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Current Applied Physics 8 (2008) 659–663

Fabrication of nano-structured polythiophene nanoparticlesin aqueous dispersion

Jung Min Lee, Sun Jong Lee, Yeon Jae Jung, Jung Hyun Kim *

Department of Chemical Engineering, Yonsei University, 134 Shinchon-Dong, Seodaemoon-Gu, Seoul 120-749, Republic of Korea

Received 30 July 2006; received in revised form 13 April 2007; accepted 27 April 2007Available online 10 October 2007

Abstract

The synthetic route of unsubstituted polythiophene (PT) nanoparticles was investigated in aqueous dispersion via Fe3+-catalyzed oxi-dative polymerization. With this new synthetic method, high conversion of thiophene monomers was obtained with only a trace of FeCl3.The dispersion state showed that the PT nanoparticles were well dispersed in many polar solvents, compared to non-polar solvents, suchas acetone, chloroform, hexane, and ethyl acetate. To compare the photoluminescence properties between PT nanoparticle dispersionand PT bulk polymers, the PL intensities were measured in the same measuring conditions. Further, core–shell poly(styrene/thiophene)(poly(St/Th)) latex particles were successfully prepared by Fe3+-catalyzed oxidative polymerization during emulsifier-free emulsion poly-merization. The different polymerization rates of each monomer resulted in core–shell structure of the poly(St/Th) latex particles. The PLdata of the only crumpled shells gave evidence that the shell component of core–shell poly(St/Th) latex particles is indeed PT, which wascorroborated by SEM data. PL intensity of the core–shell poly(St/Th) nanoparticle dispersion was much higher than that of the PTnanoparticle dispersion, due to its thin shell layer morphology, which was explained by the self-absorption effect.� 2007 Elsevier B.V. All rights reserved.

PACS: 78.55.Kz; 78.67.Bf; 82.35.Np; 82.70.Dd

Keywords: Polythiophene; Core–shell; Oxidative polymerization; Emulsifier-free emulsion polymerization

1. Introduction

Polythiophenes (PTs) have been the subject of intensiveresearch and are one of the most important classes of con-jugated luminescent polymers because they can exhibitmany interesting electrochemical, electrochromic, lumines-cent, and shielding properties due to p-conjugated carbonbackbone structures, providing bases for a wide range ofapplicable technologies [1,2]. Thiophene derivatives canbe oxidized chemically, photochemically or electrochemi-cally and polymerized to corresponding oligo- or poly-thio-phenes. Generally conjugated polymers, PTs in particular,however, are difficult to process because of their insolubil-ity in common solvents. Thus, many researches have beencarried out to improve processability [3]. Often the solubi-

1567-1739/$ - see front matter � 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.cap.2007.04.049

* Corresponding author. Tel.: +82 2 2123 7633; fax: +82 2 312 0305.E-mail address: jayhkim@yonsei.ac.kr (J.H. Kim).

lization of conjugated luminescent polymers can beachieved through functionalization of the starting materi-als with suitable side chains prior to oxidative polymeriza-tion. For PTs, intensive efforts have been made to fabricatehighly soluble and easily processable PTs by incorporatingalkyl, aryl or alkysulfonyl groups, for organic solvents andcarboxylic or sulfonic acids for water [4–6]. However, theyield of PT on introducing any additional chemical conver-sion steps was often low and the process itself is expensiveor requires the use of toxic solvents.

Preparation of PT dispersions is one of the ways to settlethese problems, since colloidal dispersions may often beapplied to a wide range of applications instead of true solu-tions. However, the oxidative polymerization of unsubstitut-ed thiophene in aqueous medium has never been reported,due to several critical problems, such as poor water-solubil-ity of PTs, low oxidizing activity of catalysts, and extremelylow conversion. An alternate route for preparing colloidal

660 J.M. Lee et al. / Current Applied Physics 8 (2008) 659–663

luminescent polymers involves coated latex particles with athin layer of conjugated polymers to form luminescent com-posites with core–shell morphology [7]. Luminescent poly-mer thin films coated onto the colloidal surfaces have beenof particular interest, owing to the expected improvementof polymer processability and unique properties intrinsic indispersed nanometer-sized materials.

