Synthesis of Calibrated Poly (3, 4-ethylenedioxythiophene) Latexes in

of PEDOT particles with a controlled size in alcohol/water dispersant media. ...... Higher Education Commission, Government of Pakistan, and Frenc...
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Langmuir 2008, 24, 11911-11920

11911

Synthesis of Calibrated Poly(3,4-ethylenedioxythiophene) Latexes in Aqueous Dispersant Media Muhammad Mumtaz, Se´bastien Lecommandoux, Eric Cloutet,* and Henri Cramail* UniVersite´ Bordeaux 1, Laboratoire de Chimie des Polyme`res Organiques, UMR 5629 UniVersite´ Bordeaux-1-CNRS-ENSCPB, ENSCPB, 16 AVenue Pey Berland, 33607 Pessac Cedex, France ReceiVed May 23, 2008. ReVised Manuscript ReceiVed June 26, 2008 The synthesis of spherical poly(3,4-ethylenedioxythiophene) (PEDOT) nanoparticles 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) such as R-(3,4-ethylenedioxythiophene) PEO, R-(N-methyl pyrrole) PEO, R-(fluorene) PEO, R,ω-(N-methyl pyrrole) PEO, R,ω-(thiophene) PEO, and R,ω-(fluorene) PEO were compared as reactive stabilizers. The molar mass and the functionality of these reactive PEOs were found to be important parameters with respect to the control of particle size and size distribution. PEDOT samples were characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM), size exclusion chromatography (SEC), and conductivity measurements.

Introduction Conjugated polymers have received a growing interest in the last two decades thanks to their specific electrical and optical properties. These materials combine the advantageous properties of commodity polymers, such as low density, with those of conventional semiconductors, for example, absorption and emission of light and tunable conductivities, making them excellent candidates for optoelectronic devices. Nowadays, these materials are extensively studied and tested for many applications including electrochromics,1 supercapacitors,2 antistatic and electrostatic coatings,3 light-emitting devices,4,5 photovoltaics,6 and sensors.7 Nevertheless, extensive use of these materials at a large scale is still limited due to their difficult processability. One convenient route to ease the process engineering of conductive polymers is to obtain these materials as stabilized nano- or microparticles. For instance, aqueous dispersions of polypyrrole and polyaniline have already been prepared by chemical oxidative polymerization of the corresponding monomers in the presence of suitable water-soluble steric stabilizers.8-16 Among conductive polymers, poly(3,4-ethylenedioxythiophene) (PEDOT) has been attracting specific attention. Indeed, PEDOT has an excellent environmental stability, a low oxidation * To whom correspondence should be addressed. E-mail: cloutet@ enscpb.fr (E.C.); [email protected] (H.C.). (1) Welsh, D. M.; Kumar, A.; Meijer, E. W.; Reynolds, J. R. AdV. Mater. 1999, 11, 1379. (2) Laforgue, A.; Simon, P.; Fauvarque, J. F.; Mastragostino, M.; Soavi, F.; Sarrau, J. F.; Conte, M.; Rossi, E.; Saguatti, S. J. Electrochem. Soc. 2003, 150, A645. (3) Leeuw, D. M.; Kraakman, P. A.; Bongaerts, P. E. G.; Mutsaers, C. M. J.; Klaassen, D. B. M. Synth. Met. 1994, 66, 263. (4) Gong, X.; Moses, D.; Heeger, A. J.; Liu, S.; Jen, A. K. Y. Appl. Phys. Lett. 2003, 83, 183. (5) McGehee, M. D.; Heeger, A. J. AdV. Mater. 2000, 12, 1655. (6) Gao, J.; Yu, G.; Heeger, A. J. AdV. Mater. 1998, 10, 692. (7) Yoon, H.; Chang, M.; Jang, J. AdV. Funct. Mater. 2007, 17, 431. (8) Armes, S. P.; Vincent, B. J. Chem. Soc., Chem. Commun. 1987, 288. (9) Armes, S. P.; Aldissi, M.; Angew, S. F. Synth. Met. 1989, 28, 837. (10) Simmons, M. R.; Chaloner, P. A.; Armes, S. P. Langmuir 1998, 14, 611. (11) Mandal, T. K.; Mandal, B. M. J. Polym. Sci., Part A: Polym. Chem. 1999, 37, 3723. (12) Pich, A.; Lu, Y.; Adler, H.-J. P.; Schmidt, T.; Arndt, K.-F. Polymer 2002, 43, 5723. (13) Chattopadhyay, S.; Banerjee, S.; Chakravorty, D.; Mandal, B. M. Langmuir 1998, 14, 1544. (14) Armes, S. P.; Aldissi, M.; Angew, S.; Gottesfeld, S. Langmuir 1990, 6, 1745.

