Carbon Nanotube Mediated Reduction in Optical Activity in Polyaniline CompositeMaterials

Marc in het Panhuis,*,† Katie J. Doherty,† Raquel Sainz,‡,§ Ana M. Benito,‡ andWolfgang K. Maser‡

School of Chemistry, UniVersity of Wollongong, Wollongong, NSW 2522, Australia and Instituto deCarboquimica (CSIC), C/Miguel Luesma Casta´n 4, E-50018, Zaragoza, Spain

ReceiVed: September 5, 2007; In Final Form: October 25, 2007

Optically active composite materials were prepared by in situ polymerization of aniline in the presence ofmultiwalled carbon nanotubes, followed by doping with (R)-(-)-10-camphorsulfonic acid. The reduction incircular dichroism intensity with increasing nanotube loading fraction is attributed to changes in the ratio of“free” polymer versus polymer interacting with carbon nanotubes. These changes are shown to correspond toa decreasing polyaniline stereoselectivity with increasing nanotube loading fraction.

1. Introduction

In the human body, all optically active molecules areenantiomerically pure, whereas the enantiomers of most syn-thetic pharmaceutically active molecules exhibit differentbiological effects. Considerable effort is directed toward efficientmethods for producing enantiomerically pure compounds.1

Examples of these include (but are not limited to) chiral filmsfor enantioseparation and chiral coatings for controlled releaseof pharmaceutical or agrochemical products.2,3

Polymer carbon nanotube composites are mainly investigatedfor potential applications involving their electrical and mechan-ical properties.4 An additional, well-known property of carbonnanotubes is their inherent optical activity (chirality). Eachindividual carbon nanotube (CNT) can be uniquely specifiedby a chiral vector, defined in terms of graphene sheet unitvectors, and a chiral angle, defined as the angle betweengraphene sheet unit vector and the chiral vector. Although atheoretical treatment of chirality effects in CNT has beenreported,5 it is extremely difficult to investigate this byexperimental means, as controlled synthesis of nanotubes is notpossible (at present).

Optical activity is not inherent to conducting polymers suchas the polyanilines and polythiophenes, but is induced throughthe addition of chiral dopants6 or through the covalent attach-ment of chiral substituents.7 It has been suggested that the opticalactivity arises from an adoption of either a one-handed helicalconformation or a helical packing of polymer chains.8

Synthesis of optically active polyaniline carbon nanotubecomposites has been achieved using in situ polymerization ofaniline in the presence of multiwalled carbon nanotubes(MWNT).9-11 We established that the optical activity is retainedin the presence of as-produced arc-discharge and chemical vapordeposition (CVD) MWNT, although somewhat reduced to thatof pure polymer.9,10 This observation was attributed to the

polymer phase coating the nanotubes. Recently, Song et al.reported high optical activity for fibrous polyaniline compositeswith embedded CVD MWNT functionalized with carboxyl andhydroxyl groups.11

In this paper, we report the synthesis and characterization ofoptically active polyaniline composite materials with nanotubeloading fractions of up to 50%. Increasing the loading fractionchanges the ratio between “free polymer” (not interacting withnanotubes) and the polymer phase coating nanotubes (interactingwith nanotubes). Our results indicate that these changes resultin a decreasing polyaniline stereoselectivity.

2. Experimental Details

MWNTs were prepared in an arc-discharge experiment bysublimation of pure graphite rods under a helium atmosphereof 66 kPa using a current of 60 A and a voltage of 25 V. Samplematerial was collected from the inner core of the formedcathodic deposit and consist of straight well-graphitized MWNTof micrometer lengths and 20-30 nm in diameter as well as afew graphitic nanoparticles as impurities.

Polyaniline carbon nanotube composites were synthesized byan in situ polymerization process of aniline in the presence ofMWNT.12,13 The general procedure is as follows: A solutionof 1 M ammonium persulfate in 1 M HCl was added slowly toa dispersion of MWNT and vacuum distilled aniline in 1 MHCl, which was then sonicated for 2 h. A solution of HCl 1 M,containing MWNT material, was stirred to disperse the carbonnanotubes. The resulting material was filtered, washed, and driedunder vacuum at room temperature for 24 h. This process yieldspolyaniline in its doped form, emeraldine salt (ES). In order totransform the emeraldine salt into emeraldine base (EB), the

* To whom correspondence should be addressed. E-mail: panhuis@uow.edu.au.