In the current study, we propose a synthetic route for PTnanoparticles via Fe3+-catalyzed oxidative polymerizationof unsubstituted thiophene monomers inside nano-sizedmonomer droplets, i.e., nano-reactors, dispersed in theaqueous medium. This novel and easy method includesan FeCl3/H2O2 (catalyst/oxidant) combination system.To investigate the factors resulting in improving the photo-luminescence (PL) efficiency of PTs, we introduced a thin(below 20 nm) layer of PTs onto the surface of monodis-perse poly(styrene/sodium p-styrene sulfonate) (poly(St/NaSS)) latex particles by using a one-step reaction. As aresult, core–shell poly(St/Th) latex particles were success-fully prepared via Fe3+-catalyzed oxidative polymerizationin emulsion polymerization and extensively characterized.

2. Experimental

2.1. Materials

Thiophene monomer (Acros Organics, USA) and sty-rene monomer (St, Junsei Chemical, Japan) were used asmonomers. Styrene monomer was purified by using aninhibitor remover column (Aldrich Co., USA). The puri-fied monomer was kept at �5 �C until use. Potassium per-sulfate (KPS, Aldrich Co., USA) used as an initiator andsodium dodecyl sulfate (SDS, Duksan Pure Chemicals,Korea) used as a surfactant were used as received. Sodiump-styrene sulfonate (NaSS, Aldrich Co., USA) was pur-chased and used as received. Sodium bicarbonate(NaHCO3, Aldrich Co., USA), iron chloride (FeCl3, KantoChemicals, Japan), and hydrogen peroxide (H2O2, DongYang Co., Korea) were analytical grades and used withoutfurther purification. Double-distilled and deionized (DDI)water was used throughout the experiments.

Scheme 1. Schematic illustration of the synthetic mechanism for thepreparation of unsubstituted PT via Fe3+-catalyzed oxidativepolymerization.

2.2. Preparation of polythiophene (PT) nanoparticles

PT nanoparticle dispersions were synthesized in a100 mL round-bottomed flask, which was fitted with areflux condenser, a nitrogen gas inlet, an ingredient inlet,and a Teflon magnetic stirrer. The reaction temperaturewas 50 �C and was maintained by a thermostat. The reac-tion procedure is as follows: DDI water (50.0 g, 2.780 mol)was added to the reactor. Thiophene monomer (12.0 g,0.143 mol) was dropped on the aqueous SDS (6.0 g,0.021 mol) solution and then several hydrogen peroxides(27 g, 0.793 mol, 50% aqueous solution) were added tothe reactant mixture solution. Anhydrous FeCl3 (0.018 g,1.1 · 10�4 mol) in DDI water was added to the reactant

mixture solution, and then this reactant mixture was stirredfor 12 h at 50 �C. The yield of PTs was ca. 99%.

2.3. Preparation of core–shell poly(styrene/thiophene)

(Poly(St/Th)) latex particles

Sodium p-styrene sulfonate (0.600 g, 2.90 mmol) andsodium bicarbonate (0.050 g, 6.0 · 10�4 mol) were dissolvedin DDI water (200 g, 11.0 mol) for 0.5 h under N2atmo-sphere. Styrene (12.0 g, 1.15 · 10�1 mol) and thiophene(6.00 g, 7.13 · 10�2 mol) were added to the mixture andheated to 80 �C. After 0.5 h, potassium persulfate (0.012 g,4.44 · 10�5 mol), FeCl3(0.009 g, 5.55 · 10�5 mol), andhydrogen peroxide (13.0 g, 3.82 · 10�1 mol) were added tothe mixture and kept at the same reaction conditions for 24 h.

2.4. Characterizations

The solid contents of the final PT and core/shell poly(St/Th) latexes, as determined by a gravimetric analysismethod, were 9.23 wt.% and 7.94 wt.%, respectively. Theparticle size and size distributions were measured byfield-emission scanning electron microscopy (FESEM;JSM-6500F, JEOL), transmission electron microscopy(TEM, CM 200, Philips) and capillary hydrodynamic frac-tionation (CHDF; CHDF-2000, Matec Applied Sciences).

