Preparation of Water-Free PEDOT Dispersions in the Presence of

Article pubs.acs.org/Langmuir

Preparation of Water-Free PEDOT Dispersions in the Presence of Reactive Polyisoprene Stabilizers Abdulkarim Charba,†,‡,§ Muhammad Mumtaz,†,‡,§ Cyril Brochon,†,‡,§ Henri Cramail,†,‡,§ Georges Hadziioannou,†,‡,§ and Eric Cloutet*,†,‡,§ †

Laboratoire de Chimie des Polymères Organiques, UMR 5629, CNRS, 16 avenue Pey Berland, Pessac, Cedex F-33607, France Laboratoire de Chimie des Polymères Organiques, Université de Bordeaux, 16 avenue Pey Berland, Pessac, Cedex F-33607, France § Laboratoire de Chimie des Polymères Organiques, UMR 5629, IPB, 16 avenue Pey Berland, Pessac, Cedex F-33607, France ‡

S Supporting Information *

ABSTRACT: Poly(3,4-ethylenedioxythiophene) nanoparticles with narrow size distribution were prepared in organic dispersant media in the presence of both iron(III) dodecylbenzenesulfonate {Fe(DBS)3}acting as both an oxidant and a stabilizerand ωfunctionalized polyisoprenes (ω-R-PI) as costabilizers. The effects of the solvent nature and concentration of Fe(DBS)3 on the size and morphology of the PEDOT particles were first studied in the absence of costabilizer. Second, the effects of the molar mass, concentration, and nature of the functional end group of the polyisoprene costabilizer were investigated. PEDOT nano-objects were characterized by transmission electron microscopy (TEM), atomic force microscopy (AFM), dynamic light scattering (DLS), and conductivity measurements.

1. INTRODUCTION After their discovery in the 1960s, conducting polymers have received a lot of interest for their potential applications in the field of electrochromic devices,1 sensors,2 supercapacitors,3 light-emitting diodes,4 photovoltaics,5,6 and electrostatic coatings.7 Despite their remarkable properties, conducting polymers are usually difficult to process, which is in part due to their molecular structure. Indeed, semiconducting polymers are known to be insoluble and infusible in most organic solvents. Several strategies were used to overcome this obstacle. For instance, processability of protonated (conducting) polyaniline was solved by introducing large counterions which decrease the strong interchain interactions.8 Another route which consists in the incorporation of side groups onto the rigid semiconducting backbone has been largely developed to substituted polythiophenes, polyphenylenes, or poly(phenylenevinylene)s, just to name a few.9−12 For processability purposes, an elegant method consists in the preparation of semiconducting polymers under the form of colloidal dispersions, also named latexes, as most dispersant phases are water-based.13−24 Pecher and Mecking13 recently reviewed strategies used to ease the processability of conducting polymers consisting in the preparation of semiconducting polymer latexes. Among the large palette of semiconducting polymers, poly(3,4-ethylenedioxythiophene) (PEDOT) is the most studied in the form of colloidal dispersions in water, particularly when associated with poly(styrenesulfonate) (PSS).25−30 It has been largely developed in optoelectronics and photovoltaics due for example to good film forming capacity and high conductivities. The latter figure was shown to be strongly © XXXX American Chemical Society

affected and enhanced by addition of various organic compounds such as ethylene glycol, dimethyl sulfoxide, or ionic liquids.29,31,32 For example, Kim et al.29 obtained conductivities of commercial PEDOT/PSS as high as 735 S cm−1 when treated with organic solvents such as ethylene glycol. Nevertheless, the major drawback of PEDOT:PSS shows up in devices after it has been deposited as films from water-based solution.33−35 Indeed, it was shown to deteriorate the performance of the organic devices in terms of efficiency as well as lifetime due to residual water in the end-product. PSS is strongly acidic and hygroscopic. It can absorb water easily leading to the deterioration of organic devices. A propose solution consist of post-treatment of PEDOT/PSS with organic solvents leading to minimization of water absorption by PSS depletion. In addition, conductivity increases because of more compact structure and rearrangement of the PEDOT polymer. The previous studies show that the removal of water is crucial for the application of PEDOT in organic electronic devices. More important is the demand from industry for lighting and display applications to develop new electronic inks in the absence of water. This prompted us to check organic solvents as dispersant media for the polymerization of EDOT. In addition, by choosing a hydrophobic stabilizer and a convenient solvent, PEDOT particles surrounded with a hydrophobic stabilizer can be protected from water and Received: June 26, 2014 Revised: August 27, 2014

