ELSEVIER Synthetic Metals 101(1999) 722-723 Polyaniline blends cast from Hexafluoro-2-propanol .I. Fraysse, T. Olinga, J. Plan&, A. Pron, M. Nechtshein, LPMS, UMR 5819, DRFMC, CEA-Grenoble, I7 av. des Martyrs 38054 Grenoble Cedes 9, France Abstract Highly transparent and conductive polymer blends were solution processed from a four component system consistingof polyaniline/camphorsulfonic acid/diethylphtalate/1,1,1,3,3,3-hexafluoro-2-propanol, Special attention waspaid to the elaboration of appropriate processing conditions, Thecalculated percolation threshold for conductivitywas 0.41weight percent. Keywords: conductivity of blends, polyaniline, hexaJIlloro-i-~ropanoI , solution processitlg, evaporation, UV- Vis-NIR absotption. 1. Introduction Blendsof polyaniline(PANI) with industrialpolymers have attracted significant researchinterest in recent years. The principalmotivation of these studies was the fabrication of highly transparent conductors with good mechanical properties. Two approaches towards the preparation of blends with low percolation threshold wereused. Banerjee and Mandal [1,2] have used colloidalPAN1 which formeda self-assembled networkof the conductive phase nanoparticles upon the formationof solid films. Alternatively Caoet al. [3,4] using the concept of so called “counterion induced solubility” proved that PANI, if doped with appropriately functionalizedacid such as camphorsulfonic acid (CSA), canbe processed from m-cresol. Since m-cresol is a very goodsolvent for variety of commercial polymers, it is possible to fabricate conductive blend from thissolventby casting. Recently, blendsof PAN1 with plasticizedcelluloseacetate (CA) were prepared and characterized [5], [6]. Unfortunately complete removalof m-cresol from the blends prepared by solution processing is extremely difficult. Significant amountof this solvent remainin the solid film which in turn makes their characterisation more difficult and limits their applications. Hopkins et al [7,8] proposed 1,1,1,3,3,3-hexafluoro-2- propanol(HFIP) as a new PANI processing solvent. Indeed, processing of PANI(CSA)a5 leads to free standing filtis which exhibit metallic type conductivity down to 200 K 191. In addition HFIP can be efficiently removed from thefilm being cast. We were therefore tempted to verify whether HFIP is a suitable solvent for the fabrication of PANI based blends with low percolation threshold. In this communication we report the results obtained for PANIICSA)&CA blends. 2. Experimental PAN1 was prepared by oxidarive polymerisation using ammonium persulfate. The exact preparation procedure can be found elsewhere [lo] It was then converted to thebase form by treatment with 0.3 M aq. ammonia solution for 72 h. The protonation of PANI base with CSA in HFIP solutionwas carried out a RT under nitrogen atmosphere. Solutions 0.15 wt % with respect to PANI base weretypically prepared. Prior to use, the solutions were filtered through a 0.45ym Milipore filter. CA together with the plasticizer (diethyl phtalatc - DEP-) were dissolved separately in HFIP. Blendswere prepared by mixing both solution followed by casting on glass subbtrates. Differe~nt contents of the plasticizers weretested anddifferent evaporation rates were applied. UV-Vis-NIR spectra of solutions and films were recorded usinga Lambda 900 PerkinElmer spectrometer. Measurements of DC conductivity have been performed with a four probe method using gold thread contacts. 3. Results and discussion Two conditions must be fulfilled in order to obtain blends exhibitingmacroscopic conductivity: i) the conductive phase must form a continuous network. ii) the inherent conductivityof the conductivephase must be as highas possible. UV-Vis-NIR spectroscopy is an extremely sensitive tool for indirectestimation of the conductivityof protonated PAN1 since it probes the degree of charge carriers (polarons) delocalization. Usually thesamples exhibitinghighconductivity (ZOO-301)-S.cm“ at RT) of metallic type give spectra with monotonically increasing absorbance towards the NIR part of the spectrum, In Fig. 1 the spectraof the PANIICSA),l,SICA/DEP blends-e presented. Since neitherCA nor DEP absorbs in this spectral region the spectra can be consideredas characteristicof PANI(CSA)~,S in the plasticized CA matrix. It is clear that the shape of the spectrum depends strongly on the solvent evaporation rate. Free evaporation la few minutes) in air atmosphere at RT leads to the spectra indicative of significantly worsepolaron delocalization which implieslower conductivity. To the contraryslow evaporation (2 days) in the atmosphere of HFIP vapourscloseto saturation at RT results in a different spectrum where charge carriers delocalization is clearly manifested in the NIR part. As a resultthe conductivity of this film is much higher (0.1 S.cm” for PANI content 1 wt %). In all subsequent experiments slow evaporation rateat RT was applied. Macroscopic conductivity is not only influenced by the processing conditions but also by the content of the plasticizer. 0379~67791991% - see frontmatter 0 1999 Elsevier Science S.A. All rights reserved. PII: SO379-6779(98)01218-1