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Synthesis of PEDOT latexes by dispersion polymerization in aqueous media
Materials Science and Engineering C 29 (2009) 377–382
Contents lists available at ScienceDirect
Materials Science and Engineering C j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m s e c
Synthesis of PEDOT latexes by dispersion polymerization in aqueous media Eric Cloutet ⁎, Muhammad Mumtaz, Henri Cramail Université de Bordeaux, Laboratoire de Chimie des Polymères Organiques, UMR5629 ENSCPB-CNRS-Université de Bordeaux, 16 Avenue Pey Berland, 33607 Pessac Cedex, France
a r t i c l e
i n f o
Article history: Received 9 June 2008 Received in revised form 18 July 2008 Accepted 19 July 2008 Available online 29 July 2008 Keywords: PEDOT Dispersion Nanoparticles Reactive stabilizers PEO
a b s t r a c t The synthesis of poly(3,4-ethylenedioxythiophene) (PEDOT) nano- and micro-objects with a narrow size distribution was achieved in a dispersant aqueous medium. Various oxidants such as ammonium persulfate, iron(III) p-toluenesulfonate and iron (III) trichloride were tested. A series of end-functionalized poly(ethylene oxide) (PEO) were compared as reactive stabilizers. The molar mass and the functionality of these reactive stabilizers were found to be important parameters with respect to the control of particle morphology, particle size and size distribution. PEDOT samples were characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM), size exclusion chromatography (SEC), UV–vis and FTIR spectroscopy and conductivity measurements. © 2008 Elsevier B.V. All rights reserved.
1. Introduction During the last two decades, conjugated polymers have received growing attention from chemists and physicists because of the valuable potential applications of these materials in numerous fields and particularly in optoelectronics [1–3]. However, the difficulty to process conjugated polymers is still preventing their common use and large scale development. In order to overcome this drawback, many research groups enhanced the solubility of conjugated polymers by introducing side chains to the rigid backbone or by preparing composites with other polymers [4]. Another route to ease the processability of conjugated polymers is to synthesize them under the form of sterically stabilized particles. For instance, aqueous dispersions of conjugated polymers have been prepared by chemical oxidative polymerization of the corresponding monomers such as pyrrole or aniline in the presence of suitable water-soluble steric stabilizers [5–14]. Among conjugated conductive polymers, poly(3,4ethylenedioxythiophene) (PEDOT) has more recently attracted specific attention because of its excellent environmental stability, its low oxidation potential and low band gap as well as its good and stable electrochromic behaviour from blue to transparent in neutral and oxidized states respectively [15,16]. The synthesis of stable PEDOT dispersions in water with the help of polystyrene-sulfonic acid was described but the authors did not give any data about the morphology of PEDOT material [17]. Oh and Im [18] have also prepared PEDOT particles in micellar solution templates using sodium dodecylbenzene sulfonic acid (DBSA) as a
⁎ Corresponding author. E-mail address: [email protected] (E. Cloutet). 0928-4931/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2008.07.024
surfactant and FeCl3/Na2S2O8 as an oxidant in aqueous media. However, in this case, PEDOT particles were poorly defined and showed a tendency to form aggregates. In another approach, the syntheses of PEDOT-coated polystyrene particles and PEDOT-coated silica particles have been reported by Armes and colleagues [19] and Han and Foulger [20] respectively. Recently, Müllen and colleagues reported the synthesis of PEDOT nanoparticles by emulsion polymerization in cyclohexane using poly(isoprene)-b-poly(methyl methacrylate) (PI-b-PMMA) as the stabilizer and iron(III) chloride as the oxidant [21]. Interestingly, Zhang and colleagues [22] also described the synthesis of PEDOT nanotubes using sodium bis(2ethylhexyl) sulfosuccinate (AOT) cylindrical micelles as the templates. To our knowledge, the synthesis of well-defined PEDOT nano-objects using steric and reactive stabilizer has never been described in the literature. In this study, we discuss the synthesis, in alcoholic media, of well-defined spherical PEDOT objects (i.e. coreshell nanoparticles and vesicles) in the presence of poly(ethylene oxide) end-functionalized with a (3,4-ethylenedioxythiophene) moiety, using ammonium persulfate or iron(III) p-toluenesulfonate hexahydrate as oxidizing agents. This paper is in the continuation of our previously described results [23]. 2. Experimental 2.1. Materials 3,4-ethylenedioxythiophene (EDOT), N-methyl-2-pyrrolecarboxylic acid, thiophene-2-carboxylic acid, fluorene-9-carboxylic acid, diisopropyl carbodiimide (DIPC) and polyethylene glycols were purchased from Aldrich and used without further purification. Tetrahydrofuran (THF) (J. T. Baker) was first distilled over CaH2 and then distilled over sodium
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Scheme 1. Synthesis of end-capped PEO by esterification reaction of α,ω-OH PEO with various carboxylic acids, i.e. N-methyl-2-pyrrolecarboxylic acid, thiophene-2-carboxylic acid and fluorene-9-carboxylic acid.
benzophenone. Dichloromethane was distilled over CaH2. Ammonium persulfate, iron (III) p-toluenesulfonate hexahydrate [Fe(OTs)3,6(H2O)] of technical grade, and p-toluenesulfonic acid (98.5%) were purchased from Aldrich and used as received. 4-(Dimethylamino)pyridinium 4-toluene sulfonate (DPTS) was prepared as described in the literature [24]. 2.2. Synthesis of poly(ethyleneoxide) based reactive stabilizers Stabilizers were prepared by the esterification of polyethylene glycol with corresponding carboxylic acid of thiophene, pyrrole and fluorene. For example α,ω-N-methyl-2-pyrrole poly(ethylene oxide) (α,ω-Py-PEO) was synthesized as follows: α,ω-OH poly(ethylene oxide) (5 g, 0.5 mmol, M n = 10,000 g mol- 1), N-methyl-2-pyrrole carboxylic acid (0.313 g, 2.5 mmol) and 4-(dimethylamino)pyridinium 4-toluene sulfonate (DPTS) (0.148 g, 0.50 mmol) were introduced in a 250 mL flame dried three necked round bottom flask under nitrogen. CH2Cl2 (60 mL) was then added and finally, diisopropylcarbodiimide (DIPC) (0.50 mL, 3.25 mmol) was introduced by syringe. Stirring under nitrogen at room temperature was continued for 60 h. The solution was filtered to remove diisopropylurea. The solvent was evaporated and the α,ω-Py-PEO was precipitated in cold ethyl ether (yield = 98%).