potential and band gap, as well as a stable electrochromic behavior, from blue to transparent in the neutral and oxidized states, respectively.17,18 Numerous groups have used different techniques to synthesize PEDOT nano-objects with the aim of improving the process engineering of this polymer. For instance, Oh and Im19 have prepared PEDOT particles in aqueous medium containing micelles of sodium dodecylbenzene sulfonic acid (DBSA) used as templates in the presence of FeCl3/Na2S2O8 as the oxidant mixture. The particles thus obtained were rather ill-defined and showed a tendency to aggregate. In another approach, the syntheses of PEDOT-coated polystyrene particles and PEDOT-coated silica particles have been reported by Armes and Khan20 and Fougler and Han,21 respectively. Mu¨llen and co-workers also reported the synthesis of PEDOT nanoparticles by emulsion polymerization in cyclohexane using poly(isoprene)-block-poly(methyl methacrylate) (PI-b-PMMA) as the stabilizer and FeCl3 as the oxidant.22 Zhang et al.23 and Yoon et al.7 also described the original synthesis of PEDOT nanotubes using sodium bis(2ethylhexyl) sulfosuccinate (AOT) cylindrical micelles as templates. We have recently described another strategy for the synthesis of core-shell PEDOT nano-objects. R-EDOT-PEO was efficiently used as a reactive stabilizer in alcoholic/water dispersant medium for the design of PEDOT nano-objects with different morphologies.24 In the continuation of our last investigations, the objective of this work is to study the role of various parameters, such as the functionality, the nature of the termini, and the molar mass of (15) Li, X.-G.; Lu¨, Q. F. Chem.sEur. J. 2006, 12, 1349. (16) Armes, S. P.; Aldissi, M.; Angew, S.; Gottesfeld, S. Mol. Cryst. Liq. Cryst. 1990, 90, 63. (17) Groenendaal, L. B.; Jonas, F.; Freitag, D.; Pielartzik, H.; Reynolds, J. R. AdV. Mater. 2000, 12, 481. (18) Heuer, H. W.; Wehrmann, R.; Kirchmeyer, S. AdV. Funct. Mater. 2002, 12, 89. (19) Oh, S.-G.; Im, S.-S. Curr. Appl. Phys. 2002, 2, 273. (20) Khan, M. A.; Armes, S. P. Langmuir 1999, 15, 3469. (21) Han, M. G.; Fougler, S. H. J. Chem. Soc., Chem. Commun. 2004, 19, 2154. (22) Mu¨ller, K.; Klapper, M.; Mu¨llen, K. Macromol. Rapid Commun. 2006, 27, 586. (23) Zhang, X.; Lee, J.-S.; Lee, G. S.; Cha, D.-K.; Kim, M. J.; Yang, D. J.; Manohar, S. K. Macromolecules 2006, 39, 470. (24) Mumtaz, M.; de Cuendias, A.; Putaux, J.-L.; Cloutet, E.; Cramail, H. Macromol. Rapid Commun. 2006, 27, 1446.

10.1021/la801591d CCC: $40.75  2008 American Chemical Society Published on Web 09/05/2008

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Mumtaz et al. Scheme 1a

a (a) Synthesis of end-capped PEO by esterification reaction of R,ω-OH-PEO with various carboxylic acids, that is, N-methyl-2-pyrrolecarboxylic acid, thiophene-2-carboxylic acid, and fluorene-9-carboxylic acid. (b) Synthesis of PEDOT latexes by dispersion polymerization in methanol/water (2:3) mixture in the presence of R-functionalized-PEO and R,ω-difunctionalized-PEO reactive steric stabilizers.

the reactive PEO stabilizers, on the formation of PEDOT particles with a controlled size in alcohol/water dispersant media. To that purpose, a series of R-functionalized and R,ω-functionalized poly(ethylene oxide) (PEO) of different molar masses, that is, R-(3,4-ethylenedioxythiophene) PEO (R-EDOT-PEO), R-(Nmethyl pyrrole) PEO (R-Py-PEO), R-thiophene PEO (R-Th-PEO), R-fluorene PEO (R-Fluo-PEO), R,ω-(N-methyl pyrrole) PEO (R,ω-Py-PEO), R,ω-(thiophene) PEO (R,ω-Th-PEO), and R,ω(fluorene) PEO (R,ω-Fluo-PEO), have been synthesized and compared as steric reactive stabilizers. End groups such as pyrrole, thiophene, or fluorene moieties have been chosen so as to react by oxidative coupling with the growing PEDOT chains. In addition, monofunctionalized or telechelic reactive stabilizers were investigated, as they are supposed to behave differently in polymerizations in dispersed media.

Experimental Section Materials. 3,4-Ethylenedioxythiophene (EDOT), N-methyl-2pyrrolecarboxylic acid, thiophene-2-carboxylic acid, and fluorene9-carboxylic acid were purchased from Aldrich and used without further purification. Tetrahydrofuran (THF) (J. T. Baker) was first distilled over CaH2 and then distilled over sodium benzophenone.

Dichloromethane was distilled over CaH2. Ammonium persulfate, iron(III) p-toluenesulfonate hexahydrate [Fe(OTs)3 · 6(H2O)] of technical grade, N-methyl-2-carboxylic acid, 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.25 Synthesis of r,ω-N-Methyl-2-pyrrole Poly(ethylene oxide) (r,ω-Py-PEO) and r-N-Methyl-2-pyrrole Poly(ethylene oxide) (r-Py-PEO) Reactive Stabilizers (see Scheme 1). R,ω-OH Poly(ethylene oxide) (5 g, 0.5 mmol, Mn ) 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 R,ω-PyPEO was precipitated in cold ethyl ether (yield ) 98%). 1H NMR (DMSO-d6): δ (ppm) ) 7.09 (s, 2H, CH), 6.83 (s, 2H, CHN), 6.08 (s, 2H, CH), 4.27 (s, 4H, CH2O), 3.84 (s, 6H, NCH3), 3.50 (m, 4H, CH2CH2O). (25) Moore, J. S.; Stupp, S. I. Macromolecules 1990, 23, 65.

