Materials Science and Engineering A334 (2002) 291–297

Synthesis and characterization of conducting copolymer nanofibrilsof pyrrole and 3-methylthiophene using the template-synthesis

method

Mei Lu, Xiao-Hong Li, Hu-Lin Li *Department of Chemistry, Lanzhou Uni�ersity, Lanzhou 730000, People’s Republic of China

Received 8 May 2001; received in revised form 23 October 2001

Abstract

Copolymer nanofibrils composed of pyrrole and 3-methylthiophene had been prepared by synthesizing the desired polymerwithin the pores of microporous anodic aluminum oxide (AAO) template membrane. The results of scanning electron microscopy(SEM) and transmission electron microscopy (TEM) revealed that the obtained copolymer nanofibrils had uniform andwell-aligned array and its diameter and length can be controlled by changing the aspect ratios of the AAO membrane. The resultsof infrared (IR) spectra, X-ray photoelectron spectroscopy (XPS) and elemental analysis indicated that both pyrrole and3-methylthiophene were involved in the copolymer and pyrrle is the major component of the prepared copolymer nanofibrils.Finally, the uses of conductivity and thermogravimetric analysis (TGA) proved that the electric conductivity and the thermalstability of copolymer nanofibrils are higher than individual homopolymers. © 2002 Elsevier Science B.V. All rights reserved.

Keywords: Copolymer; Pyrrole; 3-Methylthiophene; Template-synthesis method; Nanofibrils

www.elsevier.com/locate/msea

1. Introduction

Since the first demonstration of electropolymeriza-tion of pyrrole [1–3], the preparation of a variety ofelectronconducting polymers has been intensively inves-tigated, because of a large number of potential applica-tions such as rechargeable battery [4,5], condensers [6],semiconductor photoanodes production [7], and pro-duction of electrochemical displays [8], restoring of data[9], biochemical analysis [10] etc. Especially, much at-tention has been paid to develop strategies for improv-ing the conductivities of these polymers. Recently,attempts have been made to synthesis nanoscopic fibrilsand tubules using template synthesis method to achievethis objective [11].

This method entails synthesis of a desired materialwithin the pores of a nanoporous membrane. Since themembranes employed have linear, cylindrical pores ofuniform diameters, monodispersed nanostructures of

the desired material can be obtained in each pore.Nanotubules or fibrils of electronically conductive poly-mers, metals, semiconductors and carbon have beensynthesized via this method [12–16]. The diameter andlength of the nanostructures can be varied, at will, byvarying the characteristics of the template membrane.Previous research has shown that when the templatemethod is used to synthesize conductive polymer nan-otubules or fibrils, electronic conductivities along theaxes of these tubules or fibrils can be dramaticallyhigher than conductivities of bulk forms (e.g. powders,or thin films) of the same polymer. This enhancedconductivity resulted from preferential orientation ofthe polymer chains parallel to the tubule or fibril axis[17].

A number of nanostructure polymer such as poly-acetylene [17], polypyrrole [18], poly(3-methylthio-phene) [18] and polyaniline [11,19] have been preparedusing template-synthesis method, but use of these mate-rials has been limited by the fact most of them areintractable and have rare conjugated �-bond containingmonomers. Therefore, it seemed interesting to attemptto synthesize nanostructure copolymers from different

* Corresponding author. Tel.: +86-931-891-2517; fax: +86-931-891-1100.

E-mail address: lihl@lzu.edu.cn (H.-L. Li).

0921-5093/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved.PII: S0921 -5093 (01 )01906 -2

M. Lu et al. / Materials Science and Engineering A334 (2002) 291–297292

monomers, which would provide materials with inter-mediate properties between the two or more monomers.Consequently, copolymerization has high potential formodifying the physical properties and help to overcomethese drawbacks of the individual polymers.

