Polythiophene: Synthesis in Aqueous Medium and Controllable Morphology

Chinese Science Bulletin

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 ficult to occur due to thiophene’s high oxidation poten- environmentally and thermally stable conjugated poly- tial and poor solubility in water. mer materials, such as chemical and optical sensors, Unlike polyaniline and polypyrrole[12], there are little light-emitting diodes and displays, photovoltaic devices, literature relating the morphologies of polythiophene molecular devices, DNA detection, polymer electronic with the expectation that such materials will possess the interconnects, solar cells and transistors[2−6]. advantages of organic conductors. Gök et al.[13] prepared Three approaches to polymerization of thiophene different morphologies of polythiophene using different have been reported in the literature: (1) electropoly- surfactants in 2007. We report herein our study on this merization, (2) metal-catalyzed coupling reactions, and polymerization in aqueous medium in the presence of (3) chemical oxidative polymerization. Waltman et al.[7] the phase transfer catalyst (PTC) which could be used as prepared high conductivity polythiophene films by elec- templates to control the morphologies of polythiophene, tropolymerization in 1983, but it is rarely used in the and a mechanism for the transformation of the mor- preparation of electroluminescent materials. Yamamoto phology will also be described. et al.[1] reported the polycondensation of 2,5-dibromo- 1 Experimental thiophene catalyzed by Ni(bipy)Cl2, and similar results were also observed by Lin and Dudek[8] in their Ni, Pd, 1.1 Reagents [9] Co, and Fe catalytic system . In 1984, Yoshino et al. All chemicals were purchased from Aldrich Chemical found unsubstituted thiophene could be polymerized by Co. and Beijing Chemical Co. used as received unless ferric chloride in chloroform. Recently, Kim and his otherwise noted. Thiophene was freshly distilled prior to co-workers[10,11] prepared polythiophene in aqueous dis- use. persion via Fe3+-catalyzed oxidative polymerization and Received August 31, 2008; accepted February 3, 2009 tested its photoluminescence properties. Inspired by doi: 10.1007/s11434-009-0217-0 Kim’s research, here we are interested in polymerization †Corresponding author (email: [email protected]) Supported by the National Natural Science Foundation of China (Grant No. of thiophene by chemical oxidation, which is rather dif- 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 polythio- tion, C-C, stretching vibration of in-plane C-H, C-S, and

phene C-H out-of-plane bending vibration absorption at about ARTICLES −1 −1 −1 −1 Typically, cetyltrimethylammonium bromide (CTAB, 1433 cm ; 1216 cm ; 1072 cm and 1043 cm , 839 2.06 g), triethanolamine (TEA, 5.27 g) and thiophene cm−1, and 702 cm−1[16−20], and these absorption is weak- (2.5 mL) were dissolved in deionized water (30 mL) in ened by treatment of thiophene with ammonium persul- three-necked flask. The mixture solution was placed fate, triethanolamine and cetyltrimethylammonium bro- under ultrasonic for 30 min. Ammonium persulfate mide, supporting the formation of polythiophene. (APS, 8.26 g) was dissolved in 20 mL deionized water. Polymerization of thiophene can be monitored by Then the ammonium persulfate solution was added scanning electron microscopy. The polymerization is dropwise into the mixed solution mentioned above. The fast, and polymer forms sheet structure at the beginning mixture was heated without stirring for 24 h at 70℃. 30 min (Figure 3(a)). One hour later, some holes appear 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 nitro- Figure 2 FT-IR spectra of thiophene and polythiophene prepared gen environment. by chemical oxidative polymerization in aqueous medium.