phase. SEC for soluble PEDOT samples were performed using two PLgel 5 μm MIXED-C (300 × 7.5 mm) columns, one PLgel 5 μm (50 × 7.5 mm) guard column and a JASCO 875-UV detector with DMF as mobile phase, at 60 °C in the presence of LiBr salt. SEM images of the PEDOT samples were taken using JEOL JSM-5200 and JEOL 6700F scanning microscopes. UV–Visible–NIR spectra were recorded with a Spectramax-M2 spectrometer. FTIR spectra were taken using TENSOR 27 (BRUKER) instrument. Conductivity measurements of the PEDOT sample (pressed pellets) were performed using a Keithly 2400 Source Meter four probe instrument. 4. Results and discussion 4.1. Synthesis of reactive stabilizers based on poly(ethylene oxide) A series of PEO, end-capped by one or two functions such as pyrrole (Py), thiophene (Th) or fluorene (Fluo) and having different
2.3. Synthesis of PEDOT latexes In a typical procedure, PEDOT latexes were prepared by dispersion polymerization as the following: EDOT (1 g, 7 mmol) was charged in the flask equipped with a mechanical stirrer and containing a solution of reactive stabilizer (e.g. α-EDOT-PEO; 1 g; M n = 5200 g/mol) in 80 mL of a mixture of methanol and water (2/3, v/v). A solution of ammonium persulfate (4.36 g dissolved in 20 mL of the methanol– water mixture) was then introduced at once. The reaction mixture was stirred for 72 h at room temperature. The resulting blue dispersion was centrifuged at 10,000 rpm at 5 °C for 30 min. The supernatant was carefully decanted and the dark blue sediment was re-dispersed in methanol + water. This re-dispersion–centrifugation cycle was repeated three times in order to ensure the complete removal of inorganic material such as ammonium sulfate and eventual unattached reactive stabilizer. 3. Characterizations 1 H NMR spectra were recorded using a Bruker AC-400 NMR spectrometer. SEC of the stabilizer were performed using a JASCO HPLC pump type 9012, PL aquagel-OH MIXED 8 μm columns and a Varian (series RI-4) refractive index detector with water as mobile
Fig. 1. 1H NMR (400 MHz) spectrum of (i) hydroxyl-telechelic poly(ethylene oxide) and ¯ n = 10,000 g/mol.). (ii) α-N-methyl-pyrrole poly (ethylene oxide) in DMSO-d6 (M
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Scheme 2. Synthesis of PEDOT latexes by dispersion polymerization in methanol/water (2/3, v/v) mixture in the presence of α-functionalized PEO and α,ω-functionalized PEO reactive steric stabilizer.
molar masses were prepared. The latter was synthesized by esterification of α,ω-OH PEO with the corresponding carboxylic acids of pyrrole, thiophene, and fluorene respectively (Scheme 1). The reaction was catalyzed by the 1:1 molecular complex of 4(dimethylamino) pyridine with p-toluenesulfonic acid also known as 4-(dimethylamino)pyridinium 4-toluenosulfonate (DPTS), the role of DPTS being to suppress the side reaction leading to the formation of N-acylurea [24]. The chain-end derivatization proceeds readily at room temperature which results in the formation of mono- or di-functionalized PEO depending on the amount of the carboxylic acid used. The general synthetic pathway is shown in Scheme 1. All the functionalized PEO were characterized by 1H NMR spectroscopy. An example is shown in Fig. 1. The 1H NMR spectrum in DMSO-d6 of α-N-methyl-2-pyrrole poly(ethylene oxide) (α-PyPEO, M n = 10,000 g/mol) is shown in Fig. 1(ii) (to compare with the one of α,ω-OH PEO Fig. 1(i)). The appearance of signals at 6.08 ppm (a), 6.83 ppm (b) and 7.09 ppm (c), due to pyrrole ring protons resonance and at 4.27 ppm (d) due to CH2O– of the ester function confirms the presence of pyrrole moiety at the PEO chain-end. The
integration values of each peak are in accordance with the presence of only one pyrrole unit per PEO chain. Similarly, 1H NMR characterizations allowed us to confirm the synthesis of poly(ethylene oxide) α,ω-di-capped with different groups such as fluorene, thiophene and pyrrole. 4.2. Dispersion polymerization of EDOT EDOT is partially soluble in water, only 0.21% at 20 °C. The addition of some amount of alcohol (methanol) improves its solubility. Hence, a mixture of methanol and water (2/3, v/v) was used throughout all experiments. The oxidative polymerization of EDOT was performed in this dispersant medium in the presence of end-capped PEO used as reactive and steric stabilizers (see Scheme 2). The effect of the molar mass, functionality and concentration of the reactive PEO as well as the nature and concentration of the oxidant used, on the PEDOT particle formation and size control has been investigated. The results of the dispersion polymerization of EDOT are summarized in Table 1. As a general trend, the size of the PEDOT particles decreases with the increase in stabilizer concentration; the particle size goes from
Table 1 Synthesis of PEDOT dispersions in methanol/water mixture (2:3) using mono and di-functionalized PEO-based reactive stabilizer at 35 °C Run no.
