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Synthetic Metals 160 (2010) 1266–1272

Contents lists available at ScienceDirect

Synthetic Metals

journa l homepage: www.e lsev ier .com/ locate /synmet

ynthesis and characterization of the carbon nanotube-based compositeaterials with poly(3,4-ethylenedioxythiophene)

ran Thanh Tung, Ji Hye Yeon, Tae Young Kim ∗, Kwang S. Suh ∗

epartment of Materials Science and Engineering, Korea University, 5-1 Anam-dong, Seongbuk-gu, Seoul 137-713, Republic of Korea

r t i c l e i n f o

rticle history:eceived 29 October 2009eceived in revised form 3 February 2010

a b s t r a c t

We report a procedure to prepare a conducting nano-composites composed of multi-walled carbonnanotubes (MWNTs) and PEDOT by using a poly(sodium 4-styrenesulfonate) (PSSNa) as a inter-linkingmolecule between MWNT and PEDOT. When PSSNa chains are introduced on the MWNTs via physic-

ccepted 26 March 2010vailable online 21 April 2010

eywords:arbon nanotubesomposites

ochemical interaction, the surface of MWNT becomes negatively charged, and PSS-modified MWNTspromote the effective association of the positively charged PEDOT chains. The resulting MWNT-PSS/PEDOT composites are characterized by a better interconnection between MWNT and PEDOTcomponents.

© 2010 Elsevier B.V. All rights reserved.

elf-assemblyoly(3,4-ethylenedioxythiophene)

. Introduction

Carbon nanotubes (CNTs) have been considered promisingaterials in a range of fields, such as field emission materials,

lectrochemical devices, storage devices, nano-electronic or nano-pto-electrical devices, chemical sensors and composite materials,n account of their unique structure and outstanding properties1–5]. CNT/polymer composites have also attracted considerablettention because the individual properties of the two compo-ents can be combined to produce novel hybrid nanomaterials withnique multifunctional properties. However, the main challenge iso disperse the individual nanotubes in a polymer matrix, sinceNTs tend to aggregate, and their non-uniform dispersion in com-on solvents and organic polymer matrices often results in side

ffects. In-depth studies on a combination of CNTs with a polymerre of interest because functional polymer chains can help dissolveanotubes in certain solvents. Many studies have focused on chem-

cal functionalization techniques of CNT to improve the dispersionf CNTs in polymer matrices [6–14].

The association of conducting polymers with CNTs is aew strategy for obtaining hybrid materials with enhanced

unctionality, such as enhanced conductivity, thermal stability,nd reinforcement properties [15–21]. In particular, poly(3,4-thylenedioxythiophene) (PEDOT) is unique among conductingolymers because of its excellent processability, environmental

∗ Corresponding author. Tel.: +82 2 927 4546; fax: +82 2 929 4408.E-mail addresses: thomas75@korea.ac.kr (T.Y. Kim), suhkwang@korea.ac.kr

K.S. Suh).

379-6779/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.synthmet.2010.03.021

stability and high conductivity [22–26], and numerous attemptshave been made to prepare CNT/PEDOT composites with animproved properties. The general methods used to prepareCNT/PEDOT composites include blending, electrochemical, and insitu methods [27–31]. However, the main drawback of these meth-ods is that most of the conjugated polymers are incompatiblewith CNTs, hence the coatings are not always uniform [32]. Quiterecently, Chen et al. demonstrated in situ polymerization of CNT-PSS/PEDOT composites under hydrothermal conditions and theirelectrochemical properties were investigated as the potential elec-trodes in supercapacitors [33].

Herein, we introduce a procedure to prepare a conducting nano-composites composed of multi-walled carbon nanotubes (MWNTs)and PEDOT by using a poly(sodium 4-styrenesulfonate) (PSSNa) asa inter-linking molecule between MWNT and PEDOT. When PSSNachains are introduced on the MWNTs via physicochemical interac-tion, the surface of MWNT becomes negatively charged, which is, inturn, expected to promote the effective association of the positivelycharged PEDOT chains. The resulting MWNT-PSS/PEDOT compos-ites are characterized by a better interconnection between MWNTand PEDOT components, which will be discussed in terms of thechemical, morphological and electrical aspects.

