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Synthetic Metals 160 (2010) 1727–1732

Contents lists available at ScienceDirect

Synthetic Metals

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

aman study of polyaniline nanofibers prepared by interfacial polymerization

anu Jain, S. Annapoorni ∗

epartment of Physics and Astrophysics, University of Delhi, Delhi 110007, India

r t i c l e i n f o

rticle history:eceived 4 December 2009eceived in revised form 5 June 2010

a b s t r a c t

Polyaniline nanofibers were synthesized by interfacial polymerization of aniline. Polyaniline so formedwas studied using transmission electron microscopy, scanning electron microscopy, UV–vis and Ramanspectroscopy. TEM of polymer dispersion shows the presence of fibers having diameter around 30–80 nm.

ccepted 9 June 2010

eywords:olyaniline nanofibers

Strikingly different Raman spectra were observed from the polyaniline film, which were related to twodissimilar areas on the film. SEM of polyaniline films shows fibrous as well as flake like structures scatteredin fiber matrix. Raman spectra of the films heat treated from 25 ◦C to 300 ◦C were taken to study thestructural variations induced by the change of temperature at these two regions. Many applications ofpolymer films are dependent on the homogeneity of the systems. In this work we intend to employ Raman

ensa

nterfacial polymerizationaman spectroscopyemperature variation

Spectroscopy as an indisp

. Introduction

Polyaniline is the most studied conducting polymer because ofts ease of preparation, variety of forms, interchangeable oxida-ion states, versatility and low cost. Polyaniline’s applications rangerom LED [1], memory devices and catalysts [2], anticorrosion coat-ngs [3], membranes [4–6], detecting harmful chemicals viz. dyes7], as a glucose biosensor [8a], substituted polyaniline [8b], chem-cal sensor [9]. Polyanilines form an interesting family of polymersue to the incorporation of nitrogen in the backbone of the polymeretween two phenyl rings [10]. It has various oxidation states start-

ng from leucoemeraldine (completely reduced state), emeraldineorm (half oxidized) and pernigraniline (completely oxidized state).olyaniline can be synthesized by chemical oxidative synthesis11], electrochemical polymerization [12], etc. Polyaniline nanos-ructures like nanotubes, nanowires, nanorods and nanofibers cane prepared by various methods like rapid mixing polymerization13,14], interfacial polymerization [14], in situ seeding polymer-zation [15] or using templates and surfactants [16]. Polyanilineanofibers prove to be better candidates for various applicationswing to high surface area and better processibility, e.g. in sen-ors [17,18]. Interfacial polymerization is a general method to makeulk of polyaniline nanofibers with diameter ranging from 30 nm

o 120 nm depending upon the choice of dopant acid used [14].s pointed out by Huang and Kaner [19] polyaniline preferentially

orms nanofibers in the oxidative polymerization of aniline. Herehe polymerization occurs only at the interface, therefore once the

∗ Corresponding author. Tel.: +91 9871521718.E-mail addresses: annapoorni@physics.du.ac.in, annapoornis@yahoo.co.in

S. Annapoorni).

379-6779/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.synthmet.2010.06.008

ble tool for looking into the non-uniformities of the polyaniline films.© 2010 Elsevier B.V. All rights reserved.

formed polyaniline nanofiber leaves the polymerization site, thereis almost no chance of secondary growth.