A

dx.doi.org/10.1021/la502480r | Langmuir XXXX, XXX, XXX−XXX

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Scheme 1. Synthesis of Functionalized Polyisoprene (ω-R-PI) by Anionic Polymerization

humidity. Finally, organic solvents are usually easier to evaporate than water, leading to better film formation. There are few reports dealing with the use of organic solvents as a dispersant medium for semiconducting polymers. Müller et al.36 have prepared PEDOT nanoparticles by emulsion polymerization in cyclohexane/acetonitrile (oil-in-oil emulsion) using polyisoprene-block-poly(methyl methacrylate) (PI-bPMMA) as a stabilizer and FeCl3 as an oxidant. Selvan et al.37,38 have prepared nanoparticle composites of polypyrrole and gold in toluene by polymerizing pyrrole using tetrachloroauric acid as an oxidant in the presence of poly(vinylpyridine) and poly(vinylpyridine)-block-polystyrene as a stabilizer. Han et al.39 reported the preparation of polyaniline dispersion in hexane by reverse emulsion polymerization. In this process, aniline polymerization was initiated with ammonium persulfate (APS) in micelles of dodecylbenzenesulfonic acid (DBSA), leading to PANI particles and aggregates of diameter of 40−60 nm and 1 μm, respectively. Marcilla et al.40 reported the synthesis of PEDOT dispersions in the presence of poly(1vinyl-3-ethylimidazolium bromide) as a stabilizer. Interestingly, these PEDOT samples are redispersible in organic solvents like THF, chloroform, DMF, etc., after anion exchange. In this study, we describe PEDOT dispersions in organic media in the presence of reactive stabilizer. The effects of the solvent nature, concentration, molar mass, and terminal function (reactive moiety) of the reactive costabilizers as well as of the oxidant concentration on the size and morphology of the PEDOT particles have been investigated. Four kinds of end-capped polyisoprenes with different molar masses were used as costabilizers: ω-N-methylpyrrole−polyisoprene (ω-PyPI), ω-fluorene−polyisoprene (ω-Flu-PI), ω-3-thiophene− polyisoprene (ω-3-Th-PI), and ω-methylthiophene−polyisoprene (ω-MeTh-PI). It is noteworthy that such functionalized polyisoprenes can react with the growing PEDOT chains, leading to a control of the particle morphology.

2-carboxaldehyde (Aldrich), thiophene-3-carboxaldehyde (Aldrich), and 3-methythiophene-2-carboxaldehyde (Aldrich) were distilled before use. Toluene was dried over sodium. Isoprene was distilled over CaH2 and then over dibutylmagnesium or over n-butyllithium at 0 °C. Iron(III) chloride, technical grade, was purchased from Reidel-de Haën and used as received. Preparation of Iron(III) Dodecylbenzenesulfonate, Fe(DBS)3. The oxidant iron(III) dodecylbenzenesulfonate (Fe(DBS)3) was prepared by mixing iron(III) chloride (4.5 g, 27.7 mmol) with dodecylbenzenesulfonic acid (38.8 g, 83.2 mmol) in 2-propanol. HCl was removed by heating at 120 °C. Finally, the solvent was evaporated under vacuum. Synthesis of the polyisoprene-based reactive stabilizers (see Supporting Information for detailed description for the synthesis of ω-N-methylpyrrole−polyisoprene (ω-Py-PI), ω-fluorene−polyisoprene (ω-Flu-PI), ω-methylthiophene−polyisoprene (ω-MeTh-PI), and ω-3-thiophene−polyisoprene (ω-3-Th-PI)). Synthesis of the PEDOT Dispersion. In a typical procedure, PEDOT dispersion was prepared as follows: EDOT (400 mg, 2.8 mmol) was put in a specially designed three-necked flask equipped with mechanical stirrer and containing the solution of the required amount of the oxidant (Fe(DBS)3) and the costabilizer dissolved in 40 mL of an organic solvent. The reaction mixture was heated to 50 °C for 7 h. The PEDOT particles were separated by centrifugation, washed several times with methanol, and finally redispersed in an appropriate organic solvent. 2.2. Characterization. 1H NMR spectra were recorded using a Bruker AC-400 NMR spectrometer. Molecular weights of polymers were determined by size exclusion chromatography (SEC) that was performed using a three column set of TSK gel TOSOH (G4000, G3000, and G2000 with pore sizes of 20, 75, and 200 Å, respectively, connected in series) calibrated with polystyrene standards with THF as eluent (1 mL/min) and trichlorobenzene as a flow marker at 40 °C, using both refractometric and UV detectors (Varian). Conductivity of PEDOT product (pressed pellet) was measured by the four-probe method. TEM images of PEDOT samples were taken using a JEOL 2000 FX transmission electron microscope. AFM images were recorded in air with a Nanoscope IIIa microscope operating in tapping mode (TM).

