ethanol. A drop of the solution containing the tubes was transferred onto a copper grid covered with lacy carbon film and was dried. FT-IR spectra of fibers and tubes were recorded on Bruker Equinox 55 in the range of 4000-400 cm-1. The samples for FT-IR were prepared by grinding fiber or tube materials with KBr powder and then by pressing the mixture into pellets. Thermogravimetric analysis (TGA) was performed on a TA-2000 instrument at a heating rate of 10 °C/min in N2 atmosphere.
3. Results and Discussion 3.1. PMMA Fibers and PPy/PMMA Coaxial Fibers. PMMA is one of the polymers that can be easily electrospun into ultrafine fibers with various sizes by controlling the polymer solution properties, such as solvent, solution concentration, and polymer molecular weight.13 Beads are often formed as “byproducts” in
Figure 1. SEM images of (a) electrospun PMMA fibers (×5 K), (b) PPy/PMMA coaxial fibers obtained from 8-min deposition (×5 K), and (c) PPy/PMMA coaxial fibers obtained from 15-min deposition (×5 K).
electrospinning. Addition of a soluble organic salt such as tetrabutylammonium chloride (TBAC) to PMMA solution can reduce the bead formation by increasing solution conductivity.13 In this study, PMMA fibers were fabricated from 60 mg/mL PMMA (Mw 350 000) in DMF solution with 5 wt % (relative to PMMA) TBAC via electrospinning. The SEM image in Figure 1a shows the morphology of the electrospun PMMA fibers. Long and smooth fibers were formed uniformly as a fibrous mat without beads. The average diameter of PMMA fibers was 230 nm. The deposition of PPy on the electrospun PMMA fibers was performed by in-situ polymerization. In this process, the presence of TBAC in the template fibers was found to increase the hydrophilic surface properties of the electrospun PMMA fiber. As a result, the aqueous monomer and oxidant solutions easily penetrated through the fiber mat and wet the surface of all fibers. This leads to more effective and uniform deposition of PPy on the PMMA fibers.
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Figure 3. SEM image (a) and TEM image (b) of carbon tubes resulting from thermal treatment of PPy/PMMA coaxial fibers. Figure 2. SEM images of (a) morphology of PPy tubes obtained after removal of PMMA fiber core (×5 K) and (b) tubular structure of PPy (×10 K).
To achieve a homogeneous layer of conductive PPy, various concentrations and deposition times were tested. It was found that higher concentrations of the reactants and longer deposition times resulted in a large amount of PPy aggregates on the surface of the PMMA fibers. This may have resulted from the increased rate of the reaction and the subsequent formation of PPy aggregates in solution that were adsorbed onto the surface of the fibers. To reduce the formation of PPy aggregates, a very dilute solution with a monomer concentration of 0.008 M and a short deposition time were used in our experiment. The SEM image of PPy/PMMA coaxial fibers prepared at 8-min deposition (Figure 1b) indicates the formation of a globular layer of PPy with fewer aggregates. The average diameter of these PPy/PMMA coaxial fibers was 300 nm with a PPy wall of 35 nm. The surface roughness of the PPy layer and the PPy aggregates attached to the fibers became more pronounced when the deposition time increased. As shown in Figure 1c, 15-min deposition results in a thicker layer (∼100 nm) of PPy and more PPy aggregates on fibers. 3.2. PPy Hollow Tubes. PPy tubes can be prepared from PPy/PMMA coaxial fibers by selective removal of PMMA fiber core. Two methods, solvent extraction and thermal treatment, can be used to remove the core PMMA from the coaxial structure. Since thermal treatment is known to affect the polymer structure including a loss of conductivity, solvent exaction was applied here to dissolve the fiber core. A mat of the PPy/PMMA coaxial fibers was treated with chloroform to dissolve away PMMA and leave residual PPy as a solid. The PPy residues remaining after PMMA core removal from the PPy/PMMA coaxial fibers prepared at 8-min deposition were characterized by SEM. The SEM image in Figure 2a shows that the long fiber shape was maintained after removal of the PMMA core. The PPy also appeared ruptured in a small portion of the material. This may have been caused by some swelling of the PMMA core. The SEM image in Figure 2b clearly shows the tubular structure of PPy in the majority of the material. This indicated that by exposing the PPy/PMMA coaxial fibers to chloroform, the PMMA core could be successfully dissolved and extracted through the semipermeable PPy walls. The formation of PPy tubes after extraction of the PMMA cores also provides irrefutable evidence of the coaxial morphology of the PPy/PMMA fibers. 3.3. Conversion to Carbon Tubes. Since PPy is one of the carbon sources,17 PPy/PMMA coaxial fibers can be converted to the corresponding carbon tubes by thermal carbonization of the PPy wall and decomposition of the PMMA core. Figure 3 shows the SEM and TEM images of the resulting carbon tubes
after thermal treatment of PPy/PMMA fibers at 1000 °C for 1 h under an inert N2 atmosphere. From the SEM image, the outer surface of the tubes appears relatively smooth, though small particles can be seen on the fiber structure. The TEM image reveals the hollow structure of the carbon tubes. Carbon tubes produced by thermal treatment at 1000 °C were found to be mostly carbon. Elemental analysis showed that the N content was around 4%, and H was