The applications of polyaniline are dependent on the morphol-ogy, the oxidation state of polyaniline, surface property, its thermalstability and the history of its preparation [2]. Therefore, the studyof the structural variations and stability of the polymer is crucial forincorporating further improvements that would further increasethe device applicability. Techniques like thermogravimetric anal-ysis (TGA), differential scanning calorimetry (DSC) cannot tell usabout these variations. Raman spectroscopy is quite sensitive to theelectronic structural changes of polyaniline [20]. The oxidation andprotonation states of polyaniline affect the Raman signals collectedfrom the polymer. There are a number of reports which have usedRaman Spectroscopy as a valuable tool for analyzing behaviour ofdifferent forms of polyaniline and its oligomers [21–30]. Yan et al.[31] have given the Raman of polyaniline film at room tempera-ture. Temperature variation Raman studies have been reported inpolyaniline coated fabrics [32], polyaniline nanotubes [33]. Boyeret al. [20] have done an analysis using a model compound approach.Sedenkova et al. [34] have studied the ageing of the films pre-pared with and without dopant acid using Raman Spectroscopyand FTIR. Thermal treatment on electrochemically grown polyani-line led to new bands at 583 cm−1, 1398 cm−1, 1644 cm−1 whichwere related to the oxidized structures with oxazine like rings[28]. The Raman studies on heat treated polyaniline nanofiber filmmade by interfacial polymerization are not many. Nascimento etal. [35] have reported the resonance Raman spectra of polyaniline

−1 −1

nanofibers from 200 cm to 500 cm and found an increase in theCring–N–Cring torsion angles owing to the formation of bipolarons.

We report the Raman spectra of the polyaniline nanofibrousfilms, prepared by drop casting method, at various temperatures.There are very few reports that provide optical images of polyani-

1728 M. Jain, S. Annapoorni / Synthetic Metals 160 (2010) 1727–1732

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ig. 1. SEM of polyaniline nanofiber film drop cast from diluted nanofiber dispersib) flake-like structure.

ine films. These generally show differently coloured regions butith no Raman studies comparing them. Such type of investiga-

ions have been carried out only on polyaniline composite films.ere, an attempt has been made to investigate the structural dif-

erences in polyaniline nanofibrous films at different regions usingaman spectroscopy. Further, the effect of heat treatment on thelms is investigated in detail.

. Experimental

.1. Materials

Aniline (Thomas Baker AR 99.5% purity), methylene chlo-ide (Sigma Aldrich), ammonium peroxydisulphate (Qualigens AR9.0%), Hydrochloric acid (Qualigens 35–37% assay). All the chem-

cals were used as received without any further treatment.

.2. Synthesis

Polyaniline nanofibers were synthesized by interfacial poly-erization [14]. 3.2 mmol aniline was dissolved in 20 ml ofethylene chloride. Oxidant solution (aqueous part) consisted of

.8 mmol ammonium peroxydisulphate in 20 ml of 1 M HCl solu-ion. Monomer solution (the organic phase) was put first in 50 mleaker and on top of it the oxidant solution was poured carefullyith the help of a pipette so as not to disturb the interface. The

queous layer in contact with the interface became green in colours soon as the solution was put showing that the reaction is quiteapid. The whole aqueous layer was filled with dark green colour.he organic layer became yellowish orange in colour showing theresence of oligomers. The reaction mixture was left undisturbedor 6 h to ensure the completion of the reaction. The aqueous layeras then carefully pipetted out and centrifuged at 1000 rpm. Pre-

ipitates obtained were washed repeatedly with double distilledater for at least five times. Cake of polyaniline was taken outith double distilled water and kept as dispersion. This ensured

he sustenance of the fibrous morphology of the prepared polyani-ine. In polyaniline nanofibers preparation, filtration or drying wasvoided. Samples for study were prepared on cleaned glass slides.00 �l dispersion of polyaniline centrifuged at 1000 rpm was dropast on slides. The films, obtained by the above method, wereark green in colour. Samples for the study were pre heated at0 ◦C, 100 ◦C, 150 ◦C, 200 ◦C, 250 ◦C, 300 ◦C for about 2–3 h,cooled

own to room temperature before taking Raman measurements.ilms heated at 50 ◦C, 100 ◦C, 150 ◦C, 200 ◦C, 250 ◦C and 300 ◦C wereeferred to as P50, P100, P150, P200, P250 and P300 respectively.he sample dried in air at room temperature, i.e. at 25 ◦C waseferred to as P25.

