Polythiophene: Synthesis in aqueous medium and controllable morphology

Chinese Science Bulletin 2009 SCIENCE IN CHINA PRESS Springer Polythiophene: Synthesis in aqueous medium and controllable morphology LIU RuoChen & LIU ZhengPing Institute of Polymer Chemistry and Physics of College of Chemistry, BNU Key Laboratory of Environmentally Friendly and Functional Polymer Materials, Beijing Normal University, Beijing 100875, China Various morphologies of polythiophene have been designed and successfully prepared by chemical oxidative polymerization in the presence of phase transfer catalyst (PTC) cetyltrimethylammonium bromide (CTAB) in aqueous medium. The morphologies of polythiophene could be controlled in ribbons, fibers and spherical particles by changing the concentrations of reductant, oxidant and phase transfer catalyst. The structure, thermal stability and the conductivity have been characterized, and a mechanism for the transformation of the morphology of polythiophene has been proposed. polythiophene, preparation, aqueous medium, morphology, cetyltrimethylammonium bromide (CTAB) Since 1980 [1], polythiophene has been widely used in environmentally and thermally stable conjugated polymer materials, such as chemical and optical sensors, light-emitting diodes and displays, photovoltaic devices, molecular devices, DNA detection, polymer electronic interconnects, solar cells and transistors [2 6]. Three approaches to polymerization of thiophene have been reported in the literature: (1) electropolymerization, (2) metal-catalyzed coupling reactions, and (3) chemical oxidative polymerization. Waltman et al. [7] prepared high conductivity polythiophene films by electropolymerization in 1983, but it is rarely used in the preparation of electroluminescent materials. Yamamoto et al. [1] reported the polycondensation of 2,5-dibromothiophene catalyzed by Ni(bipy)Cl 2, and similar results were also observed by Lin and Dudek [8] in their Ni, Pd, Co, and Fe catalytic system. In 1984, Yoshino et al. [9] found unsubstituted thiophene could be polymerized by ferric chloride in chloroform. Recently, Kim and his co-workers [10,11] prepared polythiophene in aqueous dispersion via Fe 3+ -catalyzed oxidative polymerization and tested its photoluminescence properties. Inspired by Kim s research, here we are interested in polymerization of thiophene by chemical oxidation, which is rather difficult to occur due to thiophene s high oxidation potential and poor solubility in water. Unlike polyaniline and polypyrrole [12], there are little literature relating the morphologies of polythiophene with the expectation that such materials will possess the advantages of organic conductors. Gök et al. [13] prepared different morphologies of polythiophene using different surfactants in 2007. We report herein our study on this polymerization in aqueous medium in the presence of the phase transfer catalyst (PTC) which could be used as templates to control the morphologies of polythiophene, and a mechanism for the transformation of the morphology will also be described. 1 Experimental 1.1 Reagents All chemicals were purchased from Aldrich Chemical Co. and Beijing Chemical Co. used as received unless otherwise noted. Thiophene was freshly distilled prior to use. Received August 31, 2008; accepted February 3, 2009 doi: 10.1007/s11434-009-0217-0 Corresponding author (email: lzp@bnu.edu.cn) Supported by the National Natural Science Foundation of China (Grant No. 50373004) and Measuring Fund of Large Apparatus of Beijing Normal University Citation: Liu R C, Liu Z P. Polythiophene: Synthesis in aqueous medium and controllable morphology. Chinese Sci Bull, 2009, 54: 2028-2032, doi: 10.1007/s11434-009-0217-0