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-

CHEMISTRY POLYMER Figure 3 SEM images of polythiophene prepared by 57 mmol of cetyltrimethylammonium bromide, 36 mmol of ammonium persulfate Figure 1 (a) Thiophene monomer, (b) the resulting solution after and 35 mmol of triethanolamine in aqueous medium. From (a) to (g), 24 hours’ polymerization, and (c) dried precipitate. 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 lymerization occurring, some holes appear and enlarge, standing for nine hours, and the sheet structure changes and unique ‘sphere-fiber-transition’ structure presents. to fiber-like-structure (Figure 3(e)). Later, the fiber-like This structure is concentrated and splitted due to the structure turns shorter and shorter, resulting in secondary growth[22], and finally the spherical particles ‘sphere-fiber-transition’ structure (Figure 3(e) and (f)), form. These transformations are outlined in Scheme 1. and finally forms spherical particles (Figure 3(g)). The morphological transformation process of polythi- At the beginning of the reaction, cetyltrimethylam- ophene encourages us to study the controllable morpho- monium bromide forms micelles where polymerization logy. The stable morphologies, such as spherical parti- occurs. When ammonium persulfate is added to a solu- cles, nanofibers, sphere-fiber-transition, and submi- tion of cetyltrimethylammonium bromide and trietha- cro/nanoribbons, can be obtained by controlling the ratio nolamine, some white flocculent precipitate appears of oxidant to reductant. For example, keeping the ratio immediately. Few minutes later, the white precipitate of triethanolamine to cetyltrimethylammonium bromide disappears and the mixture gradually turns into black. (entries 1―5 in Table 1), the morphology of polythio- The precipitate (CTA)2S2O8, which forms lamellar phene can be controlled from ribbon to fiber, then to mesostructure and provides templates for polymerization, plays a key role in the transformation of the morphology ‘sphere-fiber-transition’, finally to sphere by increasing of polythiophene[21]. In the following, the monomer is the rate of the use of ammonium persulfate (Figure 4). Similar results have also been observed by changing the polymerized by the anion of (CTA)2S2O8, which forms sheet structure. The lamellar inorganic/organic meso- concentration of triethanolamine (entries 4, and 6―10 in structures as templates formed during polymerization Table 1) or cetyltrimethylammonium bromide (entries 8, between surfactant cations and oxidizing anions are de- 11, 12 in Table 1). Increasing the triethanolamine or graded automatically after polymerization. With the po- 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 ARTICLES 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 Figure 6 SEM images of polythiophene prepared by (a) 45 mmol, 5 10.9 5.27 1.64 59 − (b) 51 mmol, and (c) 57 mmol cetyltrimethylammonium bromide. 6 8.26 3.95 1.64 75 7.17×10−6 24 70 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 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, is decomposed by one-step over various concentration of (b) 18 mmol, (c) 29 mmol, (d) 36 mmol, and (e) 48 mmol ammonium persulfate. the triethanolamine (entries 6, 8, 10 in Table 1), which is [13,23,24] not consistent with the previous reports 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 Figure 5 SEM images of polythiophene prepared by (a) 26 mmol, polythiophene prepared by this method has potential appli-

(b) 35 mmol, (c) 44 mmol, (d) 53 mmol, (e) 71 mmol, and (f ) 88 cation to the semiconductor devices and materials. mmol triethanolamine. 3 Conclusion changes from spherical particles to submicroribbons (Figure 5) or from ‘sphere-fiber transition’ to fibers and Various morphologies of polythiophene such as spheri- CHEMISTRY POLYMER then to ribbons (Figure 6), respectively. cal particle, fiber, and ribbon have been successfully The TGA data (Figure 7) show that the polythiophene prepared by chemical oxidative polymerization in the

Liu R C et al. Chinese Science Bulletin | June 2009 | vol. 54 | no. 12 2031

presence of phase transfer catalyst in aqueous medium. changing the concentration of reductant, oxidant and The properties of the polythiophene have been tested, PTC (cetyltrimethylammonium bromide). In addition, and a mechanism for the polymerization has been pro- this chemical oxidative polymerization method is a gen- posed. The morphologies of polythiophene could be eral route to make various morphologies of conductive controlled in ribbons, fibers and spherical particles by polymer in mild condition.