2. EXPERIMENTAL SECTION

3. RESULTS AND DISCUSSION The oxidative dispersion polymerization of EDOT in organic media was studied using Fe(DBS)3 acting as both an oxidant and a stabilizer in the presence, or not, of a polyisoprene-based reactive costabilizer.

2.1. Materials. 3,4-Ethylenedioxythiophene (EDOT) and dodecylbenzenesulfonic acid 70 wt % solution in 2-propanol were purchased from Aldrich and used without further purification. Fluorene-2-carboxaldehyde (Aldrich) was dried by azeotropic distillation with benzene or toluene solution under vacuum. PyrroleB


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S5 represents the 1H NMR spectrum of ω-Py-PI (run 2, Table 1). The four signals appearing at 1.71, 2.06, 4.73, and 5.15 ppm were attributed to polyisoprene. The signals at 6.08, 6.58, and 3.69 ppm are assigned to N-methylpyrrole, confirming the functionalization of PI with N-methylpyrrole. The percentage of functionalization was calculated from the relative integration of the signal positioned at 6.08 ppm assigned to two protons of the pyrrole ring to the signal positioned at 5.15 ppm corresponding to polyisoprene. 3.2. Polymerization of EDOT Using Fe(DBS)3 Acting as Both an Oxidant and a Stabilizer. The solubility of iron(III) dodecylbenzenesulfonate (Fe(DBS)3) in nonpolar organic solvents (thanks to the presence of long aliphatic chain) allowed us to perform the dispersion polymerization of EDOT in cyclohexane, toluene etc. The general procedure for the dispersion polymerization of EDOT is shown in the Supporting Information (see Scheme S1). More than 2 equiv of Fe(DBS)3 vs EDOT is required to ensure the polymerization to proceed. It is noteworthy that dodecyl chains act as stabilizing moieties in organic media. The effects of different parameters such as the nature of the solvent and the concentration of the oxidant on the size, morphology, and conductivity of PEDOT particles were studied. According to the established definition, we made sure that the initial stage of the polymerization was homogeneous (i.e., true dispersion polymerization). 3.2.1. Effect of the Solvent. Different solvents were used for the dispersion polymerization of EDOT in organic media. Table 2 shows the effect of the solvent on the polymerization of EDOT. When THF or methanol was used as reaction media, the reaction was very slow at 50 °C, and practically no significant amount of PEDOT was obtained after 7 h of the reaction. When the temperature of the reaction medium was increased to 85 °C (run 3, Table 2), the polymerization of EDOT took place but with low yield. This PEDOT sample shows very low conductivity, which indicates the formation of oligomers only. The low efficiency of Fe(DBS)3 in methanol and THF is probably due to complexation of the oxidant with the solvent. Similar results were reported by Mandal et al. with Fe(OTS)3 in methanol.15 When cyclohexane, toluene, and dichloromethane were used as dispersant media, the reaction was faster and PEDOT was obtained in good yield (runs 4, 5, and 7, Table 2, and Figure S6 for TEM characterization). The yield obtained in these cases is higher than the theoretical yield (0.35 g) calculated by considering the formula (EDOT)n(DBS)n/3 for the PEDOT doped with DBS− (see Scheme S2 in Supporting Information). This “over yield” is due to the incorporation of some Fe(DBS)2 within PEDOT samples. As Fe(DBS)2 is not totally soluble in cyclohexane or in toluene, it cannot be removed by the classical redispersion/centrifugation “washing” cycles. However, when PEDOT prepared in cyclohexane is washed with methanol, it loses 30% of its weight, and its conductivity increases from 7.5 × 10−4 to 0.017 S/cm. In addition, PEDOT prepared in toluene loses the same quantity of its weight, and its conductivity increases from 0.2 to 1 S/cm after washing with methanol. The increase in conductivity is due to the removal of Fe(DBS)2 by methanol. The TEM image (Figure S6 in Supporting Information) shows that PEDOT product is composed of the aggregates of small particles (less than 100 nm diameter) called particulates or metastable particles. Similar results were obtained with PEDOT stabilized with SDS,41 with PANI stabilized with