showing nanofibers and flakes with Inset giving magnified view of nanofibers and

2.3. Characterization

Morphology and the size estimates were done using Transmis-sion Electron Microscope HRTEM (FEI 200KV Technai) and LEO435VP Scanning Electron Microscope operated at 15 kV. Samplesfor TEM were prepared by casting diluted dispersion of nanofiberson a carbon coated copper grid. UV–vis spectroscopy was doneon Shimadzu 2501 PC system from 200 nm to 900 nm wavelengthwith resolution of 0.5 nm. Optical path length of the quartz cuvetteis 1.0 cm. The dispersion was diluted with double distilled water.Raman spectrum was taken on Renishaw inVia Raman Micro-scope employing a 514.5 nm excitation of Ar+ ion laser with 2400lines/mm grating to analyse the scattered light from the surface ofthe film. The power of the laser was kept low around 0.025 mW inorder to avoid burning of the sample. Laser beam was focused onthe sample with the help of a 50× objective lens. All spectra weretaken from 300 cm−1 to 1800 cm−1. Wavenumber increment perdata point was set at 1.0 cm−1. The data were processed with Wire2.0 software.

3. Results and discussion

3.1. Morphology and structure

SEM images of synthesized polyaniline nanofiber film are givenin Fig. 1(a) and (b). The substrate is covered by a dense net-work of nanofibers with flake like structure scattered on the film.Fig. 1(b) gives magnified image of one such flake like structurewhich appears to form a flower. Nanofibers seem to be quite flex-ible in nature as they are curved (Fig. 1(a) inset). Sample for TEMwas prepared by dropping diluted dispersion of nanofibers in waterover carbon coated copper grid. Fig. 2 gives a typical TEM image ofthe nanofibers. It can be seen that they form interconnections whichis seen to be a characteristic of the nanofibers formed by interfacialpolymerization [31]. The diameters are in the range of 30–80 nm.TEM study confirms the formation of nanofibers and no tube likestructure was observed.

3.2. Optical study

3.2.1. UV–vis spectroscopyUV–vis spectra of polyaniline nanofiber dispersions are pre-

sented in Fig. 3. Nanofiber dispersion as prepared shows twopartially resolved absorption peaks at ∼330 nm of �–�* transition

of benzenoid structures of polyaniline and at ∼430 nm. There is anintense peak centered at around 830 nm. Absorptions at ∼430 nmand ∼830 nm correspond to the doping level of polyaniline andformation of polaron band [36–38] of emeraldine salt form ofpolyaniline nanofibers [17].

M. Jain, S. Annapoorni / Synthetic Metals 160 (2010) 1727–1732 1729

Fig. 2. TEM of diluted dispersion of polyaniline nanofibers in water prepared byinterfacial polymerization.

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3

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ig. 3. UV–vis spectrum of diluted dispersion of polyaniline nanofibers in waterrepared by interfacial polymerization.

.2.2. Raman spectroscopyRaman spectra were recorded at different regions in the film.

wo strikingly different spectra were observed. This observation

Fig. 4. Optical microscope image of the film and its Raman sp

Fig. 5. Raman spectra at 514.5 nm laser excitation in R1 region of polyanilinenanofiber film heat treated at (a) 25 ◦C and (b) 100 ◦C.

was verified by taking more spectra at different locations in thesame film. A little non-uniformity is inherent in the films preparedby drop casting method. So, one might expect a slight change inthe intensity without a major change in the nature, for the spectraobserved at different positions in the film. However, in the presentstudy, spectra having diverse nature were observed originatingfrom two optically dissimilar regions when seen under Ramanmicroscope. The film can perhaps be broadly divided into regionsdark (R1) and bright (R2) on the film. These are scattered on the filmas seen under the microscope. In view of the SEM data (Fig. 1(a) and(b)), these might be related to the distinct morphologies observed.

A typical optical image is shown in Fig. 4(a) with the Ramanspectra (at 25 ◦C) at R1 and R2 in Fig. 4(b). Raman spectra taken atregion R1 are broad in nature as compared to those collected in R2.The Raman spectra of P25 and P100 at R1 region are given in Fig. 5(a)and (b), respectively. The spectra are presented here without anysmoothening.

