emulsion synthesis of nanoparticles containing pedot ...frontier/2008, journal of polymer...emulsion...

Emulsion Synthesis of Nanoparticles Containing PEDOTUsing Conducting Polymeric Surfactant: Synergy forColloid Stability and Intercalation Doping

CHI-AN DAI,1,2 CHUN-JIE CHANG,2 HUNG-YU CHI,1 HUNG-TA CHIEN,1 WEI-FANG SU,2 WEN-YEN CHIU1,2

1Department of Chemical Engineering, National Taiwan University, Taipei, 106 Taiwan, Republic of China

2Institute of Polymer Science and Engineering, National Taiwan University, Taipei, 106 Taiwan, Republic of China

Received 17 September 2007; accepted 15 December 2007DOI: 10.1002/pola.22585Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: Poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) isa widely used conductive aqueous dispersion synthesized by using emulsion polymer-ization method. To further enhance its solution processability and conductivity ofPEDOT derivatives, we proposed to replace the nonconductive PSS with conductivepoly[2-(3thienyl)-ethoxy-4-butylsulfonate] (PTEB) as surfactant for the emulsion poly-merization of PEDOT. The reaction involved colloid stabilization and doping in onestep, and yielded PEDOT:PTEB composite nanoparticles with high electrical conduc-tivity. Contrary to its counterpart containing nonconductive surfactant, PEDOT:PTEB showed increasing film conductivity with increasing PTEB concentration. Theresult demonstrates the formation of efficient electrical conduction network formedby the fully conductive latex nanoparticles. The addition of PTEB for EDOT polymer-ization significantly reduced the size of composite particles, formed stable sphericalparticles, enhanced thermal stability, crystallinity, and conductivity of PEDOT:PTEBcomposite. Evidence from UV–VIS and FTIR measurement showed that strong molec-ular interaction between PTEB and PEDOT resulted in the doping of PEDOT chains.X-ray analysis further demonstrated that PTEB chains were intercalated in the lay-ered crystal structure of PEDOT. The emulsion polymerization of EDOT using con-ducting surfactant, PTEB demonstrated the synergistic effect of PTEB on colloid sta-bility and intercalation doping of PEDOT during polymerization resulting in signifi-cant conductivity improvement of PEDOT composite nanoparticles. VVC 2008 Wiley

Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 2536–2548, 2008

Keywords: conducting polymer; emulsion polymerization; nanoparticles

INTRODUCTION

Poly(3,4-ethylenedioxythiophene)s (PEDOT) re-present a class of conjugated polymers that canbe potentially used as active materials for flexibleorganic electronics because of their superior con-ductivity, transparency and thermal stability.1–3

Their unique optoelectronic properties makePEDOT an excellent material for various app-lications such as in electrochromics,4 antistaticcoating,5 light emitting diodes,6,7 solar cell,8,9

sensors,10–13 and biotechnology.14,15 The optoelec-tronic properties of PEDOT can be further tai-lored by controlling its synthetic processes,16–18

altering the polymer backbone conformation,19,20

changing different dopant types,21,22 and intro-ducing various functional groups onto its mainchain.23,24 However, since pure PEDOT bulk

Correspondence to: C.-A. Dai (E-mail: [email protected])

Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 46, 2536–2548 (2008)VVC 2008 Wiley Periodicals, Inc.

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material is insoluble and infusible, muchresearch attention has been focused on the de-velopment of a solution-processable PEDOT for-mulation. Currently, a commercially availableaqueous dispersion solution of PEDOT and poly(styrenesulfonate) (PEDOT-PSS, trade nameBaytron1) was developed by the Bayer AG Com-pany of Germany. PSS serves as charge balanc-ing dopant during polymerization.25 AlthoughPEDOT-PSS is solution processable, there areproblems associated with the formulation forapplications that require low surface resistanceand high optical transparency.26,27 For example,the conductivity of the PEDOT-PSS film is signif-icantly lower than that of pure PEDOT. In addi-tion, water is not particularly a good solvent forspin coating to attain large uniform films.Although electrochemical polymerization iswidely used to fabricate conductive polymer thinfilms, the requirement of using a conducting sub-strate from which a conductive polymer isformed may be too restrictive to meet the needsfor various applications.28