2. Experimental

2.1. Materials

The multi-walled carbon nanotubes, which had been synthe-sized by CVD method, were purchased from Iljin Nanotech Co.,Korea. Poly(sodium 4-styrenesulfonate) (PSS Na, 25 wt% solution

T.T. Tung et al. / Synthetic Metals 160 (2010) 1266–1272 1267

for PS

iss(

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Scheme 1. Schematic illustration of the synthetic process

n water, MW ∼100 000) was acquired from Aldrich. Polystyreneulfonic acid (PSSA, 18 wt% solution in water) and EDOT monomerolutions were obtained from Baytron. Ammonium peroxydisulfateAPS) was supplied by Junsei Chemical Co., Japan. All other reagents

ig. 1. (a) FTIR spectra of MWNT-COOH, MWNT-COCl, and PSS-MWNTs; (b) XPSpectra of MWNTs and PSS-MWNTs in the S 2p region.

S-modified MWNT and the PSS-MWNT/PEDOT composite.

and solvents were purchased from Aldrich and used without furtherpurification.

2.2. Preparation of PSS-modified MWNTs

Pristine MWNTs were acid-treated to introduce carboxylic andhydroxyl functional groups to the surface according to a conven-tional method, in which 1.0 g of pristine MWNT were agitated ina HNO3:H2SO4 (1:3) mixture and refluxed at 80 ◦C for 2 h. Thisacid-treated MWNTs were filtered and washed with large amountsof de-ionized (DI) water and vacuum dried at room tempera-ture overnight. The as-obtained MWNT-COOH (0.8 g) was thensubjected to the next step where the carboxylic acid group wasconverted to acylchloride groups by reacting them with SOCl2in N,N-dimethylformamide (DMF) at 80 ◦C for 24 h. The resultingMWNT-COCl was then obtained by evaporating solvents, washedthree times with anhydrous tetrahydrofuran (THF), and dried in avacuum at room temperature for 8 h.

PSS-MWNTs were synthesized using the following steps:100 mg of the as-prepared MWNTs, 3.0 g of PSSNa (30% aqueoussolution) and 20 mL of DI water were added into a 100 mL one-neck round bottom flask, and kept in an ultrasonic bath for 15 min.Then the mixture was heated at 60 ◦C while stirring for 24 h. After

Fig. 2. TGA thermograms of the PSS-MWNT obtained at 10 ◦C min−1 in a N2 purge.The TGA plots of pristine MWNTs and PSSNa are displayed for comparison.

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268 T.T. Tung et al. / Synthetic

he reaction, the PSS-coated MWNT sample was diluted with waternd vacuum-filtered three times through a 0.2 �m polycarbonateembrane then filtered and washed with water to ensure that no

n-grafted polymer had been removed completely from the mix.inally, the PSS-wrapped MWNT sample was dried in a vacuum at5 ◦C for 24 h.

.3. Preparation of PSS-MWNT/PEDOT composites

The synthesis of PSS-MWNT/PEDOT composites was carried outy the chemical oxidative polymerization of EDOT in the presencef PSS-MWNTs. Firstly, 4 mg of the as-received PSS-MWNTs wereispersed in 40 mL DI water and then kept in an ultrasonic bathor 15 min. To this suspension, EDOT (1.4 g), PSSA (2.0 g), and APS2.2 g) in DI water (60 mL) were added and vigorously stirred for8 h at room temperature.

.4. Characterization

The Fourier transform infrared (FTIR) spectra were obtained onNicolet FTIR-5DX spectrometer using KBr pellets. Thermal gravi-etric analysis (TGA) was carried out on a NETZSCH STA 409 PC/PG

nstrument with a heating rate of 10 ◦C min−1 under nitrogen. The

aman spectra were recorded on an RFS-100/S Raman spectrom-ter equipped with an Argon ion laser operating at 514.5 nm. TheV–vis absorption spectra were obtained using a Scinco S-3100

pectrometer. The scanning electron microscopy (SEM) imagesere recorded using a JEOL JSM-6700F instrument. The optical

Fig. 3. (a) Raman spectra of pristine MWNTs and PSS-MWNTs; (

ls 160 (2010) 1266–1272

microscope images were obtained using a Sometech instrument.High-resolution transmission electron microscopy (HRTEM) wascarried out on a TECNAI 20 microscope operating at 200 kV. The sur-face resistance was measured with 4-point probe system (Jandel,CMT-series).