Region R1 of the film at 25 ◦C is mainly characterized by anintense band around 1597 cm−1 which can be attributed to theC–C stretching modes of semiquinoid units. This band shifts to

1604 cm−1 at 100 ◦C. A weak shoulder at ∼1644 cm−1, perhapsindicating weak crosslinking in the EB units, appears at 25 ◦Cand at 100 ◦C. Weak band at 1559 cm−1 can be seen at 25 ◦C.C N stretching vibrations at ∼1489 cm−1 which may be due to

ectra at 514.5 nm laser excitation at (a) R1 and (b) R2.

1730 M. Jain, S. Annapoorni / Synthetic Metals 160 (2010) 1727–1732

Table 1Assignments of Raman bands of polyaniline nanofiber film at an excitation of 514.5 nm.

Approximate bandposition (cm−1)

Assignments Region References

1675 – R2 –1640–1644 Phenazine, Phenoxazine, safranine like segments R1 R2 30a and ref therein1620–16231629

C–C stretching in B R1 R2 31

1597–1603 C–C stretching SQR R1 R2 30a, 30b1582 C C stretching in Q R2 33, 41, 441563–15661559

C–C stretching of structures intermediate between Q and SQR R1R2R1

25, 26

1537 Phenazine, Phenoxazine, safranine-like segments R1 30a and ref therein, 391530 NH bending R2 40, 42b1489 C N stretching Q R1 26, 33, 411407 Phenazine like units R1 30a and ref therein, 341401 Protonated oxazine like units R2 30a and ref therein1396–1397 Phenoxazine like units R2 30a and ref therein1372 C–N+ stretching of SQR, phenosafranine like units R2 30a and ref therein, 39, 411343–1339,1320–1323, 1303

C–N+ stretching of radical cations, cyclised structures R1R2 26, 30a, 30b, 41

1247–1255 C–N stretching in polaronic units R2 R1 25, 26, 30b1187–1189 C–H bending B R1R2 26, 33, 411166 C–H bending Q R1 26, 40, 41878 Ring def B in ES R1 34775 Ring def Q R1 33748–755 Imine def Q R1R2 33, 42b603 B ring in plane def R1 39, 42b574–575 Ring def (6b), phenoxazine, phenazine like units R1R2 28, 30, 40, 42b515–522 Amine def in pol lattice + CNC torsion in bipolaron R1R2 39, 42b

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P25 exhibits an intense peak at around 1601 cm which can beattributed to the C–C stretching of semiquinoid units. The band at1601 cm−1 becomes broader and less intense in P200 and P250. Aweak peak at 1675 cm−1 can also be seen clearly in P25, P50 andP100 which vanishes at higher temperatures. The origin of this band

507–509 Out of plane C–H wag411–414 Out of plane C–H wag in bipolaronic + CNC torsion in

, benzenoid; Q, quinoid; SQ, semiquinoid rings; def: deformation; ES emeraldine s

he unprotonated quinoid segments has disappeared at 100 ◦C.ne can observe weak, poorly resolved bands at 1566 cm−1 of–C stretching of structures intermediate between quinoid andemiquinoid structures [25,26], 1537 cm−1 of phenazine like struc-ures, ∼1502 cm−1 at high temperature. Peak at 1407 cm−1 seemso have contributions from both C–C stretching of quinoid unitsnd ring stretching vibrations of phenazine like structures [40].ts intensity remains almost same at a higher temperature. Strong–N+ stretching vibrations at ∼1339 cm−1 signifies semiquinoneadical structures and becomes stronger at 100 ◦C. The band at247 cm−1, which might be due to the C-N stretching in polaronicnits, has shifted to 1238 cm−1 at 100 ◦C. Formation of benzenoidtructures in the polyaniline chains at elevated temperatures isisible in shift of CH in plane bending vibrations of bipolaronicorms at 1166 cm−1 to ∼1187 cm−1 of polaron lattice. Sometimeseating may force dopant and water molecules on the surface toiffuse into the chain structure and thus may cause a more effec-ive protonation at little higher temperatures. Amine deformationC–N–C bending) at ∼817 cm−1, ring deformation at 603 cm−1 and74 cm−1 (weak shoulder at 25 ◦C), amine in plane deformationt ∼515 cm−1 and out of plane ring deformations at 414 cm−1