In order for mass production, the chemicaloxidative polymerization of PEDOT is a promis-ing alternative. Such method enables the deposi-tion of films on substrates by a variety of meth-ods such as spin-coating or knife casting. Therehave been various extensive researches made onthe synthesis of PEDOT using chemical oxida-tive polymerization.25,29–32 Among the chemicaloxidative polymerization, emulsion polymeriza-tion seems to be most promising.33,34 However,since the monomer, 3,4-ethylenedioxythiophene(EDOT) is relatively insoluble in water and theonce-initiated monomer, for example, thienylcation radicals reacts in water, a typical oil-in-water emulsifier-free emulsion polymerization ofEDOT results in low yields and poor conductiv-ity. To circumvent this problem, it has recentlybeen suggested that adding surfactants such assmall molecule surfactant, for example, sodiumdodecyl sulfate (SDS) or polymeric surfactant,for example, poly (styrenesulfonate) to an aque-ous solution of EDOT significantly improves po-lymerization yield.35 The advantage of the useof surfactant containing sulfonate functionalgroups is that the surfactant stabilizes the syn-thesized colloids as well as doping in one stepwhich yields PEDOT with high electrical con-ductivity. In general, the conductivity of the la-tex particle increases with increasing dopinglevel of PEDOT from the sulfonate groups of thesurfactant. However, with excess amount of the

nonconductive surfactants on the outer surfaceof conductive particles, the conductivity of thefilm made from the core-shell latex reaches amaximum and decreases with increasing surfac-tant amount.

To overcome the aforementioned problem, atypical oil-in-water emulsion polymerizations ofPEDOT in the presence of water soluble con-ductive oligomers or polymers as surfactantmay be useful. It has been shown previouslythat core-shell nanoparticles consisting of non-conductive polymer core and conductive shell ofvarious types of conducting polymers for exam-ple, polypyrrole, polyaniline, and so forth canbe prepared.36–39 Emulsion polymerization hasalso been used to synthesize latex compositeparticles with multifunctionalities includingsensor, magnetic, and electronic properties.40,41

Although a small addition of conductive poly-mers was needed to form a conductive-fillernetwork with a high electrical conductivity, fur-ther improvement in conductivity beyond a fullcoverage of the core surface with conductivepolymer is limited.

In this article, we will present a strategy forpreparing fully conductive latex nanoparticlesusing emulsion polymerization. We propose tosynthesize a conductive polyanion, poly[2-(3thienyl)-ethoxy-4-butylsulfonate] (PTEB), as asurfactant for the emulsion polymerization ofPEDOT. Since PEDOT chains are insoluble inwater, they can be adsorbed and stabilized byPTEB chains to form composite latex particle.Since PTEB possesses conjugated structurewhich enables charge transport from one PEDOTdomain to the other, we have found that the con-ductivity of the coated PEDOT:PTEB filmincrease with increasing amount of PTEB addi-tion. In addition, the effects of addition of theconductive surfactant on the thermal property,crystallinity, and doping level of the compositenanoparticles were thoroughly studied.

EXPERIMENTAL

Materials

Thienyl ethanol, sodium hydride, 1,4-butanesul-ton, and tetrabutylammonium hydroxide (40 wt %solution in water) were used for the synthesisof PTEB and toluene was used as a solvent forthe reaction. For PEDOT:PTEB latex synthesis,3,4-ethylenedioxythiophene (EDOT) and iron

SYNTHESIS OF NANOPARTICLES CONTAINING PEDOT 2537

Journal of Polymer Science: Part A: Polymer ChemistryDOI 10.1002/pola

(III) p-toluenesulfonate (Fe(OTs)3) were pur-chased from Aldrich and used as received. Tolu-ene was distilled to remove water before usedwhile methanol was used as received.