UV–Visible absorption spectra were obtained with aUV–Visible spectrophotometer (UV-1601PC, Shimadzu).Photoluminescence spectra were recorded with a spectro-fluorophotometer (RF-5301PC, Shimadzu). The solid con-tent of the latex in the quartz cell was 0.1 wt.%. Theexcitation was incident at an angle of 0� onto the front faceof the sample, and the emission was recorded in reflectionat an angle of 90� with respect to the surface normal.

3. Results and discussion

3.1. Unsubstituted polythiophene (PT) nanoparticles

Scheme 1 represents the mechanism proposed for theFe3+-catalyzed oxidative polymerization of thiophene byanalogy to the coupling reactions of aromatic compounds.The thiophene monomers are oxidized by Fe3+ ions, andthen they transform into their cationic radical form, buton the other hand Fe3+ ions are reduced to Fe2+ ions. This

J.M. Lee et al. / Current Applied Physics 8 (2008) 659–663 661

reaction involves the coupling of two monomeric radicalsto produce a dihydro-dimer di-cation which leads to adimer after loss of two protons and re-aromatization.The dimer, which is more easily oxidized than the mono-mer, occurs in its radical form and undergoes a further cou-pling with mono- and oligo-meric cationic radicals. Theoxidation of the Fe2+ ions to Fe3+ ions occurs by a reactionwith the other oxidant, H2O2. H2O2 reacts with a couple ofH+, a kind of by-product of the polymerization, by takingtwo electrons/molecule provided from the couple of Fe2+

and produces water (H2O). This recyclic oxidation–reduc-tion step is most important because this reaction leads tothe regeneration of the Fe3+ ions and results in high con-version of thiophene monomers in spite of extremely lowconcentration of FeCl3.

Fig. 1 shows the SEM micrograph and particle size dis-tribution of the unsubstituted PT nanoparticles. Fig. 1ashows the individual particles with ca. 30 nm average par-ticle size. This result is in good agreement with CHDFdata, i.e., number average diameter (Dn = 26.4), volumeaverage diameter (Dv = 28.1), and weight average diameter(Dw = 31.6), in Fig. 1b. The CHDF data shows a unimodalpeak of a quite narrow particle size distribution. 1.89 · 1013

PT nanoparticles were used to get the statistical particle

Fig. 1. SEM images of unsubstituted PT nanoparticles and the particlesize and size distribution of the PT nanoparticles measured by usingCHDF.

Fig. 2. Dispersion state of unsubstituted PT n

Table 1Hansen solubility parameters of various organic solvents at 25 �C

Organic solvents

MeOH DM SO EtOH DMF

Solubility parameters (MPa1/2) dp 12.3 16.4 8.8 13.7dh 22.3 10.2 19.4 11.3d 29.7 26.6 26.6 24.8

size distribution data by CHDF analysis. From the particlesize and particle size distribution data, a good dispersionstate of PT nanoparticles is expected.

In Fig. 2, photographic images of PT nanoparticles invarious organic solvents are shown. Dispersion stateshowed that the PT nanoparticles were well dispersed inmany polar solvents except acetone, chloroform, hexane,and ethyl acetate. Hansen solubility parameters (d) wereintroduced to interpret the dispersibility of the preparedPT nanoparticles for various organic solvents. The solubil-ity parameter includes cohesive energy from dispersiveforce (dd), permanent dipole–dipole interaction (dp), andhydrogen bonding force (dh).

As seen in Table 1, the d value of PT nanoparticle dis-persion in various organic solvents is in accord with the dis-persion state in Fig. 2. The dh is one of the governingfactors in the index of dispersion. Thiophene is a heterocy-clic compound having one sulfur atom; therefore, hydrogenatoms from organic solvents are attracted to a lone pair ofelectrons on the negatively polarized sulfur atom of thio-phene to form a hydrogen bond. Dispersibility is affectedby the dp. The sulfur atom of thiophene is more electroneg-ative than hydrogen and has lone-pair electrons. The lone-pair orbital of the sulfur atoms in PT stick out into spaceaway from the positively charged nuclei, which gives riseto a considerable charge separation and large contributionto dp. As a result, the dispersibility of PT nanoparticles inpolar solvents, such as methanol, ethanol, dimethyl sulfox-ide (DMSO), dimethylformamide (DMF), and n-methyl-3-phenylpiperazine (NMP) is better than in non-polarsolvents, such as tetrahydrofuran (THF), chloroform(CHCl3), and hexane.