3.1. Synthesis of the Reactive PI-Based (ω-R-PI) Costabilizers. We have previously demonstrated that poly(ethylene oxide) end-capped with pyrrole, thiophene, and fluorene moieties were efficient stabilizers toward aqueous EDOT dispersion polymerization.17−20 In this study, polyisoprenes (PI) end-capped with similar terminal moieties were tested as reactive stabilizers in organic dispersant media (cyclohexane and toluene). As we have decided to “solely” investigate the influence of the stabilizer on PEDOT formation, well-defined (low dispersity) end-functional polyisoprenes of various molecular weights were targeted. Dispersity of polymeric stabilizers could indeed be another key tuning parameter during the formation of PEDOT particles as well as for their stability. Thus, anionic polymerization complies with the previous requirements. ω-N-Methylpyrrole−polyisoprene (ω-Py-PI), ω-fluorene− polyisoprene (ω-Flu-PI), ω-3-thiophene−polyisoprene (ω-3Th-PI), and ω-methylthiophene−polyisoprene (ω-MeTh-PI) were prepared by anionic polymerization of isoprene in toluene using sec-butyllithium as an initiator at 40 °C. The endfunctional unit was obtained by adding the corresponding carboxaldehyde at the end of the reaction. Scheme 1 shows the general way of preparing functionalized ω-R-PI. When Nmethylpyrrol-2-carboxaldehyde was added at the end of the reaction, it reacted with the reactive chain end, resulting in the formation of ω-N-methylpyrrole−polyisoprene (ω-Py-PI) after terminating the reaction with methanol. The same strategy was used for the preparation of ω-fluorene−polyisoprene (ω-FluPI), ω-3-thiophene−polyisoprene (ω-3-Th-PI), and ω-methylthiophene−polyisoprene (ω-MeTh-PI). Polyisoprene molecular weights were determined by SEC using THF as an eluent. The molecular weights calculated from polystyrene calibration were multiplied by 0.68 in order to get accurate values. All PI-based costabilizers show a monomodal distribution with low dispersity (Table 1 and Figure S1 in Supporting Information). Table 1. Characteristics of Functionalized Reactive Polyisoprene (ω-F-PI) Obtained by Anionic Polymerization ω-F-PI

Mn (g/mol)a

Đd

% of functionalizationb

ω-Py-PI 01 ω-Py-PI 02 ω-Py-PI 03 ω-Py-PI 04 ω-Py-PI 05 ω-Flu-PI 01 ω-Flu-PI 02 ω-Flu-PI 03 ω-Flu-PI 04 ω-MeTh-PI 01 ω-MeTh-PI 02 ω-3-Th-PI

3800 10000 16100 47000 62000 1800 11000 22000 90000 2700 34000 2000

1.1 1.1 1.1 1.1 1.1 1.1 1.1 1.1 1.2 1.1 1.1 1.1

80 99 99 ndc ndc 90 91 ndc ndc 85 ndc 90

a

Obtained by SEC. bCalculated from NMR spectrum. cNot possible to calculate because of high molecular weight polyisoprenes. dĐ: dispersity = Mw/Mn.

All functionalized ω-Py-PI were characterized by 1H NMR in CDCl3, while 1H NMR of ω-fluorene−polyisoprene (ω-FluPI), ω-3-thiophene−polyisoprene, (ω-3-Th-PI) and ω-methylthiophene−polyisoprene (ω-MeTh-PI) were performed in CD2Cl2 in order to avoid the peak of chloroform at 7.2 ppm (Supporting Information Figures S2−S5). For instance, Figure C


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Table 2. Preparation of PEDOT Particles Using Fe(DBS)3 as an Oxidant in Organic Media

a

run

solvent

T (°C)

PEDOT (g)

σ (S/cm)

1 2 3 4 5 6 7

methanol THF THF methylene chloride cyclohexane isododecane/butanol toluene

50 50 85 50 50 50 50

0 0 0.15 0.30 0.60 0.5 0.56

1 × 10−8 0.02 7.5 × 10−4 0.01 0.015

particle size (nm)

remark

sedimentation aggregation of particles sedimentation sedimentation aggregation of particles

30−80

40−100

b

0.2 g of EDOT was used in each case. [Fe(DBS)3]/[EDOT] = 2.34 mol/mol.