Chemical Synthesis of TEB

The synthesis of poly[2-(3thienyl)-ethoxy-4-butyl-sulfonate] (PTEB) from thienyl ethanol was con-ducted according to the literature procedure.39 3-thienyl ethoxybutanesulfonic acid sodium salt(TEB-Na) was synthesized by ring opening of 1,4-butanesulton as shown in Figure 1. Since the po-lymerization of TEB-Na monomer in water giveslow yield, Tran-Van et al. showed that by replac-ing sodium cation wtih tetrabutylammonium cat-ion TEB-NR4 has good solubility in chloroformand can be polymerized with FeCl3 in chloroformto obtain high yield.39 Figure 2(a,b) shows the H-NMR characterization of TEB-Na and subse-quently modified TEB-NR4 monomers, respec-tively. We performed a similar procedure and thepolymerization was carried out under argon atroom temperature for 24 h. After filtration, theblack precipitate was washed with dichlorome-thane to remove unreacted monomers, iron(II),

chlorine(I), and other salts. The dark precipitatewas then stirred in an excess amount of NH4OHsolution for 2 days to remove iron(III) by reactingwith the basic solution to form insoluble iron(III)hydroxide and to exchange the iron(III) com-plexed with the sulfonate groups for ammoniumions. The solution turns red. After filtration theaqueous solution was evaporated to obtain a solidred-brown product in 70% yield.39 PTEB of twodifferent molecular weights was synthesized byadjusting the molar ratio of TEB monomer to theoxidant, FeCl3 and the resulting PTEBs aredenoted as L-PTEB and H-PTEB for low and highmolecular weight, respectively. The weight-aver-aged molecular weight and the polydispersityindex (PDI) of the PTEB are listed in Table 1.

Characterization

Surface Tension Measurement

Surface tensions of the PTEB solution weremeasured by a tensiometer (Kruss model no.K12). A sandblasted platinum plate with dimen-sion of 24 3 10 3 0.2 mm was used for surfacetension measurement. The surface tension foreach PTEB solution sample was averaged overthree sets of measurements. The differencebetween the three measurements was usuallywithin 3%.

Thermal Gravimetric Analysis

Thermal gravimetric analysis (TGA) was con-ducted to characterize the thermal stability ofPTEB and PEDOT:PTEB latex samples usingDuPont 951 from the room temperature to600 8C with a heating rate of 10 8C/min undernitrogen atmosphere. The measurements wereconducted using 5 mg samples. The weight reten-tion versus temperature curves were recorded.Samples for the TGA measurement were firstgrinded then dried in an oven at 85 8C for a dayprior to the measurement to prevent moistureabsorption.

FTIR Measurement

The compositions and the doping level ofPEDOT:PTEB latex particles were measuredwith a Fourier transform IR spectrometer(Jasco, FTIR-615R) using a pressed CaF2 pellettechnique.

Figure 1. The synthesis scheme for conductive sur-factant, PTEB, and emulsion synthesis scheme forPTEB/PEDOT core shell latex nanoparticles. PEDOTchains adsorb onto PTEB and form a composite nano-particle.

2538 DAI ET AL.


Conductivity Measurement

The surface resistances of composite films weremeasured using a standard four-point probemethod. The thickness of copolymer film wasmeasured with a TENCOR a-Step 500 surfaceprofiler. The film samples were prepared byknife coating the latex solution onto a glasssubstrate and subsequently drying the film inan oven at 40 8C for 12 h. The thickness of allfilms after drying was maintained to be around20 lm. The conductivity, r of copolymer filmwas calculated by the equation r ¼ 1

Rd where ris the conductivity in S/cm, R is the surface re-

sistance in X, and d is the thickness of copoly-mer film. To measure the UV–Visible absorptionspectra, latex solutions were diluted to a concen-tration of 1 3 10�5 M of PEDOT:PTEB in water.The UV–Visible absorption spectra of the solu-tion were recorded with a Helios UV–VIS spec-trometer.

Transmission Electron Microscopy Measurement

To prepare samples for TEM observation, car-bon-coated copper grids were used as substratesfor drop coating a diluted latex solution on

Figure 2. H-NMR measurement on (a) TEB-Na and (b) TEB-NR4 monomers.



them. The solution covered copper grids wereslowly dried by exposure to air. TEM was per-formed on a JEOL 1230EX operating at 120 kVwith Gatan Dual Vision CCD camera.

