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Synthesis and characterization of conducting polyaniline 5-sulfosalicylate nanotubes

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2008 Nanotechnology 19 135606

(http://iopscience.iop.org/0957-4484/19/13/135606)

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Nanotechnology 19 (2008) 135606 (8pp) doi:10.1088/0957-4484/19/13/135606

Synthesis and characterization ofconducting polyaniline 5-sulfosalicylatenanotubesAleksandra Janosevic1, Gordana Ciric-Marjanovic1,Budimir Marjanovic2, Petr Holler3, Miroslava Trchova3 andJaroslav Stejskal3

1 Faculty of Physical Chemistry, University of Belgrade, Studentski Trg 12-16,11001 Belgrade, Serbia2 Centrohem, Vuka Karadzica bb, 22300 Stara Pazova, Serbia3 Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic,162 06 Prague 6, Czech Republic

E-mail: gordana@ffh.bg.ac.yu

Received 29 September 2007, in final form 30 September 2007Published 26 February 2008Online at stacks.iop.org/Nano/19/135606

AbstractConducting polyaniline 5-sulfosalicylate nanotubes and nanorods were synthesized by thetemplate-free oxidative polymerization of aniline in aqueous solution of 5-sulfosalicylic acid,using ammonium peroxydisulfate as an oxidant. The effect of the molar ratio of 5-sulfosalicylicacid to aniline on the molecular structure, molecular weight distribution, morphology, andconductivity of polyaniline 5-sulfosalicylate was investigated. The nanotubes, which have atypical outer diameter of 100–250 nm, with an inner diameter of 10–60 nm, and a lengthextending from 0.4 to 1.5 μm, and the nanorods, with a diameter of 80–110 nm and a length of0.5–0.7 μm, were observed by scanning and transmission electron microscopies. The presenceof branched structures and phenazine units besides the ordinary polyaniline structural featureswas revealed by infrared and Raman spectroscopies. The stacking of low-molecular-weightsubstituted phenazines appears to play a major role in the formation of polyaniline nanorods.The precipitation–dissolution of oligoaniline templates as a key element in the formation ofpolyaniline nanotubes is proposed to explain the crucial influence of the initial pH of thereaction mixture and its decrease during the course of polymerization.

1. Introduction

It has recently been shown that the dispersibility and process-ability of nanostructured conducting polymers, as well as theirperformance in numerous conventional applications, is signifi-cantly improved in comparison with conducting polymers hav-ing granular morphology [1–6]. The genesis of self-assembledpolyaniline (PANI) nanotubes and nanorods has received grow-ing attention during recent years [7–29]. Conducting PANInanotubes have been synthesized by the chemical oxidativetemplate-free method in the presence of various inorganicacids [10, 25], sulfonic acids [7–9, 12, 13, 17–21], carboxylicacids [11, 14, 16, 22, 24, 28], and polymeric acids [26, 27].The use of ammonium peroxydisulfate (APS) as a strong two-electron oxidant, the molar ratio aniline to acid �1 at the

beginning of oxidation, and the decrease in pH during the oxi-dation are shown to be key reaction conditions for all template-free syntheses of conducting PANI nanotubes.

The understanding of the evolution of the molecularstructure and nanotubular/nanorod morphology of PANI inthe absence of added templates is crucial for efficient designand control of self-assembly processes. A cylindrical micellemodel of aniline salts with various dopant acids [7–21, 25–28]was most frequently proposed to interpret the formation ofPANI nanotubes. This concept is not compatible with theobservation that PANI nanotubes and nanorods are formedeven when aniline has been oxidized in aqueous solutionwithout added acid [22, 23, 29]. A recent study of theformation of PANI nanofibers/nanotubes proposed a modelbased on the ‘surfactant’ role of aniline dimer cation-radicals,