Oxidant type
Stabilizer type
1 2 3 4 5 6a 7a 8a 9 10 11 12 13 14 15 16 17b 18b
(NH4)2S2O8 (NH4)2S2O8 (NH4)2S2O8 (NH4)2S2O8 (NH4)2S2O8 (NH4)2S2O8 (NH4)2S2O8 (NH4)2S2O8 (NH4)2S2O8 (NH4)2S2O8 (NH4)2S2O8 (NH4)2S2O8 (NH4)2S2O8 (NH4)2S2O8 (NH4)2S2O8 (NH4)2S2O8 Fe(OTs)3 Fe(OTs)3
PEO α-Py-PEO α-Py-PEO α-Py-PEO α-Py-PEO α-EDOT-PEO α-EDOT-PEO α-EDOT-PEO α-Flu-PEO α-Py-PEO α-Py-PEO α,ω-Th-PEO α,ω-Py-PEO α,ω-Py-PEO α,ω-Flu-PEO α,ω-Flu-PEO α,ω-Flu-PEO α,ω-Flu-PEO
a
Reference [23]. Reactions were carried at 85 °C. c Calculated by 1H NMR. b
Oxidant
Stabilizer Mn
Stabilizer introduced
Yield
PEOc incorporated
Mn (and PDI of the PEDOT samples)
Particle diameter
Mole equivalent
g/mol
wt.%
%
%
g mol− 1
nmc
1.6 1.6 1.6 1.6 2.6 2.6 2.6 2.6 1.6 1.6 1.6 1.6 1.6 1.6 1.6 1.6 2.6 2.6
10,000 10,000 10,000 10,000 10,000 9700 25,000 51,000 10,000 35,000 35,000 10,000 10,000 35,000 10,000 35,000 35,000 35,000
20 20 35 50 50 50 50 50 20 20 35 20 20 35 20 35 35 50
50 50 50 55 55 30 35 32 60 40 50 50 60 60 60 60 80 80
nd 17 22 23 10 nd 75 68 27 12 3 25 19 40 40 30 nd nd
nd nd 6000 (1.6) 5500 (1.7) 5100 (1.7) nd 5300 (1.4) 8400 (1.3) 7000 (1.8) 6200 (1.8) 6100 (1.7) 5500 (1.7) 7000 (1.5) 5800 (1.5) 5400 (1.4) 6300 (1.6) nd nd
_ 550–600 400–500 300–350 400–600 70–150 160–500 80–120 450–500 350–400 275–300 350–400 500–600 350–400 350–400 200–250 50–80 50–60
Remarks
Coagulum Particles Particles Particles Particles Particles Vesicles Particles Particles Particles Particles Particles Particles Particles Particles Particles Particles Particles
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stabilizer which prove the role played by reactive stabilizer for the synthesis of stable PEDOT latexes [23]. Finally, as already noticed in a previous publication, [23] the yield in PEDOT particles never exceed 60% irrespective of the (NH4)2S2O8 concentration. In fact, the yields have been improved up to 80–85% by the use of p-toluenesulfonate hexahydrate [Fe(OTs)3,6(H2O)] as oxidant (runs 17 and 18, Table 1). The chemical composition of the PEDOT was studied by Fourier transform infrared (FTIR) spectroscopy (see Fig. 4). The presence of peaks at 1382 and 1353 cm− 1 (C–C and CfC stretching vibrations of thiophene ring), 1214 and 1095 (C–O–C stretching in ethylene oxide unit), 986 and 694 cm− 1 (C–S bond stretching vibrations in thiophene ring) [25,26] prove the formation of PEDOT. In addition, the peaks at 840, 940, 1095, 1510 and 1618 cm− 1 are characteristics of the PEO and confirm the presence of the latter in the PEDOT samples. The presence and amount of PEO in the PEDOT samples were also determined by 1H NMR (see Table 1). UV–Visible–NIR spectra of the PEDOT samples prepared using different oxidants are shown in Fig. 5. PEDOT samples prepared using (NH4)2S2O8 as oxidant give two absorption peaks, one weak absorption at 900 nm corresponding to polarons and bipolarons due to partial doping of PEDOT during polymerization and another at 530 nm originating from π–π⁎ electronic transition in de-doped PEDOT [25]. However, a strong absorption peak at 900 nm was observed for the PEDOT samples prepared in the presence of Fe(III)(OTs)3·6(H2O) as an oxidant while absorption peak at 530 nm due to undoped PEDOT was not visible. These results corroborate the high conductivities measured for PEDOT samples obtained with Fe(III)(OTs)3·6(H2O). 4.3. Conductivity measurements of PEDOT samples
Fig. 2. SEM images of PEDOT core-shell particles prepared in the presence of (a) α-PyPEO, 10,000 g/mol, 20 wt.% and (b) α-Py-PEO, 10,000 g/mol, 50 wt.%.
550–600 nm to 300–350 nm (runs 2–4, Table 1) as the concentration of α-Py-PEO (Mn = 10,000 g/mol) varies from 20 wt.% to 50 wt.% (see Fig. 2).This phenomenon is logically explained by a higher surface coverage when using a higher amount of steric reactive stabilizer, also resulting in the formation of a larger number of stable primary particles at the beginning of the reaction. As proved by SEM images of PEDOT particles, the size distribution of the particles formed in these conditions is quite narrow, underlining the efficient role of αPy-PEO. In the case of monofunctional PEO, the substitution of pyrrole end group by fluorene moiety has nearly no effect on the particle size (run 9, Table 1). For a given PEO molar mass and concentration, particles with a comparable size and a narrow size distribution were obtained. Dispersion polymerizations performed in the presence of α,ωdifunctional-PEO reactive stabilizer did show some differences as compare to the monofunctional PEO stabilizers (runs 12–16, Table 1). As exemplified by the SEM image shown in Fig. 3, well-calibrated particles of about 300 nm average size diameter have for example been prepared using α,ω-thiophene-PEO of 10,000 g/mol while illdefined PEDOT particles and poor yields were obtained with αthiophene-PEO of 10,000 g/mol. Also, differences in particles' size could be noticed when comparing α,ω-fluorene-PEO to the other di-capped PEOs. Indeed, α,ωfluorene-PEO gave rise to smaller PEDOT particles of about 200 nm than α,ω-pyrrole-PEO or α,ω-thiophene-PEO keeping constant both the molar mass and the concentration of the stabilizers. No stable dispersion was obtained by using non-modified PEO stabilizer as a
The conductivity of PEDOT-PEO samples was measured using the conventional four probes technique on dried and compressed PEDOT particles under the form of disc pellets. Contrarily to experiments performed with Fe(III)(OTs)3·6(H2O) as an oxidant, addition of paratoluene sulfonic acid (PTSA) as an external doping agent was required for PEDOT samples prepared in the presence of (NH4)2S2O8. In the latter case, PTSA (0.67 eq vs EDOT) was added at the beginning of the polymerization. Conductivity values were found from 1.28 × 10− 6 S/cm to 1.10 × 10− 2 S/cm for PEDOT samples. Only low conductivities values were obtained in the presence of ammonium persulfate as an oxidant while high values are obtained when using Fe(III)(OTs)3·6(H2O). The conductivity values increase from 1.74 × 10− 3 S/cm to 1.10 × 10− 2 S/cm as the amount of α-Py-PEO decreases from 50 wt.% to 20 wt.% while using Fe(III)(OTs)3·6(H2O) as an oxidant. This trend is in agreement with the insulating properties of PEO within the material. A similar
Fig. 3. SEM images of PEDOT core-shell particles prepared using α,ω-Th-PEO (10,000 g/mol, 20 wt.%).