We have recently used the template to preparepyrrole/aniline copolymer nanofibrils by chemical co-polymerization technique and investigated the structureand morphology of the copolymer fibrils [20]. Later,electrochemical copolymerization technique was suc-cessfully used to prepare pyrrole/thiophene copolymernanofibrils [21], according to the method established byKwabata et al. [9]. Based on our previous work, in thisstudy, we attempted to synthesis the conductive copoly-mer nanofibrils derived from pyrrole and 3-methylthio-phene, using uniform and straight channels of anodicaluminum oxide (AAO) films as a template. The chem-ical copolymerization technique was employed forpyrrole and 3-methylthiophene loading and the copoly-mer nanofibrils were characterized by scanning electronmicroscopy (SEM), transmission electron microscopy(TEM), infrared (IR) spectroscopy, elemental analysisand X-ray photoelectron spectrophotometer (XPS) andthermogravimetric analysis (TGA). Finally, the conduc-tivity of the copolymer nanofibrils was measured by afour-probe method.

2. Experimental

2.1. Materials

Pyrrole and 3-methylthiophene were purchased fromAcros Chemical Co. and distilled under reduced nitro-gen prior to use. Acetonitrile was purified according tothe literature [22]. Fe(ClO4)3 · 9H2O was obtained fromJinshan Chemical Engineering Factory, Shanghai,

Chins. All the other ingredients were of analytical gradeand used without further purification.

2.2. Paration of AAO membranes

AAO membranes were prepared by anodic oxidationof electropolished aluminum plate at a cell voltage of80 V in 0.5 M phosphoric acid at 25 °C for 1.5 h. Thedetails of this method were described elsewhere [23]. Wehave prepared the AAO membranes with a broad rangeof pore diameters and thickness by controlling theapplied potential of anodic oxidation and the oxidizedtime. Consequently, the diameter and the length of thedesired copolymer nanofibrils can be determined avail-ably by the diameter and the thickness of the AAOmembranes. As indicated in Fig. 1(a), the pores in themembrane are arranged in a regular hexagonal lattice.Fig. 1(b) depicts the cross-section of the AAO templatewith pores parallel to each other and perpendicular tothe surface of the membrane. The AAO membraneused in this study consisted of an array of parallel andstraight channels with a diameter of about 80 nm, andthe membrane diameter and its thickness were 15 mmand �10 �m, respectively.

2.3. Preparation of copolymer nanofibrils

Gregory et al. [24] have developed a method forcoating textiles with conductive polymers. We haveused a similar procedure for chemically synthesizingcopolymer of pyrrole and 3-methylthiophene within thepores of AAO membranes. In this method, the templatemembrane is immersed in a precooled (5 °C) solutionthat contained 0.6 M pyrrole and 0.6 M 3-methylthio-phene(solvent=acetonitrile). An equal volume of pre-cooled (5 °C) oxidant solution containing 1 MFe(ClO4)3 · 9H2O (solvent=acetonitrile) was then

Fig. 1. TEM image of top surface structure (a); and SEM image of cross-section (b) of AAO template membrane.

M. Lu et al. / Materials Science and Engineering A334 (2002) 291–297 293

Fig. 2. SEM images (a); and TEM image (b) of copolymer nanofibrils.

added. The mixture was left for polymerization for 2 hat approximately 5 °C. During this period, copolymerwas produced from the monomers and deposited withinthe pores of AAO membrane.

The template-synthesis method yielded either tubulesor fibers of the desired copolymer within the pores ofAAO membrane by controlling the polymerization timeand temperature. In addition, thin films of the copoly-mer were deposited on both faces of the membrane.One or both of these surface films were removed priorto characterization. This was accomplished by simplypolishing the surface films of the membrane with 1 �malumina powder. The membrane was then ultrasoni-cated in 1 M HCl to remove the alumina powder.

2.4. Characterization of copolymer nanofibrils

SEM images of the copolymer nanofibrils were ob-tained as follows, one surface layer was removed, andthe membrane was glued (using epoxy) to a piece ofglass with the polished face up. The resulting mem-brane was immersed into 6 M NaOH solution for 2 minin order to dissolve AAO membrane. A JSM-5600LVelectron microscope was used. The copolymernanofibrils were sputtered with �10 nm of Au prior toSEM imaging.

TEM images of the samples were obtained as fol-lows, both surface layer were removed; and a piece ofthe resulting membrane was placed onto a carbon-film-coated TEM grid. The 6 M NaOH solution was thenapplied to the membrane in order to dissolve the alu-mina. The freed copolymer nanofibrils were imagedusing a HATACHI-600 electron microscope. The accel-erating voltage of the electron beam was 100 kV.