0957-4484/08/135606+08$30.00 © 2008 IOP Publishing Ltd Printed in the UK1

Nanotechnology 19 (2008) 135606 A Janosevic et al

Figure 1. In situ preparation of anilinium 5-sulfosalicylate (ASSA) and dianilinium 5-sulfosalicylate (DASSA) by reaction of aniline with5-sulfosalicylic acid (SSA) in aqueous solution.

which could aggregate to form different sizes and typesof micelle [29]. However, the mechanistic details of thismodel are not supported by quantum chemical studies ofthe early stages of the oxidative polymerization of anilinewith APS [30, 31]. Also, the surfactant role of cation-radicals of a major aniline dimer, 4-aminodiphenylamine (4-ADPA), is not consistent with their pronounced charge/spindelocalization [30] and high reactivity [32]. In contrast tothese micellar models, it has been proposed [22–24] that theinsoluble oligoanilines, produced in the early stages of theoxidation at low acidity of the medium, act as templateswhich further guide the growth of PANI nanostructures [24].The presence of branched structures and phenazine units hasalso been invariably connected with the formation of PANInanotubes [23, 24, 33].

It was reported that PANI solubility, crystallinity, thermalstability, electrochemical stability at higher potentials, andanticorrosive properties have been improved by using 5-sulfosalicylic acid (SSA) as a dopant [34–37]. In thispaper, conducting PANI nanotubes and nanorods dopedwith SSA and produced during a self-assembly processare reported. The influence of the synthetic conditionson the molecular and supramolecular structure, molecularweight distribution, and electrical properties of polyaniline5-sulfosalicylate (PANI-SSA) was investigated by Fourier-transform infrared (FTIR) and Raman spectroscopies, scanningand transmission electron microscopies (SEM and TEM),gel-permeation chromatography (GPC), and conductivitymeasurements, respectively. The evolution of molecularstructure and nanotubular morphology of PANI-SSA iscomputationally modeled by the semi-empirical quantumchemical MNDO-PM3 method [38], combined with theMM2 molecular mechanics force-field method [39], and theconductor-like screening model of solvation (COSMO) [40].

2. Experimental details

2.1. Synthesis of PANI-SSA

Aniline was distilled under reduced pressure prior to use. APSand SSA were of analytical grade and have been used asreceived from Centrohem (Serbia). In a typical procedurefor preparing PANI-SSA nanotubes and nanorods, 2.0 ml(2.2 × 10−2 mol) of aniline was dissolved in 190 ml ofaqueous solution containing 1.4 g (5.5 × 10−3 mol) of SSAand the solution was heated to boiling, and then cooled toroom temperature. The aqueous solutions of monomer and

oxidant (0.22 M APS, 100 ml) were mixed at ∼20 ◦C to startthe oxidation and the reaction mixture was stirred for 3 h. Themolar concentrations of reactants thus were [aniline] = 7.6 ×10−2 M, [SSA] = 1.9 × 10−2 M, and [APS] = 7.6 × 10−2 M.The precipitated PANI-SSA was collected on a filter, rinsedwith 5 × 10−3 M SSA, and dried in vacuum at 60 ◦C for3 h. A part of the PANI-SSA was deprotonated with excessof 5% ammonium hydroxide, and the resulting PANI base wasagain separated by filtration and dried. The effect of SSA-to-aniline molar ratio (1, 0.5, and 0.25), i.e. the influence of themonomer nature [anilinium 5-sulfosalicylate (ASSA, figure 1),dianilinium 5-sulfosalicylate (DASSA, figure 1), and mixtureof DASSA with aniline] on the yield, conductivity, molecularweights, molecular and supramolecular structures of PANI-SSA was investigated. The molar ratio [APS]/[aniline] = 1was used in all syntheses.

2.2. Characterization

A scanning electron microscope (JEOL JSM 6460 LV) and atransmission electron microscope (JEOL JEM 2000 FX) wereused to characterize the morphology of the samples.