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Fig. 4. FTIR spectra of (a) Flu-PEO-Flu and (b) PEDOT-PEO latex.
Fig. 5. UV–Visible–NIR spectra of PEDOT samples using (a) (NH4)2S2O8 and (b) Fe(III)(OTs)3·6(H2O) as oxidants.
result has been obtained and reported in the literature for dispersion polymerization of pyrrole [6,27].
Commission, Government of Pakistan, French Ministry of Education and CNRS for financial support.
5. Conclusion
References
In conclusion, very well-defined spherical particles with a controlled size and a narrow size distribution were prepared in the presence of specifically designed PEO-based reactive stabilizers in aqueous dispersant medium. The morphology and size of the nanoobjects formed were tuned by changing the functionality, molar mass, and concentration of the reactive stabilizers as well as the type of the oxidant used. PEDOT particles exhibiting high conductivity values were obtained in high yields in the presence of Fe(III)(OTs)3·6(H2O) as an oxidant. This strategy, quite efficient to prepare latex of PEDOT, is currently extended to other conductive polymers and will be discussed in forthcoming papers.
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
Acknowledgements We are grateful to the CREMEM from the University of Bordeaux for SEM observations. We are also thankful to the Higher Education
[16] [17] [18]
Y. Kudoh, K. Akami, Y. Matsuya, Synth. Met. 102 (1999) 973. S. Ghoush, O. Inganäs, Adv. Mater. 11 (1999) 1214. F. Jonas, J.T. Morison, Synth. Met. 85 (1997) 1397. A. Gök, B. Sari, J. Appl. Polym. Sci. 98 (2005) 2048. S.P. Armes, B. Vincent, J. Chem. Soc., Chem. Commun. 288 (1987). S.P. Armes, M. Aldissi, S.F. Angew, Synth. Met. 28 (1989) 837. M.R. Simmons, P.A. Chaloner, S.P. Armes, Langmuir 14 (1998) 611. T.K. Mandal, B.M. Mandal, J. Polym. Chem. Part A: Polym. Chem. 37 (1999) 3723. A. Pich, Y. Lu, H.-J.P. Adler, T. Schmidt, K.-F. Arndt, Polymer 43 (2002) 5723. S. Chattopadhyay, S. Banerjee, D. Chakravorty, B.M. Mandal, Langmuir 14 (1998) 1544. S.P. Armes, M. Aldissi, S. Angew, S. Gottesfeld, Langmuir 6 (1990) 1745. X.-G. Li, Q.F. Lü, Chem. Eur. J. 12 (2006) 1349. S.P. Armes, M. Aldissi, S. Angew, S. Gottesfeld, Mol. Cryst. Liq. Cryst. 90 (1990) 63. P. Tadros, S.P. Armes, S.Y. Luk, J. Mater. Chem. 2 (1992) 125. L.B. Groenendaal, F. Jonas, D. Freitag, H. Pielartzik, J.R. Reynolds, Adv. Mater. 12 (2000) 481. H.W. Heuer, R. Wehrmann, S. Kirchmeyer, Adv. Func. Mater. 12 (2002) 89. F. Louwet, L. Groenendaal, J. Dhaen, J. Manca, J.V. Luppen, E. Verdonck, L. Leenders, Synth. Met. 135-136 (2003) 115. S.-G. Oh, S.-S. Im, Curr. Appl. Phys. 2 (2002) 273.
382 [19] [20] [21] [22]
E. Cloutet et al. / Materials Science and Engineering C 29 (2009) 377–382
M.A. Khan, S.P. Armes, Langmuir 15 (1999) 3469. M.G. Han, S.H. Foulger, J. Chem. Soc., Chem. Commun. 19 (2004) 2154. K. Müller, M. Klapper, K. Müllen, Macromol. Rapid Commun. 27 (2006) 586. X. Zhang, J.-S. Lee, G.S. Lee, D.-K. Cha, M.J. Kim, D.J. Yang, S.K. Manohar, Macromolecules 39 (2006) 470. [23] M. Mumtaz, A. de Cuendias, J-L. Putaux, E. Cloutet, H. Cramail, Macromol. Rapid Commun. 27 (2006) 1446.
[24] [25] [26] [27]
J.S. Moore, S.I. Stupp, Macromolecules 23 (1990) 65. Y. Yang, Y. Jiang, J. Xu, J. Yu, Polymer 48 (2007) 4459. S.S. Kumar, C.S. Kumar, J. Mathiyarasu, K.L. Phani, Langmuir 23 (2007) 3401. S.P. Armes, M.N. Aldissi, M. Hawley, J.G. Beery, R. Gottesfeld, Langmuir 7 (1991) 1447.