DBSA in organic media39 and with PEDOT and polypyrrole stabilized with reactive and steric polymeric stabilizers in aqueous media.15,18 It is noteworthy that PEDOT samples thus prepared are easily redispersible in toluene. 3.2.2. Effect of the Concentration of Fe(DBS)3. Different samples of PEDOT were prepared using different concentrations of Fe(DBS)3 in toluene at 50 °C. In all experiments, the quantity of EDOT was fixed at 0.2 g (1.3 mmol). The results are summarized in Table 3.

conductivity at each stage were divided by the PEDOT concentration and conductivity at the end of the reaction (see Table 4). As no change in PEDOT concentration was observed Table 4. Kinetics of EDOT Dispersion Polymerizationa time (h)

concn of PEDOTd in the solvent (mg/g)

normalized [PEDOT]b (%)

σ (S/cm)

normalized σc (%)

0 1 2 3 5 9

0 24 29 30 34 34

0 71 86 88 100 100

0 0.90 0.95 1.00 1.05 1.1

0 82 87 91 100 100

Table 3. Preparation of PEDOT Particles Using Fe(DBS)3 as an Oxidant in Toluene at 50 °Ca run

Rb

PEDOT (g)

σd (S/cm)

particle sizec (nm)

8 9 10 11 12

1.6 1.87 2.16 2.34 2.7

0.43 0.47 0.52 0.60 0.60

8.4 × 10−4 0.001 0.04 1.0 1.2

30−90 50−100 60−120 40−100 30−80

a

The concentration of EDOT at the beginning of the reaction is 15 mg/mL. bConcentration at each stage divided by the concentration at the end of the reaction (after 5 h). cConductivity divided by the conductivity at the end of the reaction (after 5 h). dConcentration of PEDOT is more than 15 mg/g because of DBS− and some Fe(DBS)2 incorporated to the polymer.

a

Introduced EDOT = 0.2 g (10 g/L). bR = [Fe(DBS)3]/[EDOT]. Determined by TEM. dConductivity is measured after washing PEDOT with methanol several times.

c

after 5 h, we considered that the reaction was finished, and this concentration was taken as the final concentration (100% conversion). Table 4 shows that polymerization and doping process take place simultaneously with more or less same rates. TEM images show that aggregates of PEDOT particles are formed from the very first hour of the reaction. Such study shows that it is possible to obtain PEDOT dispersion in organic solvents in the presence of Fe(DBS)3 acting as both oxidant and stabilizer. Nevertheless, the obtained particles are agglomerated, which led us to add a costabilizer in order to better control the particles’ morphology. 3.3. Dispersion Polymerization of EDOT in the Presence Polyisoprene-Based Reactive Costabilizers. In order to investigate the effect of the costabilizer on the size and morphology of PEDOT particles, dispersion polymerization of EDOT was performed in the presence of Fe(DBS)3 as an oxidant using various reactive PI costabilizers either in toluene or in cyclohexane as a dispersant medium. Reactive polyisoprene was chosen because of its hydrophobicity, thus protecting PEDOT from humidity, along with its good solubility in organic solvents such as toluene, cyclohexane, etc., and its ease of preparation and functionalization by anionic polymerization. 3.3.1. Dispersion Polymerization of EDOT in Toluene in the Presence of ω-R-PI Reactive Stabilizer. PEDOT samples were prepared in the presence of different kinds of end-capped PI with different molecular weights and terminal functional units (i.e., ω-Py-PI, ω-Flu-PI, ω-3-Th-PI, and ω-MeTh-PI) using Fe(DBS)3 as an oxidant. The influence of the molecular weight and the concentration of the costabilizer on the size and morphology of PEDOT particles was studied. The data of

As it is evident from Table 3, the yields and conductivities of PEDOT increase with the increase in the concentration of Fe(DBS)3 especially when the ratio R = [Fe(DBS)3]/[EDOT] exceeds 2. In agreement with the amount of Fe(DBS)3 required for oxidative polymerization, the excess of Fe(DBS)3 gives rise to PEDOT doping as can be seen in conductivity results (see runs 10−12). All PEDOT samples were characterized by TEM (see Figure S7 in Supporting Information). TEM images show that whatever the concentration of Fe(DBS)3, the obtained PEDOT is composed of an aggregation of nano-objects. When R < 2 (runs 8 and 9, Table 3), the size of the particles decreases with decrease in the concentration of Fe(DBS)3 because the amount of Fe(DBS)3 is not sufficient to polymerize all the EDOT. As expected, when R > 2 (runs 10−12, Table 3), with the increase in the concentration of the oxidant/stabilizer (Fe(DBS)3), more stabilized nuclei were produced, leading to a decrease in the particle size. It is noteworthy that during the reaction stabilization of PEDOT particles is due to DBS− incorporated to PEDOT as a dopant counterion. 3.2.3. Kinetic Study of the Polymerization. The kinetic study of EDOT dispersion polymerization was done by gravimetric analysis and conductivity measurements. During the polymerization, PEDOT aliquots were taken out from the reaction at different times. The product was extracted by centrifugation. PEDOT samples were washed several times with methanol, and their conductivity was measured by the fourprobe method. The concentration of PEDOT and its D