Infrared spectra of the powdered samples dispersed in KBrpellets were recorded in the range 400–4000 cm−1 at 64 scansper spectrum at 2 cm−1 resolution using a Thermo NicoletNEXUS 870 FTIR spectrometer with a DTGS TEC detector.The spectra were corrected for the presence of carbon dioxideand humidity in the optical path.

Raman spectra excited in the visible range with a HeNe633 nm laser were collected on a Renishaw inVia ReflexRaman microscope. A research grade Leica DM LMmicroscope with objective magnification ×50 was used tofocus the laser beam on the sample placed on the X–Ymotorized sample stage. The scattered light was analyzed bythe spectrograph with a holographic grating with 1800 linesmm−1. A Peltier-cooled CCD detector (576 × 384 pixels)registered the dispersed light. The positioning of a samplewas controlled, and the data were processed with the Wire 2.2software.

The conductivity of polymer powders compressedbetween stainless pistons was measured at room temperatureby means of an ac bridge (Waynne Kerr Universal BridgeB 224), at fixed frequency of 1.0 kHz. During themeasurement, pressure was maintained at 124 MPa.

Molecular weights were assessed by gel-permeationchromatography using an 8 × 500 mm Labio GM 1000column operating with N-methylpyrrolidone and calibrated

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Nanotechnology 19 (2008) 135606 A Janosevic et al

Figure 2. Temperature profile during the oxidative polymerization ofaniline with APS in aqueous medium in the presence of SSA, at[SSA]/[aniline] = 1(�), 0.5 (��) or 0.25 (•).

by polystyrene standards with a spectrophotometric detectionat the wavelength of 546 nm. The PANI bases weredissolved in N-methylpyrrolidone containing 0.025 g cm−3

triethanolamine and 0.005 g cm−3 lithium bromide to preventaggregation.

2.3. Computational methods

The heat of formation (�Hf) was computed by theMNDO-PM3 method [38] (included in the molecular orbitalpackage [41] MOPAC 97, part of the Chem3D Pro 5.0package, CambridgeSoft Corporation). The solvation effectsin water were taken into account by using the COSMOmodel [40]. Input files for the semi-empirical quantumchemical computations of oligomeric species were the moststable conformers of investigated molecular structures, withminimized steric energy using the MM2 method [39]. Thegeometry optimization was performed by the EigenFollowingprocedure [42, 43]. The restricted Hartree–Fock method wasused.

3. Results and discussion

3.1. The course of the oxidative polymerization of aniline

The oxidative polymerization of aniline with APS is anexothermic process which can be easily followed by moni-toring the reaction temperature [22–24] (figure 2). The ini-tial induction period characteristic for aniline polymerizationin acidic media [22] was followed by the rapid exothermicpolymerization of aniline during the oxidations of both ASSA([SSA]/[aniline] = 1) and DASSA ([SSA]/[aniline] = 0.5)(figure 2). The oxidation of the mixture of aniline and DASSA([SSA]/[aniline] = 0.25), in contrast, proceeds in two exother-mic phases which are well separated by an athermal period (fig-ure 2). This temperature profile is similar to that observed forthe oxidative polymerization of aniline in water without addedacid [23].

Table 1. The yield, w (the weight ratio of deprotonatedPANI/aniline), number-average molecular weight, Mn,weight-average molecular weight, Mw, polydispersity index,Mw/Mn , and conductivity, σ , for PANI-SSA samples prepared byoxidative polymerization at various [SSA]/[aniline] molar ratios.

[SSA]/[aniline] w Mn Mw Mw/Mn σ (S cm−1)

1/4 0.61 7 490 51 400 6.9 0.00751/2 0.63 10 400 55 000 5.3 0.0201 0.68 14 800 68 300 4.6 0.063

The pH of ASSA, DASSA and DASSA/aniline aqueoussolutions, i.e., [SSA]/[aniline] = 1, 0.5, and 0.25, was 1.7, 3.5and 4.6, respectively. The addition of APS has negligible effecton the initial pH of the reaction mixture. The acidity of thereaction mixture continuously increases during the oxidativepolymerization of aniline, because hydrogen atoms, abstractedfrom amino groups and benzene rings of aniline molecules, arereleased as protons: nC6H5NH2+nS2O2−

8 → (–C6H4NH–)n+2nH+ + 2nSO2−

4 . The pH at the end of polymerization was∼1.5.