ave become prominent with increase in temperature signifyingdegradation in the polymer film while quinoid deformation at775 cm−1 and imine deformation of quinoid units at ∼748 cm−1

ave weakened at high temperature. The assignments of Ramanands for polyaniline nanofiber film at a 514.5 nm laser excitationre tabulated in Table 1.

Phenazine, safranine or phenoxazine like segments are identi-ed with bands at ∼415 cm−1, 575 cm−1, 1350 cm−1, 1410 cm−1,549–1537 cm−1, 1570–1560 cm−1, 1632 cm−1. Hence, their for-ation cannot perhaps be ignored due to presence of bands

−1 −1 −1 −1 −1

414 cm , 574 cm , 1407 cm , 1537 cm , 1566 cm in the R1pectra [40]. It was seen that R2 is thermally more stable than R1.he spectra maintain almost similar profiles at R2 below 200 ◦C,.e. polyaniline nanofibrous films in this domain do not degrade

uch up to 200 ◦C. So, we concentrated our studies mainly on R2.

R2 18ronic structures R1 18, 30a, 34, 39, 40

To investigate further about this region, Raman studies were car-ried out on polyaniline nanofibrous films heat treated from 50 ◦Cto 300 ◦C. We observed distinct bands at 1675 cm−1, 1530 cm−1,1448 cm−1, 1397 cm−1, 1188 cm−1, 1002 cm−1, enhanced intensityof 1249 cm−1 and fine spectral features in the lower wavenumberrange, which were not observed in R1 at room temperature.

The Raman spectra of the prepared films in the R2 region atvarious temperatures are given in Fig. 6(a)–(g). The spectrum of

−1

Fig. 6. Raman Spectra at 514.5 nm laser excitation in R2 region of polyaniline filmsheat treated at (a) 25 ◦C, (b) 50 ◦C, (c) 100 ◦C, (d), 150 ◦C, (e) 200 ◦C, (f) 250 ◦C and (g)300 ◦C.

M. Jain, S. Annapoorni / Synthetic Metals 160 (2010) 1727–1732 1731

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ig. 7. Raman spectra at 514.5 nm laser excitation in R2 region of polyaniline films bs the burned region.

n polyaniline is unclear. However, no such peak is observed in R1 oreported for polyaniline structures. Band at 1640 cm−1, which wasarlier suppressed up to 200 ◦C, can now be seen clearly in P200nd P250 signifying the formation of crosslinked structures in poly-er with increase in temperature. A weak shoulder at 1620 cm−1,

orresponding to benzene C–C stretching of emeraldine salt form,emains till 200 ◦C. Further, a shoulder at 1581–1583 cm−1 isbserved up to 150 ◦C which represents C C stretching in quinoidnits in emeraldine salt. Also a shoulder at 1563 cm−1 is visiblebove 200 ◦C. An intense band at 1530 cm−1 might have contri-utions of NH bending in the protonated amines and phenazine

ike units. This is observed below 200 ◦C and disappears at higheremperatures. Band at ∼1448 cm−1 of semiquinone rings was alsoeen to have contribution from phenazine like units bonded to 1–4henylenediamine. The relative intensity ∼1448 cm−1 as comparedo the band at 1397 cm−1 increases with temperature up to 150 ◦C.

broad band feature ∼1477–1503 cm−1 appears at temperaturesbove 200 ◦C. This might have contributions from phenazine likeegments [41] and C–C in semiquinone structures of emeraldinealt [43].

