Thermoreversible Gels of Polyaniline: Viscoelastic and Electrical Properties
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Helsinki University of Technology, Department of Engineering Physics and Mathematics, P.O. Box 2200, FIN-02015 HUT, Espoo, Finland [email protected] Neste Oy, P.O. Box 310, FIN-06101, Porvoo, Finland VTT Electronics, Microelectronics, P.O. Box 1101, FIN-02044 VTT, Espoo, Finland
ABSTRACT We demonstrate that polyaniline (PANI) dissolved in dodecyl benzene sulphonic acid (DBSA) shows thermoreversible gelation. The dissolution has been performed in formic acid which allows particle-free complexes according to optical microscopy. Below the gelation temperature the materials are rubber-elastic in compression experiments, the storage modulus G' does not essentially depend on frequency, and the samples are electronically conductive. Above the gelation temperature, G' indicates flow-like behavior and drastically lower ionic conductivity is observed. These results suggest reversible, i.e. fusible, network formation. The properties are compared with gels consisting of camphor sulphonic acid (CSA) doped PANI dissolved in m-cresol which are poorly thermoreversible. INTRODUCTION In the recent literature of electrically conducting polymers, considerable attention has been paid on finding methods to process them with controlled properties. In polyaniline, solvents have been identified [1, 2] allowing blends consisting of PANI-networks [3], and molecular level deposition processes have been described [4]. Other efforts describe plasticizers to allow fusible bulk PANI-complexes [5] and blends with reversible network formation in melt blending [6]. On the other hand, much progress has taken place in understanding (thermo)reversible gelation: In such systems polymers form solutions at certain temperatures, whereas physically connected networks are formed upon proper temperature change for particular solvents [7]. For example 9.5 %-wt of atactic polystyrene in carbon disulphide forms a solution for T > ca. -15 0C, whereas for T < -15'C a reversible network is formed. Many types of physical crosslinks have been reported, such as micellar crystallites, sidechain crystallinity, formation of triple helices, or glassy aggregates [7]. There are established methods to study the phase behavior of thermoreversible gels by, for example, dynamic rheology, "Tilted test tube method", and "Falling ball method". Various gelling studies on emeraldine base form of PANI have been reported [8]. However, thermoreversible gels based on doped conductive polymers could combine viscoelasticity and electrical conductivity of networks and fusibility, on the other hand, in a challenging way. Astonishingly enough, only very recently such systems have been described: We showed that conducting thermoreversible gel is obtained if PANI emeraldine base is dissolved in DBSA using formic acid medium [9]. As a background, we describe viscoelastic 377
Mat. Res. Soc. Symp. Proc. Vol. 488 © 1998 Materials Research Society
and conductivity properties of dissolved in m-cresol.
poorly reversible gels consisting of CSA-doped PANM
EXPERIMENTAL The emeraldine base
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