1 Yamamoto T, Sanechika K, Yamamoto A. Preparation of thermosta- polythiophenes prepared in the presence of surfactants. Synth Met, ble and electric-conducting poly(2,5-thienylene). J Polym Sci Polym 2007, 157: 23―29 Lett Ed, 1980, 18: 9―12 14 Toshima N, Hara S. Direct synthesis of conducting polymers from 2 Skotheim T, Reynolds J, Elsenbamer R. Handbook of Conducting simple monomers. Prog Polym Sci, 1995, 20: 155―183 Polymers. 2nd ed. New York: Marcel Dekker, 1998. 226 15 Ballav N, Biswas M. Preparation and evaluation of a nanocomposite

3 Perzon E, Wang X, Zhang F, et al. Design, synthesis and properties of of polythiophene with Al2O3. Polym Int, 2003, 52: 179―184 low band gap polyfluorenes for photovoltaic devices. Synth Met, 2005, 16 Wang C, Schindler J L, Kannewurf C R, et al. Poly(3,4-ethylenedi- 154: 53―56 thiathiophene). A new soluble conductive polythiophene derivative. 4 Ho H, Najari A, Leclerc M. Optical detection of DNA and proteins with Chem Mater, 1995, 7: 58―68 cationic polythiophenes. Acc Chem Res, 2008, 41: 168―178 17 Casado J, Hernandez V, Hotta S, et al. Vibrational spectra of charged 5 Mwaura J K, Zhao X, Jiang H, et al. Spectral broadening in nanocrys- defects in a series of α, α’-dimethyl end-capped oligothiophenes in-

talline TiO2 solar cells based on poly(p-phenylene ethynylene) and duced by chemical doping with iodine. J Chem Phys, 1998, 109: polythiophene sensitizers. Chem Mater, 2006, 18: 6109―6111 10419―10429 6 Zou Y, Wu W, Sang G, et al. Polythiophene derivative with phenothi- 18 Yildiz U H, Sahin E, Akhemdov I M, et al. A new soluble conducting azine-vinylene conjugated side chain: Synthesis and its application in polymer and its electrochromic devices. J Polym Sci Part A: Polym field-effect transistors. Macromolecules, 2007, 40: 7231―7237 Chem, 2006, 44: 2215―2225 7 Waltman R J, Bargon J, Diaz A F. Electrochemical studies of some 19 Kim Y H, Hotta S, Heeger A J. Infrared photoexcitation and doping conducting polythiophene films. J Phys Chem, 1983, 87: 1459―1463 studies of poly(3-methylthienylene). Phys Rev B, 1987, 36: 7486― 8 Lin J W P, Dudek L P. Synthesis and properties of poly(2,5- 7490 Thienylene). J Polym Sci Polym Chem Ed, 1980, 18: 2869―2873 20 Furukawa Y, Akimoto M, Harada I. Vibrational key bands and electrical 9 Yoshino K, Hayashi S, Sugimoto R. Preparation and properties of conductivity of polythiophene. Synth Met, 1987, 18: 151―156 conducting heterocyclic polymer films by chemical method. Jpn J 21 Zhang X, Zhang J, Liu Z, et al. Inorganic/organic mesostructure di- Appl Phys Part 2- Lett, 1984, 23: 899―900 rected synthesis of wire/ribbon-like polypyrrole nanostructures. Chem 10 Jung Y J, Lee J M, Cheong I W, et al. Luminescent polymer latex Commun, 2004, 1852―1853 particles prepared by oxidative polymerization in emulsion polym- 22 Huang J, Kaner R B. Nanofiber formation in the chemical polymeri- erization. Macromol Symp, 2007, 249: 265―269 zation of aniline: A mechanistic study. Angew Chem Int Ed, 2004, 43: 11 Lee J M, Lee S J, Jung J Y, et al. Fabrication of nano-structured 5817―5821 polythiophene nanoparticles in aqueous dispersion. Curr Appl Phys, 23 Karim M R, Lee C J, Lee M S. Synthesis and characterization of 2008, 8: 659―663 conducting polythiophene/carbon nanotubes composites. J Polym Sci 12 Huang L Y, Hou W B, Liu Z P, et al. Polypyrrole-coated styrene-butyl Part A: Polym Chem, 2006, 44: 5283―5290 acrylate copolymer composite particles with tunable conductivity. 24 Gök A, Koçak E D, Aydoğdu S. Synthesis and characterization of

Chinese Sci Bull, 2005, 50: 971―975 PT/PS/SiO2 nanocomposite in nonaqueous medium by chemical 13 Gök A, Omastová M, Yavuz A G. Synthesis and characterization of method. J Appl Polym Sci, 2005, 96: 746―752

2032 www.scichina.com | csb.scichina.com | www.springerlink.com