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Figure 1. 1H NMR spectra of (a) poly(ethylene oxide), (b) R-N-methyl-pyrrole poly(ethylene oxide), and (c) R,ω-N-methyl-pyrrole poly(ethylene oxide) in DMSO.

Similarly, R-N-methyl-2-pyrrole poly(ethylene oxide) (R-Py-PEO) was prepared using 2.3 equiv of N-methyl-2-pyrrolecarboxylic acid

with respect to PEO (the addition of only 1 equiv of N-methyl-2pyrrole carboxylic acid versus PEO was not sufficient to obtain the

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Table 1. Synthesis of PEDOT Dispersions in Methanol/Water Mixture (2:3) Using Monofunctionalized PEO-Based Reactive Stabilizer at 35°C

run no.

oxidant type

stabilizer type

1 2 3 4 5 6 7a 8a 9a 10 11 12 13 14 15b 16b 17b

(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 Fe(OTs)3

R-Py-PEO R-Py-PEO R-Py-PEO R-Py-PEO R-Py-PEO R-Py-PEO R-EDOT-PEO R-EDOT-PEO R-EDOT-PEO R-Fluo-PEO R-Py-PEO R-Py-PEO R-Py-PEO R-Py-PEO R-Py-PEO R-Py-PEO R-Py-PEO

a

Reference 24.

b

Mn (and PDI PEO of the PEDOT particle oxidant stabilizer stabilizer Mn samples) (mole introduced yield incorporated conductivity diameter (%) (S/cm) (g · mol-1) equiv) (g · mol-1) (wt %) (%) (nm)c 1.6 1.6 1.6 1.6 1.4 2.3 2.6 2.6 2.6 1.6 1.6 1.6 1.6 1.6 2.6 2.6 2.6

5000 10 000 10 000 10 000 10 000 10 000 9700 25 000 51 000 10 000 20 000 20 000 35 000 35 000 35 000 35 000 35 000

20 20 35 50 50 50 50 50 50 20 20 35 20 35 20 35 50

50 50 50 55 55 55 30 35 32 60 50 55 40 50 83 75 80

nd 17 22 23 5 10 nd 75 68 27 28 21 12 3 nd nd nd

nd nd nd nd 5.40 4.23 nd nd nd 4.40 4.36 3.36 8.38 1.28 1.10 1.90 1.74

× 10-6 × 10-6

× × × × × × × ×

10-6 10-6 10-6 10-6 10-6 10-2 10-2 10-3

nd nd 6000 5500 6000 5100 nd 5300 8400 7000 6000 5200 6200 6100

(1.6) (1.7) (2.0) (1.7) (1.4) (1.3) (1.8) (1.6) (1.7) (1.8) (1.7)

550-600 400-500 300-350 600-700 400-600 70-150 160-500 80-120 450-500 200-500 150-300 350-400 275-300

remarks coagulum particles particles particles particles particles particles vesicles particles particles particles particles particles particles desert rose desert rose desert rose

Reactions were carried out at 85 °C. c More than 95% of particles belong to the span given.

Figure 2. SEM images of PEDOT core-shell particles prepared in the presence of (a) R-Py-PEO, 10 000 g · mol-1, 20 wt %, (run 2, Table 1) and (b) R-Py-PEO, 35 000 g · mol-1, 20 wt % (run 13, Table 1).

Figure 3. 1H NMR spectra in DMSO of PEDOT samples prepared using R-Py-PEO as reactive stabilizer, Mn ) 20 000 g · mol-1, 20 wt % (run 11, Table 1).

complete monofunctionalization) (yield ) 97%). 1H NMR (DMSOd6): δ (ppm) ) 7.09 (s, 1H, CH), 6.83 (s, 1H, CHN), 6.08 (s, 1H, CH), 4.54 (s, 1H, OH), 4.27 (s, 2H, CH2O), 3.84 (s, 3H, NCH3), 3.50 (m, 4H, CH2CH2O). Synthesis of r,ω-Thiophene Poly(ethylene oxide) (r,ω-ThPEO) Reactive Stabilizers. The procedure described for the above experiment was repeated using 5 equiv of thiophene-2-carboxylic acid (yield ) 97%). 1H NMR (DMSO-d6): δ ppm ) 7.96 (d, 2H, CH), 7.81 (d, 2H, CHS), 7.22 (t, 2H, CH), 4.36 (t, 4H, CH2O), 3.50 (m, CH2CH2O).