Contribution from the phenoxazine like segments cannot per-aps be ignored in 1397 cm−1 It has shifted to ∼1400 cm−1 andas become more intense in the spectra of the films above50 ◦C. A broad band centered at 1336 cm−1 appears in P25. Asemperature increases, the band splits with local maximas at1303 cm−1, 1321 cm−1, 1334 cm−1, 1370 cm−1. Furukawa et al.

21] ascribed the splitting in 1320–1370 cm−1 region to the pres-nce of semiquinoid structures, i.e. to the C–N+ polaron band whichs a characteristic of emeraldine salt form. Nascimento et al. [44] inheir paper have associated the bands in 1324–1375 cm−1 to C–N+

f polarons having different conjugation lengths. Ueda et al. [30b]ave discussed this electron delocalization around CNC part and hasssigned lower part of the 1300–1370 cm−1 to the more delocalizedolarons while ∼1370 cm−1 has been assigned to more localizedolarons in literature. A low intensity broad peak, centered at1341 cm−1, appears in P200 and P250 instead of this broad band

eature. A strong band at 1248 cm−1 related to the C–N+. stretchingf the semiquinone radical of the emeraldine salt form increases inntensity till 150 ◦C. On further increase in temperature this bandecreases in intensity and shifts to 1253 cm−1 in films heated at00 ◦C and 250 ◦C. The spectral features indicate the deprotona-

ion of the films with little ES fractions still present. A shoulder ofhis band at ∼1235 cm−1 signifies in plane ring deformation [45]. Aow intensity band at 1188 cm−1 corresponding to the C–H bendingf the benzenoid becomes poorer and shifts to lower wavenumberndicating the presence of radical structures and on further increase

by high laser power (25 mW power approx.) White mark in the microscopic image

of temperature shifts to 1183 cm−1 with reduced intensity. Lowintensity bands 998–1000 cm−1, 862 cm−1 and ∼755 cm−1 are vis-ible in films up to 150 ◦C. The origin of the band at 998 cm−1 is notvery well understood. 755 cm−1 is related to the imine deformation(C–N C bending). Ring deformation band (6b in Wilson Notation)at 574 cm−1 becomes prominent with increase in temperature andcan be seen in the samples heated up to 250 ◦C. In P25–P150 a finestructure consisting of amine out of plane C–H wag of monosubsti-tuted benzene rings ∼509 cm−1, in plane deformations ∼521 cm−1,463 cm−1, 446 cm−1 can be observed. However, this type of finestructure is not observed in R1. 408–411 cm−1 band is present upto 150 ◦C only which might correspond to the out of plane C–H wagof monosubstituted benzene rings and to the CNC torsion.

Raman Spectra of films treated at 200 ◦C, 250 ◦C, 300 ◦C(Fig. 6(e)–(g)) consist mainly of broad peaks and have lost finestructure owing to the thermal treatment of the nanofibrous film.

As mentioned earlier, phenazine, safranine, oxazine orphenoxazine like segments are recognised with bands at∼415 cm−1, 575 cm−1, 1350 cm−1, 1395 cm−1, 1400–1416 cm−1,1549–1537 cm−1, 1570–1560 cm−1, 1630–1645 cm−1 [30,40].Presence of bands around 410 cm−1, 574 cm−1, 1343 cm−1,1370 cm−1, 1397 cm−1, ∼1400 cm−1, 1409 cm−1, 1477 cm−1,1563 cm−1, 1640 cm−1 indicates the formation of such structuresin the films in the R2 region as well. Thermal treatment of thefilms might lead to the formation of these cyclised structures inthe polyaniline chains. These structures are considered by some ofthe researchers as important for the formation of nanotubes [40].It seems that these structures might also play significant role inthe formation of the fibrous structure as well.

Sample heated at 300 ◦C, i.e. P300 and the sample burned underhigh laser power in Fig. 7 bear resemblance in their spectra. Thelong white mark in the image is the area burned because of highlaser power. Both the spectra contain two intense broad bands sep-arated by approximately 200 cm−1. P300 has peaks at 1399 cm−1

and 1597 cm−1 while the burned sample has bands at 1367 cm−1

and 1582 cm−1. This can perhaps be related to the starting of the for-mation of carbonized structures [40]. These bands can be assignedas D (disordered) band at 1370 cm−1 and G (graphitic) band at1590 cm−1 of the carbonized structures. It might be transform-ing into graphite with a fraction of disordered sp2 C–C bondingon burning [46].