RESULTS AND DISCUSSION

Synthesis and Characterization of PTEB

In a typical emulsion polymerization, surfac-tants play significant roles by forming micellesas the polymerization loci for the nucleation ofparticles as well as stabilizing the particles. Inthe current study, we proposed to use PTEB asa surfactant for the synthesis of PEDOT compos-ite nanoparticle. PTEB was synthesized in athree-step procedure from its monomer, 2-(3thienyl)-ethoxy-4-butylsulfonate as first des-cribed by Tran-Van et al.39 We followed thesame procedure as shown in Figure 1 and syn-thesized two different molecular weight ofPTEBs denoted as L-PTEB and H-PTEB for thelow and high molecular weight PTEB, respec-tively, by adjusting the molar ratio of its mono-mer to the oxidant FeCl3. The characteristics ofL-PTEB and H-PTEB are shown in Table 1.

The effect of molecular weight of PTEB on itsoptical and thermal properties as measured byUV–VIS, DSC, and TGA techniques is shown inFigure 3(a–c). UV–VIS characterization of thetwo polymers gives an absorption band at 405

and 420 nm for L-PTEB and H-PTEB, respec-tively, in water solution corresponding to the p–p* electronic transition. The red-shifting of theabsorption maximum demonstrates that withincreasing molecular weight, the conjugationlength of PTEB increases. The result is consist-ent with literature work that shows the conjuga-tion length of conductive polymers increaseswith increasing molecular weight up to a certainmolecular weight of the polymer.42 DSC mea-surement [Fig. 3(b)] of the two polymers showsthat both polymers exhibit Tg at 40 8C and nomelting transition was observed up to tempera-ture of 160 8C. TGA thermograms were recordedunder nitrogen atmosphere for the two PTEBs.A drastic weight loss was observed in the tem-perature range 200–250 8C for L-PTEB and250–300 8C for H-PTEB followed by a continu-ous gradual weight loss. Such a massive weightloss in the temperature range between 200 and300 8C (>50 wt %) should be related to thedecomposition of side chain containing sulfonategroups,43 the removal of tetrabutylammonium, acounter ion to the sulfonate group of PTEB, aswell as the degradation of polythiophene back-bone of PTEB.35 Similar thermal property wasalso observed in poly (3-alkyl thiophene) sys-tem.44 However, the onset temperature of 5%weight loss is much lower for PTEB than thosefor poly(3-alkyl thiophene)s. This is attributed tothe presence of alkoxy pendant groups whichmakes the system more reactive and easily to be

Table 1. Characteristics of the Conducting Surfactant, PTEB,and the Corresponding PEDOT:PTEB Latex Particles

Sample IDTEB:FeCl3

Ratio (mol %) Mw (g/mol) PDI Conductivity (S cm�1)

L-PTEB 1:4 3700 2.3 2.5 3 10�2

H-PTEB 1:2 5100 2.1 3.0 3 10�2

Sample IDPTEB:EDOT

Weight Ratio (wt %)

Concentration ofPTEB

in Water (g/L) Conductivity (S cm�1)

L-PTEB1 0 0.0 0.2L-PTEB2 1 0.3 1.5L-PTEB3 5 1.5 3.5L-PTEB4 10 3.0 4.0H-PTEB1 0 0.0 0.2H-PTEB2 1 0.3 1.2H-PTEB3 5 1.5 2.7H-PTEB4 10 3.0 2.8

2540 DAI ET AL.


oxidized as well as to the presence of sulfonatefunctional groups which is known for their lowerthermal stability. The second weight loss step,which took place in the temperature range of300–600 8C corresponds to the further degrada-tion of polymer backbone leaving a char yield of10 and 30% for L-PTEB and H-PTEB, respec-tively. In addition, the onset temperature of 5%weight loss for the polymer is found to increasewith increasing molecular weight. This decreas-ing thermal stability with the decreasing molec-ular weight is expected for it has been observedin many polymer systems. The relatively poorthermal property of PTEB may prevent it frombeing used as a pure material in many optoelec-tronic applications that require high tem-perature annealing. We will show later that byincorporating PTEB into PEDOT, the thermal

property of the composite particles is muchimproved.