† University of Wollongong.‡ Instituto de Carboquimica.§ Present address: Immunologie et Chimie Terapheutics, Institute de

Biologie Moleculair et Cellulair, Universite Louis PasteursCNRS, 15 RueR. Descartes, Strasbourg, France.

TABLE 1: Composition of Polymer and Composites asDetermined by Elemental Analysis (see Table S1 of theSupporting Information)

sample % polymer % MWNT

polymer 100C1 99.8 0.2C2 94.0 6.0C3 86.0 14.0C4 75.0 25.0C5 50.0 50.0

1441J. Phys. Chem. C2008,112,1441-1445

10.1021/jp077117v CCC: $40.75 © 2008 American Chemical SocietyPublished on Web 01/16/2008

composite was de-doped by stirring for 2 h with 3 wt %ammonium hydroxide, followed by a filter, wash and dryingprocedure. Polyaniline in EB form was synthesized under similarconditions without carbon nanotubes.

Thermogravimetric analysis (TGA) was carried out using aSetaram TG-DTA 92 Thermobalance, burning 10-12 mg ofpowder material in air, using a flow rate of 100 mL/min and aramp of 3°C/min up to 1250°C.

Optically active polyaniline was prepared by doping solutionsof polymer and composites (in EB form) inN-methyl-2-pyrrolidinone (NMP, Acros) with (R)-(+)-10-camphorsulfonicacid (HCSA, Aldrich). The composition of the polymer samples,and composite samples with MWNT loading fraction of up to50% are shown in Table 1. Polymer solutions were preparedwith a concentration range from 0.0417 to 0.250 mg/mL.Composites were dispersed at a constant concentration of 0.250mg/mL. The composite solutions are stable; no floating particlesand no particulate matter could be observed, even at the highestloading fraction. Electronic absorption spectra and circulardichroism spectra were recorded using a Cary 500 UV-vis-NIR and a Jasco J810 circular dichromator, respectively.

3. Results and Discussion

Thermogravimetric analysis on polymer and compositesamples prior to doping are shown in Figure 1. The polymerexhibits the decomposition pattern characteristic for the chainstructure (300-600°C) of undoped polyaniline. For compositematerials, an additional step is observed related to the decom-position of MWNT. It is clear that this is not related to thepresence of either pure MWNT (minimum at 750°C), nor purepolymer. Although this difference in thermal behavior waspreviously attributed to the existence of a new polymer/MWNTphase,10,13 additional experimental data is needed to validatethis suggestion.

The UV-vis-NIR absorption spectra (Figure 2A) of theHCSA doped polymer show the three characteristic absorptionbands, 335, 415, and 800 nm, associated withπ-π*, polaron-π*, and π-polaron band transitions of polyaniline in theemeraldine salt form, respectively. There are two distinctivepolyaniline conformations: “compact coil” (tightly coiledchains) with a characteristic localized polaron band at 800 nmand “extended coil” (expanded chains) with a characteristicintense broad absorption band in the near-infrared.10,14 Thespectral features of our polymers and composites are consistentwith a “compact coil” formation.

In a previous article, we suggested that the optical activityobserved for composites is due to “free” polyaniline chains,defined as not or loosely in contact with the carbon nanotubesurface.10

Let us now address the effect of increasing the nanotubeloading fraction while keeping the overall concentration con-stant. This means that the polymer concentration decreases withincreasing MWNT content. Figure 2A shows that dilution of apure polymer sample does not affect the position or the relativeintensity of polymer absorption bands. However, decreasing thepolymer concentration in a composite material by increasingthe MWNT loading fraction results in different behavior (Figure2B). It is well-known that the position of the polymerπ-π*band is affected by interactions with nanotubes.9,13,15-17 Ourresults show that the down-shift in this band increases withincreasing nanotube content. This band, which can be seen asindication for polymer stacking behavior, is affected by a changein the ratio of polyaniline:MWNT. The increasing down-shiftsuggests that the ratio of free polymer versus polymer interactingwith nanotubes decreases with increasing MWNT concentration.As the optical activity is most likely to be a result of “free”polymer large changes in the composite chirality with increasingnanotube loading fraction can be expected.