4. Conclusions

Polyaniline nanofibers were synthesized by interfacial poly-merization. The nanofibrous films were prepared by drop casting

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732 M. Jain, S. Annapoorni / Synth

anofibers dispersion. A dense fibrous structure of the preparedlms is clearly evident in the SEM and TEM. Flake like structuresre also visible in SEM. Nanofibers have diameter in the range of0–80 nm. Characterizations based on bulk material might not bes helpful as Raman Spectroscopy for providing us with minuteetails. Raman spectroscopy has proved to be an excellent tool fortudying the structural variations in the different regions of theame film. Raman spectra in the regions of interest show diversend very interesting features. We could identify broadly two typesf regions on the polyaniline films. Raman signals from R1, i.e.arker region are broad in nature. This region shows some dras-ic change in the intensity of some of its peaks at 100 ◦C whichhows that it is not a very stable system. The transition fromuinoid structures to benzenoid is clearly visible in the shift of theeak from 1166 cm−1 to 1188 cm−1 at higher temperature. Emeral-ine salt form becomes prominent with appearance of semiquinoidtructures and increased intensity of C–N+. stretching vibrations ofolarons at 100 ◦C in R1 region.

Raman spectra at yellowish region, i.e. at R2 is marked by sharpeaks and fine structure and peculiar spectral features in the loweravenumber region. It seems that R2 region is not affected much by

he heat treatment even up to 200 ◦C. Raman Spectra of R2 maintainimilar profiles below 200 ◦C. This region was studied in detail byaman spectroscopy. The origin of the weak band at 1675 cm−1

n R2 region, is not clear. With the increase in temperature, i.e.elow 200 ◦C, growth of differently organized polarons was evi-ent by the presence of bands ∼1300 cm−1, 1320 cm−1, 1370 cm−1.n further increasing the temperature multiple bands in region300–1350 cm−1 disappeared and presence of a single main banday perhaps indicate at the presence of polaron of uniform con-

ugation length. But due to the block nature of the polyanilinehis possibility might not be feasible. Crosslinking in polyani-ine nanofibers becomes more evident with the development of640 cm−1 band in P200 and P250. Low wavenumber region con-ains doublet structure at 446–464 cm−1 and at 508–522 cm−1. R1nd R2 both show emeraldine salt characteristics.

The Raman studies indicate the presence of phenazine, safra-ine, oxazine or phenoxazine like segments apart from otherasic units of benzenoid, quinoid and semiquinoid in polyanilinetructures in both the regions. The crosslinking effects becomeppreciable at higher temperatures which can also be held account-ble for thermal stability of the polyaniline film at temperatures of50 ◦C. Their role in the formation of polyaniline nanofibers is stillot very clear. The film treated at 300 ◦C as well as film burnedy high laser power hints at the starting of the formation of car-onized structures. The differences in the nature of the spectra inhe two regions can perhaps be related to the non-homogeneityf the film. It is difficult to assign any particular reason for theresence of inhomogeneity in a film prepared by the nanofibersispersion. Variations in Raman might be related to the presencef distinct morphologies (as viewed in SEM) or due to the formationf metallic islands. This, however, can be confirmed by performinghe Raman studies on different molarities of HCl in the polymeriza-ion medium, etc. More characterizations dependent on molaritynd nature of dopant are needed to investigate for such variationsn heat treated polymer films. This work is in progress.

cknowledgements

The authors gratefully acknowledge the financial support ofFSMS scholarship provided by UGC. We are thankful to AIIMS,

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etals 160 (2010) 1727–1732

JNU, Delhi and USIC Delhi University for SEM, TEM measurementsand recording Raman Spectra of our samples.

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