Nanostructure of polyaniline blends

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SyntheticMetals 101(1999) 789-790

Nanostructure of Polyaniline Blends J. Planes’, Y. Cheguettine’, Y. Samson* ‘Laboratoire

de Physiqlte des Me’tanx SythPtiques, SI3M and UMR 5819, ‘Laboratoire Nanostructure et Magne’tisme, SP2M, DRFMC, CEA-Grenoble, 17, me des Martyrs, F- 38054 Grenoble Cedex 9 (France)

Abstract

The nanostructureof films of conductiveblendpolyaniline/cellulose acetate(PANIKA) is investigatedby two real-spaceimaging techniques:transmission electronmicroscopy(TEM) andatomicforce microscopy(AFM). The heterogeneous natureof the disorder suggested by electronictransportmeasurements is clearly evidenced.Moreoverthe clusteringprocessof PAN1is shownto be anisotropic, with a preferentialorientationin the planeof the film. For PANI contentabove3% wt., the film surfaceis coveredby PAN1to a largeextent, andsurfacesegregation is sometimes observed. Keywords: polyaniline,phase-segregated compositeinterfaces,transmission electronmicroscopy,atomicforce microscopy.

1. Introduction

ConductivePANI, dopedwith sulphonic(CSA) or phosphonic (PPA) acid andmixedwith an insulatingpolymer- PMMA or celluloseacetate- yield conductive blendswith very peculiar properties[l-3]. First, an ultra low percolationthresholdshould be mentionned.Quasi-staticconductivity is indeedmeasurable for PAN1contentsaslow as0.1% wt. This intriguingbehavioris necessarilyrelatedto the morphology,which shoulddramatically differ from the pictureof the 3D percolationof globularandisotropic objects;in this caseindeed,the thresholdpcis around16%. First investigationswererealizedby the SantaBarbaragroup [ 11concludingthat PAN1wasdevelopinga so-called“fractal fibrillar network”. Schematically,bundlesof PANI tibrils connect blobsabovepL.; theseconnectionsvanishat pc. In Ref. [ 11,TEM observations havebeenobtainedon ultra-thinfilms madeby spin castingandfurther processed to dissolvethematrix. In this work we presentnewinvestigationsby TEM andAFM on thicker films that have beenusedfor transportmeasurements [3] pointingout variousscalesof heterogeneityandanisotropy.

3. Results and discussion

All resultspresentedhere concernequivalentsampleswith PAN1contentbetween0.13 or 0.5% wt., i.e. exceedingthe percolationthreshold,whichin thesystemstudiedis 5 0.1%. Fig. 1 showsa sectionof a 20 urn thick film in its middle part, the film planebeinghorizontal.It canbe seenthat wide regions(in light grey) of typical micrometersizeare free of PANI, whereasthe latter is “concentrated’in looseclusters(in dark grey) exhibiting anisotropy.The clustersareelongatedparallelto the planeof the film. At this magnificationlinks of 40 nm diameteraredistinguishable (thisis alsothethicknessof the cut).

2. Experimental

Films are cast from m-cresol solutions containing PANI(PPA),,,s,the matrix polymer (CA or PMMA) and the conventionalplasticizers.Thicknessof the film variesfrom 10to 80 pm. The detaileddescriptionof film fabricationcan be found in Ref. [4] For TEM, a pieceof film is embeddedin resinEPON 812 from Fluka and cut by ultramicrotomy at room temperature. Cross-sections 40 to 90 nmthick areobservedwith a Jeol200CX operatedat 200 keV. The irradiation damage,strongerfor the matrix than for PAN1favors the contrast.We have alsoimaged PAN1 clusterswith a better resolutionby dissolvingCA in acetone vapors.The stainingagentRu04 is additionallyusedwhen the matrix is PMMA. AFM in tappingmodehasbeenperformedon the film surface that waskept cleananddustfree beforeoperation.A Digital NanoscopeIIIA wasemployedwith a Si tip.