Synthesis of r,ω-Fluorene Poly(ethylene oxide) (r, ω-FluoPEO) Reactive Stabilizers. The method of preparation was the same as described above for the synthesis of R,ω-Py-PEO with the addition of 5 equiv of fluorene-9-carboxylic acid (yield ) 96%). 1H NMR (CHCl3): δ ppm ) 7.73 (d, 4H, CH), 7.66 (d, 4H, CH), 7.39 (t, 4H, CH), 7.32 (t, 4H, CH), 4.31 (t, 4H, CH2O), 3.62 (m, CH2CH2O). Synthesis of PEDOT Particles. In a typical procedure, PEDOT particles were prepared by dispersion polymerization as follows: EDOT (500 mg, 3.5 mmol) was charged in the flask equipped with a mechanical stirrer and containing a solution of reactive stabilizer (e.g., R-Py-PEO, Mn ) 10 000 g · mol-1, 125 mg) in 40 mL of a mixture of methanol and water (2:3, v/v). A solution of ammonium persulfate (4.36 g dissolved in 10 mL of the methanol/water mixture) was then introduced in one shot. 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 redispersed in the mixture of methanol and water (2:3, v/v). This redispersioncentrifugation cycle was repeated three times in order to ensure the complete removal of inorganic material such as ammonium sulfate and eventual unattached reactive stabilizer. Similarly, PEDOT samples were prepared in high yield using Fe(OTs)3 as an oxidant and heating the reaction mixture at 85 °C for 48 h. These samples were then washed as described above. Characterization. 1H NMR spectra were recorded using a Bruker AC-400 NMR spectrometer. Size exclusion chromatography (SEC) of the stabilizer was performed using 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 phase. SEC for soluble PEDOT samples was 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 dimethylformamide (DMF) as mobile phase at 60 °C in the presence of LiBr salt. Scanning electron microscopy (SEM) images of the PEDOT samples were taken using JEOL JSM-5200 and JEOL 6700F scanning microscopes. Transmission electron microscopy (TEM) images of the PEDOT samples were taken using a JEOL 2000FX transmission electron microscope. Atomic force microscopy (AFM) images were recorded in air with a Nanoscope IIIa microscope operating in tapping mode (TM). Dynamic light scattering (DLS) measurements were performed using an ALV Laser goniometer, which consists of a 22 mW HeNe linear polarized laser with a 632.8 nm wavelength, and an ALV5000/EPP Multiple tau digital correlator with 250 ns initial sample time. The samples were kept at a temperature of 25 °C. The accessible scattering angle range is from 40° up to 150°. However, most of

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Figure 4. Autocorrelation function C(q,t) and relaxation time distribution G(t) at 90° of R-EDOT-PEO reactive stabilizers at the concentration of 5 g/L in water/methanol mixture (3:2): (a) R-EDOT-PEO (Mn ) 9700 g · mol-1), (b) R-EDOT-PEO (Mn ) 25 000 g · mol-1), and (c) R-EDOT-PEO (Mn ) 51 000 g · mol-1). (d) Relaxation frequency Γ as a function of q2 for R-EDOT-PEO (Mn ) 25 000 g · mol-1).

the dynamic measurements were performed at 90°. The solutions were introduced into 10 mm diameter glass cells. Data acquisition was obtained with the ALV-Correlator Control software. Solutions used for DLS were prepared using the following method: the water/ methanol mixture used as a solvent was preliminary filtered through a 0.22 µm poly(tetrafluoroethylene) (PTFE) membrane and added to end-capped PEO. The solutions were then left under stirring for several days at 25 ( 1 °C. Conductivity measurements of the PEDOT samples (pressed pellets) were performed using a Keithley 2400 source meter four probe instrument.

Results and Discussion Synthesis of r- and r,ω-End-Functionalized PEO. A series of PEO, end-capped by one or two functions such as pyrrole (Py), thiophene (Th), or fluorene (Fluo) and having different molar masses, were prepared. The latter were synthesized by esterification of R,ω-OH PEO with the corresponding carboxylic acids of pyrrole, thiophene, and fluorene, respectively (Scheme 1a). 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), with the role of DPTS being to suppress the side reaction leading to the formation of N-acylurea.25 The chain-end derivatization proceeds readily at room temperature, which results

in the formation of mono- or difunctionalized PEO depending on the amount of the carboxylic acid used. The general synthetic pathway is shown in Scheme 1. All the functionalized PEOs were characterized by 1H NMR spectroscopy. Three examples are shown in Figure 1. The 1H NMR spectrum in dimethyl sulfoxide (DMSO) of R-Py-PEO (Mn ) 10 000 g · mol-1) is shown in Figure 1b (to compare with that of R,ω-OH PEO in Figure 1a). The appearance of signals at 6.08 (a), 6.83 (b) and 7.09 (c) ppm due to pyrrole ring proton resonance and at 4.27 (d) ppm due to CH2O- of the ester function confirms the presence of the pyrrole moiety at the PEO chainend. The integration values of each peak are in accordance with the presence of only one pyrrole unit per PEO chain. Similarly, the complete disappearance of the signal at 4.54 ppm due to the -OH function confirms the synthesis of R,ω-Py-PEO, as demonstrated in Figure 1c. In the same way, the disappearance of the signal at 4.54 ppm (-OH group) together with the appearance of signals at 7.22-7.96 ppm due to thiophene ring protons and at 4.36 ppm due to CH2Oof the ester group are in accordance with the formation of R,ωTh-PEO (Mn ) 10 000 g · mol-1) (see the Supporting Information, SI1). The 1H NMR spectrum of R,ω-Fluo-PEO was performed in CDCl3, as fluorene peaks are difficult to distinguish in DMSO. The signals at 7.31-7.74 ppm due to the resonance of fluorene

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Figure 5. (a) Relaxation time distributions G(t) at 90° and (b) relaxation frequency Γ as a function of q2 for ω-Fluo-PEO (Mn ) 10 000 g · mol-1), ω-Py-PEO (Mn ) 10 000 g · mol-1), and Py-PEO (Mn ) 20 000 g · mol-1) at 5 g/L concentration.