3. Results and discussion

The production of PSS-MWNT/PEDOT composites was achievedthrough the sequential chemical reactions in Scheme 1.

3.1. PSS-modified MWNTs

The first step involves the attachment of PSSNa onto the MWNTsurfaces. For the effective anchoring of PSSNa on the MWNT sur-faces, MWNTs were pre-treated as follows. The MWNTs werefirst treated in a mixture of HNO3:H2SO4 for partial oxidationand the creation of carboxylic groups on their defect sites, whichwere then converted to acylchloride groups using SOCl2. The poly-electrolyte, PSSNa, was then introduced onto the surface of theacylchloride-functionalized MWNT through dipole–ion interac-tions and/or covalent bonds between the carboxylic groups andsulfonated groups (SO3

−), as will be discussed later.

The FTIR spectra were recorded to confirm that the PSS had been

introduced successfully onto the surface of the MWNT (Fig. 1a).In the IR spectrum of MWNT-COOH, the peaks at approximately1722 cm−1 ( C O), 1542 cm−1 ( COO−), 1210 cm−1 ( C O) and1095 cm−1 were assigned to carboxylic acid groups. This indicates

b) proposed interaction between MWNT-COCl and PSSNa.

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T.T. Tung et al. / Synthetic

he formation of carboxylic acid groups at both ends and on theidewalls of the MWNTs. After MWNT-COOH was treated withOCl2, new and well-defined bands centered at 1724, 1625, 1586,193 and 686 cm−1 appeared, which indicates the formation ofCOCl groups [34]. PSS-MWNTs are characterized by IR peaks ofO stretching vibration (1163, 1115, 1050, 1030 cm−1) [35], S O

tretch (895 cm−1), and C S stretch (668 cm−1), which indicateshe presence of PSS on the MWNTs.

X-ray photoelectron spectroscopy (XPS) was also employed tonalyze the presence of PSS molecules on the MWNTs (Fig. 1b).n contrast to the MWNTs, the PSS-MWNTs spectra showed theppearance of S 2p peak at ∼169 eV due to the sulfonate groups inSS, which also supports the successful attachment of PSS on theWNTs.The thermal stability of the samples was examined by TGA.

ig. 2 shows the weight losses of the MWNTs, PSS-MWNTs and pureSSNa upon heating in a nitrogen atmosphere. Our TGA data revealshat at least two degradation steps are involved in pure PSSNa,hile no degradation takes place in the crude MWNTs. In PSSNa,

he weight loss starting at ∼250 ◦C can be assigned to the decompo-ition of the PSS main chain and the other at 350 ◦C corresponds tohe degradation of benzene rings. In the case of PSS-MWNTs sam-le, the onset of the decomposition temperature is slightly higher∼355 ◦C) than that of pure PSSNa. We suggest this change is causedy the formation of the chemical bonding between the PSS andWNTs.

Structural change which was undergone during the chemical

rocessing from MWNT to PSS-MWNT was monitored by Ramanpectroscopy with a laser excitation of 785 nm and the resultsre shown in Fig. 3a. The Raman spectra of the pristine MWNTsisplayed the usual D- and G-band at 1345 and 1572 cm−1, respec-

Fig. 4. SEM images of (a) MWNT-COOH and (b) PSS-MWNTs; TEM images of (c) MW

s 160 (2010) 1266–1272 1269

tively, and the D/G intensity ratio (ID/IG) was found to be 0.62. TheRaman spectrum of the PSS-MWNTs is characterized by a pres-ence of D- and G-band at 1355 and 1590 cm−1 with an increasedID/IG of 1.10. The increase in ID/IG suggests that the MWNTs werefunctionalized with PSS molecules by covalent chemical bonding,presumably due to the reaction of acylchloride on MWNTs with sul-fonate groups in PSS, as shown in Fig. 3b (left). On the other hand,the position of G-band was also found to be upshifted by 18 cm−1,which indicates the non-covalent functionalization of MWNTs withPSS molecules via dipole–ion interactions, as shown in Fig. 3b(right) [36]. From the above results, we suggest that physicochemi-cal interactions between MWNTs and PSS may be involved and thisleads to the decoration of MWNTs with PSS molecules via covalentand/or non-covalent interactions.

