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Short CommunicationReceived: 17 December 2008 Accepted: 29 May 2009 Published online in Wiley Interscience: 19 August 2009

(www.interscience.wiley.com) DOI 10.1002/jrs.2414

Raman spectroscopy of conducting poly(methyl methacrylate)/polyanilinedodecylbenzenesulfonate blendsAbdul Shakoor∗ and Tasneem Zahra Rizvi

Polyaniline soluble in organic solvents was prepared using dodecylbenzenesulphonic acid (DBSA) as functional dopant. Thesolubility parameter was calculated and the most suitable solvent chloroform was checked for the solubility and the mostcompatible polymer PMMA was selected for blending. Miscibility was maximized with 1% by weight of hydroquinone. Blendingof doped polyaniline with dodecylbenzenesulphonic acid (PAni.DBSA) in poly (methyl methacrylate) (PMMA) was explained bya change in the conformation of the polymeric chains leading to an increase in the conductivity. The electrical conductivityincreased as the weight percent of PAni.DBSA increased, showing a percolation threshold as low as 3.0% by weight andthe highest conductivity was achieved at 20% by wt of PAni.DBSA. Scanning electron micrographs showed lowest level ofphase separation. Raman spectroscopy is used to characterize the blending process of two polymers aiming to understand thetransformations in different types of charged segments. Raman results give complementary data about the blending processshowing that together with the structural change of the polymeric chains, there is also a chemical transformation of thesepolymers. Analysis of Raman spectra was done investigating the relative intensities of the bands at 574 cm−1 and 607 cm−1. Arelationship between conductivity and Raman was also proposed. Copyright c© 2009 John Wiley & Sons, Ltd.

Keywords: conductive polymers; blends; PAni.DBSA; PMMA; Raman spectroscopy; conducting blends

Introduction

Among the conducting polymers, special interest has been focusedon polyaniline dodecylbenzenesulfonate (PAni-DBSA), due to itsexcellent thermal and environment stability combined with arelatively high level of electrical conductivity. Nevertheless, afew applications have been reported based on PAni because itexhibits poor mechanical properties and is not soluble in commonsolvents. Numerous methods have been developed to overcomesuch shortcomings. It has been reported that the compositesystem by blending PAni with commodity polymers improvesits mechanical properties[1]. Despite these interesting properties,these conducting polymers suffer low strength and poor processability. As a result, there is strong interest in the preparation ofthermally processable conducting blends with insulating polymerslike rubber[2], PPy[3] and PS[4]. All these blends showed lowconductivity and high percolation threshold. It was concludedin the literature that PMMA was more miscible in the presenceof miscibility agent hydroquinone and the conductivity was alsofound to be enhanced because of hydroquinone.[5,6] Percolationthreshold was studied as low as 1% wt of PAni.DBSA in rubber poly(epichlorohydrin-co-ethylene oxide)[7]

Raman spectroscopy is a form of vibrational spectroscopy and assuch is related to near- and mid-IR absorption spectroscopy. Similarto the IR absorption techniques, Raman spectroscopy measuresthe vibrational wavenumbers of various parts of a molecule. Thesewavenumbers depend on both the bond strength and mass ofthe bound atoms as well as other factors such as intermolecularinteractions. The ‘pattern’ of vibrational wavenumbers from amolecule is, therefore, highly characteristic of a given molecularspecies and, for solid samples, of the crystalline arrangementof those molecules. Raman spectra may be readily recorded

from gases, liquids and solids. IR, percolation threshold, opticaland transmission electron micrographs of PAni.DBSA-PMMA havealready been reported in the literature.[8] Literature review revealsthat the Raman spectra of these blends are scarce. For the firsttime in the history of conducting blends Raman spectra of dopedpolyaniline is conducted with DBSA and PAni.DBSA-PMMA blendsin the presence of plasticizer hydroquinone and an attempt hasbeen made to explain it in this article.