Figure 6. SEM image of PEDOT samples prepared using R-Py-PEO reactive stabilizer (Mn ) 35 000 g · mol-1, 20 wt %, run 15, Table 1) in the presence of Fe(III)(OTs)3 · 6(H2O) as an oxidant.

ring protons and at 4.31 ppm due to the resonance of protons from CH2O- of the ester group, together with the complete disappearance of the -OH signal at 2.65 ppm prove the presence of fluorene moieties at both ends of the PEO chain (Mn ) 10 000 g · mol-1) (see the Supporting Information, SI1). This was

Mumtaz et al.

confirmed by integration of the different signals. SEC traces of all these end-functionalized PEOs remain monomodal and match the ones of the R,ω-OH PEO precursor, which confirms that the functionalization reactions did not alter the structure of the PEO chains. Dispersion Polymerization of EDOT. EDOT is partially soluble in water at only 0.21 wt % at 20 °C.26 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. 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 1b). 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 size control was investigated. Dispersion Polymerization of EDOT in the Presence of r-Py-PEO and r-Fluo-PEO Reactive Stabilizer. Dispersion polymerization data performed in the presence of R-Py-PEO and R-Fluo-PEO are gathered in Table 1. As can be seen, the size and the morphology of the PEDOT nano-objects are affected by the nature of the oxidant, the molar mass, the concentration, and the functionality of the reactive PEO. Dispersion in the Presence of (NH4)2S2O8 as an Oxidant. As already noticed with R-EDOT-PEO,24 dispersion in the presence of low molar mass R-Py-PEO (5000 g · mol-1) (run 1) does not yield well-defined nano-objects. Coagulation generally occurs in the presence of low molar mass reactive stabilizer. However, well-defined spherical particles could be obtained with the help of higher molar mass reactive stabilizer (from 10 000 to 35 000 g · mol-1) in the presence of (NH4)2S2O8. The particle size decreased from 550-600 to 350-450 nm as the molar mass of R-Py-PEO goes from 10 000 to 35 000 g · mol-1 (runs 2 and 13). This is explained by a higher surface coverage of high molar mass stabilizer as compared to a smaller one, leading to the formation of a higher number of stable primary particles at the start of the dispersion polymerization. Similar results were obtained and reported by Mandal et al. for the dispersion polymerization of pyrrole.11 As a general trend,9,27 the size of the PEDOT particles decreases with the increase in stabilizer concentration; the particle size goes from 550-600 to 300-350 nm as the concentration of R-Py-PEO (Mn ) 10 000 g · mol-1) varies from 20 to 50 wt % (see runs 2-4). Indeed, the use of a higher amount of stabilizer enables a higher surface coverage and therefore leads to the formation of a larger number of stable primary particles at the beginning of the reaction. As shown by SEM images of PEDOT particles, the size distribution of the particles formed in these conditions is quite narrow, underlining the efficient role of R-PyPEO (Figure 2) as a steric and reactive stabilizer. The substitution of pyrrole end-groups by fluorene functions has nearly no effect on the particle size. For a given PEO molar mass and concentration (runs 2 and 10), particles with a comparable size and a narrow size distribution are obtained. As already noticed in a previous publication,24 the yield in PEDOT particles never exceeds 60% irrespective of (NH4)2S2O8 concentration. This can be explained by the low efficiency of this oxidant, compared to FeIII(OTs)3 · 6(H2O) (see below). In addition, all the samples prepared using (NH4)2S2O8 were found to be soluble in DMF and DMSO, allowing the characterization of the PEDOT samples by SEC and 1H NMR. As indicated in Table 1, the molar masses determined by SEC against polystyrene (26) Provisional Product Information Sheet, Trial Product A14060, Bayer AG, Nov 1994. (27) Armes, S. P.; Aldissi, M. N.; Hawley, M.; Beery, J. G.; Gottesfeld, R. Langmuir 1991, 7, 1447.

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Table 2. Synthesis of PEDOT Dispersions in Methanol/Water Mixture (2:3) Using Bifunctionalized PEO-Based Reactive Stabilizer

run no.

oxidant type

stabilizer type

stabilizer Mn (g · mol-1)

1 2 3 4 5 6 7 8 9 10 11b 12b

(NH4)2S2O8 (NH4)2S2O8 (NH4)2S2O8 (NH4)2S2O8 (NH4)2S2O8 (NH4)2S2O8 (NH4)2S2O8 (NH4)2S2O8/FeCl3 (NH4)2S2O8/FeCl3 (NH4)2S2O8/FeCl3 Fe(OTs)3 Fe(OTs)3

R,ω-Th-PEO R,ω-Th-PEO R,ω-Py-PEO R,ω-Py-PEO R,ω-Fluo-PEO R,ω-Fluo-PEO R,ω-Fluo-PEO R,ω-Py-PEO R,ω-Fluo-PEO R,ω-Th-PEO R,ω-Fluo-PEO R,ω-Fluo-PEO