1.2 Synthesis of morphology-controlled polythiophene Typically, cetyltrimethylammonium bromide (CTAB, 2.06 g), triethanolamine (TEA, 5.27 g) and thiophene (2.5 ml) were dissolved in deionized water (30 ml) in three-necked flask. The mixture solution was placed under ultrasonic for 30 min. Ammonium persulfate (APS, 8.26 g) was dissolved in 20 ml deionized water. Then the ammonium persulfate solution was added dropwise into the mixed solution mentioned above. The mixture was heated without stirring for 24 h at 70. The resulting precipitate was collected by filtration or centrifugation. It was washed by deionized water and methanol, and then freeze-dried for 24 h. The dark brown powder was polythiophene (1.15 g). 1.3 Characterization Infrared spectra were obtained from KBr pellets on an Avatar 360 Fourier transform spectrometer. HITACHI S-4800 field emission scanning electron microscopy (SEM) samples were prepared by evaporating the aqueous dispersion on aluminum-foil-coated stages, and the morphologies with gold coating were observed at an acceleration voltage of 3.0 kv. The conductivity was measured with typical four-probe method on a BD86 instrument at room temperature. Thermogravimetric analysis was made on a Perkin-Elmer Pyris 1 TGA apparatus at a moderate heating rate (10 /min) in a nitrogen environment. tion, C-C, stretching vibration of in-plane C-H, C-S, and C-H out-of-plane bending vibration absorption at about 1433 cm 1 ; 1216 cm 1 ; 1072 cm 1 and 1043 cm 1, 839 cm 1, and 702 cm 1[16 20], and these absorption is weakened by treatment of thiophene with ammonium persulfate, triethanolamine and cetyltrimethylammonium bromide, supporting the formation of polythiophene. Polymerization of thiophene can be monitored by scanning electron microscopy. The polymerization is fast, and polymer forms sheet structure at the beginning 30 min (Figure 3(a)). One hour later, some holes appear Figure 2 FT-IR spectra of thiophene and polythiophene prepared by chemical oxidative polymerization in aqueous medium. ARTICLES 2 Results and discussion Treatment of thiophene with ammonium persulfate, triethanolamine and cetyltrimethylammonium bromide at 70 for 24 h gives a dark brown precipitate, which can be isolated by filtration. During the course of the reaction, the solution changed from colorless to opaque, and finally to black (Figure 1). The FT-IR spectra (Figure 2) of thiophene monomer and polythiophene show that the characteristic absorption bands of polythiophene are similar to those prepared by traditional method [13 15]. Their IR spectra show the typical characteristic thiophene ring stretching vibra- POLYMER CHEMISTRY Figure 1 (a) Thiophene monomer, (b) the resulting solution after 24 hours polymerization, and (c) dried precipitate. Figure 3 SEM images of polythiophene prepared by 57 mmol of cetyltrimethylammonium bromide, 36 mmol of ammonium persulfate and 35 mmol of triethanolamine in aqueous medium. From (a) to (g), the reaction time is 0.5, 1, 3, 6, 9, 12, 15 h, respectively. Liu R C et al. Chinese Science Bulletin June 2009 vol. 54 no. 12 2029

on the sheet (Figure 3(b)). These holes disappear after standing for nine hours, and the sheet structure changes to fiber-like-structure (Figure 3(e)). Later, the fiber-like structure turns shorter and shorter, resulting in sphere-fiber-transition structure (Figure 3(e) and (f)), and finally forms spherical particles (Figure 3(g)). At the beginning of the reaction, cetyltrimethylammonium bromide forms micelles where polymerization occurs. When ammonium persulfate is added to a solution of cetyltrimethylammonium bromide and triethanolamine, some white flocculent precipitate appears immediately. Few minutes later, the white precipitate disappears and the mixture gradually turns into black. The precipitate (CTA) 2 S 2 O 8, which forms lamellar mesostructure and provides templates for polymerization, plays a key role in the transformation of the morphology of polythiophene [21]. In the following, the monomer is polymerized by the anion of (CTA) 2 S 2 O 8, which forms sheet structure. The lamellar inorganic/organic mesostructures as templates formed during polymerization between surfactant cations and oxidizing anions are degraded automatically after polymerization. With the po- lymerization occurring, some holes appear and enlarge, and unique sphere-fiber-transition structure presents. This structure is concentrated and splitted due to the secondary growth [22], and finally the spherical particles form. These transformations are outlined in Scheme 1. The morphological transformation process of polythiophene encourages us to study the controllable morphology. The stable morphologies, such as spherical particles, nanofibers, sphere-fiber-transition, and submicro/nanoribbons, can be obtained by controlling the ratio of oxidant to reductant. For example, keeping the ratio of triethanolamine to cetyltrimethylammonium bromide (entries 1 5 in Table 1), the morphology of polythiophene can be controlled from ribbon to fiber, then to sphere-fiber-transition, finally to sphere by increasing the rate of the use of ammonium persulfate (Figure 4). Similar results have also been observed by changing the concentration of triethanolamine (entries 4, and 6 10 in Table 1) or cetyltrimethylammonium bromide (entries 8, 11, 12 in Table 1). Increasing the triethanolamine or cetyltrimethylammonium bromide, the morphology Scheme 1 Schematic representations of the proposed mechanism of forming spherical polythiophene. 2030 www.scichina.com csb.scichina.com www.springerlink.com

