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J. Phys. Chem. C 2009, 113, 21586–21595
Efficient Synthesis of Intrinsically Conducting Polypyrrole Nanoparticles Containing Hydroxy Sulfoaniline as Key Self-Stabilized Units Xin-Gui Li,*,†,‡ Zhen-Zhong Hou,† Mei-Rong Huang,*,†,‡ and Mark G. Moloney*,‡ Institute of Materials Chemistry, Key Laboratory of AdVanced CiVil Engineering Materials, College of Materials Science and Engineering, Tongji UniVersity, Shanghai 200092, China, and Chemistry Research Laboratory, Department of Chemistry, UniVersity of Oxford, 12 Mansfield Road, Oxford OX1 3TA, United Kingdom ReceiVed: August 24, 2009; ReVised Manuscript ReceiVed: October 12, 2009
Self-stabilized copolymer nanoparticles are easily and productively synthesized by a chemical oxidative polymerization of pyrrole (Py) and 2-hydroxy-5-sulfonic aniline (HS) in 1 M HCl without any external template. UV-vis, IR, 1D 1H NMR, 2D 1H-1H COSY NMR, and 2D 1H-13C HSQC NMR all indicate that a real copolymerization occurs between HS and Py comonomers and Py units construct the main position of the copolymer. On the basis of the elemental analysis, the reactivity ratios of HS and Py comonomers are calculated to be 0.043 and 1.14, respectively. The polymerization yield, size, morphology, and electrical conductivity of the copolymer particles can significantly be optimized by the comonomer ratio, polymerization temperature, and ammonium persulfate oxidant/comonomer ratio. The copolymer particles always keep narrow size distribution with a small polydispersity index (PDI) of 1.05-1.08. HS/Py(50/50) copolymer nanoparticles synthesized at 0 °C are found to generally have irregular granular morphology with the smallest diameter of 35-60 nm and the lowest PDI of 1.05 by laser particle-size analyzer, FE-SEM, and TEM. The mechanisms of the formation and intrinsic self-stabilization of the nanoparticles are proposed based on the powerful static repulsion from negatively charged sulfonic and hydroxyl groups on the nanoparticles. Through simple dedoping and redoping procedures, the copolymer particles exhibit a widely adjustable conductivity from 10-9 to 1.12 S cm-1. These copolymer nanoparticles show high conductivity, good self-stability, and powerful redispersibility in water and organic media. Nanocomposite films of the copolymer nanoparticles in polyvinylalcohol possess a low percolation threshold down to 0.09 wt % as well as retain 80-95% transparency and 102-108 times the conductivity of the pure polyvinylalcohol film in the nanoparticle loading from 0.09 to 3 wt %. Introduction Conducting polymers with large π-conjugated structure as multifunctional materials have received considerable attention due to their potential wide applications in chemical and biological sensors,1 rechargeable batteries,2 light-emitting diodes,3 actuators,4 photovoltaics,5 memory devices,6 and corrosion resistance.7 Polypyrrole (PPy) is one of the most important conducting polymers with high electrical conductivity,8 facile preparation,9a environmental stability, and biocompatibility.9b Unfortunately, the strong inter- and intramolecular interactions of the PPy chains lead to its infusibility and insolubility and thus poor processability, which is the major obstacle to its further widespread application. So far, two main methods have been developed to tackle these intractable problems: (1) synthesize soluble PPy by the polymerization of pyrrole (Py) derivatives with appropriate substituting groups such as long-chain alkyl and sulfonic groups at the N- or β-position,10 or by the copolymerization of Py and other monomers bearing functional groups,11,12 or (2) fabricate PPy nanoparticles to form stable colloidal dispersion in the presence of external stabilizer.13,14 For the former method, synthesis of the substituted Py derivative monomers is complicated, and requires tedious reactions, leading to high cost but low efficiency. Recently, the second method * To whom correspondence should be addressed. E-mail: adamxgli@ yahoo.com and
[email protected]. † Tongji University. ‡ University of Oxford.