3.2. Morphology

SEM images show the crucial influence of the monomernature (=[SSA]/[aniline] ratio) on the formation of PANI-SSA nanostructures (figure 3). When [SSA]/[aniline] = 1 or[SSA]/[aniline] = 0.5, granular agglomerates of PANI-SSAwere obtained (figures 3(a) and (b)). PANI-SSA nanotubesand nanorods were synthesized by the oxidation of a mixtureof DASSA and aniline at [SSA]/[aniline] = 0.25 (figures 3(c)and (d)). It can be concluded that some critical pH exists inthe range 3.5 < pH < 4.6, which represents the lowest pH ofthe monomer solution for the efficient formation of PANI-SSAnanotubes and nanorods; at pH < 3.5 the granules prevail.

The nanotubular morphology was proved by TEM(figure 4). This is the only method which is able to distinguishbetween nanotubes and nanorods. PANI-SSA nanotubes havean outer diameter of 100–250 nm, an inner diameter of 10–60 nm, and a length extending from 0.4 to 1.5 μm. Thenanotubes are accompanied by nanorods with a diameter of80–110 nm and a length of 0.5–0.7 μm.

3.3. Molecular weight distribution and conductivity

The weight-average and number-average molecular weights,Mw and Mn, decrease while the polydispersity index increaseswith the decrease in the molar ratio [SSA]/[aniline] (table 1).Granular PANI-SSA samples have unimodal molecular weightdistributions (figure 5, dotted and dashed lines). A bimodaldistribution is observed for the nanostructured PANI-SSA(figure 5, full line) and this reflects the two exothermicprocesses in the oxidation of aniline (figure 2). The anilineoligomers, with peak molecular weight Mp = 2240, producedduring the early stages of synthesis of PANI-SSA nanotubesand nanorods, at pH ∼ 4.5, are accompanied by PANI chainsproduced later at pH < 2.

The conductivity of the PANI-SSA decreases withdecreasing [SSA]/[aniline] ratio (table 1). The variations

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Nanotechnology 19 (2008) 135606 A Janosevic et al

(a) (b)

(c) (d)

Figure 3. SEM images of products of the chemical oxidative polymerization of (a) ASSA ([SSA]/[aniline] = 1); (b) DASSA([SSA]/[aniline] = 0.5), and (c), (d) mixture of DASSA and aniline ([SSA]/[aniline] = 0.25).

in morphology are reflected in conductivity: samples havinggranular morphology show conductivities in the same range,while the sample with nanotubular/nanorod morphology hasalmost one order of magnitude lower conductivity due to thepresence of a non-conducting oligomeric component (figure 5).If PANI with high conductivity is needed, then a high acidconcentration is recommended. When the preparation ofnanostructured PANI is the goal, the reduced concentration ofacid should be selected.

3.4. Molecular structure

The FTIR spectra of all PANI-SSA doped samples (fig-ure 6) show the characteristic peaks of PANI emeraldinesalt at around 1580 cm−1 (quinonoid (Q) ring stretching),1490 cm−1 (benzenoid (B) ring stretching), 1300 cm−1 (theC–N stretching of secondary aromatic amine), 1240 cm−1

(the C–N+• stretching), 1140 cm−1 (the B–NH+ = Q stretch-ing), and the band at 820 cm−1 (aromatic C–H out-of-plane deformation vibration of 1,4-disubstituted benzene ring,γ (C–H), in linear PANI backbone) [44]. The intersectionof the horizontal dashed line, placed at the level of absorp-tion at ∼1580 cm−1, with the absorption tail at wavenum-bers >2000 cm−1 moves to lower wavenumbers as the[SSA]/[aniline] ratio increases. This feature [45] is wellcorrelated with the increasing conductivity of the PANI-SSAsamples with increase of [SSA]/[aniline] ratio (table 1).