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Table 5. Preparation of PEDOT Particles Using Fe(DBS)3 as an Oxidant in Toluene at 50 °C run

costabilizer

Mn (costabilizer) (g/mol)

wt % of introduced costabilizera (%)

PEDOT particle diameterb (nm)

1 2 3 4 5 6 7 8 9 10 11 12 13

ω-Py-PI ω-Py-PI ω-Py-PI ω-Py-PI ω-Py-PI ω-Py-PI ω-Py-PI ω-Flu-PI ω-Flu-PI ω-Flu-PI ω-MeTh-PI PI PI

3800 10000 16100 47000 62000 62000 62000 11000 22000 90000 34000 20000 54000

60 50 50 50 50 30 10 50 50 50 50 50 50

aggregation 100−250 80−150 60−120 50−80 100−250 300−450 aggregation aggregation + particles (50−120 nm) 30-60 particles + rice grains aggregation aggregation

a

PEDOT particle diameterc (nm)

95−155 69−100 134−194 370−450

Versus EDOT. bDetermined from TEM images. cMeasured by DLS. dFe(DBS)3/EDOT = 2.34 mol/mol.

Figure 1. TEM images of PEDOT particles prepared in the presence of 50 wt % of ω-Py-PI having different molecular weights: (a) Mn = 10 000 g/ mol (run 2, Table 5), (b) Mn = 16 200 g/mol (run 3, Table 5), (c) Mn = 47 000 g/mol (run 4, Table 5), and (d) Mn = 62 000 g/mol (run 5, Table 5).

dispersion polymerization of EDOT are summarized in Table 5. As it is shown, in the presence of low molecular weight ω-PyPI (3800 g/mol), no stable dispersion was formed, and PEDOT particles show tendency to aggregate. The same kind of morphology is obtained when no costabilizer was used. With higher ω-Py-PI molar mass, above 10 000 g/mol, spherical nano-objects were obtained with narrow size distribution as illustrated by TEM images in Figures 1 and 2. As shown from Figure 1 and Table 5, the particles size decreases with the increase in the molecular weight and the concentration of ω-Py-PI. Similar results were reported by us in the case of PEDOT synthesis using reactive stabilizer in aqueous media.18−20 The increase in the molecular weight of ωPy-PI leads to a higher surface coverage of the PEDOT particles, which results in the formation of bigger number of stable primary particles at the beginning of the reaction. As a

consequence, smaller particles are formed. The same effect is observed when the concentration of the stabilizer increases (see Figure 2). The presence of higher amount of the costabilizer results in the formation of larger number of stable primary particles at the beginning of the reaction, leading to the formation of smaller particles at the end of the reaction. The use of ω-Py-PI of 62 000 g/mol as costabilizer at concentration of 10 wt % vs EDOT (Figure 2b) gave rise to spherical particles of 300−450 nm diameter. However, only aggregates of small particles were obtained without costabilizer. This phenomenon proved the crucial role of ω-Py-PI for the synthesis of stable and well-defined PEDOT particles. PEDOT particles were characterized by DLS measurements in toluene at a concentration of 0.5 g/L at 25 °C using ALV Correlator control software. The results are listed in Table 5 (see also Figure S8 in Supporting Information). Data show that PEDOT particle size determined with DLS measurements is E


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Figure 2. TEM images of PEDOT particles prepared in the presence of ω-Py-PI05, Mn = 62 000 g/mol. (a) No costabilizer (run 6, Table 2). (b) ωPy-PI, 10 wt % (run 7, Table 5). (c) ω-Py-PI, 30 wt % (run 6, Table 5). (d) ω-Py-PI, 50 wt % (run 5, Table 5).