IR spectra were obtained by using a NicoletAVATAR-360 fourier transform infrared (FT-IR) spec-trometer. Since the alumina had no absorption above600 cm−1 [25], the AAO membrane need not dissolve

with 6 M NaOH solution. The FT-IR spectra showedonly the absorption of the copolymer.

The X-ray photoelectron spectroscopy (XPS) analysiswere carried out using a PHI-5702 Multi-TechniqueSystem ESCA/AES with a Mg K� (1253.6 ev) source.Samples for this study were obtained as follows: firstremove both surface layers, then free the copolymernanofibrils from the AAO membrane by immersingAAO membranes into 6 M NaOH solution for 2 min,finally collect the nanofibrils and compact in an IRpellet press. The spectra were acquired with 29.35 eVpass energy.

Elemental analysis were performed by use of a Ger-man elementer Vario EL. Samples for this studies wereobtained with the similar procedure in XPS analysis,but the collected nanofibrils didnot need to be pressedpellets.

A ZRY-2P instrument was used for TGA. The sam-ples were heated up to 700 °C with a rate of 10 °Cmin−1 under nitrogen atmosphere. Conductivities ofthe prepared polymers were measured employing thefour-probe method.

3. Results and discussion

3.1. Morphology of copolymer nanofibrils

Fig. 2(a) shows the SEM images of copolymernanofibrils composed of polypyrrole and polyaniline.The AAO membrane has been dissolved to expose thecopolymer fibrils prepared in the pores. This yields anensemble of copolymer nanofibrils that protruded fromthe epoxy surface like the bristles of a brush. Someresidual copolymer surface layer is observed at the topsof these fibers. Note the high density of fibrils; thisreflects the very high porosity of the AAO templatemembrane. As would be expected, the fibrils diameter is

M. Lu et al. / Materials Science and Engineering A334 (2002) 291–297294

equivalent to the pore diameter (80 nm) of the templatemembrane. The lengths of these fibrils show that theyspan the complete thickness (10 �m) of the template.

Fig. 2(b) shows the TEM image of copolymer fibrils.The image exhibits the presence of uniform copolymernanofibrils with diameter and length of 80 nm and 10�m, respectively. Some fibrils crow together with eachother due to physical adsorption on the surface of thefibrils.

The template-synthesis method yielded either tubulesor fibers of the desired copolymer within the pores ofAAO membrane by controlling the polymerization timeand temperature. As a result, polymeric tubules areobtained at short polymerization time, although notshown here. When the copolymer is synthesized electro-chemically within the pores of the AAO membrane, thecopolymer preferentially nucleates and grows on thepore walls [3,13]. The reason is attributed to the factthat the polymers are cationic and there are anionicsites on the pore wall of the AAO membrane.

3.2. FT-IR spectra

The IR spectra of copolymer is compared with thatof polypyrrole and poly(3-methylthiophene) in Fig. 3.The most impressive characterization is the evolution ofthe absorption bands located around 1440 cm−1. In thespectrum of pure polypyrrole, the single peak located at1445 cm−1 is assigned to C�C stretch, C�N stretch and

the deformation of the five-membered ring which con-tains the C�C�N, C�C�C deformation [26]. In thespectrum of poly(3-methylthiophene), the single peaklocated at 1433 cm−1 is assigned to the C�C, and C�Sstretch mode [27]. In the spectrum of copolymer, thispeak is split into two isolated peaks located at 1475 and1442 cm−1. That is to say, a red shift was observed inthe copolymer. Another similar characterization in thespectrum of copolymer is that there is a broad peakconsisting of a peak maximum at 1176 cm−1 and ashoulder absorption at 1160 cm−1, which are assignedto the C�C stretch, C�C in-ring stretch and C�N stretchof poly(3-methylthiophene), respectively. This indicatedthat the monomers of pyrrole and 3-methylthiopheneare, indeed, incorporated into the copolymer.

The evolution of bands around 1090 cm−1 is alsointeresting. In copolymers, the two bands seem similarin shape to that of polypyrrole, while these bands donotexist in poly(3-methylthiophene). It is then judged to beassociated with the pyrrole rings of the copolymerrather than the 3-methylthiophene rings. It is then saidthat polarons and/or bipolarons seem to be localized onpyrrole ring units of the copolymer.