10 000 35 000 10 000 35 000 10 000 35 000 35 000 20 000 20 000 20 000 35 000 35 000

a

Partially soluble in DMF.

b

amount of stabilizer introduced (wt %)

yield (%)

PEO incorporated (%)

Mn (and PDI of the PEDOT samples) (g · mol-1)

particle diameter (nm)c

remarks

20 35 20 35 20 20 35 20 20 20 35 50

50 60 60 60 60 55 60 75 75 75 80 80

25 29 19 40 40 30 30 35 30 33 nd nd

5500 (1.7) 5300 (1.6) 7000 (1.5) 5800 (1.5) 5400 (1.4) 7100 (1.6) 6300 (1.6) 5400 (1.7)a 5400 (1.5)a 5300 (1.8)a nd nd

350-400 250-400 500-600 350-400 350-400 275-300 200-250 50-60 50-60 35-40 50-80 50-60

particles particles particles particles particles particles particles particles particles particles particles particles

Reaction temperature was 85 °C. c More than 95% of particles belong to the span given.

Figure 7. SEM images of PEDOT core-shell particles prepared using (a) R,ω-Th-PEO (run 1, Table 2) and (b) R,ω-Fluo-PEO (run 5, Table 2) as reactive stabilizers having same molar mass (Mn ) 10 000 g · mol-1) and concentrations (20 wt %) but different end-groups, and (c) and (d) are their corresponding TEM images.

calibration remain in the range 5000-7000 g · mol-1 whatever the experimental conditions. A typical SEC trace is shown in the Supporting Information (Figure SI2). The wt % of PEO incorporated within the PEDOT samples could be determined by means of NMR spectroscopy by comparing integrations of peaks at 3.5 and 4.4 ppm that are due to the resonances of ethylene protons of ethylene oxide and PEDOT repeating units, respectively. The PEO content within the particles is generally high (3-28 wt %) and proportional to the amount of reactive PEO introduced at the beginning of the polymerization, proving the effective participation of the reactive stabilizer in the whole polymerization (Figure 3). All the data highlight that R-Py-PEO and R-Fluo-PEO are efficient reactive stabilizers for the synthesis of calibrated PEDOT

particles in the presence of (NH4)2S2O8. In particular, a better control of the particle size and size distribution was systematically obtained as compared to experiments conducted in the presence of R-EDOT-PEO, for which a higher concentration (50 wt %) was required to finally obtain a rather broad particle size distribution (runs 7-9, Table 1).24 With the objective of better understanding the effect of the PEO functionality type on the control of PEDOT morphology, the behavior of these endfunctionalized PEOs was analyzed by DLS at different concentrations in the solvent mixture used in polymerization experiments. End-Functionalized PEO BehaVior in Methanol/Water Mixture (2:3, V/V) with Respect to the End-Group Nature. The solutions were prepared by direct dissolution of the end-functionalized

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Figure 8. SEM images of PEDOT samples prepared using R,ω-FluoPEO reactive stabilizer (Mn ) 35 000 g · mol-1, 50 wt %, run 12, Table 2) in the presence of Fe(III)(OTs)3 · 6(H2O) as an oxidant at 85 °C.

PEO. The autocorrelation functions and their CONTIN analyses28 obtained at 25 °C for different R-EDOT-PEO samples (Mn ) 9700, 25 000, and 51 000 g · mol-1) at a concentration of 5 g/L are shown in Figure 4. In the case of the lower molar mass R-EDOT-PEO samples (Mn ) 9700 and 25 000 g · mol-1) (Figure 4a and b), one main population with hydrodynamic radii RH, respectively, of 97 and 78 nm can be determined. These typical dimensions are characteristic of micellar aggregation. Concerning the higher molar mass R-EDOT-PEO (Mn ) 51 000 g · mol-1) (Figure 4c), the scattering intensity was very low, consistent with the small size observed (RH ) 7.2 nm) that corresponds to free chains (unimers) in solution. These observations can be easily understood, as a sufficient hydrophilic to hydrophobic balance is needed to ensure enough incompatibility and selfassembly process. The solution behavior in methanol/water mixture of R-EDOTPEO (Mn ) 25 000 g · mol-1) was more systematically investigated by DLS at different concentrations and different angles in water/methanol. First, no significant evolution of the apparent hydrodynamic radius has been observed with concentration. In addition, the linear dependence of the relaxation frequency Γ as a function of the square of the wave vector q2 confirms that the observed micellar aggregates are spherical and homogeneous in size (Figure 4d). Taking into account the dimension of a unimer (about 5 nm), such a morphology is not really consistent with a spherical micelle, but a vesicular structure is expected.29,30 In order to further investigate the morphology of the aggregates formed before EDOT polymerization, small-angle neutron scattering (SANS) of the R-EDOT-PEO sample was performed using D2O as a solvent. Although not very well-defined, the evolution of the intensity at q-2 and q-4 suggests again a vesicular type organization (see Figure SI3 in the Supporting Information). The formation of these spherical objects was further confirmed by TEM analysis (see Figure SI4 in the Supporting Information). The majority of these nanoparticles have a size around 80 nm which is very close to the value obtained by DLS analysis. From these analyses, one may anticipate that R-EDOT-PEO (Mn ) 25 000 g · mol-1) self-assembles to form vesicular nano-objects, thus leading to the formation of vesicles after EDOT polym(28) Provencher, S. W. Comput. Phys. Commun. 1982, 27, 213. (29) Che´cot, F.; Brulet, A.; Oberdisse, J.; Gnanou, Y.; Mondain-Monval, O.; Lecommandoux, S. Langmuir 2005, 21, 4308. (30) Che´cot, F.; Lecommandoux, S.; Gnanou, Y.; Klok, H.-A. Angew. Chem., Int. Ed. 2002, 41, 1339.