To further test the surfactant efficiency ofPTEB used in the subsequent emulsion synthesis,we measured the surface tension as a function ofdifferent concentrations of PTEB. Figure 3(d)shows that measured surface tension as a func-tion of PTEB concentration in water for bothPTEBs. The surface tension for both PTEB solu-tions decrease monotonically with increasingPTEB concentrations. For the same concentra-tion, the surface tension of L-PTEB solution isslightly lower than that of H-PTEB. Within thisconcentration range, a critical micelle concentra-tion (CMC) for both surfactant solutions was notobserved from the surface tension measurement.For emulsion polymerizations, typically, threemechanisms of particle nucleation are discussed:

Figure 3. (a) UV–VIS absorption of PTEB in aqueous solution. (b) The thermaltransition of PTEB is measured by using DSC. (c) The thermal stability of PTEB ismeasured as the weigh loss versus temperature using TGA. (d) Surface tension ofPTEB aqueous solution is measured as a function of PTEB concentration in water.L-PTEB (– –) and H-PTEB (––).



droplet nucleation, homogeneous nucleation, andmicellar nucleation. For all subsequent emulsionpolymerization, the concentrations of PTEB werebelow its CMC; for example, no micelle was pres-ent in the polymerization solution. Therefore, forthe subsequent emulsion polymerization, thelocation for particle nucleation is primarily fromthe mechanism of homogeneous nucleation.

Synthesis of PEDOT/PTEB Latex Particles

The synthesis of PTEB/PEDOT composite latexparticles follows a standard emulsion polymer-ization method in which a radical polymeriza-tion starts with water, monomer, and surfactant.The synthesis scheme is shown in Figure 1. Anaqueous surfactant solution was prepared withdifferent amount of PTEB in 100 mL of deion-ized water under stirring. Since EDOT is almostinsoluble in water (2.1 g/L at room tempera-ture), EDOT (3.1 g, 22 mmol) was first added in5 mL methanol and the solution was then addedin the surfactant solution with stirring. Theamount of PTEB added into the final aqueoussolution was adjusted so as to make PTEB con-centration of 0, 0.3, 2.1, and 3.0 g/L in waterand to make weight ratio PTEB to EDOT of 0,0.01, 0.07, 0.1 wt %, respectively. The oxidantFe(OTs)3 (29.8 g, 44 mmol) was added slowlyinto the mixture with a molar ratio of Fe(OTs)3to EDOT ¼ 1/2 for polymerization and the solu-tion was stirred for 3 days at room temperature.The initially greenish polymerization solutiongradually turns bluish. After filtration, the pre-cipitate is washed with deionized water anddried. To remove unreacted monomer, oxidantand excess PTEB, the precipitate is then againdispersed in methanol, ultrasonicated for 10 minand washed for purification.

The effect of surfactant concentration of H-PTEB on the morphology of the composite latexparticles is shown in Figure 4(a–d). TEM studiesshow that the PEDOT particles synthesizedwithout any PTEB surfactant were large and ofirregular shape with a typical particle sizelarger than 1 lm [see Fig. 4(a)]. At a relativelylow loading of 1 and 5 wt % PTEB (see sample H-PTEB2 and H-PTEB3 in Table 1), smaller but stillirregular-shaped PEDOT:PTEB composite par-ticles were synthesized as shown in Figure 4(b,c).At highest loading (10 wt %, H-PTEB4), TEMstudies show that latex particles become sphericalin shape and the average size of the particles arein the range of 100 nm as shown in Figure 4(d).

However, most spherical particles are connectedto each other, indicating H-PTEB at this concen-tration is not as effective for particle separation.By comparison, the effect of L-PTEB on the mor-phology of the latex particles is much more pro-nounced. Figure 4(e–h) show that with increas-ing L-PTEB concentration, the morphology ofthe latex particles synthesized changes fromirregular-shape to perfectly spherical particleeven at low concentration compared with that ofH-PTEB. The fact that spherical latex particleswith L-PTEB were synthesized indicates thatthe growing latex particles during polymeriza-tion were swelled by monomers and/or oligo-meric radicals. In addition, each particle wasseparated from each other as shown in Figure4(g,h). Therefore, L-PTEB was more effective instabilizing particle against coagulation.