The broad band observed at ∼3460 cm−1 correspondsto the N–H stretching vibration of secondary amine inthe polyaniline backbone [46]. The four peaks locatedat 3400–2800 cm−1 reflect mainly the hydrogen-bondedN–H stretching (H-peaks) [23, 46, 47]. The presence of5-sulfosalicylate anions as the dopants in all PANI-SSA sam-ples is confirmed by the characteristic bands observed at1675–1670 cm−1 (C=O stretching in COOH), ∼1027 cm−1

(symmetric stretching of SO3 group), 881 and ∼800 cm−1

(γ (C–H) vibrations of SSA ring), 664 cm−1 (in-planebending and/or out-of-plane bending of SSA ring) and594 cm−1 (out-of-plane bending of SSA ring) [34, 46, 48].It should be noted that the bands of SSA are most pro-nounced in the spectrum of PANI-SSA sample preparedby the oxidation of ASSA ([SSA]/[aniline] = 1). Theshoulder observed at ∼1077 cm−1 in the spectra of allPANI-SSA doped samples corresponds to SO3 symmet-ric stretching of HSO−

4 anions [46]. The coexistence ofboth 5-sulfosalicylate monovalent anions and hydrogen sul-fate anions in all doped PANI-SSA samples is a conse-quence of the complete one-proton dissociation of correspond-ing strong acids, SSA → HOOC(OH)C6H3SO−

3 + H+ andH2SO4 → HSO−

4 + H+, and suppressed second-proton disso-ciation of both acids, pKa2(SSA) = 2.5; pKa2(H2SO4) = 2.0,at the end of polymerization process when the reaction mixturebecame highly acidic, pH ∼ 1.5.

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Figure 4. TEM images of PANI-SSA nanotubes, (A), and nanorods, (B), prepared by the chemical oxidative polymerization of the mixture ofDASSA and aniline with APS in aqueous solution ([SSA]/[aniline] = 0.25). Some nanorods have a partly dissolved core (C).

Figure 5. Molecular weight distribution as obtained by GPC for thesamples prepared by the oxidation of aniline at [SSA]/[aniline] = 1(− − −), 0.5 (· · · · · ·) or 0.25 (——).

In the FTIR spectra of deprotonated samples (figure 7)the bands of SSA anions are still present, although weakened.This can be explained by the covalent bonding betweenSSA and PANI backbone [49] or by hydrophobic interactionbetween benzene rings of SSA and PANI. It is important tonote that, in the FTIR spectra of both doped and dedopednanostructured PANI-SSA, we have observed an additionalshoulder/peak at ∼1416 cm−1, which is not present in thespectra of granular PANI-SSA samples. This shoulder/peak isdue to the stretching of the phenazine ring [50], formed by theoxidative intramolecular cyclization of branched oligoanilineand PANI chains [23]. This is consistent with the findingthat the intensity of the band at 830 cm−1 (γ (C–H) vibration

Figure 6. FTIR spectra of PANI-SSA samples in salt (doped) forms,prepared by the oxidative polymerization of aniline at[SSA]/[aniline] = 1, 0.5, and 0.25.

of 1,4-disubstituted benzene ring in linear PANI backbone)with respect to the shoulder at ∼850 cm−1 (γ (C–H) vibrationof 1,2,4-trisubstituted benzene ring in branched segment ofPANI backbone) is the lowest in the spectrum of dedopednanostructured PANI-SSA, indicating the presence of lowercontent of N–C4 coupled aniline units in the nanostructuredPANI-SSA in comparison with granular PANI-SSA.