Mumtaz et al.

erization (run 8, Table 1). Indeed, the formation of vesicles was observed in some conditions with R-EDOT-PEO (run 8). We also performed DLS analysis of R-Py-PEO and R-FluoPEO reactive stabilizers of different molar masses and concentrations in the same solvent conditions. As previously observed for the highest molar mass R-EDOT-PEO (Figure 4c), one single population, corresponding to unimers with RH in the range 4-5 nm, is observed as it is clear from the relaxation time distributions [G(t)] at 90° and relaxation frequency (Γ) as a function of q2 for Py-PEO and Fluo-PEO at the concentration of 5 g/L (Figure 5). Similar results were obtained for other concentrations at different angles (Figure SI5 in the Supporting Information). Hence, we can consider that R-Py-PEO and R-Fluo-PEO reactive stabilizers do not form micellar-like aggregates in this solvent mixture. Their complete solubility in the solvent mixture ensures true dispersion polymerization conditions and may explain the quite good control of the PEDOT particle size distribution and morphology. Dispersion in the Presence of Fe(OTs)3 · 6(H2O) as an Oxidant. Let us recall that very low yield is obtained at room temperature with Fe(OTs)3 · 6(H2O).24 The polymerization of EDOT was thus performed at 85 °C in the presence of R-Py-PEO as a reactive and steric stabilizer. Unlike polymerizations conducted in the presence of (NH4)2S2O8, the solution color change “from transparent to blue purple” is quite fast with Fe(OTs)3 · 6(H2O), in accord with a higher efficiency of this latter oxidant, leading to high polymerization yields (runs 15-17). The probable high molar mass PEDOTs within the samples impede their dissolution in any common organic solvent, limiting their characterization at the molecular level. In this condition, nano-objects with desertrose-like morphology are obtained as shown in Figure 6. Such a morphology may be explained by the fact that R-Py-PEO is not effective enough to sufficiently stabilize the first nuclei, a condition to obtain well-defined spherical particles. Dispersion Polymerization of EDOT in the Presence of r,ω-Difunctionalized PEO Reactive Stabilizers. In order to further study the effect of PEO end-groups onto the control of PEDOT particles, telechelic PEOs (R,ω-Py-PEO, R,ω-Fluo-PEO, and R,ω-Th-PEO) were used as reactive stabilizers. Dispersion in the Presence of (NH4)2S2O8 as an Oxidant. The dispersion polymerization data of EDOT in the presence of R,ω-difunctionalized PEO used as reactive steric stabilizers yield very well-defined PEDOT particles, as shown in Table 2. Alike experiments performed with monofunctional PEO, a similar trend in the PEDOT particle size was observed with respect to the stabilizers’ molar mass and concentration. A comparison between the different stabilizers clearly shows that the size and size distribution of PEDOT particles obtained with R,ω-Fluo-PEO stabilizer (run 7; 200-250 nm) are slightly smaller than those obtained with R,ω-Py-PEO (run 4; 350-400 nm) and R,ω-Th-PEO (run 2; 250-400 nm), in similar experimental conditions. SEM and TEM images of the PEDOT particles prepared using these difunctional stabilizers are shown in Figure 7 that demonstrates the narrow size distribution of PEDOT particles. The PEDOT samples prepared in these conditions were soluble in DMF and DMSO and could be characterized as described above with monofunctional end-capped PEO. From these analyses, the amount of incorporated/grafted PEO could be determined. High wt % of incorporated PEO (19-40 wt %) was measured while using these difunctional PEO. This result may be logically explained by the presence of the two reactive end-groups per chain. From this investigation, it can be partially concluded that the behaviors of monofunctional-

Synthesis of Calibrated PEDOT Latexes

Langmuir, Vol. 24, No. 20, 2008 11919

Figure 9. AFM images of PEDOT core-shell particles prepared using different PEO-based reactive stabilizers of the same molar mass (20 000 g · mol-1) and concentration (20 wt %): (a) R,ω-Fluo-PEO and (b) R,ω-Th-PEO.