Fig. 3 shows photoluminescence spectra of unsubstitut-ed PT nanoparticle dispersions prepared by Fe3+-catalyzedoxidative polymerization and conventional oxidative poly-merization by using much FeCl3. As seen in Fig. 3, PLintensities of PT nanoparticle dispersions prepared bytwo different oxidative polymerization methods were ca.1.5 and 6 in the same measuring conditions, respectively.

anoparticles in various organic solvents.

NMP THF Acetone Chloroform Ethyl acetate Hexane

12.3 5.7 10.4 3.1 5.3 07.2 8.0 7.0 5.7 7.2 0

22.9 19.4 20.1 19.0 18.2 14.9

Fig. 3. PL spectra of PT nanoparticle dispersions prepared by Fe3+-catalyzed oxidative polymerization and conventional oxidative polymer-ization (excitation wave length ðkUV

maxÞ ¼ 400 nm).

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The PL intensity of PT nanoparticle dispersions in theFe3+-catalyzed system was four times higher than that inthe conventional method, resulting from the quenchingeffect of the FeCl3 [8]. A very large molar concentrationof FeCl3 (i.e., FeCl3:thiophene = 2.5:1 based on molarratio) is often used for the conventional bulk or solutionpolymerization of thiophene; however, it should beremoved from the final product to obtain a sufficient PLproperty due to the quenching effect of the FeCl3. On theother hand, there is no need to remove FeCl3 in the currentwork, because only a trace amount of FeCl3 was used. Inaddition, it would be much easier to remove trace amountsof FeCl3, since the prepared PT was in particle form [9].

3.2. Core-shell poly(St/Th) latex particles

Core-shell poly(St/Th) latex particles were prepared byoxidative polymerization of thiophene monomers duringemulsifier-free emulsion polymerization of styrene mono-mers. Fig. 4 shows representative SEM images of core–shell poly(St/Th) latex particles. This result was causedby the different polymerization rates of the monomers.Commonly, the rate of oxidative polymerization is fasterthan that of radical polymerization. PT can be polymerizedin one and a half hours, but on the other hand, polystyrene

Fig. 4. SEM images of (a) core–shell poly(St/Th) latex particles

needs more time to be polymerized entirely (i.e., 4 h) in thepresence of sodium p-styrene sulfonate (s.c. 3 wt.% basedon monomer). Pre-formed PT shell layer provides polymer-ization loci as a nano-reactor with hydrophobic domainsfor growing oligomers of styrene to be polymerized. PTshell layer is believed to be prepared by the Fe3+-catalyzedoxidative polymerization at the surface of the droplets ofmiscible monomers. Simultaneously, the inside of themonomer droplet layered by pre-formed PT shell is filledup with growing polystyrene chains, initiated by KPS,instead of monomer mixture. A high concentration of cat-ionic radicals of thiophene around the vicinity of the drop-lets is expected, due to the electrostatic attraction betweensulfonate (SO�3 ) groups from the sodium p-styrene sulfo-nate and Fe3+ ions, which are drawn into the interface ofthe droplets from the water phase [10].

As seen in Fig. 4a, the average particle size (Dn) of thelatexes was from 300 nm to 800 nm. To confirm core–shellmorphology of the resulting particles, they were exposed toa chloroform solution for 15–20 h to remove the poly(St/NaSS) in the core. As seen in Fig. 4b, crumpled PT shellswere observed.

To confirm that the shell component is indeed PT, thephotoluminescence spectrum for the dispersion of the crum-pled shells of core–shell poly(St/Th) latex particles, whichwere exposed to a chloroform solution, was measured. Asseen in Fig. 5, the distinct PL peak of the dispersion of thecrumpled shells was observed at 589 nm, which is in goodagreement with that of the PT homopolymers. From thisresult, we concluded that the shell component of core–shellpoly(St/Th) latex particles is indeed composed of PT.