has attracted much attention because nanostructured PPy demonstrating unique physicochemical characteristics can extensively be used in sensor, supercapacitor, biomedicine, and carbon nanomaterial precursors due to its better properties than bulk PPy.15 PPy spherical nanoparticle dispersions bave been prepared by a dispersion or microemulsion polymerization.13,14 However, the two approaches need external stabilizers including surfactant, cosurfactant, and/or dispersant, inevitably causing an intricate post-treatment process. Moreover, pure PPy nanoparticles with a clean surface cannot simply be obtained unless the heavy contamination of the stabilizer is carefully removed. Unfortunately, the PPy nanoparticles without the protection of stabilizers are likely to aggregate to some extent in a few minutes, leading to instability. In recent years, a unique “charged monomer copolymerization” has been applied to simply synthesize the self-stable aniline copolymer nanoparticles in water without external stabilizers.16 2-Sulfonic aniline, 4-sulfonic diphenylamine, and sulfonic m-phenylenediamine are typical comonomers with negatively charged groups to copolymerize with aniline. The electrostatic repulsive effect of the charged groups is responsible for the formation and self-stabilization of the nanoparticles, effectively avoiding the “second growth” and thus keeping the as-formed nanoparticles stable during copolymerization and subsequent purification. Actually, the units with charged groups in the resulting copolymers act as a unique internal stabilizer. This approach provides a simple and cost-
10.1021/jp9081504 2009 American Chemical Society Published on Web 11/25/2009
Intrinsically Conducting Polypyrrole Nanoparticles effective methodology to fabricate pure, clean, and strongly selfstable conducting polymer nanoparticles with an intrinsic conductivity. However, the investigation on the synthesis of the PPy nanoparticles by this method is rather scarce.17 It seems that the sulfonated aniline is a suitable comonomer, but the low polymerizability due to the presence of an electronattracting sulfonic group is a possible problem. Therefore, the introduction of an electron-donating group into the aniline ring may be used to improve reactivity of the comonomer. Here, we choose 2-hydroxy-5-sulfonic aniline (HS) with double negatively charged groups (-SO3-H+ and -O-H+) as a comonomer to copolymerize with Py by simple chemical oxidation polymerization for the preparation of pyrrole copolymer nanoparticles with a narrow size distribution. The effect of important polymerization parameters, including HS/Py ratio, oxidant/monomer ratio, and reaction temperature on the yield, structure, morphology, and properties of the nanoparticles has been systematically investigated. Moreover, the transparent nanocomposite film of the nanoparticles was also prepared and their low percolation threshold has been revealed. Experimental Section Synthesis of HS/Py Copolymer Nanoparticles. The copolymer nanoparticles were synthesized with (NH4)2S2O8 as an oxidant through a chemically oxidative copolymerization between HS and Py in HCl aqueous solution without any external template. A typical copolymerization is as follows: HS (0.946 g, 5 mmol) and Py (0.35 mL, 5 mmol) were dissolved in HCl solution (1 M, 100 mL) under magnetic stirring at 0 °C for half an hour. For an oxidant, (NH4)2S2O8 (2.284 g, 10 mmol) was dissolved in HCl (1 M, 20 mL) solution at the same temperature. The (NH4)2S2O8 solution was dropwise added into the solution of comonomers in a period of 25 min under magnetic stirring. Copolymerization was carried out for 24 h at 0 °C in air. The as-prepared products were obtained by centrifugation, and then washed with ethanol and deionized water several times until the upper layer liquid became colorless. Finally, the black copolymer powder was dried under an IR lamp for 3 days. Preparation of Nanocomposite Films. The nanocomposite films were fabricated by ultrasonically dispersing redoped HS/ Py(50/50) copolymer nanoparticles in an aqueous solution of polyvinylalcohol (PVA) for 0.5 h followed by solution casting onto a Teflon plate. After drying at 35 °C for 48 h, a freestanding film with a thickness of 15 µm was peeled off from the Teflon substrate for performance evaluation. Measurements. The OCP of the copolymerization system was monitored by using a saturated calomel electrode (SCE) as reference electrode and a Pt electrode as working electrode. The OCP and temperature of the HS/Py copolymerization solution were simultaneously detected during the drastic copolymerization of the initial ca. 320 min. The elemental analysis was conducted with an automated Elemental Analyzer (Vario ELIII, Elementar Analysensysteme GmbH, Hanau, Germany) by Stephen Boyer at London Metropolitan University. The IR spectra were recorded on a Bruker FT-IR Equinox 55 spectrophotometer with use of KBr pellets. UV-vis spectra of the copolymers in DMSO and ethanol were measured on a 760CRT spectrophotometer (Precision and Scientific Instrument Corporation, Shanghai, China) at a scanning rate of 400 nm min-1. NMR spectra were obtained on Bruker spectrometers of AV700 and DQX400 for 1D 1H NMR at 700 and 400 MHz, DQX400 for 2D 1H-1H COSY NMR at 400 MHz, and AVC500 for 1H-13C HSQC NMR at 500 MHz in DMSO-d6 at University of Oxford, UK. The size and its distribution of copolymer particles were