Experimental

Aniline was obtained from ALDRICH and vacuum distilled beforeuse. Ammonium per sulphate (APS) and dodecylbenzenesulfonate(DBSA) were obtained from FLUKA, hydroquinone and poly(methyl methacrylate) (PMMA) that was supplied by ALDRICHand used as obtained.

Preparation of PAni.DBSA

Aniline 25 ml and APS 63 g were dissolved in 200 ml and 150 ml ofHCl respectively and pre-cooled to 0 ◦C. The aniline solution wasplaced in an ice bath under magnetic stirring. APS solution wasadded gradually in 10–15 min under vigorous stirring to ensurethorough mixing. The reaction temperature was maintainedbetween 0 ◦C and 2 ◦C while PH was adjusted to 0–1 by addingmore concentrated HCl. After 6 h the solution was filtered and

∗ Correspondence to: Abdul Shakoor, Physics Department, Quaid-I-Azam Uni-versity P.O.Box 45320 Islamabad, Pakistan. E-mail: Shakoor 47@yahoo.com

Physics Department, Quaid-I-Azam University, Islamabad, Pakistan

J. Raman Spectrosc. 2010, 41, 237–240 Copyright c© 2009 John Wiley & Sons, Ltd.

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washed. Ammonia solution was added to this wet PAni-HCL andPAni-HCL was under stirring for 24 h, followed by washing andre-protonation in 1 M aqueous DBSA at a molar ratio 1 : 1. Filtrationwas done with sintered glass funnel and washed thoroughly withdistilled water till no odour of ammonia could be detected. Theproduct was vacuum dried at 60 ◦C for 24 h.[8,9]

Calculation of solubility parameter values for main materials

The solubility parameters for PMMA and PAni.DBSA weremeasured using Eqn (1) and the values of functional group molarattraction constants Fi calculated by Hoy[9]:

δp = ρ X∑

Fi/Mo (1)

where ρ is the density of polymer (kg/m3), Mo is the formulaweight of the polymer repeat unit (kg mol−1) and

∑Fi is the sum

of all groups’ molecular attraction constants in polymer repeat unit[{Jm3}0.0 5 mol−1]. The determination was based on the smallestrepeat unit of the relevant polymer, treating the co-polymers asbeing regular, for simplicity.

Blend preparation

Solutions of PMMA dissolved in the most appropriate solvent,chloroform (0.1 g/10 ml solvent); PAni.DBSA was also dissolved inthe same solvent with the same concentration. Each of the purePMMA solutions was added to PAni.DBSA solution separately inappropriate amounts (weight percent PAni.DBSA : weight percentPMMA), 1 : 99, 3 : 97, 5 : 95, 10 : 90, 20 : 80, 30 : 70, 40 : 60, 50 : 50,60 : 60, 70 : 30, 80 : 20 and 90 : 10, respectively. Hydroquinone 1%by weight was added to each solution as a miscibility agent.Each of the above solutions was ultrasonically agitated for 3 h atroom temperature prior to casting. The conductivity results andsample preparation of PAni.DBSA-PMMA blends have already beenreported in the literature[8]. In this paper we are reporting Ramanspectroscopy, percolation threshold of conductivity and scanningelectron microscopy of these blends in order to get detailedinformation of interaction between the constituent polymerswhich results in a high level of miscibility, high conductivityand low percolation threshold in these blends.

Characterization

Scanning electron microscopy (SEM) was carried out on an EVO50ZEISS instrument. For conductivity measurements fine copperwires of suitable lengths were attached at the four corners ofthe cast films of PMMA/PAni.DBSA blends using small silverpaint contacts (Acheson Electrodag 915). The DC conductivitywas measured using van der Pauw Eqn (2)

σ = 2 Ln2/[R1 + R2]d f (2)

where R1 and R2 are the resistances of the cast blend in its twoperpendicular contact configurations, d is the width of samplemeasured in micrometers, f is the geometric correction factor(usually equal to unity for symmetric contacts) and σ is the bulkconductivity in S/cm. The samples were connected to a Keithley617 programmable electrometer and Keithley 224 programmablecurrent source under computer control.