The morphological structures of the PSS-MWNTs were exam-ined by SEM and TEM to further confirm the wrapping of PSS onthe MWNT surface. A distinct difference between the morphol-ogy of MWNT-COOH and PSS-MWNTs can be perceived from theSEM images shown in Fig. 4a and b. While MWNT-COOH is com-posed of tangled ropes with a smooth surface, the PSS-MWNT isfound to have elongated tubular PSS layers on the MWNT surface.The diameters of the PSS-MWNTs increased by several nanome-ters compared to that of MWNT-COOH, which suggests that thewrapping of PSS occurs at the outer surface of the MWNTs.

Fig. 4c and d presents TEM images of the MWNT-COOH and thePSS-MWNTs. In comparison to that of MWNT-COOH, the image of

PSS-MWNT reveals extra phases on the outer surface of MWNTswith different degrees of colors, suggesting the presence of thinlayer of PSS coating.

The dispersion degree of the samples in water was monitoredby UV–vis spectra in Fig. 5 with the photographs of each sample

NT-COOH and (d) PSS-MWNT. Scale bar: (a and b) 100 nm; (c and d) 10 nm.

1270 T.T. Tung et al. / Synthetic Metals 160 (2010) 1266–1272

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ig. 5. UV–vis spectra of MWNT-COOH, MWNT-COCl and PSS-MWNTs in water atoncentrations of 1 mg sample per 5 mL of water. The inset shows the photographf (A) MWNT-COCl, (B) PSS-MWNTs, and (C) MWNT-COOH dispersed in water.

n water. As shown in the inset of Fig. 5, MWNT-COCl was not dis-ersed at all even with strong sonication, while MWNT-COOH wasbserved to be suspended in water with limited dispersing capa-ility. In contrast, PSS-MWNT was well dispersed in water with aild sonication and any sediment was not found at the bottom afterperiod of time. Such a good dispersion capability of PSS-MWNT

eads to the higher absorbance in the whole spectral ranges, indi-ating that PSS on MWNT plays a role in de-bundling of MWNTs inater solution.

.2. PSS-MWNT/PEDOT composites

The PSS-functionalized MWNTs were used as templates to fur-her adsorb EDOT chains in the polymerization and synthesizehe PSS-MWNT/PEDOT composites. Since PSS on the CNT surface

Fig. 6. SEM images of (a) PEDOT and (b) PSS-MWNT/PEDOT composite; optical m

Fig. 7. FTIR spectra of PEDOT and PSS-MWNT/PEDOT composites.

carries a negative charge, it is expected to act as a charge balanc-ing counter-ion to the positively doped PEDOT chains during thepolymerization step, thereby facilitating the association of PEDOTchains with PSS-MWNTs by electrostatic interactions as was illus-trated in Scheme 1 (step 2). Therefore, PSS plays a dual role byincreasing the solubility of MWNTs in water and providing effectiveanchoring sites for bonding with the PEDOT chains.

Fig. 6 shows representative SEM images of the pristinePEDOT:PSS (Fig. 6a) and PSS-MWNT/PEDOT composite (Fig. 6b).Compared to the pristine PEDOT:PSS, PSS-MWNT/PEDOT exhibitsthe morphology that MWNTs are embedded in the continuous PSS

phase. Fig. 6c and d presents the optical microscopy images ofthe PSS-MWNT/PEDOT and MWNT-COOH/PEDOT. While MWNT-COOH/PEDOT exhibits MWNT agglomerates in the PEDOT matrix(Fig. 6d), PSS-MWNT/PEDOT composite shows a homogenous sur-

icroscopy images of (c) PSS-MWNT/PEDOT and (d) MWNT-COOH/PEDOT.

T.T. Tung et al. / Synthetic Metals 160 (2010) 1266–1272 1271

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ig. 8. TEM images of PSS-MWNT/PEDOT composite at different magnifications. EDnd (c) 5 nm.

ace without any aggregation (Fig. 6c), suggesting that PSS not onlyisperses the CNTs well into an aqueous solution, but also makesEDOT polymer compatible with the MWNT.