Contents lists available at ScienceDirect
Materials Science and Engineering C j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m s e c
Synthesis of PEDOT latexes by dispersion polymerization in aqueous media Eric Cloutet ⁎, Muhammad Mumtaz, Henri Cramail Université de Bordeaux, Laboratoire de Chimie des Polymères Organiques, UMR5629 ENSCPB-CNRS-Université de Bordeaux, 16 Avenue Pey Berland, 33607 Pessac Cedex, France
a r t i c l e
i n f o
Article history: Received 9 June 2008 Received in revised form 18 July 2008 Accepted 19 July 2008 Available online 29 July 2008 Keywords: PEDOT Dispersion Nanoparticles Reactive stabilizers PEO
a b s t r a c t The synthesis of poly(3,4-ethylenedioxythiophene) (PEDOT) nano- and micro-objects with a narrow size distribution was achieved in a dispersant aqueous medium. Various oxidants such as ammonium persulfate, iron(III) p-toluenesulfonate and iron (III) trichloride were tested. A series of end-functionalized poly(ethylene oxide) (PEO) were compared as reactive stabilizers. The molar mass and the functionality of these reactive stabilizers were found to be important parameters with respect to the control of particle morphology, particle size and size distribution. PEDOT samples were characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM), size exclusion chromatography (SEC), UV–vis and FTIR spectroscopy and conductivity measurements. © 2008 Elsevier B.V. All rights reserved.
1. Introduction During the last two decades, conjugated polymers have received growing attention from chemists and physicists because of the valuable potential applications of these materials in numerous fields and particularly in optoelectronics [1–3]. However, the difficulty to process conjugated polymers is still preventing their common use and large scale development. In order to overcome this drawback, many research groups enhanced the solubility of conjugated polymers by introducing side chains to the rigid backbone or by preparing composites with other polymers [4]. Another route to ease the processability of conjugated polymers is to synthesize them under the form of sterically stabilized particles. For instance, aqueous dispersions of conjugated polymers have been prepared by chemical oxidative polymerization of the corresponding monomers such as pyrrole or aniline in the presence of suitable water-soluble steric stabilizers [5–14]. Among conjugated conductive polymers, poly(3,4ethylenedioxythiophene) (PEDOT) has more recently attracted specific attention because of its excellent environmental stability, its low oxidation potential and low band gap as well as its good and stable electrochromic behaviour from blue to transparent in neutral and oxidized states respectively [15,16]. The synthesis of stable PEDOT dispersions in water with the help of polystyrene-sulfonic acid was described but the authors did not give any data about the morphology of PEDOT material [17]. Oh and Im [18] have also prepared PEDOT particles in micellar solution templates using sodium dodecylbenzene sulfonic acid (DBSA) as a
⁎ Corresponding author. E-mail address: [email protected] (E. Cloutet). 0928-4931/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2008.07.024
surfactant and FeCl3/Na2S2O8 as an oxidant in aqueous media. However, in this case, PEDOT particles were poorly defined and showed a tendency to form aggregates. In another approach, the syntheses of PEDOT-coated polystyrene particles and PEDOT-coated silica particles have been reported by Armes and colleagues [19] and Han and Foulger [20] respectively. Recently, Müllen and colleagues reported the synthesis of PEDOT nanoparticles by emulsion polymerization in cyclohexane using poly(isoprene)-b-poly(methyl methacrylate) (PI-b-PMMA) as the stabilizer and iron(III) chloride as the oxidant [21]. Interestingly, Zhang and colleagues [22] also described the synthesis of PEDOT nanotubes using sodium bis(2ethylhexyl) sulfosuccinate (AOT) cylindrical micelles as the templates. To our knowledge, the synthesis of well-defined PEDOT nano-objects using steric and reactive stabilizer has never been described in the literature. In this study, we discuss the synthesis, in alcoholic media, of well-defined spherical PEDOT objects (i.e. coreshell nanoparticles and vesicles) in the presence of poly(ethylene oxide) end-functionalized with a (3,4-ethylenedioxythiophene) moiety, using ammonium persulfate or iron(III) p-toluenesulfonate hexahydrate as oxidizing agents. This paper is in the continuation of our previously described results [23]. 2. Experimental 2.1. Materials 3,4-ethylenedioxythiophene (EDOT), N-methyl-2-pyrrolecarboxylic acid, thiophene-2-carboxylic acid, fluorene-9-carboxylic acid, diisopropyl carbodiimide (DIPC) and polyethylene glycols were purchased from Aldrich and used without further purification. Tetrahydrofuran (THF) (J. T. Baker) was first distilled over CaH2 and then distilled over sodium
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E. Cloutet et al. / Materials Science and Engineering C 29 (2009) 377–382
Scheme 1. Synthesis of end-capped PEO by esterification reaction of α,ω-OH PEO with various carboxylic acids, i.e. N-methyl-2-pyrrolecarboxylic acid, thiophene-2-carboxylic acid and fluorene-9-carboxylic acid.
benzophenone. Dichloromethane was distilled over CaH2. Ammonium persulfate, iron (III) p-toluenesulfonate hexahydrate [Fe(OTs)3,6(H2O)] of technical grade, and p-toluenesulfonic acid (98.5%) were purchased from Aldrich and used as received. 4-(Dimethylamino)pyridinium 4-toluene sulfonate (DPTS) was prepared as described in the literature [24]. 2.2. Synthesis of poly(ethyleneoxide) based reactive stabilizers Stabilizers were prepared by the esterification of polyethylene glycol with corresponding carboxylic acid of thiophene, pyrrole and fluorene. For example α,ω-N-methyl-2-pyrrole poly(ethylene oxide) (α,ω-Py-PEO) was synthesized as follows: α,ω-OH poly(ethylene oxide) (5 g, 0.5 mmol, M n = 10,000 g mol- 1), N-methyl-2-pyrrole carboxylic acid (0.313 g, 2.5 mmol) and 4-(dimethylamino)pyridinium 4-toluene sulfonate (DPTS) (0.148 g, 0.50 mmol) were introduced in a 250 mL flame dried three necked round bottom flask under nitrogen. CH2Cl2 (60 mL) was then added and finally, diisopropylcarbodiimide (DIPC) (0.50 mL, 3.25 mmol) was introduced by syringe. Stirring under nitrogen at room temperature was continued for 60 h. The solution was filtered to remove diisopropylurea. The solvent was evaporated and the α,ω-Py-PEO was precipitated in cold ethyl ether (yield = 98%).