and difunctional PEO are quite comparable with respect to the control of the PEDOT morphology. However, unlike PEDOT latexes obtained with monofunctional PEO as steric stabilizer, the latexes obtained in the presence of difunctional PEO were found stable for very long time (several months without sedimentation) and were also more easily redispersible in the solvent mixture. This feature was explained by the higher wt % of incorporated PEO while using telechelic (difunctional) reactive stabilizer. We also performed DLS analysis of reactive R,ω-Py-PEO, R,ω-Th-PEO, R,ω-Fluo-PEO, and R,ω-OH-PEO of the same molar mass (20 000 g · mol-1) in the water/methanol mixture. All the systems presented a very low scattering intensity that corresponds to a small RH around 4 nm, typical of free chains in solution. It is worth noting that R,ω-Py-PEO, R,ω-Th-PEO, and R,ω-Fluo-PEO show similar behavior as hydroxytelechelic PEO in the solvent mixture. Dispersion in the Presence of Fe(III)(OTs)3 · 6(H2O) and (NH4)2S2O8/FeCl3 Mixture as Oxidants. Very small PEDOT particles with a narrow size distribution could be obtained in high yields at 85 °C (runs 11 and 12) in the presence of Fe(III)(OTs)3 · 6(H2O) as oxidant and R,ω-Fluo-PEO as stabilizer (see SEM image, Figure 8). The effect of the double functionality of the stabilizer must be underlined in this specific case, as only desert-rose-like morphologies were obtained in the presence of R-Py-PEO in similar conditions (runs 15-17, Table 1). This result proves the higher efficiency of difunctional stabilizer, compared to the monofunctional one, with respect to the formation and the stabilization of the first nuclei formed along with the dispersion polymerization. As already mentioned, PEDOT samples prepared with this oxidant could not be dissolved in any solvent limiting their characterization, explained by the high PEDOT content within the particles. Interestingly, the synthesis of small PEDOT particles and low size distribution could also be achieved in the presence of mixtures of oxidants, that is, (NH4)2S2O8 plus FeCl3 (1.6:1 mol/mol). The small particles formed (50-60 nm) were characterized by AFM as shown in Figure 9. In that case, the particles were partially soluble in DMF and could be analyzed by SEC and NMR. Data are collected in Table 2 (runs 8-10). Conductivity Measurements of PEDOT Samples. The conductivity of PEDOT-PEO samples was measured using the conventional four-probe technique on dried and compressed PEDOT particles under the form of disk pellets. Contrarily to experiments performed with Fe(III)(OTs)3 · 6(H2O) as an

oxidant, addition of p-toluene sulfonic acid (PTSA) as an external doping agent is required for PEDOT samples prepared in the presence of (NH4)2S2O8. Conductivity values were found from 1.28 × 10-6 to 1.10 × 10-2 S/cm for PEDOT samples. Only low conductivity values were obtained in the presence of ammonium persulfate as an oxidant, while high values were obtained when using Fe(III)(OTs)3 · 6(H2O). This characteristic is explained by the dual role played by iron derivatives that act as oxidant and dopant. The conductivity values increase from 1.74 × 10-3 to 1.10 × 10-2 S/cm as the amount of R-Py-PEO decreases from 50 to 20 wt % while using Fe(III)(OTs)3 · 6(H2O) as the oxidant. This trend is in agreement with the insulating properties of PEO within the material. A similar result has been obtained and reported in the literature for dispersion polymerization of pyrrole.9,27 Finally, it is noteworthy that higher conductivities could have been reached by using “extra dopant” such as iodine.

Conclusion In conclusion, spherical PEDOT 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 media. The morphology and size of the nano-objects formed were tuned by changing the functionality, molar mass, and concentration of the PEO as well as the type of the oxidant used. The DLS analysis of these endfunctionalized PEO gave no organization in the solvent mixture except for R-EDOT-PEO, which gives, in some cases, vesiclelike morphology. There was very little or no effect on the size and morphology of the PEDOT particles by changing the PEO end functionality from pyrrole to fluorene or thiophene. PEDOT particles exhibiting high conductivity values were obtained in high yields in the presence of Fe(III)(OTs)3 · 6(H2O) as the oxidant, but comparatively low yield and conductivity values were obtained using (NH4)2S2O8. A better understanding of the parameters that govern the synthesis of calibrated conductive PEDOT latexes allows us to extend this synthetic strategy to other π-conjugated polymers. It opens original routes to conductive inks for a large range of applications from PLEDs to flexible organic solar cells. Acknowledgment. The authors are thankful to Emmanuel Ibarboure (LCPO) for AFM analysis and Mathieu Urien (IMS, University of Bordeaux 1) for conductivity measurements. We are also thankful to the Higher Education Commission, Govern-

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ment of Pakistan, and French Ministry of Education for financial support. The authors are grateful to the CREMEM (Centre de Ressources en Microscopie Électronique et Microanalyse, Université de Bordeaux) for the TEM analyses. Supporting Information Available: 1H NMR (400 MHz) spectra of R,ω-Th-PEO in DMSO-d6 and R,ω-Fluo-PEO in CDCl3 (Figure SI1). SEC trace of PEDOT sample prepared using R-PyPEO, Mn ) 20 000 g · mol-1, 20 wt % (run 11, Table 1), in DMF as eluent at 60 °C in the presence of LiBr salt (Figure SI2). SANS scattering

Mumtaz et al. intensity I(q) profile of R-EDOT-PEO (Mn ) 25000 g · mol-1) solution (20 g/L) in D2O (Figure SI3). Cryo-TEM images of R-EDOT-PEO (Mn ) 25 000 g · mol-1) in water/methanol mixture (3:2) (Figure SI4). Relaxation time distributions G(t) at different angles and at 5 g/L, and relaxation frequency Γ as a function of q2 at different concentrations for R-Py-PEO (Mn ) 20 000 g · mol-1) (Figure SI5). Description of smallangle neutron scattering (SANS) experiments. This material is available free of charge via the Internet at http://pubs.acs.org. LA801591D