Fig. 7 shows the FTIR spectra of the PSS-MWNT/PEDOTomposite and pristine PEDOT:PSS. The characteristic bandsor the PSS-MWNT/PEDOT composites occurs in the range of00–1600 cm−1, showing almost identical numbers and positionsf the main peaks from pristine PEDOT:PSS. Although the character-stic IR absorption peaks of MWNTs are rather weak, the vibrationst approximately 1720 and 1620 cm−1 correspond to the stretchingodes of the C O groups on the MWNTs, indicating the presence

f MWNTs in the composites.The morphology of the PSS-MWNT/PEDOT was further inves-

igated by TEM as shown in Fig. 8. It was observed that theunctionalized MWNTs were encapsulated with PEDOT molecules,ndicating an association of PEDOT and PSS-MWNT. It should beoted here that PSS on the outer surface of MWNTs plays a vitalole in effectively anchoring EDOT monomer to the surface ofWNT. A successful PEDOT coating on the MWNTs was confirmed

y energy dispersive X-ray spectra (EDX), in which C (48.129 at%),

(47.698 at%) and O (4.172 at%) corresponding to the presence ofEDOT were observed.

To examine the electro-conductive behavior of PSS-WNT/PEDOT composites, the surface resistivity of the filmas measured using a four probe system. Films for the measure-

ctra measured on those locations indicated in (d). Scale bar: (a) 50 nm; (b) 20 nm;

ment were prepared by coating the PEDOT and PSS-MWNT/PEDOTsolutions on a polyethylene terephthalate (PET) film substrate witha drawdown bar and drying them at 80 ◦C for 2 min. The resultsshow that the PSS-MWNT/PEDOT composites exhibits a surfaceresistivity of 3.64 × 103 �/sq with a CNT loading level of 0.2 wt%,compared to the that of pristine PEDOT:PSS film (5.86 × 103 �/sq).It should be noted that the surface resistivity value achieved forPSS-MWNT/PEDOT is comparable to those in the previous litera-tures [28,37] and a decrease in sheet resistivity may be attributedto the homogeneous distribution and connection of MWNTs in thePEDOT matrix and the reduced tube-to-tube resistance.

4. Conclusions

In summary, we demonstrated a procedure to prepare aconducting nano-composites composed of multi-walled carbonnanotubes (MWNT) and PEDOT by using a poly(sodium 4-styrenesulfonate) (PSSNa) as a inter-linking molecule betweenMWNT and PEDOT. When PSSNa chains are introduced on the

MWNTs via physicochemical interaction, they not only help theMWNTs to disperse well in water, but also provide anchoring sitesto the PEDOT polymerization. The resulting PSS-MWNT/PEDOTcomposites are characterized by a better interconnection betweenMWNT and PEDOT components.

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cknowledgement

This work was supported by a Korea University Grant. We alsohank the financial support from InsCon Tech, Co. Ltd, Korea.

eferences

[1] S. Iljima, Nature 354 (1991) 56–58.[2] S. Fan, M.G. Chapline, N.R. Franklin, T.W. Tombler, A.M. Cassell, H. Dai, Science

283 (1999) 512–514.[3] H. Dai, J.H. Hafner, A.G. Rinzler, D.T. Colbert, R.E. Smalley, Nature 384 (1996)

147–150.[4] S.J. Tan, A.R.M. Verschueren, C. Dekker, Nature 393 (1998) 49–52.[5] E.W. Wong, P.E. Sheehan, C.M. Lieber, Science 277 (1997) 1971–1975.[6] Y. Ying, R.K. Saini, F. Liang, A.K. Sadana, W.E. Billups, Org. Lett. 5 (2003)

1471–1473.[7] K. Mylvaganam, L.C. Zhang, J. Phys. Chem. B 108 (2004) 15009–15012.[8] D.E. Hill, Y. Lin, A.M. Rao, L.F. Allard, Y.P. Sun, Macromolecules 35 (2002)

9466–9471.[9] Y. Liu, Z. Yao, A. Adronov, Macromolecules 38 (2005) 1172–1179.10] K. Matyjaszewski, K.J. Xia, Chem. Rev. 101 (2001) 2921–2990.11] H. Kong, C. Gao, D. Yan, J. Mater. Chem. 14 (2004) 1401–1405.12] S. Qin, D. Qin, W.T. Ford, J.E. Herrera, D.E. Resasco, S.M. Bachilo, R.B. Weisman,

Macromolecules 37 (2004) 3965–3967.13] H.X. Wu, R. Tong, X.Q. Qiu, H.F. Yang, Y.H. Lin, R.F. Cai, S.X. Qian, Carbon 45

(2007) 152–159.