phase. SEC for soluble PEDOT samples were performed using two PLgel 5 μm MIXED-C (300 × 7.5 mm) columns, one PLgel 5 μm (50 × 7.5 mm) guard column and a JASCO 875-UV detector with DMF as mobile phase, at 60 °C in the presence of LiBr salt. SEM images of the PEDOT samples were taken using JEOL JSM-5200 and JEOL 6700F scanning microscopes. UV–Visible–NIR spectra were recorded with a Spectramax-M2 spectrometer. FTIR spectra were taken using TENSOR 27 (BRUKER) instrument. Conductivity measurements of the PEDOT sample (pressed pellets) were performed using a Keithly 2400 Source Meter four probe instrument. 4. Results and discussion 4.1. Synthesis of reactive stabilizers based on poly(ethylene oxide) A series of PEO, end-capped by one or two functions such as pyrrole (Py), thiophene (Th) or fluorene (Fluo) and having different
2.3. Synthesis of PEDOT latexes In a typical procedure, PEDOT latexes were prepared by dispersion polymerization as the following: EDOT (1 g, 7 mmol) was charged in the flask equipped with a mechanical stirrer and containing a solution of reactive stabilizer (e.g. α-EDOT-PEO; 1 g; M n = 5200 g/mol) in 80 mL of a mixture of methanol and water (2/3, v/v). A solution of ammonium persulfate (4.36 g dissolved in 20 mL of the methanol– water mixture) was then introduced at once. The reaction mixture was stirred for 72 h at room temperature. The resulting blue dispersion was centrifuged at 10,000 rpm at 5 °C for 30 min. The supernatant was carefully decanted and the dark blue sediment was re-dispersed in methanol + water. This re-dispersion–centrifugation cycle was repeated three times in order to ensure the complete removal of inorganic material such as ammonium sulfate and eventual unattached reactive stabilizer. 3. Characterizations 1 H NMR spectra were recorded using a Bruker AC-400 NMR spectrometer. SEC of the stabilizer were performed using a JASCO HPLC pump type 9012, PL aquagel-OH MIXED 8 μm columns and a Varian (series RI-4) refractive index detector with water as mobile
Fig. 1. 1H NMR (400 MHz) spectrum of (i) hydroxyl-telechelic poly(ethylene oxide) and ¯ n = 10,000 g/mol.). (ii) α-N-methyl-pyrrole poly (ethylene oxide) in DMSO-d6 (M
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Scheme 2. Synthesis of PEDOT latexes by dispersion polymerization in methanol/water (2/3, v/v) mixture in the presence of α-functionalized PEO and α,ω-functionalized PEO reactive steric stabilizer.
molar masses were prepared. The latter was synthesized by esterification of α,ω-OH PEO with the corresponding carboxylic acids of pyrrole, thiophene, and fluorene respectively (Scheme 1). The reaction was catalyzed by the 1:1 molecular complex of 4(dimethylamino) pyridine with p-toluenesulfonic acid also known as 4-(dimethylamino)pyridinium 4-toluenosulfonate (DPTS), the role of DPTS being to suppress the side reaction leading to the formation of N-acylurea [24]. The chain-end derivatization proceeds readily at room temperature which results in the formation of mono- or di-functionalized PEO depending on the amount of the carboxylic acid used. The general synthetic pathway is shown in Scheme 1. All the functionalized PEO were characterized by 1H NMR spectroscopy. An example is shown in Fig. 1. The 1H NMR spectrum in DMSO-d6 of α-N-methyl-2-pyrrole poly(ethylene oxide) (α-PyPEO, M n = 10,000 g/mol) is shown in Fig. 1(ii) (to compare with the one of α,ω-OH PEO Fig. 1(i)). The appearance of signals at 6.08 ppm (a), 6.83 ppm (b) and 7.09 ppm (c), due to pyrrole ring protons resonance and at 4.27 ppm (d) due to CH2O– of the ester function confirms the presence of pyrrole moiety at the PEO chain-end. The
integration values of each peak are in accordance with the presence of only one pyrrole unit per PEO chain. Similarly, 1H NMR characterizations allowed us to confirm the synthesis of poly(ethylene oxide) α,ω-di-capped with different groups such as fluorene, thiophene and pyrrole. 4.2. Dispersion polymerization of EDOT EDOT is partially soluble in water, only 0.21% at 20 °C. The addition of some amount of alcohol (methanol) improves its solubility. Hence, a mixture of methanol and water (2/3, v/v) was used throughout all experiments. The oxidative polymerization of EDOT was performed in this dispersant medium in the presence of end-capped PEO used as reactive and steric stabilizers (see Scheme 2). The effect of the molar mass, functionality and concentration of the reactive PEO as well as the nature and concentration of the oxidant used, on the PEDOT particle formation and size control has been investigated. The results of the dispersion polymerization of EDOT are summarized in Table 1. As a general trend, the size of the PEDOT particles decreases with the increase in stabilizer concentration; the particle size goes from
Table 1 Synthesis of PEDOT dispersions in methanol/water mixture (2:3) using mono and di-functionalized PEO-based reactive stabilizer at 35 °C Run no.