Raman spectra for 633-nm exciting radiation were recorded ona Renishaw RM 1000 laser Raman (He–Ne laser) containing an

Olympus metallurgical microscope and a CCD detector. The laserpower at the sample was kept below ∼0.74 mW to avoid thermaldegradation. The laser was focused using a 50× objective lens andspatial resolution was about 1 µm.

Results and Discussions

Chloroform has the solubility parameter of 19.0 (Jm3)0.5 mol−1[10]

and solubility parameter of PAni.DBSA prepared in this work wasalso found to be 20.007 (Jm3)0.5 mol−1 which was very close to thatof chloroform. The solubility parameter of PMMA was calculatedto be 19.30 (Jm3)0.5 mol−1. So, the chloroform was selected as acommon solvent for both the polymers.

The electrical conductivity of the various PAni.DBSA contents inPMMA/PAni.DBSA blends is illustrated in Fig. 1. The conductivityresults give good agreement with the miscibility of the blends.The standard percolation theory[3,11] was used to analyse theconductivity results. The data presented in Fig. 2 were found to fitto a simple percolation model described by Eqn (3):

σ f = c (f-fp)t (3)

where c is constant, t is critical exponent, f is the volume fraction ofconductive medium, and fp is the volume fraction at percolationthreshold. By fitting the exponential data to the plot of log σ

versus log (f–fp), the values of correlation coefficient (R) and twere estimated to be 0.956 and 2.32, respectively. The estimatedelectrical conductivity percolation threshold in weight percentand volume percent are 3% and 3.04% respectively, which is verylow compared with the theoretical value of bulk conduction, i.e,16% (V/V) of the conductive volume in the mixture.

Blends with 1 wt% below conductivity threshold and 5 wt%above conductivity threshold of PAni.DBSA were studied bySEM (Figs 2a and 2b. The darker regions are again related toconductive PAni.DBSA particles in PMMA matrix as were observedin optical micrographs. The 1-wt% (Fig. 2(a)) blends appear tocontain isolated PAni.DBSA particles and so should be wellbelow the threshold for conductivity. The mixed particles inFig. 2(b) with 5 wt% of PAni.DBSA in PMMA gives the evidence ofconducting behaviour of these blends as observed, whereas thisdark region related to PAni.DBSA particles are started to clusterto form conductive pathway at 3 wt% and above of PAni.DBSAin PMMA matrix and the material come in conductive region andconductivity jumps from 10−10 S/cm (insulating region) to 10−7

S/cm (conducting region).

Figure 1. Variation of conductivity as a function of PANI.DBSA contentsthe PAni.DBSA-PMMA blend.

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Raman spectroscopy of conducting blends

Figure 2. SEM of (a) 1 : 99 and (b) 5 : 95 wt% of PAni.DBSA: PMMA blends.

Figure 3. Raman spectrum of polyaniline doped with DBSA.

The Raman spectrum of PAni doped with DBSA is shown inFig. 3. The intense band near 1500 cm−1 is assigned mainly tothe benzenoid C–C ring stretching vibration and a band near1601 cm−1 is attributed to the quinoid C C stretching mode of thepolymer chain. The bands at 1508 and 1386 cm−1 are strengthenedand new bands appear on the high/low wavenumber side for eachof these bands in the spectra of the polyaniline salts. Several newless intense bands also appear in the Raman spectra of the PAni.The 1328 cm−1 band in the PAni is assigned to the C–C stretchingmode of the quinoid ring. Due to differences in the conformation ofthe polymer and the extent of doping, the wavenumber of the C–Cstretching vibration varies. The C–C bond is strengthened in theprotonation-induced polaron lattice. In particular, the Raman bandat 1167 cm−1 is assignable to the out-of-plane–C–H bending.