mass of 11 000 g/mol leads to the formation of aggregates (run 8, Table 5) while in similar experimental conditions, in the presence of ω-Py-PI having a molar mass of 10 000 g/mol, spherical particles were obtained (run 2, Table 5). The particle size decreases from 50−120 to 30−60 nm when ω-Flu-PI molar mass increases from 22 000 to 90 000 g/mol, proving again the effect of the costabilizer chain length on the particle diameter. In order to see the influence of the different PI terminal groups on PEDOT formation, a batch of living polyisoprenyl lithium solution was divided into four lots in order to react with four different moieties, i.e., N-methylpyrrol-2-carboxaldehyde, fluorene-2-carboxaldehyde, 3-methylthiophene-2-carboxaldehyde, and thiophene-3-carboxaldehyde. Thus, four end-capped PI reactive costabilizers with identical molecular weight were obtained. PEDOT was prepared using Fe(DBS)3 in the presence of each of these four costabilizers. Table S1 in the Supporting Information shows that PEDOT particles obtained in the presence of ω-Py-PI are smaller than those obtained in the presence of other end-capped PI reactive stabilizers (see also Supporting Information Figure S10). It is noteworthy that these functional groups differ by their oxidation potential following the order N-methylpyrrole (0.6 V/SCE) < 3methylthiophene (1.2 V/SCE) < fluorene (1.24 V/SCE) < thiophene (2.3 V/SCE), meaning that ω-Py-PI can be oxidized easier than other costabilizers, thus explaining the better efficacy of ω-Py-PI. Consequently, the size of PEDOT particles increases with the oxidation potential of the terminal unit of the PI costabilizer. 3.3.2. Dispersion Polymerization of EDOT in Cyclohexane. In order to see the effect of different solvents on the dispersion polymerization of EDOT, a series of EDOT polymerizations were carried out using Fe(DBS)3 as an oxidant in the presence of ω-Py-PI as a costabilizer in cyclohexane. Among the reaction parameters that affect particles’ size (distribution) and morphology, we particularly focused on molecular weight and concentration of the costabilizer as well as the polymerization

higher than that determined by TEM images. This phenomenon can be explained by a solvent effect. Indeed, PEDOT particles are swollen by solvent during light scattering measurements as well as during dispersion polymerization. DLS measurements show that particles prepared in the presence of ω-Py-PI having a molecular weight of 62 000 g/mol have an average diameter of 80 nm with a very narrow size distribution. This confirms the results obtained with TEM and AFM images (see Figure S8). The effect of the molecular weight and the concentration of costabilizer on PEDOT particle size is also confirmed by DLS measurements. DLS measurements show that particle size decreases from 95−155 to 69−100 nm when the molecular weight of the costabilizer increases from 47 000 to 62 000 g/mol. Particle size also decreases from 370−450 to 69−100 nm when the concentration of the costabilizer increases from 10% to 50% versus EDOT. AFM image (Figure S8 in Supporting Information) shows that each PEDOT particle is formed by the aggregation of smaller nano-objects (particulates) having a diameter of 15−20 nm, as already reported.15 This result is in favor of a common mechanism of particles formation in toluene as well as in aqueous media.15,18 This mechanism already proposed by Paine saying that the combination of metastable elementary particles leads to the formation of large stable objects.42,43 In order to prove the role of the PI reactive terminal unit, EDOT was polymerized in toluene using Fe(DBS)3 as an oxidant in the presence of nonfunctionalized PI (runs 12 and 13, Table 5). TEM images show that only aggregates were obtained by addition of a non functionalized PI (see Figure S9 in Supporting Information). Same morphology was obtained in the absence of costabilizer. This trend confirms the important role of the reactive terminal group attached to the costabilizer in the stabilization process of PEDOT particles. The effect of the terminal unit (reactive moiety) can also be seen in Table 5. Data show that polymerization of EDOT in the presence of ω-fluorene-polyisoprene (ω-Flu-PI) having a molar F