14] R. Sainz, A.M. Benito, M.T. Martinez, J.F. Galindo, J. Sotres, A.M. Baro, B. Corraze,

O. Chauvet, A.B. Dalton, R.H. Baughman, W.K. Maser, Nanotechnology 16 (2005)150–154.

15] E. Kymakis, G.A.J. Amaratunga, Synth. Met. 142 (2004) 161–167.16] A. Star, J.F. Stoddart, D. Steuerman, M. Diehl, A. Boukai, E.W. Wong, X. Yang,

S.-W. Chung, H. Choi, J.R. Heath, Angew. Chem. Int. Ed. 40 (2001) 1721–1725.

[[

[

[

ls 160 (2010) 1266–1272

17] B.K. Kuila, S. Malik, S.K. Batabyal, A.K. Nandi, Macromolecules 40 (2007)278–287.

18] Y. Chen, K.S. Kang, K.J. Han, K.H. Yoo, J.H. Kim, Synth. Met. 159 (2009)1701–1704.

19] J. Wang, J. Dai, T. Yarlagadda, Langmuir 21 (2005) 9–12.20] X. Zhang, J. Zhang, R. Wang, Z. Liu, Carbon 42 (2004) 1455–1461.21] G.Z. Chen, M.S.P. Shaffer, D. Coleby, G. Dixon, W. Zhou, D.J. Fray, A.H. Windle,

Adv. Mater. 12 (2000) 522–526.22] F. Jonas, L. Schrader, Synth. Met. 41 (1991) 831–836.23] R. Kiebooms, A. Aleshin, K. Hutchison, F. Wudl, J. Phys. Chem. B 101 (1997)

11037–11039.24] H.J. Ahonen, J. Lukkari, J. Kankare, Macromolecules 33 (2000) 6787–6793.25] Y. Wang, J. Phys.: Conf. Ser. 152 (2009) 012023.26] I. Winter, C. Reece, J. Hormes, G. Heywang, F. Jonas, Chem. Phys. 194 (1995)

207–213.27] J.S. Moon, J.H. Park, Y.W. Kim, J.B. Yoo, C.Y. Park, J.M. Kim, K.W. Jin, Diam. Relat.

Mater. 14 (2005) 1882–1887.28] H.T. Ham, Y.S. Choi, M.G. Chee, M.H. Cha, I.J. Chung, Polym. Eng. Sci. 48 (2008)

1–10.29] C. Peng, J. Jin, G.Z. Chen, J. Electrochim. Acta 53 (2007) 525–537.30] C. Peng, G.A. Snook, D.J. Fray, M.S.P. Shaffer, G.Z. Chen, Chem. Commun. (2006)

4629–4631.31] S. Bhandari, M. Deepa, A.K. Srivastava, C. Lai, R. Kant, Macromol. Rapid Commun.

29 (2008) 1959–1964.32] N.A. Kumar, S.H. Kim, B.G. Cho, K.T. Lim, Y.T. Jeong, Colloid Polym. Sci. 287

(2009) 97–102.33] L. Chen, C. Yuan, H. Dou, B. Gao, S. Chen, X. Zhang, Electrochim. Acta 54 (2009)

2335–2341.

34] T.M. Wu, Y.W. Lin, Polymer 47 (2006) 3576–3582.35] X. Zhao, N. Song, X. Chen, X. Fan, Q. Zhou, J. Mater. Chem. 16 (2006)

4619–4625.36] C.O. M’Bareck, Q.T. Nguyen, M. Metayer, J.M. Saiter, M.R. Garda, Polymer 12

(2004) 4181–4187.37] D.L. Carroll, R. Czerw, S. Webster, Synth. Met. 155 (2005) 694–697.