J. Phys. Chem. C, Vol. 113, No. 52, 2009 21587 analyzed by an LS230 laser particle-size analyzer (LPA) from Beckman Coulter, Inc., USA. The real size and morphology of copolymer nanoparticles were observed by a FE-SEM (Quanta 200 FEG, FEI Company Eindhoven, The Netherlands) with an observation resolution of 500 nm). The good transparency of the composite film is mainly attributed to (1) the copolymer nanoparticles being evenly be distributed throughout the matrix to form a uniform composite film at the lower content and (2) the average size (35-45 nm) of the copolymer nanoparticles being smaller than half of the shortest wavelength (about 200 nm) of visible light, which ensures the transparency of the composite film.14 Nevertheless, the copolymer nanoparticles would agglomerate in the PVA matrix to form an inhomogeneous composite containing bigger copolymer aggregates with higher nanoparticle content up to 7 wt %, consequently leading to a significant decline in the transmittance at the whole wavelength of visible light. The transparent and flexible HS/Py copolymer nanoparticles-PVA nanocomposites have potential applications in sensors,35,36 tissue
Li et al. engineering,37 and microwave shielding38 due to their good adhesion to electrodes, excellent biocompatibility, and electrical properties. Innovation of This Study Compared with the Previous One. Our early work put forward the concept of the synthesis of Py copolymer nanoparticles using 4-sulfonic diphenylamine as a stabilizing unit.17 In this study, a combination of UV-vis, IR, 1H NMR, 1H-1H 2D COSY NMR, 1H-13C 2D HSQC NMR, FE-SEM, and TEM has thoroughly been used to carefully characterize the macromolecular and morphological structures of the Py copolymer nanoparticles, significantly demonstrating the important role of HS units in the formation and selfstabilization of more regular nanoparticles. The existence of -OH on the sulfonic aniline is beneficial for productive synthesis of Py copolymer nanoparticles with higher quality like more uniform size and regular morphology because the electrondonating -OH group can improve the reactivity of sulfonic aniline comonomer and then optimize the mechanism of the formation and self-stabilization of the copolymer nanoparticles to some extent. In addition, the well-dispersed HS/Py copolymer nanoparticles have successfully been used to fabricate the conductive nanocomposite film with excellent transparency and low percolation threshold. In contrast, the UV-vis, 1H NMR, 1 H-1H 2D COSY NMR, 1H-13C 2D HSQC NMR, FE-SEM techniques or nanocomposite film were not used or studied in our previous report.17 In short, HS as a stabilizing unit in the copolypyrrole is even better or more efficient than the 4-sulfonic diphenylamine used earlier.17 Conclusions The HS/Py copolymer nanoparticles have been directly synthesized by a chemically oxidative polymerization with (NH4)2S2O8 as oxidant in 1 M HCl medium without any external stabilizer. Three fundamental polymerization parameters including HS feed content, reaction temperature, and oxidant/comonomer ratio have been used to significantly optimize the yield, conductivity, and size of as-prepared copolymer nanoparticles. 0 °C with an HS feed content of 50 mol % and oxidant/ comonomer molar ratio of 1.0 are optimal copolymerization conditions for the synthesis of the HS/Py copolymer nanoparticles with the highest yield, largest π-conjugated structure, smallest size (35-60 nm), narrowest size distribution (PDI ) 1.05), strongest self-stability and redispersibility in water, and highest conductivity. The Py units construct the main portion of the copolymer, so the obtained copolymer nanoparticles containing HS as vital self-stabilized units can be used to replace the extrinsically stabilized PPy nanoparticles in many application fields. In particular, the nanoparticles could be an effective potential support material for enzyme immobilization and catalysis and advanced heavy-metal ion sorbents due to a large number of functional groups including sulfonic, hydroxyl, and imino groups in copolymer chains and their high specific area. The transparent flexible nanocomposite film with a low percolation threshold of 0.09 wt % could be facilely fabricated by blending the HS/Py copolymer nanoparticles into PVA. With the nanoparticles loading above 3 wt %, the nanocomposite film exhibits a high conductivity of 10-3 S cm-1 and maintains above 80% transparence of pure PVA film. This method of preparing pyrrole copolymer nanoparticles offers an effective substitute for traditional methods including the relatively complicated microemulsion and dispersion polymerizations. Acknowledgment. The project was financially supported by the Royal Society, UK, and National Natural Science Foundation of China (50773053).
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