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Figure 7. FTIR spectra of dedoped PANI-SSA samples prepared bythe oxidative polymerization of aniline at [SSA]/[aniline] = 1, 0.5,and 0.25.

The Raman bands attributed to PANI (emeraldine) saltsegments (figure 8) were observed for all PANI-SSA samplesat the following wavenumbers: 1599–1593 cm−1 (the C ∼ Cstretching vibrations of the semi-quinonoid (SQ) rings, where‘∼’ denotes the bond intermediate between the single anddouble bond), 1510–1505 cm−1 (the N–H deformation vibra-tions of protonated amine), 1342–1335 cm−1 (the C ∼ N+•stretching vibration of delocalized polaronic structures), and1173–1167 cm−1 (C–H bending in-plane vibrations of SQrings) [51, 52]. The bands at 1632, 1403, and 575 cm−1 in theRaman spectrum of PANI-SSA prepared by the oxidation ofthe mixture of DASSA and aniline are assigned to substitutedphenazine units [33, 53]. The relative intensities of these bandsin the Raman spectra of granular PANI-SSA samples, preparedfrom ASSA and DASSA, are lower than those in the spectrumof the sample containing nanotubes and nanorods as prevalentmorphology, indicating the highest amount of phenazine-likeunits in the last sample.

3.5. The mechanism of the formation of PANI-SSA nanotubesand nanorods

Equimolar quantities of aniline and anilinium cation arepresent at the beginning of the synthesis of PANI-SSAnanotubes and nanorods by the oxidation of the mixture ofaniline and DASSA (pH = 4.6), because the pKa of aniliniumcation is equal to 4.6. The oxidations of aniline moleculeswith APS and pernigraniline-like oligoanilines govern theearly stages of formation of PANI-SSA nanostructures becausethe non-protonated aniline is much more oxidizable than theanilinium cation [30]. That is why we assumed that the low-molecular-weight aniline oligomers, formed during the courseof the oxidative polymerization of the mixture of aniline andDASSA, are similar to linear and branched oligomers formedby the oxidation of aniline with APS in water without addedacid [23, 30, 31, 33]. The use of APS as an oxidant andthe presence of substantial amounts of non-protonated anilinemolecules at the start of oxidation represent the common set

Figure 8. Raman spectra (excitation wavelength 633 nm) ofPANI-SSA samples in salt forms, prepared by the oxidativepolymerization of aniline at [SSA]/[aniline] = 1, 0.5, and 0.25.

of reaction conditions of all syntheses yielding self-assembledconducting PANI nanotubes and nanorods [7–29].

It is proposed that the stacking of substituted phenazines(of pseudomauveine type, etc), formed by the oxidative in-tramolecular cyclization of branched low-molecular-weightoligoanilines [23, 24, 30, 31, 33], plays a major role in the for-mation of non-conducting, needle-like oligoaniline nanocrys-tallites, which act as a template for further growth of PANI-SSA nanostructures. We have proved by the MM2/MNDO-PM3/COSMO computational method that the stacking ofdi-pseudomauveine sulfate (figure 9(a)) and 3-(phenazin-2-ylimino)-6-phenylimino-cyclohexa-1,4-diene (figure 9(b)) isthermodynamically favorable in aqueous medium. It is furtherproved that the protonation of substituted phenazines causestheir destacking and dissolution.

The concentration of peroxydisulfate rapidly decreases,on account of both oxidative oligomerization of aniline andthe oxidation of oligoanilines to their fully oxidized forms(linear → pernigraniline; branched → phenazine), and theacidity of the reaction mixture continuously increases becausesulfuric acid is formed as a product of the reduction of perox-ydisulfate with aniline and oligoanilines. The molar ratio ofanilinium cations to aniline, [C6H5NH+

3 ]/[C6H5NH2], rapidlyincreases and the polymerization mechanism becomes basedon the redox reactions of nigraniline-like and pernigraniline-like oligoanilines with anilinium cation or aniline and low-molecular-weight leucoemeraldine-like and protoemeraldine-like oligoanilines [31]. This redox equilibrating process, auto-accelerated at pH < 2.5 during the second exothermic phaseof aniline polymerization after pernigraniline becomes proto-nated (figure 2), leads to the formation of longer PANI chainsin the conducting form of emeraldine salt with prevalent N–C4coupling mode between aniline units [31].