Fig. 1: TEM micrographof PANI(PPA)/CA, 0.5%, cut thickness 40 nm. The horizontal direction is parallel to the planeof the film. Thick white linesare dueto defaultsof the diamondknife. Imagesizeis 3.7 pm x 2.6 pm. More detailsof the clusterstructureare visible in Fig. 2. For this image,the matrix hasbeenremovedby dissolution,so that thereis no informationaboutthe originalextensionand orientation of the clusters.Nevertheless their appearance is quite comparableto that shownin Fig. 1, giving confidencein its representa-

0379-677919913 - seefrontmatter0 1999Elsevier Science S.A. All rightsreserved. PII: SO379-6779(98)00792-9

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Y. Cheguettine

et al. I Synthetic

Metals 101 (1999) 789-790

tiveness. It appears that linear aggregation of PANI occurs between “grains” of ca. 10 nm diameter, and that more compact aggregates coexist. The micrograph also shows empty regions, the size of which ranges from a few tens of nm to one pm, presumably occupied by the matrix before dissolution.

Fig. 4: AFM image in tapping mode of PANI(PPA)/CA, 0.2%. Left: height signal, middle: amplitude signal, right: phase signal; Each image is 1pm x 2um.

Fig. 2: TEM micrograph of PANI(PPA)/CA, 0.13%. CA matrix was dissolved prior to observation. Image size is 0.9pm x 0.6pm. Fig. 3 shows the surface of the film as seen by AFM with the amplitude signal in tapping mode, which gives a topographic contrast. It has been checked that a pure matrix surface exhibits the same morphology as the flat zones in the comers of Fig. 3. The emerging objects are thought to be PAN1 clusters, with comparable aggregation conformations as seen by TEM. Typical height is 5 to 10 nm and width 100 to 150 nm. This attribution is corroborated by the phase contrast image shown in Fig. 4. In this mode, a spatial modulation in the mechanical properties causes contrast. It can be seen that the limits of the object as seen in the height and amplitude images (left and middle parts of Fig. 4) exactly coincide with the corresponding phase view (right part). However the phase image does not exhibit the slope-related contrast as seen on the amplitude image, proving that the polymeric phase does change, and not only the topography.

We are speciaily motivated by the structural evolution with concentration in relationship with transport properties. The statistical analysis is not sufficient today to quantify this evolution but major trends are as follows: i) in the bulk, TEM shows that from 0.1% to a few lo, thin fibrillar aggregates (10 nm) coexist with more compact ones of several hundreds of nm. But the density of the latter decreases at Lowe concentrations. ii) at the surface AFM shows that a dense coverage by PANIclusters is reached above 3%. At this point, we must mention that a surface segregation and a concentration profile perpendicular to the plane of the film has been evidenced in PANI(CSA)/PMMA blends [S]. Due to processing conditions, the substrate side of the tilm was enriched in PANI; this leads to a strong difference between both sides in thIe 4-probe conductivity measurement, which reflects the conductivity in the plane. A more subtle effect is present-in PANIfPPA)/CA blends: the transverse conductivity seems to be systematically lower than the parallel one. This could be related to the observation of a better connectivity along the plane observed in Fig. 1. 4. Conclusion

Joint studieswith AFM andTEM of the nanostructureof PAN1blendswith ultralowpercolationthresholdseema promising tool for a betterunderstanding of the relevantscalesin those materials,andtheir dependence on the PANI content closeto pc Thesepreliminarystudieshavenot exploredall the possiblecontrastingmethods;the comparisonof AFM andTEM imageson the sameareacould be of interestin interpretingthe AFM contrast.The useof electrostaticforce or Kelvin probe microscopy may alsoconfirm our analyzes.It will alsohelp in future studies devotedto theelaborationof moreorganizedstructures. 5. Acknowledgements

We are particularly indebtedto J.J. Allegraud for ultramicrotomyandH. Chanzyfor hishelpin preparingTEM specimen. 6. References

Fig. 3: AFM image in tapping mode of PANI(PPA)/CA, 0.4%. With the amplitude signal, the contrast is due to topographic modulation. PAN1 phase is emerging. Image size is @rn x 5.6pm

[l] C.Y. Yang. et al., Synth.Met. 53, 293(1993). [2] Reghu M. et aI., Phys.Rev.B 50, 13931(1994). [3] J.Planes,A. Wolter, Y. Cheguettine,A. Pron, F. Genoudand M. Nechtschein, Phys.Rev.B 58, (vol. 11, 15Sept. 1998) [4] A. Pron, Y. Nicolau,F. GenoudandM. Nechtschein,J. Appl. Polym.Sci. 63 (1997)971. [5] M. HasikandJ. Planes,unpublished