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major role in terms of solubility and final conductivity as PEDOT once growing adsorb on polymer electrolyte backbone; in other words, PSS acts as a template. Additives that are therefore used in such dispersions allow for a better PEDOT chains orientation and π−π stacking along with increased charge carriers’ mobility (higher conductivity). In addition, aqueous media are in favor for high molecular weight PEDOT and thus higher conjugation length. In our case, once PEDOT oligomers grows they start to precipitate under the form of nuclei stabilized by both polyisoprene and dodecyl benzenesulfonate moieties. The formation of PEDOT occurs by coalescence phenomena between nuclei while the stabilization process still results from polyisoprene and dodecyl benzenesulfonate moieties. It is most probable that molecular weight, and thus conjugation length, are too small in our process while it is difficult to determine. This explains also the lower conductivity. The case of commercially available PEDOTs in organic solvent, i.e., Oligotron and Aedotron, is intermediate as they are formed of block copolymers where one of the block is made of PEDOT while the other block is polar (like poly(ethylene glycol)). Molecular weight of the PEDOT part could be tuned, and additives could affect conductivities of PEDOT materials in part due to phase segregation of block copolymers (selective solvent of the polar segment plays a role). Thus, the above original water-free conductive inks surely need further optimization/ formulation processes in order to increase their conductivity.

temperature. The results of dispersion polymerization of EDOT under different experimental conditions are reported in Table 6. On the one hand, at 50 °C PEDOT particles tend to Table 6. Preparation of PEDOT Particles Using Fe(DBS)3 as Oxidant in Cyclohexane at 50 °C run

Mn(PI-Py) (g/mol)

wt % of PI-Py introduceda

T (°C)

16 17 18 19

10000 47000 62000 62000

50 50 50 50

50 50 50 RT

a

remarksb aggregation aggregation aggregation particles (50−80 nm diameter)

Versus EDOT. bDetermined from TEM images.

aggregate. On the other hand, spherical particles were obtained when EDOT polymerization was performed at room temperature in the presence of ω-Py-PI of 62 000 g/mol (Figure 3, run 19, Table 6).

5. CONCLUSION PEDOT nano-objects were obtained using Fe(DBS)3 as an oxidant/stabilizer in cyclohexane and toluene as dispersant media. A better control of the PEDOT particle morphology and stability was obtained by the addition of PI-based reactive costabilizer. The morphology and particle size were controlled by a fine-tuning of the molecular weight, concentration, and functional end group of PI costabilizer. Pyrrole end-capped polyisoprene (ω-Py-PI) showed higher efficiency than fluorene, thiophene, and methylthiophene end-capped polyisoprene (ωFlu-PI, ω-3-Th-PI, and ω-MeTh-PI). It is noteworthy that with only 10 wt % of ω-Py-PI vs EDOT well-defined PEDOT particles could be obtained in toluene. The mechanism of formation of PEDOT particles in toluene is similar to that in aqueous media. Particles are formed by the combination of smaller particulates. These results on first PEDOT organic dispersion are of prime importance and pave the way to waterfree processes in printed organic electronic technologies as mostly required by industries in the field.

Figure 3. TEM images of PEDOT particles prepared in the presence of ω-Py-PI (62 000g/mol) 50 wt % vs EDOT in cyclohexane at RT (scale indicates 200 nm).

Kinetic study at 50 °C shows that EDOT polymerization in cyclohexane is much faster than polymerization in toluene (see Figure S11 in Supporting Information), impeding an efficient stabilization of the particles in this medium. Indeed, spherical particles can be obtained if the rate of stabilizer adsorption onto the growing particles exceeds the polymerization rate, as already reported by Hwang et al.44

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4. CONDUCTIVITY As shown in Table 3, conductivity of PEDOT increases with the concentration increase of Fe(DBS)3 thanks to the increase of doping level. A conductivity of 0.02 S/cm is obtained at [Fe(DBS)3]/[EDOT] ratio equal to 2.34 (run 6, Table 2). This conductivity increases to 1 S/cm when PEDOT is washed several times with methanol, leading to the solvation of some impurities such as Fe(DBS)2 insoluble in toluene. The conductivity decreases to 8 × 10−3 and to 1 × 10−3 S/cm when 10 and 50 wt % of stabilizer are added, respectively. This conductivity decrease is logically due to the incorporation of more insulating material (PI) to PEDOT particles. Such a system is completely different than all aqueous PEDOT:PSS dispersions or organic PEDOT dispersions that are commercially available. Usually in aqueous media, polymer electrolyte (mostly polystyrenesulfonate or PSS) plays the

ASSOCIATED CONTENT

S Supporting Information *

Experimental details; Figures S1−S11 and Table S1. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (E.C.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are thankful to HIAST (High Institute of Applied Sciences and Technologies, DAMASCUS/Syria) and for the G


Langmuir

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French Ministry of Education for financial support. The authors are grateful to CREMEM (Centre de Ressources en Microscopie Electronique et Microanalyse, Université de Bordeaux) for TEM analyses and to G. Pecastaings for AFM measurements.

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