Raman spectroscopy used in the microscope mode is veryefficient for the characterization of interactions between the com-ponents in polymer blends.[12] Figure 4 compares the resonanceRaman spectra of PAni.DBSA: PMMA blends with different pro-portion of PAni.DBSA in PMMA. It is possible to observe theintense overlapping bands at 1322 and 1337 cm−1, correspondingto C N+ •, stretching the modes of delocalized polaronic chargecarriers, which is characteristic of the protonated imine form ofpolyaniline, and are present in all blends (Fig. 4(a)–(d)). More-over two important absorptions can be highlighted: at 607 cm−1,related to benzene ring deformation[13] and at 574 cm−1, assignedto cross linking between PAni chains[14] The band at 607 cm−1 hasbeen attributed to the deformation of the benzene ring in the PAnibackbone, and according to the literature[15] it may be enhancedby the presence of other insulating polymers, such as poly(methylmethacrylate) (PMMA), polystyrene (PS) and polyethylene that

Figure 4. Raman spectra of (a) 1%, (b) 5%, (c) 10%, (d) 20% PAni.DBSA inPMMA.

show good affinity towards the conducting polymers and locatebetween the PAni chains. Therefore, the presence of the bandat 607 cm−1 in blend containing 3% and 5% of PAni.DBSA indi-cates that solution blending imparts good interaction betweenthe PMMA matrix and the polyaniline propagating species in thepresence of 1% by weight of hydroquinone. Whereas, in the blendof 1% PAni.DBSA in PMMA this band is absent. Therefore, thisblend is beyond the percolation threshold of conductivity. As thePAni.DBSA content in the blend increases, the possibility of thePAni.DBSA chains adhering to each other increases, decreasingthe conformational changes of the benzene ring. Consequently,the intensity of the band at 607 cm−1 decreases, as illustrated inFig. 4. In the same way it is also possible to observe in this figure,the increase of the band at 574 cm−1 in the Raman spectra of theblends with more content of Pani.DBSA, suggesting an increasein the formation of PAni agglomerates. The presence of theseagglomerates makes difficult the conformational changes of thebenzene ring. The intensity of the band at 574 cm−1 with respect tothe band at 607 cm−1 gives additional information regarding theinteractions between blend components. This effect is better ob-served in blends with low amount of PAni. The relative intensity ofthe band at 574 cm−1 indicates that a lower amount of PAni.DBSAcrosslinked segments due to higher interchain interaction[16,17]

that’s why the blend with 1% PAni.DBSA in PMMA. Agglomer-ates shown in the scanning electron micrographs of this blendare observed (Fig. 2(b)) and this blend is away from percolationthreshold of conductivity. Concerning resonance Raman spectraof blends containing 5 and 10% of PAni.DBSA (Fig. 4(b) and (c))

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relative lower intensity the band 574 cm−1 and the appearanceof the band at 607 cm−1; it would be possible to do a directcorrelation between relative intensities of these bands showingthe homogeneity of these blends as observed in SEM Fig.2(b) andin conductivity results.

Conclusion

PMMA/PAni.DBSA blends have been successfully prepared bysolution mixing with reasonable conductivity. The miscibility wasmaximized by using 1 wt% of hydroquinone. The percolationthreshold in conductivity was recorded as 3% wt of conductivepolymer. All these results suggest a strong interaction betweenpolyaniline and the PMMA matrix for blends due to hydroquinone.The nature of such interactions may be due to hydrogenbonding but also due to chemical interactions through graftcopolymer formation. As suggested, such interactions have alsobeen confirmed by Raman spectroscopy.

Raman spectroscopy showed strong interactions betweenpolymers, doped PAni.DBSA and PMMA, which were observedin SEM and conductivity results also.

The studies performed in this work are related to hydroquinoneas a miscibility agent in PAni.DBSA-PMMA blends. The effect ofhydroquinone on the electrical behaviour of these blends wouldbe the subject of the forthcoming paper.

Acknowledgements

The authors are grateful to Peter Foot, Kingston University, UKfor his valuable guidance and authors gratefully acknowledge

the financial support from Higher Education Commission (HEC),Pakistan, International Research Support initiative Program (IRSIP).

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www.interscience.wiley.com/journal/jrs Copyright c© 2009 John Wiley & Sons, Ltd. J. Raman Spectrosc. 2010, 41, 237–240