Oxidant type
Stabilizer type
1 2 3 4 5 6a 7a 8a 9 10 11 12 13 14 15 16 17b 18b
(NH4)2S2O8 (NH4)2S2O8 (NH4)2S2O8 (NH4)2S2O8 (NH4)2S2O8 (NH4)2S2O8 (NH4)2S2O8 (NH4)2S2O8 (NH4)2S2O8 (NH4)2S2O8 (NH4)2S2O8 (NH4)2S2O8 (NH4)2S2O8 (NH4)2S2O8 (NH4)2S2O8 (NH4)2S2O8 Fe(OTs)3 Fe(OTs)3
PEO α-Py-PEO α-Py-PEO α-Py-PEO α-Py-PEO α-EDOT-PEO α-EDOT-PEO α-EDOT-PEO α-Flu-PEO α-Py-PEO α-Py-PEO α,ω-Th-PEO α,ω-Py-PEO α,ω-Py-PEO α,ω-Flu-PEO α,ω-Flu-PEO α,ω-Flu-PEO α,ω-Flu-PEO
a
Reference [23]. Reactions were carried at 85 °C. c Calculated by 1H NMR. b
Oxidant
Stabilizer Mn
Stabilizer introduced
Yield
PEOc incorporated
Mn (and PDI of the PEDOT samples)
Particle diameter
Mole equivalent
g/mol
wt.%
%
%
g mol− 1
nmc
1.6 1.6 1.6 1.6 2.6 2.6 2.6 2.6 1.6 1.6 1.6 1.6 1.6 1.6 1.6 1.6 2.6 2.6
10,000 10,000 10,000 10,000 10,000 9700 25,000 51,000 10,000 35,000 35,000 10,000 10,000 35,000 10,000 35,000 35,000 35,000
20 20 35 50 50 50 50 50 20 20 35 20 20 35 20 35 35 50
50 50 50 55 55 30 35 32 60 40 50 50 60 60 60 60 80 80
nd 17 22 23 10 nd 75 68 27 12 3 25 19 40 40 30 nd nd
nd nd 6000 (1.6) 5500 (1.7) 5100 (1.7) nd 5300 (1.4) 8400 (1.3) 7000 (1.8) 6200 (1.8) 6100 (1.7) 5500 (1.7) 7000 (1.5) 5800 (1.5) 5400 (1.4) 6300 (1.6) nd nd
_ 550–600 400–500 300–350 400–600 70–150 160–500 80–120 450–500 350–400 275–300 350–400 500–600 350–400 350–400 200–250 50–80 50–60
Remarks
Coagulum Particles Particles Particles Particles Particles Vesicles Particles Particles Particles Particles Particles Particles Particles Particles Particles Particles Particles
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stabilizer which prove the role played by reactive stabilizer for the synthesis of stable PEDOT latexes [23]. Finally, as already noticed in a previous publication, [23] the yield in PEDOT particles never exceed 60% irrespective of the (NH4)2S2O8 concentration. In fact, the yields have been improved up to 80–85% by the use of p-toluenesulfonate hexahydrate [Fe(OTs)3,6(H2O)] as oxidant (runs 17 and 18, Table 1). The chemical composition of the PEDOT was studied by Fourier transform infrared (FTIR) spectroscopy (see Fig. 4). The presence of peaks at 1382 and 1353 cm− 1 (C–C and CfC stretching vibrations of thiophene ring), 1214 and 1095 (C–O–C stretching in ethylene oxide unit), 986 and 694 cm− 1 (C–S bond stretching vibrations in thiophene ring) [25,26] prove the formation of PEDOT. In addition, the peaks at 840, 940, 1095, 1510 and 1618 cm− 1 are characteristics of the PEO and confirm the presence of the latter in the PEDOT samples. The presence and amount of PEO in the PEDOT samples were also determined by 1H NMR (see Table 1). UV–Visible–NIR spectra of the PEDOT samples prepared using different oxidants are shown in Fig. 5. PEDOT samples prepared using (NH4)2S2O8 as oxidant give two absorption peaks, one weak absorption at 900 nm corresponding to polarons and bipolarons due to partial doping of PEDOT during polymerization and another at 530 nm originating from π–π⁎ electronic transition in de-doped PEDOT [25]. However, a strong absorption peak at 900 nm was observed for the PEDOT samples prepared in the presence of Fe(III)(OTs)3·6(H2O) as an oxidant while absorption peak at 530 nm due to undoped PEDOT was not visible. These results corroborate the high conductivities measured for PEDOT samples obtained with Fe(III)(OTs)3·6(H2O). 4.3. Conductivity measurements of PEDOT samples
Fig. 2. SEM images of PEDOT core-shell particles prepared in the presence of (a) α-PyPEO, 10,000 g/mol, 20 wt.% and (b) α-Py-PEO, 10,000 g/mol, 50 wt.%.
550–600 nm to 300–350 nm (runs 2–4, Table 1) as the concentration of α-Py-PEO (Mn = 10,000 g/mol) varies from 20 wt.% to 50 wt.% (see Fig. 2).This phenomenon is logically explained by a higher surface coverage when using a higher amount of steric reactive stabilizer, also resulting in the formation of a larger number of stable primary particles at the beginning of the reaction. As proved by SEM images of PEDOT particles, the size distribution of the particles formed in these conditions is quite narrow, underlining the efficient role of αPy-PEO. In the case of monofunctional PEO, the substitution of pyrrole end group by fluorene moiety has nearly no effect on the particle size (run 9, Table 1). For a given PEO molar mass and concentration, particles with a comparable size and a narrow size distribution were obtained. Dispersion polymerizations performed in the presence of α,ωdifunctional-PEO reactive stabilizer did show some differences as compare to the monofunctional PEO stabilizers (runs 12–16, Table 1). As exemplified by the SEM image shown in Fig. 3, well-calibrated particles of about 300 nm average size diameter have for example been prepared using α,ω-thiophene-PEO of 10,000 g/mol while illdefined PEDOT particles and poor yields were obtained with αthiophene-PEO of 10,000 g/mol. Also, differences in particles' size could be noticed when comparing α,ω-fluorene-PEO to the other di-capped PEOs. Indeed, α,ωfluorene-PEO gave rise to smaller PEDOT particles of about 200 nm than α,ω-pyrrole-PEO or α,ω-thiophene-PEO keeping constant both the molar mass and the concentration of the stabilizers. No stable dispersion was obtained by using non-modified PEO stabilizer as a
The conductivity of PEDOT-PEO samples was measured using the conventional four probes technique on dried and compressed PEDOT particles under the form of disc pellets. Contrarily to experiments performed with Fe(III)(OTs)3·6(H2O) as an oxidant, addition of paratoluene sulfonic acid (PTSA) as an external doping agent was required for PEDOT samples prepared in the presence of (NH4)2S2O8. In the latter case, PTSA (0.67 eq vs EDOT) was added at the beginning of the polymerization. Conductivity values were found from 1.28 × 10− 6 S/cm to 1.10 × 10− 2 S/cm for PEDOT samples. Only low conductivities values were obtained in the presence of ammonium persulfate as an oxidant while high values are obtained when using Fe(III)(OTs)3·6(H2O). The conductivity values increase from 1.74 × 10− 3 S/cm to 1.10 × 10− 2 S/cm as the amount of α-Py-PEO decreases from 50 wt.% to 20 wt.% while using Fe(III)(OTs)3·6(H2O) as an oxidant. This trend is in agreement with the insulating properties of PEO within the material. A similar
Fig. 3. SEM images of PEDOT core-shell particles prepared using α,ω-Th-PEO (10,000 g/mol, 20 wt.%).