Figure 1. Thermogravimetric analysis, weight loss (A) and derivative (B) of polymer, composites, and carbon nanotubes. Arrows indicate increasingMWNT loading fraction. Polymer decomposition region indicated by dashed lines.

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Figure 3A shows that the polymer solution displays opticallyactive through Cotton effects at 405 and 455 nm, respectively.These observations are consistent with the polymer adopting acompact coil conformation. As expected, the magnitude of theCD intensity decreases with decreasing polymer concentration.The decrease of the 405 and 455 nm bands is linearlyproportional with the polymer concentration (Figure 4). Theconcentration dependence of the CD data can be removedthrough the dissymmetryg-factor defined by Drake et al. asthe ratio CD/(absorbance× 32980).18 This factor provides ameasure of the stereochemical selectivity (enantiomeric purity)of the polymerization reaction. For example, evaluating theg-factor for 5 different polymer concentrations yieldsg455 )-1.58 × 10-3. The g-factor of our pure polymer is less thanthose reported by Song11 (8.8× 10-3) and Wan19 (2.3× 10-2),and can be attributed to differences in their method of synthesis,e.g., use of dimer and oligomer-assisted polymerization methods,respectively.

The optical activity of the composite materials decreases withincreasing nanotube content (see Figure 3B). This in itself isnot surprising, as all composite materials are prepared at aconstant concentration, i.e., increasing the MWNT loadingfraction reduces polymer concentration. The polymer concentra-

tion (CP) in the composite can be estimated from the sampleconcentration (CS) and sample composition (Table 1) usingCP

) CS × % polymer. For example, composite C3 withCS )0.250 mg/mL yieldsCP ) 0.215 mg/mL. How does the polymercontent relate to the magnitude of the CD intensity? Let us usecomposite C3 as an example. On the basis of the straight linefits for the polymer data (Figure 4), we can estimate the CDintensity assuming that the nanotubes would not affect thepolymers’ optical activity. For C3, we would then expect a CDintensity of-48.3 at 455 nm. However, the measured value issignificantly lower,-28.2 at 455 nm, a reduction of 42%. Thereduction in intensity at 405 nm is similar. Alternatively, therelation between polymer concentration and CD intensity canbe used to evaluate the polymer concentration needed to yieldthe measured intensity. This is 0.137 mg/mL for composite C3.This concentration can be seen as the effective concentrationof polymer in terms of optical activity. Thus, in sample C3 a14% carbon nanotubes content prohibits 42% of the polymercontent from contributing to the optical activity. A similaranalysis for sample C4 shows that a 25% loading fractionprohibits 65% of the polymer from contributing to the CDintensity. The larger than expected decrease in CD intensity is

Figure 2. UV-vis absorption spectra for polymer (A), and (B) composite samples doped with HCSA. Note: composite samples prepared atconstant sample concentration. Arrows indicate increasing polymer concentration (A) and increasing MWNT loading fraction (B). Inset: UV-visible absorption spectra (normalized to 415 nm) for composite samples.

Carbon Nanotube Mediated Reduction in Optical Activity J. Phys. Chem. C, Vol. 112, No. 5, 20081443

related to changes in the ratio of “free” polymer versus polymerinteracting with nanotubes.

These results demonstrates that the interactions between thepolymer and carbon nanotube surface affect the polymer chains’ability to adopt an optically active conformation. There are twoexplanations for this behavior, either the polyaniline chains indirect contact with MWNT are not optically active (and thismight affect subsequent polymer layers), or the chains areoptically active, but follow the chirality of the MWNT.However, as each chirality is equally likely,4 a racemic formof polyaniline is produced resulting in a zero net contributionto chirality. Unfortunately, at present it is not possible to controlnanotube chirality during their synthesis. Therefore, variationsin nanotube chirality cannot be used in determining which ofthese explanations is more plausible.

Let us now remove the effect of polymer concentrationthrough analysis of our results in terms of the aforementioneddissymmetryg-factor. This would allow us to quantify thechange in stereoselectivity of the polymer with respect toMWNT loading fraction. It has been shown above that decreas-ing polymer concentration in itself should not result in changesin g-factor, unless stereoselectivity is affected. Table 2 displaystheg-factors, evaluatured from the absorbance and CD data at405 nm (g405) and 455 nm (g455), respectively. It is clear thatthe stereoselectivity of polyaniline reduces with increasingnanotube content. For example, a MWNT of 25% reduces thestereoselectivity by more than 50%.