Property of PEDOT:PTEB Latex Particles

Thermal Gravimetric Analysis

The thermal stability of conducting polymers iscritical for their potential industrial applica-tion. TGA is an important dynamic method todetect the degradation behavior of the synthe-sized nanoparticles. The thermal stability ofPEDOT:PTEB latex nanoparticle was meas-ured by using TGA. Figure 5(a,b) show theweight loss of latex samples synthesized withL-PTEB and H-PTEB, respectively, as a func-tion of temperature. As was discussed previ-ously, both pure L-PTEB and H-PTEB haverelatively poor thermal stability with dramaticweight loss above 200–300 8C. However, the la-tex samples synthesized with PTEB surfactantshows significant improvement in the degrada-tion temperature even with 1 wt % of PTEB toEDOT monomer. For latex particles synthe-sized with L-PTEB or H-PTEB, the degrada-tion temperature and the char yield increasesignificantly with increasing PTEB to EDOTmonomer. Combined with the result from TEMmeasurement, the improvement in the thermalproperty is likely attributed to the higher mo-lecular weight of PEDOT chains synthesized aswell as better PEDOT crystalline phase forma-tion in the particle due to the presence ofPTEB. Without 1 wt % PTEB as surfactant inthe emulsion polymerization, only lower molec-ular weight of PEDOT were synthesized andthese PEDOT form aggregates and show poorthermal property. With 5 and 10 wt % L-PTEB

2542 DAI ET AL.


to EDOT in polymerization solution, the charyield is larger than that of nanoparticles syn-thesized with H-PTEB. This result is consist-

ent with TEM measurement. The nanoparticlessynthesized with L-PTEB are spherical shapeindicating those particles grow by absorption of

Figure 4. TEM micrographs of PEDOT:H-TEB latex particles for different weightfraction of PTEB to EDOT. (a) 0, (b) 1, (c) 5, and (d) 10 wt % of H-PTEB to EDOT; (e)0, (f) 1, (g) 5, and (h) 10 wt % of L-PTEB to EDOT.



oligomeric radicals which increases the molecu-lar weight of PEDOT chains synthesized.

Conductivity

Figure 6 shows the effect of the PTEB additionto EDOT on the conductivity of the PEDOT:

PTEB coated films. Without any PTEB, the con-ductivity of PEDOT film was found to be around0.5 S/cm. With increasing concentration of H-PTEB, the conductivity of the PEDOT:PTEBfilm increases. However, at 5 wt % of H-PTEB toEDOT monomer, the conductivity of PEDOT:H-PTEB films saturate around 2.5 S/cm which is�5 times increase from that of the pure PEDOTfilm. For nanoparticles synthesized with L-PTEB,the conductivity increases with increasing PTEB.The maximum conductivity achieved in this studywith 10 wt % of PTEB is close to 4 S/cm. Theimprovement of conductivity of PEDOT:L-PTEBmay be due to better surfactant property of L-PTEB which yields higher molecular weight ofPEDOT particle synthesized.

UV–VIS Measurement

The UV–VIS spectra of the latex particles pre-pared under different concentration of PTEBwere illustrated in Figure 7(a,b) for H-PTEB and

Figure 5. Thermogravimetric analysis of (a)PEDOT:H-PTEB and (b) PEDOT:L-PTEB latex particlewith different amount of initial PTEB. (– – –) purePTEB, (––) 1 wt %, (–�–�–�–) 5 wt %, and (��) 10 wt %.

Figure 6. Conductivity of PEDOT:H-PTEB (––) andPEDOT:L-PTEB (– – –) for different amount of PTEB.

Figure 7. UV–VIS measurement of (a) PEDOT:H-PTEB and (b) PEDOT:L-PTEB latex particle with dif-ferent amount of initial PTEB. (–––) 0 wt %, (– – –)1 wt %, (–�–�–�–) 5 wt %, and (��) 10 wt %. The verti-cal dash line indicates the onset of absorption of purePEDOT and the short vertical solid line indicates theshift in the onset of the absorption of PEDOT:PTEB.

2544 DAI ET AL.


L-PTEB, respectively. The trend of absorptioncurves are the same for all PEDOT:PTEB filmsprepared with different amount of PTEB whichshows that the absorption around 500 nm de-creases while the absorption >700 nm increaseswith increasing PTEB. In Figure 7, the verticaldash line denotes the onset of absorption of purePEDOT and the short vertical solid line denotesthe shift in the onset of the absorption ofPEDOT:PTEB film. The red-shifting of the onsetabsorption indicates that PEDOTwas doped withthe sulfonate groups of PTEB which results inlonger conjugation length of PEDOT. The UV–VIS measurement of PEDOT:PTEB films corre-late qualitatively well with the conductivity mea-surement.