Non-conducting needle-like nanocrystallites, with highcontent of low-molecular-weight fully oxidized oligoani-lines which show relatively low redox reactivity (substitutedphenazines and non-protonated pernigraniline-like oligoani-lines), acting as templates, become coated with a conducting

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Nanotechnology 19 (2008) 135606 A Janosevic et al

(a)

(b)

Figure 9. The most stable conformations of (a) di-pseudomauveine sulfate (�Hf = −118.9 kcal mol−1) and (b) two molecules of3-(phenazin-2-ylimino)-6-phenylimino-cyclohexa-1,4-diene in water (�Hf = 329.5 kcal mol−1), stabilized by the stacking of substitutedphenazine rings.

PANI-SSA film [44]. This leads to the formation of PANI-SSA nanorods with non-conducting core and conducting walls.It has been proposed that in this way the nanotubular growth isstarted and that it proceeds by ‘inertia’ beyond the nanocrys-tallite [24]. We propose that the dissolution of low-molecular-weight fully oxidized oligoanilines, caused by their protona-tion at pH < 2.5, from the core of PANI-SSA nanorods, in-dicated by TEM (C in figure 4), contributes to the formationof PANI-SSA nanotubes. This is also supported by the obser-vation that, while the diameter of nanotubes varies, the wallthickness is about the same (figure 4). It is still open to discus-sion how long the oligomer nanocrystallites are, and also thequestion remains unanswered whether the nanocrystallite con-stitutes only the start of a nanotube [24] or, in extreme case,the whole of its interior. It seems that the ratio of phenazineto pernigraniline structures in the core determines the ten-dency to its dissolution. We can speculate that the resistanceof the core of PANI-SSA nanorods to dissolution in acidicaqueous medium increases with increased content of substi-tuted phenazines. It could also be suspected that PANI-SSAnanorods, having much smaller outer diameter than PANI-SSA nanotubes, have a very small diameter of non-conductingcore, most probably in the range 1–5 nm. This means thatPANI-SSA nanorods may in fact be nanotubes with cavitiesof extremely small diameters invisible at the available TEMmagnification.

4. Conclusions

Aniline has been oxidized in the presence of 5-sulfosalicylicacid. The molecular structure, morphology (granular versusnanostructured), molecular weight distribution and conduc-tivity of oxidation product, polyaniline-5-sulfosalicylate, arecrucially affected by the initial [SSA]/[aniline] molar ratio.

PANI-SSA nanotubes and nanorods with outer diameters of100–250 nm and 80–110 nm, respectively, and conductivityof ∼10−2 S cm−1 were synthesized through a self-assemblyprocess by the oxidation at [SSA]/[aniline] = 0.25. FTIRspectroscopic analysis of deprotonated nanostructured PANI-SSA indicates some covalent bonding of SSA anions to thepolyaniline chain. The presence of substituted phenazine con-stitutional units, in addition to benzenoid, quinonoid and semi-quinonoid units, has been proved by Raman spectroscopy. Itis proposed that PANI-SSA nanorods are formed by the coat-ing of the non-conducting needle-like oligoaniline nanocrystal-lites with conducting PANI-SSA film. The formation of PANI-SSA nanotubes then occurs, at least in part, by the dissolutionof the cores of nanorods, initiated by the protonation of thecore constituents, substituted phenazines and pernigraniline-like oligoanilines, at pH < 2.5.

Acknowledgments

The authors thank the Ministry of Science of the Republicof Serbia (contract no. 142047) and the Czech Grant Agency(203/08/0686) for financial support.

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