The overall trends observed in this work are similar to thosereported by Song:11 i.e., increasing the nanotube content

decreases the stereochemical selectivity. There are two maindifferences in the assembly of our respective composites,“standard” versus oligomer assisted polymerization (resultingin nanofibers) and the type of nanotubes (as-prepared versusfunctionalized). Our arc-discharge produced nanotubes have adifferent surface composition compared to Song’s acid-treatedmaterials generated by chemical vapor deposition (CVD).11

Hence, the interactions during our polymerization are likely toconsist predominantly as stacking interactions between anilineand the MWNT surface. Song noted that their acid-treatednanotubes are likely to have less than straightforward interac-tions due to the introduction of surface groups.11 However,despite these differences, the same overall trends of reductionin both CD intensity and stereoselectivity (g-factor) are ob-served. This suggests that the type of nanotubes (as-preparedor functionalized) is not the determining factor in reducing thecomposites’ stereoselectivity. In contrast, the type of nanotubesis important for the composites’ electronic properties as shownin our previous work.10 Nanotubes with a highly graphitizedsurface such as arc-discharge MWNT showed a lower resistancecompared to those produced by the CVD method.10 Furtherstudies are necessary to investigate these observations; inparticular, investigations involving other types of carbon nano-tubes are needed.

4. Conclusions

The optical activity in polyaniline composite materials hasbeen investigated as a function of carbon nanotube loading

Figure 3. Circular dichroism spectra for (A) polymer concentration range 0.042-0.250 mg/mL, and (B) composite samples with loading fractions0-50% at a constant concentration of 0.250 mg/mL. The polymer spectrum at 0.250 mg/mL is shown for comparison. Arrows indicate decreasingpolymer concentration (A), and increasing MWNT loading (B).

1444 J. Phys. Chem. C, Vol. 112, No. 5, 2008 in het Panhuis et al.

fraction. The reduction in circular dichroism intensity withincreasing loading fraction was related to changes in the ratioof “free” polymer versus polymer interacting with carbonnanotubes. For example, a 25% MWNT loading fractionprohibited 65% of the polymer from contributing to the CDintensity. These changes were shown to correspond to adecreasing polyaniline stereoselectivity with increasing nanotubeloading fraction. This paper contributes to the development andunderstanding of chiral composite films.

Acknowledgment. This work is supported by University ofWollongong internal funding and the Australian ResearchCouncil. The Group of the Instituto de Carboquimica gratefullyacknowledges financial support from the Spanish Ministry ofScience and Education and the European Regional DevelopmentFund under Research Project NANOCONDPOL (MAT2006-13167-C02-02), as well as from the Regional Government ofAragon under Research Project DGA-PIP021/2005.

Supporting Information Available: Elemental analysis.This material is available free of charge via the Internet at http://pubs.acs.org.

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(12) Cochet, M.; Maser, W. K.; Benito, A. M.; Callejas, M. A.; Martnez,M. T.; Benoit, J. M.; Schreiber, J.; Chauvet, O.Chem. Commun. 2001,1450.

(13) Sainz, R.; Benito, A. M.; Teresa, Martinez, M.; Galindo, J. F.;Sotres, J.; Baro, A. M.; Corraze, B.; Chauvet, O.; Maser, W. K.AdV. Mater.2005, 17, 278.

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Figure 4. Circular dichroism (CD) intensity at 405 nm (A) and at 455 nm (B) as a function of polymer concentration. Squares and circles indicatedata for polymer (Figure 3A) and composites (Figure 3B), respectively. Equations indicate a straight-line fit for polymer data. Triangles show theexpected CD intensity calculated from the straight line fit using the polymer concentrations in composites C2-C4 (at constant sample concentration0.250 mg/mL). Data points for composite C1 have been omitted for clarity.

TABLE 2: Dissymmetry g-Factors (at 405 and 455 nm) as aFunction of Carbon Nanotube Loading Fraction

sampleMWNT loading

fraction (%) g405 × 1000 g455 × 1000

polymer 0 1.21 -1.6C1 0.2 1.16 -1.6C2 6.0 0.80 -1.20C3 14.0 0.78 -1.08C4 25.0 0.55 -0.79C5 50.0 0.43 -0.44

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