Table 1 Preparation and physical property of polythiophenes Reaction reagent Time Temp. Conv. Conductivity Entry CTAB APS (g) TEA (g) (h) ( ) (%) (S/cm) (g) 1 3.31 5.27 1.64 11 2 4.10 5.27 1.64 12 3 6.62 5.27 1.64 30 4 8.26 5.27 1.64 52 7.53 10 6 5 10.9 5.27 1.64 59 6 8.26 3.95 1.64 24 70 75 7.17 10 6 7 8.26 6.59 1.64 47 1.27 10 5 8 8.26 7.91 1.64 38 1.78 10 5 9 8.26 10.54 1.64 28 1.52 10 5 10 8.26 13.18 1.64 20 6.09 10 5 11 8.26 7.91 1.85 37 12 8.26 7.91 2.06 32 Figure 6 SEM images of polythiophene prepared by (a) 45 mmol, (b) 51 mmol, and (c) 57 mmol cetyltrimethylammonium bromide. ARTICLES Figure 7 TGA curves of polythiophene prepared by 45 mmol cetyltrimethylammonium bromide, 36 mmol ammonium persulfate and different amounts of triethanolamine. Figure 4 SEM images of polythiophene prepared by (a) 15 mmol, (b) 18 mmol, (c) 29 mmol, (d) 36 mmol, and (e) 48 mmol ammonium persulfate. Figure 5 SEM images of polythiophene prepared by (a) 26 mmol, (b) 35 mmol, (c) 44 mmol, (d) 53 mmol, (e) 71 mmol, and (f ) 88 mmol triethanolamine. changes from spherical particles to submicroribbons (Figure 5) or from sphere-fiber transition to fibers and then to ribbons (Figure 6), respectively. The TGA data (Figure 7) show that the polythiophene is decomposed by one-step over various concentration of the triethanolamine (entries 6, 8, 10 in Table 1), which is not consistent with the previous reports [13,23,24] due to the crosslink bonds between thiophene rings (α-β bonds and β-β bonds). However, the polythiophene synthesized by this method is almost as stable as it was synthesized in previous work. By increasing the concentration of the triethanolamine (entries 4, and 6 10 in Table 1), the conductivity increases as shown in Table 1 with the change of the morphologies of polythiophene from sphere particles to ribbons. Both the polymerization degree and the crosslink degree have great effects on the conductivity of polythiophene (entries 4, and 6 10 in Table 1), and the data clearly show that the polythiophene prepared by this method has potential application to the semiconductor devices and materials. 3 Conclusion Various morphologies of polythiophene such as spherical particle, fiber, and ribbon have been successfully prepared by chemical oxidative polymerization in the POLYMER CHEMISTRY Liu R C et al. Chinese Science Bulletin June 2009 vol. 54 no. 12 2031

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