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Fig. 4. FTIR spectra of (a) Flu-PEO-Flu and (b) PEDOT-PEO latex.
Fig. 5. UV–Visible–NIR spectra of PEDOT samples using (a) (NH4)2S2O8 and (b) Fe(III)(OTs)3·6(H2O) as oxidants.
result has been obtained and reported in the literature for dispersion polymerization of pyrrole [6,27].
Commission, Government of Pakistan, French Ministry of Education and CNRS for financial support.
5. Conclusion
References
In conclusion, very well-defined spherical particles with a controlled size and a narrow size distribution were prepared in the presence of specifically designed PEO-based reactive stabilizers in aqueous dispersant medium. The morphology and size of the nanoobjects formed were tuned by changing the functionality, molar mass, and concentration of the reactive stabilizers as well as the type of the oxidant used. PEDOT particles exhibiting high conductivity values were obtained in high yields in the presence of Fe(III)(OTs)3·6(H2O) as an oxidant. This strategy, quite efficient to prepare latex of PEDOT, is currently extended to other conductive polymers and will be discussed in forthcoming papers.
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
Acknowledgements We are grateful to the CREMEM from the University of Bordeaux for SEM observations. We are also thankful to the Higher Education
[16] [17] [18]
Y. Kudoh, K. Akami, Y. Matsuya, Synth. Met. 102 (1999) 973. S. Ghoush, O. Inganäs, Adv. Mater. 11 (1999) 1214. F. Jonas, J.T. Morison, Synth. Met. 85 (1997) 1397. A. Gök, B. Sari, J. Appl. Polym. Sci. 98 (2005) 2048. S.P. Armes, B. Vincent, J. Chem. Soc., Chem. Commun. 288 (1987). S.P. Armes, M. Aldissi, S.F. Angew, Synth. Met. 28 (1989) 837. M.R. Simmons, P.A. Chaloner, S.P. Armes, Langmuir 14 (1998) 611. T.K. Mandal, B.M. Mandal, J. Polym. Chem. Part A: Polym. Chem. 37 (1999) 3723. A. Pich, Y. Lu, H.-J.P. Adler, T. Schmidt, K.-F. Arndt, Polymer 43 (2002) 5723. S. Chattopadhyay, S. Banerjee, D. Chakravorty, B.M. Mandal, Langmuir 14 (1998) 1544. S.P. Armes, M. Aldissi, S. Angew, S. Gottesfeld, Langmuir 6 (1990) 1745. X.-G. Li, Q.F. Lü, Chem. Eur. J. 12 (2006) 1349. S.P. Armes, M. Aldissi, S. Angew, S. Gottesfeld, Mol. Cryst. Liq. Cryst. 90 (1990) 63. P. Tadros, S.P. Armes, S.Y. Luk, J. Mater. Chem. 2 (1992) 125. L.B. Groenendaal, F. Jonas, D. Freitag, H. Pielartzik, J.R. Reynolds, Adv. Mater. 12 (2000) 481. H.W. Heuer, R. Wehrmann, S. Kirchmeyer, Adv. Func. Mater. 12 (2002) 89. F. Louwet, L. Groenendaal, J. Dhaen, J. Manca, J.V. Luppen, E. Verdonck, L. Leenders, Synth. Met. 135-136 (2003) 115. S.-G. Oh, S.-S. Im, Curr. Appl. Phys. 2 (2002) 273.
382 [19] [20] [21] [22]
E. Cloutet et al. / Materials Science and Engineering C 29 (2009) 377–382
M.A. Khan, S.P. Armes, Langmuir 15 (1999) 3469. M.G. Han, S.H. Foulger, J. Chem. Soc., Chem. Commun. 19 (2004) 2154. K. Müller, M. Klapper, K. Müllen, Macromol. Rapid Commun. 27 (2006) 586. X. Zhang, J.-S. Lee, G.S. Lee, D.-K. Cha, M.J. Kim, D.J. Yang, S.K. Manohar, Macromolecules 39 (2006) 470. [23] M. Mumtaz, A. de Cuendias, J-L. Putaux, E. Cloutet, H. Cramail, Macromol. Rapid Commun. 27 (2006) 1446.
[24] [25] [26] [27]
J.S. Moore, S.I. Stupp, Macromolecules 23 (1990) 65. Y. Yang, Y. Jiang, J. Xu, J. Yu, Polymer 48 (2007) 4459. S.S. Kumar, C.S. Kumar, J. Mathiyarasu, K.L. Phani, Langmuir 23 (2007) 3401. S.P. Armes, M.N. Aldissi, M. Hawley, J.G. Beery, R. Gottesfeld, Langmuir 7 (1991) 1447.