Fig. 6 shows the difference of PL intensity at the sameconcentration (s.c. = 0.1 wt.%) between the PT and core–shell poly(St/Th) latex particles in aqueous dispersion.Compared to the PL intensity of the PT nanoparticle dis-persion, that of the core–shell poly(St/Th) nanoparticle dis-persion was much higher because of its thin shell layermorphology. In the case of PT homopolymer, the PL inten-sity of the PT nanoparticle dispersion decreased due to self-absorption as the concentration of PT increased to displaya highly efficient PL property. However, the PL intensity ofthe core–shell poly(St/Th) nanoparticle dispersion was sel-dom affected by self-absorption, not only in the dispersionstate but also even in the solid state (s.c. = 100 wt.%),

, and (b) crumpled PT shells of poly(St/Th) latex particles.

Fig. 5. Photoluminescence spectrum of the crumpled PT shells of poly(St/Th) latex particles in aqueous dispersion. (Excitation wavelengthðkUV

maxÞ ¼ 560 nm).

Fig. 6. Photoluminescence spectra of PT and core–shell poly(St/Th) latexparticles in aqueous dispersion. (Excitation wavelength ðkUV

maxÞ ¼ 400 nm(solid line), 560 nm (dotted line).)

J.M. Lee et al. / Current Applied Physics 8 (2008) 659–663 663

which resulted from weakening of the luminescencequenching by reducing the aggregation effect due to thethin shell layer. These results are particularly importantwhen, for example, core–shell poly(St/Th) latex particlesare desired for applications as emissive materials in LEDs.

4. Conclusions

In summary, we propose a synthetic route of PT nano-particles via Fe3+-catalyzed oxidative polymerization ofunsubstituted thiophene monomers in aqueous dispersion.This new synthetic method guarantees high conversion ofthiophene with only a trace of FeCl3, which usually deteri-orates the PL properties of PT nanoparticles.

The dispersion state showed that the PT nanoparticleswere well dispersed in many polar solvents, compared tonon-polar solvents, such as acetone, chloroform, hexane,and ethyl acetate. The fine dispersion state would beexpected to increase the processability of PT nanoparticles

in various electrical and electro-optical applications with-out introducing any additional chemical conversion steps.

The PL intensity of PT nanoparticle dispersion preparedby the Fe3+-catalyzed oxidative polymerization was muchhigher than that of bulk PT prepared by the conventionaloxidative polymerization by using much FeCl3. This isbecause the quenching effect of the FeCl3 was minimizedby using only a trace amount of FeCl3.

In addition, we have demonstrated that core–shellpoly(St/Th) latex particles were successfully prepared byFe3+-catalyzed oxidative polymerization during emulsifier-free emulsion polymerization. The self-assembled core–shellstructure of the resulting latex particles was caused by the dif-ferent polymerization rates of the monomers and the electro-static attraction between sulfonate (SO�3 Þ groups from thesodium p-styrene sulfonate and Fe3+ ions. From the PL dataof the crumpled PT shells in aqueous dispersion, one can seethat the shell component of core–shell poly(St/Th) latex par-ticle is indeed PT, which was corroborated by SEM data ofthe crumpled shells after dissolving poly(St/NaSS) core par-ticles. PL intensity of the core–shell poly(St/Th) nanoparti-cle dispersion was much higher than that of the PTnanoparticle dispersion, due to its thin shell layer morphol-ogy, which was explained by the self-absorption effect.Finally, despite the relatively limited solubility and/or pro-cessability of the unsubstituted PT homopolymers, the syn-thetic versatility of nano-structured PT nanoparticlesmakes them very attractive materials applicable in variouselectrical and electro-optical fields.

Acknowledgements

This work was financially supported by the Ministry ofEducation and Human Resources Development (MOE),the Ministry of Commerce, Industry and Energy (MO-CIE), and the Ministry of Labor (MOLAB) through thefostering project of the Lab of Excellency and the Ministryof Commerce, Industry, and Energy (MOCIE) through theproject of NGNT (No. 10023135-2005-11).

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