Another noticeable feature in the spectrum is that anabsorption band around 790 cm−1, which is character-istic of �-substituted five-membered heterocyclic com-pounds [28] appears in the copolymer, suggesting that�-positions of each monomer are involved incopolymerization.

Fig. 3. IR spectra of (a) pure polypyrrole; (b) copolymer nanofibrils; and (c) pure poly(3-methylthiophene).

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Fig. 4. Representative (a) C(1s); (b) N(1s); and (c) S(2p) core levelXPS spectra for copolymer nanofibrils.

Fig. 5. TGA of (a) copolymer; (b) polypyrrole+poly(3-methylthio-phene) mixture in a 1:1 ratio.

3.3. XPS and elemental analysis

The results of XPS intensity and elemental analysisare shown in Table 1. Charging effects were not ob-served for the more highly conductive samples in XPSspectra. In agreement with FT-IR data, both types ofanalysis results indicate that pyrrole is the major com-ponent of the prepared copolymer nanofibrils. Fig. 4(a)shows the C(1s) XPS spectra for copolymer nanofibrils.The binding energy of 284.4 eV corresponds to thestructure of R�C�C�R. Notice that a higher bindingenergy shoulder at 286.5 eV present in the spectra,which has been attributed to carbonyl. The area underthe component Gaussian centered at 286.5 eV scaleswith the intensity of the carbonyl band in the FT-IR[30]. Fig. 4(b) shows N(1s) XPS spectra for copolymernanofibrils. The binding energy of 399.7 eV corre-sponds to the structure of R�N�R. A higher bindingenergy shoulder (centered at 401.5 eV) appears in thespectra. Pfluger and Street observed that this featurearises from nitrogen atoms which bear higher positivecharge than the nitrogen atoms comprising the mainpeak [31]. Eaves et al. suggest that these more positive

Furthermore, the weak band at 1700 cm−1 appearedin polypyrrole and copolymer. This band has beenobserved in oxygen-oxidized and in electrochemicallyover-oxidized polypyrrole and has been attributed tocarbonyl [29]. This indicates that carbonyl is present inpolypyrrole and copolymer.

Table 1XPS intensity and elemental analysis of copolymer nanofibrils

Elemental analysisXPS intensity

O(1S)Cl(2P3/2)S(2P3/2) H%N(1S) S%C(1S) N%C%

64.19 1.93 4.28 18.68 50.1910.92 11.67 4.422.70

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Table 2TGA of copolymer and polypyrrole+poly(3-methylthiophene) mixture in a 1:1 ratio

Decomposition temperature (°C)Polymer

TmaxbT i

a T fc

Copolymer 175 323 620287 584132Polypyrrole+poly(3-methylthiophene) (mixture in 1:1 ratio)

T ia, initial temperature; Tmax

b , maximal decomposition temperature; T fc, final temperature.

Table 3Conductivity of copolymer nanofibrils.

Conductivity (×10−4 S cm−1)Polymer

Polypyrrole 3.061.74Poly(3-methylthiophene)3.65Copolymer

3.5. Conducti�ity measurements

Table 3 shows four-probe conductivity data for pre-pared polypyrrole, poly(3-methylthiophene) and co-polymer. From the table, we can see that thecopolymer has the highest conductivity with a conduc-tivity value of 3.65×10−4 S cm−1, whereas thepoly(3-methylthiophene) has the lowest conductivitywith a value of 1.74×10−4 S cm−1.

4. Conclusion

We conclude from this investigation that copolymernanofibrils were successfully synthesized in the poresof microporous AAO template by chemical copoly-merization technique. Electron microscopy resultsshowed that the template technique can control thelength, diameter and thickness of copolymernanofibrils, which had uniform and well-aligned ar-rays. The composition of the copolymer obtained wasdetermined using IR spectroscopy, XPS and elementalanalysis. From above, pyrrole and 3-methylthiopheneare both incorporated into the copolymer and the ma-jor component of the copolymer is pyrrole unites. Thestudy of TGA demonstrated that the copolymer wasthermally stable compared with the homopolymersand the mixtures. In addition, we found out the con-ductivity of the copolymers is higher then the homo-polymers.

Acknowledgements

This work was supported by the National NaturalScience Foundation of China (Grant No. 69890220).

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