FTIR

To give a deeper insight of the change in thechemical structure upon PTEB doping, we per-formed FTIR measurement on PEDOT:PTEBfilm. Figure 8(a,b) shows the changes in theFTIR spectra of PEDOT:PTEB powders pre-pared with H-PTEB and L-PTEB, respectively.Note that the vibrations at around 1336–1519 cm�1 are due to C��C or C¼¼C stretchingof quinoidal structure of thiophene ring and dueto ring stretching of thiophene ring (benzoidalstructure), respectively. Vibration at 1186, 1139,and 1080 cm�1 are related to ether bond stretch-ing in the ethylene dioxy group. The band ataround 1336 cm�1, which indicates the existenceof the quinoidal structure, is observed for all ofthe pure PEDOT and PEDOT:PTEB compositelatecies. Therefore all of particles synthesizedare well-doped PEDOT. However, for both H-PTEB and L-PTEB, the peaks at 1186, 1336,and 1519 cm�1 are shifted toward higher wavenumber with the addition of PTEB into PEDOT.This result is distinctly different from that ofPEDOT prepared with emulsion polymerizationusing different oxidant and emulsifier.33 In theirstudy, upon doping those peaks are shifted to alower wave number. They argued that the shiftis due to a higher degree of p-doping achieved inthe polymerization. In our system, upon dopingof PTEB those peaks shifted toward higherwave number suggest an intermolecular interac-tion between PTEB with the correspondingchemical structure. Therefore, the FTIR mea-surement shows that PTEB can be incorporatedinto PEDOT particle during polymerization aswell as effectively serves as dopant for PEDOT.

Structure Determination Using X-ray

X-ray diffraction patterns of PEDOT:PTEB com-posite nanoparticles obtained from H-PTEB andL-PTEB systems are shown in Figure 9(a,b),respectively. We believe that these X-ray diffrac-tion patterns can prove PTEB intercalation dop-ing to PEDOT. Previously, X-ray analysis ofPEDOT films synthesized with Fe(OTs)3 showsthat the corresponding crystal structure of itsfilm is of the orthorhombic type with diffractionpeaks 2h �6.18, 12.18, and 25.88. These diffrac-tion peaks correspond to (1 0 0), (2 0 0), and(0 2 0) reflections with the lattice parameter a¼ 14.3 A, b ¼ 6.8 A and c ¼ 7.8 A of the PEDOTcrystal.45 In addition, a detailed structural anal-ysis showed that the constant a corresponds to

Figure 8. FTIR measurement of (a) PEDOT:H-PTEB and (b) PEDOT:L-PTEB for different amount ofinitial PTEB. The short vertical solid line indicatesthe shift in the wavenumber associated with stretch-ing of quinoidal structure (�1336 cm�1) of PEDOT:PTEB compared with that of pure PEDOT (verticaldash line).



the interchain distance between PEDOT back-bone in the unit cell. The constant c value of theunit cell is related to the repeat unit of PEDOTchain which was previously measured by Aas-mundtveit et al.45 PEDOT chains stack on top ofeach other with a stacking distance of b/2. InFigure 9(a,b), the results of our X-ray measure-ment showed that without PTEB, pure PEDOTsamples are mainly amorphous, whereas thePEDOT:PTEB samples show their characteristicdiffraction peaks, which are indicative of the ex-istence of crystalline phase. The change in thechain structure induced by PTEB provide one ofexplanations of the different conductivities ofPEDOT:PTEB. Since the increase in the crystal-linity of conducting polymer reduces theconjugation defects occurring in the system,therefore, in general, for conducting polymer thesystem becomes more conductive if the molecu-lar order, for example, as in crystalline phase isincreased.

In addition to a more intense peak corre-sponding to the (0 2 0) reflection plane, thispeak position of the PEDOT:PTEB latex isshifted from 25.88 to lower angle with increas-ing PTEB. The amount of angle shifted in-creases with increasing PTEB added in the la-tex for both H-PTEB and L-PTEB systems. Theshift to a lower angle of the (0 2 0) reflectionplane in X-ray diffraction patterns correspondsto an increase in the stacking distance ofPEDOT chains. This result is, however, dramat-ically different from that of PEDOT synthesizedwith Fe(OTs)3 and smaller molecule emulsifierlike DBSA (dodecylbenzene sulfonic acid).35

The increase in conductivity of PEDOT inducedby organic solvent treatment also showed anincrease of the (0 2 0) diffraction peak positionwhich in turn corresponded to a decrease in thestacking distance upon doping.46 In our presentstudy, the decrease in (0 2 0) peak position,therefore the increases in stacking distance inb direction must indicate that the stackedlayers of PEDOT crystal are separated bylayers containing PTEB. In addition, uponintercalating into the stacked layers of PEDOT,PTEB also serves as a dopant for the PEDOTwhich results in the increase in the conductiv-ity with increasing PTEB concentration in theemulsion polymerization.

The growth mechanism of PEDOT nanopar-ticles with PTEB as conducting surfactant issummarized. Figure 10(a,b) shows a schematicrepresentation of nucleation and growth processof PEDOT without or with PTEB, respectively.For the emulsion polymerization without PTEBas surfactant, as PEDOT grows in chain length,it becomes highly insoluble in water and unsta-ble to suspend in water. Therefore, oligomericPEDOT chains coagulate with each other form-ing large aggregates consisting of small nucleior particles as shown in Figure 10(a). For theemulsion polymerization with PTEB, the parti-cle growth mechanism is dramatically different.As oligomeric PEDOT grows in chain length, itabsorbs onto PTEB and the composite particle isstabilized in polymerization solution due to sur-factant property of PTEB. Stabilized particlescan grow in size by the absorption of more oligo-meric radicals and/or more PTEB. Because ofthe increase in the hydrophilicity inside thegrowing particles, monomeric and oligomericPEDOT radicals and PTEB swell the growingparticle, resulting in the formation of sphericalparticle. This process is evident from the TEM

Figure 9. X-ray spectrum of (a) PEDOT:H-PTEBand (b) PEDOT:L-PTEB for different amount ofPTEB: (–––) 0 wt %, (– – –) 1 wt %, (–�–�–�–) 5 wt %,and (��) 10 wt %. The short vertical solid line indi-cates the shift in the distance of the (0 2 0) reflectionin PEDOT:PTEB compared with that in pure PEDOT(vertical dash line).

2546 DAI ET AL.


measurement. Lastly, from our UV–VIS, FTIR,and X-ray measurements, we found that PTEBis effectively intercalated into the PEDOT crys-tal as well as serves as dopant for PEDOT.

CONCLUSIONS

In this article, fully conducting latex nanopar-ticles with high conductivity were synthesizedby emulsion polymerization using water solubleconducting polymer as surfactant. The additionof PTEB in the latex plays two roles, surfactantfor stabilizing the PEDOT nanoparticles anddopant for the enhancement of its conduct-ivity. With increasing PTEB incorporation inthe latex, thermal stability, and crystallinityof PEDOT increase. Smaller and spherical

PEDOT:PTEB nanoparticles with diameter<100 nm can be synthesized with increasingamount of PTEB added during the emulsion po-lymerization. TGA measurement shows that thethermal degradation temperature of the latexincreases considerably with increasing amountof PTEB in the latex. Evidence from UV–VISand FTIR measurement showed that strong mo-lecular interaction between PTEB and PEDOTresulted in the doping of PEDOT chains. X-rayanalysis on the PEDOT crystal structure furtherdemonstrated that PTEB chains were interca-lated in the layered crystal structure of PEDOT.We proposed that alternating layers of PTEBand PEDOT were formed during particle growthin the emulsion polymerization. Contrary tononconductive surfactant covered conductivenanoparticle, the conductivity of PEDOT:PTEBincreases continuously with increasing PTEBsurfactant concentration. Fully conductive filmmade from PEDOT:PTEB nanoparticles can bemade with an improvement in conductivityclosed to 10 times better than that of purePEDOT. The present study provides a novelapproach for synthesizing highly conductive andsolution processable conducting nanoparticlesfor optoelectronic application.

The financial support of this work from the NationalScience Council (NSC 95-2120M-002-004 and 95-3114-P-002-003-MY3), the Department of Economic Affairof Taiwan, and Taiwan Textile Research Institute isgreatly appreciated.

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emulsion synthesis of nanoparticles containing pedot ...frontier/2008, journal of polymer...emulsion...

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